Magnetic Equation Of State Research Articles

Magnetic and magnetocaloric properties of amorphous Y3Fe5O12 compound

We report a theoretical model formed by two coupled magnetic sublattices of localized spins in the presence of an applied magnetic field to investigate the magnetic characteristics and magnetocaloric properties of amorphous yttrium iron garnet. The magnetic state equation is based on Handrich–Kobe´s theory, where the amorphization is taken into account by introducing fluctuations in the exchange parameters. Experimental results report that Y3Fe5O12 presents a structural phase transition from crystalline to amorphous caused by a variation of external pressure. This phase transition on Y3Fe5O12 leads to interesting results in the magnetic properties and magnetocaloric quantities.

Theoretical investigation on the magnetocaloric effect in amorphous [formula omitted] system

In this work, we investigated the magnetic and magnetocaloric properties of the amorphous system Eu80Au20. The magnetic state equation and entropy were considered in the framework of Handrich–Kaneyoshi model, which takes into account the amorphization through the symmetric exchange fluctuation in the mean field approximation. The exchange and structural fluctuations parameters were chosen based on the experimental data of Eu80Au20. The isothermal entropy change was calculated for several variations of external magnetic field. Furthermore, the adiabatic temperature change and the refrigerant capacity were calculated for a magnetic field change from 0 to 5T.

Magnetization and magnetocaloric effect in La0.7Pb0.3MnO3 ceramics and 0.85(La0.7Pb0.3MnO3)–0.15(PbTiO3) composite

Abstract

Thermal QCD transition with two flavors of twisted mass fermions

We investigate the thermal QCD transition with two flavors of maximally twisted mass fermions for a set of pion masses, 300 MeV \textless $m_\pi$ \textless 500 MeV, and lattice spacings $a$ \textless 0.09 fm. We determine the pseudo-critical temperatures and discuss their extrapolation to the chiral limit using scaling forms for different universality classes, as well as the scaling form for the magnetic equation of state. For all pion masses considered we find resonable consistency with O(4) scaling plus leading corrections. However, a true distinction between the O(4) scenario and a first order scenario in the chiral limit requires lighter pions than are currently in use in simulations of Wilson fermions.

Making the most of Taylor expansion and imaginary μ

We present preliminary results for the curvature of the pseudocritical line and susceptibilities in Nf = 2 + 1 flavor QCD. The computations are carried out on lattice sizes of 163 × 4, at matching parameters of early work of the Bielefeld group. Emphasis is placed on the control of systematic errors, by cross-validating results obtained by use of the Taylor expansion and measurements at imaginary chemical potential. To this end, we generalize the magnetic equation of state to the analysis of the number density, and we extend it to imaginary values of the chemical potential.

Response of magnetic nanoparticle assemblies

Properties of magnetic nanoparticle assemblies are analysed by employing the randomly jumping interacting moments model including quantum fluctuations, ferromagnetic inter-particle coupling and disorder. Using spatially uniform external field we find the respective magnetic state equation and phase diagram indicating an existence of spinodal regions and critical points. Possibilities to use such systems as magnetoresistive sensors are considered.

Magnetic properties and magnetocaloric effect of Ti-doped La0.7Sr0.3MnO3

We have studied the influence of Ti doping on the magnetic properties and the magnetocaloric effect of La0.7Sr0.3Mn0.92Ti0.08O3 prepared by using solid-state reaction. The temperature dependence of magnetization reveals that the presence of the Ti dopant strongly reduces the Curie temperature (TC) to a value of ∼235 K. The magnetic-entropy change (ΔSm) has been assessed by means of isothermal-magnetization data versus the magnetic field, M-H curves, recorded at different temperatures. Around TC, −ΔSm reaches maximum values of 1.1, 2.0, and 2.7 J/(kg·K) for applied fields of 10, 20, and 30 kOe, respectively. A relative cooling power (RCP) of 135 J/kg is, thus, achieved for an applied field of 30 kOe. Detailed analyses of the M-H curves around TC indicate that the sample exhibits a second-order magnetic phase transition, where the variations of the saturation magnetization (Ms) and the inverse initial susceptibility (χ0−1) versus temperature obey asymptotic relations. Using the Arrott-Noakes method, we have obtained the critical exponents β = 0.425 ± 0.016, γ = 1.017 ± 0.055, and δ = 3.393 ± 0.039. These exponents are in good agreement with the magnetic equation of state. The β value determined in our case is between those expected for the mean-field (β = 0.5) and the 3D Heisenberge (β = 0.365) models, demonstrating the existence of ferromagnetic clusters.

The quantum equations of state of plasma under the influence of a weak magnetic field

The aim of this paper is to calculate the magnetic quantum equations of state of plasma, the calculation is based on the magnetic binary Slater sum in the case of low density. We consider only the thermal equilibrium plasma in the case of nλab3≪1, where λab2=ℏ2mabKT is the thermal De Broglie wave length between two particles. The formulas contain the contributions of the magnetic field effects. Using these results we compute the magnetization and the magnetic susceptibility. Our equation of state is compared with others.

Effects of the Cu Doping on Critical Behavior of ${\rm La}_{0.7}{\rm Ca}_{0.3}{\rm MnO}_{3}$

We have studied the influence of the Cu-doping on the critical properties of a polycrystalline sample of La0.7Ca0.3Mn0.95Cu0.05O3 prepared by solid-state reaction. Analyses of static magnetization data in the vicinity of the ferromagnetic-paramagnetic phase transition based on the modified Arrott plot reveal critical parameters of TC ≈ 197.3 K, β = 0.49 ± 0.03 and γ = 1.04 ± 0.04. These parameters are in good agreement with a magnetic equation of state M(H, e) = eβf±(H/eβ+γ), where e = (T - TC)/TC (the reduced temperature), f+ for T >; TC and f_ for T <; TC. Comparing with theoretical models, the critical exponents in our case are close to those expected for the mean-field theory. This reflects that the Cu doping leads to the second-order phase transition in La0.7Ca0.3Mn0.95Cu0.05O3 while the parent compound of La0.7Ca0.3MnO exhibits the first-order phase transition. Such the phenomenon is assigned to changes in structural parameters, Mn3+/Mn4+ ratio, and magnetic interaction mechanisms caused by Cu dopants.

Self-propulsion of a planar electric or magnetic microbot immersed in a polar viscous fluid

A planar sheet immersed in an electrically polar liquid like water can propel itself by means of a plane wave charge density propagating in the sheet. The corresponding running electric wave polarizes the fluid and causes an electrical torque density to act on the fluid. The sheet is convected by the fluid motion resulting from the conversion of rotational particle motion, generated by the torque density, into translational fluid motion by the mechanism of friction and spin diffusion. Similarly, a planar sheet immersed in a magnetic ferrofluid can propel itself by means of a plane wave current density in the sheet and the torque density acting on the fluid corresponding to the running wave magnetic field and magnetization. The effect is studied on the basis of the micropolar fluid equations of motion and Maxwell's equations of electrostatics or magnetostatics, respectively. An analytic expression is derived for the velocity of the sheet by perturbation theory to second order in powers of the amplitude of the driving charge or current density. Under the assumption that the equilibrium magnetic equation of state may be used in linearized form and that higher harmonics than the first may be neglected, a set of self-consistent integral equations is derived which can be solved numerically by iteration. In typical situations the second-order perturbation theory turns out to be quite accurate.

Phase boundary for the chiral transition in (2+1)-flavor QCD at small values of the chemical potential

We determine the chiral phase transition line in $(2+1)$-flavor QCD for small values of the light quark chemical potential. We show that for small values of the chemical potential the curvature of the phase transition line can be deduced from an analysis of scaling properties of the chiral condensate and its susceptibilities. To do so we extend earlier studies of the magnetic equation of state in $(2+1)$-flavor QCD to finer lattice spacings, $aT=1/8$. We use these universal scaling properties of the chiral order parameter to extract the curvature of the transition line at two values of the cutoff, $aT=1/4$ and $1/8$. We find that cutoff effects are small for the curvature parameter and determine the transition line in the chiral limit to leading order in the light quark chemical potential. We obtain ${T}_{c}({\ensuremath{\mu}}_{q})/{T}_{c}(0)=1\ensuremath{-}0.059(2)(4)({\ensuremath{\mu}}_{q}/T{)}^{2}+\mathcal{O}({\ensuremath{\mu}}_{q}^{4})$.

Dynamics of magnetic nano-particle assembly

Ferromagnetically coupled nano-particle assembly is analyzed accounting for inter- and intra- particle electronic structures within the randomly jumping interacting moments model including quantum fluctuations due to the discrete levels and disorder. At the magnetic jump anomalies caused by quantization the magnetic state equation and phase diagram are found to indicate an existence of spinodal regions and critical points. Arrays of magnetized nano-particles with multiple magnetic response anomalies are predicted to display some specific features. In a case of weak coupling such arrays exhibit the well-separated instability regions surrounding the anomaly positions. With increasing coupling we observe further structure modification, plausibly, of bifurcation type. At strong coupling the dynamical instability region become wide while the stable regime arises as a narrow islands at small disorders. It is shown that exploring correlations of magnetic noise amplitudes represents convenient analytical tool for quantitative definition, description and study of supermagnetism, as well as self-organized criticality.

The functional renormalization group and O(4) scaling

The critical behavior of the chiral quark-meson model is studied within the Functional Renormalization Group (FRG). We derive the flow equation for the scale-dependent thermodynamic potential at finite temperature and density in the presence of a symmetry-breaking external field. We perform a set of approximations to formulate and solve the FRG flow equation in the presence of fermionic degrees of freedom and test their influence on the O(4) critical properties expected in the quark-meson model. Within this scheme, the critical scaling behavior of the order parameter, its transverse and longitudinal susceptibilities as well as the correlation lengths near the chiral phase transition are computed for vanishing baryon density. We focus on the scaling properties of these observables at non-vanishing external field when approaching the critical point from the symmetric as well as from the broken phase. We confront our numerical results with the Widom–Griffiths form of the magnetic equation of state, obtained by a systematic ε expansion of the scaling function.

Spinodal regions due to disorder in superferromagnets

AbstractInter‐ and intra‐dot structures of an assembly of ferromagnetically coupled quantum dots are analyzed within the randomly jumping interacting moments (RJIM) model including quantum effects due to the dot discrete levels and disorder. At the magnetic jump anomalies due to quantization the magnetic state equation (MSE) and phase diagrams are found to indicate an existence of spinodal regions and critical points. Arrays of dots with multiple magnetic response anomalies are predicted to display some specific features. In a case of weak coupling such arrays exhibit the well‐separated instability regions surrounding the anomaly positions. With increase in coupling we observe further structure modification, plausibly, of bifurcation type. At strong coupling the instability region becomes wide while the stable regime arises as a narrow islands at small disorders.

Self-organized criticality in superferromagnets

Superferromagnetic structures are studied accounting for quantum fluctuations due to the discrete level structure and disorder within the randomly jumping interacting moments model. The occurrence of self-organized criticality is found to indicate an existence of spinodal regions and critical points in magnetic state equation and phase diagram. The magnetodynamics show jerky behaviour displayed as erratic stochastic discontinuities for magnetic induction. Magnetic noise correlations are proposed as model-independent analytical tools employed in order to specify, quantify and analyse magnetic structure and origin of superferromagnetism.

Hybrid Mode Control in Improvement to Magnetic Suspension Ball and Beam System

The development of magnetic suspension (MS) technology has been widely applied in high-tech industry. MS actuator has features of contact-free, friction-free, low contamination and low noise. This paper describes the design and examination of the hybrid MS actuator referring to our previous works with different analysis aspect to improve the overall performance of actuator and system. The re-designed MS actuator is analyzed and controlled by deriving its magnetic force and system state equations. This paper uses commercial products for design process and simulations about the stability and robustness of system. After deriving the dynamic equation of ball and beam system, the sliding mode control (SMC) is applied in the balance and stabilize the ball and beam system. The drive circuit and MS actuators are controlled by the microcontroller in ball and beam system. The combined SMC and Fuzzy Logic Control (FLC) are also introduced to control the adjustable stable position. Different situations are tested separately with conformed results to desired responses. Finally, comparison and analyses of the results in laboratory have presented good improvements on the proposed system.

Magnetic equation of state in (2+1)-flavor QCD

A first study of critical behavior in the vicinity of the chiral phase transition of (2+1)-flavor QCD is presented. We analyze the quark mass and volume dependence of the chiral condensate and chiral susceptibilities in QCD with two degenerate light quark masses and a strange quark. The strange quark mass (m_s) is chosen close to its physical value; the two degenerate light quark masses (m_l) are varied in a wide range 1/80 \le m_l/m_s \le 2/5, where the smallest light quark mass value corresponds to a pseudo-scalar Goldstone mass of about 75 MeV. All calculations are performed with staggered fermions on lattices with temporal extent Nt=4. We show that numerical results are consistent with O(N) scaling in the chiral limit. We find that in the region of physical light quark mass values, m_l/m_s \simeq 1/20, the temperature and quark mass dependence of the chiral condensate is already dominated by universal properties of QCD that are encoded in the scaling function for the chiral order parameter, the magnetic equation of state. We also provide evidence for the influence of thermal fluctuations of Goldstone modes on the chiral condensate at finite temperature. At temperatures below, but close to the chiral phase transition at vanishing quark mass, this leads to a characteristic dependence of the light quark chiral condensate on the square root of the light quark mass.

Proposed magnetic equation of state for the three-dimensional Ising model near criticality: Is the insulating ferromagnet CrBr 3 a candidate?

In a magnetic system, consistent with Griffiths analyticity requirements one can parameterize the equation of state near criticality by writing H = r β δ h ( θ ) , T = r t ( θ ) and the magnetization M = r β m ( θ ) , where T is measured from the critical temperature. For the insulating ferromagnet CrBr 3, the experimental data of Ho and Litster [J.T. Ho, J.D. Litster, Phys. Rev. Lett. 22 (1969) 603] is well fitted by m ( θ ) as a linear function of θ [P. Schofield, J.D. Litster, J.T. Ho, Phys. Rev. Lett. 23 (1969) 1098]. Also Ho and Litster give β = 0.368 , γ = 1.215 and δ = 4.3 . Those critical experiments are very close to the recent 3D Ising results of Zhang [Z.D. Zhang, Philos. Mag. 87 (2007) 5309], namely β = 3 / 8 , γ = 5 / 4 and δ = 13 / 3 . We therefore predict that m ( θ ) will be proportional to θ as a fingerprint of the 3D Ising Hamiltonian.

Critical magnetodynamics in quantum dot arrays

Arrays of coupled quantum dots are analysed by employing the randomly jumping interacting moments model including quantum fluctuations due to the discrete level structure, ferromagnetic inter-dot coupling and disorder. The respective magnetic state equation and phase diagram are found to indicate an existence of spinodal regions and critical points. In vicinity of such points magnetic induction shows jerky behaviour displayed as erratic discontinuities associated with avalanches of the dot magnetization. The model predicts some universal scaling properties for magnetic noise. On the basis of such properties we suggest and test model-independent analytical tools employed in order to specify, quantify and analyse the system with respect to the observed magnetic structure. Some specific features for arrays of dots with singlet-triplet transitions are discussed.

Magnetic transition and equation of state of iron carbide to 400 GPa

Magnetic transition and equation of state of iron carbide to 400 GPa