Spin Fluctuation Temperature Tsf Research Articles

Intermediate Valence Behavior of Yb2Cu9Al8

Intermediate valence behavior is frequently observed in materials containing Ce, Yb, Eu, Sm, or Tm. In the current work, we report synthesis and characterization of Yb2Cu9Al8 (Th2Zn17 structure type). Its intermediate valence behavior can be described by an excitation energy Eex/kB = 319 K and a spin fluctuation temperature Tsf = 60 K. The valence state of Yb is estimated to be close to 2.04 for the low‐temperature region. The valence gradually evolves to the value of 2.80 at T = 400 K. The specific heat coefficient of γexp = 59 mJ·molYb–1·K–2 indicates a moderate effective mass enhancement, together with finite density of states at the Fermi level. The latter is also confirmed by the band structure calculations.

Local magnetism and spin fluctuation behavior of extremely dilute Fe impurity in Pd-U binary alloys: An experimental and theoretical investigation

We report the magnetic behavior of isolated Fe impurity atoms in Pd1−xUx (0 ≤ x ≤ 0.12) alloys studied through measurements of the local susceptibility (χloc) and spin relaxation time for 54Fe nuclear probe using the time differential perturbed angular distribution (TDPAD) technique. With an increase of the U concentration in Pd-U alloys, the magnetic response of Fe, exhibiting Curie-Weiss type χloc(T), reflects a steady decrease of the giant magnetic moment arising from ferromagnetic polarization of host Pd-4d band electrons. Concomitantly, we find an exponential rise in the spin fluctuation temperature TSF. The experimental observation of U induced suppression of giant magnetic moment of Fe in Pd is supported by ab intio electronic structure calculations based on density functional theory. The results presented here support the idea that sign and strength of spin polarization of host conduction band electrons play a crucial role on the spin fluctuation rate and hence moment stability.

Spin fluctuation and local magnetism of isolated Fe impurities in Pd1−xVx alloys studied by time differential perturbed angular distribution spectroscopy

The magnetic moment and spin fluctuation temperature of isolated Fe impurity atoms in Pd1−xVx (0 ≤ x ≤ 0.15) alloys have been studied by time differential perturbed angular distribution (TDPAD) technique. With increasing V content in Pd matrix, a large non-linear reduction of the local magnetic moment accompanied with an exponential increase of the spin fluctuation temperature TSF has been observed. At and beyond x = 0.12, the Fe atoms are found to be nonmagnetic. As an important new feature, TSF is observed to vary quadratically with composition dependent changes in host spin polarization.

Role of nitrogen in AlNxMn3: A density functional theory study

Recently, we successfully synthesized the nitrogen-deficient manganese antiperovskites AlNxMn3 with the very high ferromagnetic Curie temperatures TC up to 818 K [Lin et al., Appl. Phys. Lett. 98, 092507 (2011)]. In order to figure out the role of nitrogen for the magnetism, in the present work, we performed a theoretical investigation on AlNxMn3 through the first-principles calculation based on density functional theory. The results show that AlNxMn3 have the ferromagnetic ground states, and the total magnetic moments in a cell are enhanced with decreasing the nitrogen concentration. Based on the calculations of band structure and density of states, we showed the influence of nitrogen concentration on magnetism for AlNxMn3: The decreasing of nitrogen concentration from ideal AlNMn3 moves the spin-down bands towards the high energy remarkably, which enhances the exchange splitting energy ΔEex; on the other hand, nitrogen deficiency reduces the Mn-N hybridizations, which makes the 3d electrons of Mn tend to occupy the spin-up states. The nitrogen deficiency reduces the Mn-N hybridization, which narrows Mn-d bands and enhances the spin polarization. Furthermore, we estimated TC of AlNxMn3, and found the calculated TC can be scaled by spin fluctuation temperature TSF, suggesting AlNxMn3 belong to the spin fluctuation system.

Spin fluctuations in the skutterudite compound LaFe4Sb12

We report transport, magnetic and thermodynamic properties of the skutterudite compound LaFe4Sb12. The basic features are a large magnetic susceptibility χ(T), and large electronic coefficient γ of the heat capacity. In particular, a T1.35, T1.7, and T-2/3 temperature dependence of the magnetic susceptibility χ(T), resistivity ρ(T), and Gruneisen parameter Γ(T), respectively, is found at low temperature. An overall understanding of these physical properties is achieved, assuming that LaFe4Sb12 is a non-Fermi liquid system close to a ferromagnetic quantum critical point, with a spin fluctuation temperature Tsf=50±15 K.

Intermediate valence behaviour of the ternary cerium–nickel–phosphide Ce20Ni42P30

We have investigated the magnetic properties and the X-ray absorption spectra of the ternary compound Ce20Ni42P30, which crystallises in the hexagonal Sm20Ni41.6P30 structure type, space group P63/m. The cerium atom adopts an intermediate valence state ν=3.19. The high-temperature dependence of the magnetic susceptibility was analysed in terms of the interconfiguration fluctuation model resulting in a characteristic spin fluctuation temperature TSF=249K and an energy difference between the corresponding 4f0 and 4f1 states Eex/k=879K.

Effect of pressure on unconventional superconductivity in CeCoIn5

Studies of a new family of heavy fermion (HF) superconductors CeMIn5 (M: transition metal) point toward the realization of unconventional superconductivity (SC): the specific heat, thermal conductivity and nuclear spin-lattice relaxation rate of CeIrIn5 and CeCoIn5 decrease as a power law of temperature instead of exponentially for T<Tc. We report on measurements of the heat capacity of CeCoIn5 at hydrostatic pressures p⩽1.5GPa. In this compound, Tc increases with increasing pressure, while the effective mass of the quasi-particles meff decreases, as indicated by the ratio C/T (T⩾Tc). As a working hypothesis, this may be interpreted as the stabilization of the superconducting state by an increase of the characteristic spin fluctuation temperature TSF (TSF∝kF2/meff). Interestingly, in CeCoIn5 the ratio ΔC/γTc≈5 is extremely large but starts to decrease rapidly with increasing p at p≳0.8GPa where Tc(p) approaches a maximum.

Influence of lattice parameter and electronic structure on the spin fluctuations in doped UAl2

UAl2 is known as a typical spin fluctuation compound. We studied samples of U1−xMxAl2 (M = Zr, Ti, Hf, Sc) in order to learn more about the parameters important for the spin fluctuations. Results of X-ray diffraction, magnetic susceptibility and specific heat measurements are presented. The specific heat data are analyzed on the basis of the paramagnon theory. We find that a smaller lattice parameter an weakens the spin fluctuations in the case of 4-valent M = Zr, Ti, Hf, visible in a lowered spin fluctuation temperature TSF and a lower γ(=limr−0C/T), while for 3-valent M  Sc we observe an enhancement. We conclude that this is due to a change in the electronic structure.

Spin fluctuation changes in Ge doped YbPd2Si2

In YbPd2Si2, the valence of Yb is very close to 3+. Ge substitution of Si induces a negative pressure effect and the valence of Yb decreases. For the low Ge concentrations studied, the spin fluctuation temperature Tsf increases and χ4f, the Yb derived 4f susceptibility, obeys the scaling law χ4f(T) = F(T/Tsf).

Heavy fermion state and pressure-induced resistive behaviour in CeRu2Si2

Pressure-induced variation in the resistive behaviour of CeRu2Si2, a Kondo-lattice system, has been studied in the temperature interval 1.5K – 300K. At low temperatures, resistivity exhibits (i) a linear temperature dependence at low applied pressures (P ⩽2.26 kbar) and (ii) a quadratic temperature dependence ▪ (T) ∼ AT2 at higher pressures (6.5 kbar ⩽ P ⩽ 16.8 kbar). The coefficient A decrease, which in turn implies that the spin fluctuation temperature Tsf increases, with increasing applied pressure. The temperature coefficient γ of specific heat C = γ T at low temperature (0.3K ⩽ T ⩽ 1.6K) is quite large (385 mJ/mole-K2) confirming the heavy fermi liquid nature of the material.

Pressure dependence of the spin fluctuation temperature of a dilute actinide alloy: (U0.06Th0.94)S

The change with pressure of the spin-fluctuation temperature Tsf of a dilute actinide alloy, (U0.06Th0.94)S, has been determined from measurements of the low-temperature electrical resistivity rho . Between 1.4 and 4.2K, rho varies as 1-(T/Tsf)2, with Tsf increasing from 142.5+or-3.3K at 1.5 kbar to 151.0+or-4.2K at 16 kbar. These results are reasonably accounted for by the Coqblin-Schrieffer model (1969) of the Kondo effect, with a pressure dependence of the exchange integral between 5f and conduction electrons: d mod J mod / mod J mod dp=2(+or-1)10-6 bar-1.

NMR and spin/charge fluctuations in intermediate-valent SmB6

Information on spin and charge fluctuations in the non-metallic intermediate-valent compound SmB6 has been obtained via 10B and 11B NMR between 2 and 300 K. Both the isotropic shift and the quadrupole coupling constant are temperature independent over this range, as expected if the fractional valence mixing is also temperature independent. Below 4.2 K the boron spin-lattice relaxation rate T1−1 varies linearly with temperature, and yields a spin fluctuation rate τs−f1∼1013 sec−1, which corresponds to a spin fluctuation temperature Tsf∼ 50 K if characteristic of the stoichiometric compound. An increase of T1−1 above 20 K indicates the onset of Sm-spin relaxation by thermal excitations. The results are also consistent with relaxation by impurity-band states associated with vacancies, but the Korringa constant obtained on this assumption is unrealistically small. Comparison between data obtained from 10B and 11B resonances yields no indication of a contribution from electric field gradient fluctuations to the relaxation; an upper bound on the charge fluctuation time τcf≲3τsf is obtained. It is shown that correlations between Sm spin and charge fluctuations, if present, do not affect the nuclear relaxation in the extreme narrowing limit.

Thermoelectric power of Ir(Fe)

Abstract Thermoelectric power data between 4.2 and 300°K are presented for Ir containing 0.5 and 1 at.% Fe and for pure Ir. Peaks similar to those found in Pd(Ni) and Rh(Fe) occur in Ir(Fe) at the spin fluctuation temperature Tsf = 28°K, independent of solute concentration. Similarities and differences among the three alloy systems are discussed.

Resistance minima in (V-Cr)Fe alloys

Localized spin fluctuations in alloys containing a small concentration of nearly magnetic solute atoms give rise to a resistivity whose temperature dependence relative to a spin fluctuation temperature Tsf is predicted to follow a universal law whether the temperature coefficient is negative or positive. It is well known that the Kondo effect is observed in Mo-rich alloys of the (Nb-Mo)Fe system, and the spin fluctuation resistivity of alloys of the equivalent 3d system (V-Cr)Fe has now been measured. Here a negative temperature coefficient is observed and Tsf is found to decrease as the Cr content is increased. Tsf is found to be sensitive to the amount of Fe dissolved, the effect of interactions between iron atoms being to stabilize the moments (reduce Tsf) and, if sufficient iron is present, to produce spin glass ordering.

Temperature dependence of the resistivity due to localized spin fluctuations. II. Coles alloys

The resistance anomaly discovered by Coles in RhFe and occurring in several similar dilute alloys is shown to be of the same nature as the Kondo anomaly. The resistivity is due to localized spin fluctuations (LSF) and is given by a universal function of the temperature, increasing as T2, T and in T to tend finally to the unitarity limit. The characteristic knee occurs at the spin fluctuation temperature Tsf and marks the onset of the logarithmic regime. Above Tsf there is no qualitative difference between the resistivity due to LSF and that caused by a well defined localized magnetic moment.