Unai Atxitia

Ultrafast heating as a sufficient stimulus for magnetization reversal in a ferrimagnet

The question of how, and how fast, magnetization can be reversed is a topic of great practical interest for the manipulation and storage of magnetic information. It is generally accepted that magnetization reversal should be driven by a stimulus represented by time-non-invariant vectors such as a magnetic field, spin-polarized electric current, or cross-product of two oscillating electric fields. However, until now it has been generally assumed that heating alone, not represented as a vector at all, cannot result in a deterministic reversal of magnetization, although it may assist this process. Here we show numerically and demonstrate experimentally a novel mechanism of deterministic magnetization reversal in a ferrimagnet driven by an ultrafast heating of the medium resulting from the absorption of a sub-picosecond laser pulse without the presence of a magnetic field.

Micromagnetic modeling of laser-induced magnetization dynamics using the Landau-Lifshitz-Bloch equation

We present a dynamic approach to micromagnetics based on the Landau-Lifshitz-Bloch equation and Langevin dynamics. This type of modeling will be necessary at high temperatures when the magnetization length is not conserved, especially close to the Curie temperature. We model the laser-induced magnetization dynamics with various laser pulse fluences and show that the results are consistent with both experiments and atomistic modeling. Our results show different recovery rates depending on the final demagnetized state.

Ultrafast Spin Dynamics : The Effect of Colored Noise

Recent experimental results have pushed the limits of magnetization dynamics to pico- and femtosecond time scales. This ultrafast dynamics occurs in extreme conditions of strong and rapid fields and high temperatures. This situation requires a new description of magnetization dynamics, taking into account that the electron correlation time could be of the order of the inverse spin frequency. For this case we introduce a thermodynamically correct phenomenological Landau-Lifshitz-Miyasaki-Seki approach. We demonstrate the effect of the noise correlation time on the ultrafast demagnetization rate.

Landau-Lifshitz-Bloch equation for ferrimagnetic materials

We derive the Landau-Lifshitz-Bloch (LLB) equation for a two-component magnetic system valid up to the Curie temperature. As an example, we consider disordered GdFeCo ferrimagnet where the ultrafast optically induced magnetization switching under the action of heat alone has been recently reported. The two-component LLB equation contains the longitudinal relaxation terms responding to the exchange fields from the proper and the neighboring sublattices. We show that the sign of the longitudinal relaxation rate at high temperatures can change depending on the dynamical magnetization value and a dynamical polarisation of one material by another can occur. We discuss the differences between the LLB and the Baryakhtar equation, recently used to explain the ultrafast switching in ferrimagnets. The two-component LLB equation forms basis for the largescale micromagnetic modeling of nanostructures at high temperatures and ultrashort timescales.

Two-magnon bound state causes ultrafast thermally induced magnetisation switching.

There has been much interest recently in the discovery of thermally induced magnetisation switching using femtosecond laser excitation, where a ferrimagnetic system can be switched deterministically without an applied magnetic field. Experimental results suggest that the reversal occurs due to intrinsic material properties, but so far the microscopic mechanism responsible for reversal has not been identified. Using computational and analytic methods we show that the switching is caused by the excitation of two-magnon bound states, the properties of which are dependent on material factors. This discovery allows us to accurately predict the onset of switching and the identification of this mechanism will allow new classes of materials to be identified or designed for memory devices in the THz regime.

Ultrafast dynamical path for the switching of a ferrimagnet after femtosecond heating

Ultrafast laser-induced magnetic switching in rare earth, transition metal ferrimagnetic alloys has recently been reported to occur by ultrafast heating alone. Using atomistic simulations and a ferrimagnetic Landau-Lifshitz-Bloch formalism, we demonstrate that for switching to occur it is necessary that angular momentum is transferred from the longitudinal to transverse magnetization components. This dynamical path leads to magnetization switching and subsequent ultrafast precession caused by the inter-sublattice exchange field on the nanoscale.

Multiscale modeling of ultrafast element-specific magnetization dynamics in FeNi ferromagnetic alloys

Herein, to understand the differences of the magnetization dynamics of Fe and Ni in Py, we develop a model based on a hierarchical multi-scale approach to investigate the sub-lattice dynamics of ferromagnetic alloys and to obtain a deeper insight into the underlying mechanisms. First, we construct and parametrize a spin model Hamiltonian for Py on the basis of first-principles calculations. This spin model Hamiltonian in combination with extensive atomistic spin computer simulations based on the stochastic Landau-Lifshitz-Gilbert equation are used to calculate the demagnetization process after the application of a step heat pulse. The second step of the presented multiscale model links the information gained from the atomistic spin model to the macroscopic two sub-lattices Landau-Lifshitz-Bloch (LLB) equation of motion recently derived by Atxitia et al. The analytical LLB models allow for cheap simulations, and most importantly, provide insight in the element-specific demagnetization rates of Py.

Quantitative simulation of temperature-dependent magnetization dynamics and equilibrium properties of elemental ferromagnets

Atomistic spin model simulations are immensely useful in determining temperature dependent magnetic prop- erties, but are known to give the incorrect dependence of the magnetization on temperature compared to exper- iment owing to their classical origin. We find a single parameter rescaling of thermal fluctuations which gives quantitative agreement of the temperature dependent magnetization between atomistic simulations and experi- ment for the elemental ferromagnets Ni, Fe, Co and Gd. Simulating the sub-picosecond magnetization dynam- ics of Ni under the action of a laser pulse we also find quantitative agreement with experiment in the ultrafast regime. This enables the quantitative determination of temperature dependent magnetic properties allowing for accurate simulations of magnetic materials at all temperatures.

Inertia-Free Thermally Driven Domain-Wall Motion in Antiferromagnets

Domain-wall motion in antiferromagnets triggered by thermally induced magnonic spin currents is studied theoretically. It is shown by numerical calculations based on a classical spin model that the wall moves towards the hotter regions, as in ferromagnets. However, for larger driving forces the so-called Walker breakdown-which usually speeds down the wall-is missing. This is due to the fact that the wall is not tilted during its motion. For the same reason antiferromagnetic walls have no inertia and, hence, no acceleration phase leading to higher effective mobility.

Fundamentals and applications of the Landau–Lifshitz–Bloch equation

The influence of thermal excitations on magnetic materials is a topic of increasing relevance in the theory of magnetism. The Landau–Lifshitz–Bloch equation describes magnetisation dynamics at finite temperatures. It can be considered as an extension of already established micromagnetic methods with a comparable numerical effort. This review is a brief summary of this new field of research, with a focus on the fundamentals of the Landau–Lifshitz–Bloch equation, its connection with the stochastic Landau–Lifshitz equation, and its applications in modern magnetism.