Fe-porphyrin monolayers on ferromagnetic substrates: Electronic structure and magnetic coupling strength M. Bernien, X. Xu,* J. Miguel, M. Piantek, Ph. Eckhold, J. Luo, J. Kurde, W. Kuch, and K. Baberschke Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany H. Wende † and P. Srivastava ‡ Fachbereich Physik, Experimentalphysik-AG Wende, Universität Duisburg-Essen, Lotharstrasse 1, D-47048 Duisburg, Germany Received 21 June 2007; revised manuscript received 25 August 2007; published 11 December 2007 In this work, the different coupling strengths of Fe-octaethylporphyrin molecules sublimated in situ onto epitaxially grown Co and Ni films on Cu100 are studied by systematic x-ray-absorption and x-ray magnetic circular dichroism XMCD measurements. The fine structures observed in the absorption and dichroic spectra of the Fe-L 2,3 edges are explained in terms of different Fe 3d orbitals probed in angular-dependent measure- ments. The coverage of Fe-porphyrin molecules is varied from submonolayer to 1.5 ML monolayer, and measurements were carried out at room as well as at low temperatures. From the temperature dependence of the Fe-XMCD signal, a weaker coupling strength between the Fe atom and the Ni substrate is found as compared to the Co case. DOI: 10.1103/PhysRevB.76.214406 PACS numbers: 75.70.-i, 78.70.Dm I. INTRODUCTION Porphyrins have attracted the attention of the scientific community for a very long time now. Initially, this interest originated from their biological significance and catalytic ca- pabilities. Lately, 3d metal porphyrins have generated much interest due to their structural and magnetic properties. With technological advancement in various experimental tech- niques and theoretical codes, there is a substantial increase in the research efforts in this area. 1–11 In addition, recent devel- opments in the field of surface magnetism 12 and possible application of specific porphyrins in molecular spintronics as switchable magnetic molecules 13 have given a new dimen- sion to the studies that investigate their electronic structure and magnetic properties. In the recent past, porphyrins have been investigated with different techniques. 1–6 Electronic structure calculations have also been performed for bulk samples and free molecules. 7–11,14 However, if one wants to focus on the mag- netic properties of porphyrins, a technique such as x-ray magnetic circular dichroism XMCD has unique advantages over other techniques because of its element specificity and monolayer sensitivity. Furthermore, the application of sum rules to the XMCD 15,16 is nowadays widely used to deter- mine the local orbital and spin angular moments. The induced magnetic ordering in a molecular monolayer of MnIII-tetraphenylporphyrin-Cl on a thin Co film sub- strate was studied using XMCD. 6 In contrast to the Mn case, the application of sum rules for Fe is very reliable. 12,17 Re- cently, we have investigated the structural and magnetic properties of FeIII-octaethylporphyrinOEP-Cl molecules prepared by sublimation on ferromagnetic substrates using x-ray-absorption XAS and XMCD measurements. 18 We ob- served that the Fe magnetic moment can be switched in plane or out of plane by a magnetization reversal of the sub- strate. Ab initio electronic structure calculations identified a 90° super exchange coupling of the Fe ion to the surface atoms through the N ligands. In the present work, other aspects of the magnetic cou- pling of Fe-OEP molecules to Co and Ni ferromagnetic sub- strates have been tackled. We have carried out systematic XAS and XMCD investigations by preparing different cov- erages submonolayer to 1.5 ML monolayer of these mol- ecules on Co and Ni thin films. Single and double ferromag- netic layers in the range of a few atomic MLs were deposited on Cu100 to have the possibility of aligning the magneti- zation easy axis in plane and out of plane. We discuss the contribution of the different Fe 3d orbitals to the fine struc- tures observed in the absorption and dichroic spectra. In addition, temperature-dependent measurements were per- formed in order to quantify the coupling strength between Fe-OEP molecules and both substrates. They were used to estimate the coupling energies for the Co and Ni substrates. II. EXPERIMENT The sample preparation was carried out in ultrahigh vacuum conditions  p = 2.0  10 -10 mbar. The ultrathin Co and Ni films were epitaxially grown by electron-beam evaporation on a Cu100 single crystal, which was cleaned by repeated Ar + sputtering and annealing cycles. The Co and Ni layer thicknesses were chosen to be 5 and 15 ML, respec- tively, in order to obtain in-plane and out-of-plane easy mag- netization axes. A combined 5 ML Ni/5 ML Co bilayer was also prepared, displaying in-plane magnetization. In order to reduce the roughness of the Ni films, they were annealed to 420 K prior to the deposition of the molecules. The porphyrin molecules 2,3,7,8,12,13,17,18- octaethylporphyrin-FeIII chloride were adsorbed on the ferromagnetic films at room temperature by sublimating Fe- OEP-Cl molecules at 485 K. The thicknesses of the porphy- rin and ferromagnetic films were determined by a quartz mi- crobalance and medium-energy-electron-diffraction intensity oscillations, respectively, and were cross-checked by the signal-to-background ratio edge jump at the respective x-ray-absorption edges. PHYSICAL REVIEW B 76, 214406 2007 1098-0121/2007/7621/2144066 ©2007 The American Physical Society 214406-1