Field-induced phase transitions and phase diagrams in BiFeO3-like multiferroics

Abstract

The incommensurate magnetic structures and phase diagrams of multiferroics has been explored on the basis of accurate micromagnetic analysis taking into account the spin flexoelecric interaction (Lifshitz invariant). The objects of the study are BiF eO3 -like single crystals and epitaxial films grown on the < 111 > substrates. The main control parameters are the magnetic field, the magnetic anisotropy, and the epitaxial strain in the case of films. We predict novel quasi -cycloidal structures induced by external magnetic field or by epitaxial strain in the BiF eO3 -films. Phase diagrams representing the regions of homogeneous magnetic states and incommensurate structures stability are constructed for the two essential geometries of magnetic field (magnetic field oriented parallel to the principal crystal axis H C3 and perpendicular to this direction H⊥C3). It is shown that the direction of applied magnetic field substantially affects a set of magnetic phases, properties of incommensurate structures, character of phase transitions. Novel conical type of cycloidal ordering is revealed during the transition from incommensurate cycloidal structure into homogeneous magnetic state. Elaborated phase diagrams allow estimate appropriate combination of control parameters (magnetic field, magnetic anisotropy, exchange stiffness) required to the destruction of cycloidal ordering corresponding to the transition into homogeneous structure. The results show that the magnitude of critical magnetic field suppressing cycloid is lowered in multiferroics films comparing to single crystals, it can be also lowered by the selection of orientation of magnetic field. Our results can be useful for strain engineering of new multiferroic functional materials on demand.

Expansion of the spin cycloid in multiferroic BiFeO3 thin films

“…Gareeva et al have theoretically shown that the period of the cycloid in BFO can increase upon modification of specific anisotropy terms. 39 In the present case, such a change in anisotropy could arise from local deviations in the DMI and other exchange interactions, which are sensitive to surface and interface effects. Note that the ½112 propagation direction of the spin cycloid corresponds to a direction which is 36°canted out of the film plane, and, as such, the endpoints of the spin chain are exposed to proximity effects (spin pinning) at the film surface and the film-substrate interface.…”

Structure and spin dynamics of multiferroric BiFeO$_3$

“…The total energy at finite temperatures and magnetic fields can then be calculated from a large Monte Carlo sample of the system variables. Alternatively, phase transitions may be calculated through Landau-Ginzburg theory [70][71][72], where the free energy is expanded in powers of the system order parameters.…”

The Experimentalist's Guide to the Cycloid, or Noncollinear Antiferromagnetism in Epitaxial BiFeO3

“…This resulting expression can be considered as a nonuniform magnetoelectric interaction energy density and has the form of a Lifshitz invariant required to stabilize the spin cycloid. [ 96,97 ] In other words, the cycloid in BFO is the result of (DM interaction‐mediated) magnetoelectric interactions arising from the existence of spontaneous polarization.…”

“…The theories we have outlined above, namely, the thermodynamic description of BFO using Landau–Ginzburg theory including the Lifshitz invariant, and the converse spin current model as well as its implementation in the H eff formulation, collectively have been remarkably successful in describing various experimental findings in BFO. Such features include the cycloid anharmonicity, [ 96 ] calculating phase diagrams, [ 96,97 ] and predicting novel propagation vectors. [ 79,91,113 ] Moreover, the Landau formulation can be used to determine the equations of motion and thus calculate the spin wave excitations (further discussed in Section 4.3).…”