Bias Field Range Research Articles

Classification of hysteresis loops for exchange biased F/NM/F trilayer with antiferromagnetic interlayer coupling

Major hysteresis loops of the F/NM/F/AF structure are classified according to the magnetization process, with the assumption of coherent rotation for each F component and antiferromagnetic coupling between the two F layers. Magnetic stable states of the system as well as specific reversal modes of magnetization during each stage of the major hysteresis cycle are studied in detail based on energetics analysis, in the whole range of exchange bias field and interlayer antiferromagnetic coupling strength. The analysis shows 12 different types of major hysteresis loops for this system and is confirmed by simulation. Strategies to determine key material parameters for the F/NM/F/AF structure from major hysteresis loops are proposed and illustrated by numerical simulation. The research shows a subtle tuning of the magnetization process in the F/NM/F/AF structure, collectively by exchange biasing and interlayer antiferromagnetic coupling. Practically, this serves as a theoretical grounding for the identification of root causes of failure in the product analysis of GMR (giant magnetoresistance) sensors.

High-speed two-stage Hall magnetometer with increased resolution

It is demonstrated that the use of two coaxial solenoids instead of one in the magnetic field sensor cell allows one to divide the dynamic range of the magnetometer into a wide range of support magnetic field and a narrow range of bias field. This makes it possible to design, on the basis of a Hall sensor, a two-stage magnetometer with resolution of about 8·10 –11 Т. An increase in the speed of the magnetometer is achieved due to the use of a weak bias field, which represents a decaying oscillating local magnetic field (DOLMF). The leading edge of the DOLMF is used to search for a signal to be measured by a servo-system, while its decaying part after switching off the magnetic field step does not lead to magnetic relaxation in the sensitive element made of a superconducting YBa 2 Cu 3 O 7– x epitaxial film due to the fast process of vortex–antivortex annihilation. The design of the proposed magnetometer provides a high-speed measurement of not only constant but also alternating magnetic fields in the range from 10 –11 to 10 –7 Т at a frequency of up to ∼1 kHz, while preserving the high performance of the magnetometer. With the use of high-frequency magnetic field transducers, the magnetometer can operate in the frequency range of up to ∼100 kHz and measure both alternating and pulsed magnetic fields. • The use of two coaxial solenoids allows the design of a two-stage magnetometer with high resolution. • A superconducting epitaxial YBCO film is used as the sensitive element of the magnetometer. • The magnetometer provides a high-speed measurement of both constant and alternating magnetic fields.

An analytical model for nonlinear magnetoelectric effect in laminated composites

This paper presents an analytical and explicit theoretical model for the nonlinear magnetoelectric (ME) effect in laminated composites, in which the multi-field coupled properties of magnetostrictive materials are taken into account. The adopted explicit nonlinear constitutive equation is equivalent to the one in series form through expanding the magnetostriction as a Taylor series in the vicinity of the applied bias field. Then the field-dependent materials’ parameters are derived as functions of bias field and stress. Analytical solutions of nonlinear ME voltages in various modes are obtained simultaneously using mechanical differential equations, boundary conditions, and electric equations. It is predicted that the frequency-multiplying behavior observed in the composites can be tuned by bias magnetic field. Only even harmonics occur in the absence of bias field, while odd harmonics appear once a bias field is applied. An optimal bias field corresponding to the maximum linear voltage and zero nonlinear voltage is obtained, where the highest piezomagetic effect generates. It is proved that the ME distortions under a fixed ac field is actually caused by the competition results between the linear piezomagnetic effect and high-order magnetostrictive effect. Moreover, pre-stress may enhance ME voltages and reduce the optimal field due to the multi-field coupled character, and control the resonance frequency via ΔE effect. Eventually, the origin of the observed dual-peak phenomenon around resonance frequencies is revealed and explained theoretically, indicating that strong ME effect may be achieved in a wider bias field range. The analytical solutions from this study can be applied to design tunable ME sensors, energy harvesters, and some other nonlinear devices via manipulating the external bias field and pre-stress.

Two-range magnetoelectric sensor

In this study, we present a two-range magnetoelectric ME sensor design comprising of permendur (alloy of Fe-Co-V), nickel, and lead zirconate titanate (PZT) laminate composite. A systematic study was conducted to clarify the contribution of magnetostrictive layers variables to the ME response over the applied range of magnetic bias field. The two-range behavior was characterized by opposite sign of the ME response when magnetic dc bias is in different sub-ranges. The ME coefficient as a function of magnetic bias field was found to be dependent on the laminate composite structure.

Dynamic Magnetomechanical Behavior of Tb<italic>x</italic>Dy1–<italic>x</italic>Fe<italic>y</italic>Alloy Under Small-Signal AC Drive Fields Superposed With Various Bias Fields

The small-signal vibration model of Tb x Dy1– x Fe y plate was developed by mechanical-electrical analogy method. Based on the model, we demonstrated the reasonableness of measuring the small-signal magnetomechanical behaviors by a laser Doppler vibrometer. The strain coefficient of the Tb x Dy1– x Fe y plates was measured as a function of frequency and bias field ( $H_{\mathrm{ dc}})$ , and Young’s modulus, mechanical quality factor ( $Q_{m})$ , relative permeability, and magnetomechanical coupling coefficient were investigated as a function of $H_{\mathrm{ dc}}$ . Many novel characteristics were observed under the drive of a small-signal field (7.96 A/m). The change tendency of the strain coefficient at resonance differs from that at low frequency, and the resonant strain coefficients are significantly high (>85 nm/A) in a wide range of bias field from 4.78 to 55.72 kA/m. Both the negative- $\Delta E$ and positive- $\Delta E$ effects are observed, and the negative- $\Delta E$ effect in the low field range is also remarkable. In particular, $Q_{m}$ sharply decreases from the initial value of 104 to a minimum value of 11.4 and, then, increases slowly, and the ratio of the maximum variation of $Q_{m}$ over $H_{\mathrm{ dc}}$ to the minimum value of $Q_{m}$ exceeds $\sim 812.3$ %. This is an important systematic investigation on the small-signal dynamic magnetomechanical behavior of Tb x Dy1– x Fe y , and the results are highly beneficial to the designing of magnetostrictive devices.

Influence of small DC bias field on the electrical behaviour of Sr- and Mg-doped lanthanum gallate

One of the promising electrolyte materials for solid oxide fuel cells application, Sr- and Mg-doped lanthanum gallate La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM), is synthesized by conventional solid state ceramic route. X-ray Rietveld analysis confirms the formation of main orthorhombic phase at room temperature along with a few minor secondary phases. SEM micrograph reveals the grain and grainboundary morphology of the system. Electrical conductivity of the LSGM sample is measured in the temperature range 573–873 K and in the frequency range 20 Hz–1 MHz at a few small DC bias fields (at 0.0, 0.5, 1.0, 1.5 and 2.0 V). The conductivity spectra show power-law behaviour. Electrical conductivity of the sample is found to be weakly dependent on DC bias field. This is attributed to field-dependent bulk and grainboundary conduction processes. In the present system, under investigated bias field range, the possibility of formation of Schottky barrier is ruled out. The concept of grainboundary channel (pathway) modulation on the application of bias field is proposed.

Production of optically trappedRbCs87Feshbach molecules

We report the production of $^{87}$RbCs Feshbach molecules in a crossed-beam dipole trap. A mixture of $^{87}$Rb and $^{133}$Cs is cooled close to quantum degeneracy before an interspecies Feshbach resonance at 197\,G is used to associate up to $\sim5000$ molecules with a temperature of $\sim300$\,nK. The molecules are confined in the dipole trap with a lifetime of 0.21(1)\,s, long enough for future experiments exploring optical transfer to the absolute ground state. We have measured the magnetic moment of the Feshbach molecules in a magnetic bias field range between 181 and 185\,G to demonstrate the ability to control the character of the molecular state. In addition we have performed Feshbach spectroscopy in a field range from 0 to 1200\,G and located three previously unobserved resonances at high magnetic fields.

Quasilinear spin-torque nano-oscillator via enhanced negative feedback of power fluctuations

We report an approach to improving the performance of spin torque nano-oscillators (STNOs) that utilizes power-dependent negative feedback to achieve a significantly enhanced dynamic damping. In combination with a sufficiently slow variation of frequency with power this can result in a quasi-linear STNO, with very weak non-linear coupling of power and phase fluctuations over a range of bias current and field. An implementation of this approach that utilizes a non-uniform spin-torque demonstrates that highly coherent room temperature STNOs can be achieved while retaining a significant tunability.

Compact tunable bandpass filter on YIG substrate

Compact planar hairpin bandpass filters on yttrium iron garnet (YIG) substrate with a low insertion loss and large tunable range are designed, fabricated and characterised. The magnetic field tunable permeability of the YIG substrate led to a large tunable frequency range of 26% from 3.72 to 2.74 GHz when the magnetic field was varied from 0 to 800 Oe, corresponding to a large magnetically tunable frequency range of 325% of the −3dB bandwidth. In the meantime, the high permeability of the YIG substrate led to a good impedance match and a low insertion loss between 0.01–1.4 dB for the tunable bandpass filter. The operating frequency of the bandpass filter showed a non-monotonic dependence on the bias magnetic fields, which was caused by a non-linear bias field dependence of the permeability spectra. The bandpass filter showed a high insertion loss of up to 7dB within the bias field range of 200–600 Oe, which was due to the negative permeability of the YIG substrate that prohibited wave propagation.

Time-resolved investigation of magnetization dynamics of arrays of nonellipsoidal nanomagnets with nonuniform ground states

Time-resolved scanning Kerr microscopy measurements have been performed upon arrays of square ferromagnetic nanoelements of different sizes and for a range of bias fields. The experimental results were compared to micromagnetic simulations of model arrays in order to understand the nonuniform precessional dynamics within the elements. In the experimental spectra acquired from an element of length of 236 nm and thickness of 13.6 nm, two branches of excited modes were observed to coexist above a particular bias field. Below this so-called crossover field, the higher frequency branch was observed to vanish. Micromagnetic simulations and Fourier imaging revealed that modes from the higher frequency branch had large amplitude at the center of the element where the effective field was parallel to the bias field and the static magnetization. Modes from the lower frequency branch had large amplitude near the edges of the element perpendicular to the bias field. The simulations revealed significant canting of the static magnetization and effective field away from the direction of the bias field in the edge regions. For the smallest element sizes and/or at low bias field values, the effective field was found to become antiparallel to the static magnetization. The simulations revealed that the majority of the modes were delocalized with finite amplitude throughout the element while the spatial character of a mode was found to be correlated with the spatial variation in the total effective field and the static magnetization state. The simulations also revealed that the frequencies of the edge modes are strongly affected by the spatial distribution of the static magnetization state both within an element and within its nearest neighbors. Furthermore, the simulations suggest that collective modes may be supported in arrays of interacting nanomagnets, which act as magnonic crystals.

Dynamic magnetomechanical properties of epoxy-bonded (Tb 0.3Dy 0.7) 1– xPr xFe 1.55 (0 ≤ x ≤ 0.4) pseudo-1–3 magnetostrictive composites

Pseudo-1–3 magnetostrictive composites of 0.5 volume fraction are fabricated by embedding and aligning stabilizer (B, Co)-free, light rare earth (Pr)-contained magnetostrictive (Tb 0.3Dy 0.7) 1– x Pr x Fe 1.55 (0 ≤ x ≤ 0.4) particles with a size distribution of 10–300 μm in a passive epoxy matrix. The dynamic magnetomechanical properties of the composites with different Pr content x are investigated as a function of both bias field (=10–200 kA/m) and frequency (=25 Hz–70 kHz). The composites show similar qualitative trends in properties for all x with no frequency dispersion effect except for the resonance range. The dynamic relative permeability μ r 33 T demonstrates a decreasing trend with increasing x for the whole range of bias field due to the weakening of saturation magnetization with increasing Pr content. The two elastic moduli E 3 H and E 3 B , which result in negative-Δ E with maximum dynamic strain coefficient d 33 and dynamic magnetomechanical coupling coefficient k 33 near 140 kA/m bias in all composites, decrease with increasing x for all biases owing to the increasing compliance contribution from Pr. The (Tb 0.3Dy 0.7) 0.75Pr 0.25Fe 1.55 composite exhibits the largest d 33 and k 33 at 140 kA/m bias as a result of the compensation for magnetocrystalline anisotropy.

Effective bit addressing times for precessional switching of magnetic memory cells

We study the effective reversal times for bit-addressed precessional switching of the magnetization in magnetic random access memories (MRAMs). In our experiments the ultrafast magnetization dynamics of the free layer of a microscopic magnetic tunnel junction cell is accessed by means of ultrafast magnetotransport. Bit-addressed switching is studied by a fast perpendicular field pulse in combination with an easy axis bias field. For vanishing effective easy axis bias fields ultrafast quasiballistic switching of the magnetization can be achieved. Furthermore the characteristic oscillatory behavior of the precessional switching under variation of the pulse duration is observed. In a certain easy axis bias field range, the so-called addressing windows, the final magnetization state after the fast pulse is always aligned with the bias field which allows reliable bit addressing in a MRAM. Also here fast magnetization switching can be obtained. However, the decay time of the remaining magnetic excitations after bit addressing strongly depends on the initial and final orientations of the magnetization. This variation leads to different effective bit addressing times for switching and nonswitching events which could limit the MRAM operation speed.

Chain-like aggregates in magnetic fluids

Experimental investigation was carried out to detect in magnetic fluids the hypothetical aggregates analogous to polymer chains. The emphasis was placed on the method of studying low-frequency behavior of the magnetic fluids in crossed magnetic fields because this method is highly sensitive to the presence of chains in colloidal solutions. The theory of the method was improved in such a way that in the analysis of the results one can take into account the natural polydispersity of particles, interparticle interaction and demagnetization factor of the sample. The colloidal solutions of magnetite in decane with oleic acid used as a surfactant were investigated in a wide range of magnetic bias fields and magnetic phase concentration. In highly concentrated solutions no chains were detected whereas in diluted solutions the existence of short chains is the fact beyond question. The chains consist of three particles at the average and accumulate up to several percents of colloidal particles. The results of experiments were compared with predictions of the theoretical models.

Double-helix current drive revisited

The conditions required for efficient current drive in a weakly resistive plasma column, subject to a double helix traveling magnetic field, are examined in detail by using a simple non-linear Ohm's law for the plasma. In agreement with previous numerical work on the subject by Bertram [5], it is shown that field penetration is indeed strongly limited when large external bias longitudinal magnetic fields are used. However, there is a range of small external bias fields that allow for significant penetration when reasonable driving fields are used. This should be of interest for sustaining reversed-field pinches and toroidal screw pinches.

微小バブルの発生条件と安定性

Small bubbles were found in magnetic garnet films with the easy axis perpendicular to the film plane. Their diameter is smaller than that of normal bubbles. They appear after stripe domains have been chopped by a pulsed magnetic field with high intensity after a single chopping. Small bubbles are generated under a limited range of bias field and pulse field conditions. They remain stable within a narrow bias field range, above which they collapse and below which they transit irreversibly into larger elliptical bubbles or stripes. The contrast of the Faraday image of small bubbles is different from that of normal bubbles.

Formation and stability of small-size bubbles in garnet films

Anomalous magnetic bubble domains have been detected in garnet films after chopping stripe domains with a high intensity pulsed field. These bubbles have smaller diameters than typically observed bubbles under low bias fields. These small-size bubbles remain stable within a narrow bias field range, above which they collapse and below which they transit irreversibly into larger elliptical bubbles. The contrast exhibited by these bubbles in Faraday images is different from that of larger bubbles, suggesting that the magnetization of the small-size bubbles lies in the film plane.

High-frequency behavior and damping of Fe-Co-N-based high-saturation soft magnetic films

Magnetization dynamics measurements at subnanosecond time scale have been performed on Fe-Co-N high-saturation soft magnetic films with Permalloy nanolayer seeds and having a saturation magnetization of 1.9 MA/m (1900 emu/cm/sup 3/). The damping parameter /spl alpha/ varies from 0.011 to 0.018, depending on applied bias field. The peak frequencies of the imaginary permeability spectra and the zero-crossing frequencies of the real permeability spectra cannot be fitted with a fixed value of anisotropy field, indicating that the Kittel equation is only qualitatively valid for these films. A phenomenological damping criterion is established based on a small-signal solution of the Landau-Lifshitz-Gilbert equation: critical damping occurs in a soft magnetic film when the damping parameter is /spl alpha/=2//spl radic/(/spl chi//sub 0/), where /spl chi//sub 0/ is the initial permeability. The experimentally observed damping parameters are smaller than the critical value for the range of bias fields employed. The Fe-Co-N-based films have a ferromagnetic resonance frequency of 2 GHz at zero-bias field, showing great promise for applications in write heads and integrated inductors in a frequency range >1 GHz.

Quantum-well profile optimization for maximal Stark effect and application to tunable infrared photodetectors

Two procedures are described for quantum-well shape tailoring, such that both the Stark effect and the intraband absorption on the 1→2 transition in a prescribed range of bias fields are maximized. One of them relies on the isospectral transform of the Hamiltonian, and delivers quantum wells with continuously graded composition. The other uses the simulated annealing algorithm and delivers globally optimal stepwise-graded wells with a preset number of layers. Numerical calculation is performed for wells based on the AlxGa1−xAs alloy.

Design of circular patch antennas on ferrite substrates

The design and performance of circular microstrip patch antennas on biased ferrites are described. Fundamental and higher-order modes are discussed and measurements presented for two different ferrites. The high permittivity of the substrate reduces the dimensions of the patches. The cavity model is extended to include multilayer ferrite-dielectric substrates with electromagnetic feedline coupling to the patch. This provides an accurate and fast CAD tool for predicting the resonant frequency over a wide bias range. Applying a DC magnetic bias field normal to the ferrite allows tuning of the resonant frequency by about 15%. Two hands of circular polarisation are generated when the magnetic bias is applied. The resulting radiation performance of the patches, in terms of the gain and quality of circular polarisation, is presented. It is shown that the gain is not dependent on the applied bias field and that good axial ratios are measured over a wide range of bias fields, both above and below saturation magnetisation.

Harmonic generation of microwaves in semiconductors in the presence of deep repulsive traps

The characteristics of second-harmonic generation (SHG) of microwaves in a semiconductor at low lattice temperatures are investigated theoretically over a wide range of the dc bias field, beginning from a weakly heated carrier ensemble up to the onset of impurity breakdown giving due consideration to the effect of recombination of the nonequilibrium carriers in the presence of deep repulsive traps. The calculations are carried out for a parabolic law of dispersion and a scalar effective mass. In comparison to what can be obtained under the condition where the nonequilibrium carriers are captured by attractive traps or in the absence of recombination effects, the results of the quantum trapping of the nonequilibrium carriers by the repulsive traps bring in interestingly both qualitative and quantitative changes in the characteristics throughout the entire range of the bias field. The curves for the efficiency of SHG versus the bias field show a number of optimum fields for which the generation would be most efficient. The inadequacies of the present theory are pointed out and the scope for refinement of the same is discussed.