| 1. |
- Hellsvik, Johan, et al.
(författare)
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Dynamics of diluted magnetic semiconductors from atomistic spin-dynamics simulations : Mn-doped GaAs
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:14, s. 144419-
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Tidskriftsartikel (refereegranskat)abstract
- The dynamical behavior of the magnetism of diluted magnetic semiconductors (DMSs) has been investigated by means of atomistic spin-dynamics simulations. The conclusions drawn from the study are argued to be general for DMS systems in the low-concentration limit, although all simulations are done for 5% Mn-doped GaAs with various concentrations of As antisite defects. The magnetization curve M(T) and the Curie temperature T-C have been calculated and are found to be in good correspondence to the results from Monte Carlo simulations and experiments. Furthermore, equilibrium and nonequilibrium behaviors of the magnetic pair-correlation function have been extracted. The dynamics of DMS systems reveals a substantial short-ranged magnetic order even at temperatures at or above the ordering temperature, with a nonvanishing pair-correlation function extending up to several atomic shells. For the high As antisite concentrations the simulations show a short-ranged antiferromagnetic coupling and a weakened long-ranged ferromagnetic coupling. For sufficiently large concentrations we do not observe any long-ranged ferromagnetic correlation. A typical dynamical response shows that starting from a random orientation of moments, the spin correlation develops very fast (similar to 1 ps) extending up to 15 atomic shells. Above similar to 10 ps in the simulations, the pair correlation is observed to extend over some 40 atomic shells. The autocorrelation function has been calculated and compared with ferromagnets such as bcc Fe and spin-glass materials. We find no evidence in our simulations for a spin-glass behavior for any concentration of As antisites. Instead the magnetic response is better described as slow dynamics, at least when compared to that of a regular ferromagnet such as bcc Fe.
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| 2. |
- Hellsvik, Johan, et al.
(författare)
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Simulation of a spin-wave instability from atomistic spin dynamics
- 2009
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 79:18, s. 184426-
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Tidskriftsartikel (refereegranskat)abstract
- We study the spin dynamics of a Heisenberg model at finite temperature in the presence of an external fieldor a uniaxial anisotropy. For the case of the uniaxial anisotropy, our simulations show that the macromomentpicture breaks down. An effect which we refer to as a spin-wave instability results in a nondissipative Bloch-Bloembergen-type relaxation of the macromoment where the size of the macromoment changes and can evenbe made to disappear. This relaxation mechanism is studied in detail by means of atomistic spin dynamics simulations.
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| 3. |
- Skubic, Björn, et al.
(författare)
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A Comparison of Dynamic Bandwidth Allocation for EPON, GPON, and Next-Generation TDM PON
- 2009
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Ingår i: IEEE Communications Magazine. - : IEEE Communications Society. - 0163-6804 .- 1558-1896. ; 47:3, s. S40-S48
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Tidskriftsartikel (refereegranskat)abstract
- Dynamic bandwidth allocation in passive optical networks presents a key issue for providing efficient and fair utilization of the PON upstream bandwidth while supporting the QoS requirements of different traffic classes. In this article we compare the typical characteristics of DBA, such as bandwidth utilization, delay, and jitter at different traffic loads, within the two major standards for PONs, Ethernet PON and gigabit PON. A particular PON standard sets the framework for the operation of DBA and the limitations it faces. We illustrate these differences between EPON and GPON by means of simulations for the two standards. Moreover, we consider the evolution of both standards to their next-generation counterparts with the bit rate of 10 Gb/s and the implications to the DBA. A new simple GPON DBA algorithm is used to illustrate GPON performance. It is shown that the length of the polling cycle plays a crucial but different role for the operation of the DBA within the two standards. Moreover, only minor differences regarding DBA for current and next-generation PONs were found.
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| 4. |
- Skubic, Björn, et al.
(författare)
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A method for atomistic spin dynamics simulations : implementation and examples
- 2008
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 20:31, s. 315203-
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Tidskriftsartikel (refereegranskat)abstract
- We present a method for performing atomistic spin dynamic simulations. A comprehensive summary of all pertinent details for performing the simulations such as equations of motions, models for including temperature, methods of extracting data and numerical schemes for performing the simulations is given. The method can be applied in a first-principles mode, where all interatomic exchange is calculated self-consistently, or it can be applied with frozen parameters estimated from experiments or calculated for a fixed spin-configuration. Areas of potential application to different magnetic questions are also discussed. The method is finally applied to one situation where the macrospin model breaks down, magnetic switching in ultra strong magnetic fields.
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| 5. |
- Skubic, Björn, et al.
(författare)
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Competing Exchange Interactions in Magnetic Multilayers
- 2006
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 96, s. 057205-
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Tidskriftsartikel (refereegranskat)abstract
- We have studied alloying of the nonmagnetic spacer layer with a magnetic material as a method of tuning the interlayer coupling in magnetic multilayers. We have specifically studied the Fe/V(100) system by alloying the spacer V with various amounts of Fe. For some Fe concentrations in the spacer, it is possible to create a competition between antiferromagnetic Ruderman-Kittel-Kasuya-Yoshida exchange and direct ferromagnetic exchange coupling. The exchange coupling and transport properties for a large span of systems with different spacer concentrations and thicknesses were calculated and measured experimentally and good agreement between observations and theory was observed. A reduction in magnetoresistance of about 50% was observed close to the switchover from antiferromagnetic to ferromagnetic coupling.
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| 6. |
- Skubic, Björn, et al.
(författare)
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Exchange coupling and exchange bias in FM/AFM bilayers for a fully compensated AFM interface
- 2009
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Ingår i: Acta Physica Polonica. A. - 0587-4246 .- 1898-794X. ; 115:1, s. 25-29
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Tidskriftsartikel (refereegranskat)abstract
- We address the interlayer coupling in a ferromagnet/antiferromagnet bilayer where the interface of theantiferromagnet is fully compensated. We discuss the role of di®erent types of exchange interaction for theinterlayer coupling and exchange bias. We propose two types of corrections to the ideal Heisenberg Hamiltonianwhich may explain exchange bias. The ¯rst is a correction for the angular dependence of the exchange interactionsand the second a correction due to magnetostriction and interface imperfections. The ¯rst correction contributes toan anisotropy at the interface and favors either parallel or perpendicular coupling across the interface. The secondcorrection contributes to the exchange bias. Our analysis is based on atomic spin dynamics simulations, and ourresults show that small corrections to the ideal Heisenberg Hamiltonian may have macroscopic consequences insystems with frustrated interatomic interactions.
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| 7. |
- Skubic, Björn, et al.
(författare)
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Improved scheme for estimating T-CONT bandwidth demand in status reporting DBA for NG-PON
- 2009
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Ingår i: Network Architectures, Management, and Applications VII. - : SPIE - International Society for Optical Engineering. - 9780819480354 ; , s. 76330E-
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Konferensbidrag (refereegranskat)abstract
- A scheme for estimating T-CONT bandwidth demand within dynamic bandwidth allocation (DBA) for next-generation passive optical network (NG-PON) is proposed and evaluated. Estimating T-CONT bandwidth demand is considered as a main challenge in gigabit-capable passive optical network (GPON) DBA. The proposed minimum guaranteed T-CONT content (MGTC) scheme allows for a more conservative estimate of bandwidth demand. It is shown that at high load significant improvements in delay, jitter and bandwidth utilization can be achieved with the proposed scheme. For light loads the conventional scheme shows better delay performance. However, this may be overcome by controlled overgranting in the MGTC scheme.
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| 8. |
- Skubic, Björn, et al.
(författare)
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Role of magnetic impurities in Fe/V multilayers
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 77:14, s. 144408-
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the Fe/V bcc (100) multilayers and the effect of alloying the V spacer layers with various amounts of magnetic impurities (Fe, Co, Ni, and Cr). The study was performed by means of total energy electronic structure calculations. We compare the effect of the different types of impurities and discuss the interlayer exchange coupling in terms of Fermi surface topology and bulk magnetic order. The effect of interface roughness and interface intermixing on the phase diagrams was also studied.
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| 9. |
- Skubic, Björn, 1977-
(författare)
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Spin Dynamics and Magnetic Multilayers
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Theoretical studies based on first-principles theory are presented for a number of different magnetic systems. The first part of the thesis concerns spin dynamics and the second part concerns properties of magnetic multilayers. The theoretical treatment is based on electronic structure calculations performed by means of density functional theory. A method is developed for simulating atomistic spin dynamics at finite temperatures, which is based on solving the equations of motion for the atomic spins by means of Langevin dynamics. The method relies on a mapping of the interatomic exchange interactions from density functional theory to a Heisenberg Hamiltonian. Simulations are performed for various magnetic systems and processes beyond the reach of conventional micromagnetism. As an example, magnetization dynamics in the limit of large magnetic and anisotropy fields is explored. Moreover, the method is applied to studying the dynamics of systems with complex atomic order such as the diluted magnetic semiconductor MnGaAs and the spin glass alloy CuMn. The method is also applied to a Fe thin film and a Fe/Cr/Fe trilayer system, where the limits of ultrafast switching are explored. Current induced magnetization dynamics is investigated by calculating the current induced spin-transfer torque by means of density functional theory combined with the relaxation time approximation and semi-classical Boltzmann theory. The current induced torque is calculated for the helical spin-density waves in Er and fcc Fe, where the current is found to promote a rigid rotation of the magnetic order. Properties of magnetic multilayers composed of magnetic and nonmagnetic layers are investigated by means of the Korringa-Kohn-Rostocker interface Green's function method. Multilayer properties such as magnetic moments, interlayer exchange coupling and ordering temperatures are calculated and compared with experiments, with focus on understanding the influence of interface quality. Moreover, the influence on the interlayer exchange coupling of alloying the nonmagnetic spacer layers with small amounts of a magnetic impurity is investigated.
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| 10. |
- Wessely, Ola, et al.
(författare)
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Current Driven Magnetization Dynamics in Helical Spin Density Waves
- 2006
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 96:25
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Tidskriftsartikel (refereegranskat)abstract
- A mechanism is proposed for manipulating the magnetic state of a helical spin density wave using a current. It is shown that a current through a bulk metal with a helical spin density wave induces a spin transfer torque, which gives rise to a rotation of the order parameter. The use of spin transfer torque to manipulate the magnetization in bulk systems does not suffer from the obstacles seen for magnetization reversal using interface spin transfer torque in multilayered systems. The effect is demonstrated by a quantitative calculation of the current induced magnetization dynamics of a rare earth metal, Er. Finally, we propose a setup for experimental verification.
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| 11. |
- Wessely, Ola, et al.
(författare)
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Spin-Transfer Torque in Helical Spin Density Waves
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:10, s. 104433-
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Tidskriftsartikel (refereegranskat)abstract
- The current driven magnetization dynamics of a helical spin-density wave is investigated. Expressions for calculating the spin-transfer torque of real systems from first-principles density-functional theory are presented. These expressions are used for calculating the spin-transfer torque for the spin spirals of Er and fcc Fe at two different lattice volumes. It is shown that the calculated torque induces a rigid rotation of the order parameter with respect to the spin spiral axis. The torque is found to depend on the wave vector of the spin spiral and the spin polarization of the Fermi surface states. The resulting dynamics of the spin spiral is also discussed.
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