We present an overview of the current interest in non-collinear magnetism in multiferroic perovskite crystals. We first describe the different microscopic mechanisms giving rise to the non-collinearity of spins in this class of materials. We discuss, in particular, the interplay between non-collinear magnetism and ferroelectric and antiferrodistortive distortions of the perovskite structure, and how this can promote magnetoelectric responses. We then provide a literature survey on non-collinear multiferroic perovskites. We discuss numerous examples of spin cantings driving weak ferromagnetism in transition metal perovskites, and of spin-induced ferroelectricity as observed in the rare-earth based perovskites. These examples are chosen to best illustrate the fundamental role of non-collinear magnetism in the design of multiferroicity.

https://iopscience.iop.org/article/10.1088/0953-8984/28/12/123001/pdf

Non-collinear magnetism in multiferroic perovskites.

High-energy x-ray diffraction is used to investigate the bulk oxygen-ordering properties of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}.$ Four different superstructures of Cu-O chains aligned along the b axis and ordered with periodicity $ma,$ along the a axis have been observed. For $xl0.62,$ the only observed superstructure is ortho-II with $m=2.$ At room temperature, we find ortho-III $(m=3)$ for $0.72l~xl~0.82,$ ortho-V $(m=5)$ in a mixed state with ortho-II at $x=0.62,$ and ortho-VIII $(m=8)$ at $x=0.67.$ Ortho-II is a three-dimensional ordered structural phase, the remaining ones are essentially two-dimensional. None of the superstructures develops long-range ordering. The temperature dependence of the observed superstructure ordering is investigated explicitly and a structural phase diagram is presented.

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Oxygen-ordering superstructures in underdoped YBa 2 Cu 3 O 6 + x studied by hard x-ray diffraction

High purity, highly ordered and twin free ortho-II phase YBa 2 Cu 3 O 6.50 crystals have been successfully prepared and their structural properties characterized by X-ray diffraction. Removal of twin boundaries was found to be essential in order to obtain highly ordered ortho-II crystals. The ortho-II phase shows a sharp superconducting transition at T c =62 K, 5 K higher than the ortho-I phase of the same oxygen content. The ordering was found to be three-dimensional with long correlation lengths ξ a =148 A, ξ b =430 A and ξ c =58 A. No doubling of the unit lattice along the c -direction was observed, indicating an attractive interaction between oxygen ions in adjacent basal planes.

Preparation and X-ray characterization of highly ordered ortho-II phase YBa2Cu3O6.50 single crystals

An energetic expression containing four different macroscopic terms is proposed to explain and understand coupled magnetic orders (and the directions of the simultaneously occurring ferromagnetic and/or antiferromagnetic vectors) in terms of anti-phase and/or in-phase tilting of oxygen octahedra in magnetic and multiferroic perovskites. This expression is derived from a suggested simple microscopic formula, and has its roots in the Dzyaloshinsky–Moriya interaction. Comparison with data available in the literature and with first-principles calculations we conduct here confirms the validity of such a simple and general law for any tested structural paraelectric and even ferroelectric phase, and for any chosen direction of any selected primary magnetic vector.

http://iopscience.iop.org/article/10.1088/0953-8984/24/31/312201/pdf

A simple law governing coupled magnetic orders in perovskites

In this paper, we present a comprehensive study of magnetic dynamics in the rare-earth orthoferrite ${\mathrm{YbFeO}}_{3}$ at temperatures below and above the spin-reorientation (SR) transition ${T}_{\mathrm{SR}}=7.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, in magnetic fields applied along the $a,\phantom{\rule{4pt}{0ex}}b$, and $c$ axes. Using single-crystal inelastic neutron scattering, we observed that the spectrum of magnetic excitations consists of two collective modes well separated in energy: 3D gapped magnons with a bandwidth of $\ensuremath{\sim}60\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, associated with the antiferromagnetically (AFM) ordered Fe subsystem, and quasi-1D AFM fluctuations of $\ensuremath{\sim}1\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ within the Yb subsystem, with no hybridization of those modes. The spin dynamics of the Fe subsystem changes very little through the SR transition and could be well described in the frame of semiclassical linear spin-wave theory. On the other hand, the rotation of the net moment of the Fe subsystem at ${T}_{\mathrm{SR}}$ drastically changes the excitation spectrum of the Yb subsystem, inducing the transition between two regimes with magnon and spinonlike fluctuations. At $T\phantom{\rule{3.33333pt}{0ex}}l\phantom{\rule{3.33333pt}{0ex}}{T}_{\mathrm{SR}}$, the Yb spin chains have a well defined field-induced ferromagnetic (FM) ground state, and the spectrum consists of a sharp single-magnon mode, a two-magnon bound state, and a two-magnon continuum, whereas at $T\phantom{\rule{3.33333pt}{0ex}}g\phantom{\rule{3.33333pt}{0ex}}{T}_{\mathrm{SR}}$ only a gapped broad spinonlike continuum dominates the spectrum. In this work we show that a weak quasi-1D coupling within the Yb subsystem ${J}_{\text{Yb-Yb}}$, mainly neglected in previous studies, creates unusual quantum spin dynamics on the low-energy scales. The results of our work may stimulate further experimental search for similar compounds with several magnetic subsystems and energy scales, where low-energy fluctuations and underlying physics could be ``hidden'' by a dominating interaction.

V.P. Plakhty

Papers

X-ray studies of the YBa2Cu3O6+x superstructures in the range of 0.40(3) ≤ x ≤ 0.73(3)

Neutron diffraction study of weak antiferromagnetism in ytterbium orthoferrite

Magnetic ordering in the garner NaCa2Mn2V3O12