NSRRC Activity Report 2022

Physics and Materials Science 009 Fig. 1 : Non-contact optical writing of the designed AFM pattern. (a) Schematic of the designed pattern composed of two distinct AFM configurations written by laser illumination. (b) XAS spectra mapping taken by measuring I A /I B , where I A (I B ) is the intensity of the spectra taken at the energy A (B) for E//b as marked in (c) and (d). XAS polarization-dependent spectra for E//a and E//b taken at (c) Point 1 inside the word “BFO” and at (d) Point 2 outside the word “BFO”. [Reproduced from Ref. 1] Photonic-Crafting of Non-Volatile and Rewritable Antiferromagnetic Spin Textures Their remarkable properties make antiferromagnets a promising candidate in the development of next- generation spintronics devices. S pintronics has revolutionized the field of magnetic recording/reading and it is hoped that it will complement semiconductor-based microelectronics beyond Moore’s law for information technologies. Whether designed for sensing, memory, or logical applications, the most studied spintronics are based on ferromagnets. Spintronics based on antiferromagnets (AFM) is a recently emerging field in non-volatile data storage and information processing. The zero net magnetization and zero stray field of antiferromagnetic materials eliminate the interference between neighboring units, leading to high-density memory integrations. The dynamic spin behavior in AFM compared to ferromagnets can be more responsive to stimuli, leading to a shorter switching interval between two antiferromagnetic states. These remarkable properties make AFM a promising candidate in the development of next-generation spintronics devices. Further extending the concept of AFM spintronics, multiferroic systems that possess correlated ferroic orders might offer alternative solutions to renovate AFM-based spintronic devices. The coexisting order parameters and inherent couplings in multiferroics provide a playground, allowing one to manipulate their multi-functionalities with different external stimuli. Of the numerous multiferroic systems, only a few exhibit ordered spin textures at room temperature and the most promising and well-studied of these is BiFeO 3 (BFO). Bulk BFO exhibits ferroelectric polarization (P) and AFM ordering with a characteristic temperature of 640 K, far above room temperature. The correlation between the P and AFM orders endows BFO with an innate capability to achieve electrical and magnetic control of its AFM axes. To address the puzzle of the correlation between the P and AFM orders, Chang-Yang Kuo (National Yang Ming Chiao Tung University), Jan-Chi Yang (National Cheng Kung University) and their teams discovered two bistable and reversibly controllable AFM states in strained BFO film grown on an NdFaO 3 substrate. The capability of manipulating the AFM axis is the key to realizing practical AFM spintronics. They have developed an approach combining both optical and magnetic methods to write and read the reversible AFM states at desired areas on the BFO film. The concept is schematically illustrated in Fig. 1(a) . They first applied a magnetic field of 6 T along the a axis of the BFO film by using high magnetic field facility at TLS 11A , aligning the entire AFM spin along a “line” domain. An appropriate power (360 mW) for the laser beam was then chosen to thermally write the illuminated area of the BFO film over its Néel temperature. The area illuminated by the laser is able to set AFM spin to the type of cross domain. Figure 1(b) shows a sample mapping image taken by measuring the intensity ratio of the X-ray magnetic linear dichroism (XMLD). The spot size of the synchrotron beam at TPS 45A used for mapping the image is approximately 2 µm × 3 µm. The brown region