Thermal transport properties, magnetic susceptibility and neutron diffraction studies of the (Cr_100-xAl_x)₉₅Mo₅ alloy system

Previous electrical resistivity (ρ) and specific heat (C_P) studies on the ternary (Cr_100-xAl_x)₉₅Mo₅ alloy system suggested that it harbours two quantum critical points (QCPs). This study reports comprehensive investigations of this alloy system through Seebeck coefficient (S), thermal conductivity (κ), magnetic susceptibility (χ) and neutron diffraction (ND) measurements in the concentration range 0 ≤ x ≤ 8.6. The results of S and χ show that spin-density-wave (SDW) antiferromagnetism is suppressed to temperatures below 2 K for concentrations in the range 1.4 ≤ x ≤ 4.4. Plots of dS/dT in the limit as T→ 2 K depict two minima, i.e. just above x = 1.4 and 4.4. This parameter has been used as a key indicator of quantum critical behaviour (QCB) in Cr alloys. Analyses against the Nordheim-Gorter relationship demonstrates a positive slope for the incommensurate (I) SDW alloys and a negative slope for the commensurate (C) SDW alloys. Extrapolations of these two slopes intercept at a concentration of 3.2 at.% Al indicating the occurrence of band structure modifications when Al is added into the Cr₉₅Mo₅ base alloy. The Lorenz number (L) for the alloys with x = 0 and 0.5 shows interesting anomalous behaviour associated with band structure effects and SDW magnetic ordering. ND measurements as a function of temperature confirm that alloys with x < 1.4 order in the incommensurate (I) SDW phase whilst alloys with x > 4.4 show commensurate (C) SDW order. Power law fits of the form T_N∝1.40−x^0.35±0.05 for the ISDW to P phase transition and T_N∝x−4.40^0.63±0.03 for the P to CSDW phase transition rendered the critical exponents 0.35±0.05 and 0.63±0.03 respectively. Overall the results of S, κ χ and ND corroborate the existence of two QCPs at x ≈ 1.4 and 4.4.