Microwave assisted magnetic recordingMicrowave assisted magnetic recording

IntroductionIntroduction Nowadays, people pay more attention on microwave assisted magnetic recording, such as Jian-Gang Zhu and Xiaochun Zhu who came from Data storage system center Carnegie Mellon University, they use a high frequency ac field which was applied transversely to the magnetic easy axis of the medium grain. When the frequency of microwave paired with the ferromagnetic resonance frequency in the recording media, the amplitude of the magnetization could go up rapidly, and this would lead to a magnetization reversal at the writing head field’s importantly less than the normal switching fields of the medium grains. Compared with Jian-Gang Zhu and Xiaochun Zhu, we use the same mechanism that is also to use a microwave generator in the recording head to apply a microwave field to the magnetic medium, which helps to switch the magnetization of the magnetic medium. However, we are focusing on different aspects like I usually use the frequency of microwave from 15 GHz to 35 GHz or higher and I am focusing on the time of switching magnetization.

Principles and mechanismPrinciples and mechanism The main work for magnetic recording is to increase the density of magnetic media, because the volume of medium is the same, so the size of grains should be decreased, and then in order to keep thermally stability, the anisotropy constant must be increased and this will make coercivity goes up, that means the larger coercivity is needed. The coercivity depends on the magnetic materials of writing head, but the largest coercivity of material which is consisted of cobalt and iron is about 1 T to 1.5 T. This is not enough to make high density of magnetic recording. Microwave can apply an extra field and energy, so microwave may apply a new way to solve this barrier.

At first, an ac field is applied in the direction transverse to the easy axis, and then the magnetization starts to rotating and the processional angle increasing; finally, the direction of magnetization could be changed into downwards. If there is no ac field, the magnetization is also along with the easy axis. On the other hand, many properties of microwave have an influence up on switching procession, such as frequency of microwave, amplitude and so on. Because there isn’t any equations that can describe how microwave properties influence switching of,magnetization, so the main job of this,experiment is to apply the different frequencies and amplitudes cube.

Fig.2 The picture shows how the magnetization switched in magnetic grain

ExperimentsExperiments The size of cubic hard disk materials was chose as our sample.In this experiment,we used software to simulate the whole process. This software includes GID software which can be used to draw and mesh the geometry, the program AVS that is used for visualization, and CVODE what we have used it package for solving these differential equations like the Gilbert equation. Furthermore, finite element method providesus a complete grip on complex geometries e.g. magnetic media thin film, head design and soon.

Fig.3 It can be seen from this graph that the relationship between magnetization and switching time. The X axis representsthe produce time, and Y axis gives the normalized magnetization. In this experiment we chose magnetization -0.5as the

switching point, so the switching time is 0.58 ns in this condition

Results and analysisResults and analysis

Fig.4 These three graphs show the relationships between the switching time and microwave frequency in different amplitude. The X axis represents the microwave frequencies, and Y axis means the switching time. The chart a describes

that only some frequencies can switch the magnetization when amplitude was 0.1 T; the diagram b shows when the amplitude was changed into 0.2 T, all the frequencies (from 21 GHz to 36.5 GHz) can switch the magnetization; The final graph c tells us that the range of switching frequencies will increase and be stable while the amplitude went up to 0.3 T.

Fig.5 These three graphs show the relationships between the switching time and microwave frequency in different amplitude when the anisotropy energy constant is 0.82. The chart a describes that only some frequencies can switch the magnetization

when amplitude was 0.1 T; the diagram b shows when the amplitude was changed into 0.2 T, all the frequencies (from 23 GHz to 39 GHz) can switch the magnetization; The final graph c tells us that the range of switching frequencies will increase

and be stable while the amplitude went up to 0.3 T.

Fig.6 These three graphs show the relationships between the switching time and microwave frequency in different amplitude when the anisotropy energy constant is 0.78. The chart a describes that there was another very different situation when amplitude was 0.1 T; the diagram b shows when the amplitude was changed into 0.2 T, all the frequencies (from 18 GHz to 34 GHz) can switch the magnetization; The final graph c tells us that the ran

ge of switching frequencies will increase and be stable while the amplitude went up to 0.3 T.

It can be seen from all above experiment that for all the amplitudes, the magnetizationswitching process will begin from some frequency and should be end after another frequency; and there is no evidence which can prove that using the higher frequency can get the better switching time. The switching time could be influenced by many aspects not only one parameter, so we should analysis this problem all-sided. Comparing the three situations with the different anisotropy energy constants 0.8, 0.82 and 0.78, the curve of frequency-switching time had the largest scale and highest stability from frequency 17.25 GHz to 26 GHz when amplitude was 0.3 T and the anisotropy energy constant , meanwhile, the switching time was the lowest in all the experiments.

34 4 4nm

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Min Wan, Asif and Prof.Thomas