Magnetic Transition Research Articles

Interplay of the magnetic and current density field topologies in axisymmetric devices for magnetic confinement fusion

In magnetic confinement fusion devices close to axisymmetry, such as tokamaks, a key element is the winding profile of the magnetic field lines, or its inverse, the safety profile $q=q_{\boldsymbol {B}}$ . A corresponding profile, $q_{\boldsymbol {J}}$ , can be defined for the current density field lines. Ampère's law relates any mode of current perturbation $\delta \boldsymbol {J}_{m,n}$ with a mode of magnetic perturbation $\delta \boldsymbol {B}_{m,n}$ . It is shown that the knowledge of the pair $(q_{\boldsymbol {B}},q_{\boldsymbol {J}})$ allows us then to characterize the resonant, or non-resonant, nature of the modes for both the magnetic and current density field lines. The expression of $q_{\boldsymbol {J}}$ in the flux coordinate is derived. Including this calculation in real-time Grad–Shafranov equilibrium reconstruction codes would yield a comprehensive view of the magnetics. The monitoring of the pair $(q_{\boldsymbol {B}},q_{\boldsymbol {J}})$ would then allow us to investigate the role played by the resonant modes for the current density, that are current filamentary modes, in the plasma small-scale turbulence. By driving the magnetic and current density profiles apart so that the images of $q_{\boldsymbol {B}}$ and $q_{\boldsymbol {J}}$ are disjoint, these filamentary modes would not impact the magnetic field topology, being not associated with magnetic islands but with non-resonant magnetic modes. It remains to be explored to what extent such a configuration, where the spectrum of tiny current density filaments produces a spectrum of magnetic modes that has practically no effect on heat transport, is beneficial.

Numerical study of turbulent eddy self-interaction in tokamaks with low magnetic shear. Part I: Linear simulations

Abstract This study examines the impact of low and zero magnetic shear, along with rational values of safety factor, on linear mode behavior in tokamak plasmas. We focus on how magnetic field line topology and parallel domain length influence linear mode structures and growth rates as magnetic shear decreases. Our results
show that as magnetic shear is reduced to low values, linear modes can transition between different branches, significantly affecting their characteristics. At zero magnetic shear, accurate modeling requires precisely capturing magnetic topology, as small deviations near rational surfaces can greatly impact growth rates. These findings are extended to nonlinear simulations in a companion paper (Volčokas, 2024), revealing that long turbulent eddies and magnetic field line topology play a crucial role in turbulence self-interaction and plasma transport.

Supramolecular Spring-Like Fe(II) Spin-Crossover Complexes Experiencing Giant and Anisotropic Thermal Expansion Across Two Distinct Temperature Regimes.

Dynamic molecules with tunable chemical and physical properties in response to external stimuli hold great potential for applications in information storage, smart molecular machines, and biomimetics. Among them, supramolecular springs and spin-crossover (SCO) complexes can both undergo visible macroscopic changes under heat or light stimulation. In this study, we synthesized a trinuclear Fe(II)-SCO complex, [(R-L)FeII{Au(CN)2}2] (R1), using a chiral chelating ligand decorated with rotatable benzyl rings. The [FeAu2] trinuclear molecules form a 21-helical supramolecular chain via elastic Au∙∙∙Au contacts. Interestingly, the synergy between the multiple dynamic factors (SCO event, rotation of the rings, and flexibility in Au∙∙∙Au distance) endows the complex with multiple switchings in both magnetism and structure, as well as the most intriguing characteristic of giant and anisotropic "breathing" feature in thermal expansion within two distinct temperature regimes. Specifically, complex R1 undergoes two hysteretic magnetic transitions: a non-spin transition and an unsymmetric SCO transition. Both transitions are triggered/accompanied by the rotation of the benzyl rings. Correspondingly, reversible spring-like motions of the helical chains are observed in the two distinct temperature regimes. This work demonstrates a significant success in incorporating both SCO and spring-like motion in one system, paving the way for designing multifunctional dynamic materials for future devices.

Supramolecular Spring‐Like Fe(II) Spin‐Crossover Complexes Experiencing Giant and Anisotropic Thermal Expansion Across Two Distinct Temperature Regimes

Dynamic molecules with tunable chemical and physical properties in response to external stimuli hold great potential for applications in information storage, smart molecular machines, and biomimetics. Among them, supramolecular springs and spin‐crossover (SCO) complexes can both undergo visible macroscopic changes under heat or light stimulation. In this study, we synthesized a trinuclear Fe(II)‐SCO complex, [(R‐L)FeII{Au(CN)2}2] (R1), using a chiral chelating ligand decorated with rotatable benzyl rings. The [FeAu2] trinuclear molecules form a 21‐helical supramolecular chain via elastic Au∙∙∙Au contacts. Interestingly, the synergy between the multiple dynamic factors (SCO event, rotation of the rings, and flexibility in Au∙∙∙Au distance) endows the complex with multiple switchings in both magnetism and structure, as well as the most intriguing characteristic of giant and anisotropic “breathing” feature in thermal expansion within two distinct temperature regimes. Specifically, complex R1 undergoes two hysteretic magnetic transitions: a non‐spin transition and an unsymmetric SCO transition. Both transitions are triggered/accompanied by the rotation of the benzyl rings. Correspondingly, reversible spring‐like motions of the helical chains are observed in the two distinct temperature regimes. This work demonstrates a significant success in incorporating both SCO and spring‐like motion in one system, paving the way for designing multifunctional dynamic materials for future devices.

Covalent Stabilization of the Iridium-Containing Oxyhydrides Sr2Mn0.5Ir0.5O3.25H0.75 and Sr2Mn0.5Ir0.5O2.66H1.33.

Reaction between Sr2Mn0.5Ir0.5O4 and CaH2 or LiH yields the iridium-containing oxyhydride phases Sr2Mn0.5Ir0.5O3.25H0.75 or Sr2Mn0.5Ir0.5O2.66H1.33, respectively. Analysis of Mn K-edge XANES data indicate the presence of Ir3+ centers in these oxyhydride phases, whose low-spin d6 configuration is consistent with the "covalent stabilization" of the metastable oxyhydride phases, as seen previously in analogous ruthenium and rhodium containing materials. Neutron powder diffraction data indicate the hydride ions are located exclusively within the "equatorial" anion sites of Sr2Mn0.5Ir0.5O3.25H0.75. In contrast, hydride ions are observed on both the equatorial and axial anion sites of Sr2Mn0.5Ir0.5O2.66H1.33. This highly unusual anion distribution is attributed to a combination of the strong trans-influence of Ir-H σ-bonds and the stabilization of fac-IrO3H3 centers by spin-orbit coupling effects. Magnetization data indicate that Sr2Mn0.5Ir0.5O4 and Sr2Mn0.5Ir0.5O3.25H0.75 adopt spin glass states at low temperature, behavior which is attributable to the cation disorder in Sr2Mn0.5Ir0.5O4 and the cation and anion disorder in Sr2Mn0.5Ir0.5O3.25H0.75. In contrast, magnetization data collected from Sr2Mn0.5Ir0.5O2.66H1.33 show no evidence of any magnetic phase transition down to 5 K, consistent with the dilution of the magnetic network by the introduction of diamagnetic Ir3+ on the formation of the oxyhydride phase.

Charge-driven first-order magnetic transition in NiPS3.

Cross-coupling among the fundamental degrees of freedom in solids has been a long-standing problem in condensed matter physics. Despite its progress using predominantly three-dimensional materials, how the same physics plays out for two-dimensional materials is unknown. Here, we show that using31P nuclear magnetic resonance (NMR), the van der Waals antiferromagnet NiPS3undergoes a first-order magnetic phase transition due to the strong charge-spin coupling in a honeycomb lattice. Our31P NMR spectrum near the N´eel ordering temperature TN= 155 K exhibits the coexistence of paramagnetic and antiferromagnetic phases within a finite temperature range. Furthermore, we observed a discontinuity in the order parameter at TNand the complete absence of critical behavior of spin fluctuations above TN, decisively establishing the first-order nature of the magnetic transition. We propose that a charge stripe instability arising from a Zhang-Rice triplet ground state triggers the first-order magnetic transition.

DFT + U Study of Plutonium Hydrides with Occupation Matrix Control.

The magnetic order of plutonium hydrides (PuHx) has long been a subject of controversy, both experimentally and theoretically. The magnetic, structural, electronic, and thermodynamic properties of PuHx are investigated using Hubbard-corrected density functional theory (DFT + U), with U derived from linear response calculations. To address the issue of electronic metastable states within DFT + U, we employ an occupation matrix control method as well as allowing 5f orbitals to break the structural symmetry. This advanced computational approach establishes that the ground-state magnetic order for PuH2 is antiferromagnetic and that for PuH3 is ferromagnetic, consistent with Faraday and nuclear magnetic resonance experiments. Using the conventional hydrogen-vacancy model for PuHx, the hydrogen-content-induced magnetic transition and anomalous variation of the magnetic moment are successfully reproduced. In particular, the calculated transition point of x matches the experimental findings. Furthermore, the electronic configuration of the Pu atom in PuHx is determined to be 5f5, which is consistent with observations from X-ray photoemission spectroscopy. Our calculated values for enthalpy of formation, heat capacity, and entropy are in good agreement with experimental data, thereby validating the robustness of our theoretical framework.

Origin of correlated diffuse scattering in the hexagonal manganites

We use a combination of first-principles density-functional calculations and spin-dynamics simulations to explain the unusual diffuse inelastic neutron scattering in the hexagonal multiferroic yttrium manganite, YMnO3. Using symmetry considerations, we construct a model spin Hamiltonian with parameters derived from our density-functional calculations and show that it captures the measured behavior. We then show that the observed directionality in the structured diffuse scattering in momentum space is a hallmark of the triangular geometry, and that its persistence across a wide range of temperatures, both above and below the Néel temperature TN, is a result of the strong magnetic frustration. We predict that this diffuse scattering exists in a yet-to-be-observed modulated continuum of energies, that its associated spin excitations have distinct in-plane and out-of-plane character, and that vestiges of the magnetic frustration persist into the sub−TN state. Finally, we show that visualizing the magnetic order in terms of composite trimer magnetoelectric monopoles and toroidal moments, rather than individual spins, provides insight into the real-space fluctuations, revealing clusters of emerging order in the paramagnetic state, as well as collective short-range excitations in the ordered antiferromagnetic phase. Our understanding of this directional diffuse scattering in such a wide temperature range, both below and above TN, provides new insight into the magnetic phase transitions in classical frustrated systems. Published by the American Physical Society 2024

Theoretical and numerical investigation of an ultra-broadband near-perfect absorption in auxetic metamaterials

In this paper, we present and numerically investigate an auxetic metamaterial absorber based on vanadium dioxide (VO2), achieving more than 90% absorption of the incident terahertz (THz) waves between 4.15 THz to 8.43 THz with an average absorption of 98.4%. To our knowledge, the absorption bandwidth is higher than previously reported VO2-based absorbers. The proposed absorber contains two VO2 based resonator rings placed diagonally at the top of the dielectric substrate in such a way as to create an auxetic shape and provide more design space than the existing absorbers. Considering the vector nature of electromagnetic fields in three-dimensional space, numerical analysis is performed while keeping the phase-changing material VO2 in the metallic state. According to the full wave simulation, it is shown that under normal incidence, the proposed absorber provides almost flat and near-unity absorption, which covers from 4.82 THz to 7.53 THz. We describe the physical mechanism of the absorber through impedance matching theory, and the absorber performance is evaluated by observing the electric field distributions at various frequencies. The proposed structure also exhibits a satisfactory tunable range from 2% to 100%, which may satisfy the requirements of re-configurable metamaterial absorbers. We also show that the absorber provides better wide-angle absorption performance than the existing VO2-based models for both transverse polarization i.e., transverse electric (TE) and transverse magnetic (TM) modes. Owing to the higher absorption bandwidth and better tunable range, the proposed auxetic metamaterial has great potential applications in THz imaging, detectors, and sensing.

Size Effects on the Magnetic Properties of a System of ε-Fe2O3 Nanoparticles Embedded in a SiO2 Xerogel Matrix

A representative series of samples consisting of fine ε-Fe2O3 nanoparticles uniformly distributed over the SiO2 xerogel matrix with an iron oxide content of 5‒33 wt % has been synthesized and studied by mutually complementary physical methods. It has been shown by the X-ray diffractometry and high-resolution electron microscopy examination that, with increasing iron oxide concentration, the average particle size <d> increases from 4 to 11 nm. According to the X-ray diffractometry and Mӧssbauer spectroscopy data, the samples with an iron oxide content of 5–20 wt % are single-phase, while at the highest Fe2O3 concentration (33 wt %), the β-Fe2O3 and α-Fe2O3 phases arise. As the average particle size <d> increases, a monotonic increase in coercivity HC and remanent magnetization MR of the synthesized systems at room temperature is observed, which is indicative of their magnetic hysteresis. The magnetic transition known to occur in the ε–Fe2O3 oxide manifests itself in all the investigated samples as a drastic change in the HC and MR values at 150–75 K. At the same time, a thorough analysis of the temperature dependence of the real part of the ac magnetic susceptibility χ′ has shown that the particles with a size smaller than a critical value of dC ∼ 6.5 nm do not undergo the magnetic transition and have a much lower magnetic anisotropy constant as compared with coarser particles (d >dC). It has been found that, in the low-temperature region, the magnetic moments of these fine particles experience superparamagnetic blocking. It has been established that the size effects that arise in ultra-fine ε–Fe2O3 particles has a serious impact on the macroscopic magnetic properties of the highly dispersed systems based on them.

Transitioning from 0.5 to 0.9 mT: Protecting against inadvertent activation of magnet mode in active implants.

The "5 gauss line" is a phrase that is likely to be familiar to everyone working with MRI, but what is its significance, how was it defined, and what changes are currently in progress? This review explores the history of 5 gauss (0.5 mT) as a threshold for protecting against inadvertently putting cardiac pacemakers, implantable cardioverter defibrillators, and other active implantable medical devices into a "magnet mode." Additionally, it describes the background to the recent change of this threshold to 9 gauss (0.9 mT) in the International Standard IEC 60601-2-33 edition 4.0 that defines basic safety requirements for MRI. Practical implications of this change and some ongoing and emerging issues are also discussed.

Effect of Fe-doping on magnetic structures and “spin-lattice-charge” strong correlation properties in Mn3Sn1-xFexC compounds

The influences of Fe-doping on the magnetic structure, correlated lattice variation, and electronic transport of the antiperovskite compounds Mn3Sn1-xFexC have been reported for the first time. The results indicate that Fe-doping is effective in altering the magnetic behaviors in the Mn3Sn1-xFexC compounds. The magnetic phase transition point (TC) initially shifts to lower temperatures at low Fe-doping concentrations and then increases to higher temperatures at higher Fe-doping levels. This change is accompanied by the development of macroscopic ferromagnetism in the compound, which is essentially due to the emergence of a new canted ferrimagnetic phase named MCF, revealed by neutron powder diffraction (NPD). By virtue of the Fe-doping effect on the magnetism, the regulation of the negative thermal expansion (NTE) behavior and the abrupt change of resistivity in Mn3SnC are also realized. Based on these results, a magnetic phase diagram for the Mn3Sn1-xFexC series is established. This work not only provides a method for effective regulation of magnetic ordering but also strengthens our understanding of strongly correlated physical properties.

Spectral Investigation of Enhanced Orange Emission in Eu3+ Activated LaPO4 Nanophosphors Through Codoping With Na+ and Bi3+ Ions.

Lanthanum phosphate is a well-known potential host for rare earth activators for phosphor-converted white LED application. Na+ and Bi3+ codoped Eu3+ activated LaPO4 nanophosphor is successfully synthesized using citrate-based hydrothermal method at ambient condition and confirmed by XRD, EDS and XPS analysis. The excitation spectra of the sample exhibit sharp f-f characteristics peak of Eu3+ at 393 nm that is found to be the most dominant. The comparative photoluminescence studies of La1-xPO4:xEu3+(x = 0.10), La1-x-y PO4:xEu3+, yNa+(x = 0.10, y = 0.05), and La1-x-y-zPO4:xEu3+, yNa+, zBi3+ (x = 0.10, y = z = 0.05) reveal a progressive enhancement of the orange emission from Eu3+ as Na+ and (Na+, Bi3+) ions are incorporated into the LaPO4 matrix. The dominance of 5D0-7F1 magnetic dipole transition contributing a major part of orange luminescence of the nanophosphors and the symmetric site occupancy of Eu3+ was justified from the observed trend of in the Judd-Ofelt intensity parameters. The CIE studies confirm the improvement in color purity of emission from 32.5% to 90.6% due to the presence of Na+ and Bi3+ions.

Dominant role of charge ordering on high harmonic generation in Pr0.6Ca0.4MnO3

High harmonic generation (HHG) is a typical high-order nonlinear optical phenomenon and can be used to probe electronic structures of solids. Here, we investigate the temperature dependence of HHG from Pr0.6Ca0.4MnO3 in the range of 7–294 K, including the charge ordering (CO) transition and magnetic transition temperatures. The high harmonic (HH) intensity remains almost constant in the high-temperature charge-disordered phase. However, as the temperature is lowered, it starts to increase near the CO transition temperature where an optical gap related to the CO appears. The anomalous gap energy dependence resembles the one recently reported in a Mott insulator. We attribute the suppression of the HH intensity at high temperatures to the destructive interference among HH emissions from thermally activated multiple charge configurations. Our results suggest that HHG is a promising tool for probing the fluctuation of local order in strongly correlated systems. Published by the American Physical Society 2024

Structure, Optical Properties and Magnetocaloric Effects in Gd3Fe5O12 Garnet Synthesized by Solid State Method

In this work, gadolinium iron garnet (Gd3Fe5O12) was synthesized using the solid-state ceramic method. The structural analysis, confirmed through Rietveld refinement, indicated the formation of a single-phase garnet structure. Scanning electron microscopy studies revealed uniform grain size and homogeneity in the sintered material, while UV-vis absorption studies showed a peak at 300 nm, indicating strong ultraviolet interaction and an optical band gap of 3.64 eV, suggesting optoelectronic potential. The fluorescence emission spectrum shows a band at 435 nm, while the excitation spectrum exhibits bands at 275 and 360 nm. The magnetic measurements demonstrated Gd ion ordering at low temperatures, with a compensation temperature around 290 K. Arrott plots confirmed the second-order magnetic phase transition. Importantly, Gd3Fe5O12 exhibited both normal and inverse magnetocaloric effects, with a maximum magnetic entropy change of 1.05 J kg–1 K–1 at 230 K under 9 T magnetic field. These properties indicate Gd3Fe5O12 material as a promising candidate for magnetic refrigeration and optoelectronic applications.

Dirac-Like Fermions Anomalous Magneto-Transport in a Spin-Polarized Oxide 2D Electron System.

In a 2D electron system (2DES) the breaking of the inversion, time-reversal and bulk crystal-field symmetries is interlaced with the effects of spin-orbit coupling (SOC) triggering exotic quantum phenomena. Here, epitaxial engineering is used to design and realize a 2DES characterized simultaneously by ferromagnetic order, large Rashba SOC and hexagonal band warping at the (111) interfaces between LaAlO3, EuTiO3, and SrTiO3 insulators. The 2DES displays anomalous quantum corrections to the magneto-conductance driven by the time-reversal-symmetry breaking occurring below the magnetic transition temperature. The results are explained by the emergence of a non-trivial Berry phase and competing weak anti-localization/weak localization back-scattering of Dirac-like fermions, mimicking the phenomenology of gapped topological insulators. These findings open perspectives for the engineering of novel spin-polarized functional 2DES holding promises in spin-orbitronics and topologicalelectronics.

Graphene pixel based broadband polarization insensitive THz absorber

Abstract Suppressing undesired radiation and eliminating stray radiation in a high-frequency circuit might be challenging. Ultra-high frequency (UHF) electromagnetic wave absorbers are finding new applications in both the military and civilian domains. Obtaining an ultra-wide absorption band in a thin, single-layer Terahertz absorber becomes difficult. Also, achieving perfect absorption bandwidth on transverse electric (TE) and transverse magnetic (TM) mode is very sensitive. This paper presents a polarization-independent graphene-based absorbers with a near unity absorption band of 2.32 THz and 3.78 THz in a lower mid-infrared regime. The structure’s unit cell consists of cross-slot circular patches with Au as a ground separated by SiO2. To improve the absorption band another graphene layer was placed between the top and dielectric material. The absorber is polarization independent under normal incidence because of the deposited graphene structure’s fourfold symmetry. Both TE and TM polarizations were investigated, with wide-band absorption observed up to 60° incident angles in both cases. In terms of the lowest frequency of absorption, the prototype is deemed to be as thin as ∼ 0.1 λ max for both absorbers. Similarly, the effective homogeneity criteria in the subwavelength region are supported by the unit cell’s period of ∼ 0.1 λ max . The proposed absorber-2 shows 10 % higher FBW than existing work.

Magnetic sensitive mechanical response in CrSBr and its composite resonators

We study the mechanical response of bulk CrSBr in temperature using a CrSBr string resonator. We observe two abrupt changes in eigenfrequency and quality factor of the resonator with decreasing temperature, a strong one around 140 K due to an antiferromagnetic phase transition, and a weaker one around 200 K possibly related to a change of spin correlations. We find that the antiferromagnetic transition persists through a temperature window of 30 K rather than showing a narrow sharp change, indicating a gradual spin transition process. In addition, the quality factor exhibits an unexpected increase during the transition, which violates the theoretical prediction. Finally, we demonstrate that in a CrSBr/SiN composite resonator, its vibrational state is sensitively affected by the constituent CrSBr layer during the magnetic phase transitions. It reveals the potential of a composite resonator in both controlling its vibration state with and probing phase transitions of its constituent materials. Our study not only enriches the details about the antiferromagnetic phase transition in CrSBr, but also opens up possibility in magnetic sensing and in situ tuning using composite mechanical resonators.

Calorimetric Studies of Phase Transformations in the La&lt;sub&gt;1 – x&lt;/sub&gt;Y&lt;sub&gt;x&lt;/sub&gt;Mn&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;2&lt;/sub&gt; System

Differential scanning calorimetry (DSC) is used to determine the magnetic phase transformation temperatures of the La1 – xYxMn2Si2 (x = 0–1) alloys. For the compositions with х from 0 to 0.3, the temperature dependences of DSC signal exhibit λ-like endothermic effects observed near 300 K, which are related to the magnetic phase transition from the ferromagnetic to layered antiferromagnetic structure, and weak anomalies, which are observed in a temperature range of from 458 K for the composition with х = 0 to 323 К for the composition with х = 0.3 upon disordering of the layered antiferromagnetic structure. A clear endothermic peak corresponding to the disordering of interplane antiferromagnetic layered structure was found for the YMn2Si2. The data obtained are used to construct the magnetic phase diagram of the La1–xYxMn2Si2 system in a temperature range of 270–600 К. The differential scanning calorimetry is shown can be successfully used for the determination of temperatures of various magnetic phase transformations in rare-earth intermetallic compounds.

Magnetic Properties of a Nickel–Zinc Ferrite Powder with Different Degrees of Dispersion

The influence of the degree of dispersion of a nickel–zinc ferrite powder of a Ni0.7Zn0.3Fe2O4 composition on its magnetic properties has been considered. The material has been synthesized using the ceramic technology with preliminary mechanical activation of precursors. The degree of dispersion has been varied using different modes of its dry grinding in a ball mill. The patterns of the changes in saturation magnetization and the coercive force as a function of grinding modes and a specific surface area of the ferrite powder have been established. The changes in the pattern of the magnetic phase transition in the region of the Curie temperature of materials with different degrees of dispersion have been determined.