Low External Field Research Articles

Magnetism in FeNiW disordered alloys: Experiment and theory

We report here an experimental study of magnetization of FeNiW alloys at different compositions. We have studied variation of magnetization with temperature (at low external fields) and magnetic field (at low temperatures). The alloy shows para to ferromagnetic transitions across the composition range. We do not find any indication of the spin-glass phase. We have supplemented the experimental work with theoretical analysis using the first-principles tight-binding linear muffin-tin orbitals based augmented space recursion method. Our theoretical estimates of magnetic moment and Curie temperatures agree well with experiment. Our mean-field phase analysis also does not indicate the possibility of a spin-glass phase.

Abnormal Domain Evolution in Lithium Niobate with Surface Layer Modified by Cu Ion Implantation

Anomalies of domain structure evolution in congruent lithium niobate with surface layers modified by implantation of Cu ions were investigated. Formation of residual domains and growth of areas occupied by domain patterns with high density of charged domain walls were studied during cyclic polarization reversal. Growth of unusual three-rayed star domains was observed during polarization reversal in low external field much less than the threshold field for conventional congruent lithium niobate. The complicated geometry of these domains in the bulk was revealed using scanning laser confocal microscopy combined with Raman spectroscopy.

Frequency addressing of nano-objects by electrical tuning of optical antennas

We first analyze the equivalent circuit parameters of linear wire optical nano-antennas in uniaxial anisotropic media. We then exploit the electro-optic response of a bipolar nematic liquid crystal to demonstrate tuning of an optical antenna using a low frequency external electric field as the control mechanism.

Modelling the influence of hydrogen bond network on chemical shielding tensors description. GIAO-DFT study of WALP23 transmembrane α-helix as a test case

Density Functional Theory (B3LYP/6-31G(d,p)) calculations of (15)N amide and (13)C carbonyl NMR chemical shielding tensors have been performed on WALP23trans-membrane alpha-helix peptide and compared to solid state NMR experiment performed on [(13)C(1)-Ala(13), (15)N-Leu(14)] specifically labelled peptide powder sample. Using either theoretical results obtained on the whole peptide or experimental data as reference, several simplest chemical models have been explored in order to reduce the computational cost while maintaining good theoretical accuracy. From this study, it appears that the hydrogen bond (N-H...O=C) network that exists in the alpha-helix has a major influence on the chemical shielding tensor and more specifically on the carbonyl (13)C sigma(22) eigenvalue. We show that a small truncated WALP_7 model is not adequate for (13)C(1) NMR description. The application of an external electric field in order to model the hydrogen bond network allows calculating chemical shielding tensors with accurate eigenvalues while the associated eigenvectors are largely modified. Finally, a 23 residues polyglycine peptide that includes the Alanine and Leucine residues for which NMR parameters must be calculated is proposed as the chemical model. This model is sufficient to mostly reproduce the calculation performed on WALP23 with major gain in computational time. Moreover, the application of a low intensity external electric field allows reaching the experimental accuracy for the determination of the eigenvalues.

A sensitive element based on the giant magnetoimpedance effect for detecting stray fields

Features of the magnetoimpedance effect of a sensitive element in the form of an amorphous ribbon of an Fe2.5Co64.5Cr3Si15B15 alloy were studied in the absence and presence of a ferromagnetic object in the form of a parallelepiped (0.65 × 3 × 25 mm) manufactured from high-carbon steel. Measurements of the magnetic-hysteresis loops and the values of the magnetoimpedance were performed for specimens of an amorphous ribbon (0.018 × 0.75 × 56 mm). The frequency range of exciting currents was 1–20 MHz. The possibility of detecting a model object by a contactless technique at a comparatively long distance (1 mm) was shown. The frequency ranges—a low-frequency range of 1–7 MHz and a high-frequency range of 10–20 MHz—within which detection can be performed were determined. A physical model of the detection of ferromagnetic model objects with a magnetoimpedance sensitive element in the form of an amorphous ribbon was proposed. Stray fields of model objects cause significant changes in the value of the magnetoimpedance at low external fields, and the frequency of the exciting current is an additional variable parameter.

Inhomogeneous Magnet NaMnIII[BP2O7(OH)3]: Ferromagnetic Clusters Inserted in a Metamagnetic Matrix

AbstractNaMnIII[BP2O7(OH)3] (1) has been hydrothermally synthesized and characterized by single‐crystal X‐ray diffraction. The manganese(III) cations are coordinated in a tetragonal‐elongated octahedral environment, indicating the possible large magnetic anisotropy. The MnO6 octahedra are interconnected by BO4/PO4 groups, forming a 3D triclinic net of MnIII backbone. Detailed dc and ac magnetic measurements have been conducted, and a preliminary H–T phase diagram is deduced accordingly. Compound 1 adopts an unusual magnetic inhomogeneous model, consisting of ferromagnetic clusters and a metamagnetic background: At the paramagnetic temperature region, the Curie–Weiss fit shows that the manganese atoms are all in the high spin state (S = 2), and the predominant interaction is ferromagnetic. An antiferromagnetic ground state forms at approximately 10 K, the isothermal magnetizations between 6.5 and 10 K show the typical metamagnetism. At approximately 6.5 K, a short‐range ordering of the ferromagnetic cluster occurs. Below 3.5 K, the ferromagnetic clusters tend to be frozen at low external field, and the inhomogeneous magnetism can be clearly indicated by the isothermal magnetizations below 3 K. This unusual magnetism cannot be understood by the popular mechanism in doped manganese perovskites, which are well known to be magnetically inhomogeneous because of the mixed valence of manganese. In compound 1, one of the possible reasons for this unusual behaviour is the competition between magnetic superexchange interactions and the large magnetic anisotropy of MnIII.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Spin-torque driven magnetization dynamics in a nanocontact setup for low external fields: Numerical simulation study

We present numerical simulation studies of the steady-state magnetization dynamics driven by a spin-polarized current in a point contact geometry for the case of a relatively large contact diameter (D = 80 nm) and small external field (H = 30 Oe). We show, that under these conditions the magnetization dynamics is qualitatively different from the dynamics observed for small contacts in large external fields. In particular, the 'bullet' mode with a homogeneous mode core, which was the dominating localized mode for small contacts, is not found here. Instead, all localized oscillation modes observed in simulations correspond to different motion kinds of vortex-antivortex (V-AV) pairs. These kinds include rotational and translational motion of pairs with the V-AV distance d ~ D and creation/annihilation of much smaller (satellite) V-AV pairs. We also show that for the geometry studied here the Oersted field has a qualitative effect on the magnetization dynamics of a 'free' layer. This effect offers a possibility to control magnetization dynamics by a suitable electric contact setup, optimized to produce a desired Oersted field. Finally, we demonstrate that when the magnetization dynamics of the 'fixed' layer (induced only by the stray field interaction with the 'free' layer) is taken into account, the threshold current for the oscillation onset is drastically reduced and new types of localized modes appear. In conclusion, we show that our simulations reproduce semiquantitatively several important features of the magnetization dynamics in a point contact system for low external fields reported experimentally.

Magnetization, Low Field Instability and Quench of RHQT ${\rm Nb}_{3}{\rm Al}$ Strands

Since 2005, we made and tested three RHQT Nb3Al strands, one with Nb matrix and two with Ta matrix, which are fully stabilized with Cu electroplating. We observed anomalously large magnetization curves extending beyond 1 to 1.5 Tesla with the F1 Nb matrix strand at 4.2 K, when we measured its magnetization with a balanced coil magnetometer. This problem was eliminated with the Ta matrix strands operating at 4.2 K. But with these strands a similar but smaller anomalous magnetization was observed at 1.9 K. We studied these phenomena with FEM. With the F1 Nb matrix strand, it is explained that at low external field, inter-filamentary coupling currents in the outer layers of sub-elements create a shielding effect. It reduces the inside field, keeps the inside Nb matrix superconductive, and stands against a higher outside field beyond the Hc of Nb. At an even higher external field, the superconductivity of the whole Nb matrix collapses and releases a large amount of energy, which may cause a big quench. Depending on the size of the energy in the strand or the cable, a magnet could quench, causing the low field instability. Some attempt to analyze the anomaly with FEM is presented.

The Dielectric Response of Spherical Live Cells in Suspension: An Analytic Solution

We develop a theoretical framework to describe the dielectric response of live cells in suspensions when placed in low external electric fields. The treatment takes into account the presence of the cell's membrane and of the charge movement at the membrane's surfaces. For spherical cells suspended in aqueous solutions, we give an analytic solution for the dielectric function, which is shown to account for the α- and β-plateaus seen in many experimental data. The effect of different physical parameters on the dielectric curves is methodically analyzed.

Field-induced ordering phenomena and non-local elastic compliance in two-dimensionalcolloidal crystals

Ordering phenomena in colloidal dispersions exposed to external one-dimensional, periodicfields or under confinement are studied systematically by Monte Carlo computersimulations. Such systems are useful models for the study of monolayers on a substrate. Wefind that the interaction with a substrate potential completely changes the miscibility of abinary, hard disc mixture at low external field amplitudes. The underlying orderingmechanisms leading to this laser-induced de-mixing differ, depending on which componentsinteract with the substrate potential. Generic effects of confinement on crystalline order intwo dimensions are studied in a model system of point particles interacting via a potential. The state of the system (a strip of widthD) depends very sensitively on the precise boundary conditions at the two confining walls.Commensurate, corrugated boundary conditions enhance both orientational order andpositional order. In contrast, smooth repulsive boundaries enhance only the orientationalorder and destroy positional (quasi-)long range order. As external fields have astrong impact on the elastic behaviour of colloidal crystals there is a need toanalyse the elastic response in such systems for the field-free case first. To thisaim we study the strain–strain correlation functions in a two-dimensional crystalformed by super-paramagnetic colloids, as monitored by standard video microscopy.

Induction of Cell Death by Magnetic Actuation of Nickel Nanowires Internalized by Fibroblasts

Magnetic nanomaterials with multimodal functionalities have emerged as a versatile platform for biomedical applications that range from basic cellular interrogation to clinical nanomedicine. In this work, we have prepared electrodeposited ferromagnetic nickel nanowires for efficient internalization into 3T3 fibroblasts. Agitation of the nanowires by a low external field induced cell death, as assessed by MTT viability assays. The response of the interleukin-6 (IL-6) gene expression of the fibroblasts to nanowire-mediated cellular manipulation was examined by quantitative real-time polymerase chain reaction (qRT-PCR). These nanowires exhibited significant potential as therapeutic and interrogative platforms for biomedicine.

Diverging giant magnetoresistance in ferromagnet-superconductor-ferromagnet trilayers

The relevance of pair-breaking by exchange and dipolar fields, and by injected spins in a low carrier density cuprate Y$_{1-x}$Pr$_x$Ba$_2$Cu$_3$O$_7$ sandwiched between two ferromagnetic La$_{2 / 3}$Sr$_{1 / 3}$MnO$_{3}$ layers is examined. At low external field ($H_{ext}$), the system shows a giant magnetoresistance(MR), which diverges deep in the superconducting state. We establish a distinct dipolar contribution to MR near the switching field(H$_c$) of the magnetic layers. At H$_{ext} \gg$ H$_c$, a large positive MR, resulting primarily from the motion of Josephson vortices and pair breaking by the in-plane field, is seen.

InN nanotips as excellent field emitters

Unidirectional single crystalline InN nanoemitters were fabricated on the silicon (111) substrate via ion etching. These InN nanoemitters showed excellent field emission properties with the threshold field as low as 0.9V∕μm based on the criterion of 1μA∕cm2 field emission current density. This superior property is ascribed to the double enhancement of (1) the geometrical factor of the InN nanostructures and (2) the inherently high carrier concentration of the degenerate InN semiconductor with surface electron accumulation layer induced downward band bending effect that significantly reduced the effective electron tunneling barrier even under very low external field.

Crystallization and electrical characteristics of ferroelectric Bi 3.15Nd 0.85Ti 3O 12 thin films prepared by a sol–gel process

Ferroelectric Bi 3.15Nd 0.85Ti 3O 12 (BNdT) thin films were deposited on Pt/Ti/SiO 2/Si substrates using a sol–gel process and Bi(NO 3) 3 was used as one of starting materials. Upon heating the major weight loss of the wet films occurred below 320 °C and nitrogen was almost completely removed at 650 °C. The films began to crystallize around 520 °C. The films annealed at 750 °C contain grains of 50–100 nm in diameter. The BNdT film capacitors with Pt top electrodes exhibited well-saturated hysteresis loops with a remanent polarization (2 P r) of 57.4 μC/cm 2 and a coercive field of 108.3 kV/cm. The Pt/BNdT/Pt capacitors did not show any significant fatigue up to 6×10 10 read/write switching cycles, and the retained charges remained nearly constant up to 10 4 s after applying a writing pulse. The leakage current density of the BNdT films is less than 5×10 −8 A/cm 2 under a low external field below 73 kV/cm.

Nucleation in a Sheared Ising Model: Effects of External Field

Simulations using the Forward Flux Sampling method have shown a nonmonotonic dependence of the homogeneous nucleation rate on the shear rate for a sheared two dimensional Ising model [R. J. Allen et al., arXiv: 0805.3029]. For quasi-equilibrium systems (i.e. in the absence of shear), Classical Nucleation Theory (CNT) predicts the dependence of the critical cluster size and the nucleation rate on the external magnetic field. We investigate the behaviour of the sheared Ising model as a function of the external field. At low external field strength, the same nonmonotonic behaviour holds and the peak in the nucleation rate is remarkably insensitive to the field strength. This suggests that the same external field-dependence holds for the enhancement of nucleation by shear at low shear rates and the suppression of shear at high shear rates. At high field strength, the nucleation behaviour is qualitatively different. We also analyse the size and shape of the largest cluster in the transition state configurations, as a function of the external field. In the sheared system, the transition state cluster becomes larger and more elongated as the field strength decreases. We compare our results for the sheared system to the predictions of the CNT for the quasi-equilibrium case, and find that the CNT cannot easily be used to describe nucleation in the system under shear.

망간 치환된 마그네타이트 박막의 자기적 특성 연구

Mn 치환이 역스피넬 산화물 <TEX>$Fe_3O_4$</TEX>에 미치는 영향을 조사하기 위하여 졸겔 스핀코팅 방법을 이용하여 다결정 <TEX>$Mn_xFe_{3-x}O_4$</TEX> 박막을 Si(100) 기판 위에 제작하고 그 구조적, 자기적, 자기저항특성들에 대한측정 및 분석을 수행하였다. X-선 회절 측정 결과, Mn 성분비가 증가함에 따라 x = 1.78까지 입방 구조를 유지하며, 그 격자상수는 증가함을 나타내었다. 이와 같은 격자상수 증가의 주된 원인으로 사면체 자리를 선호하는 <TEX>$Mn^{2+}$</TEX> 이온이 이온반경이 상대적으로 작은 사면체 자리의 <TEX>$Fe^{3+}$</TEX> 이온을 치환함에 의한 것으로 해석된다. 박막들에 대한 진동시료자화 측정 결과, Mn 성분비 증가에 따라 포화자화량은 큰 변화를 나타내지 않았는데, Mn과 Fe 이온들의 스핀 자기능률 값 비교를 통하여 그 정성적인 설명이 가능하다. 반면 Mn의 농도가 증가함에 따라 보자력은 감소하였는데, 이는 <TEX>$Mn^{2+}$</TEX> 치환에 의한 <TEX>$Fe^{2+}$</TEX> 이온 농도의 감소에 따르는 자기 이방성의 감소에 기인하는 것으로 해석된다. 자기저항 효과측정 결과, Mn 성분비 증가에 따라 감소하는 경향을 보였으며, 자화 의존도 변화와 비교분석 결과 다결정 <TEX>$Mn_xFe_{3-x}O_4$</TEX> 박막 시료들에서 나타나는 자기저항은 스핀분극된 carrier의 grain boundary tunneling 및 spin-flip 현상에 의한 것으로 해석된다. Polycrystalline <TEX>$Mn_xFe_{3-x}O_4$</TEX> thin films were synthesized on Si(100) substrates using sol-gel method and the effects of Mn substitution on the structural, magnetic, and magnetotransport properties were analyzed. X-ray diffraction revealed that cubic structure is maintained up to x = 1.78 with increasing lattice constant for increasing x. Such increase of the lattice constant is attributable to the substitution of <TEX>$Mn^{2+}$</TEX> (with larger ionic radius) ions into tetrahedral <TEX>$Fe^{3+}$</TEX>(with smaller ionic radius) sites. VSM measurements revealed that <TEX>$M_s$</TEX> does not vary significantly with x, qualitatively explainable by comparing spin magnetic moments of Mn and Fe ions. On the other hand, <TEX>$H_c$</TEX> was found to decrease with increasing x, attributable to the decrease of magnetic anisotropy due to the decrease of <TEX>$Fe^{2+}$</TEX> density through <TEX>$Mn^{2+}$</TEX> substitution. Magnetoresistance (MR) of the <TEX>$Mn_xFe_{3-x}O_4$</TEX> films was found to decrease with increasing x. Analysis of the MR data in comparison with the VSM results gives an indication of the tunneling of spin-polarized carriers through the grain boundaries of the polycrystalline samples at low external field and spin-flip of the carriers at high external field.

Synthesis and characterization of novel photoconducting carbazole derivatives in main-chain polymers for photorefractive applications

We have prepared hole-transporting polymers that have carbazole and 3,3′-dicarbazole in the main chain by polycondensation based on Fridel–Craft reaction via cations generated by proton of p-toluene sulfonic acid catalyst. In the device, they display high diffraction efficiency at low external electric field. The DiCz composite had a high diffraction efficiency of 82.4% at 52 V/μm and a photorefractive response time constant of 1.18 s at an electric field of 50 V/μm and a writing intensity of 60 mW/cm 2. The samples have excellent properties with respect to optical quality and shelf-lifetime.

Lack of static lattice distortion inTb2Ti2O7

We investigated the possibility of temperature-dependent lattice distortions in the pyrochlore compoundTb2Ti2O7 by measuring the internal magnetic field distribution, using muon spin resonance, andcomparing it to the susceptibility. The measurements are done at temperatures as low as70 mK and external fields up to 6 kG. We find that the evolution of the width of thefield distribution can be explained by spin susceptibility only, thus ruling out atemperature-dependent hyperfine coupling. We conclude that lattice deformations are absent inTb2Ti2O7.

Synthesis and Magnetic Properties of Spherical NiO Nanocrystals

Spherical NiO nanocrystals were obtained by thermal decomposition of the precursor obtained via a hydrothermal method using urea as precipitant and polyethylene glycol (PEG) as surfactant. The structure, morphology and magnetic properties of the products were examined by XRD, TEM, ED, IR and VSM. The results of the structure and magnetic measurements on NiO nanocrystals were discussed. The results showed that the products were nanocrystalline NiO with a diameter of 21 and 50 nm, respectively, after calcined at 300 and 500 oC. The calcined NiO nanocrystals exhibited the characteristics of weak ferromagnetism by magnetic analysis at room temperature. At low external field, the hysteresis loops exhibit low coercivity, Hc=144.7 and 200.5 Oe, for the samples calcined at 300 and 500 oC, respectively.

Defect‐induced ferrimagnetism in the half‐metallic Co2CrAl and Co2CrSi compounds

AbstractCo2CrAl and Co2CrSi are amongst the most studied Heusler alloys due to their half‐metallic character. These compounds are also well‐known to present ferromagnetism with high Curie temperatures. We show using first‐principles calculations that the creation of Cr antisites (Cr atoms at the Co sites) induces ferrimagnetism in these compounds without destroying the half‐metallic character of these alloys. The reduction of the total spin moment causes lower external fields and thus smaller energy losses in realistic magnetoelectronic, also known as spintronic, devices. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)