Novel behaviour in admixed Rare Earth systems : Exchange...

Novel behaviour in admixed Rare Earth systems :

Exchange Bias, repeated magnetic compensation

and self-compensation

Department of Condensed Matter Physics & Materials Science

Tata Institute of Fundamental Research, Mumbai

*Collaborators: Dhar S. K., Kulkarni P. D., Nigam A. K., Paulose P. L.,

Rakhecha V. C., Ramakrishnan S., Sumithra S., Thamizhavel A., Venkatesh S.,

Vaidya U. V., etc.

Feb. 7, 2010, IITK:GJ Plenary

A. K. Grover*

Plan

1. Brief recap. on the characteristics of magnetic Rare Earth (RE) elements,

with special focus on Samarium .

2. A glance through investigations in admixed RE alloys belonging to the

ferromagnetic series and imbibing magnetic compensation feature in

1960s, 1970s and 1980s.

3. Contemporary interest in alloys at „zero-magnetization‟ stoichiometry.

4. Elucidation of notion of the Exchange Bias field in compensated admixed

RE based magnets, along with other interesting features.

5. Identification of self-compensation in pristine Sm magnets via fingerprint

of an Exchange Bias .

6. Synthesis of materials useful for Spintronics at ambient temperatures.

7. Discovery of novel Repeated Magnetic Compensation behavior in the

admixed rare earth inter-metallics.

Highlights of ongoing work at TIFR on „zero magnetization‟ systems

Periodic Table

3d

4f

Magnetic Rare Earth Elements: 58 - 70

gJ

The ground state properties of the magnetic rare earth ions

mL gJJz

Spin –Orbit coupling prevails and

the ground state is given by:

J = L + S, for more than half filled

J = L – S, for less than half filled.

L - orbital angular momentum,

S - spin angular momentum and

J - total angular momentum

<Lz> = Jz (gJ – 2), <Sz> = Jz (1 – gJ)

z = - B (<Lz> + 2 <Sz>)

Rare Earths Elements: The 4f- electrons give rise to magnetic moment.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14-8

-4

0

4

8

12

<z

B (<L

z>+2<S

z>)

<Lz>

Opera

tor

avera

ge v

alu

e

Number of 4f Electrons (RE3+

)

Gd

2<Sz>

z

Sm

<Lz> = -30/7, <Sz> = 25/14,

z (Sm3+) = 5/7 B

<Lz> = Jz (gJ – 2),

< Sz> = Jz (1 –gJ)

<z > = gJ Jz

For free

Sm3+:

The RKKY Interaction:

Indirect Exchange: mediated by local

s-f interactions by conduction

electrons.

H = -Jsf S.s

S is local rare earth spin

s is conduction electron spin.

Jsf is exchange coupling constant.

Variation of coupling constant jff, with distance (r)

from rare earth ion. It is oscillatory and damped

in space.

FM

AFM

Heff (two rare earth ions) = -jff S1.S2

F(x) = (x cosx – sinx) / x4

x = kF r

jff -Jsf2 Σ F(x)

H. J. Williams et al. J. Phys. Soc. of

Japan 17 (Suppl 1), 91 (1962)

Magnetization data in Pr1-xGdxAl2

1961 International Conference on Magnetism Kyoto, Japan

PrAl2: Tc ~ 35 K

μ / f.u. ~ 2.8 μB

GdAl2: Tc ~ 170 K

μ / f.u. ~ 7.7 μB

Pr1-xGdxAl2

x = 0.2

• Zero Magnetization

stoichiometry realized at x

~ 0.3

Pseudo-antiferromagnetism

• Magnetic compensation

phenomenon at x ~ 0.2 Tc

~ 80 K, Tcomp~ 40 K

• Magnetic correlation effects

present even above nominal

Tc (~ 40 K) at x ≈ 0.1

W. M. Swift et al. J. Phys Chem Solids 29, 2053 (1968)

NdAl2 : Tc ~ 65 K

HoAl2 : Tc ~ 35 K

Nd1-xHoxAl2

x = 0.25

Tc ~ 65 K

Tcomp~ 25 K

Magnetic compensation phenomenon in Nd1-xHoxAl2 : x ~ 0.25

μNd / f.u. ~ 2.5 μB

μHo / f.u. ~ 7.7 μB

1.0

0.75

0.5

0.1

0.25

NdAl2

RE based „Zero-magnetization spin -ferromagnets‟

JR′ μR′LR′SR′μR SRJR LR

R : First half R′ : Second half

ZMSF (R1-xR′xXn) : (1-x) μR ≈ x μR′

Schematic

JGd μGdSGdμPr SPrJPr LPr

Pr Gd

Pr1-xGdxAl2

R1-xR′xXn

„Zero-magnetization System‟

• Every site on a crystalline sub-lattice is randomly occupied by two

species of magnetic ions, which are coupled antiferromagnetically.

• The relative compositions of two antiferromagnetically coupled

magnetic ions is such that the net magnetization of the system is nearly

ZERO.

• Conveniently realized in a Ferromagnetic Rare Earth (RE)

Intermetallic Series

RAl2 : Cubic Cu2Mg structure, R atoms form a diamond lattice.

Hyperfine fields in Sm1–xGdxAl2 alloys - Microscopic evidence

for ferromagnetic coupling between rare earth spinsJ. Appl. Phys. Volume 50, pp. 7501-7503 (1979)

A. K. Grover, S. K. Malik and R. Vijayaraghavan

Tata Institute of Fundamental Research, Bombay 400 005, India

K. Shimizu

Faculty of Education, Toyama University, Toyama, Japan

Abstract : The results of hyperfine field studies on Sm1–xGdxAl2 alloys for 0<x<0.5 are

reported. The hyperfine field at Al, H(Al), has been measured to be +32.5 kOe in SmAl2 and

-47 kOe in GdAl2. In Sm1–xGdxAl2 alloys, we find that the magnitude of H(Al) increases with

increasing x and further H(Al) becomes negative even with small replacement of Sm by Gd.

H(Al) in these compounds is proportional to the average value of spin per rare earth ion. The

observed behaviour can be understood in terms of a ferromagnetic coupling between the spins

of Sm and Gd.

Hyperfine field at Al α < Sz >RE

SmAl2: Ferromagnet, Tc ~ 125 K

L =5, S= 5/2, J =5/2 , gJ = 2/7, <Lz> = -30/7, <Sz> = 25/14,

z(Sm3+) = gJJ = 5/7 B

GdAl2: Ferromagnet, Tc ~ 170 K

L =0, S= 7/2, J =7/2, gJ = 2

z(Gd3+) = 7 B

Magnetic Moment / formula unit of SmAl2 ~ 0.2B

Magnetic Moment / formula unit of GdAl2 ~ 7.7 B

Free Sm3+ ions

Free Gd3+ ions

d) Magnetic Interaction Studies:

The magnetic hyperfine interaction of 111Cd in Sm1-xGdxAl2 compounds

was studied at 80 K for Gd concentration of 2-10 at %. A least square fit

program has been developed to fit the TDPAC patterns in case of such

combined magnetic and quadrupole interactions. The results show that the

hyperfine magnetic field 111Cd in these compounds reverses sign at a Gd

concentration of 3 at %.

An extract from TIFR Annual Rep. 1979-80

Report of Nuclear Spectroscopy Group ( page 48)

Mössbauer and TDPAC Studies of Rare Earth Intermetallic Compounds

Can Admixed Rare Earth Intermetallics be described as Spin

Glass Systems?A. K. Grover, S. K. Dhar, S. K. Malik and R. Vijayaraghavan

Proc. of DAE N.P. and SSP Symposium, 21C, 546 (1978)

A. K. Grover et al., Proc.

DAE SSPS, 26 B, 252-254 (1983)

Presented at DAE Solid State Symposium (SSP) Mysore, December 1983

R

1%

3%

4% 2%

• Magnetic reorientation

(compensation) seen for 1% to 3% Gd

doping in DC magnetization response

• Resistance data fingerprints drop of

spin disorder contribution at Tc

No fingerprint of compensation or

reorientation in resistance data

• AC-susceptibility registers freezing of

Gd moments as peaks

Magnetic compensation phenomenon in Sm1-xGdxAl2

RRh3B2 series: Crystal structure of the type CeCo3B2

Hexagonal crystal structure of the type CeCo3B2 for the RRh3B2 compounds.

Space Group: P6/mmm

a = 5.484 Å

c = 3.087 Å

Tc = 110 K,

moment / f.u. = 0.4 μB

M versus T in zero field cooled and field cooled states in Ce0.95Gd0.05Rh3B2

Grover, A. K. & Dhar, S. K. Effect of exchange field on the anomalous behavior of 4f state of Ce in Ce1-

xGdxRh3B2 in Experimental& Theoretical aspects of Valence Fluctuations, eds. Gupta, L. C. & Malik, S. K.

(pp. 481, Plenum Press, New York

Effect of Gd substitution on anomalous ferromagnetism in CeRh3B2

Presented at International Conference on Valence Fluctuations, Bangalore, January, 1987

RRh3B2 Tc

CeRh3B2 ~ 110 K

GdRh3B2 ~ 90 K

NdRh3B2 ~ 10 K

Comparison of magnetic compensation response in

Sm0.98Gd0.02Al2 and Nd0.85Gd0.15Rh3B2

(Grover and Dhar, 1986)

“A ferromagnet having no magnetic moment”

H. Adachi, H. Ino Nature, 401, 148 (1999)

„Self-Compensation‟?

Temperature dependence of Magnetization in (pseudo) Ferrimagnetic system

Sm1-xGdxAl2

Contemporary Interest

A magnetic moment

comprising nearly equal and

opposite sub-parts, as for

example, orbital and spin

contributions to the total

magnetic moment of Sm3+.

Materials useful for (i) Detectors with no self-stray field for spin-

charge processing, (ii) Magnetic STM tips and (iii) Spintronics.

Magnetization

signal in a Sm

system

Orbital

contribution

from Sm

Spin

contribution

from Sm

Cond. Electron

Polarization

contribution

= + +

+ – –Notionally : Orbital contribution > Contribution from spins

“ Orbital Surplus” system

Occassionally : Contribution from spins > Orbital contribution

“ Spin Surplus” system

Sm ion in a metallic matrix

Admixture of higher excited states into the ground state due to strong Exchange field

and crystalline electric field.

Different temperature dependences for the „spin‟ and „orbital‟ parts of the Samarium

moments

Adachi et al. :

Differences in M-T Curves

in Ferromagnetic

Samarium compounds

H. Adachi et al, Phys. Rev. B, 59, 11445 (1999)

mtotal

(μB)

m4f

(μB)

-Lz -2Sz mcond

(μB)

SmZn 0.05 -0.36 -4.01 3.65 0.41

SmCd 0.06 -0.34 -4.19 3.85 0.40

SmAl2 0.26 0.50 4.37 -3.87 -0.24

Calculated T = 0 values

Notion of orbital and spin surplus

moments in Sm systems

Magnetic compensation in a single crystal of Nd0.75Ho0.25Al2

0 20 40 60 80 1000.00

0.08

0.16

0.24

-0.08

-0.04

0.00

0.04

0.08

H || [100]

H || [110]

H || [111]

H = 5 kOe

M (f.u.)

Temperature (K)

(a)

(b)

T* (24 K)

Tcomp

(24 K)

H = 0

Nd0.75Ho0.25Al2M (f.u

.)

H || [100]

H || [110]

H || [111]

Tc

(70 K)

CEP

Ho

Nd

H

Low field zero-crossover and the field induced reversal in magnetic momentsPrasanna D. Kulkarni et al., Europhysics Letters 86,47003 (2009)

RECENT STUDIES AT TIFR

Nd3+ and Ho3+ ions

randomly occupy the

sites on a diamond lattice.

-4 0 4

-0.10

-0.05

0.00

0.05

0.10

23.75 K

24 K

20 K

27 K

Field (kOe)

M (f.u

.)

Magnetization hysteresis loops in pseudo-ferrimagnetic Nd0.75Ho0.25Al2

M-H response is anti-ferromagnetic like very close to Tcomp

-0.8 -0.4 0.0 0.4 0.8

-0.01

0.00

0.01

(T < Tcomp

)

Nd0.75Ho0.25Al2 24 K

Magnetic Field (kOe)M

(f.u

.)

23.75 K

(T > Tcomp

)

Magnetization hysteresis loops in the very close vicinity of Tcomp

(Left/Right) Shift in M-H loop is called Exchange Bias Field

Prasanna D. Kulkarni et al., Europhysics Letters 86,47003 (2009)

Note the visible shift in the M-H loop at 23.75 K and 24 K and the symmetric M-H loop

at 23.85 K.

-0.8 -0.4 0.0 0.4 0.8

-0.01

0.00

0.01H = 23.85 K

Field (Oe)

M (f.u

.)

Tcomp

0

400

800

1200

1600

10 20 30 40 50 60

-400

0

400

Hce

ff (

Oe)

He

xch(O

e)

(f)

(e)

Temperature (K)

H || [100]

Nd0.75Ho0.25Al2

H || [100]

Exchange bias in Nd0.75Ho0.25Al2

P. Kulkarni et al., Europhysics Letters 86,47003 (2009)

The notion of exchange bias is observed in the admixed rare earth intermetallics near Tcomp

P. Kulkarni et al., IEEE Trans. Magn. 45, 2902 (2009)

Half-width of the

M-H loop

Shift in the M-H

loop

EXCHANGE BIAS FIELD IN FM/AFM LAYERS (USED IN SPIN VALVES)

J. Nogues and I. K. Schuller, JMMM 192,

203 (1999).

SPIN VALVE

Substrate

Ferromagnetic

Conductor

Ferromagnetic

Antiferromagnetic

Comparison of the multi-layer structure and the admixed

RE alloys

<Ho

>

(b)(a)

Admixed RE alloy

AF

T < TN

H

T > Tcomp

<CEP

>

<Nd

>

CEP

>

<Nd

>

<Ho

>

T < Tcomp

H

F

F/ AF Multi layers

The soft conduction electron contribution could assume a role of the

ferromagnetic layer near the magnetic compensation region

Prasanna D. Kulkarni et al., Europhysics Letters 86,47003 (2009)

0

50

100

150

200

0 20 40 60 80 100-3

-2

-1

0

0.2

0.4

0.6

(C)

(B)0 10 20 30 40 50

0

100

(b)

H (kOe)

C

p (

mJ/

g-K

)

0 20 40 60 80 100

10

15

20

Tc

( 70 K)

zero field

R (10

-5 o

hm

s)

T (K)

( 24 K)

M (f.u

.) H = 0

Temperature (K)

(10 kOe)H || [100]

H || [100]

R

/R(0

) %

Cp (

mJ/g

-K)

H || [100]

(20 kOe)

H = 10 kOe

( 24 K)

Tc

(A)

26 28 3060

70

80

C

p

( 29 K)

Cp (

mJ/g

-K)

T (K)

H || [100]

(20 kOe)

(a)

( 29 K)

( 29 K)

Tc

Tc ( 70 K)

Nd0.75Ho0.25Al2

Magnetic compensation in a single crystal of Nd0.75Ho0.25Al2

Field induced reversal in magnetic moments of Ho/Nd imprints aa a peak in the

specific heat data

Oscillatory response in the magneto-resistancePrasanna D. Kulkarni et al., Europhysics Letters 86,47003 (2009)

Magnetic compensation in (polycrystalline) Pr1-xGdxAl2

0 10 20 30 40 50 60 70 80 90 100

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

MFCC

Mag

netiz

atio

n (

f.u.

)

Temperature (K)

Pr0.8

Gd0.2

Al2

MREM

Prasanna D. Kulkarni et al., DAE-SSP 53, 1125 (2008)

Magnetization crosses M = 0 axis at a specific temperature, marked as Tcomp, in nominal

zero field

In higher values of the external magnetic field, a magnetic turn around behaviour is

observed.

0 10 20 30 40 50 60 70 80 90 100-0.01

0.00

0.01

-0.5

0.0

0.5

H ~ 1 OeMag

netiz

atio

n (

f.u.

)

Tcomp

( 38 K)

H = 14 kOe

T* ( 39 K)

Tc

( 64 K)

Mag

netiz

atio

n (

f.u.

)

Temperature (K)

Pr0.8

Gd0.2

Al2

Prasanna D. Kulkarni et al., DAE-SSP 53, 1125 (2008)

Fingerprint of Reorientation in AC-susceptibility, specific heat

data and phase reversal in the exchange bias field

400

500

600

700

800

10 20 30 40 50 60 70 80

Temperature (K)

H ~ 1 Oe

H = 20 kOe

H = 50 kOe

emu/

g)

Cp

/ T

(m

J/m

ol-K

2)

Pr0.8

Gd0.2

Al2

T* ( 39 K)

(b)

0.001

0.01

Tc T* ( 39 K)

Tc

( 64 K)

H = 50 kOe

H = 0.5 kOe

H ~ 1 Oe

(a)

0

1000

2000

0 20 40 60 80

-40

0

40

Hc

eff (

Oe)

Hex

ch(O

e)

Temperature (K)

Pr0.8Gd0.2Al2

The electrical resistance data in admixed RE systems

Prasanna D. Kulkarni et al., to be submitted

Spin-disorder resistivity drops out at the magnetic ordering temperature in H = 0 Oe

The oscillatory response in the magnetoresistance data recorded upto 50 kOe

0 20 40 60 80 100 120

1.0

1.5

zero field

50 kOe

Re

sis

tivity (

10

-4 o

hm

-cm

)

Temperature (K)

Tc ( 64 K)

Pr0.8Gd0.2Al2

Spin

disorder

resistivity

0 20 40 60 80

-4.0x10-6

0.0

4.0x10-6

T* ( 38 K)

RH

all (o

hm

)

T (K)

Tc ( 64 K)

H = 10 kOe

0 20 40 60 80 100

-4

0

4

T* ( 40 K) Tc ( 64 K)

Pr0.8Gd0.2Al2

Temperature (K)

R

MR

(%

) H = 50 kOe

The electrical resistivity measurements in the presence of the

magnetic field

I+ I-

V-V+

I+ I-

+VH

-VH

H

H

Magnetic compensation in „spin surplus‟ Sm0.98Gd0.02Al2

The „orbital-surplus‟ SmAl2 alloy is doped with Gd3+ ions to aid the spins of the Sm3+

ions.

The temperature dependence of the net magnetization display a low field zero-crossover

and a minimum at high fields.

Prasanna D. Kulkarni, S. Venkatesh et al., IEEE Trans. Magn. 45, 2902 (2009)

20 40 60 80 100 120 140

-0.04

0.00

0.04

0.08

Tc( 125 K)

ZFC

M (

B / f.u

. )

10 kOe

Tcomp

( 82 K)

Temperature (K)

Sm0.98Gd0.02Al2

-20 -10 0 10 20

-25

-20

-15

-10

-5

0

5

10

15

20

25Sm0.98Gd0.02Al2

M

(1

0-3

B / f. u

)

H (kOe)

95 K

100 K

120 K

-20 -10 0 10 20

-12

-8

-4

0

4

8

12

H (kOe)

M (

10

-3

B / f. u

)

Sm0.98Gd0.02Al2

82 K

85 K

88 K

-40 -20 0 20 40

-80

-60

-40

-20

0

20

40

60

80

M (

10

-3

B / f. u

)

H (kOe)

65 K

45 K

30 K

5 K

Sm0.98Gd0.02Al2

-40 -20 0 20 40

-20

-10

0

10

20 Sm0.98Gd0.02Al2M

(1

0-3

B / f. u

)

H (kOe)

81 K

78 K

75 K

Hysteresis loops of Sm0.98Gd0.02Al2

S. Venkatesh et al., to be submitted

0 20 40 60 80 100 120

-200

0

200

0

2000

4000

6000

Hexch(O

e)

Hce

ff (

Oe

)

(b)

Sm0.98Gd0.2Al2

(a)

Temperature (K)

The notion of exchange bias is observed in the „spin surplus‟ Gd doped SmAl2 with near

zero magnetization stoichiometry

The generic temperature dependence of the effective coercive field and the exchange

bias field in the admixed alloys

Temperature dependence of the exchange bias and effective

coercive field in „spin surplus‟ Sm0.98Gd0.02Al2

Prasanna D. Kulkarni, S. Venkatesh et al., IEEE Trans. Magn. 45, 2902 (2009)

Step change in the temperature dependence of

magnetization, concomitant with the phase

transition like attribute across Tcomp

X. H. Chen ….C. W. (Paul) Chu,

Phys. Rev. B 72, 054436(2005)

Notion of field induced spin-

orbit reorientation as a phase

transition across compensation

temperature

1%

2%

Fingerprint of Compensation

phenomenon in Specific heat of

Sm1-xGdxAl2 (x = 0.01 and

0.02)

Fingerprint of phase transition observed in the specific heat data as well as the

magnetization data ( TIFR data)

9.0

9.3

9.6

9.9

1.8

2.1

2.4

65 70 75 80 85 90 95 100

0.0

1.5

3.0

4.5

6.0

-300

-150

0

150

300

high field

7 T FCC

Sm0.98Gd0.02Al2

Temperature (K)

M (

10

-2

/

f.u

)

Hc (

kO

e)

HC

82

h

f (arb

. un

its)

He

xch

(O

e)

Hexchange

S. Venkatesh et al., to be submitted

Examples of self-compensation

The temperature dependence of the magnetization and

the hysteresis loops in ferromagnet SmPtZn

P. D. Kulkarni, S. K. Dhar, et. al., to be submitted

0 10 20 30 40 50 60 70 800.000

0.005

0.010

0.015

0.020

T (K)

M (

B / f

.u.)

SmPtZn

Tc()

H = 10 kOe

-10 -5 0 5 10

-0.02

-0.01

0.00

0.01

0.02

T = 10 K H+

M (

B /

f.u

.)H (kOe)

SmPtZn

H-

T = 30 K

Hexch

1 kOe

Sm3+ ions are on a FCC lattice

0 5 10 15 20 25 30 35 40

0

400

800

1200

1600

He

xch (

Oe

)

Temperature (K)

SmPtZn

Tc = 48 K

The temperature dependence of the exchange bias field

in SmPtZn compounds

10 20 30 40

0

800

1600

2400

3200

He

xch (

Oe)

Temperature (K)

SmCu4Pd

Tc

-40 -20 0 20 40-0.2

-0.1

0.0

0.1

0.2

0 10 20 30 40 50 600.00

0.04

0.08

0.12SmCu

4Pd

M (

B / f.u

.)

Temperature (K)

Tc

( 31 K)

SmCu4Pd

T = 5 K 20 K

Hexch

= 3.1 kOe

H (kOe)

M (

B /

f.u

.)

H+

H-

Hysteresis loops in the ferromagnet SmCu4Pd

Sm3+ ions occupy a unique site in the cubic (AuBe5) structure

K. V. Shah, …S. K. Dhar, J. Magn. Magn. Mater. 321, 3164 (2009)

P. D. Kulkarni, S. K. Dhar et. Al., to be submitted

0 10 20 30 40 500.00

0.04

0.08

0.12

x = 0.03

x = 0.015

x = 0.04

x = 0

x = 0.01

M (

B / f.u

.)

Temperature (K)

x = 0.02

Sm1-x

GdxCu

4Pd

H = 10 kOe

Temperature dependence of the magnetization in

Sm1-xGdxCu4Pd series: Sm/Gd ions are on a FCC

lattice

Magnetic compensation in Sm0.975Gd0.025Cu4Pd

0

50

0 5 10 15 20 25 30 35 40

0

500

0

0

0.0010.005

H = 50 Oe

Tc

Tc

Tcomp

Tcomp

M (

B /

f.u

.)

Temperature (K)

Hc

eff (

Oe

)

Tc

2 kOe

( 21 K)

( 31 K)

He

xch (

Oe)

Sm0.975

Gd0.025

Cu4Pd

(c)

(b)

(a)

-2 -1 0 1 2-0.004

-0.002

0.000

0.002

0.004

H (kOe)

M (

B /

f.u

.)

T = 24.5 K

-1 0 1-0.002

0.000

0.002

H (kOe)

T = 18.5 KM (

B /

f.u

.)

0 10 20 30

0

20000

Sm1-x

GdxCu

4Pd

H

exch (

Oe)

Temperature (K)

x = 0

0.01

0.015

0.02

0.04

0.03

Temperature dependence of the exchange bias

field in Sm1-xGdxCu4Pd series

0 40 80 120 160

0.1

Sm0.96

Gd0.04

Al2 H = 7 T

M (

B /

f.u

)

T (K)

Exchange bias in „spin-surplus‟ Sm1-xGdxAl2 (x = 0.03, 0.04)

The pseudo-antiferromagnetic response in

Sm0.96Gd0.04Al2 alloy

Note the monotonically increasing positive

exchange bias field at lower temperatures in

Sm0.96Gd0.04Al2 in contrast to its sign

reversal in Sm0.97Gd0.03Al2

No exchange bias in „spin-surplus‟

Sm0.94Gd0.06Al2

0 20 40 60 80 100 1200

4

8

12

16

20

24

-1

0

1

2

3

4

HC (

kO

e)

T (K)

x = 0.03

x = 0.04

x = 0.06

HE

xch

(kO

e)

x = 0.03

x = 0.04

(Sm1-x

Gdx)Al2

0 50 100 150 200 250 300

0.00

0.05

0.10

0.15

0.20

0.25

SmScGe

10 kOe

70 kOe

M (

B /

f.u

.)

Temperature (K)

H = 50 Oe

Tc

(a)

-60 -40 -20 0 20 40 60-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

200 K

100 K

H+

Field (kOe)

M (

B /

f.u.

)

50 K

SmScGe

H-

Hexch

= -(H+ + H

-) / 2

(b)

The pseudo-antiferromagnetic behavior in pristine SmScGe.

Note the magnetic ordering temperature Tc ~ 270 K, close to the ambient temperatures.

Zero magnetization ferromagnet useful at ambient temperatures:

SmScGe

P. D. Kulkarni et al., J. Phys. D: Appl. Phys. 42 (2009) 082001

FAST TRACK COMMUNICATION

“New samarium and neodymium based admixed ferromagnets with near-zero net

magnetization and tunable exchange bias field”

Magnetic compensation in „spin surplus‟ SmScGe

The magnetic compensation in „spin

surplus‟ SmScGe could be achieved with

substitution of 9 at. % Nd3+ ions.

Note the net magnetization values over

the magnetically ordered state are < 0.15

μB even upto 50 kOe.

The magnetic ordering temperature is

close to the ambient temperatures, most

suitable for applications

Prasanna D. Kulkarni et al., J. Phys. D: Applied Physics (fast track communication) 42,082001,2009

0 50 100 150 200 250 300

0.01

0.1

1

10

100

Sm0.94

Nd0.06

ScGe

SmScGe

T (K)

Hex

ch (k

Oe)

P. Kulkarni et al., Unpublished

Tuning of the exchange bias in doped SmScGe alloy

0 50 100 150 200 250 300

0

5

10

15

20

25 Sm0.94

Nd0.06

ScGe

SmScGe

T (K)

Hc

eff (k

Oe)

P. Kulkarni et al., Unpublished

Effective coercive field in doped SmScGe alloy

-10 -5 0 5 10

-0.1

0.0

0.1

T = 20 KSmAl

2

0 50 100 150 200

0.00

0.05

0.10

Tc( 125 K)

T (K)

M (

B / f.u

.)

H = 10 kOe

Field (kOe)

M (

B / f.u

.)

Hexch

( Oe)

-60 -40 -20 0 20 40 60-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0 50 100 150 200 250 3000.00

0.05

0.10

0.15

0.20( 270 K)

SmScGe

H = 10 kOe

M (

B / f.u

.)

Temperature (K)

Tc

200 K

H+

Field (kOe)

M (

B / f.u

.)

50 K

SmScGe

H-

-40 -20 0 20 40

-0.04

-0.02

0.00

0.02

0.04

0 50 100 150 2000.00

0.01

0.02

0.03

0.04

T (K)

M (

B / f.u

.)

H = 10 kOe 18 kOe

Hexch

= -(H+ - H

-)/2

SmZn

H-

H+

T = 70 K

Field (kOe)

M (

B / f.u

.)

-10 -5 0 5 10

-0.02

-0.01

0.00

0.01

0.02

0 10 20 30 40 50 60 70 800.000

0.005

0.010

0.015

0.020

T (K)

M (

B /

f.u

.)

SmPtZn

Tc()

H = 10 kOe

T = 10 K H+

M (

B /

f.u

.)

H (kOe)

SmPtZn

H-

T = 30 K

Hexch

= 1.01 kOe

-40 -20 0 20 40-0.2

-0.1

0.0

0.1

0.2

0 10 20 30 40 50 600.00

0.04

0.08

0.12SmCu

4Pd

M (

B / f.u

.)

Temperature (K)

Tc

( 31 K)

SmCu4Pd

T = 10 K 20 K

Hexch

= 6.2 kOe

H (kOe)

M (

B / f.u

.)

H+

H-M-H loops in other ferromagnetic Samarium

compounds

Prasanna D. Kulkarni et al., PRB 78, 064426 (2008)

High field magnetization response in the admixed

Nd1-xGdxRh3B2 series of alloys

Note the paramagnetic state values of the net magnetization

0 10 20 30 40 50 600.0

0.5

1.0

1.5

-0.02

0.00

0.02

0.04

0.06

(b)

M (

B /

f.u

.)

Nd0.75

Gd0.25

Rh3B

2

M (

B / f.u

.)

(a)

0.0

0.1

0.2

0.3

(~14 K)

H

2 kOe70 kOe

H = 2 kOe Tf1

T*

Tf2

50 kOeT

f2T*

Tf1

300 Oe

140 Oe

50 Oe

M (

B / f.u

.)

T*

Tf2

H =

Temperature (K)

Tf1

Nd

Gd

Gd

0

Nd

0 10 20 30 40 50 60 700.0

0.5

1.0

1.5

2.0

35

40

45

0.1

0.2

0.3

M (

B / f.u

.)

zero field

50 kOe

(c)

(b)

(

10

-5 o

hm

-cm

)

Cp/T

(m

J/g

-K2)

Temperature (K)

(a)

Nd0.75

Gd0.25

Rh3B

2Tf2

Tf1

Tf2( 23 K)

Tf2

Tf1( 48 K)

H = 10 kOe

Exploration of Zero Magnetization stoichiometry :

Nd0.75Gd0.25Rh3B2

Oscillatory magnetic response is observed in the thermomagnetic curves

of Nd0.75Gd0.25Rh3B2.

The specific heat data displays two underlying magnetic transitions

(antiferromagnetic like).Prasanna D. Kulkarni et al., PRB 78, 064426 (2008)

0 50 100

0.0

0.4

0.8

1.2

1.6

x = 0.225

x = 0.15

x = 0.1

0.175

0.25

H = 10 kOe

Nd1-x

TbxAl

2

Temperature (K)

M (

B /

f.u.

)

x = 0

Magnetization curves in the single crystal Nd0.8Tb0.2Al2

0 20 40 60 80 100 120

-0.04

-0.02

0.00

0.02

0.04

H || [100]

Tf1

150 Oe

Nd0.8Tb0.2Al2

( 88 K)

Tf2

H = 125 Oe

( 32 K)

Temperature (K)

M (f.u

.)Low field FCC curves in the single crystal Nd0.8Tb0.2Al2:

Repeated magnetic compensation

0 10 20 30 40 50 60

1E-3

0.01

35

40

45

0.04

0.08

0.12

0.5

1.0

1.5

2.0

M (

B /

f.u.

)

H ~ 1 Oe

H ~ 1 Oe

Tf2

Tf2

Temperature (K)

Tf2

(

emu/

gm)

(

10-5

ohm

-cm

)C

p / T

(m

J/g-

K2)

Nd0.75

Gd0.25

Rh3B

2

Tf1

Tf2

( 23 K) Tf1

( 48 K)

Tf1H = 2 kOe

H ~ 1 Oe

0 20 40 60 80 100

1

2

35

10

15

0.0

0.1

0.2

0.3

0

20

40

' (

10-3

em

u/g

)

Temperature (K)

Tf2

Tf1

Tf1

Tf1

( 86 K)

Tf2

Tf2

Tf1

Tf2

( 34 K)

Nd0.8Tb0.2

Al2

R (

10-6

ohm

s)

Cp

(10

3 m

J/m

ole

-K)

M (

B /

f.u.

)

H ~ 1 Oe

H ~ 1 Oe

H ~ 1 Oe

H = 5 kOe

Repeated magnetic compensation phenomenon in polycrystalline

Nd0.75Gd0.25Rh3B2 and Nd0.8Tb0.2Al2 alloys: Comparative study

• New features:

(i) Exemplification of the existence of exchange bias field on approaching Tcomp

and its phase reversal across Tcomp in admixed RE intermetallics

(ii) Evidence of the „self compensation‟ behavior in the pristine „orbital-surplus‟

and „spin-surplus‟ Samarium intermetallics

(iii) Synthesis of alloys undergoing magnetic orderings close to the ambient

temperatures and possessing large CEP, but, near-zero bulk magnetization,

and permitting easy tuning of the exchange bias field for novel applications,

etc.

(iv) Oscillatory character in the magneto-resistance response, including a change

in its sign at Tcomp.

(v) Sign reversal in Hall resistance across Tcomp.

(vi) Identification of a step change in high field magnetization and its correlation

with the fingerprint of field-induced entropic change in specific heat data .

(vii) Novel repeated magnetic compensation behavior in some specific alloys .

Summary

[1] P. D. Kulkarni et al., Europhys. Lett. 86, 47003 (2009).

[2] P. D. Kulkarni et al., J. Phys. D: Appl. Phys. 42, 082001 (2009).

[3] P. D. Kulkarni et al., IEEE Trans. Magn. 45, 2902 (2009).

[4] P. D. Kulkarni et al., Phys. Rev. B 78, 064426 (2008).

[5] U. V. Vaidya et al., J. Magn. Magn. Mat. 317, 1761 (2007).

[6] U. V. Vaidya et al., AIP Conference Proceedings 1003, 204 (2008).

[7] P. D. Kulkarni et al., J. Phys.: Conf. Ser. 150, 042102 (2009).

[8] P. D. Kulkarni et al., Solid State Physics (India) 53, 1097 (2008).

[9] S. Venkatesh et al., Solid State Physics (India) 53, 1219 (2008).

[10] A. K. Grover et al., Solid State Physics (India) 52, 27 (2007).

[11] P. D. Kulkarni et al., Solid State Physics (India) (2009).

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