Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics

Characterization of the Nickel Cobaltite, NiCo2O4, Prepared by Several Methods: An XRD, XANES, EXAFS, and XPS Study

Many interesting materials phenomena such as the emergence of high-Tc superconductivity in the cuprates and colossal magnetoresistance in the manganites arise out of a doping-driven competition between energetically similar ground states. Doped multiferroics present a tantalizing evolution of this generic concept of phase competition. Here, we present the observation of an electronic conductor-insulator transition by control of band-filling in the model antiferromagnetic ferroelectric BiFeO3 through Ca doping. Application of electric field enables us to control and manipulate this electronic transition to the extent that a p-n junction can be created, erased and inverted in this material. A 'dome-like' feature in the doping dependence of the ferroelectric transition is observed around a Ca concentration of approximately 1/8, where a new pseudo-tetragonal phase appears and the electric modulation of conduction is optimized. Possible mechanisms for the observed effects are discussed on the basis of the interplay of ionic and electronic conduction. This observation opens the door to merging magnetoelectrics and magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom already present in the multiferroic BiFeO3.

Electric modulation of conduction in multiferroic Ca-doped BiFeO3 films

The discoveries of gigantic ferroelectric polarization in BiFeO(3) (ref. 1) and ferroelectricity accompanied by a magnetic order in TbMnO(3) (ref. 2) have renewed interest in research on magnetoelectric multiferroics, materials in which magnetic and ferroelectric orders coexist, from both fundamental and technological points of view. Among several different types of magnetoelectric multiferroic, magnetically induced ferroelectrics in which ferroelectricity is induced by complex magnetic orders, such as spiral orders, exhibit giant magnetoelectric effects, remarkable changes in electric polarization in response to a magnetic field. Many magnetically induced ferroelectrics showing the magnetoelectric effects have been found in the past several years. From a practical point of view, however, their magnetoelectric effects are useless because they operate only far below room temperature (for example, 28 K in TbMnO(3) (ref. 2) and 230 K in CuO (ref. 11)). Furthermore, in most of them, the operating magnetic field is an order of tesla that is too high for practical applications. Here we report materials, Z-type hexaferrites, overcoming these problems on magnetically induced ferroelectrics. The best magnetoelectric properties were obtained for Sr(3)Co(2)Fe(24)O(41) ceramics sintered in oxygen, which exhibit a low-field magnetoelectric effect at room temperature. Our result represents an important step towards practical device applications using the magnetoelectric effects.

Low-field magnetoelectric effect at room temperature.

This review aims to summarise the progress in some materials and structures for electromagnetic applications, such as microwave absorption, electric shielding and antenna designs, which have been developed in recent years. Composites with spherical powders for microwave absorption focus mainly on those based on ferrites (especially hexagonal), carbonyl iron and related alloys and various newly emerged nanosized materials. Composites with long conductive fibres as fillers will be summarised, with speical attentions to prediction, measurment and evaluation of their performances. Metamaterials include structures for microwave absorbing applications, tunable materials or structures with reflection or transmission coefficients that are tunable by external magnetic or electric fields, and specially designed structures for microwave absorbing applications, with thickness much smaller than that of conventional composite materials and performances that can be optimised by the physical properties of substra...

Recent progress in some composite materials and structures for specific electromagnetic applications

We have prepared nonoriented and magnetically oriented specimens of Co2Z-type Ba ferrite Ba3Co2Fe24O41 (Ba3Z) and those with Sr2+ substitution for Ba2+, i.e., Ba1.5Sr1.5Co2Fe24O41 (Ba1.5Sr1.5Z) and Sr3Co2Fe24O41 (Sr3Z) with the conventional solid-state reaction method. Permeability measurements of nonoriented specimens have shown that this substitution improves the frequency characteristic of permeability, though the permeability in Sr3Z significantly decreases. X-ray diffraction (XRD) and magnetization measurements of magnetically oriented specimens have shown that the magnetic moments of iron and cobalt ions in Ba3Z and Ba1.5Sr1.5Z lie in the c plane, but that those in Sr3Z deviate from the c plane. We have studied the substitution effect of Sr2+ for Ba2+ on the crystal structures and the effective sizes and directions of magnetic moments and their temperature dependences with high-temperature neutron diffraction technique. This substitution induces the change in the distribution of cobalt ions and mome...

Crystal and magnetic structures and their temperature dependence of Co2Z-type hexaferrite (Ba,Sr)3Co2Fe24O41 by high-temperature neutron diffraction

We have made neutron diffraction measurements on powdered YMn2 and determined its spin structure to be a collinear antiferromagnet with a Mn moment of 2.7μB. By analyzing both the intensity of magnetic peaks and the NMR spin-echo spectrum at 4.2K, the direction of the spin axis was found to be parallel to the [111] direction. The Neel temperature TN of YMn2 has been determined t be about 100 K by means of magnetic susceptiblity and thermal expansion measurements. These measurement reveal that the magnetic transition is of first and shows a thermal hysteresis around TN.

Antiferromagnetism of YMn2 intermetallic compound

X-ray and neutron diffraction studies on iron-substituted Z-type hexagonal barium ferrite: Ba3Co2−xFe24+xO41 (x=0–0.6)

Electric and Magnetic Properties of “VO”

We have developed a high-pressure cell for neutron scattering. The cell uses a barrel-shaped support cylinder made of sintered ceramics. Samples 6 mm in diameter and 10 mm long can be accommodated. The cell is suitable for use at low and high temperatures, after a pressure generated at room temperature is locked. The change of pressure on cooling to cryogenic temperatures is estimated by measuring the lattice parameter of NaCl. Besides the pressure locking technique, continuous pressurization can be achieved at room temperature by using a built-in mini-press while the cell is mounted on a goniometer of neutron spectrometer.

https://iopscience.iop.org/article/10.1143/JJAP.26.152/pdf

Shinji Kawano

Papers

Crystal and magnetic structures and their temperature dependence of Co2Z-type hexaferrite (Ba,Sr)3Co2Fe24O41 by high-temperature neutron diffraction

Antiferromagnetism of YMn2 intermetallic compound

X-ray and neutron diffraction studies on iron-substituted Z-type hexagonal barium ferrite: Ba3Co2−xFe24+xO41 (x=0–0.6)

Electric and Magnetic Properties of “VO”

High-Pressure Cell for Neutron Scattering