In laser-excited ferromagnetic heterostructures, both ultrafast local magnetization dynamics and spin-transport processes can lead to a THz emission. Here, we demonstrate that the THz emission spectroscopy is a powerful tool to investigate ultrafast magnetization dynamics in laser-excited magnetic systems. The polarity of emitted THz can be used to distinguish which process, local or non-local, dominates the emission of THz in a ferromagnetic heterostructure. The measured THz radiation can be used for rigorous reconstruction of ultrafast magnetization process in the laser-excited magnetic material.
It has long been known that ferromagnets undergo a phase transition from ferromagnetic to paramagnetic at the Curie temperature, associated with critical phenomena such as a divergence in the heat capacity. A ferromagnet can also be transiently demagnetized by heating it with an ultrafast laser pulse. However, to date the connection between out-ofequilibrium and equilibrium phase transitions was not known, nor how fast the out-of-equilibrium phase transitions can proceed. In this work, by combining time- and angle-resolved photoemission (Tr-ARPES) with time-resolved transverse magneto-optical Kerr (Tr-TMOKE) spectroscopies, we show that the same critical behavior also governs the ultrafast magnetic phase transition in nickel. This is evidenced by several observations. First, we observe a divergence of the transient heat capacity of the electron spin system preceding material demagnetization. Second, when the electron temperature is transiently driven above the Curie temperature, we observe an extremely rapid change in the material response: the spin system absorbs sufficient energy within the first 20 fs to subsequently proceed through the phase transition, while demagnetization and the collapse of the exchange splitting occur on much longer timescales. Third, we find that the transient electron temperature alone dictates the magnetic response. By comparing results obtained from different methods, we show that the critical behaviors are essential for fully explaining the fluence-dependent magnetization dynamics measured using magneto-optical spectroscopy
Ultrafast laser-induced demagnetization was discovered two decades ago, however, the underlying fundamental processes of the fast magnetization dissipation continues to be intensively debated. I shall review recent achievements in the field of ultrafast magnetism with a focus on the various proposed mechanisms and on technological relevant applications, such as THz spintronics.
Ultrafast demagnetization of a ferromagnetic (FM) layer on a nonmagnetic (NM) metallic layer can cause injection of a large, ultrafast spin-polarized current into the NM layer, which can lead to a strong THz emission pulse. We quantify the amount of spin injection and find that the spin injection proceeds within femtoseconds. Studying a FM/NM/FM trilayer with orthonormal directions of the magnetization in the FM layers, we investigate how the emitted spin current pulse can excite THz spin waves in the 2nd FM layer.
Investigating further ultrafast demagnetization and all-optical switching of high-anisotropy materials such as FePt we find that element-dependent spin dynamics plays a role in the picosecond demagnetization, and that ultrafast spin transport can enhance the demagnetization of nano-sized materials.
This work was performed together with P. Maldonado, M. Berritta, R. Mondal, K. Carva, L. Salemi, U. Ritzmann, J. Hurst and P. Balaz.
Inertia is a fundamental property of a particle that can be understood from Newtons laws of motion. In a similar way, any magnetized body must possess magnetic inertia by the virtue of its magnetization. The influence of possible magnetic inertia effects has recently drawn attention in ultrafast magnetization dynamics and switching. Magnetization dynamics at the inertial regime has been investigated using thermodynamic theories that predicted the magnetic inertia can become impactful at shorter timescales. However, at the fundamental level, the origin of magnetic inertial dynamics is still unknown.
Here, we derive rigorously a description of magnetic inertia in the extended Landau-Lifshitz-Gilbert (LLG) equation starting from a fundamental and relativistic Dirac-Kohn-Sham framework. We use a unitary transformation, the so called called Foldy-Wouthuysen transformation, up to the order of 1/c4. In this way, the particle and anti-particle in fully relativistic description become decoupled and a Hamiltonian describing only the particles is derived. This Hamiltonian involves the nonrelativistic Schrodinger-Pauli Hamiltonian together with the relativistic corrections of the order 1/c2 and 1/c4.
With the thus-derived Hamiltonian, we calculate the corresponding spin dynamics leading to the LLG equation of motion. Our result exemplify that the relativistic correction terms of 1/c2 are responsible for the Gilbert damping, however, the relativistic correction terms of 1/c4 are responsible for magnetic inertial dynamics. Therefore, we predict that the intrinsic magnetic inertia is a higher-order relativistic spin-orbit coupling effect and is expected to be prominent only on ultrashort timescales (subpicoseconds). We also show that the corresponding Gilbert damping and magnetic inertia parameters are related to one another through the imaginary and real parts of the magnetic susceptibility tensor, respectively.
Spin-Orbit Coupling (SOC) is a ubiquitous phenomenon in the spintronics area, as it plays a major role in allowing for enhancing many well-known phenomena, such as the Dzyaloshinskii-Moriya interaction, magnetocrystalline anisotropy, the Rashba effect, etc. However, the usual expression of the SOC interaction
ħ/4m2c2 [E×p] • σ (1)
where p is the momentum operator, E the electric field, σ the vector of Pauli matrices, breaks the gauge invariance required by the electronic Hamiltonian. On the other hand, very recently, a new phenomenological interaction, coupling the angular momentum of light and magnetic moments, has been proposed based on symmetry arguments:
ξ/2 [r × (E × B)] M, (2)
with M the magnetization, r the position, and ξ the interaction strength constant. This interaction has been demonstrated to contribute and/or give rise, in a straightforward way, to various magnetoelectric phenomena,such as the anomalous Hall effect (AHE), the anisotropic magnetoresistance (AMR), the planar Hall effect and Rashba-like effects, or the spin-current model in multiferroics. This last model is known to be the origin of the cycloidal spin arrangement in bismuth ferrite for instance. However, the coupling of the angular momentum of light with magnetic moments lacked a fundamental theoretical basis.
Starting from the Dirac equation, we derive a relativistic interaction Hamiltonian which linearly couples the angular momentum density of the electromagnetic (EM) field and the electrons spin. We name this coupling the Angular MagnetoElectric (AME) coupling. We show that in the limit of uniform magnetic field, the AME coupling yields an interaction exactly of the form of Eq. (2), thereby giving a firm theoretical basis to earlier works. The AME coupling can be expressed as:
ξ [E × A] • σ (3)
with A being the vector potential. Interestingly, the AME coupling was shown to be complementary to the traditional SOC, and together they restore the gauge invariance of the Hamiltonian. As an illustration of the AME coupling, we straightforwardly derived a relativistic correction to the so-called Inverse Faraday Effect (IFE), which is the emergence of an effective magnetic field under illumination by a circularly polarized light.
Terahertz (THz) electromagnetic radiation is located between the realms of electronics and optics and has successfully been used to probe and even control numerous low-energy excitations including phonons, excitons and Cooper pairs. Here, we show that THz spectroscopy is also a highly useful tool in the field of ultrafast spinbased electronics (spintronics) and consider the resonant manipulation of the magnetization of an antiferromagnet (through the THz Zeeman torque) and the probing of the tailored transport of spin density from a ferromagnetic into a nonmagnetic metal (through the THz inverse spin Hall effect).