Magnetostriction, the strain induced by a change in magnetization, is a universal effect in magnetic materials. Owing to the difficulty in unraveling its microscopic origin, it has been largely treated phenomenologically. Here, we show how the source of magnetostriction—the underlying magnetoelastic stress—can be separated in the time domain, opening the door for an atomistic understanding. X-ray and electron diffraction are used to separate the sub-picosecond spin and lattice responses of FePt nanoparticles. Following excitation with a 50-fs laser pulse, time-resolved X-ray diffraction demonstrates that magnetic order is lost within the nanoparticles with a time constant of 146 fs. Ultrafast electron diffraction reveals that this demagnetization is followed by an anisotropic, three-dimensional lattice motion. Analysis of the size, speed, and symmetry of the lattice motion, together with ab initio calculations accounting for the stresses due to electrons and phonons, allow us to reveal the magnetoelastic stress generated by demagnetization.

We have used a MHz lock-in x-ray spectro-microscopy technique to directly detect changes of magnetic moments in Cu due to spin injection from an adjacent Co layer. The elemental and chemical specificity of x-rays allows us to distinguish two spin current induced effects. We detect the creation of transient magnetic moments of 3×10 −5 µB on Cu atoms within the bulk of the 28 nm thick Cu film due to spin-accumulation. The moment value is compared to predictions by Mott's two current model. We also observe that the hybridization induced existing magnetic moments on Cu interface atoms are transiently increased by about 10% or 4 × 10 −3 µB. This reveals the dominance of spin-torque alignment over Joule heat induced disorder of the interfacial Cu moments during current flow.One of the new paradigms in magnetism research is the use of spin currents to read and write static magnetic bits via the giant magneto-resistance (GMR) [1] and spin transfer torque effects [2,3]. Spin currents are also believed to play a key role in the ultrafast manipulation of the magnetization by femtosecond optical pulses, like in all optical switching [4][5][6]. They exist even during current flow through non-magnetic materials consisting of atoms with large spin-orbit coupling such as Pt, leading to spin accumulation through the spin Hall or Rashba effects [7,8]. The presence of spin currents is typically revealed through current or voltage dependent measurements, but an atomic level understanding of the detailed spin dependent scattering mechanisms requires techniques that can directly probe the electronic structure at the nanoscale.In this letter we report the direct detection of the transient magnetization in a non magnet (Cu) caused by injection of spin polarized current from an adjacent ferromagnet (Co). This is accomplished by a significant advancement in the sensitivity of scanning transmission microscope (STXM) which is achieved by time dependent modulation of a spin current synchronized with xray pulses to produce a sensitivity corresponding to the magnetic moment of about 50 Fe atoms. Using this technique, we were able to measure an extremely small transient x-ray magnetic circular dichroism (XMCD) effect in the non-magnetic Cu layer.Our experimental arrangement, shown in Fig. 1, was similar as that employed in previous studies of spin torque switching [9]. However, instead of observing directional changes of the large atomic magnetic moments of ≃ 2 µ B in a ferromagnetic (FM) Co layer we used the XMCD effect to quantitatively measure the tiny, ≃ 10 −5 µ B /atom, transient signal due to spin currents in interfacial and bulk Cu atoms.The samples consisted of a multilayer grown by sputtering, where the ferromagnetic polarizer layer [0.3Co/0.9Pd] 6 /0.3Co/[0.6Ni/0.09Co] 3 /0.21Co, was designed to have a strong perpendicular magnetic anisotropy and large spin polarization, as previously discussed in [10]. It is important to note that the Co and Ni layers were deposited at room temperature where they are immiscible so that the resulting...

Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. However, the spatial evolution of the magnetisation is not yet well understood, in part due to the difficulty in experimentally probing such rapid dynamics. Here, we find evidence of a universal rapid magnetic order recovery in ferrimagnets with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify magnon localisation and coalescence processes, whereby localised magnetic textures nucleate and subsequently interact and grow in accordance with a power law formalism. A hydrodynamic representation of the numerical simulations indicates that the appearance of noncollinear magnetisation via optical pumping establishes exchange-mediated spin currents with an equivalent 100% spin polarised charge current density of 10 7 A cm −2 . Such large spin currents precipitate rapid recovery of magnetic order after optical pumping. The magnon processes discussed here provide new insights for the stabilization of desired meta-stable states.