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Chimata, Raghuveer
Publications (6 of 6) Show all publications
Chimata, R., Delczeg-Czirjak, E. K., Szilva, A., Cardias, R., Kvashnin, Y., Pereiro, M., . . . Eriksson, O. (2017). Magnetism and ultrafast magnetization dynamics of Co and CoMn alloys at finite temperature. Physical review B, 95(21), Article ID 214417.
Open this publication in new window or tab >>Magnetism and ultrafast magnetization dynamics of Co and CoMn alloys at finite temperature
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2017 (English)In: Physical review B, ISSN 2469-9950, Vol. 95, no 21, article id 214417Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-308321 (URN)10.1103/PhysRevB.95.214417 (DOI)000404015400003 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2013.0020, 2012.0031StandUp
Available from: 2016-11-24 Created: 2016-11-24 Last updated: 2018-04-07Bibliographically approved
Chimata, R., Isaeva, L., Kadas, K., Bergman, A., Sanyal, B., Mentink, J. H., . . . Pereiro, M. (2015). All-thermal switching of amorphous Gd-Fe alloys: Analysis of structural properties and magnetization dynamics. Physical Review B. Condensed Matter and Materials Physics, 92(9), Article ID 094411.
Open this publication in new window or tab >>All-thermal switching of amorphous Gd-Fe alloys: Analysis of structural properties and magnetization dynamics
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 9, article id 094411Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-263429 (URN)10.1103/PhysRevB.92.094411 (DOI)000360884700002 ()
Available from: 2015-10-07 Created: 2015-09-30 Last updated: 2017-12-01Bibliographically approved
Chimata, R., Chico, J., Bergman, A., Berqvist, L., Sanyal, B. & Eriksson, O. (2015). Laser heated ferromagnetic simulations. In: Ultrafast Magnetism I: . Paper presented at Ultrafast Magnetization Conference, OCT 28-NOV 01, 2013, Strasbourg, FRANCE (pp. 76-78).
Open this publication in new window or tab >>Laser heated ferromagnetic simulations
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2015 (English)In: Ultrafast Magnetism I, 2015, p. 76-78Conference paper, Published paper (Refereed)
Abstract [en]

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.

Series
Springer Proceedings in Physics, ISSN 0930-8989 ; 159
National Category
Other Physics Topics
Identifiers
urn:nbn:se:uu:diva-249061 (URN)10.1007/978-3-319-07743-7_25 (DOI)000349745400025 ()978-3-319-07743-7; 978-3-319-07742-0 (ISBN)
Conference
Ultrafast Magnetization Conference, OCT 28-NOV 01, 2013, Strasbourg, FRANCE
Available from: 2015-04-22 Created: 2015-04-10 Last updated: 2017-01-25Bibliographically approved
Ganguly, S., Chimata, R. & Sanyal, B. (2015). Overcoming magnetic frustration and promoting half-metallicity in spinel CoCr2O4 by doping with Fe. Physical Review B. Condensed Matter and Materials Physics, 92(22), Article ID 224417.
Open this publication in new window or tab >>Overcoming magnetic frustration and promoting half-metallicity in spinel CoCr2O4 by doping with Fe
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 22, article id 224417Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-274283 (URN)10.1103/PhysRevB.92.224417 (DOI)000366500100006 ()
Funder
Carl Tryggers foundation Swedish Research CouncilEU, European Research CouncilSwedish National Infrastructure for Computing (SNIC)
Available from: 2016-02-03 Created: 2016-01-20 Last updated: 2017-11-30Bibliographically approved
Ramzan, M., Li, Y., Chimata, R. & Ahuja, R. (2013). Electronic, mechanical and optical properties of Y2O3 with hybrid density functional (HSE06). Computational materials science, 71, 19-24
Open this publication in new window or tab >>Electronic, mechanical and optical properties of Y2O3 with hybrid density functional (HSE06)
2013 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 71, p. 19-24Article in journal (Refereed) Published
Abstract [en]

In this paper, we have investigated the electronic, optical and mechanical properties of the Y2O3 crystal by first-principle calculations based on the density-functional theory. The generalized gradient approximation (GGA-PBE) and hybrid exchange-correlation functional (HSE06) are both used for comparative study. It is found that, the band gap of Y2O3 calculated by HSE06 method (6.0 eV) is in good agreement with the experimental band gap data (5.5 eV), and HSE06 gives better electronic structure description close to experiments. Then we calculate the elastic constants, and derive the corresponding properties i.e.; bulk, shear and Young's moduli, and Poisson's ratio. Our calculated elastic and mechanical properties correspond well with experimental data. Besides, we also obtain the equilibrium lattice and bulk modulus of yttria by fitting the Birch-Murnaghan equation of state. It is found that, the HSE06 well reproduce the experimental lattice parameters, equilibrium volume and bulk modulus of Y2O3. Based on the accurate described crystal and electronic structure and mechanical properties by HSE06 method, the optical properties of this material are also analyzed.

Keywords
Hybrid density functional, Electronic structure, Mechanical properties, Optical properties
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-198896 (URN)10.1016/j.commatsci.2012.12.026 (DOI)000316661300003 ()
Available from: 2013-05-02 Created: 2013-04-29 Last updated: 2017-12-06Bibliographically approved
Chimata, R., Bergman, A., Bergqvist, L., Sanyal, B. & Eriksson, O. (2012). Microscopic Model for Ultrafast Remagnetization Dynamics. Physical Review Letters, 109(15), 157201
Open this publication in new window or tab >>Microscopic Model for Ultrafast Remagnetization Dynamics
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2012 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 15, p. 157201-Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-185192 (URN)10.1103/PhysRevLett.109.157201 (DOI)000309590300042 ()
Available from: 2012-11-22 Created: 2012-11-21 Last updated: 2017-12-07Bibliographically approved
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