Abstract
We have studied the magneto-resistance (MR) control by use of a variation of the interparticle-coupling-form (IPCF) in half-metallic magnetite (Fe3O4) nano-particle sinter (MNPS). The average nano-particle size of the MNPS is 30nm and the measurements were carried out between 4K and 300K. The variation of the IPCF in the MNPS was performed by the heat treatment (HT) between 400°C and 800°C. The behavior of the MR and electrical resistance (ER) considerably changes at the HT temperature of around 600°C. Below around 600°C the ER indicates the variable-range-hopping conduction behavior in a wide temperature region, whereas above around 600°C the ER shows the indication of the Verwey transition near 110K like a bulk single crystal. We have considered that the electrical conduction switching in the MNPS occurs at the HT temperature of around 600°C. Namely, below around 600°C the ER is dominated by the grain-boundary conduction and above around 600°C the ER is determined by the inter-grain conduction. We have found that at room temperature the MR for the MNPS made at the HT temperature of 400°C shows approximately 20 times that of a bulk single crystal. The application of the magnetic field to grain-boundary conduction is considered to cause the large enhancement of the MR.
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