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Magnetization dynamics reveals inhomogeneity in different systems: from amorphous alloys and agglomerated nanoparticles to multiferroic crystals

Magnetic transitions in materials are often much more complicated than are the examples from the cook-books of magnetism. Therefore, additional measurements should always be performed in order to develop the realistic model of magnetic state of the system. One way of distinguishing between the magnetic states is via dynamics of the dc magnetization after change of outer conditions, i.e. the magnetic field or temperature. Magnetic transitions in multiferroic family of K3Fe5F15 and similar compounds motivated us to study the slow relaxation of magnetization of these materials on the hours time scale after change of magnetic field. This slow process was described by a logarithmic time dependence over a broad temperature and field range, similar to the behaviour of magnetic nanoparticles. The results were explained with thermal activation of magnetic moments of the magnetic nanoregions in crystal over the anisotropy barriers. Thus, from magnetization measurements the sizes of magnetic regions were estimated to be in the nanometre range, that should be of importance for the multiferroicity and magneto-electric coupling. Conducted reasoning was enlighten knowing magnetic behaviour of the nanoparticles, where slow relaxation of the magnetization comes from the activation of their magnetic moments over the anisotropy barriers. Surprisingly, in our studied FeNiB amorphous nanoparticles, both coated in SiO2 and uncoated, the relaxation of magnetization deviated from this simple picture. However, this deviation and comparison with the geometrically determined size distribution indicate that the magnetic units responsible for the observed dynamics consist of several physical nanoparticles. Similarly, the slow magnetic relaxation appeared in system of Hf100−xFex metallic glasses. For 35≤x≤43, which is above the paramagnetic regime and below the onset of the ferromagnetic state, observed behaviour was explained with the self-assembled magnetic nano-clusters embedded within a paramagnetic matrix. This reveals uniquely the development of the chemical order showing that the clusters grow rapidly with increasing x. In this way, magnetic method showed as very convenient in indicating the evolution of chemical order on nanometre scale. In three presented cases applicability of slow magnetic relaxation method teaches us how to use it for study of the nano-magnetic inhomogeneities in wide class of different materials with thenon-trivial magnetic behaviour.

Magnetization; Amorphous alloys; Multiferroic crystals

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