Magnetic Field Sweep Rate Research Articles

Effect of light ellipticity and polarization angle on the transient dynamics of the ground-state Hanle effect

We report theoretical investigations on the transient dynamics of the ground-state Hanle effect in the Voigt geometry, where a time-varying scanning magnetic field B (t) is applied perpendicular to the light propagation direction. Transient Hanle signals are calculated for different light polarizations using a theoretical model based on the density matrix approach. First, we study the ellipticity dependence of transient Hanle signals in two configurations in which the major axis of the light polarization ellipse () is aligned either perpendicular or parallel to the field B . For B , elliptical polarization of the light produces oscillations of frequencies 2ΩL(t) and ΩL(t) simultaneously, where ΩL(t) is the Larmor frequency of the corresponding field B (t). On the other hand, in the case of || B , only ΩL(t) oscillation is observed using elliptically polarized light. Second, we vary the tilt angle between the polarization vector of a linearly polarized light and the orientation of the field B and study its effect on the transient response of Hanle signals. Both the amplitude and the decay rate of the oscillations show a strong dependence on the light ellipticity and tilt angle of the polarization vector. In addition, the influence of the magnetic field sweep rate on the transient signals is demonstrated for given values of light ellipticity and polarization angle.

The thermally assisted magnetization and dielectric relaxation in spin frustrated Ca3Co2O6

The incompatibility between the antiferromagnetic exchange interaction and the triangular lattice makes Ca3Co2O6 become a frustrated system. It leads to the existence of disordered magnetic structure in the frustrated spin chains. At low temperature, the disordered spins are frozen. In this study, the Ca3Co2O6 was prepared and its low temperature dynamic behaviors were characterized. The spin freezing peak and the decay of magnetization with time well confirm the thermally assisted magnetization. Since that the flip of the spins can result in the conservation of the total angular momentum and the conserved angular momentum can make the adjacent lattice feel a transfer torque. In response to variation of magnetization, the dielectric permittivity of Ca3Co2O6 also presents relaxation feature. For the isothermal dielectric permittivity, it not only depends greatly on the sweep rates of magnetic field but also decays with time. This detectable variation of the dielectric property in Ca3Co2O6 suggests that the thermally assisted dielectric relaxation is an alternative method for characterizing the dynamic behaviors of the frustrated system.

Hysteretic magnetoresistance in superconducting SrTiO3/LaAlO3/SrTiO3 trilayer interface system

We investigated the electrical transport properties of the SrTiO3/LaAlO3/SrTiO3 (STO/LAO/STO) trilayer interface system. We found that the trilayer exhibits superconductivity at temperatures below 0.2 K. In the superconducting regime, the magnetoresistance (MR) of the system shows pronounced hysteresis, possibly due to the interplay of ferromagnetism and superconductivity. The magnitude of MR hysteresis strongly depends on the magnetic field sweep rate, and we observed a threshold field-sweep rate below which no MR is detected. At high sweep rates, the MR exhibits superconducting-normal-superconducting transition behavior. To explain these observations, we propose a model based on the ohmic heating from superconducting phase slip centers beneath Bloch-type magnetic domain walls in the ferromagnetic layer. Furthermore, we observed complex features in the MR curves that are likely due to domain wall motion in the system.

Time and temperature dependent magnetic viscosity experiments on Sr/Co nanoferrite particles

Magnetic viscosity experiments have been performed in order to investigate the magnetization reversal in Sr nanoferrite particles (nanoscale SrFe12O19) and interacting Sr/Co nanoferrite particles (SrFe12O19–CoFe2O4 nanocomposites). The magnetic viscosity S = d M ( t ) / d l n ( t ), where M ( t ) is the magnetization as a function of time, has been collected. For Sr nanoferrite S shows a maximum close to the coercive field, reflecting the relation between S and the energy barrier distribution. We evidence that magnetic viscosity experiments on Sr nanoferrite and interacting Sr/Co nanoferrite particles provide reliable qualitative results for the different magnetic field sweep rate and saturating field H s a t considered. In addition, the activation volumes extracted from the magnetic viscosity experiments performed at different temperatures on Sr nanoferrite are quantitatively correlated to anisotropy changes.

Landau theory-based relaxational modeling of first-order magnetic transition dynamics in magnetocaloric materials

The magnetocaloric effect is often largest within the neighborhood of a first-order phase transition. This effect can be utilized in magnetocaloric refrigeration, which completely eliminates the need for the greenhouse gases utilized in conventional refrigeration. However, such transitions present unique dynamical effects and are accompanied by hysteresis, which can be detrimental for such refrigeration applications. In this work, a Landau theory-based relaxational model is used to study the magnetic hysteresis and dynamics of the first-order magnetic transition of LaFe13−x Si x . Fitting the experimental magnetization data as a function of applied magnetic field under different field sweep rates with this model provided the Landau parameters (A, B, and C) and the kinetic coefficient of the studied material. We demonstrate the tendency of the magnetic hysteresis to increase with the magnetic field sweep rate, underlining the importance of studying and minimizing the magnetic hysteresis in magnetic refrigerants at practical field sweep rates. While evaluating the temperature dependence of the time required for a complete transition to occur, a nonmonotonic behavior and a sharp peak were found for temperatures near the transition temperature. Such peaks occur at the same temperature as the peak of the magnetic entropy change for low fields, whereas for higher fields the two peaks decouple. This information is critical for technological applications (such as refrigerators/heat pumps) as it provides guidelines for the optimization of the magnetic field amplitude in order to reduce the transition timescale and consequently maximize the machine operational frequency and amount of heat that is pumped in/out per second.

Thermomagnetic instability and accompanied stress intensity factor jumps in type-II superconducting bulks under various magnetization processes

For type-II superconducting bulks used as trapped-field magnets, the thermomagnetic instability, manifested as flux jumps and temperature spikes, frequently takes place, resulting in a large amount of energy dissipation in a short time and further the crack problem due to electromagnetic and thermal stresses. In this paper, based on the magnetic and heat diffusion equations and fracture theory, we develop a thermal-magnetic-mechanical coupling model to analyze the flux-jump and fracture behaviors in bulk samples of BiSrCaCuO under various magnetization processes. This model has an important advantage that the simulation domain can be restricted to the sample itself, without having to consider the air region around it, and its reliability is verified by the existing experimental and numerical results. The effects of the sample size, the ambient temperature, and the sweep rate, direction, and uniformity of the external magnetic field on the flux jumps, and Mode I and Mode II stress intensity factors are fully analyzed under different cooling conditions. It is found that as ambient temperature or field inclined angle increases or field sweep rate decreases, the first flux-jump field presents a trend of monotonically increasing for zero-field-cooling magnetization but it has an opposite trend for field-cooling magnetization. The flux jump can lead to the jump of temperature, electromagnetic force, and stress intensity factor. In addition, the sensitivity of flux-jump and fracture behaviors to different parameters and the influence of flux jump on the demagnetization behavior under crossed magnetic fields are discussed. We also find the levitation force jumping phenomenon when the bulk sample is magnetized in a nonuniform magnetic field. From the results obtained, we provide some general guidelines on how the system parameters of superconducting bulk magnets could be chosen to improve the thermal-magnetic-mechanical stability.

Slow spin dynamics and quantum tunneling of magnetization in the dipolar antiferromagnet DyScO3

We present a comprehensive study of static and dynamic magnetic properties in the Ising-like dipolar antiferromagnet (AFM) DyScO$_3$\ by means of DC and AC magnetization measurements supported by classical Monte-Carlo calculations. Our AC-susceptibility data show that the magnetic dynamics exhibit a clear crossover from an Arrhenius-like regime to quantum tunneling of magnetization (QTM) at $T^* = 10$ K. Below $T_{\mathrm{N}} = 3.2$ K DyScO$_3$ orders in an antiferromagnetic $GxAy$-type magnetic structure and the magnetization dynamics slow down to the minute timescale. The low-temperature magnetization curves exhibit complex hysteretic behavior, which depends strongly on the magnetic field sweep rate. We demonstrate that the low-field anomalies on the magnetization curve are related to the metamagnetic transition, while the hysteresis at higher fields is induced by a strong magnetocaloric effect. Our theoretical calculations, which take into account dipolar interaction between Dy$^{3+}$ moments, reproduce essential features of the magnetic behavior of DyScO$_3$. We demonstrate that DyScO$_3$ represents a rare example of inorganic compound, which exhibits QTM at a single-ion level and magnetic order due to classical dipolar interaction.

Second-Harmonic Response in Magnetic Nodal-Line Semimetal Fe3GeTe2

We experimentally investigate second-harmonic transverse voltage response to ac electrical current for a magnetic nodal-line semimetal Fe3GeTe2 (FGT). For zero magnetic field, the observed second-harmonic voltage behaves as a square of the longitudinal current, as it should be expected for nonlinear Hall effect. The magnetic field behavior is found to be sophisticated: while the first-harmonic response shows the known anomalous Hall hysteresis in FGT, the second-harmonic Hall voltage is characterized by the pronounced high-field hysteresis and flat (B-independent) region with curves touching at low fields. The high-field hysteresis strongly depends on the magnetic field sweep rate, so it reflects some slow relaxation process. For the lowest rates, it is also accomplished by multiple crossing points. Similar shape of the second-harmonic hysteresis is known for skyrmion spin textures in nonlinear optics. Since skyrmions have been demonstrated for FGT by direct visualization techniques, we can connect the observed high-field relaxation with deformation of the skyrmion lattice. Thus, the second-harmonic Hall voltage response can be regarded as a tool to detect spin textures in transport experiments.

The tuning of oxalato-bridge on magnetic interaction and field-induced phase transition in Ba2Co2Cl2(C2O4)3·4H2O

The crystal structure of the oxalato-bridged compound Ba2Co2Cl2(C2O4)3·4H2O has C2/c symmetry and contains two types of oxalate-bridged modes. We have investigated the magnetization behavior of the Ba2Co2Cl2(C2O4)3·4H2O sample with both the static and pulsed magnetic fields. The Bleaney-Bowers fitting for the temperature dependence of magnetization shows that there are two different spin-dimers with the different intra- and inter-dimer interactions. Using the high field sweep rate of the pulsed magnetic field, magnetization measurements show four continuous phase transitions (with four different critical magnetic fields) in a smaller magnetic field range from 5.5 to 7 T, which also confirms there are two different dimers. Based on the structure analysis, the four phase transitions might result from four different magnetic exchange interactions (J1, J'1, J2 and J'2) in the two different dimers. Thus, in the design of the molecular-based magnetic materials, the oxalato-bridged and its various bridging modes can be used to mediate the magnetic interaction of the intra- and inter-dimers, the electronic effect between the metal ions and induce new phases.

Sweep-Rate Dependence of the Large Magnetocaloric Effect across the Magnetostructural Transition in the Mn0.6Fe0.4(Ni,Co)Si System

A coupled magnetostructural transition (MST) from a high-temperature low-magnetic ${\mathrm{Ni}}_{2}\mathrm{In}$-type hexagonal phase to a low-temperature high-magnetic $\mathrm{Ti}\mathrm{Ni}\mathrm{Si}$-type orthorhombic phase, and the associated large magnetocaloric effect, are explored for the $({\mathrm{Mn}}_{0.6}{\mathrm{Fe}}_{0.4})({\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})\mathrm{Si}$ system. Room-temperature structural analysis is carried out to investigate the orthorhombic distortion of the hexagonal phase initiated by introducing $\mathrm{Co}$ at the $\mathrm{Ni}$ site. The indirect isothermal magnetic measurement technique is employed to estimate the magnetocaloric effect across the coupled phase transition. Fascinatingly, a relatively large value of isothermal magnetic entropy change (\ensuremath{\Delta}$S_{M}$) of (\ensuremath{-}12.02 \ifmmode\pm\else\textpm\fi{} 0.34) J ${\rm{kg}}^{-1}\;{\rm{K}}^{-1}$ at (332.5 \ifmmode\pm\else\textpm\fi{} 0.2) K and relative cooling power (RCP) of (119.6 \ifmmode\pm\else\textpm\fi{} 3.5) J ${\rm{kg}}^{-1}$ is achieved for the composition $({\mathrm{Mn}}_{0.6}{\mathrm{Fe}}_{0.4})({\mathrm{Ni}}_{0.6}{\mathrm{Co}}_{0.4})\mathrm{Si}$, for a field change of 30 kOe. The complex hysteresis behavior around the phase transition determines the efficiency of the material, which is revealed by both temperature- and field-sweep-rate dependence magnetization studies. Superheating (or supercooling) of the parent phase observed at higher temperature and field-sweep rates leads to broad thermal- and field-induced hysteresis. Lower sweep rates of temperature and magnetic field remove the quenching phenomenon across the MST and enhance the working efficiency. A large magnetocaloric effect and negligible variation of hysteresis with the temperature- and magnetic-field-sweep rates compared with other alloys makes this nominal low-cost material a promising candidate for practical solid-state cooling applications.

Systematic analysis of metamagnetic transitions in A-site disordered Sm0.5(Ca0.5−ySry)MnO3 compounds: A combined experimental and model Hamiltonian study

We investigate the ultra-sharp jump in the isothermal magnetization and the resistivity in the polycrystalline Sm0.5(Ca0.5−ySry)MnO3(y = 0, 0.1, 0.2, 0.25, 0.3, 0.5) compounds. The critical field (Hcr), required for the ultra-sharp jump, decreases with the increase of ‘Sr’ concentration, i.e. with an increase of average A-site ionic radius 〈rA〉. In addition, The magneto-transport data indicate that the phase separation increases with the increase of 〈rA〉, i.e. with ’y’. Moreover, the dependency of Hcr with magnetic field sweep rate reveals that the ultra-sharp jump from antiferromagnetic (AFM) state to the ferromagnetic (FM) state is of martensitic in nature. In parallel with experiments, we have carried out numerical calculations to understand the role of A-site disorder in the metamagnetic transition. Our two-band double exchange model Hamiltonian calculations show that the ‘Sr’ doping induces the ferromagnetic clusters in the antiferromagnetic insulating phase and in turn reduces the critical field. Overall, our combined study comprising of several materials with different composition of A-site reveal the fact that how and why certain A-type disorder produces prominent metamagnetic transition.

Influence of the martensitic transformation kinetics on the magnetocaloric effect in Ni-Mn-In

The inverse magnetocaloric effect (MCE) in Ni-Mn-based Heusler compounds occurs during the magnetostructural transition between low-temperature, low-magnetization martensite and high-temperature, high-magnetization austenite. In this study, we analyze the metamagnetic transformation of a $Ni_{49.8}Mn_{35}In_{15.2}$ compound by simultaneous adiabatic temperature change and strain measurements in pulsed magnetic fields up to 10 T. We observe an adiabatic temperature change of -10 K and a strain of -0.22 % when the reverse martensitic transition is fully induced at a starting temperature of 285 K. By a variation of the magnetic field-sweep rates between 316 Ts$^{-1}$, 865 Ts$^{-1}$ and 1850 Ts$^{-1}$, the transitional dynamics of the reverse martensitic transformation have been investigated. Our experiments reveal an apparent delay upon the end of the reverse martensitic transformation at field rates exceeding 865 Ts$^{-1}$ which is related to the annihilation of retained martensite. As a consequence, the field hysteresis increases and higher fields are required to saturate the transition. In contrast, no time-dependent effects on the onset of the reverse martensitic transformation were observed in the studied field-sweep range. Our results demonstrate that kinetic effects in Heusler compounds strongly affect the magnetic cooling cycle, especially when utilising a multicaloric "exploiting-hysteresis cycle" where high magnetic field-sweep rates are employed.

Magnetic field sweep rate influence on the critical current capabilities of a Fe(Se,Te) crystal

The relaxation phenomena of a FeSe0.5Te0.5 single crystal sample, with twinned morphology, exhibiting a second magnetization peak effect have been analyzed by means of dc magnetization measurements as a function of magnetic field (H). Using different magnetic field sweep rates for performing the superconducting hysteresis loops m(H), a change in the width of these curves has been observed together with a shift of the second peak field position. A consequent and significant variation of critical current density Jc has been extracted from the superconducting m(H) curves within the Bean critical state model. In this framework, the ratio between the critical current densities Jc calculated at 200 Oe/s and 5 Oe/s has been calculated showing interesting results in terms of transport capabilities. From the Jc(H) curves at T = 10 K for different sweep rates, the irreversibility field has been obtained for T = 10 K and fitted with a power law. The increasing trend of the irreversibility field as a function of sweep rate could suggest the use of this material for power applications where overheating due to vortices movement inside the superconductor has to be limited.

Theoretical Study of the Effect of Magnetic Field Sweep Rate and Temperature on Shielding Efficiency in a Bulk Tube-Shaped HTS

In this paper, we have studied the shielding properties of a high temperature superconducting (HTS) bulk tube-shaped cylinder in varying magnetic fields. Within this scope, we have simulated the hollow cylinder geometry in axis-symmetric finite element method (FEM) in which the electromagnetic properties of a HTS are described by a nonlinear electric field-current density (E − J) power law including a critical current density depending on magnetic field. In the first stage, we have analyzed temperature dependence of shielding factor (SF), which is described as the ratio of the applied magnetic field to the magnetic field in the center of the hollow cylinder. In the second stage, the effect of magnetic field’s sweep rate (dBapp/dt) on the magnetic shielding was numerically examined. In the final stage, using the certain lines over the superconducting tube, we have worked out the shielding field (Bsz) of the HTS tube by subtracting the z-component of magnetic field (Bz) from applied magnetic field (Bapp). Our numerical investigations indicate that the superconducting tube can provide field and temperature-controlled magnetic shielding.

Dynamic nuclear polarization by optical Stark effect in periodically pumped gallium arsenide

Optical pump-probe time- and magnetic-field-resolved Kerr rotation measurements provide a window into the mechanisms that generate dynamic nuclear polarization in bulk gallium arsenide. Previously, we have reported an unexpected dependence of the direction of the nuclear polarization on the sweep direction of the applied external magnetic field. In this paper, we present numerical calculations based on a model for this nuclear polarization due to the optical orientation and optical Stark effect produced by a train of ultrafast optical pulses. We demonstrate the correspondence of the model to our experimental measurements for different laser wavelengths and magnetic field sweep rates. Finally, we show that the model reproduces the sweep direction dependence and provides an explanation for this behavior.

Spin, charge and lattice dynamics of magnetization processes in frustrated Shastry-Sutherland system TmB4

TmB4 - the metallic tetraboride with a geometrically frustrated Shastry-Sutherland lattice (SSL) has attracted recently a lot of attention. It exhibits a strong Ising-like anisotropy along the c-axis and a rather complex phase diagram of the ordered magnetic phase below TN ≈ 11.7 K in which besides the main magnetization (M) plateau also small fractional plateaus can be observed. To learn more about the properties of fractional plateaus we have investigated the dynamics of magnetization processes in TmB4 by M vs. H (applied magnetic field) measurements at different temperatures and different field sweep rates (swr) 1.8 K. In addition, experiments of the charge carrier mobility and magnetostriction as a function of temperature and magnetic field were carried out to specify the interplay between magnetic structure and the charge dynamics, and the role of the magneto-elastic interaction in this compound, respectively. It was shown that the magnetization values of fractional plateaus become elevated with the increase of temperature at which magnetization takes place and with the decrease of magnetic field sweep rate, which points to inconsistency with theoretical predictions based on Glauber dynamics. Moreover, it turned out that the transitions between magnetic phases during magnetization processes are accompanied by an anomalous behavior of charge mobility and distinctive variations of magnetostriction.

Carbon-13 dynamic nuclear polarization in diamond via a microwave-free integrated cross effect

Color-center-hosting semiconductors are emerging as promising source materials for low-field dynamic nuclear polarization (DNP) at or near room temperature, but hyperfine broadening, susceptibility to magnetic field heterogeneity, and nuclear spin relaxation induced by other paramagnetic defects set practical constraints difficult to circumvent. Here, we explore an alternate route to color-center-assisted DNP using nitrogen-vacancy (NV) centers in diamond coupled to substitutional nitrogen impurities, the so-called P1 centers. Working near the level anticrossing condition-where the P1 Zeeman splitting matches one of the NV spin transitions-we demonstrate efficient microwave-free 13C DNP through the use of consecutive magnetic field sweeps and continuous optical excitation. The amplitude and sign of the polarization can be controlled by adjusting the low-to-high and high-to-low magnetic field sweep rates in each cycle so that one is much faster than the other. By comparing the 13C DNP response for different crystal orientations, we show that the process is robust to magnetic field/NV misalignment, a feature that makes the present technique suitable to diamond powders and settings where the field is heterogeneous. Applications to shallow NVs could capitalize on the greater physical proximity between surface paramagnetic defects and outer nuclei to efficiently polarize target samples in contact with the diamond crystal.

A differential rate meter for real-time peak tracking in optically detected magnetic resonance at low photon count rates.

We describe a real-time method to obtain normalized differential count rate signals from modulated systems with photon detection at single-photon count rates. The method is demonstrated with a real-time peak-locking and frequency control to track magnetic field using optically detected magnetic resonance of nitrogen-vacancy centers in diamond. This procedure allows us to measure the magnetic field continuously with a sensitivity of 4.1 µT/Hz1/2 and to track magnetic field sweep rates up to 50 µT/s. The differential rate meter automates the processing of voltage pulse outputs from the photon detector and provides noise levels on par with traditional photon counting methods using digital counters.

Dynamics of the magnetoelastic phase transition and adiabatic temperature change in Mn1.3Fe0.7P0.5Si0.55

The adiabatic temperature change ΔTad of a Mn1.3Fe0.7P0.5Si0.55 Fe2P-type alloy was measured under different magnetic field-sweep rates from 0.93 Ts−1 to 2870 Ts−1. We find a field-sweep-rate independent magnetocaloric effect due to a partial alignment of magnetic moments in the paramagnetic region overlapping with the magnetocaloric effect of the first-order phase transition. Additionally, the first-order phase transition is not completed even in fields up to 20 T leading to a non-saturating behavior of ΔTad. Measurements in different pulsed fields reveal that the first-order phase transition cannot follow the fast field changes as previously assumed, resulting in a distinct field-dependent hysteresis in ΔTad.

Metastable states in the frustrated triangular compounds Ca3Co2−xMnxO6 and Ca3Co2O6

The observation of unusual metastable behavior in ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2}{\mathrm{O}}_{6}$ remains an ongoing puzzle. When the magnetic field is increased at certain very slow rates, evenly spaced steps occur in the magnetization and other physical quantities as a function of magnetic field every 1.2 T. The ground state without steps is approached by slow relaxation processes over hours to days. It is a striking example of extremely slow dynamics arising from geometrical frustration in an otherwise clean and long-range ordered system, and its precise description remains controversial. Here we shed light on the mystery by reporting similar behavior in isostructural ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{6}$, albeit at magnetic field sweep rates that are six orders of magnitude faster than that in ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2}{\mathrm{O}}_{6}$. We observe these steps not only in the magnetization, but also in the magnetostriction, electric polarization, and magnetocaloric effect. We present a study of metastable behavior in both compounds across seven orders of magnitude of magnetic field sweep rates using unique magnets at the National High Magnetic Field Laboratory. The metastable steps occur for intermediate ranges of magnetic field sweep rates, vanishing or evolving for the fastest and slowest sweep rates. Our data support that these metastable steps are an intrinsic feature of the three-dimensionally frustrated geometry of Ising spins in this structural family and thus narrow the number of applicable models.