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Chemisorption on magnetic cobalt systems
Journal of Magnetism and Magnetic Materials 198}199 (1999) 312}314
Chemisorption on magnetic cobalt systems S[ te\ paH n Pick , Hugues DreysseH * J. Heyrovsky& Institute of Physical Chemistry, Dolejs\ kova 3, CZ-182 23 Prague 8, Czech Republic Institute de Physique et Chimie des Mate& riaux de Strasbourg, UMR CNRS 46, 23 rue du Loess, BP 20 CR, F-67037 Strasbourg, France
Abstract Adsorption of oxygen and carbon monoxide on several ferromagnetic cobalt systems is studied theoretically. The method is based on a self-consistent tight-binding recursion-scheme treatment and adsorption on the semi-in"nite Co(0 0 0 1) crystal and several small Co clusters is considered. It is found that atomic oxygen has only moderate e!ect on the cobalt magnetization. On the Co (0 0 0 1) surface, the magnetization is strongly suppressed by CO in the atop position but not at the bridge chemisorption site. At clusters, CO at atop geometry is also a good magnetization killer. 1999 Elsevier Science B.V. All rights reserved. Keywords: Cobalt clusters; Magnetization; Chemisorption; Carbon monoxide; Oxygen
1. Introduction Giant magnetoresistance is a unique physical phenomenon o!ering completely new perspectives in magnetic data storage devices. Presently, the granular systems attract attention. A natural choice is cobalt clusters immersed into a noble-metal host [1}4]. It is quite remarkable that in the above experimental papers, a quenching of Co particle magnetization has been found. Whereas Refs. [2,3] suggest the oxidation of clusters as a tentative explanation, the authors of Ref. [4] cast doubt on the role of impurities in their experiments. Let us stress that other presently available data disprove any tendency to magnetization reduction in cobalt layers and clusters [4}6]. On the contrary, the magnetization suppression by CO adsorption seems to be established [6] for Co clusters. Recently, the present authors performed a series of self-consistent semi-empirical tight-binding calculation to get an idea about the in#uence of simple gas adsorption on the transition-metal (TM) surface magnetism [7].
* Corresponding author. Tel.: #33-3-88107083; fax: #33-388107249. E-mail address: [email protected] (H. DreysseH )
The results obtained suggest that an atomic adsorbate does not in#uence drastically magnetization of strong ferromagnets. For CO adsorption on magnetic systems the only available theoretical data [8] for Ni clusters point to magnetization killing at the cluster surface, in accordance with experiments on nickel carbonyls.
2. Model In our calculations we employ a self-consistent tightbinding model treated by the recursion-method technique. On Co atoms, the valence s,d electrons are considered [9] whereas on O (CO), one treats explicitly the 2p (5p and 2p*) levels. In chemisorption systems a charge transfer is allowed that is controlled by Coulombic terms in the self-consistency equations. The method is essentially identical to the calculations [7,9] and details will be given elsewhere. For oxygen chemisorption, we treat an isolated O and an O(1;1) overlayer on the ideally terminated semiin"nite HCP Co(0 0 0 1) crystal, with oxygen atoms placed at three-fold FCC sites. Besides that, an isolated atom at the three-fold site on the FCC Co cluster has been studied. The Co}Co distances in the cluster are bulk-like (2.5 As ). The assumption of the Co}O distance
0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 1 1 0 7 - X
S[ . Pick, H. Dreysse& / Journal of Magnetism and Magnetic Materials 198}199 (1999) 312}314
2.0 As is based on oxygen atomic radii typical for chemisorption systems and TM oxides. For CO adsorption we study a number of geometries, First, CO in the atop or bridge site, respectively, above Co(0 0 0 1) is calculated. Further, FCC clusters Co , (N"13, 55) are treated with one or several CO molecules adsorbed in the atop position. Icosahedral Co cluster will also be mentioned. In all cases, the CO is standing upright (in the radial direction on clusters) with the carbon end oriented towards the metal. The theoretical study for CO on paramagnetic clusters [10] arrives at values 1.91 and 2.05 As for the atop and bridge sites, respectively, and experiments on clusters suggest yet greater values [11]. In the latter case, however, serious uncertainty can occur [12]. For such large distances, our calculations show but weak Co}CO interaction with the CO(5p) level position at odds with photoemission data [13,14]. Hence, we choose other distances relying on the fact that two independent reasonings suggest very similar Co}C separation: (1) The comparison with CO chemisorption at Pd and Pt surfaces, taking the di!erence in the atomic radii of particular metals into account, and (2) experimental values for Co carbonyls [15,16]. With values 1.75 As (atop) and 1.9 As (bridge site) the agreement of our results with experiment [13,14] is improved and the overall chemisorption picture resembles that for other late-TM surfaces (cf., e.g. Ref. [9]).
3. Results and discussion To study the chemisorption on Co(0 0 0 1) surface is an interesting [13,14] problem. For Co clusters the geometry is not known. According to Refs. [2,3,6] unexpected structures with a BCC centre can form, however, structures based on icosahedral motives are other candidates [12]. Nevertheless, we believe that the study of small FCC and icosahedral clusters can help in a quantitative understanding of possible physical e!ects. It is not the aim of the present study to calculate realistic carbonyl structures [8,15]. We wish rather to elucidate general conditions under which the magnetization is strongly in#uenced, and to clarify whether the e!ect remains always essentially local. For the free Co(0 0 0 1) crystal the surface magnetization we "nd is [9] 1.70 l and for bulk atoms we get 1.68 l . For FCC Co cluster we obtain 1.85 l at the 12 surface atoms and 1.61 l on the central atom; in the FCC Co cluster we predict 1.82 l on the outermost atoms and the magnetization varies in the range 1.65}1.75 l inside the cluster. The values quoted represent contribution from the d electrons; there is always a small (several hundredths of l ) negative magnetic polarization of s electrons.
313
For an isolated oxygen atom we "nd only very small local changes of magnetization both on Co(0 0 0 1) and Co . It is only the O(1;1) overlayer that produces moderate but non-negligible weakening (1.43 l ) at the Co(0 0 0 0 1) surface. For CO at the Co(0 0 0 1) surface, atop and bridge adsorption sites should be considered [10,13]. For CO at the atop site the magnetization (only) at the neighbouring Co atoms is grossly suppressed (0.48 l ) whereas for the bridge position the e!ect on neighbouring Co atoms is small (1.65 l ). The magnetization of the CO molecule is always small, which holds true also for the cluster studies below. The isolated CO molecule in the atop position allows only a weak magnetization !0.13 l of the underlying Co atom, with antiferromagnetic coupling to the rest of the cluster. There is a minor magnetization reduction on the Co atoms that are not in direct contact with Co. If a CO molecule is put on every surface atom in the atop position (CO /Co ), the whole cluster becomes para magnetic. We studied also analogous CO arrangements on the icosahedral Co cluster. The results are virtually identical. It is clear from the above results that CO in the atop position has serious but highly localized e!ect on the Co magnetization. That is why for the Co cluster we con sider arrangement with CO only on all outermost Co atoms (CO /Co ). The magnetization on the outermost Co atoms is practically killed (!0.05 l ) and couples antiferromagnetically to deeper atoms. In contrast to the CO /Co case the perturbation of inner Co atoms is not large and has an oscillatory character. To conclude, oxygen in `normala adsorption geometry has no severe e!ect on Co magnetization. If its presence is responsible for experimental observations in Ref. [2], the penetration of oxygen into clusters or oxide formation should be considered [2,3,6]. CO in the atop, but not in the bridge position can lead to a large surface-magnetization quenching. When the local magnetic moment becomes very small it has tendency to antiferromagnetic orientation with respect to its neighbours that are not in direct contact with CO. With the exception of quite small clusters CO /Co , we expect that the e!ect has a local character and the cluster core is not strongly in#uenced. Similar conclusion has also been reached experimentally [6]. Trends we obtain for CO chemisorption on strongly magnetized Co particles compare well with results for the comparatively weakly magnetic nickel clusters [8].
This study was facilitated by the common Project Scheme of Centre National de la Recherche Scienti"que and Academy of Sciences of the Czech Republic. S[ P acknowledges support by the Grant agency of the Czech Republic, No. 203/96/0951.
314
S[ . Pick, H. Dreysse& / Journal of Magnetism and Magnetic Materials 198}199 (1999) 312}314
Note added in proof Details can be found in Ref. [17]. References [1] S. Sako, K. Oshima, M. Sakai, S. Bandow, Surf. Rev. Lett. 3 (1996) 109. [2] F. Parent, Thesis, UniversiteH de Paris-Sud, 1996 (unpublished). [3] T. Ould Ely, C. Amiens, B. Chaudret, E. Snoeck, M. Verelst, P. Lecante, M. Respaud, J.-M. Broto, A. Fert, Proceedings of the "fth Colloque Louis NeH el, Banyuls sur Mer, June 1997, paper IV-P2. [4] D.A. Eastham, Y. Quing, T.H. Maddock, J. Kraft, J.P. Schille, G.S. Thompson, H. Haberland, J. Phys.: Condens. Matter 9 (1997) L497. [5] Z.Q. Li, B.L. Gu, Phys. Rev. B 47 (1993) 13611. [6] M. Respaud, J.M. Broto, H. Rakoto, A.R. Fert, L. Thomas, B. Barbara, M. Verelst, E. Snoeck, P. Lecante, A Mosset,
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
J. Osuna, T. Ould Ely, C. Amiens, B. Chaudret, Phys. Rev. B 57 (1998) 2925. S[ . Pick, H. DreysseH , Surf. Sci. 394 (1997) 192. G. Pacchioni, N. RoK sch, Acc. Chem. Res. 28 (1995) 390. S[ . Pick, Phys. Rev. B 57 (1998) 1942. M.C. Zonnevylle, J.J.C. Geerlings, R.A. van Santen, J. Catal. 148 (1994) 412. M. Respaud, Ph.D. Thesis, Institut National des Sciences AppliqueH s de Toulouse, 1997 (unpublished). M. Verelst, private communication, 1998. H. Papp, Surf. Sci. 129 (1983) 205. M. Getzla!, J. Bansmann, G. SchoK nhense, J. Chem. Phys. 103 (1995) 6691. V. Albano, P. Chini, V. Scatturin, J. Organomet. Chem. 15 (1968) 423. V.G. Albano, P.L. Bellon, P. Chini, V. Scatturin, J. Organomet. Chem. 16 (1969) 461. S[ . Pick, H. DreysseH , Phys. Rev. B 59 (1999) 4195.
Chemisorption on magnetic cobalt systems S[ te\ paH n Pick , Hugues DreysseH * J. Heyrovsky& Institute of Physical Chemistry, Dolejs\ kova 3, CZ-182 23 Prague 8, Czech Republic Institute de Physique et Chimie des Mate& riaux de Strasbourg, UMR CNRS 46, 23 rue du Loess, BP 20 CR, F-67037 Strasbourg, France
Abstract Adsorption of oxygen and carbon monoxide on several ferromagnetic cobalt systems is studied theoretically. The method is based on a self-consistent tight-binding recursion-scheme treatment and adsorption on the semi-in"nite Co(0 0 0 1) crystal and several small Co clusters is considered. It is found that atomic oxygen has only moderate e!ect on the cobalt magnetization. On the Co (0 0 0 1) surface, the magnetization is strongly suppressed by CO in the atop position but not at the bridge chemisorption site. At clusters, CO at atop geometry is also a good magnetization killer. 1999 Elsevier Science B.V. All rights reserved. Keywords: Cobalt clusters; Magnetization; Chemisorption; Carbon monoxide; Oxygen
1. Introduction Giant magnetoresistance is a unique physical phenomenon o!ering completely new perspectives in magnetic data storage devices. Presently, the granular systems attract attention. A natural choice is cobalt clusters immersed into a noble-metal host [1}4]. It is quite remarkable that in the above experimental papers, a quenching of Co particle magnetization has been found. Whereas Refs. [2,3] suggest the oxidation of clusters as a tentative explanation, the authors of Ref. [4] cast doubt on the role of impurities in their experiments. Let us stress that other presently available data disprove any tendency to magnetization reduction in cobalt layers and clusters [4}6]. On the contrary, the magnetization suppression by CO adsorption seems to be established [6] for Co clusters. Recently, the present authors performed a series of self-consistent semi-empirical tight-binding calculation to get an idea about the in#uence of simple gas adsorption on the transition-metal (TM) surface magnetism [7].
* Corresponding author. Tel.: #33-3-88107083; fax: #33-388107249. E-mail address: [email protected] (H. DreysseH )
The results obtained suggest that an atomic adsorbate does not in#uence drastically magnetization of strong ferromagnets. For CO adsorption on magnetic systems the only available theoretical data [8] for Ni clusters point to magnetization killing at the cluster surface, in accordance with experiments on nickel carbonyls.
2. Model In our calculations we employ a self-consistent tightbinding model treated by the recursion-method technique. On Co atoms, the valence s,d electrons are considered [9] whereas on O (CO), one treats explicitly the 2p (5p and 2p*) levels. In chemisorption systems a charge transfer is allowed that is controlled by Coulombic terms in the self-consistency equations. The method is essentially identical to the calculations [7,9] and details will be given elsewhere. For oxygen chemisorption, we treat an isolated O and an O(1;1) overlayer on the ideally terminated semiin"nite HCP Co(0 0 0 1) crystal, with oxygen atoms placed at three-fold FCC sites. Besides that, an isolated atom at the three-fold site on the FCC Co cluster has been studied. The Co}Co distances in the cluster are bulk-like (2.5 As ). The assumption of the Co}O distance
0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 1 1 0 7 - X
S[ . Pick, H. Dreysse& / Journal of Magnetism and Magnetic Materials 198}199 (1999) 312}314
2.0 As is based on oxygen atomic radii typical for chemisorption systems and TM oxides. For CO adsorption we study a number of geometries, First, CO in the atop or bridge site, respectively, above Co(0 0 0 1) is calculated. Further, FCC clusters Co , (N"13, 55) are treated with one or several CO molecules adsorbed in the atop position. Icosahedral Co cluster will also be mentioned. In all cases, the CO is standing upright (in the radial direction on clusters) with the carbon end oriented towards the metal. The theoretical study for CO on paramagnetic clusters [10] arrives at values 1.91 and 2.05 As for the atop and bridge sites, respectively, and experiments on clusters suggest yet greater values [11]. In the latter case, however, serious uncertainty can occur [12]. For such large distances, our calculations show but weak Co}CO interaction with the CO(5p) level position at odds with photoemission data [13,14]. Hence, we choose other distances relying on the fact that two independent reasonings suggest very similar Co}C separation: (1) The comparison with CO chemisorption at Pd and Pt surfaces, taking the di!erence in the atomic radii of particular metals into account, and (2) experimental values for Co carbonyls [15,16]. With values 1.75 As (atop) and 1.9 As (bridge site) the agreement of our results with experiment [13,14] is improved and the overall chemisorption picture resembles that for other late-TM surfaces (cf., e.g. Ref. [9]).
3. Results and discussion To study the chemisorption on Co(0 0 0 1) surface is an interesting [13,14] problem. For Co clusters the geometry is not known. According to Refs. [2,3,6] unexpected structures with a BCC centre can form, however, structures based on icosahedral motives are other candidates [12]. Nevertheless, we believe that the study of small FCC and icosahedral clusters can help in a quantitative understanding of possible physical e!ects. It is not the aim of the present study to calculate realistic carbonyl structures [8,15]. We wish rather to elucidate general conditions under which the magnetization is strongly in#uenced, and to clarify whether the e!ect remains always essentially local. For the free Co(0 0 0 1) crystal the surface magnetization we "nd is [9] 1.70 l and for bulk atoms we get 1.68 l . For FCC Co cluster we obtain 1.85 l at the 12 surface atoms and 1.61 l on the central atom; in the FCC Co cluster we predict 1.82 l on the outermost atoms and the magnetization varies in the range 1.65}1.75 l inside the cluster. The values quoted represent contribution from the d electrons; there is always a small (several hundredths of l ) negative magnetic polarization of s electrons.
313
For an isolated oxygen atom we "nd only very small local changes of magnetization both on Co(0 0 0 1) and Co . It is only the O(1;1) overlayer that produces moderate but non-negligible weakening (1.43 l ) at the Co(0 0 0 0 1) surface. For CO at the Co(0 0 0 1) surface, atop and bridge adsorption sites should be considered [10,13]. For CO at the atop site the magnetization (only) at the neighbouring Co atoms is grossly suppressed (0.48 l ) whereas for the bridge position the e!ect on neighbouring Co atoms is small (1.65 l ). The magnetization of the CO molecule is always small, which holds true also for the cluster studies below. The isolated CO molecule in the atop position allows only a weak magnetization !0.13 l of the underlying Co atom, with antiferromagnetic coupling to the rest of the cluster. There is a minor magnetization reduction on the Co atoms that are not in direct contact with Co. If a CO molecule is put on every surface atom in the atop position (CO /Co ), the whole cluster becomes para magnetic. We studied also analogous CO arrangements on the icosahedral Co cluster. The results are virtually identical. It is clear from the above results that CO in the atop position has serious but highly localized e!ect on the Co magnetization. That is why for the Co cluster we con sider arrangement with CO only on all outermost Co atoms (CO /Co ). The magnetization on the outermost Co atoms is practically killed (!0.05 l ) and couples antiferromagnetically to deeper atoms. In contrast to the CO /Co case the perturbation of inner Co atoms is not large and has an oscillatory character. To conclude, oxygen in `normala adsorption geometry has no severe e!ect on Co magnetization. If its presence is responsible for experimental observations in Ref. [2], the penetration of oxygen into clusters or oxide formation should be considered [2,3,6]. CO in the atop, but not in the bridge position can lead to a large surface-magnetization quenching. When the local magnetic moment becomes very small it has tendency to antiferromagnetic orientation with respect to its neighbours that are not in direct contact with CO. With the exception of quite small clusters CO /Co , we expect that the e!ect has a local character and the cluster core is not strongly in#uenced. Similar conclusion has also been reached experimentally [6]. Trends we obtain for CO chemisorption on strongly magnetized Co particles compare well with results for the comparatively weakly magnetic nickel clusters [8].
This study was facilitated by the common Project Scheme of Centre National de la Recherche Scienti"que and Academy of Sciences of the Czech Republic. S[ P acknowledges support by the Grant agency of the Czech Republic, No. 203/96/0951.
314
S[ . Pick, H. Dreysse& / Journal of Magnetism and Magnetic Materials 198}199 (1999) 312}314
Note added in proof Details can be found in Ref. [17]. References [1] S. Sako, K. Oshima, M. Sakai, S. Bandow, Surf. Rev. Lett. 3 (1996) 109. [2] F. Parent, Thesis, UniversiteH de Paris-Sud, 1996 (unpublished). [3] T. Ould Ely, C. Amiens, B. Chaudret, E. Snoeck, M. Verelst, P. Lecante, M. Respaud, J.-M. Broto, A. Fert, Proceedings of the "fth Colloque Louis NeH el, Banyuls sur Mer, June 1997, paper IV-P2. [4] D.A. Eastham, Y. Quing, T.H. Maddock, J. Kraft, J.P. Schille, G.S. Thompson, H. Haberland, J. Phys.: Condens. Matter 9 (1997) L497. [5] Z.Q. Li, B.L. Gu, Phys. Rev. B 47 (1993) 13611. [6] M. Respaud, J.M. Broto, H. Rakoto, A.R. Fert, L. Thomas, B. Barbara, M. Verelst, E. Snoeck, P. Lecante, A Mosset,
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
J. Osuna, T. Ould Ely, C. Amiens, B. Chaudret, Phys. Rev. B 57 (1998) 2925. S[ . Pick, H. DreysseH , Surf. Sci. 394 (1997) 192. G. Pacchioni, N. RoK sch, Acc. Chem. Res. 28 (1995) 390. S[ . Pick, Phys. Rev. B 57 (1998) 1942. M.C. Zonnevylle, J.J.C. Geerlings, R.A. van Santen, J. Catal. 148 (1994) 412. M. Respaud, Ph.D. Thesis, Institut National des Sciences AppliqueH s de Toulouse, 1997 (unpublished). M. Verelst, private communication, 1998. H. Papp, Surf. Sci. 129 (1983) 205. M. Getzla!, J. Bansmann, G. SchoK nhense, J. Chem. Phys. 103 (1995) 6691. V. Albano, P. Chini, V. Scatturin, J. Organomet. Chem. 15 (1968) 423. V.G. Albano, P.L. Bellon, P. Chini, V. Scatturin, J. Organomet. Chem. 16 (1969) 461. S[ . Pick, H. DreysseH , Phys. Rev. B 59 (1999) 4195.