Finite size effects on structure and magnetism in mass-selected CoPt ...

Eur. Phys. J. D (2013) 67: 25 DOI: 10.1140/epjd/e2012-30451-3

THE EUROPEAN PHYSICAL JOURNAL D

Regular Article

Finite size effects on structure and magnetism in mass-selected CoPt nanoparticles Véronique Dupuis1,a,b , Nils Blanc1,c , Luis E. Diaz-Sanchez2,d , Arnaud Hillion1 , Alexandre Tamion1 , Florent Tournus1 , Gustavo M. Pastor2, Andrei Rogalev3 , and Fabrice Wilhelm3 1 2 3

LPMCN, UMR 5586 CNRS/Université de Lyon, 69622 Villeurbanne Cedex, France ITP, Institut f¨ ur Theoretische Physik, Universit¨ at Kassel, 34132 Kassel, Germany European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France Received 19 July 2012 / Received in final form 8 November 2012 c EDP Sciences, Societ` Published online 26 February 2013 – a Italiana di Fisica, Springer-Verlag 2013 Abstract. We have studied size effect on the local structure and magnetism in CoPt clusters embedded in carbon matrix upon annealing in the 2–4 nm diameter range. From Co-K and Pt-L edges extended X-ray absorption fine structure (EXAFS) experiments and simulations, we report the refined quantitative non trivial structure of size-selected 3 nm in diameter CoPt magnetic clusters. In agreement with ab-initio simulation calculations, we evidenced an element-specific dependence of the local atomic relaxations in CoPt clusters leading to a strong distortion in pure Co planes. Since pure Co layers do not match the underlying Pt layer in chemically ordered L10 -like clusters, we claim that a perfect crystal can not exist at nanosize. Such result could explain the low magnetic anisotropy energy measured on chemically ordered CoPt nanomagnets assemblies.

1 Introduction After decades of systematic studies of the size and structural dependence of pure transition-metal magnetic clusters, the interest is actually moving progressively towards investigations on finite-size binary alloys. Indeed, magnetic nanoalloys attract a lot of attention because they offer the possibility to tune the magnetic moments and the magnetic anisotropy energy (MAE) probably up to the ultimate density storage limit. In particular, an extremely high magnetocrystalline anisotropy is expected from the stacking of pure Co and Pt atomic planes in the (0 0 1) direction for CoPt bulk alloys in the chemically ordered L10 phase. Nevertheless, even if a good chemical order can be observed at nanosize, a consequent MAE enhancement remains so far absent [1–5].

ISSPIC 16 – 16th International Symposium on Small Particles and Inorganic Clusters, edited by Kristiaan Temst, Margriet J. Van Bael, Ewald Janssens, H.-G. Boyen and Fran¸coise Remacle. a Present address: Institut Lumière Matière, UMR 5306 Université Lyon 1-CNRS, Université de Lyon, 69622 Villeurbanne Cedex, France. b e-mail: [email protected] c ´ Present address: Currently at Institut NEEL, CNRS and Université Joseph Fourier, BP 166, 38042 Grenoble Cedex 9, France. d Present address: Currently at Facultad de Ciencias, Universidad Aut´ onoma del Estado de México, Av. Instituto Literario 100, Col. Centro, Toluca, C.P. 50000, México.

In this paper, by using powerful X-ray absorption spectroscopy techniques at Co and Pt environment on mass-selected CoPt clusters, we propose to relate structural and local order in nanoalloys. By working at both absorption edges, we determined the first-shell Co-Co, Co-Pt and Pt-Pt coordination numbers and mean atomic distances from EXAFS simulations. The results are compared to theoretical calculations using the Vienna ab-initio simulation package (VASP).

2 Experimental procedures and results on assemblies of CoPt clusters in carbon matrix 2.1 Sample synthesis CoPt clusters are preformed in the gas phase thanks to a laser vaporization source working in the low energy clusters beam deposition (LECBD) regime. Briefly, a YAG laser (λ = 532 nm, pulse duration 8 ns, frequency 30 Hz) is used to vaporize a mixed equiatomic CoPt target rod and a continuous flow of inert gas (He, 30 mbar) is injected to rapidly cool the generated plasma and to nucleate clusters submitted to a supersonic expansion under vacuum. The apparatus is equipped with a quadrupolar electrostatic mass-deviator allowing us to deposit mass-selected clusters in an ultra-high vacuum (UHV) deposition chamber [6,7] in the 2–4 nm diameter range. The matrix is evaporated with an electron gun working under UHV conditions (base pressure of 5 × 10−10 Torr).

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(b) (b) Fig. 1. Experimental FT-EXAFS oscillations at the Co-K edge for as-prepared (a) and annealed (b) size-selected CoPt clusters embedded in carbon matrix with different diameters (HRTEM images in inset).

Both clusters and atomic matrix beams are simultaneously deposited on a silicon substrate. The cluster volumeconcentration is kept lower than 1%. 2.2 Structural characterization First of all, structural characterizations have been performed using transmission electron microscopy (TEM) on isolated CoPt clusters deposited on an amorphous carbon coated grid (then protected by a carbon thin film), before and after annealing under High Vacuum. The clusters’ size distributions are experimentally determined from transmission electron microscopy (TEM) observations and both fitted with the same Gaussian curves [7]. High resolution TEM-observations reveal that the main morphology of as-prepared CoPt-clusters consists in truncated octahedrons crystallized in the A1 fccstructure, corresponding to the equilibrium shape predicted by Wulff theory. Notice that in our range of interest, a perfect fcc-truncated CoPt octahedron with a diameter of Dm = 2.7 nm contains 586 atoms and a proportion of atoms in the first surface-monolayer equal to 46.4%. Chemical ordering has been obtained after 2 hours annealing at 750 K without any coalescence. HRTEM evidences a transition to the tetragonal chemically ordered L10 phase with a quasi perfect order parameter [7] (see also experimental HRTEM images in inset Figs. 1a and 1b).

Fig. 2. Comparison between the experimental EXAFS signal (dots, contribution of the nearest neighbours (NN) peak only) and simulated curves (solid lines) with c/a = 1.03 at the Co-K edge (a) and with c/a = 0.92 at the Pt-L edge (b) obtained on L10 CoPt clusters with 3 nm in diameter.

Then, we have studied size effects on the local environment in magnetic nanoalloys on the CRG-FAME and ID12 beamlines of the european synchrotron radiation facility (ESRF) at Grenoble. From extended X-ray absorption fine structure (EXAFS) experiments at both Co-K and Pt-L edges, in the fluorescence mode and feff fit simulations, we have been able to determine the first-shell Co-Co, Co-Pt and Pt-Pt coordination numbers and atomic distances as a function of CoPt cluster sizes and thermal treatments. By considering the Fourier transform (FT) of the experimental EXAFS-signal at the Co-K edge, for as-prepared and annealed samples, the curves exhibit a remarkable enhancement in intensity of the peak located at 2 ˚ A (distance not corrected from phase shift) corresponding to the first metallic nearest-neighbors (see Figs. 1a and 1b) as a function of diameter D. From simulations, we have found a contracted lattice parameter for the as-prepared nanoparticles in the whole sizes range. After annealing, EXAFS experiments performed at both Co-K and Pt-L edges, on the same annealed CoPt sample with 3 nm in diameter are presented in Figure 2. The best simulation has been obtained for a perfect chemically ordered CoPt packing (L10 phase) with an unique Co-Pt first neighbour distance at both edges but surprisingly with a reverse apparent c/a ratio namely for c/a = 1.03 at the Co-K edge and for c/a = 0.92 at the Pt-L edge.

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Table 1. Coordination numbers N , average interatomic disA2 ), energy shift tances (R in ˚ A), Debye Waller value (σ 2 in ˚ (ΔE in eV) and lattice parameter ratios [c/a] obtained from EXAFS simulations at both Co-K and Pt-edges on annealed CoPt clusters with 3 nm in diameter and compared to VASP calculations on L10 CoPt clusters. N Nbulk VASP (exp) (L10 ) [c/a] Co Co-Co 2.3 3 2.60 K-edge Co-Pt 4.6 6 2.63 [c/a] [1.02] Pt Pt-Co 5 6 2.63 L3 -edge Pt-Pt 2.5 3 2.69 [c/a] [0.94] Edge

Bound

R (˚ A) [c/a] 2.57 2.62 [1.03] 2.60 2.70 [0.92]

σ (˚ A2 ) 0.009 0.009 2

ΔE (eV) –3.6 –3.6

Not relaxed: a=3.7 c=3.6 c/a= 0.97

Co-Pt

Co-Co Pt-Pt

Relaxed: amean=3.68 cmean=3.76 apparent c/a= 1.02

0.004 5.98 0.004 5.98

All the parameters obtained from the EXAFS simulations at both Co-K and Pt-edges on the same annealed CoPt sample (D = 3 nm) have been reported in Table 1. The reduction of the coordination number N comes from finite size effects because we verified that N increases as a function of D. One has to mention that for the whole size range, we systematically obtained an c/a ratio greater than 1 (resp. lower than 1) from EXAFS at Co-K edge (resp. at Pt-L edge) for the L10 nanoalloys which tends to converge to an unique bulk c/a ratio equal to 0.97 (bulk value) as the clusters diameter increases. We have performed Spin-polarized density-functionnal calculations using the Vienna ab-initio simulation package (VASP) with first principles magnetic and structural optimizations [8]. From such VASP calculations on CoPt in the same size range, we find three different Gaussian relaxed distance distributions for Co-Co, Co-Pt and Pt-Pt both in chemically disordered and L10 ordered phases. In particular, while in the bulk phases, an equal Co-Co and Pt-Pt Dirac distance distribution is expected, a strong Co-Co distance dispersion has been found for relaxed clusters even in the chemically ordered phase (see Fig. 3). The tendency of Co nearest neighbour bonds to be shorter than the Pt ones results in structural stress, which in a finite CoPt cluster can be more easily distorted by moving Co atoms (than Pt atoms) in the surface faces. As seen in Table 1, the calculations fully confirm the experimental trends thus providing a detailed account of the element specific local relaxations. Indeed, we claim that we have put in evidence an element-specific dependence of the local atomic relaxations in CoPt clusters leading to a strong distortion in pure Co-Co planes. The fact that pure Co layers do not match the underlying Pt layer in chemically ordered L10 -like phase at nanosize, allows us to bring back together the EXAFS experimental results at both element edges in agreement with the VASP calculations. 2.3 Magnetic results Taking into account that the magnetization and magnetic anisotropy resulting from 3d–5d proximity effects are very sensitive to the local environment of the atoms, we have performed X-ray magnetic circular dichroism (XMCD)

Fig. 3. Gaussian relaxed first neighbour distances distributions obtained from VASP calculations on CoPt (201 atoms) in L10 ordered bulk-like phase compared to the Dirac distance distribution expected for un-relaxed one. The corresponding theoretical cluster morphologies are presented in inset. Note that the difference is very small at eye.

measurements at both edges. Such (XMCD) investigations have revealed a significant increase of both Co and Pt magnetic moments for the chemically ordered CoPt clusters compared to the bulk values. The magnetic anisotropy energy (MAE) of annealed samples has been determined from SQUID magnetometry and has been found twice that of the as-prepared samples, but one order of magnitude smaller that what is expected for the L10 CoPt bulk. Indeed from the recently developed accurate “triple fit” method where the ZFC/FC curves and a room temperature magnetization loop are simultaneously fitted [2,9,10], we have shown that the main difference comes from the MAE evolution between the as-prepared and annealed CoPt sample. In Figure 4, we present these three experimental SQUID curves and their fits obtained on both as-prepared and annealed CoPt samples. Notice that in the superparamagnetic regime, the magnetization curves at 300 K are found identical for both samples. Upon annealing the median magnetic diameter (Dm ) is conserved while the effective anisotropy constant (Keff ) has been found to increase and its corresponding MAE dispersion (ωK ) to slightly decrease (see Tab. 2). This dispersion decrease is well explained by the appearance of the chemical order at nanosize [7,11]. In a previous paper [12], we have shown that the MAE dispersion in pure Co clusters essentially comes from the effect of additional facets and is relatively small. But in nanoalloys, the size and shape dispersion is not the unique source of MAE dispersion. Indeed, since the anisotropy enhancement in as-prepared CoPt compared to pure Co clusters is due to the presence of Pt atoms, the dispersion of the magnetocrystalline anisotropy (which depends on the neighborhood of each Co atom) increases with the

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local structure on CoPt samples studied from EXAFS experiments at the Co-K and Pt-L edges. From simulations of the as-prepared samples signal, we have found a contracted lattice parameter for the particles in the whole sizes range. Upon annealing, we have clearly observed a c/a ratio greater than 1 at Co-K edge and reversely a c/a ratio lower than 1 at Pt-edge for the L10 phase in nanoalloys which tend to converge to an unique bulk c/a ratio equal to 0.97 as the clusters diameter increases. The calculations using the VASP fully confirm the experimental trends by providing a detailed account of the element specific local relaxations which bring back together the EXAFS experimental results at both edges in agreement with our previous X-ray diffraction results [8]. Nevertheless there still exist open questions concerning the fact that we find that the MAE for nearly perfect chemicaly ordered nanoparticles is one order of magnitude lower to what is expected for the bulk. The specific local relaxation at finite size could be the answer. To go further, non-colinear calculations (ab-initio + tight-binding calculations), including the spin-orbit coupling in order to obtain the MAE values are in progress for such relaxed nanoalloys.

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The authors are grateful to A. Ramos, H. Tolentino and O. Proux for stimulating discussions and for their help during experiments on the French BM30b-FAME beamlines at ESRF. Support is acknowledged from both GDR CNRS 3182 and COST-STSM-MP0903-7318 on Nanoalloys.

(b) Fig. 4. Experimental (dots) and triple fitted (line) ZFC/FC curves (a) and a room temperature magnetization loop (b) of as-prepared and annealed size-selected CoPt clusters embedded in carbon matrix with 3 nm in diameter. Table 2. Magnetic characteristic evolution of as-prepared and annealed size-selected CoPt clusters embedded in carbon matrix with 3 nm in diameter. Dm (nm) Keff (kJ m−3 ) ωK

As-prepared 3.12 ± 0.1 218 ± 20 37% ± 5%

Annealed 3.12 ± 0.1 293 ± 30 28% ± 5%

number of possible chemical arrangements. It is the reason why the MAE of chemically disordered CoPt particles is quite large even if mass-selected clusters have a small size dispersion (8%) and a highly symmetrical shape (regular truncated octahedron) [12]. As long as a well-defined and high enough degree of chemical order can be reached, the multiplicity of atomic configurations is strongly reduced and the MAE dispersion decreases while its median value increases.

3 Discussion and conclusion As a summary, in order to obtain new insights on the correlation between magnetic properties and short- or longrange chemical order in nanoalloys, we refined quantitative

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