臺灣博碩士論文加值系統

多鐵性是指一個材料在同一個狀態下有兩個或兩個以上的鐵性質，其中有一個特別的多鐵性是磁電效應目前受到很多注目，所謂的磁電效應是用外加電場來控制材料內部的磁極，或者是利用外加磁場來控制材料的電極，六方晶系的Ba2Mg2Fe12O22就是磁電效應材料，它是利用微弱的外加磁場(0.03 T)就可以控制材料的電極，然而這個現象尚未完全釐清。我們利用超導量子干涉儀(SQUID)、軟X光吸收光譜包括磁旋光二元性(MCD)以及共振軟X光磁性散射來研究Ba2Mg2Fe12O222磁性以及其相變，實驗數據顯示出Ba2Mg2Fe12O22有各向異性的自旋有序性，並且有一個commersurate-incommeersurate的磁性相變，二維的共振X光散射數據展現出Ba2Mg2Fe12O22在c軸方向的自旋與自旋之間的耦合微弱，因為如此可能會有微弱的外加磁場可以控制Ba2Mg2Fe12O22電極化的情況。

Multiferroics are materials that exhibit more than one primary ferroic order parameter
in single phase. A special kind of multiferroics with a gigantic magnetoelectric coupling has attracted much attention. Haxeferrite Ba2Mg2Fe12O22 is a particularly interesting magnetoelectric material because its electric polarization can be controlled by a low magnetic field. However, such a multiferroic feature is far from being understood. We here investigated magnetic properties and transitions of Ba2Mg2Fe12O22 by using a superconducting quantum interference device (SQUID), soft x-ray absorption including magnetic circular dichroism, and resonant soft x-ray magnetic scattering. The results indicate that Ba2Mg2Fe12O22 exhibits an anisotropic spin ordering and a commensurate-to-incommensurate magnetic transition. Two-dimension distributions of magnetic scattering reveal a weak spin-spin correlation along the c axis, leading to a possible scenario for the low-field control of electric polarization discovered in haxeferrite Ba2Mg2Fe12O22.

1 Introduction of Magnetoelectricity
1.1 Multiferroics
1.1.1 Magnetoelectric materials
1.2 Theory of ME effect
1.2.1 Landau theory
1.3 Spin current model
1.4 Exchange Interaction
1.4.1 Superexchange
2 Experimental Techniques and Setup 21
2.1 Diffractometer
2.2 Soft X-ray Absorption Spectroscopy
2.2.1 Absorption spectrum
2.2.2 The interaction between the electrons and photon
2.2.3 Electron and fluorescence yields
2.2.4 Magnetic-Circular-Dichroism
2.2.5 Sum rules
2.3 Soft x-ray scattering
2.3.1 Resonant scattering
2.3.2 Resonant scattering in quantum mechanics
2.3.3 Resonant X-ray magnetic scattering
2.4 Soft x-ray detector
3 Magnetic Transitions of Multiferroic Hexaferrite Ba2Mg2Fe12O22
3.1 Introduction
3.1.1 Haxaferrite
3.2 Experiment
3.2.1 Soft x-ray absorption
3.2.2 SQUID results
3.2.3 MCD results
3.3 Resonant soft x-ray magnetic scattering
3.3.1 Neutron diffraction
4 Conclusion

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