The understanding of how spins move and can be manipulated at pico-and femtosecond time scales is the goal of much of modern research in condensed matter physics, with implications for ultrafast and more energy-efficient data processing and storage applications. However, the limited comprehension of the physics behind this phenomenon has hampered the possibility of realising a commercial technology based on it. Recently, it has been suggested that inertial effects should be considered in the full description of the spin dynamics at these ultrafast time scales, but a clear observation of such effects in ferromagnets is still lacking. Here, we report the first direct experimental evidence of intrinsic inertial spin dynamics in ferromagnetic thin films in the form of a nutation of the magnetisation at a frequency of approximately 0.5 THz. This allows us to reveal that the angular momentum relaxation time in ferromagnets is on the order of 10 ps.

No abstract

We introduce laboratory-scale magneto-optical imaging with sub–50-nm resolution using high-harmonic radiation.

In the last decade, two revolutionary concepts in nano magnetism emerged from research for storage technologies and advanced information processing. The first suggests the use of magnetic domain walls (DWs) in ferromagnetic nanowires to permanently store information in DW racetrack memories 1 . The second proposes a hardware realisation of neuromorphic computing in nanomagnets using nonlinear magnetic oscillations in the GHz range 2,3 . Both ideas originate from the transfer of angular momentum from conduction electrons to localised spins in ferromagnets 4,5 , either to push data encoded in DWs along nanowires or to sustain magnetic oscillations in artificial neurones. Even though both concepts share a common ground, they live on very different time scales which rendered them incompatible so far. Here, we bridge both ideas by demonstrating the excitation of magnetic autooscillations inside nano-scale DWs using pure spin currents.The spin-tranfer-torque (STT) effect discovered in 1996 by Slonczewski and Berger 4,5 allows the manipulation of localised magnetic moments in a ferromagnet by the transfer of spin angular momentum from spin polarised conduction electrons. The direction of the magnetisation can either be switched permanently 6 or can be forced to oscillate at radio frequencies 7 . Quite soon thereafter it was recognised that a charge current in a ferromagnet, which is intrinsically spin polarised, can move magnetic domain walls (DWs) in nanowires 8 . This gave rise to the idea of the magnetic racetrack memory 1 and, quite recently, current induced skyrmion motion 9-11 . While these schemes target nonvolatile, long term data storage, the STT effect in spin-torque nano-oscillators can be exploited to drive magnetic auto-oscillations 12 and, eventually, to radiate spin waves 13 by compensating the intrinsic magnetic damping. Another leap was the development of spin-Hall nano-oscillators (SHNO) 26,27 in which pure spin currents are generated via the spin-Hall effect (SHE) 14-17 and by which even propagating spin waves are excited 18,19 . This puts SHNOs at the heart of magnonics 20-22 which proposes a novel type of low energy, non boolean computing based on magnons, the quanta of spin waves, as carriers of information 23 or even neuromorphic computing 3 based on the nonlinear character of magnonic auto-oscillations. In a previous work, we demonstrated that DWs can channel magnons in an effective magnetic potential well 24 . This raised the question if a magnetic DW can potentially be a self-organized, movable SHNO.In order for the STT effect to counteract the intrinsic magnetic damping, the magnetisation M has to have a component antiparallel to the polarisation P of the injected spin current I s with density J s . Due to spindependent scattering associated with the SHE, a charge current I c with density J c flowing in a heavy metal is converted into a transverse pure spin current with J s ⊥ J c ⊥ P (Fig. 1a). In a conventional nanowire, therefore, an exa b 4 µ m C r ( 1 . 5 ) / P t ( 7 ) / C o 4 0

Recent progress in multiferroic materials and spintronic devices has renewed interest in metal oxide ferromagnetic and ferrimagnetic materials. Here we report the preparation of thin films of nanocrystalline ferrimagnetic CoFe 2 O 4 (CFO) using an environmentally benign aqueous solution processing route. The evolution of the structural, optical, and magnetic properties as a function of post-deposition annealing temperature is reported. For the highest annealing temperature (800°C), the remanence, coercivity, and resistivity values are comparable with those of films fabricated by epitaxial growth methods and exceed the quality of CFO nanocrystals prepared by other wet-chemical syntheses. In addition to the ability to make high quality ferrite films, the aqueous solution processing strategy offers great flexibility for tuning film properties by incorporating or substituting additional transition metal ions.