Domain Wall Nucleation Research Articles

Impact of Dzyaloshinskii-Moriya interactions on the thermal stability factor of heavy metal/magnetic metal/oxide based nano-pillars

We studied the thermal stability of ultrathin perpendicular magnetized nanodots in the presence of the Dzyaloshinskii-Moriya interaction (DMI) using a minimum energy path method. We find that the smallest energy barrier is associated with the energy path based on domain wall nucleation and propagation down to 25 nm lateral size. We show that the DMI has a detrimental impact on the thermal stability factor of square Pt/Co/AlOx dots, which decreases linearly with the DMI amplitude. Our study reveals that the DMI limits the downscaling of MRAM cells based on heavy metal/ferromagnet/oxide trilayers.

Observation of magnetic domains in Gd-Fe thin films with complementary microscopy techniques

Abstract This paper presents a detailed microscopic observation of magnetic domains in Gd-Fe thin films using magnetic force microscopy and Kerr microscopy with longitudinal sensitivity. Two different sets of Gd-Fe films of various thicknesses were prepared where set-B is richer in Fe compared to set-A. A thickness-dependent transition from in-plane magnetization state to weak out-of-plane state with nano-scale stripe domains has been observed for the films of set-A where the critical thickness of transition was estimated to be around 50 nm . The film with thickness below the critical limit exhibits band domains along the in-plane easy axis and successively transform to the ripple-like states when the external field is applied along the hard axis. The films of set-B exhibit predominant in-plane magnetization irrespective of thicknesses. The ripple domain state to wider band domains has been observed with the increase in film thickness. The domain pattern and their angular variation confirm that the magnetization reversal is dominated by nucleation and propagation of domain walls. Micromagnetic simulation successfully complements the phenomenon of stripe domain formation at higher thicknesses.

Fixed vortex domain wall propagation in FeNi/Cu multilayered nanowire arrays driven by reversible magnetization evolution

While studying domain wall (DW) propagation in magnetic nanowires (NWs) may pave the way for future research and technological applications in recording heads and novel sensors, no attention has been paid to the investigation of magnetic reversal modes in multilayered NWs using angular first-order reversal curve (AFORC) analysis. Here, the magnetization reversal process of uniform FeNi/Cu NW arrays with a diameter of 45 nm electrodeposited in the anodic aluminum oxide template is systematically studied by AFORC analysis for the field angle θ (0° ≤ θ ≤ 90°) and compared with the average magnetic behavior of reversal modes based on conventional hysteresis loop measurements. The FeNi segment aspect ratio is kept constant at about 5, whereas the Cu segment length (LCu) increases from 2.5 to 25 nm. AFORC coercivity increases continuously with increasing θ, indicating that the NWs reverse their magnetization by nucleation and propagation of vortex DW (VDW). At θ = 0°, the respective hysteresis loop coercivity and magnetostatic coupling between FeNi segments along the NW length are reduced by increasing LCu from 2.5 to 25 nm, resulting in an enhancement in the reversible fraction of NWs from 10% to 48%. However, the VDW reversal mode is not influenced by the increase in NW reversibility as a function of θ for the different LCu, which arises from constant properties of the FeNi segments. The AFORC analysis of the reversal mechanism is also found to be in agreement with recent angle-dependent anisotropic magnetoresistance measurements in single multilayered NWs.

Current-driven domain wall motion in a planar nanowire with a square hole

Abstract The current-driven domain wall motion in a planar nanowire with a square hole located at its center is studied by micromagnetic simulations. We show the existence of four regimes that depend both on the geometrical parameters of the hole and the nanowire, as well as on the applied current density. These regimes determine a pinning-depinning pattern characterized by: i) the domain wall remains pinned in its initial position; ii) the domain wall is released from its initial position and reaches the square hole, remaining pinned in that position; iii) the domain wall reaches the hole and passes it without major obstacles, reaching the other end of the wire; iv) the domain wall reaches the hole and suffers a series of complex transformations that annihilate it, giving rise to the nucleation of a new domain wall, which is what reaches the other end of the wire. Our finding could be of practical use in the engineering of logic devices based on the movement of domain walls.

Electric field control of magnetism in Si3N4 gated Pt/Co/Pt heterostructures

In this work, we show the presence of a magnetoelectric coupling in silicon-nitride gated Pt/Co/Pt heterostructures using X-ray photoemission electron microscopy. We observe a change in magnetic anisotropy in the form of domain wall nucleation and a change in the rate of domain wall fluctuation as a function of the applied electric field to the sample. We also observe the coexistence of in-plane and out-of-plane magnetization in Pt/Co/Pt heterostructures in a region around the spin reorientation transition whose formation is attributed to substrate surface roughness comparable to the film thickness; with such domain configuration, we find that the in-plane magnetization is more sensitive to the applied electric field than out-of-plane magnetization. Although we find an effective magnetoelectric coupling in our system, the presence of charge defects in the silicon nitride membranes hampers the systematic electrostatic control of the magnetization.

Realisation of a frustrated 3D magnetic nanowire lattice

Patterning nanomagnets in three-dimensions presents a new paradigm in condensed matter physics and allows access to a plethora of phenomena including robust spin textures, magnetic metamaterials home to defects carrying magnetic charge and ultrahigh density devices that store information in three-dimensions. However, the nanostructuring of functional magnetic materials into complex three-dimensional geometries has thus far proven to be a formidable challenge. Here we show magnetic nanowires can be arranged into 3D frustrated magnetic nanowire lattices by using a combination of 3D polymer nanoprinting and metallic deposition. The fabricated nanowires are single domain and they switch via nucleation and propagation of domain walls. Deep nanoscale magnetic imaging and finite element simulations elucidate the spin texture present on the 3D nanostructured lattice. Our study demonstrates a generic platform for the production of 3D nanostructured magnetic materials allowing the realisation of racetrack memory devices and 3D nanostructured systems that mimic bulk frustrated crystals.

Superferromagnetism and Domain-Wall Topologies in Artificial "Pinwheel" Spin Ice.

For over ten years, arrays of interacting single-domain nanomagnets, referred to as artificial spin ices, have been engineered with the aim to study frustration in model spin systems. Here, we use Fresnel imaging to study the reversal process in "pinwheel" artificial spin ice, a modified square ASI structure obtained by rotating each island by some angle about its midpoint. Our results demonstrate that a simple 45° rotation changes the magnetic ordering from antiferromagnetic to ferromagnetic, creating a superferromagnet which exhibits mesoscopic domain growth mediated by domain wall nucleation and coherent domain propagation. We observe several domain-wall configurations, most of which are direct analogues to those seen in continuous ferromagnetic films. However, charged walls also appear due to the geometric constraints of the system. Changing the orientation of the external magnetic field allows control of the nature of the spin reversal with the emergence of either one- or two-dimensional avalanches. This property of pinwheel ASI could be employed to tune devices based on magnetotransport phenomena such as Hall circuits.

Field-free spin-orbit torque switching of a perpendicular ferromagnet with Dzyaloshinskii-Moriya interaction

Leveraging on interfacial Dzyaloshinskii-Moriya interaction (DMI) induced intrinsic magnetization tilting in nanostructures, a parametric window enabling field-free spin-orbit torque (SOT) magnetization switching in a perpendicular ferromagnet is established. The critical current density (Jc) bounds for SOT switching are highly dependent on the DMI, producing a distorted diamond-shaped region bounded by the Jc-DMI curves. The widest Jc interval is found for DMI values between 0.5 mJ/m2 and 0.8 mJ/m2. Geometrical modulation, of the ferromagnetic layer, reveals that the circular structure is optimum for minimizing the switching energy while maximizing the parametric window. For all the structures investigated, the SOT induced reversal process is via domain wall nucleation and propagation, and the switching is practical at room temperature.

Dipolar domain wall tuning in Fe@Py core-shell cylinders

Abstract We report a theoretical study of the impact of the dipolar interaction on the magnetic phases of Fe@Py rectangular cylindrical core-shells. Our results show that the Fe@Py core-shell geometrical parameters may be chosen to allow significant changes in magnetic phases of the isolated Fe core and Py shell. For instance, an 18 nm height rectangular (81 nm × 117 nm) and 21 nm thick Py shell inhibits the formation of the intrinsic magnetic vortex of an isolated 18 nm height (33 nm × 33 nm) square Fe core. We show that the geometric parameters of flat Fe@Py core-shell cylinders can be chosen in such a way to control the nucleation of domain walls in the Py shell. Furthermore, it allows fine-tuning the domain wall position and the domain wall width on the Py shell.

Structural and magnetic properties of nanocrystalline Zr-Co-Nb-Si-B rare earth free permanent magnets fabricated by spark plasma sintering

Abstract Rare earth free nanocrystalline Zr16Co78-xNbxSi3B3 (x = 0–3) bulk magnets were fabricated from melt spun ribbons with high density by employing spark plasma sintering (SPS) technique. The effect of SPS temperature on structural and magnetic properties was investigated. The hard magnetic Zr2Co11 phase which was present in the as-spun ribbons retained after SPS, indicating no phase decomposition. The magnetic properties were found to depend on consolidation temperature and Nb concentration. Optimum magnetic properties of coercivity of 3.3 kOe with saturation magnetization of 52 emu/g were obtained for x = 1 magnet consolidated at 600 °C. SPS magnets display a moderate magnetic texture and the coercivity mechanism is governed by combination of both domain wall pinning and nucleation of reverse domains.

Frequency dependence of the relaxor-to-ferroelectric transition under applied electrical and mechanical fields

Relaxor ferroelectrics are a unique material class with underlying microscopic processes that are yet to be fully understood. In particular, the electrical and mechanical field-modulated transition from the relaxor state to long-range ferroelectric order has been found to be the origin of the large unipolar strain response in some lead-free ferroelectric systems. Importantly, the dynamics of this transition are different than the domain wall nucleation and growth found in normal ferroelectrics, significantly changing the frequency response. In this study, the electromechanical behavior of a (Na1/2Bi1/2)TiO3-based lead-free relaxor ferroelectric is shown under applied external mechanical and electric field over several orders of magnitude in loading rate. Depending on the loading history, it is possible to directly investigate both the relaxor-to-ferroelectric transition as well as domain wall motion in the same sample. These data demonstrate the variations in frequency response between these nonlinear hysteretic processes.

Domain wall nucleation and propagation in spin-transfer torque switching with thermal effects

We conduct micro-magnetic simulations to study spin-transfer torque induced magnetization switching in perpendicular magnetic tunneling junctions. The effects of current densities and temperatures on the switching processes are studied in details. We then proposed an approach to compute the deterministic switching time by taking thermal-effect into account. The switching time is less temperature-dependent under higher current density; however, as the current density decreases, the effect of temperature on the switching time becomes more and more significant. The switching process with micro-magnetic simulations is shown to be via domain wall nucleation and propagation. The phenomena are consistent with the recent experimental found-out. We further propose a method to compute the switching time based on domain wall nucleation and propagation theory, and compare the switching time with those from macro-spin approximation. It is found the switching times from the micro-magnetic simulations are much shorter than that from the macro-spin approximations. Macro-spin approximation over-estimates the switching times due to its coherent rotation assumptions.

Large-scale 3-D interconnected Ni nanotube networks with controlled structural and magnetic properties

Large-scale, electrically interconnected three-dimensional (3-D) Ni crossed nanotube networks have been fabricated using an electrochemical dealloying method within the crossed nanopores of polymer host membranes. This method paves the way for the easy and cost-effective fabrication of 3-D magnetic NT networks with precise spatial arrangement and diameter and wall thickness of 10–100 nm controlled individually. The excellent control over geometrical parameters and morphological features of the Ni crossed nanotube networks leads to tunable magnetic and magneto-transport properties. Particularly, the low field magneto-transport behavior is consistent with the expected vortex-like states formed in different segments of the nanotube scaffold, whereas nucleation of domain walls at the intersection of the nanowire segments play a dominant role in the solid crossed nanowire networks counterpart. The present 3-D networks of nanomagnets are of special interest due to their potential for memory devices, computing architectures, sensing and biomedical applications.

Roles of Joule heating and spin-orbit torques in the direct current induced magnetization reversal

Current-induced magnetization reversal via spin-orbit torques (SOTs) has been intensively studied in heavy-metal/ferromagnetic-metal/oxide heterostructures due to its promising application in low-energy consumption logic and memory devices. Here, we systematically study the function of Joule heating and SOTs in the current-induced magnetization reversal using Pt/Co/SmOx and Pt/Co/AlOx structures with different perpendicular magnetic anisotropies (PMAs). The SOT-induced effective fields, anisotropy field, switching field and switching current density (Jc) are characterized using electric transport measurements based on the anomalous Hall effect and polar magneto-optical Kerr effect (MOKE). The results show that the current-generated Joule heating plays an assisted role in the reversal process by reducing switching field and enhancing SOT efficiency. The out-of-plane component of the damping-like-SOT effective field is responsible for the magnetization reversal. The obtained Jc for Pt/Co/SmOx and Pt/Co/AlOx structures with similar spin Hall angles and different PMAs remains roughly constant, revealing that the coherent switching model cannot fully explain the current-induced magnetization reversal. In contrast, by observing the domain wall nucleation and expansion using MOKE and comparing the damping-like-SOT effective field and switching field, we conclude that the current-induced magnetization reversal is dominated by the depinning model and Jc also immensely relies on the depinning field.

Time-resolved velocity of a domain wall in a magnetic microwire

The dynamic process of nucleation, propagation, and braking of a single domain wall (DW) has been systematically determined in a Fe-based magnetostrictive microwire under an applied axial magnetic field. While in previous reports the Sixtus-Tonks experiments have provided partial information on the process (i.e., the average velocity), in the present study we report on the instantaneous processes involved in the propagation of the DW, as well as the transient process during the DW depinning. The experimental measurements were carried out using the spontaneous Matteucci effect induced during DW propagation due to the small helical magnetization component created during the fabrication process.

Thermal gradients for the stabilization of a single domain wall in magnetic nanowires

By means of Monte Carlo simulations we studied field driven nucleation and propagation of transverse domain walls (DWs) in magnetic nanowires subjected to temperature gradients. Simulations identified the existence of critical thermal gradients that allow the existence of reversal processes driven by a single DW. Critical thermal gradients depend on external parameters such as temperature, magnetic field and wire length, and can be experimentally obtained through the measurement of the mean velocity of the magnetization reversal as a function of the temperature gradient. Our results show that temperature gradients provide a high degree of control over DW propagation, which is of great importance for technological applications.

Magnetization reversal process and evaluation of thermal stability factor in Cu doped granular L1 FePt films

Curie temperature TC distributions and magnetization reversal mechanism in Cu doped L10 FePt granular films is investigated as a function of film thickness in the range of ∼5–12 nm with Cu mol. % varying in the range of 0%–6%. It is shown that Cu doping increases the FePt tetragonality and chemical ordering. For Cu doped FePt-X films, coercivity (HC) exhibits a non-monotonic behavior with increasing film thickness, i.e., HC increases initially up to tcr ∼ 7 nm, and decreases thereafter. We attribute this behavior to the change in magnetization reversal mechanism from coherent to an incoherent (domain-wall driven) mode. While in un-doped films, the domain-walls nucleate at the grain boundaries, in doped films the Cu atoms may act as domain-wall nucleation and pinning sites, isolating magnetic spin clusters of reduced dimensionality with respect to the physical grain size. This is experimentally supported by a much poorer dependence of the AC susceptibility (both, real and imaginary components) on the film thickness above 7 nm than in the case of un-doped films. The formation of magnetic spin clusters inside the grains as a consequence of the reduced coupling between Fe-Fe and Fe-Pt-Fe atoms with increasing Cu doping can explain the experimentally evidenced reduction of both, the film Curie temperature, TC, and intrinsic anisotropy energy density, KC, with increasing Cu doping.

The influence of annealing on domain wall propagation in bistable amorphous microwire with unidirectional effect

The effect of gradual annealing on the domain wall mobility (velocity), nucleation, critical depinning and propagation fields in amorphous FeSiB microwires has been studied. A new experimental set-up, presented in this paper, allows measurement of average domain wall velocity for four different conditions and detection of the presence of unidirectional effect in wall propagation without manipulation of the microwire. The proposed interpretation is that a domain wall is considered as a relatively long object which can change its axial dimension due to inhomogeneity of damping forces acting on the wall during its propagation. It is demonstrated that unidirectional effect in domain wall propagation can be strongly reduced by annealing the wire at temperatures higher than 350 °C.

Domain Wall Nucleation in Ferromagnetic Nanowire With Perpendicular Magnetization Stimulated by Stray Field of V-Shaped Magnetic Particle

We report the results of micromagnetic simulations of domain wall (DW) nucleation and pinning/depinning processes in ferromagnetic planar structure consisting of nanowire (NW) with perpendicular anisotropy and special V-shaped nanoparticle (NP) with in-plane anisotropy located on top of NW. The magnetization reversal features of this system in an external magnetic field are investigated depending on the direction of particle magnetic moment. Possible variants of magnetic logic cells (LCs) based on such system are discussed.

Graphene nanoribbon as an elastic damper

Heterostructures composed of dissimilar two-dimensional nanomaterials can have nontrivial physical and mechanical properties which are potentially useful in many applications. Interestingly, in some cases, it is possible to create heterostructures composed of weakly and strongly stretched domains with the same chemical composition, as has been demonstrated for some polymer chains, DNA, and intermetallic nanowires supporting this effect of two-phase stretching. These materials, at relatively strong tension forces, split into domains with smaller and larger tensile strains. Within this region, average strain increases at constant tensile force due to the growth of the domain with the larger strain, at the expense of the domain with smaller strain. Here, the two-phase stretching phenomenon is described for graphene nanoribbons with the help of molecular dynamics simulations. This unprecedented feature of graphene that is revealed in our study is related to the peculiarities of nucleation and the motion of the domain walls separating the domains of different elastic strain. It turns out that the loading–unloading curves exhibit a hysteresis-like behavior due to the energy dissipation during the domain wall nucleation and motion. Here, we put forward the idea of implementing graphene nanoribbons as elastic dampers, efficiently converting mechanical strain energy into heat during cyclic loading–unloading through elastic extension where domains with larger and smaller strains coexist. Furthermore, in the regime of two-phase stretching, graphene nanoribbon is a heterostructure for which the fraction of domains with larger and smaller strain, and consequently its physical and mechanical properties, can be tuned in a controllable manner by applying elastic strain and/or heat.