X. J. Liu

Evidence of structural defect enhanced room-temperature ferromagnetism in Co-doped ZnO

We report experimentally that room-temperature ferromagnetism of Co-doped ZnO is strongly correlated with structural defects, and that carriers involved in carrier-mediated exchange are by nature by-products of the creation of the defects. On the other hand, the carrier concentration has pronounced influences on the absorption edge in optical transmission spectra of epitaxial Co:ZnO films, in which both a blue-shift and a red-shift are observed. Furthermore, high-temperature annealing results in evolution of the local Co structure from Co(2+) replacing Zn(2+) to a Co(3)O(4)-based phase, which most likely accounts for the transition from ferromagnetism to paramagnetism in the Co:ZnO films.

Influence of annealing on microstructure and magnetic properties of co-sputtered Co-doped ZnO thin films

The annealing effects on the microstructure and magnetic properties of Zn0.92Co0.08O films annealed at different temperatures (200, 400 and 600 °C) in air and vacuum have been systematically investigated. The as-grown and annealed films are all highly textured with (002) orientation and all show room temperature ferromagnetism. The as-grown film is in a state of compressive stress and the film becomes almost stress-free after an annealing treatment at 400 °C in vacuum, while the film annealed at 600 °C goes into a state of tension. The magnetic moment increases after annealing in vacuum and decreases after annealing in air at the same temperature, compared with that of as-grown film. Moreover, the magnetic moment increases with the annealing temperature up to 400 °C and then shows a rapid decline at an annealing temperature of 600 °C in vacuum. The experimental results indicate that the enhancement of ferromagnetism is strongly correlated with the increase in oxygen vacancies and that the existence of interstitial Zn atoms plays an important role in mediating the ferromagnetism in Co-doped ZnO films.

Fully epitaxial (Zn,Co)O∕ZnO∕(Zn,Co)O junction and its tunnel magnetoresistance

The authors report tunnel magnetoresistance (TMR) and its bias dependence in fully epitaxial (Zn,Co)O∕ZnO∕(Zn,Co)O magnetic tunnel junctions. A positive TMR of 20.8% is obtained at 4K, which can resist up to room temperature with the TMR ratio of 0.35% at 2T, due to improved crystallinity of barriers and electrode/barrier interfaces. The decay of TMR with bias up to 2V is significantly small leading to V1∕2, for which half of the TMR remains, well over 2V, shedding promising light on solving readout problems in gigabit-scale magnetoresistive random access memory.

Transition from diluted magnetic insulator to semiconductor in Co-doped ZnO transparent oxide

Epitaxial growth of Co-doped ZnO films has been prepared on Al2O3 (001) substrate by an efficient and low cost method, i.e., direct current reactive magnetron cosputtering. The correlation among local Co structure and magnetic, electric, and optical properties of Co:ZnO films has been carefully studied. Taking advantage of decreasing the oxygen partial pressure, we have demonstrated the transition from diluted magnetic insulators (DMIs) to semiconductors (DMSs) in Zn0.96Co0.04O films. Full multiple-scattering ab initio calculations of Co K-edge x-ray-absorption near-edge structure clearly provide a structure fingerprint to determine Co2+ replacing Zn2+ and a slight dissimilarity of local Co structure between DMIs and DMSs. Co:ZnO insulating films exhibiting more robust room temperature ferromagnetism than semiconducting films reveal that structure defects are seen to be responsible for the ferromagnetic ordering in Co:ZnO transparent oxide. Furthermore, optical transmittance measurements show a large reds...

Room temperature ferromagnetism and ferroelectricity in cobalt-doped LiNbO3 film

(5at.%) cobalt-doped LiNbO3 (Co:LN) films were prepared by combinatorial laser molecular-beam epitaxy on Si (100). The Co:LN films with Co2+ replacing Nb exhibit room temperature ferromagnetism of 1.2μB∕Co and Curie temperature of ∼540K. Through a Ag∕Co:LN∕Si metal-ferroelectric-semiconductor field effect transistor configuration, ferroelectric measurements show that the films display hysteresis loops at 300K and ferroelectric transition temperature of ∼610K. The hysteresis and the asymmetry in capacitance-voltage and leakage-voltage curves are ascribed to trapping/detrapping process of charges at the Co:LN/Si interface. The coexistence of room temperature ferromagnetism and ferroelectricity makes Co:LN a promising single-phase multiferroic.

Microstructure and nanoindentation hardness of Ag/Fe multilayers

Abstract Ag/Fe multilayers with well compositional modulation periodicity of 4–60 nm were prepared at room temperature by evaporation deposition using an ultra high vacuum (UHV) chamber. Their microstructure and hardness were investigated using XRD, TEM and nanoindentation. The fcc/bcc type multilayers show a textured polycrystalline growth with Ag (111) and Fe (110) in Ag layers and Fe layers, respectively. The hardness increases with decreasing periodicity and approaches the maximum of 6.36 GPa at the periodicity of 4 nm. The peak hardness is 1.51 times mixture value. The experimental results are well explained by the dislocation-image force-based model developed by Lehoczky.

Anomalous voltage dependence of tunnel magnetoresistance in (Zn, Co)O-based junction with double barrier

Voltage dependent tunnel magnetoresistance (TMR) has been studied in fully epitaxial (Zn,Co)O∕ZnO∕(Zn,Co)O∕ZnO∕(Zn,Co)O magnetic tunnel junctions (MTJs) with double barrier. The MTJs show extremely small voltage dependence with “half voltage” over 4V above 6K. At 5K, the TMR as a function of voltage is found to be constant up to 2V, and then decreases. “TMR transition” occurs when temperature decreases to 3–4K, and subsequently, the TMR abnormally increases with voltage at 2K. The anomalous voltage dependent TMR is discussed in terms of the large energy separation between the Fermi level and the mobility edge.