Search Results (135)

Dielectric and magnetic spherical hollow shells are employed in many applications as standard building units. These structures are commonly subjected to size reduction to obtain a high surface area/volume ratio, a property that is in favor of specific applications. However, the size reduction enhances the importance of physical mechanisms that originate from surfaces, such as the depolarization effect. Here we tackle the problem of dielectric and magnetic spherical hollow shells, consisting of a linear, homogeneous and isotropic parent material, subjected to an external potential, $U_{ext}(\mathit{r})$, of any spatial form (either dc (static) or ac of low-frequency (quasistatic limit)). By applying the method-of-linear-recursive-solution (MLRS) to the Laplace equation, we calculate analytically the internal, $U_{int}(\mathit{r})$, and total, $U_{tot}(\mathit{r})$, potentials in respect to the external one, $U_{ext}(\mathit{r})$. From $U_{int}(\mathit{r})$ and $U_{tot}(\mathit{r})$ we calculate all relevant scalar and vector physical entities of interest. The MLRS unveils straightforwardly the existence of two distinct depolarization factors, $N_l=l/(2l+1)$ and $N_{l+1}=(l+1)/(2l+1)$, both depending on the degree, $l$, however not on the order, $\mathrm{m}$, of the mode of the external potential, $U_{ext}^{(l,m)}(\mathit{r})$. These depolarization factors, $N_l$ and $N_{l+1}$, originate from the outer, $r=\mathrm{b}$, and inner, $r=\mathrm{a}$, surfaces and are accompanied by two extrinsic susceptibilities, ${\chi }_{e,l}^{ext}={\chi }_e/(1+N_l{\chi }_e)$ and ${\chi }_{e,l+1}^{ext}={\chi }_e/(1+N_{l+1}{\chi }_e)$, respectively. Importantly, $N_l+N_{l+1}=1$, irrespective of the degree, $l$, as it should. The properties of spherical hollow shells are investigated through analytical modeling and detailed simulations, with emphasis on application-relevant scenarios including resonance phenomena in scattering, quantitative materials characterization, and shielding/distortion. The generic MLRS strategy provides a flexible and reliable route for analyzing depolarization processes in other dielectric and magnetic building-unit geometries encountered in practice. Full article

Magnetic superconductor EuRbFe₄As₄ is a quite unique system in which macroscopic superconductivity and magnetic ordering coexist, with interesting interactions between Abrikosov vortices and Eu²⁺ spins that were investigated mostly by static (DC) magnetization measurements. Our aim is to study the dynamic interactions between the two sub-systems using AC susceptibility measurements in a wide range of temperatures and superimposed DC fields. In low DC fields, the magnetic transition at 15 K is clearly visible. We have observed very little difference between the AC susceptibility in different cooling regimes, but large difference for different field orientation. For field perpendicular to the superconducting planes, we have observed an anomalous dependence just below the critical temperature, which is absent in the parallel field orientation. We explained the anomaly by the interplay between the sample dimensions and the temperature dependence of the London penetration depth which may allow the paramagnetic Meissner-like response to be detected in the temperature dependence of the AC susceptibility. We stress that the newly reported phenomenon reflects an AC-susceptibility manifestation of a field-stabilized critical state rather than a thermodynamic phase. In addition, we have observed a paramagnetic AC response in the normal phase, in both field orientations, indicative of interactions between Eu²⁺ spins and flux lines. Full article

Experimental and numerical investigations of the alternating current (AC) susceptibility, $\mathsf{\chi }\left(H\right) ~ \mathrm{d}M/\mathrm{d}H,$ examined multigrain La_0.66Sr_0.34MnO₃ (LSMO) thin films (thickness d = 250 nm) grown by radio-frequency (RF) magnetron sputtering on lattice-mismatched yttria-stabilized zirconia YSZ(001) substrates. The films exhibit a columnar structure comprising two types of grains, with (001)- and (011)-oriented planes of a pseudocubic lattice aligned parallel to the film surface. Field- and angle-dependent AC susceptibility measurements at 78 K reveal characteristic peak- and tip-like anomalies, attributed to contributions from grains with three distinct directions of easy magnetization axes within the film plane. Numerical modeling based on the transverse susceptibility theory for single-domain ferromagnetic grains, incorporating first- and second-order anisotropy constants, corroborates the experimental findings and elucidates the role of different grain types in magnetization switching and AC susceptibility response. This study provides a quantitative determination of the three in-plane easy magnetization axes in LSMO/YSZ(001) films and clarifies their influence on the magnetization dynamics of multigrain thin films. The demonstrated control over multigrain LSMO/YSZ(001) thin films with distinct in-plane easy magnetization axes and well-characterized AC susceptibility suggests potential applications in magnetic memory, spintronic devices, and precision magnetic sensing. Full article

Transition metal (TM)-doped zinc oxide (ZnO) and zirconium dioxide (ZrO₂) nanocrystals exhibit complex correlations between crystal structure, defect chemistry, vibrational properties, and magnetic behavior that are strongly governed by synthesis route and dopant incorporation mechanisms. This review critically summarizes recent progress on Fe-, Mn-, and Co-doped ZnO and ZrO₂ nanocrystals synthesized by wet chemical, hydrothermal, and microwave-assisted hydrothermal methods, with emphasis on synthesis-driven phase evolution and apparent solubility limits. ZnO and ZrO₂ are treated as complementary host lattices: ZnO is a semiconducting, piezoelectric oxide with narrow solubility limits for most 3d dopants, while ZrO₂ is a dielectric, polymorphic oxide in which transition metal doping may stabilize tetragonal or cubic phases. Structural and microstructural studies using X-ray diffraction, electron microscopy, Raman spectroscopy, and Mössbauer spectroscopy demonstrate that at low dopant concentrations, TM ions may be partially incorporated into the host lattice, giving rise to diluted or defect-mediated magnetic behavior. When solubility limits are exceeded, nanoscopic secondary oxide phases emerge, leading to superparamagnetic, ferrimagnetic, or spin-glass-like responses. Magnetic measurements, including DC magnetization and AC susceptibility, reveal a continuous evolution from paramagnetism in lightly doped samples to dynamic magnetic states characteristic of nanoscale magnetic entities. Vibrational spectroscopy highlights phonon confinement, surface optical phonons, and disorder-activated modes that sensitively reflect nanocrystal size, lattice strain, and defect populations, and often correlate with magnetic dynamics. Rather than classifying these materials as diluted magnetic semiconductors, this review adopts a synthesis-driven and correlation-based framework that links dopant incorporation, local structural disorder, vibrational fingerprints, and magnetic response. By emphasizing multi-technique characterization strategies required to distinguish intrinsic from extrinsic magnetic contributions, this review provides practical guidelines for interpreting magnetism in TM-doped oxide nanocrystals and outlines implications for applications in photocatalysis, sensing, biomedicine, and electromagnetic interference (EMI) shielding. Full article

A spiral spin arrangement with a magnetic unit cell 28 times the size of the nuclear one has been reported for Fe spins below T_N = 80 K in bilayered van der Waals gapped FeOCl. In this work, we employ neutron magnetic diffraction and ac magnetic susceptibility to reveal a much smaller magnetic unit cell only 4 times the size of the nuclear one for Fe spins below T_N = 119 K, upon intercalation of 27% non-magnetic Na ions into the van der Waals gaps of FeOCl. X-ray emission spectra and X-ray absorption edge spectra reveal a charge transfer from the intercalated Na ions to the Fe sites, which partially reduces the Fe³⁺ into Fe²⁺ ions. The reduction results in a significantly increased Fe-O-Fe bond angle, which strongly enhances the antiferromagnetic superexchange (AFMSE) coupling relative to the competing ferromagnetic direct exchange (FMDE) coupling between neighboring Fe ions, thereby driving to a higher degree of magnetic symmetry and a substantially higher Neel temperature for the Fe spins in Na_0.27FeOCl. Full article

Background: Intracranial aneurysm (IA) rupture is a devastating event in neurosurgery and a leading cause of subarachnoid hemorrhage. Although aneurysm size has been traditionally emphasized, recent research has highlighted multifactorial mechanisms involving hemodynamic stress, wall degeneration, inflammation, and genetic predisposition. Methods: Evidence from animal models, human pathological studies, computational fluid dynamics analyses, genetic association studies, and advanced imaging research was reviewed to provide an integrated view of rupture mechanisms. Results: Morphological and hemodynamic studies have shown that high aspect and size ratios, coupled with low wall shear stress and an elevated oscillatory shear index, contribute to focal wall weakening. Histopathological analyses of ruptured aneurysms consistently reveal endothelial loss, smooth-muscle-cell depletion, extracellular matrix degradation, and intense inflammatory cell infiltration, with patterns such as extremely thin, hypocellular, thrombosis-lined walls. Experimental studies have identified active inflammatory pathways, including neutrophil-driven cascades via CXCL1 signaling and complement C5a–C5aR1 activation, as direct triggers of wall failure. High-resolution vessel-wall magnetic resonance imaging correlates contrast enhancement with histological evidence of inflammation and neovascularization, suggesting its utility as a biomarker of instability. Conclusions: IA rupture is driven by a dynamic interplay between adverse hemodynamic environments, inflammatory degeneration, genetic susceptibility, and pathological vascular remodeling. Integrating these mechanistic insights may improve risk stratification and guide the development of targeted preventive strategies. Full article

We investigated the effects of chemical and physical changes on the interplay between the Néel and Brown superspin relaxation mechanisms in ferrofluids containing 18 nm-diameter Co_0.2Fe_2.8O₄ magnetic nanoparticles. We attempted to tune the ferrofluid’s magnetization dynamics via three methods: (i) changing the carrier fluid from Isopar M to kerosene (ii) doubling the Co-doping level from x = 0.2 to x = 0.4, and (iii) diluting the Co_0.2Fe_2.8O₄/Isopar M nanomagnetic fluid from δ = 1 mg/mL to δ = 0.1 mg/mL. We used temperature-resolved ac-susceptibility measurements at different frequencies, χ″ vs. T|_f, to gain insight into the thermally driven superspin dynamics of the nanoparticles within the ferrofluid. Our data demonstrates that both increasing x and using a different carrier fluid quantitatively alter the temperature dependence of the Néel and Brown relaxation frequency (f_N vs. T and f_B vs. T) by changing the nanoparticles’ magnetic moments and the fluid’s viscosity. Yet, the two mechanisms remain decoupled, as indicated by the presence of two magnetic events (peaks in the χ″ vs. T|_f datasets) one corresponding to the Néel and the other to Brown relaxation. On the other hand, diluting the ferrofluid leads to a qualitative change in the collective superspin dynamics behavior. Indeed, there is just one χ″-peak in the data from the δ = 0.1 mg/mL nanofluid, and its f vs. T dependence is well-described by a model that includes coupled contributions from both the Néel and Brown relaxation: $\mathrm{f}\left(\mathrm{T}\right)=\mathrm{p}·{\displaystyle \frac{T}{{\gamma }_0}}·\mathrm{e}\mathrm{x}\mathrm{p}\left[-{\displaystyle \frac{{\mathrm{E}}^{\prime }}{{\mathrm{k}}_{\mathrm{B}}\left(\mathrm{T}-{\mathrm{T}}_0^{\prime }\right)}}\right]+$ (1 − p) ${\mathrm{f}}_0\mathrm{e}\mathrm{x}\mathrm{p}\left[-{\displaystyle \frac{{\mathrm{E}}_{\mathrm{B}}}{{\mathrm{k}}_{\mathrm{B}}\left(\mathrm{T}-{\mathrm{T}}_0\right)}}\right]$. This is a remarkable behavior that demonstrates the ability to control a ferrofluids magnetization dynamics through simple chemical and physical changes. Full article

We use Co doping to alter the magnetic relaxation dynamics in superparamagnetic nanofluids made of 18 nm average diameter Fe₃O₄ nanoparticles immersed in Isopar M. Ac-susceptibility data recorded at different frequencies and temperatures, χ″vs. T|_f, reveals a major (~100 K) increase in the superspin blocking temperature of the Co_0.2Fe_2.8O₄-based fluid (CFO) compared to its Fe₃O₄ counterpart (FO). We ascribe this behavior to the strengthening of the interparticle magnetic dipole interactions upon Co doping, as demonstrated by the relative χ″-peak temperature variation per frequency decade $\mathsf{\Phi }={\displaystyle \frac{∆\mathrm{T}}{\mathrm{T}·∆\mathrm{l}\mathrm{o}\mathrm{g}\left(\mathrm{f}\right)}}$, which decreases from Φ~0.15 in FO to Φ~0.025 in CFO. In addition, χ″vs. T|_f datasets from the CFO fluid reveal two magnetic events at temperatures T_p1 = 240 K and T_p2 = 275 K, both above the fluid’s freezing point (T_F = 197 K). We demonstrate that the physical rotation of the nanoparticles within the fluid, the Brown mechanism, is entirely responsible for the collective superspin relaxation observed at T_p1, whereas the Néel mechanism, the superspin flip across an energy barrier within the particle, is dominant at T_p2. We confirm this finding through fits of models that describe the temperature dependence of the relaxation time via the two mechanisms: ${\mathsf{\tau }}_{\mathrm{B}}(\mathrm{T})={\displaystyle \frac{3{\mathsf{\eta }}_0{\mathrm{V}}_{\mathrm{H}}}{{\mathrm{k}}_{\mathrm{B}}\mathrm{T}}}\exp \left[{\displaystyle \frac{{\mathrm{E}}^{\prime }}{{\mathrm{k}}_{\mathrm{B}}\left(\mathrm{T}-{{\mathrm{T}}_0}^{\prime }\right)}}\right]$ and ${\mathsf{\tau }}_{\mathrm{N}}\left(\mathrm{T}\right)={\mathsf{\tau }}_0\exp \left[{\displaystyle \frac{{\mathrm{E}}_{\mathrm{B}}}{{\mathrm{k}}_{\mathrm{B}}\left(\mathrm{T}-{\mathrm{T}}_0\right)}}\right]$. The best fits yield ${\mathsf{\gamma }}_0={\displaystyle \frac{3{\mathsf{\eta }}_0{\mathrm{V}}_{\mathrm{H}}}{{\mathrm{k}}_{\mathrm{B}}}}$= 1.5 × 10⁻⁸ s·K, E′/k_B = 7 03 K, and T₀′ = 201 K for the Brown relaxation, and E_B/k_B = 2818 K and T₀ = 143 K for the Néel relaxation. Full article

In this paper, we report the details of the synthesis, single crystal X-ray diffraction study, comparative crystal chemical analysis, and magnetic behavior of a new phosphate variation of the arsenate mineral zubkovaite. The title compound was obtained as a high-temperature flux product in the form of a partly ordered solid solution and was studied using scanning electron microscopy and microprobe analysis. It possesses a monoclinic symmetry with a P2₁/n space group; the unit cell parameters are a = 8.8040 (2), b = 4.8970 (1), c = 14.5772 (3), and β = 93.993(2)°. The Ca_2.62Cu_1.94Co_1.44(PO₄)₄ crystal structure exhibits some statistical disorder. Our refinement showed that two positions are mixed, being occupied by Cu/Co (M1) and Ca/Co (M2) atoms. Two types of layers that are nearly parallel to the (101) plane can be distinguished in the structure. One of them is built by sharing corners of CuO₄ squares, M1O₅ square pyramids, and PO₄ tetrahedra. The second type of layer formed from Ca²⁺- and M2⁺-centered polyhedra alternates in the [$\overline{1}$01] direction to construct a tri-periodic framework. Ca_2.62Cu_1.94Co_1.44(PO₄)₄ experiences long-range antiferromagnetic ordering at low temperatures, as evidenced by both dc— and ac—magnetic susceptibilities, as well as by the specific heat measurements. Full article

A dysprosium-based metal–organic framework (MOF), namely [DyLH₂O]_n (1) (H₃L = 4-((bis(carboxymethyl) amino)methyl)benzoic acid), was successfully synthesized via the hydrothermal method. According to the structural characterization, metal centers in this complex are linked by four bridges (two oxygens and two carboxylic groups), leading to Dy₂ units. On further connection by single carboxylic groups, the dimeric units extend to form a two-dimensional layer with a 4⁴ topological structure. Finally, the 2D layers were assembled into a 3D framework by the L⁻³ anions. A thermogravimetric test shows that [DyLH₂O]_n can maintain high thermal stability after losing water, until the temperature reaches 426 °C. Magnetic studies on 1 reveal antiferromagnetic exchange interactions of Dy³⁺…Dy³⁺ at low temperatures. Additionally, frequency-dependent out-of-phase signals were observed in alternating current (ac) magnetic susceptibility measurements for 1, indicating that it has slow magnetic relaxation features. Full article

The investigation of novel diluted magnetic semiconductors (DMSs) provides a promising platform for studying magnetism and transport characteristics, with significant implications for spintronics. DMSs based on BaZn₂As₂ are particularly noteworthy due to their high Curie temperature (T_C) of 260 K, diverse magnetic states, and potential for multilayer heterojunctions. This study investigates the magnetic evolution of carrier doping and spin dynamics in the asperomagnet (Ba,Na)(Zn,Mn)₂As₂, utilizing a combination of magnetization measurements, ac susceptibility, and muon spin rotation (µSR). Key findings include the following: (1) lower transition temperatures and coercive forces in (Ba,Na)(Zn,Mn)₂As₂ compared to the ferromagnet (Ba,K)(Zn,Mn)₂As₂; (2) a dynamic fluctuation peak around the transition temperature observed in both the ac susceptibility and longitudinal field (LF) µSR; and (3) the coexistence of static and dynamic states at low temperatures, exhibiting spin-glass-like characteristics. This study, to the best of our knowledge, may represent the first investigation of asperomagnetic order utilizing µSR techniques. It enhances the understanding of magnetic interactions in BaZn₂As₂-based systems and provides valuable insights into the exploration of high T_C DMSs. Full article

A mononuclear complex [Dy(phenN₆)(HL′)₂]PF₆·CH₂Cl₂ (H₂L′ = R/S-1,1′-binaphthyl-2,2′-diphenol) with local D_6h symmetry was synthesized. Structural determination shows that Dy³⁺ was encapsulated within the coordination cavity of the neutral hexaaza macrocyclic ligand phenN₆, forming a non-planar coordination environment. The axial positions are occupied by two phenoxy groups of binaphthol in the trans form. The local geometry of Dy³⁺ closely resembles a regular hexagonal bipyramid D_6h configuration. The axial Dy-O_phenoxy distances are 2.189(5) and 2.145(5) Å, respectively, while the Dy-N bond lengths in the equatorial plane are in the range of 2.524(7)–2.717(5) Å. The axial O_phthalmoxy-Dy-O_phthalmoxy bond angle is 162.91(17)°, which deviates from the ideal linearity. Under the excitation at 320 nm, the complex exhibits a characteristic emission peak at 360 nm, corresponding to the naphthalene ring. The AC susceptibility measurements under an applied DC field of 1800 Oe show distinct temperature-dependent and frequency-dependent AC magnetic susceptibility, typical of single-molecule magnetic behavior. The Cole–Cole plot in the temperature range of 6.0–28.0 K was fitted using a model incorporating Orbach and Raman relaxation mechanisms, giving an effective energy barrier of U_eff = 300.2 K. Theoretical calculations on complex 1 reveal that the magnetization relaxation proceeds through the first excited Kramers doublets with a calculated magnetization blocking barrier of 404.1 cm⁻¹ (581.4 K). Full article

A ditopic ligand (H₂L₁), containing picolinate subunits segmented by ethynylene bridges, has been used in the synthesis of a series of isostructural coordination polymers, formulated as [(CH₃)₂NH₂][Ln(L₁)₂]·H₂O·CH₃COOH, where Ln = Eu (1), Gd (2), Tb (3), Dy (4) and Ho (5). The single-crystal structures show that these compounds crystallise in the orthorhombic Pna2₁ space group and form 3D anionic lattices with triangular cavities. AC magnetic susceptibility measurements show that the Gd, Tb and Dy derivatives (2–4) present a slow relaxation in their magnetisation under an applied DC magnetic field. The detailed study of the AC susceptibility in compounds 2 and 4 shows that they relax following direct and Orbach mechanisms under these conditions. The Dy derivative (4) retains this behaviour in the absence of an external field, relaxing via quantum tunnelling and Orbach mechanisms. Compound 2 is one of the very few reported Gd(III) compounds showing slow relaxation in its magnetisation. Full article

Stimulation-induced sensations including the perception of flickering lights (phosphenes) and scalp tingling are commonly reported in studies on transcranial alternating current stimulation (tACS). So far, these sensations have been considered benign side-effects of stimulation that may interfere with the blinding of participants in trials. It remains unknown what shapes the susceptibility to such side-effects. We hypothesized that cortical excitability predicts their intensity. Hence, we investigated the relationship between sensations during tACS and the motor threshold measured by transcranial magnetic stimulation (TMS). Nine healthy participants aged 50 and older underwent two tACS sessions at 21 Hz and 40 Hz as part of a cross-over pilot study. The stimulation amplitude was individualized to tolerability. Sensations were assessed post-session to calculate correlation with TMS-determined motor thresholds. Stimulation sensations (a flickering light and tingling scalp sensation) correlated with brain excitability as determined by the TMS motor threshold (r = −0.51, p = 0.03, N = 9). The findings suggest a relationship between the intensity of tACS-induced sensations and cortical excitability. Tailoring tACS intensity to individual tolerability and excitability thresholds may enhance the efficacy of tACS by ensuring a more consistent and effective dose relative to endogenous cortical excitability. Full article

Spherical structures of dielectric and magnetic materials are studied intensively in basic research and employed widely in applications. The polarization, (P for dielectric and M for magnetic materials), is the parent physical vector of all relevant entities (e.g., moment, , and force, F), which determine the signals recorded by an experimental setup or diagnostic equipment and configure the motion in real space. Here, we use classical electromagnetism to study the polarization, , of spherical structures of linear and isotropic—however, not necessarily homogeneous—materials subjected to an external vector field, (E_ext for dielectric and H_ext for magnetic materials), dc (static), or even ac of low frequency (quasistatic limit). We tackle an integro-differential equation on the polarization, , able to provide closed-form solutions, determined solely from , on the basis of spherical harmonics, ${\mathrm{Y}}_l^m$. These generic equations can be used to calculate analytically the polarization, , directly from an external field, , of any form. The proof of concept is studied in homogeneous dielectric and magnetic spheres. Indeed, the polarization, , can be obtained by universal expressions, directly applicable for any form of the external field, . Notably, we obtain the relation between the extrinsic, , and intrinsic, , susceptibilities (${\chi }_{\mathrm{e}}^{\mathrm{e}\mathrm{x}\mathrm{t}}$ and ${\chi }_{\mathrm{e}}^{\mathrm{i}\mathrm{n}\mathrm{t}}$ for dielectric and ${\chi }_{\mathrm{m}}^{\mathrm{e}\mathrm{x}\mathrm{t}}$ and ${\chi }_{\mathrm{m}}^{\mathrm{i}\mathrm{n}\mathrm{t}}$ for magnetic materials) and clarify the nature of the depolarization factor, , which depends on the degree l—however, not on the order m of the mode $(l,m)$ of the applied . Our universal approach can be useful to understand the physics and to facilitate applications of such spherical structures. Full article