E. Vélu

Very large magnetoresistance effects induced by antiparallel magnetization in two ultrathin cobalt films

Magnetoresistance (MR) and magnetization measurements by SQUID have been performed on Au/Co.Au/Co/Au multilayers in which the two ultrathin cobalt films have perpendicular magnetizations and different coercive fields Hc1 and Hc2 due to their different thicknesses. The two Co layers are magnetically uncoupled, as shown by their well separated square hysteresis loops. The MR exhibits a plateau for magnetic fields H such as Hc1<H<Hc2, giving antiparallel orientation of their magnetization. The δR/R value is 1.4% at 300 K and 6% at 4.2 K. These experiments confirm the mechanism of spin‐dependent diffusion at the interfaces leading to an increase of the resistance when the two magnetic layers have antiparallel magnetizations.

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

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....

Inverse giant magnetoresistance (invited)

Inverse giant magnetoresistance (GMR) is obtained in multilayers alternating two ferromagnetic layers F1 and F2 with different asymmetry of spin scattering, α1≳1 and α2<1. This is clearly demonstrated in the simple spin‐valve system with perpendicular magnetization Fe1−xVx/Au/Co. With respect to Fe, the FeV alloys with x=0.18 and 0.29 exhibit an inversion of the spin scattering coefficients (αFe≳1, αFeV<1) due to the change of the densities of states at the Fermi level. The inverse MR of FeV/Au/Co is studied as a function of FeV layer thickness and temperature and compared to calculations based on the Camley–Barnas model. The data show that the inverse GMR is due to the bulk scattering within the FeV layer, which coexists with a substantial interface scattering favoring normal GMR.

Magnetoresistance of cobaltgold multilayers with an increasing number of periods

Abstract CoAu multilayers, eight in all, with between two and nine cobalt layers, were produced together in UHV by thermal evaporation. All cobalt layers were identically thick (0.8 nm) with easy magnetisation axis perpendicular to the film plane. The (111) gold interlayer thickness of 2.2 nm corresponds to an antiferromagnetic coupling [1]. Kerr rotation magneto-optics and magnetoresistance measurements under perpendicular magnetic field revealed separate coercive fields for each of the cobalt layers. The plateaux between two coercive fields represent different magnetisation direction configurations that could be identified by the magnetoresistance measurements. An adapted Camley—Barnas modelling of the multilayers with up to six cobalt layers was conducted which agreed well with experimental results and further confirmed the magnetisation reversal order deduced by the magnetoresistance measurements.

Magnetoresistance study of granular behaviour in Co / Au(111) ultrathin films

Abstract A study of the magnetoresistance (MR) of Au/Co( t Co )/Au and Au/Co( t Co )Au/Co/Au layers grown in UHV with one variable very small Co layer thickness, t Co , is reported. Large low-temperature MR for both sample structures was observed for t Co around the transition thickness between granular and continuous layers. A local minimum of MR was detected at t Co = 0.16 nm, corresponding to 1 atomic layer (AL) of Co epitaxially grown on Au(111).

Giant magnetoresistance of dissymmetrical Co/Au multilayers

Abstract Results are presented for the magnetoresistance (MR) of sapphire/Nb 3 /Cu 3 /Au 8 /(Co/Au 8 ) n dissymmetrical multilayers built by alternating a 0.3 nm discontinuous Co layer with a 0.7 nm continuous one. The observed enhanced MR is related to a higher spin scattering asymmetry for the granular Co layers.

Strong magnetoresistance reduction at the coercive‐field crossover of two uncoupled magnetic films

The variation of the magnetoresistance (MR) with temperature T in the range 1.4–300 K has been studied in three dissymmetric structures Au/M1/Au/M2/Au where M1 and M2 are two ultrathin films of either Co or Fe with perpendicular anisotropy. In the Fe sample, the low‐temperature MR is 7.6%, showing for the first time the spin‐valve effect in this system. In all samples, the variation of the coercive fields Hc1 and Hc2 vs T exhibits a crossover leading to a reduction by a factor one‐half of the MR at the same temperature. The fit of the MR curves versus T in the frame of the Camley–Barnas model leads to the values of the electron mean‐free‐path and spin‐dependent transmission coefficients at the interfaces in these samples.

Magnetoresistance of Co/Au multilayers as a function of the number of Co layers

The magnetoresistance (MR) of antiferromagnetically coupled and uncoupled Co/Au multilayers was studied as a function of the number of Co layers. For the antiferromagnetically coupled samples a complete antiparallel alignment of adjacent Co layers was not always observed [1] and hence the MR was sometimes not as large as could be expected. When the adjacent Co layers have different coercive fields the most antiparallel alignment of the magnetisation direction of adjacent Co layers is observed for an odd number of Co layers less than seven.

Magnetoresistance of ultrathin Co films grown in UHV on Au(111): Crossover from granular to continuous film behavior versus Co thickness (abstract)

Co/Au(111) is a model system for magnetoresistance (MR) studies due to abrupt interfaces without intermixing between Co and Au and perpendicular magnetization of Co films for Co thickness below nine atomic monolayers (AL). The high quality of Co/Au(111) multilayers is revealed by highly contrasted MR oscillations versus Au spacer thickness. We report here a detailed study of the effect of cobalt thickness done to 0.2 AL on the MR of various Au/Co structures. Experiments were performed on single Co layer sandwiched between two Au(111) films, Au/Co/Au, and on double Co films, Au/Co/Au/Co/Au, in the T range 1.5–300 K and in magnetic fields up to 10 T. For the sandwiches, two series were studied, each including eight samples grown in UHV in a single run, with Co thicknesses tCo from 0.2 to 1.6 and from 1.4 to 4 AL. The MR in the perpendicular and transverse configurations show that the easy magnetization axis is perpendicular to the films, even for low tCo. The MR value increases with tCo to the high value of...