C. Jin

Tunable magnetic and electrical properties of polycrystalline and epitaxial NixFe3-xO4 thin films prepared by reactive co-sputtering

Polycrystalline and epitaxial NixFe3−xO4 (0 ≤ x ≤ 1.03) thin films were fabricated by reactively co-sputtering Fe and Ni targets in a mixed Ar + O2 atmosphere, and the structure, magnetic and magnetotransport properties were investigated systematically. The saturation magnetization and resistivity can be tuned over a wide range. The room-temperature saturation magnetization for the polycrystalline thin films decreases linearly with x from 440 to 230 emu cm−3, due to Ni substitution. For the epitaxial thin films, the saturation magnetization and the resistivity can be tuned in the range 195–340 emu cm−3 and 10−4–10−2 Ω m by Ni substitution and the introduction of Fe vacancies, because both Ni substitution and Fe vacancies can influence the charge carrier density and the double exchange on the B sublattice.

Magnetization and Resistance Switchings Induced by Electric Field in Epitaxial Mn:ZnO/BiFeO3 Multiferroic Heterostructures at Room Temperature

Electric field induced reversible switchings of the magnetization and resistance were achieved at room temperature in epitaxial Mn:ZnO(110)/BiFeO3(001) heterostructures. The observed modulation of magnetic moment is ∼500% accompanying with a coercive field varying from 43 to 300 Oe and a resistive switching ratio up to ∼10(4)% with the applied voltages of ±4 V. The switching mechanisms in magnetization and resistance are attributed to the ferroelectric polarization reversal of the BiFeO3 layer under applied electric fields, combined with the reversible change of oxygen vacancy concentration at the Mn:ZnO/BiFeO3 interface.

Origin of the twofold and fourfold symmetric anisotropic magnetoresistance in epitaxial Fe3O4 films

The angular dependence of anisotropic magnetoresistance (AMR) in epitaxial Fe3O4 films on several kinds of substrates has been investigated to explore the nature of AMR. All the measurements show that the dependence of AMR on the angle between current and magnetic field is the superimposition of sinusoidal twofold and fourfold symmetric AMR. The AMR in epitaxial Fe3O4 films is controlled by magnetic anisotropy and antiphase boundaries (APBs). The twofold and fourfold symmetric AMR originate from the scattering far away from the APBs and that near the APBs, respectively, which is consistent with the physical picture of magnetoresistance in epitaxial Fe3O4 films. The magnetic anisotropy, such as the uniaxial anisotropy induced by the step terraces and shape geometry, is closely related to the twofold symmetric AMR. The fourfold symmetric AMR is based on magnetocrystalline anisotropy and probably not correlated with the charge order in magnetite, which was verified by the fourfold symmetric AMR in octahedral...

Enhanced exchange bias in fully epitaxial Fe3O4/tetragonal-like BiFeO3 magnetoelectric bilayers

Fully epitaxial Fe3O4/BiFeO3 bilayers were deposited on (001) LaAlO3 and SrTiO3 by magnetron sputtering. BiFeO3 remained tetragonal on highly misfit-strain LaAlO3 substrates. Obvious exchange bias with a reciprocal relation to the thickness of ferromagnetic Fe3O4 was observed in the Fe3O4/tetragonal-like BiFeO3 (Fe3O4/T-BiFeO3) bilayers below which is similar to that of Fe3O4/rhombohedral-like BiFeO3 (Fe3O4/R-BiFeO3) bilayers. An unexpected enhancement of exchange bias from in Fe3O4/R-BiFeO3 to in the Fe3O4/T-BiFeO3 bilayer was observed at 3 K, which could be attributed to the enhanced Fe–O–Fe interfacial superexchange (SE) interaction induced by distorted T-BiFeO3.

Structure, magnetic, and transport properties of epitaxial ZnFe2O4 films: An experimental and first-principles study

We investigated the structure, magnetic, and transport properties of ZnFe2O4 (ZFO) by both experimental and first-principles study. The epitaxial ZFO films prepared with various oxygen partial pressures show clear semiconducting behavior and room-temperature ferrimagnetism. A large magnetoresistance of −21.2% was observed at 75 K. The room-temperature ferrimagnetism is induced by the cation disordering. The calculated results indicate that under cation disordering, the ZFO with various oxygen vacancies is a half-metal semiconductor with both possible positive and negative signs of the spin polarization, while ZFO with no oxygen vacancies is an insulator and can be acted as the spin filter layer in spintronic devises.

Anomalous magnetic properties of the epitaxial CoFe2O4 films prepared by reactive cosputtering

Epitaxial CoFe2O4 (CFO) films with different thicknesses (6–240 nm) were fabricated on (001) SrTiO3 substrates by reactive cosputtering. Microstructure analyses indicate that the surfaces of the CFO films are covered by islands with height from 2.7 to 0.8 nm upon decreasing film thickness from 240 to 6 nm. Magnetic measurement shows that the CFO films exhibit film thickness dependence of anomalous magnetic properties, including two different “magnetic phases” reflected in the magnetization loops, and reduced saturation magnetization and coercivity with decreasing film thickness. Systematic analyses of the microstructure and magnetization loops for the CFO films with different thicknesses imply that the structure defects on the surface and at the interface, together with the antiphase boundaries in the films are responsible for the anomalous magnetic properties of the CFO films.

Experimental and first-principles study on the magnetic and transport properties of Ti-doped Fe3O4 epitaxial films

Structure, magnetic, and transport properties of the epitaxial Fe3–xTixO4 films with x ≤ 0.09 fabricated by reactive co-sputtering were investigated systematically. The lattice constant expands from 0.8360 to 0.8376 nm with x increases from 0 to 0.09. The room-temperature saturation magnetization increases by 31.7%, and the negative magnetoresistance (at 100 K) increases from 3.7% to 6.9% as x increases from 0 to 0.09. The results indicate that Ti atoms might exist at the tetrahedral sites in the spinel structure. Theoretical calculations based on the density-functional theory suggest that the doped Ti on the tetrahedral sites indeed enhances the lattice constant and magnetic moment. Meanwhile, the calculated results also imply that the Ti-doped Fe3O4 has a high spin polarization near 100%.

Magnetocrystalline anisotropy-dependent six-fold symmetric anisotropic magnetoresistance in epitaxial CoxFe3?xO4 films

The angular-dependent anisotropic magnetoresistance (AMR) of epitaxial CoxFe3−xO4 films has been investigated systematically. The six-fold symmetric AMR was observed in the (111)-oriented epitaxial CoxFe3−xO4 films in contrast to the four-fold symmetric AMR of the (100)- and (110)-oriented CoxFe3−xO4 films. The six-fold term appears at temperatures below 200 K, and becomes more pronounced with increasing x, while no six-fold term was found in epitaxial NixFe3−xO4 films. The six-fold characteristics observed in the epitaxial CoxFe3−xO4 films are considered to be induced by the large magnetocrystalline anisotropy caused by Co addition, which directly proves that the AMR of magnetite is intimately related to magnetocrystalline anisotropy.

Crystal-Orientation-Modulated Exchange Bias in Orthorhombic-YMnO3/La0.6Sr0.4MnO3 Multiferroic Heterostructures.

The magnetic properties of the all-oxide multiferroic heterostructures composed of orthorhombic YMnO3 (YMO) with E-type antiferromagnetic and double-exchange ferromagnetic (FM) La0.6Sr0.4MnO3 (LSMO) were studied. An orientation-modulated exchange bias effect, which is related to the interfacial Mn-O-Mn bond angle, was discovered. Because of the large bond angle in YMO/LSMO(100) heterostructures, a strong exchange coupling at the interface is formed. This strong exchange coupling sustains an FM phase in YMO at the interface region. The FM phase with strong magnetocrystalline anisotropy contributes to the vertical shift and exchange bias effect in (100) orientation heterostructures. When LSMO (110) and (111) were layered with YMO, the Mn-O-Mn bond angle was reduced, leading to a weakened exchange coupling at the interface, and only a relatively small exchange bias at low temperatures was visible.

Strain and Ferroelectric-Field Effects Co-mediated Magnetism in (011)-CoFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 Multiferroic Heterostructures.

Electric-field mediated magnetism was investigated in CoFe2O4 (CFO, deposited by reactive cosputtering under different oxygen flow rates) films fabricated on (011)-Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) substrates. Ascribed to the volatile strain effect of PMN-PT, the magnetization of the CFO films decreases along the [01-1] direction whereas it increases along the [100] direction under the electric field, which is attributed to the octahedron distortion in the spinel ferrite. Moreover, a nonvolatile mediation was obtained in the CFO film with low oxygen flow rate (4 sccm), deriving from the ferroelectric-field effect, in which the magnetization is different after removing the positive and negative fields. The cooperation of the two effects produces four different magnetization states in the CFO film with low oxygen flow rate (4 sccm), compared to the only two different states in the CFO film with high oxygen flow rate (10 sccm). It is suggested that the ferroelectric-field effect is related to the oxygen vacancies in CFO films.