Anisotropic Magnetocaloric Effect in Nanostructured Magnetic Clusters

Abstract

We report the first experimental observation of anisotropic magnetocaloric effect (MCE) in the Fe 8 clusters. It is found that the magnetic anisotropy plays a very important role in the determination of the magnetocaloric effect. The maximum and minimum MCE's are observed when the applied magnetic fields are parallel and perpendicular to the easy axis, respectively. The quantum spin Hamiltonian of a Fe 8 cluster is used to calculate the partition function and the magnetization in a range of temperature and magnetic field. Excellent quantitative agreement between the experimental data and calculation is observed. DOI: 10.1103/PhysRevLett.87.157203 PACS numbers: 75.30.Gw, 75.30.Sg, 75.50.Tt, 75.50.Xx It is well known that the magnetocaloric effect (MCE) has been useful in achieving very low temperatures (mK range) [1]. Up to now, most of MCE applications still remain in the low temperature range ͑,20 K͒ by using paramagnetic salts, e.g., Gd 3 Ga 3 O 12 [2]. For the MCE applications at high temperatures, different types of materials must be used [3][4][5][6]. The MCE in the superparamagnetic nanostructured materials has been recently studied by Shull and co-workers [7][8][9][10]. The results indicate that at higher temperatures the MCE is larger in the superparamagnetic nanostructures composites than in the pure paramagnetic materials. This makes the nanostructured composites appear to be very attractive candidates for the magnetic refrigeration in a very big temperature span. This enhancement was discussed by Shull et al. in considering a superparamagnetic system consisting of monosized and noninteracting magnetic clusters uniformly dispersed in a nonmagnetic media [7,10].It is well known that most magnetic nanoparticles or clusters exhibit a quite strong uniaxial magnetic anisotropy due to shape, stress, or surface effects [11], which governs the magnetic behavior of the particles [11,12]. Therefore, it is essential to understand the effect of the magnetic anisotropy in the MCE, particularly for the superparamagnetic materials. However, not much effort has been devoted to it. The reason could be that the materials used for magnetic refrigerants are either the ideal paramagnetic salts [2] or soft magnetic materials [3 -6] and therefore the effect of magnetic anisotropy is negligible. Theoretically, the anisotropic MCE was studied by using the Monte Carlo simulation for bulk ferromagnetic materials (or interacting magnetic clusters) with a uniaxial anisotropy in the vicinity of the Curie temperature [13]. Experimentally, a clean study of the anisotropic effect is generally hindered by the distribution of sizes and/or interactions. In this Letter, we report the anisotropy effect on the MCE in Fe 8 magnetic molecular crystals. With monosized clusters and well-defined anisotropic properties, this system offers, for the first time, a definite confirmation of the anisotropic MCE. It is shown that the magnetic anisotropy is important for both the qualitative and quantitative aspects of the MCE.The cluster Fe 8 with ...

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