Electrophysical properties of Sr₂FeMoO_6–δ ceramics with dielectric shells

Magnetic metal-oxide compounds with high magnetoresistance (MR) values have attracted an extraordinary interest of researchers and engineers due to their possible spintronic applications. Among these materials, the strontium ferromolybdate Sr₂FeMoO_6–δ (SFMO) has been relatively less known compared to cobaltites and manganites despite its 100% spin polarization of the conduction electrons and a high Curie temperature. In the present chapter, a stable fabrication technology and a systematic analysis of nanocomposites consisting of SFMO grains with SrMoO₄ (SMO) dielectric shells are presented. SFMO-SMO nanocomposites were fabricated as follows: synthesis of the SFMO single-phase nanopowders by a modified citrate-gel technique; compaction under high pressure; thermal treatment for the formation of dielectric shells on the grain surface. The nanocomposite exhibits a transitional behavior of the conductivity type from metallic, which is characteristic of the SFMO, to semiconducting in the temperature range from 4 to 300 K in applied magnetic fields up to 10 T. A negative MR is observed due to the tunneling of spin-polarized charge carriers through dielectric interlayers. The MR value reaches 43% in a field of 8Â Т at 10 K. The dielectric shell thickness was determined to be about 10 nm from the electrical breakdown voltage on the current-voltage characteristics. The observed electrical breakdown is found to be a reversible process determined by the impact ionization of atoms in the dielectric in strong electric fields depending on the electrons knocked-out from SrMoO₄. It was found that the MR changes the sign from negative to positive in the electrical breakdown region, revealing giant MR properties.