Noncollinear magnetism and single-ion anisotropy in multiferroic perovskites

Abstract

The link between the crystal distortions of the perovskite structure and the magnetic exchange interaction, the single-ion anisotropy (SIA) and the Dzyaloshinsky-Moriya (DM) interaction are investigated by means of density-functional calculations. Using BiFeO3 and LaFeO3 as model systems, we quantify the relationship between the oxygen octahedra rotations, the ferroelectricity and the weak ferromagnetism (wFM). We recover the fact that the wFM is due to the DM interaction induced by the oxygen octahedra rotations. We find a simple relationship between the wFM, the oxygen rotation amplitude and the ratio between the DM vector and the exchange parameter such as the wFM increases with the oxygen octahedra rotation when the SIA does not compete with the DM forces induced on the spins. Unexpectedly, we also find that, in spite of the d 5 electronic configuration of Fe 3+ , the SIA is very large in some structures and is surprisingly strongly sensitive to the chemistry of the A-site cation of the ABO3 perovskite. In the ground R3c state phase we show that the SIA shape induced by the ferroelectricity and the oxygen octahedra rotations are in competition such as it is possible to tune the wFM "on" and "off" through the relative size of the two types of distortion.

Spin-cycloid instability as the origin of weak ferromagnetism in the disordered perovskiteBi0.8La0.2Fe0.5Mn0.5O

“…Previous diffraction measurements on BiFeO 3 revealed an oxygen octahedral rotation angle of 10.6 • [32]. Importantly, the wFM canting angle is thought to be linearly related to the rotation angles in both P nma and R3c space groups up to ∼11 • [11]. In the present case, given the very similar rotational angle, it is unlikely that the increased wFM moment arises from an increased octahedral rotation, as reported in previous studies of doped BiFeO 3 compounds [7,33].…”

“…Spin canting has previously been modeled in BiFeO 3 for the P nma and R3c space groups [11]. From the results of this investigation, the direction of the spin canting angle is only dependent on the antiferrodistortive out-of-phase oxygen octahedral rotations present in both I mma and P nma symmetries since these influence the nontrivial staggered components of the resulting DM vectors (see Sec.…”

“…The general form of the model spin Hamiltonian appropriate for BiFeO 3 was recently confirmed to be similar to those describing related structures such as REMnO 3 and α-Fe 2 O 3 [11,13,14]:…”

“…A promising avenue of research to induce a sizable ferromagnetic component necessary for technical applications is cation substitution [5][6][7][8][9][10], which has been shown to both establish measurable weak-ferromagnetic (wFM) behavior and enhance ferroelectric properties. However, it is only comparatively recently that the mechanisms responsible for the magnetic ground state of pure BiFeO 3 have been investigated in detail, enabling a more systematic approach to improve the properties of BiFeO 3 [11][12][13][14]. The current work discusses the effect of substitutional doping in BiFeO 3 on the magnetic structure and properties through a combined experimental and calculation approach.…”

“…Despite this, recent developments focusing on a full theoretical description of the magnetoelectric structure of BiFeO 3 have led to a successful model Hamiltonian, which has shown through theoretical simulations to reproduce the cycloid using effective energy parameters derived from inelastic neutron scattering and light scattering techniques [11,12,22,23]. These investigations indicate the spin cycloid arises from competing interactions between nearest-neighbor (NN) and next-nearest-neighbor (NNN) superexchange interactions between the Fe ions, mediated by oxygen anions, with the propagation direction stabilized by the DM interactions.…”