Disordered Ferromagnetic Quantum Critical Point in CePd₂P₂ with Pd → Ni substitution

We report an investigation of the structural, thermodynamic, and electrical transport properties of the chemical substitution series Ce(Pd_1-xNi_x)₂P₂. Pd → Ni substitution results in (1) contraction of the unit cell volume, which tunes the relative strengths of the Kondo and RKKY interactions, and (2) the introduction of disorder through alloying. The ferromagnetic ordering temperature T_C is monotonically suppressed with increasing x, resulting in a quantum phase transition near x_cr ≈ 0.75. In this region there is a logarithmic divergence in the electronic component of the heat capacity divided by temperature that is comparable to that seen for similar Ce-based systems near antiferromagnetic quantum critical points: e.g., CeCu₂Si₂. Together with a sub-quadratic temperature dependence of the electrical resistivity, this suggests a breakdown of Fermi-liquid behavior as would be expected near a quantum critical point. This behavior is examined in the context of the Belitz-Kirkpatrick-Vojta (BKV) theory, where the disorder causes the electronic soft modes to be diffusive and, as a result, the phase transition remains second order down to zero temperature. Based on this, we suggest that this is a model system for understanding ferromagnetic quantum criticality in a disordered metal.