First detection of low field microwave absorption in the disordered multiferroic double perovskite BiFe_0.5Mn_0.5O₃

Abstract

BiFe0.5Mn0.5O3 (BFMO) is an intriguing magnetic double perovskite, only obtainable through high pressure-high temperature synthesis. It shows bulk multiferroic properties, namely the coexistence between a spin canted antiferromagnetic structure superimposed to an externally induced electric polarization at least from 77 K. In particular, the system is characterized by a significant weak ferromagnetic hysteresis loop and by a very rare phenomenon: the spontaneous magnetization reversal (MRV) versus temperature in the low field regime. To clarify the BFMO exotic magnetic phase in the low field regime, the Electron Spin Resonance (ESR) and the low field microwave absorption (LFMA) techniques were used, providing the first observation of LFMA in the bulk BFMO as an additional functionality of this material. A striking feature is that the hysteresis in LFMA signals vanishes above 45 K, while the bulk M-H loop hysteresis, measured in the same field range of LFMA, persists till room temperature. The temperature at which LFMA hysteresis vanishes qualitatively matches the position of the magnetic susceptibility’s second derivative peak, corresponding to the temperature at which the local second order mechanism responsible for MRV is maximum. The line shape of LFMA completely changes above 45 K and the ESR linewidth starts decreasing above this temperature, indicating the role of defect/disorder induced inhomogeneity. The temperature evolution of LFMA hysteresis and line shapes as a measure of the competition between Fe- and Mn-rich clusters suggests a sort of local frustration at the microscopic scale, responsible for the peculiar magnetization reversal of this system.

Microwave absorption dynamics in Fe3O4nanopowders around Verwey transition

“…Finally, for low temperature (T<Tn), MAMMAS response is approximately lineal with a small slope, where this absorption dynamics can be associated with a weak ferromagnetism in nanopowders [11]. Recently, it has been shown that a ferromagnetic resonance is accompanied with a LFMA spectrum [11,[13][14][15]; where the existence of this signal will give evidence of a magnetic order in Fe3O4 nanopowders. In Fig.…”

“…On the other hand, the ferromagnetic resonance (FMR) has been used to study the magnetic properties in nanoparticles [5,10], where this technique employs two parameters: the resonant field (Hres) and the linewidth (Hpp); giving vital information on magnetic nature in the materials [11]. Additionally, we employed two techniques based in the non-resonant microwave absorption [12]: the magnetically modulated microwave absorption spectroscopy (MAMMAS) [5,12,13] and the low-field microwave absorption (LFMA) [5,10,[13][14][15]. These techniques allow to detect the magnetic transitions in materials [11], and they can also detect the magnetic order in nanoparticles [5,16].…”