External Magnetic Eld Research Articles

Many-Channel Landauer-Like Conductance Formula in a Uniform Magnetic Field

For long time, it has been well recognized that transport in mesoscopic systems may relate to their scattering properties as it was originally proposed by Landauer [1, 2]. The single-channel conductance formula has been rst obtained heuristically as proportional to the ratio between transmission and re ection coe cients G = 2e 2 h T R [1]. Main attempts to derive this formula from linear-response theory (LRT) are known in the literature (see Ref. [3] and references therein). However, for long time, the generalization to a multichannel formula has been a great challenge for the physicists. The major di culty is that, in the LRT, the currentow in a sample is obtained as a response to an applied external perturbation inside the reservoirs in accordance to the Kubo viewpoint (KVP), whereas in the Landauer viewpoint (LVP) the current itself is considered as responsible for the induction of a position-dependent potential inside the sample. At this level, one has to note that it has been established heuristically that contacts between the reservoirs and the sample are at the origin of the di erence between the two points of view [4]. In recent works [5], a theory of transport that combines the LVP with the LRT and which does not consider the e ect of the contacts (called thereafter a self-consistent LRT) in order to calculate the relevant part of the density-operator for mesoscopic samples in the absence of an external magnetic eld, has been proposed. The obtained density-operator, necessary to evaluate all physical quantities, was used to derive a new and more plausible four-lead multichannel conductance formula that does not endure any of the inconsistencies cited in the literature for the other known formulae [3, 6]. This self-consistent LRT was recently extended to take into account the existence of a uniform magnetic eld [7, 8]. It has been shown that the density-operator may be written as a sum of two terms [7]: the rst represents the Kubo density-operator ρ [7 9] and the second denes the contribution of self-consistent e ects ρ [7, 8]: ρ = ρ + ρ. (1) Our aim in this work is the use of Eq. (1) to determine quantitatively the magnetoconductance of a four-lead mesoscopic measurement in terms of the scattering matrix elements whenever self-consistent e ects are relevant; that is, whenever the e ect of the contacts is neglected and the induced potential inside the sample is considered.

Properties of Single Molecule Magnet Ni4Mo12

At the present time two models can more or less describe the magnetic properties ofNi4Mo12. The rst one is based on the assumption of essential role of non-Heisenberg exchangeinteractions. The key feature of the second model is the symmetry breaking of exchange inter-actions. The present work is focused on the forecasting power of these spin models. Mechanismresponsible for non-Heisenberg exchange interactions is the same as mechanism resulting in in-teraction of spin chirality with external magnetic eld. An additional level splitting in magnetic eld can be observed in this case. The symmetry breaking of exchange interactions inevitablyleads to nonzero toroidal moment and related to it magnetoelectric phenomena.

Temperature Effect on the Structure of Transformer Oil Based Magnetic Fluids Using Acoustic Spectroscopy

The changes in structural arrangement in transformer oil based magnetic uids upon the e ect of an external magnetic eld and temperature were studied by acoustic spectroscopy. The attenuation of acoustic waves was measured as a function of an external magnetic eld in the range of 0 300 mT, parallel to the direction of the eld and as a function of temperature in the range of 15 35 ◦C for various magnetic nanoparticles concentrations. The strong in uence of the steeped magnetic eld on the acoustic wave attenuation was detected and its hysteresis was observed, too. When a magnetic eld is swept at a constant rate, the dominant interactions between the external magnetic eld and the magnetic moment of the nanoparticles occur, leading to the aggregation of magnetic nanoparticles and clusters formation. However, the temperature of magnetic uids has very important in uence on the obtained dependences, where the mechanism of thermal motion acts against the cluster creation. The observed in uences of magnetic eld and temperature on the investigated magnetic liquids structure are discussed.

Magnetization Reversal in Cobalt Nanocolumn Structures Obtained by Glancing Angle Deposition

An advanced deposition technique known as glancing angle deposition was used to fabricate randomly seeded magnetic cobalt columnar nanostructures. The existence of nanocolumns was con rmed by the cross-section scanning electron microscopy. The evolution in the magnetization reversal mechanism as a function of the lm thickness was investigated. The coercivity HC and MR/MS ratio (where MR and MS denote the remanent and saturation magnetization, respectively), derived from the magnetic hysteresis loops, are discussed as a function of the angle between the external magnetic eld and the surface normal. The direction of the magnetization easy/ hard axis and the columns inclination angle were determined on the basis of the angular dependences of the HC and the MR/MS. A crossover from the coherent rotation, based on the Stoner Wohlfarth model, to the curling reversal mode was observed.

Magnetic Field Dependence of Critical Attenuation in Ferromagnet

Many aspects of sound attenuation above and below critical point have been discussed in the literature [1, 2]. However, little attention has been paid so far to critical attenuation in the presence of the magnetic eld. A notable exception are the meaneld theory results [3 5]. The aim of this paper is to provide a dynamical matching formalism of the renormalization group which allows us to calculate the ultrasonic attenuation coe cient α(t, ω,H) by means of the e expansion not only in the asymptotic region H = 0 but also in the whole magneticeld range. We implement the method used in the calculation of the dynamic susceptibility in the ordered phase [6] for the evaluation of the sound attenuation coe cient in an external magnetic eld but use a modi ed matching condition suitable for H = 0. One of the best method for computation of the scaling functions is the method introduced by Nelson [7] for computation of the static correlation function above the critical temperature. It was a generalization of the renormalization group technique developed by Nelson and Rudnick [8] used to obtain the equation of state to rst order in e = 4 − d as well as the scaling functions for the susceptibility, speci c heat and etc. both in the ordered and disordered phase. Later this method was generalized by Achiam and Kosterlitz [9] to calculate the static momentum-dependent correlation function for arbitrary temperature and magnetic eld. Dengler et al. [10] were rst who generalized this method into dynamic correlation function in the disordered phase. In a recent paper Pawlak and Erdem [6] extended these results by obtaining (to rst order in e) the expressions for the dynamic susceptibility and correlation function for nonconserved Ising order parameter both above and below the critical temperature in zero magnetic eld. Generalization of this method to nonzero elds will be used for calculation of the ultrasonic attenuation.

Ultrafast Magneto-Optical and Magnetization-Induced Second Harmonic Generation Techniques for Studies of Magnetic Nanostructures

The work concerns application of titanium-sapphire femtosecond laser oscillator with 82 MHz repetition rate at 800 nm in measurements of ultrafast magneto-optical and magnetization-induced second harmonic generation e ects. Time-resolved magneto-optical Kerr e ect pump-probe method was used in investigation of ultrafast precession of magnetization in Fe/Au multilayers. Magnetization precession frequency, obtained as a function of magnetic eld applied, allows determining saturation magnetization and damping factor. The magnetization-induced second harmonic generation e ect has been used for study of garnet lms. This method allows to determine crystallographic symmetry and hysteresis loops in an external magnetic eld at 2ω laser induced frequency. The simultaneous measurements of linear and second order magneto-optical e ects enable to investigate magnetic interfaces in the studied structures.

Exchange Coupled NiFe/NiMn Bilayer Studied by a Vector Network Analyzer Ferromagnetic Resonance

Using cavity ferromagnetic resonance and vector network analyzer ferromagnetic resonance unidirectional, uniaxial and rotatable anisotropies were determined in permalloy(30 nm)/NiMn(56 nm). Resonance eld dependence on in-plane angle clearly shows unidirectional and uniaxial anisotropy. Rotatable anisotropy, which acts as internal magnetic eld independent of a direction of external magnetic eld, is observed on vector network analyzer ferromagnetic resonance as upward shift of the resonance frequency of Py/NiMn compared to Py lm. Even for a small exchange bias eld, this shift is signi cantly large.

Simulated Behavior of the 100-GHz Reentrant Linear Magnetron

The simulated behavior of a 100-GHz reentrant linear magnetron operated at a beam voltage of 2 kV is investigated using a two-dimensional particle-in-cell MAGIC code. The dimensions of the ten-vane reentrant resonator are selected to give a -mode resonance at 100 GHz, showing mode separation with the adjacent -1 mode of 0.2 %. The electron distribution and the time variation of the electric energy in each vane reveal that the -mode oscillation is dominant when the injected current density is several times higher than the start oscillation current, although the -1 mode is excited near the start oscillation current. With variation of the injected current density from 40 A/cm2 to 60 A/cm2, the generated power density is increased from 0.32 W/mm2 to 0.54 W/mm2 for an external magnetic eld of 0.78 T.

Exact solution of the mixed spin-1/2 and spin-S Ising-Heisenberg diamond chain

The geometric frustration in a class of the mixed spin-1=2 and spin-S Ising-Heisenberg diamond chains is investigated by combining three exact analytical techniques: Kambe projection method, decoration-iteration transformation and transfer-matrix method. The ground state, the magnetization process and the specic heat as a function of the external magnetic eld are particularly examined for different strengths of the geometric frustration. It is shown that the increase of the Heisenberg spin value S raises the number of intermediate magnetization plateaux, which emerge in magnetization curves provided that the ground state is highly degenerate on behalf of a sufciently strong geometric frustration. On the other hand, all intermediate magnetization plateaux merge into a linear magnetization versus magnetic eld dependence in the limit of classical Heisenberg spin S ! 1. The enhanced magnetocaloric effect with cooling rate exceeding the one of paramagnetic salts is also detected when the disordered frustrated phase constitutes the ground state and the external magnetic eld is small enough.

Radiation of ultra-wideband electromagnetic pulses by pulsed excitation of rectangular antenna

) antennas fastened to an end of a transmission line were pulse-excited by 0.5 ns rise time pulsesfor radiation of ultra-wideband signals. Wideband horn antennas were used for receiving the signals and wideband samplingoscilloscope was used for their visualization. The signals were registered in 1Œ5.6 GHz frequency range and had the quasi-Gauss envelope form. The plane antennas made of Cu, resistive Al, and resistive nanostructured Ni thin lms on supportingpolyethylene terephthalate of 100 „m thickness were used. It is shown that the radiation induced by pulsed excitation canbe used to control parameters of resistive coatings, including nanostructured ones, during a deposition process. The metallicnanostructured thin lms could be used to create plane radiating and receiving antennas having resistive elements and adaptedfor ultra-wideband signals. Ultra-wideband antennas with resistive nanostructured coatings, which have properties that canbe changed by external electric or magnetic elds or by optical radiation, can be used for modulation of radiated pulses.Further experimental studies are proposed to elaborate metallic nanostructured thin lms suitable to withstand possible damagesinduced by short high power electrical pulses.

Magnetic and thermal properties of a one-dimensional spin-1 model

We study the one-dimensional S = 1 Blume-Emery-Grifths model. Upon transforming the spin model into an equivalent fermionic model, we provide the exact solution within the Green’s function and equations of motion formalism. We show that the relevant response functions as well as thermodynamic quantities can be determined, in the whole parameters space, in terms of a nite set of local correlators. Furthermore, considering the case of an antiferromagnetic chain with single-ion anisotropy in the presence of an external magnetic eld, we nd three plateaus in the magnetization curve. In the neighborhood of the endpoints of the intermediate plateau, the specic heat shows a double peak structure.