For the first time, nanoporous Al2O3 templates with smoothed surface relief characterized by flattened interpore areas were used in the fabrication of Co/Pd and Co/Pt multilayers (MLs) with strong perpendicular magnetic anisotropy (PMA). Alternating gradient magnetometry (AGM) revealed perfectly conserved PMA in the Co/Pd and Co/Pt porous MLs (antidot arrays) with a ratio of remanent magnetization (Mr) to saturation magnetization (MS) of about 0.99, anisotropy fields (Ha) of up to 2.6 kOe, and a small deviation angle of 8° between the easy magnetization axis and the normal to the film surface. The sufficient magnetic hardening of the porous MLs with enhanced coercive field HC of up to ∼1.9 kOe for Co/Pd and ∼1.5 kOe for Co/Pt MLs, as compared to the continuous reference samples (∼1.5–2 times), is associated with the pinning of the magnetic moments on the nanopore edges. Application of the Stoner–Wohlfarth model for fitting the experimental M/MS(H) curves yielded clear evidence of the predominantly coherent rotation mechanism of magnetization reversal in the porous films.

We investigate optically induced ultrafast magnetization dynamics in [Co(0.5 nm)/Pd(1 nm)](5)/NiFe(t) exchange-spring samples with tilted perpendicular magnetic anisotropy using a time-resolved magneto-optical Kerr effect magnetometer. The competition between the out-of-plane anisotropy of the hard layer, the in-plane anisotropy of the soft layer and the applied bias field reorganizes the spins in the soft layer, which are modified further with the variation in t. The spin-wave spectrum, the ultrafast demagnetization time, and the extracted damping coefficients - all depend on the spin distribution in the soft layer, while the latter two also depend on the spin-orbit coupling between the Co and Pd layers. The spin-wave spectra change from multimode to single-mode as t decreases. At the maximum field reached in this study, H = 2.5 kOe, the damping shows a nonmonotonic dependence on t with a minimum at t = 7.5 nm. For t < 7.5 nm, intrinsic effects dominate, whereas for t > 7.5 nm, extrinsic effects govern the damping mechanisms.

The magnetization behavior, magnetic anisotropy, and domain configurations of Co/Pd multilayers with perpendicular magnetic anisotropy capped with permalloy is investigated using magnetometry, magnetic force microscopy, and ferromagnetic resonance. The thickness of the Ni80Fe20 layer in [Co/Pd](5)/NiFe (t) was varied from t = 0 to 80 nm in order to study the interplay between the anisotropy and magnetization directions of Co/Pd and NiFe. By varying the thickness of the NiFe layer, the net anisotropy changes sign, but domains with plane- normal magnetization are present even for the thickest NiFe. Ferromagnetic resonance measurements show a decrease in damping with increasing NiFe thickness. The results demonstrate how the magnetic behavior of mixed- anisotropy thin films can be controlled.

We report on the magnetization depth profile of a hybrid exchange-spring system in which a Co/Pd multilayer with perpendicular anisotropy is coupled to a CoFeB thin film with in-plane anisotropy. The competition between these two orthogonal anisotropies promotes a strong depth dependence of the magnetization orientation. The angle of the magnetization vector is sensitive both to the strength of the individual anisotropies and to the local exchange constant and is thus tunable by changing the thickness of the CoFeB layer and by substituting Ni for Pd in one layer of the Co/Pd stack. The resulting magnetic depth profiles are directly probed by element-specific x-ray magnetic circular dichroism of the Fe and Ni layers located at different average depths. The experimental results are corroborated by micromagnetic simulations.

Magnetic multilayer (ML) structures comprising a perpendicular magnetic anisotropy (PMA) layer coupled to an in-plane magnetic anisotropy (IMA) layer are promising materials for zero/low field operating spin-torque oscillators and bit patterned recording media. The magnetization tilt angle can be easily tuned by varying the IMA layer thickness due to the competition between PMA and IMA layers. To explore the underlying magnetization reversal mechanism and to further understand the control of tilt angle and uniformity of the magnetization, the IMA (NiFe, Co, and CoFeB)/PMA (Co/Pd MLs) exchange spring systems are systematically studied. Experimental data obtained from magnetometry show good agreement with 1-D micromagnetic simulations, allowing us to design tunable exchange coupled spring as a function of IMA thickness.

We study microwave signal generation as a function of drive current and applied perpendicular magnetic Field in nano contact spin torque oscillators (NC-STOs) based on orthogonal (pseudo) spin valves where the Co fixed layer has strong easy plane anisotropy, and the [Co/Nil free layer has a strong perpendicular magnetic anisotropy. The orthogonal NC-STOs exhibit a dramatic transition from the conventional ferromagnetic resonance like spin wave mode to a magnetic droplet solilon mode. In particular, the field and current dependence of the droplet solilon near threshold are discussed. Near threshold the droplet solilon undergoes complex dynamics that include mode hopping, as evident in the experimental frequency domain and rnagneloresislance response.

We present recent experimental and numerical advancements in the understanding of spin transfer torque generated magnetic droplet solitons. The experimental work focuses on nano-contact spin torque oscillators (NC-STOs) based on orthogonal (pseudo) spin valves where the Co fixed layer has an easy-plane anisotropy, and the [Co/Ni] free layer has a strong perpendicular magnetic anisotropy. The NC-STO resistance and microwave signal generation are measured simultaneously as a function of drive current and applied perpendicular magnetic field. Both exhibit dramatic transitions at a certain current dependent critical field value, where the microwave frequency drops 10 GHz, modulation sidebands appear, and the resistance exhibits a jump, while the magnetoresistance changes sign. We interpret these observations as the nucleation of a magnetic droplet soliton with a large fraction of its magnetization processing with an angle greater than 90 degrees, i.e., around a direction opposite that of the applied field. This interpretation is corroborated by numerical simulations. When the field is further increased, we find that the droplet eventually collapses under the pressure from the Zeeman energy.

A continuum of stable remanent resistance states is reported in perpendicularly magnetized pseudo spin valves with a graded anisotropy free layer. The resistance states can be systematically set by an externally applied magnetic field. The gradual reversal of the free layer with applied field and the field-independent fixed layer leads to a range of stable and reproducible remanent resistance values, as determined by the giant magnetoresistance of the device. An analysis of first-order reversal curves combined with magnetic force microscopy shows that the origin of the effect is the field-dependent population of up and down domains in the free layer.

Brillouin light scattering was applied to measure the evolution of spin waves in exchange-coupled [Co/Pd]-NiFe films with out-of-plane/in-plane magnetic anisotropy. Tunable tilting of the magnetization has important effects on the spin wave frequency gap: a substantial increase is observed with decreasing the soft NiFe thickness, while seemingly insignificant tilting angles cause a strong reduction with respect to a film with only the hard [Co/Pd] component. The spin wave frequency is reproduced also in samples, with increased thickness of the upper Pd layer, where a sudden modification in the tilting at the hard/soft interface is caused by the weakening of interlayer exchange.

By varying the Pd thickness (t(Pd)) from 0 to 8 nm in [Co/Pd](4)/Co/Pd(t(Pd))/NiFe exchange springs, we demonstrate (i) continuous tailoring of the exchange coupling between a [Co/Pd](4)/Co layer with perpendicular anisotropy, and a NiFe layer with an in-plane easy axis, (ii) tuning of the NiFe out-of-plane magnetization angle from 20 degrees to 80 degrees, and(iii) an up to two-fold increase in the NiFe damping. The partial decoupling also results in a highly uniform NiFe magnetization. These properties make [Co/Pd](4)/Co/Pd(t(Pd))/NiFe spring magnets ideal candidates for use as tilted polarizers, by combining stable and well-defined spin directions of its carriers with a high degree of angular freedom.