A new magnetic recording method using millimeter/terahertz waves !!

FUJIFILM has succeeded in verifying the principle of a new magnetic recording method using millimeter waves and terahertz waves jointly conducted with the University of Tokyo, Osaka University, etc. The results of this research have been published in the academic journal Advanced Materials. (Scheduled to be released in October 2020)

Shin-ichi Ohkoshi (Professor, Department of Chemistry, School of Science, The University of Tokyo)
Makoto Nakajima (Associate Professor, Institute of Laser Engineering, Osaka University)
Masashi Shirata (Manager, Recording Media Research & Development Laboratories, Research & Development Management Headquarters, FUJIFILM Corporation) 
Hiroaki Doshita (General Manager, Recording Media Research & Development Laboratories, Research & Development Management Headquarters, FUJIFILM Corporation) 
Hiroko Tokoro (Professor, Department of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba) 
Seiji Miyashita (Emeritus Professor, The University of Tokyo) 
Takehiro Yamaoka (Chief Engineer, Analysis Systems Solution Development Dept., Metrology and Analysis Systems Product Div., Hitachi High-Tech Corporation) 

• With increasing amounts of information, to achieve high-density recording in the future, researchers succeeded in proving the concept of “focused millimeter wave-assisted magnetic recording” as a new magnetic recording method that uses millimeter waves.
• Focusing on epsilon iron oxide (ε-Fe₂O₃) and metal-substituted ε-Fe₂O₃, which are candidates for magnetic fillers of future magnetic recording tapes and also possess high-frequency millimeter-wave absorption properties for the Beyond 5G Era, they prepared magnetic films of ε-Fe₂O₃ and constructed a focused millimeter-wave generator using terahertz (THz) light source, and experimentally demonstrated “focused millimeter wave-assisted magnetic recording”.
• The new recording method will enable the use of smaller magnetic nanoparticles for the magnetic recording media, and the recording capacity is expected to drastically increase.

A research group consisting of Professor Shin-ichi Ohkoshi of the School of Science, the University of Tokyo, Associate Professor Makoto Nakajima of Institute of Laser Engineering, Osaka University, and Manager Masashi Shirata and General Manager Hiroaki Doshita of Recording Media Research & Development Laboratories, FUJIFILM Corporation, collaborated with Professor Hiroko Tokoro of University of Tsukuba, Emeritus Professor Seiji Miyashita of the University of Tokyo, and Takehiro Yamaoka of Hitachi High-Tech Corporation, to successfully develop a new magnetic recording method, “millimeter wave magnetic recording”, using millimeter and terahertz waves.

In the era of Big Data and the Internet of Things (IoT), data archiving is a key technology. From this viewpoint, magnetic recording tapes^*1 are actively used in cloud services and data archives for business purposes because they guarantee long-term data storage, low power consumption, and low cost. Consequently, the demand for magnetic recording tapes is growing. To archive an enormous amount of data, recording density needs to be increased. In this work, Professor Ohkoshi and colleagues proposed a new magnetic recording methodology, “Focused Millimeter wave-assisted Magnetic Recording, F-MIMR,” to achieve millimeter-wave magnetic recording. To test this methodology, magnetic films were prepared using epsilon iron oxide^*2, which is drawing attention as a magnetic filler for future magnetic recording tapes and also as a millimeter-wave absorber for Beyond 5G^*3 networks, and constructed a focused millimeter wave generator using terahertz (THz) light. Irradiating the focused millimeter wave to epsilon iron oxide switched its magnetic pole direction, and magnetic field writing was confirmed. F-MIMR is an innovative magnetic recording method for the Beyond 5G Era, combining light/electromagnetic waves of Beyond 5G networks and magnetic recording. Thus, F-MIMR could contribute to raising the magnetic recording density.

The results of this research will be published online in Advanced Materials on October 8, 2020, Japan time.

• *1 Magnetic recording tapes
  Magnetic recording tapes are the earliest known magnetic recording media (since the 1950s). Japanese companies have monopolized the production of tape media and, nowadays, magnetic tapes are mainly used as the recording media for archiving. Due to the guarantee for long-term recording and low cost, demands for magnetic tapes are growing rapidly in a variety of areas, including insurance companies, banks, broadcasting stations, and web service companies.
• *2 Epsilon iron oxide (ε-Fe₂O₃)
  Professor Shin-ichi Ohkoshi and his colleagues successfully synthesized a single phase of epsilon iron oxide in 2004 by nanoparticle synthesis method. They reported that epsilon iron oxide possesses a crystal structure different from the conventional phases such as gamma (γ) and alpha (α) phases, and exhibits the largest coercive field among metal oxides at room temperature (20kOe). In addition, epsilon iron oxide has been highlighted as one of the candidates for the next generation of magnetic particles for magnetic tapes in the roadmap of the information storage industry consortium (INSIC), a global consortium of the magnetic tape industry.
• *3 Beyond 5G
  Beyond 5G is the sixth and subsequent communication standard following the fifth generation of mobile communication system (5G; 3.7GHz, 4.5GHz, 28GHz in Japan), which began to be used in 2019. Millimeter waves, which are higher frequency electromagnetic waves, are expected to be used in Beyond 5G networks.

Millimeter wave technology is expected to play a significant role in the era of Internet of Things (IoT). Millimeter waves (30-300GHz) ^*4 have potential in broadcasting wireless communications, wireless data transmissions between cellular base stations, and traffic monitoring sensors in intersection areas for advanced driver assistance systems. For example, millimeter waves at an 80-GHz frequency are widely used for car radars. Meanwhile, magnetic recording is drawing attention as a sustainable data storage system in the Big Data era. To further enhance the recording capacity to archive an exponentially increasing amount of data, the magnetic particle size must be reduced. However, as magnetic particles become smaller, the thermal stability of the magnetization decreases (the problem of superparamagnetism). In order to avoid the problem of superparamagnetism, enlargement of the magnetic anisotropy is necessary. Consequently, current magnetic recording heads cannot write against the strong magnetic anisotropy. This problem is called the “magnetic recording trilemma” and is common for magnetic recording media, including hard disk drives and magnetic tapes. To resolve the trilemma problem, several types of recording methods have been proposed such as heat-assisted magnetic recording and microwave-assisted magnetic recording.

Professor Ohkoshi and colleagues focused on epsilon iron oxide for two main reasons. First, it exhibits high-frequency millimeter wave absorption in a wide frequency range of 35 to 222 GHz due to the zero-field ferromagnetic resonance and is expected to be used for Beyond 5G applications. Second, it can also maintain spontaneous magnetization due to ferrimagnetism even with a single nanometer size particle. In this study, they prepared magnetic films based on metal-substituted epsilon iron oxide and propose a new recording methodology “Focused Millimeter Wave-Assisted Magnetic Recording (F-MIMR)” based on a novel concept of “millimeter wave magnetic recording” (Figure 1).

• *4 Millimeter waves
  Millimeter waves are electromagnetic waves with a wavelength of 1mm to 10mm and a frequency range of 30 to 300GHz. At present, millimeter-wave car radars are widely used, and practical applications of millimeter waves for wireless communication are being promoted as a next-generation high-speed wireless communication system. Epsilon iron oxide absorbs high-frequency millimeter-waves due to the precession of the magnetization, and is expected as a millimeter-wave absorber for high-speed wireless communications and automated driving support systems.

For the demonstration, magnetic films based on gallium-titanium-cobalt–substituted epsilon-iron oxide (GTC-type ε-iron oxide: ε-Ga_0.23(TiCo)_0.05Fe_1.67O₃) nanoparticles were prepared on a glass subustrate (Figure 2). Additionally, an intense millimeter-wave generator was constructed using terahertz light as a light source, and a millimeter-wave focusing ring^*5 was designed using electromagnetic field analysis simulation^*6 and fabricated on the surface of the magnetic film^*6 to focus the millimeter wave corresponding to the resonance frequency of GTC-type ε-iron oxide. As shown in Figure 3, the magnetization direction of the film was aligned along the +Z-direction. Then, an external magnetic field of 3.4kOe, slightly weaker than the coercive field value of 4.9kOe, was applied toward the opposite direction (−Z-direction). The sample was irradiated by a focused millimeter wave.

The atomic force microscopy (AFM)^*7 image of the sample after irradiation of the focused-millimeter wave showed the geometry of the ring. In the magnetic force microscopy (MFM) image, a dark shadow was observed near the focusing ring. The observed MFM image agrees well with the magnetic field distribution map simulated by electromagnetic field analysis, indicating that the magnetization was flipped by the assistance of the focused-millimeter wave. This is the first reported observation of a permanent magnetic pole flip by millimeter wave irradiation.

As an additional demonstration, a magnetic film placed face-to-face with the millimeter wave–focusing ring fabricated on Si wafer was irradiated with a millimeter wave. In the MFM image of the magnetic film after irradiation of the focused-millimeter wave, magnetization reversal was observed at the millimeter wave focused area.

To understand the spin dynamics of F-MIMR, the time evolution of all of the spins in an epsilon iron oxide nanoparticle were calculated using the stochastic Landau-Lifshitz-Gilbert model^*8. Simulation results showed that magnetization reversal instantly occurs by irradiating a millimeter wave of the resonance frequency.

Millimeter wave magnetic recording technique enables to reduce the particle size of the magnetic material and solve the magnetic recording trilemma, leading to the increase of the recording capacity. The transition energy of the millimeter wave is ca. 1/5000 compared to that visible light. Therefore, heat-up is avoided in millimeter wave–assisted magnetic recordings, which is very important for magnetic recording tapes that use organic resin for the base film.

The present research was supported in part by the “Advanced Research Program for Energy and Environmental Technologies / Development of a millimeter wave assisted magnetic recording method for magnetic tapes” project (Ohkoshi Laboratory, The University of Tokyo / Nakajima Laboratory, Osaka University / Recording Media Research & Development Laboratories, FUJIFILM Corporation) commissioned by NEDO of METI.

• *5 Focusing ring
  Focusing ring is a metallic ring with a gap. When electromagnetic waves, such as terahertz light, are irradiated to the focusing ring, a circumferential current flows, causing a resonant phenomenon that enhances the intensity of the electromagnetic waves at a particular frequency.
• *6 Electromagnetic field analysis simulation
  Electromagnetic field analysis is a simulation that theoretically calculates the electric and magnetic field distributions generated by electromagnetic waves.
• *7 Atomic force microscopy and magnetic force microscopy
  Atomic force microscopy detects the interatomic force between the probe and the sample to show the geometries of the sample surface. On the other hand, magnetic force microscopy detects the magnetic force between the probe and the sample to show the magnetic state of the sample surface.
• *8 Stochastic Landau-Lifshitz-Gilbert model
  Stochastic Landau-Lifshitz-Gilbert model is a theoretical model that incorporates stochastic noise as temperature effects into the Landau-Lifshitz-Gilbert equation, an equation that describes the precession of the magnetization in magnetic materials. F-MIMR was theoretically demonstrated using a model taking into account all of the individual spin movements in the nanoparticle.