Link between Magnetic and Dielectric Properties in Magnetite

Abstract

We have measured temperature dependence of magnetic and dielectric properties in high quality stoichiometric single-crystals of magnetite (Fe3O4). At low temperatures, below the Verwey transition, both ac susceptibility and ac permittivity reveal similar relaxation processes. We discuss these effects in connection with domain structure and variable range hopping mechanism of electrical conductivity.PACS numbers: 71.30.+h, 75.60.−d, 75.85.+t 1. Introduction Magnetite -a mixed valence iron oxide, Fe 3 O 4 , with cubic inverse spinel structure at room temperature -is a prototype ferrimagnetic system that still attracts considerable attention mainly due to the presence of the so-called Verwey transition -a structural phase transition accompanied by substantial changes of magnetic and electrical properties [1]. Although Mott's metalinsulator transition theory [2] was well suited to explain the sharp increase of resistivity of magnetite upon cooling through the Verwey temperature (T V ≈ 120 K), the exact nature of the transition is still unresolved and the charge ordering on the Fe ions has not been sufficiently clarified [3][4][5][6]. To complicate the matter even further, upon cooling through T V the structure changes to monoclinic [5] and the monocrystal splits into structural domains which are strongly linked to the instantaneous magnetic domain structure of the sample (i.e. random in weak magnetic fields). The interplay of spin, lattice and electronic degrees of freedom makes this strongly correlated system a potential candidate for novel applications [7].In this work we focus not on the Verwey transition itself but rather on the magnetic and dielectric behavior of magnetite below T V .2. Experimental Stoichiometric single crystals of magnetite were grown by floating zone technique [8] and cut to disk shape 5 mm in diameter and 1 mm thick, the disk axis corresponds to 100 crystallographic direction. Two silver electrodes (4 mm in diameter) were deposited on the flat surfaces of the disk by means of magnetron sputtering. A custom sample holder with copper and brass contacts was used to measure both electrical and magnetic properties simultaneously. To measure the magnetic properties (complex ac susceptibility, dc magnetization) a home--built continuously reading SQUID magnetometer with immobile sample was used [9]. The magnetometer oper-