Zero-field Point Research Articles

Analysis of the off-diagonal component of giant magnetoimpedance effect in Co-based (as-cast and stress-annealed) amorphous ribbons

We investigated the effect of the application of dc bias current IB on the off-diagonal magnetoimpedance (MI) components at 50 MHz in nearly-zero magnetostrictive Co66.5Fe3.5Si12.0B18.0 amorphous ribbons in as-cast and stress-annealed state. The off-diagonal MI component is anti-symmetrical with near-linear behavior within a certain field interval. For the off-diagonal response, the dc bias current is necessary to eliminate transverse domains present in the ribbon. A highly asymmetry of the off-diagonal dependence with the axial magnetic field was obtained when the bias current is applied along the ribbon. A rather high slope at the zero-field point was obtained, being more pronounced in the stress-annealed samples. This is probably related with the domain structure reorganization under the effect of the bias current. This behavior is ideal for a practical sensor circuit design.

The Magnetic Differentiation Technique for GMI Sensor

The “magnetic differentiation” technique is an original method for magnetic-field measurement. It is used for giant magnetoimpedance (GMI) sensors. The response is very different from the classical impedance measurement. To measure the external field $\text{H}_{\mathrm {ext}}$ , small increments of magnetic field, alternatively positive and negative $\pm \Delta H$ , are added to $H_{\mathrm {ext}}$ by a small coil around the GMI wire. The measured signal is the slope of the curve of the impedance $Z(H_{\mathrm {ext}})$ . The “magnetic differentiation” technique exhibits many advantages. The obtained response is a relatively simple curve with an odd symmetry. The zero of the sensor exactly corresponds to the zero magnetic field. The sensor is able to measure the “absolute zero” field, without any zero-adjustment. Around the zero-field point, the response is linear for a range of several tenths of A/m, with a very high sensitivity. The sensor response around the zero stays constant when the GMI wire is submitted to variable influence parameters, such as temperature or strain. Moreover, the measurement of high magnetic field can be easily made with an additional coil supplied by a closed-loop system; the operation point remains around the zero-field. Globally, the magnetic differentiation technique is an original way of using the GMI effect. The null signal at zero magnetic field, the linearity around the zero, and the odd response are very interesting properties, allowing the development of a new generation of GMI sensors.

Probing layer localization in twisted graphene bilayers via cyclotron resonance

Electron wavefunctions in twisted bilayer graphene may have a strong single layer character or be intrinsically delocalized between layers, with their nature often determined by how energetically close they are to the Dirac point. In this paper, we demonstrate that in magnetic fields, optical absorption (cyclotron resonance) spectra contain signatures which may be used to distinguish the nature of these wavefunctions at low energies, as well as to locate low energy critical points in the zero-field energy spectrum. Optical absorption for two different configurations -- electric field parallel and perpendicular to the bilayer -- are calculated, which are shown to have different selection rules with respect to which states are connected by the perturbation. Interlayer bias further distinguishes transitions involving states of a single layer nature from those with support in both layers. For doped systems, a sharp increase in intra-Landau level absorption occurs with increasing field as the level passes through the zero-field saddle point energy, where the states change character from single layer to bilayer.

Flexible chains of ferromagnetic nanoparticles.

We report the fabrication of flexible chains of ferromagnetic Ni nanoparticles that possess the ability to adapt other than the typically observed rigid (nearly) straight configurations in the absence of an external magnetic field. The dynamic mobility of the ferromagnetic chains originates from a layer of densely grafted polyethylene glycol macromolecules enveloping each nanoparticle in the chain. While ferromagnetic chains of unmodified Ni nanoparticles behave as stiff nickel nanorods, the chains made of the grafted nanoparticles demonstrate extreme flexibility. Upon changing the direction of the field, and inevitably going through a zero-field point, the shorter chains undergo chain-globule-chain transformation. The longer chains can bend to a high degree, attaining "snake-like" configurations.

Expanding the longitudinal magnetoimpedance sensor range by direct bias current

We investigated the effects of induced helical anisotropy and application of dc bias current IB on longitudinal magnetoimpedance (MI) in amorphous microwires both separately and together. We demonstrated that when both parameters are present, i.e., a dc bias current IB is applied to the microwire with induced helical anisotropy, the longitudinal MI sensor range can be considerably extended up to the fields considerably higher than its anisotropy field as the slope of MI curve dZ/dHE remains rather high. A highly asymmetric longitudinal MI dependence with a rather high slope at the zero-field point was obtained. Reversing the bias current IB causes reversal of the bias field direction and results in a mirroring of the MI dependence. This gives a possibility to determine both the sign and the magnitude of the external magnetic field. The obtained results can be used in development of the longitudinal MI sensors with increased sensitivity and/or range.

Manipulating the magnetoimpedance by dc bias current in amorphous microwire

We have studied the effect of the internal circumferential magnetic field HB created by the dc bias current IB on longitudinal and off-diagonal magnetoimpedance (MI) in amorphous microwire with helical anisotropy and experimentally demonstrated that by changing the dc current IB 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 IB 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 HE 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.

Symmetry breaking effect of dc bias current on magnetoimpedance in microwire with helical anisotropy: Application to magnetic sensors

We have investigated the effect of dc bias current IB on longitudinal magnetoimpedance (MI) in amorphous microwire with helical anisotropy and experimentally demonstrated that, by changing the dc current IB, it is possible to considerably affect the MI dependence. We propose to use this effect in developing a longitudinal MI sensor that overcomes the drawbacks usually associated with the longitudinal MI—namely the impossibility to determine the direction of an external axial magnetic field HE and the low sensitivity near the zero-field point. We demonstrate that a set of two measurements of longitudinal wire impedance with properly selected bias current IB allows determination of both the magnitude and direction of the magnetic field HE over an extended range, much higher than the sample anisotropy field. Additional features of the proposed method are the increased sensitivity and the detection of exceeding the operating range.

Dark region observed in fluorescence under the condition of a quadrupole magnetic field

We report an experimental observation of a dark region, around the zero-field point of a spheric quadrupole magnetostatic field, in spatial fluorescence emitted by dilute vapor atoms, which were excited by a circularly polarized traveling-wave laser tuned to a $\stackrel{\ensuremath{\rightarrow}}{F}F+1$ transition of ${}^{87}\mathrm{Rb}.$ By ruling out some possible sources, we show that the fluorescence decrease is associated with the three-dimensional nature of the magnetic field; it is a consequence of a precession of magnetic dipole moments induced by the radial magnetic field, which significantly cancels the orientation caused by optical pumping near the point of level crossing in the presence of the axial magnetic field. In an alternative way, the phenomenon can be understood as resulting from a destructive quantum interference, which is related to a coherent double-quantum coupling of atomic levels with two fields; one is the laser field, the other is the magnetic field. Analytic expressions for the half-width and contrast of the dark region have been obtained with a simplified model in the limit of low laser power. A qualitative agreement between the theory and experiment is found. The relevance of our study to laser cooling and trapping in a widely used magneto-optical trap based on such a quadrupole magnetic field is discussed. Finally, some applications of the phenomenon are also presented.

The effect of retrapping on short-circuit discharge of insulators

The basic equations relating to the release, trapping and transport of carriers in a charged insulator are introduced and specific relations relating to the zero-field point formulated. Application to a simple space charge model yields analytic solutions for the discharge current I as a function of time t. The effect of retrapping on It versus t plots is considered. An attempt is made to relate the theory to the previously obtained experimental data on high-density polythene and to existing computed solutions of other workers.

Phase diagram of the Potts model in an applied field

We use the technique of large-q expansion to investigate the phase diagram of the q-state Potts model in an applied field favoring order in one state. The method is valid for any number of dimensions. We find that for q>q/sub C/ (d) there is a first-order phase boundary starting at the zero-field transition point and terminating in a critical point in the interior of the hT plane. Using Pade approximants we obtain for d = 2 the shape and location of this line for various values of q. We also develop a lattice-gas expansion for any d which enables us, in the limit of large but finite q, to map the problem into an equivalent Ising expansion and thus find the exact location and nature of the critical point in this limit.

On the character of peculiarities in the position-space renormalization-group transformations

The image magnetic field is shown to be a discontinuous function of the object magnetic field near the zero-field point for T < T c in the thermodynamic limit in the position-space renormalization-group procedure as applied to an infinite-range interaction model.

Movement of a helical plasma segment in a magnetic configuration at a zero-field point

Using a simplified model we analyze the movement of a plasmoid injected axially into a magnetic configuration of revolution at a zero-field point of the “mono-cusp” type. One finds, in particular, that with no initial trapped field in the plasmoid, traversal, of the cusp is possible only if the injection speed of the plasmoid is greater than the Alfvén-equivalent speed. On the other hand, with an initial trapped field the cusp operates as an accelerator.