Field Dependence Research Articles

Anisotropic magnetocapacitance of antiferromagnetic cycloids in BiFeO3

Distinguishing different antiferromagnetic domains by electrical probes is a challenging task, which in itinerant compounds can be achieved, e.g., via the anisotropic magnetoresistance. Here, we demonstrate that in insulators, the anisotropic magnetocapacitance can be exploited for the same purpose. We studied the magnetic field dependence of the dielectric response in BiFeO3, one of the few room-temperature multiferroics. We observed a sizeable dielectric anisotropy upon the rotation of the modulation vector of the antiferromagnetic cycloid in the plane normal to the rhombohedral axis. Importantly, this anisotropy is characteristic of the cycloidal mono-domain state even in zero magnetic field, thus facilitating the determination of the antiferromagnetic domain population. This approach can be utilized to electrically distinguish between antiferromagnetic domains even in complex magnets, such as modulated spin structures, via the magnetodielectric coupling.

Synthesis and properties of 12442-family superconductor

We report a synthesis of two members of recently discovered high-temperature superconductors of 12442 family, with formula MCa2Fe4As4F2 (M=Rb,K) and transition temperatures of 32.7 and 34.6K, respectively. Quality of the samples was assessed using X-ray powder diffraction, superconducting transitions were identified through transport and magnetic experiments. The temperature dependence of the upper critical field and vortex activation energy was investigated under magnetic fields up to 19T. Two distinct thermally activated flux flow regimes were observed in both systems. Field dependences of activation energy U0(H) indicate a change in the properties of vortex matter in these regimes and distinctly different dissipation mechanisms, reminiscent of cuprate HTSC.

Spin glass behavior of single-crystalline ErFe2O4−δ

Abstract We investigated magnetic properties of ErFe2O4-δ single crystal containing oxygen vacancies with value of δ being estimated to be 0.066. The temperature dependence of dc magnetization in zero field cooling (ZFC) process reveals that ErFe2O4-δ undergoes a ferrimagnetic transition at 234 K followed by a broad peak at 224.3 K. The latter is attributable to the spin glass transition as indicated by the strong frequency dependence of temperature-dependent ac susceptibility. The frequency variation of transition temperature is described well by the dynamic scaling law and the critical exponent is similar to those reported for typical spin glasses. Additionally, the spin glass transition at 224.3 K is confirmed by the fact that the magnetic field dependence of irreversible transition temperature is coincident with the de Almeida-Thouless line and that the aging memory and rejuvenation effect is observed. Noticeably, ErFe2O4-δ single crystal exhibits the field-cooled hysteresis loop shift and training effect below 200 K, suggesting the occurrence of exchange bias effect originating from the spin glass phase.

Single-Molecule Magnet Behavior and Spin Structure of an FeIII7 Cartwheel Cluster Revealed by Sub-Kelvin Magnetometry and Mössbauer Spectroscopy: The Final Pieces of the Puzzle.

The spin frustration and other magnetic properties of the "cartwheel" heptanuclear cluster [FeIII7O3(O2CtBu)9(Me-dea)3(H2O)3] (Me-deaH2 = N-methyldiethanolamine) have been previously investigated; we present here a Mössbauer spectroscopic study and sub-Kelvin magnetization and ac susceptibility measurements which enable a complete magnetic picture of this frustrated cluster. 57Fe Mössbauer spectra above 150 K showed three doublets in a 1:3:3 ratio, which could be assigned by their respective quadrupole splittings to the central Fe(1) and the peripheral Fe(2) and Fe(3). The field dependence of the corresponding magnetic sextets at 3 K showed that the spins on the central Fe(1) and the three peripheral Fe(2) sites with O5N coordination are oriented mutually coparallel, while these are antiparallel to the spins on the peripheral Fe(3) sites with O6 coordination, resulting in an overall S = 5/2 ground state. This provides experimental confirmation of the previously proposed spin ground state structure. Upon cooling to sub-Kelvin temperatures, a crossover to spin blocking with TB ≈ 0.21 K could be observed. This single-molecule magnet behavior had been expected but had not been observable with a conventional SQUID. The anisotropy barrier, of 3-fold symmetry, can be described in terms of the parameter D/kB = -0.47 K and a fourth-order perturbation; the latter enables thermally activated quantum tunneling through the excited sublevel mz = ± 3/2, with an activation barrier of U/kB = 1.9 K.

Predicting proprioceptive cortical anatomy and neural coding with topographic autoencoders.

Proprioception is one of the least understood senses, yet fundamental for the control of movement. Even basic questions of how limb pose is represented in the somatosensory cortex are unclear. We developed a topographic variational autoencoder with lateral connectivity (topo-VAE) to compute a putative cortical map from a large set of natural movement data. Although not fitted to neural data, our model reproduces two sets of observations from monkey centre-out reaching: 1. The shape and velocity dependence of proprioceptive receptive fields in hand-centered coordinates despite the model having no knowledge of arm kinematics or hand coordinate systems. 2. The distribution of neuronal preferred directions (PDs) recorded from multi-electrode arrays. The model makes several testable predictions: 1. Encoding across the cortex has a blob-and-pinwheel-type geometry of PDs. 2. Few neurons will encode just a single joint. Our model provides a principled basis for understanding of sensorimotor representations, and the theoretical basis of neural manifolds, with applications to the restoration of sensory feedback in brain-computer interfaces and the control of humanoid robots.

A {Cr4Ln2} Complex with Exchange Coupled {Cr2} Units: Structural Description and Magnetic Study.

We have prepared and structurally characterized pivalate based {CrIII4LnIII2} complexes with Ln=Dy and Gd as well as the Y analogue, with the overall formula [CrIII4LnIII2(mdea)2(piv)10(OH)4], Ln= Gd, Dy and Y. We are reporting a detailed experimental magnetic properties study, including magnetization relaxation dynamics and calorimetric data, supported with quantum chemical calculations. The synthesis of the Y derivative, allowed to precisely identify the Cr(III)-Cr(III) exchange interaction magnitude which proved moderately strong and in agreement with known magneto-structural correlations. This result agrees with the observed double bridged {Cr2-mOH-mOR} units within the {Cr4Ln2} complexes.The Gd(III) complex magnetic properties can be properly described with the already established exchange coupled {Cr2} units, and a unique Gd(III)-Cr(III) exchange coupling parameter which proved anti-ferromagnetic in nature.The MCE characterization of this complexbased on magnetization and calorimetric data down to 2 K affords a moderate entropy change value, mainly affected by the strong Cr-Cr exchange interaction.The complex with Ln=Dy, showed SMM behaviour below 10 K under 0 DC applied field with negligible field dependence up to 3000 Oe and 10 kHz of AC field frequency. Two relaxation processes are clearly distinguished, with thermal barriers for an Orbach mechanism of ca. 20 cm-1 and 40 cm-1.

Exploring Elementary School Student's Computational Thinking In Terms Of Cognitive Style

Mathematics learning in the independent curriculum emphasizes the pedagogical dimension which aims to create active students so that they are able to develop their thinking patterns and get them used to finding solutions to their own problems. This study uses qualitative methods to investigate elementary school students' abilities through computational thinking, focusing on the cognitive styles of Field Independence (FI) and Field Dependence (FD). The subjects in this study amounted to 14 fifth grade students of public elementary schools in Klaten Regency, Central Java Province. Researchers used data collection instruments in the form of test, learning style questionnaires and interviews. The number of test compiled consisted of 5 items. Before use, the test were validated by 3 mathematics education experts and tested on 5 fifth grade students. After validation and testing, the test that can be used consist of 3 test. Based on the results of tests and learning style questionnaires, research took one of the students' test results with a Field Independent learning style and 1 student with a Field Dependent learning style. The results of this study indicate that there are differences in the computational thinking process of the two subjects in solving problems. At the abstraction stage, FI students are able to answer important factors that need to be considered in making conclusions, FI students are also able to explain alternative solutions to the problems given appropriately. Meanwhile, FD students tend to work directly so they tend to experience errors at the pattern recognition, algorithm thinking and generalization stages. Thus, it can be concluded that students with a field independent cognitive style are more active in using computational thinking processes than students with a field dependent cognitive style.

Signature of Superconductivity in Pressurized Trilayer-nickelate Pr4Ni3O10-δ

Abstract The discovery of high-temperature superconductivity in La3Ni2O7 and La4Ni3O10 under pressure has drawn extensive attention. Herein, we report systematic investigations on the structure, magnetism, and electrical resistance evolutions of Pr4Ni3O10-δ polycrystalline samples under various pressures. Pr4Ni3O10-δ exhibits density wave transitions on Ni and Pr sublattices at about 157.6 K and 4.3 K, respectively, and the density wave can be progressively suppressed by pressure. A structural transformation from the monoclinic P21/a space group to the tetragonal I4/mmm occurs at around 20 GPa. An apparent drop in resistance with evident magnetic field dependence is observed as pressure above 20 GPa, indicating the emergence of superconductivity. The discovery of superconductivity in Pr4Ni3O10-δ broadens the family of nickelate superconductors. Pr4Ni3O10-δ provides a new platform for investigating the mechanisms of superconductivity in the Ruddlesden–Popper phases of nickelates.

Investigating the singlet and triplet exciton recombination zone in organic light emitting diodes: Insights into field dependence, economic material usage and efficiency roll off

Investigating the singlet and triplet exciton recombination zone in organic light emitting diodes: Insights into field dependence, economic material usage and efficiency roll off

Microwave Hall measurements using a circularly polarized dielectric cavity.

We have developed a circularly polarized dielectric rutile (TiO2) cavity with a high quality-factor that can generate circularly polarized microwaves from two orthogonal linearly polarized microwaves with a phase difference of ±π/2 using a hybrid coupler. Using this cavity, we have established a new methodology to measure the microwave Hall conductivity of a small single crystal of metal in the skin-depth region. Based on the cavity perturbation technique, we have shown that all components of the surface impedance tensor can be extracted under the application of a magnetic field by comparing the right- and left-handed circularly polarized modes. To verify the validity of the developed method, we performed test measurements on tiny Bi single crystals at low temperatures. As a result, we have successfully obtained the surface impedance tensor components and confirmed that the characteristic field dependence of the ac Hall angle in the microwave region is consistent with the expectation from the dc transport measurements. These results demonstrate a significant improvement in sensitivity compared to previous methods. Thus, our developed technique allows for more accurate microwave Hall measurements, opening the way for new approaches to explore novel topological quantum phenomena, such as time-reversal symmetry breaking in superconductors.

THz generation by exchange-coupled spintronic emitters

The mechanism of THz generation in ferromagnet/metal (F/M) bilayers has been typically ascribed to the inverse spin Hall effect (ISHE). Here, we fabricated Pt/Fe/Cr/Fe/Pt multilayers containing two back-to-back spintronic THz emitters separated by a thin (tCr≤ 3nm) wedge-shaped Cr spacer. In such an arrangement, magnetization alignment of the two Fe films can be controlled by the interplay between Cr-mediated interlayer exchange coupling (IEC) and an external magnetic field. This in turn results in a strong variation of the THz amplitude A, with A↑↓ reaching up to 14 times A↑↑ (arrows indicate the relative alignment of the magnetization of the two magnetic layers). This observed functionality is ascribed to the interference of THz transients generated by two closely spaced THz emitters. Moreover, the magnetic field dependence A(H) shows a strong asymmetry that points to an additional performance modulation of the THz emitter via IEC and multilayer design.

Quantum theory of a potential biological magnetic field sensor: Radical pair mechanism in flavin adenine dinucleotide biradicals

Recent studies in vitro and in vivo suggest that flavin adenine dinucleotide (FAD) on its own might be able to act as a biological magnetic field sensor. Motivated by these observations, in this study, we develop a detailed quantum theoretical model for the radical pair mechanism (RPM) for the flavin adenine biradical within the FAD molecule. We use the results of existing molecular dynamics simulations to determine the time-varying distance between the radicals on FAD, which we then feed into a quantum master equation treatment of the RPM. In contrast to previous semi-classical models, which are limited to the low-field and high-field cases, our quantum model can predict the full magnetic field dependence of the transient absorption signal. Our model's predictions are consistent with experiments at physiological pH values.

Ferromagnetic resonance and magnetic anisotropy of 3-d metal wires with gradients of composition

We discussed experimental results concerning the ferromagnetic resonance spectra characteristics of Co-Ni and Co-Fe-Ni wires arrays with different gradients of composition deposited into porous of track etched polycarbonate membranes. The influence of interfacial boundaries and concentration gradients on the effective field of the investigated wires has been studied. An anomalous angular dependence of the FMR resonance fields is observed for wires arrays with a membrane pore density of ~18%.

Thermodynamics of black holes featuring primary scalar hair

In this work, we embark on the thermodynamic investigation concerning a family of primary charged black holes within the context of shift and parity symmetric Beyond Horndeski gravity. Employing the Euclidean approach, we derive the functional expression for the free energy and derive the first thermodynamic law, offering a methodology to address the challenge of extracting the thermal quantities in shift-symmetric scalar tensor theories characterized by linear time dependence in the scalar field. Following the formal analysis, we provide some illustrative examples focusing on the thermal evaporation of these objects. Published by the American Physical Society 2024

Tuning of magnetic properties and giant magnetoimpedance effect in multilayered microwires

We studied the magnetic properties and Giant Magnetoimpedance (GMI) effect in amorphous Co-rich microwires with similar chemical compositions and different diameters with magnetic (Co, Permalloy) and non-magnetic (Cu) layers deposited by magnetic sputtering onto glass-coating. Studies of magnetic properties and GMI effect of as-prepared microwires and the same microwires with deposited magnetic and non-magnetic layers reveal substantial impact of such layers on GMI effect and hysteresis loops. Both as-prepared samples present soft magnetic properties and high GMI effect. The contribution of magnetic layers is observed in hysteresis loop at higher magnetic field, with hysteresis loops similar to those observed in microwires with mixed amorphous-crystalline structure. Meanwhile, both magnetic and non-magnetic layers affect low field hysteresis loops of both samples. Additionally, the GMI ratio, ΔZ/Z, and magnetic field dependences of GMI ratio are substantially affected by the presence of magnetic and non-magnetic layers deposited onto glass-coating. We discussed the observed experimental dependences considering both change of the internal stresses originated by the sputtered layer as well as the magnetostatic interaction between the amorphous ferromagnetic nucleus and deposited magnetic layers.

Combined first-principles and Boltzmann transport theory methodology for studying magnetotransport in magnetic materials

Unusual magnetotransport behaviors, such as temperature-dependent negative magnetoresistance (MR) and bowtie-shaped MR, have puzzled us for a long time. Although several mechanisms have been proposed to explain these phenomena, the absence of comprehensive quantitative calculations has made these explanations less convincing. In our work, we introduce a methodology to study magnetotransport behaviors in magnetic materials. This approach integrates the anomalous Hall conductivity induced by the Berry curvature, with a multiband ordinary conductivity tensor, employing a combination of first-principles calculations and semiclassical Boltzmann transport theory. Our method also incorporates the temperature dependence of the relaxation time and the anomalous Hall conductivity, as well as the field dependence of the anomalous Hall conductivity. We initially test this approach on models, obtaining distinct behaviors of magnetoresistance and Hall resistivity across several types of magnetic materials, and then apply it to a Weyl semimetal Co3Sn2S2. The results, which align well with experimental observations in terms of magnetic field and temperature dependencies, demonstrate the efficacy of our approach. This methodology provides a comprehensive and efficient means to understand the underlying mechanisms of the unusual complex behaviors observed in magnetotransport measurements in magnetic materials. Published by the American Physical Society 2024

Enhancement of the Spin-Orbit Forbidden Transition Intensity in Nitric Oxide.

The magnetic field-induced modification of the electric-dipole forbidden transitions between the lowest rotational states of the 2Π1/2 and the 2Π3/2 substates of a 2Π diatomic radical is probed by model calculations for the ground electronic states of NO, OH and CH. Magnetic fields in the range of 0-12 T and more were considered and the intensities of the respective allowed rotational transitions for the given systems were calculated for comparison. In particular, NO shows enhanced field dependence of calculated intensities.

Anisotropic charge transport in strongly magnetized relativistic matter

We investigate electrical charge transport in hot magnetized plasma using first-principles quantum field theoretical methods. By employing Kubo’s linear response theory, we express the electrical conductivity tensor in terms of the fermion damping rate in the Landau-level representation. Utilizing leading-order results for the damping rates from a recent study within a gauge theory, we derive the transverse and longitudinal conductivities for a strongly magnetized plasma. The analytical expressions reveal drastically different mechanisms that explain the high anisotropy of charge transport in a magnetized plasma. Specifically, the transverse conductivity is suppressed, while the longitudinal conductivity is enhanced by a strong magnetic field. As in the case of zero magnetic field, longitudinal conduction is determined by the probability of charge carriers to remain in their quantum states without damping. In contrast, transverse conduction critically relies on quantum transitions between Landau levels, effectively lifting charge trapping in localized Landau orbits. We examine the temperature and magnetic field dependence of the transverse and longitudinal electrical conductivities over a wide range of parameters and investigate the effects of a nonzero chemical potential. Additionally, we extend our analysis to strongly coupled quark-gluon plasma and study the impact of the coupling constant on the anisotropy of electrical charge transport.

Electric-field-induced polarization anisotropy of interband photoluminescence in GaAs

Near-infrared photoluminescence spectra of the n-doped gallium arsenide epilayer under heating lateral electric field conditions are studied. The field dependences of hot electron and hole temperatures are determined from photoluminescence spectra and from solving the power balance equation using current–voltage characteristic. Polarization anisotropy of the photoluminescence radiation arising due to the anisotropy of hot electron distribution function and angular dependence of the interband optical matrix element in the momentum space is observed and investigated. The obtained experimental data are in satisfactory agreement with the theoretical model.

Effect of annealing conditions on giant magnetoimpedance of Co-rich glass-coated microwires

In this article we study the effect of conventional and stress annealing on the magnetic properties and giant magnetoimpedance (GMI) effect of Co65.3Si12.0B10.2Cr8.5Fe3.9Mo0.1 microwires with low and positive magnetostriction coefficient and low Curie temperature. A remarkable tuning capacity of both GMI effect and hysteresis loops is observed for a broad range of annealing conditions up to 350∘C and 320 MPa. We report appearance of double-peaks magnetic field dependencies of the GMI-ratio related with the coexistence of two different contributions originated by two different magnetic domain structure for the stress-annealed microwires. The observed experimental results are discussed in terms of the internal stresses relaxation derived from the annealing procedures, the induced magnetic anisotropy and local atomic reordering. The studies of Co-rich microwires with low and positive magnetostriction coefficient and low Curie temperature are potentially suitable for development of smart composites with embedded microwires for wireless temperature monitoring.