The influence of size on coercive field of ultra soft magnetic materials

We report here a size dependence of the coercive field in the millimeter–centimeter range length scale of ribbon like samples prepared from ultra soft amorphous and nanocrystalline alloys. A model is proposed where surface pinned domain walls are considered having an effective stiffness constant linearly increasing with the demagnetization factor.     

The stress effect on magnetic domain structure of Fe-Co amorphous alloys

The stress effect on magnetic domain structure has been studied for the Fe-Co amorphous alloy system. The domain structure was dominated by the magnetostriction-internal stress coupling. By applying the tensile stress, the domain structure was changed significantly according to the magnitude and sign of magnetostriction. The value of internal stress estimated was about 3×10⁸ dyn/cm² and nearly constant for all the alloy composition.     

Fast domain wall dynamics in amorphous glass-coated microwires

Here we report on the domain wall dynamics in amorphous glass-coated FeCuNbSiB microwires measured in the temperature range from 77 up to 400 K. At low temperatures below 200 K, the domain wall velocity is proportional to the applied magnetic field. At temperatures above 200 K, two regions have been found: one with low domain wall mobility at low fields and another one with high domain wall mobility at high fields. The different regions of the domain wall dynamics are treated in terms of the change of the domain wall configuration from transversal to vortex one. Moreover, non-linear regime is shown at low fields at the temperature 373 K as a result of the domain wall interaction with the local defects.     

横向偏置场作用的非晶带巨磁阻抗效应理论

采用多畴结构模型,考虑非晶带具有180°畴壁的磁畴及其两面的偏置磁场方向的不同,根据自由能最小原理,Maxwell方程组及带阻尼项的Landau-Lifshitz方程,建立了非晶态合金带在横向偏置磁场作用下的巨磁阻抗效应的理论计算公式. 提出并采用四状态平均磁导率代替单畴模型获得的磁导率,得到了更符合实际的处于偏置场作用的阻抗随外磁场变化的理论结果. 关键词： 偏置磁场 四状态 多畴结构 巨磁阻抗     

Magnetic relaxations in amorphous soft magnetic alloys

The paper reviews selected results of the extended experimental investigations of magnetic properties, time–temperature stability and workability of the soft magnetic amorphous alloys controlled by structural magnetic relaxation. Complex approach to the magnetic relaxations in multicomponent amorphous alloys is presented. The transition from magnetic after-effect to a new MAE spectrometry is illustrated on ternary amorphous CoSiB alloy.     

The effect of surface crystalline layers on asymmetric off-diagonal magnetoimpedance in field-annealed CoFeSiB amorphous ribbons

The off-diagonal magnetoimpedance in field-annealed CoFeSiB amorphous ribbons was measured using a pick-up coil wound around the sample. One side of a ribbon was etched in hydrofluoric acid solution during various times in order to change the thickness of the surface crystalline layer appearing after annealing. The asymmetric two-peak field dependence of the off-diagonal impedance was observed for all samples. The evolution of the off-diagonal magnetoimpedance with the change in the ribbon thickness is analyzed.     

Magnetic contrast from domain walls in BaFe12O19 by tunneling stabilized magnetic force microscopy

The magnetic structure of BaFe₁₂O₁₉ is imaged with a scanning tunneling microscope having a flexible, magnetic tip. We find that Fe thin films evaporated on a silicon tip, integrated with a cantilever, behave as magnetically soft tips. Therefore, we are able to image domain walls with high lateral resolution. A different contrast along the domain wall due to surface magnetic charges is observed. We explain the data using previously established models for wavy domain walls. The obtained images are the first experimental evidence of magnetic charges induced on the wavy domain walls in BaFe₁₂O₁₉.     

Development of a magnetic measurement device for thin ribbon samples

This paper presents a magnetic measurement device for thin ribbon samples, which are produced by rapid cooling technique. This device enables us to measure magnetic properties easily by only inserting a ribbon sample into a sample holder. The sample holder was made by bakelite to fix any width sample. A long solenoid coil was used to generate a uniform magnetic field and the sample holder was placed at the mid part of the solenoid. The magnetic field strength was measured using a shunt resistor and the magnetic flux density and magnetization in sample ribbons were evaluated by using search coils. The accuracy of measurement was verified with an amorphous metal ribbon sample. Next, we have measured magnetic properties of some magnetic shape memory alloys, which have different compositions. The measured results are compared and we clarified the effect of Sm contents on the magnetic properties.     

Magnetic properties and magnetic entropy change of amorphous and crystalline GdNiAl ribbons

The structure and magnetic properties of amorphous melt-spun and subsequently crystallized GdNiAl ribbons were investigated. An amorphous phase was formed after the quenching process by melt spinning with a copper wheel having a surface speed of 30 m/s. A hexagonal phase with lattice parameters a=7.023 ? and c=3.916 ? was formed in the GdNiAl ribbon after annealing above its crystallization temperature. Magnetic entropy change was calculated directly from isothermal magnetic measurements. The results show that both the amorphous and annealed samples have a high magnetocaloric effect, indicating that these alloys can be considered as candidates for magnetic refrigeration applications. Received: 14 August 2001 / Accepted: 18 September 2001 / Published online: 23 January 2002     

Soft X-ray resonant magnetic scattering study of magnetization reversal in low dimensional magnetic heterostructures

Soft X-ray resonant magnetic scattering (SXRMS) has been used to investigate the microscopic magnetization reversal behavior of complex magnetic systems. SXRMS is a unique technique, providing chemical, spatial and magnetic sensitivity, which is not affected by external magnetic fields. The study of two selected thin magnetic heterostructures is presented, amorphous rare-earth transition metal alloys and perpendicular exchange coupled antiferromagnetic/ferromagnetic films. In the first system, the internal structure of magnetic stripe domains on nanometer length scales is obtained by measuring bi-dimensional (2D) scattering images. In the second system, the element specificity is exploited to identify the role of the uncompensated spins in the antiferromagnetic layer on the exchange coupling phenomena. Future trends are also discussed.     

Damping of the domain walls motion in Co-based amorphous ribbons with helical magnetic anisotropy: Part III

The damping of the motion of domain walls of a sandwich domain structure by the eddy currents magnetic fields, the stray fields and the hysteresis friction fields is investigated. The blocking of the motion of domain walls by the eddy currents magnetic fields is discovered.     

Influence of current amplitude on the nonlinear asymmetric ac volt-ampere characteristics in amorphous ribbons with GMI effect

Asymmetric giant magnetoimpedance (GMI) effect in amorphous ribbons is very promising due to its possible application in developing the highly sensitive linear magnetic-field sensors. In this paper, the model of the nonlinear asymmetric volt-ampere characteristics in field-biased Co-based amorphous ribbon has been established through the time derivative of the longitudinal magnetization component in the ribbon. Its harmonics are also developed by Fourier analysis. The influence of the current amplitude on the nonlinear asymmetric effect has been studied. It is found that when the current amplitude is 8.54 mA, the sensitivity of the first harmonic voltage on the external field is equal to that of the second harmonic voltage. The results obtained are useful for developing the high-sensitive magnetic-field sensors.     

Biased magnetization reversal in bi-phase multilayer microwires

We report on the magnetic behaviour of a novel family of two-magnetic-phase multilayer microwires consisting of: (i) a bistable FeSiB glass-coated amorphous microwire as soft nucleus, and (ii) a polycrystalline CoNi outer microtube as harder layer. Such bi-phase microwires are prepared by combined quenching and drawing plus sputtering and electroplating techniques. The stray field produced by the harder outer layer after premagnetizing it to saturation is used to bias the magnetization reversal process of the soft nucleus via dipolar magnetostatic coupling. A detailed analysis of the asymmetric low-field magnetization reversal process of the soft nucleus is presented together with the study of the fluctuating switching field and its asymmetric behaviour. The study of the domain wall characteristics under the presence of a nucleation coil at one end of the microwire allows us to draw conclusions on the role of the bias field generated by the premagnetized hard outer layer.     

The influence of planar defects on the coercive field of hard magnetic materials

Planar defects are found to act as strong pinning centres in hard magnetic materials. The interaction of the domain wall with planar defects is assumed to originate in a local perturbation of exchange coupling and crystalline anisotropy. The coercive field is calculated within the framework of micromagnetic continuum theory as well as with a discrete lattice theory which accounts for the interaction between the individual atomic spins. The theory has been applied to discuss the influence of grain boundaries and antiphase boundaries in the intermetallic compound Co₅Sm.     

Study of surface magnetic properties in Co-rich amorphous microwires

Magneto-optical investigations carried out on a Co-rich glass-covered amorphous microwire is presented. The appearance of circular magnetic bistability and the influence of tensile stress and high-frequency electric current on the surface magnetization reversal have been studied. The change of the mechanism of the magnetization reversal induced by the high-frequency electric current is also discussed.     

外磁场与带轴夹角对非晶FeSiB/Cu/FeSiB三明治薄带巨磁阻抗特性的影响

采用层间胶合方法制备了淬态非晶FeSiB/Cu/FeSiB三明治薄带,研究了同尺寸单层薄带和三明治薄带的巨磁阻抗(giant magneto-impedance, GMI)随外磁场与带轴夹角β的变化特性.结果表明,FeSiB单层薄带在7.0 MHz最佳响应频率下,GMI仅约30%,外磁场与带轴夹角对单层薄带GMI几乎没有影响;三明治薄带的GMI效应则十分显著,在0.6 MHz最佳响应频率下,纵、横向GMI比分别达到272%和464%, GMI随β的增大而增强;所有β角的三明治薄带GMI曲线都出现各向异性峰,各向异性峰随β的增大而展宽.根据磁畴转动模型推导了薄带横向磁导率与各向异性场及β之间的函数关系式.结果显示,三明治薄带GMI随夹角β变化的特性与理论推算的横向磁导率变化有较好的一致性,而单层薄带则不然.该磁畴转动模型能定性解释三明治薄带GMI随外磁场方向变化特性.     

Temperature dependence of magnetic stripe domain period in ultrathin films

In ultrathin films, due to the thermal activation and temperature dependencies of the magnetic parameters, magnetization reversal processes are strongly affected by thermal effects. We analyze changes of domain periods of ultrathin cobalt and L₁₀$\mathrm{L}{1}_0$ films in a wide temperature range. With regard to the temperature dependencies of the film magnetic parameters we calculate the equilibrium stripe period as a function of temperature. It is shown that on film heating the equilibrium domain structure (DS) period decreases and at the reorientation phase transition (RPT) approaches its minimal value corresponding to the temperature independent period of the sinusoidal domain structure. Just below the RPT temperature (or thickness) the stripe domain period was found to exponentially decrease with temperature. Irreversible temperature changes of the domain period affected by coercivity are also discussed.     

Recent results in magnetic force microscopy

We present domain wall images obtained by using Magnetic Force Microscope (MFM) on magnetic samples like: double layer of permalloy alloy, magnetic hard disk, BaFe₁₂O₁₉ single crystal and YGdTmGa/YSmTmGa magnetic garnet. We have imaged topography and magnetic forces of the same area. The Fe double- and single-layer thin film tips have been prepared to achieve high sensitivity (10^–12N) and high resolution of MFM.     

High-resolution magnetic imaging by local tunneling magnetoresistance

We give an overview over our recent efforts of high-resolution magnetic imaging using scanning tunneling microscopy with a ferromagnetic tip. Magnetic sensitivity is obtained on the basis of local tunneling magnetoresistance between a soft magnetic tip and the sample. The magnetisation of the tip is switched periodically with a small coil, leading to variations of the tunneling current due to the tunneling magnetoresistance effect. These variations are detected with a lock-in amplifier to separate spin-dependent parts from the topographic parts of the tunneling current such that the topography and the magnetic structure of the sample can be recorded simultaneously. Crucial for this method is to avoid mechanical vibrations of the tip, that may also lead to variations in the tunneling current. Exemplary studies of polycrystalline Ni and the closure domain pattern of Co(0001) are presented, showing high contrast at acquisition times as low as 3 ms/pixel and a lateral resolution of the order of 1 nm. Further it is demonstrated that besides topography and magnetisation, also local information about the magnetic susceptibility can be obtained. Received: 28 April 2000 / Accepted: 15 May 2000 / Published online: 7 March 2001