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The Magnetocaloric Properties of CoSiMn0.95Fe0.05
K. Morrison1, Y.Miyoshi1, J.Moore1, K. Sandeman2, A. Barcza2, D. Caplin1, L. F. Cohen1
1Blackett Laboratory, Imperial College, London SW7 2BZ2Department of Materials Science, University of Cambridge, CB2 3QZ
Funded by EPSRC ©Imperial College
Outline
Introduction
1. What can we learn from magnetic imaging?
– Does fragmentation lower Hc in this system?
2. We can correlate magnetisation, heat capacity and latent heat measurements.
Summary & Future work
©Imperial College
Motivation
• RT refrigeration
• Application of inverse MCE?
• Understanding and improving the magnetic and thermal behaviour of this family of materials
©Imperial College
Sample Prep & Background• Samples supplied by K. Sandeman,
University of Cambridge
• Induction melted
• Annealed – 950°C, 60 hours, slow cooled
[1]S. Niziol et al., J. Mag. Mag. Mater. 79, 333-337 (1989)
[2]K. G. Sandeman et al., PRB 74, 224436 (2006)
CoMnGe
CoMnSi
Material
0.89
0.4
mCo
(µB)
3.16
2.6
mMn
(µB)
FM-PMc plane
Metamagnetic
(AFM-FM-PM)
ab plane
TransitionLocalised
Moment
[2]
[1]
Degree of covalent bonding
©Imperial College
1 - CoMnSi0.92Ge0.08:
Bulk M-H Loops
• 1st Order?
• 2nd Order?
The magnetic processes
involved in this system can be
uncovered by hall imaging
©Imperial College
Imaging of CoMnSi0.92Ge0.08
From 1st order to 2nd order
2nd Order: (RT)
1.2T 1.5T 1.8T 2.1T 3.5T
1st Order: (230K)
2.6T 2.9T 3.2T 3.5T 3.8T
©Imperial College
CoSiMn0.95Fe0.05 : Bulk M-H Loops
1st Order2nd Order
• Similar behaviour to the Ge-doped sample
• Phase diagram closes up where 1st order transition disappears
©Imperial College
Gd5Ge4: Fragmentation
J. Moore et al., APL 88, 072501 (2006)
Fragmentation reduced Hc by ~ 20%
©Imperial College
CoSiMn0.95Fe0.05 : Fragmentation
• Change in Hc from fragmentation less obvious
• Hc varies by ~0.2T(peak-peak)
for a single fragment
©Imperial College
CoSiMn0.95Fe0.05 : Entropy change
∆S starts to drop off BEFORE the field limited temperature is reached
T
HMTS
c
H
∂
∂∆−=∆ )(dH
T
H)M(T,∆H)(T,∆S
H
H
H
M
2
1
∫
∂
∂=
©Imperial College
Summary of Magnetisation Data
• 1st order to 2nd order magnetic phase transition observed in 2 similar materials
• Hall imaging reveals the mechanism for ferromagnetic growth
• Fragmentation lowers Hc by ~ 0.1T
• Gradual decrease in ∆S at low temperatures
©Imperial College
2 - SiN Membrane Sensor: (A.C.) Heat
Capacity Measurements
• A.C power supplied to the heater of a commercial (TCG 3880) Xensornanocalorimeter
• Sensitive to small samples (< 100x50µm)
• Heat equation solved for a point heat source
Minakov et al., Rev. Sci. instrum., 76, 043906 (2005)
ωφωω i
h
eVi
KPC =
©Imperial College
SiN Membrane Sensor: (D.C.) Latent
Heat Measurements
Y.Miyoshi - to be published
• Adiabatic set-up
• Measure Vth as field is ramped
• Latent heat causes jump in Vth
• Use heat capacity data to convert to ∆T and ∆S
©Imperial College
SiN Membrane Sensor: Observing the
Structural Transformation
A volume change of the sample stretches/relaxes the SiNmembrane
*J. Moore et al., APL 88, 1 (2006)
Minakov et al., Rev. Sci. instrum., 76, 043906 (2005)©Imperial College
� The heater resistance changes
� RH can be measured to give a
qualitative description of the
magnetostriction
*
∆C switches sign over the region 90-120K
CoSiMn0.95Fe0.05 : Heat Capacity
Ht (DOWN) Ht(UP)
©Imperial College
CoSiMn0.95Fe0.05 : Latent Heat
200K
90K 120K
215K205K
(Plotted on same scale)
Y.Miyoshi - to be published
UP field
DOWN field
©Imperial College
CoSiMn0.95Fe0.05 : Correlation of
Measurements
5.5 6.0 6.5 7.0 7.5-1.0
-0.5
0.0
0.5
1.0
Q,
µJ
µ0H, T
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
∆R
/R, %
0.696
0.698
0.700
0.702
C,
µJ/K
0.0006
0.0008
0.0010
M, em
u
Y.Miyoshi - to be published
• Magnetisation
• Heat capacity
• Structural change
• Latent heat
T = 90K
©Imperial College
Entropy Measurements: Correlation
Y.Miyoshi - to be published
T
LS
LH−=∆
T
HMTS
c
HM
∂
∂∆−=∆ )(
1
20
max),(
H
H
T
H
HCdT
T
HTCS ∫=∆
©Imperial College
Comparison of CoMnSi series
K. G. Sandeman et al., PRB 74, 224436 (2006)
• Enhanced ∆Scompared to similar materials in this family
• Reduction of Hc
and shift in Tt
©Imperial College