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Abstract
Thermomagnetic recording is a technique used as a writing process for magneto-optical (MO) drives. Despite their significant advantages, such as rewritability, nonvolatility, reliability, and large cycling endurance, MO drives are rarely used today because of the complex drive systems that must deal with magnetic field and lightwave simultaneously. This study reports on the light-induced thermomagnetic recording of a ferromagnetic thin-film CoFeB on a Si photonic platform. Lightwave guided in the Si waveguide evanescently coupled to the thin-film magnet and underwent optical absorption, resulting in heating and a decrease in coercive force. Therefore, we observed magnetization reversal with an applied magnetic field for both continuous and modulated light pulses using a magneto-optical Kerr effect microscope, and the light-induced thermomagnetic recording was experimentally demonstrated on a Si photonic platform. The proposed scheme enables the realization of on-chip MO memories on the Si photonic platform in which neither bulky free-space optics nor mechanical rotation systems are required.
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1. Introduction
Magneto-optical (MO) recording is a mature technology used to back up files in personal computers and audio and video recording systems, and it utilizes thermomagnetic recording for writing and MO readout techniques [1–3]. In the thermomagnetic recording, the magnetic recording medium is locally heated with a laser pulse near the Curie temperature or compensation temperature. Then, the coercive force dramatically decreases, and its magnetization is easily aligned along the direction of the applied external magnetic field after cooling to ambient temperature. MO drives are standard optical disks with several advantages, such as rewritability, removability, nonvolatility, reliability, and large cycling endurance. Unfortunately, MO drives have been replaced by hard disk drives or other optical storage media, such as digital versatile disks and Blu-ray disks, to avoid their complex and bulky MO recording system that includes a disk, an electromagnet, a servomotor, and an optical head assembly composed of a laser diode, an objective lens, a photodetector, and a beam splitter.
Silicon (Si) photonics is an up-and-coming platform for photonic integrated circuits (PICs) owing to its compatibility with mature complementary metal-oxide-semiconductor (CMOS) technology that offers high-volume and low-cost manufacturing [4]. A large number of optical components are integrated on a silicon-on-insulator (SOI) chip, including Si/III-V lasers [5], germanium photodetectors [6], and optical modulators [7]; therefore, Si photonics enables the miniaturization of bulky optical systems. In the field of PICs, MO materials have been employed to realize nonreciprocal optical devices, such as optical isolators and optical circulators [8–13]. To magnetize the MO material in such MO devices, a static magnetic field generated from a permanent external magnet or an integrated electromagnet is required while some MO materials can hold the magnetization without external magnetizing elements [14]. In addition, by combining an MO material and a stripe array of thin-film magnets with large remanent magnetization, a nonvolatile optical switch has been demonstrated [15,16]. The switch state is dynamically controlled by a current-induced magnetic field from an integrated electromagnet and maintained without any power supply owing to the nonvolatile magnetization of thin-film magnets. In addition to nonreciprocal devices and optical switches, magnetic materials are excellent candidates for realizing on-chip optical memory owing to the nonvolatile nature of magnetization. The integrated phase-change memory has already been demonstrated as a nonvolatile on-chip photonic memory [17,18]. Phase-change memories generally have less cycling endurance due to atomic migration and compositional changes during the amorphous–crystalline phase transition [19]. However, atoms do not move for data storage, and only the reversal of the angular momentum occurs in the magnetic memories. Therefore, magnetic memories have larger cycling endurance.
To realize solid-state MO memories in PICs, controlling the magnetic state of a medium with a lightwave propagating in the waveguide is required. However, magnetization of magnetic materials in the on-chip MO devices reported so far is directly flipped by an external magnetic field. This paper first shows light-induced thermomagnetic recording using light guided in a Si photonic waveguide. In our proposal, magnetization was optically flipped while a bias external magnetic field was required. A ferromagnetic material, CoFeB, was used as the magnetic recording material. To observe the magnetic properties using a magneto-optical Kerr effect (MOKE) microscope, we fabricated a Si waveguide with a broad width of 5 µm, on which the CoFeB thin-film was placed. With an increase in the coupled optical power to the waveguide, the coercive force of the integrated thin-film magnet decreases because the guided light interacts with the CoFeB film via its evanescent field, and the film is heated due to optical absorption. Therefore, magnetization reversal occurs along the applied magnetic field once light couples to the Si waveguide, and light-induced photomagnetic recording is experimentally demonstrated on the Si platform. Our proposed method enables nonvolatile MO memory on a Si photonics platform.
2. Device design and fabrication
2.1 Magnetic property and device structure
A soft magnetic material, CoFeB, possesses a weak coercive force and a high saturation magnetization and is widely used for applications such as magnetic recording media or sensors [20,21]. A 40-nm-thick CoFeB film was deposited on a Si/SiO2 substrate deposited with a 10-nm-thick Ru underlayer by an RF facing target sputtering method at room temperature. The deposition conditions were optimized so that CoFeB had a large in-plane anisotropy. Using Ru as an interlayer, the crystal orientation of CoFeB was configured to reduce the coercive force [22]. Figure 1 shows the magnetic properties of the CoFeB film measured using a superconducting quantum interference device (SQUID) at temperatures of 300, 350, and 400 K. The magnetization curves were measured along the direction of the film plane. An excellent squareness ratio was obtained at every temperature, and a decrease in the coercive force Hc of ∼17 Oe was observed when the temperature was increased by 100 K from 300 to 400 K.
Fig. 1. Magnetic property of CoFeB measured using SQUID. The inset shows the structure of the measured sample.
To test the thermomagnetic magnetic recording, a CoFeB thin film with a footprint of 5 µm ×15 µm was deposited on a Si waveguide. Schematics of both perspective and cross-sectional views of the thin-film magnet placed on the Si waveguide are illustrated in Figs. 2(a) and (b), respectively. The waveguide was fabricated on an SOI substrate with a 220-nm-thick Si layer and a 3-µm-thick buried oxide layer, and a 40-nm-thick and 15-µm-long CoFeB film was deposited on the top with a 150-nm-thick SiO2 interlayer. By forming a rectangular shape of the CoFeB film, its magnetization is easily aligned along the longer side owing to the magnetic shape anisotropy. On the CoFeB film, a 100-nm-thick SiO2 cladding layer was deposited to prevent degradation of the magnet, such as oxidation. The waveguide has a width of 5 µm, which is sufficiently broad to observe the magnetic domain of the CoFeB film using a MOKE microscope. To avoid the excitation of undesirable higher-order optical modes on the broader waveguide, tapered waveguides connect adiabatically with a 450-nm-width single-mode waveguide.
Fig. 2. Images of the thin-film magnet placed on Si waveguide for testing thermomagnetic recording. Schematics of (a) 3D prospect view, and (b) 2D cross-sectional view along the dot line in (a), and (c) microscopic image of the fabricate device.
3. Simulation of light propagation and heating
Multiphysics simulations combined with electromagnetic field and heat transfer analyses were performed to investigate light-induced heating in the thin-film magnet on the waveguide. First, both two-dimensional (2D) cross-sectional mode profiles and three-dimensional (3D) propagation of light in the thin-film magnet-loaded Si waveguide are calculated using a finite-element method (FEM)-based mode solver and a finite-difference time-domain (FDTD) method, respectively. The refractive index and extinction coefficient of the material used in these simulations are summarized in Table 1 of the Appendix B. As a polarization of the light source, the fundamental transverse magnetic (TM) mode at a wavelength of 1550 nm is used instead of the transverse electric (TE) mode. In our structure, where the CoFeB film is deposited on the top of the waveguide, the electric field of the TE mode is strongly confined in the Si waveguide and less field penetrates into the magnet than the TM mode. Thus, the optical power is absorbed more efficiently in the TM mode than in the TE mode. The simulated mode profile of the fundamental TM mode is shown in Fig. 3(a). The simulated results calculated the absorption loss due to evanescent coupling to the magnetic layer to be ∼1 dB/µm. This absorption loss is changed by varying the thickness of the SiO2 layer between the Si waveguide and the CoFeB film, and higher absorption occurs with a thinner SiO2 layer. Figure 3(b) shows a color map of the electric field distribution of the TM-mode light traveling from left to right along the z-axis in the Si waveguide. The guiding light evanescently couples to the CoFeB film, and the injected optical power is mainly absorbed by the film. Subsequently, the absorbed optical energy generates heat within the CoFeB film, increasing its temperature [23]. Second, to analyze the photothermal effect, a 3D heat transport simulation based on FEM was performed. In this finite-element heat transfer solver, the absorbed optical power distribution of Ru/CoFeB shown in Fig. 3(b) was imported as a heat source. The initial ambient temperature was set to 300 K. A 10-mW continuous-wave (CW) light was assumed to be input to the waveguide. The parameters used for the heat transfer simulations are listed in Table 1 of Appendix B. The color map of the steady-state temperature distribution on the surface of the CoFeB film is shown in Fig. 4(a). Since the optical power is attenuated along the propagation direction owing to the absorption of Ru/CoFeB, the heat generated within the magnetic film is not uniform. The temperature of the CoFeB layer becomes the highest at a position close to the center of the edge of the film. In addition, the highest temperature along the z-direction at x = 0 µm for various optical input powers is shown in Fig. 4(b). The temperature decreases along the light propagation direction, and the maximum increase in temperature is ∼180 K for a 10-mW CW light input, which sufficiently reduces the coercive force of the thin-film magnet. By reducing the Si waveguide's width and the CoFeB film's volume, a higher increase in temperature can be obtained with the same optical power. For example, the highest temperature rise of ∼1000 K is achieved for a 10-mW CW light input when a 450-nm wide Si waveguide with a CoFeB film with a footprint of 0.45 µm × 2 µm is used.
Fig. 3. (a) 2D cross-sectional profile for Ey of TM mode simulated by FEM, and (b) color map of the E field distribution of light guided in the waveguide simulated by FDTD method.
Fig. 4. Simulated temperature rise due to optical absorption of CoFeB. (a) Color map of the 2D temperature distribution of the surface of the CoFeB film for a 10-mW CW light input to the waveguide. (b) Increases in temperature along the z axis at x = 0 µm with different optical powers from 1, 2.5, 5, 7.5, 10 to 12.5 mW. The area surrounded by dot lines (−7.5 µm ≤ z ≤ 7.5 µm) is within the surface of CoFeB.
4. Experimental results
The absorption loss of CoFeB on the waveguide was characterized experimentally. A microscopic image of the fabricated thin-film magnet-loaded Si waveguide is shown in Fig. 2(c). The polarization of the amplified spontaneous emission light was adjusted to the TM mode. As shown in Fig. 5, the transmittance of the waveguide with the 15-µm-long CoFeB film is −15 dB lower than that of a reference waveguide with no magnetic film at a wavelength of 1550 nm; thus, the absorption loss of the CoFeB film is estimated to be ∼1 dB/µm, which agrees well with the simulated result in Section 2. Most of the coupled optical power is absorbed by the thin-film magnet and used for heating.
To investigate the thermal effect induced by optical absorption, we measured the optical power-dependent magnetic property of CoFeB on the waveguide using a MOKE microscope (NEOARK Corp. BH-762I-TIT) equipped with a 50× objective lens. The sample was illuminated with linearly polarized light. Then, the MOKE microscope collected the reflected polarized light from the device, and the magnetic domains of CoFeB in the in-plane direction were obtained by analyzing the difference in the brightness of the measured image, with the reference image based on the MO Kerr rotation. Schematic and microscopic images of the measurement setup for the light-induced thermomagnetic recording are shown in Fig. 6. The device under test was set on a stage with an electromagnetic coil, which applied a magnetic field along the longer side of the CoFeB film. The power of CW light from a tunable laser diode (TLD) at a wavelength of 1550 nm was amplified by an erbium-doped fiber amplifier (EDFA) and varied from 1 to 12.5 mW. The TM-polarized light was launched from a polarization-maintaining lens-tipped fiber into the device through a cleaved facet of the waveguide. The position of the lens-tipped fiber at the input side was adjusted by maximizing the brightness of the near-field pattern (NFP) of the optical output from the cleaved waveguide facet. The NFP was observed using an objective lens and an infrared camera, as shown in the inset of Fig. 6(a). Taking the images of the magnetic domains for each applied magnetic field using the MOKE microscope, the relative magnetization was observed as the difference in the black-and-white contrast. The magnetic properties for different optical powers were measured as shown in Fig. 7(a). The maximum magnetic field applied to the device was 1 kOe, which was sufficient to saturate the magnetization of CoFeB in the film plane. The measured magnetization was normalized to the remanent magnetization. The powers shown in Fig. 7(a) are the optical powers coupled to the input waveguide, and the black line corresponds to the case without optical input. The plots in Fig. 7(a) are measured in sequence along the short-dashed arrow applying round-trip magnetic field. Clear hysteresis loops were observed for each result, and the coercive force changed depending on the optical power. Figure 7(b) shows the coercive force extracted from the measured hysteresis loops. It is clearly shown that the coercive force decreases as the input optical power increases. This is consistent with the results discussed in Section 2 that the temperature of the CoFeB film is increased owing to the optical absorption, and the coercive force of CoFeB decreases at higher temperatures. Figure 7(c) shows the magnified graph of the left half of Fig. 7(a) for optical powers of zero and 12.5 mW together with the observed magnetic domains at several measured points. The black-and-white contrast corresponded to each direction of the magnetic domain. When there is no input light, the brightness of the magnetic domain becomes dark because of magnetization reversal at a magnetic field below −100 Oe. Conversely, when a 12.5-mW lightwave is an input to the waveguide, the brightness of the magnetic domain starts to darken on the lower side of the CoFeB film at a magnetic field of ∼ −70 Oe. This means that the temperature of the magnetic film is higher at the lower side, which is close to the optical input, as shown in Fig. 4(b). Although the temperature distribution of the thin-film magnet is not uniform, its magnetization is entirely reversed by applying a magnetic field below ∼ −85 Oe.
Fig. 6. Measurement setup of light-induced thermomagnetic recording. (a) Schematic image. Insets are the microscopic image of the device with the CoFeB film and the photographic image of the near field pattern (NFP) of the optical output from the cleaved facet observed using the magneto-optical Kerr effect (MOKE) microscope and the IR camera, respectively. (b) Photographic image.
Fig. 7. Experimental results of optical power dependent magnetic properties of the CoFeB film on the Si waveguide measured using the MOKE microscope. (a) Hysteresis loops, and (b) coercive force Hc for different optical powers. (c) Magnified hysteresis loops of (a) for optical powers of zero and 12.5 mW. Insets show the magnetic domains of the CoFeB film at each measured point.
We then experimentally demonstrated the thermomagnetic recording in the thin-film magnet on the Si waveguide triggered by light. Figure 8(a) shows the magnetic property of the CoFeB film magnified from Fig. 7(a) for input optical powers of zero and 12.5 mW. When the coercive force is reduced by light-induced heating, magnetization reversal occurs under a magnetic field within the magenta-colored region in Fig. 8(a). Therefore, in the following study, after magnetization was reset downward by applying a magnetic field of −1 kOe, a constant magnetic field of ∼80 Oe was applied upward to prepare the recording. The photothermal magnetic changes were confirmed from the domain patterns before and after the optical input. Dummy patterns of the CoFeB magnet were also formed around the magnet on the waveguide so that the magnetic changes of both magnets could be compared. In the initial state before coupling light to the waveguide, the brightness of the magnetic domains of both the CoFeB film on the waveguide and the dummy CoFeB were dark, as shown in Fig. 8(b). Subsequently, CW light with a 12.5-mW optical power was launched into the waveguide. The brightness of the magnetic region on the waveguide turned bright, as shown in Fig. 8(c), while that of the dummy CoFeB films remained dark. This indicates that the magnetization direction of the CoFeB film only on the waveguide is flipped upward from downward by CW light input. Consequently, light-induced heating and subsequent thermomagnetic recording were experimentally demonstrated. The highest temperature of the CoFeB film obtained from the simulation in Fig. 4 is ∼530 K, which sufficiently reduces the coercive force and reverses the magnetization direction under an external magnetic field of ∼80 Oe.
Fig. 8. Experimental demonstration of the light-induced thermomagnetic recording. (a) Principle of the light-induced thermomagnetic recording, and magnified hysteresis loops of Fig. 7(a) for optical powers of zero and 12.5 mW. (b) and (c) show magnetic domains of CoFeB on the Si waveguide before and after coupling a 12.5-mW CW light to the waveguide, respectively. (d) and (e) show magnetic domains of CoFeB placed on the Si waveguide before and after coupling a light pulse to the waveguide, respectively. (f) Simulated time-domain temperature change in CoFeB for light pulse with a peak power of 50 mW and a duration of 50 ns.
Finally, a rectangular pulse light was input instead of CW light for thermomagnetic recording. An optical modulator modulated the optical output from the TLD to generate rectangular pulses with a peak power of 50 mW for a duration of 50 ns at a repetition period of 20 µs. Each optical pulse supplies the energy of 2.5 nJ. Since the repetition period is sufficiently long to relax the light-induced heat generated in the device, the thermal effect generated by each optical pulse could be characterized. Figures 8(d) and (e) show the magnetic domains before and after coupling the light pulse, respectively. Similar to the result of the magnetic domain change in Figs. 8(b) and (c) with CW light input, the brightness of the CoFeB film only on the waveguide was turned bright. Therefore, the magnetization direction of the CoFeB film on the waveguide was reversed by an optical pulse. We simulated the temperature evolution of CoFeB using an optical pulse. In the transient heat transfer simulation, the absorbed optical power calculated in Section 2 B is modulated using a shutter function to mimic an optical pulse and used as a heat source. Figure 8(f) shows the temporal change in the highest temperature of the CoFeB film. We observe that the temperature of the CoFeB magnet on the Si waveguide rises quickly owing to optical absorption-induced heating and reaches ∼570 K at the end of the optical pulse. We expect that a faster magnetization reversal less than a few ns is obtained using a shorter optical pulse with higher peak power. However, photothermal magnetization reversal of less than a few nanoseconds could not be observed. We deduce that the coupled optical power was attenuated by the nonlinear absorption of Si, such as two-photon absorption and subsequent free-carrier absorption. This might be solved using photonic waveguides with lower nonlinear absorptive materials, such as silicon nitride [24].
In future work, to realize on-chip MO memories based on our proposed scheme, the stored magnetic data should be read out using a lightwave guided in the waveguide. In the MO disk, the stored information is read out with the polar magneto-optical Kerr effect. When linearly polarized light is typically incident on a magnetic medium, its polarization plane is rotated slightly upon reflection. This rotation angle depends on the magnetization direction in the medium, and the reflected light passes through the detector. Such a scheme using free-space optics does not apply to PICs. Instead, the phase shift based on the transverse MO effect can be employed in PICs. By flipping the magnetization direction of the MO material, the phase of light guided in the waveguide is changed owing to the MO effect. The phase change induces light intensity change by combining additional optical components such as a microring resonator or a Mach–Zehnder interferometer [25,26]. Figure 9(a) shows the proposed structure in the microring configuration, consisting of a Si microring resonator and an MO material (e.g., Ce;YIG) for the storage layer. A stripe array of a CoFeB film for the storage layer is placed on an additional waveguide located in the lower side of the mirroring resonator. This adjacent waveguide is designed so as not to couple with the microring resonator and used only for writing. The data were stored in the magnetization direction of the CoFeB film using the thermomagnetic recording scheme described in this article. By modulating the peak power of the writing pulse, the number of CoFeB stripes whose magnetization is flipped can be controlled. This means that intensity-modulated multi-bit data can be stored in a single microring resonator. The magnetization of the MO material located in the microring was set to a designated state by the magnetic field generated by the CoFeB stripes. The performance example of the on-chip MO memory is shown in Fig. 9(c). The resonant wavelength of the microring resonator is changed by the phase change given by the MO material. Thus, the stored information is read out as the intensity of the optical pulse with an appropriate wavelength propagating through the busline coupled to the microring resonator.
Fig. 9. Schematics of the proposed structure of the on-chip MO memory in the microring configuration. (a) Top view and (b) cross-sectional view. (c) Performance example of the on-chip MO memory. Blue and red lines are examples of wavelength spectra for bits “1” and “0”, respectively.
5. Conclusion
For the first time, we demonstrated a thin-film magnet's light-induced thermomagnetic recording, which was integrated on a Si photonic platform. The proposed scheme is based on heating due to the optical absorption of the evanescent light field coupled to the thin-film magnet on the Si waveguide. We experimentally showed that the coercive force of the CoFeB film with a footprint of 5 µm × 15 µm placed on a 5-µm-wide silicon waveguide was controlled by a light guided in the waveguide. When CW light with a 12.5-mW optical power was launched into the waveguide, the magnetization direction was flipped by applying an appropriate magnetic field. Furthermore, thermomagnetic recording using a light pulse with 50 ns duration and 2.5 nJ energy was achieved. The required energy can be reduced to approximately ∼10 pJ using a thin-film magnet with a smaller volume, shorter light pulse less than ∼1 ns. In this study, the footprint of the integrated magnet was relatively large because the magnetic properties were characterized using a MOKE microscope with limited resolution. The footprint can be reduced using a narrower waveguide (∼450 nm). Further reduction is possible using alternative structures such as plasmonic nano-waveguides, which strongly confines light in a nanoscale region beyond the diffraction limit [29,30]. In addition, magnetic materials with perpendicular magnetic anisotropy can be explored to reduce the size of the magnet. Our work proves that compact solid-state MO recording systems can be realized on a Si photonic platform by utilizing the proposed method. We believe that the proposed on-chip MO memory is the promising candidate for on-chip optical memories with nonvolatility and large cycling endurance.
Appendix A: fabrication process
The waveguide device in this work was fabricated using an SOI substrate with a 220-nm-thick Si layer and a 3-µm-buried oxide layer. First, a 200-nm-thick SiO2 layer was deposited as a hard mask to protect the Si layer, and a 300-nm-thick positive resist (ZEP-520A) was coated onto the SOI substrate. Subsequently, the waveguide patterns were exposed to the resist using an electron beam lithography (EBL) system. The waveguide patterns were transferred to a SiO2 hard mask via reactive ion etching using CF4, and Si waveguides were subsequently formed using SF6. A 150-nm-thick SiO2 layer was deposited on the waveguide core using plasma-enhanced chemical vapor deposition (PE-CVD). EBL was performed to transfer the magnet patterns. A 10-nm-thick Ru buffer layer followed by a 40-nm-thick CoFeB thin-film magnet was deposited using an RF facing target sputtering method at room temperature with Ar. Subsequently, patterned thin-film magnets were formed using a lift-off process. Finally, a 100-nm-thick SiO2 protection layer was deposited on top of the thin-film magnet using PE-CVD.
Appendix B: material parameters
The material parameters used for the simulations of light propagation and heat conduction in this work are summarized in Table 1 [27,28].
Table 1. Material Parameters for Simulations of Light Propagation and Heat Conduction
Funding
New Energy and Industrial Technology Development Organization (JPNP13004, JPNP16007); Core Research for Evolutional Science and Technology (JPMJCR15N6, JPMJCR18T4); Japan Society for the Promotion of Science (19H02190, 20J11225).
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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