| 1. |
- Chimata, Raghuveer, et al.
(författare)
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All-thermal switching of amorphous Gd-Fe alloys : Analysis of structural properties and magnetization dynamics
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:9
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Tidskriftsartikel (refereegranskat)abstract
- In recent years there has been an intense interest in understanding the microscopic mechanism of thermally induced magnetization switching driven by a femtosecond laser pulse. Most of the effort has been dedicated to periodic crystalline structures while the amorphous counterparts have been less studied. By using a multiscale approach, i.e., first-principles density functional theory combined with atomistic spin dynamics, we report here on the very intricate structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd and Fe atomic concentrations at the nanoscale level. Both structural and dynamical properties of Gd-Fe alloys reported in this work are in good agreement with previous experiments. We calculated the dynamic behavior of homogeneous and inhomogeneous amorphous Gd-Fe alloys and their response under the influence of a femtosecond laser pulse. In the homogeneous sample, the Fe sublattice switches its magnetization before the Gd one. However, the temporal sequence of the switching of the two sublattices is reversed in the inhomogeneous sample. We propose a possible explanation based on a mechanism driven by a combination of the Dzyaloshinskii-Moriya interaction and exchange frustration, modeled by an antiferromagnetic second-neighbor exchange interaction between Gd atoms in the Gd-rich region. We also report on the influence of laser fluence and damping effects in the all-thermal switching.
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| 2. |
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| 3. |
- Chimata, Raghuveer, et al.
(författare)
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Laser heated ferromagnetic simulations
- 2015
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Ingår i: ULTRAFAST MAGNETISM I. - Cham : Springer International Publishing. - 9783319077437 - 9783319077420 ; , s. 76-78
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Konferensbidrag (refereegranskat)abstract
- In this work, we show a model of ferromagnetic material heated by a laser pulse. The laser creates a pattern of circles on the ferromagnetic materials with hot regions heated up to 3000K and cold regions at 100K and 400K. In our model the Landau-Lifshitz-Gilbert equation for a macrospin and spin temperature is passed through stochastic field. We show that the difference of magnon dispersion in the cold regions of the material.
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| 4. |
- Chimata, Raghuveer, et al.
(författare)
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Magnetism and ultrafast magnetization dynamics of Co and CoMn alloys at finite temperature
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 95:21
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Tidskriftsartikel (refereegranskat)abstract
- Temperature-dependent magnetic experiments such as pump-probe measurements generated by a pulsed laser have become a crucial technique for switching the magnetization in the picosecond time scale. Apart from having practical implications on the magnetic storage technology, the research field of ultrafast magnetization poses also fundamental physical questions. To correctly describe the time evolution of the atomic magnetic moments under the influence of a temperature-dependent laser pulse, it remains crucial to know if the magnetic material under investigation has magnetic excitation spectrum that is more or less dependent on the magnetic configuration, e.g., as reflected by the temperature dependence of the exchange interactions. In this paper, we demonstrate from first-principles theory that the magnetic excitation spectra in Co in fcc, bcc, and hcp structures are nearly identical in a wide range of noncollinear magnetic configurations. This is a curious result of a balance between the size of the magnetic moments and the strength of the Heisenberg exchange interactions, that in themselves vary with configuration, but put together in an effective spin Hamiltonian results in a configuration-independent effective model. We have used such a Hamiltonian, together with ab initio calculated damping parameters, to investigate the magnon dispersion relationship as well as ultrafast magnetization dynamics of Co and Co-rich CoMn alloys.
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| 5. |
- Chimata, Raghuveer, 1984-
(författare)
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Magnetization dynamics of complex magnetic materials by atomistic spin dynamics simulations
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In recent years, there has been an intense interest in understanding the microscopic mechanism of laser induced ultrafast magnetization dynamics in picosecond time scales. Magnetization switching on such a time scale has potential to be a significant boost for the data storage industry. It is expected that the writing process will become ~1000 times faster by this technology, compared to existing techniques. Understanding the microscopic mechanisms and controlling the magnetization in such a time scale is of paramount importance at present.In this thesis, laser induced ultrafast magnetization dynamics has been studied for Fe, Co, GdFe, CoMn and Heusler alloys. A multiscale approach has been used, i.e., first-principles density functional theory combined with atomistic spin dynamics utilizing the Landau –Lifshitz-Gilbert equation, along with a three-temperature phenomenological model to obtain the spin temperature. Special attention has been paid to the calculations of exchange interaction and Gilbert damping parameters. These parameters play a crucial role in determining the ultrafast magnetization dynamics under laser fluence of the considered materials.The role of longitudinal and transversal excitations was studied for elemental ferromagnets, such as Fe and Co. A variety of complex temporal behavior of the magnetic properties was observed, which can be understood from the interplay between electron, spin, and lattice subsystems. The very intricate structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd and Fe atomic concentrations at the nanoscale was studied. We have shown that the ultrafast thermal switching process can happen above the compensation temperature in GdFe alloys. It is demonstrated that the exchange frustration via Dzyaloshinskii-Moriya interaction between the atomic Gd moments, in Gd rich area of these alloys, leads to Gd demagnetization faster than the Fe sublattice. In addition, we show that Co is a perfect Heisenberg system. Both Co and CoMn alloys have been investigated with respect to ultrafast magnetization dynamics. Also, it is predicted that ultrafast switching process can happen in the Heulser alloys when they are doped with heavy elements. Finally, we studied multiferroic CoCr2O4 and Ca3CoMnO4 systems by using the multiscale approach to study magnetization dynamics. In summary, our approach is able to capture crucial details of ultrafast magnetization dynamics in technologically important materials.
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| 6. |
- Chimata, Raghuveer, et al.
(författare)
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Microscopic Model for Ultrafast Remagnetization Dynamics
- 2012
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 109:15, s. 157201-
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Tidskriftsartikel (refereegranskat)abstract
- In this Letter, we provide a microscopic model for the ultrafast remagnetization of atomic moments already quenched above the Stoner-Curie temperature by a strong laser fluence. Combining first-principles density functional theory, atomistic spin dynamics utilizing the Landau-Lifshitz-Gilbert equation, and a three-temperature model, we analyze the temporal evolution of atomic moments as well as the macroscopic magnetization of bcc Fe and hcp Co covering a broad time scale, ranging from femtoseconds to picoseconds. Our simulations show a variety of complex temporal behavior of the magnetic properties resulting from an interplay between electron, spin, and lattice subsystems, which causes an intricate time evolution of the atomic moment, where longitudinal and transversal fluctuations result in a macrospin moment that evolves highly nonmonotonically.
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| 8. |
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| 9. |
- Ganguly, Shreemoyee, et al.
(författare)
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Overcoming magnetic frustration and promoting half-metallicity in spinel CoCr2O4 by doping with Fe
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:22
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present a systematic study of the effects of Fe doping on the electronic and magnetic structures of spinel CoCr2O4 by ab initio density functional theory and atomistic spin dynamics calculations. Our calculated magnetic structure for pristine CoCr2O4 correctly reproduces the experimental one with a q-vector of (0.67, 0.67,0.0), establishing the accuracy of the calculated interatomic exchange interactions. We show that the noncollinear spin structure with a nonzero q-vector in the spinel structure is driven towards collinearity by Fe doping by a complex interplay between interatomic exchange interactions. In the inverse spinel structure with 100% Fe doping, a collinear antiferromagnetic order develops along with a half-metallic electronic structure, which evolves due to the chemical disorder between Fe and Co in the B sites described by the coherent potential approximation. This is a comprehensive theoretical study to understand the evolution of magnetic and electronic properties of multiferroic CoCr2O4 doped with Fe.
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| 10. |
- Jana, Somnath, et al.
(författare)
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Analysis of the linear relationship between asymmetry and magnetic moment at the M edge of 3d transition metals
- 2020
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Ingår i: Physical Review Research. - : American Physical Society. - 2643-1564. ; 2:1
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Tidskriftsartikel (refereegranskat)abstract
- The magneto-optical response of Fe and Ni during ultrafast demagnetization is studied experimentally and theoretically. We have performed pump-probe experiments in the transverse magneto-optical Kerr effect (T-MOKE) geometry using photon energies that cover the M absorption edges of Fe and Ni between 40 and 72 eV. The magnetic asymmetry was obtained by forming the difference of reflected intensities obtained for two opposite orientations of the sample magnetization. Density functional theory (DFT) was used to calculate the magneto-optical response of different magnetic configurations, representing different types of excitations: long wavelength magnons, short wavelength magnons, and Stoner excitations. In the case of Fe, we find that the calculated asymmetry is strongly dependent on the specific type of magnetic excitation. Our modeling also reveals that during remagnetization Fe is, to a reasonable approximation, described by magnons, even though small nonlinear contributions could indicate some degree of Stoner excitations as well. In contrast, we find that the calculated asymmetry in Ni is rather insensitive to the type of magnetic excitations. However, there is a weak nonlinearity in the relation between asymmetry and the off-diagonal component of the dielectric tensor, which does not originate from the modifications of the electronic structure. Our experimental and theoretical results thus emphasize the need to consider a coupling between asymmetry and magnetization that may be more complex than a simple linear relationship. This insight is crucial for the microscopic interpretation of ultrafast magnetization experiments.
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