Magnetic ordering with reduced cerium moments in hole-doped CeOs2Al10

Abstract

The lightly hole-doped system CeOs1.94Re0.06Al10 has been studied by muon spin relaxation and neutron diffraction measurements. A long-range antiferromagnetic ordering of the Ce-sublattice with substantially reduced value of the magnetic moment 0.18(1)µB has been found below TN = 21 K. Similar to the undoped parent compound, the magnetic ground state of CeOs1.94Re0.06Al10 preserves the anomalous direction of the ordered moments along the c-axis. The obtained result reveals the crucial difference between electron-and hole-doping effects on the magnetic ordering in CeOs2Al10. The former suppresses the anisotropic c − f hybridization and promotes localized Ce moments. On the contrary, the latter increases the hybridization and shifts the system towards delocalized non-magnetic state.PACS numbers: 75.25.-j Unusual magnetic order in the Kondo semiconductors CeOs 2 Al 10 and CeRu 2 Al 10 with an orthorhombic cage structure has attracted much attention in last few years.1-11 The order takes place at unexpectedly high temperature T N ∼ 29 K if one takes into account the large separation distance (∼ 5.2Å) between Ce ions in the structure and involvement of a very small ordered moment ∼ 0.3 -0.4µ B along the c-crystallographic direction 9,12,13 which is not the easy axis in the static susceptibility measurements above the transition temperature. In addition, a spin gap formation takes place at a temperature slightly higher T N in these materials as well as in the paramagnetic state of CeFe 2 Al 10 .7,11,14,15 The latter does not show any sign of magnetic order and the Ce ions in this compound are believed to be in the valence fluctuating regime. The mechanism providing the high ordering temperature and the gap formation has not been clarified yet. Based on optical conductivity measurements, Kimura et al.16 suggested a charge density wave as the primary instability which then induces the unusual magnetic ordering at T N . An indirect confirmation of this idea is the presence of the structural modulation with the (0, − 2 3 , 2 3 ) propagation vector revealed by electron diffraction in CeOs 2 Al 10 at 15 K.3 The critical temperature where this modulation comes in was not clarified and later X-ray and neutron diffraction measurements failed to find evidence of the symmetry lowering below and above T N . 9,11,12The reduced ordered moment found in the neutron diffraction experiments has been attributed by Strigari et al. 17,18 to the crystal field effects studied by polarizationdependent soft x-ray absorption measurements. The proposed ground state wave function provides quantitative agreement with the experimental data and has been confirmed by inelastic neutron scattering.19 The nature of the strongly anisotropic exchange parameters stabilizing the ordered state with the moments being along the caxis is however unclear so far.Very recently, a great sensitivity of CeRu 2 Al 10 and CeOs 2 Al 10 to electron-doping has been revealed. [20][21][22] In both compounds a small substitution with 4d and 5d transition metals carrying e...

Contrasting carrier doping effects in the Kondo insulatorCeOs2Al10: The influential role ofc−fhybridization in spin-gap formation

“…In this caged-type structure, the Ce atom is surrounded by a polyhedron formed by 4 Os/Ir and 16 Al atoms and forms a zigzag chain along the orthorhombic c−axis. [24,25] The refined lattice parameters and unit cell volume are given in table I. It is clear from neutron diffraction refinement that small amount of electron or hole doping does not change lattice parameters and volume drastically.…”

Contrasting effect of La substitution on the magnetic moment direction in the Kondo semiconductorsCeT2Al10(T=Ru,Os)

“…To see the magnetic Bragg peaks clearly, we plotted the difference between the 1.5 K and 35 K data for both compounds, which are shown in Fig. 4, along with their 3% Re doped counterparts (see Refs [10,25] for details of the neutron diffraction experiment of these materials). The important observation is the qualitatively similar diffraction patterns between the La and Re doped compositions.…”

“…The structural parameters for both samples are listed in Table I. A comparison of the [10,25]. To investigate the magnetic structure, we carried out neutron diffraction measurements on the D20 instrument at 1.5 K and 35 K (12 hours each temperature) on Ce 0.9 La 0.1 Ru 2 Al 10 and Ce 0.9 La 0.1 Os 2 Al 10 .…”

“…On hole-doping the spin gap and the antiferromagnetic order with an anomalous direction of the magnetic moment (i.e. moment either along c−axis or b−axis) not governed by the single ion crystal field anisotropy (this prefers moment along a−axis) survive with small ordered state moments of 0.18−0.23µ B [6,24,25]. In contrast to this, electron doping destabilizes the spin gap formation and the antiferromagnetic ordering becomes normal with moment directions along the a−axis e.g.…”

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“…3), have generated considerable interest in both theoretical and experimental condensed matter physics [28,[29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. This interest arose due to the various interesting ground states observed in this family of Ce-compounds as well as their caged type crystal structure that is important for the enhanced thermoelectric properties.…”