Magnetic Anisotropy and Magnetocaloric Efect (MCE) in NiFe 2 O 4 Nanoparticles
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Magnetic Anisotropy and Magnetocaloric Efect (MCE) in NiFe2O4 Nanoparticles J. Gass1, N. A. Frey1, M. B. Morales1, M. J. Miner1, S. Srinath1,2, and H. Srikanth1 1 Department of Physics, University of South Florida, 4202 East Fowler Ave, Tampa, FL, 33620 2 School of Physics, University of Hyderabad, Hyderabad, 500 046, India ABSTRACT Magnetic refrigeration based on the magnetocaloric effect (MCE) has been proposed as an attractive alternative to gas compression technology. Some calculations suggest that MCE can be obtained with higher efficiencies than compressor driven refrigeration. We examine the MCE in a system of nickel ferrite nanoparticles with size range of 6 to 15 nm. A peak in the MCE at 55 K is observed that increases with higher magnetic fields. This gives this system a relatively high peak entropy change compared to other ferrite systems. A sensitive radiofrequency (RF) transverse susceptibility measurement has also been used to study the magnetic anisotropy. We show that the MCE peak is not associated with the blocking temperature and is likely a field-driven surface spin reorientation which also has a signature in the transverse susceptibility. INTRODUCTION Magnetic nanoparticles are of current interest because they exhibit behavior very different from bulk systems. Magnetic nanoparticles whose size has been reduced below the single domain limit (~15 to 20 nm for iron oxide) can exhibit superparamagnetism at room temperature and transition to a spin-glass like state at low temperature [1]. The effects of surface states also become extremely important because the surface atoms become a substantial fraction of the total atoms in nanoparticle systems. Theoretical investigations suggest nanoparticle-based systems could be useful for magnetic refrigeration [2]. The magneto-caloric effect (MCE) is due to the coupling of the magnetic sublattice with the external applied field and is seen in all magnetic materials. While the concept of magnetic refrigeration has been around for a long time, it has not found much use except at low temperatures. However, the discovery of giant MCE in Gd5(SixGe1-x)4 alloys [3] displaying promising refrigeration characteristics in the intermediate temperature (40 K to 200 K) range has created renewed interest in MCE in general. DISCUSSION Magneto-caloric effect (MCE) in ferrite nanoparticles MCE in conventional ferromagnets is the result of spin disorder-order transition and is largest near the paramagnetic to ferromagnetic transition associated with the Curie temperature (Tc). Typical entropy changes range in the 0.1 to 1 J/kg-K using fields of 1 to 2 Tesla. In the giant MCE systems the transition is accompanied by a meta-magnetic transition and the strong coupling between the magnetic and structural degrees of freedom cause an increase in the MCE by one to two orders magnitude. These entropy changes are only large in a very narrow temperature range around the Curie point. It is not a trivial matter to change the Curie temperature of a material through variations in
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