H.L. Bai

Fabrication of ultrathin epitaxial γ-Fe2O3 films by reactive sputtering

Ultrathin epitaxial ?-Fe2O3 (0?0?1) and (0?1?1) films are synthesized by reactive sputtering on corresponding MgO substrates. The average roughness of the epitaxial films increases with decreasing film thickness, and that of ~5?nm thick films is ~4.2??. The easy axis of the epitaxial ?-Fe2O3 films is verified to be 1?1?1 by angular dependence of hysteresis loops measured by a vibrating sample magnetometer with a sample rotator. The saturation magnetization of the ultrathin epitaxial ?-Fe2O3 films is close to the bulk value of ~390?emu?cm?3 and independent of film thickness when the thickness is above 5?nm, which is crucial for their practical applications in spin filter devices.

Microstructure, magnetic, and optical properties of sputtered Mn-doped ZnO films with high-temperature ferromagnetism

The microstructure, magnetic, and optical properties of Mn-doped ZnO films have been examined. It has been found that Mn doping could improve the growth of ZnO (002) orientation without Mn oxide formation. All the films are ferromagnetic with a Curie temperature of above 350K. The ferromagnetism comes from the ferromagnetic interaction activated by oxygen vacancies between the Mn ions that replace Zn ions, but not from Mn oxide impurities. At an atomic fraction of 2.2% Mn, the average moment per Mn ion reaches a maximum of 0.55μB. With the further increase of Mn atomic fraction, the average moment per Mn ion decreases because the antiferromagnetic energy is lower than the ferromagnetic one due to the reduced distance between the adjacent Mn ions. Meanwhile, the optical band gap value increases from 3.120to3.162eV with the increase of Mn atomic fraction from 0% to 7.5%.The microstructure, magnetic, and optical properties of Mn-doped ZnO films have been examined. It has been found that Mn doping could improve the growth of ZnO (002) orientation without Mn oxide formation. All the films are ferromagnetic with a Curie temperature of above 350K. The ferromagnetism comes from the ferromagnetic interaction activated by oxygen vacancies between the Mn ions that replace Zn ions, but not from Mn oxide impurities. At an atomic fraction of 2.2% Mn, the average moment per Mn ion reaches a maximum of 0.55μB. With the further increase of Mn atomic fraction, the average moment per Mn ion decreases because the antiferromagnetic energy is lower than the ferromagnetic one due to the reduced distance between the adjacent Mn ions. Meanwhile, the optical band gap value increases from 3.120to3.162eV with the increase of Mn atomic fraction from 0% to 7.5%.

Structure and magnetic properties of facing-target sputtered Co–C granular films

We studied the structure and magnetic properties of as-deposited and subsequently annealed CoxC100−x granular films fabricated by a DC facing-target magnetron sputtering system at room temperature using atomic force microscopy, x-ray diffraction (XRD), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy and a vibrating sample magnetometer. The average roughness, Rα, of the as-deposited CoxC100−x granular films is smaller than that of the Si(100) substrates. XRD and TEM analyses indicate that the as-deposited CoxC100 − x granular films are composed of ~2 nm amorphous cobalt grains dispersed in an amorphous carbon matrix, and their morphology is composition independent. The high resolution TEM image of the as-deposited Co30C70 film shows that cobalt and carbon have already separated during the deposition, even if the aggregation of cobalt is not complete. Annealing at 300–450°C causes the crystallization of amorphous cobalt followed by an increase in grain size and the graphitization of the amorphous carbon matrix. The constriction arising from the structural environment results in the coexistence of the hcp and fcc Co phases at temperatures higher than the phase transformation point of 425°C. Magnetic measurements reveal that the coercivity of the as-deposited CoxC100−x granular films decreases with the increase in cobalt concentration, and increases with decrease in film thickness. The enhanced coercivity can be attributed to the weakened intergrain interaction because of the increased percolation threshold and/or the destruction of long-range domain structures caused by the reduction in film thickness.

Evolution of structure, magnetic and transport properties of sputtered films from Fe to Fe3O4

Polycrystalline iron oxide films were fabricated using the reactive sputtering method without substrate heating. Structure characterization indicates that the dominant phases in the films evolve from α-Fe to pure Fe3O4 with the increasing O2 flow rate. In polycrystalline Fe3O4 films, disordered atoms exist at the grain boundaries. Magnetic properties analyses reveal that the room-temperature magnetization first decreases and later increases due to the variation of the volume fraction of the paramagnetic FeO phase with a Neel temperature of 198 K. The magnetoresistance MR (= [R(H) − R(0)]/R(0)) of the films increases from 0.1% for pure Fe film to 10.6% for the Fe3O4 film at 80 K under a 90 kOe field. The transport mechanism of FeO–Fe3O4 and Fe3O4 films is suggested to be the tunnelling process, which satisfies the log ρ ~ T−1/2 relation. The Hall resistivity of the Fe3O4 film decreases with increasing temperature. The ordinary and extraordinary Hall coefficients of the Fe3O4 film at 300 K are about 100 and 420 times larger than those of bulk Fe.

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.

Electrical transport properties and magnetoresistance of polycrystalline Fe3O4/p-Si heterostructures

Polycrystalline Fe3O4 films were deposited on the p-Si wafers using reactive sputtering to form the polycrystalline Fe3O4/p-Si heterostructures. A rectifying behavior was observed in the polycrystalline Fe3O4/p-Si heterostructures due to the formation of p-n junction between Fe3O4 and p-Si. The metal-insulator transition was observed, and the transition temperature decreases from 250 K at 100 mA to 110 K at 1 mA in the reverse range, but it keeps at 100 K in the forward range. The current-dependent magnetoresistance (MR) crossovers from negative to positive with different manners for the forward and reverse currents. The characteristic MR is thought to be caused by the rectifying effect and band structure of the Fe3O4 near the interface of the heterostructures.

Fourfold symmetric anisotropic magnetoresistance based on magnetocrystalline anisotropy and antiphase boundaries in reactive sputtered epitaxial Fe3O4 films

The fourfold symmetric anisotropic magnetoresistance (AMR) at high fields in epitaxial Fe3O4 films, which is incompatible with the traditional twofold symmetry, was found to be independent with the current direction but associated with their magnetocrystalline anisotropy. (001)-, (110)-, and (112)-oriented Fe3O4 films show fourfold symmetry in AMR while twofold symmetry appears for (111)-oriented Fe3O4 films. The cubic magnetocrystalline anisotropy field superimposed onto the external magnetic field modifies the alignment of the spins near antiphase boundaries, leading to the oscillating scattering rate for the transport electrons across antiphase boundaries and thus the corresponding fourfold symmetry in AMR.

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...

Microstructure of amorphous carbon nitride films fabricated by facing-target reactive magnetron sputtering

Carbon nitride films have been fabricated by a dc facing-target reactive sputtering system for various N2 fractions (PN) in the gas mixture. Complementary measurement techniques, including atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and high-resolution transmission electron microscopy (HRTEM), were used to systematically study the morphology and microstructure of the films. AFM images show that the average surface roughness increases with increasing PN. XPS analyses indicate that the concentration of N is not directly proportional to PN, and it rises quickly to a saturated value of ~33 at% at a PN of 20%, which can be attributed to the chemical sputtering effect. The ratio N–C(sp2)/N–C(sp3) increases with increase in PN from 0% to 20%, and then decreases with further increase in PN. However, the number of sp2-hybridized C atoms continues to increase over the whole range of PN, as evidenced by Raman and FTIR measurements. The growth of a disordered sp2 C structure at PN below and above 20% can be attributed to the incorporation of N and the compressive stress relaxing, respectively. Raman scattering and HRTEM analyses reveal an incomplete ordering process of the sp2 C structure with increase in PN.

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.