There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Atomistic modelling of magnetic materials provides unprecedented detail about the underlying physical processes that govern their macroscopic properties, and allows the simulation of complex effects such as surface anisotropy, ultrafast laser-induced spin dynamics, exchange bias, and microstructural effects. Here we present the key methods used in atomistic spin models which are then applied to a range of magnetic problems. We detail the parallelization strategies used which enable the routine simulation of extended systems with full atomistic resolution.

/pdf/atomistic-spin-model-simulations-of-magnetic-nanomaterials-31j7wcva63.pdf

Atomistic spin model simulations of magnetic nanomaterials.

Atomistic modelling of magnetic materials provides unprecedented detail about the underlying physical processes that govern their macroscopic properties, and allows the simulation of complex effects such as surface anisotropy, ultrafast laser-induced spin dynamics, exchange bias, and mi- crostructural effects. Here we present the key methods used in atomistic spin models which are then applied to a range of magnetic problems. We detail the parallelisation strategies used which enable the routine simulation of extended systems with full atomistic resolution.

Atomistic spin model simulations of magnetic nanomaterials

This paper reviews the state of the art in spin-torque and spin-Hall-effect-driven nano-oscillators. After a brief introduction to the underlying physics, the authors discuss different implementations of these oscillators, their functional properties in terms of frequency range, output power, phase noise, and modulation rates, and their inherent propensity for mutual synchronization. Finally, the potential for these oscillators in a wide range of applications, from microwave signal sources and detectors to neuromorphic computation elements, is discussed together with the specific electronic circuitry that has so far been designed to harness this potential.

Spin-Torque and Spin-Hall Nano-Oscillators

Dynamics induced by spin-transfer torque is a quickly developing topic in modern magnetism, which has initiated several new approaches to magnetic nanodevices. It is now well established that a spin-polarized electric current injected into a ferromagnetic layer through a nanocontact exerts a torque on the magnetization, leading to microwave-frequency precession detectable through the magnetoresistance effect. This phenomenon provides a way for the realization of tunable nanometre-size microwave oscillators, the so-called spin-torque nano-oscillators (STNOs). Present theories of STNOs are mainly based on pioneering works predicting emission of spin waves due to the spin torque. Despite intense experimental studies, until now this spin-wave emission has not been observed. Here, we report the first experimental observation and two-dimensional mapping of spin waves emitted by STNOs. We demonstrate that the emission is strongly directional, and the direction of the spin-wave propagation is steerable by the magnetic field. The information about the emitted spin waves obtained in our measurements is of key importance for the understanding of the physics of STNOs, and for the implementation of coupling between individual oscillators mediated by spin waves. Analysis shows that the observed directional emission is a general property inherent to any dynamical system with strongly anisotropic dispersion.

https://www.uni-muenster.de/imperia/md/content/physik_ap/demokritov/papers/nmat2882.pdf

Direct observation and mapping of spin waves emitted by spin-torque nano-oscillators

We present atomistic calculations of the magnetic phase transition behavior in an L10 FePt system to study the effect of grain size distribution on the Curie temperature (Tc) dispersion with relevance to heat assisted magnetic recording. Identifying the relation between the size and Tc of a grain by means of finite size scaling analysis of the differentiated magnetization versus T data allows to show that a lognormal size distribution transforms into a lognormal Tc distribution with moments dependent on the critical exponents. We also address the question of the universality class of FePt.

/pdf/the-curie-temperature-distribution-of-fept-granular-magnetic-1wsabjgt1d.pdf

The Curie temperature distribution of FePt granular magnetic recording media

The electron transport in magnetic tunnel junction containing pinholes is studied, where ferromagnetic layers make direct contact through narrow conducting channels. The transport is treated within the Landauer formalism using a tight-binding Hamiltonian. It is found that a narrow pinhole induces a resonant tunneling behavior causing a magnetoresistance that oscillates with the bias voltage. After averaging over pinholes of various shapes, the oscillation and magnitude of magnetoresistance (MR) are reduced, which can explain how barrier roughness diminishes the MR. For a pinhole of a large radius, where the number of atoms inside is much larger than 1, an enhanced MR is obtained whose value is similar to an all-metal giant magnetoresistance spin valve.

Effect of pinholes in magnetic tunnel junctions

A simple torque-based method for calculating the transient behavior of temperature-dependent magnetic anisotropy is introduced. Using this method we obtain several physical quantities at finite temperature, including effective anisotropy, anisotropy field, and their fluctuations, that are crucial to heat-assisted magnetic recording (HAMR). We show that use of a composite grain that includes a high Curie temperature soft layer reduces the anisotropy fluctuations.

Time Dependence of Magnetic Anisotropy at Finite Temperature for Homogeneous and Composite Media

In general, the invention is directed to techniques for reducing the amount of switching current that is utilized within a magnetic storage (e.g., MRAM) device. An example apparatus includes a fixed magnetic layer that provides a fixed direction of magnetization, an exchange-coupled magnetic multi-layer structure, and a non-magnetic layer placed between the fixed magnetic layer and the exchange-coupled magnetic multi-layer structure. The exchange-coupled magnetic multi-layer structure includes a recording layer configured to record information and an assisting layer having a lower anisotropy than the recording layer. The exchange coupling between the recording and assisting layers is operable to switch a magnetization direction of the recording layer. In some cases, the exchange-coupled magnetic multi-layer structure may further include a spacer separating the recording and assisting layers and configured to weaken an exchange coupling between the recording and assisting layers.

http://absimage.aps.org/image/MAR09/MWS_MAR09-2008-006571.pdf

Exchange-assisted spin transfer torque switching

Electronic transport with surface scattering in metallic nanowires is studied theoretically based on an atomistic tight-binding approach. It is shown that the mean free path (MFP) strongly depends on the size of the wire and the scattering potential at the surface. In the weak scattering regime, the MFP grows with the wire diameter in an oscillatory manner. A perturbation theory is developed to explain this finite size effect. For narrow wires with small roughness, we show that the surface can be the dominant source of scattering and increases the resistivity well above the bulk value, which will adversely impact the future application of nanowire.

Surface scattering in metallic nanowires

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