香港科技大学Hong Kong University of Science and Technology

(HKUST)

Magnetic and transport properties in the field-induced phase

transitions in Ni50Mn50-xInx Heusler alloys

Zhang XixiangPhysics, Hong Kong University of Science and Technology

Introduction

The search for ferromagnetic materials suitable for application in semiconductor spintronics devices.

To allow efficient spin injection into the semiconductor, these materials must satisfy at least the following

1) Curie temperature significantly higher than the room temperature, the working temperature of semiconductors used industrially.

2) A very high spin polarization of the electron states at the Fermi level.

Heusler alloys were predicted to be Half-metals

Half metalkey material candidate for spintronics

%100==↓↑

↓↑

+−

nnnnonpolarizati

↑n ↓nand are electron density

↑n ↓nOr =0

Nonmagnetic vs. Magnetic Materials

N(E)

E

EF

N(E)

E

Nonmagnetic Magnetic

Half metals

EF

N(E)

E

Spins of all the conducting electrons pint to the same direction “Up”

Heusler alloys of Higher-spin-polarization(much higher than the conventional ferromagnetic metals)

• spin polarization of the Co2MnSi Heuslercompound has been estimated to be 61% at 10 K.

(Kammerer et al. APL85, 79,(2004))

• Point-contact Andreev reflection measurements of the spin polarization yield polarization values for Co2MnSi and NiMnSb of 56% and 45%, respectively (L. Ritchie et al., Phys. Rev. 68, 104430(2003)).

Polarization of Ferromagnetic materials

Ferromagnetic shape memory alloys

Ferromagnetic Heusler alloys are well known ferromagnetic shape memory alloys– Martensitic transformation– Magnetic field induced strain – Applications in actuators – ….

Typical Heusler alloys -Ni2MnGa

Characteristics

275 280 285 290 295 300 305

-12000-10000-8000-6000-4000-2000

0200040006000

c

b

a

Stra

in(p

pm)

T(K)

Spontaneous shape memory effect in single crystals

Spontaneous shape memory effect in single crystals

Thermal-elastic martensitic transformation Shape memory effect

-20 -15 -10 -5 0 5 10 15 200.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

T=250 K

T=270 K

T=275 K

T=280 K

T=285 K

MFI

S (%

)

Magnetic field (kOe)

H=0 H≥HA

Mechanism of field induced strain

Characteristics of Ni2MnGa

• High temperature phase (or austenite) : ferromagnetic, low magnetic anisotropy cubic lattice

• Low temperature phase (martensite):ferromagnetic, strong magnetic anisotropy, tetragonal lattice

• Both phase are ferromagnetic metals

Transition temperature shifts very slightly

M(T) curves with different applied magnetic fieldsF. Zuo et al PRB58,11127(1998)

Temperature depedennt reistivityTemperature-dependent resistivity

Zuo et al, J. Phys.: Condens. Matter 11 (1999) 2821–2830.)

CoNiGa alloys

0 100 200 3000.0012

0.0016

0.0020

250 260 270

1.65

1.70

Res

ista

nce

(mΩ

)

Temperature (K)

Co5Ni2Ga3

R

esis

tanc

e (Ω

)

Temperature (K)

0 50 100 150 200 250 300

0.5

1.0

1.5

2.0

2.5

-60 -40 -20 0 20 40 60

1.54

1.55

1.56

1.57

T = 250 K

ρ (m

Ω)

H (kOe)

Co5Ni2Ga3H = 50 kOe

MR

(%)

T (K)

Magnetoresistance

Ni2MnCoIn crystalsNature 439, 957(2006)

MBErr

•−=

Ni50Mn50-xInx single crystals(x=14-16.3)

• single crystals were grown at a rate of 5–30 mm/h using a Czochralski instrument with a cold crucible system

• X-ray: High temperature phase is L21-type ordered structure with a lattice constant a=6.006Å.

• Cooling to 93K, the crystal structure changes to an orthorhombic structure with a rather complex martensitic modulated sub-structure.

Magnetic, electrical and thermal transport measurements

• Magnetic measurements: Quantum Design SQUID magnetometer, 2-400 K, 5 Tesla

• Transport measurements: Quantum Design Physical Property Measurement system (PPMS), 2-400 K, 9Tesls

• Specific Heat: PPMS

0

20

40

60

80

100

120

140

0 40 80 120 160 200 240 280 320 360

0.4

0.8

1.2

1.6

2.0

2.4

a

c

d

a

b

T (K)

ρ (μΩ

.m)

x10

H (T) 0.01 3.0 5.0 7.0

M (e

mu/

g)

bTC

d

c

b

a

H (T) 0.0 3.0 5.0 7.0

M(T) and ρ(T) in different fields

Features:

(1) The martensitic transformation with thermal hysteresis.

(2) Martensitic transformation is magnetic field dependent.

(3) Magnetic field shift transition to lower temperature.

(4) The transformation can be totally suppressed by magnetic field.

(5) Currie temperature Tc=315K

(6) Low-T martensite is ferrimagnetic, poor metal.

(7) High T, ferromagnetic austenite, metal

(8) Martensitic transformation is accompanied by a metal-poor metal transition.

GMR in a broad temperature range

(APL91,2007)

0 50 100 150 200 250 300 350 4000

20

40

60

80

100

-Δρ/ρ(

0)

T (K)

At 5K, for Crystal 1 MR =86%

GMR in a broad temperature range

Giant MR and Tunneling MR

H

ρ

ρΤ ρ||

Giant MR (GMR) ρΤ = ρ||

-120

-80

-40

0

40

80

120

-10 -8 -6 -4 -2 0 2 4 6 8 10-80

-60

-40

-20

0

0 3 6 935

40

a

H (T)

MR

(%)

M (e

mu/

g)

b

20 K 70 K 90 K 170K

20 K

M(H) and MR(H) in different fields

M(H) curves indicate a metamagneticbehavior or field-induced phase transition

ρ(H) or MR (H) has the similar transition fields

Superzone GapSuperzone gap: The antiferromagnetic lattice does not

commensurate with the crystal lattice, which leads to a new Brillouin boundary

(a gap appearing on the Fermi surface).

• In intermetallic alloys, the large MR results from the collapsing of the “superzone gap” due to field induced first order phase transition. (UNiGa, PRL77,5253).

0 50 100 150 200 250 300 3500

5

10

15

0

50

100

150

0.0

0.5

1.0

1.4

1.9

T (K)

c

3 T

0 T

a

0.01 T

3 T x12

M (e

mu/

g)

b

κ (w

/K.m

)

3 T

0 T

ρ (μΩ

.m)

Ni50Mn33.7In16.3 single crystal

Showing the similar martensitictransformation, metal-poor metal transition, GMR and Giant Thermalconductivity.

Free electron Wiedemann-Franz law

281045.2 −− Ω== KWxLTkσ

the upper bound for Δκel

The sum of Δκel and zero-field κgives total κ

0.0 2.0 4.0 6.0 8.00

20406080

100

-60

-40

-20

0

H (T)

T=60 K

Δκ/κ

(%)

Δρ/ρ

(%)

M(H) and MTC(H) in different fields

The largest magneto-thermal conductivity

0 50 100 150 200 250 300 3500

20

40

60

80

100

120 -Δρ/ρ(0) Crystal 1 -Δρ/ρ(0) Crystal 2 -Δk/k(0) Crystal 2

-Δρ/ρ(

0) &

Δk/

k(0)

(%)

T (K)

At 5K, for Crystal 1 MR =86%

Specific heat measurement

0 50 100 150 200 250 300 350 4000

100

200

300

400

500

600

Curie temperature

Martensitic transition

0 T 9 T

C p (μJ

/K.m

g)

T (K)

Specific heat measurement

0 200 400

0.2

0.4

0.6γ9T=0.076(mJ/g.K2)

γ0=0.039(mJ/g.K2)

T2 (K2)

C P/T (μ

J/K2 .m

g)

CP=γT+AT3 ( ) ( ) 3/1*23/191 nmV Bk

m hπγ =

MR obtained from Specific Heat measurement

3/1)(0

90

9n

n TT =γγ 4.7

09 ≈n

n T

090 ]./1/1[)0(/)]9()0([ nnnTMR T−=−= ρρρ

τρ 2nem=

MR =86.4%

Excellent agreement

Conclusion

Large magnetoresistance (MR) and magnetothermal conductivity are due to the collapse of Superzone gap, when the magnetic field induced ferrimagnetic to the ferromagnetic phase transformation happens

A combined giant inverse and normal magnetocaloric effect for

room-temperature magnetic cooling

(PRB76,2007)

Magnetocaloric effect (MCE)H, T2Spin ordered

H=0, T1Spin disordered

H

dHT

THMTHSTHSSH

HHMMM ∫ ∂

∂=−=Δ

2

1

)),((),(),( 12

Ferromagnet

TC

M

agne

tizat

ion

Temperature (K)

Only near the TC, dM/dT is large, leads to a significant MCERoom temperature application: GdGeSi , LAFeSi alloys

Ni50Mn33.13In13.90 single crystal

0 50 100 150 200 250 300 350 4000

20

40

60

80

0 100 200 300 400

0500

10001500200025003000

9 T

5 T

1 T

T (K)

C (J

kg-1K-1

)

M (e

mug

-1)

T (K)

Martensite, Ferrimagnet with Tc = 183 KAustenite , Ferromagnet with Tc = 315 K

Tp=310K

Metamagnetism vs Phase-transition

0.0 2.0 4.0 6.0 8.0 10.0

0

10

20

30

40

50

60

70

80

0.0 5.0 10.00

204060

312.5

307.5 302.5 297.5

295

292.5

290

M (e

mug

-1)

H (T)

310 315 320 325

Calculation of entropy change

)1(1

1∑ Δ+−

−+=Δ−

i iHiMiMiTiT

SM

consantSM

H=

ΔΔ

=ΔΔθ

Clausius-Clapeyron equation

Entropy change vs temperature

290 300 310 320 330 340-35

-30

-25

-20

-15

-10

-5

0

5

10

15

C-C equation

2 T 3T 4T 5T 6T 7T 8T 9T

9T

2T

a

-ΔS

(JK

g-1K-1

)

T (K)

Loading

Heat exchanger

H

Cooling water Cooling water

Valve

Tube Loading

Heat exchanger

H

Cooling water Cooling water

Loading

Heat exchanger

H

Cooling water Cooling water

Valve

Tube

Acknowledgement

• Collaborators:• Bei Zhang, HKUST• Guangheng Wu, Institute of Physics, Beijing• Jinglan Chen, Institute of Physics, Beijing• Shuyun Yu Institute of Physics, Beijing• ….

Financial supports 1) Hong Kong RGC 2) National Natural Science Foundation of China

(50571113)