Amorphous Ferromagnets Research Articles

Stress-dependent Magnetization Processes in Co based Amorphous Microwires

Soft magnetic materials with high magnetic susceptibility are sensitive to changing magnetic fields and generate electrical voltage signals whose spectra contain higher harmonics. Magnetic susceptibility and saturation field are largely determined by magnetoelastic interactions in amorphous ferromagnets, respectively, the amplitudes of higher harmonics should depend on external mechanical stresses. In this work, we study the processes of magnetization reversal in amorphous microwires of two compositions: Co71Fe5B11Si10Cr3 and Co66.6Fe4.28B11.51Si14.48Ni1.44Mo1.69 under the action of external tensile stresses. For the first composition, mechanical stresses exceeding a certain limit (more than 350 MPa) lead to the transformation of the magnetic hysteresis from a bistable type to an inclined one. In this case, a sharp change of the harmonic spectrum is observed. In microwires of the second composition with an initially inclined loop, external stresses cause a monotonous increase in the slope of the hysteresis loop (a decrease in susceptibility). In this case, the amplitudes of higher harmonics change significantly at low stresses, less than 100 MPa. The results were obtained by remagnetization of microwire samples using a system of flat coils, which demonstrates the potential of using these materials as wireless sensors of mechanical stresses with remote reading.

Nonlinear magnetoelectric effect in a ring composite heterostructure

Objectives. The relevance of the study of magnetoelectric (ME) effect in ring ferromagnetic–piezoelectric heterostructures is due to the possibility of creating various ME devices having improved characteristics. A detailed investigation of the nonlinear ME effect in a ring composite heterostructure based on lead zirconate titanate (PZT) piezoceramics and Metglas® amorphous ferromagnetic (FM) alloy under circular magnetization is presented.Methods. The ME effect was measured by the low-frequency magnetic field modulation method. Excitation alternating- and constant magnetic bias fields were created using toroidal coils wound on a ring heterostructure for circular magnetization of the FM layer.Results. When excited with circular magnetic fields in a non-resonant mode, the ME ring heterostructure generates a nonlinear ME voltage of higher harmonics. The field and amplitude dependencies of the first three ME voltage harmonics were investigated. ME coefficients were obtained for the linear ME effect α(1) = 5.2 mV/(Oe·cm), the nonlinear ME effect α(2) = 6 mV/(Oe2·cm), and α(3) = 0.15 mV/(Oe3·cm) at an excitation magnetic field frequency f = 1 kHz. The maximum amplitudes of the 1st and 3rd harmonics were observed at a constant bias magnetic field H ~ 7 Oe, which is almost two times smaller than in planar PZT–Metglas® heterostructures.Conclusions. A nonlinear ME effect was observed and investigated in a ring heterostructure based on PZT piezoceramics and Metglas® amorphous FM alloy. Due to the absence of demagnetization during circular magnetization of the closed FM layer, nonlinear ME effects are detected at significantly lower amplitudes of the exciting alternating and constant bias magnetic fields as compared to planar heterostructures. The investigated ring heterostructures are of potential use in the creation of frequency multipliers.

Antiferromagnetic fourfold anisotropy induced exchange bias and magnetization reversal behaviors in CoFeB/IrMn bilayers

Bilayers of an amorphous CoFeB ferromagnet coupled to an epitaxially grown IrMn antiferromagnet, whose anisotropy is fourfold symmetric, are modeled to study the angular (φ) dependence of magnetization reversal mechanism and exchange bias behavior, based on a modified Monte Carlo method. It is found that positive and negative exchange-bias field (HE) maximum values appear in the antiferromagnetic easy-axis directions, and the HE zero-value transition/return points exist in the antiferromagnetic hard-axis directions. Furthermore, the value of HE(φ) is well fitted by considering cos3φ/sin3φ terms with larger or comparable coefficients as compared to the first-order terms in the Fourier expansion. Finally, the microscopic spin configuration results evidence these phenomena originating from the tailoring of antiferromagnetic spins by simply rotating the field-cooling direction between antiferromagnetic easy- and hard-axis directions. We open an exchange-bias mode governed by antiferromagnetic fourfold anisotropy and propose a method to precisely detect antiferromagnetic higher-order anisotropies on an exchange-bias support.

Nonlinear resonant magnetoelectric effect in a circumferentially magnetized ferromagnetic–piezoelectric ring heterostructure

The nonlinear magnetoelectric (ME) effect of voltage harmonic generation in a ring heterostructure, comprising a magnetostrictive layer of an amorphous ferromagnet and a piezoelectric layer of lead zirconate titanate is investigated. The structure was circumferentially magnetized with a permanent field H = 0–40 Oe and excited by a circular alternating magnetic field h = 0–3.6 Oe. At the radial acoustic resonance frequency of the structure of ∼54.2 kHz, the first voltage harmonic generation efficiency was 2.9 V/(Oe cm), the second was 0.95 V/(Oe2 cm), and the third was 0.21 V/(Oe3 cm). The absence of demagnetization effects in the ring structure, in comparison with a planar one, leads to a decrease in the optimal biasing magnetic field as well as to a change in the dependence of the second and third ME voltage harmonics on the magnetic field. The discovered nonlinear effect can be used to create frequency doublers.

Магнитоимпеданс в планарной магнитоэлектрической гетероструктуре аморфный ферромагнетик-пьезоэлектрик: модуляция электрическим полем

The magnetoimpedance effect and the converse magnetoelectric effect in a planar heterostructure consisting of mechanically bonded layers of amorphous ferromagnet FeBSiC and piezoelectric lead zirconate titanate are studied. Magnetoimpedance was observed in the frequency range of 0.1-40 MHz and bias dc magnetic field of 0-300 Oe; the maximum magnitude of the effect at a frequency of 10 MHz reached 12%. The converse magnetoelectric effect was observed in magnetic fields of 0-50 Oe; the maximum value of the effect at the structure acoustic resonance frequency of 40 kHz was 1.45 G/(V/cm). Amplitude modulation of the magnetoimpedance by an electric field at the resonance frequency of the structure with a coefficient of ~1∙10─2 is found. Modulation occurs as a result of a combination of the piezoelectricity and magnetostriction of the layers, which leads to a change in the transverse magnetic permeability and the thickness of the skin-layer of the ferromagnet.

Measurement of the spin-wave stiffness and energy gap in the magnon spectrum of amorphous ferromagnets by small-angle scattering of polarized neutrons

This work studies the magnetic excitations of amorphous ferromagnetic alloys (Fe40Ni40P14 11B6 and Fe48Ni34P18) by small-angle polarized neutron scattering in the geometry when the magnetic field is inclined towards the neutron beam direction. Polarized neutrons are used in order to extract the scattering arising from the spin waves only. In this case, the energy-integrated neutron cross section contains a component which depends on neutron polarization and has a left–right asymmetry in the plane determined by the directions of the field and the neutron beam. Small-angle polarized neutron scattering measurements on spin waves in amorphous iron–nickel alloys were performed at different values of the external magnetic field H and neutron wavelength λ. The spin-wave spectrum is quadratic in terms of the momentum transfer and contains both a field gap and an inherent gap Δ of a non-field nature: E q = Aq 2 + gμB H + Δ. Only by measuring simultaneously two dependencies of the cut-off angle θc – on the applied magnetic field and on the neutron wavelength – is it possible to obtain reliable information about the spin-wave stiffness and the spin-wave gap in these amorphous ferromagnetic systems.

Fourfold magnetic anisotropy induced in CoFeB/IrMn bilayers by interfacial exchange coupling

Exchange bias (EB) occurring in ferromagnetic (FM)/antiferromagnetic (AFM) bilayers conventionally can lead to a unidirectional magnetic anisotropy () as well as an accompanied uniaxial magnetic anisotropy (). We observed an additional fourfold magnetic anisotropy () induced by interfacial exchange coupling in amorphous CoFeB/epitaxial IrMn bilayers with an EB. Because of the combined effect of the three kinds of magnetic anisotropies, one- and two-step magnetic switching processes were observed at different magnetic field orientations, which usually appear in single-crystal FM layer with an intrinsic magnetocrystalline anisotropy but not in amorphous FM layer. The angular dependent magnetic switching fields can be nicely fitted by a phenomenological model based on domain wall nucleation and propagation with the in-plane along <100>. The ferromagnetic resonance measurements indicate that the specific strength of for EB along [100] is larger than that for EB along [110]. The induced can be understood by considering two types of AFM domains caused by both monatomic steps and defects and their induced net uncompensated spins along the in-plane <100> axes. The different dependence of on the EB direction are because of the different effects of growth magnetic field on the presence of AFM domains.

Magnetoelectric effects in a composite ferromagnet–piezoelectric heterostructure under pulsed excitation

Magnetoelectric (ME) effects in multiferroic ferromagnet–piezoelectric (PE) composite heterostructures result in a change in the electric polarization under the action of magnetic field or a change in the magnetization under the action of electric field and, therefore, are interesting from scientific point of view and for applications. In this work, the direct and converse ME effects are experimentally studied when a planar amorphous ferromagnet–PE lead zirconate–titanate structure is excited by magnetic and electrical field pulses with duration from microseconds to tens of milliseconds. Under the action of magnetic field pulses, the structure generates damped voltage oscillations with a frequency equal to the acoustic resonance frequency of the structure, as well as an exponentially decreasing dc voltage. By choosing the duration of magnetic pulses, it is possible selectively excite the linear ME effect in the structure or generate the second voltage harmonic at the resonance frequency. Electrical field pulses result in the excitation of damped magnetization oscillations at the resonance frequency of the heterostructure. The pulsed excitation method allows to estimate the ME conversion coefficients for both the direct and converse ME effects.

Magnetoimpedance modulation in a planar magnetoelectric ferromagnet – piezoelectric heterostructure

The effect of a giant change in the impedance of ferromagnetic materials under the action of an external magnetic field is widely used to elaborate highly sensitive magnetic field sensors. The purpose of this work was to demonstrate the possibilities of controlling the magnitude of the magnetoimpedance in a ferromagnet-piezoelectric structure using an electric field. Method. In the measurements, we used a planar heterostructure containing a strip of amorphous ferromagnet Metglas, 25 µm thick and 25 mm long, mechanically connected to a bimorph, 0.5 mm thick and 30 mm long, made of piezoceramic lead zirconate titanate. An alternating current with a frequency of 30 kHz...10 MHz was passed through the strip, the structure was placed in a longitudinal permanent magnetic field of 0...500 Oe, an alternating electric field up to 400 V/cm with a frequency of 60 Hz...50 kHz was applied to the piezobimorph, and the change in the impedance of the strip was recorded. Results. In the absence of electric field, a narrowing of the magnetoimpedance magnetic fields region with a decrease in the current frequency and saturation of the magnetoimpedance in magnetic fields above 334 Oe were observed. The maximum value of the magnetoimpedance reached 18% at a current frequency of 1 MHz. The application of electric field to the piezobimorph led to the appearance of side components in the frequency spectrum of the voltage on the ferromagnetic layer, which indicates the amplitude-phase modulation of the magnetoimpedance. The amplitude modulation coefficient reached a maximum value of 6 · 10−3 for the electric field frequency of 11.2 kHz and decreased monotonically with an increase in the magnetic field. The modulation of the magnetoimpedance occurs due to the converse magnetoelectric effect in the heterostructure, which leads to the modulation of the magnetization of the ferromagnetic layer, and the subsequent change in the relative magnetic permeability and thickness of the skin layer in the ferromagnet. The results obtained can be used to create magnetic fields sensors controlled by an electric field.

Magnon spectrum of the amorphous ferromagnet Co4P from atomistic spin dynamics

An anomaly in the magnon dispersion of the amorphous ferromagnet Co$_{4}$P, often referred to as a `roton-like' excitation, attracted much attention half a century ago. With the current interest in heat and spin currents in amorphous magnets, we apply modern simulation methods, combining reverse Monte Carlo to build the atomic structure and the stochastic Landau-Lifshitz equation for spin dynamics, to re-investigate the magnetic excitation spectrum. We find two magnon valleys, one at the origin and another at a finite wavenumber close to the observations, but without a magnon gap. We conclude that the second dip is due to Umklapp scattering caused by residual long-range order, which may be an alternative explanation of the putative roton excitation. Our study paves the way to study magnon transport in amorphous magnets and related spintronic applications.

Study of the Spin-Wave Dynamics of Amorphous Ferromagnets and Helimagnets with the Dzyaloshinskii‒Moriya Interaction

Study of the Spin-Wave Dynamics of Amorphous Ferromagnets and Helimagnets with the Dzyaloshinskii‒Moriya Interaction

Determination of small magnitudes of magnetostriction in amorphous microwires with an arbitrary type of magnetic anisotropy

The magnetic anisotropy and magnetic structure of amorphous ferromagnets are largely determined by magnetoelastic interactions due to the absence of magnetocrystalline anisotropy, e.g., in amorphous microwires with a glass sheath, the source of anisotropy is the mechanical stresses that arise in the ferromagnetic core upon manufacturing. Hence, to control the magnetic structure and magnetization reversal processes occurred in amorphous magnetics, it is necessary to know the magnetostriction coefficient of the material. We propose an improved approach to measuring extremely small values of the magnetostriction coefficient of ferromagnetic microwires with an arbitrary type of magnetic anisotropy and magnetic microstructure. The samples of amorphous wires in a glass sheath made of Co67Fe5B12Si14Cr3 alloys were studied. The type of magnetic anisotropy of the samples (from axial to circular) was changed using current annealing. The developed method is based on small-angle precession of magnetization around the wire axis, resulted from the effect of the axial magnetic field induced by an alternating current passed through the wire. A voltage signal generated in a detection coil wound around the sample at a frequency doubled with respect to the frequency of the alternating current was recorded using a lock-in amplifier. When exposed to external mechanical loads, the voltage signal changes, and an additional axial magnetic field (bias field) is required to maintain a constant level of this signal. The value of magnetostriction is determined from the dependence of the displacement field on mechanical loads. The maximum sensitivity of measurements in the range of 10–8 – 10–7 is achieved at a uniform magnetization, increased frequency of the alternating current, and high value of the ratio between the length and diameter of tested wire samples. The sign and magnitude of the magnetostriction constant change upon current annealing which correlates with modification of the magnetization curves. The results obtained can be used to determine and adjust the parameters of the actuators developed on the basis of the considered microwires (in particular, microsensors of mechanical stresses and microactuators).

Some Thermomagnetic and Mechanical Properties of Amorphous Fe75Zr4Ti3Cu1B17 Ribbons.

The microstructure, revealed by X-ray diffraction and transmission Mössbauer spectroscopy, magnetization versus temperature, external magnetizing field induction and mechanical hardness of the as-quenched Fe75Zr4Ti3Cu1B17 amorphous alloy with two refractory metals (Zr, Ti) have been measured. The X-ray diffraction is consistent with the Mössbauer spectra and is characteristic of a single-phase amorphous ferromagnet. The Curie point of the alloy is about 455 K, and the peak value of the isothermal magnetic entropy change, derived from the magnetization versus external magnetizing field induction curves, equals 1.7 J·kg−1·K−1. The refrigerant capacity of this alloy exhibits the linear dependence on the maximum magnetizing induction (Bm) and reaches a value of 110 J·kg−1 at Bm = 2 T. The average value of the instrumental hardness (HVIT) is about 14.5 GPa and is superior to other crystalline Fe-based metallic materials measured under the same conditions. HVIT does not change drastically, and the only statistically acceptable changes are visibly proving the single-phase character of the material.

Light and microwave driven spin pumping across FeGaB–BiSb interface

3-D topological insulators (TI) with large spin Hall conductivity have emerged as potential candidates for spintronic applications. Here, we report spin to charge conversion in bilayers of amorphous ferromagnet (FM) Fe_{78}Ga_{13}B_{9} (FeGaB) and 3-D TI Bi_{85}Sb_{15} (BiSb) activated by two complementary techniques: spin pumping and ultrafast spin-current injection. DC magnetization measurements establish the soft magnetic character of FeGaB films, which remains unaltered in the heterostructures of FeGaB-BiSb. Broadband ferromagnetic resonance (FMR) studies reveal enhanced damping of precessing magnetization and large value of spin mixing conductance (5.03 x 10^{19} m^{-2}) as the spin angular momentum leaks into the TI layer. Magnetic field controlled bipolar dc voltage generated across the TI layer by inverse spin Hall effect is analyzed to extract the values of spin Hall angle and spin diffusion length of BiSb. The spin pumping parameters derived from the measurements of the femtosecond light-pulse-induced terahertz emission are consistent with the result of FMR. Kubo-Bastin formula and tight-binding model calculations shed light on the thickness-dependent spin-Hall conductivity of the TI films, with predictions that are in remarkable agreement with the experimental data. Our results suggest that room temperature deposited amorphous and polycrystalline heterostructures provide a promising platform for creating novel spin orbit torque devices.

Field-induced vortex-like textures as a probe of the critical line in reentrant spin glasses

We study the evolution of the low-temperature field-induced magnetic defects observed under an applied magnetic field in a series of frustrated amorphous ferromagnets (Fe_{1-x}Mn_{x})_{75}P_{16}B_{3}Al_{3} (“a-Fe_{1-x}Mn_{x}”). Combining small-angle neutron scattering and Monte Carlo simulations, we show that the morphology of these defects resemble that of quasi-bidimensional spin vortices. They are observed in the so-called “reentrant” spin-glass (RSG) phase, up to the critical concentration x_{mathrm{C}} approx 0.36 which separates the RSG and “true” spin glass (SG) within the low temperature part of the magnetic phase diagram of a-Fe1−xMnx. These textures systematically decrease in size with increasing magnetic field or decreasing the average exchange interaction, and they finally disappear in the SG sample (x = 0.41), being replaced by field-induced correlations over finite length scales. We argue that the study of these nanoscopic defects could be used to probe the critical line between the RSG and SG phases.

Noise generation in a bilayer ferromagnet-piezoelectric heterostructure at the converse magnetoelectric effect

The paper describes first observation of parametric generation of noise in a composite multiferroic resonator. Resonator of a disk shape contains two mechanically coupled layers, one of which is the amorphous ferromagnet (FM) FeBSiC and the other is the piezoelectric (PE) lead zirconate titanate. DC magnetic field of 0–100 Oe is applied parallel to the plane of the resonator. Resonator is excited in the frequency range f = 2–10 kHz by a harmonic electric field with amplitude of up to 330 V cm−1 applied to the PE layer. Changes in the magnetization of the resonator caused by the converse magnetoelectric effect were recorded using an electromagnetic coil. With an increase in the excitation field to the threshold value, the parametric generation of harmonics and subharmonics with a discrete spectrum is observed, which then turns into a stochastic mode and a continuous spectrum of frequencies is generated. Noise density is hysterically dependent on the excitation field and non-monotonically depends on the dc magnetic field. Theory of parametric generation of the noise in a multiferroic resonator is developed, taking into account excitation of acoustic resonances and magnetoacoustic nonlinearity of the FM layer of the resonator. Theory, qualitatively describes the main characteristics of noise generation.

Spontaneous magnetization reversal and positive exchange bias effect in CoFeSiB/FeCrO3 heterostructures

The exchange bias (EB) phenomenon is an interface-driven effect; it depends on the interface spin configuration, which in turn is immensely related to antiferromagnetic spin configuration. Here, we chose an antiferromagnet, FeCrO3, with a corundum structure with randomly occupied cations. We report EB in the bilayer heterostructures that are composed of an amorphous CoFeSiB ferromagnet interfaced with an epitaxial FeCrO3 layer fabricated in-situ by pulsed laser deposition. The bilayers exhibit crossover from conventional negative EB to positive EB from room temperature down to 5 K. The crossover occurs for a cooling field that exceeds the interfacial coupling strength at the interface. Systems that show positive EB effects sometimes also display spontaneous magnetization reversal; the present bilayers showed spontaneous EB below 50 K. Temperature and field-dependent magnetisation studies reveal that the prepared heterostructure consists of uncompensated interface spins due to the disorder in the FeCrO3 layer that gets frozen at low temperatures, pins the spins in the ferromagnetic layer and causes the spontaneous EB effect.

Resonance magnetoelectric effect in a composite ferromagnet–dielectric–piezoelectric Langevin-type resonator

The direct and converse magnetoelectric (ME) effects in a Langevin-type resonator are observed and investigated for the first time. The resonator comprised a disk made of lead–zirconate titanate ceramic, located between two sapphire cylinders, with layers of amorphous ferromagnet glued to its side surfaces. ME effects were observed at frequencies of bending (∼70 kHz) and longitudinal (∼200 kHz) oscillations of the resonator upon its excitation by ac magnetic and electric fields. The field conversion efficiency for the direct ME effect was α E ≈ 31 V (Oe−1 cm−1). The magnetization modulation efficiency for the converse ME effect reached α B ≈ 0.72 G cm−1 V−1. The nonlinear ME effects such as frequency doubling, frequency mixing, and resonance frequency shift are observed at high field amplitudes. The magnitude of both the direct and converse ME effects in the resonator is controlled by an external dc magnetic field. The Langevin-type ME resonators can be used in highly sensitive magnetic field sensors, controlled resonators and transformers, and other components of electronics and microsystem technology.

Effects of ferromagnetic-material thickness on magnetoelectric voltage transformation in a multiferroic heterostructure

A magnetoelectric (ME) voltage transformer is fabricated on the basis of a ferromagnetic (FM)–piezoelectric (PE) heterostructure comprising two equally thick laminated layers of an amorphous FM alloy and a piezoceramic lead zirconate-titanate layer sandwiched between them. The structure, placed inside an excitation coil, is electrically poled and magnetized in the direction of the long axis. The primary voltage is applied to the coil and the secondary voltage is measured between the electrodes of the PE material. It is shown for the first time that the change in the total thickness of magnetic layers significantly influences the transformer´s characteristics. At the largest total thickness of FM layers of 138 µm, the open-circuit voltage transformation ratio K has a maximum value of about 20, and the power transfer efficiency η at a matched resistive load of about 20 kΩ reaches 45%. The variation of the control magnetic field in the range of 0–21.6 kA m−1 makes it possible to change the voltage transformation ratio K from zero to the maximum value. A simple model allows one to calculate the dependence of the characteristics of the ME transformer on the frequency of the primary voltage, thickness of the FM layers, control magnetic field, and the load.

Planar magnetoelectric voltage transformer based on a composite ferromagnet-piezoelectric heterostructure

A planar magnetoelectric voltage transformer based on a layered composite structure containing layers of amorphous ferromagnet and piezoelectric lead zirconate-titanate has been fabricated and investigated. The transformer uses a coilless system (the so-called "magnetic capacitor") to excite the direct magnetoelectric effect at the frequency of the acoustic resonance of the structure, which made it possible to significantly simplify the design. The voltage transformation ratio of the magnetoelectric transformer, in contrast to the known electromagnetic or piezoelectric transformers, can be tuned by an external magnetic field from zero to 13.2 with an increase in the field from zero to 30 Oe. The operating range of the output voltage of the transformer is 0−400 mV, and the output power reaches 9 μW at optimal load resistance. The technique for calculating the characteristics of the transformer is described.