Ảnh Hưởng Của Trường Tương Tác Lên Độ Nhạy Của Cảm Biến Hall Phẳng

I HC QUC GIA H NI

TRNG I HC CNG NGH

L c Anh

NH HNG CA TRNG TNG TC LN NHY CA CM BIN HALL PHNG

KHO LUN TT NGHIP I HC H CHNH QUY

Ngnh: Vt l k thut

H NI - 2010

I HC QUC GIA H NI TRNG I HC CNG NGH

L c Anh

NH HNG CA TRNG TNG TC LN NHY CA CM BIN HALL PHNG

KHO LUN TT NGHIP I HC H CHNH QUY

Ngnh: Vt l k thut

Cn b hng dn: TS. Trn Mu Danh

Cn b ng hng dn: ThS. Bi nh T

H NI - 2010

Li cm n

Trc ht em xin c by t lng bit n su sc ti thy gio TS. Trn Mu Danh. Thy du dt em trn con ng khoa hc, thy lun ng vin gip em trong nhng lc kh khn nht. Em xin cm n nhng kinh nghim qu gi m thy dy bo em em c th t hon thin mnh tr thnh ngi c ch.

Em xin c gi li cm n n Thc s Bi nh T, ngi thy, ngi anh rt mc knh trng. Nu khng c s hng dn tn tnh, nhng li ng vin, nhc nh v gip ca anh th em khng th hon thnh kha lun tt nghip ny.

Trong sut nhng nm thng hc tp v nghin cu ti khoa Vt l k thut v cng ngh nano, em c to mi iu kin thun li thc hin cng vic hc tp v nghin cu ca mnh, ng thi em cng nhn c s quan tm ca cc thy, c gio. Em xin gi li cm n chn thnh ti s gip .

Em cng xin chn thnh cm n ti cc thy c gio, anh ch trong phng th nghim Vt liu v Linh kin t tnh nano trng i Hc Cng Ngh to iu kin v gip em rt nhiu trong thi gian va qua.

Cui cng vi lng bit su sc v tnh yu chn thnh nht, em xin c gi ti nhng ngi thn trong gia nh em, c bit l cha m em lun bn cnh em trong hc tp cng nh trong cuc sng.

Tm tt ni dung

Kha lun ny cp n cc loi cm bin t in tr. Trong chng ti tp trung i su vo vic m phng, nghin cu cc thng s ca cm bin da trn hiu ng Hall phng. Bng cch ch to cm bin c cu trc spin-valve vi cc gi tr ca trng tng tc khc nhau, chng ti kho st hiu ng Hall phng tm ra cu trc hot ng tt nht. Kt t vic m phng nhy theo s thay i ny, cho thy rng nng lng tng tc cng nh th nhy cng cao. Chng ti cng so snh vi kt qua o thc nghim. t tm ra ch lm vic n nh cho cm bin. Cm bin Hall phng vi nhy cao, n nh, t s tn hiu trn nhiu ln em li nhiu ha hn trong ng dng y sinh.

Mc lc Trang

M u............................................................................................................................1 Chng I. Tng quan v cm bin sinh hc................................................................3 1.1. Gii thiu chung .......................................................................................................3 1.2. Nhng kiu biosensor truyn thng .........................................................................4 1.3. Cm bin sinh hc theo cng ngh in t hc spin ................................................5

1.3.1. Nguyn l chung: ..............................................................................................5 1.3.2. u im ca cm bin sinh hc s dng cng ngh in t hc spin ..............6 1.3.3. Nhng kiu cm bin sinh hc da trn cng ngh in t hc spin ...............7

1.3.3.1. Cm bin sinh hc da trn hiu ng t in tr d hng (AMR Biosensor) ...............................................................................................................7 1.3.3.2. Cm bin sinh hc da trn hiu ng t in tr khng l (GMR Biosensor) ...............................................................................................................8 1.3.3.3. Cm bin sinh hc da trn hiu ng Hall phng (Planar Hall Biosensor)................................................................................................................................9 1.3.3.4. Cm bin sinh hc da trn hiu ng van-spin (Spin-valve Biosensor) .11 1.3.3.5. Cm bin sinh hc da trn hiu ng t in tr xuyn ngm (TMR Biosensor) .............................................................................................................12

1.4 Tng kt ...................................................................................................................13 Chng II. Tng quan v cm bin Hall Phng .......................................................15 2.1. Hiu ng Hall phng...............................................................................................15 2.2. Nng lng t v m hnh Stonner Wohlfarth....................................................16

2.2.1. Cc dng nng lng t ..................................................................................16 2.2.1.1. Nng lng trao i .................................................................................16 2.2.1.2. Nng lng d hng t tinh th .............................................................16 2.2.1.3. Nng lng t n hi .............................................................................19 2.2.1.4. Nng lng tnh t...................................................................................22

2.3. Cm bin Hall phng vi cu trc khc nhau.........................................................22 2.3.1. Cm bin Hall phng vi cu trc spin-vale...................................................22 2.3.2. Cm bin Hall phng vi cu trc GMR ........................................................24

2.4. Tng kt ..................................................................................................................25 Chng III. Kt qu m phng s nh hng ca trng tng tc ln nhy ca cm bin v gii thch...........................................................................................25

3.1. M phng s ph thuc ca th VPHE vo t trng ngoi khi thay i t trng dch HJ. ...................................................................................................................26 3.2. M phng s ph thuc ca th VPHE vo t trng ngoi khi thay i t trng d hng HK .............................................................................................................30 3.3. S nh hng ca vic thay i gc gia t trng ngoi H v dng qua cm bin I ..............................................................................................................34 3.4. So snh kt qu m phng v kt qu thc nghim ...............................................36 Kt lun chung .............................................................................................................38 Ti liu tham kho.......................................................................................................39

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M u S nhn bit c tnh chn lc v m t nh lng ca tt c cc loi phn t sinh

hc ng vai tr quan trng trong khoa hc sinh hc, trong chun on lm sng, nghin cu y t, v c trong vic kim sot nhim mi trng. Cho ti nay th, phng php ph bin vn l ly mu ti hin trng cn phn tch, sau bo qun v a v cc phng th nghim phn tch. thc hin c iu ny, yu cu phi c nhng phng th nghim hin i, m trong phi trang b cc thit b hin i v t tin. Cng vi l phi c nhng b phn cn b c nng lc chuyn mn cao c th thc hin, nh gi cc kt qu ca vic phn tch. V d: phn tch mt mu mu ta cn phi tri qua rt nhiu bc v s dng nhiu loi my mc hin i bao gm phn tch v cng nh nhng chuyn i v d tm ca nhng phn t ho hc m ta nghi vn .v.v.

Gn y, tng ca vic tch hp tt c nhng qu trnh phn tch trn thnh mt thit b cm tay d s dng, c th cho kt qu ngay lp tc ti v tr cn phn tch, nhn c rt nhiu s quan tm t cc nh nghin cu v cc cng ty cng ngh sinh hc. T mt h thng dng lab-on-chip c tn biosensor c a ra n gin ho c hiu qu nhiu nhim v trong cc lnh vc iu tr y t hoc nghin cu sinh hc, v thm ch c th m ra nhng ng dng hon ton mi.

Biosensor l mt thit b pht hin, nhn dng, v truyn thng tin v mt s thay i sinh-l, hay s c mt ca cc cht ha hc khc nhau, hoc nhng vt liu sinh hc trong mi trng. K thut hn, biosensor l mt my d bao gm mt phn t sinh hc (ging nh enzyme hay cc khng th), v mt phn t in c th chuyn tn hiu thnh tn hiu o c. biosensor c rt nhiu hnh dng v kch thc khc nhau, c thay i ty theo iu kin mi trng. Chng c th pht hin v o chnh xc nhng ni tp trung ca vi khun hay nhng cht ha hc nguy him.

Biosensor s dng nhiu phng php d tm khc nhau, tuy nhin trong tt c th vic da trn nguyn l ca s lai ha, cho php mt s phn tch song song cao ca nhiu phn t sinh hc khc nhau v mi vng ca sensor c mt chc nng c th khc nhau. Biosensor c th c chia lm hai kiu chnh: mt l vn s dng phng php nh du, mt l th s dng phng php pht hin s lai ha trc tip.

Trc y, phng php chnh l s dng phng php d tm hunh quang (biosensor hunh quang). Tuy nhin mt vi nm tr li y, vi s pht trin mnh m ca mt cng ngh mi: spintronic (in t hc spin). to ra mt s pht trin mi cho cc chp sinh hc spintronic vi u im vt tri l nhy cao hng ng nhanh d tch hp, d t ng ha thay th vic nh du bng hunh quang truyn thng t tin. Bng cch s dng ht t c iu khin bi dng in ta c th phn tch c nhiu mu sinh hc.

Chng ta c th s dng ht t pht hin cc tng tc sinh hc. Vic d tm cc ht t c th s dng cm bin t in tr d hng (AMR), cm bin t in tr khng l (GMR), cm bin spin-valve, cm bin in tr Hall mt phng (PHR), cm

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bin t in tr xuyn ngm (TMR). Hu ht cc cm bin t in tr u da trn hiu ng t - in tr. c bit, khi d tm cc ht t chng ta quan tm n t s tn hiu trn nhiu (signal-to-noise), th cm bin Hall phng chim u th hn hn (S/N=1450) [2]. Nn trong bi kha lun, chng ti l tp trung nghin cu cu trc nguyn tc hot ng, cc thng s ca cm bin Hall phng.

Bi kha lun gm 3 chng. - Chng 1. S ni v cc phng php d tm cc phn t sinh hc, trong tp

trung vo 2 phng php chnh: D tm bng phng php hunh quang v phng php d tm da trn hiu ng spintronic. Cc loi cm bin t in tr, nguyn tc hot ng v c im tn hiu li ra cng c th hin trong chng ny.

- Chng 2. Chung ti i su vo nghin cu hiu ng Hall phng, cc thng s cho hiu ng ny, cng thc tnh th ni ra v nhy.

- Chng 3. Chng ti s dng cc cng thc chng 2 i vo m phng s ph thuc ca nhy vo trng tng tc.

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Chng I. Tng quan v cm bin sinh hc

1.1. Gii thiu chung S nhn dng phn t sinh hc v ang ng mt vai tr quan trng trong vic

chm sc sc khe, cng nghip dc phm, phn tch mi trng v nhng ng dng cng ngh sinh hc rng ri khi c ng dng cho s lai ha DNA-DNA (chun on bnh di truyn, pht hin t bin gen) v s tng tc gia khng th v khng nguyn (pht hin vi sinh vt, pht hin tc nhn sinh hc gy chin tranh v.v). Trong nhng trng hp ny, vic pht hin chnh xc s tng tc gia hai phn t sinh hc vi cu trc ging nhau l c thc hin nh s dng cm bin sinh hc (biosensor).

Cc cm bin sinh hc gm c 2 thnh phn chnh l thnh phn nhn bit tn hiu sinh hc v thnh phn chuyn i. Phn nhn bit tn hiu sinh hc ging nh mt phn t sinh hc, n nhn dng cc tng tc sinh hc. Ngc li b chuyn i s bin i tn hiu nhn c thnh tn hiu in o c. V hai thnh phn ny s c tch hp vo thnh mt cm bin ta c th thy trn Hnh 1.1., s kt hp ny cho php n c th o mc tiu cn phn tch m khng cn s dng thuc th. V d: Lng ng trong mt mu mu c th c o trc tip bi mt biosensor, bi ch cn nhng cm bin vo mu th. iu ny th l tri ngc vi phng php phn tch thng thng l phi tri qua nhiu bc v mi bc li cn phi dng n thuc th x l mu. S n gin v tc ca php o l mt thun li ca biosensor.

Mu cn phn tch Tn hiu ra

Hnh 1.1. S mt cm bin sinh hc

Trc y, biosensor thnh cng vi phng php nh du hunh quang.

Tuy nhin nh c s pht trin ca in t hc spin. Thay v nhn bit cc phn t sinh hc bng cc cng c t tin nh cc h qut hunh quang quang hc hay lade, chng ta c th s dng cc loi cm bin ng dng cng ngh in t hc spin. Da trn cc hiu ng GMR, AMR, TMR, Hall and Planar Hall, v.v.

Phn t

Sinh hc Phn t in

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Trong chng ny ti s a ra mt ci nhn tng quan v mt s kiu cm bin sinh hc (biosensor)in hnh c pht trin cho nhng ng dng sinh hc. 1.2. Nhng kiu biosensor truyn thng

Trc y loi cm bin ph bin nht l cm bin sinh hc s dng phng php hunh quang v cu to chung ca mt cm bin sinh hc s dng phng php hunh quang in hnh s nh sau:

- Mt dy cc u d c gn c nh trn b mt cm bin bng nhng chm micro (thng l cc ht hunh quang).

- Bung lai ha (thng l l mt h thng vi rnh hay cn gi l vi knh cha cht lng c kch thc micro).

- Mt c cu sp xp cc DNA ch ty chn theo dy (to in trng cho cc phn tch phn t tch in nh DNA hoc cc dy ng dn to t trng cho cc DNA ch gn ht t).

- Cc ht d tm. Trn Hnh 1.2. m t qu trnh d tm bng phng php nh du hunh quang,

gm 3 giai on: - C nh u d trn b mt chip. - Nh dung dch c cha cc DNA ch cn d tm. - Cc phn t sinh hc l phn b ca nhau s lin kt vi nhau, qu trnh lai ha

xy ra v sau ra sch cc phn t khng lin kt.

Hnh 1.2. S d tm qu trnh lai ha s dng ht hunh quang gn vo cc i tng

sinh hc v my qut hunh quang laze d tm.

Phng php ny ta c th bit c s lng gen xc nh v so snh s khc nhau gia cc mu cn phn tch. S d tm ny khng nhng bit c s c mt ca phn t b bnh hay khng m ta c th bit thm c s lng ca cc phn t ny.[1]

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1.3. Cm bin sinh hc theo cng ngh in t hc spin 1.3.1. Nguyn l chung:

Mt chip sinh hc (biochip) s dng cng ngh spin in t c bn gm c mt dy cc phn t cm bin (nh cc cm bin t-in tr); mt dy cc u d (cc phn t sinh hc bit nh cc chui nucleotide c trng ca cc gen hoc cc khng th) c c nh trn b mt ca cc sens (thng qua cc chm c kch thc micro hoc cc dy c sp xp theo c trng in hoc t); mt bung lai ha (thng l mt b rp ni cc rnh cha cht lng c kch thc micro); v mt c cu dng sp xp cc bia (target) ty chn theo dy (to in trng cho cc phn tch phn t tch in nh DNA hoc cc dy ng dn to t trng cho cc bia c gn ht t) (Hnh 1.3).

Hnh 1.3. S mt biochip s dng cng ngh in t hc spin. Cc i tng d tm (phn t sinh hc trong mu dng nhn dng nh chui

DNA phn b ph hp ca u d DNA c nh, hoc cc khng nguyn tng ng vi cc khng th c nh) c nh ln trn b mt chip qu trnh nhn dng c tin hnh. Cc phn t sinh hc c th c gn ht t tnh trc hoc sau bc lai ha (recognition). Cc ht t thng l cc ht siu thun t hoc st t khng c t d trong thin nhin vi kch c nano hoc micro mt v c kh nng gn kt vi cc phn t sinh hc. Di tc dng ca t trng, cc ht ny s b t ha v t tng cng xut hin. T trng sinh ra t cc ht t b t ha c th thay i in tr ca cm bin s dng cng ngh spin in t, do c th gip chng ta nhn bit c cc phn t sinh hc cn phn tch.

Cc chp sinh hc (biochips) da trn hiu ng t in tr c gii thiu ln u vo nm 1998 phng th nghim nghin cu hi qun (NRL) ca M. Sau trn th

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gii pht trin thm nhiu phng nghin cu v cc cng ty pht trin h thng ny. Vic nhn bit ht t c hon thin bng cch s dng cc cm bin tch hp t in tr c cu trc v hnh dng khc nhau nh GMR hnh que, cu trc GMR hnh gp khc (meander GMR structures) v GMR hnh xoy c; cc cu trc van spin ng thng, hnh rng lc v hnh ch U; cc vng AMR; cm bin hnh ch thp s dng hiu ng Hall mt phng; v cc tip xc t xuyn ngm. Cc cu trc ny cn cho php s dng t trng iu khin chnh xc v cc thao tc trn chip, kt hp s truyn dn tn hiu vi vic d tm.

Nguyn l ca biochip s dng cng ngh spin in t c s dng d tm cc biu hin ca cc phn t sinh hc (bao gm c cc lin kt sinh hc) trong cc m hnh lin kt nh lin kt biotin-streptavidin, immunoglobulinG - Protein A v AND - cADN (v d cystic fibrosis - bnh x nang), trong cc pht trin ng dng dng cho vic d tm cc cht c trong v kh sinh hc v gn y nht l ng dng trong vic d tm cc t bo t vi sinh vt gy bnh. Cu trc ca hai chip s dng s lai ha c h tr ca t trng v vic d tm cc ADN cn d c lin quan ti bnh x nang l kt qu thu c trong qu trnh nghin cu th nghim chip vi cc DNA phn b vi cc DNA cn d tm. Sau khi nh cc phn t sinh hc c nh ht t ln b mt cm bin, mt dng in c t vo trong khong 3 pht thu ht cc ht vo khu vc cm nhn, sau cc ht t c gi n nh trong vng 3 pht qu trnh lai ha din ra. Chip c ra loi b cc ht t khng c lin kt ring hoc lin kt yu. Khi ngi ta thu c tn hiu cn li vo khong 1mV do lai ha. Tn hiu ny tng ng vi khong 50 ht nano lin kt vi b mt. Khi s dng cc phn t sinh hc cn d khng phi l phn b ca u d, tn hiu tr li vi ng nn ngha l khng c s lai ha xy ra. Cc cm bin c nh (2 6 mm2) c di hot ng nh cha c vo khong 200 ht nano vi ng knh 250mm, nhng cho tn hiu trn tng ht ln hn. [5] 1.3.2. u im ca cm bin sinh hc s dng cng ngh in t hc spin

Tt c cc thit b in t hc spin (spintronics) bao gm c nhng cm bin in t hc spin u da trn vic iu khin cc spin ca in t, ln c nhng thun li nh sau:

- Tiu th t nng lng: do qu trnh bin i trong cc thit b spintronics da trn s i chiu ca cc spin.

- Do tnh cht phi t ca cc phn t sinh hc nn gim tn hiu nhiu. - C n nh cao, php o c th thc hin c nhiu ln, v loi b tn hiu

nn khng mong mun. - Tc nhanh v khng phi mt thi gian truyn in tch. Thi gian o

cc spin t trng thi up v down ngn.

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1.3.3. Nhng kiu cm bin sinh hc da trn cng ngh in t hc spin 1.3.3.1. Cm bin sinh hc da trn hiu ng t in tr d hng (AMR Biosensor)

Miller l ngi u tin gii thiu phng php d tm cc ht s dng hiu ng AMR vo nm 2002. Hiu ng t in tr d hng (AMR) l hin tng tng in tr di tc dng ca t trng ( hay ni chnh xc hn l di tc dng ca cm ng t B) do lc Lorentz tc dng ln cc ht ti in. V bn cht hiu ng AMR chnh l s ph thuc in tr vo gc gia vect t v chiu dng in. Nguyn nhn xut hin hiu ng ny l do xc sut tn x in t s-d s khc nhau theo phng t trng tc dng. Hiu ng ny ln nht khi t trng tc dng song song vi chiu dng in.

Hnh 1.4. Vng cm bin AMR d ht t (a); Trng thi in tr nh nht khi dng in I song song vi t M ca vng (b); Trng thi in tr ln nht khi dng in

I vung gc vi t M ca vng (c).

Nguyn tc hot ng ca cm bin AMR l da vo s tn x ca in t theo hng mmen t ca vt liu lm cm bin. Trong trng hp ny, cm bin AMR c cu trc l mt vng kim loi st t (NiFe), khi khng c t trng ngoi tc dng t ca vng l mt ng trn khp kn nh Hnh 1.3. (b), trong trng hp ny nu t mt dng in chy qua cm bin th dng in c th chy qua d dng, do hiu ng AMR ca vng s l ln nht. Ngc li, khi c ht t vi mmen t vung gc vi b mt ca cm bin, t ti tm ca cm bin th t ca vng s hng tm nh hnh 1.3c, vung gc vi dng in v cn tr s di chuyn ca cc in tch khi chy qua vng cm bin, hiu ng AMR ca vng lc ny l nh nht.

Trn cc vt liu st t nh Fe, Co, Ni v hp kim ca chng hiu ng ny thng kh ln so vi vt liu khng t.

Thit b ny thch hp trong vic d tm cc ht n l. Cc ht t t trung tm ca vng trn NiFe vi bn knh bn trong ca vng trn ph hp vi bn knh ca ht. S chuyn i ra tn hiu in ca cm bin c xc nh: VS = -(R/R)s I Rsq (2Rav/h) (/Hk)2 (1.1)

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Trong : - R/Rs l t s t in tr bo ha (l s khc bit gia in tr ca cm bin

khi cc lp t sp xp phn song song v song song chia cho in tr nh nht).

- h = Rout - Rin - Rav l bn knh trung bnh. - I l cng dng qua sensor. - Rsq =/t in tr mt (in tr vung). - l in tr ca sensor. - t l dy ca sensor. - Hk l hng s d hng ca lp st t. - l gi tr trung bnh t trng ca ht t.[1]

1.3.3.2. Cm bin sinh hc da trn hiu ng t in tr khng l (GMR Biosensor)

Nm 1998, Baselt l ngi u tin xut ra cm bin t in tr d tm s c mt ca ht c kch thc micro. Cu trc ca 1 cm bin GMR chun bao gm 3 lp vt liu (lp st t (FM)/ lp phi t (NM)/ lp st t (FM)). trng thi ban u (khi cha b t ha theo t trng ngoi) mmen t ca 2 lp st t nh hng phn song song vi nhau. trng thi ny cc in t b tn x nhiu khi i qua cc lp vt liu ca cm bin do in tr ca cm bin ln nn tn hiu in mch ngoi l nh (Hnh 1.5.a). Di tc dng ca t trng ngoi, t ca lp Fe t c xu hng nh hng li song song vi nhau theo phng ca t trng. ng thi vi qu trnh quay ca vector t , in tr ca mu gim mnh (in t khi chy qua cc lp ca cm bin s t b tn x ) nn to ra c tn hiu in ln mch ngoi (Hnh 1.5. (b)).[1]

Hnh 1.5. Cm bin GMR , a) trng thi in tr thp v b)trng thi in tr cao ca

cm bin GMR .

ng cong p ng ca cm bin c bin din nh hnh v.

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Hnh 1.6. Hiu ng t in tr khng l c biu din bng t s R/R(H=0) ca mng mng a lp (Fe/Co).

S chuyn i ra tn hiu in ca cm bin c xc nh : VS= -( R/R)s)IRspW(/hHk) (1.2) Trong : - R/Rs l t s t in tr bo ha. - W, h tng ng l chiu rng v dy ca sensor. - I l dng qua sensor. - Rsp=/t : vi l in tr sut ca sensor, t l dy ca sensor. - Hk l hng s d hng ca lp st t. - l gi tr trung bnh t trng ca ht t. Cm bin GMR biu din hng s Hooge cao hn so vi cm bin Spin-valve v

AMR, c sinh ra bi s lng ln ca ht t ln b mt v s phc tp hn ca cu trc vi t tnh. Hng s Hooge c tnh l 1. T s S/N ti tn s thp l khong 382, v t trng nh nht m cm bin c th cm nhn c l khong 93nT.[1] 1.3.3.3. Cm bin sinh hc da trn hiu ng Hall phng (Planar Hall Biosensor)

Da vo s tn x ca in t theo phng t ca lp st t. Khi cho dng in I chy qua cm bin theo hng x, th in t s b tn x theo hng ca t M to ra in trng E theo hng ca t M. in trng E ny to ra hiu in th V theo hng y vung gc vi dng in (Hnh 1.7. - 1.8.).

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Hnh 1.7. Cu trc hnh hc ca cm bin Hall phng.

Hnh 1.8. ng c trng ca in p Hall phng theo t trng c m phng

theo m hnh Stonner wohlfarth. Vi m hnh ny, t ca lp NiFe trng thi tnh phi nm dc theo hng

ca dng in. Tr khng thay i R/R khong 23% vi lp NiFe dy 2030 nm. ng cong p ng c biu din Hnh 1.8.

Trong vng t trng nh, sensor lm vic trong vng tuyn tnh. Ch cn mt t trng nh ta d dng nhn c gi tr ln nht ca in th PHE. Do vy ta c th chn vng lm vic ca cm bin l on tuyn tnh ca ng c trng t - in tr v thng qua tn hiu u ra ta c th tnh ton nh lng c s lng cc ht.

S chuyn i ra tn hiu in ca cm bin c xc nh:

VS = - I R (/Hk) (1.3) Trong :

- R = (// - )/t ,

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- // , l in tr ca dng qua cm bin song song v vung gc vi vector t , t l dy ca mng mng t.

- Hk l hng s d hng ca lp st t. - l gi tr trung bnh t trng ca ht t. Gi tr trung bnh t trng ca ht t trn mt ht t l 0.38 Hmax. Vi mt dng

tng ng vi dng s dng trong cm bin spin-valve th th ra ca cm bin thp hn 6 ln. Hng s Hooge l 10-2, thp hn 5-10 ln so vi cm bin spin-valve. T s tn hiu trn nhiu ti tn s thp l 1450. N c th nhn bit trong vng t trng nh nht l 32nT.[1] 1.3.3.4. Cm bin sinh hc da trn hiu ng van-spin (Spin-valve Biosensor)

Cu trc chun ca cm bin van-spin bao gm 4 lp vt liu (lp phn st t/ lp st t b ghim/ lp phi t/ lp st t t do). Hai lp st t c ngn cch nhau bi mt lp kim loi khng t, trong 1 lp st t t do, 1 lp c ghim bng tng tc trao i vi 1 lp vt liu phn st t. Khi cha c t trng ngoi tc dng, t ca lp st t t do ngc chiu vi t ca lp st t b ghim, do in t khng di chuyn qua cc lp ca cm bin c, v vy in tr ca cm bin l ln. (Hnh 1.9. (a)). Khi c t trng ngoi (t trng ca ht t), mmen t ca lp st t t do s quay theo hng t trng ngoi, lm cho t ca lp st t t do v t ca lp st t b ghim nh hng song song vi nhau, do cc in t c th truyn qua cc lp ca cm bin (Hnh 1.9. (b)) v in tr ca cm bin gim.

Trong trng thi tnh, t ca lp ghim nm theo chiu ngang, c ghim bi lin kt trao i gia lp ghim vi lp phn st t, cn t ca lp t do hng theo chiu dc. S nh hng theo chiu dc ca lp t do v trng thi n domain l do d hng hnh dng.

Hnh 1.9. Cm bin spin van d ht t.

12

S chuyn i ra tn hiu in ca cm bin c xc nh: VS = -(1/2)( R/R)s I Rsq W (/hHk) (1.4) Trong : - R/Rs l t s t in tr bo ha. - W, h tng ng l chiu rng v dy ca sensor. - I l cng dng qua sensor. - Rsq =/t in tr mt (in tr vung). - l in tr ca sensor. - t l dy ca sensor. - Hk trng d hng hiu dng. - l gi tr trung bnh t trng ca ht t.[1]

1.3.3.5. Cm bin sinh hc da trn hiu ng t in tr xuyn ngm (TMR Biosensor)

Cu trc chun ca cm bin TMR bao gm 3 lp vt liu (lp st t/lp in mi/lp st t). Hot ng tng t nh cm bin GMR, khi cha c t trng ngoi, th t ca 2 lp st t ban u l phn song song vi nhau, do in t b tn x nhiu v khng th truyn qua cm bin (Hnh 1.10. (a)). Khi c t trng ngoi, t ca 2 lp st t s nh hng song song vi nhau, nn in t t b tn x v c th xuyn qua cc lp ca cm bin, to ra tn hiu in (Hnh 1.10. (b)).

Hnh 1.10. S ca cm bin TMR c bn tm cc ht t vi t song song vi b mt ca cm bin.

S chuyn i ra tn hiu in ca cm bin c xc nh : VS = -(1/2)( R/R)s I Rsq RA (/WhHk) (1.5) Trong :

13

- R/Rs l t s t in tr bo ha. - W, h tng ng l chiu rng v dy ca sensor. - I l cng dng qua sensor. - Rsq =/t in tr mt (in tr vung) vi l in tr ca sensor, t l dy

ca sensor. - Hk trng d hng hiu dng. - l gi tr trung bnh t trng ca ht t. - R l in tr ca cm bin. - A l din tch tip xc. Trong cm bin cu trc xuyn ngm, dng chy qua cm bin c gii hn bi

th nh thng. Ch tip xc phi c ti u ha sao cho R*A l thp v duy tr c t s t tr xuyn hm cao trong khi mc nhiu l thp nht.[1] 1.4 Tng kt

Trong chng ny, ti nu mt s nhng kiu cm bin sinh hc in hnh theo kiu truyn thng v hin i. Trong ti nhn mnh v chi tit vo phn cm bin sinh hc da trn ng dng ca cng ngh in t hc spin. thy c s khc bit gia cc loi cm bin sinh hc. V vi mc ch chnh l nhm tm ra c kiu cm bin sinh hc thch hp vi mc ch nghin cu ca kha lun ny. Chi tit hn, mc ch l tm ra kiu cm bin sinh hc cho nhy cao, t s tn hiu trn nhiu ln. Cng vi vic tham kho cc kt qu ca nhng nh nghin cu khc qua Bng 1.1.

Bng 1.1. Cc thng s c trng ca cm bin t in tr.[1]

Loi cm bin I (mA)

S (V/Oe) S/N

Bmin (nT)

Vng AMR 10 2 50 26

Hall phng 10 15 1450 32

Spin van 10 87 442 54

GMR 5 13 382 93

MTJ 1 10 114 202

14

Ti thy rng cm bin Hall phng l s la chn thch hp nht. V t cc thng s a ra ta thy cm bin sinh hc theo kiu ny c nhy ln v t s tn hiu trn nhiu cng ln. V phn tip theo ti s i vo kho st kiu cm bin sinh hc ny vi nhng cu trc khc nhau tm ra cu trc tt nht.

15

Chng II. Tng quan v cm bin Hall Phng

2.1. Hiu ng Hall phng Cm bin Hall phng l c da trn hiu ng Hall phng ca nhng vt liu

st t. C cu hnh o 4 mi d (dng hnh hc) ging vi hiu ng Hall thng v hiu ng Hall d thng, nhng v bn cht th hiu ng Hall phng, t trng ngoi phi t song song vi mt phng mu. V n ph thuc vo gc gia t ca mu v chiu dng in.

Hiu ng Hall phng c tm thy trong vt liu t khi in tr ca vt liu ph thuc vo gc gia phng ca mt dng in J v t ca mu M. Di tc dng ca dng Ix t theo phng x (ban u khi cha c t trng ngoi th t M ca mu s song song vi dng in Ix), v t trng ngoi B hp vi dng in Ix mt gc th vc t t ca mu M nm trong mt phng ca cm bin s lch mt gc so vi phng mt dng in Jx, khi s c th ra Vy xut hin theo phng y.

Hnh 2.1. Cu trc hnh hc cm bin Hall phng.

Theo nh lut Ohm in p Hall phng sinh ra trong cu trc n domain theo

hng y l:

Vy = Jxw Rsin.cos (2.1) R = (// - )/t vi // , ln lt l in tr sut ca mu o theo phng

song song v vung gc vi t , t l chiu dy tng cng ca mng.

16

Tuy nhin nghin cu v hiu ng Hall phng trong cc cm bin Hall, ngi ta thng s dng m hnh Stonner Wohlfarth. Do vy phn tip theo ta s i tm hiu cc dng nng lng t v m hnh Stonner Wohlfarth tng qut. 2.2. Nng lng t v m hnh Stonner Wohlfarth 2.2.1. Cc dng nng lng t 2.2.1.1. Nng lng trao i

Nng lng tnh in ca tng tc trao i c m t bi phng trnh:

ij

ijW 2 ( )td i jJ S S=

(2.2)

Hay nu gi thit i jS S S= = :

2

ij ijij

W 2 ostd J S c = (2.3)

y l cc vecto spin in t ca cc nguyn t ,i j trong cc n vh .

Tch phn trao i ijJ ph thuc vo t s gia hng s mng a v ng knh hiu dng d ca lp v in t d. Khi ijJ >0, Wtd cc tiu khi / /i jS S (trng thi st t). biu thc trn c th vit li:

2

ij ijij

W 2 ostd J S c = (2.4)

y ij l gc gia cc vecto iS vi jS v ta gi thit l tch phn trao i J nh nhau i vi tt c cc cp ion. Tng c ly theo tt c cc nguyn t ln cn.

Nng lng trao i l ng hng (isotrophic), n ch ph thuc vo tng tc tnh in ca cc in t v khng ph thuc vo gc gia cc momen sin v phng trc tinh th.[3] 2.2.1.2. Nng lng d hng t tinh th

ng cong t ha dc theo cc phng khc nhau ca cc n tinh th Fe, Ni, Co l khc nhau.

Phng m t ha t n bo ha d dng nht gi l phng t ha d, hay phng d (easy direction). Phng m s t ha kh t c bo ha nht (ch l bo ha t trng cao) g l phng t ha kh, hay phng kh (hard direction).

Cc tinh th c mt phng t ha d c gi l st t n trc (uniaxial). Cc tinh th c nhiu phng t ha d gi l st t a trc.

Th d: St (Fe) l st t 3 trc v c cc phng d [100], [010], [001]; Nickel (Ni) l st t 4 trc vi 4 trc d l cc phng loi [111]; Cobalt (Co) l st t n trc vi trc d l phng loi [001]. Hp cht R-Co, R-Fe (R=t him) thng c cu trc lc gic (hexagonal) hoc t gic (tetragonal) v c d hng t cao. V d:

5 17 2, ,SmCo SmCo Nd Fe c d hng rt cao v l cc tinh th n trc t (trc d l trc c hay [001]).

17

Nh vy cng t ha vt liu ph thuc vo phng t trng ngoi i vi trc tinh th, tc l quay vecto M theo phng H ta phi thng nng lng lin kt ca M vi trc tinh th. Nng lng lin kt ny gi l nng lng d hng t tinh th (magnetocrystalline anisotropy), k hiu l aW .

HA M [100] Hnh 2.2. Mmen t di s nh hng ca t trng ngoi v d hng t. Xt nng lng d hng t tinh th mt cch v m (hay mt cch hin tng

lun). Ta biu din nng lng d hng t tinh th theo cc cosin ch phng ca gc gia vecto t t pht sI v cc phng ca trc tinh th. Gi thit phng trc x, y, z trng vi phng tinh th ca mng lp phng: [ ] [ ] [ ]100 , 010 , 001x y z , nng lng aW c dng:

( ) ( ) ( )( ) ( )( ) ( )( )

2 2 20 1 1 2 3 2 1 2 2 3 3 4 3 1 2 3

2 2 2 3 2 2 3 3 34 1 2 1 3 2 1 2 3 3 1 3 2 5 1 2 3

4 4 4 2 2 2 2 2 26 1 2 3 7 1 2 2 3 1 3

3 3 3 3 3 38 1 2 1 3 2 1 2 3 3 1 3 2

...

aW B B B B

B B

B B

B

= + + + + + + + + + ++ + + + + + + +

+ + + + + ++ + + + +

+ + (2.4) Trong mt tinh th lp phng do hiu ng chn, khi +M-M nng lng t

khng i tc l khng ph thuc vo du ca cc cosin ch phng 1 , 2 , 3 , do cc s hng cha 1 bc l phi bng 0 v:

( )( )( )

2 2 20 3 1 2 3

4 4 46 1 2 3

2 2 2 2 2 27 1 2 2 3 1 3 ...

aW B B

B

B

= + + + ++ + ++ + + + (2.5)

18

Ta cn c th n gin ha biu thc trn, nu xt n ( ) ( )22 2 2 2 2 21 2 3 1 2 31 + + = = + +

V ( ) ( ) ( )22 2 2 4 4 4 2 2 2 2 2 21 2 3 1 2 3 1 2 2 3 1 32 1 + + = + + + + + = , tc l hai s hng ny c th thay bng mt s hng v do thay cho ba s hng bc 4 ta gi li mt s hng bc 6 ca biu thc trn ta ch cn gi li mt s hng bc 6 (chng hn 2 2 2 2 2 21 2 2 3 1 3 + + ). Tng t trong ba s hng bc 6 ca biu thc trn ta ch cn gi li mt s hng bc 6 (chng hn 2 2 21 2 3 ). Biu thc d hng t tinh th do c th c vit di dng:

( )2 2 2 2 2 2 2 2 20 1 1 2 2 3 1 3 2 1 2 3aW K K K = + + + + (2.6) y aW l nng lng t do d hng t ca tinh th khng bin dng hay l

nng lng d hng t tinh th t nhin ca cc tinh th c cu trc lp phng; 0 1 2, ,K K K l cc hng s d hng t (anisotropy constant) v c th nguyn nng

lng. Vi tinh th lc gic c th chng minh:

2 4

0 1 2sin sinaW K K K = + + (2.7) y l gc gia SI v trc lc gic. Ta cn c th vit di dng:

' ' 2 ' 40 1 2aW K K K = + + (2.8)

Vi =cos Bng cch xt iu kin cc tiu nng lng ca cc biu thc trn, c th chng

minh rng du v ln tng i ca cc hng s d hng quyt nh phng d l phng no.

Ngoi vic c m t thng qua cc hng s d hng nh trnh by trn, d hng t tinh th cn c th c m t nh mt trng hiu dng gi l trng d hng hiu dng. gi thit vecto M

uurb lch khi phng t d mt gc do nh

hng ca t trng ngoi H chng hn. D hng t s biu hin y nh mt hin tng gy ra bi tc dng ca mt trng hiu dng c xu hng quay M

uurtr v

phng 0 = . Moomen xon ca trng d hng ny l:

2 2

2 20 0 0

W W W Wa a a a

= = = = + = (2.9)

y aW l nng lng d hng t tinh th. Trong biu thc trn, ta t

0

Wa =

=0 v phng =0 ng vi nng lng cc tiu (phng d). Nh vy c th coi nh c mt trng hiu dng AH tc dng theo phng d m n tha mn iu kin:

19

2

20

Wsin aS A S AM H M H

= = (2.10)

Vy

2

20

W1 aA

S

HM =

= (2.11) Th d vi phng [ ]001 ca tinh th lp phng hoc trc c ca mt tinh th n

trc ta c, nu ch xt hng s d hng u tin:

22

1 12

WW ; 2aa K K = = (2.12)

Tc l:

21

2

W 21 aA

S S

KHM M

= = (2.13) Vi cc tinh th khc nhau theo cc phng khc d khc nhau, ta c cc gi tr

ca trng tinh th d hng hiu dng AH nh trn.[3] 2.2.1.3. Nng lng t n hi

Mng tinh th ca cht rn t khng hon ton cng, khi b t ha n thay i khin cho hnh dng v kch thc vt thay i hin tng ny gi l hin tng t gio (magnetostriction). cc vt liu st t, t gio ch c quan st CT T> v do trng thi thun t. Khi CT T< vt liu st t c moomen t t pht (cc spin xp sp song song nhau). H thng c xu hng gim nng lng. nng lng trao i Wtd gim, tch phn trao i A phi tng. Xt ng cong Bethe. Nu vt liu tng ng vi mt im pha tri ng cong Bethe, A tng th t s a/d phi tng hay khong cch gia cc nguyn t tng (im 1 dch n im 2 hay ht n ra). Cn nu h bn phi ng cong Bethe, tng A th t s a/d phi gim (ht co li).

V trong h cn c nng lng d hng t nn s bin dng theo cc phng khc nhau phi khc nhau. Nng lng d hng t nh hn nng lng trao i hai bc do n khng lm thay i th tch m ch lm thay i hnh dng ca vt (xem hnh). Hin tng ny gi l hin tng t gio t pht.

Hy xt biu thc ton hc ca t gio t pht. Do t gio CT T< ta c:

' u = + (2.14) y u c cc thnh phn i ik ku = vi = tenxo hng 2 v

1 2 3( ) ( ); cosS iI f in = = = ch phng ca mooment t. V c d hng, ni chung ' khng song song vi , ta k hiu:

' = (2.15)

20

V:

=

i lng c gi l t gio t pht ca tinh th theo hng . Nu l

vecto n v, ta c th vit:

( )2 2't i i i

i iu = =

(2.16) y i l cosin ch phng ca vecto Khai trin biu thc trn theo chui Taylor, ta c:

t i iu =

(2.17)

y:

i ik ku = (2.18) (Ta b qua cc thnh phn bc cao trong khai trin trn) nu xt n tnh i

xng lp phng ca tinh th (hiu ng chn), ta c:

ij ji = (2.19) t iu bit vo biu thc trn ca t ta c:

2 2 211 1 22 2 33 3 12 1 2 23 2 3 31 3 12 2 2t = + + + + + (2.20)

C th chng minh rng ij c th vit di dng:

2

0 1ii i = + (2.21)

2ij i j = (2.22) Trong 0 1 2, , l cc hng s. Khi phng trnh c vit li l:

( )( )

2 2 2 2 2 20 1 1 1 2 2 3 3

2 1 2 1 2 2 3 2 3 1 3 3 12t = + + + +

+ + (2.23)

Cng thc trn cho ta t gio t pht theo mt hng bt k CT T< vi i - hng ca M i - hng xt t gio

Mt tinh th c kch thc bt k c t ha bo trong t trng cng c th c xem l mt n men v do cc kt qu trn c th p dng c cho trng hp ny.

21

By gi ta hy xt mt n tinh th dng cu gm nhiu men (Hnh). Khi CT T< mu chuyn trangj thi 1 n trng thi 2 (tinh th thay i kch thc, nhng

khng thay i hnh dng). tinh th c 6 nhm men (6 pha t) tng ng vi 6 hng c trng bi cc cosin ch phng nh sau:

Cc pha 1 v 2 1 2 31, 0, 0 = = = Cc pha 3 v 4 1 2 30, 1, 0 = = = Cc pha 5 v 6 1 2 30, 0, 1 = = = Tt c cc hng l ng xc sut v c gi tr nh nhau, nn t gio vi mi pha

c dng:

Pha 1 v 2 21 0 1 1a a = + Pha 3 v 4 22 0 1 2a a = + Pha 5 v 6 23 0 1 3a a = + ln ca t gio t pht ca ton tinh th l :

( )2 2 21 2 3 10 1 2 33 3aa + += = + + + (2.24)

V ( )2 2 21 2 3 1 + + = nn :

10 3

aa = + (2.25)

V khng ph thuc vo cosin ch phng na. Nh vy tinh th thay i kch thc nhng khng thay i hnh dng v c t ha bng 0.

By gi nu 0H , ln ca t gio ph thuc vo hng t ha vi cc trc tinh th. Nu SH H= l trng t ha tinh th n bo ha hon ton, mu b thay i c v hnh dng v s bin i kch thc mu t trng thi (1) n trng thi (3) trn hnh c m t bi phng trnh trn. T , ta xc nh c ln ca t gio khi t ha bo ha mu :

V sau khi s dng:

( )2 2 2 2 2 2

1 1 1 2 2 3 3

2 1 2 1 2 2 3 2 3 3 1 3 1

13

2

S a

a

= + + + + + (2.26)

y, i l cosin ch phng ca vecto t , i l cosin ch phng ca phng kho st t gio. Cc hng s 1 2,a trong phng trnh trn c th biu din qua cc i lng t gio dc ca phng [ ]100 v phng [ ]111 (k hiu qua 100 , 111 ,) tc l t gio c xt theo phng t t trng ( i i = ). Tht vy, bi v :

22

1 1 2 3 2 31, 0 = = = = = = (theo phng [ ]100 ) 1 2 3 1 2 3

13

= = = = = = (theo phng [ ]111 ) Nn khi ta c :

100 1

23

a = v 111 223 a = (2.27)

Nh vy t gio theo mt phng bt k c vit li l :

( )2 2 2 2 2 2

100 1 1 2 2 3 3

111 1 2 1 2 2 3 2 3 3 1 3

3 12 3

3

s

= + + + + (2.28)

Cc gi tr 100 , 111 trong (2.27) c th c xc nh t thc nghim.[3] 2.2.1.4. Nng lng tnh t

Nng lng tng tc ca vt liu st t c t I vi t trng ngoi 0H hay nng lng tnh t (magnetostatic energy) c m t bi:

0WH IH= (2.29) Khi vt th c kch thc hu hn c t ha, cc cc t t do c cm ng

hai u gy ra mt trng ngc hng vi vecto t . T trng ny t l vi ln I v c gi l trng kh t (demagnetizing field). Nh vy c s tn ti tng tc ca vt liu vi trng kh t ca chnh n. Nng lng tng tc ny c gi l nng lng trng kh t.[3] 2.3. Cm bin Hall phng vi cu trc khc nhau 2.3.1. Cm bin Hall phng vi cu trc spin-vale

Cu hnh van spin l mt mng mng ba lp cha 2 lp st t (F1 v F2) ngn cch nhau bi mt lp kim loi khng t tnh (NM), trong lp F1 thng c ghim bng tng tc trao i vi mt lp vt liu phn st t v lp F1 ny c gi l lp st t b ghim. Cu trc van-spin c m t nh Hnh 2.3.

Hnh 2.3. Cu trc spin-vale

23

Ta c nng lng t ca mi lp nh sau: Lp st t t do tn ti: - Nng lng d hng t tinh th - Nng lng tnh t (khi c t trng ngoi) Lp st t b ghim: - Nng lng d hng t tinh th - Nng lng tnh t (khi c t trng ngoi) Lp st t v lp st t b ghim: - Nng lng tng tc trao i lin phn mng (interlayer magnetostatic

coupling) Lp st t b ghim v lp phn st t: - Nng lng tng tc trao i (exchange bias) p dng m hnh Stonner-Wohlfarth, di tc dng ca t trng ngoi H, nng

lng t trn mt n v din tch ca lp st t t do c cho bi cng thc: E = - Hex Ms tp cos( p) + Kup tp sin2p - Msp tp H cos( p) + Kuf tf sin2f - Msf tf H cos ( - f) J cos(f - p) (2.30)

Trong : E l nng lng t trn mt n v din tch ca lp st t t do, H l

cng t trng ngoi tc dng ln mu, tf v tp l dy lp st t t do v lp st t b ghim, f, p l gc gia t ca lp st t t do v lp st t b ghim i vi phng trc d ca lp st t t do. Msf, Msp ln lt l t bo ha ca lp st t t do v lp st t b ghim. Kuf v Kup l hng s d hng t hiu dng ca lp st t t do v lp st t b ghim. Hex l t trng ghim (trng trao i dch c sinh ra do tng tc gia lp phn st t vi lp st t b ghim). J l h s tng tc trao i gia lp st t b ghim v lp st t t do; l gc gia t trng ngoi vi trc t ha d ca lp b ghim. l gc gia t trng trao i dch vi trc d ca lp b ghim.

in p Hall phng s c vit li nh sau:

1/ 2. sin 2yK J

HV I R I RH H

= + (2.31)

Vi l gc gia t ca mu v mt dng. Trong sf J fJ M H t= vi JH l trng tng tc trao i lin phn mng ca

lp st t t do v lp st t b ghim. Nu tng tc trao i gia lp st t b ghim v lp phn st t mnh, gc

gia t v trc t ha d ca lp st t b ghim c c nh vng t trng thp p tin ti 0.

24

Vi gc nh th cos = 0. nhy ca cm bin trong trng hp ny l:

yK J

V RSIH H H

= = + (2.32)

2.3.2. Cm bin Hall phng vi cu trc GMR Trong m hnh cu trc GMR kch thc mi lp l kch thc n men. V

cu trc GMR c m t nh Hnh 2.4.

Hnh 2.4. Cu trc GMR Ta c nng lng t ca mi lp nh sau: Lp st t 1 tn ti: - Nng lng d hng t tinh th - Nng lng tnh t (khi c t trng ngoi) Lp st t 2 tn ti: - Nng lng d hng t tinh th - Nng lng tnh t (khi c t trng ngoi) Lp st t 1 v lp st t 2: - Nng lng tng tc trao i lin phn mng (interlayer magnetostatic

coupling) p dng m hnh Stonner-Wohlfarth, di tc dng ca t trng ngoi H, nng

lng t trn mt n v din tch ca lp st t t do c cho bi cng thc:

Lp st t 1

Lp khng t

Lp st t 2

25

E = Ku1 t1 sin21 - Ms1 t1 H cos( 1) + Ku2 t2 sin22 - Ms2 t2 H cos ( 2) J cos(1 2) (2.32) Trong : E l nng lng t trn mt n v din tch ca lp st t t do, H l

cng t trng ngoi tc dng ln mu, tf v tp l dy lp st t t do v lp st t b ghim, (1- 2) l gc gia phng t 1M v 2M ca hai lp st t. Ms1, Ms2 ln lt l t bo ha ca lp st t th nht v lp st t th 2. Ku1 v Ku2 l hng s d hng t hiu dng ca hai lp st t. J l h s lin kt trao i lin phn mng gia hai lp st t; l gc gia t trng ngoi vi trc t ha d.

Suy ra in p Hall phng c vit li nh sau:

jKy HH

HRIRIV += sin (2.33)

Trong jss HtMtMttJ )..(1 2211

21

++= vi jH l trng tng tc trao i lin phn mng ca hai lp st t.

Vi gc nh th cos = 0. nhy ca cm bin trong trng hp ny l:

yK J

V RSIH H H

= = + (2.32)

2.4. Tng kt Trong Chng II. ta tm hiu k hn v cm bin da trn hiu ng Hall

phng vi cc cu trc khc nhau. Ta bit c nguyn l ca cm bin Hall phng, m hnh ca cm bin. Chi tit hn ta bit cc dng nng lng t c trong cm bin v m hnh nng lng ca ca cm bin, cch tnh tn hiu li ra v nhy ca cm bin. Trong chng tip theo ta s i vo vic m phng cho mt cm bin c th v so snh vi thc nghim.

Chng III. Kt qu m phng s nh hng ca trng tng tc ln nhy ca cm bin v gii thch

Nh trnh by trong cc chng trc. Loi cm bin da trn hiu ng Hall

phng c c im l nhy cao ti c nano- Tesla v t s tn hiu trn nhiu ln. V vy n c dng pht hin cc ht t ng dng trong y-sinh hc. Nhm mc ch to ra cm bin c nhy cao, n nh chng ti kho st nh hng ca trng tng tc ti nhy ca cm bin.

26

Cc cm bin d nh s c ch to trn Si c xi ho vi SiO2 c dy 500 nm nhm mc ch cch in hon ton gia lp cm bin v . Cm bin d nh ch to c cu trc nh sau:

Si /SiO2 (500) /Ta(5)/NiFe(10)/Cu(1.2 )/CoFe(10nm)/IrMn(15)/Ta(5) (nm) Ta s m phng s ph thuc ca tn hiu, nhy vo s thay ca lp i ca

trng tng tc trong cu trc trn. 3.1. M phng s ph thuc ca th VPHE vo t trng ngoi khi thay i t trng dch HJ.

Nghin cu s ph thuc ca th Hall phng vo s thay i ca t trng ngoi trong cu trc van-spin khi trng tng tc lin phn mng gia lp st t b ghim v lp st t t do (HJ) thay i. Ta bit rng trong cc cu trc t in tr khng l (cu trc van-spin v cu trc GMR), cc lp st t c kh nng tng tc vi nhau qua lp khng t tnh (tng tc RKKY). Trng tng tc ny gi l trng tng tc trao i lin phn mng HJ. Trng tng tc trao i ny ph thuc rt nhiu vo cu trc, tnh cht t ca cc lp vt liu v ph thuc vo khong cch gia cc lp st t ( dy ca lp khng t). Khi s dng cc cu trc ny nghin cu v ng dng lm cc cm bin Hall mt phng chng ta s nghin cu s ph thuc ca trng tng tc trao i lin phn mng ny i vi ng cong th Hall.

Nng lng E da theo m hnh Stoner-Wohlfarth : E = - Hex Ms tp cos( p) + Kup tp sin2p - Msp tp H cos( p)

+ Kuf tf sin2f - Msf tf H cos ( - f) J cos(f - p) (3.1) Trong trng hp lc tng tc gia lp st t b ghim (CoFe) v lp phn st t

(IrMn) ln th c th coi nh lp st t b ghim hon ton. Khi gc gia t v phng trc t ha d ca lp st t b ghim s c gi c nh trong vng t trng nh: p=0, =0, =900. Nng lng E s c vit li l: E= - Msp tp H + Kuf tf sin2f - Msf tf H cos ( - f) J cos(f - p) (3.2)

Da theo iu kin cc tiu nng lng ta c:

0=ddE

= 2Kuf tf sin fcos f - Msf tf H sin( - f) J sin(f - p)

H = (2Kuf tf sin fcos f - J sin(f - p))/ Msf tf sin( - f) (3.3) Mt khc:

Vy = Jx w R sin cos (3.4) tin hnh m phng ta gi thit: - Cm bin c cp dng mt chiu c nh I = 1 mA - T trng ngoi H c t vung gc vi dng, =90. - T M c t cng phng vi dng I.

27

- in tr R0=0.0000900. - MS=0.486675. - Trng d hng HK=5 (Oe). S dng hai phng trnh (3.3) v (3.4). Khi thay i gi tr ca t trng tng

tc trao i lin phn mng HJ, ta cho HJ thay i trong khong t 1 Oe n 50 Oe ta c s thay i ca th VPHE vo t trng ngoi nh sau.

Hnh 3.1. Trng d hng HJ=1 (Oe)

28

Hnh 3.2. Trng d hng HJ=10 (Oe)

Hnh 3.3. Trng d hng HJ=20 (Oe)

29

Hnh 3.4. Trng d hng HJ=50 (Oe)

Nh ni cc phn trn, th Hall phng ph thuc vo qu trnh tn x dng spin in t theo phng t . Nhn trn Hnh 3.1. khi trng tng tc trao i lin phn mng HJ = 1 Oe, th Hall phng d dng t gi tr bo ha, c ngha l di tc dng ca t trng ngoi cc mmen t ca mu quay theo phng t trng, t trng bo ha ch vo khong 30 Oe, khi t trng ln hn 30 Oe th cc mmen t quay hon ton theo phng t trng, trng thi ny s tn x ca dng theo phng t l nh nht, in tr Hall bng khng hay th Hall bng khng (0). Khi gim dn t trng t 30 Oe xung n 0 Oe ta nhn thy c mt gi tr cc i ca VPHE, iu ny c th gii thch nh sau: khi gim dn t trng t 30 Oe xung 7 Oe, di tc dng ca trng d hng, cc mmen t ca mu quay tr v theo phng ca trc d (theo chiu mi tn nh hnh v (Hnh 3.5.). Dng in tn x theo phng t tng dn, tc l th Hall tng dn v t gi tr cc i t trng 7 Oe.

Theo cng thc tnh th Hall )2sin(21 RJwV = gi tr cc i ny tng ng

vi gc 045= tc l phng t hp vi dng mt gc 450. iu ny cho ta bit c dng in tn x theo phng t l ln nht khi t hp vi dng mt gc 450.

Cc mmen t s quay tr v theo phng trc t ha d khi t trng gim v n 0 Oe, qu trnh tn x cng gim dn v 0, th Hall phng bng 0 (vn)

30

Hnh 3.5. S quay ca vecto M theo hng t trng ngoi H

Khi tng gi tr HJ ta nhn thy rng th Hall kh t gi tr bng khng vng t trng thp (khong 30 50 Oe). Cc nh cc i ca th Hall b dch theo s tng ca trng tng tc trao i lin phn mng, tng HJ th im cc i th Hall s t c vng t trng cao hn. iu ny c th c gii thch nh sau: khi HJ tng, c ngha l, trong vng tip xc gia hai lp st t s xut hin cu trc xon do tng tc gia hai lp st t (RKKY) thng qua lp khng t tng. Dng spin in t s tn x qua lp xon ny lm cho th Hall kh t trng thi bo ha khi t trng thp. ph v cu trc xon ny ta cn mt t trng ln (H >> HJ) cc mmen t trong cu trc quay theo phng t trng.

Vi nhy ca cm bin c tnh bi cng thc HVS = / (V/Oe) ta thy rng khi tng gi tr HJ dc trong vng tn hiu tuyn tnh ca cm bin gim dn, tc l nhy ca cm bin gim.

T cc kt qu trn ta thy trong vng t trng nh, tn hiu o VPHE thu c thay i rt tuyn tnh vi t trng. iu ng ni y l tn hiu ny phn nh c c ln v du ca t trng o. Trong vng tuyn tnh, tn hiu o rt n nh th hin qua tnh i xng tuyt i v ln so vi ln ca t trng m khng ph thuc vo du m hay dng ca t trng ngoi. Hn th na, ng tn hiu o ny hu nh khng c s tr t khi o theo chiu tng t trng v theo chiu gim ca t trng o. iu ny c ngha rt quan trng i vi vic ng dng m bo tnh n nh v lp li cao ca kt qu o. 3.2. M phng s ph thuc ca th VPHE vo t trng ngoi khi thay i t trng d hng HK

Suy lun tng t nh phn 3.1. Ta bit rng trong cc cu trc t in tr khng l (cu trc van-spin v cu trc GMR), ngoi vic cc lp st t c kh nng tng tc vi nhau qua lp khng t tnh (tng tc RKKY). Hay cn gi l trng tng tc trao i lin phn mng HJ. Bn thn cc lp st t cn tn ti mt nng lng khc nh ch ra trong phn l thuyt l nng lng d hng t tinh th. Nng lng d hng ny to ra trng d hng HK trong mi lp st t. Trng d hng ny ph thuc rt nhiu vo cu trc, tnh cht t ca cc lp vt liu v ph thuc vo dy ca cc lp st t. Sau y ta s kho st s ph thuc ca tn hiu ra VPHE vo trng d hng ny.

M , IgM

gH

31

tin hnh m phng ta gi thit: - Cm bin c cp dng mt chiu c nh I = 1 mA - T trng ngoi H c t vung gc vi dng, =90. - T M c t cng phng vi dng I. - in tr R0=0.0000900. - Ms=0.486675. - Trng d hng HJ=5 (Oe). S dng phng trnh (3.3) v (3.4). Khi thay i gi tr ca t trng d hng

HK, ta cho HK thay i trong khong t 1 Oe n 50 Oe ta c s thay i ca th VPHE vo t trng ngoi nh sau.

Hnh 3.6. Trng d hng HK=1 (Oe)

32

Hnh 3.7. Trng d hng HK=10 (Oe)

Hnh 3.8. Trng d hng HK=20 (Oe)

33

Hnh 3.9. Trng d hng HK=50 (Oe) Nhn vo cc th trn ta thy c v s thay i tng t nh cc phn trn.

Nhn trn Hnh 3.6. khi trng tng tc trao i lin phn mng HK = 1 Oe, th Hall phng d dng t gi tr bo ha, c ngha l di tc dng ca t trng ngoi cc mmen t ca mu quay theo phng t trng, t trng bo ha ch vo khong 30 Oe, khi t trng ln hn 30 Oe th cc mmen t quay hon ton theo phng t trng, trng thi ny s tn x ca dng theo phng t l nh nht, in tr Hall bng khng hay th Hall bng khng (0). Khi gim dn t trng t 30 Oe xung n 0 Oe ta nhn thy c mt gi tr cc i ca VPHE, iu ny c th gii thch nh sau: khi gim dn t trng t 30 Oe xung 7 Oe, di tc dng ca trng d hng, cc mmen t ca mu quay tr v theo phng ca trc d (theo chiu mi tn nh hnh v (Hnh 3.5.). Dng in tn x theo phng t tng dn, tc l th Hall tng dn v t gi tr cc i t trng 7 Oe.

Khi tng gi tr HK ta nhn thy rng cng ging phn trn, th Hall kh t gi tr bng khng vng t trng thp (khong 30 50 Oe). Cc nh cc i ca th Hall b dch theo s tng ca trng d hng, tng HK th im cc i th Hall s t c vng t trng cao hn. iu ny c th c gii thch nh sau: Khi HK tng, c ngha l nng lng d hng K tng, m ta bit quay vecto M theo phng H ta phi thng nng lng lin kt ca M vi trc tinh th. Hay ni cch khc l thng c nng lng d hng trn. Nng lng ny cng cao th vecto M cng kh quay theo H. Mt khc ta bit tn hiu li ra ph thuc vo gc gia t v dng I. Ln khi t kh thay i th gc gia t v dng I cng kh thay i. Ln trng thi cc i kh t c ngay.

34

Suy ra ging vi trng hp thay i trng tng tc trao i. nhy gim khi tng trng d hng HK. Kt lun: T kt qu phn tch trn ta thy rng r rng l nhy ca cm bin thay i tuyn tnh v t l nghch vi s thay i ca HJ v HK trong vng t trng nh. Do ta c s ph thuc ca nhy theo s thay i ca trng tng tc HJ v HK nh sau:

K JCS

H H= +

Trong C l bin s: 3.3. S nh hng ca vic thay i gc gia t trng ngoi H v dng qua cm bin I

Sau y ta s kho st vic thay i gc t gia t trng ngoi H v dng mt chiu I nh hng ti nhy ca cm bin.

Cc thng s cho qu trnh m phng nh sau : - Cm bin c cp dng mt chiu c nh I = 1 mA - T M c t cng phng vi dng I. - in tr R0=0.0000900. - Ms=0.486675. - Trng d hng HK=5 (Oe). - Trng tng tc trao i lin phn mng HJ=10 (Oe). S dng hai phng trnh (3.3) v (3.4). Sau khi m phng ta s c nhng kt

qu nh sau:

Hnh 3.10. Gc ban u =150

35

Hnh 3.11. Gc ban u =450

\

Hnh 3.12. Gc ban u =750

36

Hnh 3.13. Gc ban u =900

Ta thy rng khi thay i tng dn gc t 150 n 900 vng tuyn tnh ca cm bin tng dn. Ngha l vi gc 150 th vng tuyn tnh ca cm bin nh nht v gc 900 vng tuyn tnh ca cm bin l ln nht. Hay ni cch khc khi t trng ngoi c t vung gc vi dng ngoi th ta c nhy ca cm bin thu c ln nht. 3.4. So snh kt qu m phng v kt qu thc nghim

Thc hin vic ch to mu nh d nh. Sau ta o s ph thuc ca tn hiu VPHE vo t trng ngoi H ly cc kt qu o c ca thc nghim ny so snh vi cc ng m phng trn ta c s so snh gia ng thc nghim v ng m phng nh sau:

37

Hnh 3.14. So snh ng cong thc nghim v m phng

Trn Hnh 3.10. ta thy rng ng cong thc nghim khng trng kht hon

ton vi ng m phng ca ta, ta thy ti thi im ban u im tn hiu bng 0 ca ng thc nghim l dch vi ng m phng (ni cch khc dch vi gc ta ) mt khong H . iu ny c th c gii thch nh sau: Ta bit vic xc nh chnh xc hng ca vecto t M khi ch to l khng hon ton. Do m trong qu trnh ch to khng phi lun t c t M cng phng vi dng ngoi I. M s c mt gc lch ban u gia t M v dng I. Do mu thc t khi o s khng to c vi t trng ngoi mt gc chnh xc l 900.

Tuy nhin iu ta quan tm y l nhy v tn hiu thu c. Nh ta thy trn th th tn hiu v dc thu c ca thc nghim l gn nh trng vi ng m phng. S dng cng thc tnh nhy cho thc nghim v m phng ta thy nhy theo thc nghim gn nh hon ton ging vi qu trnh m phng.

38

Kt lun chung

Trong qu trnh thc hin kha lun ny chng ti t c nhng kt qu

chnh nh sau: M phng c s nh hng ca trng tng tc ln cm bin c cu trc

spin-vale (cu trc c th ch ra Chng III). Trong chng ti m phng c s nh hng ca trng tng tc trao i lin phn mng HJ ln nhy ca cm bin, s nh hng ca trng d hng HK ln nhy ca cm bin, nh hng ca vic thay i gc gia t trng ngoi H v dng qua cm bin I ln nhy ca cm bin, v cui cng l so snh vi nhng kt qu thc nghim thu c t vic o mu.

T chng ti thy c rng nhy S ca cm bin t l nghch vi trng tng tc trao i lin phn mng HJ v trng d hng HK. ng thi chng ti nhn thy rng cm bin lm vic tuyn tnh vi nhng vng t trng nh.

Vi vic thay i gc gia t trng ngoi H v dng qua cm bin I th chng ti thy rng cm bin c nhy S v tuyn tnh cao nht khi t trng ngoi c t vung gc vi dng qua cm bin. Sau chng ti em so snh vi kt qu thu c t thc nghim v thy rng gn nh hon ton ging nhau v nhy.

Do vy vic ch to v s dng cm bin Hall phng theo nhng kt qu m phng trn em li nhiu u im nh: nhy cao, tuyn tnh cao, v gim tri.

39

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