Markus Fuger

Perpendicular FePt-based exchange-coupled composite media

Exchange-coupled composite media were realized by combining perpendicular hard magnetic FePtCu alloy films with perpendicular Co/Pt multilayers which are magnetically softer. We demonstrate that the switching field of the hard layer can be efficiently altered by modifying the material properties of the soft layer by varying the number of Co/Pt bilayers. Moreover, the possibility of effectively tuning the interlayer exchange coupling using rapid thermal annealing was shown. These studies were supported by theoretical modeling revealing the relevant factors to reduce the switching field of the hard layer which are important for future media design.

FePt L10/A1 graded media with a rough interphase boundary

A graded media consisting of FePt L10(hard) and A1(soft) phases separated by a rough wedge-shaped interphase boundary, the “phase graded media” is suggested. The rough interface helps domain wall propagation from the soft to the hard phase, owing to the easily reversed wedge tips of the hard phase. The reversed domain expands in the hard phase with a small additional field. As a result, the switching field of the phase graded media was reduced to 13 kOe which is 16% of FePt L10 single phase (79 kOe), the reduction is comparable with the stacked graded media.

Calculation of coercivity of magnetic nanostructures at finite temperatures

We report a finite temperature micromagnetic method (FTM) that allows for the calculation of the coercive field of arbitrary shaped magnetic nanostructures at time scales of nanoseconds to years. Instead of directly solving the Landau-Lifshitz-Gilbert equation, the coercive field is obtained without any free parameter by solving a non linear equation, which arises from the transition state theory. The method is applicable to magnetic structures where coercivity is determined by one thermally activated reversal or nucleation process. The method shows excellent agreement with experimentally obtained coercive fields of magnetic nanostructures and provides a deeper understanding of the mechanism of coercivity.

Thermal switching field distribution of a single domain particle for field-dependent attempt frequency

We present an analytical derivation of the switching field distribution (SFD) at finite temperature for a single domain particle from the Neel-Brown model in the presence of a linearly swept magnetic field. By considering the field dependence of the attempt frequency f0 in the rate equation, we find enhancement of coercivity compared to models using constant f0. The contribution of thermal fluctuations to the standard deviation of the switching field HC derived here reaches values of 10% HC. Considering this contribution, which has been neglected in previous work, is important for the correct interpretation of measurements of switching field distributions.

Validation of the transition state theory with Langevin-dynamics simulations

Finite-element Langevin-dynamics simulations are performed in order to extract the attempt frequency of small magnetic particles as a function of an applied perpendicular field. The obtained values of the attempt frequency are in excellent agreement with the analytical results of [Kalmykov, J. Appl. Phys. 96, 1138 (2004)]. It is shown that an external field that is applied perpendicularly to the easy axis with a strength of just about 1% of the anisotropy field is strong enough that the framework of the transition state theory (TST) for broken symmetries can be applied. It is concluded that for most realistic structures, the attempt frequency can be numerically calculated by broken symmetry—TST formulism.

Theory and micromagnetics of pinning mechanism at cylindrical defects in perpendicular magnetic films

A theoretical model is developed to describe the pinning energy and the pinning field as a function of pore density, pore diameter and material parameters of a magnetic film. It is shown that the pinning energy and pinning field increases monotonically with increasing pore diameter. A magnetic storage concept is introduced which combines percolated media and graded media which allows to write on magnetic films with ultra high anisotropies.

Three-dimensional magneto-resistive random access memory devices based on resonant spin-polarized alternating currents

Selective switching of a magneto-resistive random access memory (MRAM) multilayer stack is demonstrated using resonant spin-polarized alternating currents (AC) superimposed on spin-polarized direct currents. Finite element micromagnetic simulations show that the use of frequency triggered AC allows one to maximize the transferred spin transfer torque selectively in order to merely reverse the magnetization of a single storage layer in a stack. Using layers with different resonance frequencies, which are realized by altering the anisotropy constants, allows one to address them by tuning the AC frequency. A rapid increase of the storage density of MRAM devices is shown by using three-dimensional sandwich structures.