Pavel Borisov

Crystal structure and multiferroic properties of Gd-substituted BiFeO3

Room-temperature crystal structure, local ferroelectric, and magnetic properties of the Bi1−xGdxFeO3 (x=0.1,0.2,0.3) polycrystalline samples have been investigated by x-ray diffraction, piezoresponse force microscopy, and magnetometry techniques. Performed measurements have revealed a sequence of the composition-driven structural phase transitions R3c→Pn21a (occurs at x∼0.1) and Pn21a→Pnma (takes place within the concentrational range of 0.2<x<0.3). The latter structural transformation is attributed to the substitution-induced suppression of the polar displacements. Gd substitution has been shown to effectively induce the appearance of the spontaneous magnetization, thus indicating a promising way for improving multiferroic properties of antiferromagnetic BiFeO3.

Magnetoelectric exchange bias systems in spintronics

Magnetoelectric switching of perpendicular exchange bias is observed in a Co∕Pt multilayer attached to single crystalline magnetoelectric antiferromagnetic Cr2O3(111). The exchange bias field HEB can be set to positive or negative values by applying an electric field Efr either parallel or antiparallel to the magnetic freezing field Hfr while cooling to below the Neel temperature. Based on this result, the antiferromagnetic spin state can be used as a medium for data storage. The authors propose magnetic random access memory cells and magnetic logic devices, which are purely voltage controlled.

Tilt engineering of spontaneous polarization and magnetization above 300 K in a bulk layered perovskite

Tilting toward two properties Opposing electronic and symmetry constraints can make it difficult to combine some pairs of material properties in a single crystalline material. Magnetization and electrical polarization are such a pair, but their combination could be useful for applications such as magnetoelectric information storage. Pitcher et al. now show that careful design of chemical substitutions in a layered perovskite are both electrically polar and weakly ferromagnetic at temperatures up to 330 K. Science, this issue p. 420 Chemical substitutions produce atomic displacements in a crystal that lead to both electrical polarization and magnetization. Crystalline materials that combine electrical polarization and magnetization could be advantageous in applications such as information storage, but these properties are usually considered to have incompatible chemical bonding and electronic requirements. Recent theoretical work on perovskite materials suggested a route for combining both properties. We used crystal chemistry to engineer specific atomic displacements in a layered perovskite, (CaySr1–y)1.15Tb1.85Fe2O7, that change its symmetry and simultaneously generate electrical polarization and magnetization above room temperature. The two resulting properties are magnetoelectrically coupled as they arise from the same displacements.

Electric in-plane polarization in multiferroic CoFe2O4/BaTiO3 nanocomposite tuned by magnetic fields

Ferrimagnetic CoFe2O4 nanopillars embedded in a ferroelectric BaTiO3 matrix are an example for a two-phase magnetoelectrically coupled system. They operate at room temperature and are free of any resource-critical rare-earth element, which makes them interesting for potential applications. Prior studies succeeded in showing strain-mediated coupling between the two subsystems. In particular, the electric properties can be tuned by magnetic fields and the magnetic properties by electric fields. Here we take the analysis of the coupling to a new level utilizing soft X-ray absorption spectroscopy and its associated linear dichroism. We demonstrate that an in-plane magnetic field breaks the tetragonal symmetry of the (1,3)-type CoFe2O4/BaTiO3 structures and discuss it in terms of off-diagonal magnetostrictive-piezoelectric coupling. This coupling creates staggered in-plane components of the electric polarization, which are stable even at magnetic remanence due to hysteretic behaviour of structural changes in the BaTiO3 matrix. The competing mechanisms of clamping and relaxation effects are discussed in detail.

Electrically controlled exchange bias for spintronic applications

Exchange coupling between a magnetoelectric (111)-oriented Cr2O3 single crystal and a CoPt multilayer with perpendicular anisotropy exhibits an exchange bias field proportional to the applied axial electric field. Extrapolation from bulk to thin film magnetoelectric pinning system suggests promising spintronic applications due to coupling between the electric field-controlled magnetization and the magnetization of a neighbor ferromagnetic layer. Pure voltage control of magnetic configurations of tunneling magnetoresistance spin valves is an attractive alternative to current-induced magnetization switching.

Converse magnetoelectric effect in CoFe2O4–BaTiO3 composites with a core–shell structure

Multiferroic composites were prepared by covering CoFe2O4 nanoparticles with a shell of BaTiO3 using a sol–gel technique. Scanning probe microscopy confirmed the formation of a core–shell structure with a magnetic core and a piezoelectric shell. The converse magnetoelectric effect was studied at different temperatures and bias fields. The magnetoelectric coefficient peaks at approximately 270 K and reaches the value αC≈(2.2 ± 0.1)10 − 11 s m − 1, which surpasses those reported previously for similar structures. A change of the sign of the magnetoelectric coefficient observed for an increasing magnetic bias field is related to the non-monotonic field dependence of magnetostriction in polycrystalline CoFe2O4.

Superconducting quantum interference device setup for magnetoelectric measurements.

A commercial superconducting quantum interference device (SQUID) setup (MPMS 5S from Quantum Design), equipped with a magnetic ac susceptibility option, is modified for measurements of the linear magnetoelectric (ME) effect, i.e., of the magnetic moment induced by an applied external electric field in a ME sample. Test measurements on a Cr(2)O(3) (111) single crystal are in excellent agreement with previously reported data of its ME susceptibility. The main advantages of the proposed setup are the improved precision due to the high sensitivity of the SQUID magnetometer in combination with the lock-in technique and a relatively simple experimental realization.

Designing switchable polarization and magnetization at room temperature in an oxide

Ferroelectric and ferromagnetic materials exhibit long-range order of atomic-scale electric or magnetic dipoles that can be switched by applying an appropriate electric or magnetic field, respectively. Both switching phenomena form the basis of non-volatile random access memory, but in the ferroelectric case, this involves destructive electrical reading and in the magnetic case, a high writing energy is required. In principle, low-power and high-density information storage that combines fast electrical writing and magnetic reading can be realized with magnetoelectric multiferroic materials. These materials not only simultaneously display ferroelectricity and ferromagnetism, but also enable magnetic moments to be induced by an external electric field, or electric polarization by a magnetic field. However, synthesizing bulk materials with both long-range orders at room temperature in a single crystalline structure is challenging because conventional ferroelectricity requires closed-shell d0 or s2 cations, whereas ferromagnetic order requires open-shell dn configurations with unpaired electrons. These opposing requirements pose considerable difficulties for atomic-scale design strategies such as magnetic ion substitution into ferroelectrics. One material that exhibits both ferroelectric and magnetic order is BiFeO3, but its cycloidal magnetic structure precludes bulk magnetization and linear magnetoelectric coupling. A solid solution of a ferroelectric and a spin-glass perovskite combines switchable polarization with glassy magnetization, although it lacks long-range magnetic order. Crystal engineering of a layered perovskite has recently resulted in room-temperature polar ferromagnets, but the electrical polarization has not been switchable. Here we combine ferroelectricity and ferromagnetism at room temperature in a bulk perovskite oxide, by constructing a percolating network of magnetic ions with strong superexchange interactions within a structural scaffold exhibiting polar lattice symmetries at a morphotropic phase boundary (the compositional boundary between two polar phases with different polarization directions, exemplified by the PbZrO3–PbTiO3 system) that both enhances polarization switching and permits canting of the ordered magnetic moments. We expect this strategy to allow the generation of a range of tunable multiferroic materials.

Crystal structure and magnetic properties of Bi0.8(Gd1?xBax)0.2FeO3 (x = 0, 0.5, 1) multiferroics

Investigations of crystal structure and magnetic properties of Bi0.8(Gd1−xBax)0.2FeO3 (x = 0, 0.5, 1) samples have been performed. The Bi0.8Gd0.2FeO3 and Bi0.8Ba0.2FeO3 compounds have been shown to crystallize in the polar space groups Pn21a and R3c, respectively. It has been found that no continuous series of solid solutions is formed in the Bi0.8(Gd1−xBax)0.2FeO3 system: the crystal structure of the Bi0.8Gd0.1Ba0.1FeO3 sample is characterized by a coexistence of Pnma and R3c structural phases which differ in their chemical compositions. All of the Bi0.8(Gd1−xBax)0.2FeO3 (x = 0, 0.5, 1) compounds have been found to possess a spontaneous magnetization at room temperature. For Gd-containing samples, a significant enhancement of the magnetization takes place with decreasing temperature.

Study of Ni2–Mn–Ga phase formation by magnetron sputtering film deposition at low temperature onto Si substrates and LaNiO3∕Pb(Ti,Zr)O3 buffer

Film deposition of Ni2MnGa phaselike alloy by radio frequency (rf) magnetron sputtering was performed onto bare Si(100) substrates and LaNiO3∕Pb(Ti,Zr)O3 (LNO/PZT) ferroelectric buffer layer near room temperature. The prepared samples were characterized using conventional x-ray diffraction (XRD), superconducting quantum interference device, and electron dispersive x-ray spectroscopy from scanning electron microscope observations. The optimized films deposited under high rf power and low argon pressure present good surface quality and highly textured phase crystallization. The positioning distance between the substrate and the target-holder axis has some limited effect on the film’s composition due to the specific diffusion behavior of each element in the sputtering plasma. Extended four pole high resolution XRD analysis allowed one to discriminate the intended Ni–Mn–Ga tetragonal martensitic phase induced by the (100) LNO/PZT oriented buffer. This low temperature process appears to be very promising, allo...