Temperature dependence of giant magnetoresistance and magnetic properties in electrodepositedCo−Cu∕Cumultilayers: The role of superparamagnetic regions

Abstract

We have shown recently that both the magnetization and the magnetoresistance of electrodeposited Co-Cu/ Cu multilayers can be decomposed by assuming the presence of both ferromagnetic ͑FM͒ and superparamagnetic ͑SPM͒ regions in the magnetic layers. In the present work, for two selected samples, one with a large SPM and another one with a large FM contribution to the giant magnetoresistance, low temperature magnetic and magnetoresistance measurements were performed in order to reveal the evolution of the FM and SPM terms with temperature. The average apparent magnetic moment of the SPM regions deduced from the two sets of data showed a good agreement. The role of electrochemical processes in the formation of the SPM regions is discussed. An attempt has also been made to elaborate on some models for the spatial distribution of the constituent elements ͑Co and Cu͒ leading to the occurrence of SPM regions. The results are discussed also in the framework of interacting SPM regions.

Magnetic and magnetoresistance studies of nanometric electrodeposited Co films and Co/Cu layered structures: Influence of magnetic layer thickness

“…The typical SPM moments obtained previously on electrodeposited Co/Cu multilayers [32][33][34][35][36] Previous reports by other researchers on both electrodeposited [36,37] and sputtered [38][39][40] Co/Cu multilayers with very thin Co layers around 1 nm and below gave further support to our findings and conclusions concerning the appearance of SPM regions in such multilayers. Namely, it turned out from these studies when increasing the Co layer thickness in the subnanometer range that whereas the M(H) curves evolve from a hysteresis-less behavior to the appearance of a clear hysteresis, i.e., the formation of FM regions, the MR(H) curves remain still strongly non-saturating with negligible hysteresis.…”

Giant magnetoresistance in electrodeposited Co-Cu/Cu multilayers: Origin of the absence of oscillatory behavior

“…An important consequence of the presence of SPM regions in multilayers is that there will be another contribution called GMR SPM which is due to spin-dependent scattering for electron paths "SPM region  NM region  FM region" (or in opposite order). The occurrence of electron paths "SPM region 1  NM region  SPM region 2" was found to be negligible in the multilayer systems 44,46 as opposed to conventional granular metals. 62,63 It has been found for electrodeposited Co-Cu/Cu multilayers with non-saturating MR(H) behaviour 44,46 that beyond technical saturation of the magnetization at about H s = 2 to 3 kOe, the field dependence of the magnetoresistance MR(H) can be described by the Langevin function L(x) where x = µH/kT with µ constituting the average magnetic moment of a SPM region.…”

“…The occurrence of electron paths "SPM region 1  NM region  SPM region 2" was found to be negligible in the multilayer systems 44,46 as opposed to conventional granular metals. 62,63 It has been found for electrodeposited Co-Cu/Cu multilayers with non-saturating MR(H) behaviour 44,46 that beyond technical saturation of the magnetization at about H s = 2 to 3 kOe, the field dependence of the magnetoresistance MR(H) can be described by the Langevin function L(x) where x = µH/kT with µ constituting the average magnetic moment of a SPM region. Beyond the saturation of ferromagnetic regions (H > H s ), the GMR FM and the AMR terms are saturated and, hence, their contributions remain constant for magnetic fields above H s , apart from a small, linearly decreasing term (due to the paraprocess).…”

“…This decomposition method has been recently successfully applied to analyze the Colayer thickness dependence of electrodeposited Co-Cu/Cu multilayers. 64 It should be noted that the occurrence of SPM regions is not restricted to electrodeposited magnetic/nonmagnetic multilayers 25,26,28,33,39,[41][42][43][44][45][46] since magnetic measurements revealed the presence of an SPM contribution to the magnetization also in multilayers prepared by physical deposition methods. [65][66][67][68][69][70][71][72][73][74][75] Specifically, the field dependence of the magnetoresistance in MBE-grown Co/Cu multilayers 72 could be well fitted by a Langevin function which implies the same magnetoresistance mechanisms as described above for the case of electrodeposited multilayers.…”

“…Even a monotonic decrease of the GMR magnitude with Cu layer thickness, with a levelling off at around 2 nm, was reported for Co-Ni-Cu/Cu multilayers. 38 On the other hand, there has been recently a significant progress in understanding the electrochemical processes governing deposit formation 21,25,26,[39][40][41][42] and the factors influencing the GMR characteristics, with special reference to the appearance of a possible superparamagnetic (SPM) contribution to the total GMR 26,28,[41][42][43][44][45][46] in electrodeposited multilayers. This instigated us to undertake a thorough study of the evolution of the GMR magnitude in electrodeposited Co/Cu multilayers with Cu layer thicknesses from 0.5 nm to 4.5 nm in steps of about 0.1 nm.…”