phys. stat. sol. (c) 2, No. 10, 3572– 3575 (2005) / DOI 10.1002/pssc.200461772
Magnetic properties of bimetallic Co-Pd nanostructures Javier Guevara∗1,2 , Ana Mar´ıa Llois2,3 , Faustino Aguilera-Granja4 , and Juan Mart´ın Montejano-Carrizales4 1 2
3 4
Escuela de Ciencia y Tecnolog´ıa, Universidad de San Mart´ın, Alem 3901, 1563 San Mart´ın, Argentina U.A. F´ısica, C.A.C., Comisi´on Nacional de Energ´ıa At´omica, Avda Gral Paz 1499, 1560 San Mart´ın, Argentina Departamento de F´ısica, FCEN-UBA, Cdad Universitaria, Cdad Autonoma de Buenos Aires, Argentina Instituto de F´ısica, ”Manuel Sandoval Vallarta”, Universidad Aut´onoma de San Luis Potos´ı, 78000 SLP, M´exico
Received 11 October 2004, revised 27 May 2005, accepted 27 May 2005 Published online 29 July 2005 PACS 36.40.Cg, 61.46.+w, 71.10.Fd, 75.75.+a We study the dependence of the magnetic properties of Co-Pd nanoclusters (Co cores coated by Pd atoms) on size and relative composition. We consider Co-Pd clusters having closed shell cubo-octahedral structure with an increasing Co core size. The electronic and magnetic properties are calculated with a parametrized Hubbard Hamiltonian within the unrestricted Hartree-Fock approximation. We show that, depending on the relative composition, the Pd coating can give rise to an enhancement of the average magnetic moment of the Co core. We compare with results from slabs by using an ab initio calculation method. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Pd is a widely used element in catalytic processes and it frequently appears in its low dimensional manifestations, such as clusters and small particles. Actually, Pd is an expensive element and in order to lower costs different possibilities have been proposed so that the active Pd component constitutes only a small fraction of the whole particles used for catalytic purposes. The suggested systems are mainly core/coated arrangements which can be used in place of pure Pd particles. Systems like Co (core) or Ni (core) coated by Pd have been already synthesized and could be candidates for substitution of pure Pd clusters, while preserving the surface properties of Pd pure particles and showing also additional ones, such as core induced magnetism [1–3]. It is also shown that interatomic distances within clusters of these sizes are very close to the corresponding bulk ones and can be grown as fcc structures. Few attempts have been done till now in the direction of understanding the physical behavior of coated particles. As ab-initio calculations are limited to clusters of small sizes, even for highly symmetric ones, we use a tight-binding Hamiltonian solved within the unrestricted Hartree-Fock approximation [4]. Taking advantage of symmetry properties we are able to handle clusters of up to 561 atoms, size which is already within the real scale of the experimentally produced clusters. We have recently studied Ni-Pd bimetallic clusters [5] and in this contribution we focus on Co-Pd clusters and slabs. In the case of slabs we perform ab initio calculations, by using the Wien2k code [6]. ∗
Corresponding author: e-mail:
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phys. stat. sol. (c) 2, No. 10 (2005) / www.pss-c.com
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2 Method of calculation For the electronic calculations of the clusters a tight-binding Hamiltonian with spd orbitals is used. Spin magnetism is obtained from a Hubbard-like term solved in the unrestricted Hartree-Fock approximation, as in previous works for pure and mixed systems [4]. All many-body contributions appear in the diagonal terms. Extra s orbitals outside the clusters are added to account for the electronic spill over at the surface. The formalism and the parameters are detailed in References [4]. We do calculations for fcc-like aggregates of 561 atoms and assume bulk-like parameters for both core and coating atoms, and average values for the Co-Pd bonds [4]. The ab initio calculation are done for slabs 7 layers thick grown in the (111) direction, with different relative compositions. For the slab lattice constants we have used in each case the weighted average between the ones of FCC Co (6.47 au) and FCC Pd (7.30 au).
3 Results 3.1 Cluster calculations In Fig. 1 we show the results obtained for the average magnetic moment per atom as a function of Co content (NCo ) for the coated clusters. The total number of atoms being fixed at 561, we increase the size of the Co core (decreasing the number of Pd atoms at the surface). The upper nearly-constant curve gives the average magnetic moment per atom corresponding to the Co atoms as a function of Co content, the lower curve gives the corresponding values for Pd, while the one with the empty squares gives the total average magnetic moment per atom in the system. The enhancement in the magnetic moment of the Co atoms, as compared to the bulk value, is due to finite size effects and also to the hybridization with the Pd atoms. In the case with just one Co atom in the core we are in the presence of a magnetic impurity, this explains the large local moment obtained and shown in Fig. 1. This value agrees with previous theoretical [7] and experimental results [8]. The average magnetic moment per Co atom tends towards the free standing cluster value as a function of increasing Co concentration. 3.2 Slab calculations It is worth comparing the cluster results with the ones obtained for the 7-layer slabs grown in the (111) direction, by doing ab initio calculations [6] for different layered Co-Pd arrangements. We consider in this case not only Co coated situations but also arrangements in which Pd lies in the central layers. In Fig. 2 we show the results of the magnetic moments as a function of Co concentration, xCo , it can be clearly seen that the slab results show a similar behavior to the cluster ones. For xCo =1/7, we have a 3Pd/Co/3Pd slab, for xCo =2/7 three different layered arrangements are studied which are schematically shown in Fig. 3, for xCo =3/7 the slab has the layer sequence 2Pd/3Co/2Pd, for xCo =4/7 the sequence is Pd/2Co/Pd/2Co/Pd, for xCo =5/7 it is Pd/5Co/Pd, and for xCo =6/7 it is 3Co/Pd/3Co. The pure Pd slab shows no magnetization and the pure Co slab presents a slightly higher average magnetic moment than the bulk. We can clearly see that Co reaches magnetic values close to 2µB at low Co concentration. In the case of xCo =2/7, 4/7, and 6/7, the central layer is always made of Pd. In the case of xCo =2/7 we consider three different layer arrangements (see Fig. 3). For 2Pd/Co/Pd/Co/2Pd (”A” in Fig. 3) we obtain the lowest value for the magnetic moment of Co, namely µCo = 1.93µB , while µP d = 0.27µB . In the arrangement Co/5Pd/Co, the Co atoms are at the surface (”B” in Fig. 3), we obtain µCo = 1.95µB and µP d = 0.15µB , we have an slightly enhanced magnetization for Co due to surface effects and, on the contrary, a lower Pd magnetization. The most interesting case is the one when the Co atoms are in the sub-surface layer, Pd/Co/3Pd/Co/Pd, (”C” in Fig. 3). In this case µCo = 2.00µB and µP d = 0.30µB , both, hybridation and surface effects, act together to enhance the magnetization of both, Co and Pd atoms. It is also the system with the lowest total energy among the three arrangements. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. Guevara et al.: Magnetic properties of bimetallic Co-Pd nanostructures
µ (µB/atom)
2
Co
1.6 1.2
Cluster Average 0.8 0.4
Pd
0
0
100
300
200
500
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NCo Fig. 1 Magnetic moment per atom as a function of the number of Co atoms. The total number of atoms is 561.
µ (µB/atom)
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Co
1.6 1.2
Slab Average
0.8
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0.4 0
0
0.2
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xCo Fig. 2 Magnetic moments per atom as a function of Co content and for the 7 layer slabs.
4 Discussion and conclusions In this contribution we study the magnetic behavior of segregated Co-Pd nanoclusters and Co-Pd slabs as a function of Co concentration and show that there is an interplay between hybridization and dimensionality © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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A
B
C
µCo=1.93
µCo=1.95
µCo=2.00
µPd=0.27
µPd=0.15
µPd=0.30
Pd Co
Fig. 3 Three different slab arrangements for xCo =2/7 shown in a schematic form. The corresponding average magnetic moments for Co and Pd atoms are given in units of µB .
effects which contribute to the enhancement of the average magnetic moments of Co and Pd atoms within the nanostructures. In the cluster calculations the values of the magnetic moments for the different Co concentrations are very similar to the ones obtained for the corresponding layered slab structures. In particular the Pd magnetic moment lies around 0.2µB , both in the slabs as in the coated clusters. Acknowledgements We acknowledge Fundaci´on Antorchas, UBACyT-X115, PICT 03-10698 and Proyecto IM40, both of Agencia Nacional de Promoci´on Cient´ıfica y Tecnol´ogica for partial support. Javier Guevara and Ana Mar´ıa Llois are also researchers of CONICET. This work was partially funded by the PROMEP-SEP-CA, M´exico.
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