Magnetic Domain Patterns Research Articles

Manipulation of Magnetic Skyrmion Density in Continuous Ir/Co/Pt Multilayers

We show that magnetic skyrmions can be stabilised at room temperature in continuous [Ir/Co/Pt]5 multilayers on SiO2/Si substrates without the prior application of electric current or magnetic field. While decreasing the Co thickness, a transition of the magnetic domain patterns from worm-like state to separated stripes is observed. The skyrmions are clearly imaged in both states using magnetic force microscopy. The density of skyrmions can be significantly enhanced after applying the "in-plane field procedure". Our results provide means to manipulate magnetic skyrmion density, further allowing for the optimised engineering of skyrmion-based devices.

Unveiling the anisotropic fractal magnetic domain structure in bulk crystals of antiskyrmion host (Fe,Ni,Pd)3P by small-angle neutron scattering

Intermetallic Pd-doped (Fe,Ni)3P, which crystallizes in a non-centrosymmetric tetragonal structure with S 4 symmetry, has recently been discovered to host magnetic antiskyrmions, antivortex-like topological spin textures. In this material, uniaxial magnetic anisotropy and dipolar interactions play a significant role, giving rise to finely branched magnetic domain patterns near the surface of bulk crystals, as revealed by a previous magnetic force microscopy (MFM) measurement. However, small-angle neutron scattering (SANS) is a more suitable method for characterizing bulk properties and fractal structures on the mesoscopic length scale. In this study, using SANS and MFM, the magnetic domain structure in bulk single crystals of (Fe0.63Ni0.30Pd0.07)3P is quantitatively investigated. The SANS results demonstrate that the magnetic domain structure exhibits anisotropic fractal character on length scales down to the width of the magnetic domain walls. The fractal features are gradually lost in magnetic fields, and different field dependencies are observed at 300 and 2 K due to a temperature-dependent anisotropy. This study quantifies the fractality of the highly anisotropic magnetic domain structures in an antiskyrmion material, and highlights the versatility of SANS for the study of fractal structures in magnetic systems.

Macroscopic magnetic hardening due to nanoscale spinodal decomposition in Fe–Cr

The Fe–Cr alloy system is the basis of ferritic steels, which are important structural materials for many applications, including their use in future fusion reactors. However, when exposed to elevated temperatures and radiation, the Fe–Cr system can undergo phase separation, resulting in Fe-rich (α) and Cr-rich (α’) nanoscale regions. This in turn generates the so-called “475 °C embrittlement” and modifies the magnetic properties. The correlation between the microstructural and magnetic changes is however poorly understood, which currently prevents the possibility of assessing the material in a non-destructive way by magnetometry. Here, we study the microstructural decomposition of an Fe–40Cr alloy induced by annealing at 500 °C for extensive time scales and its impact on the magnetic properties using magnetometry and advanced experimental methods, such as atom probe tomography, transmission electron microscopy (TEM), and micromagnetic simulations. Upon annealing, the alloy rapidly exhibits a spinodal decomposition morphology with a typical length scale of about 10 nm. With increasing annealing time, the hardness assessed by Vickers testing, the magnetic saturation, and the coercivity increase, which correlates with an increase in α-volume fraction and the system's heterogeneity. The magnetic domain patterns imaged by TEM and interpreted with the help of micromagnetic simulations reveal at the nanometer scale the impact of decomposition on the magnetic response of Fe–Cr.

Structural and magnetic properties of Pt/Co/Mn-based multilayers

Magnetic multilayers are a rich class of material systems with numerous highly tunable physical parameters that determine both their magnetic and electronic properties. Here, we present a comprehensive experimental study of the system Pt/Co/Mn, which extends the group of $\mathrm{Pt}/\mathrm{Co}/X$ ($X=$ metal) multilayers that have been investigated thus far. We demonstrate that an increasing Co layer thickness changes the magnetic anisotropy from out of plane to in plane, whereas the deposition of thicker Mn layers leads to a decrease in the saturation magnetization. Temperature-dependent magnetometry measurements reinforce the hypothesis of antiferromagnetic coupling at the Co/Mn interfaces being responsible for the observed Mn thickness dependence of the magnetization reversal. Moreover, magneto-optical imaging experiments indicate systematic changes in magnetic domain patterns as a function of the Co and Mn layer thickness, suggesting the existence of bubblelike domains---potentially even magnetic skyrmions---in the case of sufficiently thick Mn layers, which are expected to contribute to a sizable Dzyaloshinskii-Moriya interaction in the multilayer stacks. We identify Pt/Co/Mn as a highly complex multilayer system with strong potential for further fundamental studies and possible applications.

Laser MBE growth of cobalt-platinum multilayers with perpendicular magnetic anisotropy

Periodic multilayers with perpendicular magnetic anisotropy (PMA) attract a lot of attention nowadays being fundamentally interesting from the point of view of interface magnetism. They also provide a suitable test bench for the studies of spatially resolved magnetization dynamics at picosecond time scale conducted at x-ray free electron laser (XFEL) facilities. In the present paper we have developed a novel laser molecular beam epitaxy (LMBE) approach to grow Co/Pt multilayers on crystalline Al2O3 (0001) substrates. By applying Kerr-effect magnetometry to the samples fabricated at different conditions, we have found it essential for the appearance of PMA, to perform metal deposition in 0.03 mbar of argon and to keep Co layer thickness below 4 Å. Along with magnetic studies, we have investigated crystal structure and depth resolved layer composition of the fabricated Co/Pt multilayers by applying electron diffraction, x-ray diffraction and x-ray reflectometry. The combined atomic-force/magnetic-force microscopy studies have revealed a very low surface roughness of down to 1 Å (for the samples grown at properly optimized growth conditions) and a pronounced labyrinth pattern of magnetic domains with a periodicity of 250 nm appropriate for XFEL studies of ultrafast optical magnetization. The presented LMBE approach to fabricate magnetically ordered heterostructures with a pronounced PMA is supposed to be an important milestone on the way of facilitating future design of nanomaterials for applications in magnetic memories and for fundamental studies of ultrafast magnetization dynamics.

Defect Nanostructure and its Impact on Magnetism of α-Cr2 O3 Thin Films.

Thin films of the magnetoelectric insulator α-Cr2 O3 are technologically relevant for energy-efficient magnetic memory devices controlled by electric fields. In contrast to single crystals, the quality of thin Cr2 O3 films is usually compromised by the presence of point defects and their agglomerations at grain boundaries, putting into question their application potential. Here, the impact of the defect nanostructure, including sparse small-volume defects and their complexes is studied on the magnetic properties of Cr2 O3 thin films. By tuning the deposition temperature, the type, size, and relative concentration of defects is tailored, which is analyzed using the positron annihilation spectroscopy complemented with electron microscopy studies. The structural characterization is correlated with magnetotransport measurements and nitrogen-vacancy microscopy of antiferromagnetic domain patterns. Defects pin antiferromagnetic domain walls and stabilize complex multidomain states with a domain size in the sub-micrometer range. Despite their influence on the domain configuration, neither small open-volume defects nor grain boundaries in Cr2 O3 thin films affect the Néel temperature in a broad range of deposition parameters. The results pave the way toward the realization of spin-orbitronic devices where magnetic domain patterns can be tailored based on defect nanostructures without affecting their operation temperature.

Study of Magnetic Stripe Domain Patterns in High-Coercive MnBi 3D Hexagonal Microstructures

Study of Magnetic Stripe Domain Patterns in High-Coercive MnBi 3D Hexagonal Microstructures

Magnetic and geometric control of spin textures in the itinerant kagome magnet Fe3Sn2

Magnetic materials with competing magnetocrystalline anisotropy and dipolar energies can develop a wide range of domain patterns, including classical stripe domains, domain branching, and topologically trivial and nontrivial (skyrmionic) bubbles. We image the magnetic domain pattern of ${\mathrm{Fe}}_{3}{\mathrm{Sn}}_{2}$ by magnetic force microscopy and study its evolution due to geometrical confinement, magnetic fields, and their combination. In ${\mathrm{Fe}}_{3}{\mathrm{Sn}}_{2}$ lamellae thinner than 3 $\ensuremath{\mu}\mathrm{m}$, we observe stripe domains whose size scales with the square root of the lamella thickness, exhibiting classical Kittel scaling. Magnetic fields turn these stripes into a highly disordered bubble lattice. Complementary micromagnetic simulations quantitatively capture the magnetic field and thickness dependence of the magnetic patterns, reveal strong reconstructions of the patterns between the surface and the core of the lamellae, and identify the observed bubbles as skyrmionic bubbles. Our results imply that geometrical confinement together with competing magnetic interactions can provide a path to fine-tune and stabilize different types of topologically trivial and nontrivial spin structures in centrosymmetric magnets.

Dependence of magnetic domain patterns on plasma-induced differential oxidation of CoPd thin films

We demonstrate the evolution of the micro-patterned magnetic domains in CoPd thin films pretreated with e-beam lithography and O2 plasma. During the days-long oxidation, significantly different behaviors of the patterned magnetic domains under magnetization reversal are observed via magneto-optic Kerr effect microscopy on different days. The evolution of the magnetic behaviors indicate critical changes in the local magnetic anisotropy energies due to the Co oxides that evolve into different oxide forms, which are characterized by micro-area X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. The coercive field of the area pre-exposed to plasma can decrease to a value 10 Oe smaller than that unexposed to plasma, whereas after a longer duration of oxidation the coercive field can instead become larger in the area pre-exposed to plasma than that unexposed, leading to an opposite magnetic pattern. Various forms of oxidation can therefore provide an additional dimension for magnetic-domain engineering to the current conventional lithographies.

Appearance of inverted hysteresis, pinning effect in amorphous FeCoB ribbon with annealing: A Kerr microscopy investigation

A systematic investigation has been carried out on the magnetic domain evolution and local hysteresis behavior in iron based amorphous ribbons (FeCoB) by Kerr microscopy with annealing. Explicitly, we have studied the ‘stress’ magnetic domain structure at the strained region of the ribbon, which is likely to be caused by local residual stress present in the ribbon. It is observed that the stress magnetic domain pattern gradually vanishes at elevated annealing temperature due to thermally relieving of compressive stress. Interestingly, in the same region of the ribbon, but at different ranges of temperature annealing, we have encountered different magnetic domains mechanisms and hysteresis behavior, namely domain wall pinning, inverted hysteresis behavior, and domain wall propagation. The results are explained in terms of competition between different source of anisotropy in the ribbon emerging at different annealing temperature.

Understanding thickness dependent magnetic properties of Tb-Fe thin films

In this paper, we have attempted to understand the magnetic behaviour of Tb-Fe thin films grown with different thicknesses through detailed electron microscopy, magnetization measurements and magnetic domain imaging studies. Tb-Fe thin films over a range of thicknesses such as 50, 100, 200, 400, 600 and 800 nm were deposited on Si substrates using electron beam evaporation technique. Grazing incidence X-ray diffraction studies revealed that all the films were amorphous in nature. Microstructural studies using transmission electron microscopy showed columnar channel of voids extending from substrate to film surface. Magnetization measurements along the in-plane and out-of-plane direction indicated presence of out-of-plane magnetic anisotropy in all the films. Saturation magnetization and coercivity measured along the out-of-plane direction were found to vary with the thickness of the films. A very high coercivity of ~2000 mT was obtained for the film having thickness of 400 nm. Magnetic anisotropy calculations indicated a strong perpendicular magnetic anisotropy for the 400 nm thick film. The variation of magnetic anisotropy with film thickness was correlated with the magnetic domain patterns observed in the films using magnetic force microscopy.

Phase prediction method for pattern formation in time-dependent Ginzburg-Landau dynamics for kinetic Ising model without a priori assumptions of domain patterns

We propose a phase prediction method for the pattern formation in the uniaxial two-dimensional kinetic Ising model with the dipole-dipole interactions under the time-dependent Ginzburg-Landau dynamics. Taking the effects of the material thickness into account by assuming the uniformness along the magnetization axis, the model corresponds to thin magnetic materials with long-range repulsive interactions. We propose a new theoretical basis to understand the effects of the material parameters on the formation of the magnetic domain patterns in terms of the equation of balance governing the balance between the linear- and nonlinear forces in the equilibrium state. Based on this theoretical basis, we propose a new method to predict the phase in the equilibrium state reached after the time-evolution under the dynamics with a given set of parameters, by approximating the third-order term using the restricted phase-space approximation [R. Anzaki, K. Fukushima, Y. Hidaka, and T. Oka, Ann. Phys. 353, 107 (2015)] for the $\phi^4$-models. Although the proposed method does not have the perfect concordance with the actual numerical results, it has no arbitrary parameters and functions to tune the prediction. In other words, it is a method with no a priori assumptions on domain patterns.

Systematic survey of magnetic configurations in multilayer ferromagnet system with dipole-dipole interaction

We investigate various magnetic configurations caused by the dipole--dipole interaction in thin-film magnets with perpendicular anisotropy under open boundary conditions. Two different approaches are simulated: one starts from a random magnetic configuration and decreases temperatures stepwisely; the other starts from the saturated out-of-plane ferromagnetic state to evaluate its metastability. As typical patterns of magnetic configuration, five typical configurations are found: an out-of-plane ferromagnetic, in-plane ferromagnetic, vortex, multidomain, and canted multidomain states. Notably, the canted multidomain forms a concentric magnetic-domain pattern with an in-plane vortex structure, resulting from the open boundary conditions. Concerning the coercivity, a comparison of the magnetic configurations in both processes reveals that the out-of-plane ferromagnetic state exhibits metastability in the multidomain state, but not in the vortex state. We also confirm that the so-called Neel-cap magnetic-domain-wall structure, which is originally discussed in the in-plane anisotropy system, appears at the multidomain state.

Obtaining Underlying Parameters from Magnetic Domain Patterns with Machine Learning

The question of how to understand the underlying mechanisms of complicated spatiotemporal patterns formed in various physical, chemical, and biological systems is still and open issue. Here, we give a successful example using the machine learning methods of a convolutional neural network (CNN) and a kernel ridge regression (KRR). By using these methods, we could quantitively identify an underlying parameter, the anisotropy coefficient α, from images of magnetic domain patterns formed in ferromagnetic thin films. We demonstrated that models based on both machine learning methods successfully identify α, with a mean absolute error of less than 0.077, from data synthesized with the time-dependent Ginzburg–Landau (TDGL) equation. Further, we showed that the CNN model outperformed the KRR model, but the KRR model provided significant pattern features for representing α more interpretably than the CNN could. Our results provide new insights into pattern analysis of magnetic materials from a machine learning viewpoint.

Multiple domain structure and symmetry types in narrow temperature and magnetic field ranges in layered Cr2Ge2Te6 crystal measured by magnetic force microscope

Intrinsic magnetism has recently been confirmed to survive in bilayer Cr2Ge2Te6. In this work, multiple domain structure and symmetry types and their transformations are firstly discovered in bulk layered Cr2Ge2Te6 crystal using a homebuilt magnetic force microscope (MFM). These transitions occur within a surprisingly narrow temperature range of 2 K and magnetic field range of 0.07 T, which colorful phenomenon hasn't been discovered in other materials but Cr2Ge2Te6. The MFM images show a series of basic domain patterns. Some of them also hasn't been found before, such as self-fitting disks and fine ladder structure within Y-connected walls. The symmetry of these magnetic domain patterns is discussed. The results not only are important in developing new magnetic domain theories but also may show some potential applications in storage depending on the future investigations.

Ultrafast phononic switching of magnetization

Identifying an efficient pathway to change the order parameter via a subtle excitation of the coupled high-frequency mode is the ultimate goal of the field of ultrafast phase transitions. This is an especially interesting research direction in magnetism, where the coupling between spin and lattice excitations is required for magnetization reversal. Despite several attempts however, the switching between magnetic states via resonant pumping of phonon modes has not yet been demonstrated. Here we show how an ultrafast resonant excitation of the longitudinal optical phonon modes in magnetic garnet films switches magnetization into a peculiar quadrupolar magnetic domain pattern, unambiguously revealing the magneto-elastic mechanism of the switching. In contrast, the excitation of strongly absorbing transverse phonon modes results in thermal demagnetization effect only.

Robustness of the remanent magnetic domain pattern formation and associated stripe-bubble transitions in Co/Pt multilayers against field sequencing

Thin ferromagnetic [Co/Pt] multilayers with perpendicular magnetic anisotropy exhibit a variety of nanoscopic magnetic domain patterns at remanence, from long interlaced stripes to lattices of bubbles, depending on the multilayer structure but also on the magnetic history of the sample. For optimized structural parameters, stripe-bubble transitions accompanied by drastic increases in domain density have been observed when the magnitude of the previously applied perpendicular field Hm is finely tuned throughout the hysteresis loop. Here, we investigate the robustness of these morphological transitions against field sequencing and field cycling. We conducted this study on [Co(x)/Pt(7Å)]N=50 where x varies from 4 to 60 Å. We mapped the morphological transition with Hm varying from 0 to 9 T, following both an ascending sequence (0 → 9 T) and a descending sequence (9 T → 0). We found that the optimal field Hm = H* at which the domain density is maximized and its associated maximal density n* are not significantly affected by the field sequencing direction. We have also investigated possible pumping effects when cycling the applied field at the value H*. We found that n* remains relatively stable through field cycling, and much more stable in the bubble state, compared to longer stripe states. The observed robustness of these morphological transitions against field sequencing and field cycling is of crucial importance for potential magnetic recording applications.

Uncorrelated magnetic domains in decoupled SrFe12O19/Co hard/soft bilayers

Composites of magnetically hard and soft phases are present in multiple and diverse applications, ranging from bulk permanent magnets in motors and generators to state-of-the-art recording media devices. The nature of the magnetic coupling between the hard and soft phases is of great technological relevance, as the macroscopic properties of the functional composite material ultimately depend on the atomic-scale interactions between phases. In this work, the hard/soft bilayer system SrFe12O19/Co has been studied based on photoemission electron microscopy combined with x-ray absorption and magnetic circular dichroism. Our experiments show that the magnetization of the hard magnetic oxide has a direction perpendicular to the layer plane, whereas the magnetization of the soft metallic overlayer remains in-plane. As a consequence, the magnetic domain patterns observed for the hard and soft phases are very different and completely uncorrelated to one another, indicating that no soft spins align with the hard phase by pure magnetodipolar arguments. The results are understood as the consequence of an absence of exchange-coupling between phases, in a scenario in which the shape anisotropy of the soft layer overcomes the Zeeman energy of the perpendicular magnetic field generated by the hard ferrite. Micromagnetic simulations of our system predict that low degrees of exchange-coupling effectively prevent substantial softening of the composite and lead to the alignment of soft and hard magnetic moments. A strategy thus emerges for the development of future hard-soft magnets, based on minimizing the degree of exchange-coupling while avoiding complete uncoupling.

Tunable and attractive magnetic properties of FeBPSiCu alloys

Novel Fe76-xB10P5Si9Cux (x = 0, 0.5, 1 and 1.5 at.%) amorphous alloys with low Fe content were successfully synthesized with aim of investigating the crystallization behaviors and magnetic properties. Thermodynamic behavior revealed that Cu substitution triggered the transition of primary precipitation phase from Fe23B6 to α-Fe, which favored the high saturation magnetic flux density (Bs) of alloys after annealing. Through adjustment of Fe/Cu ratio, the Fe75B10P5Si9Cu1 alloy exhibited the attractive and tunable magnetic properties, i.e. the high effective permeability (μe) of 25000 and low coercivity (Hc) of 1.3 A/m in amorphous state as well as the high Bs of 1.62 T and μe of 18000 in crystallization state. The high Bs could be attributed to the uniform refined composite microstructure, whereas the excellent magnetic softness characterized by magnetic domain patterns was determined by magnetic anisotropy induced via different atomic structures. Such tunable performance can promise alloy a potential soft-magnetic material to meet the requirement of most electric devices.

From stripes to bubbles: Deterministic transformation of magnetic domain patterns in Co/Pt multilayers induced by laser helicity

The optical control of magnetism offers an attractive possibility to manipulate small magnetic domains for prospective memory devices on ultrashort time scales. Here, we report on the local deterministic transformation of the magnetic domain pattern from stripes to bubbles in out-of-plane magnetized Co/Pt multilayers controlled only by the helicity of ultrashort laser pulses. Relying on the experimentally determined average size of stripe domains and the magnetic layer thickness, we calculate the temperature and characteristic fields at which the stripe-bubble transformation occurs. Furthermore, we demonstrate that in the narrow range of the laser power, the helicity induces a drag on domain walls.