Basal Plane Anisotropy Research Articles

Edge and Basal Plane Anisotropy of a Preanodized Pencil Graphite Electrode Surface Revealed Using Scanning Electrochemical Microscopy and Electrocatalytic Dopamine Oxidation as a Molecular Probe.

Pencil graphite (PGE), an ultralow-cost and ready-to-use disposable-type electrode, has been used for various electrochemical and electroanalytical applications after its surface anodization (PGE*, * means preanodized surface). Indeed, systematic studies on mechanistic and surface features of PGE* have not yet been explored. Herein, we report anodized pencil graphite as a model system to study molecular level insights into the surface using a scanning electrochemical microscopy (SECM) technique and dopamine (DA) electrocatalytic oxidation reaction as a molecular probe. The as-prepared PGE* showed an appreciable electronic conductivity similar to the edge-plane graphitic sites (EPPG) of the highly pyrolytic graphitic electrode (HOPG) but without any surface deterioration that occurs with HOPG due to the instability of the EPPG. Physicochemical characterizations by FESEM, FTIR, Raman, and XPS techniques revealed a flake-like exfoliated PGE* surface with higher contents of carbon-oxygen especially phenolic/alcoholic functional groups than the PGE surface. Based on the chronocoulometric experiment, the number of functional groups formed on the PGE* was calculated as 10.9 × 10-10 mol cm-2. An independent SECM technique using ferricyanide as a redox probe showed the existence of a heterogeneous surface and exhibited an improved electron transfer activity due to the flake-like graphitic island on the PGE* surface. Investigated DA electrochemical oxidation on PGE* yielded about three times enhancement in the peak current signal and about 200 mV reduction in the oxidation potential over the PGE without any serious surface fouling feature that is related to the intermediate polydopamine formation on the basal-plane graphitic surface of the underlying electrode. As an independent electroanalytical study, a prototype electrochemical sensor using PGE* as a working electrode for instant detection of DA-containing pharmaceutical samples in a 1 mL Eppendorf vial has been demonstrated.

Magnetic Domain State and Anisotropy in Hematite (α‐Fe2O3) From First‐Order Reversal Curve Diagrams

AbstractHematite carries magnetic signals of interest in tectonic, paleoclimatic, paleomagnetic, and planetary studies. First‐order reversal curve (FORC) diagrams have become an important tool for assessing the domain state of, and magnetostatic interactions among, magnetic particles in such studies. We present here FORC diagrams for diverse hematite samples, which provide a catalog for comparison with other studies and explain key features observed for hematite. Ridge‐type signatures typical of uniaxial single‐domain particle assemblages and “kidney‐shaped” FORC signatures, and combinations of these responses, occur commonly in natural and synthetic hematite. Asymmetric features that arise from the triaxial basal plane anisotropy of hematite contribute to vertical spreading in kidney‐shaped FORC distributions and are intrinsic responses even for magnetostatically noninteracting particles. The dominant FORC distribution type in a sample (ridge, kidney‐shaped, or mixture) depends on the balance between uniaxial/triaxial switching. The identified signals explain magnetization switching and anisotropy features that are intrinsic to the magnetic properties of hematite and other materials with multiaxial magnetic anisotropy.

Nanocalorimetric evidence for nematic superconductivity in the doped topological insulator Sr0.1Bi2Se3

Spontaneous rotational-symmetry breaking in the superconducting state of doped $\mathrm{Bi}_2\mathrm{Se}_3$ has attracted significant attention as an indicator for topological superconductivity. In this paper, high-resolution calorimetry of the single-crystal $\mathrm{Sr}_{0.1}\mathrm{Bi}_2\mathrm{Se}_3$ provides unequivocal evidence of a two-fold rotational symmetry in the superconducting gap by a \emph{bulk thermodynamic} probe, a fingerprint of nematic superconductivity. The extremely small specific heat anomaly resolved with our high-sensitivity technique is consistent with the material's low carrier concentration proving bulk superconductivity. The large basal-plane anisotropy of $H_{c2}$ is attributed to a nematic phase of a two-component topological gap structure $\vec{\eta} = (\eta_{1}, \eta_{2})$ and caused by a symmetry-breaking energy term $\delta (|\eta_{1}|^{2} - |\eta_{2}|^{2}) T_{c}$. A quantitative analysis of our data excludes more conventional sources of this two-fold anisotropy and provides the first estimate for the symmetry-breaking strength $\delta \approx 0.1$, a value that points to an onset transition of the second order parameter component below 2K.

Uniaxial and basal plane anisotropy caused by Cr2+ impurity ions in CrBO3 crystals

In this study, the uniaxial and basal anisotropy governed by Cr2+ impurity ions was treated based on the Cr2+– BO33− ion vacancy model using single ion approximation. The anomalous behavior of the basal anisotropy at low temperatures was predicted.

Analysis of the crystal electric field parameters of YbNi4P2

The crystal electric field (CEF) scheme of YbNi4P2 is determined, based on experimental data from inelastic neutron scattering, heat capacity, susceptibility and NMR measurements. Despite the tetragonal crystal structure, 9 parameters are needed to describe the crystal field in YbNi4P2 due to the orthorhombic site symmetry of the Yb ion. A large basal plane anisotropy is detected by the local probe NMR. Our analysis yields CEF excitation energies of 8.5, 12.5 and roughly 30 meV and a ground state wave function that is dominated by the state. Furthermore, we present an analysis of the CEF scheme based on density functional theory calculations, which confirms the large basal plane anisotropy.

High-pressure study of the basal-plane anisotropy of the upper critical field of the topological superconductor SrxBi2Se3

We report a high-pressure transport study of the upper-critical field, $B_{c2}(T)$, of the topological superconductor Sr$_{0.15}$Bi$_2$Se$_3$ ($T_c = 3.0$ K). $B_{c2}(T)$ was measured for magnetic fields directed along two orthogonal directions, $a$ and $a^*$, in the trigonal basal plane. While superconductivity is rapidly suppressed at the critical pressure $p_c \sim 3.5$ GPa, the pronounced two-fold basal-plane anisotropy $B_{c2}^a/B_{c2}^{a^*} = 3.2$ at $T=0.3$ K, recently reported at ambient pressure (Pan et al., 2016), is reinforced and attains a value of $\sim 5$ at the highest pressure (2.2 GPa). The data reveal that the unconventional superconducting state with broken rotational symmetry is robust under pressure.

Magnetic properties of a Ho2Fe14Si3 single crystal

Magnetization of a Ho2Fe14Si3 single crystal was measured in a steady magnetic field of up to 9 T and in pulsed fields of up to 60 T applied along the principal axes. Ho2Fe14Si3 is a ferrimagnet below TC = 480 K, has a spontaneous magnetic moment of about 8 μB/f.u. (at T = 4.2 K) and exhibits a large easy-plane magnetic anisotropy. There is also a certain anisotropy within the basal plane, the b axis [120] being the easy-magnetization direction. In fields applied along the a and b axes field-induced first-order phase transitions are observed at 29 T and at 22 T, respectively. Along the easy axis b we observe also an S-shaped anomaly at about 47 T, which does not correspond to a phase transition. A simple model predicts that the two observed first-order transitions are the only ones taking place in Ho2Fe14Si3; the magnetization should subsequently grow continuously and arrive at saturation at ∼100 T. This is in stark contrast to the behavior of the parent compound Ho2Fe17, where as many as three sequential first-order transitions are expected for H||b. The reason for the disparity is that the basal-plane anisotropy constant KHo is at least one order of magnitude smaller in Ho2Fe14Si3 than it is in Ho2Fe17.

Domain structure transformation and magnetic susceptibility of Ho2Fe17 single crystals

For the first time, an investigation of the domain structure (DS) of Ho2Fe17 single crystals in the temperature range between 4 K and the Curie temperature of the crystals was carried out. Despite having ‘easy plane’ anisotropy in all temperature ranges and, therefore, no spin reorientation transitions and no expected changes in the DS, the Ho2Fe17 single crystals demonstrated a considerable transformation of magnetic domain structure at temperatures near 110 K caused by an increasing role of the magnetic anisotropy in the (001) basal plane at low temperatures. The domain walls of this low-temperature DS demonstrated coercivity to some degree, and this results in anomalies on temperature dependences of AC susceptibilities χ′(T) and χ″(T) measured in the excitation field μ0Hac = 1 mT (so-called non-SRT anomalies).

Effect of atomic modulation on the J-mixing-dominant magnetic anisotropy in SmFe7 epitaxial films

The magnetocrystalline anisotropy in TbCu7-type SmFe7 epitaxial films has been verified in relation to the 4f-electron character. The reinforcement of the 4f intermultiplet mixing by nitrogenation is verified from an enlarged basal-plane anisotropy. The sign of the uniaxial anisotropy constant is varied from positive to negative by nitrogenation, in contradiction to behaviors in Th2Zn17-type Sm2Fe17Nx. 57Fe Mössbauer spectroscopy and first-principles calculation revealed that the transformation of the Sm-neighboring Fe-atom plane from a rippling to a flat shape, which characterizes the Th2Zn17- and TbCu7-type local structures, markedly modulates the magnetic anisotropy, owing to the 4f semilocalized character.

Observing and tuning the density distribution of localized states of monolayer graphene oxide by using external electric field

The density distribution of localized states (DDLS) in monolayer graphene oxide (mGO) was observed and tuned under applied external electric field. The fluorescence anisotropy in mGO basal plane was observed which revealed the inhomogeneous DDLS. The enhanced and/or quenched fluorescence trajectories under electric field demonstrated the tuning of DDLS. Using the first-principles density-functional calculations, we demonstrated that the DDLS tuned by electric field could be attributed to the polarity effect of oxidized sp3 functional groups. The tunable DDLS is beneficial to improve and control the mGO's optoelectronic properties.

High frequency electromagnetic properties of interstitial-atom-modified Ce2Fe17NX and its composites

The magnetic and microwave absorption properties of the interstitial atom modified intermetallic compound Ce2Fe17NX have been investigated. The Ce2Fe17NX compound shows a planar anisotropy with saturation magnetization of 1088 kA/m at room temperature. The Ce2Fe17NX paraffin composite with a mass ratio of 1:1 exhibits a permeability of μ′ = 2.7 at low frequency, together with a reflection loss of −26 dB at 6.9 GHz with a thickness of 1.5 mm and −60 dB at 2.2 GHz with a thickness of 4.0 mm. It was found that this composite increases the Snoek limit and exhibits both high working frequency and permeability due to its high saturation magnetization and high ratio of the c-axis anisotropy field to the basal plane anisotropy field. Hence, it is possible that this composite can be used as a high-performance thin layer microwave absorber.

Magnetic field dependence of the basal-plane superconducting anisotropy in YBa2Cu4O8from small-angle neutron scattering measurements of the vortex lattice

We report a study of the basal-plane anisotropy of the superfluid density in underdoped YBa${}_{2}$Cu${}_{4}$O${}_{8}$ (Y124), showing the effects of both the CuO${}_{2}$ planes and the fully occupied CuO chains. From small-angle neutron scattering measurements of the vortex lattice, we can infer the superconducting (SC) properties for a temperature ($T$) range $T$ = 1.5 K to ${T}_{\text{c}}$ and magnetic induction $B$ from 0.1 to 6 T. We find that the superfluid density along a has a simple $d$-wave $T$ dependence. However, along b (the chain direction) the superfluid density falls much more rapidly with $T$ and also with increasing field. This strongly suggests the suppression of proximity-effect-induced superconductivity in the CuO chains. In addition, our results do not support a common framework for the low-field in-plane SC response in Y124 and related YBa${}_{2}$Cu${}_{3}$O${}_{7}$, and also indicate that any magnetic field induced charge-density-wave order in Y124 exists only for fields above 6 T.

Homogeneous and Goldstone modes of spin excitations in Cr1/3NbS2 helimagnet

Contributions of homogeneous and Goldstone modes of the spin precession were distinguished in FMR spectra of Cr1/3NbS2 chiral helimagnet. The resonance field of homogeneous mode is determined by uniaxial magnetic anisotropy. The resonance field of Goldstone mode is determined by six-fold anisotropy in basal plane. For the first time, it has been shown experimentally that effective excitation of Goldstone mode is realized only when microwave magnetic field vector h is perpendicular to wave vector of magnetic structure Q.

High-field magnetization of a DyFe5Al7 single crystal

The magnetization of a DyFe5Al7 single crystal with the tetragonal ThMn12-type structure was measured in pulsed fields up to 60T along the principal axes. The compound orders ferrimagnetically at TC=231K and exhibits a compensation of the Dy and Fe sublattices at Tcomp=93K. DyFe5Al7 displays a high magnetic anisotropy of the easy-plane type, the [100] axis is the easy magnetization direction with the spontaneous magnetic moment Ms=2.1μB/f.u. at T=2K. In a field applied along the easy axis, two field-induced hysteretic transitions are observed at 30 and 53T at T=2K. The critical field Hcr1 of the first transition depends strongly on temperature, whereas Hcr2 is practically temperature-independent. Above the transitions the magnetization continues to grow as a function of field, as the sublattice moments continue to rotate. To reach the full saturation (forced ferromagnetic state), fields of several Tesla higher than 60T are required. The field dependence of magnetization along the easy [100] axis was analyzed within a model for a ferrimagnet with an anisotropic dominant sublattice. The model correctly reproduces the behavior of magnetization in a magnetic field. The intersublattice molecular field on Dy and the basal-plane anisotropy constant of the Dy sublattice have been determined.

Field dependence of the superconducting basal plane anisotropy of TmNi2B2C

The superconductor TmNi2B2C possesses a significant four-fold basal plane anisotropy, leading to a square Vortex Lattice (VL) at intermediate fields. However, unlike other members of the borocarbide superconductors, the anisotropy in TmNi2B2C appears to decrease with increasing field, evident by a reentrance of the square VL phase. We have used Small Angle Neutron Scattering measurements of the VL to study the field dependence of the anisotropy. Our results provide a direct, quantitative measurement of the decreasing anisotropy. We attribute this reduction of the basal plane anisotropy to the strong Pauli paramagnetic effects observed in TmNi2B2C and the resulting expansion of vortex cores near Hc2.

Surface spin slips in thin dysprosium films

We report a theoretical investigation of new magnetic phases of thin dysprosium films. At the surfaces, the balance between exchange and anisotropy energies favors the alignment of spins along the basal plane easy axis directions. Near the Curie temperature, the helimagnetic phase is strongly modified, allowing surface spins to lock into the basal plane anisotropy easy directions. As the temperature raises, we find transitions to other surface lockin patterns, which are tunable by external field strengths of a few hundred milli-Oersteds.

Investigation of the c-axis and basal plane anisotropy in epitaxial NdCo5 thin films

The hexagonal NdCo5 phase has been prepared as an epitaxial film with an alignment of the c-axis perpendicular to the film plane and a unique orientation of the basal plane with respect to the substrate. In this highly textured film, the change in the magnetocrystalline anisotropy from easy c-axis to easy plane was observed by temperature dependent magnetization measurements. The specific orientation of the basal plane allows the investigation of the magnetic anisotropy in the easy plane state below 200 K without interference from the shape anisotropy of the thin film sample. We observe a difference between the a-axis and b-axis in saturation magnetization as well as in the magnetization reversal.

Anisotropic electron spin resonance ofY bIr2Si2

A series of electron spin resonance (ESR) experiments were performed on a single crystal of the heavy fermionmetal Y bIr2Si2 to map out the anisotropy of the ESR-intensityIESR which is governed by the microwave field component of theg-factor. The temperaturedependencies of IESR(T) and g(T) were measured for different orientations and compared within the range2.6 K ≤ T ≤ 16 K. The analysis of the intensity dependence on the crystal orientation with respect to boththe direction of the microwave field and the static magnetic field revealed remarkablefeatures: the intensity variation with respect to the direction of the microwavefield was found to be one order of magnitude smaller than expected from theg-factor anisotropy. Furthermore, we observed a weak basal plane anisotropy of the ESRparameters which we interpret to be an intrinsic sample property.

Pronounced basal plane anisotropy in the magnetoresistance of YbCo2Si2

AbstractWe have investigated the antiferromagnetic (AFM) state of YbCo2Si2 by means of magnetoresistance measurements for different field and current directions. We observed huge magnetoresistance effects, up to ±60% even at low fields B < 2 T, and a very rich variety of features. The results evidence a complex magnetic phase diagram, with a pronounced anisotropy even for different field directions within the basal plane. The analysis of these results gives valuable insight into the different AFM phases of YbCo2Si2.

Violation of the isotropic mean free path approximation for overdopedLa2−xSrxCuO4

Magnetotransport measurements on the overdoped cuprate ${\text{La}}_{1.7}{\text{Sr}}_{0.3}{\text{CuO}}_{4}$ are fitted using the Ong construction [Phys. Rev. B 43, 193 (1991)] and band parameters inferred from angle-resolved photoemission. Within a band picture, the low-temperature Hall data can only be fitted satisfactorily by invoking strong basal-plane anisotropy in the mean free path $\ensuremath{\ell}$. This violation of the isotropic-$\ensuremath{\ell}$ approximation supports a picture of dominant small-angle elastic scattering in cuprates due to out-of-plane substitutional disorder. We conjecture that both band anisotropy and anisotropy in the elastic-scattering channel strongly renormalize the Hall coefficient in ${\text{La}}_{2\ensuremath{-}x}{\text{Sr}}_{x}{\text{CuO}}_{4}$ across the entire overdoped regime.