Stable microwave-assisted magnetization switching for nanoscale exchange-coupled composite grain
© Tanaka et al.; licensee Springer. 2013
Received: 19 July 2013
Accepted: 22 October 2013
Published: 5 November 2013
Magnetization mechanisms of nanoscale magnetic grains greatly differ from well-known magnetization mechanisms of micrometer- or millimeter-sized magnetic grains or particles. Magnetization switching mechanisms of nanoscale exchange-coupled composite (ECC) grain in a microwave field was studied using micromagnetic simulation. Magnetization switching involving a strongly damped or precessional oscillation was studied using various strengths of external direct current and microwave fields. These studies imply that the switching behavior of microwave-assisted magnetization switching of the ECC grain can be divided into two groups: stable and unstable regions, similar to the case of the Stoner-Wahlfarth grain. A significant reduction in the switching field was observed in the ECC grain when the magnetization switching involved precessional oscillations similar to the case of the Stoner-Wohlfarth grain. This switching behavior is preferred for the practical applications of microwave-assisted magnetization switching.
KeywordsMicrowave-assisted magnetization reversal Exchange-coupled composite grain Micromagnetic simulation
Nanoscale magnetic grains are essential for extending the areal density of hard disk drives. These nanoscale grains are found in hard disk drives, in which the problem of writability still remains to be solved. Energy-assisted magnetic recording schemes [1, 2] have already been proposed for solving the writability problems in magnetic recordings. In these recording schemes, microwave-assisted magnetization reversal (MAMR) has recently attracted much attention as an alternative technique for future ultrahigh density recordings. In the case of MAMR, a microwave field is tuned to the ferromagnetic resonance frequency of the recording medium, during which a quasi-direct current (dc) field is also applied, wherein the quasi-dc field is smaller than the switching field in the absence of microwaves. Resonant magnetic precession drives the magnetization over the energy barrier imposed by anisotropy provided that the microwave field amplitude is sufficiently large. Recent experiments [3–6] and simulations [7–13] have demonstrated a reduction in the switching field by applying a large amplitude microwave field with frequencies in the order of gigahertz. To realize ultrahigh density recordings for hard disk drives, magnetic materials with a strong perpendicular magnetic anisotropy (such as L 10-FePt) are required to overcome thermal fluctuations. However, for magnetization reversal, these materials require a strong magnetic head field and microwave field  at extremely high frequencies. This is an issue concerning MAMR that needs to be resolved. Recent micromagnetic analysis has shown that an exchange-coupled composite (ECC) structure  with both soft and hard magnetic materials effectively reduces the strengths of dc and microwave fields as well as the optimum microwave frequency for magnetization reversal [16–20].
The analytical treatment for the magnetization of a single magnetic vector under circular microwave fields was discussed [14, 21, 22]. In these articles, various steady states of precessional magnetization motions were studied by solving the Landau-Lifshitz-Gilbert (LLG) equation. However, there are so far no reports about the steady state of precessional magnetization motions of ECC structured grain. This study presents the magnetization switching behavior of a nanoscale ECC grain using microwave assistance by drawing comparison with the magnetization motions of Stoner-Wohlfarth grain using LLG simulation.
Results and discussion
The theoretical treatment is very useful when analyzing the MAMR process. However, applicable field situations of the treatment are limited . Hence, a numerical integration of the LLG equation is necessary for analyzing MAMR processes under various field situations.
Although the data is not shown, a great reduction in HSW was also confirmed at T = 400 K when the incident angle was large. These advantages ensure magnetization switching of high Ku materials by magnetic fields that are practical in device applications such as hard disk drives.
Magnetization switching behavior of a nanoscale ECC grain under microwave assistance has been numerically analyzed by comparing it with that of a Stoner-Wohlfarth grain. The computational simulation indicated that significant switching field reduction due to relatively large microwave field excitation is observed in the ECC grains. Therefore, the magnetization switching in the ECC grain under microwave assistance seems to be divided into two regions of stable and unstable switching depending on applied dc and microwave field strength. Stable switching is more favorable for practical applications when using microwave-assisted magnetization switching in the ECC grain. These results qualitatively agree with the theoretical analysis and the LLG simulation for the Stoner-Wohlfarth grain.
TT is an assistant professor in ISEE, Kyushu University. His research interests include micromagnetics, magnetic recording, and high frequency magnetic devices. SK received a B.S. degree in Electrical Engineering from Kyushu University in 2013. YF received an M.S. degree in ISEE from Kyushu University in 2013. YO received a B.S. degree in Electrical Engineering from Kyushu University in 2012. KM is a professor in ISEE, Kyushu University. His research interests include magnetic devices.
This research was partially supported by the Storage Research Consortium (SRC) and a Grant-in-Aid for Young Scientists (A) (grant no. 25709029) 2013 from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.
- Rottmayer RE, Batra S, Buechel D, Challener WA, Hohlfeld J, Kubota Y, Li L, Lu B, Mihalcea C, Mountfield K, Pelhos K, Peng C, Rausch T, Seigler MA, Weller D, Yang X: Heat-assisted magnetic recording. IEEE Trans Magn 2006, 42: 2417–2421.View Article
- Zhu JG, Zhu X, Tang Y: Microwave assisted magnetic recording. IEEE Trans Magn 2008, 44: 125–131.View Article
- Thirion C, Wernsdorfr W, Mailly D: Switching of magnetization by nonlinear resonance studied in single nanoparticles. Nature Mater 2003, 2: 524–527. 10.1038/nmat946View Article
- Moriyama T, Cao R, Xiao JQ, Lu J, Wang XR, Wen Q, Zhang HW: Microwave-assisted magnetization switching of Ni80Fe20 in magnetic tunnel junctions. Appl Phys Lett 2007, 90: 152503. 10.1063/1.2720746View Article
- Nozaki Y, Ohta M, Taharazako S, Tateishi K, Yoshimura S, Matsuyama K: Magnetic force microscopy study of microwave-assisted magnetization reversal in submicron-scale ferromagnetic particles. Appl Phys Lett 2007, 91: 082510. 10.1063/1.2775047View Article
- Yoshioka T, Nozaki T, Seki T, Shiraishi M, Shinjo T, Suzuki Y, Uehara Y: Microwave-assisted magnetization reversal in a perpendicularly magnetized film. Appl Phys Express 2010, 3: 013002. 10.1143/APEX.3.013002View Article
- Rivkin K, Ketterson JB: Magnetization reversal in the anisotropy-dominated regime using time-dependent magnetic fields. Appl Phys Lett 2006, 89: 252507. 10.1063/1.2405855View Article
- Nozaki Y, Matsuyama K: Numerical study for ballistic switching of magnetization in single domain particle triggered by a ferromagnetic resonance within a relaxation time limit. J Appl Phys 2006, 100: 053911. 10.1063/1.2338128View Article
- Okamoto S, Kikuchi N, Kitakami O: Magnetization switching behavior with microwave assistance. Appl Phys Lett 2008, 93: 102506. 10.1063/1.2977474View Article
- Scholz W, Batra S: Micromagnetic modeling of ferromagnetic resonance assisted switching. J Appl Phys 2008, 103: 07F539. 10.1063/1.2838332View Article
- Gao KZ, Benakli M: Energy surface model and dynamic switching under alternating field at microwave frequency. Appl Phys Lett 2009, 94: 102506. 10.1063/1.3097229View Article
- Wang X, Gao KZ, Hohlfeld J, Seigler M: Switching field distribution and transition width in energy assisted magnetic recording. Appl Phys Lett 2010, 97: 102502. 10.1063/1.3486167View Article
- Tanaka T, Kato A, Furomoto Y, Md Nor AF, Kanai Y, Matsuyama K: Microwave-assisted magnetic recording simulation on exchange-coupled composite medium. J Appl Phys 2012, 111: 07B711. 10.1063/1.3678450
- Okamoto S, Igarashi I, Kikuchi N, Kitakami O: Microwave assisted switching mechanism and its stable switching limit. J Appl Phys 2010, 107: 123914. 10.1063/1.3436570View Article
- Victora RH, Shen X: Composite media for perpendicular magnetic recording. IEEE Trans Magn 2005, 41: 537–542.View Article
- Bashir MA, Schrefl T, Dean J, Goncharov A, Hrkac G, Bance S, Allwood D, Suess D: Microwave-assisted magnetization reversal in exchange spring media. IEEE Trans Magn 2008, 44: 3519–3522.View Article
- Li S, Livshitz B, Bertram HN, Schabes M, Schrefl T, Fullerton EE, Lomakin V: Microwave assisted magnetization reversal in composite media. Appl Phys Lett 2009, 94: 202509. 10.1063/1.3133354View Article
- Igarashi M, Suzuki Y, Miyamoto H, Maruyama Y, Shiroishi Y: Mechanism of microwave assisted magnetic switching. J Appl Phys 2009, 105: 07B907. 10.1063/1.3075850View Article
- Li H, Hou F, Li P, Yang X: Influences of switching field rise time on microwave-assisted magnetization reversal. IEEE Trans Magn 2011, 47: 355–358.View Article
- Tanaka T, Narita N, Kato A, Nozaki Y, Hong YK, Matsuyama K: Micromagnetic study of microwave-assisted magnetization reversals of exchange-coupled composite nanopillars. IEEE Trans Magn 2013, 49: 562–566.View Article
- Bertotti G, Serpico C, Mayergoyz D: Nonlinear magnetization dynamics under circularly polarized field. Phys Rev Lett 2001, 86: 724–727. 10.1103/PhysRevLett.86.724View Article
- Bertotti G, Mayergoyz ID, Serpico C, d’Aquino M, Bonin R: Nonlinear-dynamical-system approach to microwave-assisted magnetization dynamics. J Appl Phys 2009, 105: 07B712. 10.1063/1.3072075
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