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Register a new userPressure dependence of the Verwey transition in magnetite: an infrared spectroscopic point of view
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Added by
Christine Kuntscher
Publication date
2012
fields
Physics
and research's language is
English
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We investigated the electronic and vibrational properties of magnetite at temperatures from 300 K down to 10 K and for pressures up to 10 GPa by far-infrared reflectivity measurements. The Verwey transition is manifested by a drastic decrease of the overall reflectance and the splitting of the phonon modes as well as the activation of additional phonon modes. In the whole studied pressure range the down-shift of the overall reflectance spectrum saturates and the maximum number of phonon modes is reached at a critical temperature, which sets a lower bound for the Verwey transition temperature T$_{mathrm{v}}$. Based on these optical results a pressure-temperature phase diagram for magnetite is proposed.
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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 identified. They induce the phase transformation from the high-temperature cubic to the low-temperature monoclinic structure. The on-site Coulomb interaction $U$ between 3d electrons at Fe ions plays a crucial role in this transition -- it amplifies the coupling of phonons to conduction electrons and thus opens a gap at the Fermi energy. {it Published in Phys. Rev. Lett. {bf 97}, 156402 (2006).}
We have studied the electronic structure of bulk single crystals and epitaxial films of magnetite Fe$_3$O$_4$. Fe $2p$ core-level spectra show clear differences between hard x-ray (HAX-) and soft x-ray (SX-) photoemission spectroscopy (PES), indicative of surface effects. The bulk-sensitive spectra exhibit temperature ($T$)-dependent charge excitations across the Verwey transition at $T_V$=122 K, which is missing in the surface-sensitive spectra. An extended impurity Anderson model full-multiplet analysis reveals roles of the three distinct Fe-species (A-Fe$^{3+}$, B-Fe$^{2+}$, B-Fe$^{3+}$) below $T_V$ for the Fe $2p$ spectra, and its $T-$dependent evolution. The Fe $2p$ HAXPES spectra show a clear magnetic circular dichroism (MCD) in the metallic phase of magnetized 100-nm-thick films. The model calculations also reproduce the MCD and identify the magnetically distinct sites associated with the charge excitations. Valence band HAXPES shows finite density of states at $E_F$ for the polaronic metal with remnant order above $T_V$, and a clear gap formation below $T_V$. The results indicate that the Verwey transition is driven by changes in the strongly correlated and magnetically active B-Fe$^{2+}$ and B-Fe$^{3+}$ electronic states, consistent with resistivity and bulk-sensitive optical spectra.
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.
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 the 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.
The optical properties of magnetite at room temperature were studied by infrared reflectivity measurements as a function of pressure up to 8 GPa. The optical conductivity spectrum consists of a Drude term, two sharp phonon modes, a far-infrared band at around 600 cm$^{-1}$, and a pronounced mid-infrared absorption band. With increasing pressure both absorption bands shift to lower frequencies and the phonon modes harden in a linear fashion. Based on the shape of the MIR band, the temperature dependence of the dc transport data, and the occurrence of the far-infrared band in the optical conductivity spectrum the polaronic coupling strength in magnetite at room temperature should be classified as intermediate. For the lower-energy phonon mode an abrupt increase of the linear pressure coefficient occurs at around 6 GPa, which could be attributed to minor alterations of the charge distribution among the different Fe sites.
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