Ji-Rong Sun

Influence of negative lattice expansion and metamagnetic transition on magnetic entropy change in the compound LaFe11.4Si1.6

Magnetization of the compound LaFe11.4Si1.6 with the cubic NaZn13-type structure was measured as functions of temperature and magnetic field around its Curie temperature TC of ∼208 K. It is found that the magnetic phase transition at TC is completely reversible. Magnetic entropy change ΔS, allowing one to estimate the magnetocaloric effect, was determined based on the thermodynamic Maxwell relation. The achieved magnitude of |ΔS| reaches 19.4 J/kg K under a field of 5 T, which exceeds that of most other materials involving a reversible magnetic transition in the corresponding temperature range. The large entropy change is ascribed to the sharp change of magnetization, which is caused by a large negative lattice expansion at the TC. An asymmetrical broadening of |ΔS| peak with increasing field was observed, which is resulted from the field-induced itinerant-electron metamagnetic transition from the paramagnetic to ferromagnetic state above the TC.

Magnetic entropy change in Ni51.5Mn22.7Ga25.8 alloy

A considerable magnetic entropy change has been observed in Ni51.5Mn22.7Ga25.8 alloys under a field of 0.9 T. This change originates from a sharp magnetization jump which is associated with a martensitic to austenitic structure transition. The large low field entropy change and the adjustable martensic–austensic transition temperature indicate a great potential of Ni–Mn–Ga as working materials for magnetic refrigerants in a wide temperature range.

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.

Very large magnetic entropy change near room temperature in LaFe11.2Co0.7Si1.1

A very large magnetic entropy change ΔS has been observed in Fe-based cubic NaZn13-type compound LaFe11.2Co0.7Si1.1 near the Curie temperature TC of 274 K. The value of the entropy change is ∼20.3 J/kg K under a magnetic field of 5 T at TC=274 K. It markedly exceeds that of pure Gd at the corresponding temperature range [V. K. Pecharsky & K. A. Gschneidner, Jr., Phys. Rev. Lett. 78, 4494 (1999)]. The great entropy change produced by the sharp change of magnetization is associated with a large negative lattice expansion at TC. The very large magnetic entropy change and low cost suggest that the compound LaFe11.2Co0.7Si1.1 has great potential for applications as magnetic refrigerants near room temperature.

Metallic and Insulating Interfaces of Amorphous SrTiO3-Based Oxide Heterostructures

The conductance confined at the interface of complex oxide heterostructures provides new opportunities to explore nanoelectronic as well as nanoionic devices. Herein we show that metallic interfaces can be realized in SrTiO(3)-based heterostructures with various insulating overlayers of amorphous LaAlO(3), SrTiO(3), and yttria-stabilized zirconia films. On the other hand, samples of amorphous La(7/8)Sr(1/8)MnO(3) films on SrTiO(3) substrates remain insulating. The interfacial conductivity results from the formation of oxygen vacancies near the interface, suggesting that the redox reactions on the surface of SrTiO(3) substrates play an important role.

Effects of magnetic field on the manganite-based bilayer junction

An oxide bilayer junction has been fabricated by growing a La0.32Pr0.35Ca0.33MnO3 film on 0.5 wt % Nb-doped SrTiO3 crystal, and its behavior under magnetic field is experimentally studied. It is found that external field greatly affected the rectifying property and the resistance of the junction, causing an extremely large magnetoresistance. The most striking observation of the present work is that the magnetoresistance of the junction can be either positive or negative, depending on temperature and applied current, and is asymmetric with respect to the direction of the bias current. These results reveal the great potential of the manganites in configuring artificial devices.

Current-induced effect on the resistivity of epitaxial thin films of La0.7Ca0.3MnO3 and La0.85Ba0.15MnO3

Electric-current-dependent resistance has been studied in epitaxial thin films of La0.7Ca0.3MnO3 and La0.85Ba0.15MnO3. Attention was focused at the influence of the applied dc current on the resistance of these epitaxial thin films in the absence of a magnetic field. A significant change in the ratio of the peak resistance at different currents or current resistance was found to be ∼23%–26% with a current density up to 8×104 Acm−2. For both La0.7Ca0.3MnO3 and La0.85Ba0.15MnO3 compounds, the dependence of the measured resistance on the current revealed a good linear relationship. Although the nature behind such an effect has not been well understood yet, the feature that the resistance in doped manganese oxides could be easily controlled by the electric current should be of interest for various applications such as field effect devices.

Doping effects arising from Fe and Ge for Mn in La0.7Ca0.3MnO3

Structural, magnetic, and transport properties of polycrystalline La0.7Ca0.3Mn1−xFexO3 and La0.7Ca0.3Mn1−xGexO3 are experimentally studied. Single-phase samples are obtained in the range x=0–0.12 for Fe, and x=0–0.06 for Ge. There are no appreciable structure changes due to the introduction of Fe and Ge. The Mn-site doping favors a reduced magnetic/resistive transition, at rates of ∼22 K for 1% Fe and ∼28 K for 1% Ge, and an elevated resistivity. No metal–insulator transition occurs when the content of Fe exceeds ∼0.1. The enhanced doping effects in La0.7Ca0.3Mn1−xGexO3 can be ascribed to the reduced hole concentration noting that the presence of Fe and Ge influence the contents of mobile eg electrons and holes in the compounds, respectively. Equivalence of the effects from Fe and Ge doping, respectively, to those due to eg electron and hole trapping and the relation between Mn- and O-site doping are discussed.

Magnetism and magnetic entropy change of LaFe11.6Si1.4Cx(x=0−0.6) interstitial compounds

Chemically stable LaFe11.6Si1.4Cx (x=0−0.6) interstitial compounds were prepared. The Curie temperatures increases from 195 to 250 K with an increase in X from 0 to 0.6 due to obvious lattice expansion. The maximal magnetic entropy changes |ΔS| with a change in magnetic field of 0–5 T, are 24.8, 24.2, 18.8 and 12.1 J/kg K for x=0, 0.2, 0.4, and 0.6, respectively, notably exceeding that of Gd (|ΔS|∼9.8 J/kg K at TC=293 K). The magnetic transition varies from first order to second order with an increase in carbon concentration, which has a decisive influence on |ΔS|. The large |ΔS|, chemical stability, and low cost make LaFe11.4Si1.6Cx a promising candidate as a magnetic refrigerant in the corresponding temperature ranges.

Giant negative thermal expansion in bonded MnCoGe-based compounds with Ni2In-type hexagonal structure.

MnCoGe-based compounds undergo a giant negative thermal expansion (NTE) during the martensitic structural transition from Ni2In-type hexagonal to TiNiSi-type orthorhombic structure. High-resolution neutron diffraction experiments revealed that the expansion of unit cell volume can be as large as ΔV/V ∼ 3.9%. The optimized compositions with concurrent magnetic and structural transitions have been studied for magnetocaloric effect. However, these materials have not been considered as NTE materials partially due to the limited temperature window of phase transition. The as-prepared MnCoGe-based compounds are quite brittle and naturally collapse into powders. By using a few percents (3-4%) of epoxy to bond the powders, we introduced residual stress in the bonded samples and thus realized the broadening of structural transition by utilizing the specific characteristics of lattice softening enforced by the stress. As a result, giant NTE (not only the linear NTE coefficient α but also the operation-temperature window) has been achieved. For example, the average α̅ as much as -51.5 × 10(-6)/K with an operating temperature window as wide as 210 K from 122 to 332 K has been observed in a bonded MnCo0.98Cr0.02Ge compound. Moreover, in the region between 250 and 305 K near room temperature, the α value (-119 × 10(-6)/K) remains nearly independent of temperature. Such an excellent performance exceeds that of most other materials reported previously, suggesting it can potentially be used as a NTE material, particularly for compensating the materials with large positive thermal expansions.