Interplay between interfacial and structural properties on the magnetism of self-organized core-shell Co/Pt supported nanodots

P. Campiglio, N. Moreau, V. Repain, C. Chacon, Y. Girard, J. Klein, J. Lagoute, S. Rousset, H. Bulou, F. Scheurer, C. Goyhenex, P. Ohresser, E. Fonda, and H. Magnan

Phys. Rev. B 84, 235443 – Published 27 December 2011

Abstract

We have studied the influence of Pt capping on the magnetic properties of self-organized Co nanodots by means of complementary structural and magnetic investigation techniques. The growth of monodisperse 5 nm diameter Co dots on a Au(111) surface and its progressive coverage by Pt were performed under ultrahigh vacuum conditions and imaged by scanning tunneling microscopy, revealing a weak mixing at room temperature. The Co/Pt core-shell structure was studied both by molecular-dynamics simulations and surface extended x-ray-absorption fine structure, showing a global dilatation of the Co core. Both magnetic moments and hysteresis cycles at various temperatures were measured using x-ray magnetic circular dichroism, revealing a slight increase of the magnetic moments and the out-of-plane magnetic anisotropy after Pt capping. Moreover, the variation of magnetic anisotropy as a function of capping was followed in situ by the magneto-optical Kerr effect. Our investigations demonstrate that interfacial hybridization between Co and Pt is dominant over magnetoelastic contribution in this system.

Received 25 October 2011

DOI:https://doi.org/10.1103/PhysRevB.84.235443

Authors & Affiliations

P. Campiglio^*, N. Moreau, V. Repain^†, C. Chacon, Y. Girard, J. Klein, J. Lagoute, and S. Rousset

Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot Paris 7, Sorbonne Paris Cité, Centre National de la Recherche Scientifique, Joint Research Units 7162 Case Courrier 7021, FR-75205 Paris 13, France

H. Bulou, F. Scheurer, and C. Goyhenex

Institut de Physique et Chimie des Matériaux de Strasbourg, Université Louis Pasteur and Centre National de la Recherche Scientifique, Joint Research Units 7504, Boîte Postale 43, FR-67034 Strasbourg, France

P. Ohresser and E. Fonda

Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, FR-91192 Gif sur Yvette, France

H. Magnan

Service de Physique et de Chimie des Surfaces et des Interfaces, IRAMIS, Commissariat a l’Energie Atomique Saclay, FR-91191 Gif sur Yvette, France

^*Also at Department of Materials Science, Universitá degli Studi di Milano-Bicocca, Via R. Cozzi 53 Milano IT-20125, Italy.
^†vincent.repain@univ-paris-diderot.fr.

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Vol. 84, Iss. 23 — 15 December 2011

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Images

Figure 1
Three-dimensional views of STM images $(100 \times 100 {nm}^{2})$ showing the growth of Pt around and on the top of Co nanoislands. The color scale is intended to represent the different elements of the system (yellow: Au; blue: Co; gray: Pt). (a) 0.5 ML of Co. (b) 0.5 ML of Pt surrounding Co islands. (c) STM image ( $30 \times 15 {nm}^{2})$ and profile analysis of a typical Pt-rim/Co-core dots. (d) 1.5 ML of Pt deposited on Co.Reuse & Permissions
Figure 2
(Top) Raw Co K edges absorption of a 0.5 ML Co covered by 3 ML of Pt in normal and grazing geometry. (Bottom) Fourier backtransforms of the signal (open dots) in the normal (left) and grazing (right) geometry with the fits (red lines) as described in the text.Reuse & Permissions
Figure 3
(a) Different sketches of the encapsulation process. 1: Raw 585 Co atoms cluster. 2: Surrounded by 200 Pt atoms ( $θ_{Pt} = 0.125$ ML). 3: Surrounded by 2000 Pt atoms ( $θ_{Pt} = 1.25$ ML). 4: Surrounded by 2900 Pt atoms ( $θ_{Pt} = 1.8$ ML). (b) Change of the mean in-plane Co-Co distance with encapsulation. (c) Change of the distance between Co planes with encapsulation. The vertical dotted lines indicate the completion of the first and second Pt layer around the Co core.Reuse & Permissions
Figure 4
(a) Normalized XMCD signal at the $L_{3}$ and $L_{2}$ Co edges recorded on the 0.4 ML Co/Au(111) (black curve) and on the 2.5 ML Pt on 0.4 ML Co/Au(111) (gray curve, red online, shifted by 3 eV for clarity). (b) Experimental magnetization curves determined with XAS at the $L_{3}$ Co edge (open circles) for Pt ( $2.5 ML)$ /Co ( $0.4 ML$ )/Au(111) at different temperatures $T$ and angles $α$ . The zero-field anomaly is due to a reproducible experimental overdetection in the sample electron current at zero field. The full lines represent the best fit according to the model described in the text.Reuse & Permissions
Figure 5
(a) Change in the temperature dependence of the magnetic susceptibility of Co nanodots (black circles) upon Pt capping (colored diamonds); the coverage of the Pt overlayer is reported in the legend. The experimental data (open symbols) were fitted with the model described in the text (continuous line). (b) Mean activation energy for magnetization reversal as a function of Pt coverage as deduced from Eq. (2). The statistical dispersion around mean values is $σ_{E} ≃ 0.25 {\bar{E}}_{a}$ . All the measurements have been repeated with a good reproducibility, the standard deviations being the error bars. The dashed line is drawn as a guide for the eye. In the inset, a schematic representation of the system is reported for each $θ_{Pt}$ studied.Reuse & Permissions
Figure 6
(a) Mean activation energy for magnetization reversal of Co nanodots as a function of $θ_{Pt}$ , if the overlayer is deposited in successive steps. As a comparison, the results for the single steps case [as in Fig. 5b] are also reported in dotted lines. (b) STM images ( $80 \times 80 {nm}^{2}$ ) showing the different morphology of a 2.5 ML thick Pt overlayer when deposited in single (left) or in successive steps (right).Reuse & Permissions

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