The theoretical foundation for a nonvolatile memory device based on magnetic vortices is presented. We propose a realization of a vortex random-access memory (VRAM) containing vortex cells that are controlled by alternating currents only. The proposed scheme allows to transfer the vortex into an unambiguous binary state regardless of its initial state within a subnanosecond time scale. The vortex handedness defined as the product of chirality and polarization as a bit representation allows direct mechanisms for reading and writing the bit information. The VRAM is stable at room temperature.

Time-resolved X-ray microscopy is used to image the influence of alternating high-density currents on the magnetization dynamics of ferromagnetic vortices. Spin-torque induced vortex gyration is observed in micrometer-sized permalloy squares. The phases of the gyration in structures with different chirality are compared to an analytical model and micromagnetic simulations, considering both alternating spin-polarized currents and the current's Oersted field. In our case the driving force due to spin-transfer torque is about 70% of the total excitation while the remainder originates from the current's Oersted field. This finding has implications to magnetic storage devices using spin-torque driven magnetization switching and domain-wall motion.PACS numbers: 68.37. Yz, 72.25.Ba , 75.25.+z, 75.40.Mg, The discovery that spin-polarized electrons traveling through ferromagnets apply a torque on the local magnetization 1 opened up a new field of research in solid state physics that could potentially result in new magnetic storage media. It is now understood that the spin-transfer torque acts on inhomogeneities in the magnetization, e.g., on interfaces between magnetic layers, 2 on domain walls, 3,4 i.e., interfaces between regions of uniform magnetization, or on magnetic vortices. 5,6,7,8 Magnetic domain walls, usually vortex walls, can be driven by spin-polarized currents to store information in bit registers. 10Vortices appear in laterally confined thin films when it is energetically favorable for the magnetization to point in-plane and parallel to the edges. In the center the magnetization is forced out-of-plane to avoid large angles between magnetic moments that would drastically increase the exchange energy. The region with a strong out-of-plane magnetization component is called the vortex core and is only a few nanometers in diameter. 11,12 The direction of the magnetization in the vortex core, also called the core polarization p, can only point out-ofor into-the-plane (p=+1 or p=−1, respectively). Hence ferromagnetic thin films containing vortex cores have been suggested as data storage elements. The chirality c = +1(−1) denotes the counterclockwise (clockwise) in-plane curling direction of the magnetization. It is known that vortices can be excited to gyrate around their equilibrium position by magnetic fields. 13,14 Recently it has been shown that field excitation can also switch the core polarization. 15,16,17,18,19,20 Micromagnetic simulations predict that spin-polarized currents can cause vortices both to gyrate 5,7 and to switch their polarization. 8,21,22 Both for field-and spin-torque-driven excitation, the direction of gyration is governed by the vortex polarization according to the right-hand rule (see Fig. 2 of Ref.14 ). The phase of field-driven gyration depends also on the chirality, while spin-torque driven gyration is independent of the chirality as the spin-transfer torque is proportional to the spatial derivative of the magnetization.7 Time-and spatially averaging experimental techniques indicate...

The strength of the non-adiabatic spin torque is currently under strong debate, as its value differs by orders of magnitude as well in theoretical predictions as in measurements. Here, a measurement scheme is presented that allows to determine the strength of the non-adiabatic spin torque accurately and directly. Analytical and numerical calculations show that the scheme allows to separate the displacement due to the Oersted field and is robust against uncertainties of the exact current direction. PACS numbers: 75.60.Ch, 72.25.Ba A spin-polarized current flowing through a ferromagnetic sample interacts with the magnetization and exerts a torque on the local magnetic moments. For conduction electron spins that follow the local magnetization adiabatically it has been shown that the interaction via spin transfer can be described by adding a current-dependent term to the Landau-LifshitzGilbert equation.[1] This equation has been extended by an additional term that takes the non-adiabatic influence of the itinerant spins into account.[2] Theoretically, several mechanisms have been proposed as the origin of the non-adiabatic spin torque, leading to different orders of magnitude for its strength. [2,3,4,5,6] Thus a precise measurement of the non-adiabatic spin torque is necessary to give insight into its microscopic origin. A determination of its strength is further important for a reliable prediction of the current-driven domain-wall velocity. [2] Currently measured values of the non-adiabatic spin torque for permalloy differ by one order of magnitude, [7,8,9,10] thus the strength of the non-adiabatic spin torque is under strong debate. In these experiments the observed motion of a domain wall was compared with micromagnetic simulations to determine the non-adiabatic spin torque. This analysis is highly susceptible to surface roughness and Oersted fields.Due to its high symmetry and spacial confinement a vortex in a micro-or nanostructured magnetic thin-film element is a promising system for the investigation of the spin-torque effect. [11,12,13] Vortices are formed when the in-plane magnetization curls around a center region. In this few-nanometerlarge center region, called the vortex core, the magnetization turns out of plane to minimize the exchange energy. There are four different ground states of a vortex. These states are labeled by the direction of the out-of-plane magnetization, called polarization p, and the sense of rotation of the in-plane magnetization, called chirality c. Polarizations of p = 1 and p = −1 denote a core that points parallel or antiparallel to the z axis, respectively. A chirality of c = 1 denotes a counterclockwise curling of the in-plane magnetization while c = −1 denotes a clockwise curling.It is known that vortices are displaced from their equilibrium position when excited by spin-polarized electric current pulses. [12,13,14,15,16,17,18,19,20] The spatial confinement of the vortex core within the film element yields an especially accessible system for measurements with scanning probe...

The spin-transfer torque between itinerant electrons and the magnetization in a ferromagnet is of fundamental interest for the applied physics community. To investigate the spin-transfer torque, powerful simulation tools are mandatory. We propose a micromagnetic standard problem including the spin-transfer torque that can be used for the validation and falsification of micromagnetic simulation tools. The work is based on the micromagnetic model extended by the spin-transfer torque in continuously varying magnetizations as proposed by Zhang and Li. The standard problem geometry is a permalloy cuboid of 100 nm edge length and 10 nm thickness, which contains a Landau pattern with a vortex in the center of the structure. A spin-polarized dc current density of 10 12 A/m 2 flows laterally through the cuboid and moves the vortex core to a new steady-state position. We show that the new vortex-core position is a sensitive measure for the correctness of micromagnetic simulators that include the spin-transfer torque. The suitability of the proposed problem as a standard problem is tested by numerical results from four different finite-difference and finite-element-based simulation tools.

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