The spin structure of head-to-head domain walls in Ni 80 Fe 20 structures is studied using high-resolution photoemission electron microscopy. The quantitative phase diagram is extracted from these measurements and found to exhibit two phase boundaries between vortex and transverse domain walls. The results are compared with available theoretical predictions and micromagnetic simulations and differences to the experiment are explained, taking into account thermal excitations. Temperature-dependent measurements show a thermally activated transformation of transverse to vortex domain walls in 7 nm thick and 730 nm wide structures at a transition temperature between 260°C and 310°C, which corresponds to a nucleation barrier height for a vortex wall between 6.7ϫ 10 −21 J and 8.0ϫ 10 −21 J.

We present an experimental study of domain wall motion induced by current pulses as well as by conventional magnetic fields at temperatures between 2 and 300 K in a 110 nm wide and 34 nm thick Ni 80 Fe 20 ring. We observe that, in contrast with field-induced domain wall motion, which is a thermally activated process, the critical current density for current-induced domain wall motion increases with increasing temperature, which implies a reduction of the spin torque efficiency. The effect of Joule heating due to the current pulses is measured and taken into account to obtain critical fields and current densities at constant sample temperatures. This allows for a comparison of our results with theory. PACS numbers: 72.25.Ba, 75.60.Ch, 75.75.+a, 85.70.Kh The interplay between spin currents and domain walls in magnetic nanostructures has been studied intensively in the last decade, driven by fundamental interest in the basic physical mechanisms involved. Furthermore, currentinduced magnetization reversal by domain wall motion is a promising alternative to the conventional field-induced reversal for technological applications in nonvolatile memories and sensors, which has lead to an increase in research in this field [1]. The phenomenon of current-induced domain wall motion has been long known [2,3] and recently controlled current-induced motion of single domain walls in magnetic nanostructures has been achieved. Several important aspects such as domain wall velocities [4,5], critical current densities [6 -8], thermally assisted motion [9], and the deformation of the domain wall spin structure due to current [4] have been addressed. Current-induced switching has been also investigated in a trilayer pillar geometry at variable temperatures [10,11]. The underlying theory of interaction between current and magnetization is still controversial. Different approaches have been suggested in the ballistic limit [12,13] as well as in the diffusive limit [2,12]. An adiabatic spin torque has been introduced into the Landau-Lifshitz-Gilbert equation of magnetization dynamics [12,14,15]. Motivated by large discrepancies between experiment and theory, a nonadiabatic term was included [16,17]. The relative importance of the two torques in domain wall motion is still the subject of much debate [16 -18]. In order to gain information on the (non)-adiabaticity of the spin torque, a study of domain wall motion as a function of current and field at a constant sample temperature is needed. Using combinations of current and field allows one to compare the theoretical calculations [18] of the dependence of the critical current on the applied field with the experimental results. Of particular importance for comparison of experiment and theory is a constant sample temperature to separate spin torque and temperature effects, because existing theory so far neglects heating effects. Since significant Joule heating due to injected current pulses was observed [19], this effect must be quantitatively measured and taken into account. The possibi...

We report observations of the effect of electrical currents on the propagation and spin structure of vortex walls in NiFe wires. We find that magnetic vortices are nucleated and annihilated due to the spin torque effect. The velocity is found to be directly correlated with these transformations and decreases with increasing number of vortices. The transformations are observed in wide elements, while in narrower structures the propagation of single vortex walls prevails.Current-induced domain wall motion ͑CIDM͒ has recently prompted much attention, since it can be an alternative to the conventional field-induced switching. In particular, a number of memory and sensor devices based on this effect have been suggested, a prominent example being the racetrack design envisaged by Parkin.1 To use currentinduced domain wall motion in such a device, reliable and reproducible wall propagation has to be achieved.While CIDM has long been known theoretically 2,3 as well as experimentally, 4 only recently controlled currentinduced motion of single domain walls in magnetic nanostructures has been observed. [5][6][7][8][9][10][11][12] The underlying theory of interaction between current and magnetization is still controversial. Different approaches have been suggested with adiabatic 13-15 and non-adiabatic contributions. 16,17 While these theories qualitatively reproduce CIDM, quantitative calculations of the wall velocities do not agree with the experimental observations and this has initiated a discussion about the importance of the nonadiabatic contribution. 7,8,16,17 It was recently shown 7 that vortex head-to-head domain walls are not only displaced but also the wall spin structures can be transformed to a transverse wall with consequences for the wall velocity and the reliability of the switching. Theoretically it was predicted that the vortex core is not only expelled but also renucleated periodically, 17 and the latter should also have an effect on the wall velocity. The observation of such a nucleation of a vortex core is of interest for a further theoretical understanding, since it depends crucially on the nonadiabatic term; 17 it has been also predicted that for sufficiently high current densities the single domain state is not the stable ground state and nucleation occurs. 18In this letter we investigate current-induced domain wall propagation and wall transformations in 28 nm thick zigzag line elements to determine the wall-type transformations which occur by the nucleation and annihilation of vortex cores and their influence on the wall velocities.28 nm thick Permalloy ͑Ni 80 Fe 20 ͒ zigzag structures capped with 2 nm Au with widths W between 100 nm and 2 m and lengths of up to 100 m have been fabricated as detailed in Ref. 19. In Fig. 1͑a͒, a scanning electron microscopy ͑SEM͒ image of such a zigzag structure is shown ͑W =1 m͒. To image the magnetization configuration, x-ray magnetic circular dichroism photoemission electron microscopy ͑XMCDPEEM͒ is used. 20,21 In Fig. 1͑b͒ we present an XMCDPEEM image of the sta...

Dynamic magnetic properties of a single micrometer-sized magnetic element consisting of a permalloy and a partially patterned CoFe layer separated by an intervening Cu spacer layer have been studied by means of a micro-focus Brillouin light scattering setup, which allows for local measurements of the magnetization dynamics on the submicrometer scale. It is shown that quantized spin-wave modes excited in the magnetic element act as radiation sources for spin waves in the surrounding magnetic film. It is found that the intensities of spin waves excited by different quantized modes follow different distance laws when traveling away from the region of excitation.

Ferromagnetic metal rings of nanometre range widths and thicknesses exhibit fundamentally new spin states, switching behaviour and spin dynamics, which can be precisely controlled via geometry, material composition and applied f eld. Following the discovery of the 'onion state', which mediates the switching to and between vortex states, a range of fascinating phenomena has been found in these structures. In this overview of our work on ring elements, we f rst show how the geometric parameters of ring elements determine the exact equilibrium spin conf guration of the domain walls of rings in the onion state, and we show how such behaviour can be understood as the result of the competition between the exchange and magnetostatic energy terms. Electron transport provides an extremely sensitive probe of the presence, spatial location and motion of domain walls, which determine the magnetic state in individual rings, while magneto-optical measurements with high spatial resolution can be used to probe the switching behaviour of ring structures with very high sensitivity. We illustrate how the ring geometry has been used for the study of a wide variety of magnetic phenomena, including the displacement of domain walls by electric currents, magnetoresistance, the strength of the pinning potential introduced by nanometre size constrictions, the effect of thermal excitations on the equilibrium state and the stochastic nature of switching events.