J. B. Yang

Comprehensive study of the resistance switching in SrTiO3 and Nb-doped SrTiO3

We have demonstrated that the resistance switching (RS) effect can be controlled by the modification of the electrode configurations and the carrier densities in the Ag/SrTiO3 and Ag/Nb-doped SrTiO3(Nb:STO) structures. The elimination of the Schottky junction in the metal/Nb:STO completely destroys the RS effect, which suggests that the RS effect originates from the modification of Schottky-like barrier formed at the interface of metal/Nb:STO. The rectifying I-V curves revealed that the change in resistance was attributed to the trapping or detrapping carriers at the interface. The carrier density plays an important role in the determination of RS effect. The presence of the RS in SrTiO3 requires an appropriate doping level to provide conditions for trapping carriers at the interface.

Anisotropic nanocrystalline MnBi with high coercivity at high temperature

Magnetic hard nanocrystalline MnBi has been prepared by melt spinning and subsequent low temperature annealing. A coercivity of 2.5 T can be achieved at 540 K for MnBi with an average grain size of about 20-30 nm. The coercivity iHc, mainly controlled by the coherent magnetization rotation, shows a strong dependence on the time of grinding and exhibits a positive temperature coefficient from 100 up to 540 K. The unique temperature dependent behavior of the coercivity (magnetocrystalline anisotropy) has a relationship with the variations in the crystal lattice ratio of c/a with temperatures. In addition, discontinuity can not be found in the lattice parameters of a, c, and c/a ratio at the magnetostructural transition temperature. The nanocrystalline MnBi powder fixed in an epoxy resin and under an applied magnetic field of 24 kOe shows a maximum energy product of 7.1 MGOe at room temperature and shows anisotropic characteristics with high Mr/Ms ratio up to 560 K.

Preparation and magnetic properties of MnBi

MnBi with low temperature phase was fabricated by melt-spinning and subsequently annealing. The influence of quenching speeds, compositions and annealing conditions on the formation of low temperature phase MnBi was systematically investigated. It was found the amorphous MnBi ribbons could transform into low temperature phase by heat treatment in a temperature range of 533–593 K. The coercivity of MnBi was greatly improved by porphyrization, and exhibited a positive temperature coefficient. The maximum energy product BHmax of the anisotropic bonded magnet is 7.1 MGOe (56 kJ/m3) and 4.0 MGOe (32 kJ/m3) at room temperature and 400 K.

τ-MnAl with high coercivity and saturation magnetization

In this paper, high purity τ-Mn54Al46 and Mn54−xAl46Cxalloys were successfully prepared using conventional arc-melting, melt-spinning, and heat treatment process. The magnetic and the structural properties were examined using x-ray diffraction (XRD), powder neutron diffraction and magnetic measurements. A room temperature saturation magnetization of 650.5 kAm-1, coercivity of 0.5 T, and a maximum energy product of (BH)max = 24.7 kJm-3 were achieved for the pure Mn54Al46 powders without carbon doping. The carbon substituted Mn54−xAl46Cx, however, reveals a lower Curie temperature but similar saturation magnetization as compared to the carbon-free sample. The electronic structure of MnAl shows that the Mn atom possesses a magnetic moment of 2.454 μB which results from strong hybridization between Mn-Al and Mn-Mn. We also investigated the volume and c/a ratio dependence of the magnetic moments of Mn and Al. The results indicate that an increase in the intra-atomic exchange splitting due to the cell volume ex...

Trap-assisted tunneling resistance switching effect in CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3 heterostructure

We reported the resistance switching (RS) behavior in the epitaxially grown CeO2/ La0.7(Sr0.1Ca0.9)0.3MnO3 (CeO2/LSCMO) heterojunctions on SrTiO3 substrate. The CeO2/LSCMO device displayed improved switching characteristics as compared to that of metal/manganite device. The switching threshold voltage showed a strong dependence on the thickness of the CeO2 layer, where a minimum/maximum thickness was required for the appearance of the resistance switching. Both set and reset threshold voltages increase with the increase of the CeO2 layer thickness due to the trap-assisted electron tunneling effect. In the meantime, the defects or vacancies in the CeO2 films, in particular, the concentration of the defects or vacancies in the interface between CeO2 and LSCMO, have a significant impact on the switching effect. These results suggest that the electron tunneling accompanied by a trapping/detrapping process at the interface is likely responsible for the RS effect in the insulator/manganite system.

Structure and exchange bias of Ni50Mn37Sn13 ribbons

Ni50Mn37Sn13 ribbons were produced by the melt-spinning method. Structure and magnetic measurements on the Ni50Mn37Sn13 ribbons indicated that it is ferromagnetic below 340 K and undergoes an austenitic-to-martensitic phase transition just below room temperature. The austenitic phase has the cubic L21 structure, with the excess manganese atoms occupying the 4(b) sites. The martensitic phase has an orthorhombic structure. With increasing applied magnetic field, the martensite start temperature Ms and martensite finish temperature Mf shift to lower temperatures because of the field-induced phase transition. The exchange-bias effect is observed and strongly varied with different cooling fields at low temperature. At 5 K, the exchange-bias field reaches a maximum of 290 Oe when the cooling field increases to 200 Oe, and then reduces sluggishly with further increase of the cooling field because of weakening of exchange coupling between antiferromagnetic and ferromagnetic interfaces.

High frequency electromagnetic properties of interstitial-atom-modified Ce2Fe17NX and its composites

The magnetic and microwave absorption properties of the interstitial atom modified intermetallic compound Ce2Fe17NX have been investigated. The Ce2Fe17NX compound shows a planar anisotropy with saturation magnetization of 1088 kA/m at room temperature. The Ce2Fe17NX paraffin composite with a mass ratio of 1:1 exhibits a permeability of μ′ = 2.7 at low frequency, together with a reflection loss of −26 dB at 6.9 GHz with a thickness of 1.5 mm and −60 dB at 2.2 GHz with a thickness of 4.0 mm. It was found that this composite increases the Snoek limit and exhibits both high working frequency and permeability due to its high saturation magnetization and high ratio of the c-axis anisotropy field to the basal plane anisotropy field. Hence, it is possible that this composite can be used as a high-performance thin layer microwave absorber.

Structural properties and large coercivity of bulk Mn3−xGa (0 ≤ x ≤ 1.15)

The tetragonal DO22 phase of the Mn3−xGa compounds, with x varying from 0 to 1.15, has been successfully synthesized by annealing the ingots with the cubic γ-phase. It is found that the lattice parameter a of the tetragonal cell remains almost constant, while the lattice parameter c increases significantly as x varies from 0 to 1.15. In the meantime, the magnetization of Mn3−xGa (at 7 T) increases dramatically with increasing x. A substitution model, which is different to the model with manganese vacancies, is proposed to explain these changes. According to this model, the increase of the lattice parameter c and the magnetization with x can be explained by an assumption that Mn atoms at 2b sites are preferentially substituted by larger and nonmagnetic Ga atoms. A coercivity which is higher than that of other bulk Mn3−xGa alloys ever reported is achieved. Coercivities as large as iHc = 21.4 kOe and 18.2 kOe are obtained for Mn3.0Ga at 5 K and 300 K, respectively.

The asymmetric magnetization reversal in exchange biased granular Co/CoO films

A special kind of asymmetric magnetization reversal characterized by a sharp step in the second quadrant of the hysteresis loop has been investigated in the exchange biased Co/CoO films. This asymmetry only exists in the film where the small clusters grow to form islands. A bimodal distribution of the first-order reversal curves diagram demonstrates that the irreversible magnetization reversal is composed of the free part and exchange biased part, originating from the grains smaller and larger than 6 nm in the film, respectively. The finite-size effect of the antiferromagnetic anisotropy in the ultrafine grains is responsible for this asymmetry.

Temperature dependence of exchange bias and training effect in Co/CoO film with induced uniaxial anisotropy

The exchange bias effect and training effect of the Co/CoO film with induced uniaxial anisotropy were investigated as functions of temperature. It was found that both effects exhibited drastic differences along the easy and the hard axes. Along the easy axis, the magnetization reversal was dominated by domain wall motion throughout the whole temperature range. However, along the hard axis, the magnetization reversal was dominated by domain wall motion and domain rotation at temperatures below and above 150 K, respectively. The crossover of the two reversal modes characterized with significant asymmetry in the hysteresis loop was observed along the hard axis at 150 K due to the interplay between the exchange and uniaxial anisotropies. Significant difference of training effect in the two directions was observed and ascribed to the differences of the duration and intensity of the interaction between ferromagnetic and antiferromagnetic spins in the two magnetization reversal modes.