  The search for energy-efficient and environmentally friendly cooling
technologies is a key driver for the development of magnetic refrigeration
based on the magnetocaloric effect (MCE). This phenomenon arises from the
interplay between magnetic and lattice degrees of freedom that is strong in
certain materials, leading to a change in temperature upon application or
removal of a magnetic field. Here we report on a new material,
Mn$_{1-x}$Fe$_x$NiSi$_{0.95}$Al$_{0.05}$, with an exceptionally large
isothermal entropy at room temperature. By combining experimental and
theoretical methods we outline the microscopic mechanism behind the large MCE
in this material. It is demonstrated that the competition between the
Ni$_2$In-type hexagonal phase and the MnNiSi-type orthorhombic phase, that
coexist in this system, combined with the distinctly different magnetic
properties of these phases, is a key parameter for the functionality of this
material for magnetic cooling.
