Decomposition of the magnetoresistance of multilayers into ferromagnetic and superparamagnetic contributions

Abstract

An analysis of both magnetic and magnetotransport properties is presented for electrodeposited multilayers prepared intentionally under conditions to make the superparamagnetic (SPM) magnetization contribution comparable to or larger than the ferromagnetic term. Based on a model elaborated for the giant magnetoresistance (GMR) of granular metals [N. Wiser, J. Magn. Magn. Mater. 159, 119 (1996); B. Hickey et al., Phys. Rev. B. 51, 667 (1995)], it is shown that in such multilayers both the magnetization and the GMR can be decomposed into ferromagnetric and superparamagnetic contributions where the latter term is described by a Langevin function. The size of the SPM regions estimated from the experimental data is in the nanoscale regime. It is believed that the method applied here gives a quantitative answer to the problem of the often observed nonsaturating behavior of GMR in multilayers. Electrodeposited multilayers are particularly prone to this feature although the occurrence of SPM regions is quite common in multilayers prepared by any technique. Therefore, this type of analysis should help better understanding of the factors influencing the GMR of multilayer films.

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).…”

“…In such cases, the GMR effect arises from spin-dependent scattering originating from electron paths of the type "FM region 1  NM region  FM region 2" and this is the conventional GMR FM term observed in physically deposited multilayers [1][2][3][4][5] which saturates at the above mentioned magnetic fields. However, it has been shown recently 43,44 that a non-saturating behaviour frequently observed in magnetic/non-magnetic multilayers produced by any methods can be successfully explained by the presence of SPM regions in the magnetic layers. 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).…”

“…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.…”

Магнитооптические, оптические и магнитотранспортные свойства сверхрешеток Co/Cu с ультратонкими слоями кобальта

“…В отличие от слоевых пленок такие " квазигранулированные" (кластерно-слоистые) пленки имеют изотропный эффект ГМС, пренебрежимо малый гистерезис намагниченности и показывают суперпара-магнитное (SPM) поведение. Дальнейшие исследования магнитных и магнитотранспортных свойств Co/Cu пока-зали, что толщина слоев Co, способы и условия напыле-ния образцов существенно влияют на формирование и свойства " квазигранулированных" пленок [11][12][13][14][15]. Объектами исследования в настоящей работе явля-ются мультислойные наноструктуры Co(t x , ¦ )/Cu(9.6 ¦ ) (0.3 ¦ ≤ t Co ≤ 15 ¦ ), обладающие гигантским магнито-сопротивлением.…”