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

Abstract

─ A detailed study of the evolution of the magnetoresistance was performed on electrodeposited Co/Cu multilayers with Cu layer thicknesses ranging from 0.5 nm to 4.5 nm. For thin Cu layers (up to 1.5 nm), anisotropic magnetoresistance (AMR) was observed whereas multilayers with thicker Cu layers exhibited clear giant magnetoresistance (GMR) behaviour. The GMR magnitude increased up to about 3.5 to 4 nm Cu layer thickness and slightly decreased afterwards. According to magnetic measurements, all samples exhibited ferromagnetic (FM) behaviour. The relative remanence turned out to be about 0.75 for both AMR and GMR type multilayers. This clearly indicates the absence of an antiferromagnetic (AF) coupling between adjacent magnetic layers for Cu layers even above 1.5 nm where the GMR effect occurs. The AMR behaviour at low spacer thicknesses indicates the presence of strong FM coupling (due to, e.g., pin-holes in the spacer and/or areas of the Cu layer where the layer thickness is very small). With increasing spacer thickness, the pin-hole density reduces and/or the layer thickness uniformity improves which both lead to a weakening of the FM coupling. This improvement in multilayer structure quality results in a better separation of magnetic layers and the weaker coupling (or complete absence of interlayer coupling) enables a more random magnetization orientation of adjacent layers, all this leading to an increase of the GMR. Coercive field and zero-field resistivity measurements as well as the results of a structural study reported earlier on the same multilayers provide independent evidence for the microstructural features established here. A critical analysis of former results on electrodeposited Co/Cu multilayers suggests the absence of an oscillating GMR in these systems. It is pointed out that the large GMR reported previously on such Co/Cu multilayers at Cu layer thicknesses around 1 nm can be attributed to the presence of a fairly large superparamagnetic (SPM) fraction rather than being due to a strong AF coupling. In the absence of SPM regions as in the present study, AMR only occurs at low spacer thicknesses due to the dominating FM coupling.

Influence of Ag Additive to the Spacer Layer on the Structure and Giant Magnetoresistance of Electrodeposited Co/Cu Multilayers

“…7a), the peak positions of the MR(H) curves correspond well to the coercivity values measured for ED Co/Cu multilayers with similarly dominant GMR FM term. 8 On the other hand, the almost complete absence of coercivity (or hysteresis) in the MR(H) curves shown in Fig. 7b indicates a dominant SPM behavior.…”

“…By contrast, in ED Co/Cu multilayers, [5][6][7][8][9] the GMR shows a monotonous increase with increasing Cu layer thickness up to about 4 to 5 nm; for thicker Cu layers, the GMR decreases again. The lack of oscillatory GMR was evidenced as a sign for the absence of a significant AF coupling.…”

“…The lack of oscillatory GMR was evidenced as a sign for the absence of a significant AF coupling. 8 At Cu layer thicknesses below about 2 nm, most studies on ED Co/Cu multilayers 1,8 indicated a complete absence of GMR: an anisotropic magnetoresistance (AMR) 10,11 effect occurred instead. This latter feature hints at the bulk-like behavior of the ED Co/Cu multilayers due to a direct coupling of adjacent magnetic layers via pinholes for small spacer thicknesses.…”

Giant Magnetoresistance and Structure of Electrodeposited Co/Cu Multilayers: The Influence of Layer Thicknesses and Cu Deposition Potential

“…By looking at former reports, [5][6][7][8][9][10][11][12][13][14][15] it can be established that in most cases not completely systematic studies on layer thicknesses have been carried out for ED Co/Cu multilayers from the sulfate bath. For example, there were several studies of the GMR dependence on layer thicknesses, where one of the layer thicknesses was fixed and the other layer thickness was only varied.…”

“…Over the last two decades, numerous reports have been published on studying the GMR characteristics of ED Co/Cu multilayers from the pure sulfate bath (containing at most some buffering agents). [5][6][7][8][9][10][11][12][13][14][15] In this list of references, we have included only those works from the much larger number of reports 4 in which the ED Co/Cu multilayers were prepared from the sulfate bath at or close to the electrochemically optimized Cu deposition potential E EC Cu 5,10,16 where neither Co dissolution, nor Co codeposition can occur during the Cu pulse. These features ensure that the actual layer thicknesses will be fairly close to the nominal values and that the spacer layer will not contain magnetic Co atoms.…”