Magnetic thin films with perpendicular magnetic anisotropy have localized excitations that correspond to reversed, dynamically precessing magnetic moments, which are known as magnetic droplet solitons. Fundamentally, these excitations are associated with an attractive interaction between elementary spin-excitations and have been predicted to occur in perpendicularly magnetized materials in the absence of damping. Although damping suppresses these excitations, it can be compensated by spin-transfer torques when an electrical current flows in nanocontacts to ferromagnetic thin films. Theory predicts the appearance of magnetic droplet solitons in nanocontacts at a threshold current and, recently, experimental signatures of droplet nucleation have been reported. However, to date, these solitons have been observed to be nearly reversible excitations, with only partially reversed magnetization. Here, we show that magnetic droplet solitons exhibit a strong hysteretic response in field and current, proving the existence of bistable states: droplet and non-droplet states. In the droplet soliton state we find that the magnetization in the contact is almost fully reversed. These observations, in addition to their fundamental interest, are important to understanding and controlling droplet motion, nucleation and annihilation.

Orthogonal spin-transfer magnetic random access memory ͑OST-MRAM͒ uses a spin-polarizing layer magnetized perpendicularly to a free layer to achieve large spin-transfer torques and ultrafast energy efficient switching. We have fabricated and studied OST-MRAM devices that incorporate a perpendicularly magnetized spin-polarizing layer and a magnetic tunnel junction, which consists of an in-plane magnetized free layer and synthetic antiferromagnetic reference layer. Reliable switching is observed at room temperature with 0.7 V amplitude pulses of 500 ps duration. The switching is bipolar, occurring for positive and negative polarity pulses, consistent with a precessional reversal mechanism, and requires an energy of less than 450 fJ.

In a combined experimental and numerical study, we determine the details of the pinning of domain walls at constrictions in permalloy nanostructures. Using high spatial-resolution ͑Ͻ10 nm͒ electron holography, we image the spin structure of geometrically confined head-to-head domain walls at constrictions. Low-temperature magnetoresistance measurements are used to systematically ascertain the domain-wall depinning fields in constrictions down to 35 nm width. The depinning fields increase from 60 to 335 Oe with decreasing constriction width and depend on the wall spin structure. The energy barrier to depin the wall from the constriction is quantitatively determined and comparison with the depinning field strength allows us to gauge the energy barrier height of the pinning potential.The recent upsurge of interest in magnetic domain walls has been fuelled partly by a fundamental interest in the spin structure of nanoscale domain walls and in particular by possible novel logic and memory applications based on domain walls.1-3 Control of the domain wall behavior, and in particular of the magnetic switching, can be achieved through pinning centers, which provide well defined stable locations for doma in walls and can be used to confine the wall propagation.3-6 Pinning centers can result from imperfections in the material, 7 which are inherently hard to control. Instead, artificially structured variations in the geometry of an element have been introduced to engineer such pinning. 5,[8][9][10][11][12] In addition to applications, such as domain-wall diodes in logic devices, 8 constrictions have also allowed the determination of more fundamental properties of domain walls, such as magnetoresistance ͑MR͒ effects associated with domain walls and their quasiparticle spin-block behavior.10,11 To probe this landscape for the energy potential, rings have proven to be an apt geometry since, due to the curved geometry, domain walls can be positioned easily using uniform fields applied along appropriate directions. In earlier work on narrow rings, we observed indirectly by MR measurements that constrictions create an attractive potential well for transverse walls, but vortex walls are repelled from a constriction. 10,11 While qualitatively the domain-wall behavior could be determined using MR measurements, the details of the wall positions and in particular the nanoscale wall spin structure-in structures with constrictions-has not been elucidated. An adequate characterization of this energy potential requires the determination of its width and amplitude, since for applications it is the strength of the pinning, which corresponds to the depth of the potential well, that is critical for engineering of devices with reliable switching, thermal stability, etc. From a fundamental point of view, the strength of the pinning is directly correlated with the wall spin structure, so that only the determination of its nanoscale spin structure will enable an in-depth understanding of the energetics governing the pinning strength.In this lette...

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 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...