Electrical transport properties of stoichiometric and nonstoichiometric magnetites

Randall John Rasmussen, Purdue University

Abstract

A systematic investigation of the electrical transport properties of single-crystal Fe$\sb{3(1-\delta)}$O$\sb4$ was undertaken for various compositions, specified by the parameter $\delta$, which spanned the high-temperature (1400$\sp\circ$C) stability field. From dc electrical resistivity ($\rho$) and thermopower (Q) experiments carried out between 77K and 300K, it was found that the Verwey transition temperature (T$\sb{\rm v}$) and the associated changes in these properties decreased with increasing departures from the ideal or stoichiometric composition ($\delta$ = 0; T$\sb{\rm v}$ = 122K). From the behavior of $\rho$ near T$\sb{\rm v}$, two regimes were noted: Fe$\sb{3(1-\delta)}$O$\sb4$ compositions for which $\rho$ changed discontinuously at T$\sb{\rm v}$, and those for which the change in $\rho$ was gradual and prominently marked by thermal hysteresis. This result was consistent with the existence of a critical composition ($\delta\sb{\rm c}$ $\simeq$ 0.0039) reported earlier, where for $\delta$ $>$ $\delta\sb{\rm c}$, the thermodynamic signature of the Verwey transition changes from first- to second- (or higher) order. The low-temperature (T $<$ T$\sb{\rm v}$) anisotropy in $\rho$ and Q was investigated by employing a field-cooling technique. For Fe$\sb3$O$\sb4$ ($\delta$ = 0) the anisotropy in these properties was considerable and temperature-dependent, attaining a maximum value for $\rho$ of ca. 2.25 at 93K. The effect of c axis switching on $\rho$ was also investigated. Temperature-dependent (a, b) twinning was observed. For increasing $\delta$, the anisotropy in $\rho$ and Q and its temperature dependence diminished. Impedance measurements for 5Hz $\leq$ f $\leq$ 10 MHz were carried out between 77K and 300K for Fe$\sb{3(1-\delta)}$O$\sb4$ specimens characterized by first- ($\delta$ = 0) and second order ($\delta$ = 0.0050; T$\sb{\rm v}$ $\simeq$ 100K) Verwey transitions. For T $>$ T$\sb{\rm v}$, both samples exhibited a frequency-dependent impedance which increased for frequencies greater than ca. 1MHz. The degree of this dependence and its variation with temperature were correlated with the magnetic permeability. For T $<$ T$\sb{\rm v}$, the impedance of Fe$\sb3$O$\sb4$ was frequency-independent, which is consistent with an adiabatic hopping process. The low-temperature impedance of nonstoichiometric magnetite decreased for frequencies greater than ca. 1MHz; this behavior was interpreted as arising from impurity hopping. The resistivity and thermopower of Fe$\sb{3(1-\delta)}$O$\sb4$ were rationalized in terms of a mean field theory of transitions sufficiently flexible to handle both first- and second-order phase changes.

Degree

Ph.D.

Advisors

Honig, Purdue University.

Subject Area

Chemistry

COinS