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

I. Bakonyi, E. Simon, B. G. Tóth, L. Péter, and L. F. Kiss

Phys. Rev. B 79, 174421 – Published 14 May 2009

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 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) behavior. The GMR magnitude increased up to about 3.5–4 nm Cu-layer thickness and slightly decreased afterward. According to magnetic measurements, all samples exhibited ferromagnetic (FM) behavior. 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 behavior at low spacer thicknesses indicates the presence of strong FM coupling (due to, e.g., pinholes in the spacer and/or areas of the Cu layer where the layer thickness is very small). With increasing spacer thickness, the pinhole 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 in 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 of 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.

Received 10 September 2008

DOI:https://doi.org/10.1103/PhysRevB.79.174421

Authors & Affiliations

I. Bakonyi^*, E. Simon, B. G. Tóth, L. Péter, and L. F. Kiss

Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary

^*Corresponding author; bakonyi@szfki.hu

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Vol. 79, Iss. 17 — 1 May 2009

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Images

Figure 1
(Color online) Room-temperature electrical resistivity $(ρ_{0})$ of electrodeposited Co/Cu multilayers in zero external magnetic field as a function of the Cu-layer thickness with constant magnetic layer thickness $d_{Co} \approx 2.7 nm$ . The symbols ○ represent experimental data after correction for the shunting effect of the Cr(5 nm)/Cu(20 nm) metallic substrate layers (see text for details). The error bar for each data point is at most twice the size of the data symbol. The solid line through the corrected experimental data indicates a trendline only. The dashed line represents the resistivity of a Co/Cu multilayer in a simple parallel-resistor model (Refs. 48, 49) calculated with bulk resistivity values of the individual layers. The dotted line is just a linear extrapolation of the experimental data to $d_{Cu} = 0$ .Reuse & Permissions
Figure 2
Longitudinal (L, open symbols) and transverse (T, full symbols) components of the MR for two electrodeposited Co/Cu multilayers: one exhibiting AMR (triangles) and one exhibiting GMR (circles). The spacer layer thickness is also indicated for both samples. The saturation field $(H_{s})$ is defined as the magnetic field above which the $MR (H)$ variation can be considered as nearly linear. An extrapolation to $H = 0$ yields the saturation magnetoresistance $({MR}_{s})$ . The inset shows the $MR (H)$ curve for the multilayer exhibiting GMR where the definition of $H_{p}$ , the $MR (H)$ peak position, is also given.Reuse & Permissions
Figure 3
Evolution of the longitudinal (LMR) and transverse (TMR) saturation components of the MR for the investigated electrodeposited Co/Cu multilayers as a function of the Cu-layer thickness $d_{Cu}$ . For multilayers with $d_{Cu}$ not exceeding about 1.5 nm, the observed magnetoresistance is of the AMR type ( $LMR > 0$ ; $TMR < 0$ ). For larger Cu-layer thicknesses, the total observed magnetoresistance is dominated by GMR ( $LMR < 0$ ; $TMR < 0$ ). The vertical dashed line separates the AMR and GMR thickness ranges. The vertical arrows denote the approximate positions of the GMR maxima reported for fcc(111) Co/Cu multilayers prepared by physical deposition methods (Refs. 5, 56, 57).Reuse & Permissions
Figure 4
(Color online) High-field magnetization curves normalized with the values measured at $H = 50 kOe$ and displayed above the remanence values $(M_{r} / M_{s} \approx 0.75)$ for electrodeposited Co/Cu multilayers with AMR behavior $(d_{Cu} = 0.9 nm)$ and with GMR behavior $(d_{Cu} = 3.0 nm)$ . The inset shows the corresponding low-field magnetization curves with coercive field $(H_{c})$ values of 25 and 63 Oe, respectively.Reuse & Permissions
Figure 5
(Color online) Magnetoresistance peak position $H_{p}$ and coercive field $H_{c}$ values for the investigated electrodeposited Co/Cu multilayers as functions of the Cu-layer thickness $d_{Cu}$ .Reuse & Permissions
Figure 6
(Color online) Evolution of the saturation GMR $({GMR}_{s})$ in electrodeposited Co/Cu multilayers with Cu-layer thickness $d_{Cu}$ . The figures in brackets indicate literature data references. It is noted that, according to a study reported in Ref. 41, in electrodeposited Co/Cu multilayers obtained under galvanostatic control, the actual layer thicknesses for the samples from Ref. 27 can be by as much as 1 nm higher than the original nominal thicknesses specified in that work.Reuse & Permissions

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