d⁰/d¹⁰\,B^-site doping enables magnetic and magnetocaloric tuning in gadolinium double perovskites

In A₂BB^ O₆ A 2 B B ′ O 6 double perovskites (DPs) with 3 d and 5 d magnetic ions, the magnetic behavior of the B sublattice can be adjusted by selecting nonmagnetic isovalent B^ B ′ cations with d 0 or d 10 electron configuration. Here we show that d⁰/d¹⁰\,B^ d 0 / d 10 B ′ -site doping also enables magnetic tuning in 4 f 7 gadolinium DPs. We substitute Nb 5+ ( d 0 ) for Sb 5+ ( d 10 ) in the solid solution Ba 2 GdNb x Sb 1− x O 6 (0 ≤ x ≤ 1) and find tunable local magnetic correlations evolving from weak antiferromagnetic ( x = 0) to almost suppressed ( x = 0.75) to minimal ferromagnetic ( x = 1) interactions. We find that B^ B ′ -site doping tunes the superexchange between nearest-neighbor magnetic spins, enabling controlled suppression of the magnetic ordering temperature. In the x = 0.75 compound with almost no net magnetic interactions, we calculate a magnetocaloric effect with >99% of the maximum theoretical magnetic entropy for Heisenberg Gd 3+ spins at 1.8 K and 9 T. This work demonstrates that B^ B ′ -site doping tunes magnetic interactions in A₂GdB^ O₆ A 2 Gd B ′ O 6 DPs, highlighting its potential for the rational design of efficient adiabatic demagnetization refrigeration (ADR) materials.