C. T. Tanaka

Spin-polarized tunneling in a half-metallic ferromagnet

Direct measurement of the conduction electron spin polarization (P) in epitaxial NiMnSb was performed to test the prediction of half metallicity in this material. Spin-polarized tunneling in NiMnSb/Al2O3/Al junctions showed P of 28%, contrary to the predicted value of 100%. Magnetoresistance measurements in NiMnSb/Al2O3/Ni80Fe20 junctions concurred with this result. The discrepancy between theory and experiment is discussed. Also, the latter junctions show four nonvolatile remanent states due to the NiMnSb magnetocrystalline anisotropy, which has potential as four-level logic elements.

Andreev reflection: A new means to determine the spin polarization of ferromagnetic materials

We have developed a simple method to measure the transport spin polarization of ferromagnetic materials. This technique relies on the fact that the Andreev reflection process at the interface between a superconductive and normal is influenced by the spin polarization P of the normal metal. In a very short time we have been able to measure the spin polarization of several metals: NixFe1−x, Ni, Co, Fe, NiMnSb, La0.7Sr0.3MnO3, and CrO2, whose spin polarization ranges from 35% to 90%. Our results compare well with other methods for measuring P.

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 NiMnSb/Al2O3/Ni0.8Fe0.2 junctions at 77 K and 5.7% for NiMnSb/Al2O3/Co0.5Fe0.5. 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 NiMn...

Spin-polarized tunnelling, magnetoresistance and interfacial effects in ferromagnetic junctions

Abstract The pioneering studies of spin-polarized tunnelling by Meservey and Tedrow in the early 1970s showed that the conduction electrons in ferromagnetic (FM) metals are spin polarized and that the spin is conserved in the tunnelling process. Only recently (1995) improved material fabrication techniaues have permitted realization of the Julliere quantitative model, showing that tunnelling in ferromagnet/insulator/ferromagnet (FM/I/FM) junctions should lead to a large junction magnetoresistance (JMR); JMR values greater than 30% have been achieved at room temperature. This recent success has led to several fundamental auestions regarding the phenomenon of spin tunnelling and also the development of JMR devices. In this paper, experimental results, such as the dependence on bias, temperature and barrier characteristics of FM/I/FM tunnelling are reviewed briefly. The influence of inelastic tunnelling processes, metal at the interface and material properties on the JMR is discussed. The future direction from both the physics and the applications viewpoints, is also covered.

Spin polarized tunneling in half‐metallic ferromagnets (abstract)

We present a study of spin polarized tunneling in tunnel junctions in which one of the conducting layers is a half‐metallic ferromagnet (HMF). HMF are unique in that the Fermi level of these materials intersect the majority spin electron band, while the minority band has an energy gap near the Fermi level. Hence, HMF simultaneously have both metallic and semiconducting characteristics, and theory predicts that the conduction electrons are 100% spin polarized. As a result, the magnetoresistance in magnetic multilayers or trilayer tunnel junctions is expected to be significantly higher than with conventional ferromagnetic materials. Two important parameters affecting the performance of these junctions are the smoothness of the HMF surface and its surface composition. Tunnel junctions consisting of a layer of NiMnSb, a barrier layer of Al2O3, and a layer of aluminum were prepared and studied for their tunneling properties. Surface analysis of HMF films was done using Auger depth profiling and AFM.

Conduction across the interface between a superconductor and a spin polarized metal

We have studied Andreev reflection in point contacts formed between several low T/sub c/ superconductors and various spin polarized normal metals. We have also developed a theoretical model for the phenomenon. There is good accordance between theory and experiment.