Denise Hinzke

Magnetization switching in nanowires: Monte Carlo study with fast Fourier transformation for dipolar fields

Abstract For the investigations of thermally activated magnetization reversal in systems of classical magnetic moments numerical methods are desirable. We present numerical studies which base on time quantified Monte Carlo methods where the long-range dipole–dipole interaction is calculated with the aid of fast Fourier transformation. As an example, we study models for ferromagnetic nanowires comparing our numerical results for the characteristic time of the reversal process also with numerical data from Langevin dynamics simulations where the fast Fourier transformation method is well established. Depending on the system geometry different reversal mechanism occur like coherent rotation, nucleation, and curling.

Micromagnetic modeling of laser-induced magnetization dynamics using the Landau-Lifshitz-Bloch equation

We present a dynamic approach to micromagnetics based on the Landau-Lifshitz-Bloch equation and Langevin dynamics. This type of modeling will be necessary at high temperatures when the magnetization length is not conserved, especially close to the Curie temperature. We model the laser-induced magnetization dynamics with various laser pulse fluences and show that the results are consistent with both experiments and atomistic modeling. Our results show different recovery rates depending on the final demagnetized state.

Length Scale of the Spin Seebeck Effect

The observation of the spin Seebeck effect in insulators has meant a breakthrough for spin caloritronics due to the unique ability to generate pure spin currents by thermal excitations in insulating systems without moving charge carriers. Since the recent first observation, the underlying mechanism and the origin of the observed signals have been discussed highly controversially. Here we present a characteristic dependence of the longitudinal spin Seebeck effect amplitude on the thickness of the insulating ferromagnet (YIG). Our measurements show that the observed behavior cannot be explained by any effects originating from the interface, such as magnetic proximity effects in the spin detector (Pt). Comparison to theoretical calculations of thermal magnonic spin currents yields qualitative agreement for the thickness dependence resulting from the finite effective magnon propagation length so that the origin of the effect can be traced to genuine bulk magnonic spin currents ruling out parasitic interface effects.

Magnetization switching in a Heisenberg model for small ferromagnetic particles

We investigate the thermally activated magnetization switching of small ferromagnetic particles driven by an external magnetic field. For low uniaxial anisotropy the spins can be expected to rotate coherently, while for sufficient large anisotropy they should behave Ising-like, i.e., the switching should then be due to nucleation. We study this crossover from coherent rotation to nucleation for a classical three-dimensional Heisenberg model with finite anisotropy. The crossover is influenced by the size of the particle, the strength of the driving magnetic field, and the anisotropy. We discuss the relevant energy barriers which have to be overcome during the switching, and find theoretical arguments which yield the energetically favorable reversal mechanisms for given values of the quantities above. The results are confirmed by Monte Carlo simulations of Heisenberg and Ising models.

Constrained Monte Carlo method and calculation of the temperature dependence of magnetic anisotropy

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 two-ion anisotropy in L10 FePt, and recover the experimentally observed M 2.1 scaling. The method is newly applied to evaluate the temperature-dependent effective anisotropy in the presence of the Neel 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.

Monte Carlo simulation of magnetization switching in a Heisenberg model for small ferromagnetic particles

Using Monte Carlo methods we investigate the thermally activated magnetization switching of small ferromagnetic particles driven by an external magnetic field. For low uniaxial anisotropy one expects that the spins rotate coherently while for sufficiently large anisotropy the reversal should be due to nucleation. The latter case has been investigated extensively by Monte Carlo simulation of corresponding Ising models. In order to study the crossover from coherent rotation to nucleation we use a specially adjusted update algorithm for the Monte Carlo simulation of a classical three-dimensional Heisenberg model with a finite uniaxial anisotropy. This special algorithm which uses a combined sampling can simulate different reversal mechanisms efficiently. It will be described in detail and its efficiency and physical validity will be discussed by a comparison with other common update-algorithms.

Linear and elliptical magnetization reversal close to the Curie temperature

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.

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

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/square-cm 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.

Magnetisation switching of FePt nanoparticle recording medium by femtosecond laser pulses

Manipulation of magnetisation with ultrashort laser pulses is promising for information storage device applications. The dynamics of the magnetisation response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage are FePt nanoparticles, for which switching of the magnetisation with optical angular momentum was demonstrated recently. The mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetisation switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to switch the magnetisation of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.

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

In order for the current increase in magnetic storage density to continue, one must overcome the so-called magnetic recording trilemma; namely, that smaller grains are required for higher data densities and to ensure their thermal stability, materials with a high anisotropy are required. The higher coercive field that this produces also becomes a limiting factor as the maximum field produced by the recording head is constrained by the saturation magnetization of the pole. One proposed solution to the trilemma is the use of heat assisted magnetic recording HAMR, which utilizes the temperature dependence of the anisotropy to enable writing of materials with a high coercivity. For the highest anisotropy media, this will require heating to the Curie temperature TC of the material. Close to TC, longitudinal fluctuations in the magnetization can have a significant impact on the expected energy barriers and therefore the relaxation time of the magnetization. These effects become especially important when attempting to minimize the time to reverse the magnetization state of the media that will be important at higher storage densities.