Zhi-qi Kou

Tuning of magnetocaloric effect in a La0.69Ca0.31MnO3 single crystal by pressure

We report a study of the effect of hydrostatic pressure on the magnetocaloric effect in a La0.69Ca0.31MnO3 single crystal. The single crystal exhibits a much larger magnetic entropy change (ΔSm) than the corresponding polycrystalline samples, reaching 5.2J∕kgK and 8.5J∕kgK for a magnetic field variation of 1T and 5T, respectively. Under hydrostatic pressure, the peak position of ΔSm significantly moves to higher temperatures due to the shift of the magnetic phase transition, from 213.5K under ambient pressure up to 236.5K under a pressure of 1.1GPa, while the maximum value of ΔSm remains nearly the same. These exceptional results demonstrate that the magnetocaloric effect in magnetic materials with strong spin-lattice coupling can be effectively tuned by pressure.

Applied field Mossbauer study of shape anisotropy in Fe nanowire arrays

In order to investigate the magnetic anisotropy of Fe nanowire arrays from microscopic point of view, Fe57 Mossbauer spectra were measured at various magnetic fields along and perpendicular to the nanowire axis, respectively. On the basis of the absorption intensities of the second and the fifth lines of sextet, the orientation of Fe magnetic moments can be detected. It was found that the shape anisotropy dominates the overall magnetic anisotropy in the Fe nanowires. Furthermore, the longitudinal and transverse demagnetizing fields of Fe nanowire, respectively, were deduced from the effective hyperfine field at Fe57 nuclei as a function of applied field. The chain-of-spheres model in conjunction with symmetric fanning mechanism was adopted to interpret the domain structure and the parallel coercivity of magnetic nanowire arrays.

Mössbauer effect probe of local Jahn-Teller distortion in Fe-doped colossal magnetoresistive manganites

The local structure of the Fe-doped La1−xCaxMnO3 (x=0.00–1.00) compounds has been investigated by means of Mossbauer spectroscopy. 57Fe Mossbauer spectra provide direct evidence of Jahn–Teller distortion in these manganites. On the basis of the Mossbauer results, the Jahn–Teller coupling was estimated. It is noteworthy that the Ca-concentration dependence of the Jahn–Teller coupling strength is very consistent with the magnetic phase diagram. Our results reveal that Mossbauer spectroscopy cannot only detect the local structural distortion, but also provide a technique to investigate the Jahn–Teller coupling of Fe-doped La1−xCaxMnO3 colossal magnetoresistive perovskites.

Rectifying and magnetotransport properties of the heterojunction of Co-doped and undoped TiO2−δ with La0.69Ca0.31MnO3 single crystal

Electron-doped wide-band-gap dilute magnetic semiconductor Ti0.93Co0.07O2−δ and TiO2−δ were grown on a hole-doped La0.69Ca0.31MnO3 single crystal to form heterojunctions. These junctions exhibit good rectifying properties and magnetoresistance effect over a relatively wide temperature range. The results for TiO2−δ were similar to that of Ti0.93Co0.07O2−δ in all respects. A schematic band structure of the junction was proposed to account for the results. This work indicates that manganite single crystals can be used as substrates for integration with other materials, which may open an alternative avenue for the exploitation of the manganite-based devices.

Mössbauer effect probe of field-induced magnetic phase transitionin LaFe13−xSix intermetallic compounds

Direct evidence of a field-induced magnetic phase transition in LaFe13−xSix intermetallics with a large magneticaloric effect was provided by Fe57 Mossbauer spectra in externally applied magnetic fields. Moreover, Mossbauer spectra demonstrate that a magnetic structure collinear to the applied field is abruptly achieved in LaFe11.7Si1.3 compound once the ferromagnetic state appears, showing a metamagnetic first-order phase transition. In the case of LaFe11.0Si2.0, the Fe magnetic moments rotate continuously from a random state to the collinear state with increasing applied field, showing that a second-order phase transition is predominant. The different types of phase transformation determine the magnetocaloric effects in response to temperature and field in these two samples.

Voltage induced ultrasharp current jump and magnetic tunability of CaMnO3−δ∕La0.69Ca0.31MnO3 heterojunction

The authors report the current-voltage characteristics of CaMnO3−δ∕La0.69Ca0.31MnO3 heterojunctions prepared under different oxygen pressures. The most interesting observation is that the heterojunctions made under low oxygen pressure shows an ultrasharp current jump in the current-voltage curves and the nonlinear coefficient can reach ∼2×104. They also show remarkable magnetoresistance. The results can be understood in terms of the oxygen vacancy related defects at the junction interface. This work shows that all-manganite-based heterojunctions can show giant nonlinear coefficient, which may have potential applications.

Atomic occupancy preference of Ga and Ti and its effect on the Mn spin arrangements in YMn6Sn6: Evidence from119Sn Mossbauer spectroscopy

Abstract The effects of Ga and Ti substitutions for Sn on the Mn spin arrangements in the YMn6Sn6 compound have been investigated by means of X-ray diffraction (XRD), magnetization measurement and Mössbauer spectroscopy. XRD refinement indicates that Ga and Ti atoms prefer to occupy 2d and 2e sites, respectively. Mössbauer spectra indicate incommensurate magnetic structures for the YMn6Sn6, YMn6Sn5.4Ga0.6 and YMn6Sn5.4Ti0.6 compounds. The substitution of Ga for Sn at the 2d sites is found to significantly decrease the turn angle in the Mn–Sn3–Sn2–Sn3–Mn layer and to subsequently increase the net magnetization. On the other hand, the substitution of Ti for Sn at 2e sites increases the turn angles in the Mn–(Y,Sn1)–Mn and Mn–Sn3–Sn2–Sn3–Mn layers, and consequently decreases the exchange coupling of Mn–Mn between different layers. The resulting ferromagnetic and ferrimagnetic behavior, as has been observed for YMn6Sn5.4Ga0.6 and YMn6Sn5.4Ti0.6, respectively, can be explained in terms of these spin interactions.