Alexander Barcza

From Sensitivity to Tunability in CoMnSi-based
magnetocaloric compounds
A new family of potential magnetocaloric materials
A Barcza, K G Sandeman
Department of Materials Science
Device Materials Group
University of Cambridge
Tuesday, 14 August 2007 / MMMR, Baden
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 1 / 27

Outline
1 Introduction
Motivation
Inverse/negative magnetocaloric effect
Thermodynamic quantities describing the magnetocaloric effect
The magnetocaloric effect in metamagnets
2 CoMnSi-based metamagnets
CoMnSi
CoMnSi1−xGex
CoMn1−xNixSi
3 Summary and Outlook
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 2 / 27

Why use Mn based alloys?
Mn based compounds are:
relatively cheap
Introduction Motivation
environmentally friendly (not all of them)
show first-order phase transitions (as well as higher order)
MnAs 1) , TC = 318 K, ∆SM = −32 J kg −1 K −1 in 5 T
Mn1.1Fe0.9P0.47As0.53 2) : TC 310 K, ∆SM = −20 J kg −1 K −1 in 5 T
Ni0.5Mn0.5−xSnx 3) : Tt 300 K ∆SM = 18 J kg −1 K −1 in 5 T
(x=0.13)
Mn3GaC 4) : Tt = 164 K, ∆SM = 12 J kg −1 K −1 in 5 T
properties are tunable by substitution of elements
exchange interactions between Mn-Mn are crucial for magnetism of
Mn-based compounds
1) Wada H, Appl. Phys. Lett. 79 3302-3304 (2001)
2) Tegus O, J. Magn. Magn. Mater. 272 2389-2390 (2004)
3) Krenke T, Nature Mat. 4 450-454 (2005)
4) Tohai T, J. Appl. Phys. 94 1800-1802 (2003)
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 3 / 27

Studied inverse MCE materials
Introduction Inverse/negative magnetocaloric effect
Mn2Sb 1) is ferrimagnet with T C=550 K
two Mn sites with different magnetic moments
substitution of Mn with Cr/V leads to first order antiferro- to
ferrimagnetic transition
Mn1.95Cr0.05Sb 2) : ∆SM = 7 J kg −1 K −1 in 5 T at ∼198 K
Mn1.82V0.18Sb 3) : ∆SM = 5.5 J kg −1 K −1 in 5 T at ∼280 K
Fe0.49Rh0.51 4) :
highest observed inverse MCE of 20 J kg −1 K −1 at ∆H = 1.95 T
MCE declines when material is cycled through phase transition
1) Wijngaard J H, Phys. Rev. B 45 5395-5405 (1992) (and references therein)
2) Tegus O, Physica B 319 174-192 (2002)
3) Zhang Y Q, J. Alloys and Compounds 365 35-38 (2004)
4) Annaorazov et al. J. Appl. Phys. 79 1689 (1996)
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 4 / 27

Inverse MCE in FeRh
Kouvel J S J. Appl. Phys. 37 1257 (1966)
Annaorazov et al. J. Appl. Phys. 79 1689 (1996)
Introduction Inverse/negative magnetocaloric effect
first order magnetostructural phase
transition from antiferro- to
ferromagnetic state at 330 K for FeRh
development of magnetic moment on
Rh at the phase transition
crystal volume changes at the phase
transition ∼ 0.3 % [2]
spontaneous magnetostriction is a
measure for lattice entropy
contributions
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 5 / 27

Introduction Thermodynamic quantities describing the magnetocaloric effect
Isothermal entropy change ∆ST and adiabatic temperature change ∆Tad
∆ST =
H2
H 1
H2
∂M
dH ∆Tad = −
∂T p,H
H1 T
CH,p
∆Tad and ∆ST are large near a phase transition,
∂M
dH
∂T p,H
the sharper the phase transition the larger ∆Tad and ∆ST ,
∆Tad and ∆ST are larger for higher magnetic fields.
For application in a room temperature magnetic refrigerator materials
should have
phase transitions around room temperature and a large MCE,
phase transition that can be induced in small magnetic fields,
should be environmentally friendly and cheap.
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 6 / 27

Introduction Thermodynamic quantities describing the magnetocaloric effect
Isothermal entropy change ∆ST and adiabatic temperature change ∆Tad
∆ST =
H2
H 1
H2
∂M
dH ∆Tad = −
∂T p,H
H1 T
CH,p
∆Tad and ∆ST are large near a phase transition,
∂M
dH
∂T p,H
the sharper the phase transition the larger ∆Tad and ∆ST ,
∆Tad and ∆ST are larger for higher magnetic fields.
For application in a room temperature magnetic refrigerator materials
should have
phase transitions around room temperature and a large MCE,
phase transition that can be induced in small magnetic fields,
should be environmentally friendly and cheap.
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 6 / 27

Metamagnets as refrigerants
M
S T
(∂M/∂T) > 0
H
H =0
T t
H 1
T C
Introduction The magnetocaloric effect in metamagnets
T
T
Remember:
H2
∆Tad =
H1 ∆ST =
− T
∂M
dH
CH,p ∂T p,H
H2
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 7 / 27
H 1
∂M
dH
∂T p,H
At a metamagnetic transition
∆ST is positive,
∆Tad is negative.

CoMnSi-based metamagnets CoMnSi
The metamagnet – CoMnSi
sample was quenched from 1000 ◦ C
60.0
M (Am 2 /kg)
50.0
40.0
30.0
20.0
10.0
4000 Oe
0.0
100 200 300 400 500 600
T (K)
Bińczycka et al., phys. stat. sol. (a) 35 K69 (1976)
Johnson et al., phys. stat. sol (a) 20 331 (1973)
Nizioł et al., phys. stat. sol. (a) 45 591 (1978)
There are 3 phase transitions in CoMnSi
antiferro- to ferromagnetic
(metamagnetic) phase transition
at Tt = 200 − 360 K,
ferro- to paramagnetic phase
transition at T C 410 K,
orthorhombic to hexagonal
structural phase transition at
Tstructural = 917 ◦ C.
! phase transition temperatures are very
sample dependent !
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 8 / 27

CoMnSi-based metamagnets CoMnSi
The metamagnet – CoMnSi
sample was quenched from 1000 ◦ C
60.0
M (Am 2 /kg)
50.0
40.0
30.0
20.0
10.0
4000 Oe
0.0
100 200 300 400 500 600
T (K)
Bińczycka et al., phys. stat. sol. (a) 35 K69 (1976)
Johnson et al., phys. stat. sol (a) 20 331 (1973)
Nizioł et al., phys. stat. sol. (a) 45 591 (1978)
There are 3 phase transitions in CoMnSi
antiferro- to ferromagnetic
(metamagnetic) phase transition
at Tt = 200 − 360 K,
ferro- to paramagnetic phase
transition at T C 410 K,
orthorhombic to hexagonal
structural phase transition at
Tstructural = 917 ◦ C.
! phase transition temperatures are very
sample dependent !
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 8 / 27

CoMnSi-based metamagnets CoMnSi
The metamagnet – CoMnSi
sample was quenched from 1000 ◦ C
60.0
M (Am 2 /kg)
50.0
40.0
30.0
20.0
10.0
4000 Oe
0.0
100 200 300 400 500 600
T (K)
Bińczycka et al., phys. stat. sol. (a) 35 K69 (1976)
Johnson et al., phys. stat. sol (a) 20 331 (1973)
Nizioł et al., phys. stat. sol. (a) 45 591 (1978)
There are 3 phase transitions in CoMnSi
antiferro- to ferromagnetic
(metamagnetic) phase transition
at Tt = 200 − 360 K,
ferro- to paramagnetic phase
transition at T C 410 K,
orthorhombic to hexagonal
structural phase transition at
Tstructural = 917 ◦ C.
! phase transition temperatures are very
sample dependent !
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 8 / 27

CoMnSi1−xGex-system
Nizioł et al., J. Magn. Magn. Mat. 79 333-337 (1989)
CoMnSi-based metamagnets CoMnSi 1−x Gex
Objectives in making CoMnSi1−xGex:
change sample composition and
gain control over Tt
separate Tt and T C
measure the magnetocaloric effect
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 9 / 27

CoMnSi-based metamagnets CoMnSi 1−x Gex
Magnetization M(H) and MCE of CoMnSi1−xGex
Ge substitution decreases the metamagnetic transition temperature
transition is sharpest in CoMnSi0.95Ge0.05 yielding ∆ST=9 J kg −1 K −1
at ∆H = 5 T
change of sign around room temperature
Sandeman et al., Phys. Rev. B 74 224436 (2006)
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 10 / 27

CoMnSi-based metamagnets CoMnSi 1−x Gex
Magnetization M(H) and MCE of CoMnSi1−xGex
Ge substitution decreases the metamagnetic transition temperature
transition is sharpest in CoMnSi0.95Ge0.05 yielding ∆ST=9 J kg −1 K −1
at ∆H = 5 T
change of sign around room temperature
interested in the microscopic magnetization?
– see next talk by Kelly Morrison
Sandeman et al., Phys. Rev. B 74 224436 (2006)
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 10 / 27

CoMn1−xNixSi – Aims
CoMnSi-based metamagnets CoMn 1−x Nix Si
Objectives of making Ni substituted CoMn1−xNixSi are
increase T C, far away from Tt
tune Tt near room temperature
obtain large values for ∆Tad and ∆ST
get single phase material
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 11 / 27

CoMnSi-based metamagnets CoMn 1−x Nix Si
Synthesis and characterization of CoMn1−xNixSi
quasi contact free co-melting of elements in an induction furnace,
subsequent annealing of ingots in a box furnace at 1123 - 1373 K,
measurement of structural transition temperatures with
calorimetry,
determination of crystal structure with X-ray powder diffraction,
magnetic measurements with VSM and SQUID
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 12 / 27

CoMnSi-based metamagnets CoMn 1−x Nix Si
High temperature structural transition temperatures
heating:
cooling:
Thermal analysis (SDT) signals for heating (upper
figure) and cooling (lower figure):
Ni (%) T heating
struc T cooling
7 926
struc
842 84
9 924 846 78
11 920 826 94
∆Tstruc
value of Tstruc is important to choose
annealing conditions
three different hold temperatures were
choosen (850 ◦ C, 950 ◦ C, 1100 ◦ C)
all samples were slowly cooled
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 13 / 27

CoMnSi-based metamagnets CoMn 1−x Nix Si
Crystal structure of CoMn1−xNixSi
orthorhombic Pnma phase with
TiNiSi-type structure
a=5.83 Å, b=3.68 Å, c=6.86 Å
atoms in Wyckoff pos. 4c (x, 1
4 , z)
not all reflections belong to the space
group Pnma
ac-plane viewed along b-axis of the orthorhombic unit cell
Mn (pink), Co (blue), Si (yellow)
atom x y z
Co 0.159(1) 0.250000 0.559(1)
Mn 0.015(2) 0.250000 0.184(1)
Ni 0.015(2) 0.250000 0.184(1)
Si 0.773(3) 0.250000 0.620(2)
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 14 / 27

CoMnSi-based metamagnets CoMn 1−x Nix Si
Second hexagonal crystallographic phase
high temperature hexagonal
structure is metastable at room
temperature
hexagonal P63/mmc phase with
Ni2In type
a=3.93 Å, c=5.22 Å
ac-plane viewed along b-axis of the orthorhombic unit cell
Mn in ( 1 3 , 2 3 , 3 4 ) (pink), Co in ( 1 3 , 2 3 , 1 ) (blue), Si in (0, 0, 0)
4
(yellow)
axis-transformation during orthorhombic to hexagonal unit cell
transition:
a =
hex b ortho
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 15 / 27
a hex
1/2
3
a = hex
c ortho

CoMnSi-based metamagnets CoMn 1−x Nix Si
A metastable high temperature hexagonal phase
amount of second phase:
weight fraction of hexagonal Phase (%)
24
22
20
18
16
14
12
10
8
6
4
2
Annealing temperature
1100 °C
950 °C
850 °C
0
6 8 10 12
degree of Ni substitution (%)
unit cell volume:
147.4
6 8 10 12
degree of Ni substitution (%)
annealing at a temperature below Tstruc for a long time
(200 h) decreases the weight fraction of second phase
samples annealed away from Tstruc show similar
behaviour of unit cell volume versus Ni concentration
orthorhombic unit cell volume (Å 3 )
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 16 / 27
147.9
147.8
147.7
147.6
147.5
Annealing temperature
1100 °C
950 °C
850 °C (Si powder)

CoMnSi-based metamagnets CoMn 1−x Nix Si
Magnetization M(H) of CoMn1−xNixSi at low
temperatures
M (Am 2 /kg)
140.0
120.0
100.0
80.0
60.0
40.0
20.0
330 K
CoMn 0.93 Ni 0.07 Si
70 K
330 K
280 K
250 K
230 K
210 K
190 K
170 K
150 K
130 K
110 K
90 K
70 K
0.0
0 1 2 3 4 5
µ H (T)
0
sample was annealed at 950 ◦ C
at low temperatures there are
two regions where magnetization
changes
metamagnetic phase transition
at ∼3 T
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 17 / 27

CoMnSi-based metamagnets CoMn 1−x Nix Si
Magnetization M(H) of CoMn1−xNixSi at low
temperatures
M (Am 2 /kg)
140.0
120.0
100.0
80.0
60.0
40.0
20.0
330 K
CoMn 0.93 Ni 0.07 Si
70 K
330 K
280 K
250 K
230 K
210 K
190 K
170 K
150 K
130 K
110 K
90 K
70 K
0.0
0 1 2 3 4 5
µ H (T)
0
sample was annealed at 950 ◦ C
at low temperatures there are
two regions where magnetization
changes
metamagnetic phase transition
at ∼3 T
Does the second phase have an influence on M(H)?
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 17 / 27

CoMnSi-based metamagnets CoMn 1−x Nix Si
Curie transitions above room temperature
samples annealed at 850 ◦ C
Ni (%) T C1 (K)
7 431
9 437
11 445
two transitions at temperatures
above room temperature
relative drop of magnetization
above T C1 corresponds to the
amount of second phase
T C increases with Ni content
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 18 / 27

CoMnSi-based metamagnets CoMn 1−x Nix Si
The magnetocaloric effect in CoMn1−xNixSi
magnetisation data
M (Am 2 /kg)
140.0
120.0
100.0
80.0
60.0
40.0
20.0
330 K
CoMn 0.93 Ni 0.07 Si
70 K
330 K
280 K
250 K
230 K
210 K
190 K
170 K
150 K
130 K
110 K
90 K
70 K
0.0
0 1 2 3 4 5
µ H (T)
0
second hexagonal phase was
identified to be ferromagnetic.
magnetic field induced
metamagnetic phase transition
at ∼3 T (70 K).
from M versus T curves: Tt
around 170 K in 1 T.
T C of both phases are above
430 K
isothermal magnetisation versus magnetic field curves were used to
calculate the magnetocaloric effect
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 19 / 27

CoMnSi-based metamagnets CoMn 1−x Nix Si
The magnetocaloric effect in CoMn1−xNixSi
Temperature dependence of ∆ST at various Ni concentrations
M (Am 2 /kg)
∆ S T (Jkg -1 K -1 )
140.0
120.0
100.0
80.0
60.0
40.0
20.0
0.0
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
11 %
CoMn 0.91 Ni 0.09 Si
1 2 3 4 5
µ H (T)
CoMn 0 Ni Si 1-x x
9 %
7 %
330 K
280 K
250 K
230 K
210 K
190 K
170 K
150 K
130 K
110 K
90 K
70 K
M (Am 2 /kg)
0 50 100 150 200 250
T (K)
140.0
120.0
100.0
80.0
60.0
40.0
20.0
0.0
330 K
10 K
CoMn 0.89 Ni 0.11 Si
1 2 3 4 5
µ 0 H (T)
maximal entropy change ∼2.5 J/kgK
in a magnetic field change from 0 -
5 T at ∼100 K
∆ST decreases with Ni concentration
ferromagnetic background of second
phase has negative influence
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 20 / 27
330 K
280 K
250 K
230 K
210 K
190 K
170 K
150 K
130 K
110 K
90 K
70 K
50 K
30 K
10 K

CoMnSi-based metamagnets CoMn 1−x Nix Si
Order of the metamagnetic phase transition
7 %
M 2 (Am 2 /kg) 2
9 %
M 2 (Am 2 /kg) 2
11 %
M 2 (Am 2 /kg) 2
20000
18000
16000
14000
12000
10000
8000
6000
4000
2000
20000
18000
16000
14000
12000
330 K
CoMn 0.93 Ni 0.07 Si
70 K
330 K
280 K
250 K
230 K
210 K
190 K
170 K
150 K
130 K
110 K
90 K
70 K
0.00 0.01 0.02 0.03 0.04
µ H/M (T kg/ Am
0
0.05 0.06
2 0
CoMn Ni Si )
0.91 0.09
0.00 0.01 0.02 0.03 0.04 0.05
µ H/M (T kg/Am
0
0.06 0.07
2 10000
330 K
280 K
8000
250 K
230 K
210 K
6000
190 K
170 K
4000
150 K
130 K
2000
110 K
90 K
70 K
0
CoMn Ni Si)
0.89 0.11
20000
18000
16000
14000
12000
10000
8000
6000
4000
2000
330 K
10 K
330 K
280 K
250 K
230 K
210 K
190 K
170 K
150 K
130 K
110 K
90 K
70 K
50 K
30 K
10 K
0.00 0.01 0.02 0.03 0.04
µ H/M (T kg/Am
0
0.05 0.06
2 0
)
around phase transitions thermodynamic potentials
can be expanded in a Taylor series with respect
to the order parameter:
Φ = Φ0 + aM 2 + bM 4 − MH
at equilibrium (∂Φ/∂M = 0):
αM + βM 3 = H −→ H
M
= α + βM2
≥ 0, higher order phase transitions
β
< 0, first order phase transitions
Banerjee S K, Phys. Lett. 12 16 (1964)
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 21 / 27

Summary
Summary and Outlook
Samples of CoMn1−xNixSi with x = 0.07, 0.09, 0.11 were
synthesised.
A second ferromagnetic phase was identified and partial control
over it was obtained.
Maximum ∆ST of 2.5 J/kgK at 100 K in 5 T was calculated from
isothermal M(H) curves.
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 22 / 27

Conclusion
Conclusion and further work
Values for ∆ST are too small at a too low temperature for room
temperature applications.
Second phase broadens the metamagnetic phase transition and
decreases the magnetocaloric effect.
Further work
Employment of element and crystallographic phase specific
experimental techniques to study the influence of the second phase
(SEM with EBSD, XPEEM).
Try other substitutions (e.g. Ni on the Co site and Cr on the Mn
site).
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 23 / 27

Conclusion
Conclusion and further work
Values for ∆ST are too small at a too low temperature for room
temperature applications.
Second phase broadens the metamagnetic phase transition and
decreases the magnetocaloric effect.
Further work
Employment of element and crystallographic phase specific
experimental techniques to study the influence of the second phase
(SEM with EBSD, XPEEM).
Try other substitutions (e.g. Ni on the Co site and Cr on the Mn
site).
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 23 / 27

Acknowledgements
Acknowledgements
Karl G. Sandeman for supervising me and my work.
Sibel Özcan for sample synthesis.
Mary E. Vickers and Andrew Moss for a lot of help with X-ray
diffraction and data analysis.
EPSRC and Camfridge Ltd. for funding.
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 24 / 27

Appendix
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 25 / 27

Appendix
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 26 / 27

Appendix
A Barcza, K G Sandeman (DMG) CoMnSi-based magnetocaloric compounds 14th August 2007 27 / 27