K. Usadel - Academia.edu (original) (raw)
Papers by K. Usadel
Physical Review B, 2006
The dynamical response of nanoparticles as probed by ferromagnetic resonance ͑FMR͒ is studied wit... more The dynamical response of nanoparticles as probed by ferromagnetic resonance ͑FMR͒ is studied within a classical spin model using Landau-Lifshitz-Gilbert dynamics. The FMR signal is calculated numerically for different temperatures. The dependence of both the shift of the resonance signal and the linewidth on temperature is obtained, yielding information about the microscopic parameters of the particles.
Advances in Solid State Physics 42, 2002
The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/... more The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/CoO bilayers can be intentionally enhanced by a factor of up to 4 if the antiferromagnetic CoO layer is diluted by non-magnetic defects in its volume part away from the interface. Monte Carlo simulations of a simple model consisting of a ferromagnetic layer exchange coupled to a diluted antiferromagnetic layer show bias of the right order of magnitude and qualitatively reproduce the experimentally observed dependence of the bias field on the Humber of defects. The exchange bias results from a domain state in the antiferromagnet, which is formed during field cooling and carries an irreversible domain state magnetization. Apart from intentionally introduced defects, also structural defects can enhance the exchange bias coupling. Twin boundaries in undiluted CoO increase the exchange bias coupling in Co/CoO by more than a factor of 2 cornpared to untwined samples. This observation indicates that structural defects in the antiferromagnet, such as twin or gTain boundaries, might also stabilize a domain state, that the domain state model for exchange bias is more generally applicable to understand the origin of the bias phenomenon.
Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 1999
We analyze the depinning transition of a driven interface in the three-dimensional (3D) random fi... more We analyze the depinning transition of a driven interface in the three-dimensional (3D) random field Ising model (RFIM) with quenched disorder by means of Monte Carlo simulations. The interface initially built into the system is perpendicular to the [111] direction of a simple cubic lattice. We introduce an algorithm which is capable of simulating such an interface independent of the considered dimension and time scale. This algorithm is applied to the 3D RFIM to study both the depinning transition and the influence of thermal fluctuations on this transition. It turns out that in the RFIM characteristics of the depinning transition depend crucially on the existence of overhangs. Our analysis yields critical exponents of the interface velocity, the correlation length, and the thermal rounding of the transition. We find numerical evidence for a scaling relation for these exponents and the dimension d of the system.
Physical review. B, Condensed matter, 1993
ABSTRACT
Physical review. B, Condensed matter, 1991
A recent calculation of Yokota on the phase boundary for the Sherrington-Kirkpatrick version of t... more A recent calculation of Yokota on the phase boundary for the Sherrington-Kirkpatrick version of the quantum Ising spin glass in a transverse field seems to lead to unphysical results. Various other approaches on this subject are quoted and compared with the exact Trotter-Suzuki method to resolve the controversies about the critical line in this model.
Physical review. B, Condensed matter, 1990
ABSTRACT
Physical review. B, Condensed matter, 1989
ABSTRACT
Magnetics International Conference, 2005
We report on results obtained with a mean-field type calculation for an exchange bias (EB) system... more We report on results obtained with a mean-field type calculation for an exchange bias (EB) system consisting of a ferromagnetic (FM) monolayer in contact with an antiferromagnetic (AFM) thin film. Both the bias field Heb and the coercivity Hc are calculated. The AFM layer is magnetically diluted and different degrees of dilution ranging from zero to 50% are considered. The
Physical Review Letters, 2000
The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/... more The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/CoO layers can intentionally be increased by a factor of up to 3 if the antiferromagnetic CoO layer is diluted by nonmagnetic defects in its volume part away from the interface. Monte Carlo simulations of a simple model of a ferromagnetic layer on a diluted antiferromagnet show exchange bias and explain qualitatively its dilution and temperature dependence. These investigations reveal that diluting the antiferromagnet leads to the formation of volume domains, which cause and control exchange bias.
Physical Review Letters, 2003
Experimentally an asymmetry of the reversal modes has been found in certain exchange bias systems... more Experimentally an asymmetry of the reversal modes has been found in certain exchange bias systems. From a numerical investigation of the domain state model evidence is gained that this effect depends on the angle between the easy axis of the antiferromagnet and the applied magnetic field. Depending on this angle the ferromagnet reverses either symmetrically, e. g. by a coherent rotation on both sides of the loop, or the reversal is asymmetric with a non uniform reversal mode for the ascending branch, which may even yield a zero perpendicular magnetization. PACS numbers: 75.70.Cn, 75.40.Mg, 75.50.Lk, For compound materials consisting of a ferromagnet (FM) in contact with an antiferromagnet (AFM) a shift of the hysteresis loop along the magnetic field axis can occur which is called exchange bias (EB). Often, this shift is observed after cooling the entire system in an external magnetic field below the Néel temperature T N of the AFM (for a review see ). The role of the AFM is to provide at the interface a net magnetization which is stable during reversal of the FM consequently shifting the hysteresis loop. The key for understanding EB is to understand this stability.
Physical Review B, 2006
A numerical investigation of exchange coupled ferromagnetic/antiferromagnetic multilayers with a ... more A numerical investigation of exchange coupled ferromagnetic/antiferromagnetic multilayers with a twinned crystal structure for the antiferromagnet is presented. Motivated by recent experimental findings we focus on the influence of the directions of the magnetic field during the initial cooling procedure and during the hysteresis. Upon variation of these directions the ferromagnet displays different reversal modes or even an asymmetric reversal with different kinds of reversal mechanisms for the decreasing and increasing branch of a single hysteresis loop. These findings can be explained within the context of the domain state model for exchange bias.
Physical Review B, 2005
For a model system consisting of a ferromagnetic layer exchange coupled to an antiferromagnetic l... more For a model system consisting of a ferromagnetic layer exchange coupled to an antiferromagnetic layer with a compensated interface detailed mean field type calculations are performed. Both the coercive field and the exchange bias field are calculated. For the coercive field a rather broad enhancement around the Néel temperature TN of the antiferromagnetic layer is found irrespectively of whether the antiferromagnetic layer is structurally disordered or not, while exchange bias is only found for disordered systems. We show that the observed enhancement of the coercivity around TN also found experimentally and the occurrence of exchange bias are of different origin.
Physical Review B, 2006
The thermodynamic behavior of flat circular nanomagnets with a vortex domain configuration is stu... more The thermodynamic behavior of flat circular nanomagnets with a vortex domain configuration is studied using Langevin dynamics simulations for the dynamical behavior as well as local mean-field calculations for equilibrium properties. Our studies show that the vortex core becomes thermally unstable with increasing temperature, acting like a superparamagnetic system. On time scales where the vortex core remains within one of the metastable states it still has a stronger temperature dependence than the magnetization far away in the bulk of a domain.
physica status solidi (b), 1997
Physica A: Statistical Mechanics and its Applications, 1997
The kinetic roughening of a driven interface between three dimensional spin-up and spin-down doma... more The kinetic roughening of a driven interface between three dimensional spin-up and spin-down domains in a model with non-conserved scalar order parameter and quenched disorder is studied numerically within a discrete time dynamics at zero temperature. The exponents characterizing the morphology of the interface are obtained close to the depinning transition.
Zeitschrift f�r Physik B Condensed Matter, 1991
The infinite range X Y spin glass model in a transverse field is investigated by means of the Tro... more The infinite range X Y spin glass model in a transverse field is investigated by means of the Trotter-Suzuki approach. The exact phase diagram is obtained showing that a spin glass transition takes place for non-zero values of the transverse field 7 up to a critical value 7c= 1.44+0.01. The present numerically exact calculations are in good agreement with our previous approximate results and they clear remaining discrepancies from previous work.
Physical Review B, 2006
The dynamical response of nanoparticles as probed by ferromagnetic resonance ͑FMR͒ is studied wit... more The dynamical response of nanoparticles as probed by ferromagnetic resonance ͑FMR͒ is studied within a classical spin model using Landau-Lifshitz-Gilbert dynamics. The FMR signal is calculated numerically for different temperatures. The dependence of both the shift of the resonance signal and the linewidth on temperature is obtained, yielding information about the microscopic parameters of the particles.
Advances in Solid State Physics 42, 2002
The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/... more The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/CoO bilayers can be intentionally enhanced by a factor of up to 4 if the antiferromagnetic CoO layer is diluted by non-magnetic defects in its volume part away from the interface. Monte Carlo simulations of a simple model consisting of a ferromagnetic layer exchange coupled to a diluted antiferromagnetic layer show bias of the right order of magnitude and qualitatively reproduce the experimentally observed dependence of the bias field on the Humber of defects. The exchange bias results from a domain state in the antiferromagnet, which is formed during field cooling and carries an irreversible domain state magnetization. Apart from intentionally introduced defects, also structural defects can enhance the exchange bias coupling. Twin boundaries in undiluted CoO increase the exchange bias coupling in Co/CoO by more than a factor of 2 cornpared to untwined samples. This observation indicates that structural defects in the antiferromagnet, such as twin or gTain boundaries, might also stabilize a domain state, that the domain state model for exchange bias is more generally applicable to understand the origin of the bias phenomenon.
Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 1999
We analyze the depinning transition of a driven interface in the three-dimensional (3D) random fi... more We analyze the depinning transition of a driven interface in the three-dimensional (3D) random field Ising model (RFIM) with quenched disorder by means of Monte Carlo simulations. The interface initially built into the system is perpendicular to the [111] direction of a simple cubic lattice. We introduce an algorithm which is capable of simulating such an interface independent of the considered dimension and time scale. This algorithm is applied to the 3D RFIM to study both the depinning transition and the influence of thermal fluctuations on this transition. It turns out that in the RFIM characteristics of the depinning transition depend crucially on the existence of overhangs. Our analysis yields critical exponents of the interface velocity, the correlation length, and the thermal rounding of the transition. We find numerical evidence for a scaling relation for these exponents and the dimension d of the system.
Physical review. B, Condensed matter, 1993
ABSTRACT
Physical review. B, Condensed matter, 1991
A recent calculation of Yokota on the phase boundary for the Sherrington-Kirkpatrick version of t... more A recent calculation of Yokota on the phase boundary for the Sherrington-Kirkpatrick version of the quantum Ising spin glass in a transverse field seems to lead to unphysical results. Various other approaches on this subject are quoted and compared with the exact Trotter-Suzuki method to resolve the controversies about the critical line in this model.
Physical review. B, Condensed matter, 1990
ABSTRACT
Physical review. B, Condensed matter, 1989
ABSTRACT
Magnetics International Conference, 2005
We report on results obtained with a mean-field type calculation for an exchange bias (EB) system... more We report on results obtained with a mean-field type calculation for an exchange bias (EB) system consisting of a ferromagnetic (FM) monolayer in contact with an antiferromagnetic (AFM) thin film. Both the bias field Heb and the coercivity Hc are calculated. The AFM layer is magnetically diluted and different degrees of dilution ranging from zero to 50% are considered. The
Physical Review Letters, 2000
The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/... more The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/CoO layers can intentionally be increased by a factor of up to 3 if the antiferromagnetic CoO layer is diluted by nonmagnetic defects in its volume part away from the interface. Monte Carlo simulations of a simple model of a ferromagnetic layer on a diluted antiferromagnet show exchange bias and explain qualitatively its dilution and temperature dependence. These investigations reveal that diluting the antiferromagnet leads to the formation of volume domains, which cause and control exchange bias.
Physical Review Letters, 2003
Experimentally an asymmetry of the reversal modes has been found in certain exchange bias systems... more Experimentally an asymmetry of the reversal modes has been found in certain exchange bias systems. From a numerical investigation of the domain state model evidence is gained that this effect depends on the angle between the easy axis of the antiferromagnet and the applied magnetic field. Depending on this angle the ferromagnet reverses either symmetrically, e. g. by a coherent rotation on both sides of the loop, or the reversal is asymmetric with a non uniform reversal mode for the ascending branch, which may even yield a zero perpendicular magnetization. PACS numbers: 75.70.Cn, 75.40.Mg, 75.50.Lk, For compound materials consisting of a ferromagnet (FM) in contact with an antiferromagnet (AFM) a shift of the hysteresis loop along the magnetic field axis can occur which is called exchange bias (EB). Often, this shift is observed after cooling the entire system in an external magnetic field below the Néel temperature T N of the AFM (for a review see ). The role of the AFM is to provide at the interface a net magnetization which is stable during reversal of the FM consequently shifting the hysteresis loop. The key for understanding EB is to understand this stability.
Physical Review B, 2006
A numerical investigation of exchange coupled ferromagnetic/antiferromagnetic multilayers with a ... more A numerical investigation of exchange coupled ferromagnetic/antiferromagnetic multilayers with a twinned crystal structure for the antiferromagnet is presented. Motivated by recent experimental findings we focus on the influence of the directions of the magnetic field during the initial cooling procedure and during the hysteresis. Upon variation of these directions the ferromagnet displays different reversal modes or even an asymmetric reversal with different kinds of reversal mechanisms for the decreasing and increasing branch of a single hysteresis loop. These findings can be explained within the context of the domain state model for exchange bias.
Physical Review B, 2005
For a model system consisting of a ferromagnetic layer exchange coupled to an antiferromagnetic l... more For a model system consisting of a ferromagnetic layer exchange coupled to an antiferromagnetic layer with a compensated interface detailed mean field type calculations are performed. Both the coercive field and the exchange bias field are calculated. For the coercive field a rather broad enhancement around the Néel temperature TN of the antiferromagnetic layer is found irrespectively of whether the antiferromagnetic layer is structurally disordered or not, while exchange bias is only found for disordered systems. We show that the observed enhancement of the coercivity around TN also found experimentally and the occurrence of exchange bias are of different origin.
Physical Review B, 2006
The thermodynamic behavior of flat circular nanomagnets with a vortex domain configuration is stu... more The thermodynamic behavior of flat circular nanomagnets with a vortex domain configuration is studied using Langevin dynamics simulations for the dynamical behavior as well as local mean-field calculations for equilibrium properties. Our studies show that the vortex core becomes thermally unstable with increasing temperature, acting like a superparamagnetic system. On time scales where the vortex core remains within one of the metastable states it still has a stronger temperature dependence than the magnetization far away in the bulk of a domain.
physica status solidi (b), 1997
Physica A: Statistical Mechanics and its Applications, 1997
The kinetic roughening of a driven interface between three dimensional spin-up and spin-down doma... more The kinetic roughening of a driven interface between three dimensional spin-up and spin-down domains in a model with non-conserved scalar order parameter and quenched disorder is studied numerically within a discrete time dynamics at zero temperature. The exponents characterizing the morphology of the interface are obtained close to the depinning transition.
Zeitschrift f�r Physik B Condensed Matter, 1991
The infinite range X Y spin glass model in a transverse field is investigated by means of the Tro... more The infinite range X Y spin glass model in a transverse field is investigated by means of the Trotter-Suzuki approach. The exact phase diagram is obtained showing that a spin glass transition takes place for non-zero values of the transverse field 7 up to a critical value 7c= 1.44+0.01. The present numerically exact calculations are in good agreement with our previous approximate results and they clear remaining discrepancies from previous work.