Elsevier

Electrochimica Acta

Volume 138, 20 August 2014, Pages 56-61
Electrochimica Acta

Neutron reflectivity of electrodeposited thin magnetic films

https://doi.org/10.1016/j.electacta.2014.06.086Get rights and content

Highlights

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    Electrodeposited magnetic bi-layers were measured by polarised neutron reflectivity.

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    When growing a CoNiCu alloy from a single bath a Cu rich region is initially formed.

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    This Cu rich region is formed in the first layer but not subsequent ones.

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    Ni deposition is inhibited in thin film growth and Co deposits anomalously.

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    Alloy magnetism and neutron scattering length give a self-consistent model.

Abstract

We present a polarised neutron reflectivity (PNR) study of magnetic/non-magnetic (CoNiCu/Cu) thin films grown by single bath electrodeposition. We find that the composition is neither homogeneous with time, nor consistent with bulk values. Instead an initial, non-magnetic copper rich layer is formed, around 2 nm thick. This layer is formed by the deposition of the dilute, but rapidly diffusing, Cu2+ ions near the electrode surface at the start of growth, before the region is depleted and the deposition becomes mass transport limited. After the region has been depleted, by growth etc., this layer does not form and thus may be prevented by growing a copper buffer layer immediately preceding the magnetic layer growth. As has been previously found, cobalt deposits anomalously compared to nickel, and even inhibits Ni deposition in thin films. The layer magnetisation and average neutron scattering length are fitted independently but both depend upon the alloy composition. Thus these parameters can be used to check for model self-consistency, increasing confidence in the derived composition.

Introduction

Magnetic multilayers have a huge range of applications across industry and research, historically as giant magneto-resistive (GMR) read heads and currently with the new field of current-induced spin transfer torque spintronics [1], [2], as well as other fields [3], [4]. Many papers exist trying to optimise MR values for such layers in electrodeposited systems [5], [6], [7], [8], [9]. However, these mostly focus on the interfaces and improving the MR through different layer thicknesses and using large numbers of layer repeats; little work has been done on the magnetic properties of the individual layers. The ideal tool to study such magnetic and structural properties is polarised neutron reflectivity due to its sensitivity to magnetic and structural information as well as ability to probe buried interfaces.

Electrodeposition is widely used to grow a wide range of (predominantly metallic) alloys [10], [11], [12] but can also be used to create multilayer films [13], [14]. Whilst it is possible to grow many different materials with multiple baths, their use may introduce contaminants during transfer. Single bath techniques are hence often preferred [15]. In the single bath method diffusion limiting is the most commonly used tool to control alloy composition using applied potential as the sole varied parameter, albeit with carefully chosen electrolyte concentrations. Diffusion limited alloy growth requires that the least noble ion species is present in a lower concentration than the more noble species, so that at more negative cathodic potentials its growth becomes limited by mass transport while the flux of the more noble ions are much higher. With this method it is impossible to grow pure films of the most noble species, though purity levels over 99% can be achieved [13]. At potentials between the standard electrode potentials (SEPs) of the species only the least noble component will be deposited. The voltage window for alloy growth is then defined by the SEP of the least noble species and the solvent breakdown potential (i.e. due to hydrogen evolution etc.).

In a previous study [16] on this system we found that for large scale film deposition the absolute values of the electrolyte concentrations as well as their ratios was important. Specifically, using weak concentrations of electrolytes enables a wide range of alloy compositions but introduces inhomogeneity in film thickness. The inhomogeneity can be overcome by using more concentrated electrolyte species, though this comes at the cost of the composition range which can be achieved solely by changing the applied potential.

In this paper we examine the magnetic as well as the structural properties of homogeneous bilayers of magnetic/non-magnetic films. We use polarised neutron reflectivity (PNR) which is sensitive to both structural and magnetic properties of thin films and is also able to probe buried interfaces, making it a perfect tool for this study. We explore the voltage window of these solutions and its effect on the magnetic properties. We are using CoNiCu alloys and multilayers due to both the reported enhancement in GMR compared to CoCu [5], [17], [18] and the increased window for homogeneous and flat films which accompany the addition of small amounts of Ni in the electrolyte solution [19].

Section snippets

Experimental

The electrolyte consisted of 100 ml of Millipore Milli-Q water with 500 mM boric acid (H3BO3), 500 mM CoSO4•7H2O, 500 mM NiSO4•7H2O, 20 mM CuSO4 all obtained from Sigma-Aldrich and >99.995% purity level. The pH of this solution was then adjusted to pH3.6 with the addition of aqueous ammonia chloride in order to decrease hydrogen evolution at the cathode. The cathode was either polished glass or single crystal (100) Si wafer with a 10 nm conducting layer of Pt sputter coated onto the surface. The

Results and Discussion

The recovery of the real space structure of a film from the neutron reflectivity curve is an inverse problem and in general requires additional information to be uniquely solved. This is usually knowledge of growth conditions/calibrations and XRR or ellipsometry. Based on this knowledge a test model is created and the structural parameters are refined using a program such as xpolly (based on [20]). In general there will be no unique solution for a given reflectivity curve [21]; as such many of

Conclusions

We have demonstrated the growth of high quality thin magnetic (CoxNiyCuz) films and multilayers, via electrodeposition, suitable for measurement using polarised neutron reflectivity, having previously developed the methodology to do so [16]. PNR's unique abilities allow analysis of the growth mechanisms of this system. As expected the nature of the substrate had little or no effect. The composition of thin film samples was found to be markedly different from thicker samples used for calibration

Acknowledgements

The authors would like to thank the EPSRC and the STFC for funding (RB1010521); JFKC and KNV would also like to thank the Cambridge Philosophical society for funding.

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