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Phase-resolved x-ray ferromagnetic resonance (XFMR) has been measured in fluorescence yield, extending the application of XFMR to opaque samples on opaque substrates. Magnetization dynamics were excited in a Co 50 Fe 50 (0.7)/Ni 90 Fe 10 (5) bilayer by means of a continuous wave microwave excitation, while x-ray magnetic circular dichroism (XMCD) spectra were measured stroboscopically at different points in the precession cycle. By tuning the x-ray energy to the L 3 edges of Ni and Fe, the dependence of the real and imaginary components of the element specific magnetic susceptibility on the strength of an externally applied static bias field was determined. Precessional dynamics are exploited in the operation of high frequency magnetic devices such as magnetic disk drives, nonreciprocal microwave devices, and spin transfer oscillators. The trajectory of precession and its damping are of crucial importance, and x-ray magnetic circular dichroism (XMCD) has the potential to provide new insight. The chemical specificity of XMCD means that it is possible to isolate the precession in different regions of the sample. By measuring the relative amplitude and phase of oscillation at each chemical species and at distinct sites in the crystal structure, one may in principle determine whether the relaxation takes place preferentially at a specific location within the structure.X-ray ferromagnetic resonance (XFMR) combines XMCD and ferromagnetic resonance (FMR) phenomena in a technique capable of measuring the FMR response of an alloy or a multilayer with both chemical and site specificity. Previous XFMR studies can be divided into two distinct categories: phase-resolved measurements 1-3 and time-averaged measurements.4-6 Both types of measurements are generally performed in transmission, although an earlier phaseresolved study was performed in reflection at grazing incidence.7 Transmission measurements are usually limited to samples grown on substrates such as Si 3 N 4 that are transparent in the soft x-ray regime. In this paper, we present the first phase-resolved XFMR measurements made in fluorescence yield. A Ta(5)[Cu(25)/Ta(3)] 3 /Cu(25)/Ta(10)/Ru(5)/ Co 50 Fe 50 (0.7)/Ni 90 Fe 10 (5)/Ru(3) stack (thicknesses in nanometers) was magnetron sputtered onto an insulating sapphire substrate. A second stack with a Dy(1) layer inserted between the Ni 90 Fe 10 (5) and the Ru(3) layers was also deposited. For today's thin sensor layers containing NiFe, the Fe concentration is typically in the range of 8-12%. It is for this reason that we look at Ni 90 Fe 10 as opposed to the more commonly studied Ni 80 Fe 20 . Elements with 490 lm  400 lm of the ferromagnetic bilayer were formed on the Cu underlayer by electron-beam lithography and ion beam milling. Photolithography was then used to pattern the exposed Cu into a coplanar waveguide structure with 50 X characteristic impedance. The experimental geometry for the XFMR measurement is shown in Fig. 1. Measurements were performed in the portable octupole magnet system (POMS) on beam lin...

The microwave emission of magnetic tunnel junction nanopillars with symmetric CoFeB/MgO/CoFeB interfaces and a 9-nm-thick free layer was studied as a magnetic field was applied in plane at different angles H with respect to the easy axis. As H was increased, a more complicated mode spectrum containing modes of larger amplitude was observed. The character of the different modes was deduced by screening many devices fabricated from the same wafer, observing the variation of frequency both between different junctions and with H, and through comparison with micromagnetic simulation. Within selected junctions at certain H values, the dependence of the frequency, amplitude, and linewidth of the lowest-frequency edge-localized mode upon the applied field and current can be interpreted in terms of combined thermal and spin-transfer-torque (STT) excitation. Fitting to a simple analytical model yielded values for the in-plane and out-of-plane spin STT. The out-of-plane STT was fo und to contain terms with both a linear and a quadratic dependence on the bias current. The linear term was often found to dominate, as expected for a tunnel junction with asymmetric rather than symmetric interfaces, suggesting that the fuller structure of the free and reference layers should be taken into account. The amplitude of the in-plane STT was somewhat larger than expected, suggesting that the dominant edge mode occupies only part of the area of the free layer but is subject to a proportionately larger fraction of the torque exerted upon the free layer as a whole. This might be expected when the resonant modes of the free layer interact and the lowest-frequency mode dominates

Precessional dynamics of a Co50Fe50(0.7)/Ni90Fe10(5)/Dy(1)/Ru(3) (thicknesses in nm) thin film have been explored by low temperature time-resolved magneto-optical Kerr effect and phase-resolved x-ray ferromagnetic resonance measurements. As the temperature was decreased from 300 to 140 K, the magnetic damping was found to increase rapidly while the resonance field was strongly reduced. Static x-ray magnetic circular dichroism measurements revealed increasing ferromagnetic order of the Dy moment antiparallel to that of Co50Fe50/Ni90Fe10. Increased coupling of the Dy orbital moment to the precessing spin magnetization leads to significantly increased damping and gyromagnetic ratio of the film while leaving its magnetic anisotropy effectively unchanged.

Vortex dynamics within arrays of square ferromagnetic nano-elements have been studied by time-resolved scanning Kerr microscopy (TRSKM), while x-ray photoemission electron microscopy has been used to investigate their equilibrium state. An alternating field demagnetization process was found to initialize a distribution of equilibrium states within the individual elements of the array, including quasi-uniform states and vortex states of different chirality and core polarization.Repeated initialization revealed some evidence of stochastic behaviour during the formation of the equilibrium state. TRSKM with a spatial resolution of ~300 nm was used to detect vortex gyration within arrays of square nano-elements of 250 nm lateral size. Two arrays were studied consisting of a 9×9 and 5×5 arrangement of nano-elements with 50 nm and 500 nm element edge-to-edge separation to encourage strong and negligible dipolar interactions respectively. In the 5×5 element array, TRSKM images, acquired at a fixed phase of the driving microwave magnetic field, revealed differences in the gyrotropic phase within individual elements. While some phase variation is attributed to the dispersion in the size and shape of elements, the vortex chirality and core polarization are also shown to influence the phase. In the 9×9 array, strong magneto-optical response due to vortex gyration was observed across regions with length equal to either one or two elements.Micromagnetic simulations performed for 2×2 arrays of elements suggest that particular combinations of vortex chirality and polarization in neighbouring elements are required to generate the observed magneto-optical contrast.