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Register a new userSpin transfer in ultrathin BiFeO3 film under external electric field
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Hong-Jian Feng
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First-principals calculations show that up-spin and down-spin carriers are accumulating adjacent to opposite surfaces of BiFeO3(BFO) film with applying external bias. The spin carriers are equal in magnitude and opposite in direction, and down-spin carriers move to the direction opposing the external electric field while up-spin ones along the field direction. This novel spin transfer properties make BFO film an intriguing candidate for application in spin capacitor and BFO-based multiferroic field-effect device.
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First-principles density-functional theory calculations show switching magnetization by 90 degree can be achieved in ultrathin BFO film by applying external electric-field. Up-spin carriers appear to the surface with positive field while down-spin ones to the negative field surface, arising from the redistribution of Fe-t2g orbital. The half-metallic behavior of Fe-3d states in the surface of R phase film makes it a promising candidate for AFM/FM bilayer heterostructure possessing electric-field tunable FM magnetization reversal and opens a new way towards designing spintronic multiferroics. The interface exchange-bias effect in this BFO/FM bilayer is mainly driven by the Fe-t2g orbital reconstruction, as well as spin transferring and rearrangement.
Using density functional theory (DFT), we study how the stability of individual magnetic skyrmions in an ultrathin transition-metal film can be controlled via the external electric fields. For applied electric fields of $mathcal{E}$= $pm 0.5$ V/{AA}, we find changes from 8 to 30$%$ of the Heisenberg exchange, the Dzyaloshinskii-Moriya interaction, the magnetocrystalline anisotropy energy, and the higher-order exchange interactions. Based on atomistic spin simulations using the DFT parameters, we find that the energy barriers for electric field assisted skyrmion writing and deleting can vary by up to a factor of three more than the variations of the interactions. This unexpected result originates from the electric field induced shifts of the critical magnetic field, marking the onset of the field-polarized phase, which exhibits metastable skyrmions. The shift leads to an electric field dependent change of the skyrmion radius at a fixed magnetic field and explains the enhanced energy barrier variations.
Bismuth ferrite, BiFeO3, is the only known room-temperature multiferroic material. We demonstrate here, using neutron scattering measurements in high quality single crystals, that the antiferromagnetic and ferroelectric orders are intimately coupled. Initially in a single ferroelectric state, our crystals have a canted antiferromagnetic structure describing a unique cycloid. Under electrical poling, polarisation re-orientation induces a spin flop. We argue here that the coupling between the two orders may be stronger in the bulk than that observed in thin films where the cycloid is absent.
The Landau theory of phase transitions of Ba0.8Sr0.2TiO3 thin film under external electric field applied in the planar geometry is developed. The interfacial van-der-Waals field Ez=1.1x10^8 V/m oriented normal to the film-substrate interface was introduced in to the model calculation to explain experimentally observed behavior of the polarization as a function of planar electric field. The Ez - misfit strain phase diagram of the film is constructed and discussed.
We propose a way to use electric-field to control the magnetic ordering of the tetragonal BiFeO3. Based on systematic first-principles studies of the epitaxial strain effect on the ferroelectric and magnetic properties of the tetragonal BiFeO3, we find that there exists a transition from C-type to G-type antiferromagnetic (AFM) phase at in-plane constant a ~ 3.905 {AA} when the ferroelectric polarization is along [001] direction. Such magnetic phase transition can be explained by the competition between the Heisenberg exchange constant J1c and J2c under the influence of biaxial strain. Interestingly, when the in-plane lattice constant enlarges, the preferred ferroelectric polarization tends to be canted and eventually lies in the plane (along [110] direction). It is found that the orientation change of ferroelectric polarization, which can be realized by applying external electric-field, has significant impact on the Heisenberg exchange parameters and therefore the magnetic orderings of tetragonal BiFeO3. For example, at a ~ 3.79 {AA}, an electric field along [111] direction with magnitude of 2 MV/cm could change the magnetic ordering from C-AFM to G-AFM. As the magnetic ordering affects many physical properties of the magnetic material, e.g. magnetoresistance, we expect such strategy would provide a new avenue to the application of multiferroic materials.
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