Wall Pinning Research Articles

Magnetic and electrical properties of postannealed Co2MnSi Heusler alloy films

The Co2MnSi (CMS) Heusler alloy compound with ordered crystalline structure is attractive for a number of applications due to soft ferromagnetic properties, high Curie temperature (TC ≅ 985 K), high saturation magnetization (Ms ≅ 5.1 µB/f.u), and predicted full spin polarization of carriers. In this work, we report preparation and properties of nanocrystalline CMS films with thickness d = 120 nm magnetron sputtered onto Si(100) substrates followed by annealing in vacuum at Ta = 300 ÷ 500 °C. Nanocrystalline structure of partially ordered cubic B2 phase with grain diameter d = 15 ÷ 30 nm increasing gradually with Ta has been indicated after annealing at Ta ≥ 400 °C. The angle-dependent differential susceptibility measurements revealed an interesting evolution of magnetic properties of the films in a course of postdeposition annealing at Ta=500 °C. Formation of isolated FM particles demonstrating the angle-dependent coercivity Hc(α) following the well-known Stoner–Wohlfarth model has been indicated after 0.5 h annealing. However, the Kondorsky low Hc ~ 1/cosα demonstrating a strong pinning of domain walls has been certified for similar films after 1.5 h annealing. At the same time, long-term (~ 5 h) annealing at 500 °C revealed reduced magnetization of the films due to a decomposition of the material. Nevertheless, electrical transport measurements revealed low field magnetoresistance of these films of about 0.1% and 0.05% at 80 and 295 K, respectively, demonstrating tunneling of spin-polarized carriers between nanocrystalline grains.

Effect of the Initial States, the Anisotropy, and Structural Defects on a Nonequilibrium Critical Behavior of the Three-Dimensional Heisenberg Model

A numerical Monte Carlo study has been performed on the influence of various initial states, easy-axis-type magnetic anisotropy, and structural defects on a nonequilibrium critical behavior of the classical three-dimensional Heisenberg model. An analysis of the time dependence of the magnetization and the autocorrelation function for the isotropic Heisenberg model shows substantial influence of the initial states on the relaxation of the magnetization and the aging effects in the behavior of the autocorrelation function, which are characterized by anomalous slowing down of the relaxation and correlation in the system as the waiting time increases. The study of the anisotropic Heisenberg model shows that the behavior of the magnetization and the autocorrelation function in a long-time regime is characterized by the critical exponents of the three-dimensional Ising model with a faster time decay of the autocorrelation function than that for the isotropic model. It has been revealed that the existence of structural defects in the case of the evolution of the system from the low-temperature initial state leads to an anomalously significant slowing down of the autocorrelation function. These features in the behavior of the autocorrelation function are characterized by the “superaging” effect with “superaging” exponent μ = 2.6(1) and related to the pinning of domain walls on structural defects during a nonequilibrium change in the domain structure of the system. In the case of evolution from the high-temperature initial state, the structural defects enhance the aging effect in the aging regime but their influence is irrelevant in the long-time regime.

Metamagnetism and kinetic arrest in a long-range ferromagnetically ordered multicaloric double perovskite Y2CoMnO6

A systematic magnetic study of the double perovskite oxide Y2CoMnO6 (YCMO) has been performed. A monoclinic P21/n phase of YCMO was synthesized using a sol-gel method. Neutron diffraction (ND) measurements evidence the onset of long-range ferromagnetic (FM) ordering at TC ~ 76 K, which persists down to 5 K. The presence of 25% antisite disorder, estimated from the ND data, leads to the appearance of short-range antiferromagnetic (AFM) interactions. The existence of thermal hysteresis due to competing interactions between FM and AFM phases is observed in the M vs. T measurements. The pinning of magnetic domain walls at the Co/Mn antiphase boundaries results in a metamagnetic-like behavior. The field dependence of thermomagnetic irreversibility and the nature of virgin curves indicate the occurrence of a kinetic arrest phenomenon, which is further verified via cooling and heating of the system in an unequal fields (CHUF) protocol. In full agreement with the ND data, a detailed analysis of critical exponents near the paramagnetic (PM)-FM phase transition also establishes YCMO as a long-range interacting mean-field system. A careful examination of the temperature- and field-dependent magnetic entropy change yields an in-depth understanding of coexisting magnetically ordered and disordered phases in YCMO, leading to a comprehensive magnetic phase diagram of this multifunctional double perovskite system.

High-Speed Piezoresponse Force Microscopy and Machine Learning Approaches for Dynamic Domain Growth in Ferroelectric Materials.

Domain dynamics has been one of the hottest research topics for ferroelectric materials in order to understand the ferroelectric mechanisms and to develop the related applications. By using high-speed piezoresponse force microscopy (HSPFM), it is possible to observe the dynamic domain evolution in an ultrashort time increment. This paper combines the HSPFM experiments and machine learning to study the domain growth under a weak AC field in ferroelectric materials. Here, the Bayesian optimized support vector machine is employed to classify the switching domain and stable domain. The results indicate that the machine learning classifier is capable of discerning the switching area. In addition, the domain associated characteristics, such as domain pinning and domain wall pinning, can also be observed and analyzed by combining experiments and machine learning. The machine learning approach can fast and deeply extract the complicated features related to free energy from the multidimensional signals obtained by HSPFM.

Superior polarization retention through engineered domain wall pinning

Ferroelectric materials possess a spontaneous polarization that is switchable by an electric field. Robust retention of switched polarization is critical for non-volatile nanoelectronic devices based on ferroelectrics, however, these materials often suffer from polarization relaxation, typically within days to a few weeks. Here we exploit designer-defect-engineered epitaxial BiFeO3 films to demonstrate polarization retention with virtually no degradation in switched nanoscale domains for periods longer than 1 year. This represents a more than 2000% improvement over the best values hitherto reported. Scanning probe microscopy-based dynamic switching measurements reveal a significantly increased activation field for domain wall movement. Atomic resolution scanning transmission electron microscopy indicates that nanoscale defect pockets pervade the entire film thickness. These defects act as highly efficient domain wall pinning centres, resulting in anomalous retention. Our findings demonstrate that defects can be exploited in a positive manner to solve reliability issues in ferroelectric films used in functional devices.

Исследование влияния начальных состояний, анизотропии и дефектов структуры на неравновесное критическое поведение трехмерной модели Гейзенберга

A numerical Monte Carlo study of different initial states, magnetic anisotropy of “easy axis” type, and structural defects influence on non-equilibrium critical behavior of the classical three-dimensional Heisenberg model has been carried out. Analysis of time dependence of the magnetization and the autocorrelation function for isotropic Heisenberg model has shown strong influence of initial states on relaxation of the magnetization and aging effects in behavior of the autocorrelation function, which are characterized by anomalous slowing down of relaxation and correlation in system with increasing waiting time. Study of the anisotropic Heisenberg model with magnetic anisotropy of “easy axis” type has revealed that behavior of the magnetization and autocorrelation function in long-time regime is characterized by critical exponents of the three-dimensional Ising model and more fast decay of the autocorrelation function than for isotropic Heisenberg model. It has been revealed that in structurally disordered isotropic Heisenberg model relaxing from the low-temperature initial state, the significant changes of non-equilibrium aging phenomena in comparison with pure model are observed due to the pinning of domain walls on defects. It has been shown that during this relaxation process a strong slowing-down of time dependence of the autocorrelation function is realized. As a result, superaging effects occur in long-term behavior of the autocorrelation function with values of the superaging exponent μ=2.6(1). During evolution of system from high-temperature initial state, the presence of structural defects leads to increase of aging effects in aging regime, but their influence is found irrelevant in long-time regime.

Investigation of the pinning and mobility of domain walls in cobalt micro- and nanowires by the nuclear spin echo method under the additional influence of a magnetic video pulse

The dynamics of domain walls in magnetic cobalt microwires synthesized using electroless technology was studied by the nuclear spin echo method of 59Co nuclei located at the centers of the domain walls of the FCC cobalt phase under the influence of an additional magnetic video pulse. The measured dependences of the intensity of the echo signals on the amplitude of the magnetic video pulse allow one to estimate the pinning and mobility of the domain walls in micro- and nanowires, the anisotropy of its effect during rotation of the sample containing transversely polarized magnetic wires, and also to establish the possible type of domain walls moving in them.

Magnetic Properties of Copper–Indium Doped Spinel FeCr2S4

Magnetic properties of Fex(Cu0.5In0.5)1– xCr2S4 (x = 0.6–1) solid solutions between ferrimagnet FeCr2S4 (TC = 170 K) and antiferromagnet Cu0.5In0.5Cr2S4 (TN = 28 K) have been studied at temperatures in the range 4–300 K in permanent magnetic fields H = 7 960 A/m (100 Oe) and 3.58 × 106 A/m (45 kOe). Magnetic transition temperatures have been determined. The divergence between ZFC and FC curves below the magnetic ordering temperature has been observed for all compositions. An Fe0.7Cu0.15In0.15Cr2S4 sample has the highest coercive force. The interpretation of results is based on the account for the system’s coercivity and the domain wall pinning in the test samples by nonmagnetic copper and indium ions.

Antiphase boundaries, magnetic domains, and magnetic vortices in Ni–Mn–Ga single crystals

The interplay between antiphase boundaries (APBs), magnetic domain structure and functional properties was investigated in the martensitic state of Ni–Mn–Ga single crystals which showed magnetically induced martensite reorientation (MIR) with 6% strain. The APB density was controlled by different heat treatments. The APBs and magnetic domains were observed by Lorentz transmission electron microscopy (LTEM). Slow cooling at ~1 K/min resulted in a low density (<1/µm), air quenching in a medium density (≈8/µm), and water quenching in a high density (≈15/µm) of APBs. Abundant pinning of domain walls on APBs was observed, which resulted in close correspondence between the magnetic domain walls and antiphase boundaries (APBs), magnetic domain memory, and finer domain patterns for the high APB density. For low APB density, a labyrinth domain structure was established between domain walls pinned on APBs. For low and medium density of APBs the magnetization was oriented mostly parallel to the out-of-plane easy magnetization axis. For high APB density the magnetization switched to the in-plane orientation, indicating that the effective magnetic anisotropy decreased and became lower than the stray field energy. In addition, magnetic vortices appeared. The novel functionalities based on a combination of MIR and interactions of the magnetic structure with APBs are feasible since the MIR was observed even for the highest density of APBs.

Antiphase Boundaries, Magnetic Domains, and Magnetic Vortices in Ni-Mn-Ga Single Crystals

The interplay between antiphase boundaries (APBs), magnetic domain structure and functional properties was investigated in the martensitic state of Ni-Mn-Ga single crystals which showed magnetically induced martensite reorientation (MIR) with 6% strain. The APB density was controlled by different heat treatments and the APBs and magnetic domains were observed by Lorentz transmission electron microscopy (LTEM). Slow cooling at ~1 K/min resulted in a low density (<1/μm), air quenching in a medium density (≈8/μm), and water quenching in a high density (≈15/μm) of APBs. Abundant pinning of domain walls on APBs was observed, which resulted in one-to-one correspondence between the magnetic domain walls and antiphase boundaries (APBs), magnetic domain memory, and finer domain patterns for the high APB density. For low APB density, a labyrinth domain structure was established between domain walls pinned on APBs. For low and medium density of APBs the magnetization was oriented mostly parallel to the out-of-plane easy magnetization axis. For high APB density the magnetization switched to the in-plane orientation, indicating that the effective magnetic anisotropy became lower than the stray field energy, and magnetic vortices additionally appeared. The novel functionalities based on a combination of MIR and interactions of the magnetic structure with APBs are feasible since the MIR was observed even for the highest density of APBs.

Generation and stability of structurally imprinted target skyrmions in magnetic multilayers

Target Skyrmions (TSks) are extended topological spin textures with a constant chirality where the rotation of the z component of the magnetization is larger than π. TSks have topological charge 1 or 0, if the z component of the magnetization Mz goes through a rotation of nπ where n is an odd or even integer, respectively. TSks with a rotation of the z component of up to 4π have been imaged via high spatial resolution element-specific x-ray imaging. The TSks were generated by weakly coupling 30 nm thin Permalloy (Ni80Fe20, PY) disks with a 1 μm diameter to asymmetric (Ir 1 nm/Co 1.5 nm/Pt 1 nm) × 7 multilayers that exhibit Dzyaloshinskii-Moriya interaction. The PY disks stabilize the TSks in the multilayers due to reduced stray field energy and enforced circular symmetry from pinning of domain walls at the edges of the disks. Upon applying external magnetic fields, it is the skyrmion core at the center that ensures stability of the TSk, whereas the collapse of the extended structures in the TSk does not depend on the topological charge.

Magnetoelastic coupling behavior at the ferromagnetic transition in the partially disordered double perovskite La2NiMnO6

The magnetocapacitance and magnetoresistance properties near room temperature of partially disordered double perovskite $\mathrm{L}{\mathrm{a}}_{2}\mathrm{NiMn}{\mathrm{O}}_{6}$ are related, at least in part, to coupled ferroelastic and magnetic instabilities that are responsible for a ferromagnetic phase transition near 280 K. A systematic analysis of this coupling from the perspectives of strain and elasticity has revealed a system with biquadratic coupling among three order parameters belonging to irreducible representations of ${X}_{3}^{+}, {\mathrm{\ensuremath{\Gamma}}}_{\text{4}}^{\text{+}}$ and $\text{m}{\mathrm{\ensuremath{\Gamma}}}_{\text{4}}^{\text{+}}$ of the parent space group $Fm\overline{3}m$. Classical octahedral tilting drives the structural transitions at high temperatures and strong acoustic attenuation through the temperature interval \ensuremath{\sim}300--500 K, observed by resonant ultrasound spectroscopy from a polycrystalline sample, is consistent with pinning of ferroelastic twin walls by point defects. Below room temperature, stiffening of the shear modulus by up to \ensuremath{\sim}40% can be understood in terms of biquadratic coupling of the ferromagnetic order parameter with strain. Acoustic attenuation with Debye-like patterns of loss in the temperature interval \ensuremath{\sim}150--280 K yielded activation energies and relaxation times which match up with AC magnetic and dielectric spectroscopy data reported previously in the literature. The dynamic loss mechanism, perhaps related to hopping of electrons between $\mathrm{N}{\mathrm{i}}^{2+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$, is potentially multiferroic, therefore. In addition to the possibilities for tailoring the intrinsic properties of $\mathrm{L}{\mathrm{a}}_{2}\mathrm{NiMn}{\mathrm{O}}_{6}$ by controlling oxygen content, $B$-site order or by choice of substrate for imposing a strain on thin films, it should be possible also to engineer extrinsic properties which would respond to applied electric, magnetic, and stress fields.

Ultrafast depinning of domain walls in notched antiferromagnetic nanostructures

The pinning and depinning of antiferromagnetic (AFM) domain wall is certainly the core issue of AFM spintronics. In this work, we study theoretically the N\'eel-type domain wall pinning and depinning at a notch in an antiferromagnetic (AFM) nano-ribbon. The depinning field depending on the notch dimension and intrinsic physical parameters are deduced and also numerically calculated. Contrary to conventional conception, it is revealed that the depinning field is remarkably dependent of the damping constant and the time-dependent oscillation of the domain wall position in the weakly damping regime benefits to the wall depinning, resulting in a gradual increase of the depinning field up to a saturation value with increasing damping constant. A one-dimensional model accounting of the internal dynamics of domain wall is used to explain perfectly the simulated results. It is demonstrated that the depinning mechanism of an AFM domain wall differs from ferromagnetic domain wall by exhibiting a depinning speed typically three orders of magnitude faster than the latter, suggesting the ultrafast dynamics of an AFM system.

Effect of Nanoscale Defect Structure on Magnetic Properties of Tb–Co Films with Perpendicular Magnetic Anisotropy

Magnetization reversal is studied for Ta/Tb29Co71/Ta thin films with different types of nanostructure. The films are deposited onto structuring substrates of anodized aluminum with unpolished surfaces, polished surfaces, and barrier layers. Nanoholes are found to triple the coercivity, due likely to the pinning of domain walls. Continuous and antidot films reveal the strong effect nanoconvexities have on the slope of the hysteresis loops. Such surface features could be one of the main reasons for the attenuation of perpendicular anisotropy in films deposited onto porous substrates.

Phenomenological Approach to Analyzing the Magnetization Reversal of Uniaxial Highly Anisotropic Ferromagnetic Materials

The reversal of specific features of the hysteresis behavior of alloys for permanent magnets based on SmCo5 and Nd2Fe14B is modeled using a phenomenological approach to elemental magnetization. The model of magnetization reversal in Sm(Co, Cu, Fe, Zr)7.4 alloys is based on the assumption that it is determined in these materials by domain wall pinning. Predictions are compared to experimental results.

Crystal Structure and Magnetic Properties of Cu-Substituted La0.90Ag0.10MnO3 Compounds

In the present work, a series of (La0.90Ag0.10)(Mn1 − yCuy)O3 compounds (y = 0–0.20) have been prepared by solid-state reaction route to study the influence of Cu substitution at Mn site of (La0.90Ag0.10)MnO3 compound on crystal structure and magnetic property. From the refinement of XRD patterns, it is found that no significant structural change has taken place by Cu substituion and all compounds are found to crystallize to rhombohedral structure ($$ R\overline{3}c $$ space group). However, it is observed that the Cu substitution into (La0.90Ag0.10)MnO3 compound has led to a drastic decrease of ferromagnetic (FM) transition temperature (TC), from 245.8 K for y = 0 to 174.3 K for y = 0.20 compound. The reduction of FM TC has been explained in terms of dilution of double exchange interaction across Mn3+-O2−-Mn4+ networks. The measurement of magnetization data under zero field cooling and field cooled warming conditions exhibits strong magnetic irreversibility, which is explained in terms of domain wall pinning. The hysteresis (M-H) curves measured for these compounds at 5 K indicate that the magnetization was found to saturate even at very low field and the value of saturation magnetization (MS) is found to decrease with increase in Cu doping and it is in correlation with theoretically estimated value of saturation magnetization. The measurement of Arrott plots (M2 versus H/M) for 10, 15, and 20% Cu-doped compounds indicates that these compounds exhibit second-order magnetic phase transition.

Tilted magnetisation for domain wall pinning in racetrack memory

Abstract The interest in spintronics devices based on domain wall (DW) motion has gained attention for many years. However, the stochastic behaviour of DW motion is still a fundamental issue for the practical implementation of DW devices. In this study, we demonstrate that effective domain wall pinning can be achieved by using exchange interaction between Co/Ni multilayer with perpendicular magnetic anisotropy (PMA) and Co layer with in-plane magnetic anisotropy (IMA) to create locally tilted magnetisation. The strength of exchange interaction is tuned by varying the thickness of spacer layer Pt between the PMA and IMA layers, thus forming different tilt angles. Micromagnetic simulations were performed to verify the relation between pinning field and magnetisation tilt angle. Polar Kerr microscopy shows the current-driven DW pinning and depinning in the Co/Ni multilayer device with Co crossbars, where the thickness of spacer layer Pt is 1 nm. The proposed approach can potentially be used in future DW memory device applications.

Effect of heterogeneous microstructure on magnetization reversal mechanism of hot-deformed Nd-Fe-B magnets

The hot-deformed (HD) Nd-Fe-B magnets show heterogeneous microstructure composed of coarse and fine grain regions. It is significant to fully understand the influence of this complex microstructure on the magnetization reversal process which can give the guidance for the enhancement of the magnetic properties. In this paper, the heterogeneous microstructure of the (HD) Nd-Fe-B magnets were characterized from the morphology, size, macro-texture and micro-structure. In addition, the magnetization reversal process of the HD Nd-Fe-B magnets was systematically analyzed by magnetic measurement, in-situ domain evolution observation and micromagnetic simulation. The results indicate that the HD Nd-Fe-B magnets mainly consist of fine grain regions (FGRs) and coarse grain regions (CGRs). The FGRs show plate-like grains with fine grain size and strong c-axis texture, while the CGRs show equiaxial grains with large grain size and weak c-axis texture. In particular, it is worth noting that the texture in homogeneity exists not only between FGRs and CGRs, but also inside both the FGRs and CGRs. The dominant coercivity mechanism of the HD Nd-Fe-B magnets is domain wall pinning. Also, the experimental analysis shows that the reverse domain is formed and expanded in the CGRs at low reverse applied field, while the reverse domain occurs in the FGRs at higher reverse applied field. The micromagnetic simulation results also confirm the above magnetization reversal process. In addition, micromagnetic simulation results also show that the orientation of the grains also affects the pinning strength, besides the grain size.

Investigation of domain wall pinning by square anti-notches and its application in three terminals MRAM

In this work, we perform investigations of the competition between domain-wall pinning and attraction by antinotches and finite device borders. The conditions for optimal geometries, which can attain a stable domain-wall pinning, are presented. This allows the proposition of a three-terminal device based on domain-wall pinning. We obtain, with very small pulses of current applied parallel to the nanotrack, a fast motion of the domain-wall between antinotches. In addition to this, a swift stabilization of the pinned domain-wall is observed with a high percentage of orthogonal magnetization, enabling high magnetoresistive signal measurements. Thus, our proposed device is a promising magnetoresistive random access memory device with good scalability, duration, and high speed information storage.

Domain wall pinning and hard magnetic phase in Co-doped bulk single crystalline Fe3GeTe2

We report the effects of cobalt doping on the magnetic properties of two-dimensional van der Waals ferromagnet Fe3GeTe2. Single crystals of (Fe{1-x}Cox)3GeTe2 with x=0-0.78 were successfully synthesized and characterized with x-ray diffraction, energy dispersive x-ray spectroscopy and magnetization measurements. Both the Curie-Weiss temperature and ferromagnetic (FM) ordered moment of Fe3GeTe2 are gradually suppressed upon Co doping. A kink in zero-field-cooling low field M(T) curve which is previously explained as an antiferromagnetic transition is observed for samples with x=0-0.58. Our detailed magnetization measurements and theoretical calculations strongly suggest that this kink is originated from the pinning of magnetic domain walls. The domain pinning effects are suddenly enhanced when the doping concentration of cobalt is around 50%, both the coercive field Hc and the magnetic remanence to saturated magnetization ratio MR/MS are largely improved and a hard magnetic phase emerges in bulk single crystal samples. The strong doping dependent magnetic properties suggest more spintronic applications of Fe3GeTe2.