Quantitative Analysis Of Polarized Neutron Specular Reflectivity From a Co/Cu(111) Superlattice at the Second Antiferrom
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QUANTITATIVE ANALYSIS OF POLARIZED NEUTRON SPECULAR REFLECTIVITY FROM A Co/Cu(111) SUPERLATTICE AT THE SECOND ANTIFERROMAGNETIC MAXIMUM J.F. Ankner*, A. Schreyer-, C.F. Majkrzak*, K. Br6hl**, Th. Zeidler**, P. B6deker**, and H. Zabel** *National Institute of Standards and Technology, Gaithersburg, MD 20899 **Ruhr Universitit Bochum, D-4630 Bochum, Federal Republic of Germany
ABSTRACT We have used polarized neutron reflectivity to prove the existence of oscillatory coupling in MBE-grown Co/Cu(lll) superlattices. For a sample of composition [3.4 nm Co/2.0 nm Cu]10 , we find that only a small fraction of the sample volume exhibits coherent antiferromagnetic order, with the structure of the remainder of the film being attributable to ferromagnetic domains and small disordered structures.
THE Co/Cu(111) CONTROVERSY The magnetic coupling of ferromagnetic cobalt layers across nonmagnetic copper interlayers is an extensively studied phenomenon and consequently the field is characterized by a certain amount of disharmony. The first observations of antiferromagnetic (AF) coupling [1] and coupling dependent on Cu interlayer thickness [2] were made on films grown in the (100) orientation (cobalt assumes the fcc structure of copper). Oscillatory coupling with a period of about 1.0 nm was first seen [3, 4] in sputtered (111) multilayers. Subsequent attempts to observe oscillations in single-crystal (111) superlattices grown by molecular-beam epitaxy (MBE) have generally yielded negative or ambiguous results [5-8]. A recent paper [9] reports oscillatory coupling between isolated cobalt monolayers in a copper matrix, but with a much longer period than in the sputtered multilayers. We have grown a series of Co/Cu(111) superlattices by MBE and have measured oscillations in saturation field as a function of Cu interlayer thickness with peaks around dcu = 1.0,1.9, and 2.8 nm [10], in agreement with theoretical predictions [11]. Similar results have subsequently been obtained by Dupas, et al [12] from magnetoresistance measurements. Using polarized neutron diffraction, we have determined that the films exhibit coherent AF order at the first two maxima and no measurable AF order at an intermediate thickness [10]. In this paper, we will present a detailed investigation of the nature of the magnetic order present in a superlattice with dc, = 2.0 nm and characterize this order by means of quantitative refinement of polarized neutron specular reflectivity data. Combining this information with x-ray and magneto-optic Kerr effect (MOKE) measurements, we will comment on the influence of structural disorder on the observed magnetic behavior.
MEASURING MAGNETIC STRUCTURE The nature of the magnetic order in Co/Cu(111) films grown by molecular-beam epitaxy (MBE) has been controversial because of the coexistence of remanent ferromagnetism with a generally small or nonexistent antiferromagnetic volume fraction. Our samples were grown by MBE on A12 0 3 (1120) substrates with a Nb(110) buffer layer (see refs. [10, 13, 14] for details). The MOKE loo
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