| 1. |
- Berritta, Marco, et al.
(författare)
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Ab Initio Theory of Coherent Laser-Induced Magnetization in Metals
- 2016
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 117:13
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Tidskriftsartikel (refereegranskat)abstract
- We present the first materials specific ab initio theory of the magnetization induced by circularly polarized laser light in metals. Our calculations are based on nonlinear density matrix theory and include the effect of absorption. We show that the induced magnetization, commonly referred to as inverse Faraday effect, is strongly materials and frequency dependent, and demonstrate the existence of both spin and orbital induced magnetizations which exhibit a surprisingly different behavior. We show that for nonmagnetic metals (such as Cu, Au, Pd, Pt) and antiferromagnetic metals the induced magnetization is antisymmetric in the light's helicity, whereas for ferromagnetic metals (Fe, Co, Ni, FePt) the imparted magnetization is only asymmetric in the helicity. We compute effective optomagnetic fields that correspond to the induced magnetizations and provide guidelines for achieving all-optical helicity-dependent switching.
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| 2. |
- Cherkasskii, Mikhail, et al.
(författare)
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Theory of inertial spin dynamics in anisotropic ferromagnets
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 106:5
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Tidskriftsartikel (refereegranskat)abstract
- Recent experimental observation of inertial spin dynamics calls upon holistic reevaluation of the theoretical framework of magnetic resonance in ferromagnets. Here, we derive the secular equation of an inertial spin system in analogy to the ubiquitous Smit-Beljers formalism. We find that the frequency of precessional ferromagnetic resonances is decreased as compared to the noninertial case. We also find that the frequency of nutational resonances is generally increased due to the presence of magnetic anisotropy and applied magnetic field. We obtain exact solutions of the secular equation and approximations that employ the terminology of noninertial theory and thus allow for convenient estimates of the inertial effects.
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| 3. |
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| 4. |
- John, R., et al.
(författare)
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Magnetisation switching of FePt nanoparticle recording medium by femtosecond laser pulses
- 2017
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Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- Manipulation of magnetisation with ultrashort laser pulses is promising for information storage device applications. The dynamics of the magnetisation response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage are FePt nanoparticles, for which switching of the magnetisation with optical angular momentum was demonstrated recently. The mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetisation switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to switch the magnetisation of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.
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| 5. |
- Li, Jingwen, et al.
(författare)
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A perspective on nonlinearities in coherent magnetization dynamics
- 2022
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 120:5
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Tidskriftsartikel (refereegranskat)abstract
- The recent thrust in ultrafast magnetization dynamics aims at extending spintronic functionalities to terahertz frequencies. Deterministic manipulation of magnetization at the corresponding ultrashort timescales requires minute control not only over the magnetization itself but also the reservoirs it is interacting with. Although the various intricate couplings between spins, phonons, and electrons-all of which are susceptible to ultrashort laser pulses-lead to many (often nonlinear) coupling routes, magnetization-dynamical nonlinearities have remained largely underexplored. In this Perspective, we highlight recent advances and foresee future developments in the rapidly evolving field of nonlinear magnetization dynamics. Given the elementary character of coherent excitations, we put particular emphasis on their nonlinearities. We briefly review theoretical aspects and assess excitation mechanisms to reach the nonlinear regime of magnetic excitations in a broad class of magnetic materials, such as ferromagnets, antiferromagnets, and ferrimagnets. We present an overview of the groundbreaking experiments that showcase the unique insights provided by magnetic nonlinearities. We conclude by discussing open challenges and opportunities that underpin the potential of nonlinear magnetization dynamics for the advancement of spintronics and cavity quantum electrodynamics with spin waves at terahertz frequencies.
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| 6. |
- Mondal, Ritwik, et al.
(författare)
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Ab initio investigation of light-induced relativistic spin-flip effects in magneto-optics
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:17
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Tidskriftsartikel (refereegranskat)abstract
- Excitation of a metallic ferromagnet such as Ni with an intensive femtosecond laser pulse causes an ultrafast demagnetization within approximately 300 fs. It was proposed that the ultrafast demagnetization measured in femtosecond magneto-optical experiments could be due to relativistic light-induced processes. We perform an ab initio investigation of the influence of relativistic effects on the magneto-optical response of Ni. To this end, first, we develop a response theory formulation of the additional appearing ultrarelativistic terms in the Foldy-Wouthuysen transformed Dirac Hamiltonian due to the electromagnetic field, and second, we compute the influence of relativistic light-induced spin-flip transitions on the magneto-optics. Our ab initio calculations of relativistic spin-flip optical excitations predict that these can give only a very small contribution (<= 0.1%) to the laser-induced magnetization change in Ni.
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| 7. |
- Mondal, Ritwik, et al.
(författare)
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Dynamics of the relativistic electron spin in an electromagnetic field
- 2020
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Ingår i: Journal of Physics. - : IOP PUBLISHING LTD. - 0953-8984 .- 1361-648X. ; 32:45
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Tidskriftsartikel (refereegranskat)abstract
- A relativistic spin operator cannot be uniquely defined within relativistic quantum mechanics. Previously, different proper relativistic spin operators have been proposed, such as spin operators of the Foldy-Wouthuysen and Pryce type, that both commute with the free-particle Dirac Hamiltonian and represent constants of motion. Here we consider the dynamics of a relativistic electron spin in an external electromagnetic field. We use two different Hamiltonians to derive the corresponding spin dynamics. These two are: (a) the Dirac Hamiltonian in the presence of an external field, and (b) the semirelativistic expansion of the same. Considering the Foldy-Wouthuysen and Pryce spin operators we show that these lead to different spin dynamics in an external electromagnetic field, which offers possibilities to distinguish their action. We find that the dynamics of both spin operators involve spin-dependent and spin-independent terms, however, the Foldy-Wouthuysen spin dynamics additionally accounts for the relativistic particle-antiparticle coupling. We conclude that the Pryce spin operator provides a suitable description of the relativistic spin dynamics in a weak-to-intermediate external field, whereas the Foldy-Wouthuysen spin operator is more suitable in the strong field regime.
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| 8. |
- Mondal, Ritwik, et al.
(författare)
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Generalisation of Gilbert damping and magnetic inertia parameter as a series of higher-order relativistic terms
- 2018
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Ingår i: Journal of Physics. - 0953-8984 .- 1361-648X. ; 30:26
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Tidskriftsartikel (refereegranskat)abstract
- The phenomenological Landau-Lifshitz-Gilbert (LLG) equation of motion remains as the cornerstone of contemporary magnetisation dynamics studies, wherein the Gilbert damping parameter has been attributed to first-order relativistic effects. To include magnetic inertial effects the LLG equation has previously been extended with a supplemental inertia term; the arising inertial dynamics has been related to second-order relativistic effects. Here we start from the relativistic Dirac equation and, performing a Foldy-Wouthuysen transformation, derive a generalised Pauli spin Hamiltonian that contains relativistic correction terms to any higher order. Using the Heisenberg equation of spin motion we derive general relativistic expressions for the tensorial Gilbert damping and magnetic inertia parameters, and show that these tensors can be expressed as series of higher-order relativistic correction terns. We further show that, in the case of a harmonic external driving field, these series can be summed and we provide closed analytical expressions for the Gilbert and inertial parameters that are functions of the frequency of the driving field.
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| 9. |
- Mondal, Ritwik, et al.
(författare)
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Inertial effects in ultrafast spin dynamics
- 2023
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier. - 0304-8853 .- 1873-4766. ; 579
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Forskningsöversikt (refereegranskat)abstract
- The dynamics of magnetic moments consists of a precession around the magnetic field direction and a relaxation towards the field to minimize the energy. While the magnetic moment and the angular momentum are conventionally assumed to be parallel to each other, at ultrafast time scales their directions become separated due to inertial effects. The inertial dynamics gives rise to additional high-frequency modes in the excitation spectrum of magnetic materials. Here, we review the recent theoretical and experimental advances in this emerging topic and discuss the open challenges and opportunities in the detection and the potential applications of inertial spin dynamics.
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| 10. |
- Mondal, Ritwik, et al.
(författare)
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Influence of intersublattice coupling on the terahertz nutation spin dynamics in antiferromagnets
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:10
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Tidskriftsartikel (refereegranskat)abstract
- Spin-nutation resonance has been well explored in one-sublattice ferromagnets. Here, we investigate the spin nutation in two-sublattice antiferromagnets as well as, for comparison, ferrimagnets with inter- and intrasublattice nutation coupling. In particular, we derive the susceptibility of the two-sublattice magnetic system in response to an applied external magnetic field. To this end, the antiferromagnetic and ferrimagnetic (sub-THz) precession and THz nutation resonance frequencies are calculated. Our results show that the precession resonance frequencies and effective damping decrease with intrasublattic nutation coupling, while they increase with intersublattice nutation in an antiferromagnet. However, we find that the THz nutation resonance frequencies decrease with both the intra- and intersublattice nutation couplings. For ferrimagnets, conversely, we calculate two nutation modes with distinct frequencies, unlike antiferromagnets. The exchangelike precession resonance frequency of ferrimagnets decreases with intrasublattice nutation coupling and increases with intersublattice nutation coupling, as antiferromagnets, but the ferromagneticlike precession frequency of ferrimagnets is practically invariant to the intra- and intersublattice nutation couplings.
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