W.B. Mi

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.

Polycrystalline iron nitride films fabricated by reactive facing-target sputtering: Structure, magnetic and electrical transport properties

Structure, magnetic and electrical transport properties of the reactive sputtered iron nitride films fabricated at different substrate temperatures (Ts) and nitrogen flow rates (FN2) were investigated systematically. The single-phase polycrystalline γ′-Fe4N film was obtained at FN2 = 20 sccm and Ts = 450 °C. The films are soft ferromagnetic at room temperature. The electrical resistivity (ρ) of the films fabricated at different FN2 and Ts decreases with the decreasing temperature, showing a metallic behavior. Meanwhile, a variety of magnetoresistance (MR) behaviors were observed depending on the applied magnetic field and measuring temperature. It can be deduced that the complex MR behaviors are dominated by Lorentz force, spin-orbit interaction, and weak localization effect.

Electronic and magnetic structure of Fe3O4/BiFeO3 multiferroic superlattices: First principles calculations

Fe3O4/BiFeO3(001) superlattices comprising multiferroic BiFeO3 and ferrimagnetic half-metallic Fe3O4 have been investigated using first principles calculations. Two models were simulated: Model (a) contains the interfaces of Fe(A)−BiO and Fe2O4(B)−FeO2; Model (b) contains the interfaces of Fe(A)−FeO2 and Fe2O4(B)−BiO. The magnetization enhances 13% and 8% for models (a) and (b) due to the interfacial bonding between Fe(A)/Fe(B) and Bi atoms, respectively. The much larger enhancement in model (a) is ascribed to the facts that the Fe(A) atoms are surrounded by relatively less O atoms than Fe(B) in model (b), which increases the hybridization between Fe(A) and Bi atoms. The calculated results suggest that the number of oxygen atoms at the interfaces plays an important role on determining the interfacial coupling strength. Meanwhile, the interfacial bonding also affects the spin polarization of the Fe3O4 at the interface.

A comparative study of transport properties in polycrystalline and epitaxial chromium nitride films

Polycrystalline CrNx films on Si(100) and glass substrates and epitaxial CrNx films on MgO(100) substrates were fabricated by reactive sputtering with different nitrogen gas flow rates (fN2). With the increase of fN2, a lattice phase transformation from metallic Cr2N to semiconducting CrN appears in both polycrystalline and epitaxial CrNx films. At fN2= 100 sccm, the low-temperature conductance mechanism is dominated by both Mott and Efros-Shklovskii variable-range hopping in either polycrystalline or epitaxial CrN films. In all of the polycrystalline and epitaxial films, only the polycrystalline CrNx films fabricated at fN2 = 30 and 50 sccm exhibit a discontinuity in ρ(T) curves at 260–280 K, indicating that both the N-vacancy concentration and grain boundaries play important roles in the metal-insulator transition.

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%.

Structure, optical, and magnetic properties of facing-target reactive sputtered Ti1−xFexO2−δ films

Structure, optical, and magnetic properties of the anatase Ti1−xFexO2−δ films fabricated by facing-target reactive sputtering were investigated. Structural analyses indicate that there are no impurity phases in the films and the grain size reaches maximum when Fe content x is 0.017. The optical band gap of the Ti1−xFexO2−δ films decreases with the increasing x. Photoluminescence (PL) spectra show that direct and indirect band-to-band transitions coexist, and the indirect radiative recombination can be regarded as a one photon and two phonon coupling courses. The PL intensity due to oxygen vacancies enhances and the refractive index of the fims increases with the increasing x. The Ti1−xFexO2−δ films exhibit room-temperature ferromagnetism due to the oxygen vacancies.

Characterization of facing-target reactive sputtered polycrystalline Fe3O4 films

Polycrystalline half-metallic Fe3O4 films fabricated by facing-target reactive sputtering were investigated systemically. Structural results reveal that typical Fe3O4 grains were well isolated by grain boundaries and grew with columnar structure. The room-temperature magnetization of Fe3O4 films at 50kOe field is much lower than that of bulk Fe3O4 (471emu∕cm3), due to the presence of strong antiferromagnetic coupling within grain boundary and the amorphous bottom and top layers. The insaturation magnetization and loop shift in high field region at low temperature also stem from the strong antiferromagnetic coupling. The resistivity increases with the decreasing temperature and is consistent with fluctuation-induced tunneling mechanism in a wide temperature ranging from 50to300K. The negative magnetoresistances up to −9.7% at 150K and −6.4% at 300K under 90kOe magnetic field perpendicular to the film plane were observed and discussed.

Anomalous Hall effect and magnetoresistance behavior in Co/Pd1−xAgx multilayers

In this paper, we report anomalous Hall effect (AHE) correlated with the magnetoresistance behavior in [Co/Pd1−xAgx]n multilayers. For the multilayers with n = 6, the increase in Ag content from x = 0 to 0.52 induces the change in AHE sign from negative surface scattering-dominated AHE to positive interface scattering-dominated AHE, which is accompanied with the transition from anisotropy magnetoresistance (AMR) dominated transport to giant magnetoresistance (GMR) dominated transport. For n = 80, scaling analysis with Rs∝ρxxγ yields γ ∼ 3.44 for x = 0.52 which presents GMR-type transport, in contrast to γ ∼ 5.7 for x = 0 which presents AMR-type transport.

Electrical transport and magnetic properties of reactive sputtered polycrystalline Ti1−xCrxN films

The magnetoresistance (MR) and magnetic properties of (2 0 0) preferentially oriented Ti1−xCrxN films have been investigated systematically. All the films are ferromagnetic. The Curie temperature of the films is above 305 K, which is much higher than the previously reported value of 140 K in the Ti1−xCrxN systems. MR shows a weak saturation trend with the applied magnetic field, and increases significantly with a decrease in temperature below 50 K following the relation log |MR| = a − bT1/2. The MR–T curves are fitted using MR = P2/(1 + P2) with the spin polarization P of , where the fitted P0 is 55% at x = 0.14. The enhanced coercivity and low-temperature MR are related to the pinned surface moments of the Cr-doped TiN grains.

Large negative magnetoresistance in reactive sputtered polycrystalline GdNx films

Polycrystalline ferromagnetic GdNx films were fabricated at different N2 flow rates (fN2) to modify N-vacancy concentration so as to study its influence on electrotransport. Metal-semiconductor transition appears at Curie temperature (TC) of ∼40 K. Temperature-dependent magnetoresistance (MR) shows a peak at TC. The films at fN2 = 5, 10, 15, and 20 sccm show MR of −38%, −42%, −46%, and −86% at 5 K and 50 kOe, respectively. Above 15 K, MR is from colossal MR and from both colossal and tunneling MR below 15 K. The enhanced MR at fN2 = 20 sccm is attributed to large spin polarization of half-metallicity in GdNx with low N vacancies.