Magnetoresistance in ferromagnet-insulator-ferromagnet tunnel junctions with half-metallic ferromagnet NiMnSb compound

Ferromagnet-insulator-ferromagnet tunnel junctions with one NiMnSb electrode were studied to test the 100% conduction electron spin polarization predicted from band structure calculations performed on this compound. This phenomenon, known as half-metallic ferromagnetism (HMF), should result in significantly larger junction magnetoresistance (JMR) than in junctions using only conventional ferromagnetic materials such as Ni, Co, and Fe alloys which show JMR of up to 32%. Analysis by x-ray diffraction, Rutherford back scattering, SQUID, AFM, and STM confirm that the NiMnSb has the desired physical properties. A maximum JMR of 8.1% was observed in ${\mathrm{NiMnSb/Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}{\mathrm{/Ni}}_{\mathrm{0.8}}{\mathrm{Fe}}_{\mathrm{0.2}}$ junctions at 77 K and 5.7% for ${\mathrm{NiMnSb/Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}{\mathrm{/Co}}_{\mathrm{0.5}}{\mathrm{Fe}}_{\mathrm{0.5}}$ . Maximum JMR for these two types of junctions at room temperature was 2.4% and 3.7%, respectively. The JMR observed is much lower than that expected for an HMF-I-FM junction. This could be due to scattering of the spins at the FM-I interfaces resulting from surface degradation of the NiMnSb, since the film growth requires deposition at elevated temperatures of >400 °C.