Phase Transitions in Magneto-Electric Hexaferrites

Abstract

Hexaferrites have long been the object of extensive studies because of their great possibility for applications�permanent magnets, highdensity recording media, microwave devices, in biomedicine, to name but a few. Lately, many researchers' efforts have been focused on the existence of the magneto-electric effect in some hexaferrite systems and the appealing possibility of them being used as single-phase multiferroic and magnetoelectric materials. As indicated by theoretical analyses, the origin of the large magneto-electric effect can be sought in the strong interaction between the magnetization and the electric polarization that coexist in insulators with noncollinear magnetic structures. The hexaferrites' magnetic structure and, particularly, the specific magnetic spin ordering are the key factors in observing magneto-electric phases in hexaferrites. Some of these phases are metastable, which hampers their direct practical use. However, as the hexaferrites' phase diagrams reveal, chemical doping can be used to prepare a number of noncollinear stable magnetic phases. Since the magneto-electric effect has to do with the magnetic moments ordering, it seems only logical that one should study the cation substitutions' influence on the magnetic phase transition temperature. In this paper, we summarize recent examples of advances in the exploration of magnetic phase transitions in Y-type hexaferrites. In particular, the effect is emphasized by substituting in Y-type hexaferrites the nonmagnetic Me 2+ cations with magnetic ones and of the magnetic Fe 3+ cations with nonmagnetic ones on their magnetic properties and magnetic phase transitions. The work deals with the structural properties of and the magnetic phase transitions in a specific Y-type hexaferrite, namely, Ba(Sr) 2 Me 2 Fe 12 O 22 .

Influence of nonmagnetic cation substitution on magnetic order temperature in Y-type hexaferrites: Ba0.5Sr1.5Zn2Fe12O22 and Ba0.5Sr1.5Zn2Al0.08Fe11.92O22

“…The results were elaborated by magnetic, diffraction, and Mössbauer studies. As commented in the recent review [22], due to the competition of exchange interactions and magnetic anisotropies several specific magnetic phases were determined to stabilize in particular temperature ranges, which can further be modified by applied magnetic field. Some of these phases exhibit room temperature spin-driven ferroelectricity [23] that according to the classification of Khomskii [24] corresponds to a type-II magnetoelectric coupling originated from complex magnetic orders.…”