Phase‐change materials offer a promising route for the practical realisation of new forms of general‐purpose and ‘brain‐like’ computers. An experimental proof‐of‐principle of such remakable capabilities is presented that includes (i) the reliable execution by a phase‐change ‘processor’ of the four basic arithmetic functions of addition, subtraction, multiplication and division, (ii) the demonstration of an ‘integrate and fire’ hardware neuron using a single phase‐change cell and (iii) the expostion of synaptic‐like functionality via the ‘memflector’, an optical analogue of the memristor.

The picosecond magnetization dynamics of arrays of square Ni 88 Fe 12 /Co 80 Fe 20 bilayer nanoelements were studied by optical pump-probe measurements. Experimentally observed modes were found to fall upon two branches, with a crossover from the high-to low-frequency regime as the element size was reduced to less than 220 nm. Micromagnetic simulations revealed that the branches are associated with center and edge modes. The edge mode is found to dominate as the element size is reduced so that the magnetic response to a pulsed field becomes less spatially uniform. DOI: 10.1103/PhysRevB.71.220409 PACS number͑s͒: 75.40.Gb, 75.30.Ds, 75.50.Ss, 78.47.ϩp Increased storage densities in magnetic data storage technology require the use of nanoscale magnetic elements.1 A full understanding of the spin-wave ͑SW͒ spectrum is also essential if higher data rates are to be achieved, for example, by the implementation of ultrafast precessional switching. 2Spatial confinement leads to quantized modes with frequency and spatial character that have a complicated dependence upon the exchange interaction and nonuniform demagnetizing field within the element.3-8 Interest in switching processes has led to the development of experimental techniques that allow SW excitations to be observed within the time domain.9,10 Although a number of studies have been performed upon microscale elements, nanoscale elements have not yet been extensively explored. Continued progress therefore requires the study of high-quality arrays of elements of identical shape and size. 3,5,8,11 In this paper, time-resolved scanning Kerr microscopy ͑TRSKM͒ measurements 9 are used to investigate magnetization dynamics in arrays of magnetic elements with size ranging from 64 to 630 nm. Specifically, we study elements with a composition similar to that used in the free layer of a spin or tunnel valve recording sensor or a magnetic random access memory element. Using the TRSKM as a probe of the magnetization dynamics at the center of an array, we record its time-dependent response to a pulsed magnetic field. We show that the measured precession undergoes a crossover to a lower-frequency regime as the element size is reduced below a certain value. Numerical simulations performed with the object oriented micromagnetic framework ͑OOMMF͒ ͑Ref. 12͒ reproduce the observed variations in the mode frequencies and lead to the surprising conclusion that the magnetic response of the smallest nanoscale elements is less, rather than more, spatially uniform.A Ta͑50 Å͒ /Co 80 Fe 20 ͑10 Å͒ /Ni 88 Fe 12 ͑27 Å͒ /Ta͑100 Å͒ film was sputtered onto a Si substrate and patterned, using a combination of electron-beam lithography and ion milling, into square arrays of square elements. The element lengths ͑edge-to-edge separations͒ were 630 ͑37.5͒, 425 ͑21.9͒, 220 ͑95͒, 120 ͑37.5͒, and 64 ͑48.4͒ nm, while the length of each array was about 4 m. Scanning electron microscope images 13 showed that the corners of the 64 and 120 nm elements were slightly rounded. The compositions of the Ni 88 Fe...

An experimental scheme for studying spin wave propagation across thin magnetic film samples is proposed. The scheme is based upon the creation of picosecond pulses of strongly localized effective magnetic field via ultrafast optical irradiation of a specially deposited exchange bias or exchange spring layer. The spin waves are excited near the irradiated surface before propagating across the thickness of the sample. They are then detected near the other surface either within the finite optical skin depth using the linear magneto-optical Kerr effect in metallic samples or by the magnetic second harmonic generation. The experiment can facilitate investigations of propagating spin waves with wavelengths down to several nanometers and frequencies in excess of hundreds of Gigahertz. An experiment upon a periodically layered nanowire (a finite cross-section magnonic crystal) is numerically simulated, although the sample might equally well be a continuous film or an array of elements (e.g. nanowires) that either have uniform composition or are periodically layered as in a magnonic crystal. The experiments could be extended to study domain wall-induced spin wave phase shifts and can be used for the creation of spin wave magnetic logic devices. r

Despite recent progress in spin-current research, the detection of spin current has mostly remained indirect. By synchronizing a microwave waveform with synchrotron x-ray pulses, we use the ferromagnetic resonance of the Py (Ni_{81}Fe_{19}) layer in a Py/Cu/Cu_{75}Mn_{25}/Cu/Co multilayer to pump a pure ac spin current into the Cu_{75}Mn_{25} and Co layers, and then directly probe the spin current within the Cu_{75}Mn_{25} layer and the spin dynamics of the Co layer by x-ray magnetic circular dichroism. This element-resolved pump-probe measurement unambiguously identifies the ac spin current in the Cu_{75}Mn_{25} layer.

Time-resolved scanning Kerr microscopy measurements have been performed upon arrays of square ferromagnetic nanoelements of different sizes and for a range of bias fields. The experimental results were compared to micromagnetic simulations of model arrays in order to understand the nonuniform precessional dynamics within the elements. In the experimental spectra acquired from an element of length of 236 nm and thickness of 13.6 nm, two branches of excited modes were observed to coexist above a particular bias field. Below this so-called crossover field, the higher frequency branch was observed to vanish. Micromagnetic simulations and Fourier imaging revealed that modes from the higher frequency branch had large amplitude at the center of the element where the effective field was parallel to the bias field and the static magnetization. Modes from the lower frequency branch had large amplitude near the edges of the element perpendicular to the bias field. The simulations revealed significant canting of the static magnetization and effective field away from the direction of the bias field in the edge regions. For the smallest element sizes and/or at low bias field values, the effective field was found to become antiparallel to the static magnetization. The simulations revealed that the majority of the modes were delocalized with finite amplitude throughout the element while the spatial character of a mode was found to be correlated with the spatial variation in the total effective field and the static magnetization state. The simulations also revealed that the frequencies of the edge modes are strongly affected by the spatial distribution of the static magnetization state both within an element and within its nearest neighbors. Furthermore, the simulations suggest that collective modes may be supported in arrays of interacting nanomagnets, which act as magnonic crystals.