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The influence of Y- and La-substitution for Ce on the competing Kondo effect and magnetic ordering, as well as on spin dynamics in the Kondo semiconductor CeRu

YFe2Al10 forms in a well-ordered crystal structure, with a unique site for the magnetic atom Fe. At elevated temperatures YFe2Al10 behaves electronically as a good metal. At low temperatures on the other hand, signatures of correlated behavior develop and eventually non-Fermi-liquid scaling dominates all of the physical properties: the electrical resistivity develops a negative temperature coefficient below 20 K, and the specific heat Cp(T)/T and χ(T) both assume a power-law increase upon lowering the temperature below ≃3 K. Lowest temperature studies have failed to find magnetic ordering in YFe2Al10 in spite of compelling evidence that there is impending order in this compound and that it is indeed of ferromagnetic character. Here we report on measurements of thermal and electronic transport as well as heat capacity studies on the series of compounds YFe2 + δAl10 with δ ≤ |0.1|, with the purpose of testing the stability of the low-temperature divergences in physical properties against off-stoichiometric Fe.

We have investigated the contribution of Ce $4f$ states to the electronic structure of the intermetallic ferromagnet ${\mathrm{CePdIn}}_{2}$ by means of x-ray absorption spectroscopy and resonant and nonresonant photoemission spectroscopy. The line shape of the Ce ${M}_{5,4}$ absorption edge reveals the localized nature of the $4f$ states, and is consistent with a predominantly 3+ ionic state for Ce ions. Fitting of the Ce $3d$ core level gives a Ce $4f$ occupation number at room temperature of 0.92, which is in good agreement with the Ce effective magnetic moment of $2.20{\ensuremath{\mu}}_{B}$ (corresponding to $\ensuremath{\sim}87%$ of the free-electron moment) as calculated from the inverse magnetic susceptibility. Moreover, the hybridization strength between $4f$ and conduction electrons is found to be $\ensuremath{\sim}180$ meV, revealing that ${\mathrm{CePdIn}}_{2}$ is a strongly hybridized system. This is consistent with the results from the analysis of the resonant valence band photoemission measurements at both the ${N}_{5,4}$ and the ${M}_{5}$ edges, showing that the Ce $4f$ states are composed of the features predicted by the single-impurity Anderson model, i.e., a broad $4{f}^{0}$ peak centered at 1.9 eV and two $4{f}^{1}$ spin-orbit states much closer to the Fermi level. The same spectra also show that the Ce $4f$ resonant spectral weight extends over a wide binding energy range, overlapping with that presumably occupied by the Pd $4d$ ligand states. This energy overlap is interpreted as a signature of the strong hybridization governing the system, which could possibly favor the emergence of long-range ferromagnetism through the indirect exchange between localized $4f$ states mediated by highly dispersive $d$ electrons.

The opening of a spin gap in the orthorhombic compounds CeT$_2$Al$_{10}$ (T = Ru and Os) is followed by antiferromagnetic ordering at $T_N$ = 27 K and 28.5 K, respectively, with a small ordered moment (0.29$-$0.34$\mu_B$) along the $c-$axis, which is not an easy axis of the crystal field (CEF). In order to investigate how the moment direction and the spin gap energy change with 10\% La doping in Ce$_{1-x}$La$_x$T$_2$Al$_{10}$ (T = Ru and Os) and also to understand the microscopic nature of the magnetic ground state, we here report on magnetic, transport, and thermal properties, neutron diffraction (ND) and inelastic neutron scattering (INS) investigations on these compounds. Our INS study reveals the persistence of spin gaps of 7 meV and 10 meV in the 10\% La-doped T = Ru and Os compounds, respectively. More interestingly our ND study shows a very small ordered moment of 0.18 $\mu_B$ along the $b-$axis (moment direction changed compared with the undoped compound), in Ce$_{0.9}$La$_{0.1}$Ru$_2$Al$_{10}$, however a moment of 0.23 $\mu_B$ still along the $c-$axis in Ce$_{0.9}$La$_{0.1}$Os$_2$Al$_{10}$. This contrasting behavior can be explained by a different degree of hybridization in CeRu$_2$Al$_{10}$ and CeOs$_2$Al$_{10}$, being stronger in the latter than in the former. Muon spin rotation ($\mu$SR) studies on Ce$_{1-x}$La$_x$Ru$_2$Al$_{10}$ ($x$ = 0, 0.3, 0.5 and 0.7), reveal the presence of coherent frequency oscillations indicating a long$-$range magnetically ordered ground state for $x$ = 0 to 0.5, but an almost temperature independent Kubo$-$Toyabe response between 45 mK and 4 K for $x$ = 0.7. We will compare the results of the present investigations with those reported on the electron and hole$-$doping in CeT$_2$Al$_{10}$.

We present magnetization, specific heat, and $^{27}\mathrm{Al}$ NMR investigations on ${\mathrm{YbFe}}_{2}{\mathrm{Al}}_{10}$ over a wide range in temperature and magnetic field. The magnetic susceptibility at low temperatures is strongly enhanced at weak magnetic fields, accompanied by a $\mathrm{ln}({T}_{0}/T)$ divergence of the low-$T$ specific heat coefficient in zero field, which indicates a ground state of correlated electrons. From our hard-x-ray photoemission spectroscopy study, the Yb valence at 50 K is evaluated to be 2.38. The system displays valence fluctuating behavior in the low to intermediate temperature range, whereas above 400 K, ${\mathrm{Yb}}^{3+}$ carries a full and stable moment, and Fe carries a moment of about $3.1{\ensuremath{\mu}}_{B}$. The enhanced value of the Sommerfeld-Wilson ratio and the dynamic scaling of the spin-lattice relaxation rate divided by $T[^{27}(1/{T}_{1}T)]$ with static susceptibility suggests admixed ferromagnetic correlations. $^{27}(1/{T}_{1}T)$ simultaneously tracks the valence fluctuations from the $4f$ Yb ions in the high temperature range and field dependent antiferromagnetic correlations among partially Kondo screened Fe $3d$ moments at low temperature; the latter evolve out of an Yb $4f$ admixed conduction band.

We present magnetization, specific heat, and (27)Al NMR investigations on YbFe2Al10 over a wide range in temperature and magnetic field. The magnetic susceptibility at low temperatures is strongly enhanced at weak magnetic fields, accompanied by a ln(T0/T) divergence of the low-T specific heat coefficient in zero field, which indicates a ground state of correlated electrons. From our hard-x-ray photoemission spectroscopy study, the Yb valence at 50 K is evaluated to be 2.38. The system displays valence fluctuating behavior in the low to intermediate temperature range, whereas above 400 K, Yb(3+) carries a full and stable moment, and Fe carries a moment of about 3.1 μB. The enhanced value of the Sommerfeld-Wilson ratio and the dynamic scaling of the spin-lattice relaxation rate divided by T[(27)(1/T1T)] with static susceptibility suggests admixed ferromagnetic correlations. (27)(1/T1T) simultaneously tracks the valence fluctuations from the 4f Yb ions in the high temperature range and field dependent antiferromagnetic correlations among partially Kondo screened Fe 3d moments at low temperature; the latter evolve out of an Yb 4f admixed conduction band.

YFe2Al10 and YbFe2Al10 are new additions to the currently investigated rare-earth (R) series of compounds RT2Al10 in which T is Fe, Ru, or Os. Unusual physical properties are drawing considerable attention to this group of compounds. Intriguingly, very small moments order in CeRu2Al10 at as high as 27 K, whereas CeFe2Al10 is a Kondo insulator with very strong hybridization of the Ce magnetic moments and no magnetic ordering at all. Here we present the first results of two further iron-based compounds YFe2Al10 and YbFe2Al10 as a timely contribution to the knowledge of the physics at work in this system. In YFe2Al10 we find signatures of correlated electrons and the low-temperature results of specific heat, magnetic susceptibility and electrical resistivity are indicative of ferromagnetic quantum criticality. In YbFe2Al10 localized f-electron spins of Yb become involved in correlations with a moderately enhanced electronic specific heat that classifies it as a non-ordered heavy-fermion compound. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have carried out muon spin relaxation (mu SR), neutron diffraction, and inelastic neutron scattering (INS) investigations on polycrystalline samples of Ce(Ru1-xFex)(2)Al-10 (x = 0, 0.3, 0.5, 0.8, and 1) to investigate the nature of the ground state (magnetic ordered versus paramagnetic) and the origin of the spin-gap formation as evident from the bulk measurements in the end members. Our zero-field mu SR spectra clearly reveal coherent two-frequency oscillations at low temperature in x = 0, 0.3, and 0.5 samples, which confirm the long-range magnetic ordering of the Ce moment with Neel temperature T-N = 27, 26, and 21 K, respectively. On the other hand, the mu SR spectra of x = 0.8 and x = 1 down to 1.4 K and 0.045 K, respectively, exhibit a temperature-independent Kubo-Toyabe term, confirming a paramagnetic ground state. The long-range magnetic ordering in x = 0.5 below 21 K has been confirmed through the neutron diffraction study. INS measurements of x = 0 clearly reveal the presence of a sharp inelastic excitation near 8 meV between 5 K and 26 K, due to an opening of a gap in the spin excitation spectrum, which transforms into a broad response at and above 30 K. Interestingly, at 4.5 K, the spin-gap excitation broadens in x = 0.3 and exhibits two clear peaks at 8.4(3) and 12.0(5) meV in x = 0.5. In the x = 0.8 sample, which remains paramagnetic down to 1.2 K, there is a clear signature of a spin gap of 10-12 meV at 7 K, with a strong wave-vector-dependent intensity. Evidence of a spin gap of 12.5(5) meV has also been found in x = 1. The observation of a spin gap in the paramagnetic samples (x = 0.8 and 1) is an interesting finding in this study, and it challenges our understanding of the origin of the semiconducting gap in CeT2Al10 (T = Ru and Os) compounds in terms of a hybridization gap opening only a small part of the Fermi surface, gapped spin waves, or a spin-dimer gap. (Less)

The low temperature state of CeRu2Al10 has been studied by neutron powder diffraction and muon spin relaxation (muSR). By combining both techniques, we prove that the transition occurring below T*~27K, which has been the subject of considerable debate, is unambiguously magnetic due to the ordering of the Ce sublattice. The magnetic structure with propagation vector k=(1,0,0) involves collinear antiferromagnetic alignment of the Ce moments along the c-axis of the Cmcm space group with a reduced moment of 0.34(2)mu_B. No structural changes within the resolution limit have been detected below the transition temperature. However, the temperature dependence of the magnetic Bragg peaks and the muon precession frequency show an anomaly around T2~12 K indicating a possible second transition.

We report the results of a comprehensive physical and magnetic property study of the new isotypic aluminides SmT₂Al₁₀ (T = Fe, Ru). These two compounds are members of a rare-earth based system which has become an exemplary case study of the interplay of magnetism and correlated electron phenomena. SmFe₂Al₁₀ and SmRu₂Al₁₀ are found to order in a putative antiferromagnetic spin arrangement at T(N) = 14.5 K and 12.5 K, respectively. Moreover, SmRu₂Al₁₀ shows a further phase transition at T(SR) = 5 K which is likely due to spin reorientation. The susceptibility of SmFe₂Al₁₀ points to a valence instability of the Sm ionic state at intermediate temperatures well above T(N). Electronic and thermal transport confirm that SmFe₂Al₁₀ undergoes an antiferromagnetic superzone gap formation below T(N), whereas SmRu₂Al₁₀ suffers a lattice anomaly driven magnetoelastic coupling at T(N). Below T(N), the physical properties of SmT₂Al₁₀ (T = Fe, Ru) are governed by magnons with an antiferromagnetic spin-wave spectrum that reveals spin-gap opening. Our findings in this work have exposed a new anomalous correlated compound in the RT₂Al₁₀ series. SmFe₂Al₁₀ has a magnetic ordered ground state in spite of an unstable valence at higher temperature. This is comparable with CeRu₂Al₁₀, which is a unique and controversial Kondo insulator that orders antiferromagnetic at T(N) = 27 K. Among the series of rare-earth RT₂Al₁₀ compounds, the presented Sm compounds are two new members with anomalously high magnetic ordering temperatures, and it is envisaged that together with the two very well studied compounds CeRu₂Al₁₀ and CeOs₂Al₁₀ our presented studies will enable a broader approach towards understanding the fascinating properties of this materials class.

We report on physical properties of the two new iron aluminides YFe 2 Al 10 and YbFe 2 Al 10 . They are members of a broader rare-earth based family of intermetallics in which unusual properties have been the subject of considerable recent interest. We argue that in both the title compounds electronic correlations are causing a non-Fermi-liquid ground state, but with different consequences for the ground state. Neither of the compounds are magnetically ordered above 0.4 K, but in YFe 2 Al 10 we find evidence from heat capacity, magnetic susceptibility and 27 Al-NMR results that point toward a low-temperature ferromagnetic instability which may be driving this compound into ferromagnetic quantum criticality.