Electrocaloric Effect of (110) Oriented KNbO3 Film

The room temperature electrocaloric effect is researched for (110) oriented KNbO3 film based on Landau-Devonshire theory. The phase map with different ferroelectric states is built at room temperature with the considerations of thermodynamic equilibrium conditions and minimum of thermodynamic potential. Five ferroelectric structural phases are obtained theoretically. The negative in-plane misfit strains are conducive to form the tetragonal c phase and the positive strains are in favor of the stability of tetragonal a1 and a2 phases. The electrocaloric effect relies on both misfit strain and electric field. Moreover, large electrocaloric effect is achieved in the orthorhombic phases.

Tunable Planar Hall Effect in (Ga,Mn)(Bi,As) Epitaxial Layers

We have thoroughly investigated the planar Hall effect (PHE) in the epitaxial layers of the quaternary compound (Ga,Mn)(Bi,As). The addition of a small amount of heavy Bi atoms to the prototype dilute ferromagnetic semiconductor (Ga,Mn)As enhances significantly the spin–orbit coupling strength in its valence band, which essentially modifies certain magnetoelectric properties of the material. Our investigations demonstrate that an addition of just 1% Bi atomic fraction, substituting As atoms in the (Ga,Mn)As crystal lattice, causes an increase in the PHE magnitude by a factor of 2.5. Moreover, Bi incorporation into the layers strongly enhances their coercive fields and uniaxial magneto-crystalline anisotropy between the in-plane ⟨110⟩ crystallographic directions in the layers grown under a compressive misfit strain. The displayed two-state behaviour of the PHE resistivity at zero magnetic field, which may be tuned by the control of applied field orientation, could be useful for application in spintronic devices, such as nonvolatile memory elements.

Anisotropic chemical expansion due to oxygen vacancies in perovskite films

In scientifically intriguing and technologically important multifunctional ABO3 perovskite oxides, oxygen vacancies are most common defects. They cause lattice expansion and can alter the key functional properties. Here, it is demonstrated that contrary to weak isotropic expansion in bulk samples, oxygen vacancies produce strong anisotropic strain in epitaxial thin films. This anisotropic chemical strain is explained by preferential orientation of elastic dipoles of the vacancies. Elastic interaction of the dipoles with substrate-imposed misfit strain is suggested to define the dipolar orientation. Such elastic behavior of oxygen vacancies is anticipated to be general for perovskite films and have critical impacts on the film synthesis and response functions.

Cooperative evolution of polar distortion and nonpolar rotation of oxygen octahedra in oxide heterostructures

Polarity discontinuity across LaAlO3/SrTiO3 (LAO/STO) heterostructures induces electronic reconstruction involving the formation of two-dimensional electron gas (2DEG) and structural distortions characterized by antiferrodistortive (AFD) rotation and ferroelectric (FE) distortion. We show that AFD and FE modes are cooperatively coupled in LAO/STO (111) heterostructures; they coexist below the critical thickness (tc) and disappear simultaneously above tc with the formation of 2DEG. Electron energy-loss spectroscopy and density functional theory (DFT) calculations provide direct evidence of oxygen vacancy (VO) formation at the LAO (111) surface, which acts as the source of 2DEG. Tracing the AFD rotation and FE distortion of LAO reveals that their evolution is strongly correlated with VO distribution. The present study demonstrates that AFD and FE modes in oxide heterostructures emerge as a consequence of interplay between misfit strain and polar field, and further that their combination can be tuned to competitive or cooperative coupling by changing the interface orientation.

Doping Effect on Piezoelectric, Magnetic and Magnetoelectric Properties of Perovskite—Ferromagnetic Magnetoelectric Composites

This chapter explains the effect of compositional modification on the magnetoelectric coefficient in sintered piezoelectric – magnetostrictive composites. It was found that 15 at% doping of Pb(Zn1/3Nb2/3)O3 [PZN] in Pb(Zr0.52Ti0.48)O3 [PZT] enhances the piezoelectric and magnetoelectric properties of a PZT – 20 at% Ni0.8Zn0.2Fe2O4 [NZF] composite. The effect of doping on the ferromagnetic phase was also investigated. With increases in Zn concentration, it was found that the coercive field and Curie temperature of Ni(1-x)ZnxFe2O4 [NZF] decreases, while its saturation magnetization has a maxima at 30 mole% Zn. X-ray diffraction revealed that the lattice constant of NZF increases from 8.32 Å for 0 at% Zn to 8.39 Å for 50 at% Zn. The magnetoelectric coefficient was found to have a maxima of 144 mV/cm.Oe at 30 at% Zn. To understand better, the effect of 40% (by mole) Zn substitution on structural, piezoelectric, ferromagnetic and magnetoelectric properties of Pb(Zr0.52Ti0.48)O3 - CoFe2O4 (PZT - CFO) sintered composite is also explained. X-ray diffraction of Co0.6Zn0.4Fe2O4 (CZF) showed the shift in almost all diffraction peaks to lower diffraction angle confirming the increase in lattice parameter in all three direction from 8.378 (for CFO) to 8.395 Å for (Co,Zn)Fe2O4 (CZF). SEM and TEM results showed defect structure (cleavage, twins, strain fields) in the CZF particle, which is a clear indication of misfit strain developed due to lattice expansion. Magnetic properties measured over temperature (5 K – 1000 K) showed increased magnetization but lower magnetic Curie temperature in PZT - CZF particle. Magnetoelectric coefficient measured as function of ferrite concentration showed an increase of more than 100% after doping the CFO phase with 40% Zn. This enhancement can be attributed to increase in the lattice strain, magnetic permeability and decrease in coercivity.

Effect of misfit strain on the thermal expansion coefficient of graphene/MoS2 van der Waals heterostructures

Because of their advanced properties inherited from their constituent atomic layers, van der Waals heterostructures such as graphene/MoS 2 are promising candidates for many optical and electronic applications. However, because...

Structural Quality and Magnetotransport Properties of Epitaxial Layers of the (Ga,Mn)(Bi,As) Dilute Magnetic Semiconductor

Structural analysis of epitaxial layers of the (Ga,Mn)(Bi,As) quaternary dilute magnetic semiconductor (DMS), together with investigations of their magnetotransport properties, has been thoroughly performed. The obtained results are compared with those for the reference (Ga,Mn)As layers, grown under similar conditions, with the aim to reveal an impact of Bi incorporation on the properties of this DMS material. Incorporation of Bi into GaAs strongly enhances the spin-orbit coupling strength in this semiconductor, and the same has been expected for the (Ga,Mn)(Bi,As) alloy. In turn, importantly for specific spintronic applications, strong spin-orbit coupling in ferromagnetic systems opens a possibility of directly controlling the direction of magnetization by the electric current. Our investigations, performed with high-resolution X-ray diffractometry and transmission electron microscopy, demonstrate that the (Ga,Mn)(Bi,As) layers of high structural quality and smooth interfaces can be grown by means of the low-temperature molecular-beam epitaxy method, despite a large difference between the sizes of Bi and As atoms. Depending on the applied buffer layer, the DMS layers can be grown under either compressive or tensile misfit strain, which influences their magnetic properties. It is shown that even small 1% Bi content in the layers strongly affects their magnetoelectric properties, such as the coercive field and anisotropic magnetoresistance.