Luděk Kraus

GMI modeling and material optimization

Abstract A review of theoretical models of giant magnetoimpedance (GMI) is presented. The whole frequency range from kHz to GHz is covered starting from the simple quasistatic models and ending with the complex dynamic models based on the simultaneous solution of linearized Maxwell and Landau–Lifshitz equations. The similarity between GMI and FMR is emphasized. The asymmetric GMI (AGMI) behavior is discussed in more detail. Three different mechanisms of asymmetry are outlined. Based on the theoretical results the rules for obtaining high performance GMI materials are put forward.

Correlation between saturation magnetization, magnetostriction and creep-induced anisotropy in amorphous (FeCo)/sub 85/B/sub 15/ alloys

Compositional dependence of saturation magnetization, magnetostriction and creep-induced magnetic anisotropy is investigated in amorphous (Fe/sub 1-x/Co/sub x/)/sub 85/B/sub 15/ alloys. A close correlation of the three quantities was found. An anomaly centred at x=0.25 may indicate chemical short-range order of the Fe/sub 3/Co type in this concentration range. An explanation of the creep-induced anisotropy behaviour based on the bond-orientational anisotropy model is presented. It takes into account both the short-range ordering in the shear centres and medium-range deformations of their surroundings. >

Continuous stress annealing of amorphous ribbons for strain sensing applications

Abstract Stress-annealed amorphous Co 69 Fe 2 Cr 7 Si 8 B 14 ribbons are suitable materials for strain sensors required in civil engineering applications. The equipments for the continuous stress-annealing of ribbons and the automatic homogeneity test of magnetic properties are described. The magnetic properties of stress-annealed ribbons are discussed. The use of ribbon in an inductive strain sensor is also illustrated.

Amorphous CoFeCrSiB ribbons for strain sensing applications

Abstract Magnetoelastic properties of amorphous Co 71− x Fe x Cr 7 Si 8 B 14 ribbons, with 2⩽ x ⩽12, were investigated. The magnetostriction constant λ s changes linearly from −0.8×10 −6 to +4.0×10 −6 with increasing x . Stress-annealing at 350°C induces an easy plane magnetic anisotropy perpendicular to the ribbon axis. Magnetization reversal takes place by the moment rotation and the magnetizing curves are linear up to the anisotropy field, which is proportional to the stress applied during annealing. Simple strain sensors based on these materials show practically constant sensitivity nearly up to the mechanical strength of the ribbon and acceptable reproducibility of measurements.

Ferromagnetic resonance of transversally magnetized amorphous microwires and nanowires

Ferromagnetic resonance of very thin glass covered wires with diameters ranging from 25 μm down to 133 nm was investigated at microwave frequencies 49.1 and 69.7 GHz at room temperature. The static magnetic field was applied perpendicular to the wire axis. The resonance spectra substantially change when the wire diameter decreases below the electromagnetic skin depth. Depending on the wire thickness and the experimental arrangement, various resonance modes can be excited. In thick wires, an inhomogeneously broadened resonance curve, with two distinct peaks at minimum and maximum resonance fields, can be seen. In submicron wires, generally three narrow resonances can be observed. The weak central resonance peak corresponds to the uniform precession (Kittel) resonance mode and is excited by the uniform component of microwave magnetic field. The other two belong to non-uniform magnetostatic modes, which are excited by the strong circumferential magnetic field due to electric polarization of the wire. The experimental results are explained using a strong skin effect limit and a quasistatic approximation for the bulk and submicron wires, respectively.Ferromagnetic resonance of very thin glass covered wires with diameters ranging from 25 μm down to 133 nm was investigated at microwave frequencies 49.1 and 69.7 GHz at room temperature. The static magnetic field was applied perpendicular to the wire axis. The resonance spectra substantially change when the wire diameter decreases below the electromagnetic skin depth. Depending on the wire thickness and the experimental arrangement, various resonance modes can be excited. In thick wires, an inhomogeneously broadened resonance curve, with two distinct peaks at minimum and maximum resonance fields, can be seen. In submicron wires, generally three narrow resonances can be observed. The weak central resonance peak corresponds to the uniform precession (Kittel) resonance mode and is excited by the uniform component of microwave magnetic field. The other two belong to non-uniform magnetostatic modes, which are excited by the strong circumferential magnetic field due to electric polarization of the wire. The exp...

Magnetostatic coupling in soft/hard biphase magnetic systems based on amorphous alloys

We report on magnetic bias in biphase systems arising from by dipolarlike interaction. Composites consist of amorphous ribbons and wires prepared by rapid solidification techniques, surrounded by an electroplated layer. Amorphous metallic core is (Co0.94Fe0.06)72.5Si12.5B15 with ultrasoft magnetic behavior (coercivity of 10–10−1A∕m), and the electroplated coating is crystalline Co85Ni15 alloy with harder magnetic character (coercivity of 104A∕m. After premagnetizing in a dc saturating field (106A∕m), the low-field (±103A∕m) hysteresis loops are ascribed to the magnetization process of the soft core and exhibit a typical shift towards the direction of the premagnetizing field. This shift is ascribed to the magnetostatic field generated by uncompensated poles at the edges of the hard layer, which remains close to its remanence state. The influence of the geometry of both phases on magnetostatic energy term allows us to tailor the bias field and susceptibility of the core. These results open other possibilit...

Ferromagnetic resonance in submicron amorphous wires

Ferromagnetic resonance in glass-coated amorphous wires with the diameter of metallic core varying from 25 μm to 133 nm is investigated. The microwave frequencies of 49.1 and 69.7 GHz are used and static magnetic field is applied either parallel or perpendicular to the long wire axis. In agreement with theoretical predictions the resonance curves of submicron wires substantially differ from the curves of the bulk wires. Depending on the symmetry and intensity of microwave electric and magnetic fields in the sample vicinity the circumferential and/or dipolar resonance modes can be excited. In bulk wires the resonance fields of the two modes coincide. In submicron wires, however, their resonance fields differ, indicating the metallic character of the circumferential mode and the insulator character of the dipolar mode. In wires with diameters 717 and 869 nm radial standing spin wave resonances are observed in parallel field configuration. The experimental results for the parallel field configuration can be ...

AN AMORPHOUS MAGNETIC BIMETALLIC SENSOR MATERIAL

An amorphous bimetal ribbon consisting of magnetostrictive (Fe40Ni40B20) and nonmagnetostrictive (Co67Fe4Cr7Si8B12) layers was prepared by planar flow casting from a double‐chamber crucible. The effect of applied tensile stress on hysteresis loops and the surface domain structures of the stress‐relieved bimetal was investigated at room temperature. The hysteresis loops can be well explained by superpositions of hysteresis loops of the individual layers. Only the magnetostrictive layer is responsible for the influence of applied stress on magnetic behavior. At a certain stress, the magnetic anisotropy of the magnetostrictive layer abruptly changes from a hard‐ribbon‐axis to an easy‐ribbon‐axis type. This transition is accompanied by a change of domain structure and a sharp maximum of the coercive field. A simple model taking into account an interplay of the applied tensile stress with the compressive stress produced by thermal contraction after stress relief and/or by bending of the ribbon has been develop...

Magnetostatic bias in a composite hard/soft/hard microlayer

A composite magnetically hard/soft/hard trilayer ribbon with micrometric thickness has been prepared by combined melt-spinning and electroplating techniques. Its main peculiarities relate to the existence of a magnetic coupling between hard and soft magnetic phases resulting in bias magnetic field acting on the soft layer. A systematic study of the low-field hysteresis loops has been performed after premagnetizing under a direct current saturating field as a function of the geometrical dimensions of both types of layers. The analysis of the observed bias effect has allowed us to conclude the magnetostatic origin of the tailorable coupling. The present investigation in microscale system points out to the potential technological applications that in a further step can be developed in nanomagnetic systems.

Magneto-optic vector magnetometry of CoFeCrSiB amorphous ribbons

A magneto-optic method suitable for investigation of soft magnetic ribbons is proposed that combines the internal magnetic field induced by a current flowing through the ribbon axis with the external magnetic field generated by an air coil. The method eliminates effects of demagnetizing factor and shape anisotropy on measured hysteresis loops. Both in-plane magnetization components are measured by means of magneto-optic vector magnetometry. The method is applied to study magnetic properties of CoFeCrSiB amorphous ribbons showing asymmetric giant magnetoimpedance effects. A coherent rotation of magnetization and in-plane easy axis inclined from the ribbon axis is observed. It corresponds to helical magnetic anisotropy originating from the preparation processes.