In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Magnetocaloric effect: From materials research to refrigeration devices

In the past 20 years, there has been a surge in research on the magnetocaloric response of materials, due mainly to the possibility of applying this effect for magnetic refrigeration close to room temperature. This review is devoted to the main families of materials suitable for this application and to the procedures proposed to predict their response. Apart from the possible technological applications, we also discuss the use of magnetocaloric characterization to gain fundamental insight into the nature of the underlying phase transition.

The Magnetocaloric Effect and Magnetic Refrigeration Near Room Temperature: Materials and Models

The magnetic refrigeration technique based on the magnetocaloric effect (MCE) has attracted increasing interest because of its high efficiency and environment friendliness. in this article, our recent progress in exploring effective MCE materials is reviewed with emphasis on the MCE in the LaFe13-xSix-based alloys discovered by us. These alloys show large entropy changes over a wide temperature range near room temperature. The effects of magnetic rare-earth doping, interstitial atoms and high pressure on the MCE have been systematically studied. Special issues, such as appropriate approaches to determining the MCE associated with the first-order magnetic transition, the depression of magnetic and thermal hysteresis, and the key factors determining the magnetic exchange in alloys of this kind, are discussed. The applicability of giant MCE materials to magnetic refrigeration near ambient temperature is evaluated. A brief review of other materials with significant MCE is also presented.

Recent Progress in Exploring Magnetocaloric Materials

This review paper discusses the influence of morphology and crystallographic structure on gas-sensing characteristics of metal oxide conductometric-type sensors. The effects of parameters such as film thickness, grain size, agglomeration, porosity, faceting, grain network, surface geometry, and film texture on the main analytical characteristics (absolute magnitude and selectivity of sensor response (S), response time (τres), recovery time (τrec), and temporal stability) of the gas sensor have been analyzed. A comparison of standard polycrystalline sensors and sensors based on one-dimension structures was conducted. It was concluded that the structural parameters of metal oxides are important factors for controlling response parameters of resistive type gas sensors. For example, it was shown that the decrease of thickness, grain size and degree of texture is the best way to decrease time constants of metal oxide sensors. However, it was concluded that there is not universal decision for simultaneous optimization all gas-sensing characteristics. We have to search for a compromise between various engineering approaches because adjusting one design feature may improve one performance metric but considerably degrade another.

The role of morphology and crystallographic structure of metal oxides in response of conductometric-type gas sensors

A series of BaTiO3 thin films with various thicknesses from 10 to 400 nm were epitaxially grown under various oxygen pressures from 2×10−4 to 12 Pa on SrTiO3 (001) substrates using laser molecular beam epitaxy. Being confirmed by reflection high energy electron diffraction, atomic force microscopy, x-ray diffraction, and high resolution transmission electron microscopy the epitaxial single crystal BaTiO3 thin films are highly c-axis or a-axis oriented with a root-mean-square surface roughness of 0.14 nm. The observed thickness and oxygen pressure dependent structural characteristics of the BaTiO3 thin films are discussed by taking into account both the misfits in thermal expansion and lattice constants between BaTiO3 films and SrTiO3 substrates, and the effect of the energy of the sputtered particles, which is consistent with the established strain relaxation mechanism. An abnormal expansion of lattice volume of a BaTiO3 unit cell is found and attributed to the effect of oxygen vacancies in the BaTiO3 films.

Thickness and oxygen pressure dependent structural characteristics of BaTiO3 thin films grown by laser molecular beam epitaxy

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.

Effects of magnetic field on the manganite-based bilayer junction

Second-harmonic generation (SHG) of a 1.064-\ensuremath{\mu}m incident beam was investigated on ${\mathrm{BaTiO}}_{3}{/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}$ superlattices prepared by laser molecular-beam epitaxy. The incidence angle and the polarization angle dependence of the SHG coefficients were measured. The SHG coefficients were greatly enhanced by the superlattice structure with the maximum value of ${d}_{33}=156.5\mathrm{p}\mathrm{m}/\mathrm{V}$ being more than one order of magnitude larger than that of bulk ${\mathrm{BaTiO}}_{3}$ crystal. The mechanism of SHG enhancement was discussed.

Enhancement of second-harmonic generation in BaTiO3/SrTiO3 superlattices

A series of BaTiO3−x (x=0, 0.07, 0.18, and 0.48) thin films with c-axis oriented tetragonal perovskite structures were epitaxially grown on SrTiO3 (100) and MgO (100) substrates by laser molecular-beam epitaxy under various oxygen pressures. The electrical conductivity of the deposited thin films increases with decreasing the oxygen pressure. A metallic conduction is observed in the oxygen most deficient BaTiO2.52 thin film whose oxygen content (2.52) and electrical resistivity (6.0×10−5 Ω cm) are both the minimum values among literature. An optical absorption peak is observed in each oxygen deficient BaTiO3−x thin film. The peak position shifts to lower energy, and the peak width becomes larger with decreasing the oxygen pressure. The influence of oxygen pressure on the electrical and optical properties in the reduced BaTiO3−x thin films is discussed.A series of BaTiO3−x (x=0, 0.07, 0.18, and 0.48) thin films with c-axis oriented tetragonal perovskite structures were epitaxially grown on SrTiO3 (100) and MgO (100) substrates by laser molecular-beam epitaxy under various oxygen pressures. The electrical conductivity of the deposited thin films increases with decreasing the oxygen pressure. A metallic conduction is observed in the oxygen most deficient BaTiO2.52 thin film whose oxygen content (2.52) and electrical resistivity (6.0×10−5 Ω cm) are both the minimum values among literature. An optical absorption peak is observed in each oxygen deficient BaTiO3−x thin film. The peak position shifts to lower energy, and the peak width becomes larger with decreasing the oxygen pressure. The influence of oxygen pressure on the electrical and optical properties in the reduced BaTiO3−x thin films is discussed.

Electrical and optical properties of strongly reduced epitaxial BaTiO3−x thin films

A method introducing stress is suggested to enhance optical nonlinearity Three stress induced samples of BaTiO3 thin film, BaTiO3/SrTiO3 superlattice, and cerium doped BaTiO3/SrTiO3 superlattice were epitaxially grown on SrTiO3 (100) substrates by laser molecular beam epitaxy The measurement of the dependence of the effective second-harmonic generation (SHG) coefficients on the fundamental polarization angle and incident angle indicates that SHG coefficients d33 of the three samples are greatly enhanced, with the maximum value being one order of magnitude larger than that of the bulk BaTiO3 crystal Our results clearly show that the introduction of stress is a very powerful means to enhance nonlinear optical susceptibility

Tong Zhao

Papers

Thickness and oxygen pressure dependent structural characteristics of BaTiO3 thin films grown by laser molecular beam epitaxy

Effects of magnetic field on the manganite-based bilayer junction

Enhancement of second-harmonic generation in BaTiO3/SrTiO3 superlattices

Electrical and optical properties of strongly reduced epitaxial BaTiO3−x thin films

Stress-induced enhancement of second-order nonlinear optical susceptibilities of barium titanate films