It is now well established that the paramagnetic-to-ferromagnetic transition in the magnetocaloric La(FeSi)13 is a cooperative effect involving spin, charge, and lattice degrees of freedom. However, the influence of this correlated behavior on the ferromagnetic state is as yet little studied. Here we measure the specific heat at low temperatures in a systematic set of LaFexMnySiz samples, with and without hydrogen, to extract the Sommerfeld coefficient, the Debye temperature, and the spin-wave stiffness. Substantial and systematic changes in magnitude of the Sommerfeld coefficient are observed with Mn substitution and introduction of hydrogen, showing that over and above the changes to the density of states at the Fermi energy there are significant enhanced d-band electronic interactions at play. The Sommerfeld coefficient is found to be 90–210mJmol−1K−2, unusually high compared to that expected from band-structure calculations. The Debye temperature determined from the specific heat measurement is insensitive to Mn and Si doping but increases when hydrogen is introduced into the system. The Sommerfeld coefficient is reduced in magnetic field for all compositions that have a measurable spin-wave contribution. These results move our understanding of the cooperative effects forward in this important and interesting class of materials significantly and provide a basis for future theoretical development.