H. Sang

Magnetocaloric effect in perovskite manganites La0.7−xNdxCa0.3MnO3 and La0.7Ca0.3MnO3

In perovskite manganites La0.7−xNdxCa0.3MnO3 (x=0, 0.05, 0.1, 0.15, and 0.20) prepared by the sol–gel technique, large magnetocaloric effects have been observed. The maximum of the magnetic entropy change peaks at the magnetic ordering temperature. Compared to La0.7Ca0.3MnO3, an enhancement of the magnetocaloric effect has been obtained in the samples with Nd3+ dopant (for La0.5Nd0.2Ca0.3MnO3, the maximum of ΔSM is about −2.31 J/kg K).

Low-field magnetoresistance in nanosized La0.7Sr0.3MnO3/Pr0.5Sr0.5MnO3 composites

Nanosized La0.7Sr0.3MnO3/Pr0.5Sr0.5MnO3 (LSMO1−xPSMOx) ceramic composites are prepared using solid-sate sintering. Their microstructural, electro- and magnetotransport properties are characterized by means of various techniques. It is found that the antiferromagnetic/ferromagnetic coupling between PSMO/LSMO at low temperature and the weak ferromagnetic order of PSMO at high temperature results in enhanced low-field magnetoresistance (LFMR) of the composites. With increasing temperature up to 250 K the observed LFMR decays more slowly than that for pure LSMO and this behavior may be explained by the spin coupling near boundaries between LSMO and PSMO grains.

Array of nickel nanowires enveloped in polyaniline nanotubules and its magnetic behavior

An array of nickel nanowires enveloped in polyaniline nanotubules was prepared by the “second-order-template” method. As the first step, an array of the polyaniline nanotubules was synthesized in the pores of an alumina membrane, then nickel nanowires were fabricated by electrochemical deposition of the nickel into the polyaniline nanotubules. The composite nanostructure was characterized by x-ray powder diffraction, and scanning electron micrography. Its magnetic properties were measured by a vibrating sample magnetometer. The coercivity and remanence are of high anisotropy with a maximum at θ=90° (parallel to the nanowires).

Multiferroic properties and dielectric relaxation of BiFeO3∕Bi3.25La0.75Ti3O12 double-layered thin films

BiFeO3 (BFO)∕Bi3.25La0.75Ti3O12 (BLT) films were prepared on (111) Pt∕Ti∕SiO2∕Si substrates via metal organic decomposition method. The multiferroic and dielectric properties of the films were studied. It was found that the ferroelectric polarization and dielectric constant of the films were enhanced by introducing BLT as a barrier layer between BFO and Pt bottom electrode, while the ferromagnetism of BFO was not influenced. More interestingly, the films showed dielectric relaxor behavior, and the possible causes of which were discussed.

Magnetic domain structures of Co22Ag78 granular films observed by magnetic force microscopy

A magnetic force microscope (MFM) was used to image the topography and magnetic microstructures of Co22Ag78 granular films. The observed morphology shows isolated nanometer-scale cobalt particles (granules) embedded in the silver matrix. Stripe magnetic domains with much larger size (typically of ∼100 nm wide) than that of cobalt particles are resolved clearly on MFM micrographs for the annealed samples. It is demonstrated that the domain width and the relative magnetic force strength first increases and then decreases with annealing temperature with a maximum at about 600 K. We suggest that the appearance of the stripe domains is attributed to magnetic correlation among many of the isolated single-domain cobalt particles and is dependent on the microstructure of the samples.

Magnetic entropy change in La0.54Ca0.32MnO3−δ

A La-deficient manganite perovskite sample La0.54Ca0.32MnO3−δ was prepared by conventional solid-state reaction method. The Curie temperature TC is 272 K, about 10 K higher than that of La1−xCaxMnO3. A large magnetic entropy change has been observed and the maximum −ΔSM≈2.9 J/kg K appears at its Curie temperature upon a 0.9 T magnetic field change. The easy fabrication and higher chemical stability make La0.54Ca0.32MnO3−δ a suitable candidate as a working substance in magnetic refrigeration technology.

Study on GMR in CoAg thin granular films

Abstract Giant magnetoresistance has been observed in CoxAg1−x(8⩽ x ⩽ 46 at%) thin granular films. The samples were prepared on glass substrates using ion-beam consputtering technique. It is shown that GMR depends on x, substrate temperature, and annealing treatment. For x = 22 at% film after annealing 0.5 at 500 K, the maximum of δϱ/ϱ is ∼ 13.4% at room temperature.

Microstructure investigation of Fe/Al2O3/Fe tunneling junction

A series of Fe/Al2O3/Fe tunneling junction samples were prepared by ion beam sputtering technique. After deposited, the Al layers in a series of samples were exposed to the air for 24–100 h to obtain Al2O3 layers (100 A) with different oxidation quality. A tunneling magnetoresistance (TMR) value of 5.89% was observed in the sample with the Al layer oxidized for 72 h. The I–V curve of this sample shows a nonlinear relationship, indicating the existence of well metal–insulator contact in the sample. The microstructures of these samples were analyzed by using x-ray photoelectron spectroscopy. The results show that good Al2O3 layers are formed in all samples, but there exists an interdiffusion interface between the Fe and Al2O3 (with Al metal inclusions) layer which greatly influences the TMR effect.

Microstructural investigation of as-deposited Co-Ag nano-granular films

The microstructures of as-deposited Co-Ag nano-granular films are characterized by X-ray diffraction, transmission electron microscopy and high-resolution electron microscopy. In as-deposited films, there is a strong tendency for phase separation to occur; Co and Ag particles coexist in the films. The average size of Co grains varies with the Co concentration. Ag particles have the preferential lattice orientation and Co particles have lattice orientations , , , and in the direction of the film surface normal. There is a tendency for two connecting grains to have two groups of parallel lattice planes.

MAGNETOOPTICAL KERR EFFECT IN FE-SIO2 GRANULAR FILMS

Polar Kerr rotation spectra of Fe‐SiO2 granular films were measured at room temperature in the wavelength region between 400 nm and 850 nm. A maximum Kerr rotation angle (θk) of 18 min was obtained when a magnetic field of 10 kOe was applied perpendicular to the film plane. We have measured θk as a function of Fe volume fraction (fv) of the films. It was found that θk increases with increasing fv and peaks at fv≊0.4. We also found that θk decreases as the Fe particle size increases. Our results indicate that the density of Fe particles and the interfaces between Fe particles and SiO2 matrix may play an important role in the Kerr rotation of granular films.