TMR and Half-Metals

For a half-metallic ferromagnet/non-magnet/half-metallic ferromagnet junction, only one spin channel is reserved for electron transport as shown in the right figure. Because of this spin-filtering effect, the half-metals can become an ideal spin source for both ohmic and ballistic transport.

In theory, a magnetic tunnel junction with half-metallic ferromagnets should therefore show infinite tunneling magnetoresistance (TMR). However, there has been no such a report presented to date. As shown in the left figure, coherent tunneling through an epitaxial MgO barrier shows very large TMR at room temperature (RT), while the junctions with the half-metallic electrodes show much lower TMR ratios. This departure from the theory is predominantly due to the interface disorder of the half-metals, which is calculated to reduce the spin polarisation significantly. It is therefore crucial to characterise the interfacial properties and to find out a robust condition to maintain the high spin polarisation at the interface.

For further reading

  • Reviews on Heusler alooys: A. Hirohata et al., "Heusler Alloys: Experimental Approach towards Room-Temperature Half-Metallicity," in "Epitaxial ferromagnetic films and spintronic applications" A. Hirohata et al. (Eds.) (Research Signpost, Kerala, India, 2009).

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