Spin Fluctuation Temperature Research Articles

Temperature and field dependence of the magnetic susceptibility of TiBe2under high pressure

The magnetic susceptibility $\ensuremath{\chi}(T,P)$ of Ti${\mathrm{Be}}_{2}$ is determined at a field strength of 2.1 T as a function of both temperature from 3 to 300 K and hydrostatic pressure up to 1.3 GPa (13 kbar). At low temperatures, $\ensuremath{\chi}$ is found to decrease under pressure, the degree of depression diminishing rapidly with increasing temperature from $\frac{\ensuremath{\partial}\mathrm{ln}\ensuremath{\chi}}{\ensuremath{\partial}P}=(\ensuremath{-}9.6\ifmmode\pm\else\textpm\fi{}0.4)%/\mathrm{GPa}\mathrm{at} 3 \mathrm{K}\mathrm{to} (0\ifmmode\pm\else\textpm\fi{}0.3)%/\mathrm{GPa}\mathrm{at} 300 \mathrm{K}$. Pressure is also found to suppress strongly the low-temperature anomaly in the field dependence of $\ensuremath{\chi}(H)$ at 5.6 T. An increase in the spin-fluctuation temperature with pressure is indicated, $\frac{\ensuremath{\partial}{T}_{\mathrm{SF}}}{\ensuremath{\partial}P}\ensuremath{\simeq}+3.5$ K/GPa, where ${T}_{\mathrm{SF}}(0)\ensuremath{\simeq}24$ K, although it is not possible to account for the present results in terms of the variation of a single characteristic energy. The Stoner factor appears to decrease rapidly under pressure. The pressure dependence of the lattice parameter for Ti${\mathrm{Be}}_{2}$ at room temperature is determined up to 27 GPa, yielding the initial compressibility $K=(7.6\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ G${\mathrm{Pa}}^{\ensuremath{-}1}$. It is shown that the ferromagnetism of $\mathrm{Ti}{\mathrm{Be}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{x}$ for $x\ensuremath{\ge}0.15$ cannot be due solely to the volume expansion associated with increasing $x$. Preliminary measurements indicate that substituting Mn for Ti in Ti${\mathrm{Be}}_{2}$ causes a strong decrease in the total magnetization in analogy with previous studies involving Fe substitution. A simple qualitative model of the electronic structure of Ti${\mathrm{Be}}_{2}$ near ${E}_{F}$ is suggested. Ti${\mathrm{Be}}_{2}$ is not superconducting at temperatures above 150 \ensuremath{\mu}K.

Specific heat, magnetic susceptibility, and resistivity of anU0.85La0.15Al2alloy

The compound U${\mathrm{Al}}_{2}$ exhibits ferromagnetic spin fluctuations at low temperatures. When 15 at.% La is substituted for U in U${\mathrm{Al}}_{2}$, we find the low-temperature magnetic susceptibility increases by more than a factor of 2. We have also measured specific heat on this sample at low temperatures. We find that the electronic density of states has increased by 60% and the spin-fluctuation contribution has increased by a factor of 2. We conclude from these measurements that the Stoner enhancement factor has increased by \ensuremath{\sim}30% and the spin-fluctuation temperature has increased by \ensuremath{\sim}50% as compared with U${\mathrm{Al}}_{2}$. In addition, we have measured the electrical resistivity of ${\mathrm{U}}_{0.85}$${\mathrm{La}}_{0.15}$${\mathrm{Al}}_{2}$. We find the residual resistivity to be approximately an order of magnitude larger than that of U${\mathrm{Al}}_{2}$. We attribute this effect to the breaking of U---U bonds by the La atoms.

Magnetic property changes in epitaxial metal film sandwiches

Epitaxial metal film sandwiches (EMFS) containing Pd or Cr, have been prepared between single crystal Ag or Au. The Modified Pd/Cr show major changes in physical properties. Pd has a stretched lattice parameter in Au-Pd-Au, which combines with a tetragonal distortion to cause exchange enhancements up to 28 000 and spin fluctuation temperatures of 1–10 K. In Au-Cr-Au, Cr takes up the fcc structure, leading to superconductivity due to a high N( E F). These results are contrasted to data for Ag-Pd-Ag and Ag-Cr-Ag EMFS.

Quenching of spin fluctuations in Sc 3In by high magnetic fields

The effect of magnetic field on the low-temperature heat capacity and electrical resistivity in three Sc 3In specimens with Curie temperatures in zero field of T c(0) = 5.5, 6.0 and 6.3 K are systematically and reasonably analyzed by assuming that the characteristic spin-fluctuation temperature, T s, is equal to T c(0).

Gapless superconductivity and variation of Tc in the heavy-fermion system CeCu 2Si 2

Polycrystalline samples of CeCu 2Si 2 exhibit bulk superconductivity carried by “heavy fermions”. While usually T c ≊ (0.55 ± 0.15) K, a few samples showed considerably lower transition temperatures ( T c ⩾ 0.11 K), which could, however, be raised to usual values by proper heat treatment. We found that lower T c's are accompanied by both slightly reduced values of the spin-fluctuation temperature of the Ce 3+ ions and the phenomenon of “gapless superconductivity”. In agreement with recent results by Aliev et al. [5], a CeCu 2Si 2 single crystal did not become superconducting above 20 mK, but showed a 20% deficiency in Cu occupation.

Influence of high magnetic fields on the low temperature heat capacity of the itinerant electron ferromagnet Sc 3In

The low temperature (1.3–20 K) heat capacity of the weak itinerant electron ferromagnet Sc 3In was measured in magnetic fields up to 10 T. The measurements were made on three different samples containing 24.1, 24.3 and 24.4 at.% In with Curie temperatures at H = 0 of 5.5, 6.0 and 6.3 K, respectively. The heat capacity peak at T c becomes smaller with increasing fields and at 9.98 T the magnetic entropies are only 11 to 19% of the zero field value. Above T c the spin fluctuation contribution to heat capacity, which is enhanced by the magnetic field at low fields (<≈ 5 T), is quenched at high fields (⩾ ≈ 5 T). Our results show that the spin fluctuations in Sc 3In are almost completely quenched by the magnetic fields of ≈ 10 T, and that the characteristics spin fluctuation temperature, T s , of this itinerant electron ferromagnet is about the same as T c .

Expansion of the Laves phase TiBe 2 by addition of gallium with anomalous results

Substitution of gallium for part of the beryllium in the Laves phase TiBe 2 forms a stable cubic C15 phase over the general composition range TiBe 2-xGa x where 0 ≤ × ≤ resulting in a linear expansion of the lattice from aino = 6.4532 rA for the TiBe 2 to an a o = 6.596 Å for the limiting composition. No ferromagnetic transition is observed for measurements to 2.1 K. Low-temperature specific heat data show that the temperature-dependent T 3lnT/T SF term, related to spin fluctuations, decreases with increasing Ga content. This decrease is shown to be due to a decrease in the density of electronic states at the Fermi energy rather than a decrease in the spin fluctuation temperature.

Ti NMR Study of the Nearly Ferromagnetic System TiBe2

NMR data on Ti${\mathrm{Be}}_{2}$ have been taken from 1.3 to 270 K in magnetic fields up to 60 kG. The linear dependence of the Knight shift and the NMR linewidth with $\ensuremath{\chi}=\frac{M}{H}$ show that the increase of $\ensuremath{\chi}$ versus applied field at low temperatures is due to a homogeneous property of the electron gas. The $d$-electron contribution to the spin-lattice relaxation rate is found to scale linearly with $\ensuremath{\chi}T$ in a wide temperature range spanning the spin-fluctuation temperature.

Quenching of spin fluctuations by high magnetic fields in the heat capacity of CeSn3

The low-temperature (1.3-20.0-K) heat capacity of the mixed-valent compound Ce${\mathrm{Sn}}_{3}$ was measured in magnetic fields up to \ensuremath{\sim} 10 T. The electronic specific heat constant $\ensuremath{\gamma}$ decreases with increasing fields and at 9.98 T reaches $\ensuremath{\sim}\frac{3}{4}$ of the zero-field value. The nearly constant value of $\ensuremath{\gamma}$ for $H&gt;7.5\mathrm{T}$ suggests that the spin fluctuations are probably completely quenched by 10 T. The ${T}^{3} \mathrm{ln}T$ term of the heat capacity observed in zero field disappears in fields &gt; 2.5 T. Furthermore, the applied magnetic field induces a magnetic moment on the Ce atoms, and this leads to a magnetic contribution to the heat capacity which has a ${T}^{3}$ temperature dependence. Analysis of the heat-capacity data of Ce${\mathrm{Sn}}_{3}$ yields the many-body enhancement factor due to paramagnons (${\ensuremath{\lambda}}_{\mathrm{spin}}$) as 0.65, the Stoner enhancement factor ($S$) as 4.23, the spinfluctuation temperature (${T}_{s}$) as 5.8 K, and the effective field sufficient to quench spin fluctuations (${H}_{\mathrm{eff}}$) as 8.6 T. The field dependence of the electronic contribution to the heat capacity is compared with that predicted by two theoretical models and the agreement is fair in one case and poor in the second case.

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.

Magnetic ordering in the presence of fast spin fluctuations: A neutron scattering study of CeIn3

We report results of elastic and inelastic neutron scattering experiments in the trivalent spin-fluctuation metal Ce${\mathrm{In}}_{3}$. Antiferromagnetic order occurs at 10.23 \ifmmode\pm\else\textpm\fi{} 0.01 K. The magnetic reflections can be indexed on a doubled chemical cell of cubic symmetry. The saturated ordered moment $0.65\ifmmode\pm\else\textpm\fi{}0.1{\ensuremath{\mu}}_{B}$ per cerium atom is comparable to the value $0.71{\ensuremath{\mu}}_{B}$ expected for ordering within the ${\ensuremath{\Gamma}}_{7}$ doublet, which is the ground level expected for $J=\frac{5}{2}$ cerium moments in a cubic crystal field. The critical behavior of the order parameter is of the form $\frac{{M}_{\mathrm{st}}^{2}(T)}{{M}_{\mathrm{st}}^{2}(0)}=A{[\frac{({T}_{N}\ensuremath{-}T)}{{T}_{N}}]}^{2\ensuremath{\beta}}$ where ${M}_{\mathrm{st}}$ is the staggered magnetization, $A=2.2\ifmmode\pm\else\textpm\fi{}0.2$ and $\ensuremath{\beta}=0.42\ifmmode\pm\else\textpm\fi{}0.02$; in addition the critical fluctuations are extremely weak. We discuss this nearly mean-field behavior in the context of recent theories which describe critical behavior in systems where the critical temperature is much smaller than a characteristic spin-fluctuation temperature. The inelastic scattering measurements provide evidence for the existence of such a characteristic energy. At low temperatures the inelastic cross section is dominated by an anomalously broad magnetic scattering peak, centered near 13 meV and with a half-width of about 10 meV. We interpret the large linewidth as arising from fast spin fluctuations, which arise from strong Kondo-type exchange coupling of the $4f$ spins to the conduction electrons.

Radiation Damage Studies of USn

ABSTRACTUranium stannide, the most stable intermetallic compound in the Uranium/Sn system, has the Cu3Au(LI2 ) structure, and is paramagnetic to liquid He temperature. The magnetic susceptibility data are consistent with 5f band states rather than a localized electronic structure. Samples of this compound have been prepared both with depleted and 235U enriched, as well as 119Sn enriched starting material. Detailed 119mSn Mössbauer effect studies have been carried out over the temperature range 78≤T≤300 K on 60Co gamma ray irradiated, thermal neutron irradiated (depleted), and thermal neutron irradiated (enriched) samples, using the V 10 position of the Brookhaven HFR facility for the neutron irradiations (thermal/fast &gt;3.6×102). The Mössbauer hyperfine parameters are: IS(78) = 2.45 mm/sec, Q.S.(78) 1.38 mm/sec for the unirradiated samples, consistent with the metallic nature of Sn in the USn3 structure. No anomalous charge states have been identified in the Mössbauer spectra of the gamma or neutron irradiated samples. The free-atom lattice temperature is ∼250 K for the unirradiated sample, and shows only minor changes with 60Co–γ radiation to a total dose of 1000 Megarad. Thermal neutron radiation (nvt ∼5×1016 ) of the depleted sample results in a lowering of the lattice temperature to ∼240 K. Two hours heating at 800°C are sufficient to anneal the radiation damage as judged by lattice dynamical data. A sample enriched to 99.94% 235U and irradiated to a fluence of ∼5×1015 slow neutrons/cm2 showed an increase in the freeatom lattice temperature to ∼275 K. A two hour anneal at 800°C restored this parameter to its original value. Resistivity data for these samples and their relationship to spin fluctuation temperatures will also be discussed.

5f virtual bound states in (UxTh1-x)S and (UxTh1-x)SE alloys at low uranium concentration (metallic conductors)

The electrical resistivity and magnetic susceptibility of dilute solutions of uranium in ThS and ThSe have been measured between 4 and 300K. The magnetic susceptibilities follow the law chi = chi 0+C/(T+ theta p) with theta p approximately=140K for sulphides and 40K for selenides. The uranium contribution to electrical resistivity in the low-temperature limit is rho = rho 0(1-(T/Tf)2) with Tf approximately=160K for sulphides and 58K for for selenides. These results, which disagree with localized 5f behaviour, are well described by strong localized spin fluctuations associated to 5f(j=5/2) virtual bound states on uranium atoms. The decrease of spin fluctuation temperature Tf approximately= theta p going from sulphides to selenides is correlated to the increase in interatomic distances.

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.

Temperature dependence of the thermal conductivity in nearly magnetic metals: Application to plutonium

In nearly magnetic metals, if one takes into account the temperature dependence of the Stoner susceptibility, then the electron-par-amagnon thermal resistivity, after the low-temperature enhanced linear T behavior, goes through a maximum around the spin-fluctuation temperature and then decreases as T−1 at higher temperatures. A comparison is made with the corresponding behavior of the electron-paramagnon electrical resistivity and the thermal dependence of the Lorenz number is deduced. The theory can account for the extremely large value of the thermal resistivity of plutonium and for its decrease at high temperatures.

Hyperfine Interaction Studies of Local Moments in Metals

The aim of this article is to describe hyperfine measurements on dilute magnetic alloys. We discuss only single impurity effects. The study of a paramagnetic impurity in a non-magnetic host is split into two parts: first the weak resonant coupling corresponds to a well isolated magnetic moment, secondly the strong resonant coupling corresponds to a situation where the Kondo or the spin fluctuation temperature is an order of magnitude greater than the experimental temperature. After a brief summary of the theoretical problems and some remarks on the possibility of the radioactive measurements, we restrict the experimental review to few cases: study of the Au Yb and Cu Fe alloys by Mössbauer effect (M.E.), study of the Pt CO, Au CO and La Ce alloys by nuclear orientation experiments (N.O.).

Magnetic and electrical properties of very dilute Zn-3d alloys

Magnetic and electrical properties of very dilute (7-120 at ppm) alloys of Cr, Mn, and Fe in Zn are presented. Both Cr and Mn display local moment behaviour with logarithmic temperature dependence in the low temperature (1.5-4.2K) resistivity and a Curie-Weiss susceptibility (1.3-15K). The authors obtain peff=3.6 and 4.1 mu B, respectively, for Cr and Mn solute, and in both cases the Curie Weiss theta is about 0.5K. Non interacting Fe atoms contribute a temperature independent enhanced Pauli component to the susceptibility of dilute ZnFe alloys which corresponds to the characteristic spin fluctuation temperature of 180K.

Thermal variation of the spin relaxation rate in nearly magnetic metals

Considering either the nuclear spin resonance on the nucleus of a nearly magnetic atom or the electron spin resonance of a magnetic impurity in a nearly magnetic host, it is shown that taking into account an explicit temperature dependence of the Stoner susceptibility of the interacting electrons yields an important thermal decrease of 1/T1T around the spin-fluctuation temperature. The physical case of the nuclear spin relaxation rate of platinum is discussed.

Transport properties of local spin fluctuation systems

The dynamical susceptibility due to local spin fluctuations in transition metals has been calculated in RPA, employing a generalized Wolff model with different Coulomb interactions at host and impurity sites, modified hopping from or into the impurity sites, and including potential scattering. The electrical and thermal resistivities and the diffusion part of the thermopower have been calculated, assuming well-localized spin fluctuations. The results for resistivity and thermopower turn out to be surprisingly similar to those obtained for Kondo systems. In particular, a giant peak in the thermopower is found which is of the same magnitude and of similar shape in both systems. The deviations from the Wiedmann-Franz law are also strongly temperature dependent, with a maximum of about 16% near the spin fluctuation temperature.