Expansion of the spin cycloid in multiferroic BiFeO3 thin films

Abstract

Understanding and manipulating complex spin texture in multiferroics can offer new perspectives for electric field-controlled spin manipulation. In BiFeO 3 , a well-known room temperature multiferroic, the competition between various exchange interactions manifests itself as non-collinear spin order, i.e., an incommensurate spin cycloid with period 64 nm. We report on the stability and systematic expansion of the length of the spin cycloid in (110)-oriented epitaxial Co-doped BiFeO 3 thin films. Neutron diffraction shows (i) this cycloid, despite its partly out-of-plane canted propagation vector, can be stabilized in thinnest films; (ii) the cycloid length expands significantly with decreasing film thickness; (iii) theory confirms a unique [112] cycloid propagation direction; and (iv) in the temperature dependence the cycloid length expands significantly close to T N. These observations are supported by Monte Carlo simulations based on a first-principles effective Hamiltonian method. Our results therefore offer new opportunities for nanoscale magnonic devices based on complex spin textures.

Revisiting spin cycloids in multiferroic BiFeO3

“…Further insight for the C 1 /C 2 values can be obtained from the existence of type-2 cycloid found in experiments in slightly strained thin films 8,17,18 . Within the considered C values based on the critical magnetic field, the best C 1 and C 2 that allow the energy to be close to < 112 > or < 111 > cycloid is around C 1 ∼ 150×10 −7 Hartree/(Bohr•µ 2 B ) and C 2 ∼ 30×10 −7 Hartree/(Bohr•µ 2 B ), which is close to the triple point, where the AFM phase, type-1 cycloid and < 111 > cycloid have the same energies.…”

“…This cycloid phase appears to be energetically competing with the G-AFM state, as evidenced by a transition from type-1 cycloid to G-AFM under small compressive strain 8 , magnetic field [9][10][11][12] , electric field 13,14 , doping 15 , and hydrostatic pressure 16 , strongly suggesting that these two phases are close in energy. Recently, a different (type-2) cycloid has been found in slightly tensile-strained (001) film 8 and (110) film on SrTiO 3 substrate 17,18 , which was proposed to propagate in the [112] direction. Type-2 cycloid is therefore also expected to be energetically close to the other two aforementioned magnetic arrangements.…”

Atomic‐Scale Interface Structure in Domain Matching Epitaxial BaBiO3 Thin Films Grown on SrTiO3 Substrates

“…In addition to single crystals, high‐quality thin films provide an ideal playground to investigate the essential structural and physical properties of materials, due to the possibility of tuning their structural parameters and chemical compositions. Such films are also of great importance for studies of materials with emergent phenomena, induced by the presence of interfaces (either to a substrate or to neighboring layers as in a multilayer or superlattice system), which adds more degrees of freedom for controlling the materials properties, e.g., by lattice strain or interface/surface manipulations.…”

Interfacial Strain Gradients Control Nanoscale Domain Morphology in Epitaxial BiFeO3 Multiferroic Films

“…[ 60 ] The length of this cycloid in films is typically λ ≈ 70–80 nm (Refs. [ 32,47,61 ] ), and various works have considered the so‐called “anharmonicity” of the cycloid. [ 26,62 ] Anharmonicity refers to a “bunching” of the spins towards a preferred direction, usually induced by an external magnetic field or by a strain induced anisotropy.…”

“…Since strain and interfacial effects tune both ferroelectricity and magnetism, one may envisage that these strain and interface effects in multiferroics—materials with coexisting polar and magnetic orders—can affect both ferroic orders, and even the magnetoelectric coupling. Some imminent examples of this include electric control of the (anti‐) ferromagnetic order driven by the interface coupling of both order parameters, [ 27 ] interface strain enhanced multiferroism, [ 28 ] ferroelastic switching of magnetoelectric order, [ 29 ] electrostatic modulation of the magnetic order parameter, [ 30,31 ] finite size effects on the magnetic long range order, [ 32 ] etc.…”