Nowak, Ulrich

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## Suchergebnisse Publikationen

#### Dependence of training effect on the antiferromagnetic structure of exchange-bias bilayers within the domain-state model

2014, Biternas, Andreas G., Chantrell, Roy W., Nowak, Ulrich

The influence of the antiferromagnetic thickness and dilution on the training effect is investigated with the use of an atomistic model for the magnetic interaction for constant temperature. We analyze the phenomenology in both ferromagnet and antiferromagnet in terms of hysteresis loop quantities and stable spin populations during training. While for small antiferromagnetic layer thicknesses we observe thermal training, an increase in the AFM thickness leads to athermal training. In contrast an increase in the AFM dilution leads to athermal training, while in low dilution we observe thermal training. At a value of dilution in the range of 30–40%, we observe the largest exchange-bias field with the smallest training effect. The domain structure of the antiferromagnet changes rapidly with dilution, which is shown to give large changes in the training effect.

#### Atomistic spin model based on a spin-cluster expansion technique: Application to the IrMn3/Co interface

2011, Szunyogh, Laszlo, Udvardi, László, Jackson, Jerome, Nowak, Ulrich, Chantrell, Roy W.

In order to derive tensorial exchange interactions and local magnetic anisotropies in itinerant magnetic systems, an approach combining the spin-cluster expansion with the relativistic disordered local moment scheme is introduced. The theoretical background and computational aspects of the method are described in detail. The exchange interactions and site-resolved anisotropy contributions for the IrMn3/Co(111) interface, a prototype for an exchange bias system, are calculated including a large number of magnetic sites from both the antiferromagnet and ferromagnet. Our calculations reveal that the coupling between the two subsystems is fairly limited to the vicinity of the interface. The magnetic anisotropy of the interface system is discussed, including effects of the Dzyaloshinskii-Moriya interactions that appear due to symmetry breaking at the interface.

#### Constrained Monte Carlo Method and Calculation of the Temperature Dependence of Magnetic Anisotropy

2010, Asselin, Pierre, Evans, Richard Francis L., Barker, Joe, Chantrell, Roy W., Yanes Díaz, Rocio, Chubykalo-Fesenko, Oksana, Hinzke, Denise, Nowak, Ulrich

We introduce a constrained Monte Carlo method which allows us to traverse the phase space of a classical spin system while fixing the magnetization direction. Subsequently we show the method's capability to model the temperature dependence of magnetic anisotropy, and for bulk uniaxial and cubic anisotropies we recover the low-temperature Callen-Callen power laws in M. We also calculate the temperature scaling of the 2-ion anisotropy in L10 FePt, and recover the experimentally observed M2.1 scaling. The method is newly applied to evaluate the temperature dependent effective anisotropy in the presence of the N'eel surface anisotropy in thin films with different easy axis configurations. In systems having different surface and bulk easy axes, we show the capability to model the temperature-induced reorientation transition. The intrinsic surface anisotropy is found to follow a linear temperature behavior in a large range of temperatures.

#### Training effect of exchange-bias bilayers within the domain state model

2009, Biternas, Andreas G., Nowak, Ulrich, Chantrell, Roy W.

An investigation of the temperature dependence of the training effect of various exchange coupled bilayers with different types of anisotropy is presented. We use an atomistic model for the magnetic interactions within a classical Heisenberg spin Hamiltonian. In general, the behavior of the exchange-bias field is separated into low- and high-temperature regions. This separation is made according to the trend of exchange-bias field after the second hysteresis loop and the parameters of the power-law fit for these fields. It is found that with increasing antiferromagnetic thickness, systems follow the same temperature trend but with lower values of the exchange-bias field and a weaker training effect. This is due to the fact that thicker antiferromagnetic layers lead to increased stability of the antiferromagnetic domains. Also, the behavior of the coercive fields is investigated, concluding that the training effect occurs predominantly in the first half of the hysteresis loop.

#### Stochastic form of the Landau-Lifshitz-Bloch equation

2012, Evans, Richard F. L., Hinzke, Denise, Atxitia, Unai, Nowak, Ulrich, Chantrell, Roy W., Chubykalo-Fesenko, Oksana

The Landau-Lifshitz-Bloch equation is a formulation of dynamic micromagnetics valid at all temperatures, treating both the transverse and longitudinal relaxation components important for high-temperature applications. In this paper we discuss two stochastic forms of the Landau-Lifshitz-Bloch equation. Both of them are consistent with the fluctuation-dissipation theorem. We derive the corresponding Fokker-Planck equations and show that only the stochastic form of the Landau-Lifshitz-Bloch equation proposed in the present paper is consistent with the Boltzmann distribution at high temperatures. The previously used form does not satisfy this requirement in the vicinity of the Curie temperature. We discuss the stochastic properties of both equations and present numerical simulations for distribution functions and the average magnetization value as a function of temperature.

#### Multiscale modeling of magnetic materials: Temperature dependence of the exchange stiffness

2010, Atxitia, Unai, Hinzke, Denise, Chubykalo-Fesenko, Oksana, Nowak, Ulrich, Kachkachi, Hamid, Mryasov, Oleg N., Evans, Richard Francis L., Chantrell, Roy W.

For finite-temperature micromagnetic simulations the knowledge of the temperature dependence of the exchange stiffness plays a central role. We use two approaches for the calculation of the thermodynamic exchange parameter from spin models: (i) based on the domain-wall energy and (ii) based on the spin-wave dispersion. The corresponding analytical and numerical approaches are introduced and compared. A general theory for the temperature dependence and scaling of the exchange stiffness is developed using the classical spectral density method. The low-temperature exchange stiffness A is found to scale with magnetization as m1.66 for systems on a simple cubic lattice and as m1.76 for an FePt Hamiltonian parametrized through ab initio calculations. The additional reduction in the scaling exponent, as compared to the mean-field theory (A~ m2), comes from the nonlinear spin-wave effects.

#### Behavior of the antiferromagnetic layer during training in exchange-biased bilayers within the domain state model

2010, Biternas, Andreas G, Chantrell, Roy W., Nowak, Ulrich

An analysis of the antiferromagnetic spins is used to study the training effect in exchange bias bilayers. We use an atomistic model for the magnetic interactions within a classical Heisenberg spin Hamiltonian for the investigation of the temperature dependence of the training effect. Various exchange bias bilayers with different kinds of anisotropy are presented. The spins in the antiferromaget are grouped as stable and unstable according to their behavior during the reversal. It is found that their population changes during the training effect. The stable spins result in a magnetization which is shown to be related to the exchange bias field behavior. The behavior of the interface spins is shown to be determined predominantly by the degree of geometric spin frustration at the interface.

#### The thermodynamic limits of magnetic recording

2012, Richter, H. J., Lyberatos, Andreas, Nowak, Ulrich, Evans, Richard Francis L., Chantrell, Roy W.

Thermal stability of the recorded information is generally thought to set the limit of the maximum possible density in magnetic recording. It is shown that basic thermodynamics always cause the probability of success of the write process to be less than 100%. This leads to a thermally induced error rate, which eventually limits the maximum possible density beyond that given by the traditional thermal stability limit. While the thermally induced error rate is negligible for recording of simple single domain particles, it rapidly increases in the presence of a write assist, in particular if the write assist is accomplished by an increased recording temperature. For the ultimate recording system that combines thermally assisted writing with a recording scheme that uses one grain per bit, the upper bound for the maximum achievable density is 20 Tbit/inch² for a bit error rate target of 10ˉ².

#### Atomistic spin model simulation of magnetic reversal modes near the Curie point

2010, Barker, Joe, Evans, Richard Francis L., Chantrell, Roy W., Hinzke, Denise, Nowak, Ulrich

The so-called linear reversal mode is demonstrated in spin model simulations of the high anisotropy material L10 FePt. Reversal of the magnetization is found to readily occur in the linear regime despite an energy barrier (KV/kBT) that would conventionally ensure stability on this timescale. The timescale for the reversal is also established with a comparison to the Landau Lifshitz Bloch equation showing good agreement.

#### Ultrafast Path for Optical Magnetization Reversal via a Strongly Nonequilibrium State

2009, Vahaplar, K., Kalashnikova, A. M., Kimel, A. V., Hinzke, Denise, Nowak, Ulrich, Chantrell, Roy W., Tsukamoto, A., Itoh, A., Kirilyuk, A., Rasing, Theo

Using time-resolved single-shot pump-probe microscopy we unveil the mechanism and the time scale of all-optical magnetization reversal by a single circularly polarized 100 fs laser pulse. We demonstrate that the reversal has a linear character, i.e., does not involve precession but occurs via a strongly nonequilibrium state. Calculations show that the reversal time which can be achieved via this mechanism is within 10 ps for a 30 nm domain. Using two single subpicosecond laser pulses we demonstrate that for a 5 µm domain the magnetic information can be recorded and readout within 30 ps, which is the fastest write-read event demonstrated for magnetic recording so far.