  We applied a tabletop, ultrafast, high-harmonic generation (HHG) source to
measure the element-specific ferromagnetic resonance (FMR) in ultra-thin
magnetic alloys and multilayers on an opaque Si substrate. We demonstrate a
continuous wave bandwidth of 62 GHz, with promise to extend to 100 GHz or
higher. This laboratory-scale instrument detects the FMR using ultrafast,
extreme ultraviolet (EUV) light, with photon energies spanning the M-edges of
most relevant magnetic elements. An RF frequency comb generator is used to
produce a microwave excitation that is intrinsically synchronized to the EUV
pulses with a timing jitter of 1.1 ps or better. We apply this system to
measure the dynamics in a multilayer system as well as Ni-Fe and Co-Fe alloys.
Since this instrument operates in reflection-mode, it is a milestone toward
measuring and imaging the dynamics of the magnetic state and spin transport of
active devices on arbitrary and opaque substrates. The higher bandwidth also
enables measurements of materials with high magnetic anisotropy, as well as
ferrimagnets, antiferromagnets, and short-wavelength (high wavevector)
spinwaves in nanostructures or nanodevices. Furthermore, the coherence and
short wavelength of the EUV will enable extending these studies using dynamic
nanoscale lensless imaging techniques such as coherent diffractive imaging,
ptychography, and holography.
