Ideal Paramagnet Research Articles

Synthesis, Structure, and Magnetic and Biological Properties of Copper(II) Complexes with 1,3,4-Thiadiazole Derivatives.

New coordination compounds of copper(II) with 2,5-bis(ethylthio)-1,3,4-thiadiazole (L1) and 2,5-bis(pyridylmethylthio)-1,3,4-thiadiazole (L2) with compositions Cu(L1)2Br2, Cu(L1)(C2N3)2, Cu(L2)Cl2, and Cu(L2)Br2 were prepared. The complexes were identified and studied by CHN analysis, infrared (IR) spectroscopy, powder X-Ray diffraction (XRD), and static magnetic susceptibility. The crystal structures of Cu(II) complexes with L1 were determined. The structures of the coordination core of complexes Cu(L2)Cl2 and Cu(L2)Br2 were determined by Extended X-ray absorption fine structure (EXAFS) spectroscopy. Magnetization measurements have revealed various magnetic states in the studied complexes, ranging from an almost ideal paramagnet in Cu(L1)2Br2 to alternating-exchange antiferromagnetic chains in Cu(L1)(C2N3)2, where double dicyanamide bridges provide an unusually strong exchange interaction (J1/kB ≈ -23.5 K; J2/kB ≈ -20.2 K) between Cu(II) ions. The cytotoxic activity of copper(II) complexes with L2 was estimated on the human cell lines of breast adenocarcinoma (MCF-7) and hepatocellular carcinoma (HepG2).

Magnetocaloric Properties of an Ising Antiferromagnet on a Kagome Lattice

Owing to a high degree of geometrical frustration an Ising antiferromagnet on a kagome lattice (IAKL) is known to exhibit no long-range ordering at any temperature, including the ground state. Nevertheless, at low temperatures it shows a strongly correlated, highly fluctuating regime known as a cooperative paramagnet or classical spin liquid. In the ground state it is characterized by a macroscopic degeneracy which translates to a relatively large value of the residual entropy. It has been shown that the presence of a macroscopic degeneracy associated with geometrical frustration below the saturation field can facilitate an enhanced magnetocaloric effect (MCE), which can exceed that of an ideal paramagnet with equivalent spin by more than an order of magnitude. In the present study we investigate magnetic and magnetocaloric properties of IAKL by Monte Carlo simulation. In particular, we calculate the entropy of the system using the thermodynamic integration method and evaluate quantities which characterize MCE, such as the isothermal entropy and adiabatic temperature changes in a varying magnetic field. It is found that IAKL shows the most interesting magnetocaloric properties at low temperatures and moderate magnetic fields, suggesting its potential to be used in technological applications for low-temperature magnetic refrigeration.

Experimental study of magnetic anisotropy in a layered CsNd(MoO4)2

Ground-state properties of Nd3+ ion in the magnetically concentrated CsNd(MoO4)2 lattice of orthorhombic symmetry have been investigated at low temperatures. Magnetic-field dependence of magnetization measured at 5K in the magnetic field applied along the a, b, c crystallographic axes reflects slight deviations from the tetragonal symmetry of a local surrounding of Nd3+ ion. The analysis of the data performed within a model of an ideal paramagnet provided g-factor values, ga=3.08, gb=1.90, and gc=1.95. Angular dependence of the electron paramagnetic resonance spectra studied at temperature 2.5K was investigated in the ab, ac and bc planes. The spectra are dominated by a broad asymmetric line which is strongly deformed due to the formation of a fine structure ascribed to the presence of nonequivalent Nd3+ sites. The analysis of the spectra confirmed the easy-axis character of a magnetic anisotropy. In addition, nontrivial tilting of the local anisotropy axes from the crystallographic axes was revealed, indicating lowering of crystal symmetry at low temperatures.

Synthesis and characterization of copper complexes with the 2,3-dicyano-5,6-dimercaptopyrazine ligand: Magnetic properties of a ferrocenium salt

Copper complexes with the 2,3-dicyano-5,6-dimercaptopyrazine ligand (dcdmp), Cu(dcdmp)2, were obtained in both monoanionic and dianionic forms as the tetrabutylammonium salts and were characterised, as well as a decamethylferrocenium salt of the dianion. (n-Bu4N)2Cu(dcdmp)2 crystallises in the triclinic system, space group P1¯, Z = 1, with unit cell parameters a = 9.9512(6) Å, b = 10.541(1) Å, c = 13.221(1) Å, α = 69.756(9)°, β = 88.112(6)° and γ = 79.658(7)°. Its crystal structure consists of layers of well isolated square planar Cu(dcdmp)2− anions alternating with layers of n-Bu4N+ cations along b + c, and the magnetic properties in the range 2–300 K are close to those of an ideal paramagnet with an effective magnetic moment of 1.7 μB/f.u. n-Bu4NCu(dcdmp)2 crystallises in the monoclinic space group C2/c, Z = 8, with unit cell parameters a = 35.281(4) Å, b = 9.501(1) Å, c = 21.885(2) Å and β = 109.842(9)°, being isostructural with the Au analogue previously described. The salt [Fe(Cp*)2]2[Cu(dcdmp)2] crystallises in the monoclinic system, space group C2/c, Z = 4, with unit cell parameters a = 15.891(1) Å, b = 10.5133(7) Å, c = 30.264(3) Å and β = 91.105(2)°. Its crystal structure consists of out-of-registry columns, parallel to b, of alternated dianions and side-by-side pairs of cations, … A2−(D+D+)A2−(D+D+) … This compound behaves as a paramagnet with weak AF interactions and no ordering down to 1.7 K. The magnetic properties, due to both cation and anion S = 1/2 contributions, show a large anisotropy, ascribed essentially to the g-factor anisotropy of the Fe(Cp*)2cations.

Enhanced magnetocaloric effect in frustrated magnets

The magnetothermodynamics of strongly frustrated classical Heisenberg antiferromagnets on kagome, garnet, and pyrochlore lattices is examined. The field induced adiabatic temperature change (dT/dH)_S is significantly larger for such systems compared to ordinary non-frustrated magnets and also exceeds the cooling rate of an ideal paramagnet in a wide range of fields. An enhancement of the magnetocaloric effect is related to presence of a macroscopic number of soft modes in frustrated magnets below the saturation field. Theoretical predictions are confirmed with extensive Monte Carlo simulations.

Magnetic Impurities in Glass and Silver Powder at Milli- and Microkelvin Temperatures

In search of an ideal paramagnet for thermometry at very low temperatures, we have studied the magnetic behaviour of impurity moments (localized Fe3+, impurity concentration x = (180±10) ppm) dissolved in a structural borosilicate glass, in the temperature range 0.07 ≤ T ≤ 300 mK by means of dynamic ac susceptibility, and at 1.6≤T ≤ 300 K by static dc magnetization. In order to improve the thermal coupling of the insulating glass at the lowest temperatures, it was pulverized and mixed with silver powder of submicron grain size; the composite was subsequently compacted to a cylindrical sample by applying a pressure of a few kbar. This contact method which is applicable to other materials with bad thermal conductivity as well improved the accessible minimum temperature for the glass down to ≃0.1 mK. At low temperatures, we observe a Curie Weiss law for the dynamic susceptibility χ of the magnetic Fe3+ impurities in the glass down to ≈0.6 mK, a broad maximum of χ at T ≃ 0.38 mK and a decrease towards even lower temperatures. Compared to the frequently used, highly diluted metallic spin glasses, the magnetic behaviour of the glass makes inductance thermometry applicable in a much larger temperature range. In addition, we have investigated the low temperature magnetic properties of a sample of compacted silver powder. The small amount of paramagnetic impurities in the Ag particles (x = (4 ± 1) ppm) exhibits a low temperature susceptibility which can be described by the Kondo effect with an unexpectedly small Kondo temperature of TK ≃ 1 mK.

The dynamics of a Genetic Algorithm for simple random Ising systems

A formalism is presented for analysing Genetic Algorithms. It is used to study a simple Genetic Algorithm consisting of selection, mutation and crossover which is searching for the ground states of simple random Ising-spin systems: a random-field ideal paramagnet and a spin-glass chain. The formalism can also be applied to other population based search techniques and to biological models of micro-evolution. To make the problem tractable, it is assumed that the population dynamics can be described by a few macroscopic order parameters and that the remaining microscopic degrees of freedom can be averaged out. The macroscopic quantities that are used are the cumulants of the distribution of fitnesses (or energies) in the population. A statistical mechanics model is presented which describes the population configuration in terms of the cumulants, this is used to derive equations of motion for the cumulants. Predictions of the theory are compared with experiments and are shown to predict the average time to convergence and the average fitness of the final population accurately. A simplified version of the equations is produced by keeping only leading nonlinear terms, and truncating the cumulant expansion. This is shown to give a novel description of the role of genetic operators in search, e.g. it is argued that an important role of crossover is to reduce the skewness of the population.

Localized-spins ideal paramagnet within non-extensive thermostatistics

Abstract A Within a recently generalized thermostatistical framework which allows for non-extensive effects, we discuss numerically the ideal paramagnet (N non-interacting localized spins in the presence of an external magnetic field) in order to illustrate the existence of a well behaved thermodynamic limit for the canonical ensemble. Both the specific heat (generalized Schottky anonnaly) and the isothermal magnetic susceptibility (generalized Curie law) are calculated and data collapse is exhibited for N >> 1. In addition to this, and within a molecular-field approximation, we have generalized the Curie-Weiss law.

Entropy and heat along reversible paths for fluids and magnets

An identity for the volume derivative of entropy along an arbitrary reversible path is derived and used to show that the heat to a fluid can attain both positive and negative values along certain negatively sloped paths in the pressure–volume plane. A temperature–entropy plot graphically illustrates this point. An analogous entropy identity for magnetic systems is derived and used together with a temperature–entropy plot to gain insights into paramagnetic behavior. In contrast with the ideal gas, a path for an ideal paramagnet can have both positive and negative heats only if it attains both positive and negative slopes in a magnetization vs magnetic field plot.

Instability of the QED vacuum in the presence of condensed matter

The possibility that the interaction of condensed matter with radiation may give origin to non-trivialclassical configurations for the transverse electromagnetic field is investigated. The analysis demonstrates that, in a stationary state (e.g. the ground state), the transverse electric field cannot develop a non-vanishingclassical configuration. On the contrary the magnetic field can develop, in general, non-trivialclassical configurations. The analysis of an ideal paramagnet shows the possible instauration of a ferromagnetic order consisting of a spontaneous alignment of spins, due to the interaction with a spontaneously created static magnetic field. In this approach, magnetic order is viewed as the formation of a bound state between condensed matter and radiation field.

Magnetism in cupric oxide

Single-crystal neutron diffraction measurements have been used to study the long-range magnetic ordering in cupric oxide, CuO. An incommensurate antiferromagnetic structure forms below the Neel temperature on 230(1) K, with a propagation vector (0.506(1)a*-0.483(1)c*) which remains constant down to a magnetic phase transition at 213(1) K. Below the latter temperature, the structure remains antiferromagnetic with a commensurate propagation vector (1/2 0-1/2), and this structure persists to the lowest temperature reached in the investigation (20 K). The arrangement of the copper moments in both phases is such that the n-glide perpendicular to the b axis of the monoclinic cell, space group C2/c, does not reverse the direction of the spin. The two magnetic sublattices related to the C-face-centring scatter in phase quadrature and the relative directions of the spin on them could not be determined. Good agreement is obtained for the commensurate phase with a multipole model for the copper magnetisation density and spins of 0.65(3) mu B directed parallel to b. The lower sublattice magnetisation in the incommensurate phase precluded a meaningful multipole fit, but a reasonable agreement is obtained with a model in which the spins rotate in the a-c plane following an elliptical envelope with major axis directed 33(2) degrees to c in beta obtuse and a maximum moment of 0.38(2) mu B at 215 K. The paramagnetic scattering at ambient temperature and 550 K was measured to try to find the origin of the peak in the susceptibility. No significant paramagnetic scattering could be obtained from a powdered sample although the sensitivity of detection was some five times that required to observe the scattering from a Cu2+ ion in an ideal paramagnet.

Paramagnetism of Liquid Helium Three

The elaboration of a theoretical model describing the paramagnetic properties of liquid Heg throughout the region of existence of this phase is presented. The model is based on a molecular field theoretical type of approach. The ratios of the actual paramagnetic susceptibility of liquid He/sup 3/ to the one it would have if it were an ideal paramagnet are predicted to be representable through a unique function depending on the reduced temperature variable. The latter contains the characteristic temperature of the nuclear spin system, which, at the present time, is only available empirically through the susceptibility ratio data. In the susceptibility ratio-reduced temperature representation all susceptibility ratios of liquid He/sup 3/ fall on a single curve. This theoretical ratio curve describes very closely the experimental ratios up to values of the reduced temperature of 0.90 --1.0. Beyond this range the experimental susceptibility ratios become systematically larger than the calculated ratios, the differences between them being small. Quantitative arguments are advanced that appear to explaln satisfactorily these discrepancies and to indicate that the theory should be valid throughout the whole range of the natural reduced temperature variable. The spin entropyspin susceptibility relation, established and used previously, yields, with the theoretical paramagnetismmore » model, rigorous lower limits of the entropy, heat capacity, and expansion coefficient of the liquid throughout the region of existence of this phase. With the recent extension of the melting pressure data to quite low temperatures, the rigorous spin entropy of the liquid along the melting line allows one to estimate the entropy of the solid along the melting line and at low temperatures. Here the solid entropy turns out to be less than R In2, per mole, yielding the temperature of its heat capacity anomaly to be below one hundredth of a degree Kelvin. The discussion of the liquid He/sup 3/-solid He/sup 3/ equilibrium, on the basis of these results, seems to render questionable any analysis of the thermal properties of the solid that ignores the existence of its nuclear spin system even at medium temperatures. A semiquantitative description of the entropy-pressure diagram of He/sup 3/ discloses various singular characteristics of the liquid entropy along the melting line in its pressure dependence, as well as the peculiar features of the solid entropy at and around the melting pressure anomaly. (auth)« less