  The prospect of efficient solid state refrigeration at room temperature is
driving research into magnetic cooling engine design and magnetic phase
transition-based refrigerants. In this Viewpoint an Ashby-style map of magnetic
refrigerant properties is constructed, comparing popular materials with limits
derived from an idealised first order transition model. This comparison
demonstrates the potential for new magnetocaloric material systems to be
established through structural control and optimisation at the atomic-, nano-
and micro-scale.
