Magnetic Peak Research Articles

Relevance and Assessment of Molecular Diffusion Coefficients in Liquid Chromatography

Molecular diffusion plays an important role in high-performance liquid chromatography, especially in fundamental column performance studies. An accurate knowledge of the molecular diffusion coefficients (D m) of compounds selected for column evaluation is therefore crucial. In this review, a general overview is presented of the advantages and drawbacks of correlation-based and experimental methods that can be employed to determine molecular diffusion coefficients. The former include the Wilke–Chang, Scheibel, Reddy–Doraiswamy, Lusis–Ratcliff and Hayduk–Laudie equations, and other empirical correlations based on the Wilke–Chang equation. It is discussed how the association factor (ψ) that is required in several of these correlations can be obtained from the solubility parameter (δ). Frequently used experimental methods include the light scattering, nuclear magnetic resonance, peak parking and Taylor–Aris method, and methods employing microfluidic devices. The principles of these experimental methods are elucidated in detail. Moreover, the influence of several parameters, such as solute characteristics, solvent viscosity, temperature and pressure on the molecular diffusion coefficient is described.

Effects of proton irradiation on flux-pinning properties of underdoped Ba(Fe0.96Co0.04)2As2 pnictide superconductor

We study the effect of proton irradiation on Ba(Fe0.96Co0.04)2As2 superconducting single crystals from combined magnetisation and magnetoresistivity measurements. The study allows the extraction of the values of the apparent pinning energy U0 of the samples prior to and after irradiation, as well as comparison of the values of U0 obtained from the flux-flow reversible region with those from the flux-creep irreversible region. Irradiation reduces Tc modestly, but significantly reduces U0 in both regimes: the critical current density Jc is modified, most strikingly by the disappearance of the second magnetisation peak after irradiation. Analysis of the functional form of the pinning force and of the temperature dependence of Jc for zero field, indicates that proton irradiation in this case has not changed the pinning regime, but has introduced a high density of shallow point-like defects. By considering a model that takes into account the effect of disorder on the irreversibility line, the data suggests that irradiation produced a considerable reduction in the average effective disorder overall, consistent with the changes observed in U0 and Jc.

Magnetocaloric effect suppression induced by mechanical milling on intermetallic DyCO2.

Structural and magnetic properties of mechanically milled samples of intermetallic DyCo2 were investigated by x-ray diffraction and magnetization measurements. The results show that the average crystallite size is reduced down to 35 nm for the sample milled for 8 h. The milling process induces structural disorder as evidenced by x-ray diffraction data, leading to a weakening of Itinerant Electron Metamagnetism (IEM) mechanism and suppression of the magnetocaloric effect. The RCP values of milled samples were not significantly reduced in comparison with the bulk sample due to the broadening of magnetic entropy change peak.

Long Periodic Helimagnetic Ordering in CrM3S6 (M = Nb and Ta)

We report long periodic chiral helimagnetic orderings in ferromagnetic inorganic compounds CrM3S6 (M = Nb and Ta) with a chiral space group of P6322. Magnetization in polycrystalline samples and high resolution powder neutron diffraction were measured. Our powder neutron diffraction measurements in CrM3S6 successfully separated nuclear and magnetic satellite peaks, having the period of hundreds of angstroms along the c— axis. Therefore, we propose that the magnetic ordering in ferromagnetic CrM3S6 is not ferromagnetic, but long periodic chiral helimagnetic ordering.

From an Anomalous Peak Effect to a Second Magnetization Peak in Nb-rich Nb-Ti Alloys

We investigated the evolution of the DC magnetic hysteresis curves of Nb 0.89Ti 0.11 alloys thermo-mechanically processed by intermediate heat treatments at 900 ∘C in vacuum and cold rolling (∼ 50 % thickness reduction). Starting with a rectangular piece (∼ 0.4 mm thick) cut from the as-grown alloy, after the first thermo-mechanical treatment, the specimen exhibits over a wide temperature T interval a peak effect not far from the DC irreversibility line. With a supplemental thermo-mechanical treatment, the peak effect disappears and is substituted by a second magnetization peak (where the characteristic fields are significantly lower) induced by pinning enhancement. The second magnetization peak was clearly seen at high temperatures only, due to the occurrence of thermo-magnetic instabilities in the low-T domain. In both cases, analysis of magnetic relaxation evidences a crossover towards plastic vortex creep accompanying the maximum in the effective critical current density. These results suggest a common nature of the observed effects, related to the disordering of the vortex system induced by pinning.

Infarct size assessment by cardiac magnetic resonance and peak troponin I after aspiration thrombectomy and intracoronary abciximab assisted primary percutaneous coronary intervention in a real-world cohort of patients with ST-segment elevation myocardial infarction: A single-center study

To assess the effect of manual thrombectomy on infarct size by cardiac magnetic resonance (CMR) and peak troponin I (TnI) levels. Use of manual thrombectomy during primary percutaneous coronary intervention (primary PCI) and its effect on infarct size is still debatable. 70 patients (30 patients with thrombectomy and 40 without) who underwent primary PCI for ST-elevation myocardial infarction (STEMI) with adjunct intracoronary abciximab between January 2007 and August 2013 and had CMR afterwards were included. No significant difference in the baseline characteristics except for a higher baseline TnI (11.6 ± 16.7 vs. 2.4 ± 7.9, P = 0.009) and more visible thrombus and or TIMI 0 flow (P = 0.04) in the thrombectomy group. No significant difference was found in infarct size assessed by CMR (18.1 ± 13.2 vs. 16.45 ± 11.7, P = 0.6) or peak TnI (75.9 ± 126 vs. 51.3 ± 50.4, P = 0.26) between the two groups. A moderate positive correlation was found between Peak as well as TnI at 24 hours (TnI-24 h) and CMR-determined infarct size (r = 0.5 and r = 0.7 respectively, P < 0.001). TnI-24 h (B = 0.152, 95.0% Confidence Interval (CI) 0.116–0.187, P < 0.001) as well as final TIMI grade (B = −10,848, 95.0% CI −15.109 to −6.587, P < 0.001) predicts infarct size. In a retrospective real world cohort of patients with STEMI, no difference was found in infarct size assessed by CMR or peak TnI between the groups with and without thrombectomy. TnI-24 h as well as final TIMI flow predicts infarct size.

Itinerant and Localized Magnetization Dynamics in Antiferromagnetic Ho.

Using femtosecond time-resolved resonant magnetic x-ray diffraction at the Ho L_{3} absorption edge, we investigate the demagnetization dynamics in antiferromagnetically ordered metallic Ho after femtosecond optical excitation. Tuning the x-ray energy to the electric dipole (E1, 2p→5d) or quadrupole (E2, 2p→4f) transition allows us to selectively and independently study the spin dynamics of the itinerant 5d and localized 4f electronic subsystems via the suppression of the magnetic (2 1 3-τ) satellite peak. We find demagnetization time scales very similar to ferromagnetic 4f systems, suggesting that the loss of magnetic order occurs via a similar spin-flip process in both cases. The simultaneous demagnetization of both subsystems demonstrates strong intra-atomic 4f-5d exchange coupling. In addition, an ultrafast lattice contraction due to the release of magneto-striction leads to a transient shift of the magnetic satellite peak.

Spin-Lattice-Coupled Order in Heisenberg Antiferromagnets on the Pyrochlore Lattice.

Effects of local lattice distortions on the spin ordering are investigated for the antiferromagnetic classical Heisenberg model on the pyrochlore lattice. It is found by MonteCarlo simulations that the spin-lattice coupling (SLC) originating from site phonons induces a first-order transition into two different types of collinear magnetic ordered states. The state realized at the stronger SLC is cubic symmetric characterized by the magnetic (1/2,1/2,1/2) Bragg peaks, while that at the weaker SLC is tetragonal symmetric characterized by the (1,1,0) ones, each accompanied by the commensurate local lattice distortions. Experimental implications to chromium spinels are discussed.

Magnetic and Microwave Properties of 3-D Nanocomposites With Transition Metal and Palladium Particles

The microwave properties of magnetic 3-D nanocomposites, based on opal matrices, modified by the introduction of 3-D transition and palladium metal particles inside the intersphere voids, have been studied. The presence of palladium in the nanocomposites under study leads to a number of distinct features. The frequency dependence of transmission and reflection coefficients at room temperature, in the frequency range of 26–38 GHz, was measured, and magnetic resonance and antiresonance have been studied for several opal-based nanocomposites, containing Fe, Ni, and Pd. The magnitude of the magnetic resonance peak height is less for the nanocomposites containing Pd particles than that of the nanocomposites with transition metals without Pd. For the nanocomposites with only one metal, Ni or Fe, the antiresonance is also observed. Due to the large variations of the microwave transmission and reflection coefficients, this class of nanocomposite materials seems to be applicable in magnetic field driven devices in the millimeter wavelength band.

Technogenic magnetic particles in soils as evidence of historical mining and smelting activity: A case of the Brynica River Valley, Poland

In the area of Brynica River basin (Upper Silesia, southern Poland) the exploitation and smelting of iron, silver and lead ores was historically documented since early Middle Ages. First investigations showed that metallurgy industry had a large impact from 9th century (AD) until the Second World War. The aim of the study was to use magnetic prospection to detect traces of past mining and ore smelting in Brynica River Valley located in Upper Silesia (southern Poland). The field screening was performed by measurement magnetic susceptibility (κ) on surface and in vertical profiles and was supported locally by gradiometric measurements. Vertical distribution of magnetic susceptibility values was closely associated with the type of soil use. Historical technogenic magnetic particles resulting from exploitation, processing, and smelting of iron, silver, and lead ores were accumulated in the soil layer at the depth 10 to 25cm. They were represented by sharp-edged particles of slag, coke, as well as various mineralogical forms of iron minerals and aggregates composed of carbon particles, aluminosilicate glass, and single particles of metallic iron. The additional geochemical study in adjacent peat bog supported by radiocarbon dating was also performed. The application of integrated geochemical-magnetic methods to reconstruct the historical accumulation of pollutants in the studied peat bog was effective. The magnetic peak, which was pointed out by magnetic analyses, is consistent with the presence of charcoal and pollution from heavy metals, such as Ag, Cd, Cu, Fe, Pb, or Sn. The results of this work will be helpful for the further study of human's impact on the environment related to the historical and even pre-historical ore exploitation and smelting and also used for better targeting the archeological excavations on such areas.

Distinct doping dependence of critical temperature and critical current density in Ba1-xKxFe2As2 superconductor.

Since the high transition temperature (High-Tc) superconductivity was discovered in the series of materials containing iron (Fe), their potential for the applications has been extensively scrutinized. In particular, a lot of effort has been made in achieving the high current-carrying ability by revealing the vortex pinning behavior. Here, we report on the critical current density (Jc) for the pristine Ba1−xKxFe2As2 single crystals with various K concentrations (0.25 ≤ x ≤ 0.52) determined by the magnetization hysteresis loop measurements. The x-dependence of Jc is characterized by a spike-like peak at x ~ 0.30, which corresponds to the under-doped region. This behavior is distinct from a moderate Tc dome with a broad maximum spanning from x ~ 0.3 to 0.5. For the under-doped samples, with increasing magnetic field (H), a second magnetization peak in Jc is observed, whereas for the optimally- and over-doped samples, Jc monotonically decreases with H. This result emphasizes that fine tuning of doping composition is important to obtain strong flux pinning. The origin of the characteristic doping dependence of Jc is discussed in connection with the orthorhombic phase domain boundary, as well as the chemical inhomogeneity introduced by the dopant substitutions.

Premartensitic transition and relevant magnetic effects in Ni50Mn34In15.5Al0.5 alloy.

Resistance measurement, in situ optical microscopic observation, thermal and magnetic measurements have been carried out on Ni50Mn34In15.5Al0.5 alloy. The existence of a pronounced premartensitic transition prior to martensitic transition can be characterized by microstructure evolution as well as exothermic peak and smooth decrease of resistance and magnetization with obvious hysteresis over a wide temperature range upon cooling. Consequently, the alloy undergoes two successive magneto-structural transitions consisting of premartensitic and martensitic transitions. Magnetoelastic coupling between magnetic and structural degrees of freedom would be responsible for the appearance of premartensitic transition, as evinced by the distinct shift of transitions temperatures to lower temperature with external applied field of 50 kOe. The inverse premartensitic transition induced by magnetic field results in large magnetoresistance, and contributes to the enhanced inverse magnetocaloric effect through enlarging the peak value and temperature interval of magnetic entropy change ΔSm.

Incommensurate spin correlations in highly oxidized cobaltates La2-xSrxCoO4.

We observe quasi-static incommensurate magnetic peaks in neutron scattering experiments on layered cobalt oxides La2−xSrxCoO4 with high Co oxidation states that have been reported to be paramagnetic. This enables us to measure the magnetic excitations in this highly hole-doped incommensurate regime and compare our results with those found in the low-doped incommensurate regime that exhibit hourglass magnetic spectra. The hourglass shape of magnetic excitations completely disappears given a high Sr doping. Moreover, broad low-energy excitations are found, which are not centered at the incommensurate magnetic peak positions but around the quarter-integer values that are typically exhibited by excitations in the checkerboard charge ordered phase. Our findings suggest that the strong inter-site exchange interactions in the undoped islands are critical for the emergence of hourglass spectra in the incommensurate magnetic phases of La2−xSrxCoO4.

Magnetic Fluctuations in Pair-Density-Wave Superconductors.

Pair-density-wave superconductivity constitutes a novel electronic condensate proposed to be realized in certain unconventional superconductors. Establishing its potential existence is important for our fundamental understanding of superconductivity in correlated materials. Here we compute the dynamical magnetic susceptibility in the presence of a pair-density-wave ordered state and study its fingerprints on the spin-wave spectrum including the neutron resonance. In contrast to the standard case of d-wave superconductivity, we show that the pair-density-wave phase exhibits neither a spin gap nor a magnetic resonance peak, in agreement with a recent neutron scattering experiment on underdoped La_{1.905}Ba_{0.095}CuO_{4} [Z. Xu etal., Phys. Rev. Lett. 113, 177002 (2014)].

Observation of magnetic fragmentation in spin ice

Fractionalised excitations that emerge from a many body system have revealed rich physics and concepts, from composite fermions in two-dimensional electron systems, revealed through the fractional quantum Hall effect, to spinons in antiferromagnetic chains and, more recently, fractionalisation of Dirac electrons in graphene and magnetic monopoles in spin ice. Even more surprising is the fragmentation of the degrees of freedom themselves, leading to coexisting and a priori independent ground states. This puzzling phenomenon was recently put forward in the context of spin ice, in which the magnetic moment field can fragment, resulting in a dual ground state consisting of a fluctuating spin liquid, a so-called Coulomb phase, on top of a magnetic monopole crystal. Here we show, by means of neutron scattering measurements, that such fragmentation occurs in the spin ice candidate Nd$_2$Zr$_2$O$_7$. We observe the spectacular coexistence of an antiferromagnetic order induced by the monopole crystallisation and a fluctuating state with ferromagnetic correlations. Experimentally, this fragmentation manifests itself via the superposition of magnetic Bragg peaks, characteristic of the ordered phase, and a pinch point pattern, characteristic of the Coulomb phase. These results highlight the relevance of the fragmentation concept to describe the physics of systems that are simultaneously ordered and fluctuating.

Measuring Parameters of Defects using Alternate Metal Magnetization Method for In-line Inspection

Background/Objectives: The purpose of this work is to develop a new method of non-destructive inspection for measuring defect parameters during the in-line inspection, without prior preparation of internal surface of the pipe. Methods: The basic principle behind this method is to form a magnetic field inside the pipe. In cases which show mechanical damage on the pipeline inner or outer surface, increased magnetic field amplitudes appear. Measurements of the value of the magnetic field amplitude along the pipeline inner surface enable to detect the defect occurrence and to evaluate the defect transverse dimensions and depth based on the magnetic peak height and width. Findings: The article provides a detailed description of the magnetic field changes in the defect localization area and presents an algorithm for determining the pipeline wall loss. A measuring sensor is selected; the choice of sinusoidal alternating current passing through the pipe body is justified and the principle of the developed method implementation is described. Also, an experiment has been conducted in which it was found that the magnetic field amplitude depends on the defect depth. The smaller pipeline wall loss in the defect zone, the higher the magnetic field variation is. Thus, the experiment confirms the validity of the noncontact magnetic method for pipeline NDT with alternating magnetization of metal. The novelty of the method lies in the possibility to conduct the non-destructive inspection of the pipeline without prior surface preparation, through a layer of corrosion and slime deposits, providing the clearance up to 20 mm between the pipe wall and the measuring sensor, with the probability of defect detection making 0.9. Applications/Improvements: This method provides solution for timely inspection of corrosion damage in subsurface pipelines of small-diameter heating networks (DN200, DN400) of housing and public utilities in places inaccessible for external inspection. Keywords: Alternate Magnetization, Defect, In-Line Inspection, Magnetic Field, Non-destructive Testing, Pipeline, Slime Deposits, Thickness Measurement

Neutron diffraction study of microstructural and magnetic effects in fine particle NiO powders

Nickel oxide powders with grain sizes ranging from 100 to 1500 nm have been studied by high-resolution neutron diffraction. We have found that the atomic structure, the antiferromagnetic ordering, and the value of the nickel magnetic moments inherent in the bulk material of NiO are still preserved and are nearly independent of the average size of the grains. The sizes of the coherently scattering atomic and magnetic domains were estimated independently owing to a complete separation of the nuclear and magnetic peaks in the neutron diffraction patterns. It is shown that the finite-size and surface disorder effects in particles at the submicron scale have a more pronounced influence on the magnetism than on their structural properties. We conclude that the core–shell model suggested earlier for nanosized particles can be successfully extended to particles whose sizes are in the submicron range.

Excellent magneto-caloric effect of a low cost Gd34Ni22Co11Al33 metallic glass

In the present work, we reported a low cost Gd34Ni22Co11Al33 metallic glass containing less Gd element but exhibiting excellent magneto-caloric effect (MCE). The Gd34Ni22Co11Al33 metallic glass prepared into the shape of ribbon shows typical amorphous characteristics in X-ray diffraction pattern and a magnetic entropy change peak (−ΔSmpeak) up to 9.9Jkg−1K−1 at about 52.5K under a field of 5T. The mechanism for the rather high −ΔSmpeak as well as low Curie temperature of the Gd34Ni22Co11Al33 metallic glass was investigated.

Second magnetization peak effect, vortex dynamics, and flux pinning in 112-type superconductor Ca0.8La0.2Fe(1-x)CoxAs2.

Investigation of vortex pinning and its relaxation is of great importance for both basic physics and technological applications in the field of superconductivity. We report a great improvement of superconducting properties in the recently discovered 112-type superconductors (Ca, La)FeAs2 through Co co-doping. High critical current density Js(5 K) > 2*106 A/cm2 is obtained and pronounced second peak effect is observed in magnetization hysteresis loops. Both the dynamic and static relaxation studies result in comparable and sizable relaxation rates S or Q, indicating a fast vortex creep. The second magnetization peak (SMP) is found to be strongly associated with a crossover from elastic to plastic vortex creep. Above the crossover, plastic vortex creep governs the vortex dynamics in a wide range of temperatures and fields. A good scaling behavior of the normalized pinning force density fp by formula fp = hp(1−h)q (p = 1.44, q = 1.66, h = 0.44) is revealed, which demonstrates an important contribution from core normal point-like pinning sites. To better understand the SMP phenomenon, we discuss the related physical scenario as well as the affecting factors in the SMP occurrence.

Magnetocaloric response of amorphous and nanocrystalline Cr-containing Vitroperm-type alloys

The broad compositional range in which transition metal (TM) based amorphous alloys can be obtained, yields an easily tunable magnetocaloric effect (MCE) in a wide temperature range. In some TM-based alloys, anomalous behaviors are reported, as a non-monotonous trend with magnetic moment (e.g. FeZrB alloys). Moreover, in certain Cr-containing Vitroperm alloys anomalously high values of the magnetic entropy change were published. In this work, a systematic study on MCE response of Cr-containing amorphous alloys of composition Fe74-xCrxCu1Nb3Si15.5B6.5 (with x=2, 8, 10, 12, 13, 14 and 20) has been performed in a broad Curie temperature range from 100K to 550K. Curie temperature and magnetic entropy change peak of the amorphous alloys decrease with the increase of Cr content at rates of −25.6K/at% Cr and −54mJkg−1K−1/at% Cr, respectively, following a linear trend with the magnetic moment in both cases. The presence of nanocrystalline phases has been considered as a possible cause in order to explain the anomalies. The samples were nanocrystallized in different stages, however, the magnetocaloric response decreases as crystallization progresses due to the large separation of the Curie temperatures of the two phases.