Magnetic Peak Research Articles

Vortex phases and glassy dynamics in the highly anisotropic superconductor HgBa2CuO4+\u03b4

We present an extensive study of vortex dynamics in a high-quality single crystal of HgBa2CuO4+δ, a highly anisotropic superconductor that is a model system for studying the effects of anisotropy. From magnetization M measurements over a wide range of temperatures T and fields H, we construct a detailed vortex phase diagram. We find that the temperature-dependent vortex penetration field Hp(T), second magnetization peak Hsmp(T), and irreversibility field Hirr(T) all decay exponentially at low temperatures and exhibit an abrupt change in behavior at high temperatures T/Tc >~ 0.5. By measuring the rates of thermally activated vortex motion (creep) S(T, H) = |dlnM(T, H)/dlnt|, we reveal glassy behavior involving collective creep of bundles of 2D pancake vortices as well as temperature- and time-tuned crossovers from elastic (collective) dynamics to plastic flow. Based on the creep results, we show that the second magnetization peak coincides with the elastic-to-plastic crossover at low T, yet the mechanism changes at higher temperatures.

Magnetic properties and large cryogenic magneto-caloric effect of Er0.2Tm0.2Ho0.2Cu0.2Co0.2 amorphous ribbon

In this paper, the glass forming ability (GFA), magnetic properties and magneto-caloric effect (MCE) performances in Er0.2Tm0.2Ho0.2Cu0.2Co0.2 amorphous ribbon were reported. A relatively big supercooled liquid region ~43.1 K and a broad typical ferromagnetic to paramagnetic phase transition around TC ~ 13.5 K accompanied by a large cryogenic MCE were observed in Er0.2Tm0.2Ho0.2Cu0.2Co0.2 amorphous ribbon. The magnetic entropy change peak and refrigerant capacity (relative cooling power) values in Er0.2Tm0.2Ho0.2Cu0.2Co0.2 amorphous ribbon for ΔH = 0–5 T are calculated to be 15.73 J/kgK and 354.3 (448.1) J/kg, respectively. The large MCE parameters together with good GFA, making the Er0.2Tm0.2Ho0.2Cu0.2Co0.2 amorphous ribbon is also attractive for cryogenic magnetic refrigeration (MR).

Disordering Induced Second Magnetization Peak Effect and the Vortex Pinning Mechanism in V0.0007NbSe2 Single Crystal

The consequences of the random pinning in the presence of weak disorder are studied systematically based on the appearance of the second magnetization peak (SMP) and peak effect in the magnetic measurement. Here, we have observed the system dependency of the SMP effect. We have analysed the dc magnetization curves and the associated vortex pinning mechanisms of V0.0007NbSe2 single crystal at different temperatures and magnetic fields to estimate its pinning strengths in the external magnetic field H oriented parallel to the crystallographic ab plane. The characteristics of critical current density result in the presence of weak collective pinning near zero-magnetic field and the large bundle collective pinning along with SMP effect at the intermediate magnetic fields. The field dependence characteristics of the pinning force density highlights the presence of Bean-Livingstone’s zero-field peak with the presence of intermediate peak and high-field peak effect phenomena. The temperature dependence of the critical current density shows the dominating nature of δl pinning in the intermediate fields region. The vortex phase diagram of V0.0007NbSe2 is drawn showing different vortex phases at altered temperatures and magnetic fields. Thus, this study depicts the SMP effect and the diverse pinning characteristics in the quasi-two-dimensional superconductor associated with the external defects.

Magnetic properties and excellent cryogenic magnetocaloric performances in B-site ordered RE2ZnMnO6 (RE = Gd, Dy and Ho) perovskites

The solid-state magnetic cooling (MC) relying on magnetocaloric effect (MCE) of magnetic materials is recognized as a promising energy efficiency and environmentally friendly technology. However, there is still a large gap between the fundamental materials in the laboratory developing stage and the practically realizable applications. In this paper, a new series of B-site ordered RE2ZnMnO6 (RE = Gd, Dy and Ho) perovskites are successfully fabricated, and their structural and magnetic properties together with the cryogenic magnetocaloric performances are studied in detail. All the RE2ZnMnO6 compounds are found to crystallize in the B-site ordered orthorhombic (double perovskites) structure with the space group of P121/C1. The magnetic ground state are confirmed to be a ferromagnetic (FM) coupling for Gd2ZnMnO6, whereas, ferrimagnetic (FIM) coupling for Dy2ZnMnO6 and Ho2ZnMnO6 based on the magnetic measurements and first principle calculations. The ordering temperatures (TM) are determined to be ~6.4 K for Gd2ZnMnO6, ~10.4 K for Dy2ZnMnO6, and ~6.8 K for Ho2ZnMnO6, respectively. Furthermore, several vital parameters for determining the magnetocaloric performances including -ΔSMmax (magnetic entropy change peak values), TEC(2) (temperature-averaged magnetic entropy change) and RCP (relative cooling powers) are evaluted to be 15.17, 14.44 J/(kgK) and 226 J/kg for Gd2ZnMnO6, 8.32, 8.17 J/(kgK) and 208 J/kg for Dy2ZnMnO6, and 13.19, 12.68 J/(kgK) and 246 J/kg for Ho2ZnMnO6, with the field change ΔH = 50 kOe, respectively. It is worthly noted that Gd2ZnMnO6 exhibits relatively great magnetocaloric performances below 10 K, which suggests that Gd2ZnMnO6 is a competitive candidate for cryogenic magnetic cooling.

Theoretical Analysis of Strain‐Optoelectronic Properties in Externally Deformed Ge/GeSi Quantum Well Nanomembranes via Neutral Plane Modulation

For indirect‐bandgap germanium (Ge), strain‐engineering provides a controllable means to transform Ge into direct‐bandgap material with strongly enhanced light emission efficiency. Flexible electronics offer a powerful platform for introducing external strain into semiconductor nanomembranes (NMs) through deformation with flexible substrates. So far, the Ge/GexSi1−x alloy quantum well (QW) NMs have not been sufficiently investigated to comprehend how the external deformation tunes the wavelength and light‐emitting properties. Herein, a theoretical model based on the elasticity theory and k·p method is established to analyze the strain‐optoelectronic properties of the externally deformed Ge/GexSi1−x QW NMs via neutral plane (NP) modulation in both embedded and sticked form. Calculated results reveal that the elastic strain is linearly distributed, inducing tilt of the band edge similar to the electric field. Considering the fracture limit of the material, the NP layout determines the critical bending curvature that the NM can achieve. Large tunable range of wavelength Δλ of 1615 nm and enhanced transverse electric (TE)/transverse magnetic (TM) peak gain of 949/3778 cm−1 are predicted for r = 1000 nm (the distance between NP and QW) under the bending limit. Further calculations conclude that larger r allow for smaller bending curvature to achieve wider range of wavelength and more enhanced optical gain.

Helical and collinear spin density wave order in the S=12 one-dimensional frustrated chain compound NaCuMoO4(OH) investigated by neutron scattering

We performed neutron diffraction experiments on a single crystal of an $S=\frac{1}{2}$ frustrated ferromagnetic chain compound ${\mathrm{NaCuMoO}}_{4}$(OH) at zero and finite magnetic fields. Three magnetic Bragg peaks with the propagation vector of $\mathbit{q}=(0,\ensuremath{\delta},0)$ $[\ensuremath{\delta}=0.481(1)]$ were observed at the zero field, which means that an incommensurate helical magnetic order was realized. Using the ratio of the intensities between these magnetic peaks, a proper-screw structure was suggested, where the spiral plane is the crystallographic $ac$ plane. While the $\ensuremath{\delta}$ is unchanged at lower fields, it decreases linearly with the field where ${\ensuremath{\mu}}_{0}H\ensuremath{\ge}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. The field dependence is consistent with that of the field-induced spin density wave state that originated from the formation of the bound state of two magnons in the $S=\frac{1}{2}$ frustrated ferromagnetic chain.

Stripe Discommensuration and Spin Dynamics of Half-Doped Pr_{3/2}Sr_{1/2}NiO_{4}.

We present inelastic neutron scattering measurements of magnetic excitations in stripe ordered Pr_{3/2}Sr_{1/2}NiO_{4} at T∼10 K. For the observed magnetic incommensurability ε=0.4, we have incorporated a stripe discommensuration model in our linear spin wave calculation and obtained best agreement with the measured spin wave dispersion, especially to explain the symmetrical outward shift of the magnetic peaks from Néel ordered zone center in energy range 35 to 45meV. Our study indicates the prerequisite to consider a discommensurated spin stripe unit with proper out-of-plane and in-plane exchange interactions in between Ni^{2+} spins to describe the observed spin wave characteristics in Pr_{3/2}Sr_{1/2}NiO_{4}.

Magnetic field induced antiferromagnetic cone structure in multiferroic BiFeO3

Neutron diffraction measurements were performed under high magnetic fields up to 17 T in a multiferroic ${\mathrm{BiFeO}}_{3}$ single crystal, in which an intermediate magnetic (IM) phase has been found between the cycloid and canted antiferromagnetic phases [S. Kawachi et al., Phys. Rev. Mater. 1, 024408 (2017)]. We clearly found that the incommensurate magnetic peaks, which split perpendicular to the magnetic field in the cycloid phase, rotate by 90 deg to align parallel to the field in the IM phase. The magnetic structure in the IM phase can be best described by an antiferromagnetic cone (AF cone) structure. The transition from the cycloid to AF cone is of first order and the direction of the magnetic wave vector and the easy plane of the cycloidal component are rotated by 90 deg without changing the cycloidal modulation period, whereas the transition from the AF cone to canted antiferromagnetic phase is gradual and the cone angle becomes smaller gradually without changing the modulation period. Interestingly, the cycloidal component as well as the cone angle in the IM phase shows a large hysteresis between the field increasing and decreasing processes. This result, combined with the magnetostriction with a large hysteresis previously reported in the IM phase, suggests a strong magnetoelastic coupling.

Neutron diffraction study of magnetic ordering in high pressure phases of rare earth metal holmium

The magnetic ordering in rare earth metals is well established for ambient pressure crystal structures, however, little is known about the magnetic ordering in their corresponding high-pressure crystalline modifications. Holmium (Ho) was studied in a large-volume diamond anvil cell at the Spallation Neutron Source to high-pressure up to 20 GPa and to low-temperature to 10 K. We have conducted two independent high-pressure low-temperature experiments under non-hydrostatic and quasi-hydrostatic pressure conditions respectively. The ambient pressure hexagonal close packed (hcp) phase of holmium shows two magnetic transitions below 10 GPa one to an incommensurate Antiferromagnetic (AFM) phase and another to a conical-Ferromagnetic (c-FM) phase. In contrast, alpha-Samarium-type (α-Sm) phase above 10 GPa and the double hexagonal close packed (dhcp) phase above 19 GPa show only one FM transition marked by the appearance of a magnetic peak at 3 Å and the concurrent enhancement of nuclear peaks below 30 K. These new transitions observed by neutron diffraction can be accounted by a commensurate superlattice formation along c-axis in both the α-Sm-type phase and the dhcp phase.

Second magnetization peak effect and the vortex phase diagram of V0.0015NbSe2 single crystal

Effect of the weak point disorder on vortex matter phase diagram is studied by incorporation of V atoms through magnetic and magnetoresistance measurement in layered NbSe2 single crystal. We observe that the point disorder introduces fishtail effect and the second magnetization peak (SMP) in the M-H curve of V0.0015NbSe2 at magnetic field far below upper critical field (Hc2). 3D Collective creep of elastic flux line lattice (FLL) support the vortex motion below the onset of SMP. However, the crossover from collective to plastic deformation of FLL support the occurrence of SMP effect. Presence of the peak effect in pinning force density near irreversible magnetic field (Hirr) indicates the 2D to 3D distortion of FLL due to lattice softening associated with the rapid reduction of elastic modulus. Magneto transport measurement shows the glassy transition near the zero resistance region. In the glassy region, the flux line shows 2D characteristics. Above zero resistive region, thermally activated flux flow region is well described by Arrhenius relation. It is seen that the activation energy is decreased significantly by the incorporation of V atoms in NbSe2. The magnetic field dependence of activation energy follows a power law of U0(H) ~ H-α where the exponent α changed from 0.5 to a 0.9 at a crossover field of Hcr = ~ 0.7 T, indicating the transition from weak plastic deformation of FLL to the strong entangled state in vortex liquid phase. Finally a vortex phase diagram is constructed demonstrating all the different phases of vortex lattice of V0.0015NbSe2 at different magnetic fields and temperatures.

Impact of Magnetorotational Instability on Grain Growth in Protoplanetary Disks. I. Relevant Turbulence Properties

Abstract Turbulence in protoplanetary disks induces collisions between dust grains, and thus facilitates grain growth. We investigate the two fundamental assumptions about the turbulence in obtaining grain collisional velocities—the kinetic energy spectrum and the turbulence autocorrelation time—in the context of the turbulence generated by the magnetorotational instability (MRI). We carry out numerical simulations of the MRI, as well as driven turbulence, for a range of physical and numerical parameters. We find that the convergence of the turbulence α-parameter does not necessarily imply the convergence of the energy spectrum. The MRI turbulence is largely solenoidal, for which we observe a persistent kinetic energy spectrum of k −4/3. The same is obtained for solenoidal driven turbulence with and without a magnetic field, over more than 1 dex near the dissipation scale. This power-law slope appears to be converged in terms of numerical resolution, and to be due to the bottleneck effect. The kinetic energy in the MRI turbulence peaks at the fastest growing mode of the MRI. In contrast, the magnetic energy peaks at the dissipation scale. The magnetic energy spectrum in the MRI turbulence does not show a clear power-law range, and is almost constant over approximately 1 dex near the dissipation scale. The turbulence autocorrelation time is nearly constant at large scales, limited by the shearing timescale, and shows a power-law drop close to k −1 at small scales, with a slope steeper than that of the eddy crossing time. The deviation from the standard picture of the Kolmogorov turbulence with the injection scale at the disk scale height can potentially have a significant impact on grain collisional velocities.

Influence of carbon concentration and magnetic transition on the austenite lattice parameter of 30Mn-C steel

In this study, the effects of carbon concentration and magnetic transition on the austenite lattice parameter of Fe-30Mn-xC alloys were assessed. A linear relationship was obtained between the carbon content and the lattice parameter determined by neutron diffraction and neutron Bragg-edge transmission imaging for austenitic steels bearing 0.007–1.20 mass% C. The results obtained through both methods showed good agreement. In addition, carbon partitioning was evaluated in artificially piled-up discs with different carbon contents in order to develop a new technique for advanced multi-phase steels. The analysis of asymmetry and broadening of the diffraction line profile revealed that the carbon partitioning could be determined. In addition, the Néel temperature was determined by in situ neutron diffraction upon heating and cooling with magnetic diffraction peaks and the change in the lattice parameter with temperature being monitored. The temperature dependence of the austenite lattice parameter was found to become smaller below the Néel temperature. In conclusion, the results presented herein expand the current knowledge on the properties of high Mn austenite.

Experimental validation of the planktic foraminifera fragmentation index as proxy for the end-Cretaceous Ocean Acidification

Abstract The final ~50 ky of the Maastrichtian leading up to the Cretaceous-Paleogene (K-Pg) boundary mass extinction at Bidart (France) and Gamsbach (Austria) record poor carbonate preservation indicative of ocean acidification preceding the mass extinction. Further evidence includes enhanced planktic foraminifera test fragmentation, anomalously low bulk-rock magnetic susceptibility (acidification) and peak mercury content (toxicity) related to peak Deccan volcanism in India. Here we present new experimental data that validates the Fragmentation Index of planktic foraminifera as a reliable proxy for the end-Cretaceous acidification. Pristine Cretaceous planktic foraminiferal shells (from DSDP 525A, palaeodepth ~1000 m) were exposed to buffers of pH 8.0, 7.5, 7.0 and 6.5 for 15 days each, and their preservation state was quantified as a function of time. The critical variables affecting test taphonomy are morphology, pH and time of exposure. Thin-walled fragile biserial species constitute an average ~60% of typical Late Maastrichtian assemblages (e.g Heterohelix globulosa and H. planata) and are most susceptible to dissolution, followed by simple coiled forms such as Rugoglobigerina sp. (~19% of the assemblage) and Hedbergella sp. (~6% of the assemblage). The globotruncanids (Globotruncana, Globotruncanita, ~12%) are least susceptible to chemical and physical damage. Lower pH conditions render tests more vulnerable to physical fragmentation. Caution is needed to assess taphonomic inflation of morphologically robust yet environmentally sensitive Cretaceous species (e.g. globotruncanids), that may result in an underestimation of the degree/nature of the faunal crisis and tempo of extinctions in the pre-extinction acidification interval.

EDM ASSISTED BY MAGNETIC FIELD FOR ARMOURED STEEL

The present paper presents a study on the performance of electrical discharge machining (EDM) assisted by magnetic field for armoured steel (HV500). Response surface methodology (RSM) has been utilized to model, analyze and determine the optimal parameters setting in the EDM process assisted by magnetic field. The process performance criteria such as; material removal rate (MRR), tool wear rate (TWR), were evaluated. Peak current, magnetic flux density, duty factor, and dielectric fluid pressure have been considered the main factors affecting EDM assisted by magnetic field performance. RSM was employed to develop the experimental models. The EDM process assisted by magnetic field process has proved its adequacy to machine armoured steel alloy under acceptable metal removal rate and minimum tool wear rate. The metal removal rate generally increases with the increase of the magnetic flux density and peak current value. The effect of duty factor was limited and dielectric fluid pressure has a moderate effect. Further, the minimum tool wear rate has been obtained at the parametric combination of higher magnetic flux density and higher duty factor when machined by EDM assisted by magnetic field with copper electrodes. It has also been clarified that the effect of magnetic flux density was limited on metal removal rate with graphite electrodes. It has also been found that the minimum tool wear rate was obtained at the parametric combination of higher magnetic flux density, higher peak current and lower duty factor, when machined by EDM assisted by magnetic field, with graphite electrodes.

Current and Magnetic Field of a Simplex Wave Winding in a DC Generator

The DC generator is widely used in the fields of industry motor control and energy transfer, and the simplex wave winding (SWW) is the main winding pattern of the DC generator. In this paper, a brief introduction to the structure of the SWW is presented first. Then the characteristics of the current and electromagnetic fields of the SWW are analyzed particularly in a commutation cycle and two typical concepts of the “central slot” (CS) and the “widest magnetic peak area” (WMPA) are proposed respectively. The characteristic frequency which is 63 and 21 times higher than the fundamental frequency (FF) of the voltage between two brushes is obtained and verified from 6 conditions of different speeds and currents. Furthermore, with the help of an induction coil sensor between two main poles, the characteristic frequency 84 times higher than the FF of the WMPA is also verified in 6 conditions and the results coincides with the theory well.

Magnetic Interactions, Superconductivity, and Spin-Resonance Peak in Iron-Based Materials

Different mechanisms of superconductivity result in specific symmetries and structures of the gap in multiband systems. Here we review the spin fluctuation mechanism of Cooper pairing and discuss the spin resonance feature in the superconducting state of iron-based materials.

Electron‐Scale Magnetic Structure Observed Adjacent to an Electron Diffusion Region at the Dayside Magnetopause

AbstractIn this paper, we present observations made by the Magnetospheric Multiscale (MMS) mission of a small‐scale magnetic structure adjacent to a reconnecting current sheet at the dayside magnetopause. While MMS crosses the current sheet, it observes signatures of an electron diffusion region (EDR), including crescent shaped electron velocity distribution functions. Right after the EDR crossing, all spacecraft encounter an electron‐scale magnetic structure that can roughly be divided into two parts: a force‐free flux rope‐like part and a nonforce‐free magnetic enhancement. Within the leading force‐free part of the structure, the magnetic field component normal to the current sheet exhibits a bipolar signature, suggesting that the structure is flux rope‐like. In the trailing edge of the bipolar signature, three spacecraft observe a magnetic enhancement that has an amplitude almost twice the ambient magnetic field. The magnetic peak adjacent to an EDR can be associated with an electron vortex, where the perpendicular current is carried by E×B drifting electrons. The structure is advected tailward along the magnetopause while in the plasma frame, it propagates upstream suggesting that this reconnection event is highly three‐dimensional. Fluctuations in the plasma density and magnetic field and large peaks in the parallel electric field observed on the magnetospheric side of the EDR suggest that the current sheet is corrugated due to an electromagnetic drift instability. This or another instability of a thin reconnecting current sheet (e.g., tearing) is a likely cause of the formation of the observed magnetic structure.

Magnetic detection of paleoflood layers in stalagmites and implications for historical land use changes

Flooding events are major natural hazards that present significant risk to communities worldwide. Calculations of flood recurrence rate through time are important tools for regulating land use, determining insurance rates, and for the design and construction of levees and dams. Typically, flood recurrence rates are based on limited historical data or on evidence preserved in the geologic record as overbank deposits, tree ring scars, or high water scour marks. However, these approaches are either limited in their ability to produce continuous time series of flooding events or do not consider the effects of regional land use change. Here we use scanning superconducting quantum interference device (SQUID) microscopy to rapidly image the magnetization associated with flood layers in a polished surface of an annually laminated stalagmite from Spring Valley Caverns (SVC) in southeastern Minnesota. A time series of magnetization peaks, each of which corresponds to a flooding event, yields an average flood recurrence rate of ≤5 events per century for the last 500 years. This rate increases to ∼7 events per century since 1900, coincident with historical timber and agricultural land-use changes in Minnesota. This approach produces a continuous record of well-dated, extreme-precipitation events that can be examined within the context of land use change.

Thermodynamic Properties of Mirror Structures in the Magnetosheath: MMS Observations and Double-polytropic MHD Simulations

Abstract Mirror structures are manifested as a wave train of magnetic peaks and/or dips, which are anticorrelated with plasma density. The evolution behavior of the ion temperatures perpendicular and parallel to the local magnetic field, T ⊥ and T ∥, in the magnetic peaks and dips is rare to be theoretically studied. In this paper, the thermodynamic properties of mirror structures are investigated using two-dimensional magnetohydrodynamic simulations with double-polytropic laws. Two polytropic exponents, γ ∥ and γ ⊥, are used as parameters to describe various thermodynamic conditions in the anisotropic plasma—for example, γ ∥ = 1, γ ⊥ = 1 for double-isothermal and γ ∥ = 3, γ ⊥ = 2 for double-adiabatic. Using empirical values of γ ∥ = 1.14 and γ ⊥ = 0.94 for magnetosheath plasma, the variations of T ∥ and T ⊥ in the magnetic dips and peaks observed by the Magnetospheric Multiscale Mission in the Earth’s magnetosheath can be reproduced. This consistent result cannot be achieved by use of γ ∥ = 3, γ ⊥ = 2 and γ ∥ = 0.5, γ ⊥ = 2, where the latter exponents can lead to the same mirror instability threshold as kinetic theory. In magnetic dips, T ∥ is found to be increased for γ ∥ > 1 but decreased for γ ∥ < 1, while T ⊥ is decreased for γ ⊥ > 1 but increased for γ ⊥ < 1. For magnetic peaks, the variation features of T ∥ and T ⊥ are opposite to that of the magnetic dips. It is concluded that the thermodynamic properties of mirror structures are further from double-adiabatic but closer to double-isothermal conditions.

Magnetic ordering in rare earth metal dysprosium revealed by neutron diffraction studies in a large-volume diamond anvil cell

ABSTRACTA high pressure low-temperature neutron diffraction study has been carried out on rare earth metal Dysprosium (Dy) to 10.8 GPa and 8 K in a large-volume diamond anvil cell using a spallation neutron source. Dy crystallizes in an ambient pressure hexagonal close packed phase and forms an incommensurate helical antiferromagnetic phase, marked by the appearance of superlattice magnetic peaks below 176 K with a turn-angle between the hexagonal layers that increases both with increasing temperature and pressure. The ferromagnetic transition below 87 K is marked by an increase in intensity of nuclear peaks with disappearance of magnetic superlattice reflections. Dy transforms to an alpha-Samarium (α-Sm) phase at 7 GPa and only ferromagnetic ordering is observed for this high pressure phase. The ferromagnetic transition is observed at 59 K in the α-Sm phase at 10.8 GPa in close agreement with the magnetic ordering temperature obtained from electrical transport measurements. In the entire pressure-temperature range of this study, Dy shows a negative thermal expansion coefficient of as much as −2.5% in the magnetically ordered phases between 200 and 8 K.