313 research outputs found
Unified model of hyperthermia via hysteresis heating in systems of interacting magnetic nanoparticles
No full textWe present a general study of the frequency and magnetic field dependence of the specific heat power produced during field-driven hysteresis cycles in magnetic nanoparticles with relevance to hyperthermia applications in biomedicine. Employing a kinetic Monte-Carlo method with natural time scales allows us to go beyond the assumptions of small driving field amplitudes and negligible inter-particle interactions, which are fundamental to the applicability of the standard approach based on linear response theory. The method captures the superparamagnetic and fully hysteretic regimes and the transition between them. Our results reveal unexpected dipolar interaction-induced enhancement or suppression of the specific heat power, dependent on the intrinsic statistical properties of particles, which cannot be accounted for by the standard theory. Although the actual heating power is difficult to predict because of the effects of interactions, optimum heating is in the transition region between the superparamagnetic and fully hysteretic regimes
The role of interfacial intermixing on HAMR dynamics in bilayer media
No full textWe use an atomistic spin model to simulate FePt-based bilayers for heat assisted magnetic recording (HAMR) devices and investigate the effect of various degrees intermixing that might arise throughout the fabrication, growth and annealing processes, as well as different interlayer exchange couplings, on HAMR magnetisation dynamics. Intermixing can impact the device functionality, but interestingly does not deteriorate the properties of the system. Our results suggest that modest intermixing can prove beneficial and yield an improvement in the magnetisation dynamics for HAMR processes, also relaxing the requirement for weak exchange coupling between the layers. Therefore, we propose that a certain intermixing across the interface could be engineered in the fabrication process to improve HAMR technology further
Ultrafast magnetism I: Proceedings of the international conference umc2013 strasbourg, france, october 28th – november 1st
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Ultrafast spectroscopy with spin polarization
No full textA project for the design and r.alization of an experimental station dedicated to ultrafast spin polarization dynamics allowing for spin polarization measurements of photoelectron yield as excited by free electron laser pulses is presented
Atomistic investigation of the temperature and size dependence of the energy barrier of CoFeB/MgO nanodots
No full textThe balance between low power consumption and high efficiency in memory devices is a major limiting factor in the development of new technologies. Magnetic random access memories (MRAMs) based on CoFeB/MgO magnetic tunnel junctions (MTJs) have been proposed as candidates to replace the current technology due to their non-volatility, high thermal stability, efficient operational performance. Understanding the size and temperature dependence of the energy barrier and the nature of the transition mechanism across the barrier between stable configurations is a key issue in the development of MRAM. Here, we use an atomistic spin model to study the energy barrier to reversal in CoFeB/MgO nanodots to determine the effects of size, temperature, external field. We find that for practical device sizes in the 10-50 nm range, the energy barrier has a complex behavior characteristic of a transition from a coherent to domain wall driven reversal process. Such a transition region is not accessible to simple analytical estimates of the energy barrier preventing a unique theoretical calculation of the thermal stability. The atomistic simulations of the energy barrier give good agreement with experimental measurements for similar systems, which are at the state of the art and can provide guidance to experiments identifying suitable materials and MTJ stacks with the desired thermal stability
Multiscale modeling of ultrafast magnetization dynamics
No full textAtomistic and mesoscopic-scale spin dynamics have proven to be a powerful tool for studying magnetization dynamics on the sub-picosecond time-scale. These approaches are ideal for combining different length and time-scales. In this paper we focus on atomistic spin dynamics in the ultrafast regime, in particular on recent results of heat-induced switching in the transition-metal rare-earth ferrimagnets
MODELS OF SPIN TORQUE USING SELF-CONSISTENT SOLUTIONS OF THE MAGNETISATION AND SPIN ACCUMULATION
No full textA model of spin accumulation (m) is proposed to develop theoretical approaches to calculate the m in any arbitrary magnetic structure. The model is based on generalising
the approach of Zhang, Levy and Fert (PRL 88, 236601, 2002). The calculation involves the layer-wise discretisation of the structure and the development of semi-analytical approaches to solve for the equilibrium m throughout the structure. Interestingly, the layer discretisation allows the treatment of diff�use interfaces using a gradual variation of the magnetic and transport properties across the interface. The e�ffect of the interfaces
between a ferromagnet and a nonmagnet and between two ferromagnets on spin injection is investigated. The formalism for calculating the m is first generalised by taking m as the di�fference of spin-up and spin-down density of states, which is necessary for treating the interface between diff�erent ferromagnets. Then, the e�ffect of atomic species interdiffusion at the interface is included by using Ficks's law. It is shown that the discontinuity of the m at the interface depends strongly on the degree of interface mixing.
Subsequently, current-induced domain wall (DW) motion in a ferromagnetic thin fi�lm driven by a spin-polarised current is investigated using an atomistic model coupled with a standard Landau-Lifshitz-Gilbert equation. The inclusion of the spin-transfer torque is represented as an additional �field. The m is calculated self-consistently and naturally includes the adiabatic and non-adiabatic contributions depending on the rate of change of magnetisation relative to the spin di�ffusion length. In this work, it is importantly found that the constants �x and �x used in the standard micromagnetic model do not provide a good description of the spin torque phenomenon due to the non-physical behaviour. Therefore, it is suggested to describe the spin-transfer torque directly from the m.
Finally, the evolution of the magnetisation and m are demonstrated by introducing a spin-polarised current into a material containing a DW whose width is varied by changing the anisotropy constant. It is found that the adiabatic spin torque tends to develop in the direction of the magnetisation whereas the non-adiabatic spin torque
arising from the mistracking of conduction electrons and local magnetisation results in out-of-plane magnetisation components. However, the adiabatic spin torque signifi�cantly
dominates the dynamics of magnetisation. The total spin torque acting on the magnetisation increases with anisotropy constant due to the increasing magnetisation gradient. This leads to increasing DW displacement
Dimensional scaling effects on critical current density and magnetization switching in CoFeB-based magnetic tunnel junction
No full textIn this work, we theoretically investigate the size dependence of the magnetization reversal behavior in CoFeB-MgO-CoFeB magnetic tunnel junctions (MTJs) by employing an atomistic spin model coupled with the spin accumulation model. The former and the latter are used to construct the magnetic structure and to model the spin transport behavior, respectively. The accuracy of the approach is confirmed by investigating the dependence of the magnetic properties on the size of the MTJ. Perpendicular magnetic anisotropy (PMA) is observed for thickness less than 1.3 nm, which is in an excellent agreement with experiment. To investigate the magnetization dynamics induced by spin-polarized current, a charge current is injected into the MTJ structure perpendicular to the stack leading to a spin-transfer torque acting on the magnetization of the CoFeB layer. The results show that the critical current density to reverse the magnetization is lower for PMA-MTJ and in addition for the same injected current density the time required to switch the magnetization is shorter than for an in-plane MTJ. The results can be used as a guideline to optimize the design of high performance MTJs for STT-MRAM applications
Models of advanced recording systems: A multi-timescale micromagnetic code for granular thin film magnetic recording systems
No full textMicromagnetic modelling provides the ability to simulate large magnetic systems reliably without the computational cost limitation imposed by atomistic modelling. Through micromagnetic modelling it is possible to simulate systems consisting of thousands of grains over a time range of nanoseconds to years, depending upon the solver used. Here we present the creation and release of an open-source multi-timescale micromagnetic code combining three key solvers: Landau-Lifshitz-Gilbert; Landau-Lifshitz-Bloch; Kinetic Monte Carlo. This code, called MARS (Models of Advanced Recording Systems), is capable of accurately simulating the magnetisation dynamics in large and structurally complex single- and multi-layered granular systems as is shown by comparison to established atomistic simulation results. The short timescale simulations are achieved for systems far from and close to the Curie point via the implemented Landau-Lifshitz-Gilbert and Landau-Lifshitz-Bloch solvers respectively. This enables read/write simulations for general perpendicular magnetic recording and also state of the art heat assisted magnetic recording (HAMR). The long timescale behaviour is simulated via the Kinetic Monte Carlo solver, enabling investigations into signal-to-noise ratio and data longevity. The combination of these solvers opens up the possibility of multi-timescale simulations within a single software package. For example the entire HAMR process from initial data writing and data read back to long term data storage is possible via a single simulation using MARS. The use of atomistic parameterisation for the material input of MARS enables highly accurate material descriptions which provide a bridge between atomistic simulation and real world experimentation. Thus MARS is capable of performing simulations for all aspects of recording media research and development. This ranges from material characterisation and optimisation to system design and implementation. The object orientated nature of MARS is structured to facilitate quick and simple development and easy implementation of user defined custom simulation types which can utilise either timescale or a combination of both timescales. Program summary: Program title: MARS CPC Library link to program files: https://doi.org/10.17632/8mx7cndcdx.1 Developer's repository link: https://bitbucket.org/EwanRannala/mars/ Code Ocean capsule: https://codeocean.com/capsule/2549929 Licensing provisions: MIT Programming language: C++ Supplementary material: MARS testing methodology (PDF), HAMR simulation example video. Nature of problem: A combined model that enables the complete modelling of magnetic recording processes at elevated temperatures covering all time scales from writing (nanoseconds) up to long term data storage (years). The model must also accurately describe the granular nature of the recording media as grain sizes are reduced to a few nanometres. Solution method: Short timescale behaviours are captured via the Landau-Lifshitz-Gilbert and Landau-Lifshitz-Bloch solvers for low and high temperature systems respectively. The long time scale behaviours are captured via a kinetic Monte Carlo solver. To enable complex models which account for mixed timescale behaviours the solvers are implemented as a single class structure which allows for dynamic solver selection. The granular structure is generated via a Laguerre-Voronoi tessellation with a custom implemented packing algorithm to produce highly realistic grain size distributions. Complex thermal dependencies of materials can be incorporated via atomistic parameterisation forming a multi-timescale model of the material
Granular micromagnetic HAMR model: investigation of damping dependence and parametric optimization for high performance
No full textHeat assisted magnetic recording (HAMR) is a novel high-density magnetic recording technology that relies on thermal assist from a laser during the writing process. To achieve high writing performance, it is important to study and optimize the crucial factors affecting the magnetization reversal mechanism at elevated temperature. In this work, we use a multiscale approach that combines atomistic and micromagnetic models to study the magnetization reversal behavior in the recording medium. The atomistic model allows to parameterize accurately the macroscopic approach, which is utilized to model the system and its dynamics. We perform a parametric investigation of the switching properties as a function of the HAMR setup characteristics as well as the material properties, such as magnetic damping. The results show that high damping and moderate external fields can achieve high-performance HAMR media characterized by high switching probability, short switching time and low peak temperature. We demonstrate that switching occurs via the linear reversal mechanism. By systematic variation of the longitudinal susceptibility we force a transition to coherent reversal and demonstrate that this reduces the switching probability, showing linear reversal to be an important component within the HAMR process
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