High-Frequency Dynamics Modulated by Collective Magnetization Reversal in Artificial Spin Ice

Abstract

Spin-torque ferromagnetic resonance (ST-FMR) arises in heavy metal/ferromagnet heterostructures when an alternating charge current is passed through the bilayer stack. The methodology to detect the resonance is based on the anisotropic magnetoresistance, which is the change in the electrical resistance due to different orientations of the magnetization. In connected networks of ferromagnetic nanowires, known as artificial spin ice, the magnetoresistance is rather complex owing to the underlying collective behavior of the geometrically frustrated magnetic domain structure. Here, we demonstrate ST-FMR investigations in a square artificial spin-ice system and correlate our observations to magnetotransport measurements. The experimental findings are described using a simulation approach that highlights the importance of the correlated dynamics response of the magnetic system. Our results open the possibility of designing reconfigurable microwave oscillators and magnetoresistive devices based on connected networks of nanomagnets.

Mutual influence between macrospin reversal order and spin-wave dynamics in isolated artificial spin-ice vertices

“…We have identified the two main curves as corresponding to modes a) and b), in good agreement with the predictions. However, no evidence was found for mode d), possibly because it is strongly confined within the narrow macrospin, and thus generates a comparatively weaker signal [ 34,39 ].…”

“…Each macrospin is modeled as a permalloy dot with an elliptical cross section, with major and minor axis of 500 and 200 nm, and a uniform thickness of 15 nm. This choice was guided by the fact that overly elongated ellipses are known to have low-frequency excitations localized in narrow areas at the ends of the sample (longitudinally), and those modes are unlikely to give strong ferromagnetic resonance (FMR) signals, especially because in real samples imperfections affect the particle edges randomly in the array [ 34,35,36,37 ]. Hence, our choice of elliptical macrospins with aspect ratio 2/5 exhibits many lowfrequency excitations ("bulk" modes) that occupy most of the dot surface.…”

Collective magnetization dynamics in nanoarrays of thin FePd disks

“…They arise from the idealised circular shape of the discs in the simulations. In real samples the lithographically fabricated elements are imperfect, leading to the absence of these modes 17 . The origin of spin wave maps for these simulated modes is discussed in more detail in the Appendix C.…”

Advances in artificial spin ice

“…In addition to using spin waves as a means to study defect formation in artificial spin systems, artificial spin-ice geometries may offer a platform for programmable spin-wave devices 3 . In this regard, detection of spin waves in artificial spin ice with spintorque spectroscopy may be especially useful 117 . A connected geometry able to transmit spin-wave information through the lattice is particularly suited to the control of the manifold of spin-wave states 118 .…”

“…For the quasicrystal lattices, we highlight connected geometries, although quasicrystal systems made up of individual magnets have also been investigated. In addition to the connected quasicrystal lattices, examples of other lattices with connected elements exist in, for example, square 117 , kagome 54 and brickwork 197 geometries. These are topologically equivalent to antidot lattices 125 that are interesting as magnonic crystals and as networks for the observation of domain-wall motion.…”