Magnetic Transition Temperature TM Research Articles

Smectic pair-density-wave order in EuRbFe4As4.

The pair density wave (PDW) is a superconducting state in which Cooper pairs carry centre-of-mass momentum in equilibrium, leading to the breaking of translational symmetry1-4. Experimental evidence for such a state exists in high magnetic field5-8 and in some materials that feature density-wave orders that explicitly break translational symmetry9-13. However, evidence for a zero-field PDW state that exists independent of other spatially ordered states has so far been elusive. Here we show that such a state exists in the iron pnictide superconductor EuRbFe4As4, a material that features co-existing superconductivity (superconducting transition temperature (Tc) ≈ 37 kelvin) and magnetism (magnetic transition temperature (Tm) ≈ 15 kelvin)14,15. Using spectroscopic imaging scanning tunnelling microscopy (SI-STM) measurements, we show that the superconducting gap at low temperature has long-range, unidirectional spatial modulations with an incommensurate period of about eight unit cells. Upon increasing the temperature above Tm, the modulated superconductor disappears, but a uniform superconducting gap survives to Tc. When an external magnetic field is applied, gap modulations disappear inside the vortex halo. The SI-STM and bulk measurements show the absence of other density-wave orders, indicating that the PDW state is a primary, zero-field superconducting state in this compound. Both four-fold rotational symmetry and translation symmetry are recovered above Tm, indicating that the PDW is a smectic order.

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.

Transport and thermodynamic properties of the novel compound Nd5CuSn3

Electrical resistance R and specific heat C were measured to study the physical properties of the novel compound Nd5CuSn3 which crystallizes in the hexagonal Hf5CuSn3-type structure. Nd ions occupy two non-equivalent sites. The values of R and C showed distinct anomalies at the magnetic transition temperature TM of 56.3 K. The increments of TM in the magnetic fields are consistent with the material being in a ferro- or ferri-magnetic state at temperatures below TM.

Low temperature Mössbauer spectroscopic studies on Sm3+ doped Zn-Mn ferrites

For the first time, we report on the low temperature Mössbauer spectroscopic study of Zn2+0.5Mn2+0.5Sm3+xFe3+2−xO4 (where x=0.01–0.05) prepared by the modified solution combustion method using a mixture of urea and glucose as a fuel. The Mössbauer spectroscopy at room and low temperatures was applied to understand the magnetic properties of the samples. The room temperature Mössbauer spectroscopy results suggest that the occupation of the octahedral sites by Sm3+ ions leads to the distortion enhancement of 57Fe nuclei environments, which leads to an increase in quadrupole splitting Δ values of D2 and D3 doublets. The low temperature Mössbauer spectroscopy results indicate that the presence of Sm3+ ions in the octahedron sites causes the decrease in the number of Fe–O–Fe chains. The transformation of Mössbauer spectra doublets into Zeeman sextets is accompanied by a significant decrease in the magnitude IM of Mössbauer spectra intensity within the 0–1.2mm/s velocity range normalized to its value at 300K. This drop in the temperature dependence of IM allows one to obtain the magnetic phase transition temperature TM from the Mössbauer experiment.

Electron microscopy, X-Ray diffraction and Mossbauer studies of PbFe0.5Nb0.5O3, PbFe0.5Ta0.5O3 and BaFe0.5Nb0.5O3 ceramics sintered from mechanoactivated nanopowders

ABSTRACTThe effect of high-energy mechanical activation on magnetic phase transition temperature TM of PbFe0.5Nb0.5O3, PbFe0.5Ta0.5O3, and BaFe0.5Nb0.5O3 perovskites has been studied. Mossbauer studies have shown that TM values in all the perovskites studied can be changed substantially by mechanical activation and subsequent annealing. The increase of TM by 40-50 K, as compared to corresponding single crystals and ceramics sintered from the non-activated powders, was achieved in the mechanoactivated powders of both lead-containing and lead-free perovskites studied.

High Magnetic Transition Temperature and Semiconductor like Transport Properties of Mn-doped α-YbAlB4

We succeeded in growing single crystals of α-YbAl1–xMnxB4 by the substitution of Mn for the Al site in the intermediate-valence rare-earth heavy fermion system α-YbAlB4. We performed specific heat and resistivity measurement on single crystals of α-YbAl1–xMnxB4 (x = 40). In the specific heat measurement on α-YbAl0.60Mn0.40B4, a sharp peak was observed at magnetic transition temperature TM = 9.7(6) K. The resistivity measurements on x = 0.40 also show a semiconductor-like behavior in the entire temperature range from 2 - 300 K. An inclination change was also observed at 10(1) K in resistivity measurement. These results indicate a magnetic order. The magnetic transition temperature is quite high compared to its temperatures observed in the other Yb-based heavy fermion compounds.

Ru-NQR Study for Novel Phase Transition in CeRu2Al10

We have performed Ru-NQR measurements on CeRu2Al10 exhibiting novel phase transition at an abnormally high temperature T0 = 27.3 K and on NdRu2Al10 with a magnetic transition temperature Tm = 2.4 K as a reference RKKY system. The splitting of the NQR line due to internal fields below T0 shows a mean-field-like monotonic increase, indicating no change in the magnetic structure below T0. The internal field strength is one order larger than those at Al sites in CeRu2Al10, and is comparable to that at the Ru site in NdRu2Al10 despite the sevenfold smaller magnitude of the 4f moment, being indicative of an enhanced conduction electron polarization at the Ru site. One of the causes of the high T0 might be the enhanced exchange coupling through the Ce–Ru–Ce path mediated by the enhanced conduction electron polarization. Being similar to that in the Al site, the nuclear spin–lattice relaxation rate 1/T1 shows a gap-like decrease below T0 without enhancement owing to a critical slowing down at T0, in contrast to t...

Effect of Fe substitution on magnetocaloric effect in metamagnetic boron-carbide ErNi2−xFexB2C compounds

The magnetic properties and magnetocaloric effect (MCE) in ErNi2−xFexB2C have been studied. Substitution of Fe for Ni lowered the magnetic transition temperature TM, and reduced the magnetic hysteresis of ErNi2B2C. An inverse MCE was observed under low magnetic field and at low temperatures, which is attributed to the nature of antiferromagnetic state for the present ErNi2−xFexB2C compounds. A normal MCE was observed under higher magnetic field changes, which is related to a field-induced first order metamagnetic transition from antiferromagnetic to ferromagnetic state. The maximum values of magnetic entropy change −ΔSMmax are 14.5, 12.7, and 10.6J/kgK with a magnetic field change of 0–70kOe for x=0, 0.1, and 0.2 in ErNi2−xFexB2C, respectively.

The Application of a New Simulation Approach to Ferrimagnetic Nanowires

The quantum simulation model and the self-consistent computational algorithm we proposed two years ago are utilized to investigate the physical properties of a magnetic nanowire consisting of 3d ions which are coupled antiferromagnetically. In the absence of the external magnetic field, all simulations are started from a spin configuration with all moments in the nanosample randomly oriented and performed from a temperature above the magnetic transition temperature TM down to very low temperature as carried out by previous researchers using the Monte Carlo method, and such obtained results are all physically reasonable, verifying the correctness of the simulation model and computing algorithm. In addition, our calculated results suggest that increasing the surface anisotropy enables an increase in the magnetic transition temperature, although less effectively than by enhancing the Heisenberg exchange strength directly.

Commensurate and incommensurate states of a spin density wave in a quasi-two-dimensional system with an anisotropic energy spectrum in an external magnetic field of arbitrary direction relative to magnetization

A theory of thermodynamic properties of a spin density wave (SDW) in a quasi-two-dimensional system (with a preset impurity concentration x) is constructed. We choose an anisotropic dispersion relation for the electron energy and assume that external magnetic field H has an arbitrary direction relative to magnetic moment MQ. The system of equations defining order parameters MQz, MQσ, Mz, and Mσ is constructed and transformed with allowance for the Umklapp processes. Special cases when H ‖ MQ and H ⊥ MQ (HZHσ = 0) are considered in detail as well as cases of weak fields H of arbitrary direction. The condition for the transition of the system to the commensurate and incommensurate states of the SDW is analyzed. The concentration dependence of magnetic transition temperature TM is calculated, and the components of the order parameter for the incommensurate phase are determined. The phase diagram (T,~x) is constructed. The effect of the magnetic field on magnetic transition temperature TM is analyzed for HZHσ = 0, and longitudinal magnetic susceptibility χ‖ is calculated; this quantity demonstrates the temperature dependence corresponding to a system with a gap for x xc. In the immediate vicinity of the critical impurity concentration (x ∼ xc), the temperature dependence of the magnetic susceptibility acquires a local maximum. The effect of anisotropy of the electron energy spectrum on the investigated physical quantities is also analyzed.

Effect of external pressure on the magnetic properties of LnFeAsO (Ln = La, Ce, Pr, Sm)

We investigate the effect of external pressure on magnetic order in undoped LnFeAsO (Ln =La, Ce, Pr, Sm) by using muon spin relaxation measurements and ab initio calculations. Both the magnetic transition temperature Tm and the Fe magnetic moment decrease with external pressure. The effect is observed to be lanthanide dependent, with the strongest response for Ln = La and the weakest for Ln = Sm. The trend is qualitatively in agreement with our density functional theory calculations. The same calculations allow us to assign a value of 0.68(2) μB to the Fe moment, obtained from an accurate determination of the muon sites. Our data further show that the magnetic lanthanide order transitions do not follow the simple trend of Fe, possibly as a consequence of the different f-electron overlap.

Magnetocaloric effect in metamagnetic borocarbide DyNi2 − xAxB2C (A = Co and Cr) compounds

The magnetic properties and magnetocaloric effect (MCE) in DyNi2−xAxB2C (A=Co and Cr, x=0.1, and 0.2) compounds have been studied. Substitution of Co or Cr for Ni lowered the magnetic transition temperature TM, and reduced the magnetic hysteresis of DyNi2B2C. An inverse MCE was observed, which is attributed to the nature of antiferromagnetic state under low magnetic field and at low temperatures for the present DyNi2−xAxB2C compounds. A normal large MCE was observed under higher magnetic field changes, which is related to a field-induced first order meatmagnetic transition from antiferromagnetic to ferromagnetic state. The values of −ΔSMmax are 16.3, 10.2 16.1, and 13.7Jkg−1K−1 with a magnetic field change of 5T for x=0.1 (Co), 0.2 (Co), 0.1 (Cr), and 0.2 (Cr) in DyNi2−xAxB2C system, respectively. The magnetic transition, the origin of large MCE as well as its potential application in DyNi2−xAxB2C system were discussed.

Effect of Nonmagnetic Ion Lu Substitution for Dy on Superconductivity and Magnetism in Antiferromagnetic Superconductor Dy1-xLuxNi2B2C Compound

A series of Dy1-xLuxNi2B2C (x=0–1) compounds have been investigated by X-ray diffraction and by examining the temperature dependences of electrical resistivity, magnetization and heat capacity. All the samples exhibit the same LuNi2B2C-type structure; and lattice parameters depend almost linearly on the Lu concentration x. The superconducting transition temperature Tc decreases rapidly with increasing x, and superconductivity disappears at approximately x=0.2, then increases with a further increase in x. The magnetic transition temperature TM and the effective magnetic moment Peff decrease gradually with increasing x. The ferrimagnetic-like behavior emerges at about x=0.2; which can be the main reason causing the suppression of superconductivity. The temperature dependence of the magnetic entropy Sm originated from Dy ions has been evaluated on the basis of heat capacity data. The saturated values of Sm(T)/(1-x) are approximately 90% of Rln 4 and the effective magnetic moment normalized by Dy concentration is almost unchanged, which indicates that the magnetic ground state of Dy3+ does not show a pronounced change.

Magnetic history dependence of the AC susceptibility of GdSr2RuCu2Oz

AbstractThe AC susceptibility of GdSr2RuCu2Oz samples has been measured as function of the temperature, at different AC field amplitudes (from 1 to 16 G) and frequencies (in the range [10‐103 Hz]), applied parallel to the major axis of the sample, and without any additional DC magnetic field. In particular we have analysed the temperature region above the Superconducting critical temperature Tc, and around the magnetic transition temperature Tm. Our attention has been focused on the differences between the curves measured after cooling the sample in zero AC field (Zero AC Field Cooling or ZAFC) and those measured after cooling in presence of a non zero AC field (AC Field Cooling or AFC). Clear differences between the ZAFC and AFC susceptibility curves appear in the temperature range between Tc and Tm, and they depend on the measuring field amplitude and frequency. Finally, indications of a spin glass behaviour in Ru‐1212 system observed from its AC magnetic response are reported (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase-transition-related crystal properties: the Neumann principle revisited

The primary properties acquirable by a crystal upon undergoing a displacive structural phase transition (DSPT) and/or magnetic and/or electric transition are defined with the aid of a Gibbs free-energy expansion, and a microscopic group-theoretical analysis of them is carried out. The properties are, apart from those which define the transitions, piezomagnetism, piezoelectricity, magnetoelectricity and piezomagnetoelectricity. It is shown that the macroscopic tensors characterizing them are sums of atomic property tensors, and, as a result, a crystal may exhibit spontaneous piezomagnetism and/or piezoelectricity below its DSPT point TD and/or spontaneous magnetoelectricity below its ferroelectric transition point TE. A crystal with a magnetic transition temperature TM<TD or TE (or one with TE<TD) may therefore be in a weak or secondary magnetic (or electric) state in the temperature region TM<T<or=TD or TE (or TE<T<or=TD). Thus it is found, in particular, that a crystal with TM<TD or TE may exhibit macroscopic properties characterized by axial c-tensors at temperatures T<or=TD or TE, and not just at T<or=TM. Among the crystals cited as being capable of going into the secondary magnetic and/or electric state are the transforming A-15 crystals. It is shown that they may exhibit not only the linear magnetoelectric effect, but also spontaneous piezomagnetism and piezoelectricity, and that their transition into the superconducting state and their high critical fields Hc may be connected with these properties. The analysis brings to the fore the limited scope of the Neumann principle.

Two-dimensional af ordering or spin-glass freezing at 2.23K in oxygen-doped Ba 2GdCu 3O 7−y

Specific heat was measured with adiabatic method for samples of Ba 2GdCu 3O 7−y with different oxygen compositions. Oxygen doping was no effects on magnetic transition temperature ( Tm) and specific heat above Tm but a little effects on specific heat below Tm. At the temperatures well below Tm, specific heat follows a law, Cm = AT s , with s ∼ 2.0, which is not compatible with 3-D antiferromagnetic or ferromagnetic ordering but with 2-D antiferromagnetic ordering or spin-glass freezing. The unaffected magnetic ordering temperature by oxygen doping indicates that the dominant magnetic interaction in these materials is the dipole-dipole interaction.