Xu Zuo

Oxygen-defect-induced magnetism to 880 K in semiconducting anatase TiO2−δ films

We demonstrate a semiconducting material, TiO2??, with ferromagnetism up to 880?K, without the introduction of magnetic ions. The magnetism in these films stems from the controlled introduction of anion defects from both the film?substrate interface as well as processing under an oxygen-deficient atmosphere. The room-temperature carriers are n-type with n~3 ? 1017?cm?3. The density of spins is ~1021?cm?3. Magnetism scales with conductivity, suggesting that a double exchange interaction is active. This represents a new approach in the design and refinement of magnetic semiconductor materials for spintronics device applications.

Ferromagnetism in pure wurtzite zinc oxide

The ferromagnetism induced by the intrinsic point defects in wurtzite zinc oxide is studied by using ab initio calculation based on density functional theory. The calculations show that both oxygen interstitial and zinc vacancy may induce ferromagnetism into this material. The calculations also show that zinc oxide with oxygen interstitial may be a ferromagnetic semiconductor. Based on the simplified electronic configuration of the defect molecules, we explain the total magnetic moment, electronic structure, and ferromagnetism.

Computational study of copper ferrite (CuFe2O4)

Magnetic properties and electronic structure of copper ferrites in both normal and inverse spinel structures are studied using a principle spin-polarized band structure calculation method with a modified Becke’s three-parameter exchange correlation. The calculated exchange constants show that the Neel configuration may be unstable for both normal and inverse structures. The local magnetic moments are calculated using Mulliken population analysis and show that the normal structure may achieve very high magnetization. The calculated density of states show that copper ferrite in both normal and inverse spinel structure may be half metallic.

MMIC circulators using hexaferrites

MMIC circulators operating at K/sub A/-band have been fabricated from single crystal and polycrystalline hexaferrite platelets integrated into glass-microwave integrated circuit wafers. Measurements on circulators utilizing single crystal BaFe/sub 11.1/Sc/sub 0.9/O/sub 19/ showed two-band circulation, below-FMR (at 36 GHz) and above-FMR (at 24 GHz), while those using single crystal BaFe/sub 12/O/sub 19/ showed above-FMR circulation at 32-35 GHz. Self-biased circulators using SrFe/sub 12/O/sub 19/ ceramics showed above-FMR circulation at 32 GHz. All circulators showed reasonable insertion losses and isolation, with the best insertion loss of 1.2 dB at an isolation of 18 dB being found for a BaFe/sub 11.1/Sc/sub 0.9/O/sub 19/ circulator operating below-FMR.

Large induced magnetic anisotropy in manganese spinel ferrite films

The oxygen pressure dependence of magnetic anisotropy in pulse laser deposited manganese ferrite (MnFe2O4) films was investigated. Magnetic anisotropy fields (Ha) are shown to exceed 5kOe when films were processed at oxygen pressures below 5mTorr. Further, it is shown that the magnetically preferred direction of Ha can be aligned either along the film plane (pO2 8mTorr). The ability to induce large perpendicular magnetic anisotropy in spinel ferrites allows for new applications (i.e., phase shifters, filters, isolators, and circulators) near or above X-band frequencies to be considered.

Cation-disorder-enhanced magnetization in pulsed-laser-deposited CuFe2O4 films

Copper ferrite films have been deposited on (100) MgO substrates by pulsed-laser deposition. The oxygen pressure used in deposition was varied from 1to120mTorr with the substrate temperature fixed at 700°C. Magnetization values are measured to increase with oxygen pressure, reaching a maximum value of 2480G, which is a 42% increase over the bulk equilibrium value. Extended x-ray absorption spectroscopy shows that the Cu cation inversion δ [defined as (Cu1−δFeδ)tet[CuδFe2−δ]octO4] decreases monotonically from 0.72 to 0.55 with increasing saturation magnetization.

Microwave properties of pulsed laser deposited Sc-doped barium hexaferrite films

The properties of thick BaScxFe12−xO19 (0.3<x<0.6) scandium hexaferrite films were measured by static and microwave field techniques. Films were deposited by pulsed laser ablation onto c-plane sapphire at oxygen pressures of 20 and 50 mTorr. Vibrating sample magnetometer measurements as corroborated by x-ray data showed that the films below 3 μm had easy axis of magnetization (c-axis) normal to the film plane with saturation magnetization values of 3.0–3.8 kG. From the ferrimagnetic resonance frequency versus external magnetic field, we deduced a g value of 1.96±0.03 and uniaxial anisotropy field of ∼10 kOe. The ferrimagnetic resonance linewidth for the film thicker than 5 μm was maximum at 32 GHz and decreased with increasing frequency, indicating evidence for nonuniform magnetic field scattering internally. However, the linewidths were lower for films having thickness below 3 μm.

Single crystal hexaferrite phase shifter at Ka band

A phase shifter was fabricated from a scandium doped M-type (SCM) barium hexaferrite single crystal substrate, and tested over microwave frequencies from 20 to 40 GHz and bias magnetic fields from 0 to 2 kOe. At a microwave frequency of 40 GHz and bias magnetic field of 1.3 kOe, the differential phase-shift rate was 42°/kOe mm and the loss rate was 0.7 dB/mm. The phase shifter utilized the large uniaxial anisotropy field inherent in ScM single crystals, where the crystallographic c axis and the magnetic bias field were parallel to each other and in the substrate plane. The performance of the phase shifter was modeled by the conjugate method for transmission electron microscopy wave and compared with experimental results.

Development of high frequency ferrite phase-shifter

A phase-shifter was fabricated on a c-texture Zn/sub 2/Y (Zn/sub 2/Ba/sub 2/Fe/sub 12/O/sub 22/) single crystal substrate, and tested over microwave frequencies from 2 to 35 GHz and for biasing magnetic fields from 0.2 to 5 kOe. The differential phase-shift rate was 90/spl deg//kOe.mm at a microwave frequency of 20 GHz and a magnetic bias-field of 1.3 kOe. This device showed that it is possible to reduce the bias-field requirement efficiently and simply by utilizing the large in-plane anisotropy in Y-type hexaferrites and magnetostatic modes. A simple model based on the conjugate method for a TEM wave gave a reasonable match with the measured performance of the device, when the uniaxial magnetic anisotropy and magnetostatic modes were included.

Ab initio calculation on ferromagnetic reduced anatase TiO2−δ

The room-temperature ferromagnetism of reduced anatase (TiO2−δ) is studied using density functional calculation. Two kinds of point defects, titanium interstitial and oxygen vacancy (Ov), have been studied. The calculated magnetization shows that Ov is the point defect substantially related to the observed ferromagnetism. A model of the ferromagnetism is given based on the spin-polarized Ti3F+ cluster and the double exchange between the clusters. The calculated density of states supports the model and is in agreement with the donor impurity band exchange model.