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تسجيل مستخدم جديدInfrared and Raman studies of the Verwey transition in magnetite
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We present infrared and Raman measurements of magnetite (Fe_3O_4). This material is known to undergo a metal-insulator and a structural transition (Verwey transition) at T_V=120K. At temperatures below T_V, we observe a strong gap-like suppression of the optical conductivity below 1000 cm^-1. The structural aspect of the Verwey transition demonstrates itself by the appearance of additional infrared- and Raman-active phonons. The frequencies of the infrared-active phonons show no significant singularities at the transition whereas their linewidths increase. The frequency and linewidth of the Raman-active phonon at 670 cm^-1 changes abruptly at the transition. For T<T_V, we observe fine structures in the infrared and Raman spectra which may indicate strong anharmonicity of the system below the transition. Our estimate of the effective mass of the carriers above the transition to be about 100 m, where m is the free electron mass. Our measurements favor a polaronic mechanism of conductivity and underline the importance of the electron-phonon interaction in the mechanism of the Verwey transition.
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By combining {it ab initio} results for the electronic structure and phonon spectrum with the group theory, we establish the origin of the Verwey transition in Fe$_3$O$_4$. Two primary order parameters with $X_3$ and $Delta_5$ symmetries are identifi
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We incorporate single crystal Fe$_3$O$_4$ thin films into a gated device structure and demonstrate the ability to control the Verwey transition with static electric fields. The Verwey transition temperature ($T_V$) increases for both polarities of th
e electric field, indicating the effect is not driven by changes in carrier concentration. Energetics of induced electric polarization and/or strain within the Fe$_3$O$_4$ film provide a possible explanation for this behavior. Electric field control of the Verwey transition leads directly to a large magnetoelectric effect with coefficient of 585 pT m/V.
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