Magnetism of ordered and disordered alloys of R2Fe14B (R = Nd, Er) type

Magnetic susceptibility, magnetization and neutron diffraction measurements have been performed to study structure and magnetic states of crystalline and amorphous Nd2Fe14B and Er2Fe14B alloys. In the crystalline state there exists a large (about 20%) anisotropy of Er-sublattice magnetization. Values of the magnetic anisotropy constant of Er ions and the Er-Fe exchange-coupling parameter were estimated using temperature dependence of the Er-and Fe-sublattice magnetizations. Amorphous state of the samples was obtained by irradiation of fast (Eeff≥1MeV) neutrons with a fluence up to the 1.2 Ч 1020 n/cm-2 at 340 K. It is shown that antiferromagnetic coupling between the rare-earth and iron spins is kept in the amorphous state. Amorphization of the samples is found to result in reduction of the Curie temperature (TC) by about 200 K and almost total absence of coercivity. We suggest that the strong decrease of TC is a consequence of enhancement of negative Fe-Fe interactions as a result of dispersion of interatomic distances, which is a characteristic feature of the amorphous state. © 2013 Elsevier B.V. All rights reserved

Some aspects of antiferromagnetic ordering in LiMnP 0.85V 0.15O 4: Neutron diffraction and DC-magnetization studies

V-substituted LiMnPO 4 has been synthesized by the solid state reaction route. Combined Rietveld refinement of neutron and X-ray data revealed that all vanadium ions are located in the same positions as the phosphorus ions. The magnetic structure of LiMnP 0.85V 0.15O 4 was found to be the same as that described for undoped antiferromagnetic LiMnPO 4 (T N=34.5 K). DC-magnetization measurements were carried out to study the peculiarities of magnetic ordering in LiMnP 0.85V 0.15O 4. An irreversible behavior of DC magnetization was found at magnetic fields less than ∼2 kOe. It was demonstrated that an increase in the magnitude of the applied field leads to a reduction of the discrepancy between zero-field-cooled and field-cooled processes. These effects were explained by the movement of domain walls and by transition of the system to a monodomain state. The anomaly in the magnetization vs field dependence attributed presumably to the spin-flop transition was observed at ∼40 kOe. The existence of magnetic inhomogeneity in the paramagnetic phase of LiMnP 0.85V 0.15O 4 was proved by the analysis of the χT product. It was concluded that the observed changes in χT are indicative of a competition between ferromagnetic and antiferromagnetic correlations at temperatures slightly exceeding T N. © 2012 Elsevier B.V. All rights reserved

Irradiation-induced amorphized state of rapidly quenched R12Fe82B6 alloys (R = Nd, Er)

Rapidly quenched (RQ) alloys R 12Fe 82B 6 (R = Nd, Er) have been prepared by melt spinning. Their magnetic properties, structure, and phase composition have been studied before and after fast-neutron irradiation to a fluence of 1.2 × 10 20 n/cm 2. Before irradiation, it is the Nd 2Fe 14B-type phase (space group P4 2/mnm) that is mainly present in the RQ alloys at 295 K. In the Nd 12Fe 82B 6 alloy, this phase has a ferro- magnetic structure with magnetic moments at the Nd and Fe atoms oriented parallel to the c axis; the Er 2Fe 14B phase found in the Er12Fe82B6 alloy is characterized by a ferrimagnetic order of Er and Fe magnetic moments parallel to the basal plane. After irradiation, the crystalline state of both RQ alloys transforms into an amorphous state. The ferro- and ferrimagnetic orders are retained in them, but the Curie temperature of the alloys with Nd and Er decreases by 100 and 200 K, respectively. The spontaneous magnetization of the irradiated alloys at 5 K is approximately equal to their magnetization in the crystalline state, whereas their coercive force is lower than that for the alloys before irradiation by two orders of magnitude. The effects observed are explained by the dispersion of Fe-Fe exchange interactions resulting from the dispersion of interatomic spacings in the amorphous alloys. © Pleiades Publishing, Ltd., 2012