Giant Magnetoresistance in Annealed Fe/Cr Multilayers
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GIANT MAGNETORESISTANCE IN ANNEALED Fe/Cr MULTILAYERS Noa More Rensing * and Bruce M. Clemens ** *Department of Applied Physics, Stanford University, Stanford CA 94305 "*Department of Materials Science and Engineering, Stanford University, Stanford CA 94305 ABSTRACT The giant magnetoresistance effect in antiferromagnetically coupled Fe/Cr multilayers has been attributed to spin dependent scattering at the interfaces between the constituents. One possible source of this spin dependent scattering is chromium impurities in the iron layers due to intermixing at the interfaces. Annealing the films can promote the diffusion of the components, increasing the impurity concentration and therefore the magnetoresistance. For this study Fe/Cr multilayers were annealed at several temperatures and for several durations. Annealing at moderate temperatures (-' 350"C) increases the magnetoresistance, while higher temperature anneals (- 600'C) cause the magnetoresistance to disappear completely. Long anneals at 330'C (> 100 hours) also reduce the magnetoresistance. VSM measurements indicate that the antiferromagnetic coupling is reduced in the annealed samples but show no evidence of magnetically "dead" alloy layers. Low angle Xray diffraction indicates that the structural effect of annealing is very subtle in comparison to the significant magnetic effect. INTRODUCTION The influence of the structure of the interfaces on the magnetoresistance in antiferromagnetically coupled multilayers and spin valves has been a subject of discussion since the effect was discovered. Baibich et a/.[1] pointed out that perfectly smooth interfaces could
not contribute to the in-plane magnetoresistance, but that steps and other imperfections in the surfaces could contribute to a spin dependent resistance. Camley and Barnas[2] parameterized the interface scattering without attributing its origin, and more detailed theories have attempted to incorporate the possibility of both bulk and interfacial spin dependent scattering[3, 4, 5]. Fits to experimental data, however, largely indicate that interface
scattering dominates. The importance of interface scattering was proven by Baumgart et al.[6] in a study of doping interfaces of Fe/Cr multilayers by different spin scatterers.
They found that when the interfaces were doped with elements that have the same spin scattering asymmetry as chromium, the magnetoresistance was unchanged, but that elements with the opposite asymmetry reduced the magnetoresistance, in effect canceling the influence of some of the chromium scattering sites. Experimental data on the effect of interface structure has thus far been inconclusive. Petroff et al.[7] reported that MBE grown Fe/Cr samples grown under conditions that roughened the interfaces had increased magnetoresistance over smooth samples, increasing from 14% to 33%. Additionally, annealing the smooth sample at 300'C increased in magnetoresistance to 27.3%, but when it was annealed at 350 0 C the effect decreased to 1.15%. These results were attributed to thermally induce
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