Polarized Neutron Reflectivity Measurements of Collinear and Non-Collinear Magnetic Structures in Fe/Cr(100) Superlattic
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POLARIZED NEUTRON REFLECTIVITY MEASUREMENTS OF COLLINEAR AND NON-COLLINEAR MAGNETIC STRUCTURES IN Fe/Cr(100) SUPERLATTICES J.F. Ankner*, A. Schreyer*, Th. Zeidler**, C.F. Majkrzak*, H. Zabel**, J.A. Wolft, and P. Griinbergt National Institute of Standards and Technology, Gaithersburg, MD 20899 **Ruhr Universitat Bochum, D-4630 Bochum, Federal Republic of Germany tForschungszentrum Jillich, D-5170 Jiilich, Federal Republic of Germany
ABSTRACT We have used polarized neutron reflectivity to study the magnetic microstructure of two Fe/Cr(100) superlattices grown by molecular beam epitaxy. The first film, of nominal composition [0.9 nm Cr/5.5 nm Fe]s, and atomically sharp interfaces, exhibits classical collinear antiferromagnetic structure. The other film ([1.2 nm Cr/5.5 nm Fe]5), grown at a higher temperature, shows the field-dependent non-collinear features attributed to biquadratic coupling. We will describe these measurements and our preliminary structural analysis. HISTORY AND MOTIVATION The Fe/Cr system possesses a rich history and has served as the point of discovery for most of the novel aspects of magnetic coupling in transition-metal multilayers, wedges, and films. The observations of the antiferromagnetic coupling of iron across a chromium interlayer [1], the giant magnetoresistive effect [2, 3], and oscillatory interlayer coupling [4, 5] preceded similar discoveries in other systems and have spawned a large industry devoted to the study of transition-metal layers. Recent work has maintained this trend, with the measurement of oscillations with a period of two Cr layers [6-8] and non-collinear ("biquadratic") coupling [9]. We present polarized neutron reflectivity data from samples exhibiting both simple collinear antiferromagnetic order and noncollinear structure. Other workers have studied the Fe/Cr system using neutrons [10-15], but have not drawn a clear distinction between collinear and non-collinear structure. The samples used in this study were prepared by molecular-beam epitaxy (MBE) on the same GaAs/Fe/Ag substrate-buffer system described in ref. [9]. Instead of wedge structures, two superlattices of nominal composition [z nm Cr/5.5 nm Fe]s, with x = 0.9 and 1.2 nm, were grown at T = 293 and 523 K, respectively (see refs. [16-18] for details). In the following sections, we will present a qualitative comparison of the two different types of structure by means of magneto-optic Kerr effect and neutron reflectivity measurements. POLARIZED-NEUTRON REFLECTIVITY By measuring the specular reflectivity of neutrons, one can determine the layer-averaged, depth-dependent chemical and magnetic structure of planar samples. The theory of polarizedneutron reflectivity has been described by a number of authors [19-23], so we will simply outline it here. Figure 1 shows a beam of neutrons of wavelength Aincident onto and specularly reflected from a flat surface. The reflectivity depends on the specular glancing angle 0 and is conventionally described in terms of the wavevector transfer, Q = -(47r sin 0/A)\, where i is
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