Magnetization dynamics in an artificial square spin-ice lattice made of Ni80Fe20 with magnetic field applied in the lattice plane is investigated by broadband ferromagnetic resonance spectroscopy. The experimentally observed dispersion shows a rich spectrum of modes corresponding to different magnetization states. These magnetization states are determined by exchange and dipolar interaction between individual islands, as is confirmed by a semianalytical model. In the low field regime below 400 Oe a hysteretic behavior in the mode spectrum is found. Micromagnetic simulations reveal that the origin of the observed spectra is due to the initialization of different magnetization states of individual nanomagnets. Our results indicate that it might be possible to determine the spin-ice state by resonance experiments and are a first step towards the understanding of artificial geometrically frustrated magnetic systems in the high-frequency regime.Frustrated magnetic systems, such as spin ices, have been of scientific interest for a long time due to their highly degenerated ground states, which result in complex magnetic ordering and collective behavior [1][2][3][4][5]. In contrast to the prototypical crystalline materials that started the exploration of spin-ice systems, such as the pyrochlores Dy 2 Ti 2 O 7 , Ho 2 Ti 2 O 7 and Ho 2 Sn 2 O 7 [6,7], artificially structured spin-ice lattices offer the unique opportunity to control and engineer the interactions between the elements by their geometric properties and orientation [1,8,9]. Another outstanding advantage of artificial spin ices is that the magnetization state of each individual spin (i.e., macrospin/single domain magnetic particle) is directly accessible through magnetic microscopy (e.g., scanning probe, electron, optical or X-ray microscopy). The 16 possible magnetization configurations of a square spin ice are shown in Fig. 1(a).Spin dynamics in magnonic crystals, materials with periodic perturbations or variations in one of the magnetic properties of the system, have been extensively investigated [10-13]. One-and two-dimensional magnonic crystals were studied and the research community paid particular attention to nano-structured materials [10], such as chains of dots or arrays of discs [14], antidot lattices with different shapes and alignments [15][16][17], gratings or nanostripes [18], etc.Although artificial spin ices offer a fascinating playground to investigate how specific magnetization states of individual islands or defects can affect the collective spin dynamics, there are only very few works on dynamics in the GHz-regime [19,20] reported. Sklenar et al. show broadband ferromagnetic resonance (FMR) measurements on an artificial bicomponent square spin-ice lattice utilizing a macroscopic meanderline approach and find a field-dependent behavior in remanence where interactions between individual elements presumably play a less important role. Furthermore, the geometrical arrangement of the structures in the artificial lattice leads to frustration by d...

Novel mechanisms for electromagnetic wave emission in the terahertz frequency regime emerging at the nanometer scale have recently attracted intense attention for the purpose of searching next-generation broadband THz emitters. Here, we report broadband THz emission, utilizing the interface inverse Rashba-Edelstein effect. By engineering the symmetry of the Ag/Bi Rashba interface, we demonstrate a controllable THz radiation (∼0.1-5  THz) waveform emitted from metallic Fe/Ag/Bi heterostructures following photoexcitation. We further reveal that this type of THz radiation can be selectively superimposed on the emission discovered recently due to the inverse spin Hall effect, yielding a unique film thickness dependent emission pattern. Our results thus offer new opportunities for versatile broadband THz radiation using the interface quantum effects.

Due to its transverse nature, spin Hall effects (SHE) provide the possibility to excite and detect spin currents and magnetization dynamics even in magnetic insulators. Magnetic insulators are outstanding materials for the investigation of nonlinear phenomena and for novel low power spintronics applications because of their extremely low Gilbert damping. Here, we report on the direct imaging of electrically driven spin-torque ferromagnetic resonance (ST-FMR) in the ferrimagnetic insulator Y3Fe5O12 based on the excitation and detection by SHEs. The driven spin dynamics in Y3Fe5O12 is directly imaged by spatially-resolved microfocused Brillouin light scattering (BLS) spectroscopy. Previously, ST-FMR experiments assumed a uniform precession across the sample, which is not valid in our measurements. A strong spin-wave localization in the center of the sample is observed indicating the formation of a nonlinear, self-localized spin-wave 'bullet'.

The Rashba-Edelstein effect stems from the interaction between the electron's spin and its momentum induced by spin-orbit interaction at an interface or a surface. It was shown that the inverse Rashba-Edelstein effect can be used to convert a spin-into a charge current. Here, we demonstrate that a Bi/Ag Rashba interface can even drive an adjacent ferromagnet to resonance. We employ a spin-torque ferromagnetic resonance excitation/detection scheme which was developed originally for a bulk spin-orbital effect, the spin Hall effect. In our experiment, the direct Rashba-Edelstein effect generates an oscillating spin current from an alternating charge current driving the magnetization precession in a neighboring permalloy (Py, Ni80Fe20) layer. Electrical detection of the magnetization dynamics is achieved by a rectification mechanism of the time dependent multilayer resistance arising from the anisotropic magnetoresistance.Conventional spintronics relies on the exchange interaction between conduction electrons on one side and localized spins in magnetic materials on the other side [1]. Stimulated by the experimental demonstration of spinto charge current conversion using bulk spin Hall effects (SHE), these kind of spin-orbital phenomena were actively investigated in the last decade and opened up the door to the research field of spin-orbitronics [2][3][4][5]. SHEs can be investigated by means of spin-current injection from a ferromagnet (FM) into materials with large spinorbit coupling, usually normal metals (NM) such as Pt or Pd [6], and sensing the generated voltage generated by means of the inverse spin Hall effect (ISHE) [7][8][9][10][11][12][13][14]. Other interesting applications of SHEs are the effective magnetization switching of nanomagnets or the movement of domain walls [15][16][17]. Furthermore, the ferromagnetic linewidth modulation as well as the excitation of spin waves and ferromagnetic resonance by SHE was demonstrated in ferromagnetic metals and insulators [18][19][20][21][22]. The SHE is a bulk effect occurring within a certain volume of the NM determined by the spin-diffusion length. The conversion efficiency can be expressed by a materialspecific parameter, the spin Hall angle γ SHE [4].Very recently, it has been shown that the inverse Rashba-Edelstein effect (IREE) can also be used for transformation of a spin-into a charge current [23][24][25][26]. The IREE is the inverse process to the Rashba-Edelstein effect (REE) [27]. The REE originates from spin-orbit interaction in a 2D electron gas at interfaces or surfaces, which effectively produce a steady non-equilibrium spin polarization from a charge current driven by an electric field. The Hamiltonian of this interaction is given by [23]: H R = α R (k ×ê z ) · σ, where α R is the Rashba coefficient, e z is the unit vector in z-direction [see Fig. 1(b,c)] and σ is the vector of Pauli matrices. As a result of this interaction the dispersion curves of the 2D electron gas are spin-split if α R = 0, as illustrated in Fig. 1(a). Analogous to the spin Ha...

a) saglam@anl.gov b) weizhang@oakland.edu c) hoffmann@anl.gov AbstractWe investigate spin transport through metallic antiferromagnets using measurements based on spin pumping combined with inverse spin Hall effects in Ni80Fe20/FeMn/W trilayers. The relatively large magnitude and opposite sign of spin Hall effects in W compared to FeMn enable an unambiguous detection of spin currents transmitted through the entire FeMn layer thickness. Using this approach we can detect two distinctively different spin transport regimes, which we associate with electronic and magnonic spin currents respectively. The latter can extend to relatively large distances (≈ 9 nm) and is enhanced when the antiferromagnetic ordering temperature is close to the measurement temperature.

Using first-principles transport calculations, we predict that the anisotropic magnetoresistance (AMR) of single-crystal Co x Fe 1−x alloys is strongly dependent on the current orientation and alloy concentration. An intrinsic mechanism for AMR is found to arise from the band crossing due to magnetization-dependent symmetry protection. These special k points can be shifted towards or away from the Fermi energy by varying the alloy composition and hence the exchange splitting, thus allowing AMR tunability. The prediction is confirmed by delicate transport measurements, which further reveal a reciprocal relationship of the longitudinal and transverse resistivities along different crystal axes.

In his classic paper on thermal grain boundary grooving Mullins [W.W. Mullins, J. Appl. Physics 28, 333 (1957)] assumes that the dihedral angle at the groove root remains constant and predicts that the groove width and depth grow αt0.25. Here, we derive models describing groove growth while the dihedral angle changes. In our grooving experiments with tungsten at 1350 °C in which the dihedral angle decreased, the growth exponent for the groove depth reached values as high as 0.44 while the growth exponent for the width decreased slightly from Mullins' value of 0.25. Hence groove width data alone are not sufficient for verifying Mullins' growth law unless the dihedral angle is constant. The observed changes in the dihedral angle are used as an input for numerical simulations. With the simulations we are able to extract the surface diffusion constants. Atomic force microscope observations of groove widths and depths in tungsten are in excellent agreement with the simulations.

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