Atomistic spin dynamics
Atomistic spin dynamics is a microscopic method of modelling magnetization dynamics in solid state combining first principle (ab initio) calculations with Landau-Lifshitz-Gilbert equation. By means of this method one assign a magnetic moment to each atom in the studied system. These magnetic moments interact via the exchange interactions, calculated using ab initio methods. Finally, the exchange interactions are included in the local effective magnetic fields, which govern the manetization dynamics
Atomistic spin dynamics can be used to examine magnetic properties of various materials as well as complex behaviour of magnetic moments influeced by spin transfer torque or temperature gradients studied in spintronics.
Copper magmanese arsenide is an antiferromagnetic material, which rystallizes in the orthorhombic phase. However, using molecular-beam epitaxy, it can be prepared in a tetragonal phase having interesting propertie for applications in spintronics.
CuMnAs is an room-temperature antiferromagnetic material with significant spin-orbit interaction. Due to the symmetry of the crystal lattice, the ordering parameter of CuMnAs can be efficiently manipulated due to so called staggered current-induced fields (Phys. Rev. Lett. 113, 157201).
Domain wall dynamics
In magnetism, a domain wall is an interface separating magnetic domains, i.e. areas where magnetization is homogeneous. Domain wall is a transition area between different magnetic moments and usually undergoes an angular displacement of 90 or 180 degrees (Wikipedia).
Apart from magnetic field, domain walls can be moved by spin current. Current-induced domain wall dynamics is an important effect for spintronic applications. The best known concept of domain-wall-based magnetic memory is the racetrack memory (Wikipedia).
Magnetism is a class of physical phenomena that are mediated by magnetic fields (Wikipedia).
A large part of the research in our group is focused on study of magnetic materials, especially ferromagnets and antiferromagnetis. Both types of materials are important for spintronics since some of them can be used for generation of spin currents and/or their magnetic moment can be manipulated by spin currents.
Magnetization dynamics is extensively studied in physics due to its importance for magnetic memories, data processing, and field sensors. In solid state, magnetic degrees of freedom are influenced by various phenomena to some extent. Thus, apart from magnetic fields, magnetic moments can be manipulated by spin-polarized current, thermal fluctuations, temperature gradients, laser pulses and spin-orbit torques. Importantly, mutual interactions between magnetic moments via exchange coupling and magnetostatic fields becomes important in larger structures and diluted magnetic systems with disorder.
Micromagnetics is a field of physics dealing with the prediction of magnetic behaviors at sub-micrometer length scales. The length scales considered are large enough for the atomic structure of the material to be ignored (the continuum approximation), yet small enough to resolve magnetic structures such as domain walls or vortices (Wikipedia).
Monte Carlo methods are a broad class of computational algorithms that rely on repeated random sampling to obtain numerical results. Their essential idea is using randomness to solve problems that might be deterministic in principle. They are often used in physical problems and are most useful when it is difficult or impossible to use other approaches (Wikipedia).
In condensed matter physics, Monte Carlo methods are used to study magnetic properties of materials at elevated temperatures. In combination with atomistic spin models, one can study magnetic phase transitions in different systems.
Multiferroics are defined as materials that exhibit more than one of the primary ferroic order parameters, ferromagnetism, ferroelectricity, or ferroelasticity in the same phase (Wikipedia).
In our group we focus on multilayer systems consisted of ferroelectric and ferromagnetic layers. It has been shown, that the adjacent ferroelectric layer can modify magnetic anisotropy in the ferromagnetic layer. Thus the magnetic moments can be manipulated by spin current as well as by electric field applied to the ferroelectric part.
Quantum Monte Carlo
Quantum Monte Carlo encompasses a large family of computational methods whose common aim is the study of complex quantum systems. One of the major goals of these approaches is to provide a reliable solution of the quantum many-body problem. Quantum Monte Carlo approaches all share the common use of the Monte Carlo method to handle the multi-dimensional integrals that arise in the different formulations of the many-body problem (Wikipedia).
When electric current passes through a magnetic conductor, the electron flux becomes spin-polarized. We talk about spin current or spin-polarized current. Apart from electric charge, spin current transfer also momentum. This momentum transfer can be observed eg. in magnetic multilayers with noncollinear magnetizations, where spin transfer torques, trasfered by the spin current, act on the localized magnetic moments.
Today, number of different methods of spin current generations are known: spin filtering, spin Seebeck effect, spin Hall effect, spin pumping, or ultrafast laser-induced demagnetization.
Spin transfer torque
Spin-transfer torque is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current (Wikipedia).
The effect of spin transfer torque is extensively studied in spintronics due to its potential for magnetic random access memories and data processing. Apart from the magnetic multilayers, spin transfer torque appears also in magnetic thin films with nonhomogeneous magnetization textures and can be used to manipulate with magnetic domain walls, vortices, or skyrmions.
A spin valve is a device, consisting of two or more conducting magnetic materials, whose electrical resistance can change between two values depending on the relative alignment of the magnetisations in the layers. The resistance change is a result of so called giant magnetoresistive effect (Wikipedia).
When current density flowing through a spin valve is large enought, it exerts spin transfer torque on the magnetizations, which can lead to magnetization dynamics and magnetization switching.
Spin waves are propagating disturbances in the ordering of magnetic materials. These low-lying collective excitations occur in magnetic lattices with continuous symmetry. From the equivalent quasiparticle point of view, spin waves are known as magnons, which are boson modes of the spin lattice (Wikipedia).
In our group we study spin waves and their potential utilization in spintronics. Using analytical and numerical methods we study, how spin waves can be generated by means of spin current, laser pulses, and domain wall dynamics.
Spin–orbit interaction (a.k.a. spin–orbit coupling) is a relativistic interaction of a particle's spin with its motion inside a potential. In solid state, spin-orbit interaction can be understood as a momentum-dependent magnetic field acting on the spin of the electron. As a result, when electric current flows through a single magnetic layer, spin-orbit interaction can generate torques, known as spin-orbit torques acting on the localized magnetic moments.
In many magnetic systems lacking bulk or structure inversion symmetry spin-orbit torques substantially influence magnetization dynamics. In some systems, spin-orbit torques can lead to magnetization switching.
Spintronics, also known as spin electronics, is the scope of physics which study the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices. In the last decades, spintronics is one of the most developing area in solid state physics connecting physics with material science and engineering.
A topological insulator is a material with non-trivial topological order that behaves as an insulator in its interior but whose surface contains conducting states, meaning that electrons can only move along the surface of the material (Wikipedia).
In our group we continuously study bulk conductivity and magnetic properties of Bismuth chalcogenides (Bi2Se3 and Bi2Te3) doped by magnetic atoms like Mn, Fe or Cr.
Transition metal is an element whose atom has a partially filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell (Wikipedia).
In spintronics, transition metals like Fe, Co, or Ni are widely used due to their ferromagnetic properties and conductivity. Due to exchange interactions between the condutive (s) electrons and localized (d) electrons, these materials can be used for generation of spin current. On the other hand, spin current flowing through these metals can exert spin transfer torque on the localized magnetic moments.
In 1996 Beaurepaire et al. have shown that a laser pulse can induced a significant reduction of magnetization of a Nickel thin film, which occur in less than a picosecond (Phys. Rev. Lett. 76, 4250). This observation demonstrated that one can manipulate with magnetization in ultrafast way, which is very promising for the future evolution of magnetic memories and data processing.
In our group we study this effect by ab initio calculations as well as by various phenomenological models. We focus mainly on ultrafast demagnetization due to superdiffusive spin transport of hot electrons excited by the laser pulse (Phys. Rev. Lett. 105, 027203).
phone: +420 95155 1397
- Ultrafast demagnetization in metals with focus on laser-induced electronic transport in metallic multilayers
- Magnetization dynamics including numerical simulations and analytical models of magnetization dynamics induced by magnetic fields, spin currents, spin-orbit torques or laser pulses; with special interest in the dynamics of magnetic domain walls and other magnetic topological defects
- Magnetic properties of disordered alloys ranging from ferro- and antiferromagnets up to noncollinear topological insulators doped by magnetic atoms; the main method of research is the atomistic spin dynamics based on ab initio calculations
- F. Máca, J. Kudrnovský, P. Baláž, et al.
Tetragonal CuMnAs alloy: Role of defects
- P. Baláž, S. J. Hämäläinen, and S. Van Dijken
Static properties and current-induced dynamics of pinned magnetic domain walls under applied fields: An analytic approach
- P. Baláž, M. Žonda, K. Carva, et al.
Transport theory for femtosecond laser-induced spin-transfer torques
Available projects for students
- Study of the dynamic properties of 90-degree magnetic domain wallsStudium dynamických vlastností 90-stupnových magnetických doménových stěn
Projects for students of 1st and 2nd year
- Magnetic dynamics in spintronic nanostructuresMagnetická dynamika v spintronických nanostrukturách