Session Z23: Domains Walls, Spin Textures and Artificially Structured Materials

Quantum Barkhausen Noise Induced by Domain Wall Co-Tunneling

Most macroscopic magnetic phenomena are typically understood classically. Here, we examine the dynamics of a uniaxial rare-earth ferromagnet deep within the quantum regime, so that domain wall motion, and the associated hysteresis, is dominated by large-scale quantum tunneling of spins, rather than classical thermal activation over a potential barrier. The domain wall motion is found to exhibit avalanche dynamics, observable as an unusual form of Barkhausen noise . We observe non-critical behavior in the avalanche dynamics that only can be explained by going beyond traditional renormalization group methods or classical domain wall models. We find that this quantum Barkhausen noise exhibits two distinct mechanisms for domain wall movement, each of which is quantum-mechanical, but with very different dependences on an external magnetic field applied transverse to the spin (Ising) axis. These observations can be understood in terms of the correlated motion of pairs of domain walls, nucleated by co-tunneling of plaquettes, with plaquette pairs correlated by dipolar interactions; this correlation is suppressed by the transverse field. Similar macroscopic correlations may be expected to appear in the hysteresis of other systems with long-range interactions   

Multipolar order in antiferromagnetic domain walls: application to MnPS3 moiré superlattices

We study multipolar order in domain walls in easy-plane antiferromagnets in various geometries. We show that while the dipolar order vanishes for simple geometries, it stays finite for angled domain walls. Among the multipoles that we consider, we find that the octupolar order is largest regardless of the geometry of the domain wall. Next we perform atomistic simulations on twisted bilayer MnPS3 and show that the noncolinear order at small twist angles support both dipolar and octupolar order.   

Domain Wall Depinning in the Presence of Multiple Paths and Multiple Pinning Sites

The stochastic behavior of magnetic domain walls (DW) may be an impediment to their use in racetrack memories. Here, we investigate the path of DWs in Co/Pt multilayers with perpendicular anisotropy, after depinning from photolithographically patterned notches that act as strong pinning sites. We focus on the effects of strain in the form of surface acoustic waves with a frequency of 244.75 MHz. Measurements were performed using a scanning MOKE stage focused on the notches. This allowed for measurements of the domain wall transit time after depinning over a range of strain values. The corresponding probabilities indicate that the DWs could traverse a variety of paths, each of which consisted of multiple pinning sites. Strain altered the pinning landscape, with increasing strain resulting in shorter characteristic depinning time for all paths. Hence, paths with short depinning times, showed little to no discernible pinning as strain increased, whereas paths with very long characteristic times (and thus, small probabilities) instead became more visible at higher strain.   

Topological quasiparticle dynamics in constricted nanomagnets

The dynamic properties in nanostructured magnets are conventionally described by domain wall motion. However, when the geometrical size is constricted to the limiting domain wall length scale, the competing energetics between anisotropy, exchange and dipolar interactions can cause emergent new kinetic events that can be viewed as quasi-particles. Using neutron scattering and simulations, we study ferro- and antiferromagnetic dynamics on a nanoscale honeycomb structure. In both types of materials topological defects in the spin texture are found to behave as dynamical quasi-one-dimensional particles, freely moving along honeycomb joints at picosecond timescales. The dynamic phenomena, arising due to chiral vortex-type topological quasi-particle kinetics, take place in the absence of any external stimuli and persist to much lower temperature than the boundary crossing energy for magnetization reversal, and persist well above the Néel temperature of the antiferromagnetic ordering. This quasi-particle behavior appears to be universal in constricted nanomagnets, in contrast with the conventional understanding of domain wall transport as the only possible mode of dynamics at this scale.   

Imaging Spin Textures in Momentum Space with QSPLEEM

The ability to measure and control the spin degree of freedom in materials has both uncovered new phases of matter and incited the information age. Here, we discuss the newly developed quantum spin-polarized low-energy electron microscope, (QSPLEEM) available at the Molecular Foundry. Using a technique dubbed Spin- and Angle- Resolved Reflection Electron Spectrosopy (SP-ARRES), we present measurements of spin textures in the unoccupied electronic structure of magnetic materials, and compare with first principles calculations.   

Time-resolved secondary electron microscope with polarization analysis

Magnetization dynamics has drawn a lot of attention in spintronics due to its application in spintonic device. Motion of spin textures or magnetization switching needs a dynamic profile to study the mechanism to improve its characteristics. One of the most difficult parts in magnetization dynamics study is maintaing high spacial resolution with high temporal resolution. We have used exchange-scattering type spin detector in our microscpe and the signal to noise ratio can be reached by at least one order of magnitude larger compared to conventional Mott spin detector or SPLEED type detector. Our set-up so far, shows a temporal resolution of 40 ns and spatial resolution of 10 nm . The main reason is we are using channeltron now, and this time resolution can be reached down to 1 ns if we replace the detector with channel plate. We have shown the motion of magnetic skyrmion (size less than 100 nm) by current pulses.   

A quantitative comparison between iterative and non-iterative phase retrieval for non-collinear spin textures visualized with Lorentz microscopy

Phase contrast imaging with electron holography takes advantage of the wave properties of coherent electrons and their interaction with electromagnetic fields, e.g., matter. In comparison to conventional microscopy, it provides an enhanced sensitivity and resolution and enables the visualization of spin degrees of freedom in the form of the in-plane magnetic induction. These experiments are based on the detection of the electron phase using off-axis or in-line holography. The former requires a biprism, the latter works in Fresnel mode/Lorentz mode with post processing involving a phase retrieval algorithm, such as transport-of-intensity or Gerchberg-Saxton. In this talk, I will discuss the strength of both iterative and non-iterative algorithms through application to modeled and real materials systems hosting non-collinear spin textures. One focus will be on chiral spin textures in amorphous materials where exit wave reconstruction allows to disentangle electrostatic, magnetization, and magnetic field contributions to the phase shift.   

Navigating Magnetic Chiral States with Autoencoder

In this study, we employed Autoencoder, a type of neural network, to interpolate and extrapolate between two distinct magnetic structures: the labyrinth and the skyrmion. The Autoencoder, acting as a computational observer, learned from instances of these magnetic structures, converting each instance into a compact representation known as a latent code. Analyzing this latent code space enabled the generation of novel magnetic structures responsive to external magnetic fields not included in the training dataset. We introduced two algorithms for modifying the latent codes. The first algorithm involved inversion and translation operations within the latent space, preserving the inherent chiral characteristics of the original data. The second algorithm, employing recursive flow with a modification bias, induced topological changes, leading to a diverse array of statistically plausible.

 

 Our research aims to delve into the intersection of artificial intelligence and condensed matter physics, highlighting the potential applications of artificial intelligence technologies in advancing physics research.   

Analysis of the magnetization dynamics near the Curie temperature in CoPt multilayer granular media: case study for heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) has been shown to satisfy an increased data storage need of hard disk drives by enabling the writing of high-anisotropy materials such as FePt. Modelling the magnetization dynamics near the Curie temperature (T_c) is crucial when predicting the limits of HAMR or any technology that relies on fast, high temperature magnetization control. However, there are some differences in the behavior of magnetic grains under conditions important to HAMR, such as cooling rate and magnetic field angle. Such differences arise from how thermal fluctuations are modeled in the Landau-Lifshitz-Bloch equation. Therefore, it is important to benchmark these results against experiments to validate theoretical models. Here, we model the switching probability of granular CoPt media with VAMPIRE’s atomistic simulator as preparation for experiments. Unlike FePt, CoPt thin-films serve as a good benchmark due to their tunable composition, allowing us to more freely change parameters like saturation magnetization, anisotropy and T_c.   

Characterization of magnetic nanoparticle ensembles via Thermal Noise Magnetometry

Accurate characterization of magnetic nanoparticles (MNPs) is essential because of their biomedical applications, such as magnetic particle hyperthermia, magnetic particle imaging, and drug targeting. Most magnetic methods characterize MNPs by measuring their response to an externally applied excitation field, which can affect the state of the particles in the ensemble, thereby complicating the characterization of the individual particle properties.

We recently proposed a passive method called Thermal Noise Magnetometry (TNM) which captures the dynamics of the particles in thermal equilibrium without external excitation. This purely observative measurement reduces the impact of external influences on the measurement results to a minimum, thereby giving further insights into the system's fundamental magnetization dynamics. 

In this contribution, we discuss our recent advances that turned this emerging technique into a more practical and accessible magnetic characterization method. The scaling behavior of the stochastic TNM signal with sample properties [1] and the impact of temperature on the TNM signal [2] are discussed and verified with experiments and simulations. Until now, all TNM experiments have been performed with SQUID sensors, requiring cryogenic cooling and limiting the geometrical freedom of the experiment. We propose an alternative TNM setup based on Optically Pumped Magnetometers that is ideally suited to monitor clustering and aggregation processes in biological media. The performance of the tabletop setup is compared with an established SQUID-based TNM setup [3].   

Characterization of synthesized iron-based core-shell nanoarchitectures in carbon matrix annealed by oxygen and nitrogen.

The carbon microspheres filled with clusters of few layer graphene nanostructures were produced by solid phase pyrolysis using as precursors a metal free porphyrin and phthalocyanine. The morphology, structure and size of multilayered graphene nanostructures were investigated using XRD, XPS, Raman, SEM and HR TEM microscopy images, magnetometry and EPR measurements. In recent years we have synthesized and realized structural and magnetic investigation of Fe-Fe₃O₄ and Fe-Fe₃C nanoalloys having “core-shell” architectures. Oxygen and nitrogen annealed phthalocyanine and porphyrin (graphene) influence magnetic properties as compared with those of pyrolysis of iron-based graphene. We conducted investigations of structural and magnetic properties of materials annealed by oxygen and nitrogen at different temperatures using X-ray diffraction (XRD), measurements of XPS spectra, high resolution SEM/STEM images, magnetometry PPMS measurements. The measured magnetization of magnetic saturation and coercivity as well as the specific absorption rate (SAR) show that these materials are attractive for magnetic hyperthermia medical applications. Under external electromagnetic fields the magnetic heating of these materials shows unique properties, and they may consider promising materials for magnetic hyperthermia of cancer cells. Hysteresis loop of the (Fe-Fe₃C)@C and (Fe-Fe₃O₄)@C nanocomposites are of special interest because of high Mr/Ms ratio.   

Imaging of magnetization response of soft magnetic thin film using diamond quantum sensors with wide frequency range

 In power electronics, a core loss of inductors in the high-frequency range is a bottleneck for the miniaturization of the system. To develop soft magnetic materials with low loss at high frequency, AC magnetic imaging is effective. Here, we developed a diamond quantum imaging with a wide frequency range.

 Using nitrogen-vacancy centers, we performed simultaneous imaging of the amplitude and phase of the AC stray field from a CoFeB-SiO₂ thin film, developed for high-frequency inductors. This film has low conductivity derived from the structure of nanomagnetic columns dispersed in an insulator matrix and in-plane uniaxial anisotropy. We imaged a 100-10 kHz AC field by continuous-wave optically-detected magnetic resonance with frequency modulation of the microwave, where the AC signal is down-converted to DC and captured by the camera's speed. The magnetic response correlated with the anisotropy. When the external field was parallel to the easy axis, the phase delayed as frequency increased, implying the loss increased. When perpendicular, the phase didn't delay up to 5 kHz, implying the loss was almost zero. Diamond quantum sensors will help evaluate loss in inductors.   

Correlation between magneto-transport and magnetization reversal in CoFeB/Pt bilayers

Hall effect measurements on out-of-plane magnetized materials provide means to study the magnetization reversal process by analogy with, e.g., magneto-optical Kerr effect (MOKE) magnetometry. Chiral non-collinear spin textures may contribute to the so-called topological Hall effect as the difference between ordinary plus anomalous Hall effect and M(H) curves. In this talk, I will present results for soft-magnetic CoFeB/Pt bilayer films with perpendicular magnetic anisotropy and compare as-grown with annealed films. The differing behavior of MOKE and magneto-transport for hysteresis loops, first-order reversal curves, and angle-dependent relaxation indicates the presence of a strong topological effect, a rationale corroborated by structural characterizations and magnetic imaging.   

Single crystal growth of SmCrGe3 with Cr Linear Chain in Hexagonal Lattice

SmCrGe3 was discovered in a polycrystalline form, with a reported Curie temperature of 155 K, the step-like M(H) at 2 K, and a low-temperature transition at 55 K. [1] Compared with itinerant magnet LaCrGe3, SmCrGe3 offers the opportunity to understand the interaction between localized 4f elections and itinerant electrons in this quasi-one-dimensional system and the role this interaction plays in domain pining changing, [2] which may explain “one domain” behavior in LaCrGe3 single crystals.  Based on these points, single crystal SmCrGe3, with a hexagonal structure and the face-sharing Cr-centered octahedra aligned along the c-axis, was grown using the self-flux. Single crystal and powder X-ray diffraction were used to characterize the sample. Transport, magnetization, and specific heat measurements were taken. There is no second transition observed around 55 K. According to the measurements, SmCrGe3 behaves like a hard ferromagnet, which is different from other RECrGe3 (RE = La - Nd). The coercivity field as a function of temperature and field-dependent pinning change is given, and the results will be discussed in detail. 

[1], Haiying Bie, et al., Chem. Mater. 2007, 19, 4613-4620.

[2], M Xu, et al., Physical Review B 107 (13), 134437.