Magnetization Reversal Mechanism Research Articles

Micromagnetic Simulations of Chaotic Ferromagnetic Nanofiber Networks.

Electrospinning can be used to create nanofibers with diameters of typically a few tens to a few hundred nanometers. While pure polymers are often electrospun, it is also possible to use polymer blends or to include nanoparticles. In this way, e.g., magnetic nanofiber networks can be created with a certain diameter distribution, random fiber orientations, and random crossing positions and angles. Here we present for the first time micromagnetic simulations of small parts of stochastically oriented nanofiber networks. Magnetization reversal mechanisms are investigated for different local spatial distributions; mutual influences of neighboring magnetic fibers due to dipolar interactions are depicted. This study serves as a base for the possible use of such stochastic nanofiber networks in the research area of neuro-inspired materials.

Magnetization switching induced by magnetic field and electric current in perpendicular TbIG/Pt bilayers

Magnetic insulators (MIs) have attracted great attention because of their low Gilbert damping, long spin transmission length, and no Ohmic loss. In this study, the high quality TbIG films with perpendicular magnetic anisotropy were epitaxially grown on GGG (111) substrates. In TbIG/Pt bilayers, the angular dependence of coercivity is found to obey the Kondorsky model, suggesting the magnetization reversal mechanism of magnetic domain nucleation and expansion. The transverse component of spin Hall magnetoresistance (SMR), which is analogous to the planar Hall resistance in a ferromagnetic metal, is found to be about seven times larger than the SMR-induced anomalous Hall resistance (analogous to the anomalous Hall resistance in a ferromagnetic metal). Moreover, the phase diagrams of the current-induced magnetization switching with different angles and magnitudes of the assisting magnetic field were drawn for the TbIG/Pt bilayers. The current-induced damping-like effective field (HDL) characterized by the harmonic measurements was evaluated to be about 164 Oe/108 A cm−2. By providing a comprehensive investigation of magnetization switching behaviors in MIs, our results will promote the application of ultralow-dissipation MI based spintronic devices.

Improved magnetic performance of Cobalt-based ribbons by nanocrystallization through magnetic annealing

Phase transformation driven soft magnetic properties have been correlated through different stages of nano-crystallization of Co-based amorphous alloys, via transverse magnetic annealing, by combining structural, magnetothermal, domain imaging, and AC/DC magnetometry techniques. The nano-crystallization starts by nucleation and growth process of soft magnetic meta-stable, thermodynamically favored, Co23B6 phase with less nucleation activation energy compared to other stable phases. In the second crystallisation stage, Co2B and Co3B, as a semi-hard magnetic phase, are identified in the alloys, magnetically annealed at 525 and 550 °C, respectively. Field-induced anisotropy dominates over the residual contributions of magneto-crystalline anisotropy of the phases, precipitated after field annealing at 510, 515, and 525 °C. The anomalous loss is significantly reduced as by annealing in a transverse magnetic field due to the reorientation of the preferred magnetisation axis, and consequently, change in dominant magnetization reversal mechanism from domain wall motion to magnetization rotation. In addition, magnetic annealing causes a measurable decrease in the domain width, which, in turn, promotes pinning and inhibits domain wall motion, thus further favours coherent domain rotation as the primary mechanism of magnetization. The combined mechanism of nanocrystallisation and coherent magnetisation rotation accounts for a 70% decrement in the anomalous loss in the so-processed ribbons at 525 °C, which renders them attractive for applications in mid- and high-frequency power supplies and inverters.

Tailoring dual reversal modes by helicity control in ferromagnetic nanotubes

We investigate the effects of the competition between exchange ($J$) and dipolar ($D$) interactions on the magnetization reversal mechanisms of ferromagnetic nanotubes. Using first atomistic Monte Carlo (MC) simulations for a model with Heisenberg spins on a cylindrical surface, we compute hysteresis loops for a wide range of the $\gamma=D/J$ parameter, characterizing the reversal behavior in terms of the cylindrical magnetization components and the vorticity parameter along the tube length. For $\gamma$'s close to the value for which helical (H) states are stable at zero applied field, we show that the hysteresis loops can occur in four different classes that are combinations of two reversal modes with well-differentiated coercivities with probabilities that depend on the tube length and radius. This variety in the reversal modes is found to be linked to the metastability of the $H$ states during the reversal that induce different paths followed along the energy landscape as the field is changed. We further demonstrate that reversal by either of the two modes can be induced by tailoring the nanotube initial state so that vortices with equal or contrary chirality are formed at the ends, thus achieving low or high coercive fields at will without changing $\gamma$. Finally, the results of additional micromagnetic simulations performed on tubes with similar aspect ratio show that dual switching modes and its tailoring can also be observed in tubes with more microscopic dimensions.

Magnetic domain structure and magnetization reversal in (Co/Ni) and (Co/Pd) multilayers

The temporal behavior of magnetization reversal mechanism and domain structures of (Co/Ni) and (Co/Pd) multilayers have been studied using magneto-optical Kerr effect microscopy and magnetometry measurements. The domain size and nucleation field of (Co/Ni) multilayers were found to be reduced as the number of bilayers increases. On the other hand, (Co/Pd)×10 multilayer has a much larger domain size, faster magnetization switching, and a higher nucleation field than that in (Co/Ni). From the imaging of magnetic domains, two different types were observed, namely, dendritic domains for (Co/Ni) and blocks type for (Co/Pd). From time dependence of magnetization reversal at a fixed applied magnetic field, it was revealed that the reversal mechanism in (Co/Ni) multilayer occurs by nucleation followed by domain propagation. Whereas, for (Co/Pd) multilayers, few large domains were observed, and they were found to expand at a faster rate until a complete saturation.

First-order reversal curves (FORCs) of nano-engineered 3D Co-Fe structures

In this paper we present first-order reversal curve (FORC) diagrams of ensembles of three-dimensional Co3Fe nanostructures as 2 × 2 arrays of nano-cubes and nano-trees. The structures are fabricated and investigated by an advanced platform of focused electron beam induced deposition combined with high-resolution detection of magnetic stray fields using a home-built micro-Hall magnetometer based on an AlGaAs/GaAs heterostructure. The experimental FORC diagrams are compared to macrospin simulations for both geometries at different angles of the externally applied magnetic field. The measured FORC diagrams are in good agreement with the simulated ones and reflect non-uniform magnetization reversal dominated by multi-vortex states within, and strong magnetic coupling between, the building blocks of our nanostructures. Thus, a FORC analysis of small arrays of 3D magnetic nanostructures provides more detailed insights into the mechanisms of magnetization reversal beyond standard major hysteresis loop measurements.

Magnetization reversal mechanisms in Fe/NiO bilayers grown onto nanoporous alumina membranes and Si wafers

We report on the magnetization reversal measured in two ferromagnetic/antiferromagnetic (F/AF) bilayer series: Fe/NiO/Al2O3 (nanoporous membranes, series N) and Fe/NiO/Si (wafers, series W). The Fe and NiO layers were deposited by pulsed laser ablation and magnetron sputtering, respectively. In both series different oxygen partial pressures were implemented in the Ar plasma during the NiO growth. The morphologies of both series (as imaged by atomic force microscopy) reflect those of their substrates and, particularly, the series N samples exhibit a six-fold columnar growth around each one of the membranes nanopores. The in-plane hysteresis loops measured upon field cooling the samples down to different temperatures in the range from 50 K up to 290 K evidenced i) 50 K, 0% oxygen coercivities that decreased markedly in both series samples with the increase of the Fe layer thickness (particularly the Fe 5 nm, series N sample exhibited a coercivity larger than the Fe magneto-crystalline anisotropy field), ii) a decrease of the coercivity with the increase of the NiO deposition oxygen partial pressure, observed in both series independently of the Fe layer thickness, iii) low temperature coercivities larger in the series N samples than in the series W ones. Our data are analyzed in correlation to the deposits morphology and in terms of the occurrence of either propagation mediated reversal (collective mode linked to spatially averaged interactions at the F/AF interface) or localized switching (defect ruled mechanism taking place in a spatially confined environment). It is concluded that i) the magnetization reversal mechanism active in series W samples corresponds to a weak pinning regime propagation of walls interacting with uncompensated moments at the F/AF interface, ii) in series N samples, the magnetization reversal does not involve propagation, and iii) in the latter series the reversal events are spatially restricted to the dot-like tops of the NiO columns surrounding the membrane pores.

Dominance of Shape Anisotropy among Magnetostatic Interaction and Magnetocrystalline Anisotropy in Electrodeposited (FeCo)1−xCux (X = 0.1–0.5) Ternary Alloy Nanowires

The (FeCo)1−xCux (X = 0.1–0.5) nanowire (NWs) arrays were synthesized through AC-electrochemical deposition in anodized aluminum oxide (AAO) templates. The morphology and composition of the synthesized (FeCoCu) NWs were found from scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDS) that confirmed the diameter of the NWs is in the range of 50–60 nm and the composition of FeCoCu alloy NWs, respectively. The XRD measurement of the NWs was carried out in AAO templates along with Al-substrate. The FeCoCu alloy NWs show BCC phase along with reflection peak (311) in all samples which corresponds to Al-substrate. The crystallite size displays overall increasing trend confirmed through XRD using Scherrer equation. The magnetic analysis reveals that by increasing the Cu concentration in FeCoCu alloy NWs, the saturation magnetization and coercivity (Hc) decreased. The results of MH loops confirmed that easy axis lies along the NWs and hard axis perpendicular to the NWs due to dominance of shape anisotropy. Magnetization reversal mechanism occurs by nucleation mode with the motion of domain walls. The frequency-dependent dielectric analysis investigated thoroughly using impedance and the behavior of dielectric parameters has been examined using Maxwell-Wagner’s model and Koop’s theory. The value of dielectric constant decreases progressively with increasing frequency and vice versa. It is also observed that with increase in Cu concentration, the dielectric constant also decreases due to higher conductivity of Cu than Fe and Co.

Crystal structure and magnetic properties of wafer-size Co nanostrip arrays self-organized on a step-bunched surface of Si(111)-5.55 × 5.55-Cu/(Cu, Cu2Si, W-O) buffers

Structure and magnetization reversal of Co nanostrips self-organized on a Si(111)-5.55 × 5.55-Cu step-bunched surface covered by Cu, Cu2Si and W-O buffer layers are systematically investigated and characterized in details. Optimal conditions for growth of well-ordered arrays of mm-long straight nanostrips with hexagonal close-packed and face-centered cubic crystal phases are defined. We find that the Co nanostrips with epitaxial and multiepitaxial structure grows on monocrystalline buffer layers of Cu and Cu2Si, respectively. The amorphous W-O buffer promotes the growth of polycrystalline Co nanostrips. We reveal the origin and compute the magnitude of magnetic anisotropy in Co nanostrips with different crystal phases. Structure-dependent magnetization reversal modes of nanostrip arrays are determined analytically using angular dependences of the coercive force. Our analysis of distributions of the switching and interaction fields by the first-order reversal curve diagram method specifies contributions of global and local interaction fields. Micromagnetic simulations considering the magnetostatic interaction between nanostrips demonstrate the crystal structure-dependent magnetization reversal mechanisms based on propagation of domain walls, whose type (symmetric or asymmetric transverse domain wall, vortex domain wall) and velocity are determined by the magnetic parameters, interaction fields and driving magnetic fields.

Diameter dependent structural and magnetic properties of CoNi alloy nanotubes

Cobalt-Nickel (CoNi) alloy nanotubes (NTs) with diameters 50 nm (D1), 100 nm (D2), 150 nm (D3), and 200 nm (D4) have been fabricated using Anodic Aluminum Oxide (AAO) membranes. X-ray diffraction spectroscopy (XRD), field-emission scanning electron microscopy (FE-SEM), vibrating-sample magnetometer (VSM), and physical property measurement system (PPMS) analysis methods were employed to systematically investigate the structural, morphological and magnetic properties of CoNi NTs samples. Magnetization reversal mechanism has been explained by angular dependence of coercivity. Low temperature studies have shown that coercivity Hc of nanotubes follows the thermal activation model. Temperature dependence of Hc follows the 3/2 power law for the field dependence of energy barriers.

Wave reversal mode: A new magnetization reversal mechanism in magnetic nanotubes

We have investigated the magnetic properties of 14 nm thick and 1 μm long nickel and permalloy nanotubes with external diameters of 40 and 100 nm as a function of the angle θ at which the external magnetic field is applied. Our results show that the coercivity of 40 nm diameter nickel nanotubes follows a non-monotonic behavior from θ = 0° up to θ = 60°, while that corresponding to permalloy displays an increasing monotonic trend at the same angular range. At θ = 90°, both materials evidence a sharp drop of the coercivity to zero, indicating that the reversal mechanism has changed to a pseudo-coherent rotation. On the other hand, nickel and permalloy nanotubes with 100 nm in diameter exhibit a similar angular dependence of the coercivity, reversing their magnetization through the nucleation and propagation of vortex domain walls for angles lower than 75°. For θ = 90°, a novel striking mechanism, the wave reversal mode (W), arises. This phenomenon leads to an unusual S-type shape in the hysteresis curves at those given parameters, which is until now an effect that has not been reported for these nanostructures.

A two-step fabrication and characterization of 1D hybrid ferromagnetic-multiferroic Ni – BiFe1-xCoxO3 core-shell nanostructures

Hybrid core-shell nanowires (NWs) with nickel as core and Co doped BiFeO3 (BFCO) as shell were synthesized by employing a two-step methodology. A noticeable exchange bias Hex phenomenon has been observed which clearly demonstrated the antiferromagnetic functionality of BiFeO3 shell material, was also observed. Both coercivity and exchange bias has shown slight increase after Co doping in BFO. This increase might be due to reduction in domain sizes of ferromagnetic material between layers of core-shell nanowires. At lower temperatures, the super-paramagnetic contributions play very important role to contribute to the overall magnetic behavior of core-shell nanostructures. It has been observed that easy magnetization axis was aligned along the NWs’ axis because of dominant shape anisotropy. Angular dependence of coercivity Hc has shown the coherent mode of magnetization reversal mechanism. This two-step methodology can be used to fabricate other similar hybrid nanostructures which can further be investigated for their possible implementation in device applications.

Structural and Uniaxial Magnetic Anisotropy of Co1-XMgX(X = 0.04–0.12) Nanowires in Alumina Templates

The nanoporous anodized aluminum oxide (AAO) templates were synthesized by two-step anodization in a solution of 0.5 M phosphoric acid at 60 V and temperature T ~ 0 °C. The CoMg nanowires were synthesized by template-based AC Electrodeposition technique at different voltages by AC source 13–17 V at room temperature. These CoMg nanowires were investigated via different characterization techniques such as scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and vibrating sample magnetometer (VSM). The SEM result shows that nanowires have average diameter about 150 nm and length about 23 μm. The EDX reveals that nanowires only consisted of Mg and Co atoms, and atomic ratio of Mg/(Co + Mg) showed increasing tendency with deposition voltage which depicts that higher AC electrodeposition voltage is more suitable for Mg-rich deposition. The XRD results show that CoMg nanowires have hexagonal crystal structure with polycrystalline nature. The MH loops confirm that nanowires are soft ferromagnetic with uniaxial magnetic anisotropy. It is observed from angular dependence of coercivity that magnetization reversal mechanism occurs by a combination of curling and nucleation mode. The squareness (Mr/Ms) is found to be 0.24 along nanowires and 0.16 perpendicular to nanowire axis suggesting that easy magnetization direction lies along the nanowires. The effective anisotropy is observed along the nanowire axis due to dominance of shape anisotropy and suppression of dipole-dipole interactions among nanowires.

Effect of CoSi2 interfacial layer on the magnetic properties of Si|CoSi2|Sm-Co thin films

Abstract Magnetic thin films with a layer sequence of Si|CoSi2|Sm-Co were grown by direct sputter deposition at elevated temperatures, through interfacial diffusion between Si (1 0 0) substrate and the overlying Sm-Co layer. HR-TEM analysis revealed the occurrence of CoSi2-interfacial layer close to the Si-substrate surface, with controllable thicknesses of ~20 and 35 nm at deposition temperatures: 450 and 500 °C, respectively. XRD studies confirmed the crystallization of Sm2Co17 and SmCo5 magnetic phases accompanied by the other phases such as CoSi2 and SmCoSi2 due to the intermixing of Co and Si -atoms at higher deposition temperatures. The measured coercivity values are found to be increased from 8.7 to 11.6 kOe at higher CoSi2-layer thickness. The angular-dependent hysteresis measurements demonstrated a distinct isotropic and uniaxial magnetic anisotropy characteristics for the Sm-Co films consisting of 35 and 20-nm thick CoSi2 interfacial layers, respectively and the associated magnetization reversal mechanisms are discussed using the Stoner-Wohlfarth model. The temperature coefficients of remanence (α) and coercivity (β) were determined from the temperature-dependent hysteresis curves. The Sm-Co films consisting of 35-nm thick CoSi2-layer exhibited a better thermal stability with ‘α’ and ‘β’ values of 0.35 ± 0.05%/°C and −0.13 ± 0.02%/°C, respectively. The results of present study provide splendid opportunities for exploiting the potential of CoSi2 as an under layer, for growing the Sm-Co films towards high-temperature applications.

Fabrication and temperature-dependent magnetic properties of large-area L10-FePt/Co exchange-spring magnet nanopatterns

Abstract Exchange-spring (ES) magnets L10-FePt/Co nanopatterns with out-of-plane texture have been fabricated by using nanoimprint lithography (NIL) in combination with inductively coupled plasma reactive etching process. The patterns are regularly arranged 60 nm diameter size nanomagnets, and different thickness of the Co soft magnet layers hysteresis loops of the different ratio of Co soft magnet addition on L10-FePt hard nanomagnet was measured by superconducting quantum interference device (SQUID) magnetometry in the temperature range from 50 K to 350 K. The influence of exchange-coupling between hard and soft magnet by thickness control on magnetic properties of these nanocomposite patterns was investigated. The magnetic property results have proved to be possible tuned magnetic properties of magnet nanopatterns. According to exchange-coupling between two-phase quantities, the magnetic properties and magnetization reversal mechanism depend significantly. The exchange-spring magnet nanopatterns have a smaller coercivity, remanence, and magnetic anisotropy, while a higher saturation polarization than pure hard magnet nanopattern due to the soft magnetic layer. The temperature dependence of the coercivity was analyzed by the universal relation within the framework of micromagnetism. The obtained microstructural parameters α and Neff give evidence for a reduction of the coercivity by the exchange coupling in the exchange-spring magnets.

Nanoscale magnetization inhomogeneity within single phase nanopillars.

We report observation of a radial dependence in the magnetic anisotropy of epitaxially strained CoFe2O4 nanopillars in a BaTiO3 matrix. This archetypal example of a multiferroic heterostructure with a self-assembling three-dimensional architecture possesses significant out-of-plane uniaxial magnetic anisotropy. The anisotropy originates from the large magnetostriction of CoFe2O4 and the state of stress within the nanocomposite. Magnetometry suggests the existence of two magnetic phases with different anisotropies. Micromagnetic simulations of a core-shell magnetic anisotropy qualitatively reproduce features of the magnetic hysteresis and elucidate the magnetization reversal mechanism: The magnetization initially reorients within the pillar core, followed by that of the shell. This is consistent with polarized small-angle neutron scattering which can be described by a CoFe2O4 magnetization that is nonuniform on nanometer length scales. As the length scale of inhomogeneity of the magnetic anisotropy is similar to estimates of the relaxation of the displacement field from the CoFe2O4-BaTiO3 interface, stress and its influence on structure provide an important route to new functionality of vertically aligned nanopillars for applications in low-power memory, computing, and sensing.

Control of vortex chirality in bi-component magnetic nanodisks

In the last decade, special attention has been paid to the nucleation and control of the vortex state in magnetic nanostructures, for possible applications in nonvolatile magnetic random access memories and logic networks. Here, we study bicomponent magnetic nanodisks, in both single and 2D array arrangements, as potential storage systems where the information unit is represented by vortex chirality (magnetization rotational direction). We mainly focus on 300 nm diameter disks made of a permalloy lens and an iron crescent, with variable locations of the permalloy-iron interface. Through an extensive micromagnetic modeling analysis, we demonstrate the possibility of tuning, via the application of a magnetic field parallel to the interface, the nucleation of the vortex (always occurring in the permalloy region), its motion up to the expulsion site, and its chirality. Moreover, we find that the vortex stability is preserved over a wide field range also for strongly packed disks, due to the reduced effects of interdisk magnetostatic coupling. The main features of the magnetization reversal mechanism are preserved by reducing the disk diameter up to about 60 nm, making possible their exploitation in concept devices for magnetic encoding and logic computation.

Influence of the magnetic field sweeping rate on magnetic transitions in synthetic ferrimagnets with perpendicular anisotropy

The evolution of the switching field between stable states in MgO/CoFeB/Ta/CoFeB/MgO/GaAs and Pt/Co/Ir/Co/Pt/Ta/SiO2 synthetic ferrimagnets with perpendicular anisotropy as a function of the magnetic field sweeping rate (MFSR) of 0.1–104 Oe/s has been studied. The most significant effect of the MFSR is an inversion of the interstate transition sequence. In the MgO/CoFeB/Ta/CoFeB/MgO/GaAs heterostructure, the increase in MFSR switches the dominant mechanism of magnetization reversal from propagation of domain walls to a nucleation of reversed magnetization areas. In Pt/Co/Ir/Co/Pt/Ta/SiO2, the MFSR affects the final domain state of the transition, starting from the initial antiparallel configuration of the synthetic ferrimagnet.

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.

Magnetic behaviour of $$^{57}$$Fe/Co/Al multilayers deposited on a glass substrate

Multilayers of \(^{57}\hbox {Fe/Co/Al}\) 20 and 40 trilayers (TLs) prepared on a glass substrate using ion beam sputtering deposition techniques have been studied. Magnetization curves obtained at different angles show the anisotropic nature of both the samples. Both samples show strong in-plane uniaxial anisotropy. Negative \(\delta m\) values suggest that dipolar type interactions between grains contribute to the in-plane uniaxial anisotropy. Angular dependence of coercivity indicates that the magnetization reversal mechanism is different for both samples. The variation in coercivity with azimuthal angle is well reproduced in the 20 TL sample using the modified Kondorsky model and by the Stoner–Wohlfarth model in the 40 TL sample.