Exploiting High Order Magnetic Anisotropy for Advanced Magnetocaloric Refrigerants.

Three new 3 d-4 f complexes of general formula trans-[MF(py)][LnDOTA] (M=Cr or Co, Ln=Dy or Y, py=pyridine and DOTA=tetraazacyclododecane-N,N',N'',N'''-tetraacetate) have been synthetised. The fluoride-bridged systems were designed to achieve perfect tetragonal symmetry by combining four-fold symmetric lanthanide and transition metal building blocks. From single crystal measurements, an unprecedented switch of the tetragonal anisotropy of the Dy complexes has been observed. A combination of spectroscopic observations, magnetometry measurements and ab initio calculations allowed us to pinpoint the origin of this phenomenon, which is related to the peculiar energy level structure of the complexes. Moreover, the fourfold anisotropy of the complex has been exploited to design a rotating magnetocaloric experiment showing that tetragonal anisotropy holds great potential to engineer a new class of more efficient magnetic refrigerants.