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

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Motivation

• RT refrigeration

• Application of inverse MCE?

• Understanding and improving the magnetic and thermal behaviour of this family of materials

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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

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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

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CoMnSi0.92Ge0.08:

From 1st order to 2nd order

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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

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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

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Gd5Ge4: Fragmentation

J. Moore et al., APL 88, 072501 (2006)

Fragmentation reduced Hc by ~ 20%

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CoSiMn0.95Fe0.05 : Fragmentation

• Change in Hc from fragmentation less obvious

• Hc varies by ~0.2T(peak-peak)

for a single fragment

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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

∫

∂

∂=

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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

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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 =

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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

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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)

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CoSiMn0.95Fe0.05 : Phase Diagram

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CoSiMn0.95Fe0.05 : Latent Heat

200K

90K 120K

215K205K

(Plotted on same scale)

Y.Miyoshi - to be published

UP field

DOWN field

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CoSiMn0.95Fe0.05 : ∆T

Y.Miyoshi - to be published

),( THC

LT =∆

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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

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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 ∫=∆

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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

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Summary

• Iron doping decreases Hc and Tt enhancing the low-field behaviour of ∆S

• Comparison of Maxwell and Clausius Clapeyron relations

• Decrease in entropy appears to not be limited by field

• Correlation of magnetometry, calorimetry and latent heat data

• Future work…

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