Speaker
Description
Multiferroic materials with magnetic, ferroelectric and ferroelastic ordering have drawn significant attention because of the fundamental physics as well as their potential applications for future electronic devices. A sample of polycrystalline BaErFeO$_4$ was prepared by conventional solid-state method. It has an orthorhombic crystal structure with $Pnma$ space group and lattice parameters $a=13.11080(3)$ Å, $b=5.68412(1)$ Å, $c=10.22506(2)$ Å [1] at room temperature. Fe$^{3+}$ ions are located in two different crystal environments, FeO$_5$ square pyramids (atom Fe$1$) and FeO$_6$ octahedra (atom Fe$2$).
At room temperature, the Mössbauer spectrum of BaErFeO$_4$ recorded in the velocity range of $\pm4$ mm/s, consists of two doublets D$_1$ and D$_2$ with close values of isomer shift ($IS$) of $0.37$ and $0.30$ mm/s but with markedly different quadrupole splittings ($QS$) of $0.25$ and $0.50$ mm/s, respectively. The first doublet D$_1$ with larger $IS=0.37$ mm/s corresponds to the ferric ions in the octahedral surrounding (Fe$_2$O$_6$) and the second doublet D$_2$ with smaller $IS=0.30$ mm/s can be attributed to the ferric ions in square pyramids (Fe1O$_5$). Furthermore, Mössbauer spectra of BaErFeO$_4$ were collected at low temperatures ranging from $4.2$ to $55$ K. The paramagnetic doublets remain unchanged down to $T_{N_1}\sim 49$ K [1, 2], below which the spectra are split into magnetic sextets, indicating an onset of the magnetic ordering of iron ions. The spectrum at $40$ K was fitted by two sextets with very broad continuous distributions of the hyperfine magnetic field, which indicate an antiferromagnetic ordering of the iron magnetic moments in the form of an incommensurate spin-density wave [2], where the magnitudes of the iron magnetic moments (and thereby of the hyperfine magnetic field) change dramatically. Below the transition temperature $T_{N_2}\sim 34$ K [1, 2] at temperatures $30$ K, $20$ K and $10$ K in zero external magnetic field, the spectral shapes are narrow and well resolved, and they can be decomposed into two well-separated sextets S$_1$ and S$_2$ corresponding to the two nonequivalent Fe sites, which is consistent with a commensurate antiferromagnetic structure below $T_{N_2}$ [2]. At liquid helium temperature ($4.2$ K) in zero external magnetic field, the width of the distribution of the hyperfine magnetic field in the sextet S$_2$ belonging to pyramidal sites increases to the value $\Delta B_{eff}=1.7$ T, compared with the value $\Delta B_{eff}=0.5$ T observed at $10$ K. The broadening of the lines in the sextet is probably caused by a change in the orientation of the magnetic moments in the sublattice by the influence of spontaneous noncollinear ordering of the Er magnetic moments.
Acknowledgements
The support by the project GA ČR 25-16097S is gratefully acknowledged. The authors thank Prof. N. T. Dang for providing the sample.
References
[1]A. A. Belik et al., “Different magnetic and magnetodielectric behavior of BaRFeO4 ferrites with R = Ho, Er, Tm, and Yb,” Journal of Alloys and Compounds, vol. 922. Elsevier BV, p. 166297, Nov. 2022. https://doi.org/10.1016/j.jallcom.2022.166297
[2] A. Dönni et al., "Er-driven incommensurate to commensurate magnetic phase transition of Fe in the spin-chain compound BaErFeO4" Physical Review B, vol. 109, no. 6. American Physical Society (APS), Feb. 02, 2024. https://doi.org/10.1103/physrevb.109.064403
