In this paper, we review some of the key concepts in ultrathin film magnetism which underpin nanomagnetism. We survey the results of recent experimental and theoretical studies of well characterized epitaxial structures based on Fe, Co and Ni to illustrate how intrinsic fundamental properties such as the magnetic exchange interactions, magnetic moment and magnetic anisotropies change markedly in ultrathin films as compared with their bulk counterparts, and to emphasize the role of atomic scale structure, strain and crystallinity in determining the magnetic properties. After introducing the key length scales in magnetism, we describe the 2D magnetic phase transition and survey studies of the thickness dependent Curie temperature and the critical exponents which characterize the paramagnetic-ferromagnetic phase transition. We next discuss recent experimental and theoretical results on the determination of the exchange constant, followed by an overview of measurements of the magnetic moment in the elemental 3d transition metal thin films in the various crystal phases that have been successfully stabilized, thereby illustrating the sensitivity of the magnetic moment to the local symmetry and to the atomic environment. Finally, we discuss briefly the magnetic anisotropies of Fe, Co and Ni in the fcc crystalline phase, to emphasize the role of structure and the details of the interface in influencing the magnetic properties. The dramatic effect that adsorbates can have on the magnetic anisotropies of thin magnetic films is also discussed. Our survey demonstrates that the fundamental properties, namely, the magnetic moment and magnetic anisotropies of ultrathin films have dramatically different behaviour compared with those of the bulk while the comparable size of the structural and magnetic contributions to the total energy of ultrathin structures results in an exquisitely sensitive dependence of the magnetic properties on the film structure.

Direct observations of current-induced domain-wall propagation by spin-polarized scanning electron microscopy are reported. Current pulses move head-to-head as well as tail-to-tail walls in submicrometer Fe20Ni80 wires in the direction of the electron flow, and a decay of the wall velocity with the number of injected current pulses is observed. High-resolution images of the domain walls reveal that the wall spin structure is transformed from a vortex to a transverse configuration with subsequent pulse injections. The change in spin structure is directly correlated with the decay of the velocity.

We present the results of a study of the magnetic properties of an array of 34-nm thick Co(100) epitaxial ring magnets, with inner and outer diameters of d(in) = 1.3 microm and d(out) = 1.6 microm, respectively. Magnetic measurements and micromagnetic simulations show that a two step switching process occurs at high fields, indicating the existence of two different stable states. In addition to the vortex state, which occurs at intermediate fields, we have identified a new bi-domain state, which we term the onion state, corresponding to opposite circulation of the magnetization in each half of the ring. The onion state is stable at remanence and undergoes a simple and well characterized nucleation free switching.

The high-symmetry ring geometry is shown to exhibit a wide range of intriguing magnetostatic and magnetodynamic properties, which we survey in this topical review. We consider first the patterning and deposition techniques, which are used to fabricate ring structures (diameters between 0.1 and 2 µm) and discuss their respective advantages and disadvantages. The results of direct nanoscale imaging of the novel magnetization configurations present in rings with different geometrical parameters (including discs) are discussed. These results give valuable insight into the influence of the magnetic anisotropies governing the magnetic states. The different types of domain walls that arise are compared quantitatively to micromagnetic simulations. The magnetodynamic switching between the different magnetic states is described in detail. In particular we elaborate on the different geometry-dependent magnetic switchings, since the different transitions occurring allow us to determine which energy terms govern the reversal process. We discuss a process by which fast (sub-ns) and controlled switching can be achieved, therefore making rings an attractive geometry for applications, in addition to studying fundamental issues of nanomagnetism.

In a combined numerical and experimental study, we demonstrate that current pulses of different polarity can reversibly and controllably displace a magnetic domain wall (DW) in submicrometer permalloy (NiFe) ring structures. The critical current densities for DW displacement are correlated with the specific spin structure of the DWs and are compared to results of micromagnetic simulations including a spin-torque term. Using a notch, an attractive local pinning potential is created for the DW resulting in a highly reproducible spin structure of the DW, critical for reliable current-induced switching.PACS numbers: 72.15. Gd, 75.60.Ch, 75.60.Ej, 85.70.Kh Switching by domain wall motion [1] induced by spinpolarized currents rather than by external fields is a promising approach to the switching of magnetic nanostructures, since it entails simple fabrication processes without the need for strip lines, combined with the possibility of achieving fast and reproducible switching [2 -6]. The current-induced magnetization switching mechanism has been shown to be able to switch multilayer giant magnetoresistance structures [7] and to reverse simple single layer elements as observed for L-shaped elements [2,3], for ring-shaped [4] and straight [5] structures with constrictions, and also in multilayer wires [6]. This currentinduced domain wall motion is due to a spin-torque effect, where the electrons transfer angular momentum to the domain wall when passing through it, pushing it in the direction of the electron flow [8]. Since the original paper by Berger [8], a number of different theories have been suggested that treat the interaction between the spinpolarized current and the magnetization in the ballistic limit [9,10] or in the diffusive limit [9,11]. For wide inplane domain walls with widths of hundreds of nanometers [12] the spins of the electrons are expected to follow the magnetization adiabatically, and thus the diffusive description is expected to apply. While there are a number of experimental results, no direct quantitative comparison to the theoretical predictions has been made available.In this Letter we demonstrate that different types of head-to-head domain walls present in ring structures can be reversibly and controllably displaced by spin-polarized current pulses. Direct comparison with the results of our micromagnetic simulations that include a diffusive spintorque term allows us to determine to what extent the experimentally observed effects can be described by a purely diffusive spin-torque theory.Rings are a particularly apt geometry to investigate the effect of pulses on domain walls, since head-to-head walls with different spin structures can be obtained for different ring geometries [12]. For thin film rings, transverse walls