Cheng Zhao-Hua

Large magnetic entropy change near room temperature in the LaFe11.5Si1.5H1.3 interstitial compound

The LaFe11.5Si1.5H1.3 interstitial compound has been prepared. Its Curie temperature TC (288 K) has been adjusted to around room temperature and the maximal magnetic entropy change (|?S|~17.0 J?kg-1?K-1 at TC) is larger than that of Gd (|?S|~9.8 J?kg-1?K-1 at TC=293 K) by ~73.5% under a magnetic change from 0 to 5 T. The origin of the large magnetic entropy change is attributed to the first-order field-induced itinerant-electron metamagnetic transition. Moreover, the magnetic hysteresis of LaFe11.5Si1.5H1.3 under the increase and decrease of the field is very small, which is favourable to magnetic refrigeration application. The present study suggests that the LaFe11.5Si1.5H1.3 compound is a promising candidate as a room-temperature magnetic refrigerant.

Structure and superconductivity of Mg(B1-xCx)2 compounds

In this paper, we report on the structural properties and superconductivity of Mg(B1-xCx)(2) compounds. Powder X-ray diffraction results indicate that the samples crystallize in a hexagonal AlB2-type structure. Due to the chemical activity of Mg powders, a small amount of MgO impurity phase is detected by X-ray diffraction. The lattice parameters decrease slightly with the increasing carbon content. Magnetization measurements indicate that the non-stoichiometry of MgB2 has no influence on the superconducting transition temperature and the transition temperature width. The addition of carbon results in a decrease of T-c and an increase of the superconducting transition width, implying the loss of superconductivity.In this paper, we reported the structural properties and superconductivity of Mg(B1-xCx)2 compounds. Powder x-ray diffraction results indicate that the samples crystallize in a hexagonal AlB2-type structure. Due to the chemical activity of Mg powders, a small amount of MgO impurity phase was detected by x-ray diffraction. The lattice parameters decrease slightly with increasing carbon content. Magnetization measurements indicate the non-stoichiometry of MgB2 has no influence on the superconducting transition temperature and the transition temperature width. The addition of carbon results in a decrease of Tc and an increase in the superconducting transition width, implying the loss of superconductivity.

Magnetic properties of TbMn6Sn6-xGax (x=0.0-1.2) compounds

Effects of Ga substitution for Sn on the structure and magnetic properties of TbMn6Sn6-xGax (x=0.0-1.2) compounds have been investigated by means of x-ray diffraction, magnetization measurement and Sn-119 Mossbauer spectroscopy. The substitution of Ga for Sn results in a decrease in lattice constants and unit-cell volumes. The magnetic ordering temperature decreases monotonically with increasing Ga content from 423 K for x=0.0 to 390 K for x=1.2. At room temperature, the easy magnetization direction changes from the c-axis to the ab-plane. This variation implies that the substitution of Ga for Sn leads to a decrease in the c-axis anisotropy of the Tb sublattice. An increase in the non-magnetic Ga concentration results in a monotonic decrease of the spontaneous magnetization M-s at room temperature. Since there are three non-equivalent Sn sites, 2c (0.33, 0.67,0), 2d (0.33, 0.67,0.5) and 2e (0,0,0.34) in the TbMn6Sn6-xGax compounds, the Sn-119 Mossbauer spectra of the TbMn6Sn6 and TbMn6Sn5.4Ga0.6 compounds can be fitted by three sextets. The hyperfine fields (HFs) decrease in the order of HF(2d)>HF(2e)>HF(2c), which is in agreement with the magnetic structure.

Magnetoresistances and magnetic entropy changes associated with negative lattice expansions in NaZn13-type compounds LaFeCoSi

Magnetoresistances and magnetic entropy changes in NaZn13-type compounds La(Fe1−xCox)11.9Si1.1 (x=0.04, 0.06 and 0.08) with Curie temperatures of 243 K, 274 K and 301 K, respectively, are studied. The ferromagnetic ordering is accompanied by a negative lattice expansion. Large magnetic entropy changes in a wide temperature range from ~230 K to ~320 K are achieved. Raising Co content increases the Curie temperature but weakens the magnetovolume effect, thereby causing a decrease in magnetic entropy change. These materials exhibit a metallic character below TC, whereas the electrical resistance decreases abruptly and then recovers the metal-like behaviour above TC. Application of a magnetic field retains the transitions via increasing the ferromagnetic ordering temperature. An isothermal increase in magnetic field leads to an increase in electrical resistance at temperatures near but above TC, which is a consequence of the field-induced metamagnetic transition from a paramagnetic state to a ferromagnetic state.

Mossbauer study and magnetic properties of electrochemical material LiFePO4

Magnetic properties and crystal symmetry of electrochemical material LiFePO4 have been investigated by Mossbauer spectroscopy and magnetization measurement. Magnetization reveals the antiferromagnetic nature of LiFePO4. Temperature dependence of inverse susceptibility and that of hyperfine field confirm that there is an antiferromagnetic-paramagnetic transition at about 50K.

Photoinduced Magnetization Change in Multiferroic YbFe2O4

We investigate the influence of laser illumination on the magnetization in multiferroic YbFe2O4 single crystals. A photoinduced magnetization change is confirmed in both ab plane and the c-axis directions. The temperature dependence of the photoinduced magnetization reduction excludes laser heating as the cause. In terms of the breakdown of charge order driven by laser illumination, the photoinduced magnetization change provides strong evidence for the spin-charge coupling in YbFe2O4. This photomagnetic effect based on charge-order-induced multiferroicity could be used for the non-thermal optical control of magnetization.We have studied the influence of laser illumination on the magnetization in multiferroic YbFe2O4 single crystals. A photoinduced magnetization change has been confirmed in both ab plane and c axis direction. The temperature dependence of the photoinduced magnetization reduction excludes laser heating as the cause. In terms of the breakdown of charge order driven by laser illumination, the photoinduced magnetization change provides a strong evidence for the spin-charge coupling in YbFe2O4. This photomagnetic effect based on charge-order-induced multiferroicity could be used for non-thermal optical control of magnetization.

Structure and superconductivity of Mg1-xLixB2

The effects of the substitution of Li for Mg in Mg1-xLixB2(x=0, 0.1, 0.2) on their structure and superconductivity have been investigated. It has been found by X-ray diffraction that the substitution of Li for Mg with x=0.1 and 0.2 does not cause phase transformation in these samples. However, the measurements of temperature-dependent normalized magnetization indicate the loss of superconductivity with the increase of the Li content in these samples.

Electron spin resonance investigation of the substitution of Fe3^＋ for Ti4^＋ ions in rutile TiO2 single crystal

A Fe doped rutile TiO2 single crystal is grown in an O2 atmosphere by the floating zone technique. Electron spin resonance (ESR) spectra clearly demonstrate that Fe3+ ions are substituted for the Ti4+ ions in the rutile TiO2 matrix. Magnetization measurements reveal that the Fe:TiO2 crystal shows paramagnetic behaviour in a temperature range from 5 K to 350 K. The Fe3+ ions possess weak magnetic anisotropy with an easy axis along the c axis. The annealed Fe:TiO2 crystal shows spin-glass-like behaviours due to the aggregation of the ferromagnetic clusters.

Magnetization reversal of ultrathin Fe film grown on Si（111） using iron silicide template

Ultrathin Fe films were epitaxially grown on Si(111) by using an ultrathin iron silicide film with p(2 × 2) surface reconstruction as a template. The surface structure and magnetic properties were investigated in situ by low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), and surface magneto-optical effect (SMOKE). Polar SMOKE hysteresis loops demonstrate that the Fe ultrathin films with thickness t < 6ML (monolayers) exhibit perpendicular magnetic anisotropy. The characters of M-H loops with the external magnetic field at difference angles and the angular dependence of coercivity suggest that the domain-wall pinning plays a dominant role in the magnetization reversal process.

Vortex Pinning due to Dynamic Spin–Vortex Interaction in a Superconductor/Ferromagnet Multilayer

We investigate the mutual interaction between superconductivity and ferromagnetism in a Nb/Ni81Fe19 multilayer by ac susceptibility measurements. Compared with a pure superconducting Nb film, the critical current density of the multilayer is apparently enhanced in a low magnetic field region but remains nearly the same in high magnetic fields, which indicates that a continuous ferromagnetic layer with in-plane magnetization can produce strong vortex pinning in a low field region. We interpret this unusual vortex-pinning phenomenon as a consequence of dynamic spin–vortex interaction which induces a spin rotation following vortex movement. In addition, we propose that this dynamic interaction could be used for spin manipulation via a superconductor.