Invited Paper MI+SP+AS-ThM5
Impact of Interfacial Magnetism on Magnetocaloric Properties of Thin Film Heterostructures

Thursday, November 1, 2012, 9:20 am, Room 006

In an effort to understand the impact of nanostructuring on the magnetocaloric effect, we have grown and studied gadolinium in W(5nm)/Gd(30nm)/W(5nm)]₈ heterostructures. The entropy change associated with the second-order magnetic phase transition was determined from the isothermal magnetization for numerous temperatures and the appropriate Maxwell relation. The entropy change peaks at a temperature of 284 K with a value of approximately 3.4 J/kg K for 30 kOe field change; the full width at half max of the entropy change peak is about 70 K, significantly wider than that of bulk Gd under similar conditions. The relative cooling power of this nanoscale system is about 240 J/kg, somewhat lower than that of bulk Gd (410 J/kg). Polarized neutron reflectometry was used to determine the depth profile of the magnetic moment per Gd atom, m_Gd. Despite sharp interfaces observed by transmission electron microscopy, m_Gd is systematically suppressed near the Gd-W interfaces. Because the peak magnetic entropy change is proportional to m^2/3, the maximum achievable magnetocaloric effect in Gd-W heterostructures is reduced. By extension, our results suggest that creating materials with Gd-ferromagnet interfaces may increase the m_Gd relative to the bulk, leading to enhanced magnetocaloric properties. Together, these observations suggest that nanostructuring may be a promising route to tailoring the magnetocaloric response of materials.