physica B 165&166 (1990) 1365-1366North-Holland

MAGNETORESISTANCE MEASUREMENTS ON SINTERED YBa2Cu30 7_S

Magnus ANDERSSON and Osten RAPP

Solid State Physics, The Royal Institute of Technology,S-100 44 Stockholm, Sweden

We have measured the magnetoresistanceup to 0.16 in magnetic fields up to 12method. Qualitative agreement is foundveloped theories for magnetoresistance

1. INTRODUCTIONA lot of work has been made in trying

to understand the superconducting tran­sition of high temperature superconduc­tors. In this respect, the analysis ofthe excess conductivity just above thesuperconducting transition temperatureT plays an important role. By an extra­p51ation of the normal-state resistivityfrom temperatures far above T down toT , it is not possible to tak~ the de­t&iled temperature dependence of thenormal-state resistivity into account.Magnetoresistivity measurements do notsuffer from this drawback.

There are two contributions to themagnetoconductivity; the Aslamazov-Larkinterm1 (AL) and the Maki-Thompson term2- 3(MT). Recently, theoretical expressionfor these terms have been calculated forlayered high-temperature superconductorswith an applied magnetic field B perpen­dicular to the conducting Cuo-planes. 4- 5

According to this theory, the total mag­netoconductivity ~a = (j (B) -a (0) consistsof four contributions; the orbital andZeeman contribution of the AL and MTterm respectively.

Measurements of magnetocon~uctivity

on YBa 2Cu30 7 _S single crystals have showngood agreement between theory and exper­iment in magnetic fields up to 12 T andtemperatures close to T , E=(T-T )/T lessthan 0.04. In polycryst&lline m&terlals,the measured magnetoconductivity has tur­ned out to be lower than eXgected fromtheoretical considerations. Severalfactors contribute to this effect: (i)the temperature derivative of the resi­stivity is lower than in single crystals(ii) the incomplete packing of the sint­ered material, which leads to an apparentlarger resistivity p in sintered samples.

Furthermore, the directional averagingof p and ~a(B) both reduce the observed~a(B) .

of sintered YBa2Cu30 7_S for E=(T-T )/TT, by using an ordlnary resistiv~ cbetween measured data and recently de­in high-temperature superconductors.

We here present our preliminary re­sults for ~a(B) of sintered YBa2Cu30 7_Sin magnetic fields up to 12 T measuredby a direct resistive method. Earlier,Matsuda et al have measured the magneto­conductivity up to 4 T using a field­modulation technique. 7 Their method ismore sensitive than ours, but probablylimited to lower magnetic fields due tothe use of ac currents.

2. EXPERIMENTALThe measurements were performed in a

commercial flowing gas cryostat equippedwith a 12 T superconducting magnet. Thetemperature was controlled to betterthan several mK by a magnetic field in­dependent capacitive sensor. To furtherguarantee a stable temperature, thesample was protected from the flowinggas in the cryostat by an extra temp­erature shielding. By regulating the

0.70 r-----.-----,--------r----,

0.65

0.60

0.55

0.50

0.45

0.40 '--__.....L .L- ----I._----'

100 120 140

Temperature (K)

FIGURE 1Normalized resistivity versus temperaturecurve for YBa2Cu30 7_ near T . The regionused for magnetocon8uctivity measurementsis marked in the figure.

0921-4526/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)

1366 M. Andersson, 6. Rapp

10000 ..------,.-----,.-------,6

~ B =2TE 5

";"C;

4~ab = 15A

0- 0 ~ =3A1- 3/>. C

Ol />.

2- o />.

2 o/>.

0 00.60 0 0

1-2.5 -2.0 -1.5 -1.0 -0.5

log «T-T)fT)

ACKNOWLEDGEMENTSWe are grateful to Mats Nygren and

Zsolt Hegedus, Stockholm University, forhelp and advice in sample preparation.This work has been supported by theSwedish National Research Foundation andby the Board for Technical Development.

as ba (B) .In the e-region from about 0.007 to

0.07 the temperature dependence of ba(B)is in good agreement with theory. Theexperimental data are a factor 1/75 be­low the theoretical ones. This is part­ly due to arbitrary grain directions(1/3) and resistivity anisotropy (1/2)9,but still a factor 1112.5 remains. Thisis attributed to inclusion of voids inthe sample and the lower temperature de­rivative of the resistivity compared tosingle crystals.

FIGURE 3Temperature dependence of the magneto­conductivity for two YBa2Cu301_5sampleswith an applied field of 2T. rne solidline represent the theoretical express­ion including the leading terms in theAl- and MT-terms with Bile-axis .

15

,.

E = 0.010

E = 0.160

E = 0.032

. "

..

. . .

, ' E = 0.016

......•• • £ = 0.007

..... ." ... " ",..

..

..

.. "..'.'.' '.',.'.,

o --" "o

500

2000 1-0'

1000

1500

8000

4000

6000

~ ..E .'

0 .'-9-~

2500

2000

5 10

Magnetic field (T)FIGURE 2

Magnetoconductivity of YBa2Cul 0 7_5 atdifferent temperatures above T . Notethat the scales of the ordinat~s are notthe same in the figures.

temperature of this shield, the coolingcapacity to the sample could be accura­tely controlled.

Resistance was measured by a dc cur­rent comparator bridge with a resolutionof a few nV.

Sintered samples were prepared accor­ding to sta~dard methods as describedpreviously. Two different samples wereused as a check of the reproducibilityof results for sintered samples.

3. RESULTS AND DISCUSSIONIn fig 1, we have marked the region

used for magnetoresistivity measurements.The magnetoconductivity for some reducedtemperatures e are shown in fig 2. We seea clear change in behaviour from temper­atures close to T , where the magneto­conductivity changes more rapidly at lowapplied fields, than at higher t~mper­

atures, where we have almost a B depend­ent ba(B). This is in good qualitativeagreement with theoretical expressions.

To simplify the anlaysis we extractedin fig 3 the temperature dependence ofba(B) at 2T from results such as in

fig 2. For high temperatures, we reachthe limit where temperature fluctuationsin the sample is at the same magnitude

REFERENCES(1) L.G.Aslamazov and A.I.Larkin, Phys.

Lett. 26A (1968) 238.(2) K.Maki, Prog.Theor.Phys. 39 (1968) 897.(3) R.S.Thompson, Phys.Rev. Bl (1970) 327.(4) S.Hikami and A.I.Larkin, Mod.Phys.

Lett. B2 (1988) 693.(5) A.G. Aronov, S.Hikami and A.I.Larkin

Phys.Rev.Lett 62 (1989) 965.(6) M_Hikita and M.Suzuki, Phys.Rev. B41

(1990) 834.(7) Y.Matsuda et al., Solid State Comm.

68 (1988) 4756.(8) M.Andersson et al., Physica C 160

(1989) 65.(9) T-K.Xia and D.Stroud, Phys.Rev. B37

(1988) 118