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The Verwey Phase of Magnetite - a Long-running Mystery in Magnetism
Mark S. Senn,Jon P. Wright,J. Paul Attfield 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.62 No.10
Magnetite (Fe3O4) is the original magnetic material and the parent of ferrite magnets, with modern applications ranging from spintronics to MRI contrast agents. At ambient temperatures magnetite has a cubic spinel-type crystal structure, but it undergoes a complex structural distortion and becomes electrically insulating below the 125 K Verwey transition. The electronic ground state of the Verwey phase has been unclear for over 70 years as the low temperature structure was unknown, but the full superstructure was recently determined by high energy microcrystal x-ray diffraction. An analysis of 168 frozen phonon modes in the acentric (and hence multiferroic)low temperature magnetite structure is presented here. Differences between the amplitudes of centric and acentric branches of , X and W modes all contribute to the significant off-center atomic distortions in the low temperature structure.
R.S. Liu,T.S. Chan,S. Mylswamy,G.Y. Guo,J.M. Chen,J.P. Attfield 한국물리학회 2008 Current Applied Physics Vol.8 No.1
The chemical pressure control in (Sr2. xCax)FeMoO6 (06 x 6 2.0) with double perovskite structure has been investigated systemat-x = 0 and x = 2.0. The increasing Ca con-tent in (Sr2. xCax)FeMoO6 samples increases the magnetic moment close to the theoretical value due to reduction of Fe/Mo anti-sitedisorder. An increasing Ca content results in increasing (Fe2+ +Mo6+)/(Fe3+ +Mo5+) band overlap rather than bandwidth changes.This is explained from simple ionic size arguments and is supported by X-ray absorption near edge structure (XANES) spectra and bandstructure calculations.