Four 52-metal-ion 3d-4f cluster complexes featuring a common core of Ln(42)M(10) (Ln = Gd3+, Dy3+; M = Co2+/3+, Ni2+) were obtained through self-assembly of the metal ions templated by mixed anions (ClO4- and CO32-). Magnetic studies revealed that the Gd42Co10 and Gd42Ni10 clusters exhibit the largest magnetocaloric effect (MCE) among any known 3d-4f complexes. Replacement of Gd3+ ions with anisotropic Dy3+ ions caused significant changes in the magnetic behavior of the clusters; both DY42Co10 and Dy42Ni10 displayed slow relaxation of the magnetization.