Magnetization Reversal Mode Research Articles

New insights into nano-magnetism by spin-polarized scanning tunneling microscopy

We study the magnetization reversal and the position dependence of the spin-dependent electronic properties of nm small bilayer Co islands on Cu(111) by spin-polarized scanning tunneling microscopy in magnetic fields at low temperatures of 8K. The analysis of the energy barrier of magnetization reversal from measurements of the switching field suggests a crossover of the magnetization reversal mode with increasing island size around 7500 atoms from exchange-spring behavior to domain wall formation. The quantitative analysis of the island size dependence of the energy barrier indicates an inhomogeneous magnetic anisotropy of the island. The island rim is magnetically soft, whereas the center shows a pronounced effective anisotropy of 0.148meV/atom. We speculate that this inhomogeneity of the magnetic anisotropy might be a consequence of the spatial dependence of the spin-dependent electronic properties. We measure a spin-polarization and a tunnel magneto resistance ratio of opposite sign at the rim as compared to the island center.

Magnetic anisotropy in ordered textured Co nanowires

The magnetization reversal in ordered arrays of Co nanowires with tailored hcp-phase texture, controlled by pH synthesis and nanowires length, has been investigated. The angular dependence of coercivity has been experimentally determined for different crystal textures, and the corresponding magnetization reversal mode is interpreted by analytical modelling. The results show that reversal takes place by propagation of a transverse-like domain wall mode. The fitting of experimental and calculated data allows us the quantitative evaluation of the magnetocrystalline anisotropy constant strength whose magnetization easy direction evolves from parallel to the wires toward in-plane orientation with the change of hcp-phase texture.

Domain wall configuration and magneto-transport properties in dual spin-valve with nanoconstriction

We investigated the effect of external field on magneto-transport properties of synthetic antiferromagnet dual spin-valve with nanoconstriction with focus on domain wall (DW) configuration and magnetization reversal process. As magnetic field rotated from in-plane to out-of-plane along hard axis configuration, the magnetization reversal mode changed from a vortex to a transverse type, and a multistep switching process appeared due to the development of a transverse magnetization reversal mode with DW pushing towards the higher anisotropy region. The difference in the shape of nanoconstriction made an asymmetric energy barrier to the DW propagation which resulted in an asymmetry depinning field.

Magnetic anisotropy and magnetization reversal in Co/Cu multilayers nanowires

The Co/Cu multilayer nanowires fabricated in an array using anodized aluminum oxide (AAO) template by electrodeposition method, have been investigated. It has been observed that the magnetization reversal mode and magnetic anisotropy depend upon the Co and Cu layer thicknesses. Magnetization reversal occurs by curling mode at around Co = 400 nm and Cu = 10 nm, while for Co = 30 nm and Cu = 60 nm, magnetization reversal occurs by nucleation mode. A change of magnetic anisotropy from out of plane to in plane is observed when thickness of Cu layer tCu = 60 nm and that of Co tCo = 30 nm. Magnetic anisotropy is lost when thickness of the Co layer tCo = 400 nm and that of Cu tCu= 10 nm. Magnetic properties have been explained by the competition among shape anisotropy, magnetostatic interactions and magnetocrystalline anisotropy. Magnetic properties can be tuned accordingly depending upon the thickness of the Co and Cu nanodisks.

Angular dependence of the coercivity and remanence of ordered arrays of Co nanowires

The angular dependence of the coercivity and remanence of ordered hexagonal arrays of Co nanowires prepared using anodic aluminum oxide templates was investigated. The experimental evolution of coercivity as a function of the angle, in which the external field is applied, is interpreted considering micromagnetic simulations. Depending on the angle between the axis of the wire and the applied magnetic field direction our results show that the magnetization reversal mode changes from vortex to a transverse domain wall. Besides, we observed that the dipolar interactions cause a reduction in coercive fields, mainly in the direction of easy magnetization of the nanowires. Good agreement between numerical and experimental data is obtained.

Magnetization Reversal and Anisotropy with Chains of Superparamagnetic Ni Nanoparticles

Chains of superparamagnetic (SPM) Ni nanoparticles of 20 and 33 nm in diameter are synthesized by a facile, template-free, wet chemical method in magnetic field. The blocking temperature for the sample of 20 nm appears at T(B) approximately 255 K, above which the sample exhibits SPM behaviors by a M(H) measurement. At T < T(B), the temperature dependent coercivity, Hc(T), is beyond the description by the simple two-well model proposed by Neel and Brown in the early day. In contrast, for the Ni nanochains of diameter larger than the coherence length, Lcoh approximately 25 nm, the magnetization reversal properties are well described by this model. The magnetization reversal behavior of the SPM Ni nanochains with the diameter much smaller than the coherence length is one of the interesting points to study. Possible transition from the localized magnetization reversal mode in the high temperature end to the coherently rotational mode at low temperature is discussed.

Long-range interlayer-coupled magnetization reversal mediated by the antiferromagnetic layer in Py/FeMn/CoFe trilayers

With a combination of vector magneto-optical Kerr effect magnetometry and polarized neutron reflectometry, the detailed magnetization reversal mechanism of the exchange-biased Py(30-nm)/FeMn[${t}_{\text{AFM}}$ $=$ (0--30)-nm]/CoFe(30-nm) trilayers was studied. We found that Py/FeMn(15-nm) and FeMn(15-nm)/CoFe bilayers show completely different magnetization reversal modes, whereas they become very similar to each other in the corresponding Py/FeMn/CoFe trilayers. This is convincing evidence that the 15-nm FeMn layer mediates the magnetization reversal behaviors of both Py and CoFe layers through interlayer exchange bias coupling. Furthermore, magnetization reversal of Py and CoFe layers are decoupled for ${t}_{\text{AFM}}=30$ nm, indicating that the exchange length the magnetization reversal between two adjacent ferromagnetic layers is correlated over is less than 30 nm. This is in reasonable agreement with the theoretically predicted domain-wall width such as 28 nm for the polycrystalline FeMn/Co bilayer and 50 nm for the perfect Fe${}_{50}$Mn${}_{50}$ crystal.

Nonlinear stationary ac response of the magnetization of uniaxial superparamagnetic nanoparticles

The nonlinear stationary ac response of the magnetization of assemblies consisting of (i) noninteracting uniaxial superparamagnetic nanoparticles with aligned easy axes and (ii) randomly oriented nanoparticles subjected to superimposed ac and dc bias magnetic fields of arbitrary strength and orientation is calculated by averaging Gilbert’s equation augmented by a random field. The magnetization dynamics of uniaxial particles driven by a strong ac field applied at an angle to the easy axis of the particle (so that the axial symmetry is broken) alters drastically leading to new nonlinear effects due to coupling of the thermally activated magnetization reversal mode with the precessional modes via the driving ac field. In particular, the high frequency response reveals significant nonlinear effects in the precessional motion with significant consequences for the dynamic hysteresis and ultra-fast switching of the magnetization following an ultrafast change in the applied field.

Magnetization reversal in Co‐base nanowire arrays

AbstractThe magnetic behaviour of ordered arrays of magnetic nanowires is controlled by the combined action of anisotropy energy terms determined by geometrical shape, crystalline structure and magnetostriction, as well as by the magnetostatic interactions among them. A number of works have been reported on the role of magnetostatic interaction in arrays of nanowires, sometimes focusing on nanowires with vanishing crystalline and magnetostrictive energy terms. Here, in turn we are interested in Co‐base nanowires whose magnetocrystalline anisotropy plays a very important role. Arrays of Co‐base nanowires offer novel, sometimes unexpected behaviour that is not fully understood. Such results are connected to their magnetocrystalline anisotropy, and to their complex magnetization reversal mode. Arrays of Co and Co‐base nanowires have been fabricated by template‐assisted method, by electroplating into highly ordered self‐assembled pores of nanoporous alumina membranes. Their hcp or fcc crystal structure is confirmed to depend on length of nanowires as well as on the presence of selected elements in the composition. The magnetic behaviour of these arrays has been experimentally investigated as a function of (i) geometry (e.g. diameter and, particularly short length) and (ii) composition (e.g. adding Ni or Pd). A complementary study on its temperature dependence reveals the nature of the dominant anisotropy, while the angular dependence of hysteresis loops (e.g. coercivity and remanence) enables a deeper analysis of the magnetization reversal process. Micromagnetic reversal modes are reviewed and modelling and simulations procedures introduced. Complex rotational mechanisms are concluded to be responsible for the reversal process following micromagnetic modes distinct from those pure classically established.magnified image SEM image of an array of Co‐base nanowires.

Experimental evidence for an angular dependent transition of magnetization reversal modes in magnetic nanotubes

We report on the experimental and theoretical investigation of the magnetization reversal in magnetic nanotubes that have been synthesized by a combination of glancing angle and atomic layer deposition. Using superconducting quantum interference device magnetometry the angular dependence of the coercive fields is determined and reveals a nonmonotonic behavior. Analytical calculations predict the crossover between two magnetization reversal modes, namely, the movement of different types of domain boundaries (vortex wall and transverse wall). This transition, already known in the geometrical dependences of the magnetization reversal in various nanotubes, is found within one type of tube in the angular dependence and is experimentally confirmed in this work.

Magnetic anisotropy and magnetization reversal of (Ga,Mn)As dot-array

The present article describes the magnetic anisotropy and magnetization reversal of (Ga,Mn)As dot-arrays with 1 μm and 200 nm in diameter fabricated by electron-beam lithography and Ar ion etching. The magnetization reversal mode is due to the magnetization rotation in the dot-arrays fabricated in the present study; they probably have a single-domain structure. The microfabrication of the (Ga,Mn)As dot-arrays modifies magnetic anisotropy through the relaxation of strain in the dots as previously reported; the 200-nm-dot-array exhibits magnetically isotropic features. The cubic anisotropy constant of the 200-nm-dot-array is estimated to be reduced by approximately a tenth of that of the (Ga,Mn)As epitaxial film by the calculation from the measured M-H loops based on a phenomenological model related to magnetic energy.

Cooperative motion of domain walls in magnetic multilayers

It is well known that the sequence of induced emf spikes in a Barkhausen spectrum bears no resemblance to the sequence of spikes in successive field cycles; in general, no two spectra are ever alike. The present study reports the observation of remarkably reproducible Barkhausen spectra over repeated field cycling in TbFe/FeCo exchange spring multilayers. Their reproducibility is shown to have origins in the highly synchronized and cooperative motion of domain walls in different layers. Whereas a handful of previous studies have shown limited reproducibility in small field excursions, in small samples, or over small distances, the reproducibility of magnetic avalanches in the present study occurs along the entire major magnetization loop, and the emf avalanches can be reproducibly transmitted over macroscopic distances (\ensuremath{\sim}1 cm). The complexity of these highly reproducible spectra varies with the in-plane field direction, and reveals a new mode of magnetization reversal along the hard axis. Results also provide an avenue for information transmission over macroscopic distances in a magnetic medium.

Effect of a dc bias magnetic field on the magnetization relaxation of antiferromagnetic nanoparticles

The relaxation of the magnetization of antiferromagnetic nanoparticles owing to thermal agitation is treated via the Fokker-Planck equation describing the evolution of the distribution function of the magnetization orientations of an individual nanoparticle. By solving this equation using matrix-continued fractions, the correlation function of the longitudinal component of the magnetization, its characteristic relaxation times, and dynamic susceptibility are calculated for arbitrary dc field orientations across wide ranges of frequencies, temperatures, and damping. Furthermore, a simple analytic equation for the dynamic susceptibility at low frequencies is also proposed. It is shown that a dc field applied at an angle to the easy axis of the particle alters essentially the magnetization dynamics of the particle owing to coupling of the magnetization reversal mode with the precessional modes.

Atomistic spin model simulation of magnetic reversal modes near the Curie point

The so-called linear reversal mode is demonstrated in spin model simulations of the high anisotropy material L10 FePt. Reversal of the magnetization is found to readily occur in the linear regime despite an energy barrier (KV/kBT) that would conventionally ensure stability on this timescale. The timescale for the reversal is also established with a comparison to the Landau–Lifshitz–Bloch equation showing good agreement.

Nonlinear magnetization relaxation of superparamagnetic nanoparticles in superimposed ac and dc magnetic bias fields

The nonlinear ac response of the magnetization $\mathbf{M}(t)$ of a uniaxially anisotropic superparamagnetic nanoparticle subjected to both ac and dc bias magnetic fields of arbitrary strengths and orientations is determined by averaging Gilbert's equation augmented by a random field with Gaussian white-noise properties in order to calculate exactly the relevant statistical averages. It is shown that the magnetization dynamics of the uniaxial particle driven by a strong ac field applied at an angle to the easy axis of the particle (so that the axial symmetry is broken) alters drastically leading to different nonlinear effects due to coupling of the thermally activated magnetization reversal mode with the precessional modes of $\mathbf{M}(t)$ via the driving ac field.

Magnetic-field-orientation dependent magnetization reversal and spin waves in elongated permalloy nanorings

The field-orientation dependence of magnetization reversal and spin wave modes in nanorings has been investigated by focused magneto-optical Kerr effect spectroscopy, Brillouin spectroscopy, and micromagnetic simulations. The samples studied are periodic arrays of 150 nm wide Ni80Fe20 rings with long axes of 850, 1200, and 1500 nm, and respective short axes of 550, 700, and 900 nm. For the smallest ring, the onion-to-vortex transition is observed for both the applied magnetic field directed along, and 5° from its magnetization easy axis. In contrast for the two larger rings, this transition is not observed for the field in the latter orientation. Splitting of spin wave modes is observed for the transition from the onion to vortex state. Micromagnetic simulations of magnetization reversal and spin waves are in good agreement with experiments. The simulated temporal recording of magnetization distributions during transition reveals that different switching mechanisms result from different applied magnetic field orientations.

Magnetization reversal processes of single nanomagnets and their energy barrier

Micromagnetic simulations were performed to investigate the influence of geometry and magnetic anisotropy constant on energy barrier and magnetization reversal mechanism of individual bits important for the bit patterned media concept in magnetic data storage. It is shown that dependency of the energy barrier on magnetic and geometric properties of bits can be described by an analytical approach in the case of quasi-coherent magnetization rotation process. However, when the bit size exceeds a critical size, for which an incoherent magnetization reversal is preferred, the analytical approach becomes invalid and no self-consistent theory is available. By systematically investigating the influence of bit size on the magnetization reversal mode, it was found that the transition from quasi-coherent to incoherent magnetization reversal mode can still be described analytically if an activation volume is considered instead of the bit volume. In this case, the nucleation volume is an important parameter determining thermal stability of the bit. If the volume of the bit is larger than twice the activation volume, the energy barrier stays nearly constant; with further increase in bit size, no gain in thermal stability can be achieved.

Fabrication and Magnetization Reversal Processes for Co/Cu Multilayer Nanowires

Co/Cu multilayer nanowires fabricated in an array using anodized aluminium oxide (AAO) template has been investigated. Experimental conditions are optimized to fabricate Co/Cu multilayer systems with fixed Cu and variable Co layer thicknesses. Magnetization reversal mode is found to depend on the Co layer thickness. A transition occurs from coherent rotation to a combination of coherent and curling rotation at around t(Co) = 60 nm with increasing t(Co). The reversal modes have been investigated using the magnetic hysteresis loops measured at room temperature for Co/Cu nanowires placed at various angles between the directions of the nanowire axis and external fields using a vibrating sample magnetometer. The magnetic easy axis changes from the direction perpendicular to nanowires to that parallel to the nanowires at around t(Co) = 60 nm, indicating a change in the magnetization reversal mode. The reversal mode for the nanowires with thin disk-shaped Co layers is of a coherent rotation type, while that for long rod-shaped Co layers can be explained by a combination of coherent rotation and a curling mode.

Antiferromagnetic spin and twin domain walls govern hysteretic expressions of exchange anisotropy

The present study shows that antiferromagnetic spin and twin domain walls govern the hysteretic expressions of exchange anisotropy at low and high fields, respectively, using annealed NiO single crystals and Co. In the presence of twin walls, spin walls are shown to be a geometrical necessity in the antiferromagnetic NiO. A threshold field $(\ensuremath{\sim}10\text{ }000\text{ }\text{Oe})$ exists below which twin walls are frozen, and rotational hysteresis is dominated by losses due to spin walls. Above the threshold field, twin walls become mobile, resulting in a sharp increase in rotational hysteresis. Remarkably, rotational hysteresis associated with spin walls is similar to that of an ordinary ferromagnet---as the field strength increases, rotational hysteresis tends toward zero. However, unlike an ordinary ferromagnet where rotational hysteresis becomes zero above its saturation field, rotational hysteresis in antiferromagnet drops but then sharply increases once the threshold field for twin wall motion is exceeded. In crystals without spin walls, low-field rotational hysteresis is zero or negligible. Domain imaging of twin walls in antiferromagnet and Weiss walls in ferromagnet reveals a one-to-one spatial correlation even though twin walls are considered to have no net dipoles. This surprising result is explained by the fact that crystallographic interfaces in real crystals are not atomically sharp or ideal, and the defective interface invariably results in net moment across the finite width of the twin wall. The field dependence of domain walls in Co film exchange coupled to NiO shows global similarities to previously reported behavior of Co films deposited on nanocrystalline NiO [H. D. Chopra, D. X. Yang, P. J. Chen, H. J. Brown, L. J. Swartzendruber, and W. F. Egelhoff, Jr., Phys. Rev. B 61, 15312 (2000)]. In both cases, domain wall motion is not the dominant mode of magnetization reversal (wall motion is entirely absent in the present study while wall motion was only occasionally observed in the previous study), reversal occurs abruptly and different regions switch at different fields, and the observed pinning sites are frozen in the applied field range. Despite vastly different underlying microstructure of NiO in these two studies (annealed NiO single crystal with a fixed orientation across the NiO-Co interface in the present study versus nanocrystalline NiO in the previous study), the observed similarities may be understood by recognizing that, just as crystallographically imperfect twin interfaces serve as pinning sites for walls in Co film, other defects can play a similar role. Practically, results provide means of tuning rotational hysteresis by varying the type and the density of domain walls in NiO and by tuning the saturation field of the ferromagnet relative to the threshold field for the twin wall motion.

Highly asymmetric magnetic behavior in exchange biased systems induced by noncollinear field cooling

A detailed study of the angular dependence of the magnetization reversal in polycrystalline ferromagnetic (FM)/antiferromagnetic Co/IrMn bilayers with noncollinear FM and unidirectional anisotropies shows a peculiar asymmetric magnetic behavior. The anisotropy configuration is set via a field cooling (FC) procedure with the magnetic field misaligned with respect to the easy magnetization direction of the FM layer. Different magnetization reversal modes are observed for either positive or negative angles with respect to the FC direction. The angular dependence of both coercivity and exchange bias also clearly displays the broken symmetry of the induced noncollinearity. Our findings are reproduced with a modified Stoner–Wohlfarth model including the induced anisotropy configuration. Our results highlight the importance of the relative angle between anisotropies in exchange bias systems, opening a new path for the tailoring of their magnetic properties.