Xisong Zhou

Processing and characterization of TiO2 film prepared on glass via pulsed laser deposition

Nanocrystalline titanium dioxide films with anatase structures were prepared on glass by pulsed laser deposition. Our results demonstrated that deposition temperature and oxygen pressure were important parameters in optimization of the microstructure and conductivity of TiO2 anatase film. The conductivity of the TiO2 film increased as the substrate temperature increased. It did not, however, increase further even though the depositions were performed above 600 °C. It was also confirmed that the conductivity showed a unique dependence on ambient oxygen pressure, which demonstrated the significant influence of oxygen pressure on the carrier concentration in thin oxide films.

Reverse sequence of formation of titanium nitrides by nitrogen implantation

Room‐temperature implantation was conducted for the thin titanium films by 80‐keV nitrogen ions. It was found that TiN began to appear at a dose around 2×1017 N/cm2, and the titanium film converted entirely into TiN after 1×1018 N/cm2 implantation. Surprisingly, Ti2N, which has a lower N/Ti ratio than TiN, was only detected at an even higher implantation dose, e.g., as high as 2×1018 N/cm2. This reverse sequence of titanium nitride formation was attributed to the structural compatibility between the matrix and new phase being formed. Viewed in this light, a shearing mechanism is proposed, which can explain the titanium nitride formation, and is also applicable to other metal nitrogen systems.

Electrical control of Co/Ni magnetism adjacent to gate oxides with low oxygen ion mobility

We investigate the electrical manipulation of Co/Ni magnetization through a combination of ionic liquid and oxide gating, where HfO2 with a low O2− ion mobility is employed. A limited oxidation-reduction process at the metal/HfO2 interface can be induced by large electric field, which can greatly affect the saturated magnetization and Curie temperature of Co/Ni bilayer. Besides the oxidation/reduction process, first-principles calculations show that the variation of d electrons is also responsible for the magnetization variation. Our work discloses the role of gate oxides with a relatively low O2− ion mobility in electrical control of magnetism, and might pave the way for the magneto-ionic memory with low power consumption and high endurance performance.

Grain-boundary-controlled impedances of electroceramics: Generalized effective-medium approach and brick-layer model

In every electroceramic there exist variations throughout the microstructure of both grain size (and shape) and electrical properties of individual grain boundaries. To calculate the effects of such microstructural variations on the frequency-dependent impedance/dielectric behavior, we describe a generalized statistical effective-medium approach based on effective-medium theory. To illustrate the predictions of the approach, the effect of various distributions in the grain boundary conductivity and the grain size, as well as the grain shape and porosity, are considered. The calculations show that such variations lead to depression of the boundary arc and deviations of the impedance spectra from the well-known brick-layer model with an idealized microstructure consisting of identical grains and grain boundaries.

Anomalous afterglow from Y2O3-based phosphor

Abstract Eu 3+ -activated Y 2 O 3 phosphor co-doped with Ti 4+ or/and Mg 2+ ions were prepared by calcinating the precursor powders in the 98%N 2 –2%H 2 atmosphere. Red long afterglow was observed in the Y 2 O 3 :Eu 3+ , Ti 4+ , Mg 2+ after excitation by 254 nm light, and the duration can reach nearly 1.5 h in the light perception of the dark-adapted human eyes (0.32 mcd/m 2 ). The main emission peaks were ascribed to Eu 3+ ions transition from 5 D J (J=0,1,2) to 7 F J (J=0,1,2,3,4) , and the afterglow may be ascribed to some suitable complex trap centers produced by the introduction of Ti 4+ and Mg 2+ ions in the Y 2 O 3 host lattice.

Structural correlation in tantalum nitride formation by direct nitrogen implantation

Abstract Tantalum targets were implanted with 80 keV nitrogen ions at doses from 5 × 10 16 to 5 × 10 17 N cm −2 at room temperature. X-ray analyses of the implanted layers show a correlation between the types of nitrides formed and the implantation dosage. As the nitrogen content increases nitrogen-rich nitrides start forming. Ta 2 N first forms at the lower implantation level; at higher doses TaN starts forming. Possible shearing mechanisms responsible for nitride formation are proposed.

Spin-orbit torque in MgO/CoFeB/Ta/CoFeB/MgO symmetric structure with interlayer antiferromagnetic coupling

Spin current generated by the spin Hall effect in a heavy metal that would diffuse up and down to adjacent ferromagnetic layers and exert torque on their magnetization is called spin-orbit torque. Antiferromagnetically coupled trilayers, namely, the so-called synthetic antiferromagnets usually are employed to serve as the pinned layer of spintronic devices based on spin valves and magnetic tunnel junctions to reduce the stray field and/or increase the pinning field. Here we investigate the spin-orbit torque in a MgO/CoFeB/Ta/CoFeB/MgO perpendicularly magnetized multilayer with interlayer antiferromagnetic coupling. It is found that the magnetization of two CoFeB layers can be switched between two antiparallel states simultaneously. This observation is replicated by the theoretical calculations by solving the Stoner-Wohlfarth model and the Landau-Lifshitz-Gilbert equation. Our findings combine spin-orbit torque and interlayer coupling, which might advance the magnetic memories with a low stray field and low power consumption.

Electrochemical control of the phase transition of ultrathin FeRh films

We investigate the electrical manipulation of the phase transition in ultrathin FeRh films through a combination of ionic liquid and oxide gating. The 5 nm-thick FeRh films show an antiferromagnetic-ferromagnetic transition at around 275 K with in-plane magnetic field of 70 kOe. A negative gate voltage seriously suppresses the transition temperature to ∼248 K, while a positive gate voltage does the opposite but with a smaller tuning amplitude. The formation of electric double layer associated with a large electric field induces the migration of oxygen ions between the oxide gate and the FeRh layer, producing the variation of Fe moments in antiferromagnetic FeRh accompanied by the modulation of the transition temperature. Such a modulation only occurs within several nanometers thick scale in the vicinity of FeRh surface. The reversible control of FeRh phase transition by electric field might pave the way for non-volatile memories with low power consumption.

Transport properties of SnTe-Bi2Te3 alloys

Abstract Bismuth telluride based alloys have been a class of important thermoelectric materials widely used in cooling devices around room temperature. In this work, a series of polycrystalline samples of ternary compounds in the SnTe–Bi2Te3 system were prepared by rapid quenching of the melt followed by a sintering procedure of compacted pellets. The electrical conductivity and Seebeck coefficient of samples with various compositions were measured from room temperature up to about 600 K. The measured results show that the transport properties strongly vary with the composition and temperature. The microstructure in this system shows randomly oriented fine plate-shaped grains with multi-layered structure.

Tunneling anisotropic magnetoresistance driven by magnetic phase transition

The independent control of two magnetic electrodes and spin-coherent transport in magnetic tunnel junctions are strictly required for tunneling magnetoresistance, while junctions with only one ferromagnetic electrode exhibit tunneling anisotropic magnetoresistance dependent on the anisotropic density of states with no room temperature performance so far. Here, we report an alternative approach to obtaining tunneling anisotropic magnetoresistance in α′-FeRh-based junctions driven by the magnetic phase transition of α′-FeRh and resultantly large variation of the density of states in the vicinity of MgO tunneling barrier, referred to as phase transition tunneling anisotropic magnetoresistance. The junctions with only one α′-FeRh magnetic electrode show a magnetoresistance ratio up to 20% at room temperature. Both the polarity and magnitude of the phase transition tunneling anisotropic magnetoresistance can be modulated by interfacial engineering at the α′-FeRh/MgO interface. Besides the fundamental significance, our finding might add a different dimension to magnetic random access memory and antiferromagnet spintronics.Tunneling anisotropic magnetoresistance is promising for next generation memory devices but limited by the low efficiency and functioning temperature. Here the authors achieved 20% tunneling anisotropic magnetoresistance at room temperature in magnetic tunnel junctions with one α′-FeRh magnetic electrode.