Giant magnetoresistance as a probe of interfacial electronic character (invited) (abstract)

Abstract

The giant magnetoresistance (GMR) effect exhibited by magnetic multilayers and related structures is very sensitive to the nature of the interfaces between the magnetic and nonmagnetic layers. We have explored the dependence of GMR on the electronic character of these interfaces by inserting additional thin magnetic layers at the interfaces. Insertion of thin Co layers in Ni–Fe/Cu/Ni–Fe or thin Ni–Fe layers in Co/Cu/Cu exchange biased sandwiches leads to an increase or decrease, respectively, in the magnitude of the GMR effect; the variation is monotonic with thickness of the inserted layer. In contrast, insertion of thin layers of Fe at the Ni–Fe/Cu interfaces in Ni–Fe/Cu/Ni–Fe exchange biased sandwich structure results in a nonmonotonic variation of GMR with Fe layer thickness; for intermediate thicknesses in particular the GMR is substantially reduced. The magnetism and structure of the Fe layers is explored in related sputter-deposited single-crystalline Ni/Fe/Cu thin-film structures prepared using thin Fe/Pt seed layers grown at high temperatures on single crystalline polished wafers of MgO(100), MgO(110), and (0001)Al2O3. The magnetism of the Ni, Fe, and Cu layers is examined using x-ray magnetic circular dichroism (XMCD) and the structure of the Fe is analyzed using extended x-ray absorption fine structure (EXAFS) studies. These experiments show that the Fe moment varies nonmonotonically with thickness resulting from a structural phase transition from fcc to bcc Fe with increasing thickness. The Fe displays a very small magnetic moment for an intermediate range of Fe thickness for which the Fe structure is fcc. This range of thickness depends on the crystalline orientation of the Fe. It is within this same range of thickness that the GMR is suppressed in related exchange biased sandwich structures. Interestingly we find that such nonferromagnetic layers of fcc Fe can be used as spacer layers in magnetic multilayers of, for example, Ni/Fe. These multilayers display both antiferromagnetic interlayer coupling of the Ni layers and giant magnetoresistance, which each oscillate as a function of Fe thickness.