Nanoscale control of temperature operation ranges for magnetocaloric applications

We devised a proof-of-concept materials design that addresses the necessary requirements for magnetocaloric materials to have a constant magnetocaloric effect (MCE) over a large temperature range. For this purpose, we have fabricated epitaxial Co1-x(z)Ru x(z) films engineered to have a triangular gradient in exchange strength J along the thickness. Different from homogeneous Co1-x Ru x layers, where the maximum value of magnetic entropy change ΔS m falls rapidly with temperature away from the ferromagnetic (FM)-paramagnetic (PM) phase transition, the Co1-x(z)Ru x(z) graded structures exhibit high MCE over a large temperature range, leading to an improved cooling capacity. Theoretical modeling results confirm the enhanced temperature range and highlight a core aspect of our exchange graded materials approach, namely the ability to control and manipulate magnetism at nanoscale dimensions. As we demonstrate, this control is reliant on the fact that the temperature driven PM-FM phase transition does not occur in the entirety of the material system but only in well-defined nanoscopic regions of our samples at any given temperature, enabling us to significantly extend the useful temperature range for magneto-caloric utilization.