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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).
Publications:
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