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Magnetic materials encompass a rich variety of spin alignments. It is well known that in case of ferromagnetic (FM) small particle systems, as the particle size decreases below the single domain value, the spins are increasingly affected by thermal fluctuations in comparison with anisotropy energy and the system becomes super paramagnetic (SPM). Further, the presence of large magnetic moment in ferro or ferri nano particles makes difficult to study the size effects. On the other hand, only few works have been reported focusing on the AFM particles even though it is suitable to study the surface and interface effects (1,2). It is also known that in the nano structured FM/AFM interfaces, the exchange coupling at the interface enhance the anisotropy energy, providing the magnetization stability for nano particles, there by overcoming the superpara-magnetism3.This makes materials suitable for practical application like recording media etc. Further, the study of AFM nano particle is interesting from physics point of view, in view of magnetization reversal by quantum tunneling, magnetic behavior with surface effect due to large surface/volume ratio, induced magnetic moments, SPM, Exchange bias effect, unidirectional anisotropy due to exchange coupling between core-shell etc. Therefore, AFM ordering in small particle in comparison with bulk required detailed study. Focusing on our work, we have shown in spinel CoRh¬2O4 that AFM ordering exists at TN ~ 25K both in bulk and nano particle samples~ 15nm. The interest in this materials stems from the fact that AFM spinel having only moment at tetrahedral site is interesting for the study of geometrical frustration effect ,the absence of site exchange of cations (Co 2+ and Rh3+ ) in nano particle, AFM ordering TN ~ 27K remains same for bulk and ~ 15 nm particles. We have proposed a core-shell model with a core retaining its AFM order and the enhancement of magnetization below TN is due to increasing number of frustrated shell(surface) spins with unconventional relaxation at 2K in AFM nanoparticle1. Subsequently, the concept of core-shell model proposed by us for AFM nano particles was further extended not only to understand AFM in Cr2O3 nano particles through ESR experiments but also to understand the interplay between charge and spin degrees of freedom in perovskite nano particles of LaCaMnO 4.In this talk we will review theoretical and experimental status of core shell spin model for AFM and FM nanoparticles.
References
1. Bhowmik Ranganathan, Nagarajan (Physical Rev B 69 054430 (2004) (Physical.Rev.B75 012410 (2007)
2. D.Tobia etal Phy.Rev B 78 104412 (2008)
3. Vassil Skumryev etal Nature 423 850 (2003)
4. T.Zhang etal Phy. Rev B 76 174415 (2007)
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