J. R. Sun

A phenomenological fitting curve for the magnetocaloric effect of materials with a second-order phase transition

The magnetic entropy change of polycrystalline samples Gd, La(Fe0.92Co0.08)11.83Al1.17, LaFe10.8Si2.2, Mn5Ge2.7Ga0.3, Nd2AlFe13Mn3, and TbCo2 with a second-order phase transition has been investigated. A uniform phenomenological function that describes the magnetic entropy change is found for these materials. This could be of great benefit for the design of magnetic refrigerators. The field dependence of the critical exponent for the variation in the maximum entropy change with field is studied. The critical exponent value of 2/3, which is predicted by the mean field theory, is only satisfied for moderate field values. The refrigerant capacity is analyzed and compared to the predictions of the fitting function.

Surface spin-glass behavior in La2/3Sr1/3MnO3 nanoparticles

The low-temperature magnetic and transport properties of La2/3Sr1/3MnO3 nanoparticles have been investigated. It is found that a surface spin-glass behavior exists in La2/3Sr1/3MnO3 nanoparticles, which undergo a magnetic transition to a frozen state below 45 K. The low-temperature surface spin-glass behavior exists even at the highest field used (H=50u200akOe). Moreover, the spin-glass-like transition disappears for particles above 50 nm. In addition, the suppressed low-field magnetoconductivity (LFMC) observed at low temperature for nanosized La2/3Sr1/3MnO3 is obviously lower than the expected upper limit of LFMC, 1/3, for polycrystalline manganites, which is proposed to arise from the higher-order tunneling through the insulating spin-glass-like surface layers.

Strain-dependent vacuum annealing effects in La0.67Ca0.33MnO3−δ films

The effects of vacuum annealing on the structural and transport properties of the La0.67Ca0.33MnO3−δ films grown on SrTiO3 (LCMO/STO) and NdGaO3 (LCMO/NGO) substrates have been studied. A lattice expansion due to oxygen release during the annealing is observed. Under the same condition, the change of the out-of-plane lattice parameter in LCMO/STO is two to three times larger than that in LCMO/NGO, indicating a strong tendency for the oxygen in the former to escape. Correspondingly, the metal-to-semiconductor transition shifts to lower temperatures, linearly with lattice constant until a critical value, Δd=0.03 A for LCMO/STO and Δd=0.05 A for LCMO/NGO, after which a sudden drop of the transition temperature to zero occurs. The different lattice strains in both films are presumably responsible for the different critical oxygen contents for the occurrence of the resistive transition.

Crystal structure and magnetoresistance of Na-doped LaMnO3

The crystal structure and magnetoresistance of are investigated. La1-xNaxMnO3 crystallizes in a rhombohedrally distorted perovskite structure and exhibits a sharp ferromagnetic transition as well as a negative magnetoresistance at around room temperature. On the basis of alternating-current susceptibility and resistivity measurements as well as a comparison with La1-xNaxMnO3 compounds, it is proposed that Na doping tends to drive the system from a regime characterized by strong Hund coupling and strong electron-phonon coupling to one characterized by weak Hund coupling and weak electron-phonon coupling.

Magnetic, electric and thermal properties of La0.7Ca0.3Mn1-xFexO3 compounds

Magnetization, resistivity and specific heat of La0.7Ca0.3Mn1-xFexO3 (0.0 less than or equal to x less than or equal to 0.12) are investigated between 77 and 300 K. The substitution of Fe for Mn does not alter the lattice constants and structural symmetry of the parent compound significantly, but results in a reduction of the magnetization, an increase of the resistivity, decreases of the magnetic and metal-insulter transition temperatures. The substitution also introduces inhomogeneities in the compounds. The experimental results indicate that Fe ions exist in the form of trivalent ions in the compounds, acting as trapping centers and thus blocking the site-percolation path of the e(g) electrons

Magnetic entropy change and its temperature variation in compounds La(Fe1−xCox)11.2Si1.8

Magnetic entropy change ΔS of compounds La(Fe1−xCox)11.2Si1.8 with the cubic NaZn13-type structure was investigated around their Curie temperature TC. It is found that the phase transition is completely reversible, indicating a nature of second order phase transition. The maximum value of |ΔS|∼13.0 J/kgu200aK under a field of 5 T was achieved in compound LaFe11.2Si1.8 at its TC of ∼222 K, which exceeds that of most materials involving a second order transition at the corresponding temperature. With increasing substitution of Co for Fe from x=0 to x=0.8, TC shifts from 222 to 307 K and entropy change decreases. However, |ΔS| still has a considerable magnitude near room temperature. The large magnetic entropy change is believed to be due to the abrupt change of magnetization at TC, which is associated with the strong structural and magnetic interplay in the compounds.

Reduction of hysteresis loss and large magnetic entropy change in the NaZn13-type LaPrFeSiC interstitial compounds

Magnetic properties and magnetic entropy change of the NaZn13-type La0.5Pr0.5Fe11.5Si1.5Cx compounds have been investigated. Both the lattice parameter and the Curie temperature increase linearly with increasing carbon concentration. The maximum hysteresis loss at TC reduces remarkably from 94.8J∕kg for x=0to23.1J∕kg for x=0.3 because of the weakening of the itinerant electron metamagnetic transition. However, the magnetic entropy change remains at the large values of 32.4J∕kgK for x=0 and 27.6J∕kgK for x=0.3 under a field change of 0–5T, which implies that a large magnetocaloric effect and a small hysteresis loss have been simultaneously achieved in the La0.5Pr0.5Fe11.5Si1.5Cx carbides.

Relation between magnetic entropy and resistivity in La/sub 0.67/Ca/sub 0.33/MnO/sub 3/

We present an investigation of the magnetic entropy /spl Delta/S and resistivity /spl rho/, and the relation between them, for La/sub 0.67/Ca/sub 0.33/MnO/sub 3/ film. We found that the maximum /spl Delta/S is approximately 6 J/kg/spl middot/K, and the maximum magnetoresistance (defined as /spl rho/(0)//spl rho/(H)-1) is about 400% under a field /spl mu//sub o/H of 5 T. A careful analysis gives an equation of the form /spl Delta/S=-/spl alpha//spl int//sub 0//sup H/[/spl part/ln(/spl rho/)//spl part/T]/sub H/dH (/spl alpha/=21.74 emu/g), which relates magnetic order to transport behavior of the compounds. This result reveals the predominant role of magnetic polarons on the magnetoresistive property of manganites. It also provides an alternative method to determine magnetic entropy change on the basis of resistive measurements.

Photoresponse of the Schottky junction Au/SrTiO3:Nb in different resistive states

A systematic study on photovoltaic effects has been performed for the Schottky junction Au/SrTiO3:0.05u2002wtu2009%Nb, the resistance of which can be tuned, by applied electric pulses, between ∼1 and ∼200u2002MΩ. It is found that, despite the great change in junction resistance, the photocurrent across the junction is constant when the power and wavelength of incident light are fixed. The corresponding Schottky barrier, deduced from the photoresponse data is ∼1.5u2002eV, independent of junction resistance. This result suggests the invariance of the interfacial barrier during resistance switching and the occurrence of filamentary conduction channels.

Magnetocaloric effect in itinerant electron metamagnetic systems La(Fe1-xCox)11.9Si1.1

The NaZn13-type compounds La(Fe1−xCox)11.9Si1.1 (x=0.04, 0.06, 0.08) were successfully synthesized, in which the Si content is the limit that can be reached by arc-melting technique. TC is tunable from 243 to 301 K with Co doping from x=0.04 to 0.08. Great magnetic entropy change ΔS in a wide temperature range from ∼230 to ∼320K has been observed. The adiabatic temperature change ΔTad upon changing magnetic field was also directly measured. ΔTad of sample x=0.06 reaches ∼2.4K upon a field change from 0 to 1.1 T. The temperature hysteresis upon phase transition is small, ∼1K, for all samples. The influence of Co doping on itinerant electron metamagnetic transition and magnetic entropy change is discussed.