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

Multiferroic bismuth ferrite-based materials for multifunctional applications: Ceramic bulks, thin films and nanostructures

Bismuth ferrite (BiFeO3), a perovskite material, rich in properties and with wide functionality, has had a marked impact on the field of multiferroics, as evidenced by the hundreds of articles published annually over the past 10 years. Studies from the very early stages and particularly those on polycrystalline BiFeO3 ceramics have been faced with difficulties in the preparation of the perovskite free of secondary phases. In this review, we begin by summarizing the major processing issues and clarifying the thermodynamic and kinetic origins of the formation and stabilization of the frequently observed secondary, nonperovskite phases, such as Bi25FeO39 and Bi2Fe4O9. The second part then focuses on the electrical and electromechanical properties of BiFeO3, including the electrical conductivity, dielectric permittivity, high-field polarization, and strain response, as well as the weak-field piezoelectric properties. We attempt to establish a link between these properties and address, in particular, the macroscopic response of the ceramics under an external field in terms of the dynamic interaction between the pinning centers (e.g., charged defects) and the ferroelectric/ferroelastic domain walls.

BiFeO3 Ceramics: Processing, Electrical, and Electromechanical Properties

In order to elucidate room-temperature (RT) ferromagnetism (FM) in ZnO, we have analyzed a multitude of experimental publications with respect to the ratio of grain-boundary (GB) area to grain volume. FM only appears if this ratio exceeds a certain threshold value ${s}_{\text{th}}$. Based on these important results nanograined pure and Mn-doped ZnO films have been prepared, which reveal reproducible RT FM and magnetization proportional to the film thickness, even for pure ZnO films. Our findings strongly suggest that grain boundaries and related vacancies are the intrinsic origin for RT ferromagnetism.

Magnetization study of nanograined pure and Mn-doped ZnO films: Formation of a ferromagnetic grain-boundary foam

Single-phase Bi5Ti3FeO15 and Bi6Ti3Fe2O18 ceramics have been synthesized by solid state reaction. The ferroelectric transition of the compounds was studied by differential scanning calorimetry, high-temperature x-ray diffraction, and temperature-dependent dielectric measurements. Two solid-state structural transitions were observed in both compounds, one is the orthorhombic↔tetragonal transition (ferroelectric transition) at 1021 K for Bi5Ti3FeO15 and 973 K for Bi6Ti3Fe2O18, and the other is accompanied by an abrupt lattice expansion of the tetragonal phase at about 1110 K for Bi5Ti3FeO15 and about 1090 K for Bi6Ti3Fe2O18.

Ferroelectric transition of Aurivillius compounds Bi5Ti3FeO15 and Bi6Ti3Fe2O18

We report the crystal structure and magnetic properties of the Zn1-xMnxO compounds synthesized by a combustion method. The Zn1-xMnxO compounds with x=0-0.1 crystallize in the wurtzite ZnO structure. The lattice parameters a and c of Zn1-xMnxO increase linearly with the Mn content, indicating that Mn2+ ions substitute for Zn2+ ions. Scanning electron microscopy shows that the average particle radius of Zn0.95Mn0.05O is about 40 nm. From the Curie-Weiss behavior of susceptibility at high temperature, it was found that the Mn-Mn interaction is dominated by antiferromagnetic coupling with effective nearest-neighbor exchange constant J about -32 K and the large negative value of the Curie-Weiss temperature. (C) 2005 American Institute of Physics.

Structure and magnetic properties of Mn-doped ZnO nanoparticles

Single-phase samples of Bi(Fe1−xCrx)O3 with x=0, 01, and 02 were synthesized by a combustion method X-ray diffraction reveals that the lattice parameters of Bi(Fe1−xCrx)O3 perovskites decrease linearly with the Cr content, indicating that Cr ions substitute for Fe ions to form a solid solution X-ray photoelectron spectroscopy investigation shows that Cr ions have the Cr3+ valence state in Bi(Fe1−xCrx)O3 The frequency dependence of dielectric constants was investigated at room temperature Magnetic measurements show hysteresis loops at both 5 and 300K and the substitution of Cr for Fe enhances the magnetization

Magnetic properties of Bi(Fe1−xCrx)O3 synthesized by a combustion method

The crystal structure and magnetic properties of SmCo7−xSix (x=0.1–0.9) compounds were studied by means of x-ray powder diffraction and magnetic measurements. Rietveld refinement of x-ray powder diffraction pattern shows that the as-cast compound SmCo7−xSix with x=0.9 crystallizes in the TbCu7-type structure with the space group P6/mmm, and the doping element Si has a distinct preference to occupy the 3g site. According to the refinement result, the composition of the compound is derived as SmCo5.85Si0.90. The compound SmCo5.85Si0.90 exhibits ferromagnetic order with the Curie temperature of about 717 K and a saturation moment of about 6.58±0.05 μB/f.u. The SmCo5.85Si0.90 compound shows a strong uniaxial magnetocrystalline anisotropy, and an anomalous increase of magnetization at low temperature is observed in an external field applied perpendicular to the easy direction of magnetization.

Crystal structure and magnetic properties of SmCo5.85Si0.90 compound

The crystal structure and magnetic properties of SmCo7-xHfx (x=0.05,0.1,0.2,0.3) compounds were studied by means of x-ray powder diffraction and magnetic measurements. The as-cast compounds SmCo7-xHfx with x=0.1 and 0.2 crystallize in the TbCu7-type structure with the space group P6/mmm. According to the structure refinement, the doping element Hf prefers to occupy the 2e site. The compounds have high Curie temperatures (1080 K for x=0.1, 1075 K for x=0.2), large saturation magnetization (104 emu/g for x=0.1, 102 emu/g for x=0.2 at 5 K), and large magnetic anisotropy fields (244 kOe for x=0.1, 282 kOe for x=0.2 at 5 K) with strong uniaxial magnetocrystalline anisotropy. (C) 2004 American Institute of Physics.

Guanghui Rao

Papers

Ferroelectric transition of Aurivillius compounds Bi5Ti3FeO15 and Bi6Ti3Fe2O18

Structure and magnetic properties of Mn-doped ZnO nanoparticles

Magnetic properties of Bi(Fe1−xCrx)O3 synthesized by a combustion method

Crystal structure and magnetic properties of SmCo5.85Si0.90 compound

Crystal structure and magnetic properties of SmCo7−xHfx compounds