Department of Physics

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Now showing 1 - 5 of 27
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    Electronic correlation effects in LnFe2Al10 (Ln = Y, Yb)
    (Journal of the Physical Society of Japan, 2011) Strydom, A. M.; Peratheepan, P.; Sarkar, R.; Baenitz, M.; Steglich, F.
    We report on physical properties of the two new iron aluminides YFe2Al10 and YbFe2Al10. 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 YFe2Al10 we find evidence from heat capacity, magnetic susceptibility and 27Al􀀀NMR results that point toward a low-temperature ferromagnetic instability which may be driving this compound into ferromagnetic quantum criticality. DOI: 10.1143/JPSJS.80SA.SA043
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    Fluctuations and the ferromagnetic instability in YFe2Al10: the role of Fe stoichiometry
    (Physica Status, 2013) Strydom, A. M.; Khuntia, P.; Baenitz, M.; Peratheepan, P.; Steglich, F.
    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. DOI:
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    Competing 4f-electron dynamics in Ce(Ru1-xFex) 2Al10 (0≤x≤1.0): Magnetic ordering emerging from the Kondo semiconducting state
    (Physical Review, 2013) Adroja, D. T.; Hillier, A. D.; Muro, Y.; Kajino, J.; Takabatake, T.; Peratheepan, P.; Strydom, A. M.; Deen, P. P.; Demmel, F.; Stewart, J. R.; Taylor, J. W.; Smith, R. I.; Ramos, S.; Adams, M.A.
    We have carried out muon spin relaxation (muSR), neutron diffraction and inelastic neutron scattering (INS) investigations on polycrystalline samples of Ce(Ru1-xFex)2Al10 (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 muSR spectra clearly reveal coherent two-frequency oscillations at low temperature in x=0, 0.3 and 0.5 samples, which confirms the long-range magnetic ordering of the Ce-moment with TN=27, 26 and 21 K respectively. On the other hand the muSR 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 Q-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 hybridization gap opening only a small part of the Fermi surface, gapped spin waves, or a spin-dimer gap.
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    Contiguous 3d and 4f Magnetism: strongly correlated 3d electrons in YbFe2Al10
    (Physical Review, 2014) Khuntia, P.; Peratheepan, P.; Strydom, A. M.; Utsumi, Y.; Tsuei, K. D.; Tjeng, L. H.; Steglich, F.; Baenitz, M.
    We present magnetization, specific heat, and 27 Al NMR investigations on YbFe 2 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 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, Yb 3 + carries a full and stable moment, and Fe carries a moment of about 3.1 μ 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 4 f Yb ions in the high temperature range and field dependent antiferromagnetic correlations among partially Kondo screened Fe 3 d moments at low temperature; the latter evolve out of an Yb 4 f admixed conduction band. DOI:
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    Electronic, magnetic, and transport properties of the isotypic aluminides SmT2Al10 (T = Fe, Ru)
    (Journal of Physics, 2015) Peratheepan, P.; Strydom, A. M.
    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 TN = 14.5 K and 12.5 K, respectively. Moreover, SmRu2Al10 shows a further phase transition at TSR = 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 TN. Electronic and thermal transport confirm that SmFe2Al10 undergoes an antiferromagnetic superzone gap formation below TN, whereas SmRu2Al10 suffers a lattice anomaly driven magnetoelastic coupling at TN. Below TN, 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 TN = 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. DOI: