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

Abstract

The opening of a spin gap in the orthorhombic compounds CeT2Al10 (T = Ru and Os) is followed by antiferromagnetic ordering at TN = 27 K and 28.5 K, respectively, with a small ordered moment (0.29−0.34µB ) 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 Ce1−xLaxT2Al10 (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 µB along the b−axis (moment direction changed compared with the undoped compound), in Ce0.9La0.1Ru2Al10, however a moment of 0.23 µB still along the c−axis in Ce0.9La0.1Os2Al10. This contrasting behavior can be explained by a different degree of hybridization in CeRu2Al10 and CeOs2Al10, being stronger in the latter than in the former. Muon spin rotation (µSR) studies on Ce1−xLaxRu2Al10 (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 CeT2Al10.

Pr- and La-doping effects on the magnetic anisotropy in the antiferromagnetic phase of Kondo semiconductorCeRu2Al10

“…In the former, T 0 is estimated to be ∼7 K from the specific heat and electrical resistivity. The long-range order below 5 K in Ce 0.5 La 0.5 Ru 2 Al 10 was recently confirmed through the μSR experiment [40]. The temperature dependence of χ b and χ c is weak below ∼20 K. Very small temperature dependence of χ b is consistent with the AFM order with m AF b expected from the magnetic phase diagram for H b.…”

“…In Ce 0.9 La 0.1 Ru 2 Al 10 , the AFM order with m AF b is realized at the ambient pressure but m AF is easily rotated from b to c axis by a small magnitude of pressure. m AF b at the ambient pressure was directly confirmed by the neutron diffraction experiment [40]. This clearly indicates that the enhancement of the c-f hybridization makes the AFM order with m AF c more stable than that with m AF b.…”

“…Very recently, it was reported that in Re-doped CeRu 2 Al 10 , the rotation of m AF from c to b axis is induced and in Re-doped CeOs 2 Al 10 , the direction of m AF is maintained along the c axis, with a reduced magnitude of m AF [39]. In Ce 0.9 La 0.1 Os 2 Al 10 , m AF c is still maintained [40]. These results are difficult to understand simply by the variation of the c-f hybridization due to Re doping, because Re doping was considered to enhance the c-f hybridization and have a tendency to change the system into the intermediate valence regime.…”

“…The doping effects in CeRu 2 Al 10 have been investigated extensively [15,17,[32][33][34][35][36][37][38][39][40]44,46,47,55]. The doping effects often provide unexpected results, especially in the magnetic anisotropy in the AFM phase.…”

“…Furthermore, m AF is easily rotated from c to b or from c to a axis by a small magnetic field, a small amount of Ce-or Ru-site substitution and pressure, etc. [27][28][29][30][31][32][33][34][35][36][37][38][39][40]. (5) The spin gap and also charge gap appear in the AFM-ordered phase, whose origin has not yet been clarified [6,[40][41][42][43][44][45][46][47][48][49][50][51][52][53].…”