Interplay between Charge and Magnetic Orderings in YbPd

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The pressure--temperature phase diagram of an intermediate-valence compound YbPd was revealed via simultaneous ac-calorimetry and electrical resistivity measurements. Two successive structural phase transition temperatures, T_{1} = 125 K and T_{2} = 110 K, are found to decrease with increasing pressure. The magnetic ordering transition at T_{\text{N}} \sim 1.9 K decreases monotonically up to 1.7 GPa and disappears discontinuously at P_{\text{c}}\sim 1.9 GPa, where the structural phase transition at T_{2} is also suppressed. At P_{\text{c}}, enhancement of the effective mass and residual resistivity are observed. Another probable magnetic phase transition, however, is found to take place at T_{\text{ML}}\sim 0.3 K and smoothly varies even when crossing P_{\text{c}}. Structural phase transitions at ambient pressure were also studied by a single crystal X-ray diffraction measurement. Below T_{2}, a tetragonal lattice distortion with doubling of the unit cell along the c-axis is observed, and thus there are two inequivalent Yb-sites below P_{\text{c}}. These results indicate that one Yb-site, having larger valence configuration and thus smaller Kondo temperature, orders at T_{\text{N}}. The other phase transition at T_{\text{ML}} seems to couple to the crystal structure for T_{2} < T < T_{1}. YbPd is a unique system exhibiting magnetic orderings in metallic charge-ordered Yb systems.
2013-08-15