Magnetocaloric effect (MCE)

What is MCE?
• MCE is the change in temperature associated with the entropy change due to ordering in a magnetic material via an applied field
• Discovered by Warburg, 1881
• Original application was for low temperature experimentation in the 1930's to present
• Current Interest is for refrigeration at higher temperatures due to high efficiency: 60% efficiency in a working apparatus and due to lack of pressurized gases

Magnetic Nanoparticles for MCE
• Theoretical entropy change for superparamagnetic particles is order of magnitude larger than bulk counterpart
• Entropy change in traditional materials at low temperatures limited by Tc
• For nanoparticle system, entropy change is limited by the Tb
• Reproducible, cost-effective, non-hazardous and it can be scaled up
• Easy to assemble as thin films by Langmuir-Blodgett and spin-coating
• Nanoparticle size, interactions can be tuned to vary the operating temperature
• Nanoparticles also have more surface area to facilitate effective heat exchange
MCE in Superparamagnetic Particles

H = 0, Random thermal fluctuations, Disordered state --> Higher entropy

All lined up: Lower disorder:Lower entropy
The entropy change (ΔS) in this process is several orders of magnitude more than that for bulk samples.
How data is taken and analyzed
• An M vs. H curve is measured at a fixed temperature using PPMS.This is done for a range of temperatures. The data is rearranged into M vs. T curves at fixed H. Then the change in entropy equation is performed numerically by the Maxwell relation.
Entropy Change in NiFe2O4
Relevant Papers
• “Magnetic anisotropy and magneto-caloric effect (MCE) in NiFe2O4 nanoparticles” –J. Gass, N. A. Frey, M. B. Morales, M. J. Miner, S. Srinath and H. Srikanth, MRS Proceedings Vol. 962, 0962-P05-03 (2007)
• Pecharsky, Gschneider. APL. 70. 3299. 1997.
• E. Warburg. Ann. Phys. Chem. 13. 141. 1881.
• “Magnetocaloric effect in ferrite nanoparticles” –P. Poddar, J. Gass, D. Rebar, S. Srinath, H. Srikanth, S. A. Morrison and E. E. Carpenter, Journal of Magnetism and Magnetic Materials 307, 227 (2006)
• R. D. Shull, IEEE Trans. Magn. 29, 2614 (1993); R. D. Shull, AIP Conf. Proc. 273, 628 (1993).
• “Surface Spin Disorder in NiFe2O4 Nanoparticles” R. H. Kodama, A. E. Berkowitz, E. J. McNiff Jr. ,S. Foner, Phys. Rev. Lett. 77, 394 - 397 (1996).
Back to FML home