Session G41: Magnetic Characterization and Imaging

Optical characterization of in-plain spin texture in 10-nm-scale Neel-type magnetic domain wall

We present that dark-field longitudinal magneto-optical Kerr effect (MOKE) microscopy can visualize the in-plain spin texture of the subwavelength-scale Neel-type magnetic domain wall. Conventional MOKE microscopy has never resolved the spin texture at the nanoscale due to not only the optical diffraction limit but also extremely small magneto-optic (MO) susceptibility. The dark-field illumination isolates the MOKE signal of the domain wall from those of the magnetic domains effectively with a high signal-to-background ratio. Also, we employed a perfect optical absorption multilayer to vanish the non-MO signals dramatically and boost the measurement efficiency. We examined the Neel-type magnetic domain wall of the Pt/Co/Pt/Ta film and measured its MO scattering width to be ~9.14 pm, experimentally. The intensity of the longitudinal MOKE signal from the domain wall depends on the orientation of in-plane magnetization relative to the azimuthal angle of incidence. Angle-dependent, precise measurement of the longitudinal MOKE scattering enables us to identify spatial magnetization distribution along the 10-nm-scale magnetic domain wall. We expect our work will offer a new possibility in research on complex in-plane spin textures such as vertical Bloch lines and skyrmions.   

Microsphere-assisted super-resolution magneto-optic imaging

The diffraction limit [1] is a fundamental barrier in optical imaging. It can be surpassed by the photonic nanojet phenomenon, generated by a lens-like dielectric micro-object [2]. Microspheres have been demonstrated as a powerful platform to challenge the diffraction limit by creating a virtual image with enhanced resolution and magnification [3,4]. Here we report on wide-field MOKE (Magneto-Optical Kerr effect) measurements harnessing this resolution enhancement. In-plane and perpendicular magnetized domains, probed with the longitudinal and polar Kerr effects, respectively, are imaged at increase spatial resolution. Our findings may serve as a way to combine the advantages of fast time resolution and versatile sample environment of optical measurements with smaller length scales.
[1] Lord Rayleigh, Philosophical Magazine Series 5, 8(49):261– 274 (1879)
[2] Z. Chen, A. Taflove & V. Backman Optics Express, 12(7), 1214 (2004)
[3] Z. Wang, W. Guo, L. Li et al., Nature Communications, 2(1), 218 (2008)
[4] G. Huszka & M. A. M. Gijs, (2018) Scientific Reports, 8(1), 601 (2018)   

Development of a Multichannel Atomic Magnetometer for Parallel Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) in the ultra-low field (ULF) regime is a promising method for anatomical imaging with multiple advantages over conventional several-Tesla MRI. The leading high-sensitivity multichannel ULF MRI was realized only with multichannel superconducting quantum interference devices (SQUIDs); however, the need for cryogenic infrastructure is a serious drawback. We aim to develop an alternative, more practical ULF MRI based on a high-sensitivity multichannel atomic magnetometers (AMs) and multiple flux transformers (FTs). AMs are currently the most sensitive non-cryogenic magnetic-field sensors. Our approach represents the first ULF MRI implementation using the novel technique, which has many innovative features such as acceleration of MRI, a large array of sensors based on a single large atomic vapor cell, reducing the cost of sensors ~10 times, and flexible design based on FTs. In this talk, we will describe the basic principle and the design of our ULF MRI system.   

Ionically-controlled phase separation in cobaltite heterostructures

Complex oxide heterostructures provide access to emergent functional and structural phases which are not present in the bulk constituent materials. Controlling ionic distribution and stoichiometry in complex oxide heterostructures has been utilized to significantly alter and tune the electronic, magnetic, and structural properties. Recently, deposition of a strong oxygen getter on top of an oxide thin film has emerged as a novel way to tailor oxygen stoichiometry and nanoscale functional properties. However, the role played by nanoscale morphology, i.e. phase separation and defects in these oxide thin films and heterostructures remains largely unknown. In this talk, I will focus on Gd/La_0.67Sr_0.33CoO₃ (LSCO) heterostructures due to the high oxygen ion conductivity, as well as the coupled magnetic and electronic properties of LSCO, which are strongly dependent on the oxygen stoichiometry. This combination of properties enable the ionic control of the functional properties of LSCO thin films through the presence of oxygen getter layers such as Gd. We utilize x-ray nanodiffraction to directly image the nanoscale morphology of LSCO thin films as they are progressively transformed from the equilibrium perovskite phase to the metastable brownmillerite (BM) phase with increasing Gd thickness. Our studies show the coexistence of perovskite and BM phases with a critical oxygen vacancy concentration threshold which leads to the formation of extended BM filaments. Strain maps reveal that the perovskite phase changes from compressive to tensile strain on opposite sides of a BM filament. Our studies provide an unprecedented nanoscale survey of the phase separation in the cobaltites and shed light on the formation of the metastable BM phase.   

"Magnetic Etch-a-Sketch" using the 1st-order phase transition in FeRh

We demonstrate a novel approach for room-temperature rewritable magnetic patterning using the 1^st-order phase transition from antiferromagnet (AF) to ferromagnet (FM) in FeRh. We employ epitaxial Fe_0.52Rh_0.48 films designed such that both phases are metastable at room temperature. Starting with the film in a uniform AF state, we write arbitrary patterns of FM phase using a focused pulsed laser with ~650 nm resolution. We image the FM patterns with anomalous Nernst microscopy and show that they are stable under magnetic field – at least up to 3 kOe – as well as elevated temperature up to ~315 K. The FM patterns can be written using a single picosecond laser pulse per pixel and can be fully erased by cooling the film below room temperature.

Ref: A. B. Mei et al, arXiv:1906.07239 (2019)   

Novel sample preparation and detection method for magnetic resonance force imaging of electron spins

Magnetic resonance force microscopy (MRFM) is a technique that opens up the possibility of extremely sensitive detection and imaging of electron spins, with sensitivity capable of detecting a single radical. In our experiment, we detect a change in magnetization of nitroxide radical probes as a shift in the frequency of a magnet-tipped cantilever. We discuss recent developments that have allowed us to overcome difficulties with this measurement. One difficulty is frequency noise from sample dielectric fluctuations interacting with charges on the cantilever tip. We have found that we can reduce this noise by applying a 10 nm metal coating over the sample. However, deposition of this metallic coating produces a “dead layer” in which nitroxide labels are no longer active. We show a new sample preparation technique which applies this coating without exposing sample radicals during metal deposition. Another difficulty is that the motion of the cantilever magnet causes a change in the resonance frequency of spins during the microwave pulses, which leads to image blurring and reduction in sensitivity. We show ways to “pause” cantilever motion during microwave application to prevent blurring and improve sensitivity.   

NV- center magnetometry using Bayesian statistics

In magnetometry using NV^- centers, we demonstrate order-of-magnitude speed up with Bayesian experimental design as compared to conventional frequency-swept measurements.
The NV^- center is a quantum defect with spin 1 and coherence time up to several milliseconds at room temperature. Zeeman splitting of NV^- energy levels allows detection of the magnetic field via photoluminescence. It is an excellent platform for magnetometry with potential spatial resolution down to few nanometers and demonstrated sensitivity down to nT/Hz^1/2.
We compare conventional magnetic field measurement of fluorescence under pre-determined sweeps of microwave frequency with the measurement using a Bayesian statistics methodology. In the Bayesian experimental design, the frequency of each measurement is determined in real-time from utility predictions based on the accumulated experimental data. We report order of magnitude increase in the precision of magnetic field and hyperfine splitting determination for the same measurement time in the Bayesian scheme, compared with the conventional scheme.   

Imaging nanoscale magnetization using scanning-probe magneto-thermal microscopy*

High resolution, time-resolved magnetic microscopy is crucial for understanding novel magnetic phenomenon such as skyrmions, spinwaves, and domain walls. Currently, achieving 10-100 nanometer spatial resolution with 10-100 picosecond temporal resolution is beyond the reach of table-top techniques. To break free of the far-field diffraction limit, we have developed a near-field magnetic microscope based on magneto-thermo interactions: the time-resolved anomalous Nernst effect and the time-resolved longitudinal spin Seebeck effect. Our technique involves scanning a sharp gold tip within a near-field optical excitation. The resulting tip-sample interaction creates a nanoscale thermal gradient for magneto-thermal microscopy, and its extension to imaging an applied current density. We study the characteristics of near-field thermal excitation with a picosecond laser and demonstrate magnetic imaging of a multi-domain state. We present sub-100 nm spatial resolution from imaging current density around a nano-constriction. Our results suggest a new approach to nanoscale spatiotemporal magnetic microscopy in an accessible, table-top form to aid in the development of high-speed magnetic devices.   

Multi-axis cavity optomechanical torque characterization of magnetic microstructures and the contribution of the Einstein-de Haas effect

Significant new functionality is reported for torsion mechanical tools aimed at full magnetic characterizations of both spin statics and dynamics in micro- and nanostructures. Specifically, multiple torque directions utilizing higher order mechanical modes are monitored, as is essential for study of anisotropic three-dimensional structures. The approach is demonstrated through application to shape and microstructural disorder-induced magnetic anisotropies in lithographically patterned permalloy, and will have utility for the determination of important magnetic thin-film and multilayer properties including interface anisotropy and exchange bias.

With the extension of torque magnetometry measurements into radio frequencies, the contribution of the Einstein-de Haas (EdH) effect can become comparable to the conventional magnetic cross-product signal. Through sensitive optomechanical detection of higher order mechanical modes, the torques owing to the EdH effect are elucidated, and offer a new method for exploring high frequency magnetic susceptibility in anisotropic structures.   

Direct Imaging of Nanostructured Magnetic Metalattices in 3D

Nano-engineered magnetic materials are of great interest for tuning magnetic interactions and because of their potential applications for next-generation spintronic devices. Nanoscale 3D magnetization textures such as inverse nickel metallatices can be fabricated by infiltrating close-packed silica nanospheres with nickel. Such structures can support many distinct magnetic phases and topologies, leading to a new class of nanostructured magnetic metamaterials. Despite recent success in theoretical predicting and fabricating these materials, 3D characterization of their structure and properties remains very challenging due to the lack of suitable microscopy tools. We developed a new 3D nanoscale magnetic imaging capability that combines x-ray magnetic circular dichroism, ptychographic imaging, and vector tomography. Using this new magnetic vector ptycho-tomography method on a synchrotron beamline, we imaged a nickel magnetic metallatice sample to unveil the underluing structure and spin texture.   

Scaling of domain cascades in stripe and skyrmion phases

The origin of deterministic macroscopic properties often lies in microscopic stochastic motion. Magnetic fluctuations that manifest as domain avalanches and chaotic magnetization jumps exemplify such stochastic motion and have been studied in great detail. Here we report Fourier space studies of avalanches in a system exhibiting competing magnetic stripe and skyrmion phase using a soft X-ray speckle metrology technique. We demonstrate the existence of phase boundaries and underlying critical points in the stripe and skyrmion phases. We found that distinct scaling and universality classes are associated with these domain topologies. The magnitude and frequency of abrupt magnetic domain jumps observed in the stripe phase are dramatically reduced in the skyrmion phase. Our results provide an incisive way to probe and understand phase stability in systems exhibiting complex spin topologies.   

Ultra-Short-Period Undulators for the Next Generation X-ray Free Electron Lasers

There is a growing interest on the compact XFELs to be used in applications such as medical science (surgery, fat removal), material science and military. In order to build a compact XFEL, constructing an undulator with sub-millimeter period (λ_u) is mandatory. Many problems arise due to miniaturization of undulators; such as keeping the magnetic field high while still having a considerable gap to minimize beam scraping. In this work, RADIA program has been employed for the modeling of sub millimeter period undulators (20 µm < λ_u < 400 µm) with three different magnet configuration; namely Up-Down (↑↓↑↓…), Halbach (Wiggler) (↑→↓←↑…) and Hybrid (→↑←↓→…). Effects of sub-millimeter undulator period and gap on the magnetic field pattern of the undulator have been discussed.