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Cobalt-based full-Heusler compounds with composition Co2M′ Z (where M ′ is a transition metal and Z is a main group element) are attracting attention due to their predicted half-metallic behaviour, a considerably desired property for spin-dependent electron transport devices. Knowledge of the basic magnetic properties of these materials, in particular in the form of thin films, is required both to fully exploit these promising materials, and to understand their underlying electronic structure and establish structure-property relationships. In this Topical Review, we present a survey of the magnetic anisotropy, exchange, and damping of Co2M ′ Z compounds. These properties are directly related to spin-spin and spin-orbit interactions.
We report on the influence of the improved L2 1 ordering degree on the magnetic properties of Co 2 MnSi Heusler films. Different fractions of the L2 1 phase are obtained by different post-growth annealing temperatures ranging from 350 • C to 500 • C. Room temperature magneto-optical Kerr effect measurements reveal an increase of the coercivity at an intermediate annealing temperature of 425 • C, which is a fingerprint of an increased number of pinning centers at this temperature.Furthermore, Brillouin light scattering studies show that the improvement of the L2 1 order in the Co 2 MnSi films is correlated with a decrease of the saturation magnetization by about 9 %. The exchange stiffness constant of Co 2 MnSi, however, increases by about 8 % when the L2 1 order is improved. Moreover, we observe a drop of the cubic anisotropy constant K 1 by a factor of 10 for an increasing amount of the L2 1 phase.
Co 2 FeSi(100) films with L2 1 structure deposited onto MgO(100) were studied exploiting both longitudinal (LMOKE) and quadratic (QMOKE) magneto-optical Kerr effect. The films exhibit a huge QMOKE signal with a maximum contribution of up to 30 mdeg, which is the largest QMOKE signal in reflection that has been measured thus far. This large value is a fingerprint of an exceptionally large spin-orbit coupling of second or higher order. The Co 2 FeSi(100) films exhibit a rather large coercivity of 350 and 70 Oe for film thicknesses of 22 and 98 nm, respectively. Despite the fact that the films are epitaxial, they do not provide an angular dependence of the anisotropy and the remanence in excess of 1% and 2%, respectively.
Brillouin light scattering spectroscopy from so-called standing spin waves in thin magnetic films is often used to determine the magnetic exchange constant. The data analysis of the experimentally determined spin-wave modes requires an unambiguous assignment to the correct spin wave mode orders. Often additional investigations are needed to guarantee correct assignment. This is particularly important in the case of Heusler compounds where values of the exchange constant vary substantially between different compounds. As a showcase, we report on the determination of the exchange constant (exchange stiffness constant) in Co 2 MnSi, which is found to be A = 2.35 ± 0.1 µerg/cm (D = 575 ± 20 meVÅ 2 ), a value comparable to the value of the exchange constant of Co.
Magnetic anisotropies and magnetization reversal properties of the epitaxial Heusler compound Co 2 Cr 0.6 Fe 0.4 Al (CCFA) deposited on Fe and Cr buffer layers are studied. Both samples exhibit a growth-induced fourfold anisotropy, and magnetization reversal occurs through the formation of stripy domains or 90 • domains. During rotational magnetometric scans the sample deposited on Cr exhibits about 2 • sharp peaks in the angular dependence of the coercive field, which are oriented along the hard axis directions. These peaks are a consequence of the specific domain structure appearing in this particular measurement geometry. A corresponding feature in the sample deposited on Fe is not observed.
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