Coherent Rotation Of Magnetization Research Articles

Localized magnetization reversal processes in cobalt nanorods with different aspect ratios

We present results of the synthesis of cobalt nanorods using the polyol process and the mechanism of magnetization reversal. We show that the nucleation step is significantly dependent on the nature of the ruthenium chloride used as the nucleating agent. This allows varying the diameter and aspect ratio of the cobalt nanorods independently. Co nanorods with aspect ratio, mean diameter, and mean length in the ranges ARm = 3–16, D m = 7–25 nm, and L m = 30–300 nm, respectively, were produced using this method. X-ray diffraction and electron microscopy showed that a strong discrepancy between the structural coherence and morphological aspect ratio can exist because of stacking faults. The coercivity of assemblies of different nanorods was systematically measured, and the highest values were obtained for the smallest diameter and the largest structural coherence length. Micromagnetic simulations were performed to account for the dependence of the coercive field on the diameter. An important observation is that simple coherent magnetization rotation models do not apply to these magnetic nano-objects. Even for very small diameters (D m = 5–10 nm) well below the theoretical coherent diameter D coh(Co) = 24 nm, we observed inhomogeneous reversal modes dominated by nucleation at the rod edges or at structural defects such as stacking faults. We conclude that, in order to produce high-coercivity materials based on nanowires, moderate aspect ratios of 5–10 are sufficient for providing a structural coherence similar to the morphological aspect ratio. Thus, the first priority should be to avoid the formation of stacking faults within the Co nanowires.

Magnetization jumps in nanostructured Nd–Fe–B alloy at low temperatures

Abstract Magnetic properties of the nanostructured isotropic alloy on the base of Nd 2 Fe 14 B type phase were investigated at low temperatures. The evaluated average grain size of this phase was much smaller than its critical single domain diameter. Hence the magnetization and demagnetization processes were expected to be performed by coherent magnetization rotation. For such coercivity type system magnetization jumps were revealed on the demagnetization hysteresis loop branch in the vicinity of the coercive force at temperatures below 4 K. It was shown that magnetization jumps have a stochastic behavior and their number strongly depends on the temperature and the mass of measured samples. High temperature spikes corresponding to magnetization discontinuities were observed. All these results allowed to propose that magnetization jumps in nanostructured magnetics with magnetization rotation reversal processes comply with the local heating model.

Magnetic Domain Control of Exchange Biased Magneto-Electric Thin Film Composites

Magneto-electric composites constitute a promising class of materials for the application as highly sensitive magnetic field sensors. Their compatibility to miniaturization, their passive nature and their large dynamic range are just some of the numerous advantages of these composites compared to alternative magnetic field sensors, as e.g. flux-gates or magneto-resistive sensors. However, the highest sensitivities are observed in the presence of well-defined magnetic bias fields that are of significant disadvantage in terms of miniaturization and noise performance.Self-biased magneto-electric composites, which are fabricated using the exchange bias effect [1], exhibit an increased total anisotropy in comparison to systems without exchange bias. As a consequence, small exchange bias fields are favourable because of a minor reduction of the magneto-electric voltage coefficient. However, weakly biased magneto-electric composites might loose their self-biasing properties and possibly show an increase of discontinuities in magnetization reversal due to the formation of magnetic domains. By a thickness variation of the ferromagnetic layer a maximum voltage coefficient αME ≈ 430 V/cmOe was found for a magnetostrictive multilayer of 3 x (5 nm Ta / 3 nm Cu / 8 nm Mn-Ir / 333 nm Fe-Co-Si-B). Yet, a stable single domain state indicating a well defined magnetization reversal by coherent magnetization rotation was achieved for layer thicknesses up to 100 nm Fe-Co-Si-B with a slightly reduced magneto-electric cofficient of αME ≈ 340 V/cmOe. This slight reduction is overcompensated by the improved control of the magnetic domain pattern that is highly desirable for magnetic field sensors and of special importance when frequency conversion techniques are applied [2].[1] Lage, E., Kirchhof, C., Hrkac, V., Kienle, L., Jahns, R., Knöchel, R., Quandt, E. & Meyners, D. Exchange biasing of magnetoelec¬tric composites. Nature materials 11, 523–9 (2012).[2] Jahns, R., Greve H., Woltermann, E., Quandt, E. & Knöchel, R. Sensitivity enhancement of mag¬netoelectric sensors through frequency-conversion. Sensors and Actuators A: Physical 183, 16-21 (2012).This work was funded by the German Research Foundation (DFG) as part of the Collaborative Research Center 855 “Magnetoelectric Composites–Future Biomagnetic Interfaces”

Rotatable anisotropy driven training effects in exchange biased Co/CoO films

The training effect for exchange bias in field-cooled Co/CoO bilayers films is investigated. Previous experiments on the same system have shown that, starting from the ascending branch of the first hysteresis loop, coherent magnetization rotation is the dominant reversal mechanism. This is confirmed by the performed numerical simulations, which also indicate that the training is predominantly caused by changes of the rotatable anisotropy parameters of uncompensated spins at the Co/CoO interface. Moreover, in contrast with what is commonly assumed, the exchange coupling between the rotatable spins and the ferromagnetic layer is stronger than the coupling between the ferromagnet and the spins responsible for the bias. Thus, uncompensated spins strongly coupled to the ferromagnet contribute to the coercivity rather than to the bias, whatever the strength of their magnetic anisotropy.

Magnetic Behavior of Single Ni Nanowires and its Arrays Embedded in Highly Ordered Nanoporous Alumina Templates

We report on magnetization reversal and geometry dependent magnetic anisotropy of Ni nanowire arrays electrodeposited in nanoporous alumina templates. Using micromagnetic simulation we have found that magnetization reversal mechanism in arrays with different nanowire diameters is curling. This magnetic behavior appears with propagation of the domain wall along a nanowire. The calculations have been proven by the analysis of hysteresis curves. To explain magnetic properties of closely-spaced nanowire arrays we have taken into consideration the magnetostatic interaction between adjacent nanowires and their structural defects, like as boundary grains. The investigated magnetic domain pattern of individual bended nanowires confirms rather complicated magnetization reversal mechanism than either coherent rotation of magnetization or its curling. Competition between the shape and magnetoelastic anisotropies can induce an unusual zigzag-like domain pattern in a single nanowire.

Magnetic domain control and voltage response of exchange biased magnetoelectric composites

Self-biased magnetoelectric composites, which are realized with the exchange bias effect, hold an increased total anisotropy field compared to systems without exchange bias. Thus, small exchange bias fields are favorable because of a minor reduction of magnetic permeability and magneto-electric voltage coefficient. However, weakly biased magnetoelectric composites lose their self-biasing properties and possibly show an increase of discontinuities in magnetization reversal due to the formation of magnetic domains. By a thickness variation of the ferromagnetic layer, a maximum voltage coefficient αME ≈ 430 V/cm Oe was found for a magnetostrictive multilayer of 3 × (5 nm Ta/3 nm Cu/8 nm Mn-Ir/333 nm Fe-Co-Si-B). Yet, a stable single domain state indicating a well defined magnetization reversal by coherent magnetization rotation was achieved for layer thicknesses up to 100 nm Fe-Co-Si-B with αME ≈ 340 V/cm Oe. This slight reduction is overcompensated by the improved control of the magnetic domain pattern which is highly beneficial for magnetic field sensing applications and of special importance, when frequency conversion techniques are applied.

Magnetostrictive Performance in Py/TbFe Coupled Bilayers: Dependence on Hard Layer Thickness

Magnetostrictive TbFe was chosen as hard layer, and the spin dynamics in the coupled bilayers Py(10 nm)/TbFe(t nm) (Py-permalloy; t=4, 7, 10, 30) were investigated by taking advantage of the engineering magnetostrictive strain under the constraints of the measurement. The dependence of the engineering magnetostrictive strain on TbFe layer thickness reflects a competitive process between the interfacial exchange interaction and the TbFe anisotropy. For the dominant interfacial exchange interaction in the coupled bilayers with 4 and 7 nm TbFe layer, coherent magnetization rotation of Py and TbFe layers is very similar to the behavior described by the Stoner-Wolfarth model. Magnetic hysteresis in the coupled bilayers with 10 nm TbFe layer and the decrease in the maximum engineering magnetostrictive strain in the coupled bilayers with 30 nm TbFe layer were observed.

Frequency dependence of magnetization reversal in thin Fe(100) films

We have studied the magnetization reversal of epitaxial Fe(100) films in an ac magnetic field as a function of frequency between 0.1 and 1.2 MHz by polarized neutron reflectivity. Different modes of magnetization reversal are excited depending on the ac field frequency, those with domain nucleation and propagation, and those with a coherent magnetization rotation. At low frequencies the magnetization follows the external field adiabatically via nucleation and propagation of antiparallel 180${}^{\ensuremath{\circ}}$ domains. At higher frequencies additional 90${}^{\ensuremath{\circ}}$ domain walls are being nucleated and propagate. Beyond 0.5 MHz, the magnetization reversal becomes progressively suppressed and finally comes to a stop at about 1 MHz.

Magnetization reversal studies of continuous and patterned exchange biased NiFe/FeMn thin films

In this article we present a detailed investigation of the structural and magnetic properties of exchange biased NiFe (ferromagnet)/FeMn (antiferromagnet) thin films. The influence of the shape anisotropy on exchange bias and the magnetization reversal mechanism in a sample with patterned lines is compared with a continuous two-dimensional reference sample. Polarized neutron reflectivity (PNR) is employed to study the magnetization reversal by analyzing the spin-flip and non-spin-flip reflectivities. PNR measurements show that the magnetization reversal in the reference two-dimensional film and patterned lines is by domain wall motion rather than coherent rotation of magnetization.

Efficient hysteresis loop simulations of nanoparticle assemblies beyond the uniaxial anisotropy

We propose a modified Stoner-Wohlfarth model combined with the geometrical approach of the coherent rotation of magnetization for simulating the hysteresis loops of an assembly of magnetic nanoparticles. The temperature and the size distribution are taken into account. This combined model enables the computation of hysteresis loops at low temperatures for assemblies of particles having an arbitrary type of anisotropy. The applicability of this model for fitting experimental data is discussed and results are compared to the zero-field-cooled and field-cooled fits. As an application, the hysteresis loops measured on Co clusters embedded in carbon and germanium matrices are fitted revealing unambiguously the presence of a biaxial anisotropy.

Anisotropic nanocrystalline MnBi with high coercivity at high temperature

Magnetic hard nanocrystalline MnBi has been prepared by melt spinning and subsequent low temperature annealing. A coercivity of 2.5 T can be achieved at 540 K for MnBi with an average grain size of about 20-30 nm. The coercivity iHc, mainly controlled by the coherent magnetization rotation, shows a strong dependence on the time of grinding and exhibits a positive temperature coefficient from 100 up to 540 K. The unique temperature dependent behavior of the coercivity (magnetocrystalline anisotropy) has a relationship with the variations in the crystal lattice ratio of c/a with temperatures. In addition, discontinuity can not be found in the lattice parameters of a, c, and c/a ratio at the magnetostructural transition temperature. The nanocrystalline MnBi powder fixed in an epoxy resin and under an applied magnetic field of 24 kOe shows a maximum energy product of 7.1 MGOe at room temperature and shows anisotropic characteristics with high Mr/Ms ratio up to 560 K.

Influence of magnetostatic interactions on the magnetization reversal of patterned magnetic elements

The magnetization reversal in patterned thin-film arrays of elliptical submicron permalloy elements has been investigated by magnetic-force microscopy and micro-magneto-optic Kerr effect. Three different spatial arrangements of chains are considered, namely chains aligned parallel to the long axis of the ellipse, chains aligned parallel to the short axis of the ellipse, and arrays with roughly equal element-to-element spacings in both directions. Comparison of the hysteresis loops in an in-plane field perpendicular to the ellipses’ long axes shows that the magnetization reversibility is highest for chains along the long axis. This is due to the nearly coherent magnetization rotation in the applied magnetic field and to the formation of a head-to-tail domain arrangement. Other arrangements, such as chains of ellipses aligned parallel to short axis, yield flux-closure domains as the applied magnetic field is changed.

Effect of Anisotropy and Direction of Magnetization on Complex Permeability of Ferromagnetic Rectangular Thin Slabs

Ultrasoft magnetic materials, such as amorphous, nanocrystalline, and polycrystalline alloys, have been successfully used for power electronic applications during recent years. However, enhancements are needed for the integration of power electronic features, which involves high power densities and operating frequencies up to a few megahertz. Complex permeability spectra, which are used to describe material behavior in these applications, depend mainly on domain-wall motions and coherent magnetization rotation mechanisms. In this paper, we present a model describing these mechanisms, in terms of magnetic anisotropies and domain structure of the material. To validate the model, we measured permeability spectra of polycrystalline Ni-Fe alloys under mechanical stress using a specific setup. These measurements are suitable for comparing the physical model according to different magnetoelastic anisotropies. Results are useful for correlating high-frequency magnetic behavior and the annealing process of materials.

Interpretation of hysteresis loops of GaMnAs in the framework of the Stoner–Wohlfarth model

We have used the magnetooptical Kerr effect to investigate the shape of the hysteresis loops of thin GaMnAs films grown on substrates with different buffer layers. Depending on whether the easy axis of magnetization is in the plane of the thin film or out of the plane, and depending on the orientation of the external magnetic field with respect to the crystallographic axes, a great variety of hysteresis loops is observed. Because magnetooptical effects depend linearly on specific components of the magnetization, it has been possible to determine the orientation of the magnetization with varying magnetic field. The experimental findings are very well described by the Stoner–Wohlfarth model of coherent magnetization rotation, yielding precise values for the anisotropy constants. We present this model and its use in the context of magnetooptical measurements as a relatively simple and straightforward method for establishing magnetization parameters of ferromagnetic semiconductors in thin film form.

10.1007/s11448-008-1004-x

This work is devoted to a small-angle polarized neutron scattering study of the structure and magnetic properties of nickel inverted photonic crystals. Depending on the intensity of the small-angle scattering, diffraction maximums up to fourth-order reflections, which correspond to scattering from the highly ordered structures of the test samples, are observed. Several contributions to the scattering are analyzed: a nuclear contribution; a magnetic contribution; a contribution depending on an external magnetic field; and a nuclear magnetic interference, which shows a correlation between magnetic and nuclear structures. It is found that a magnetization reversal process, which was represented by a standard hysteresis curve, for weak fields was accompanied by both domain formation and coherent magnetization rotation from the field direction to directions caused by geometric structure peculiarities.

Cone magnetization state and exchange bias in IrMn∕Cu∕[Co∕Pt]3 multilayers

The effects of a Cu interlayer on the perpendicular exchange bias in IrMn∕Cu∕[Co∕Pt]3 multilayers were investigated by focusing on the correlation between the exchange bias and the magnetic anisotropy. The in-plane magnetization hysteresis loops were interpreted in the framework of a phenomenological model based on the coherent magnetization rotation, indicating that the easy magnetization axis makes an angle of approximately 15° with the normal of the IrMn∕[Co∕Pt]3 film. This angle decreases with the introduction of the Cu spacer thickness in the IrMn∕Cu∕[Co∕Pt]3 system, indicating that the Cu interlayer leads to a predominant perpendicular anisotropy. Although a maximum of the out-of-plane anisotropy is found for Cu layer thickness between 4 and 5Å, the maximum of the perpendicular exchange bias was found at 3Å of Cu, which could be attributed to the interplay between two effects, mainly the increase in the effective perpendicular anisotropy with the Cu spacer thickness due to the reorientation of the Co moment toward the normal to the film’s plane direction, and the exponential decrease in the ferromagnet/antiferromagnet exchange coupling (and, consequently, of the exchange bias shift field) with the Cu interlayer thickness.

Magnetization Reversal in Arrays of Ni Nanowires With Different Diameters

Arrays of Ni ferromagnetic nanowires of three different diameters (20, 40, and 170 nm) are obtained by electrodeposition into nanoporous alumina templates. Hysteresis curves parallel and perpendicular to the applied field are studied by angle dependent vector vibrating sample magnetometry. Hysteresis curves, from high remanence and coercivity for the smallest diameter, to nearly anhysteretic curves for the largest diameter, can be tuned by varying the diameter of the nanowires. The results show that the magnetic response of these arrays is a combination of coherent and incoherent rotation of magnetization of the nanowires.

Ultrafast dynamics of four-state magnetization reversal in (Ga,Mn)As

The ultrafast dynamics of in-plane four-state magnetization reversal from compressively strained (Ga,Mn)As film was investigated by magneto-optical Kerr rotation measurement. The magnetization reversal signal was dramatically suppressed upon pumping, and recovered slowly with time evolution. The low switching field Hc1 increased abruptly from 30 to 108 G on the first several picoseconds and recovered back to the value before optical pumping within about 500 ps, whereas the high switching field Hc2 did not change obviously upon pumping, implying a domain-wall nucleation/propagation at low fields and coherent magnetization rotation at high fields in the magnetization reversal process.

Hyper-domains in exchange bias micro-stripe pattern

A combination of experimental techniques, e.g. vector-MOKE magnetometry, Kerr microscopy and polarized neutron reflectometry, was applied to study the field induced evolution of the magnetization distribution over a periodic pattern of alternating exchange bias (EB) stripes. The lateral structure is imprinted into a continuous ferromagnetic/antiferromagnetic EB bilayer via laterally selective exposure to He-ion irradiation in an applied field. This creates an alternating frozen-in interfacial EB field competing with the external field in the course of the re-magnetization. It was found that in a magnetic field applied at an angle with respect to the EB axis parallel to the stripes the re-magnetization process proceeds via a variety of different stages. They include coherent rotation of magnetization towards the EB axis, precipitation of small random (ripple) domains, formation of a stripe-like alternation of the magnetization, and development of a state in which the magnetization forms large hyper-domains comprising a number of stripes. Each of those magnetic states is quantitatively characterized via the comprehensive analysis of data on specular and off-specular polarized neutron reflectivity. The results are discussed within a phenomenological model containing a few parameters, which can readily be controlled by designing systems with a desired configuration of magnetic moments of micro- and nano-elements.

Magnetization reversal in permalloy ferromagnetic nanowires investigated with magnetoresistance measurements

The magnetization reversal process in single Permalloy $({\text{Ni}}_{81}{\text{Fe}}_{19})$ nanowires has been investigated by magnetoresistance measurements as a function of the angle between the applied field and the wire direction. The Permalloy nanostructures fabricated on an ultrathin film by atomic force microscopy consist of two large rectangular pads connected by a nanowire with the shape of a long thin narrow tape. For each field direction in the plane of the film the dependence of the magnetoresistance on the field value exhibits two main contributions: one from the pads and one from the nanowire. The contribution from the pads is due to a usual anisotropic magnetoresistance characteristic of coherent magnetization rotation, whereas the contribution from the nanowire is an abrupt transition at the switching field. The dependence of the switching field on the in-plane field angle is quantitatively described by a model of nucleation field with the buckling magnetization rotation mode.