Finite size scaling effects in giant magnetoresistance multilayers

Abstract

We report on the findings of a study conducted on Co/Cu giant magnetoresistance multilayerswith a wide variation in the number of bilayer repeats at temperatures from 4.2 to 300 K.The total film thickness varied from 77 to 2700 Å. When the data were treated within aFuchs–Sondheimer approximation for finite size effects at any one temperature an excellentagreement with the observed variation in giant magnetoresistance ratio was found. Over∼50 bilayers were required before the full giant magnetoresistance is observed. Unexpectedly,the length scale on which the GMR rises was found to be the same at all temperaturesmeasured. This leads to unphysical temperature dependences in the Fuchs–Sondheimermean free paths. Subsequent structural characterization showed that this length scale isrelated to the vertical coherence length of the grains within the sample. The same lengthscale was found for other related multilayer systems grown in the same chamber. Thebreakdown of the Fuchs–Sondheimer model is discussed in terms of modern theories ofphase-coherent transport in the giant magnetoresistance: full giant magnetoresistance isonly obtained when electrons can coherently sample a large enough number of layers.