Temperature Dependence Of Susceptibility Research Articles

Stochastic ferrimagnetic Landau-Lifshitz-Bloch equation for finite magnetic structures

Precise modeling of the magnetization dynamics of nanoparticles with finite size effects at fast varying temperatures is a computationally challenging task. Based on the Landau-Lifshitz-Bloch (LLB) equation we derive a coarse grained model for disordered ferrimagnets, which is both fast and accurate. First, we incorporate stochastic fluctuations to the existing ferrimagnetic LLB equation. Further, we derive a thermodynamic expression for the temperature dependent susceptibilities, which is essential to model finite size effects. Together with the zero field equilibrium magnetization the susceptibilities are used in the stochastic ferrimagnetic LLB to simulate a $5\times10$ nm$^2$ ferrimagnetic GdFeCo particle with 70 % FeCo and 30 % Gd under various external applied fields and heat pulses. The obtained trajectories agree well with those of an atomistic model, which solves the stochastic Landau-Lifshitz-Gilbert equation for each atom. Additionally, we derive an expression for the intergrain exchange field which couple the ferromagnetic sublattices of a ferrimagnet. A comparison of the magnetization dynamics obtained from this simpler model with those of the ferrimagnetic LLB equation shows a perfect agreement.

Investigation of re-entrant relaxor behaviour in lead cobalt niobate ceramic

The temperature dependent dielectric properties revealed re-entrant relaxor behaviour (Tm ∼130 K and 210 K for 1 kHz) below a high temperature diffused ferroelectric phase transition, Tc ∼270 K in lead cobalt niobate (PCN). Multiple positive/negative magnetodielectric effect and deviation from straight line at ∼130 K is observed in temperature dependence of inverse susceptibility, which might hint at origin of frustration. Microstructure examination depicts closely packed grains with grain size ∼8-10 μm and XRD pattern revealed single phase pseudo cubic crystal structure having Pm3m symmetry with lattice constant ∼4.0496(2) Å. Rietveld Refinement on XRD data yields larger value of thermal parameters, implying Pb and O are disordered along <111> and <110> directions, respectively. Observation of A1g (780 cm−1) mode in Raman spectroscopy and F-spot in SAED pattern along <110> unit axis in TEM suggests presence of nano scale 1:1 Co and Nb non-stoichiometric chemical ordering (CORs), akin to lead magnesium niobate (PMN). K-edge XANES spectra reveals the presence of cobalt in two oxidation states (Co2+ and Co3+); whereas, niobium exists in Nb3+ and Nb5+ states. Therefore, these local-average structural properties suggest chemical, structural and spatial heterogeneities. Such multiple heterogeneities are believed to play a crucial role in producing re-entrant relaxor behaviour.

Effect of magnetic field on specific heat and magnetic susceptibility of biased bilayer graphene: A full band approach

We have addressed the behaviors density of states and thermodynamic properties of doped biased bilayer graphene for AA stacking in the context of tight binding model hamiltonian. Specially we have studied the temperature dependence of spin susceptibility and specific heat of simple bilayer graphene in the presence of external magnetic field and bias voltage. The effects of electron doping and magnetic field on specific heat have been investigated. Green’s function approach has been implemented to find the behavior of electronic density of states and thermodynamic properties of bilayer graphene. We have found the temperature dependence of specific heat and spin susceptibility for different values of magnetic field and bias voltage in the presence of doping effects. Our results for density of states of bilayer graphene in the presence of bias voltage show the increase of magnetic field reduces the band gap width. Also there is a peak in temperature dependence of specific heat for all values of magnetic field and bias voltage. Moreover the effects of magnetic field and chemical potential on the behaviors of Pauli paramagnetic spin susceptibility have been addressed in details.

Effect of praseodymium doping on the phase transitions of barium orthoferrites

In this article, an effort was made to explore the properties of praseodymium-substituted barium orthoferrites with a better performance in absorption of microwaves. Orthoferrites with BaPrxFe2−xO4 (0 ≤ x ≤ 0.1) were prepared using an ultrasound-assisted sonochemical reactor. In this study, ultrasonic technique was used for the preparation and characterization of BaPrxFe2−xO4 (0 ≤ x ≤ 0.1) materials. X-ray diffraction (XRD) pattern was used to determine the structural properties such as phase purity and crystallite parameters. Analysis of the XRD pattern indicated that the prepared BaPrxFe2−xO4 ferrites have orthorhombic structure with the Bb21 m space group. The temperature-dependent AC susceptibility (χac) and in situ ultrasonic measurements were carried out to explore the Curie temperature of the BaPrxFe2−xO4 samples. Magnetic properties of the prepared BaPrxFe2−xO4 orthoferrites showed narrow M − H hysteresis loops. The coercivity of barium-based samples was found to increase with praseodymium substitution whereas the remenance and saturation magnetization decreased.

Dynamics of magnetism in Fe-Cr alloys with Cr clustering

The dynamics of magnetic moments in iron-chromium alloys with different levels of Cr clustering show unusual features resulting from the fact that even in a perfect body-centred cubic structure, magnetic moments experience geometric magnetic frustration resembling that of a spin glass. Due to the long range exchange coupling and configuration randomness, magnetic moments of Cr solutes remain non-collinear at all temperatures. To characterise magnetic properties of Fe-Cr alloys, we explore the temperature dependence of magnetisation, susceptibility, Curie temperature and spin-spin correlations with spatial resolution. The static and dynamic magnetic properties are correlated with the microstructure of Fe-Cr, where magnetisation and susceptibility are determined by the size of Cr precipitates at nominal Cr concentrations. The Curie temperature is always maximised when the solute concentration of Cr in the $\alpha$ phase is close to 5 to 6 at.\%, and the susceptibility of Fe atoms is always enhanced at the boundary between a precipitate and solid solution. Interaction between Cr and Fe stimulates magnetic disorder, lowering the effective Curie temperature. Dynamic simulation of evolution of magnetic correlations shows that the spin-spin relaxation time in Fe-Cr alloys is in the 20 to 40 ps range.

Persistent radical anions in the series of peri-arylenes: broadband light absorption until far in the NIR and purely organic magnetism

Stable radicals in organic conjugated molecules are of great interest due to their magnetic signals and broad optical absorptions. In this paper, we report on naphthalene, benzoperylene, perylene, terrylene, and quaterrylene carboximides, reduced under controlled conditions, where stable metal-free solid salts of radical anions could be obtained forming darkly colored solutions with line-rich UV/Vis/NIR spectra and exhibiting special magnetic properties. The most bathochromic shift of the absorption maxima extend from 760 until 1700 nm. Persistent paramagnetic properties of the solids were observed and temperature-dependent susceptibilities are measured.Graphical abstract

Magnetic behaviour of (C2H5NH3)2CuCl4 type multiferroic

Abstract Metal organic compound (C2H5NH3)2CuCl4 belongs to the family of layered perovskite multiferroic materials. It consists of ferromagnetic layers of corner sharing [CuCl4]2− octahedra connected by organic ions (two layers of [C2H5NH3]+). In this work we have studied magnetic properties of (C2H5NH3)2CuCl4 using SQUID magnetometer and AC susceptometer. Temperature dependence of magnetization showed existence of the antiferromagnetic transition at 10.2 K which changes to ferromagnetic with the application of moderate magnetic field. Near structural transition temperatures the anomaly of magnetization was observed. Hysteresis loops showed ferromagnetic behaviour for T T N and existence of anisotropy. Results of AC measurements showed no frequency dependence of the susceptibility and obtained temperature dependence of nonlinear susceptibility can be interpreted as a result of successive crossovers between different regimes.

Good to the last drop: The emergence of coffee ringspot virus.

Two and a half billion times per day a human hand reaches for a fresh cup of coffee. Although arguably dispensable for life per se, with an industry value of US$174 billion, coffee provides the lifeblood that sustains economies of producing countries located in the “coffee belt” situated between the Tropics of Cancer and Capricorn. As a “solvent” in which many human interactions take place, coffee is witness to the broad spectrum of human activities from the mundane to the pleasurable and personal. However, in opposition to its economic, cultural, and physiological importance, diseases such as coffee rust (caused by the fungus Hemileia vastatrix) dictate activity on stock markets with their periodic epidemics, which in turn affects the migration patterns of displaced farm workers [1]. Other diseases, such as those caused by coffee ringspot virus (CoRSV), currently fly mostly under the radar of many integrated pest management systems. The unique biology of this and related viruses offers exciting research opportunities ranging from cell biology, plant pathology and physiology, conservation ecology, to climate change-related epidemiology. This review highlights important aspects of CoRSV, including its unique features, and examines the potential role of climate change in its emergence (Fig 1). Open in a separate window Fig 1 Diverse array of research projects related to understanding the effect of CoRSV on coffee. (A) Whether enjoyed alone or in the company of others, coffee is an integral component of daily life in all countries around the world. Only one small-scale study has been done to determine the effect of CoRSV on coffee quality [34]. (B) The effect of CoRSV on the yield of coffee plants has not been examined. (C) No formal investigations have been made to determine how CoRSV influences the development of coffee cherries. (D) Only one study has investigated the population structure of CoRSV [8, 35]. It remains to be determined if phylogenetic trees derived from different CoRSV genes or from viral RNA isolated from plants or mites are congruent. Furthermore, evidence for recombination or reassortment of CoRSV genomes has not been investigated in detail. (E) The reservoir of CoRSV in wild species, particularly in the Cerrado of Brazil, has not been investigated. It is unknown if the population structure of CoRSV in the wild is similar to that in coffee plants. (F) The molecular basis for temperature dependent susceptibility to systemic infections has not been determined [23]. (G) The cell biology of CoRSV beyond generation of protein interaction and localization maps is poorly characterized, particularly with respect to identification of host factors required for replication and cell-to-cell movement, viroplasm formation, and nucleocytoplasmic trafficking of CoRSV nucleocapsids and proteins [23]. CoRSV, coffee ringspot virus; NPC, nuclear pore complex; VP, viroplasm; XPO1, Exportin 1.

Magnetic behaviors in a bilayer graphene nanoisland structure: A Monte Carlo study

The Monte Carlo simulation has been applied to study the magnetic behaviors of a bilayer graphene nanoisland structure on a triangular lattice by modeling the ferrimagnetic Ising system with spin-5/2 and spin-2. The temperature dependence of magnetization, susceptibility and internal energy curves of the system and two sublattices with the influence of the anisotropy, the exchange coupling, the external magnetic field and the size index have been discussed in detail. The diagrams of the blocking temperature under the effects of different parameters have been obtained. The results show the anisotropy plays a significant role in controlling the magnetic properties of the bilayer graphene nanoisland structure.

Magnetism and magnetoresistance in Al half doped LaNiO3

Abstract The rhombohedral perovskite LaAl0.5Ni0.5O3 (or La2AlNiO6), synthesized by a combustion method is found to contain Al3+ and Ni3+ cations in the rhombohedral lattice. The material exhibits a temperature dependent (thermally excited) paramagnetic susceptibility and follows the modified Curie-Weiss law implying an antiferromagnetic ordering at very low temperatures. A non-saturating hysteresis loop with a coercivity of 108 Oe is observed at 2 K which is arising from the magnetic contribution from Ni in the lattice. The conduction mechanism in LaAl0.5Ni0.5O3 for 80–300 K follows the Mott's variable range hopping mechanism. A negative magnetoresistance of 1.75% is observed at 2 K under 90 kOe field which decreases linearly by increasing the temperature up to 18 K and then changes to positive with comparatively low value. Both the negative and positive magnetoresistance increases linearly with an applied magnetic field.

Effect of annealing temperature on structural and magnetic properties of Co2TiO4 ceramics prepared by sol-gel method

In this paper, polycrystalline Co2TiO4 ceramics have been synthesized using a sol-gel process followed by annealing at different temperatures. The lattice size and the average grain size of the samples increases with rise in annealing temperature. The temperature-dependent inverse paramagnetic susceptibilities recorded under zero-field-cooling condition have been fitted according to the Néel's expression for ferrimagnets. Subsequently, the molecular field constants and the corresponding exchange constants have been calculated. The fitting result shows that the magnetic interaction in the system becomes weaker as the annealing temperature rises. In addition, negative magnetization is observed during field-cooling process. The higher annealing temperature is beneficial to the growth of tetrahedral sublattice, leading to a decrease on compensation temperature. Furthermore, magnetization hysteresis loops for all the samples demonstrate the crucial role of grain size on the magnetic properties.

Dipolar-coupled moment correlations in clusters of magnetic nanoparticles

Here, we investigate the nature of the moment coupling between 10-nm DMSA-coated magnetic nanoparticles, in both colloidal dispersion and in powder form. The individual iron oxide cores were composed of > 95% maghemite and agglomerated to clusters. At room temperature the ensemble behaved as a superparamagnet according to M\"ossbauer and magnetization measurements, however, with clear signs of dipolar interactions at low temperatures. Analysis of temperature-dependent AC susceptibility data in the superparamagnetic regime indicates a tendency for dipolar coupled anticorrelations of the core moments within the clusters. To resolve the directional correlations between the particle moments we performed polarized small-angle neutron scattering and determined the magnetic spin-flip cross-section of the powder in low magnetic field at 300 K. We extract the underlying pair distance distribution function of the magnetization vector field by an indirect Fourier transform of the cross-section, and which suggests positive as well as negative correlations between nearest neighbor moments, with anticorrelations clearly dominating for next-nearest moments. These tendencies are confirmed by Monte Carlo simulations of such core-clusters.

Probing the magnetic properties of ultrathin Pt/Mn bilayers

We report on the interplay between surface morphology and the magnetic properties of Pt/Mn ultra-thin bilayers by measuring the temperature dependence of the resistivity of Hall bars cooled under application of an external magnetic field. The resistivity (ρ) versus temperature (T) curves measured under a magnetic field applied along the x-, y- and z- directions exhibit a clear difference between the zero-field-cooled (ZFC) and field-cooled (FC) traces. The disparity between the ZFC and FC traces allowed us to determine the direction of the magnetic easy-axis. The ρ vs. T characteristic of the Pt/Mn bilayers revealed the existence of antiferromagnetic (AF) order below 79 K, in agreement with the Néel temperature observed in temperature dependent susceptibility curves. We also investigated the morphological evolution of the Mn films as a function of Mn thickness by atomic force microscopy (AFM). We observed the formation of a Mn layered structure on a 2nm-thick Al2O3 buffer layer for films thicker than 2.5 nm. Our results suggest that bilayer structures consisting of non-magnetic noble metals on ultra-thin antiferromagnetic metals can be used as a probe to explore the complex magnetic properties of the AF layer.

Understanding of metal-insulator transition in VO2 based on experimental and theoretical investigations of magnetic features

The metal-insulator transition temperature Tc in VO2 is experimentally shown to be almost the same as a magnetic transition temperature Tm characterized by an abrupt decrease in susceptibility, suggesting the evidence of the same underlying origin for both transitions. The measurement of susceptibility shows that it weakly increases on cooling for temperature range of T > Tm, sharply decreases near Tm and then unusually increases on further cooling. A theoretical approach for such unusual observations in susceptibility near Tm or below is performed by modeling electrons from each two adjacent V4+ ions distributed along V-chains as a two-electron system, which indicates that the spin exchange between electrons could cause a level splitting into a singlet (S = 0) level of lower energy and a triplet (S = 1) level of higher energy. The observed abrupt decrease in susceptibility near Tm is explained to be due to that the sample enters the singlet state in which two electrons from adjacent V4+ ions are paired into dimers in spin antiparallel. By considering paramagnetic contribution of unpaired electrons created by the thermal activation from singlet to triplet levels, an expression for susceptibility is proposed to quantitatively explain the unusual temperature-dependent susceptibility observed at low temperatures. Based on the approach to magnetic features, the observed metal-insulator transition is explained to be due to a transition from high-temperature Pauli paramagnetic metallic state of V4+ions to low-temperature dimerized state of strong electronic localization.

Hirschfeld surface analysis and short-range ferromagnetic ordering in La0.5Ca0.4Ag0.1MnO3 based on critical behavior study

We investigated the Hirschfeld surface analysis and critical behaviour in La0.5Ca0.4Ag0.1MnO3 near the second-order ferromagnetic phase transition at Curie temperature TC. We determined the critical exponents, β, γ and δ corresponding to the temperature dependence of spontaneous magnetization, initial susceptibility and isothermal magnetization, respectively. The study of Hirshfeld surface reflects that both the electrostatic and intermolecular interactions are short range. The values for critical exponents obtained from magnetic measurements are very close to those predicted by the mean field tri-critical model. The critical-exponent values deduced here were in a good conform to those obtained using the modified Arrott plot and Widom scaling relation.

Absence of a multiglass state in some transition metal doped quantum paraelectrics

We critically investigate the purported existence of a multiglass state in the quantum paraelectrics $\mathrm{SrTiO}{}_{3}$ and $\mathrm{KTaO}{}_{3}$ doped with magnetic $3d$ transition metals. We observe that the transition metals have limited solubility in these hosts, and that traces of impurity magnetic oxides persist even in the most well-processed specimens. Our dielectric measurements indicate that the polar nanoregions formed as a consequence of doping appear to lack co-operativity, and the associated relaxation process exhibits a thermally activated Arrhenius form. At lower temperatures, the dielectric susceptibility could be fit using the Barrett's formalism, indicating that the quantum-paraelectric nature of the host lattices is unaltered by the doping of magnetic transition metal oxides. All these doped quantum paraelectrics exhibit a crossover from the high-temperature Curie-Weiss regime to one dominated by quantum fluctuations, as evidenced by a $T{}^{2}$ dependence of the temperature-dependent dielectric susceptibility. The temperature dependence of the magnetic susceptibility indicates that magnetic signatures observed in some of the specimens could be solely ascribed to the presence of impurity oxides corresponding to the magnetic dopants used. Hence, the doped quantum paraelectrics appear to remain intrinsically paramagnetic down to the lowest measured temperatures, ruling out the presence of a multiglass state.

Visualization and analysis of magnetothermal convection of paramagnetic liquid in the Rayleigh-Benard model

We examined the heat transfer of magnetothermal convection in a Rayleigh-Benard model (height 9.2 mm, vessel diameter 20 mm, aspect ratio 2.17). The working fluid was an aqueous gadolinium nitrate solution of 0.15 mol/kg ($\mathrm{pH}=4.52$ at 305.5 K, paramagnetic substance). Not only the magnetic body force but also the temperature dependence of paramagnetic susceptibility according to Curie's law provides the driving body force of convection and exerts a decisive influence over the heat transfer performance. The visual observation of the isothermal contour of convection was realized by the addition of a thermochromic liquid crystal (TLC). Using a large upward magnetic body force, i.e., $(\stackrel{P\vec}{b}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{P\vec}{\ensuremath{\nabla}})\phantom{\rule{0.16em}{0ex}}{b}_{z}=83.31\phantom{\rule{0.16em}{0ex}}{\mathrm{T}}^{2}/\mathrm{m}$ at the vessel center, we succeeded in visualizing the horizontal isothermal illuminant of the TLC, which revealed the realization of a quasiweightless condition in the Rayleigh-Benard model. The heat transfer on convection was analyzed by the method of Churchill and Ozoe. Its performance was enhanced by the downward magnetic body force and was suppressed by the upward magnetic body force, as compared with Rayleigh-Benard convection. The convective flows in the experiment ($\mathrm{Prandtl}\phantom{\rule{0.16em}{0ex}}\mathrm{number}=5.17$, $\mathrm{Ra}=1.53\ifmmode\times\else\texttimes\fi{}1{0}^{5}$, aspect ratio 2.0) were numerically simulated by three-dimensional computation. All the experimental and numerical results were arranged by Rayleigh number (Ra) and Nusselt number (Nu). In addition, we introduced the magnetic Rayleigh number (${\mathrm{Ra}}_{\mathrm{m}}$) instead of Ra. The results of Nu plotted versus the ${\mathrm{Ra}}_{\mathrm{m}}$ were closely distributed in the vicinity of the Silveston curve. This relationship reveals that the heat transfer on magnetothermal convection is controlled by the use of ${\mathrm{Ra}}_{\mathrm{m}}$.

Dielectric susceptibility of quantum Ising model: Simulated monoclinic A2BX4-type ferroelectrics

To explain the anomalous increase in the permittivity of monoclinic Rb2ZnI4 at low temperatures, the dielectric susceptibilities of modulated structures are calculated. A quantum Ising model is reduced from a lattice Hamiltonian by the mean field approximation and two-quantum-level approximation. The susceptibilities of modulated phases remain to be finite values at zero temperature as in the uniform ferroelectric phase, if the quantum effect works. Furthermore, the susceptibilities of the incommensurate and high-order commensurate phases increase at low temperatures. Since the ferroelectric phase can be realized with accompanying strong discontinuity, such an increase is different from the phenomenon during the lock-in transition. The phase diagram and the temperature dependence of susceptibility are calculated. With the selection of a proper set of parameters, the theoretical results agree qualitatively with experimental reports of (Rb1−xKx)2ZnI4.

Diluted magnetic Dirac-Weyl materials: Susceptibility and ferromagnetism in three-dimensional chiral gapless semimetals

We theoretically investigate the temperature-dependent static susceptibility and long-range magnetic coupling of three-dimensional (3D) chiral gapless electron-hole systems (semimetals) with arbitrary band dispersion [i.e., $\varepsilon(k) \sim k^N$, where $k$ is the wave vector and $N$ is a positive integer]. We study the magnetic properties of these systems in the presence of dilute random magnetic impurities. Assuming carrier-mediated Ruderman-Kittel-Kasuya-Yosida indirect exchange interaction, we find that the magnetic ordering of intrinsic 3D chiral semimetals in the presence of dilute magnetic impurities is ferromagnetic for all values of $N$. Using finite-temperature self-consistent field approximation, we calculate the ferromagnetic transition temperature ($T_{\rm c}$). We find that $T_{\rm c}$ increases with increasing $N$ due to the enhanced density of states, and the calculated $T_{\rm c}$ is experimentally accessible assuming reasonable coupling between the magnetic impurities and itinerant carriers.

Evolution from Griffiths like phase to non-Griffiths like phase with Y doping in [formula omitted

Detailed analysis of the temperature dependent dc susceptibility data shows the modification from the Griffiths like phase (GP) to non-Griffiths like phase (non-GP) in (La1-xYx)0.7Ca0.3MnO3(x=0,0.3,0.4,0.5,0.6) compounds with x. The existence of non-Griffiths phase is revealed from the upward deviation of the inverse susceptibility (1/χ) versus temperature plot from the Curie-Weiss behavior for the compounds with x⩾0.5. Whereas Griffiths phase is identified from the downturn of 1/χ versus T plot from the Curie-Weiss line in the compounds with x=0.3,0.4. The increased lattice distortion (with x) decreases the M-O-Mn bond angle and at the same time increase the electron-phonon (e-ph) coupling which in turn decreases the ferromagnetic double exchange (DE) interactions and enhances the effective short-range antiferromagnetic superexchange interactions between Mn3+/Mn3+ and Mn4+/Mn4+ ions above the Curie temperature (TC). The enhancement of this short-range antiferromagnetic (SR-AFM) interactions have been attributed to the conversions from GP to non-GP phase. The increased SR-AFM interactions with Y doping has also been substantiated from the magnetocaloric study.