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.     

Cylindrical magnetization model for glass-coated microwires with circular anisotropy: Statics

The static magnetization profile of glass-coated microwires with effective circular anisotropy is investigated using micromagnetics. In this family of microwires, the ferromagnetic nucleus with an amorphous character presents a magnetic structure composed of an inner region with axial domains and an outer region with circular domains, due to magnetoelastic anisotropy. A one-dimensional micromagnetic model is developed, taking into account both the exchange and magnetoelastic anisotropy energies, and solved quasi analytically. The total energy, magnetization profiles and magnetization curves are investigated as a function of radius and anisotropy constant of the nucleus. This work represents a fundamental study of the magnetization process in these amorphous microwires and provides guidelines for the production of microwires with tailored magnetic properties. En passant, the nucleation problem in an infinite cylinder, introduced by W.F. Brown, is revisited.     

Recent advances in studies of magnetically soft amorphous microwires

In this paper, we present the giant magneto-impedance (GMI) effect (real part of longitudinal impedance, Z, and of the off-diagonal impedance) and hysteretic magnetic properties of amorphous glass-coated microwires with different compositions possessing nearly zero, positive and negative magnetostriction constant and metallic nucleus diameter ranging between 6 and 16 μm. Enhanced soft magnetic properties (low coercivity of about 4 A/m) and high-GMI effect have been observed in Co-rich microwires with vanishing magnetostriction constant. The magnetic anisotropy field of these microwires depends on the ratio between metallic diameter, d and total microwires diameter, D. Stress-sensitive magnetic properties have been obtained in Fe-rich microwires after stress annealing: hysteresis loop stress-annealed (SA) microwires drastically changes under applied stress. A variety of hysteresis loops with different hysteresis loops can be obtained in Fe-rich microwires changing the conditions (time and/or temperature) of the stress annealing. The obtained results allow us to tailor the microwire magnetic properties for magnetic sensors applications through selection of their composition and/or geometry and by thermal treatment.     

Giant magnetoimpedance in thin amorphous and nanocrystalline microwires

We present the results on Giant magneto-impedance effect (GMI) in amorphous and nanocrystalline microwires at frequencies until 4 GHz, paying special attention to tailoring the frequency and magnetic field dependence of the GMI effect. Correlation between magnetoelastic anisotropy and magnetic field dependences of diagonal and off-diagonal impedance components are observed.     

Dimensional resonances of elastic and magnetoelastic vibrations in two layered structure

Peculiarities of dimensional resonances of elastic and magnetoelastic waves in bi-layered insulator structure: ferromagnetic film–non-magnetic elastic substrate have been investigated. Dependences of resonant frequencies of vibration modes upon thicknesses of magnetic and non-magnetic layers, elastic and magnetoelastic parameters, applied magnetic field have been calculated. The presence of peculiarities of resonant frequencies harmonics behavior with change of magnetic layer thickness have been shown.     

Thermal expansion and magnetostriction of GdAg2, and relations to the magnetoelastic paradox

The antiferromagnet GdAg₂ has been shown to be a good model system for the magnetoelastic paradox (MEP), because it exhibits large symmetry conserving magnetoelastic strains and the antiferromagnetic propagation vector breaks the tetragonal lattice symmetry (therefore a large symmetry breaking magnetoelastic strain can be expected in a single magnetic structure). As in many similar Gd based compounds no symmetry breaking strain has been found in the experiment. In order to investigate this MEP further, we have measured magnetostriction and magnetization on a textured polycrystal. The behaviour closely resembles that of GdNi₂B₂C, the prototype system for the magnetoelastic paradox (MEP). Our forced magnetostriction data indicate that the crystal distorts in applied magnetic field and gives further evidence that the MEP is a low field effect. The observed phase transitions are in agreement with available specific heat and neutron diffraction data. Moreover, the saturation magnetic field was measured in high pulsed magnetic fields and agrees well with the value calculated from the Standard Model of Rare Earth Magnetism (SMREM).     

Study of thickness-dependent magnetic and transport properties of Fe/Al nanostructures

The paper presents magnetic and transport properties of compositionally modulated Fe/Al multilayer structures (MLS), with an overall atomic concentration ratio of Fe:Al = 3:1, 2:1 and 1:1. All MLS show soft ferromagnetic behaviour at room temperature (RT) with an in-plane easy axis of magnetization. In each case, coercivity increases continuously and magnetization decreases with an increase in temperature due to enhancement in the anisotropy as a result of non-uniform and disordered formation of thin intermixed (dead) FeAl layer at the interfaces. The Curie temperature obtained for the MLS is much less than that of bcc Fe but is well above RT. The observed magnetic behaviour is mainly attributed to the formation of different FeAl phases and increase in anti-ferromagnetic interlayer coupling with addition of Al. The formation of these phases is also supported by resistivity results. The results of this research enabled us to understand that by controlling of layers thickness and temperature in multilayer systems, the nanogranular thin films with good resistive and soft magnetic properties can be obtained.     

Magnetic properties of Fe-based glass-coated microwires

Axial hysteresis loops of glass-coated amorphous Fe₇₀B₁₅Si₁₀C₅ microwire have been measured as a function of both the diameter of metallic nucleus (from 3.7–14.9 μm) and the thickness of the coating (4.0–9.6 μm). They exhibit low-field rectangular hysteresis loops with a single and large Barkhausen jump even for samples as short as 5 mm long. Coercivity remarkably increases (roughly from 1 to 10 Oe) and remanence decreases (from 1 to 0.45 T), respectively, as the ratio of metallic nucleus radius to the total radius of the wire decreases from 0.63 to 0.16. The strength of the internal stresses induced during the fabrication depends on this ratio, and the easy axes of the corresponding magnetoelastic anisotropies determine the actual value of coercivity and remanence for these microwires.     

Magnetoelastic contribution in domain wall dynamics of amorphous microwires

We studied the domain wall (DW) propagation of magnetically-bistable Fe–Co-rich microwires paying attention to the effect of applied and internal stresses. Magnetic field, H, dependences of DW velocity, v, were measured in Co_41.7Fe_36.4Si_10.1B_11.8 microwires with metallic nucleus diameters (from 13 μm to 18 μm) and with different ρ-ratio between the metallic nucleus diameter, d, and total microwire diameter, D. DW velocity decreased under the application of stresses. From measured dependences we evaluated DW mobility, S, dependence on the applied stresses. The results obtained for Co_41.7Fe_36.4Si_10.1B_11.8 sample show that S decreases with the increasing of applied stresses, σ_a. The observed dependences manifest that increasing of magnetoelastic anisotropy results in the decreasing of DW mobility and DW velocity     

Giant magnetoimpedance for biosensing: Advantages and shortcomings

Biosensors are an important area of modern sensor applications. Magnetoimpedance (MI) phenomenon was proposed for biosensing in 2003. Since that MI biosensor prototypes based on amorphous ribbons, multilayered structures, amorphous rapidly quenched wires and glass covered microwires were designed and tested. In this paper the advantages and shortcomings of MI-based devices for magnetic labels or label-free biodetection are discussed in view of recent results.     

Specific features of the magnetoresistance in multilayer systems of magnetic nanoislands in weak magnetic fields

New magnetic structures such as multilayer systems of magnetic nanoislands being alternating layers of nanoislands of various magnets have been proposed. The electric, magnetic, and magnetooptical properties of the systems have been studied. The magnetoresistance of ～2% related to the anisotropic effect has been revealed. In multilayer structures of magnetic nanoislands, a unidirectional axis of predominant magnetization has been found, which changes its orientation depending on the structure parameters. The magnetic field required to reorient the axis in the opposite direction has been estimated to be 2 kOe < H _A < 20 kOe. The periodic multilayer structures of magnetic nanoislands are very sensitive to hyperweak magnetic fields (to 10^?6 Oe).     

Magnetoelastic effects at the interface of CoP nanostructured multilayers

Summary The role of magnetoelastic energy to determine the causes which affect the magnetic anisotropy are reviewed. The simple model used to describe magnetostriction in the new nanostructured materials is reported. Starting from this model the magnetostriction in multilayer samples, of the kind magnetic material/non-magnetic material, is discussed. In particular the fundamental influence of the interlayers is shown. Finally it is demonstrated that by measurement of saturation magnetostriction it is possible to have information on the interlayers thickness and average composition.     

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

Magnetic reversal mechanism of the Sub/AlN5 nm/[CoPt2 nm/AlN5 nm]₅ nano multilayer film, which shows strong perpendicular magnetic anisotropy (K_u=6.7×10⁶ erg/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.     

Origin of magnetic and magnetoelastic tweedlike precursor modulations in ferroic materials

Based on experimental observations of modulated magnetic patterns in a Co0.5Ni0.205Ga0.295 alloy, we propose a model to describe a (purely) magnetic tweed and a magnetoelastic tweed. The former arises above the Curie (or Néel) temperature due to magnetic disorder. The latter results from compositional fluctuations coupling to strain and then to magnetism through the magnetoelastic interaction above the structural transition temperature. We discuss the origin of purely magnetic and magnetoelastic precursor modulations and their experimental thermodynamic signatures.     

Tb0.3Dy0.7Fe2合金磁畴偏转的滞回特性研究

研究了不同载荷作用下Tb_0.3Dy_0.7Fe₂合金在压磁和磁弹性效应中磁畴偏转的滞回特性. 基于Stoner-Wolhfarth模型的能量极小原理, 采用绘制自由能-磁畴偏转角度关系曲线的求解方法, 研究了压磁和磁弹性效应中载荷作用下的磁畴角度偏转和磁化过程, 计算分析了不同载荷作用下磁畴偏转的滞回特性. 研究表明, 压磁和磁弹性效应中磁畴偏转均存在明显的滞回、跃迁效应, 其中磁化强度的滞回效应来源于磁畴偏转的角度跃迁; 压磁效应中预加磁场的施加将增大磁化强度的滞回, 同时使滞回曲线向大压应力方向偏移; 磁弹性效应中磁畴偏转的滞回存在两个临界磁场强度, 不同磁场强度下合金具有不同的磁畴偏转路径和磁化滞回曲线, 临界磁场强度的大小取决于预压应力的施加. 理论分析对类磁致伸缩材料磁畴偏转模型的完善和材料器件的设计应用非常有意义.     

Magnetic and magnetoelastic properties of GdZn single crystals

The magnetic and magnetoelastic properties of GdZn have been investigated by various ways: magnetization, magnetic susceptibility, magnetocaloric effect, pressure dependence of the Curie temperature, spontaneous magnetostriction, parastriction and sound velocity measurements. The γ-mode strain is large for an S-state ion, and two orders of magnitude larger than the ?-one. Both the first-order B^γ,2 and the three isotropic second-order magnetoelastic coefficients exhibit a m²(1 ? 0.5m²) thermal variation (where m(T, H) is the reduced magnetization), which differs from the classical behaviour. All the results suggest a noticeable contribution of the conduction band to the magnetism of GdZn.     

Size effects in thin antiferromagnetic layers and “ferromagnet-nonmagnetic metal” multilayer magnetic structures

The specific features of the spin-flop and spin-flip transitions in thin antiferromagnetic layers and “ ferromagnet-nonmagnetic metal” multilayer magnetic structures are considered. The dependence of the magnetic fields corresponding to these phase transitions on the thickness of the antiferromagnet or on the number of layers in the multilayer is determined.