Stress oscillations and surface alloy formation during the growth of FeMn on Cu„001… Wei Pan, 1,2 Dirk Sander, 2 Minn-Tsong Lin, 1, * and Ju ¨ rgen Kirschner 2 1 Department of Physics and Center for Nanostorage Research, National Taiwan University, Taipei106, Taiwan 2 Max-Planck-Institut fu ¨r Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany Received 16 May 2003; published 19 December 2003 In situ stress and medium-energy electron-diffraction MEED measurements have been performed simul- taneously during the deposition of FeMn on Cu001. For a thickness above 5 layers, stress and MEED exhibit coherent layer-by-layer oscillations with a period of one atomic layer, where the largest compressive stress corresponds to the filled layer. In this thickness regime, the average stress is -0.590.02 GPa. From this, we deduce the biaxial modulus of FeMn layers as 148 ( 5) GPa, which agrees well with the respective bulk value. For a FeMn thickness below 1.5 layers, the resulting stress is qualitatively ascribed to the sum of the individual stress contributions from Fe on Cu001 and Mn on Cu001.A c (2 2) low-energy electron diffraction pattern in this thickness regime indicates the formation of a c (2 2) MnCu surface alloy in the initial growth of FeMn on Cu001, which induces a compressive surface stress of -0.7 N/m for the initial deposition of the FeMn alloy. This surface alloy formation leads to a Fe-rich FeMn alloy near the Cu interface. This compositional change might modify the antiferromagnetic coupling of the 1:1 FeMn alloy. DOI: 10.1103/PhysRevB.68.224419 PACS numbers: 75.70.-i, 68.35.Gy, 68.35.Ct, 68.55.-a I. INTRODUCTION Fe x Mn 1 -x alloys are prototypes for antiferromagnetic ma- terials with a high Ne ´ el temperature T N . 1 FeMn thin films are widely used in a spin-valve structure in combination with magnetoresistive materials for application in magnetic sen- sors and magnetic data storage. 2 The lattice constant of the Fe x Mn 1 -x alloys us depends on the alloy composition x. 1 This allows to choose a composition x, which leads to a moderate misfit between alloy film and substrate. For the 1:1 composition, i.e., x =0.5, the lattice constant of bulk FeMn is 3.629 Å. 1 This alloy has a face-centered cubic structure and its deposition on Cu001 could lead to epitaxially ordered films, as the misfit between FeMn and Cu is small and it amounts to  =( a Cu -a FeMn )/( a FeMn ) =-0.4 %. This paper focuses on combined stress measurements and diffraction experiments to correlate atomic structure and re- sulting stress in the thickness range from 0 to 18 layers of FeMn on Cu001. We find that in contrast to a simplistic growth model where FeMn alloy layers grow pseudomorphi- cally strained on Cu001, the formation of a surface alloy between Mn and Cu is clearly observed by the stress and diffraction experiments. Only for an alloy thickness above 5 layers, we find experimental evidence from stress oscilla- tions that the alloy grows in a layer-by-layer fashion. The formation of the MnCu surface alloy has important conse- quences for the magnetic properties of ultrathin FeMn films, as due to the surface alloy formation, Mn atoms are incorpo- rated into the Cu surface. This intermixing might affect the antiferromagnetic coupling near the FeMn-Cu interface. II. EXPERIMENT The experiment was performed in an ultrahigh vacuum chamber with a base pressure of 1 10 -10 mbar. A 124  m thin Cu001 crystal width: 3 mm, length: 12 mm was used as a substrate. The long edge of the crystal was oriented along the 110 direction. It was clamped at one end along its width, and the other end was free. This mounting allows us to measure the stress during FeMn deposition by measuring the change of curvature of the crystal during film growth. 3 The surface of the Cu001 was cleaned by Ar + sputtering and followed by short annealing to 720 K 30 s by radiation from a W shield, which was heated by a W filament. The surface cleanliness was checked by Auger electron spectros- copy AES and sharp diffraction spots of low-energy electron-diffraction LEED indicated good crystalline order- ing. The FeMn alloy was grown by codeposition from indi- vidual Fe and Mn evaporators. 4,5 The purity of Mn and Fe is 99.5% and 99.99% at.%, respectively. The deposition rate of the alloy was set to a 1:1 atomic ratio by adjusting the evaporation rate of each evaporator, as confirmed by indi- vidual MEED measurements and by AES. 6–9 The error for the alloy composition is estimated to be 5%. Both evapo- rators were adjusted to give a deposition rate close to 0.25 ML/min, which results in a growth rate of the FeMn alloy of 0.5 ML/min. Here ML stands for monolayer. The stress measurement was carried out during film growth at room temperature. The mechanical stress in the alloy film  f induces a change of curvature of the crystal  (1/R ),  (  f t f ) = Yt s 2 /6(1 - )   (1/R ), where t f and t s are the thicknesses of the alloy film FeMn and the substrate Cu, respectively, Y and  are the Young’s modulus and Poisson ratio of Cu001, and R is the radius of the curvature. 10,11 The slope of the curvature signal as a function of film thickness gives the film stress  f in gigapascal. The curvature was obtained by a highly sensitive optical beam deflection technique, which is schematically shown in Fig. 1. Two laser beams are reflected from two points of the surface, which are separated by a distance d, onto two position- sensitive split-photodiode detectors. The detectors are mounted at a distance L away from the substrate. Thus the curvature  (1/R ) is derived from the difference of the posi- tion signal as  (1/R ) =(  P 1 - P 2 )/2dL , where  P 1 and  P 2 are the changes of the position signals for the bottom PHYSICAL REVIEW B 68, 224419 2003 0163-1829/2003/6822/2244195/$20.00 ©2003 The American Physical Society 68 224419-1