In magnetic memory devices, logical bits are recorded by selectively setting the magnetization vector of individual magnetic domains either 'up' or 'down'. In such devices, the fastest and most efficient recording method involves precessional switching: when a magnetic field B(p) is applied as a write pulse over a period tau, the magnetization vector precesses about the field until B(p)tau reaches the threshold value at which switching occurs. Increasing the amplitude of the write pulse B(p) might therefore substantially shorten the required switching time tau and allow for faster magnetic recording. Here we use very short pulses of a very high magnetic field to show that under these extreme conditions, precessional switching in magnetic media supporting high bit densities no longer takes place at well-defined field strengths; instead, switching occurs randomly within a wide range of magnetic fields. We attribute this behaviour to a momentary collapse of the ferromagnetic order of the spins under the load of the short and high-field pulse, thus establishing an ultimate limit to the speed of deterministic switching and magnetic recording.

Torques appear between charge carrier spins and local moments in regions of ferromagnetic media where spatial magnetization gradients occur, such as a domain wall, owing to an exchange interaction. This phenomenon has been predicted by different theories 1-7 and confirmed in a number of experiments on metallic and semiconductor ferromagnets 8-19. Understanding the magnitude and orientation of such spin-torques is an important problem for spin-dependent transport and currentdriven magnetization dynamics, as domain-wall motion underlies a number of emerging spintronic technologies 20,21. One outstanding issue concerns the non-adiabatic spin-torque component β, which has an important role in wall dynamics, but no clear consensus has yet emerged over its origin or magnitude. Here, we report an experimental measurement of β in perpendicularly magnetized films with narrow domain walls (1-10 nm). By studying thermally activated wall depinning, we deduce β from the variation of the Arrhenius transition rate with applied currents. Surprisingly, we find β to be small and relatively insensitive to the wall width, which stands in contrast to predictions from transport theories 2,5-7. In addition, we find β to be close to the Gilbert damping constant α, which, in light of similar results on planar anisotropy systems 15 , suggests a universal origin for the non-adiabatic torque. The adiabatic torque, which accounts for transport processes in which the conduction spin follows the local spatial magnetization variation by remaining in either the majority or minority state, is well understood and has been reproduced by a number of different transport theories. In contrast, the non-adiabatic contribution, characterized by a dimensionless parameter β (ref. 22), remains the subject of much debate. Various mechanisms have been put forward to explain its origin, such as momentum transfer 2,7 , spinmistracking 4,6 or spin-flip scattering 3. It is predicted that large nonadiabatic effects should appear in narrow domain walls because of large magnetization gradients 2,5,6 , whereby the wall width becomes comparable to important transport scales such as the spin-diffusion length 2 or the Larmor precession length 6 , which are of the order of a few nanometres in ferromagnetic transition metals. The presence of a non-adiabatic term is of fundamental importance, because its existence implies that current-driven wall motion is possible for any finite current in a perfect system, even in the absence of an applied magnetic field. Difficulty in characterizing β experimentally therefore stems in part from being able to distinguish between extrinsic sources of wall pinning, due to structural defects, for example, from the intrinsic finite threshold current predicted 2 for β = 0. We have studied current-driven domain wall dynamics in two different pseudo spin-valve systems based either on CoNi

Transition metal ferromagnetic films with perpendicular magnetic anisotropy (PMA) have ferromagnetic resonance (FMR) linewidths that are one order of magnitude larger than soft magnetic materials, such as pure iron (Fe) and permalloy (NiFe) thin films. A broadband FMR setup has been used to investigate the origin of the enhanced linewidth in Ni|Co multilayer films with PMA. The FMR linewidth depends linearly on frequency for perpendicular applied fields and increases significantly when the magnetization is rotated into the film plane. Irradiation of the film with Helium ions decreases the PMA and the distribution of PMA parameters. This leads to a great reduction of the FMR linewidth for in-plane magnetization. These results suggest that fluctuations in PMA lead to a large two magnon scattering contribution to the linewidth for in-plane magnetization and establish that the Gilbert damping is enhanced in such materials (α ≈ 0.04, compared to α ≈ 0.002 for pure Fe).PACS numbers: 75.47.-m,85.75.-d,75.70.-i,76.50.+g Magnetic materials with perpendicular magnetic anisotropy (PMA) are of great interest in information storage technology, offering the possibility of smaller magnetic bits [1] and more efficient magnetic random access memories based on the spin-transfer effect [2]. They typically are multilayers of transition metals (e.g., Co|Pt, Co|Pd, Ni|Co) with strong interface contributions to the magnetic anisotropy [3], that render them magnetically hard. In contrast to soft magnetic materials which have been widely studied and modeled [4,5,6,7], such films are poorly understood. Experiments indicate that there are large distributions in their magnetic characteristics, such as their switching fields [1]. An understanding of magnetization relaxation in such materials is of particular importance, since magnetization damping determines the performance of magnetic devices, such as the timescale for magnetization reversal and the current required for spin-transfer induced switching [2,8].Ferromagnetic resonance (FMR) spectroscopy provides information on the magnetic damping through study of the linewidth of the microwave absorption peak, ∆H, when the applied field is swept at a fixed microwave frequency. FMR studies of thin films with PMA show very broad linewidths, several 10's of mT at low frequencies ( 10 GHz) for polycrystalline alloy [9], multilayer [10] and even epitaxial thin films [11]. This is at least one order of magnitude larger than the FMR linewidth found for soft magnetic materials, such as pure iron (Fe) and permalloy (FeNi) thin films [5]. Further, it has recently been suggested that the FMR linewidth of perpendicularly magnetized CoCrPt alloys cannot be explained in terms of Landau-Lifshitz equation with Gilbert damping [12], the basis for understanding magnetization dynamics in ferromagnets:Here M is the magnetization and γ= |gµ B / | is the gyromagnetic ratio. The second term on the right is the damping term, where α is the Gilbert damping constant. This equation describes precessional motion of th...

We report the first observation of high wave vector magnon excitations in a ferromagnetic monolayer. Using spin-polarized electron energy loss spectroscopy, we observed the magnon dispersion in one atomic layer (ML) of Fe on W(110) at 120 K. The magnon energies are small in comparison to the bulk and surface Fe(110) excitations. We find an exchange parameter and magnetic anisotropy similar to that from static measurements. Our results are in sharp contrast to theoretical calculations, indicating that the present understanding of magnetism of the ML Fe requires considerable revision.

We present the first surface spin-wave (SW) dispersion measurements up to the surface Brillouin zone boundary of a two monolayer Fe film on W(110) by using spin-polarized electron energy loss spectroscopy. Pronounced features of SW peaks are observed in the spectra at room temperature. We found that the SW energies in the Fe film are strongly reduced compared to spin waves in bulk Fe and to theoretical predictions. Our results suggest that this reduction is caused by the reduction of exchange interaction within the 2 ML Fe on W(110) as compared to bulk Fe.