Spin Fluctuation Temperature Research Articles

Magnetic Gruneisen parameters in chromium

Gruneisen parameters are defined and determined for Cr in the paramagnetic phase, at and below the Neel temperature, at low temperatures and at zero temperature, by comparing the temperature dependence of the thermophysical properties, as well as from direct pressure measurements (the latter also for dilute antiferromagnetic alloys of Cr with V or Mo). The values are all large and negative, and in particular in the paramagnetic phase the logarithmic volume dependence of the characteristic spin fluctuation temperature is 155.

151Eu-Mossbauer investigations in a new Eu-based mixed valent material EuIr2Si2

151 Eu-Mossbauer and magnetic susceptibility measurements on EuIr 2 Si 2 suggest that this material is an intermediate valent system. Variation of average valence V is very much pronounced compared to EuCu 2 Si 2 ; V = 2.3 and 2.8 at 300K and 4.2K respectively. Excitation energy, E ex = ( E 2+ g − E 3+ g ) and the spin fluctuation temperature, T sf , are strongly temperature dependent.

Mixed Valence Properties of CeM2Si2 (M  Mn, Fe, Co, Ni, Cu)

AbstractLattice parameter measurements in the temperature range 12 to 300 K and LIII edge measurements in the range 4.2 to 300 K are performed on the compounds CeM2Si2 (M  Mn, Fe, Co, Ni, Cu) to study the valence changes. Magnetic measurements (1.6 to 600 K) for fields up to 1 T confirm antiferromagnetism of CeMn2Si2 (TN ≈ 370 K) and show enhanced Pauli paramagnetism of mixed valence compounds with M  Fe, Co, Ni with spin fluctuation temperatures around 150 K with Co, Ni. Transport properties measurements show that the compounds with M  Co, Ni, Fe do not become superconducting down at 18 mK and that a Fermi liquid behaviour exists in the range 40 to 70 K.

Spin-fluctuation effects in CeSix alloys: High-field heat-capacity, magnetic susceptibility, and electrical resistivity studies, and a comment on TiBe2.

The low-temperature (1.5--20 K) heat capacity in zero and applied fields up to 9.8 T, the magnetization in a field of 1.8 T in the range 1.5--300 K, the low-field ac magnetic susceptibility (50 mK--10 K), and electrical resistivity (2.4--300 K) have been measured for ${\mathrm{CeSi}}_{\mathrm{x}}$ (x=1.83, 1.85, and 1.90) alloys. A contribution of the form ${T}^{3}$lnT to the heat capacity, predicted for spin-fluctuation compounds by the paramagnon theory, occurs for all three compounds. In ${\mathrm{CeSi}}_{1.83}$ and ${\mathrm{CeSi}}_{1.85}$ the deviation from ${T}^{3}$lnT behavior occurs below approximately 8 and 2.4 K, respectively, due to the onset of magnetic order at 5.5 and 1 K, respectively. The transition of the spin-fluctuation regime to a magnetically ordered state in these two alloys is reported for the first time. In ${\mathrm{CeSi}}_{1.83}$ the entropy associated with magnetic ordering accounts for approximately 17% of the theoretically expected R ln2 entropy; the rest is presumably associated with the spin fluctuations. Spin fluctuations in ${\mathrm{CeSi}}_{1.90}$ are not quenched in fields up to 7.5 T, but the data suggest that the quenching may be starting at 10 T. The heat capacity in applied fields develops a bump which is presumably due to the development of a magnetic moment induced by the field. A similar such bump in ${\mathrm{TiBe}}_{2}$ may also occur. A gradual progression from paramagnetic spin or valence fluctuations to magnetically ordered Kondo regime takes place with decreasing silicon concentration. An analysis of these data indicate that the ${\mathrm{CeSi}}_{\mathrm{x}}$ alloys are moderately heavy fermions with \ensuremath{\gamma} values of 184.6--269.6 mJ/mol Ce ${\mathrm{K}}^{2}$, ${T}_{s}$ (spin fluctuation temperature) values of 28 and 29 K, and Stoner enhancements varying from 135 to 645.

Nuclear magnetic resonance studies in rare earth ternary phosphides

The results of31P Knight shift (KS) and spin-lattice relaxation time (T1) measurements in the temperature interval 4.2–300 K are reported for the compounds RENi2P2(RE=Ce, Eu, Yb) in order to understand the nature of the ground state in these compounds. KS results conclusively establish that all these compounds exhibit non-magnetic ground states. The temperature dependence of spin-fluctuation temperature (Tsf) in each case is estimated from the measured data. For EuNi2P2 the values ofTsf above 77 K qualitatively agree with those obtained from Mossbauer and susceptibility data employing ionic interconfigurational fluctuation model, but disagree at lower temperatures.

Magnetism changes in Ge and Os substituted CeRu2Si2

In Ge substituted CeRu2Si2, normal chemical pressure effects push the system from the parent heavy fermion towards antiferromagnetism to ferromagnetism. On the other hand, with Os substitution, electronic effects dominate and the system becomes less magnetic and the spin fluctuation temperature increases. This can be understood as consequences of changes in (Ce-Si) and (Ce-M) overlapping.

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).

Magnetoresistivity of the spin-fluctuation materials TiBe2 and UAl2.

High-field magnetoresistivity (MR) measurements are presented for high-purity samples of ${\mathrm{TiBe}}_{2}$ and ${\mathrm{UAl}}_{2}$. The MR of ${\mathrm{TiBe}}_{2}$ gives strong evidence for scattering due to spin fluctuations. The peculiar features in the MR around 5 T are apparently connected to the maxima in the susceptibility and may provide a clue to a theoretical description of the contributions of spin fluctuations to the resistivity. A value for the spin-fluctuation temperature is obtained and it is in agreement with earlier estimates. The MR of ${\mathrm{UAl}}_{2}$ is consistent with the behavior of a compensated metal. From the high-field behavior of the MR we expect to observe de Haas--van Alphen oscillations above approximately 10 T, but up to 40 T no signals are found. A small negative MR at higher temperatures may be due to spin-fluctuation scattering, but the evidence is not as clear as for ${\mathrm{TiBe}}_{2}$.

Investigation of 4f-magnetism in CeNi 2P 2, EuNi 2P 2 and YbNi 2P 2 by susceptibility and NMR studies

The results of 31P NMR Knight shift (KS) and spin-lattice relaxation time (T 1) as well as magnetic susceptibility ( ξ) measurements in the temperature interval 4.2–300K are reported for the compounds RENi 2 P 2( RE = Ce, Eu and Yb) in order to understand the nature of the 41 magnetism in these compounds. The KS results conclusively establish that all these compounds exhibit non-magnetic ground states. There is a low-temperature upturn in ξ for CeNi 2P 2 and YbNi 2P 2 presumably due to magnetic impurities. The estimated spin fluctuation temperatures (T sf) for EuNi 2P 2 above 50K agree qualitatively with those obtained earlier from Mössbauer and ξ data employing ionic interconfigurational fluctuation model, but seem to disagree at low temperatures. Implications of this result are discussed.

Spin fluctuations in RuFe alloys with Si impurities

Resistivity studies on spin fluctuating (Ru 1− x Si x ) 98.93Fe 1.07 alloys are presented over a wide range of temperature between 50 mK and 300 K. Analyses of the incremental resistivity arising from Fe atoms indicate that the spin fluctuation temperatures in these systems are decreased by the presence of Si impurities over the pure Ru Fe ( x=0 ) value. The same trend is also observed for the superconducting transition temperature which decreases to 53 mK for the 3.5 at.% alloy from the T c value of 263 mK for the pure Ru Fe alloy.

Conduction Electron Scattering in Alloys

The aim of this review is to present a simple physical picture which shows how the electrical resistivity of a system depends upon the spatial extent and lifetime of the scattering disturbance measured in relation to the conduction electron mean free path and relaxation time. The contribution from spin fluctuations associated with isolated magnetic impurities is discussed on the basis of this model and it is shown that at temperatures below the characteristic spin fluctuation temperature the impurity acts as though it were nonmagnetic. Some results are given for both 'Kondo' (Anderson) and exchange enhanced (Wolfi) systems. Spin glasses are also discussed and the resistivity behaviour is shown to result from a competition between the RKKY interaction and spin fluctuation effects. Ordered magnetic clusters are shown to be static for periods comparable with the conduction electron relaxation time, so that there is no resistivity anomaly expected at the superparamagnetic blocking temperature. The observed temperature dependence of the resistivity then follows simply from the change in magnetic correlations within the cluster.

Dilute PdNi-a homogeneous magnetic system of fluctuating moments

It is shown that published magnetic susceptibility and magnetisation data for PdNi alloys are consistent with a homogeneous model in which the Ni atoms have a spin fluctuation temperature Ts approximately 55K and are coupled by ferromagnetic Ni-Ni interactions. Using a molecular-field model the equivalent classical RMS moment per Ni atom square root ( mu 2) is shown to be concentration independent for concentrations between 1.3 and 16.7 at.% Ni and for temperatures between the ordering temperature Tc and 700K. square root ( mu 2) exhibits a temperature-dependent exchange enhancement at high temperatures, and at low temperatures falls to zero due to intrinsic spin fluctuations. The exchange enhancement and the ferromagnetic coupling between Ni moments are shown to be related in accordance with the indirect-coupling model of Takahashi and Shimizu (1965). The critical concentration for the onset of ferromagnetism and the concentration dependence of the ordering temperature Tc are determined from the curve for square root ( mu 2) and are shown to be in excellent agreement with experiment. A simple model is presented which qualitatively reproduces the magnetisation curves for paramagnetic and ferromagnetic alloys.

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.

Spin-fluctuation effects in the resistivity of Fe-Zr metallic glasses.

We report for the first time clear evidence of spin-fluctuation effects in the resistivity of a metallic glass. The system studied is Fe-Zr close to, and on either side of, the critical concentration ${x}_{c}$ for ferromagnetism. A strong positive anomaly, which reaches its greatest extent around ${x}_{c}$, is seen superposed on a nonmagnetic, monotonically decreasing background. The data may be scaled into a universal curve and fitted at low temperatures by the universal expression of Kaiser and Doniach, yielding spin-fluctuation temperatures which pass through a minimum at ${x}_{c}$. .AE

Pressure-induced variation in the resistive behaviour of the Kondo-lattice system CeRu2Si2

Abstract Pressure-induced variation in the resistive behaviour of CeRu 2 Si 2 , a Kondo-lattice system, has been studied in the temperature interval 1.5–300 K. At low temperatures, resistivity exhibits (i) a linear temperature dependence for low applied pressures ( P T ) ∼ AT 2 at higher pressures (6.5 kbar P A decreases, which in turn implies that the spin fluctuation temperature T sf increases, with increasing applied pressure.

Effect of pressure on spin fluctuations and superconductivity in heavy-fermion UPt3.

We have determined the effect of hydrostatic pressure on the susceptibility, on the ${T}^{2}$ temperature dependence of the spin-fluctuation resistivity, and on superconductivity in ${\mathrm{UPt}}_{3}$. The spin-fluctuation temperature ${T}_{s}$, derived from the slope of resistivity versus ${T}^{2}$, is used within a Fermi-liquid picture to calculate the susceptibility \ensuremath{\chi} at T=0 K. The depression of this calculated \ensuremath{\chi} with pressure agrees with the directly measured value \ensuremath{\partial} ln\ensuremath{\chi}/\ensuremath{\partial}P=-24 ${\mathrm{Mbar}}^{\mathrm{\ensuremath{-}}1}$. Both the superconducting transition temperature ${T}_{c}$ and the initial slope of the upper critical field also decrease under pressure. We find that \ensuremath{\partial} ln${T}_{c}$/\ensuremath{\partial}P=-25 ${\mathrm{Mbar}}^{\mathrm{\ensuremath{-}}1}$ and speculate upon correlations between \ensuremath{\chi} and ${T}_{c}$.

Influence of spin fluctuations on the electrical resistance ofUAl2andUPt3at high pressures and low temperatures

Influence of spin fluctuations on the electrical resistance of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">UAl</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">UPt</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>at high pressures and low temperatures

Intermediate valence and Kondo effect in Ce 1−xSc xAl 2

Results of lattice parameter, thermal expansion, thermopower and magnetic susceptibility measurements are reported for Ce 1 − x Sc x Al 2, a system exhibiting a solubility gap over a wide composition range. Two isostructural phases are identified within the gap. For the Sc-rich systems, both phases contain intermediate valent (IV) Ce ions, whereas in the Ce-rich systems ( x < 0.5), a low-volume IV phase coexists with a high-volume phase containing Ce 3+ ions. The temperature-induced overall valence change of the IV ions amounts to 15–20%. A factor-of-three increase of the spinfluctuation temperature is found upon increasing the Sc concentration from x = 0.5 to 0.95. For the Ce-rich systems, we have discovered a hitherto not known “Kondo anomaly” in the temperature dependence of the thermal expansivity.

Quenching of spin fluctuations in the highly enhanced paramagnetsRCo2(R=Sc,Y,orLu)

The low-temperature (1.3-20.0 K) high-magnetic-field (0-10 T) heat capacity and the magnetization and magnetic susceptibility (1.7-300 K) of the strongly Pauli paramagnetic $R{\mathrm{Co}}_{2}$ ($R=\mathrm{S}\mathrm{c},\phantom{\rule{0ex}{0ex}}\mathrm{Y},\phantom{\rule{0ex}{0ex}}\mathrm{o}\mathrm{r}\phantom{\rule{0ex}{0ex}}\mathrm{L}\mathrm{u}$) compounds with the Mg${\mathrm{Cu}}_{2}$-type structure were measured. The heat-capacity results for Sc${\mathrm{Co}}_{2}$, Y${\mathrm{Co}}_{2}$, and Lu${\mathrm{Co}}_{2}$ show that the electronic specific-heat constant decreases with increasing magnetic fields (by 7%, 4%, and 10%, respectively, at 10 T). For Y${\mathrm{Co}}_{2}$ the coefficient of the ${T}^{3}$ term ($\ensuremath{\beta}$) in the heat capacity is found to increase by 18% at 10 T, but for Sc${\mathrm{Co}}_{2}$ and Lu${\mathrm{Co}}_{2}$ $\ensuremath{\beta}$ remains constant within experimental error. Analyses based on several theoretical models of the quenching of spin fluctuations by high magnetic fields suggest that the characteristic spin-fluctuation temperature is \ensuremath{\sim}20 K for Sc${\mathrm{Co}}_{2}$, \ensuremath{\sim}35 K for Y${\mathrm{Co}}_{2}$, and \ensuremath{\sim}16 K for Lu${\mathrm{Co}}_{2}$. The magnetization and the field dependence of the magnetic susceptibility of the same samples as used in the heat-capacity measurements indicate the presence of ferromagnetic impurities in the samples, but the estimated concentrations are sufficiently low that they probably have no effect on the observed heat capacities. Maxwell's thermodynamics relationship between the field dependence of the heat capacity and the temperature dependence of the magnetic susceptibility has been examined.

Magnetoresistance of Sc 3In

The magnetoresistance of the weak itinerant-electron ferromagnet Sc 3In was measured from 1.5 to 40 K in magnetic fields up to 14 T. The measurements were made on three samples containing 24.1, 24.3 and 24.4 at% In with Curie temperatures at H = 0 of T c(0) = 5.5, 6.0 and 6.3 K, respectively. By assuming that the characteristic spin-fluctuation temperature is equal to T c(0), the negative magnetoresistance in Sc 3In was systematically and reasonably analyzed in terms of the quenching of spin fluctuations by magnetic fields.