Nowak, Ulrich

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Ulrich
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Laser induced magnetization switching in films with perpendicular anisotropy : a comparison between measurements and a multi-macrospin model

2009, Bunce, Christopher, Wu, Jing, Ju, Ganping, Lu, Bin, Hinzke, Denise, Kazantseva, Natalia, Nowak, Ulrich, Chantrell, Roy W.

Thermally-assisted ultra-fast magnetization reversal in a DC magnetic field for magnetic multilayer thin films with perpendicular anisotropy has been investigated in the time domain using femtosecond laser heating. The experiment is set-up as an optically pumped stroboscopic Time Resolved Magneto-Optical Kerr Effect magnetometer. It is observed that a modest laser fluence of about 0.3 mJ/cm2 induces switching of the magnetization in an applied field much less than the DC coercivity (0.8 T) on the sub-nanosecond time-scale. This switching was thermally-assisted by the energy from the femtosecond pump-pulse. The experimental results are compared with a model based on the Landau Lifschitz Bloch equation. The comparison supports a description of the reversal process as an ultra-fast demagnetization and partial recovery followed by slower thermally activated switching due to the spin system remaining at an elevated temperature after the heating pulse.

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Spin excitations in a monolayer scanned by a magnetic tip

2009, Magiera, Martin P., Brendel, Lothar, Wolf, Dietrich E., Nowak, Ulrich

Energy dissipation via spin excitations is investigated for a hard ferromagnetic tip scanning a soft magnetic monolayer. We use the classical Heisenberg model with Landau-Lifshitz-Gilbert (LLG) dynamics including a stochastic field representing finite temperatures. The friction force depends linearly on the velocity (provided it is small enough) for all temperatures. For low temperatures, the corresponding friction coefficient is proportional to the phenomenological damping constant of the LLG equation. This dependence is lost at high temperatures, where the friction coefficient decreases exponentially. These findings can be explained by properties of the spin polarisation cloud dragged along with the tip.

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Giant magnetic anisotropy of the bulk antiferromagnets IrMn and IrMn3 from first principles

2009, Szunyogh, László, Lazarovits, Bence, Udvardi, László, Jackson, Jerome, Nowak, Ulrich

We perform an ab initio study of the ordered phases of IrMn and IrMn3, the most widely used industrial antiferromagnets. Calculation of the form and the strength of the magnetic anisotropy allows the construction of an effective spin model, which is tested against experimental measurements regarding the magnetic ground state and the Néel temperature. Our most challenging result is the extremely strong second-order anisotropy for IrMn3 appearing in its frustrated triangular magnetic ground state, which is surprising since the ordered L12 phase has a cubic symmetry. We explain this large anisotropy by the fact that cubic symmetry is locally broken for each of the three Mn sublattices.

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Ultrafast Spin Dynamics : The Effect of Colored Noise

2009, Atxitia, Unai, Chubykalo-Fesenko, Oksana, Chantrell, Roy W., Nowak, Ulrich, Rebei, A.

Recent experimental results have pushed the limits of magnetization dynamics to pico- and femtosecond time scales. This ultrafast dynamics occurs in extreme conditions of strong and rapid fields and high temperatures. This situation requires a new description of magnetization dynamics, taking into account that the electron correlation time could be of the order of the inverse spin frequency. For this case we introduce a thermodynamically correct phenomenological Landau-Lifshitz-Miyasaki-Seki approach. We demonstrate the effect of the noise correlation time on the ultrafast demagnetization rate.

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Magnetic Friction and the Role of Temperature

2009, Magiera, Martin P., Wolf, Dietrich E., Brendel, Lothar, Nowak, Ulrich

A magnetic dipole moving parallel to a ferromagnetically interacting surface is subject to a friction force due to the conversion of kinetic energy into spin excitations. This phenomenon is studied in the framework of the classical anisotropic Heisenberg-model, using the stochastic Landau-Lifshitz-Gilbert equation. The friction force is calculated from dissipation rates, which are obtained directly from energy functions. For small velocities, magnetic friction increases linearly (like Stokes' law for laminar flow). The characteristic low- and high-temperature behavior is analyzed and explained by a relaxation time ansatz.

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Exchange bias for a ferromagnetic film coupled to a spin glass

2009, Usadel, Klaus-Dieter, Nowak, Ulrich

For a model system consisting of a ferromagnetic layer exchange coupled to a spin glass, extensive Monte Carlo simulations are performed. For the spin glass the standard short-range Gaussian model is used. Exchange bias is observed as a result of a frozen spin-glass state. The exchange bias fields are calculated for different temperatures, cooling fields, and thicknesses of the spin-glass layer and the training effect is investigated. A major result of our simulations is that the bias field decreases with increasing strength of the cooling field in qualitative agreement with recent experiments.

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Linear and elliptical magnetization reversal close to the Curie temperature

2009, Kazantseva, Natalia, Hinzke, Denise, Chantrell, Roy W., Nowak, Ulrich

For further improvement of magnetic information storage density and writing speed, laser-induced writing procedures have been extensively explored recently. Within the framework of the Landau-Lifshitz-Bloch equation of motion, which does not conserve the length of the magnetization vector, we investigate thermally assisted switching analytically. We show that for temperatures close to (but still below) the Curie temperature two reversal modes appear, an elliptical mode and a linear one. We calculate the coercive fields and energy barriers for both elliptical and linear switching. Investigating the dynamics of linear reversal, which is the more relevant case close to the Curie temperature, we calculate the temperature dependence of the minimal time and field needed for thermally assisted switching below and above the Curie temperature.

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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.

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Current-induced domain wall motion including thermal effects based on Landau-Lifshitz-Bloch equation

2009, Schieback, Christine, Hinzke, Denise, Kläui, Mathias, Nowak, Ulrich, Nielaba, Peter

We employ the Landau-Lifshitz-Bloch (LLB) equation to investigate current-induced domain wall motion at finite temperatures by numerical micromagnetic simulations. We extend the LLB equation with spin torque terms that account for the effect of spin-polarized currents and we find that the velocities depend strongly on the interplay between adiabatic and non-adiabatic spin torque terms. As a function of temperature, we find non-monotonous behavior, which might be useful to determine the relative strengths of the spin torque terms experimentally.

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Temperature dependence of the current-induced domain wall motion from a modified Landau-Lifshitz-Bloch equation

2009, Schieback, Christine, Hinzke, Denise, Kläui, Mathias, Nowak, Ulrich, Nielaba, Peter

We employ the Landau-Lifshitz-Bloch (LLB) equation to investigate current-induced domain wall motion at finite temperatures by numerical micromagnetic simulations. We extend the LLB equation with spin torque terms that account for the effect of spin-polarized currents and we find that the velocities depend strongly on the interplay between adiabatic and nonadiabatic spin torque terms. As a function of temperature, we find nonmonotonous behavior, which might be useful to determine the relative strengths of the spin torque terms experimentally.