H. L. Du

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.

Effect of excess PbBr2 on photoluminescence spectra of CH3NH3PbBr3 perovskite particles at room temperature

The organic-inorganic halide perovskites have promising applications in light-emitting devices besides solar cells. We here prepare CH3NH3PbBr3 perovskite particles on SiO2 substrates and find that the photoluminescence (PL) spectrum of the particles at room temperature has two peaks, locating at 529 and 549 nm, respectively, much different from that of the corresponding films prepared on the oxygen plasma-cleaned SiO2 substrates, which has a single peak. The double peaks have different temperature-dependence behaviors. By the x-ray diffraction and energy dispersive x-ray spectroscopy analyses, excess PbBr2 is detected inside the particles. We deduce that such excess PbBr2 has introduced shallow level defects. It is concluded that band-to-band recombination and these defects result in the double-peaked feature of the PL spectra of CH3NH3PbBr3 particles at room temperature.

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.

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.

Magnetic and transport properties of cobalt doped La0.7Sr0.3MnO3

The magnetic and transport properties of La0.7Sr0.3Mn1−xCoxO3 (0 ≤ x ≤ 1) samples were systematically studied. A magnetic phase diagram was constructed according to the observed results. At the two ends (x ≤ 0.3 and x > 0.9), the samples exhibit the conventional ferromagnetism with the metallic conducting behavior, which can be interpreted in terms of the double exchange coupling between Mn (Co) ions. As for the intermediate Co-doping region (0.3 < x ≤ 0.9), the samples enter a mixed magnetic phase state, which displays the characteristics of the glassy states. According to the frequency-dependent results from the ac susceptibility, a representative frequency sensitivity factor K was calculated to be ∼0.1 for the typical La0.7Sr0.3Mn0.6Co0.4O3 sample, corresponding to the coexistence of superparamagnetic-like free spins and the reentrant spin glass. The results of the Curie-Weiss fitting of the dc magnetization suggest that the Co3+ ions are in the intermediate spin state with the increase of Co-doping le...

Epitaxial growth of Y3Fe5O12 thin films with perpendicular magnetic anisotropy

Here, we report the realization of epitaxial Y3Fe5O12 (YIG) thin films with perpendicular magnetic anisotropy (PMA). The films are grown on the substituted gadolinium gallium garnet substrate (SGGG) by pulsed laser deposition. It was found that a thin buffer layer of Sm3Ga5O12 (SmGG) grown on top of SGGG can suppress the strain relaxation, which helps induce a large enough PMA to overcome the shape anisotropy in YIG thin films. The reciprocal space mappings analysis reveals that the in-plane strain relaxation is suppressed, while the out-of-plane strain relaxation exhibits a strong dependence on the film thickness. We found that the PMA can be achieved for both bilayer (YIG/SmGG) and tri-layer (SmGG/YIG/SmGG) structural films with YIG layer thicknesses up to 20 nm and 40 nm, respectively.

Wide temperature span of entropy change in first-order metamagnetic MnCo1?xFexSi

The crystal structure and magnetic properties of MnCoxFe1?xSi (x?=?0?0.5) compounds were investigated. With increasing Fe content, the unit cell changes anisotropically and the magnetic property evolves gradually: Curie temperature decreases continuously, the first-order metamagnetic transition from a low-temperature helical antiferromagnetic (AFM) state to a high-temperature ferromagnetic state disappears gradually and then a spin-glass-like state and another AFM state emerge in the low-temperature region. The Curie transition leads to a moderate conventional entropy change. The metamagnetic transition not only yields a larger negative magnetocaloric effect at lower applied fields than in MnCoSi but also produces a very large temperature span (103?K for ??0H?=?5?T) of ?S(T), which results in a large refrigerant capacity. These phenomena were explained in terms of crystal structure change and magnetoelastic coupling mechanism. Because of the large isothermal entropy change, the wide working temperature span and the low cost, MnCo1?xFexSi compounds are promising candidates for near room-temperature magnetic refrigeration applications.