Ac Magnetization Research Articles

Ferromagnetic Exchange and Slow Magnetic Relaxation in Cobalt Bis(1,2-dithiolene)-Bridged Dilanthanide Complexes.

The construction of multinuclear lanthanide-based molecules with significant magnetic exchange interactions represents a key challenge in the realization of single-molecule magnets with high operating temperatures. Here, we report the synthesis and magnetic characterization of two series of heterobimetallic compounds, (Cp*2Ln)2(μ-Co(pdt)2) (Ln = Y3+, Gd3+, Dy3+; pdt2- = 1,2-diphenylethylenedithiolate) and [K(18-crown-6)][(Cp*2Ln)2(μ-Co(pdt)2)] (Ln = Y3+, Gd3+), featuring two lanthanide centers bridged by a cobalt bis(1,2-dithiolene) complex. Dc magnetic susceptibility data collected for the Gd congeners indicate significant Gd-Co ferromagnetic exchange interactions with fits affording J = +11.5 and +7.33 cm-1, respectively. Magnetization decay and ac magnetic susceptibility measurements carried out on the single-molecule magnet (Cp*2Dy)2(μ-Co(pdt)2) reveal full suppression of quantum tunneling and open-loop hysteresis persisting up to 3.5 K. These results, along with those of high-field EPR spectroscopy, suggest that transition metalloligands can enforce strong exchange interactions with adjacent lanthanide centers while maintaining a geometry that preserves molecular anisotropy. Furthermore, the magnetic properties of [K(18-crown-6)][(Cp*2Gd)2(μ-Co(pdt)2)] show that increasing the spin of the ground state of the bridging complex may be a viable alternative to increasing J in obtaining well-isolated, strongly coupled magnetic ground states.

Magnetostriction related to skyrmion-lattice formation in chiral magnet FeGe

The B20 type chiral magnet FeGe exhibits the formation of skyrmion-lattice (SkL) phases in the vicinity of the magnetic ordering temperature. The SkL is a magnetic superlattice composed of vortex-type topological spin objects, and it has experimentally been known that its formation requires the existence of an intermediate (IM) phase between SkL and the paramagnetic (PM) phases. We take interest in how the crystal lattice experiences the formation of these topological spin texture. In this study, we observed the so-called spin–orbit coupling induced magnetostriction related to these topological spin texture formation, in addition to the ac magnetization anomalies. The temperature and magnetic field dependences of the lattice parameter reflected the transformation of phases, such as helimagnetic (HM), SkL, IM, conical (CM), and PM phases. In the PM region, a phase characterized as gaseous skyrmions was detected similarly to the case of the same B20 type MnSi. Furthermore, the HM, CM, and IM phases were also divided into two regions. Thus, the precise phase diagram near Tc was reconstructed from the prospect of the magnetostriction such that we demonstrated that the stabilization of skyrmions needs a finite magnetic field.

A Rutherford MgB2 cable with resistive NbTi barriers and a CuNi30 sheath

Abstract The Rutherford cable was made of 12 single-core MgB2in-situ wires with an NbTi resistive barrier and a CuNi outer sheath. The uniformity of the wires and cables was analyzed by X-ray microtomography. Critical currents of wire’s and cable’s samples were measured at 4.2 K in the outer fields 4.0 - 8.0 T and at 15-25 K and fields 1.0 - 5.5 T. Magnetization AC losses were measured at temperatures of 20-40 K, external fields 0.001-0.1 T and frequencies of 72 and 144 Hz. The obtained results show high engineering current density and reduced losses in MgB2 cable, which can be especially interesting for windings of superconducting motors or generators.

Influence of Remote Laser Cutting on Magnetic Loss and Mechanical Properties in 0.2 mm Silicon Steel

Energy loss due to eddy current generation is a significant factor in reduced efficiency of electrical machines, therefore motor cores with thinner sheets are required due to their effect in reducing eddy currents. However, reducing the thickness of laminations is challenging due to the high tooling costs in blanking due to the small tolerances required, whilst other methods such as conventional laser cutting are too slow to be commercially viable. In this paper the influence of remote laser cutting on thin electrical steel sheets (Cogent/Tata Steel NO20 3.2% Si) has been investigated on the tensile strain to fracture, fatigue life, edge hardness, and energy loss in an AC magnetisation cycle for two laser power levels. The laser power has found to have no statistically significant influence on the bulk mechanical properties of the material, but significantly affects the measured specific loss. The latter was associated with the increased edge hardness at higher laser powers. Based on the parameters selected it is highly questionable whether RLC offers a viable method for lamination cutting, significant refinement of the laser parameters and/or the use an alternative laser such as an ultra-fast pulsed laser is required to be competitive with existing methods. Additionally, it is shown that energy loss increases linearly with respect to length of the cut edge - to the minimum cut spacing tested of 6 mm. These results can be used to optimise laser parameters to reduce the degradation of magnetic properties by decreasing laser power where possible.

Magnetic properties of Co1-xFexCr2S4 (x = 0–0.4) solid solutions

The magnetic properties of the Co1-xFexCr2S4 solid solutions for compositions adjacent to CoCr2S4 were investigated using various techniques, including magnetization measurements and AC susceptibility. The results revealed that substituting Fe for Co in the CoCr2S4 compound led to changes in the magnetic behavior, such as the presence of ferrimagnetic ordering and the emergence of a cluster spin glass phase. The study found that the temperature of the ferrimagnetic transition (TC) decreased with increasing Fe concentration, indicating the influence of iron on the magnetic properties of the solid solutions. Additionally, the discovery of a cluster spin glass phase for specific compositions (x = 0, 0.1, 0.2, 0.4) suggests complex magnetic behavior in these materials. In summary, the study offers valuable insights into the magnetic properties of Co1-xFexCr2S4 solid solutions and highlights the potential for tuning their magnetic behavior through compositional variations.

Effects of particle surface modification on magnetic behavior of soft magnetic Fe@SiO2 composites and Fe compacts

The study aims to evaluate the influence of surface modification of Fe powder on the magnetic behavior of soft magnetic compacts and composites that can possibly enhance their properties. The smoothing of ferromagnetic particle surfaces led to a decrease in the total energy loss as the most evident positive impact in all investigated classes (max. by 11% for small, 63–125 μm particle-based annealed Fe compacts, at max. induction 0.5 T and frequency 100 Hz) and to a partial increase in specific electrical resistivity (max. by 47% for small particle-based Fe@SiO2 composites) and resonant frequency (max. by 48% for large, 200–400 μm particle-based Fe@SiO2 composites) as well as partial decrease in coercivity (max. by 14% for small particle-based annealed Fe compacts). Removing surface irregularities negatively affected the maximum total permeability (max. drop by 28% for large particle-based Fe@SiO2 composites) due to increased inner demagnetizing fields. Applying the Bertotti theory for loss separation, we obtained parameters of loss components and assumed the domain structure using simultaneously active magnetic objects as predictors. The total loss decrease observed after the smoothing process originates from the significantly increased numbers of active magnetic objects, facilitating AC magnetization reversal so that domain wall displacements are accompanied by lower energy loss, manifested as a decrease in the excess loss component (max. by 61% for small particle-based Fe@SiO2 composites).

Quantum spin liquid ground state in the trimer rhodate Ba4NbRh3O12

Frustrated magnets offer a plethora of exotic magnetic ground states, including quantum spin liquids (QSLs), in which enhanced quantum fluctuations prevent a long-range magnetic ordering of the strongly correlated spins down to lowest temperature. Here we have investigated the trimer based mixed valence hexagonal rhodate Ba4NbRh3O12 using a combination of dc and ac magnetization, electrical resistivity, specific heat, and muon spin rotation/relaxation (μSR) measurements. Despite the substantial antiferromagnetic exchange interactions, as evident from the Weiss temperature (θW∼−35 to −45K), among the Rh-local moments, neither long-range magnetic ordering nor spin freezing is observed down to at least 50 mK, in ac-susceptibility, specific heat, and zero-field μSR measurements (down to 0.26 K). We ascribe the absence of any magnetic transition to enhanced quantum fluctuations as a result of geometrical frustration arising out of the edge-sharing equilateral Rh-triangular network in the structure. Our longitudinal-field μSR result evidences persistent spin fluctuations down to 0.26 K, thus stabilizing a dynamic QSL ground state in Ba4NbRh3O12. Furthermore, the magnetic specific heat data at low T reveal a significant T-linear contribution plus a quadratic T dependence, which may indicate the gapless Dirac QSL phenomenology of the spinon excitations with a linear dispersion. Published by the American Physical Society 2024

Understanding Ion Dynamics in Closoborate-Type Lithium-Ion Conductors on Different Time-Scales.

The lithium-ion transport mechanism in 0.7Li(CB9H10)-0.3Li(CB11H12) complex hydride solid electrolyte was studied over a wide time-scale (ns-ms) by choosing appropriate techniques for assessing ionic motion on the desired time-scale using nuclear magnetic resonance (NMR) relaxation, AC impedance, and pulsed field gradient-NMR (PFG-NMR) measurements. The 7Li NMR line width decreased with increasing temperature, and the spin-lattice relaxation time T1 for the cation and anions showed a minimum near 303 K, indicating that the lithium ions and the anions were highly mobile. The activation energy estimated from the analysis of the NMR relaxation time matched well with the values estimated from the AC impedance and PFG-NMR. This confirms that the lithium-ion motion in 0.7Li(CB9H10)-0.3Li(CB11H12) is the same over a wide time-scale, suggesting steady Li-ion motion over a wide transport range. This understanding offers insights into strategies for designing complex hydride lithium superionic conductors.

Resonant anomalous Hall effect in a ferromagnetic Weyl semimetal

The anomalous Hall effect (AHE) has been widely studied and is well-known in ferromagnetic metals. It is usually investigated in the static regime with the magnetization at equilibrium. In this work, we study the AHE in the dynamic regime where the magnetization is resonantly excited. The microwave-induced ac current and magnetization precession both at GHz frequencies can cooperatively generate a dc voltage. In conventional ferromagnets, this effect, dubbed as resonant AHE (RAHE), is often overwhelmed by other effects such as spin pumping and spin rectification induced by anisotropic magnetoresistance. Here, we observe the RAHE in Co2MnGa, a ferromagnetic Weyl semimetal. In this material, the RAHE dominates over other effects, as observed in angle-dependent measurements. The RAHE in Co2MnGa is an order of magnitude larger than in Permalloy, a conventional ferromagnet. This enhancement is induced by the Berry curvature in the topological band structure of Co2MnGa. The large RAHE demonstrated in this work provides a viable methodology to convert microwave signals into dc voltages for telecommunication applications.

Preparation of Fe@Fe3O4/ZnFe2O4 Powders and Their Consolidation via Hybrid Cold-Sintering/Spark Plasma-Sintering.

Our study is focused on optimizing the synthesis conditions for the in situ oxidation of Fe particles to produce Fe@Fe3O4 core-shell powder and preparation via co-precipitation of ZnFe2O4 nanoparticles to produce Fe@Fe3O4/ZnFe2O4 soft magnetic composites (SMC) through a hybrid cold-sintering/spark plasma-sintering technique. XRD and FTIR measurements confirmed the formation of a nanocrystalline oxide layer on the surface of Fe powder and the nanosized nature of ZnFe2O4 nanoparticles. SEM-EDX investigations revealed that the oxidic phase of our composite was distributed on the surface of the Fe particles, forming a quasi-continuous matrix. The DC magnetic characteristics of the composite compact revealed a saturation induction of 0.8 T, coercivity of 590 A/m, and maximum relative permeability of 156. AC magnetic characterization indicated that for frequencies higher than 1 kHz and induction of 0.1 T, interparticle eddy current losses dominated due to ineffective electrical insulation between neighboring particles in the composite compact. Nevertheless, the magnetic characteristics obtained in both DC and AC magnetization regimes were comparable to those reported for cold-sintered Fe-based SMCs.

A highly conducting tetrathiafulvalene-tetracarboxylate based dysprosium(iii) 2D metal-organic framework with single molecule magnet behaviour.

The synthesis and whole characterization by a multitechnique approach of an unprecedented dysprosium(iii) 2D metal organic framework (MOF), involving the redox-active tetrathiafulvalene (TTF)-based linker TTF-tetracarboxylate (TTF-TC), are herein reported. The single-crystal X-ray structure, formulated as [Dy6(TTF-TC)5(H2O)22]·21H2O (1), reveals a complex 2D topology, with hexanuclear Dy6 clusters as secondary building units (SBUs) interconnected by five linkers, stacked almost parallel in each layer and eclipsed along the [111] direction, leading to the formation of 1D channels filled by water molecules. The mixed valence of the TTF units is confirmed by both bond distance analysis, Raman microscopy and diffuse reflectance spectroscopy, and further supported by band structure calculations, which also predict activated conductivity for this material. Thanks to efficient TTF stacking and partial oxidation, 1 shows semiconducting behavior, with, however, a record conductivity value of 1 mS cm-1 at room temperature, when compared to the previously reported TTF-based MOFs. Furthermore, temperature and magnetic field dependent ac (alternative current) magnetic susceptibility measurements demonstrate field induced slow relaxation of magnetization, accounting for two independent relaxation processes, with an energy barrier (U eff/K) of around 12 K, typical for dysprosium carboxylate complexes. The herein reported 2D Dy-MOF provides a valuable master plan for coexistence of conducting π-TTF stacks and highly anisotropic DyIII SMM properties.

Alkyl chain length influence of the functionalized diethanolamine ligand on the slow relaxation of the magnetization in {CoIII3DyIII3} complexes.

We report the synthesis, structural characterization and magnetic properties of a series of Co(III)/Dy(III) complexes built up from an N-alkyl derivatized amino-alcohol ligand diethanolamine, functionalized with CnH2n+1 alkyl chains (n = 3, 4, 6, 8 and 10). While n = 3 afforded a butterfly {CoIII2DyIII2} core, the other alkyl chains (n = 4, 6, 8) afforded hexanuclear triangle-in-triangle {CoIII3DyIII3} complexes as shown by single-crystal X-ray determinations. Infrared spectroscopy allows us to characterize the C10 derivative complex, which did not crystallize. Magnetic ac susceptibility data collected below 10 K at driving frequencies up to 10 kHz under zero-dc field and up to 1 T provide insight into the SMM behaviour of the studied complexes. The characteristic time of the magnetization dynamics can be understood in terms of the competing Raman, Direct and Orbach-like mechanisms. A relationship between the magnetization dynamics within the family of complexes and the increasing alkyl chain length is discussed.

Design and Control of a Novel Wireless Permanent Magnetic AC Motor With Compact Structure

Design and Control of a Novel Wireless Permanent Magnetic AC Motor With Compact Structure

Magnitnye svoystva dvoynogo perovskita Sr2CrNbO6

A double perovskite Sr2CrNbO6 powder compound was studied by using X-ray diffraction, AC and DC magnetization, and ESR measurements. Two transitions in antiferromagnetically ordered regimes were observed through magnetization measurements at T = 5 and 2 K and were confirmed by the linear dependence of the magnetization on the applied magnetic field at these temperatures, approximations of the temperature dependence of the ESR linewidth, and AC magnetization. The zero field cooling curve was approximated by Bonner–Fisher law for quasi one dimensional chain with the exchange integral J/kB = 1 K between chromium spins. An approximation of linear part of magnetic susceptibility temperature dependence was performed using Curie–Weiss law. To describe the obtained effective moment μeff = 3.577μB, the presence Cr3+ and Cr4+ ions is estimated at a respective ratio about 0.8 : 0.2.

Magnetism and specific heat of ludwigites Mn1.17Co1.83BO5 and Mn1.39Co1.61BO5

Mn1.17Co1.83BO5 and Mn1.39Co1.61BO5 ludwigites were synthesized by the flux technique and investigated by means of X-ray diffraction, X-ray fluorescence, DC and AC magnetic susceptibility, and specific heat analysis. The crystal structure of both ludwigites belongs to the Pbam space group with a = 9.25 Å, b = 12.41 Å, and c = 3.05 Å for Mn1.17Co1.83BO5 and a = 9.27 Å, b = 12.45 Å, and c = 3.05 Å for Mn1.39Co1.61BO5. The simultaneously observed negative values of the Curie-Weiss temperatures and ferromagnetic-type hysteresis loops allow us to assume that the ferrimagnetic ordering is realized in Mn1.39Co1.61BO5 below TF = 60.8 K, while in Mn1.17Co1.83BO5 in addition to the above mentioned experimental facts the frequency dependencies of the real and imaginary parts of the AC magnetization were observed assuming the presence of the canonical spin-glass state below TSG = 44.5 K. The observed difference in coercive forces of M-H curves at low temperatures can be associated with presence of two spin subsystems for Mn1.17Co1.83BO5.

ESR and magnetization studies of double perovskite Sr[formula omitted]FeNbO6

By pyrolysis of nitrate-organic mixtures, we synthesized double perovskite Sr2Fe0.6NbO5.4. We performed the study of the temperature dependencies of the EPR spectra, DC and AC magnetization, and magnetization isotherms in a wide temperature range of 4–300K. The EPR spectrum is described by the sum of two contributions below 125 K and indicates phase separation of the sample. The paramagnetic part of the inverse magnetic susceptibility is well approximated by the Curie–Weiss law with ΘCW≈−60K. The exchange integrals between iron spins, calculated from the temperature dependence of the ESR integral intensity and the Curie–Weiss temperature, coincide and are equal to J/kB≈10K. The negative sign of the Curie–Weiss temperature and the hysteresis loops indicate the antiferromagnetic canted nature of exchange interactions, which is further confirmed by the presence of a peak in the real part of the AC magnetization and its absence in the imaginary part.

Magnetic ac susceptibility of superconducting Ta films for quantum computing

Recently, breakthroughs were reported in the long coherence time (>0.5 ms) transmon qubit using tantalum films (Ta). Identifying the loss mechanism in Ta films will help further improve the performance of Ta-based superconducting qubits. Here, we report a study of superconducting properties in two sets of Ta films grown on c-cut and a-cut sapphires respectively using contact transport and noncontact magnetic ac susceptibility measurements. Although the resistive transition appears to be similar in both films, we found strikingly different responses in their complex magnetic susceptibilities . in the c-cut films exhibits a sharp peak at superconducting transition T c and becomes featureless below T c indicating a strongly coupled superconducting state. In contrast, in the a-cut films exhibits a broad peak near T c and a second peak appears below T c, indicating granular superconductivity behavior. This second peak in is associated with the hysteresis loss at weak-link-type grain boundaries, which is believed to be a leading source of decoherence in the a-cut Ta films. The derived magnetic loss tangent in the a-cut films is about one order of magnitude higher than in the c-cut films. This study demonstrates that ac susceptibility is an excellent probe for characterizing bulk superconducting properties of the constituent superconducting materials used in qubits.

Unveiling the structural phase transition and magnetic structure of ternary boride PrIr3B2

We present a comprehensive study of PrIr3B2, which includes a detailed investigation of its crystal and magnetic structure using neutron diffraction. AC and DC magnetization and heat capacity data reveal antiferromagnetic ordering at T N = 10 K. The heat capacity measurements further exhibit a broad peak near 270 K which is related to a structural transition from P6/mmm to C2/m seen in low temperature x-ray diffraction and neutron diffraction. High intensity neutron diffraction data confirm the long-range ordering of Pr3+ spins, with no apparent magnetic moment on either of the Iridium sites. Two possible magnetic structures with either k 1 = [1,0,0] or k 2 = [½,½,0] fit nearly equally well the neutron diffraction data. However, based on previous magnetization studies on a single crystalline sample it is argued that the second solution with k 2 corresponds to the appropriate magnetic structure of PrIr3B2 below 10 K. In this magnetic structure, the Pr3+ moments are oriented at ∼45° to both the a and b axes, with the c-axis being the hard axis of magnetization. Overall, our results provide new insights into the magnetic and structural properties of PrIr3B2.

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.

Heating efficiency of Gd- and Co-doped γ-Fe2O3 nanoparticles measured by AC magnetometer for magnetic-mediated hyperthermia

Most research groups, including us, utilize calorimetric methods to determine the heat dissipation by magnetic nanoparticles (MNPs) under an alternating magnetic field (AMF). Herein, we report the heating efficiencies of γ-Fe2O3 and doped γ-Fe2O3 NPs using AC magnetometry, which allows us to directly calculate the AC hysteresis loop area from which the heating abilities can be deduced. First, all NPs were prepared and thoroughly characterized both structurally (XRD, Rietveld, and TEM) and magnetically (DC and AC magnetization measurements). Structural analysis indicated the phase purity (γ-Fe2O3) and crystallite sizes (∼10 nm) of the as-prepared γ-Fe2O3 NPs. Both DC and AC measurements indicated the superparamagnetic behavior for γ-Fe2O3 and Gd-doped γ-Fe2O3(Gd-5%) NPs, while Co-doped γ-Fe2O3(Co-5%) NPs exhibited ferrimagnetic nature. The heating abilities and specific absorption rate (SAR) values were then analyzed at frequency, f = 132 kHz and several AC field amplitudes (µ0HAC) ranging from 0 to 88 mT. From AC magnetometry calculations, the SAR values were found to be 20 W/g and 17 W/g for γ-Fe2O3 and γ-Fe2O3(Gd-5%) NPs, respectively, while that of γ-Fe2O3(Co-5%) NPs reached SAR of 120 W/g, almost 6 times higher. This high heating efficiency of γ-Fe2O3(Co-5%) sample is attributed to their higher effective anisotropy and saturation magnetization where the heat release is mainly dominated by Neel relaxation. Finally, a viability assay against metastatic breast cancer cells was conducted, indicating the biocompatibility and low toxicity of the as-synthesized γ-Fe2O3 and doped γ-Fe2O3 NPs. These results strongly suggest the promising utilization of γ-Fe2O3 NPs, particularly Co-doped, as a potential candidate for magnetic-mediated hyperthermia.