Maximum In Chi Research Articles

Hidden magnetic order in the triangular-lattice magnet Li2MnTeO6

The manganese tellurate Li2MnTeO6 consists of trigonal spin lattices made up of Mn4+ (d3, S = 3/2) ions. The magnetic properties of this compound were characterized by several experimental techniques, which include magnetic susceptibility, specific heat, dielectric permittivity, electron spin resonance (ESR), nuclear magnetic resonance (NMR) and neutron powder diffraction (NPD) measurements, and by density functional calculations (DFT). The magnetic susceptibility chi(T) demonstrates very unusual behavior. It isdescribed by the Curie-Weiss law at high temperature with Curie-Weiss temperature of Theta = -74 K, exhibits no obvious anomaly indicative of a long-range magnetic ordering at low magnetic fields. At high magnetic fields, however, the character of chi(T) changes showing a maximum at about 9 K. That this maximum of chi(T) reflects the onset of an antiferromagnetic order was confirmed by specific heat measurements, which exhibit a clear lambda-type anomaly at TN around 8.5 K even at zero magnetic field, and by 7Li NMR and dielectric permittivity measurements. The magnetic structure of Li2MnTeO6, determined by neutron powder diffraction measurements at 1.6 K, is described by the 120-degree non-collinear spin structure with the propagation vector k = (1/3, 1/3, 0). Consistent with this finding, the spin exchange interactions evaluated for Li2MnTeO6 by density functional calculations are dominated by the nearest-neighbor antiferromagnetic exchange within each triangular spin lattice. This spin lattice is strongly spin frustrated with f = |Theta|/TN around 8 and exhibits a two-dimensional magnetic character in a broad temperature range above TN.

Relationship between bond-breakage correlations and four-point correlations in heterogeneous glassy dynamics: Configuration changes and vibration modes

We investigate the dynamic heterogeneities of glassy particle systems in the theoretical schemes of bond breakage and four-point correlation functions. In the bond-breakage scheme, we introduce the structure factor S(b)(q,t) and the susceptibility χ(b)(t) to detect the spatial correlations of configuration changes. Here χ(b)(t) attains a maximum at t=t(b)(max) as a function of time t, where the fraction of the particles with broken bonds φ(b)(t) is about 1/2. In the four-point scheme, treating the structure factor S(4)(q,t) and the susceptibility χ(4)(t), we detect superpositions of the heterogeneity of bond breakage and that of thermal low-frequency vibration modes. While the former grows slowly, the latter emerges quickly to exhibit complex space-time behavior. In two dimensions, the vibration modes extending over the system yield significant contributions to the four-point correlations, which depend on the system size logarithmically. A maximum of χ(4)(t) is attained at t=t(4)(max), where these two contributions become of the same order. As a result, t(4)(max) is considerably shorter than t(b)(max).

Synthesis and structural and magnetic characterization of a hexadecanuclear cobalt phosphonate compound

A wheel-like hexadecanuclear cobalt phosphonate [Co(16)(OH)(6)(chp)(22)(O(3)PC(6)H(9))(2)(H(2)O)(4)] has been synthesized by reactions of cobalt perchlorate with cyclohexenephosphonic acid (H(2)O(3)PC(6)H(9)) and 6-chloro-2-hydroxypyridine (Hchp) in the presence of Et(3)N. Magnetic studies on the compound show a general decline of chi(M)T with decreasing T. The appeared maximum of chi(M)T at low temperature suggests the presence of a non-diamagnetic ground state. The alternating current susceptibilities show frequency-dependent signals.

Density scaling in viscous liquids: From relaxation times to four-point susceptibilities

We present numerical calculations of a four-point dynamic susceptibility, chi(4)(t), for the Kob-Andersen Lennard-Jones mixture as a function of temperature T and density rho. Over a relevant range of T and rho, the full t-dependence of chi(4)(t) and thus the maximum in chi(4)(t), which is proportional to the dynamic correlation volume, are invariant for state points for which the scaling variable rho(gamma)/T is constant. The value of the material constant gamma is the same as that which superposes the relaxation time tau of the system versus rho(gamma)/T. Thus, the dynamic correlation volume is a unique function of tau for any thermodynamic condition in the regime where density scaling holds. Finally, we examine the conditions under which the density scaling properties are related to the existence of strong correlations between pressure and energy fluctuations.

Strong H···F Hydrogen Bonds as Synthons in Polymeric Quantum Magnets: Structural, Magnetic, and Theoretical Characterization of [Cu(HF2)(pyrazine)2]SbF6, [Cu2F(HF)(HF2)(pyrazine)4](SbF6)2, and [CuAg(H3F4)(pyrazine)5](SbF6)2

Three Cu(2+)-containing coordination polymers were synthesized and characterized by experimental (X-ray diffraction, magnetic susceptibility, pulsed-field magnetization, heat capacity, and muon-spin relaxation) and electronic structure studies (quantum Monte Carlo simulations and density functional theory calculations). [Cu(HF(2))(pyz)(2)]SbF(6) (pyz = pyrazine) (1a), [Cu(2)F(HF)(HF(2))(pyz)(4)](SbF(6))(2) (1b), and [CuAg(H(3)F(4))(pyz)(5)](SbF(6))(2) (2) crystallize in either tetragonal or orthorhombic space groups; their structures consist of 2D square layers of [M(pyz)(2)](n+) that are linked in the third dimension by either HF(2)(-) (1a and 1b) or H(3)F(4)(-) (2). The resulting 3D frameworks contain charge-balancing SbF(6)(-) anions in every void. Compound 1b is a defective polymorph of 1a, with the difference being that 50% of the HF(2)(-) links are broken in the former, which leads to a cooperative Jahn-Teller distortion and d(x(2))(-y(2)) orbital ordering. Magnetic data for 1a and 1b reveal broad maxima in chi at 12.5 and 2.6 K and long-range magnetic order below 4.3 and 1.7 K, respectively, while 2 displays negligible spin interactions owing to long and disrupted superexchange pathways. The isothermal magnetization, M(B), for 1a and 1b measured at 0.5 K reveals contrasting behaviors: 1a exhibits a concave shape as B increases to a saturation field, B(c), of 37.6 T, whereas 1b presents an unusual two-step saturation in which M(B) is convex until it reaches a step near 10.8 T and then becomes concave until saturation is reached at 15.8 T. The step occurs at two-thirds of M(sat), suggesting the presence of a ferrimagnetic structure. Compound 2 shows unusual hysteresis in M(B) at low temperature, although chi vs T does not reveal the presence of a magnetic phase transition. Quantum Monte Carlo simulations based on an anisotropic cubic lattice were applied to the magnetic data of 1a to afford g = 2.14, J = -13.4 K (Cu-pyz-Cu), and J(perpendicular) = -0.20 K (Cu-F...H...F-Cu), while chi vs T for 1b could be well reproduced by a spin-1/2 Heisenberg uniform chain model for g = 2.127(1), J(1) = -3.81(1), and zJ(2) = -0.48(1) K, where J(1) and J(2) are the intra- and interchain exchange couplings, respectively, which considers the number of magnetic nearest-neighbors (z). The M(B) data for 1b could not be satisfactorily explained by the chain model, suggesting a more complex magnetic structure in the ordered state and the need for additional terms in the spin Hamiltonian. The observed variation in magnetic behaviors is driven by differences in the H...F hydrogen-bonding motifs.

Polynuclear Cation−Cation Complexes of Pentavalent Uranyl: Relating Stability and Magnetic Properties to Structure

Reaction of {[UO2Pys][mu-KI2Py2]}n (1) with 2 equiv of potassium dibenzoylmethanate (Kdbm) in pyridine or acetonitrile affords, respectively, the corresponding tetranuclear complexes of pentavalent uranyl ([UO2(dbm),2]2[mu-K(Py)2]2[mu8-K(Py)]}2I2 x Py2 (2) (in 70% yield) and {[UO2(dbm)2]2[mu-K(MeCN)2][mu8-K]}2 (3) (in 40% yield) in which four UO2+ are mutually coordinated (T-shaped "cation-cation" interaction). The X-ray structures of 2 and 3 show also the presence of, respectively, six and four potassium cations involved in UO2+...K+ interactions. Reaction of 2 with an excess of 18-crown-6 (18C6) affords the dimeric complex [UO2(dbm)2K(18C6)]2 (4) presenting a diamond-shaped interaction between two UO2+ groups, in 45% yield. 1H and PFGSTE diffusion NMR spectroscopy of 2 and 3 in pyridine show unambiguously the presence of UO2+...UO2+ and UO2+...K+ interactions (tetrametallic species) in solution, which leads to a rapid (7 days) disproportionation of pentavalent uranyl to afford [U(dbm)4] and [UO2(dbm)2] species. The UO2+...K+ interaction plays an important synergistic role in the stabilization of the UO2+...UO2+ interactions. Accordingly, the lower affinity of (K(18C6))+ for the uranyl(V) oxygen in complex 4 results in a lower number of coordinated K+ and therefore in a weakened UP2+...UO2+ interaction. The UO24+...UO2+ interactions is completely disrupted in dmso or in the presence of Kdbm, preventing disproportionation of pentavalent uranyl. Solid-state variable-temperature magnetic susceptibility studies showed the unambiguous presence of antiferromagnetic coupling between the two oxo-bridged uranium centers of complex 4, with the appearance of a maximum in chi versus T at approximately 5 K. The different behavior of the tetrameric complex 3, which probably involves a magnetic coupling occurring at lower temperature, can be ascribed to the different geometric arrangement of the interacting uranyl(V) groups.

Magnetic Properties of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ Manganite Bulk and Nanometer-Sized Crystals

AC and DC magnetic measurements in temperature interval 5-300 K and at external magnetic field Hles 5 T were employed to probe and compare magnetic ordering in bulk and nanometer-sized (15plusmn2 nm) La0.7Ca0.3MnO3 single crystals. The respective Curie temperatures (TC)~239 K and 233 K, close to each other are observed in bulk and nano samples. While, their low T spontaneous magnetizations are strongly different, being about 3.6 and 1.2 muB/f.u., respectively. AC susceptibility (chi) of a bulk sample, measured at Hdc=0, shows a maximum of chi(T) at T~TC. In a contrast, chi(T) of the nano-sample demonstrates a broad maximum far below its TC. In the nano-sample both real and imaginary parts of chi are frequency dependent within the 5-280 K range. In a contrary, the frequency dependence is observed only for imaginary part of chi in bulk crystal. It is concluded that the similar ferromagnetic ground state exists in both considered samples; however, it is strongly frustrated in agglomerated nano particles due to surface magnetic disorder and interparticle interactions.

Synthesis, structure and magnetism of new single molecule magnets composed of MnII2MnIII2 alkoxo-carboxylate bridged clusters capped by triethanolamine ligands.

A family of tetranuclear mixed-valent Mn(II)(2)/Mn(III)(2) complexes of type [Mn(4)(LH(2))(2)(LH)(2)(H(2)O)(x)(RCO(2))(2)](Y)2.nS has been synthesised and structurally characterised, where LH(3) = triethanolamine (N(CH(2)CH(2)OH)(3)), (R=CH(3), x=2, Y = CH(3)CO(2)-, n=2, S = H(2)O; 1), (R=C(6)H(5), x=0, Y=C(6)H(5)CO(2)-, n=1, S = CH(3)CN; 2), (R=C(2)H(5), x=0, Y=ClO(4)(-), n=0; 3). A common structural core was deduced from X-ray crystallography and consists of a rhomboidal (planar-diamond) array with two 7-coordinate Mn(II) "wingtip (w)" centres and two 6-coordinate Mn(III) "body (b)" centres. The Mn(III) ions are bridged to the Mn(II) ions by mu3-oxygen atoms from a deprotonated alcohol "arm" of each tridentate LH(2-) ligand and by mu2-oxygen atoms from each tetradentate LH(2)(-) ligand. The four nitrogen atoms from LH(2-) and LH(2)(-) groups, together with bridging and terminal carboxylates oxygens complete the outer coordination sites around the Mn atoms. A feature of these clusters is that they are linked together in the crystal lattice by hydrogen-bonding interactions involving a non-coordinated hydroxyl arm on each LH(2-) group. Detailed DC and AC magnetic susceptibility measurements and magnetisation isotherms have been made on the three complexes and show that intra-cluster ferromagnetic coupling is occurring between the S = 2 Mn(III) and S = 5/2 Mn(II) ions to yield S = 9 ground states. The g, J(bb) and J(wb) parameters have been deduced. Inter-cluster antiferromagnetic coupling was noted in and this influences the magnetisation versus field behaviour and the temperature and magnitude of the out-of-phase AC chi"M maxima in comparison to those observed for and. An Arrhenius plot of the reciprocal temperature of the maxima in chi"M obtained at different frequencies (10 to 1500 Hz), in the range 1.75 K to 4 K, against the natural logarithm of the magnetization relaxation rate (1/tau) yielded values of the activation energies and pre-exponential factors for two of these new tetranuclear single-molecule magnets (SMMs), and. The activation energies were compared with the potential energy barrier height, U, for magnetisation direction reversal (U = DS(2)) using the axial zero-field splitting parameter, D, deduced from the DC M/H isotherm analysis for these S = 9 species. The very small separation of S = 9 and 8 levels for these clusters highlights the limitations in the determination of D values from M/H data at low temperatures.

Structures and magnetic behavior of 1-, 2-, and 3D coordination polymers in the Cu(II)-dicyanamide-pyrimidine family.

Using aqueous conditions, three new coordination polymers containing Cu(2+) cations, dicyanamide (dca) anions, and pyrimidine (pym) were isolated and structurally and magnetically characterized. Comprising the bulk of the product yield, Cu(dca)(2)(pym)(2), 1, crystallizes in the monoclinic space group P2(1)/c with a = 7.3569(5) A, b = 13.4482(9) A, c = 7.4559(5) A, beta = 98.984(3) degrees, and V = 728.6(1) A and forms linear 1D chains. The second compound, Cu(dca)(NO(3))(pym)(H(2)O), 2, is also monoclinic, P2(1)/n, with a = 7.6475(3) A, b = 12.2422(5) A, c = 11.0286(4) A, beta = 106.585(2) degrees, and V = 989.6(1) A(3). A 2D network structure consisting of both bridging mu-dca and pym ligands is formed while the NO(3)(-) and H(2)O are axially bonded to the Cu center. Cu(3)(dca)(6)(pym)(2).0.75H(2)O, 3, is triclinic, Ponemacr;, with a = 7.7439(4) A, b = 9.3388(5) A, c = 10.1779(5) A, alpha = 86.014(2) degrees, beta = 88.505(2) degrees, gamma = 73.623(2) degrees, and V = 704.46(9) A(3). The structure of 3 is quite unique in that [Cu(3)(pym)(2)](6+) trimers are interconnected via mu-dca ligands affording a complex 3D self-penetrating framework. Magnetically, 1 exhibits extremely weak exchange interactions along the Cu-(dca)(2)-Cu ribbons while 2 and 3 display very strong magnetic couplings mediated by the mu-bonded pym ligands. Moreover, 2 shows a broad maximum in chi(T) at 40 K and behaves as a uniform 1D antiferromagnetic chain with g = 2.09(1), J/k(B) = -42.6(1) K, and TIP = -66 x 10(-)(6) emu/mol. An S = (1)/(2) trimer model that includes intertrimer interactions successfully described the magnetic behavior of 3, yielding g = 2.10(1), J/k(B) = -69.4(5) K, theta = -0.28(3) K, and TIP = -180 x 10(-)(6) emu/mol. It is found that mu-bonded dca and pym ligands mediate very weak and very strong exchange interactions, respectively, between Cu(2+) centers.

Analysis of eta i mode by reactive and dissipative descriptions and the effects of magnetic q and negative shear on the transport

Ion temperature gradient driven modes are studied in the short wavelength region by using a three pole reactive fluid model and an adiabatic E*B convection model, as well as the gyro-Landau fluid model. In all of the above models the parallel ion dynamics is included and the results have been compared both analytically and numerically. Furthermore, the effects of the magnetic safety factor q on the growth rate and transport coefficient chi i have been investigated. The maximum of chi i as a function of the mode number is computed, and the stabilizing effect of the negative magnetic shear has been studied

Effect of sample geometry of ceramic high Tc superconductors on the critical state and the intergranular critical current densities

To study the penetration of magnetic fields into ceramic YBCO samples and consequently local intergranular critical current densities, measurements of the complex AC susceptibility were performed, not only for different amplitudes of the AC field but also for different sample geometries. The experimental data are discussed in the framework of a modified critical state model. In the analysis it is taken into account that the position of the intergranular maximum in chi " depends on the Jc(Hi,T) function as well as on the sample geometry.

Low-field magnetic properties of CoCl2-graphite intercalation compounds

The authors report low-field (0-1200 Oe) magnetic properties of the CoCl2-graphite intercalation compounds (GIC). Using high-quality stage-1, CoCl2-GIC samples, they determine that there is only one low-temperature field-induced transition at Ht(0)=380 Oe in these compounds. They estimate that the interplanar exchange interaction J'=0.025 K, and hence the interlayer-to-intralayer exchange coupling ratio is mod J'/J mod approximately 10-3 for the stage-1 CoCl2-GIC. The Ht-T phase diagram for the stage-1 CoCl2-GIC has been mapped out using both in-plane susceptibility chi ' and c-axis resistivity rho c measurements. No field-induced transition in the susceptibility that could be ascribed to the presence of a sixfold in-plane anisotropy field H6 was observed. Zero-field susceptibility chi = chi '-i chi " measurements on CoCl2 intercalation compounds with larger c-axis repeat distances than the stage-1 compounds showed one maximum in the real component chi '(T) at Tmax, and two maxima in the imaginary component chi "(T) at Tcl and Tcu, where Tcl<Tmax<Tcu. Additionally, the magnitude of the chi '(T) maximum at Tmax is approximately 10-15 times greater than the magnitude of the maxima in chi "(T). For the stage-2 compounds they obtain mod J'/J mod approximately 10-4, representing an order of magnitude decrease with respect to the stage-1 compound.

Magnetic susceptibility of AuFe alloy films

AuFe alloys containing 2 to 12 at.% Fe are produced by quench condensation from the vapour phase onto a cold substrate. The paramagnetic susceptibility chi is measured in situ in an AC magnetic field of 22 Oe and 1100 Hz at temperatures between 2.5K and 150K. The paramagnetic Curie temperature, the effective magnetic moment and the temperature Tf of the maximum of chi depend on the Fe concentration and on annealing, Tf increases with annealing. Films annealed at 1100K show nearly the same Tf values as bulk AuFe. Most of the increase of Tf with annealing can be attributed to the rising mean free path of the conduction electrons. A rounded peak of chi is observed for films annealed at 80K; a sharper peak after annealing at 295K or 1100K. Small DC fields reduce only the susceptibility peak of films annealed at 295K or at higher temperature. A sharp peak of chi and its reduction in a DC field is apparently due to Fe clusters produced during annealing.

On the logarithmic terms of the susceptibility

It is shown that the coefficients of the T2 ln T and H2 ln mod H mod terms in the temperature, chi (T), and the field dependence, chi (H), of the susceptibility of normal paramagnetic metals cannot vanish simultaneously. All contributions to these non-analytic terms are taken into account to show (subject to a certain identification between two quantities) that the coefficients of T2 ln T and of H2 ln mod H mod are at least proportional to S and S4 respectively where S is the Stoner enhancement factor. This explains the fact that while the maximum in chi (T) is observed in so many cases, the anomaly in the Arrott plot (the 'S effect') has so far been observed only in very strongly enhanced paramagnets.

Gold-iron spin glasses in low DC fields. I. Susceptibility and thermoremanence

Detailed studies of low-field susceptibility chi (T), thermoremanence and isothermal remanence in gold-iron alloys containing 0.25, 2, 4, 7at% Fe are presented. The susceptibilities show sharp but not discontinuous reversals of slope at Tf. Below the maximum in chi (T), the zero-field cooled susceptibility shows a marked shoulder at 0.6Tf which coincides with a maximum in the incremental isothermal remanence as a function of temperature. Below Tf the total magnetisation decreases slowly, flattening out to a constant value of approximately 0.9M(Tf). Thermoremanence increases almost linearly with decreasing temperature from zero at Tf. Both total magnetisation M(T)/M(Tf) and TRM/M(Tf) follow functions of temperature which are almost independent of composition. The results suggest a finite susceptibility at T=0, approximately 0.35 chi (Tf) indicating the presence of a reversible component to the magnetisation at absolute zero. Analogies with the behaviour of rock magnetism are indicated and the variation of TRM is analysed in terms of a spectrum of blocking temperatures deduced from isothermal remanence experiments.

Microscopic Fermi liquid theory of the temperature dependence of the susceptibility of normal paramagnetic metals

A microscopic theory of Fermi liquids is used to study the temperature dependence of the susceptibility chi (T) of normal paramagnetic metals. A T2 ln T variation is proved to exist in the case of spin-independent forces. This variation agrees with Misawa's calculations based on phenomenological Landau theory. Previous theories by Stoner (1938), Beal-Monod et al. (1968) and Kawabata (1974) are critically discussed. The T2 ln T term provides an explanation for the maximum in chi (T) observed in many metals as well as for the overall temperature dependence at low temperatures. Comparison with experiments on Pd, U2C3, YCo2, LuCo2, He3 and Ni3Ga is given. The coefficient of the T2 ln T term is calculated within the 'paramagnon model'. The effects of impurities and imperfect order are studied and are found to suppress the maximum in chi (T). This provides an explanation to the observed behaviour in different palladium alloys as well as to the lack of a maximum in Ni3Ga.