Dynamics of AC susceptibility and coercivity behavior in nanocrystalline TbAl1.5Fe0.5 alloys

Abstract

The static and dynamic magnetic macroscopic properties of bulk and nanocrystalline TbAl1.5Fe0.5 alloys have been investigated. In bulk state, this alloy is understood as a reentrant ferromagnet. This is characterized by a ferromagnetic Curie transition at 114K, as deduced from magnetization including Arrott plots, higher than that of TbAl2. The reentrance is found at lower temperatures, below 66K, with a cluster glass behavior setting in, deduced from the magnetization irreversibility. This is accompanied by an abrupt increase in the coercivity from 0.08kOe to 15kOe at 5K, with respect to the TbAl2 alloy. Room temperature Mössbauer spectroscopy confirms the paramagnetic state of such a bulk alloy. The spin dynamics within the disordered magnetic state is described by the AC-susceptibility which shows a Vogel–Fulcher law for the slowing down process. This is caused by a random anisotropy affecting the existing clusters. The production of milled TbAl1.5Fe0.5 alloys enhances the presence of magnetic disorder and results in the particle downsizing toward the nanocrystalline state (close to 10nm). In this case, two frequency-dependent contributions exist, with different activation energies, one of them cannot be described by ideal spin glass nor blocking/unblocking (nanoparticle) processes. In addition, the coercivity reduces to 1kOe with the decrease in the size as a consequence of the existence of single domain particles. The results are explained by the intricate interplay between exchange interactions and magnetocrystalline anisotropy with disorder and size effects.