We report specific heat (C) and magnetization (M) of single crystalline Ce4Pt12Sn25 at temperature down to 50 mK and in fields up to 3 T. C/T exhibits a sharp anomaly at 180 mK, with a large jump in a Sommerfeld coefficient γ C/T of Δγ 30 J/molK2-Ce, which, together with corresponding cusp-like magnetization anomaly, indicate antiferromagnetic (AFM) ground state with Nel temperature TN 0.18 K. Numerical calculations based on Heisenberg model reproduce well zero field specific heat data, and point to a very small Kondo scale TK, clearly placing Ce4Pt12Sn25 in the weak exchange coupling J < Jc limit of the Doniac diagram. Magnetic field suppresses AFM state at H* 0.7 T, much more rapidly than indicated by theoretical calculations. © Published under licence by IOP Publishing Ltd.

Many superconducting materials allow the penetration of magnetic fields in a mixed state in which the superfluid is threaded by a regular lattice of Abrikosov vortices, each carrying one quantum of magnetic flux. The phenomenological Ginzburg-Landau theory, based on the concept of characteristic length scales, has generally provided a good description of the Abrikosov vortex lattice state. We conducted neutron-scattering measurements of the vortex lattice form factor in the heavy-fermion superconductor cerium-cobalt-indium (CeCoIn5) and found that this form factor increases with increasing field-opposite to the expectations within the Abrikosov-Ginzburg-Landau paradigm. We propose that the anomalous field dependence of the form factor arises from Pauli paramagnetic effects around the vortex cores and from the proximity of the superconducting state to a quantum critical point.