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The electrical and magnetic properties of magnetite at the low temperature phase transition Cheeke, John David Nicholas

Abstract

The electrical and magnetic properties of magnetite from 100-300°K have been studied. Specimens were obtained from natural crystals of local origin and fabricated in the form of rectangular bars. When this was not possible, due to excessive cracks in the crystal, the electrical properties of the resulting irregularly shaped disc were measured by use of the Van der Pauw theorem. Observation of the temperature variation of the resistivity showed that the resistivity increased as the temperature was decreased and that it increased by a factor 24 over a 10°K temperature interval when the specimen was cooled below 115°K. This phenomenon is related to an order-disorder transition at this temperature, in which the crystalline symmetry changes from cubic to orthorhombic. One specimen did not show the resistivity transition, probably due to excessive impurity content, which would prevent ordering from occurring. The results are explained qualitatively by the Verwey model, which postulates a conduction mechanism involving the jumping of electrons between octahedral sites of the Fe₃O₄ unit cell, as opposed to the conventional band type of conduction. An attempt was made to measure the Hall mobility of the specimens, but a Hall voltage was not detected within the sensitivity of the apparatus. An upper limit of 1 cm²/volt-sec. for the mobility was established. A negative magnetoresistance effect was observed in both specimens and was measured as a function of temperature for both transverse and longitudinal magnetic fields. A pronounced minimum at the transition temperature was observed for one specimen, while the other showed no change here. The AC permeability was observed over the transition region and again a sharp decrease was observed on cooling through the transition. Normal induction curves were obtained for the specimen at various fixed temperatures from the permeability data, from which it was deduced that the specimen was much harder to saturate below the transition. The permeability on warming was found to be independent of the magnetic state of the sample while cooling through the transition, and it was again observed that one crystal did not undergo the phase transition at 115°K.

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