Unusual Magnetic Properties of Fe/Ni Bilayers on Cu(100)
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Unusual Magnetic Properties of Fe/Ni Bilayers on Cu(100) B. Schirmer, X. Liu and M. Wuttig I. Physikalisches Institut der RWTH Aachen, D-52056 Aachen, Germany and Institut für Grenzflächenforschung und Vakuumphysik, Forschungszentrum Jülich, D-52428 Jülich, Germany ABSTRACT Ultrathin iron films grown on Cu(100) have been found to exhibit a rich variety of structural and magnetic phases. In the present work, Fe/Ni bilayers have been prepared by molecular beam epitaxy to explore novel magnetic phenomena introduced by the ferromagnetic (FM) Ni underlayer. Unusual properties have been observed by measuring the temperature dependent magnetic properties. For 5.3 ML Fe on 7 ML Ni, a temperature dependent exchange coupling in the Fe film has been observed between the FM surface layer and FM interface layer. INTRODUCTION A close correlation between structure and magnetism has been revealed in ultrathin Fe films on Cu(100). For the room temperature grown ultrathin films, two different fcc iron phases are stabilized on Cu(100)[1]. The first one exists for iron layer thickness up to 5 monolayers (ML). This iron phase is characterized by a ferromagnetic order and an enlarged atomic volume of 12.1 Å3 with (4×1) or (5×1) superstructure. For iron films on Cu(100) with a thickness between 5 and 11 ML, only the first two layers are coupled ferromagnetically and show an atomic volume of 12.1 Å3. The interior part of the film has an atomic volume of 11.4 Å3 and presumably shows antiferromagnetic (AFM) coupling [1,2,3]. In this case the (2×1)pm2g phase is the frequently observed superstructure. The transition to the stable bcc ground state of iron is observed above 11 ML [4]. On the other hand, ultrathin Ni films can be pseudomorphically grown on Cu(100) below 11 ML [5]. Such a situation is ideal to explore the possible role of a magnetic interface on the structure and magnetism of thin films by investigating Fe/Ni bilayers on Cu(100). Indeed, we have discovered several unusual magnetic phenomena by this strategy. A temperature dependent magnetic coupling, which can be FM or AFM in different temperature ranges, was found to exist between the Fe surface magnetic live layer and the Fe/Ni interface in 5.3 ML Fe/7 ML Ni/Cu(100) films. EXPERIMENTAL DETAILS The experiments were performed in an ultrahigh vacuum chamber equipped with several facilities for the preparation and characterization of thin films including Auger electron spectroscopy (AES), medium energy electron diffraction (MEED), LEED, and magneto-optic Kerr ellipsometry. The base pressure of the chamber is 6×10-9 Pa. Prior to film deposition the copper (100) crystal was cleaned by Ar sputtering and annealing cycles. The films of iron and nickel were deposited from small disks of high purity (Fe 99.99%, Ni 99.98%) by thermal evaporation with a typical evaporation rate of 0.3 ML/min. During the evaporation the residual gas pressure was below 2×10-8 Pa. The thickness of the films was controlled by AES and MEED oscillations, which allow a high precision in thickness determination. For
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