Temperature dependence of the magnetic properties and magnetoimpedance of nanocrystalline Fe<Subscript>73.5</Subscript>Si<Subscript>16.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> ribbons

The temperature dependences of the magnetic properties and the magnetoimpedance effect of soft magnetic nanocrystalline Fe_73.5Si_16.5B₆Nb₃Cu₁ alloy ribbons are studied in the temperature range 24–160°C. A high temperature sensitivity of the impedance and the magnetoimpedance effect of the ribbons are detected in the ac frequency range 0.1–50 MHz. At an ac frequency of 50 MHz, the change in the impedance reaches 0.2 Ω/°C (0.5%/°C) in the temperature range 85–160°C. When the temperature increases, a monotonically decreasing character of the dependence of the magnetoimpedance effect on the applied magnetic field changes into a dependence having an increasing initial segment. The effect of temperature on the magnetoimpedance properties of the soft magnetic nanocrystalline ribbons is shown to result from temperature-induced changes in their electrical conductivity, magnetization, and effective magnetic anisotropy.     

Frequency spectrum of the nonlinear magnetoimpedance of multilayer film structures

Magnetization reversal by high-frequency current in FeCuNbSiB/Al/FeCuNbSiB three-layer film structures is studied. The frequency spectrum of the voltage arising in a coil wound on the sample as a function of a permanent magnetic field (nonlinear magnetoimpedance) is taken. It is shown that the frequency spectra of the voltage are qualitatively different for the longitudinal and transverse orientations of the field with respect to the direction of the current. Frequency spectrum harmonics are demonstrated to be highly sensitive to a magnetic field. A simple electrodynamic model to describe experimental data is suggested.     

Ac magnetotransport in La0.7Sr0.3Mn0.95Fe0.05O3 at low dc magnetic fields

We report the ac electrical response of La_0.7Sr_0.3Mn_1−xFe_xO₃(x=0.05) as a function of temperature, magnetic field (H) and frequency of radio frequency (rf) current (). The ac impedance (Z) was measured while rf current directly passes through the sample as well as in a coil surrounding the sample. It is found that with increasing frequency of the rf current, Z(T) shows an abrupt increase accompanied by a peak at the ferromagnetic Curie temperature. The peak decreases in magnitude and shifts down with increasing value of H. We find a magnetoimpedance of for at around room temperature when the rf current flows directly through the sample and when the rf current flows through a coil surrounding the sample. It is suggested that the magnetoimpedance observed is a consequence of suppression of transverse permeability which enhances skin depth for current flow. Our results indicate that the magnetic field control of high frequency impedance of manganites is more useful than direct current magnetoresistance for low-field applications.     

Structural,magnetic and electrical transport properties in cold-drawn thin Fe-rich wires

Microstructural (X-ray diffraction), magnetic properties (hysteresis loop), electrical resistivity, magneto-impedance and stress impedance effects have been investigated in cold-drawn Fe_77.5B₁₅Si_7.5 amorphous wire. Initial amorphous wire (obtained by the in-rotating-water technique) with diameter of 125 μm was submitted to cold-drawn process decreasing the diameter to 50 μm. Such cold-drawn wire was treated by current annealing (currents of 190, 210, 220 and 230 mA during times between 1 and 45 min) for tailoring the magnetic and electrical transport properties. A qualitative analysis of the magnetoimpedance and stress impedance effects is given by considering the influence of the magnetoelastic anisotropy and frequency of the AC driving electrical current on the circular permeability.     

Magnetic transition in Co/(Gd–Co) multilayers

[Co/Gd_0.36Co_0.64]₄/Co multilayers with Co termination layer have been prepared by rf sputtering. They form macroscopic ferrimagnets with a compensation temperature (T_comp) determined by the thickness ratio of the layers. In low fields the magnetization of Co and Gd–Co layers are along the axis of the applied field. Increasing field makes the moments of both the Co and Gd–Co layers deviate from the axis of the field giving rise to a transition into a twisted state. These magnetic transitions were studied by vibrating sample magnetometer (VSM), magneto-optic Kerr effect and magnetoresistance measurements at various temperatures. The nucleation and evolution of surface- and bulk-twisted magnetic states were also observed in these multilayers.     

Radiofrequency magnetoresistance of Fe/Cr superlattices

The rf magnetoresistance of Fe/Cr superlattices is studied for two orientations of the current: parallel and across the superlattice layers. A mutually single-valued correspondence is established between the relative magnetoresistance measured at dc current and the change in the transmission coefficient of electromagnetic waves in the magnetic field. When rf currents flow across the layers, the relative change in the signal amplitude is proportional to twice the change in the electrical resistance of the superlattice and is of opposite sign. It is shown that the rf losses are determined by the surface resistance which is proportional to the superlattice thickness and inversely proportional to its conductivity. An equation is derived for the rf electric field distribution in the superlattice. It is established that when the thickness of the superlattice is small compared with the skin layer depth, field and current components which penetrate through the entire superlattice exist.     

Optical properties and the magnetoimpedance effect in amorphous Co-Ni-B metal alloys

A magnetoimpedance effect is observed in Co_80–xNi_xB20 (x = 4, 6, 8, 10, 12) amorphous metal alloy ribbons when an alternating current at frequencies ranging from 1–3000 kHz is passed through samples in an external magnetic field. Spectra of the permittivity and optical conductivity of the surface of ribbons of these alloys are obtained for incident photon energies of 1.0–5.0 eV. The relationships between the magnitude of the magnetoimpedance effect, and the magnetic properties, electronic structure parameters, and optical characteristics of the amorphous alloy samples are determined.     

Radio-frequency annealing effects in amorphous Fe40Ni40B20

The influence of radio frequency (rf) magnetic fields on the properties of amorphous Fe₄₀Ni₄₀B₂₀ was studied using Mössbauer spectroscopy. The measurements were performed with frequencies of 67 and 53 MHz and rf field intensity in the range of 1 to 12 Oe. The narrowing of the hyperfine spectra due to the rf field and the formation of rf sidebands were observed. The effect of instability and crystallization of the amorphous metal enhanced by the rf field at temperatures much lower than the crystallization temperature was observed.     

Current distribution and giant magnetoimpedance in composite wires with helical magnetic anisotropy

The giant magnetoimpedance effect in composite wires consisting of a non-magnetic inner core and soft magnetic shell is studied theoretically. It is assumed that the magnetic shell has a helical anisotropy. The current and field distributions in the composite wire are found by means of a simultaneous solution of Maxwell equations and the Landau–Lifshitz equation. The expressions for the diagonal and off-diagonal impedance are obtained for low and high frequencies. The dependences of the impedance on the anisotropy axis angle and the shell thickness are analyzed. Maximum field sensitivity is shown to correspond to the case of the circular anisotropy in the magnetic shell. It is demonstrated that the optimum shell thickness to obtain maximum impedance ratio is equal to the effective skin depth in the magnetic material.     

Influence of the special features of the effective magnetic anisotropy on the temperature dependences of the magnetoimpedance of nanocrystalline Fe<Subscript>73.5</Subscript>Si<Subscript>16.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> strips

The influence of the thermal treatment type on the temperature dependences of the magnetoimpedance of nanocrystalline Fe_73.5Si_16.5B₆Nb₃Cu₁ alloy strips is investigated. The main mechanisms determining the temperature behavior of the magnetoimpedance of strips with induced magnetic anisotropy having various special features are established. The prospects for application of the alloy strips nanocrystallized in the presence of a magnetic field as sensitive elements of temperature sensors and special magnetic field detectors are demonstrated.     

FeCuCrVSiB多层膜巨磁阻抗效应的研究

研究了结构为 (FM/SiO₂)₃/Ag/(SiO₂/FM)₃ 多层膜的巨磁阻抗(GMI)效应（这里的F M≡FeCuCrVSiB）．多层膜采用射频溅射法沉积在单晶Si衬底上，沉积过程中，沿膜面长方 向施加约72kA/m的磁场，然后在不同的温度下对样品进行了退火处理．结果表明，该多层膜 样品即使在沉积态便具有相当好的软磁性能和GMI效应，在7MHz的频率下，最大纵向和横向 巨磁阻抗比分别为45%和44%．在230℃下经90min退火处理后的样品具有最佳的GMI效应，在85MHz的频率下，最大纵向和横向巨磁阻抗比分别达到251%和277%．与磁性层总厚度相同的FeCuCrVSiB/Ag/FeCuCrVSiB三层膜相比较，在这种多层结构中出现的GMI效应更强． 关键词： 多层膜 巨磁阻抗效应 趋肤效应 有效磁导率     

Off-diagonal magnetoimpedance in stress-annealed amorphous ribbons

The influence of creep-induced magnetic anisotropy on the off-diagonal magnetoimpedance in amorphous Co₆₇Fe₄Cr₇Si₈B₁₄ ribbon is investigated. Hard-ribbon-axis anisotropy is produced by continuous stress annealing. On applying DC bias current, the off-diagonal impedance becomes an antisymmetric function of applied field with a quasi-linear part around the zero field. Theoretical explanation of the phenomenon is based on classical electrodynamics. It is shown that the maximum on the frequency dependence of off-diagonal impedance, observed around 1 MHz, results from competition between the electromagnetic induction and the skin effect. The quasi-linear characteristic can be utilized in low-cost high-sensitive magnetic field sensors.     

Magnetoresistance anisotropy of strontium ruthenate films grown coherently on a TiO2-terminated SrTiO3(001) substrate

The SrRuO₃ films with a thickness of 80 nm have been coherently grown on a TiO₂-terminated SrTiO₃(001) substrate. Biaxial mechanical stresses induce a considerable difference between the unit cell parameters of the SrRuO₃ layer in the substrate plane (??3.904 ?) and along the normal to the substrate surface (??3.952 ?). The electrical resistivity of the SrRuO3 film decreases practically linearly with increasing magnetic field strength H when the latter is parallel to the current I _b and the projection of the easy magnetization axis in the substrate plane. At T = 4.2 K, ??₀ H = 14 T, and the magnetic field oriented along the hard magnetization axis, the negative anisotropic magnetoresistance of the grown layers reaches 16% and exerts a notice-able effect on the response of electrical resistivity of the SrRuO₃ film to the magnetic field.     

Giant magnetoimpedance in amorphous (Co0.93Fe0.7)63Ni10Si11B16 glass-coated microwire

Giant magnetoimpedance (GMI) effect has been measured in a glass-coated amorphous (Co_0.93Fe_0.7)₆₃Ni₁₀Si₁₁B₁₆ microwire as a function of DC magnetic field and up to the frequency of 11 MHz. The sample shows single peak GMI characteristics within the whole range of frequency. The domain structure of the above sample has been changed by applying tensile stresses up to 603 MPa and current annealing with a DC current of 50 mA for various time durations, and the corresponding effect on GMI has been studied in detail. A maximum change of 8.85% in MI of the as-quenched sample has been observed around a frequency of 5.05 MHz. Application of an external tensile stress reduces the GMI value by increasing the inner core domain, whereas heat treatment of the sample enhances the same. The square-shaped magnetic hysteresis loop of the as-quenched sample helps us understanding the MI results.     

Tb0.3Dy0.7Fe2合金磁畴偏转研究

研究了Tb_0.3Dy_0.7Fe₂合金在压磁和磁 弹性效应中的磁畴偏转和磁导率特性. 基于Stoner-Wolhfarth 模型能量极小原理, 绘制了自由能与磁畴偏转角度的关系曲线, 研究了压应力和磁场载荷作用下磁畴角度的偏转特性, 计算分析了不同载荷作用下磁畴偏转的磁导率特性, 并与实验数据进行比较论证. 研究表明,应力和磁场的作用都将使磁畴方向[111]和[111]发生角度跃迁, 直观有效地解释了材料巨磁致伸缩效应的机理; 应力和磁场作用下磁畴的偏转将使材料磁导率呈减小趋势, 其中磁场能对磁导率的影响大于应力能, 这一现象在小载荷作用下尤为明显. 实验结果表明, 磁导率的计算数据与实验数据符合得较好, 验证了计算方法的正确性. 理论分析对Terfenol-D磁畴偏转模型的完善 和磁化过程中磁滞回线的绘制非常有意义.     

Manipulating the magnetoimpedance by dc bias current in amorphous microwire

We have studied the effect of the internal circumferential magnetic field H_B created by the dc bias current I_B on longitudinal and off-diagonal magnetoimpedance (MI) in amorphous microwire with helical anisotropy and experimentally demonstrated that by changing the dc current I_B it is possible to considerably change the MI dependencies. The bias current applied to such microwire transforms the symmetric and hysteretic MI curve to asymmetrical and anhysteretic. The minimum of longitudinal MI curve shifts from the zero-field point. Reversing the bias current causes reversal of the bias field direction and results in a mirroring of the MI curves. It is proposed to apply a cross-checking of two MI curves with I_B of different polarity for magnetic field sensing. In particular, this method allows to overcome the drawbacks usually associated with longitudinal MI—namely the impossibility to determine the direction of an external axial magnetic field H_E and the low sensitivity near the zero-field point. Moreover, the operating range of the longitudinal MI sensor, in contrast to the off-diagonal one, can be much extended as it exhibits a quite high sensitivity in the field range up to one order of magnitude higher than the anisotropy field.     

Magnetic-field-induced changes in the photorefractive sensitivity of lithium niobate

The effect of a magnetic field on the average photorefractive sensitivity of an undoped LiNbO₃ crystal is studied by phase-mismatched second-harmonic generation. The experimental data obtained show the photorefractive sensitivity to reverse sign as the external magnetic field exceeds B ₁=?0.38±0.04 T. The magnetic field is oriented perpendicular to the crystal optical axis and to the plane of laser radiation polarization. The variation of the photorefractive sensitivity is associated with paramagnetic iron centers, whose photoionization probability depends on the direction of their magnetic moment relative to the optical axis.     

Magnetoimpedance in La0.67Pb0.33MnO3 manganite prepared by sol–gel method

In the present work, manganite La_0.67Pb_0.33MnO₃ was prepared by the sol–gel method. The difference between metal–insulator transition temperature T_MI (217 K) and Curie temperature T_c (342 K) in the sol–gel nanocrystalline manganite is mainly due to the grain boundary effect. The breaking of Mn–O–Mn bonds and strong scattering at the grain boundary cumber the transport. At room temperature 300 K, impedance and resistance increase with increasing frequency of ac currents. The observed dc magnetoresistance in sol–gel La_0.67Pb_0.33MnO₃ is related to the spin-polarized inter-grain tunneling and spin-dependent scattering at grain boundaries. The sol–gel manganite shows the magnetoimpedance characteristics, which are different from those of traditional sintered manganites and metallic giant magnetoimpedance materials. For sol–gel La_0.67Pb_0.33MnO₃ at low frequencies, the impedance experiences a peak under a low longitudinal field. In contrast, at high frequencies the peak phenomenon disappears, and the impedance drops sharply with low fields, which is due to the inter-grain or grain boundary effect. The permeability also sensitively varies with an application of transverse field. The magnetoimpedance effect in sol–gel nanocrystalline manganite is influenced by both field-induced permeability change and dc magnetoresistance.     

The modulated magnetic structure of α-Fe2O3: Ga weak ferromagnet

The domain structure of a diluted weak α-Fe₂O₃: Ga ferromagnet is considered. Magnetic fields of specific amplitudes and orientations applied in the basal plane of the crystal are found to produce nonuniform magnetic states due to a periodic deviation of the antiferromagnetic vector from the easy axis of crystallographic anisotropy. A phase diagram of the modulated magnetic state with the triad axis is constructed, and a dependence of the spatial period of modulation on an external magnetic field is studied. A phenomenological model and the nature of the magnetic superstructure discovered are discussed.     

Barium titanate single crystal electrooptic modulator

In this work, the magnitudes of the critical voltages are determined for various mutual orientations of the optical axis of the crystal, the vector of the applied electrical field intensity, and the wave vector of the optical radiation being modulated both for the linear and quadratic electrooptic effects in BaTiO₃ crystals. It is shown that the critical voltages are considerably lower for the linear electrooptic effect than for the quadratic. The minimal critical voltage which ensures a modulation depth of 100% for the linear electrooptic effect is equal to 30 V.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 7, pp. 35—38, July, 1973.