Zhenjun Wang

Extraordinary Hall effect and ferromagnetism in Fe-doped reduced rutile

Room-temperature ferromagnetism is observed in reduced rutile TiO2−δ by Fe doping. The epitaxial films grown by pulsed-laser deposition are carefully examined by x-ray diffraction, transmission electron microscopy, and magnetic and transport measurements. The films exhibit the extraordinary Hall-effect and thin-film magnetic shape anisotropy. The magnetic moments and anticipated Curie temperatures of the films rule out Fe particles, iron oxides, and Ti–Fe oxides as possible sources for the observed magnetic signals. The carriers of the Fe-doped reduced rutile are p-type, with a carrier density of 1×1022/cm3. This room-temperature dilute magnetic semiconductor should find potential applications in spintronics.

Room-temperature ferromagnetism in manganese doped reduced rutile titanium dioxide thin films

MnxTi1−xO2−δ (x=0.02–0.12) thin films grown on α-Al2O3 substrates by pulsed-laser deposition have been investigated. X-ray diffraction and transmission electron microscopy results indicate that the films are single phase and reduced rutile-type. Superconducting quantum interference device magnetometer measurements show the films are ferromagnetic at room temperature with nonzero coercivity up to 170 Oe. The saturation magnetization of the reduced films is as high as 0.83 μB per Mn atom at room temperature. The temperature dependence of the resistivity shows semiconducting behavior with p-type carriers. The nature of the p-type conduction and its significance to the ferromagnetism are discussed.

Ferromagnetism and transport properties of Fe-doped reduced-rutile TiO2−δ thin films

We have investigated FexTi1−xO2−δ (x=0.02, 0.06, and 0.08) thin films grown on α-Al2O3 substrates by pulsed-laser deposition. X-ray diffraction results indicate that the films are single phase and of reduced-rutile type. Detailed microstructural observations reveal no measurable magnetic impurities in the films. Vibrating sample magnetometer measurements show the films are ferromagnetic at room temperature with coercivity ranging from 340 to 770 Oe. The temperature dependence of the resistivity shows nearly metallic behavior at room temperature but semiconducting behavior at lower temperatures. The extraordinary Hall effect with coercivities similar to those in magnetic hysteresis curves was observed at room temperature. The carriers are p type with a carrier density of about 1022/cm3.

Anomalous magnetoresistance in NiMnGa thin films

The origin of anomalous negative magnetoresistance and its temperature dependence in polycrystalline Ni–Mn–Ga films prepared by pulse laser deposition was studied. The investigation of structural, transports, magnetic, and ferromagnetic resonance properties of the films suggests contributions of different mechanisms in magnetotransport. At low magnetic fields the main contribution to magnetoresistance is due to the transport between the areas with different orientation of magnetic moments, while at high fields it is an electron scattering of in spin-disordered areas.

Structure and magnetic properties of Cr∕Fe-doped SnO2 thin films

Cr- and Fe-doped SnO2 films were deposited on Al2O3 substrates by pulsed-laser deposition. X-ray-diffraction patterns show that the films have rutile structure and grow epitaxially along the (101) plane. The diffraction peaks of Cr-doped samples exhibit a systematic shift toward higher angles with increasing Cr concentration. This indicates that Cr dissolves in SnO2. On the other hand, there is no obvious shift of the diffraction peaks of the Fe-doped samples. The magnetization curves indicate that the Cr-doped SnO2 films are paramagnetic at 300 and 5K. The Fe-doped SnO2 samples exhibit ferromagnetic behavior at 300 and 5K. Zero-field-cooled and field-cooled curves indicate superparamagnetic behavior above the blocking temperature of 100K, suggesting that it is possible that there are ferromagnetic particles in the Fe-doped films. The anisotropic magnetoresistance was not observed in any of the samples.

Seebeck coefficient and thermal conductivity in doped C60

Pressed bulk samples of C60 doped with P, Co, Al, and Bi have been investigated for their thermoelectric properties. These samples show extremely low thermal conductivity, typically in the range of 0.1–0.3W∕Km at room temperature. The Seebeck coefficients of Co, Al, and Bi doped C60 solids are in the tens of μV∕K; however, for P doped C60 samples, a very large Seebeck coefficient in the order of 103μV∕K was observed. The value of the Seebeck coefficient seems to depend sensitively on the P concentration and changes sign upon annealing at 100°C. Ab initio density functional theory calculations show that the calculated electronic structures and the activation energies strongly depend on the dopants in C60 solids. The high Seebeck coefficient in studied P doped C60 is due to the system’s unique dopant and concentration.

Direct observation of strain fields in epitaxial growth Fe3O4 thin films on MgO substrates

Fe3O4 thin films with a thickness of about 140 nm have been deposited on MgO (001) substrates by reactive sputtering. Microstructural studies indicated that monocrystalline Fe3O4 thin films were epitaxially grown on the MgO substrates with a cubic-on-cubic orientation relationship with respect to the substrates. Strain field due to slight lattice mismatch between the films and substrates was observed. The saturation moment and the coercivity for in-plane magnetization hysteresis loop were 235 emu/cc and 500 Oe, respectively. The lower saturation moment value of the Fe3O4 thin film suggested that the strain field may play a role in the reduction of magnetization.

Magnetotransport in NiMnGa thin films

The influence of substrate temperature and annealing conditions on structure, magnetic, and magnetotransport properties of NiMnGa films was investigated, and the crucial effect of substrate temperature was confirmed. It was found that amorphous disordered films are formed during deposition on a substrate held at room temperature. Postdeposition annealing leads to partial recrystallization and recovery of magnetic properties. Annealing at 773 K does not allow the formation of crystal structure, which is created during deposition on substrates held at the same temperature. The highest values of magnetoresistance were observed in the films with a well-defined crystal structure.

Magnetic and transport properties of NiMnAl thin films

The magnetic and transport properties of Ni2MnAl thin films prepared using pulse laser deposition were investigated. It was shown that the films are granular and multiphase. Contrary to the data reported earlier we observe nonmonotonic temperature dependence of resistance with minimum in the vicinity of 100 K for the films deposited on substrates held at 773 K and negative magnetoresistance (MR) values of about 2.5% at 5 K and 1.6% at RT in magnetic field of 50 kOe. These values of MR are the highest reported up to date for Ni2MnAl films. Large negative MR in our films arises from a nonhomogeneous structure and its origin is the same as for granular films.

Room temperature ferromagnetism of manganese doped rutile titanium dioxide thin films

In this paper, the magnetic properties of Mn/sub x/Ti/sub 1-x/O/sub 2/ thin films were reported. The M-H curve of Mn/sub 0.15/Ti/sub 0.85/O/sub 2/ films was measured at 300K. The hysteresis with the coercive force (Hc) about 22 Oe was showed. The films exhibit ferromagnetic behaviors at room temperature.