Spin transfer in magnetic multilayers offers the possibility of ultrafast, low-power device operation. We report a study of spin pumping in spin valves, demonstrating that a strong anisotropy of spin pumping from the source layer can be induced by an angular dependence of the total Gilbert damping parameter, α, in the spin sink layer. Using lab- and synchrotron-based ferromagnetic resonance, we show that an in-plane variation of damping in a crystalline Co_{50}Fe_{50} layer leads to an anisotropic α in a polycrystalline Ni_{81}Fe_{19} layer. This anisotropy is suppressed above the spin diffusion length in Cr, which is found to be 8 nm, and is independent of static exchange coupling in the spin valve. These results offer a valuable insight into the transmission and absorption of spin currents, and a mechanism by which enhanced spin torques and angular control may be realized for next-generation spintronic devices.

We demonstrate spin pumping, i.e. the generation of a pure spin current by precessing magnetization, without application of microwave radiation commonly used in spin pumping experiments. We use femtosecond laser pulses to simultaneously launch the magnetization precession in each of two ferromagnetic layers of a Galfenol-based spin valve and monitor the temporal evolution of the magnetizations. The spin currents generated by the precession cause a dynamic coupling of the two layers. This coupling has dissipative character and is especially efficient when the precession frequencies in the two layers are in resonance, where coupled modes with strongly different decay rates are formed.The generation of a spin current (SC) by magnetization precession (MP) is known as spin pumping (SP) [1]. Thereby, the precessing magnetization of a ferromagnetic (FM) film transfers angular momentum to an adjacent material, representing a pure SC that is not accompanied by the flow of charges. SCs generated by SP contain an ac-component at the precession frequency and carry also the MP phase. Conceptually, SP offers a new way of building spintronic devices by flexibly combining conducting and insulating materials [2][3][4][5][6][7][8]. This has stimulated intense efforts aimed at demonstrating SCs in a robust way [9].Conventional SP experiments exploit a ferromagnetic resonance (FMR) where the MP is driven by a microwave field [10]. The transfer of angular momentum to the adjacent material results in enhanced damping of the FMR [11,12] and thus to a broadening of the corresponding resonance spectrum [13,14]. In turn, the SC injected into the adjacent layer can be detected by, for example, the inverse spin Hall effect [2][3][4][5][6][7][8][15][16][17][18][19][20][21][22]. In a spin valve structure consisting of two FM layers separated by a nonmagnetic spacer, the SC generated by one layer drives the magnetization precession of the other layer [23][24][25][26]. At resonance, when the precession frequencies of the FM layers coincide, a strongly coupled collective precessional mode forms [27,28].This conventional approach has a drawback, however: applying monochromatic microwave fields for driving the MP lacks the flexibility required for nanoscale applications, it strictly sets the MP and SC phase, and requires exact matching to the FMR frequency. Ultrafast optical excitation, widely used nowadays in ultrafast optomagnetism for launching MPs [29], is a promising alternative. In metallic FMs, ultrashort laser pulses trigger MP by rapidly alternating the magnetic anisotropy [30]. While laser pulses have been utilized for SC generation via the transport of spin-polarized electrons from an opticallyexcited magnetic region [31][32][33][34][35][36], no evidence of pure SCs generated by optically launched MP has been reported.In this Letter, we report optically excited SP in a pseudo spin-valve (PSV) consisting of two FM layers separated by a normal metal spacer. By femtosecond laser pulses we simultaneously excite MP in the two magnetic layers...

Article:Beevers, J. E., Love, C. J., Lazarov, V. K. orcid.org/0000-0002-4314-6865 et al. The magnetoelectric effect in M-type Ti-Co doped strontium hexaferrite has been studied using a combination of magnetometry and element specific soft X-ray spectroscopies. A large increase (> x30) in the magnetoelectric coefficient is found when Co 2+ enters the trigonal bi-pyramidal site. The 5-fold trigonal bi-pyramidal site has been shown to provide an unusual mechanism for electric polarization based on the displacement of magnetic transition metal (TM) ions. For Co entering this site, an offcentre displacement of the cation may induce a large local electric dipole as well as providing an increased magnetostriction enhancing the magnetoelectric effect. INTRODUCTIONThe control of magnetism using applied electric fields offer the possibility of a new generation of ultralow power, high density storage. In this respect, magnetoelectric (ME) multiferroic materials are intensely studied in order to understand how different symmetry breaking orders exist in the same material and how these orders can be coupled. Among the few room temperature single-phase ME multiferroics reported, hexaferrites show potential for device applications as they exhibit a low field ME effect at room temperature [1]. M-type hexaferrites are arranged in different repeating sequences of basic building blocks; the R and S layers [2]. Fe 3+ cations occupy both octahedral (Oh) and tetrahedral (Td) co-ordinated sites in the S block (Wyckoff positions 2a and 4f1) and octahedral sites (12k and 4f2) in the R block, see online supplementary materials. The Ba ion located at positions 2d strongly distorts the octahedral site located at the 2b positions giving rise to a bi-pyramidal 5 fold co-ordination which induces a large uniaxial magnetic anisotropy parallel to the c-axis [3]. However, Co 2+ and Ti 4+ substitutions for Fe 3+ dramatically alters the magnetic properties [4]. Ti substitutions at the 12k sites decrease the exchange coupling between spins in the R and S blocks whilst Co substitutions change the magnetic anisotropy from uniaxial to an easy cone of magnetization tilted away from the c-axis. The result is to stabilize a non-collinear conical magnetic structure [2,5] which is of high interest in the field of multiferroics [6]. The ME effect at room temperature in SrFe8Ti2Co2O19 was first reported in bulk [7] and, thereafter, in thin films [8]. Co 2+ substitutions in ferrite structures are a well-known source of magnetoelastic coupling due to the large orbital moment of Co 2+ [9]. The linear ME coupling, , which is the change in magnetization with an applied electric field is directly proportional to the magnetoelastic coupling, or magnetostriction, , implying that an increase in  increases  [10]. However, a mechanism such as the piezoelectric effect or electrostriction for coupling the applied electric field into strain is also required. Several mechanisms for magnetically induced ferroelectricity have been proposed in recent years [11] with the sp...