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Marked Change in the Ground State of CeRu2Al10 Induced by Small Amount of Rh Substitution

M. Sera, T. Nishioka, and K. Kindo

The recently discovered Kondo semiconductor CeT2Al10 (T = Ru, Os, Fe) has attracted much attention because of their unusual ground states. Among many Kondo semiconductors, CeRu2Al10 and CeOs2Al10 are very rare systems exhibiting antiferromagnetic (AFM) order at T0 ~ 30 K [1,2]. As has been revealed by extensive studies up to now, the AFM ordered phases in these two compounds are strange. The AFM moment (MAF) is parallel to the c-axis, which is not expected from the large anisotropy in magnetic susceptibility (χa > χc > χb) in the paramagnetic region. Here, χa is the magnetic susceptibility along the a-axis, etc. The transition temperature is relatively high considering the long distance (5.2 Å) between Ce ions. Although T0 is high, the ordered moment is as small as 0.3 ~ 0.4μB/Ce. In our previous paper, we reported the high field magnetization curve of CeRu2Al10 and CeOs2Al10 for the magnetic field (H) along the a-axis, and proposed that the unusual AFM ordered phase in this system is caused by the large hybridization between the conduction band and the nearly localized 4f shell (c-f hybridization) along the a-axis, leading to the AFM order with MAF // c [3].

Fig. 1. (a) Temperature dependence of magnetic susceptibility of Ce(Ru1-xRhx)2Al10 (x = 0, 0.05) along the a-, b-, and c-axes measured at 1 T. (b) Magnetization curve of Ce(Ru0.95Rh0.05)2Al10 under various temperatures for H // a axis. The inset shows the MH curve of 
Ce(Ru1-xRhx)2Al10 (x = 0, 0.05) at T = 1.3 K for the H // a-axis.

Fig. 2. Temperature dependence of magnetic susceptibility for H // a-, b-, and c-axes at 1 T calculated by mean field calculation using CEF level scheme obtained by Strigari et al. The solid lines represent the calculated results with the anisotropic exchange interaction of (Jaex, Jbex, Jcex) = (-7.3 K, -27 K, -40 K). The symbols are experimental results of Ce(Ru0.95Rh0.05)2Al10.

Quite recently, Kobayashi et al. have reported the marked change in the magnetic susceptibility of CeRu2Al10 by Rh substitution [Ce(Ru1-xRhx)2Al10, x = 0.1, 0.2, and 0.3]. They found in these compounds that, below ~ 150 K down to T0, χa is strongly enhanced and shows a very large decrease with decreasing temperature after showing a sharp peak at T0, and thus suggested a possible AFM order with MAF // a. To clarify the origin of the marked change in the ground state by Rh substitution, we studied the magnetic susceptibility and high field magnetization of Ce(Ru0.95Rh0.05)2Al10.

Figure 1(a) represents the temperature (T) dependence of χ of Ce(Ru1-xRhx)2Al10 (x = 0, 0.05) along the a-, b-, and c-axes measured at 1 T. In Ce(Ru0.95Rh0.05)2Al10, χa shows a large decrease with decreasing temperature after showing a sharp peak at T0 ~ 24 K and the decrease in χc below T0 observed in CeRu2Al10 originating from MAF // c disappears by Rh substitution. These results suggest that the AFM order with MAF // a occurs below T0. In Fig. 1(b), we show the magnetization (M) curves of Ce(Ru0.95Rh0.05)2Al10 under various temperatures for the H // a-axis. In Ce(Ru0.95Rh0.05)2Al10, at 1.3 K, M exhibits a spin-flop transition at ~ 13 T. M at 50 T is ~ 1.4μB/Ce, which is larger than ~1.3μB/Ce for x = 0. This suggests a more localized nature of Ce ions in Ce(Ru0.95Rh0.05)2Al10 than in CeRu2Al10. The experimental results were analyzed by mean field calculation for the two-sublattice model using the crystalline electric field (CEF) level scheme obtained by Strigari et al. and the anisotropic exchange interaction [4,5]. The calculated results of χ along the three crystal axes are shown in Fig. 2. χa in the paramagnetic region and a sharp peak at T0 are reproduced well by the calculation. χc and χb, which are nearly independent of temperature, are also reproduced. On the basis of both experimental and calculated results, we conclude that, by a small amount of Rh substitution with one extra 4d electron, the large c-f hybridization along the a-axis, leading to the unusual ground state of CeRu2Al10, is markedly suppressed and the Ce ion in a Rh-substituted sample becomes close to the normal localized magnetic ion.


References
  • [1] T. Nishioka et al., J. Phys. Soc. Jpn. 78, 123705 (2009).
  • [2] D. D. Khalyavin et al., Phys. Rev. B 82, 100405 (2010).
  • [3] A. Kondo et al., Phys. Rev. B 83, 180415 (2011).
  • [4] F. Strigari et al., Phys. Rev. B 86, 081105 (2012).
  • [5] K. Kunimori et al., Phys. Rev. B 86, 245106 (2012).
Authors
  • A. Kondo, K. Kindo, K. Kunimoria, H. Noharaa, H. Tanidaa, M. Seraa, R. Kobayashib, T. Nishiokac, and M. Matsumurac
  • aHiroshima University
  • bJapan Atomic Energy Agency
  • cKochi University