Research Profile - Department of Materials Science and Metallurgy ...

Karl Sandeman
Royal Society University Research Fellow
MA University of Cambridge
PhD University of Cambridge
+44 (0) 1223 762982
kgs20@cam.ac.uk
www.msm.cam.ac.uk/dmg/
Magnetic Phase Transitions and Cooling
My work in the Device Materials Group aims to improve
fundamental understanding of the transitions between magnetic
states in materials. In these days of environmentally aware
science, one potential application of this research is in a new,
high-efficiency, gas-free cooling device: the magnetic refrigerator.
Magnetic refrigeration
A conventional refrigerator uses the energetic difference
between gas and liquid to provide cooling via the expansion
and contraction of a volatile refrigerant. In an analogous way, we
can harness the energetic differences between magnetic states
in suitable solid refrigerants by reversible magnetization and
demagnetization. The resulting “magnetocaloric effect” manifests
itself by a change in temperature of a magnetic material on
adiabatic (de)magnetization. In most cases, magnetization is
accompanied by a temperature increase, largest around magnetic
phase transitions accompanied by a sharp change in the
magnetic state of the material.
Negative magnetocaloric effect
I have focused on an unusual subset of magnetocaloric effects,
where magnetization brings about a decrease in the temperature
of the magnetic refrigerant. Work to-date is pioneering the study
of a new material in this area, and we are now combining ab-initio
models, neutron-diffraction experiments and bulk magnetization
studies to understand the subtle interplay of magnetic
interactions which brings about the negative magnetocaloric
effect in our material and in other related systems. This work has
led to the foundation of a spin-out company, Camfridge Ltd.,
which has built several prototype magnetic-cooling engines.
Minimizing magnetic fields
A question of technological importance, and one that also creates
interesting scientific possibilities is how to minimize the magnetic
fields that are needed to drive the changes of magnetic state we
examine. To this end, I am currently combining different applied
fields (magnetic, stress etc.) in single phase and multi-phase
systems to explore and drive phase transitions in new ways.
KG Sandeman, R Daou, S Özcan, JH Durrell, ND Mathur & DJ Fray,
“Negative magnetocaloric effect from highly sensitive metamagnetism in
CoMnSi 1–x
Ge x
” Phys. Rev. B 74, 224436 (2006).
JHT Ransley, SH Mennema, KG Sandeman, G Burnell, EJ Tarte, JE
Evetts & MG Blamire, JI Kye & B Oh, “The normal-state resistivity of grain
boundaries in YBa 2
Cu 3
O 7–δ
" Appl. Phys. Lett. 84, 4089–4091 (2004).
KG Sandeman, GG Lonzarich & AJ Schofield, “Ferromagnetic
superconductivity driven by changing Fermi surface topology” Phys. Rev.
Lett. 90, 167005 (2003).
KG Sandeman & AJ Schofield, “Model of anisotropic scattering in a quasitwo-dimensional
metal” Phys. Rev. B 63, 094510 (2001).
Research Profile 35