Cluster Spin Glass Formation in the Double Double Perovskite CaMnFeTaO₆

CaMnFeTaO 6 has been synthesized at 1200 °C under 10 GPa pressure. Powder neutron diffraction shows that CaMnFeTaO 6 adopts a double double perovskite structure (tetragonal space group P 4 2 / n, lattice parameters a = 7.683(3) and c = 7.685(7) Å) with cation disorder at all transition-metal sites. Magnetization measurements reveal an apparent ferro- or ferrimagnetic transition at T m1 = 51 K and a susceptibility peak at T m2 = 20 K, but no long-range magnetic order is observed by neutron diffraction down to 1.5 K. This is attributed to the formation of superparamagnetic clusters of ferrimagnetically ordered spins below T m1 that freeze into a cluster spin glass at T f = T m2 . ac magnetization measurements confirm the cluster spin glass ground state. Disorder from the substitution of Fe 2+ for Mn 2+ and Fe 3+ /Ta 5+ inversion disrupts the network of superexchange interactions leading to the cluster spin glass ground state, in contrast to other P 4 2 / n double double perovskites where long-range magnetic order is stabilized.