ABSTRACT
The temperature (T) dependences of magnetization dynamics, especially for magnetic damping anisotropy, have been systematically investigated in well-ordered Co2FeAl films with a biaxial anisotropy. It is found that the damping anisotropy factor Q, defined as the fractional difference of damping between the hard and easy axes, changes from 0.35 to -0.09 as T decreases from 300 to 80 K, performing a distinctive reorientation transition at T â¼ 200 K. Through the thickness-dependent damping measurement results, the damping anisotropy reorientation is verified to originate from the competitions between the intrinsic anisotropic distribution of bulk spin orbit coupling and the interfacial two-magnon scattering. The former governs the effective damping at high temperatures, while the latter with an opposite fourfold symmetry gradually plays a dominant role at reduced temperatures, leading to the transition of the Q value from positive to negative. The clear clarification of damping anisotropy variation as well as the underlying mechanism in this study would be of great importance for designing key spintronic devices with optimized dynamic magnetic properties.
ABSTRACT
Magnetization dynamics of the epitaxially-grown Co2FeAl (CFA) thin films have been systematically investigated by the time-resolved magneto-optical Kerr effect (TR-MOKE). The dependences of precession frequency f, relaxation time τ and magnetic damping factor α upon the orientation of applied magnetic field are found to have a strong four-fold symmetry. Two series of samples with various substrate temperatures (Ts) and thickness (tCFA) were prepared and a large Gilbert damping difference between the hard and easy axes is extracted to be 3.3 × 10-3 after subtracting the extrinsic contributions of spin pumping, two-magnon scattering and magnetic inhomogeneities. The four-fold variation of Gilbert damping relates closely to the in-plane magnetocrystalline anisotropy and can be attributed to the anisotropic distribution of spin-orbit coupling. Our findings provide new insights into the anisotropic properties of magnetization and damping, which is very helpful for designing and optimizing advanced spintronic devices on different demands.
ABSTRACT
High frequency magnetic precessions with strong intensity are strongly desired in material systems for high performance magnetic memory or nano-oscillator applications with ultrafast manipulation speed. Here, we demonstrate an exchange-coupled asymmetric composite film structure of Ta/Pd/[Pd/Co]5/Cu(tCu)/[Co/Ni]5/Ta with adjustable strong perpendicular magnetic anisotropy and interlayer coupling strength, in which the dynamic magnetic properties are systematically studied by using time-resolved magneto-optical Kerr effect spectroscopy. It is demonstrated that the in-phase precession frequency is between those of the single hard magnetic [Pd/Co]5 and soft [Co/Ni]5 multilayers, which can be significantly enhanced for the strongly coupled case at tCu < 1 nm. Moreover, in the weakly coupled samples with tCu = 1.0-3.0 nm, besides the common in-phase acoustic mode, an out-of-phase optical mode occurs simultaneously with a frequency even higher than that of the hard magnetic [Pd/Co]5 layer. The optical mode precession frequency and amplitude show an unusual non-monotonic variation trend with the increase of tCu, which has been theoretically analyzed and attributed to the co-effect of decreased coupling strength and increased magnetic anisotropy field difference between the two multilayer stacks. Moreover, by adjusting tCu and the [Co/Ni] repetition number N, an optical mode of strong intensity can be actively achieved, even reaching 80% as compared to the acoustic mode. These results provide effective control and better understanding of magnetic dynamics in perpendicular composite films, which are of key importance for developing ultrafast spintronics-based devices.
