| The coupling between magnetization(M) and electric polarization(P) is referred to as magnetoelectric(ME) effect. ME materials have been attracted extensive attention due to its potential application in the field of information storage and signal sensor. Although the research on ME effect can go back to 19 th century, it undergoes a very slow progress. Further achievement has been received on the theory, synthesis and measurement for ME materials from 1990 s. However, in most discovered ME materials, the ME effect can only be observed under extreme conditions, such as low temperature(< 30 K) and high magnetic field(> 1 T), which brings a limitation to their practical application. Until now, ME effect has been found in all of the six kinds of hexaferrite except X-type. The Z-type hexaferrite Sr3Co2Fe24O41(Sr3Co2Z), which is a novel ME material discovered in recent years, can exhibit the ME effect above room temperature and in a low magnetic field. It has a great potential for practical application.Because of the complexity of crystal structure, it is a tremendous challenge to obtain the pure phase of Sr3Co2 Z. Strictly speaking, all the samples adopted for its ME effect research in literature are not single phase. In addition, all crystal structure analysis of hexaferrites that contain Sr is directly referenced to those contain Ba due to their similarity in component elements and crystal structure. As a result, many ambiguous or even conflicting descriptions exist in literatures about phase content, ME effect and mechanism for Sr3Co2 Z.It is a meaningful work to study the ME effect of Sr3Co2 Z using pure phase samples and compare the property with that of impure samples with known phase content. This will improve the synthesis techniques and benefit the research on the intrinsic physical properties. This work focuses on Sr3Co2 Z. Its solid state synthesis technology, magnetic properties, electrical properties and ME coupling effect are systematically studied. The research has a great significance and provides an instructive guidance to the development of novel functional materials.Enormous difficulties are existed in the synthesis of single phase Sr3Co2 Z. In this work, the influence of ratio between raw materials, calcining temperature, sintering temperature as well as sintering atmosphere etc. on the phase content of Sr3Co2 Z in final product was studied on the basis of thermodynamic analysis. During the reaction, spinel and M-type hexaferrite are proposed to be formed prior to Sr3Co2 Z. Insufficient oxygen during the synthesis process will lead to its decomposition. After optimization, ceramic samples with single-phase of Sr3Co2 Z are fabricated. The optimal temperature and dwelling time for Sr3Co2 Z synthesis in oxygen are 1150 °C and 4 h, respectively. In addition, through the careful X-ray analysis and crystal structure refinement for various products during the synthesis of Sr3Co2 Z, crystallographic information files(CIFs) are firstly made for Sr3Co2 Z, Sr4Co2 U, Sr2Co2 X and SrCo2 W.Secondly, magnetic field and temperature dependence of magnetization and magnetoresistive effect were studied for Sr3Co2 Z. Its magnetization is 52.1 emu/g under magnetic field of 6 kOe, and its coercive field is 53.2 Oe. Its magnetization decreases with the rising temperature, the magnetic structure transition gives rise to anomalies at ~370 K and ~505 K. The magnetoresistive effect is-85% at 200 K under 70 kOe, which could be related to the spin-polarized intergrain tunneling effect.Then, electrical properties such as temperature and frequency dependence of dielectricity, electrical hysteresis, volt-ampere characteristic and pyroelectric property for Sr3Co2 Z were studied. Dielectric anomalies appear in which the one at ~370 K is consistent with magnetic structure transition while the other at ~600 K is caused by the relaxation of oxygen vacancies. It cannot confirm that whether there exists the contribution from spontaneous polarization in the electrical hysteresis and volt-ampere characteristic for Sr3Co2 Z. The electrical hysteresis are significantly affected by leakage current, while the negative differential resistive effect observed in volt-ampere characteristic is strongly related to the absorbed water molecules. The ferroelectric effect could be derived from its pyroelectric property in which a peak exhibits at ~370 K.Finally, Sr3Co2 Z was studied on its ME coupling effect. At the magnetic structure transition temperature, the dielectric anomaly was observed as well as the current peak in the pyroelectric curve. Besides, the pyroelectric current is dependent on the external magnetic field. At 300 K, the magnetodielectric effect is ~1% under 30 kOe and the magnetic field induced electrical polarization is ~1.2 μC/m2 at 200 K under 4 kOe. The ME coupling effect is intimately related to the magnetic structure. Inhomogeneous distribution of Co ions in the crystal structure of Sr3Co2 Z gives rise to its specific magnetic anisotropy, which could stabilize its ME coupling effect above room temperature. |
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