Reversible Rotation Of Magnetization Research Articles

Thin Films of BaM Hexaferrite with an Inclined Orientation of the Easy Magnetization Axis: Crystal Structure and Magnetic Properties

Thin (~50 nm thick) BaM hexaferrite (BaFe12O19) films were grown on (1–102) and (0001) cut α-Al2O3 (sapphire) substrates via laser molecular beam epitaxy using a one- or two-stage growth protocol. The advantages of a two-stage protocol are shown. The surface morphology, structural and magnetic properties of films were studied using atomic force microscopy, reflected high-energy electron diffraction, three-dimensional X-ray diffraction reciprocal space mapping, powder X-ray diffraction, magneto-optical, and magnetometric methods. Annealed BaFe12O19/Al2O3 (1–102) structures consist of close-packed islands epitaxially bonded to the substrate. The hexagonal crystallographic axis and the easy axis (EA) of the magnetization of the films are deflected from the normal to the film by an angle of φ~60°. The films exhibit magnetic hysteresis loops for both in-plane Hin-plane and out-of-plane Hout-of-plane magnetic fields. The shape of Mout-of-plane(Hin-plane) and Min-plane(Hin-plane) hysteresis loops strongly depends on the azimuth θ of the Hin plane, confirming the tilted orientation of the EA. The Mout-of-plane(Hout-of-plane) magnetization curves are caused by the reversible rotation of magnetization and irreversible magnetization jumps associated with the appearance and motion of domain walls. In the absence of a magnetic field, the magnetization is oriented at an angle close to φ.

Surface crystallization and magnetic properties of Fe84.3Cu0.7Si4B8P3 soft magnetic ribbons

The magnetic microstructure and hysteresis properties of Fe84.3Cu0.7Si4B8P3 soft magnetic ribbons have been investigated by Kerr-microscopy, Kerr-magnetometry and inductive hysteresis measurements. Crystalline surface layers were already present after quenching, causing an out-of-plane anisotropy in the amorphous bulk. Annealing at elevated temperature leads to nanocrystallization in the inner part of the ribbon and to a gradual decline of the perpendicular anisotropy with increasing temperature. The surface magnetization process is determined by the coercive motion of 180° domain walls along the ribbon axis, while the bulk magnetization process is dominated by reversible rotation of magnetization.

Magnetic reversal mechanism in strong perpendicular magnetic anisotropic CoPt/AlN multilayer film

Magnetic reversal mechanism of the Sub/AlN5nm/[CoPt2nm/AlN5nm]5 nano multilayer film, which shows strong perpendicular magnetic anisotropy (Ku=6.7×106erg/cm3), has been studied. The angle-dependent magnetic hysteresis loops of this highly perpendicular anisotropic CoPt/AlN multilayer film were measured in the present work, applying a magnetic field along different angles φ with respect to the film normal. It demonstrates that the magnetic reversal of the CoPt ultrathin layers in the CoPt/AlN multilayer film is occurred by the reversible magnetization rotation and the irreversible displacement of domain walls. The φ-dependent part of coercive field is resulted from the internal stress according to the Kondorsky and Kersten model. The φ-independent part of coercive field implies some random and isotropy pinning centers (e.g., vacancies, dislocations, grain boundaries) in the ultrathin CoPt layers. Our work is useful for coercivity control of metal/ceramics layered structures, in particular the perpendicular magnetic tunneling junctions.

Electrically controlled reversible and hysteretic magnetic domain evolution in nickel film/Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32 (011) heterostructure

We report direct Lorentz microscopy observations of electrically induced magnetic domain motion in a nickel film/Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32 (PMN-PT (011)) heterostructure. The 0.5 mm-thick PMN-PT substrate contains a 10 μm-wide, 60 nm-thick Ni/Pt electron-permeable observation region. Stress from the substrate creates magnetoelastic anisotropy of up to 4 kJ m−3 in the nickel film resulting in reversible magnetization rotation as well as non-reversible domain wall jumps (i.e., Barkhausen jumps). The observed magnetization of the film is directly related to the local strain gradient as computed by the finite element method, providing strong evidence of the effectiveness of the strain-mediated magnetoelectric approach for device applications.

Revealing the reversible rotation of magnetization in exchange-coupled composite media switching

We present a study of the magnetic reversal mechanism for exchange coupled composite perpendicular media, i.e., hard granular layer/exchange-coupling layer (ECL)/soft layer (SL) systems, without soft underlayer. Using a vibrating sample magnetometer, the out-of-plane and in-plane components of the magnetization are monitored simultaneously, while sweeping an external out-of-plane magnetic field. The in-plane signal reveals the reversible rotation of the magnetization during the switching process. The amplitude of this rotation is correlated with the variations in coercivity and angular dependence of the remanent coercivity for different ECL and SL thicknesses.

Complex susceptibility measurements in amorphous glass-coated microwires

We have applied the measurement of the amplitude dependence of the complex AC susceptibility to study the magnetization processes in amorphous glass-coated microwires with positive, negative and vanishing magnetostriction. The features of the magnetization process (reversible domain wall movement, irreversible domain wall movement, reversible magnetization rotation or irreversible magnetization rotation) can be recognized from the shape of the amplitude dependence of complex AC susceptibility, which confirms the domain structures predicted in microwires with different magnetostriction constant.

Reversible magnetization processes and energy density product in Sm–CoFe and Sm–Co/Co bilayers

The hysteresis properties of epitaxial SmCo/Co and SmCo/Fe bilayers are calculated by the solution of the Landau–Lifshitz Gilbert equation. The thin film grain structure is taken into account using appropriate finite element techniques. The J(H) curve shows the typical exchange spring behavior for the bilayer if the soft magnetic layer thickness exceeds 10 nm. However, the reversible rotations of the magnetization for low external field deteriorate the maximum energy density product. Straight B(H) curves are obtained only for a Fe layer thickness of 5 nm. Magnetization reversal starts with the reversible rotation of the soft layer magnetization. Initially, the magnetization rotates in opposite directions in different regions of the film. The reversible rotations penetrate substantially into the hard layer.

Decomposition of susceptibility spectra in a torsion-stressed Fe-based amorphous wire

The complex susceptibility spectra are measured as functions of the alternating-current field amplitude and the torsion angle in an Fe77.5Si7.5B15 amorphous wire. The susceptibility spectra show dispersion with a relaxation frequency of 40 kHz due to irreversible motion of the inner core domain walls when the driving field is larger than the threshold field of 10 mOe. The spectra for a small driving field can be decomposed into two relaxation dispersions by using the nonlinear curve fitting, one originating from reversible wall motion of the inner core domains and with a relaxation frequency of 0.36 MHz, and the other originating from reversible magnetization rotation in the outer shell domains and with relaxation frequency of 1.82 MHz. The static susceptibilities resulting from the reversible and the irreversible magnetization processes show an asymmetric change with positive and negative torsion angles.

Matteucci and inverse Wiedemann effects in amorphous wires with enhanced circumferential domains

Nonmagnetostrictive CoFeSiB alloy wires were subjected to cold drawing processing followed by annealing under tension, in order to induce the formation of circular magnetic anisotropy. The axial (MZ–HZ) and circular (Mφ–Hφ) hysteresis loops indicate the existence of induced transverse anisotropy, with the magnitude of the anisotropy field being dependent on the processing conditions, while circular loops are quasibistable for samples processed under stress. Very peculiar hysteresis loops are obtained for the Matteucci (MI, Mφ–HZ) and inverse Wiedemann (IWE, MZ–Hφ) effects corresponding to those samples exhibiting the mentioned circular bistability. The circular shape of the MI loop is interpreted as produced by reversible magnetization rotation within circumferential domain. The effective hysteresis arises from the existence of a helical distribution of anisotropy in the transition region between the inner axial domain and the external shell. IWE loop confirms the presence of this helicity.

Characterization of Magnetization Reversal Mechanism for Magnetic Recording Media by Rotational Hysteresis Loss Analysis

Experimental characterization of magnetic anisotropy and intergranular magnetic coupling for currently developed recording media was carried out with magnetic torque analysis. In thin film media, the magnitude of the magnetic fieled where rotational hysteresis loss vanishes showes an almost constant value in each meterial independent of coercive force Hc. It is concluded that this field nearly corresponds to magentic anisotropy field of grains. Experimental results shows that the increase of Hc in almost all thin film media is mainly caused by the decrease of the strength of intergranular exchange coupling. On the other hand, with decreasing the strength of intergranular coupling, the number of grains where Hc is determined by reversible rotation of magnetization gradually increases. Futhermore, the existence of incoherent magnetization reversal in grains is suggested in the real films especially in CoCrTa media.

Process of magnetization in amorphous ribbons carrying an electric current

Hysteresis loops were measured for as-quenched Fe 60Ni 20B 18Si 2 and Co 74Fe 6B 20 amorphous ribbons using an induction method, with direct current ( J) flowing along the samples. The decrease of the coercive field ( H c ) and the maximum magnetization ( M m) reached at a certain amplitude of the applied field ( H) as a function of the current was investigated. In addition, the influence of an alternating current on the process of magnetization of the samples was investigated. The splitting of peaks in the derivative of magnetization with a time ( M) versus H curve was also investigated. The experimental results show that the reversible rotation of magnetization is responsible for the decrease of M m. A linear variation of H c with J was observed for low J, whereas for higher J this linearity vanishes. The observed non-linear decrease of H c with J for high J and the peak splitting of M can be explained by local variation of the direction of magnetization in the samples and by the reversible rotation of magnetization when J is applied.

Magnetic Properties of Alnico Alloy Phases and Temperature Instability of Permanent Magnets

Analysis of the structural constituents of Alnico alloys with high coercive force showed that the saturation magnetization of the α′ phase is quite high, and for Alnico VIII and Alnico VS55 is close to the saturation of binary Fe–Co (Is∼1800 emu) At the same time, the saturation magnetization of the α phase is only several scores of emu. The relative volume of α′-phase decreases from Alnico V to Alnico VIII and VS55. This result is in qualitative agreement with the corresponding decrease of Is and increase in Hc for these alloys. Comparison of the Ku(T) and Hc(T) experimental curves for these alloys reveals their correspondence: a maximum in Ku and Hc in the temperature interval of +20°-–+100°C and a relatively small decrease of Ku and Hc over a wide temperature range. Calculation of the Ku(T) dependence from the equation Ku = ½(Is1 − Is2)2N·ν1·ν2, taking into account the temperature dependence of Is1 and Is2, shows good agreement between the experimental and calculated curves. These results prove that it is the shape anisotropy of the strongly magnetic phase particles that explains the high coercive force of the Alnico alloys. Anisotropy fields HA=(2Ku/Is) were calculated using the Ku and Is values of the alloys measured at room temperature. Comparison of the HA values with the coercive force shows that the anisotropy field is three-times higher than the coercive force values. Causes of the variation in anisotropy field in various alloys may be explained by analyzing the data. The increase in HA in Alnico VIII in comparison with Alnico V is explained by changing alloy-phase characteristics: an increase in ΔIs causes a 35% increase in HA, and an increase in ΔN causes a 17% increase; an increase in ν1+ν2 results in a 9% increase of HA, and decreased saturation Is causes a 9% increase in HA. All this results in a value of HA for Alinco VIII 70% higher than for Alnico V. When comparing corresponding characteristics of Alnico VIII and VS55, one can see that the ΔIs value is very small, and the increase in ΔN is negligible. The increase in HA is mainly due to a decrease in Is in Alnico VS55 in comparison with Alinco VIII, which increases HA by 30%. The decrease in Is is mainly due to a smaller ν1 (relative volume of the α′-phase). Thus, the Ku and HA values achieved in single-crystal alloys Alnico VIII and VS55 are nearly as large as possible because of the following: (a) further increase in Is (at a given composition) and increase in ΔN are practically impossible; (b) Is2 values are low enough so that further decrease has only a small effect. Part II. Special features of temperature changes in the induction of permanent magnets are qualitatively explained on the basis of analysis of results of investigation of the temperature dependence of the main alloy magnetic properties: saturation magnetization, coercive force and uniaxial anisotropy constant. Irreversible induction losses during the first temperature cycles may be related to the process of irreversible rotation in some precipitates of strongly magnetic phase under the influence of the internal field because of a decrease of anisotropy constant. Reversible magnet induction changes may be accounted for by the following: (a) Reversible temperature dependence of alloy saturation; (b) Temperature dependence of reversible rotation of magnetization in single-domain particles in the internal demagnetizing field. Induction changes when cooling magnets in the interval of +100° to −180°C are probably caused by two opposite effects, namely: (a) an increase in the saturation of the alloy results in an increase in induction, and (b) increasing reversible rotation of magnetization of strongly magnetic phase particles decreases the induction.1 Thus, a maximum is observed on the curve of reversible induction vs temperature. Reversible induction changes with temperatures above +100° may be accounted for by the influence of the following factors: (a) the decrease in alloy saturation results in a decrease of induction; (b) diminishing reversible rotation of particle magnetization causes some increase in induction. Because of the second mechanism, reversible temperature changes in magnet induction are always less than relative temperature changes in alloy saturation. It can be shown that the higher the internal demagnetizing field, the greater is this effect. This explains, first, the decrease in ΔB/B rev and temperature coefficient of induction with lowering the magnet working point, and second, the relatively high stability (i.e., small ΔB/B rev and temperature coefflcient values) of Alnico VIII and VS55 magnets. The (BH)max point for these high coercive-force alloys corresponds to high values of the internal demagnetizing field. This causes a significant reduction in the decrease of induction with temperature. Both parts of this abstract will be published in the I.E.E.E. Transactions on Magnetics as individual papers.