First order reversal curves and intrinsic parameter determination for magnetic materials; limitations of hysteron-based approaches in correlated systems

Imaging Nucleation and Propagation of Pinned Domains in Few-Layer Fe5-xGeTe2

Engineering nontrivial spin textures in magnetic van der Waals materials is highly desirable for spintronic applications based on hybrid heterostructures. The recent observation of labyrinth and bubble domains in the near room-temperature ferromagnet Fe5-xGeTe2 down to a bilayer thickness was thus a significant advancement toward van der Waals-based many-body physics. However, the physical mechanism responsible for stabilizing these domains remains unclear and requires further investigation. Here, we combine cryogenic scanning diamond quantum magnetometry and field reversal techniques to elucidate the high-field propagation and nucleation of bubble domains in trilayer Fe5-xGeTe2. We provide evidence of pinning-induced nucleation of magnetic bubbles and further show an unexpectedly high layer-dependent coercive field. These measurements can be easily extended to a wide range of magnetic materials to provide valuable nanoscale insight into domain processes critical for spintronic applications.

Exchange Bias Effect of Ni@(NiO,Ni(OH)2) Core/Shell Nanowires Synthesized by Electrochemical Deposition in Nanoporous Alumina Membranes

Tuning and controlling the magnetic properties of nanomaterials is crucial to implement new and reliable technologies based on magnetic hyperthermia, spintronics, or sensors, among others. Despite variations in the alloy composition as well as the realization of several post material fabrication treatments, magnetic heterostructures as ferromagnetic/antiferromagnetic coupled layers have been widely used to modify or generate unidirectional magnetic anisotropies. In this work, a pure electrochemical approach has been used to fabricate core (FM)/shell (AFM) Ni@(NiO,Ni(OH)₂) nanowire arrays, avoiding thermal oxidation procedures incompatible with integrative semiconductor technologies. Besides the morphology and compositional characterization of these core/shell nanowires, their peculiar magnetic properties have been studied by temperature dependent (isothermal) hysteresis loops, thermomagnetic curves and FORC analysis, revealing the existence of two different effects derived from Ni nanowires' surface oxidation over the magnetic performance of the array. First of all, a magnetic hardening of the nanowires along the parallel direction of the applied magnetic field with respect their long axis (easy magnetization axis) has been found. The increase in coercivity, as an effect of surface oxidation, has been observed to be around 17% (43%) at 300 K (50 K). On the other hand, an increasing exchange bias effect on decreasing temperature has been encountered when field cooling (3T) the oxidized Ni@(NiO,Ni(OH)₂) nanowires below 100 K along their parallel lengths.

Narrow Segment Driven Multistep Magnetization Reversal Process in Sharp Diameter Modulated Fe67Co33 Nanowires

Magnetic nanomaterials are of great interest due to their potential use in data storage, biotechnology, or spintronic based devices, among others. The control of magnetism at such scale entails complexing the nanostructures by tuning their composition, shape, sizes, or even several of these properties at the same time, in order to search for new phenomena or optimize their performance. An interesting pathway to affect the dynamics of the magnetization reversal in ferromagnetic nanostructures is to introduce geometrical modulations to act as nucleation or pinning centers for the magnetic domain walls. Considering the case of 3D magnetic nanowires, the modulation of the diameter across their length can produce such effect as long as the segment diameter transition is sharp enough. In this work, diameter modulated Fe₆₇Co₃₃ ferromagnetic nanowires have been grown into the prepatterned diameter modulated nanopores of anodized Al₂O₃ membranes. Their morphological and compositional characterization was carried out by electron-based microscopy, while their magnetic behavior has been measured on both the nanowire array as well as for individual bisegmented nanowires after being released from the alumina template. The magnetic hysteresis loops, together with the evaluation of First Order Reversal Curve diagrams, point out that the magnetization reversal of the bisegmented FeCo nanowires is carried out in two steps. These two stages are interpreted by micromagnetic modeling, where a shell of the wide segment reverses its magnetization first, followed by the reversal of its core together with the narrow segment of the nanowire at once.

Magnetization Reversal Process and Magnetostatic Interactions in Fe56Co44/SiO2/Fe3O4 Core/Shell Ferromagnetic Nanowires with Non-Magnetic Interlayer

Nowadays, numerous works regarding nanowires or nanotubes are being published, studying different combinations of materials or geometries with single or multiple layers. However, works, where both nanotube and nanowires are forming complex structures, are scarcer due to the underlying difficulties that their fabrication and characterization entail. Among the specific applications for these nanostructures that can be used in sensing or high-density magnetic data storage devices, there are the fields of photonics or spintronics. To achieve further improvements in these research fields, a complete understanding of the magnetic properties exhibited by these nanostructures is needed, including their magnetization reversal processes and control of the magnetic domain walls. In order to gain a deeper insight into this topic, complex systems are being fabricated by altering their dimensions or composition. In this work, a successful process flow for the additive fabrication of core/shell nanowires arrays is developed. The core/shell nanostructures fabricated here consist of a magnetic nanowire nucleus (Fe₅₆Co₄₄), grown by electrodeposition and coated by a non-magnetic SiO₂ layer coaxially surrounded by a magnetic Fe₃O₄ nanotubular coating both fabricated by means of the Atomic Layer Deposition (ALD) technique. Moreover, the magnetization reversal processes of these coaxial nanostructures and the magnetostatic interactions between the two magnetic components are investigated by means of standard magnetometry and First Order Reversal Curve methodology. From this study, a two-step magnetization reversal of the core/shell bimagnetic nanostructure is inferred, which is also corroborated by the hysteresis loops of individual core/shell nanostructures measured by Kerr effect-based magnetometer.

Relation of the average interaction field with the coercive and interaction field distributions in First order reversal curve diagrams of nanowire arrays

First-order reversal curve diagrams, or FORC diagrams, have been studied to determine if the widths of their distributions along the interaction and coercivity axes can be related to the mean-field magnetization dependent interaction field (MDIF). Arrays of nanowires with diameters ranging from 18 up to 100 nm and packing fractions varying from 0.4 to 12% have been analyzed. The mean-field MDIF has been measured using the remanence curves and used as a measuring scale on the FORC diagrams. Based on these measurements, the full width of the interaction field distribution and the full width at half maximum (FWHM) of the FORC distribution profile along the interaction field direction are shown to be proportional to the MDIF, and the relation between them is found. Moreover, by interpreting the full width of the coercive field distribution in terms of the dipolar induced shearing, a simple relation is found between the width of this distribution and the MDIF. Furthermore, we show that the width of the FORC distribution along the coercive field axis is equal to the width of the switching field distribution obtained by the derivation of the DC remanence curve. This was further verified with the switching field distribution determined using in-field magnetic force microscopy (MFM) for very low density nanowires. The results are further supported by the good agreement found between the experiments and the values calculated using the mean-field model, which provides analytical expressions for both FORC distributions.

Magnetization Reversal Mechanism in Exchange-Biased Spring-like Thin-Film Composite

Development of modern spintronic devices requires materials exhibiting specific magnetic effects. In this paper, we investigate a magnetization reversal mechanism in a [Co/Pd_x]₇/CoO/[Co/Pd_y]₇ thin-film composite, where an antiferromagnet is sandwiched between a hard and a soft ferromagnets with different coercivities. The antiferromagnet/ferromagnet interfaces give rise to the exchange bias effect. The application of soft and hard ferromagnetic films causes exchange-spring-like behavior, while the choice of the Co/Pd multilayers provides large out-of-plane magnetic anisotropy. We observed that the magnitude and the sign of the exchange bias anisotropy field are related to the arrangement of the magnetic moments in the antiferromagnetic layer. This ordering is induced by the spin orientation present in neighboring ferromagnetic films, which is, in turn, dependent on the orientation and strength of the external magnetic field.

3D interacting magnetic multilayered nanowire arrays: the emergence and evolution of new first-order reversal curve features

The crucial role of magnetostatic interactions in tuning properties of storage devices based on magnetic nanowires (NWs) has recently been highlighted by advanced characterization techniques including the first-order reversal curve (FORC) analysis, evaluating physical entities constituting conventional 2D NW systems. Herein, FORC diagrams of ferromagnetic (FM)/non-magnetic (NM) multilayered NW arrays are simulated using Monte Carlo calculations, involving magnetostatic interactions between segments in 3D space. The FM length is constant to 6 µm whereas the NM length (L _NM) varies from 10 to 300 nm, significantly influencing interwire and intrasegment interactions of neighboring NWs and coupled segments along the NW length. Intriguingly, this is accompanied with the emergence of two new FORC diagram features in addition to the typical demagnetizing-type feature, indicating complex behavior of the 3D interacting NWs with the same anisotropy field for each FM segment. The FORC coercivity of the emerging features is tracked individually, presenting evolution as a function of L _NM. Our results also evidence an increase in interwire and intrasegment interactions when increasing NW diameter, being in accordance with total magnetostatic energy calculations.

Misaligned anisotropies in spin-valve films studied through magnetoresistance and magnetization measurements

Magnetization and magnetoresistance properties of Py/Cu/Py/IrMn spin valve (SV) films are studied in the framework of the modified antiferromagnetic domain-wall model applied to a granular multidomain system. In the simulations, a misalignment between the in-plane easy magnetization axes of the Py and IrMn is considered. Magnetization and magnetoresistance data are simulated for a number of field orientations and a fairly good agreement with the experiment is found. The same holds for the respective distributions of the coercive and exchange-bias field values determined from magnetoresistance first-order reversal curves (MR-FORC) obtained for magnetic field parallel to the direction of that applied during the sample deposition. Both experimental and theoretical data of the angular variations of the magnetoresistance at constant fields are successfully used to obtain the misalignment angles. For some samples, mostly those with thinner Py layer coupled to IrMn one, our results indicate that the misalignment is due to interfacial magnetic frustration. Moreover, it is shown that MR-FORC diagrams are useful to extract information about the alignment of the grains along the field-cooling directions in SVs, as well as that these can be used to determine the threshold of the continuity of the pinned magnetic layer.

Time resolved FORC analysis and magnetic anisotropy in K0.4[Cr(CN)6][Mn(S)-pn] (S)-pnH0.6 chiral molecular magnets

Effect of the delay between stabilization of magnetic field and recording of the reversal curve on the first order reversal curves (FORC) has been found and analyzed in K_0.4[Cr(CN)₆][Mn(R/S)-pn](R/S)-pnH_0.6 chiral magnet. The difference between the 'delayed' and 'on time' FORC diagrams manifests the effect of magnetic relaxation on switching field of the nonlinear spin configurations presented in crystals (spin solitons, domain walls, etc). The domain walls and spin solitons with different relaxation rates contribute to reversal magnetization. The temperature dependence of magnetic anisotropy governing nonlinear spin excitations is discussed in the frames of the Callen-Callen formalism.

Two-Step Magnetization Reversal FORC Fingerprint of Coupled Bi-Segmented Ni/Co Magnetic Nanowire Arrays

First Order Reversal Curve (FORC) analysis has been established as an appropriate method to investigate the magnetic interactions among complex ferromagnetic nanostructures. In this work, the magnetization reversal mechanism of bi-segmented nanowires composed by long Co and Ni segments contacted at one side was investigated, as a model system to identify and understand the FORC fingerprint of a two-step magnetization reversal process. The resulting hysteresis loop of the bi-segmented nanowire array exhibits a completely different magnetic behavior than the one expected for the magnetization reversal process corresponding to each respective Co and Ni nanowire arrays, individually. Based on the FORC analysis, two possible magnetization reversal processes can be distinguished as a consequence of the ferromagnetic coupling at the interface between the Ni and Co segments. Depending on the relative difference between the magnetization switching fields of each segment, the softer magnetic phase induces the switching of the harder one through the injection and propagation of a magnetic domain wall when both switching fields are comparable. On the other hand, if the switching fields values differ enough, the antiparallel magnetic configuration of nanowires is also possible but energetically unfavorable, thus resulting in an unstable magnetic configuration. Making use of the different temperature dependence of the magnetic properties for each nanowire segment with different composition, one of the two types of magnetization reversal is favored, as demonstrated by FORC analyses.

Ultra-narrow Band Perfect Absorber and Its Application as Plasmonic Sensor in the Visible Region

We propose and numerically investigate a perfect ultra-narrowband absorber with an absorption bandwidth of only 1.82 nm and an absorption efficiency exceeding 95% in the visible region. We demonstrate that the perfect ultra-narrowband absorption is ascribed to the coupling effect induced by localized surface plasmon resonance. The influence of structural dimensions on the optical performance is also investigated, and the optimal structure is obtained with the extremely low reflectivity (0.001) of the resonance dip. The perfect absorber can be operated as a refractive index sensor with a sensitivity of around 425 nm/RIU and the figure of merit (FOM) reaching 233.5, which greatly improves the accuracy of the plasmonic sensors in visible region. Moreover, the corresponding figure of merit (FOM*) for this sensor is also calculated to describe the performance of the intensity change detection at a fixed frequency, which can be up to 1.4 × 10⁵. Due to the high sensing performance, the metamaterial structure has great potential in the biological binding, integrated photodetectors, chemical applications and so on.