Elsevier

Applied Surface Science

Volume 267, 15 February 2013, Pages 196-199
Applied Surface Science

Magnetic anisotropy of cobalt nanoparticle 2D arrays grown on corrugated MnF2(1 1 0) and CaF2(1 1 0) surfaces

https://doi.org/10.1016/j.apsusc.2012.10.006Get rights and content

Abstract

Cobalt nanoparticle 2D arrays with different effective thicknesses of cobalt layer (2 nm < deff < 10 nm) were grown by molecular beam epitaxy on CaF2(1 1 0)/Si(0 0 1) and MnF2(1 1 0)/CaF2(1 1 0)/Si(0 0 1) substrates with corrugated morphology of the surface. Surface morphology analysis showed that for effective thickness of cobalt layer deff = 5 nm the lateral dimensions of cobalt islands are about 5–10 nm and the distances between the islands differs in a half along and across the grooves. In both types of the heterostructures the shape of hysteresis loops measured by LMOKE depend on orientation of in-plane magnetic field relative to the direction of the grooves. The azimuthal dependence of coercive field Hc in Co/CaF2(1 1 0)/Si(0 0 1) structures corresponds to Stoner–Wohlfarth model's predictions, which takes into account the anisotropy of individual particles. In contrast to that, in Co/MnF2(1 1 0)/CaF2(1 1 0)/Si(0 0 1) structures these dependences are analogous to those predicted by the model based on account of magnetic–dipole interaction between particles which are placed in chains (chain-of-spheres-model). Possible explanations of the difference in magnetic anisotropy are suggested.

Highlights

Corrugated surface of substrate affect magnetic anisotropy of Co films. ► Cobalt nanoparticles forms linear chains. ► The anisotropy is measured by LMOKE. ► The anisotropy is due to magnetic dipole interaction between nanoparticles.

Introduction

It was earlier found [1], [2], [3], [4], [5] that physical properties of substrates, such as the symmetry of crystal structure, orientation of crystallographic directions, morphology of the surface layer, and magnetic properties of substrates, can strongly influence the magnetic anisotropy of planar magnetic heterostructures. In particular, the study of magnetic properties of 2D arrays of cobalt nanoparticles grown on CaF2(1 1 0)/Si(0 0 1) substrates with a corrugated surface has shown [6], [7], [8] that the shape of magnetic hysteresis loops strongly depends on the orientation of in-plane magnetic field H relative to the direction of the grooves U on the surface. For orientation of magnetic field along U wide hysteresis loops with approximately rectangular shape were observed. In contrast to that for orientation of the magnetic field H perpendicular to U the hysteresis loops are narrow with much smaller loop squareness. Such anisotropy has been observed on both continuous cobalt films with coalesced Co nanoparticles [8] and on 2D arrays of individual cobalt nanoparticles separated by some distance to each other [6], [7].

The mechanism responsible for the magnetization reversal in such structures is the rotation of the magnetization. Observed anisotropy of the hysteresis loops can be approximately described in the framework of Stoner–Wohlfarth (SW) model [9] assuming presence in the system of some distribution of in-plane easy directions around the corrugation direction U [8]. In principle, in-plane magnetic anisotropy of the arrays of separated nanoparticles can be caused by the magnetic anisotropy of the individual nanoparticles and/or magnetic dipole interaction between somehow ordered magnetic particles. Magnetic anisotropy of planar magnetic systems also can strongly depend on magnetic properties of the substrate. In particular, the exchange shift and change of the width and shape of hysteresis loops can be observed in the ferromagnetic films grown on antiferromagnetic substrates below Neel temperature (TN) of the antiferromagnet [10]. This so-called exchange bias effect is related to manifestation of unidirectional magnetic anisotropy and has been observed in a number of magnetic heterostructures [11]. Because of important applications of this effect in spin valve based devices [12], investigation of its microscopic mechanisms is in demand. It was shown, however, that competition of unidirectional magnetic anisotropy with other magnetic anisotropies existing above the Neel temperature of the antiferromagnet, can result in rather complicated magnetic behavior [13]. For this reason, magnetic anisotropy studies of ferromagnet–antiferromagnet heterostructures above the Neel temperature are needed.

In this work, we undertook comparative study of magnetic anisotropy of Co nanoparticle arrays grown on corrugated ridged and grooved MnF2(1 1 0) and CaF2(1 1 0) surfaces. It was found that magnetic properties of these two systems are quite different. Possible reasons of the difference are discussed in the paper.

Section snippets

Experimental

Heterostructures were grown by molecular beam epitaxy (MBE) on atomically clean Si(0 0 1) substrates. Calcium fluoride buffer layer was grown at 440 °C with thin wetting layer, which resulted in orientation of its (1 1 0) crystallographic plane parallel to (0 0 1) plane of Si substrate [14]. The surface of this buffer layer demonstrated grooved and ridged morphology with the ridges running along [−1 1 0] direction of the substrate.

Epitaxial MnF2 layer as thick as 35 nm was grown at 100 °C on the top of CaF

Results and discussion

The hysteresis loops of Co nanoparticle arrays grown on both CaF2 and MnF2 corrugated surfaces were found to considerably depend on the orientation of the magnetic field H relative to the groove direction U. Fig. 2 shows the hysteresis loops in the sample #5613 (Co/MnF2, deff = 5 nm) for UH and UH.

For HU the hysteresis loop has practically rectangular shape with close values of coercive Hc (200 Oe) and saturation field Hs (220 Oe). The hysteresis loop for HU is characterized by smaller value of H

Conclusions

Experimental studies of hysteresis loops in 2D cobalt nanoparticle arrays grown on CaF2 and MnF2 corrugated surfaces show considerable difference in magnetic properties. In the arrays grown on CaF2 layer magnetic anisotropy is determined mainly by that of individual particles. However considerable contribution of magnetic dipole interaction is observed in the arrays grown on MnF2. One can suppose that stronger manifestation of magnetic dipole interaction in these structures as compared to CaF2

Acknowledgements

This study was partially supported by Russian Ministry of Education and Science (State contract No. 16.513.11.3095) and European Commission (grant FP7-PEOPLE-2009-IRSES GA n°: 247518). The authors appreciate stimulating discussions with J. Camarero and J. Nogues.

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