Magnetic Peak Research Articles

Direct observation of electron density reconstruction at the metal-insulator transition in NaOsO3

The $5d$ transition-metal oxides offer further opportunities to test our understanding of the interplay of correlation effects and spin-orbit interactions in materials in the absence of a single dominant interaction. The subtle balance between solid-state interactions can result in mechanisms that minimize the interaction energy, and in material properties of potential use for applications. We focus here on the $5d$ transition-metal oxide $\mathrm{NaOs}{\mathrm{O}}_{3}$, a strong candidate for the realization of a magnetically driven transition from a metallic to an insulating state exploiting the so-called Slater mechanism. Experimental results are derived from nonresonant and resonant x-ray single-crystal diffraction at the Os $L$ edges. A change in the crystallographic symmetry does not accompany the metal-insulator transition in the Slater mechanism and, indeed, we find no evidence of such a change in $\mathrm{NaOs}{\mathrm{O}}_{3}$. An equally important experimental observation is the emergence of the (300) Bragg peak in the resonant condition with the onset of magnetic order. The intensity of this space-group-forbidden Bragg peak continuously increases with decreasing temperature in line with the square of intensity observed for an allowed magnetic Bragg peak. Our main experimental results, the absence of crystal symmetry breaking, and the emergence of a space-group-forbidden Bragg peak with developing magnetic order, support the use of the Slater mechanism to interpret the metal-insulator transition in $\mathrm{NaOs}{\mathrm{O}}_{3}$. We successfully describe our experimental results with simulations of the electronic structure and with an atomic model based on the established symmetry of the crystal and magnetic structure.

Stability of the spiral spin liquid in MnSc2S4

We investigate the stability of the spiral spin liquid phase in ${\mathrm{MnSc}}_{2}{\mathrm{S}}_{4}$ against thermal and quantum fluctuations as well as against perturbing effects of longer-range interactions. Employing ab initio density functional theory (DFT) calculations we propose a realistic Hamiltonian for ${\mathrm{MnSc}}_{2}{\mathrm{S}}_{4}$, featuring second $({J}_{2})$ and third $({J}_{3})$ neighbor Heisenberg interactions on the diamond lattice that are considerably larger than previously assumed. We argue that the combination of strong ${J}_{2}$ and ${J}_{3}$ couplings reproduces the correct magnetic Bragg peak position measured experimentally. Calculating the spin-structure factor within the pseudofermion functional-renormalization group technique, we find that close to the magnetic phase transition the sizable ${J}_{3}$ couplings induce a strong spiral selection effect, in agreement with experiments. With increasing temperature the spiral selection becomes weaker such that in a window around three to five times the ordering temperature an approximate spiral spin liquid is realized in ${\mathrm{MnSc}}_{2}{\mathrm{S}}_{4}$.

Vortex phase diagram study in the superconductor Ca3Ir4Sn13

We present the vortex phase diagram for a single crystal of a low Tc superconductor, Ca3Ir4Sn13 explored via detailed ac magnetic susceptibility () and ac electrical resistivity (ρ (H, T)) measurements. We observe a sharp peak effect (PE) phenomenon close to the superconducting-normal phase transition. This is in contrast to the findings for a second crystal of Ca3Ir4Sn13 in which a second magnetization peak was observed. Across the PE region, both the magnetization as well as the resistivity response depend on the thermomagnetic history of the sample. Further, the two measurement techniques (ac susceptibility and resistivity) reveal the occurrence of PE in different regions of the H−T phase space. In the high-field low-temperature region (H > 42 kOe and T < 3 K), the scans reveal a PE feature which is not observed in the ρ (H, T) data. On the other hand, the ρ (H, T) data display a sharp PE in the low-field high-temperature region (H < 23 kOe and T > 5 K) wherein the measurements do not show any signature of the PE. Two characteristic H–T regions have been identified in which only one of the two measurement techniques ( and ρ (H, T)) assists to identify the PE boundary.

Solar radius and luminosity variations induced by the internal dynamo magnetic fields

Although the occurrence of solar irradiance variations induced by magnetic surface features (e.g., sunspots, faculae, magnetic network) is generally accepted, the existence of intrinsic luminosity changes due to the internal magnetic fields is still controversial. This additional contribution is expected to be accompanied by radius variations, and to be potentially significant for the climate of the Earth. We aim to constrain theoretically the radius and luminosity variations of the Sun that are due to the effect of the variable magnetic fields in its interior associated with the dynamo cycle. We have extended a one-dimensional stellar evolution code to include several effects of the magnetic fields on the interior structure. We investigate different magnetic configurations, based on both observational constraints and on the output of state-of-the-art mean field dynamo models. We explore both step-like and simply periodic time dependences of the magnetic field peak strength. We find that the luminosity and radius variations are in anti-phase and in phase, respectively, with the magnetic field strength. For peak magnetic field strengths of the order of tens of kilogauss, luminosity variations ranging between 10^{-6} and 10^{-3} (in modulus) and radius variations between 10^{-6} and 10^{-5} are obtained. Modest but significant radius variations (up to 10^{-5} in relative terms) are obtained for magnetic fields of realistic strength and geometry, providing a potentially observable signature of the intrinsic variations. Establishing their existence in addition to the accepted surface effects would have very important implications for the understanding of solar-induced long-term trends on climate.

Local probe of irradiation-induced structural changes and orbital magnetism in Fe60Al40 thin films via an order-disorder phase transition

Hard x-ray absorption and magnetic circular dichroism spectroscopy have been applied to study the consequential changes of the local environment around Fe atoms and their orbital polarizations in 40 nm thick ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ thin films along the order-disorder $(B2\ensuremath{\rightarrow}A2)$ phase transition initiated by 20-keV ${\mathrm{Ne}}^{+}$ ion irradiation with fluences of $(0.75--6)\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{4pt}{0ex}}\text{ions}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The analysis of the extended x-ray absorption fine structure spectra measured at the Fe K edge at room temperature revealed an increased number of Fe-Fe nearest neighbors from 3.47(7) to 5.0(1) and $\ensuremath{\sim}1%$ of volume expansion through the transition. The visualization of the Fe and Al nearest-neighbor rearrangement in the first coordination shell of Fe absorbers via the transition was carried out by wavelet transformations. The obtained changes in Fe coordination are evidently reflected in the x-ray magnetic circular dichroism spectra which show an increased orbital magnetic moment of Fe atoms and a pronounced magnetic multielectronic excitations peak at $\ensuremath{\sim}60$ eV above the edge. The amplitudes of both peaks demonstrated similar dependencies on the irradiation fluence. The results of self-consistent density functional calculations on relaxed ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ model structures for the ordered $(B2)$ and the disordered $(A2)$ phases are consistent with the experimental findings and point to the formation of Fe-rich regions in the films studied.

Natural anisotropic nanoparticles with a broad absorption spectrum for solar energy harvesting

Optical properties of spherical nanoparticles, made of natural hyperbolic material-Bi2Te3, are studied numerically based on the Mie theory from the ultraviolet to the near-infrared region. The absorption efficiency factor of an anisotropic spherical nanoparticle is calculated by superimposing contributions from different polarizations. The numerical results show that Bi2Te3 nanoparticles exhibit broadband absorption in the wavelength range of 300–2400 nm with several absorption peaks, owing to the simultaneous excitation and superposition of electric and magnetic dipole resonances. In order to elucidate the mechanism of resonance absorption peaks of Bi2Te3 nanoparticles, the Mie theory is used to calculate the electric field inside these nanoparticles by assuming the nanoparticle to be isotropic, having the same dielectric function as either parallel or perpendicular dielectric function of Bi2Te3. It is shown that increasing the size of the Bi2Te3 nanoparticle causes a redshift of the resonance wavelength, an increase in the magnetic dipole absorption peak, and a decrease in the electric dipole absorption peak. The numerical results also show that Bi2Te3 nanoparticles have high absorption efficiency factor in the wavelength of 300–2400 nm, demonstrating their potential to be used for harvesting solar energy.

Electron mirror instability: particle-in-cell simulations

Properties of the electron mirror instability and its competition with the usually dominant whistler (electron cyclotron) instability driven by the electron perpendicular temperature anisotropy are investigated at the linear level using a Vlasov linear solver and at the nonlinear level using a two-dimensional full particle code. The simulation results show that the linearly subdominant electron mirror instability may compete at the nonlinear level with the whistler instability and may even eventually become the dominant mode that generates robust non-propagating sub-ion-scale coherent structures in the form of magnetic peaks.

Effect of Cr atoms in vortex dynamics of NbSe2 superconductor and study of second magnetization peak effect

We report on the high quality single crystal of CrxNbSe2 growth to study the effect of Cr impurity on the vortex dynamics of NbSe2 superconductor using M–H curves with different relaxation rates. The field dependence nature of critical current density (Jc) is well investigated using Larkin-Ovchinnikov collective pinning model and Anderson-Kim’s plastic creep model. We have observed significant improvement in the field dependence nature of Jc with increasing Cr impurities in NbSe2. This reveals the crossover of vortex lattice from elastic (E) to plastic (P) creep region causing second magnetization peak (SMP) or fishtail effect at highest Cr concentration, which has not been yet observed. Also addition of Cr impurity enhances the Jc up to 4 × 105 A/cm2. However, the interaction between core region of vortices and pinning centres ensure the δl pinning in Cr0.0005NbSe2 and significantly improved with increment of Cr impurity, which allowing both δl and δTc mechanism. Both, δl and δTc core pinning strongly enhance the coupling between vortices in pinning potential of nonstatistical distribution of Cr impurity, which makes 3D magnetic fluxes into 2D nature. Furthermore, the point pining and surface pinning mechanisms are responsible for pinning at low magnetic field region in Cr0.0005NbSe2. The appearance of prominent peak in pinning force density near upper critical field (Hc2) indicates the lattice softening at higher Cr impurity. These studies not only provide fundamental study of vortex dynamic but also open the door for further study in low Tc superconducting materials.

Growth and physical properties of Bi2Sr2CaCu2O8+x crystals grown by a simple pressure technique and comparison with regrowth self-flux technique

Single crystals of a high temperature superconductor Bi2Sr2CaCu2O8+x (2212) were grown using two different techniques, namely, regrowth technique as well as a pressure technique and the results are compared. Single crystal X-ray diffraction (XRD) performed on crystals grown using both techniques show no impurity or intergrowth phase. However, powder XRD reveals the presence of only Bi2Sr2CuO6+x(2201) intergrowth phase in crystals grown using pressure technique, with additional impurity phases also found in the crystals grown using regrowth technique. Rietveld refinement showed the weight fraction of 2212 as 93% in crystals grown from pressure technique, confirming the excellent quality of the as-grown crystals. The transition temperature of crystals grown using both techniques was ∼90 K. It was found that pressure technique gives crystals that have a higher superconducting volume fraction (∼100) than that of crystals grown using regrowth technique. Structural and magnetization measurements reveal that the as-grown crystals grown using pressure technique are of high quality, comparable to those grown using traveling solvent floating zone technique. Crystals grown by both the techniques exhibit second magnetization peak anomaly.

A modified residual stress dependent Jile-Atherton hysteresis model

Non-oriented electrical steel is widely used in the manufacturing of motor stators. The magnetic property will be influenced by the residual stress induced from manufacturing processes. The classical Jile-Atherton hysteresis model with constant parameters is widely used to describe the magnetization characteristic of ferromagnetic materials. In the model, the parameter a represents the domain density, k is the pinning factor reflecting the strength of the pinning effect, α is the domain coupling parameter affecting the slope of the mid-segment of the hysteresis loop and C is the coefficient of reversibility. However, all the four parameters are influenced by residual stress. Furthermore, the influence of magnetic peak induction on parameters can’t be ignored according to our work. The classical Jile-Atherton hysteresis model cannot reflect the influence of residual stress and magnetic peak induction. In this paper, a modified residual stress dependent Jile-Atherton hysteresis model was proposed, whose parameters are extended to functions related to residual stress and magnetic peak induction. A novel device was designed to carry out magnetic property testing experiments under large stress. The parameters were identified by a MATLAB program and the influence of residual stress and magnetic peak induction was analyzed. The accuracy of parameters was validated by experiments, which showed that the modified model worked well in describing the magnetization characteristic under different stresses and magnetic peak inductions. This could provide some guidance for the manufacturing processes of motor stators.

Tuning Magnetic Soliton Phase via Dimensional Confinement in Exfoliated 2D Cr1/3NbS2 Thin Flakes.

Thin flakes of Cr1/3NbS2 are fabricated successfully via microexfoliation techniques. Temperature-dependent and field-dependent magnetizations of thin flakes with various thicknesses are investigated. When the thickness of the flake is around several hundred nanometers, the softening and eventual disappearance of the bulk soliton peak is accompanied by the appearance of other magnetic peaks at lower magnetic fields. The emergence and annihilation of the soliton peaks are explained and simulated theoretically by the change in spin spiral number inside the soliton lattice due to dimensional confinement. Compared to the conventional magnetic states in nanoscale materials, the stability and thickness tunability of quantified spin spirals make Cr1/3NbS2 a potential candidate for spintronics nanodevices beyond Moore's law.

Manifolds of magnetic ordered states and excitations in the almost Heisenberg pyrochlore antiferromagnet MgCr2O4

In spinels ACr2O4 (A=Mg, Zn) realisation of the classical pyrochlore Heisenberg antiferromagnet model is complicated by a strong spin-lattice coupling: the extensive degeneracy of the ground state is lifted by a magneto-structural transition at TN=12.5 K. We study the resulting low-temperature low-symmetry crystal structure by synchrotron x-ray diffraction. The consistent features of x-ray low-temperature patterns are explained by the tetragonal model of Ehrenberg et. al (Pow. Diff. 17, 230( 2002)), while other features depend on sample or cooling protocol. Complex partially ordered magnetic state is studied by neutron diffraction and spherical neutron polarimetry. Multiple magnetic domains of configuration arms of the propagation vectors k1=(1/2 1/2 0), k2=(1 0 1/2) appear. The ordered moment reaches 1.94(3) muB/Cr3+ for k1 and 2.08(3) muB/Cr3+ for k2, if equal amount of the k1 and k2 phases is assumed. The magnetic arrangements have the dominant components along the [110] and [1-10] diagonals and a smaller c-component. By inelastic neutron scattering we investigate the spin excitations, which comprise a mixture of dispersive spin waves propagating from the magnetic Bragg peaks and resonance modes centered at equal energy steps of 4.5 meV. We interpret these as acoustic and optical spin wave branches, but show that the neutron scattering cross sections of transitions within a unit of two corner-sharing tetrahedra match the observed intensity distribution of the resonances. The distinctive fingerprint of cluster-like excitations in the optical spin wave branches suggests that propagating excitations are localized by the complex crystal structure and magnetic orders.

Nonvolatile Control of Magnetocaloric Operating Temperature by Low Voltage.

The limited operating temperature is the main obstacle for the practical applications of magnetic refrigeration. In this work, the voltage control of magnetocaloric effect (MCE) is investigated in a La0.7Sr0.3MnO3 (LSMO)/CeO2/Pt device. Different from the conventional method of volatile manipulating MCE by large-voltage-induced strain, nonvolatile manipulation of magnetocaloric operating temperature with good stability is realized in the LSMO film by applying low voltages of less than 2.3 V. The experimental results demonstrate that the magnetic entropy change peak temperature for the LSMO film can be extended from 302 to 312 K by voltage. This nonvolatile effect can be well-understood with the resistive switching mechanism and has potential in promoting microscale refrigeration technology.

Reduction of the ordered magnetic moment and its relationship to Kondo coherence in Ce1−xLaxCu2Ge2

The microscopic details of the suppression of antiferromagnetic order in the Kondo-lattice series Ce$_{1-x}$La$_{x}$Cu$_{2}$Ge$_{2}$ due to nonmagnetic dilution by La are revealed through neutron diffraction results for $x=0.20$, $0.40$, $0.75$, and $0.85$. Magnetic Bragg peaks are found for $0.20\le x\le0.75$, and both the N\'{e}el temperature, $T_{\textrm{N}}$, and the ordered magnetic moment per Ce, $\mu$, linearly decrease with increasing $x$. The reduction in $\mu$ points to strong hybridization of the increasingly diluted Ce $4f$ electrons, and we find a remarkable quadratic dependence of $\mu$ on the Kondo-coherence temperature. We discuss our results in terms of local-moment- versus itinerant-type magnetism and mean-field theory, and show that Ce$_{1-x}$La$_{x}$Cu$_{2}$Ge$_{2}$ provides an exceptional opportunity to quantitatively study competing magnetic interactions in a Kondo lattice.

Neutron diffraction study and theoretical analysis of the antiferromagnetic order and the diffuse scattering in the layered kagome system CaBaCo2Fe2O7

The hexagonal swedenborgite, CaBaCo$_2$Fe$_2$O$_7$, is a chiral frustrated antiferromagnet, in which magnetic ions form alternating Kagome and triangular layers. We observe a long range $\sqrt{3} \times \sqrt{3}$ antiferromagnetic order setting in below $T_N = 160$ K by neutron diffraction on single crystals of CaBaCo$_2$Fe$_2$O$_7$. Both magnetization and polarized neutron single crystal diffraction measurements show that close to $T_N$ spins lie predominantly in the $ab$-plane, while upon cooling the spin structure becomes increasingly canted due to Dzyaloshinskii-Moriya interactions. The ordered structure can be described and refined within the magnetic space group $P31m^\prime$. Diffuse scattering between the magnetic peaks reveals that the spin order is partial. Monte Carlo simulations based on a Heisenberg model with two nearest-neighbor exchange interactions show a similar diffuse scattering and coexistence of the $\sqrt{3} \times \sqrt{3}$ order with disorder. The coexistence can be explained by the freedom to vary spins without affecting the long range order, which gives rise to ground-state degeneracy. Polarization analysis of the magnetic peaks indicates the presence of long-period cycloidal spin correlations resulting from the broken inversion symmetry of the lattice, in agreement with our symmetry analysis.

Low temperature ferromagnetic properties, magnetic field induced spin order and random spin freezing effect in Ni1.5Fe1.5O4 ferrite; prepared at different pH values and annealing temperatures

We present the low temperature magnetic properties in Ni1.5Fe1.5O4 ferrite as the function of pH at which the material was prepared by chemical route and post annealing temperature. The material is a ferri/ferromagnet, but showed magnetic blocking and random spin freezing process on lowering the measurement temperature down to 5 K. The sample prepared at pH ∼12 and annealed at 800 °C showed a sharp magnetization peak at 105 K; the superparamagnetic blocking temperature of the particles. The magnetization peak remained incomplete within measurement temperature up to 350 K for rest of the samples, although peak temperature was brought down by increasing applied dc magnetic field. The fitting of temperature dependence of coercivity data according to Kneller′s law suggested random orientation of ferromagnetic particles. The fitting of saturation magnetization according to Bloch′s law provided the exponent that largely deviated from 3/2, a typical value for long ranged ferromagnet. An abrupt increase of saturation magnetization below 50 K suggested the active role of frozen surface spins in low temperature magnetic properties. AC susceptibility data elucidated the low temperature spin freezing dynamics and exhibited the characters of cluster spin glass in the samples depending on pH value and annealing temperature.

Sirius-Details of the New 3.2 T Permanent Magnet Superbend

An all permanent magnet superbend is proposed for the new Sirius optics and it can now provide hard X-rays in a critical energy of 19 keV. In the new design, the superbend has a stronger magnetic peak field of 3.18 T and it is composed of two low field parts designed with a transverse gradient on each side of the high field pole. The full magnet is joined into a single permanent magnet named BC and a floating pole links all three parts. The low field poles, the floating poles, and the return flux in the back can be adjusted to correct the integrated dipole and quadrupole components. The magnetic and mechanical design, assembly, magnetic measurements, and production strategy will be presented.

Discrimination of Holocene tephra units in Lake Van using mineral magnetic analysis

Detailed magnetic analysis of the four sediment cores from Lake Van, Eastern Turkey dating back to 9.4 cal ka BP were carried out for discriminating and correlating tephras and laminated sediments in four different cores. Six tephra units (T0, T1, T2, T3, T4 and T5) with distinct magnetic properties were identified in the cores. The tephra units are characterized by ferrimagnetic material, with a grain size in the pseudo single domain (PSD) range. There is no significant correlation between magnetic susceptibility peaks of the different tephra units except for the tephra T1 and T2. On the contrary ARM profiles show significant correlations as remanent magnetization indicators. The tephra units T1 and T2, have a higher magnetic susceptibility and a higher intensity of remanent magnetization, and finer grain size than the other tephra units. The results suggest that there is a clear difference between the magnetic properties of the different tephra units and the lake sediments. Our findings show that also differential deposition of volcanic material including magnetic mineral occurs during the transport with distance from the volcanic source.

Epitaxially grown monolayer VSe2: an air-stable magnetic two-dimensional material with low work function at edges

Recent experimental breakthroughs open up new opportunities for magnetism in few-atomic-layer two-dimensional (2D) materials, which makes fabrication of new magnetic 2D materials a fascinating issue. Here, we report the growth of monolayer VSe2 by molecular beam epitaxy (MBE) method. Electronic properties measurements by scanning tunneling spectroscopy (STS) method revealed that the as-grown monolayer VSe2 has magnetic characteristic peaks in its electronic density of states and a lower work-function at its edges. Moreover, air exposure experiments show air-stability of the monolayer VSe2. This high-quality monolayer VSe2, a very air-inert 2D material with magnetism and low edge work function, is promising for applications in developing next-generation low power-consumption, high efficiency spintronic devices and new electrocatalysts.

Achieving better glass-forming ability and magnetocaloric effect in the minor Zr-substituted Gd50Co50 amorphous alloy

The glass-forming ability (GFA) and magnetic properties of the minor Zr-substituted Gd[Formula: see text]Co[Formula: see text] amorphous alloy were investigated. The Gd[Formula: see text]Co[Formula: see text]Zr2amorphous ribbons prepared by melt-spinning show better GFA. With increasing minor Zr addition, the Gd[Formula: see text]Co[Formula: see text]Zr2amorphous ribbons possessed higher magnetic entropy change peak ([Formula: see text]S[Formula: see text] = 3.9 Jkg[Formula: see text]K[Formula: see text], under 5 T) but lower Curie temperature (T[Formula: see text] = 231 K) than the Gd[Formula: see text]Co[Formula: see text] amorphous alloy. The mechanism for the improved GFA, magnetocaloric effect (MCE) and the decrease in T[Formula: see text] was investigated. Finally, combined with other Gd-based amorphous ribbons, the Gd[Formula: see text]Co[Formula: see text]Zr2amorphous alloy was employed to construct amorphous composites to achieve a table-like magnetic entropy change profile, which can provide optimal efficiency when utilizing an Ericsson thermodynamic cycle.