Ultrathin Ferromagnetic Films Research Articles

Stochastic dynamics of Galton board based on magnetic skyrmions

Conventional Galton board is an iconic model in statistics to study the stochastic dynamics of particle with ideal hypothesis of equal probability. To test this hypothesis, we introduce an artificial spin network by utilizing skyrmion clusters in ultrathin ferromagnetic films, resembling a nanoscale magnetic Galton board. We show that the board output exhibits a standard Gaussian distribution when the external magnetic field oscillates with the lowest frequency of skyrmion gyration, recovering the results of conventional Galton board. The stochastic response of magnetic Galton board can be tailored by tuning the direction of external magnetic field and modifying the local geometry. Notably, the distribution deviates the Gaussian profile when applies magnetic fields with higher frequencies. We interpret it in terms of the breaking of equal-probability hypothesis due to the phase gradient between adjacent skyrmions. Our work provides theoretical reference for understanding the random properties of skyrmion clusters and for their potential applications in future neural network computing.

Significant modulation of Gilbert damping in ultrathin ferromagnetic films by altering the surface magnetic anisotropy

Significant modulation of Gilbert damping in ultrathin ferromagnetic films by altering the surface magnetic anisotropy

Tailoring Magnetic Anisotropy in Ultrathin Cobalt by Surface Carbon Chemistry

AbstractThe ability to manipulate magnetic anisotropy is essential for magnetic sensing and storage tools. Surface carbon species offer cost‐effective alternatives to metal‐oxide and noble metal capping layers, inducing perpendicular magnetic anisotropy in ultrathin ferromagnetic films. Here, the different mechanisms by which the magnetism in a few‐layer‐thick Co thin film is modified upon adsorption of carbon monoxide (CO), dispersed carbon, and graphene are elucidated. Using X‐ray microscopy with chemical and magnetic sensitivity, the in‐plane to out‐of‐plane spin reorientation transition in cobalt is monitored during the accumulation of surface carbon up to the formation of graphene. Complementary magneto‐optical measurements show weak perpendicular magnetic anisotropy (PMA) at room temperature for dispersed carbon on Co, while graphene‐covered cobalt exhibits a significant out‐of‐plane coercive field. Density‐functional theory (DFT) calculations show that going from CO/Co to C/Co and to graphene/Co, the magnetocrystalline and magnetostatic anisotropies combined promote out‐of‐plane magnetization. Anisotropy energies weakly depend on carbidic species coverage. Instead, the evolution of the carbon chemical state from carbidic to graphitic is accompanied by an exponential increase in the characteristic domain size, controlled by the magnetic anisotropy energy. Beyond providing a basic understanding of the carbon‐ferromagnet interfaces, this study presents a sustainable approach to tailor magnetic anisotropy in ultrathin ferromagnetic films.

Magnetic Skyrmions Under Confinement

We present a variational treatment of confined magnetic skyrmions in a minimal micromagnetic model of ultrathin ferromagnetic films with interfacial Dzylashinksii-Moriya interaction (DMI) in competition with the exchange energy, with a possible addition of perpendicular magnetic anisotropy. Under Dirichlet boundary conditions that are motivated by the asymptotic treatment of the stray field energy in the thin film limit we prove existence of topologically non-trivial energy minimizers that concentrate on points in the domain as the DMI strength parameter tends to zero. Furthermore, we derive the leading order non-trivial term in the Γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Gamma $$\\end{document}-expansion of the energy in the limit of vanishing DMI strength that allows us to completely characterize the limiting magnetization profiles and interpret them as particle-like states whose radius and position are determined by minimizing a renormalized energy functional. In particular, we show that in our setting the skyrmions are strongly repelled from the domain boundaries, which imparts them with stability that is highly desirable for applications. We provide explicit calculations of the renormalized energy for a number of basic domain geometries.

Onset of pattern formation in thin ferromagnetic films with perpendicular anisotropy

We consider the onset of pattern formation in an ultrathin ferromagnetic film of the form Omega _t:= Omega times [0,t] for Omega Subset mathbb {R}^2 with preferred perpendicular magnetization direction. The relative micromagnetic energy is given by E[M]=∫Ωtd2|∇M|2+Q∫Ωt(M12+M22)+∫R3|H(M)|2-∫R3|H(e3χΩt)|2,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\begin{aligned} \\mathcal {E}[M]= \\int _{\\Omega _t} d^2 |\ abla M|^2+ Q \\int _{\\Omega _t} (M_1^2+M_2^2) + \\int _{\\mathbb {R}^3} |\\mathcal {H}(M)|^2 - \\int _{\\mathbb {R}^3} |\\mathcal {H}(e_3 \\chi _{\\Omega _t})|^2, \\end{aligned}$$\\end{document}describing the energy difference for a given magnetization M: mathbb {R}^3 rightarrow mathbb {R}^3 with |M| = chi _{Omega _t} and the uniform magnetization e_3 chi _{Omega _t}. For t ll d, we derive the scaling of the minimal energy and a BV-bound in the critical regime, where the base area of the film has size of order |Omega |^{{frac{1}{2}}} sim (Q-1)^{{-frac{1}{2}}} d e^{frac{2pi d}{t} sqrt{Q-1}}. We furthermore investigate the onset of non-trivial pattern formation in the critical regime depending on the size of the rescaled film.

Ferromagnetic resonance excited by interfacial microwave electric field: the role of current-induced torques

Excitation of magnetization dynamics in magnetic materials, especially in ultrathin ferromagnetic films, is of utmost importance for developing various ultrafast spintronics devices. Recently, the excitation of magnetization dynamics, i.e. ferromagnetic resonance (FMR) via electric field-induced modulation of interfacial magnetic anisotropies, has received particular attention due to several advantages, including lower power consumption. However, several additional torques generated by unavoidable microwave current induced because of the capacitive nature of the junctions may also contribute to the excitation of FMR apart from electric field-induced torques. Here, we study the FMR signals excited by applying microwave signal across the metal-oxide junction in CoFeB/MgO heterostructures with Pt and Ta buffer layers. Analysis of the resonance line shape and angular dependent behavior of resonance amplitude revealed that apart from voltage-controlled in-plane magnetic anisotropy (VC-IMA) torque a significant contribution can also arises from spin-torques and Oersted field torques originating from the flow of microwave current through metal-oxide junction. Surprisingly, the overall contribution from spin-torques and Oersted field torques are comparable to the VC-IMA torque contribution, even for a device with negligible defects. This study will be beneficial for designing future electric field-controlled spintronics devices.

Quantifying symmetric exchange in ultrathin ferromagnetic films with chirality

The symmetric (Heisenberg) exchange interaction is fundamental to magnetism and assumes critical importance in designing magnetic materials for novel emergent phenomena and device applications. However, quantifying exchange is extremely challenging for ultrathin magnetic films, as techniques and approximations reliably used for bulk materials are largely inapplicable in the two-dimensional limit. Here we present and contrast the measurement of exchange stiffness $A$ by several methods on a series of five Co/Pt-based ultrathin ($1--2$ nm) films. We compare results from (a) spin-wave spectroscopy by Brillouin light scattering, (b) three analytical models describing the temperature dependence of magnetization obtained by magnetometry, (c) microscopic domain periodicity measurements and simulations, and (d) ab initio density functional theory calculations. While different methods present some qualitatively consistent trends across samples, we note, for any given sample, considerable differences (up to $5\ifmmode\times\else\texttimes\fi{})$ in the absolute values of $A$ across the techniques, consistent with discrepancies of $A$ reported in literature for nominally similar samples. We analyze possible sources of the discrepancies across various methods, notably including their relationship to the spin-wave dispersion, and the wave-vector ranges probed. We compare the strengths and limitations of the techniques, and outline directions for their future use in characterizing exchange interactions in ultrathin films.

Light‐Induced Mott‐Insulator‐to‐Metal Phase Transition in Ultrathin Intermediate‐Spin Ferromagnetic Perovskite Ruthenates (Adv. Mater. 12/2023)

Insulator–Metal Transition In article number 2211612, Zhicheng Zhong, Xuefeng Wang, and co-workers report that UV-light irradiation on perovskite-based heterostructures can drive the photocharge transfer from SrTiO3 substrates to intermediate-spin ferromagnetic SrRuO3 ultrathin films containing oxygen vacancies. As a consequence, a reversible Mott-insulator-to-metal phase transition occurs in the SrRuO3 layer, as revealed by systematic light-irradiation transport experiments and dynamical mean-field theory calculations.

Influence of Dzyaloshinskii–Moriya interaction and perpendicular anisotropy on spin waves propagation in stripe domain patterns and spin spirals

Texture-based magnonics focuses on the utilization of spin waves in magnetization textures to process information. Using micromagnetic simulations, we study how (1) the dynamic magnetic susceptibility, (2) dispersion relations, and (3) the equilibrium magnetic configurations in periodic magnetization textures in a ultrathin ferromagnetic film in remanence depend on the values of the Dzyaloshinskii–Moriya interaction and the perpendicular magnetocrystalline anisotropy. We observe that for large Dzyaloshinskii–Moriya interaction values, spin spirals with periods of tens of nanometers are the preferred state; for small Dzyaloshinskii–Moriya interaction values and large anisotropies, stripe domain patterns with over a thousand times larger period are preferable. We observe and explain the selectivity of the excitation of resonant modes by a linearly polarized microwave field. We study the propagation of spin waves along and perpendicular to the direction of the periodicity. For propagation along the direction of the periodicity, we observe a bandgap that closes and reopens, which is accompanied by a swap in the order of the bands. For waves propagating in the perpendicular direction, some modes can be used for unidirectional channeling of spin waves. Overall, our findings are promising in sensing and signal processing applications and explain the fundamental properties of periodic magnetization textures.

Tunable logic of complex variables and quantum networks on its basis

Continuous - additive-multiplicative (AM) logic is considered, in which logical operations are replaced by algebraic operations («×» and «+») or operations with vectors, and binary variables «0» and «1» are replaced by continuous scalar ones («0- 1») or complex variables. To build this logic, a continuous analogue of the canonical form of Boolean logic is used in the form of a perfect disjunctive or conjunctive normal form (KAM logic). A feature of KAM logic is a continuous dependence on input variables and a potential variety of continuous logic functions. Based on the previously proposed «fuzzy» (distributed) continuous function, in the form of a superposition of «clear» functions, and the tunable QAM element circuit that implements it, this element is generalized to a network QAM element with several tunable outputs. The multiplication functions in this QAM element can be performed using a known memristor, which can be replaced by a memtransistor based on a field effect transistor. In quantum QAM networks, these elements are, respectively: «k-memristor» and «k-memtransistor». One of the options for a k-memtransistor is a composite hybrid spin-field-effect transistor based on a planar spin valve with magnetic memory and a field-effect transistor with a ferroelectric memory. It is noted that the main technological problem of quantum QAM networks based on such a hybrid spin transistor is the creation of planar conducting and ferromagnetic elements based on ultrathin metallic and ferromagnetic films on silicon.

Magnetic Thin Films used for Memory Devices: A Scientometric Analysis

Ultra-thin ferromagnetic films coupled with nonmagnetic layers are extensively studied for their potential applications in data storage. The conventional memory industry is currently in the spotlight, with researchers worldwide contributing toward better data storage technologies that can tackle the data storage problems posed by this era of big data. Various technologies such as perpendicular magnetic recording (PMR) and heat-assisted magnetic recording (HAMR) have been introduced since Poulson’s idea of storing signals was conceptualized. With the introduction of new technologies, like PMR and HAMR, various materials have to be restudied for potential recording media implementation. It is vital to carry out scientometric/bibliometric studies before extensive research. It gives us an insight into the significance of the topic and the gap in the research work. This paper presents a scientometric study from 2010 to 2022 of all the magnetic thin films used for various applications. It provides an overview of the technologies and literature study of their development in data storage applications. The study discusses the analytic results from different comprehensive high indexed databases such as Scopus, Web of Science (WoS), Google Scholar, and the Institute of Electrical and Electronics Engineers (IEEE). To contribute meaningful insights and the perfect representation of the extracted data, various tools like iMapbuilder, Gephi and RAWgraphs are used in this article.

A micromagnetic theory of skyrmion lifetime in ultrathin ferromagnetic films

We use the continuum micromagnetic framework to derive the formulas for compact skyrmion lifetime due to thermal noise in ultrathin ferromagnetic films with relatively weak interfacial Dzyaloshinskii-Moriya interaction. In the absence of a saddle point connecting the skyrmion solution to the ferromagnetic state, we interpret the skyrmion collapse event as "capture by an absorber" at microscale. This yields an explicit Arrhenius collapse rate with both the barrier height and the prefactor as functions of all the material parameters, as well as the dynamical paths to collapse.

Renormalization group analysis of the finite two-dimensional XY model with fourfold anisotropy: Application to the magnetic susceptibility of a ferromagnetic ultrathin film

The topological phase transition introduced by Kosterlitz and Thouless has become a central concept in condensed matter physics. With the increasing relevance of topological phases to applications in spintronics, it is important to extend the description of the Kosterlitz-Thouless transition in an infinite, isotropic two-dimensional XY ferromagnet to experimentally realizable systems such as a finite, anisotropic ultrathin film with fourfold in-plane anisotropy. Using a renormalization group analysis, this description is used to calculate the magnetic susceptibility for comparison to experiment.

Anomalous Hall Effect and Magneto-Optic Kerr Effect in Pt/Co/Pt Heterostructure

Magnetic multilayer with large perpendicular magnetic anisotropy (PMA) has attracted sustained interest owing to its importance to fundamental physics and applications. In this work, the high quality of Pt/Co/Pt heterostructures with large PMA was successfully achieved to exhibit a large anomalous Hall effect (AHE) with squared Hall loops. By calculating the proportional relationship between the longitudinal resistivity (ρxx) and the abnormal Hall coefficient (Rs), it is confirmed that the basic mechanism of AHE comes from the external skew scattering (SS) and side jump (SJ), while SS contribution, related to asymmetric scattering from impurities, is dominant in the AHE. Furthermore, the obvious magneto-optical Kerr effect (MOKE) was also observed using the polar MOKE microscopy. The obviously circular magnetic domain can form and propagate in response to the applied out-of-plane magnetic field, resulting in the magnetization reversal of the entire film. This work offers important information in terms of both AHE and MOKE in the ultrathin ferromagnetic films with perpendicular anisotropy, establishing the application foundation for the nonvolatile memories and spintronics.

Ultrafast time-evolution of chiral N\xe9el magnetic domain walls probed by circular dichroism in x-ray resonant magnetic scattering

Non-collinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fueled by possible fine engineering of several magnetic interactions, notably the interfacial Dzyaloshinskii-Moriya interaction. This allows for the stabilization of complex chiral spin textures such as chiral magnetic domain walls (DWs), spin spirals, and magnetic skyrmions among others. We report here on the behavior of chiral DWs at ultrashort timescale after optical pumping in perpendicularly magnetized asymmetric multilayers. The magnetization dynamics is probed using time-resolved circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). We observe a picosecond transient reduction of the CD-XRMS, which is attributed to the spin current-induced coherent and incoherent torques within the continuously varying spin texture of the DWs. We argue that a specific demagnetization of the inner structure of the DW induces a flow of spins from the interior of the neighboring magnetic domains. We identify this time-varying change of the DW texture shortly after the laser pulse as a distortion of the homochiral Néel shape toward a transient mixed Bloch-Néel-Bloch texture along a direction transverse to the DW.

Thickness-dependence of magnetic damping related to two-magnon scattering in ultrathin Ni0.81Fe0.19 films

Magnetic damping is of critical importance for devices due to its influences on magnetization dynamics. When the ferromagnetic layer goes into the nanometer scale, the magnetic damping related to the two-magnon scattering (TMS) effect becomes the main damping in nonmagnetic/ferromagnetic films. Here, we experimentally demonstrated the thickness-dependence of TMS damping, one part dependent on ferromagnetic thickness (), another part independent, in both Cu/NiFe and Pt/NiFe films. The scale of TMS damping can be explained based on the ‘Arias–Mills’ mode, and its origin is the distribution of surface magnetic anisotropy due to interface defects. The rest of TMS damping is independent on is attributed to the more generic defects creating inhomogeneity, and its origin can be sorted into the ‘McMichael–Krivosik’ mechanism. The quantitative results reveal that the -dependent and -independent TMS damping are of equal importance in ultrathin NiFe films. Meanwhile, the approximately equal intrinsic damping was displayed for Cu/NiFe and Pt/NiFe thin films. Our results add a clearer understanding of TMS damping in ultrathin ferromagnetic films.

Simultaneous measurement of the exchange parameter and saturation magnetization using propagating spin waves

The exchange interaction in ferromagnetic ultra thin films is a critical parameter in magnetization-based storage and logic devices, yet the accurate measurement of it remains a challenge. While a variety of approaches are currently used to determine the exchange parameter, each has its limitations, and good agreement among them has not been achieved. To date, neutron scattering, magnetometry, Brillouin light scattering, spin-torque ferromagnetic resonance spectroscopy, and Kerr microscopy have all been used to determine the exchange parameter. Here, we present a method that exploits the wavevector selectivity of Brillouin light scattering to measure the spin wave dispersion in both the backward volume and Damon–Eshbach orientations. The exchange, saturation magnetization, and magnetic thickness are then determined by a simultaneous fit of both dispersion branches with general spin wave theory without any prior knowledge of the thickness of a magnetic “dead layer.” In this work, we demonstrate the strength of this technique for ultrathin metallic films, typical of those commonly used in industrial applications for magnetic random-access memory.

Hard x-ray photoelectron spectroscopy of tunable oxide interfaces

The tunability of the oxygen content in complex oxides and heterostructures has emerged as a key to designing their physical functionalities. Controlling the interface reactivity by redox reactions provides a powerful means to deliberately set distinct oxide phases and emerging properties. We present routes on how to control oxygen-driven redox mechanisms in ultrathin ferro(i)magnetic and ferroelectric oxide films and across oxide interfaces. We address the growth and control of metastable EuO oxide phases, the control of phase transitions of binary Fe oxides by oxygen migration, the in operando determination of NiFe2O4/SrTiO3 interface band alignments, as well as the role of interfacial oxide exchange in ferroelectric HfO 2-based capacitors—uncovered by the unique capabilities of photoelectron spectroscopy and, in particular, using hard x-rays.

Magnetic Properties of Ultrathin As-deposited and Annealed Ta/CoFeB/TaOx Heterostructures

In order to meet the ever-increasing demand of magnetic recording industry, it is important to developferromagnetic thin film heterostructure compatible for magnetic memory device. Here, we have developed ultrathin ferromagnetic film of transition metal borides(CoFeB) which has huge potential to be integrated in magnetic memory devices. In particular, we have studied the surface roughness and magnetic properties of sputter deposited Substrate/1 nm Ta/1.5 nm CoFeB/1nm TaOx heterostructures. Magnetic properties investigation of as-deposited and 300 ⁰C annealed Ta/CoFeB/TaOx heterostructure using vibrating sample magnetometer indicates the presence of in-plane anisotropy in both the film stacks and a reasonable increase in the saturation magnetization of annealed film stack. Importantly, possible boron diffusion as well as partial crystallization of CoFeB layer due to annealing play crucial roles in governing the magnetic properties in these film stacks. These results provide in-depth insight into the factors affecting saturation magnetization of such ultrathin film heterostructures.

Sign-tunable anomalous Hall effect induced by two-dimensional symmetry-protected nodal structures in ferromagnetic perovskite thin films.

Magnetism and spin-orbit coupling are two quintessential ingredients underlying topological transport phenomena in itinerant ferromagnets. When spin-polarized bands support nodal points/lines with band degeneracy that can be lifted by spin-orbit coupling, the nodal structures become a source of Berry curvature, leading to a large anomalous Hall effect. However, two-dimensional systems can possess stable nodal structures only when proper crystalline symmetry exists. Here we show that two-dimensional spin-polarized band structures of perovskite oxides generally support symmetry-protected nodal lines and points that govern both the sign and the magnitude of the anomalous Hall effect. To demonstrate this, we performed angle-resolved photoemission studies of ultrathin films of SrRuO3, a representative metallic ferromagnet with spin-orbit coupling. We show that the sign-changing anomalous Hall effect upon variation in the film thickness, magnetization and chemical potential can be well explained by theoretical models. Our work may facilitate new switchable devices based on ferromagnetic ultrathin films.