magnetic phase competition in off-stoichiometric heusler...
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Magnetic Phase Competition in Off-Stoichiometric HeuslerAlloys: The Case of Ni50-xCoxMn25+ySn25-y
K. P. Bhatti, D.P. Phelan, C. LeightonChemical Engineering and Materials Science, University of Minnesota
V. Srivastava, R.D. JamesAerospace Engineering and Mechanics, University of Minnesota
S. El-KhatibPhysics, American University of Sharjah
NIST Center for Neutron Research
S. Yuan, P.L. Kuhns, A.P. Reyes, J.S. Brooks, M.J.R. HochNational High Magnetic Field Laboratory
-
Multiferroic alloys (ferroelastic + magnetic)
> Magnetoelasticity, magnetic shape memory,field-induced phase transformations, magnetocaloric and barocaloric effects
> Actuators/sensors, magnetic refrigeration, energy conversion, heat-assisted recording?
Thermal hysteresis: Vital for applications, but fundamentals elusive….
Introduction: Martensitic (Magnetic) Alloys
T
TM(first-order
MPT)
AusteniteMartensite
T (K)
Krenke et al.Nat. Mater. (2005)
-
Introduction: Minimization of Thermal Hysteresis
Recent theoretical work by James et al.: Geometrical compatibility between austenite and martensite unit cells vital for T.
An invariant plane occurs at their interface if, 2 = 1, where 1,2,3 are the ordered eigenvalues of the transformation stretch matrix, U.
Cui et al. Nat. Mater. (2006)Zhang et al. Acta. Mat. (2009)
Experiment: Remarkable correlations between T and2. Direct observation of “ideal” interface by HRTEM
T
-
Application to Magnetic Alloys: The Route to Ni50-xCoxMn40Sn10
Step I Full Heusler alloys Ni2MnZ, Z = Sn, In, Ga, etc. (e.g. Ni2MnSn, F, TC = 350 K)
[ ]
[ ]
[ ]
-
Application to Magnetic Alloys: The Route to Ni50-xCoxMn40Sn10
Step I Full Heusler alloys Ni2MnZ, Z = Sn, In, Ga, etc. (e.g. Ni2MnSn, F, TC = 350 K)
Step II Ni50Mn25+yZ25-y:Tunable magnetic interactions (F vs. AF)*MPT for y > 52 1. [ ]
[ ]
[ ]
(001) plane
excess Mn*Krenke, Acet, Wasserman, Moya, Manosa, Planes, PRB (2005, 2006)
-
Application to Magnetic Alloys: The Route to Ni50-xCoxMn40Sn10
Step I Full Heusler alloys Ni2MnZ, Z = Sn, In, Ga, etc. (e.g. Ni2MnSn, F, TC = 350 K)
Step II Ni50Mn25+yZ25-y:Tunable magnetic interactions (F vs. AF)*MPT for y > 52 1.
*Krenke, Acet, Wasserman, Moya, Manosa, Planes, PRB (2005, 2006)
?
-
Application to Magnetic Alloys: The Route to Ni50-xCoxMn40Sn10
Step I Full Heusler alloys Ni2MnZ, Z = Sn, In, Ga, etc. (e.g. Ni2MnSn, F, TC = 350 K)
Step II Ni50Mn25+yZ25-y:Tunable magnetic interactions (F vs. AF)*MPT for y > 52 1.
Step III Ni50-xCoxMn40Sn10: Enhanced TC, MSConvenient TM2 1 (1.0051 @ x = 6)
*Krenke, Acet, Wasserman, Moya, Manosa, Planes, PRB (2005, 2006)
?
-
Application to Magnetic Alloys: The Route to Ni50-xCoxMn40Sn10
Step I Full Heusler alloys Ni2MnZ, Z = Sn, In, Ga, etc. (e.g. Ni2MnSn, F, TC = 350 K)
Step II Ni50Mn25+yZ25-y:Tunable magnetic interactions (F vs. AF)*MPT for y > 52 1.
Step III Ni50-xCoxMn40Sn10: Enhanced TC, MSConvenient TM2 1 (1.0051 @ x = 6)
Ni46Co6Mn40Sn10: TC 440 K TM 380 K M 1000 emu/cm3
T < 10 K
*Krenke, Acet, Wasserman, Moya, Manosa, Planes, PRB (2005, 2006)
?
-
Magnetism in Ni50-xCoxMn40Sn10 (and Related Systems)
High TC, high M, convenient TM, low T
F austenite, non-F martensite (AF or P?)
Nominally non-F martensite reveals some significant surprises:
> Superparamagnetic-like behavior at low T (in a bulk solid!)> Exchange bias in blocked low T state (in a single phase)> Collective freezing. Super-spin-glass?> ZFC exchange bias
Acute sensitivity of magnetism to composition
Nanoscale MagneticInhomogeneity?
-
Experimental Details
0
100
200
300
400
Data
Model
Residual
(a) T = 410K
cubic, Fm-3m
Inte
nsity [
arb
. u
nits]
Ni44Co6Mn40Sn10
20 30 40 50 60 70 80 90 100
0
200
400
(b) T = 300K
5M monoclinic, P21
2[deg]
Arc-melted polycrystalline bulk ingots [Srivastava et al APL (2010);Bhatti et al PRB (2012)]
Characterized by:
> DSC
> T-dependent XRD
> T-dependent Neutron PowderDiffraction (NPD)
> Magnetometry
> Small-Angle Neutron Scattering (SANS)
> 55Mn zero-field NMR
-
Ni50-xCoxMn40Sn10 Phase Diagram
0 2 4 6 8 10 12 14
0
100
200
300
400
500
600
0 5 10 15 20 25
0
100
200
300
400
500
600
700
(b)
F (Aust.)
TB
TC
TM
P (Aust.)
x
AF/SP (Mart.)
EB Clustered
(Mart.)
Clustered
(Mart.)
e/a
T [K
]
e/a
Ni50-x
CoxMn
40Sn
10
TC
TM
P (Aust.)
T [K
]
y
F (Aust.)
AF (Mart.)
Ni50
Mn25+y
Sn25-y
(a)
8.20 8.15 8.10 8.067.8 8.0 8.2 8.4
?
-
Ni50-xCoxMn40Sn10 Phase Diagram
0 2 4 6 8 10 12 14
0
100
200
300
400
500
600
0 5 10 15 20 25
0
100
200
300
400
500
600
700
(b)
F (Aust.)
TB
TC
TM
P (Aust.)
x
AF/SP (Mart.)
EB Clustered
(Mart.)
Clustered
(Mart.)
e/a
T [K
]
e/a
Ni50-x
CoxMn
40Sn
10
TC
TM
P (Aust.)
T [K
]
y
F (Aust.)
AF (Mart.)
Ni50
Mn25+y
Sn25-y
(a)
8.20 8.15 8.10 8.067.8 8.0 8.2 8.4
?
-
Ni50-xCoxMn40Sn10 Phase Diagram
0 2 4 6 8 10 12 14
0
100
200
300
400
500
600
0 5 10 15 20 25
0
100
200
300
400
500
600
700
(b)
F (Aust.)
TB
TC
TM
P (Aust.)
x
AF/SP (Mart.)
EB Clustered
(Mart.)
Clustered
(Mart.)
e/a
T [K
]
e/a
Ni50-x
CoxMn
40Sn
10
TC
TM
P (Aust.)
T [K
]
y
F (Aust.)
AF (Mart.)
Ni50
Mn25+y
Sn25-y
(a)
8.20 8.15 8.10 8.067.8 8.0 8.2 8.4
?
-
Ni50-xCoxMn40Sn10 Phase Diagram
0 2 4 6 8 10 12 14
0
100
200
300
400
500
600
0 5 10 15 20 25
0
100
200
300
400
500
600
700
(b)
F (Aust.)
TB
TC
TM
P (Aust.)
x
AF/SP (Mart.)
EB Clustered
(Mart.)
Clustered
(Mart.)
e/a
T [K
]
e/a
Ni50-x
CoxMn
40Sn
10
TC
TM
P (Aust.)
T [K
]
y
F (Aust.)
AF (Mart.)
Ni50
Mn25+y
Sn25-y
(a)
8.20 8.15 8.10 8.067.8 8.0 8.2 8.4
?
-
Ni50-xCoxMn40Sn10 Phase Diagram
0 2 4 6 8 10 12 14
0
100
200
300
400
500
600
0 5 10 15 20 25
0
100
200
300
400
500
600
700
(b)
F (Aust.)
TB
TC
TM
P (Aust.)
x
AF/SP (Mart.)
EB Clustered
(Mart.)
Clustered
(Mart.)
e/a
T [K
]
e/a
Ni50-x
CoxMn
40Sn
10
TC
TM
P (Aust.)
T [K
]
y
F (Aust.)
AF (Mart.)
Ni50
Mn25+y
Sn25-y
(a)
8.20 8.15 8.10 8.067.8 8.0 8.2 8.4
?
-
0 100 200 300 400 5000
10
20
30
0 100 200 300 400 5000.00
0.02
0.04
0.06
0.08
M
[e
mu
/cm
3]
T [K]
Ni48
Co2Mn
40Sn
10
H = 5000 Oe
M [em
u/c
m3]
FCW
FCC
ZFCW
T [K]
H = 10 Oe
M vs. T (x = 2)
-
0 2 4 6 8 10 12 14
0
100
200
300
400
500
600
0 5 10 15 20 25
0
100
200
300
400
500
600
700
(b)
F (Aust.)
TB
TC
TM
P (Aust.)
x
AF/SP (Mart.)
EB Clustered
(Mart.)
Clustered
(Mart.)
e/a
T [K
]
e/a
Ni50-x
CoxMn
40Sn
10
TC
TM
P (Aust.)
T [K
]
y
F (Aust.)
AF (Mart.)
Ni50
Mn25+y
Sn25-y
(a)
8.20 8.15 8.10 8.067.8 8.0 8.2 8.4
M vs. T (x = 6)
?
-
0 100 200 300 400 500 6000
2
4
6
8
10
0 100 200 3000.00
0.05
0.10
0.15
0.20
FCC
ZFCW
M [
em
u/c
m3]
T [K]
H = 10 Oe
Ni44
Co6Mn
40Sn
10
M [
em
u/c
m3]
T [K]
M vs. T (x = 6)
-
0 2 4 6 8 10 12 14
0
100
200
300
400
500
600
0 5 10 15 20 25
0
100
200
300
400
500
600
700
(b)
F (Aust.)
TB
TC
TM
P (Aust.)
x
AF/SP (Mart.)
EB Clustered
(Mart.)
Clustered
(Mart.)
e/a
T [K
]
e/a
Ni50-x
CoxMn
40Sn
10
TC
TM
P (Aust.)
T [K
]
y
F (Aust.)
AF (Mart.)
Ni50
Mn25+y
Sn25-y
(a)
8.20 8.15 8.10 8.067.8 8.0 8.2 8.4
M vs. T (x = 14)
?
-
-75 -50 -25 0 25 50 75
-900
-600
-300
0
300
600
900
0 100 200 300 400 500 6000
2
4
6
8
10
5 K
300 K
550 K
H [kOe]
M [
em
u/c
m3]
FCC
FCW
ZFCW
T [K]
Ni36
Co14
Mn40
Sn10
M [
em
u/c
m3] H = 10 Oe
M vs. T (x = 14)
-
0 2 4 6 8 10 12 14
0
100
200
300
400
500
600
0 5 10 15 20 25
0
100
200
300
400
500
600
700
(b)
F (Aust.)
TB
TC
TM
P (Aust.)
x
AF/SP (Mart.)
EB Clustered
(Mart.)
Clustered
(Mart.)
e/a
T [K
]
e/a
Ni50-x
CoxMn
40Sn
10
TC
TM
P (Aust.)
T [K
]
y
F (Aust.)
AF (Mart.)
Ni50
Mn25+y
Sn25-y
(a)
8.20 8.15 8.10 8.067.8 8.0 8.2 8.4
M vs. H (x = 2)
?
-
M vs. H (x = 2)
-30 -15 0 15 30-80
-40
0
40
80-60 -40 -20 0 20 40 60
-50
0
50
-30
0
30
(c)
5 K
50 K
H [kOe]
Langevin fit
M [
em
u/c
m3]
(b)
200 K
175 K
150 K
125 K
M [
em
u/c
m3]
500 K
400 K
Ni48
Co2Mn
40Sn
10
(a)
-
M vs. H (x = 2)
-30 -15 0 15 30-80
-40
0
40
80-60 -40 -20 0 20 40 60
-50
0
50
-30
0
30
(c)
5 K
50 K
H [kOe]
Langevin fit
M [
em
u/c
m3]
(b)
200 K
175 K
150 K
125 K
M [
em
u/c
m3]
500 K
400 K
Ni48
Co2Mn
40Sn
10
(a)
T > TM: P
T < TM: AF/P
-
M vs. H (x = 2)
-30 -15 0 15 30-80
-40
0
40
80-60 -40 -20 0 20 40 60
-50
0
50
-30
0
30
(c)
5 K
50 K
H [kOe]
Langevin fit
M [
em
u/c
m3]
(b)
200 K
175 K
150 K
125 K
M [
em
u/c
m3]
500 K
400 K
Ni48
Co2Mn
40Sn
10
(a)
T > TM: P
T < TM: AF/P
300 - 100 K: Langevin-like M(H)
HTHT
Tk
Tk
HTTTnTHM BG
C
B
B
C
CC )()(
)(coth)()(),(
-
M vs. H (x = 2)
-30 -15 0 15 30-80
-40
0
40
80-60 -40 -20 0 20 40 60
-50
0
50
-30
0
30
(c)
5 K
50 K
H [kOe]
Langevin fit
M [
em
u/c
m3]
(b)
200 K
175 K
150 K
125 K
M [
em
u/c
m3]
500 K
400 K
Ni48
Co2Mn
40Sn
10
(a)
T > TM: P
T < TM: AF/P
300 - 100 K: Langevin-like M(H)
T < 100 K: M(H) gradually opens“Static” FZFC exchange bias*
HTHT
Tk
Tk
HTTTnTHM BG
C
B
B
C
CC )()(
)(coth)()(),(
*Wang et al PRL (2011)
-
0 2 4 6 8 10 12 14
0
100
200
300
400
500
600
0 5 10 15 20 25
0
100
200
300
400
500
600
700
(b)
F (Aust.)
TB
TC
TM
P (Aust.)
x
AF/SP (Mart.)
EB Clustered
(Mart.)
Clustered
(Mart.)
e/a
T [K
]
e/a
Ni50-x
CoxMn
40Sn
10
TC
TM
P (Aust.)
T [K
]
y
F (Aust.)
AF (Mart.)
Ni50
Mn25+y
Sn25-y
(a)
8.20 8.15 8.10 8.067.8 8.0 8.2 8.4
M vs. H (x = 6)
?
-
M vs. H (x = 6)
-75 -50 -25 0 25 50 75
-100
0
100
-0.6
0.0
0.6
-100
0
100
-0.6
0.0
0.6
-1000
-500
0
500
1000
-5.6
-2.8
0.0
2.8
5.6
-500
0
500
-2.8
0.0
2.8
-0.1 0.0 0.1-100
-50
0
50
100
(d)
H [kOe]
5K
30K
(c)M
[
B/f
.u.]
M [
em
u/c
m3]
80K
100K
150K
200K
300K
370K
(b)
400K
395K
390K
415K
550K
(a)
415K
-
M vs. H (x = 6)
-75 -50 -25 0 25 50 75
-100
0
100
-0.6
0.0
0.6
-100
0
100
-0.6
0.0
0.6
-1000
-500
0
500
1000
-5.6
-2.8
0.0
2.8
5.6
-500
0
500
-2.8
0.0
2.8
-0.1 0.0 0.1-100
-50
0
50
100
(d)
H [kOe]
5K
30K
(c)M
[
B/f
.u.]
M [
em
u/c
m3]
80K
100K
150K
200K
300K
370K
(b)
400K
395K
390K
415K
550K
(a)
415K
T > TC: P
T < TC: Soft F
-
M vs. H (x = 6)
-75 -50 -25 0 25 50 75
-100
0
100
-0.6
0.0
0.6
-100
0
100
-0.6
0.0
0.6
-1000
-500
0
500
1000
-5.6
-2.8
0.0
2.8
5.6
-500
0
500
-2.8
0.0
2.8
-0.1 0.0 0.1-100
-50
0
50
100
(d)
H [kOe]
5K
30K
(c)M
[
B/f
.u.]
M [
em
u/c
m3]
80K
100K
150K
200K
300K
370K
(b)
400K
395K
390K
415K
550K
(a)
415K
T > TC: P
T < TC: Soft F
400 - 380 K: Hysteresis region(H-induced MPT)
-
M vs. H (x = 6)
-75 -50 -25 0 25 50 75
-100
0
100
-0.6
0.0
0.6
-100
0
100
-0.6
0.0
0.6
-1000
-500
0
500
1000
-5.6
-2.8
0.0
2.8
5.6
-500
0
500
-2.8
0.0
2.8
-0.1 0.0 0.1-100
-50
0
50
100
(d)
H [kOe]
5K
30K
(c)M
[
B/f
.u.]
M [
em
u/c
m3]
80K
100K
150K
200K
300K
370K
(b)
400K
395K
390K
415K
550K
(a)
415K
T > TC: P
T < TC: Soft F
400 - 380 K: Hysteresis region(H-induced MPT)
380 - 60 K: Langevin-like M(H)
-
M vs. H (x = 6)
-75 -50 -25 0 25 50 75
-100
0
100
-0.6
0.0
0.6
-100
0
100
-0.6
0.0
0.6
-1000
-500
0
500
1000
-5.6
-2.8
0.0
2.8
5.6
-500
0
500
-2.8
0.0
2.8
-0.1 0.0 0.1-100
-50
0
50
100
(d)
H [kOe]
5K
30K
(c)M
[
B/f
.u.]
M [
em
u/c
m3]
80K
100K
150K
200K
300K
370K
(b)
400K
395K
390K
415K
550K
(a)
415K
T > TC: P
T < TC: Soft F
400 - 380 K: Hysteresis region(H-induced MPT)
380 - 60 K: Langevin-like M(H)
T < 60 K: M(H) gradually opens“Static” F
-
Superparamagnetic-like Behavior
102
103
104
50 100 150 200 250 300
1017
1018
1019
x = 2
x = 3
x = 4
x = 6
c [
B]
(a)
Ni50-x
CoxMn
40Sn
10
(b)
nc [
cm
-3]
T [K]
c 200-250 B
nc 2-3 x 1019 cm-3
dc 2-6 nm
Implication: Dense ensemble ofnm-scale spin clusters
HTHT
Tk
Tk
HTTTnTHM BG
C
B
B
CCC )(
)(
)(coth)()(),(
-
Exchange Bias
0
1000
2000
0 50 100 150 2000
200
400
600
800 Ni48
Co2Mn
40Sn
10
Ni44
Co6Mn
40Sn
10
Hc
[Oe]
HFC
= 70 kOe
(a)
(b)
HE [O
e]
T [K]
x = 2:
> SP blocking at 100-150 K> EB blocking at 60 K> Large HC peak
x = 6:
> SP blocking at 60 K> EB blocking at 60 K> No HC peak
-
Phase Diagram
-
SANS (NIST)
Detectorchamber
ReactorNeutronguide
Cryostat/magnet
-
0.01 0.1
0.1
1
10
100
q [Å-1]
(a) 500 K
d/d
[cm
-1]
n
P
q
Td
d
Tqd
d)(
,
2
2
)(
1
)(
,
Tq
Td
d
Tqd
d L
Low q (large length-scale)Porod Scattering
High q (short length-scale)Lorentzian Scattering
SANS (x = 6)
-
0.01 0.10.01
0.1
1
10
100
0.1
1
10
100
0.1
1
10
100
0.1
1
10
100
0.1
1
10
100
(e) 30 K
q [Å-1]
(d) 380 K
Ni44
Co6Mn
40Sn
10
(c) 390 K
(b) 425 K
(a) 500 K
d/d
[cm
-1]
SANS: q Dependence
-
0.01 0.10.01
0.1
1
10
100
0.1
1
10
100
0.1
1
10
100
0.1
1
10
100
0.1
1
10
100
(e) 30 K
q [Å-1]
(d) 380 K
Ni44
Co6Mn
40Sn
10
(c) 390 K
(b) 425 K
(a) 500 K
d/d
[cm
-1]
SANS: q Dependence
Low q Porod scattering:
> Increases below TC> Magnetic domain scattering > Proof of long-range F order (>> 100 nm)
High q Lorentzian scattering:
> Peaks at TC> F spin correlations
Intermediate q scattering:
> A peak develops! 2/q 120-200 Å> Structure factor peak. Liquid-like spatial
distribution of magnetic clusters.
2
2
)(2
))((exp)(
)(
),(T
TqqT
d
d
q
Td
d
Tqd
d G
G
n
P
Tot
2
2
)(
1
)(
Tq
Td
d
L
-
SANS: T Dependence
100 200 300
0.02
0.03
0.03
0 100 200 300 400 500
0.0
0.1
0.2
0.3
0
50
100
150
d/d
[cm
-1]
T [K]
(b) q = 0.1 Å-1
d/d
[cm
-1]
T [K]
Ni44
Co6Mn
40Sn
10
d/d
[cm
-1]
Heating
Cooling
(a) q = 0.005 Å-1 q = 0.005 Å
-1 (1200 Å):
> Sharp onset of domainscattering at TC
> Hysteretic loss at TM
q = 0.1 Å-1 (60 Å):
> Critical scattering peak at TC
-
SANS: T Dependence
100 200 300
0.02
0.03
0.03
0 100 200 300 400 500
0.0
0.1
0.2
0.3
0
50
100
150
d/d
[cm
-1]
T [K]
(b) q = 0.1 Å-1
d/d
[cm
-1]
T [K]
Ni44
Co6Mn
40Sn
10
d/d
[cm
-1]
Heating
Cooling
(a) q = 0.005 Å-1 q = 0.005 Å
-1 (1200 Å):
> Sharp onset of domainscattering at TC
> Hysteretic loss at TM
q = 0.1 Å-1 (60 Å):
> Critical scattering peak at TC
> 130 K transition (!), clearly associated with clusters
> Blocking at 10-12 s time-scales!
-
SANS T Dependence: Porod
0 100 200 300 400 500
2
3
410
-9
10-6
10-3
T [K]
n (d/d
) P
[cm
-1]
Scattering in martensite strong, T-dependent
Paramagnetic, n = 4 (grains); Ferromagnetic, n = 2 (pinned domains); Martensitic, n = 3 (AF domains?)
Additional (indirect) evidence for AF order in the martensite
-
0
1
0 100 200 300 400 500
0.03
0.04
0.05
(d/d
) G
[cm
-1]
[cm
-1]
qG[c
m-1]
T [K]
0.02
0.04
SANS T Dependence: Gaussian
Gaussian peak intensity actually emerges at high T (even above TC?)
Peak position weakly T-dependent. Peak width decreases on cooling
Correlation length > 5 inter-particle spacings! Collective response*?
*Cong et al APL (2010); APL (2010) and Wang et al PRL (2011)
Å Å
-
0
1
2
3
0 100 200 300 400 5000
20
40
(d
/d
) L
[cm
-1]
T [K]
[Å
-1]
SANS T Dependence: Lorentzian
Only present in significant intensities in paramagnetic phase
Magnetic correlation length diverges as T TC+
-
Qualitative Model
-
Qualitative Model
P
-
Qualitative Model
P
F
-
Qualitative Model
P
F
AF?
-
Qualitative Model
P
F
AF?
AF?
-
AF in the Martensite: Neutron Diffraction Problems
AF known at Ni50Mn50, suspected in someform in Ni50Mn25+ySn25-y
Indirect evidence:
> (T)> Exchange bias> SANS (Porod scattering)
Why not just do neutron diffraction?
> Texture an issue> Looking for the onset of AF order
simultaneous with MPT to lowsymmetry state (very challenging refinement)
-
AF in the Martensite: Neutron Diffraction Problems
AF known at Ni50Mn50, suspected in someform in Ni50Mn25+ySn25-y
Indirect evidence:
> (T)> Exchange bias> SANS (Porod scattering)
Why not just do neutron diffraction?
> Texture an issue> Looking for the onset of AF order
simultaneous with MPT to lowsymmetry state (very challenging refinement)
-
AF in the Martensite: Neutron Diffraction Problems
AF known at Ni50Mn50, suspected in someform in Ni50Mn25+ySn25-y
Indirect evidence:
> (T)> Exchange bias> SANS (Porod scattering)
Why not just do neutron diffraction?
> Texture an issue> Looking for the onset of AF order
simultaneous with MPT to lowsymmetry state (very challenging refinement)
Weak evidence for an AF order parameter?
Polarized neutrons…….
-
AF in the Martensite: 55Mn Zero Field NMR (/2 = 10.5 MHz/T)
150 200 250 300 350 400 450 500
Frequency f (MHz)
Ni50-xCoxMn40Sn10
x=14
x=7
x=0
AF F
AF x 0.1 F
F
T = 1.6 K
48 T14 T
NMR signal enhancement factor, , measured by calibration to 19F
Typically: High in Fs, low in AFs
225 10
-
AF in the Martensite: 55Mn Zero Field NMR (/2 = 10.5 MHz/T)
Unambiguous F/AF assignment
The martensite indeed has ordered AF, decreasing with Co substitution
-10.5 MHz / TNi50Mn40Sn10 (x = 0)
-
AF in the Martensite: 55Mn Zero Field NMR
AF/F phase coexistence at low T
Volume fractions
-
Conclusions
Bhatti, El-Khatib, Srivastava, James, Leighton, Phys. Rev. B. 85, 134450 (2012)
Bhatti, Srivastava, Phelan, James, Leighton, Heusler Alloys, Eds. Hirohata and Felser, Springer (2015)
Yuan, Kuhns, Reyes, Brooks, Hoch, Srivastava, James, El-Khatib, Leighton, Phys. Rev. B. 91, 214421 (2015)
Yuan, Kuhns, Reyes, Brooks, Hoch, Srivastava, James and Leighton, Phys. Rev. B., submitted (2015)
Ni50-xCoxMn25+ySn25-y: Physically very interesting, potentially useful set of alloys
High TC, convenient TM, exceptionally low T, high M
Nanoscale magneto-electronic inhomogeneity:
> SP-like freezing of nm-scale spin clusters> Exchange bias in a nominally single-phase system> First direct observation by SANS
(liquid-like distribution, dc 2-6 nm, dc-c = 12 nm, strong interactions)> First observation by zero-field 55Mn NMR
(proof of short-range AF order, dc 8.5 4.0 nm)
New magnetic phase diagram
Qualitative model based on (unavoidable) compositional fluctuations
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