Magnetic and structural properties of rf-sputtered Co/Fe and Co/Cr multilayers

Abstract

Multilayers alternating cobalt with different spacers such as iron and chromium at nanometric scale have been deposited by rf diode sputtering. The structures have been characterized in situ by kinetic ellipsometry and ex situ by grazing x-ray reflection, x-ray diffraction, Auger profile analysis, and transmission electron microscopy. Nuclear magnetic resonance (NMR) and high-field SQUID magnetometry have been used to determine the magnetic properties. In Co/Fe multilayers the structure strongly depends on the cobalt thickness tCo. For tCo below 2 nm, the cobalt layers exhibit a bcc crystalline structure, and the crystalline coherence extends over many periods. For tCo above 2 nm, for Co a mixed fcc-hcp phase has been observed. In this case the crystalline coherence is destroyed from one iron layer to the other. The NMR frequency (198 MHz), and the magnetic moment (1.58 μB/atom) of bcc cobalt have been evaluated. A similar behavior is observed in Co/Cr multilayers, the bcc phase disappearing around tCo=1.5 nm. Moreover, interdiffusion is observed at the Cr-on-Co interface: The samples have zero magnetization up to tCo=0.7 nm. Preliminary experiments show some antiferromagnetic coupling between Co layers. <lz>

Growth of epitaxial bcc Co(001) electrodes for magnetoresistive devices

“…11,12,[22][23][24][25][26] The fact that thicker layers were not obtainable agrees well with the theoretical prediction that bcc Co is not metastable, but strain induced. 16 We also tried to grow bcc Co directly on GaAs͑001͒ instead of on an Fe buffer layer.…”

“…NMR studies of bcc Co grown on Cr͑110͒, 11 Fe͑110͒, 11,22 Au, 12 and MnAl ͑Ref. 26͒, show that in zero applied field, pure bcc Co is characterized by a resonance line at a frequency of 199 MHz, with a full width at half maximum ͑FWHM͒ of about 10-15 MHz for well-grown layers.…”

“…less than 3 nm͒ for various substrates ͑including GaAs͒ and crystal orientations. 11,12,[22][23][24][25][26] This implicates that the stability of the grown layers has to be checked carefully every time. In Sec.…”

“…11,12,[22][23][24][25][26] The exact value depends sensitively on the growth method, temperature and speed, and on the specific substrate. The most frequently used substrates, apart from GaAs, are Fe and Cr.…”

“…The first report of bcc Co, made by Prinz, 10 used growth on GaAs͑110͒. Later, bcc Co was stabilized in between Fe layers and Cr layers, 11 in between Au layers, 12 and on some alloy substrates such as TiAl 13 and CoSi 2 /Si(001). 14 In the following section, we will give a short overview of the reports on bcc Co in the literature.…”