Magnetic Relaxation Data Research Articles

Tuning the Superspin Dynamics in Inverse Spinel Ferrite Nanoparticle Ensembles via Indirect Cation Substitution

We used magnetic and synchrotron X-ray diffraction measurements to investigate the possibility of tuning the strength of magnetic interparticle interactions in nanoparticle ensembles via chemical manipulation. Our main result comes from temperature-resolved in-phase ac-susceptibility data collected on 8 nm average-diameter Ni0.25Zn0.75Fe2O4 (Ni25) and Ni0.5Zn0.5Fe2O4 (Ni50) nanoparticles at different frequencies, χ′ vs. T|f. We found that the relative peak temperature variation per frequency decade, ϕ=∆TT·∆log(f)—a known measure of interparticle interaction strength—exhibits a four-fold increase, from ϕ = 0.04 in Ni50 to ϕ = 0.16 in Ni25. This corresponds to a fundamental change in the nanoparticles’ superspin dynamics, as proven by the fit of phenomenological models to magnetic relaxation data. Indeed, the Ni25 ensemble exhibits superparamagnetic behavior, where the temperature dependence of the superspin relaxation time, τ, is described in the Dorman–Bessais–Fiorani (DBF) model: τT=τrexp⁡EB+EadkBT, with parameters τr = 4 × 10−12 s, and (EB + Ead)/kB = 1473 K. On the other hand, the nanoparticles in the Ni50 ensemble freeze collectively upon cooling in a spin-glass fashion according to a critical dynamics law: τ(T)=τ0TTg−1zν, with τ0 = 4 × 10−8 s, Tg = 145 K, and zν = 7.2. Rietveld refinements against powder X-ray diffraction data reveal the structural details that underlie the observed magnetic behavior: an indirect cation replacement mechanism by which non-magnetic Zn ions are incorporated in the tetrahedral sites of the inverse spinel.

Range and sensitivity of 17O nuclear spin-lattice relaxation as a probe of aqueous electrolyte dynamics.

The study of electrolytic solutions is of relevance in many research fields, ranging from biophysics, materials, and colloid science to catalysis and electrochemistry. The dependence of solution dynamics on the nature of electrolytes and their concentrations has been the subject of many experimental and computational studies, yet it remains challenging to obtain a full understanding of the factors that govern solution behavior. Here, we provide additional insights into the behavior of aqueous solutions of alkali chlorides by combining 17O relaxation data with diffusion and viscosity data and contrast their behavior with 1H nuclear magnetic resonance relaxation data. The main findings are that 17O relaxation correlates well with viscosity data but not with diffusion data, while 1H relaxation correlates with neither. Certain ionic trends match known ion-specific series behavior, especially at high concentrations. Notably, we also examine the ranges of the interactions and conclude that the majority of the effects are tied to local water reorientation dynamics.

Vortex Dynamics and Pinning in CaKFe4As4 Single Crystals from DC Magnetization Relaxation and AC Susceptibility

Among various “families” of iron-based superconductors, the quite recently discovered AeAFe4As4 (where Ae is an alkali-earth metal and A is an alkali metal) has high critical current density, a very high upper critical field, and a low anisotropy, and has recently received much interest for the possibility of high magnetic field applications at the liquid hydrogen temperature. We have performed DC magnetization relaxation and frequency-dependent AC susceptibility measurements on high-quality single crystals of CaKFe4As4 with the aim of determining the pinning potential U*. The temperature dependence of U* displays a clear crossover between elastic creep and plastic creep. At temperatures around 27–28 K, U* has a very high value, up to 1200 K, resulting in an infinitesimally small probability of thermally activated flux jumps. From the dependence of the normalized pinning potential on irreversible magnetization, we have determined the creep exponents in the two creep regimes, which are in complete agreement with theoretical models. The estimation of the pinning potential from multifrequency AC susceptibility measurements was possible only near the critical temperature due to equipment limitations, and the resulting value is very close to the one that resulted from the magnetization relaxation data. Magnetic hysteresis loops revealed a second magnetization peak and very high values of the critical current density.

A study on multi-exponential inversion of nuclear magnetic resonance relaxation data using deep learning

Nuclear magnetic resonance (NMR) is a powerful tool for formation evaluation in the oil industry to determine parameters, such as pore structure, fluid saturation, and permeability of porous materials, which are critical to reservoir engineering. The inversion of the measured relaxation data is an ill-posed problem and may lead to deviations of inversion results, which may degrade the accuracy of further data analysis and evaluation. This paper proposes a deep learning method for multi-exponential inversion of NMR relaxation data to improve accuracy. Simulated NMR data are first constructed using a priori knowledge based on the signal parameters and Gaussian distribution. These data are then used to train the neural network designed to consider noise characteristics, signal decay characteristics, signal energy variations, and non-negative features of the T2 spectra. With the validation from simulated data, the models introduced by multi-scale convolutional neural network (CNN) and attention mechanism outperform other approaches in terms of denoising and T2 inversion. Finally, NMR measurements of rock cores are used to compare the effectiveness of the attention multi-scale convolutional neural network (ATT-CNN) model in practical applications. The results demonstrate that the proposed method based on deep learning has better performance than the regularization method.

Pinning potential in highly performant CaKFe4As4 superconductor from DC magnetic relaxation and AC multi-frequency susceptibility studies

We have investigated the pinning potential of high-quality single crystals of superconducting material CaKFe4As4 having high critical current density and very high upper critical field using both magnetization relaxation measurements and frequency-dependent AC susceptibility. Preliminary studies of the superconducting transition and of the isothermal magnetization loops confirmed the high quality of the samples, while temperature dependence of the AC susceptibility in high magnetic fields show absolutely no dependence on the cooling conditions, hence, no magnetic history. From magnetization relaxation measurements were extracted the values of the normalized pinning potential U*, which reveals a clear crossover between elastic creep and plastic creep. The extremely high values of U*, up to 1200 K around the temperature of 20 K lead to a nearly zero value of the probability of thermally-activated flux jumps at temperatures of interest for high-field applications. The values of the creep exponents in the two creep regimes resulted from the analysis of the magnetization relaxation data are in complete agreement with theoretical models. Pinning potentials were also estimated, near the critical temperature, from AC susceptibility measurements, their values being close to those resulted (at the same temperature and DC field) from the magnetization relaxation data.

Investigations on asphaltene aggregate formation by high-field diffusion NMR and low-field ghost solvent NMR relaxometry

Aggregation of asphaltenes remains a scientific challenge. In the present contribution, at first, transverse magnetization relaxation (T2) data from low-field NMR measurements of the asphaltenes from heavy, medium-heavy, or light crude oil samples were recovered. Following this, diffusion ordered spectroscopy nuclear magnetic resonance (NMR) spectra of these asphaltenes were acquired to obtain average molecular weights (M w) of asphaltene aggregates. Analyzing the relaxation and diffusion behavior of asphaltene aggregates as a function of solution percentage and hence molecular weights are utilized to explain the difference between light, medium-heavy and heavy crude oil asphaltenes in aggregate formation. The main results suggest that light crude oil asphaltenes form a small but significant number of aggregates. In contrast, heavy crude oil asphaltenes demonstrate a more complicated picture at high concentrations with small, medium, and large aggregates. The relaxation behavior of medium-heavy asphaltenes is similar to those of heavy crude oil asphaltenes in diluted solutions and similar to light crude oil asphaltenes in concentrated solutions. Diffusion NMR results showed that the light crude oil asphaltene samples have a relatively smaller aggregate size, higher diffusion coefficient, and hence a smaller molecular weight than the samples extracted from heavy crude oils.

Vortex dynamics and second magnetization peak in the iron-pnictide superconductor Ca0.82La0.18Fe0.96Ni0.04As2

We report the studies of detailed magnetic relaxation and isothermal magnetization measurements in the vortex state of the 112-type iron-pnictide Ca0.82La0.18Fe0.96Ni0.04As2 superconductor with 22 K. In the isothermal M(H), a well defined second magnetization peak (SMP) feature is observed in the entire temperature range below T c for measurements with -axis. However, for -planes, the SMP feature is suppressed at low temperatures, which might be due to 2D Josephson vortices forming at low temperatures and high magnetic fields in such an anisotropic system. A rigorous analysis considering the magnetic relaxation data for -axis suggests an elastic to plastic pinning crossover across H p , which also seems accompanied with a possible phase transition in vortex lattice near . Moreover, point disorder and surface defects are likely to be the dominant sources of pinning, which contribute to the -type of pinning in the sample. A high J c , in access of 105 A cm−2 observed could potentially make this material technologically important.

Magnetization relaxation dynamics in [Co/Pt]3 multilayers on pico- and nanosecond timescales

We experimentally investigated magnetization relaxation dynamics in the largely unexplored time window extending from few picoseconds up to two nanoseconds following femtosecond laser pulse excitation. We triggered magnetization dynamics in ${[\mathrm{Co}(0.4\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Pt}(0.7\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]}_{3}$ multilayers and measured the resulting magneto-optic response by recording both transient hysteresis loops as well as transients of magnetization dynamics. We observe that the coercive field of the sample is still strongly suppressed even \ensuremath{\sim}1 ms after the laser excitation, which is three orders of magnitude longer than the recovery time of the magnetization amplitude. In addition, we succeeded to fit the magnetization relaxation data in the entire experimentally observed time window by considering two phenomenological time constants ${\ensuremath{\tau}}_{f}^{*}$ and ${\ensuremath{\tau}}_{s}^{*}$ describing fast (ps) and slow (ns) magnetization relaxation processes, respectively. The fits of the data suggest a magnetic field dependent relaxation slowdown beyond 100 ps after excitation. We observe an explosion of the ${\ensuremath{\tau}}_{f}^{*}$ and ${\ensuremath{\tau}}_{s}^{*}$ values when the magnetization is completely quenched and relaxes intrinsically in the absence of an external magnetic field. We interpret the phenomenological time constants ${\ensuremath{\tau}}_{f}^{*}$ and ${\ensuremath{\tau}}_{s}^{*}$ using an intuitive physical picture based on the Landau-Lifshitz-Bloch model and numerical solutions of the extended three-temperature model [Shim et al., Sci. Rep. 10, 6355 (2020)].

Vortex dynamics and phase diagram in the electron-doped cuprate superconductor Pr0.87LaCe0.13CuO4

Second magnetization peak (SMP) in hole-doped cuprates and iron pnictide superconductors has been widely explored. However, similar feature in the family of electron-doped cuprates is not common. Here, we report the vortex dynamics study in the single crystal of an electron-doped cuprate Pr$_{0.87}$LaCe$_{0.13}$CuO$_4$ superconductor using dc magnetization measurements. A SMP feature in the isothermal $M(H)$ was observed for $H$$\parallel$$ab$-planes. On the other hand, no such feature was observed for $H$$\parallel$$c$-axis in the crystal. Using magnetic relaxation data, a detailed analysis of activation pinning energy via collective creep theory suggests an elastic to plastic creep crossover across the SMP. Moreover, for $H$$\parallel$$ab$, a peak in the temperature dependence of critical current density is also observed near 7 K, which is likely be related to a dimensional crossover (3D-2D) associated to the emergence of Josephson vortices at low temperatures. The anisotropy parameter obtained $\gamma$ $\approx$ 8-11 indicates the 3D nature of vortex lattice mainly for $H$$\parallel$$c$-axis. The $H$-$T$ phase diagrams for $H$$\parallel$$c$ and $H$$\parallel$$ab$ are presented.

Features of Relaxation of the Remanent Magnetization of Antiferromagnetic Nanoparticles by the Example of Ferrihydrite

The relaxation of the remanent magnetization of antiferromagnetically ordered ferrihydrite nanoparticles at the exchange bias effect implemented in these systems has been investigated. The magnetization relaxation depends logarithmically on time, which is typical of the thermally activated hoppings of particle magnetic moments through the potential barriers caused by the magnetic anisotropy. The barrier energy obtained by processing of the remanent magnetization relaxation data under the field cooling conditions significantly exceeds the barrier energy under standard (zero field cooling) conditions. The observed difference points out the possibility of using the remanent magnetization relaxation to analyze the mechanisms responsible for the exchange bias effect in antiferromagnetic nanoparticles and measure the parameters of the exchange coupling of magnetic subsystems in such objects.

{Re6Q8}(SO3)6]10- (Q = S or Se): Facile Synthesis and Properties of the Most Highly Charged Octahedral Cluster Complexes and High Magnetic Relaxivity of Their Colloids with Gd3+ Ions.

New octahedral rhenium cluster complexes [{Re6Q8}(SO3)6]10- (Q = S or Se) were synthesized starting from [{Re6Q8}(H2O)4(OH)2]·12H2O. The complexes were crystallized as sodium salts and characterized by X-ray single-crystal diffraction and elemental analyses, IR, UV/vis and luminescence spectroscopies. Magnetic relaxation data demonstrate the complex formation of the cluster units with gadolinium ions. The analysis of the magnetic relaxation rates measured at various Gd:cluster ratios and different concentrations revealed the conversion of the aggregates (Gdx[{Re6Se8}(SO3)6]y)n- into a nanoparticulate form even at x = 1 and y ≥ 1. Thus, the self-assembly of the cluster units into the nanoparticles is greatly facilitated by counterion binding with sodium cations. The concentration conditions were optimized for the formation and hydrophilization of NaxGdy[{Re6Q8}(SO3)6]-based colloids with the magnetic relaxivity values of r1(2) = 21.0(24.1) and r1(2) = 25.9(29.8) mM-1 s-1 for the {Re6S8}2+ and {Re6Se8}2+ derivatives, respectively.

Magnetic Relaxation Experiments in CNT-Based Magnetic Nanocomposite

In this work, we discuss the relaxation of the magnetic moments in a novel carbon nanotube (CNT)-based nanocomposite synthesized by using chemical vapor deposition process. The material consists of a matrix of CNT filled by Fe-based nanoparticles. This structure is seen clearly by scanning and transmission. X-ray diffraction and Raman spectroscopy are used to detect the predominant Fe3C phase and the CNT presence in the sample, respectively. The results obtained from both hysteresis cycles, M(H), and zero field cooled-field cooled (ZFC-FC) measurements confirm that the material is characterized by both a strong ferromagnetic exchange and random magnetic anisotropy. For the first time, we have been able to fit the magnetic relaxation data, M(t), by using both the two distributions of nanoparticles data deduced from the ZFC-FC data and the temperature dependence of the magnetic anisotropy obtained from the law of approach to saturation in random magnets.

Deviation from Canonical Collective Creep Behavior in Li0.8Fe0.2OHFeSe

We present an extensive study of the vortex dynamics in Li$_{0.8}$Fe$_{0.2}$OHFeSe single crystal, which is prepared by intercalating insulating spacer layer Li$_{0.8}$Fe$_{0.2}$OH into FeSe. Absence of fishtail effect and the crossover from elastic to plastic creep are observed similar to the parent compound FeSe. However, when we apply collective creep theory to the magnetic relaxation data, the obtained creep exponent $\mu$ is found to be $\sim$ 4.1 much larger than the predicted maximum value. Besides, the elastic creep in the vortex phase diagram of Li$_{0.8}$Fe$_{0.2}$OHFeSe is found to reside only in an extremely small region. Such a large value of $\mu$ and the small elastic creep region may be originated from the weakening of coupling between the vortices in neighbouring layers by elongating the layer distance. It may indicate that the vortex structure of Li$_{0.8}$Fe$_{0.2}$OHFeSe is in the crossover regime between elastic Abrikosov vortices to stacks of pancake vortices. Our study also suggests that the intercalated FeSe is a promising candidate for searching intrinsic Josephson phenomena.

Influence of Pluronic F127 microenvironments on the photochemical nitric oxide release from S-nitrosoglutathione

Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) (F127) hydrogels have been used to deliver nitric oxide (NO) topically in biomedical applications. Here, the effect of F127 microenvironments on the photochemical NO release from S-nitrosoglutathione (GSNO) was investigated in F127 solutions 7.6 wt% 15 wt% and 22.5 wt% at 15 °C and 37 °C. Small-angle X-ray Scattering (SAXS) and Differential Scanning Calorimetry (DSC) measurements, along with proton Nuclear Magnetic Resonance (1H NMR) spectral shifts and T2 relaxation data at six different concentration-temperature conditions, allowed identifying F127 microphases characterized by: a sol phase of unimers; micelles in non-defined periodic order, and a gel phase of cubic packed micelles. Kinetic measurements showed that GSNO photodecompositon proceeds faster in micellized F127 where GSNO is segregated to the intermicellar microenvironment. Real time kinetic monitoring of NO release and T2 relaxation profiles showed that NO is preferentially partitioned into the hydrophobic PPO cores of the F127 micelles, with the consequent decrease in its rate of release to the gas phase. These results show that F127 microphases affect both the kinetics of GSNO photodecomposition and the rate of NO escape and can be used to modulate the photochemical NO delivery from F127/GSNO solutions.

Doping Dependence of the Second Magnetization Peak, Critical Current Density, and Pinning Mechanism in BaFe2–xNixAs2 Pnictide Superconductors

A series of high quality BaFe$_{2-x}$Ni$_x$As$_2$ pnictide superconductors were studied using magnetic relaxation and isothermal magnetic measurements in order to study the second magnetization peak (SMP) and critical current behaviour in Ni-doped 122 family. The temperature dependence of the magnetic relaxation rate suggests a pinning crossover, whereas, it's magnetic field dependence hints a vortex-lattice structural phase-transition. The activation energy ($U$) estimated using the magnetic relaxation data was analyzed in detail for slightly-underdoped, slightly-overdoped and an overdoped samples, using Maley's method and collective creep theory. Our results confirm that the SMP in these samples is due to the collective (elastic) to plastic creep crossover as has been observed for the other members of 122-family. In addition, we also investigated the doping dependence of the critical current density ($J_c$) and the vortex-pinning behaviour in these compounds. The observed $J_c$ is higher than the threshold limit (10$^5$ A/cm$^2$) considered for the technological potential and even greater than 1 MA/cm$^2$ for slightly underdoped Ni-content, x = 0.092 sample. The pinning characteristics were analyzed in terms of the models developed by Dew-Hughes and Griessen $et$ $al$, which suggest the dominant role of $\delta l$-type pinning.

Irreversibility in Rolled Tantalum

We investigated the magnetic and thermodynamic properties of rolled tantalum in superconducting state. The magnetization shows a crossover from type I to type II superconductivity as the magnetic field increases beyond the thermodynamic critical field. In both regimes, the sample shows a substantial irreversibility of the magnetization which we attribute to the pinning of the entities that characterize the intermediate and mixed states. Magnetic relaxation data taken in the intermediate state shows an important contribution of the quantum tunnelling below 3.7 K.

Study of the second magnetization peak and the pinning behaviour in Ba(Fe0.935Co0.065)2As2 pnictide superconductor

Isothermal magnetic field dependence of magnetization and magnetic relaxation measurements were performed for the axis on a single crystal of Ba(Fe0.935 Co0.065)2As2 pnictide superconductor having Tc = 21.7 K. The second magnetization peak (SMP) for each isothermal M(H) was observed in a wide temperature range from Tc to the lowest temperature of measurement (2 K). The magnetic field dependence of relaxation rate R(H), showed a peak (Hspt) between Hon (onset of SMP in M(H)) and Hp (peak field of SMP in M(H)), which is likely to be related to a vortex-lattice structural phase transition, as suggested in the literature for a similar sample. In addition, the magnetic relaxation measured for magnetic fields near Hspt showed some noise, which might be the signature of the structural phase transition of the vortex lattice. Analysis of the magnetic relaxation data using Maley’s criterion and the collective pinning theory suggested that the SMP in the sample was due to the collective (elastic) to plastic creep crossover, which was also accompanied by a rhombic to square vortex lattice phase transition. Analysis of the pinning force density suggested a single dominating pinning mechanism in the sample, which did not showing the usual and nature of pinning. The critical current density (Jc), estimated using the Bean critical state model, was found to be 5 × 105 A cm−2 at 2 K in the zero magnetic field limit. Surprisingly, the maximum of the pinning force density was not responsible for the maximum value of the critical current density in the sample.

Evolution of structure, magnetism, and electronic transport in the doped pyrochlore iridate Y2Ir2−xRuxO7

The interplay between spin-orbit coupling (SOC) and electron correlation ($U$) is considered for many exotic phenomena in iridium oxides. We have investigated the evolution of structural, magnetic and electronic properties in pyrochlore iridate Y$_2$Ir$_{2-x}$Ru$_{x}$O$_7$ where the substitution of Ru has been aimed to tune this interplay. The Ru substitution does not introduce any structural phase transition, however, we do observe an evolution of lattice parameters with the doping level $x$. X-ray photoemission spectroscopy (XPS) study indicates Ru adopts charge state of Ru$^{4+}$ and replaces the Ir$^{4+}$ accordingly. Magnetization data reveal both the onset of magnetic irreversibility and the magnetic moment decreases with progressive substitution of Ru. These materials show non-equilibrium low temperature magnetic state as revealed by magnetic relaxation data. Interestingly, we find magnetic relaxation rate increases with substitution of Ru. The electrical resistivity shows an insulating behavior in whole temperature range, however, resistivity decreases with substitution of Ru. Nature of electronic conduction has been found to follow power-law behavior for all the materials.

Surface spins disorder in uncoated and SiO2 coated maghemite nanoparticles

We studied the surface spins disorder in uncoated and silica (SiO2) coated maghemite (γ-Fe2O3) nanoparticles using temperature and time dependent magnetization. The average crystallite size for SiO2 coated and uncoated nanoparticles was about 12 and 29nm, respectively. Scanning electron microscopy (SEM) showed that the nanoparticles are spherical in shape and well separated. Temperature scans of zero field cooled (ZFC)/field cooled (FC) magnetization measurements showed lower average blocking temperature (TB) for SiO2 coated maghemite nanoparticles as compared to uncoated nanoparticles. The saturation magnetization (Ms) of SiO2 coated maghemite nanoparticles was also lower than the uncoated nanoparticles and is attributed to smaller average crystallite size of SiO2 coated nanoparticles. For saturation magnetization vs. temperature data, Bloch's law (M(T)= M(0).(1− BTb)) was fitted well for both uncoated and SiO2 coated nanoparticles and yields: B =3×10−7K-b, b=2.22 and B=0.0127K-b, b=0.57 for uncoated and SiO2 coated nanoparticles, respectively. Higher value of B for SiO2 coated nanoparticles depicts decrease in exchange coupling due to enhanced surface spins disorder (broken surface bonds) as compared to uncoated nanoparticles. The Bloch's exponent b was decreased for SiO2 coated nanoparticles which is due to their smaller average crystallite size or finite size effects. Furthermore, a sharp increase of coercivity at low temperatures (<25K) was observed for SiO2 coated nanoparticles which is also due to contribution of increased surface anisotropy or frozen surface spins in these smaller nanoparticles. The FC magnetic relaxation data was fitted to stretched exponential law which revealed slower magnetic relaxation for SiO2 coated nanoparticles. All these measurements revealed smaller average crystallite size and enhanced surface spins disorder in SiO2 coated nanoparticles than in uncoated γ-Fe2O3 nanoparticles.

Pb- and Sm-doping effects on the vortex dynamics in Bi4O4S3 superconductor

Polycrystalline samples of Bi4O4S3, Bi3.94Pb0.06O4S3, and Bi3.94Sm0.06O4S3 were synthesized using a conventional solid-state reaction method. The vortex dynamics of these samples are studied and compared through measuring the critical current density (Jc) and magnetic relaxation. The estimated Jc value of three samples at temperature 2 K decreases with doping Pb and Sm elements due to the depression of the superconductivity. The normalized pinning force response with reduced field follows the Kramer scaling law f(h)∝ hp(1−h)q, and the maximum of the reduced field (hmax) is observed at 0.31, 0.24, and 0.18 for Bi4O4S3, Bi3.94Pb0.06O4S3, and Bi3.94Sm0.06O4S3, respectively, indicating the presence of both surface and point pinning centers in these samples. The barrier energy estimated by the magnetic relaxation data depends on the field as a negative power law U0∝Hn and the magnetic relaxation rate S exhibits a considerable monotonic increase with magnetic field, which corroborates a plastic nature of the creep vortex dynamics in these compounds.