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Register a new userControlled co-excitation of direct and indirect ultrafast demagnetization in Co/Pd multilayer with large perpendicular magnetic anisotropy
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Ever since its discovery in 1996, ultrafast demagnetization has ignited immense research interest due to its scientific rigor and technological potential. A flurry of recent theoretical and experimental investigations has proposed direct and indirect excitation processes in separate systems. However, it still lacks a unified mechanism and remains highly debatable. Here, for the first time, we demonstrate that instead of either direct or indirect interaction, simultaneous and controlled excitation of both direct and indirect mechanisms of demagnetization are possible in a multilayers composed of repeated Co/Pd bi-layers. Moreover, we were able to modulate demagnetization time (from ~350 fs to ~750 fs) by fluence and thickness dependent indirect excitation due to heat current flowing vertically downward from top layers, which is combined with an altogether different scenario of direct irradiation. Finally, by regulating the pump wavelength we could effectively control the contribution of indirect process, which gives a confirmation to our understanding of the ultrafast demagnetization process.
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We have studied the magnetic properties of multilayers composed of ferromagnetic metal Co and heavy metals with strong spin orbit coupling (Pt and Ir). Multilayers with symmetric (ABA stacking) and asymmetric (ABC stacking) structures are grown to study the effect of broken structural inversion symmetry. We compare the perpendicular magnetic anisotropy (PMA) energy of symmetric Pt/Co/Pt, Ir/Co/Ir multilayers and asymmetric Pt/Co/Ir, Ir/Co/Pt multilayers. First, the interface contribution to the PMA is studied using the Co layer thickness dependence of the effective PMA energy. Comparison of the interfacial PMA between the Ir/Co/Pt, Pt/Co/Ir asymmetric structures and Pt/Co/Pt, Ir/Co/Ir symmetric structures indicate that the broken structural inversion symmetry induced PMA is small compared to the overall interfacial PMA. Second, we find the magnetic anisotropy field is significantly increased in multilayers when the ferromagnetic layers are antiferromagnetically coupled via interlayer exchange coupling (IEC). Macrospin model calculations can qualitatively account for the relation between the anisotropy field and the IEC. Among the structures studied, IEC is the largest for the asymmetric Ir/Co/Pt multilayers: the exchange coupling field exceeds 3 T and consequently, the anisotropy field approaches 10 T. Third, comparing the asymmetric Ir/Co/Pt and Pt/Co/Ir structures, we find the IEC and, to some extent, the interface PMA are stronger for the former than the latter. X-ray magnetic circular dichroism studies suggest that the proximity induced magnetization in Pt is larger for the Ir/Co/Pt multilayers than the inverted structure, which may partly account for the difference in the magnetic properties. These results show the intricate relation between PMA, IEC and the proximity induced magnetization that can be exploited to design artificial structures with unique magnetic characteristics.
We use time-resolved x-ray resonant magnetic scattering (tr-XRMS) at the Co M$_{2,3}$- and Tb O$_1$-edges to study ultrafast demagnetization in an amorphous Co$_{88}$Tb$_{12}$ alloy with stripe domains. Combining the femtosecond temporal with nanometer spatial resolution of our experiment, we demonstrate that the equilibrium spin texture of the thin film remains unaltered by the optical pump-pulse on ultrashort timescales ($<$1 ps). However, after $simeq$ 4 ps, we observe the onset of a significant domain wall broadening, which we attribute to a reduction of the uniaxial magnetic anisotropy of the system, due to energy transfer to the lattice. Static temperature dependent magnetometry measurements combined with analytical modeling of the magnetic structure of the thin film corroborate this interpretation.
Current induced spin-orbit torques have been studied in ferromagnetic nanowires made of 20 nm thick Co/Pd multilayers with perpendicular magnetic anisotropy. Using Hall voltage and lock-in measurements, it is found that upon injection of an electric current both in-plane (Slonczewski-like) and perpendicular (field-like) torques build up in the nanowire. The torque efficiencies are found to be as large as 1.17 kOe and 5 kOe at 108 A/cm2 for the in-plane and perpendicular components, respectively, which is surprisingly comparable to previous studies in ultrathin (~ 1 nm) magnetic bilayers. We show that this result cannot be explained solely by spin Hall effect induced torque at the outer interfaces, indicating a probable contribution of the bulk of the Co/Pd multilayer.
Depth-grading of magnetic anisotropy in perpendicular magnetic media has been predicted to reduce the field required to write data without sacrificing thermal stability. To study this prediction, we have produced Co/Pd multilayers with depth-dependent Co layer thickness. Polarized neutron reflectometry shows that the thickness grading results in a corresponding magnetic anisotropy gradient. Magnetometry reveals that the anisotropy gradient promotes domain nucleation upon magnetization reversal - a clear experimental demonstration of the effectiveness of graded anisotropy for reducing write-field.
Magnetometry and neutron scattering have been used to study the magnetic properties of pressure graded Co/Pd multilayers. The grading of the multilayer structure was done by varying the deposition pressure during sputtering of the samples. Magnetic depth profiling by polarized neutron reflectometry directly shows that for pressure-graded samples, the magnetization changes significantly from one pressure region to the next, while control samples sputtered at uniform pressure exhibit essentially uniform magnetic depth profiles. Complementary magnetometry results suggest that the observed graded magnetic profiles are due in part to a decrease in saturation magnetization for regions deposited at progressively higher pressure. Increased deposition pressure is shown to increase coercivity, and for graded samples, the absence of discrete steps in the hysteresis loops implies exchange coupling among regions deposited at different pressures.
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