Exchange Spring Media for advanced magnetic recording

The optimization of materials for advanced magnetic recording with high recording density has a great impact on the development of information technologies. Advanced numerical techniques to describe the time evolution of the magnetization based on Browns theory of micromagnetics and the Landau-Lifshitz-Gilbert equation. Micromagnetics reveal the necessary information to determine the limits of the thermal stability of written bits on magnetic media. For too high aerial densities the grains are no longer stable. This effect is called superparamagnetic limit. In our previous work we have developed a micromagnetic code based on finite element discretization in order to study the influence of realistic microstructures on the magnetic switching behaviour of nano- and mesoscopic structures. The aim of the project is to design future magnetic storage media with an aerial density of 1 TBit/inch and beyond. In order to achieve this goal new concepts for magnetic recording are introduced such as exchange spring media, lateral exchange spring media. These new concepts have one in common. They rely on strongly exchange coupled soft region which improve the writeability of the high coercive data layer. Exchange spring media bases on perpendicular recording with a granular soft layer on top. Lateral exchange spring media is the further stage of percolation media. In percolation media a soft magnetic track is introduced that acts as a nucleation centre for the domains. This nucleation moves into the hard magnetic recording layer during the writing process. The main objective is to find structures that can be written with state of the art recording heads which give maximum head field of 1400 kA/m. Sophisticated design of the material composition should allow to design media where the introduction of the soft layer does not significantly reduces the thermal stability but decreases the coercive field of the entire structure up to a factor of three. As a consequence magnetic extremely hard materials can be written with moderate external fields. In order to design these structures the finite element micromagnetics fully integrated recording simulator which was implemented at our Institute is embedded in a global optimization routine. For the optimization a neuronal network is used in combination with efficient global optimizer such as simulated annealing, genetic algorithms and hysteretic optimization. The fully integrated recording simulator is expanded in order to calculate the signal to noise ratio of a written bit pattern. The calculation of the energy barrier and the calculated signal to noise ratio are used for the optimization. The objective function is the aerial density under the constraint of a given thermal stability and a given signal to noise ratio. An other concept that is investigated are patterned elements embedded in a soft magnetic matrix. Materials will be selected which have a switching field that is insensitive to defects (intragrain exchange interactions and anisotropy variations) and the field angle of the external field. Different concepts of the microstructure are used to design elements that have a small switching field distribution. The close cooperation with international partners allows to validate the designed structures with experiments. As an input for the experimentalist an expanded Sharrock`s law will be developed which is required for the measurements of the thermal stability of exchange spring recording media.

The optimization of materials for advanced magnetic recording with high recording density has a great impact on the development of information technologies. Advanced numerical techniques to describe the time evolution of the magnetization based on Brown`s theory of micromagnetics and the Landau-Lifshitz-Gilbert equation. Micromagnetics reveal the necessary information to determine the limits of the thermal stability of written bits on magnetic media. For too high aerial densities the grains are no longer stable. This effect is called superparamagnetic limit. In our previous work we have developed a micromagnetic code based on finite element discretization in order to study the influence of realistic microstructures on the magnetic switching behaviour of nano- and mesoscopic structures. The aim of the project is to design future magnetic storage media with an aerial density of 1 TBit/inch and beyond. In order to achieve this goal new concepts for magnetic recording are introduced such as exchange spring media, lateral exchange spring media. These new concepts have one in common. They rely on strongly exchange coupled soft region which improve the writeability of the high coercive data layer. Exchange spring media bases on perpendicular recording with a granular soft layer on top. Lateral exchange spring media is the further stage of percolation media. In percolation media a soft magnetic track is introduced that acts as a nucleation centre for the domains. This nucleation moves into the hard magnetic recording layer during the writing process. The main objective is to find structures that can be written with state of the art recording heads which give maximum head field of 1400 kA/m. Sophisticated design of the material composition should allow to design media where the introduction of the soft layer does not significantly reduces the thermal stability but decreases the coercive field of the entire structure up to a factor of three. As a consequence magnetic extremely hard materials can be written with moderate external fields. In order to design these structures the finite element micromagnetics fully integrated recording simulator which was implemented at our Institute is embedded in a global optimization routine. For the optimization a neuronal network is used in combination with efficient global optimizer such as simulated annealing, genetic algorithms and hysteretic optimization. The fully integrated recording simulator is expanded in order to calculate the signal to noise ratio of a written bit pattern. The calculation of the energy barrier and the calculated signal to noise ratio are used for the optimization. The objective function is the aerial density under the constraint of a given thermal stability and a given signal to noise ratio. An other concept that is investigated are patterned elements embedded in a soft magnetic matrix. Materials will be selected which have a switching field that is insensitive to defects (intragrain exchange interactions and anisotropy variations) and the field angle of the external field. Different concepts of the microstructure are used to design elements that have a small switching field distribution. The close cooperation with international partners allows to validate the designed structures with experiments. As an input for the experimentalist an expanded Sharrock`s law will be developed which is required for the measurements of the thermal stability of exchange spring recording media.