B. G. Shen

Determination of the entropy changes in the compounds with a first-order magnetic transition

Entropy changes in the compounds of La1−xPrxFe11.5Si1.5 (x=0.3 and 0.4) have been experimentally studied. A tower-shaped entropy change of the height of ∼27J∕kgK is obtained based on the analyses of heat capacity, while the Maxwell relation predicts an extra entropy peak of the height of ∼99J∕kgK, slightly varying with Pr content. A careful study indicates that the Maxwell relation cannot be used in the vicinity of the Curie temperature because of the coexistence of paramagnetic and ferromagnetic phases, and the huge entropy peak is a spurious result. Similar conclusions are applicable to MnAs and Mn1−xFexAs, for which huge entropy changes have been reported. Appropriate methods for the determination of entropy change of the compound with phase separation are discussed based on the magnetic data.

Bipolar Resistance Switching in Fully Transparent ZnO:Mg-Based Devices

Transparent indium–tin-oxide/ZnO:Mg/F-doped SnO2 devices that show bipolar resistance switching have been successfully fabricated. In addition to the transmittance above 80% for visible light, the devices show a high-to-low resistance ratio greater than 2.5, an endurance more than 105 cycles, and a resistance retention longer than 5000 s even at the temperature of 110 °C. The field-induced resistance change can be explained based on the formation/rupture of conduction filaments, due to the migration of structural defects in electric field. The present work shows the potential application of resistive random access memory to invisible electronics.

Electronic transport and colossal electroresistance in SrTiO3:Nb-based Schottky junctions

Current-voltage characteristics and colossal electroresistance (CER) have been experimentally investigated in the temperature range from 293 to 454 K for the Schottky junctions Au/SrTiO3:0.5 wt % Nb and Au/SrTiO3:0.05 wt % Nb. Both junctions show electron tunneling-dominated transport behavior. Postannealing of SrTiO3:0.05 wt % Nb in oxygen atmosphere causes a transition of the transport behavior from electron tunneling to thermionic emission. The CER effect appears in the junctions with the transport behavior dominated by electron tunneling and greatly weakens when thermionic emission prevails after postannealing. This result reveals the presence of a close relation between CER and electron tunneling.

Entropy changes due to the first-order phase transition in the Gd5SixGe4−x system

Entropy changes due to magnetostructrual phase transition in Gd5SixGe4−x intermetallics have been studied based on a systematic analysis of experiment data and mean-field theory calculations. It is found that the magnetic and lattice entropy changes have the same sign. Further analysis indicates that the main entropy change (∼60%–∼80%) comes from the field-induced change of the magnetic order, while the rest arise from the entropy difference of the two crystallographic modifications joined by the structural transition, probably due to the variation of the lattice vibration mode. The present work reveals the importance of lattice entropy for a system experiencing a first-order transition.

Magnetovolume effect in intermetallics LaFe13−xSix

Based on the Bean–Rodbell model, which assumes a linear variation of exchange coupling with atom spacing, the magnetovolume effects in LaFe13−xSix (x = 1.2–2.0) have been systematically studied. A relation between phase volume and magnetization is first obtained by comparing the structural and magnetic data collected at various temperatures. The maximum spontaneous magnetostriction thus derived is dependent on the content of Si, linearly decreasing from ~2.15% for x = 1.2 to ~1.12% for x = 2. Based on these results and limited experimental data, the parameters involved in the Bean–Rodbell model are determined for the LaFe13−xSix compounds. Further analysis indicates that the Bean–Rodbell model equipped with these parameters gives a satisfactory description of the magnetovolume effects produced by interstitial hydrogen for the LaFe11.44Si1.56 hydride. To explain the pressure effects, in contrast, changes of the parameters under pressure, which are a result of the enhancement of the first-order character of the phase transition, have to be taken into account. These results indicate that either the increase or the decrease of the Curie temperature is simply a consequence of the variation of the phase volume due to the introduction of interstitial atoms or the application of a high pressure, and can be described well by the Bean–Rodbell model.

Reversible electroresistance at the Ag∕La0.67Sr0.33MnO3 interface

We report the observation of reversible electroresistance in a metallic film La0.67Sr0.33MnO3 (LSMO). The transport behavior of the Ag/LSMO system is found to be sensitive to external electric field, and a switching of the resistance between two definite resistive states can be induced by electric pulses of different polarity. The current-voltage relation, measured by tuning bias voltage, is significantly nonlinear, asymmetric against field polarity, and hysteretic in the field increase/decrease processes. This relation can be well described by a simple equation I=σV+kVn with n being a number between 1.8 and 2.4. It is found that the rough surface of the LSMO film favors the electroresistance effects.

Magnetic field-induced entropy change in phase-separated manganites

Magnetic field-induced entropy change has been experimentally studied for phase-separated manganites Eu0.55Sr0.45MnO3 and La0.27Nd0.40Ca0.33 MnO3. The entropy derived from the measured heat capacity exhibits a significant decrease under applied field in a broad temperature range below Curie temperature, and the maximum change is ∼8J∕kgK for Eu0.55Sr0.45MnO3 and is ∼4.2J∕kgK for La0.27Nd0.40Ca0.33MnO3 for a field change of 0–5T. In comparison with the calorimetric technique, Maxwell relation fails to give a proper description for the entropy change. It underestimates the entropy reduction in the low temperature range and even anticipates an entropy increase for Eu0.55Sr0.45MnO3. Failures of the Maxwell relation could be ascribed to the coexistence of two phases in the compound and to the variation of the proportion of each phase with applied field.

Change in photovoltage due to an external magnetic field in a manganite-based heterojunction

The effect of magnetic field on the photovoltaic effect (PVE) has been investigated for a manganite-based heterojunction composed of a La0.7Ce0.3MnO3 film and a 0.5 wt % Nb-doped SrTiO3 substrate. A linear decrease in the photovoltage (Voc) with magnetic fields is observed and the relative variation of Voc [defined as 1−Voc(H)∕Voc(0)] is larger than 10% under a field of 0.5 T at a temperature T=16.5K. There is a proportional relation between ΔVoc and the magnetoresistance of the heterojunction. We attribute the modulation of PVE to variation of magnetization and resistance of depletion layer by external magnetic fields.

Pressure enhancement of the giant magnetocaloric effect in LaFe11.6Si1.4

The authors have studied the effects of pressure on the magnetocaloric effect in a polycrystalline LaFe11.6Si1.4 sample. The Curie temperature TC of the sample rapidly decreases from 191K at ambient pressure to 80K under 8.3kbar pressure. The metamagnetic transition induced by field at temperatures above TC becomes extremely sharp under high pressure and the critical field Hc of the transition increases fast with increasing temperature. As a result, the giant magnetocaloric effect in LaFe11.6Si1.4 is greatly enhanced by pressure, especially at low magnetic fields. For a field variation of 1T only, the maximum value of the entropy change is as high as 34J∕kgK.

Volume dependence of the magnetic coupling in LaFe13−xSix based compounds

The effects of pressure and hydrogenation, the former causes a lattice contraction while the latter causes an expansion of the sample, have been systematically studied for LaFe13−xSix (x=1.3–2.1). It is found that the typical change of the Curie temperature is ∼150K when ∼1.6H∕f.u. is absorbed and ∼−106K as the pressure sweeps from 0to1GPa. One of the most remarkable results of the present work is the presence of a universal relation between Curie temperature and phase volume. The former linearly grows with the increase of lattice constant (∼1510K∕A), irrespective of how the phase volume is modified. This result implies the exclusive dependence of the magnetic coupling in LaFe13−xSix on the Fe–Fe distances and the interstitial hydrogen does not affect the electronic structure of the compounds.