Magnetic Properties Tuning via Broad Range Site Deficiency in Square Net Material UCuBi.

HfCuSi-type pnictogen compounds have recently been shown to be a versatile platform for designing materials with topologically nontrivial band structures. However, these phases require strict control over the electron count to tune the Fermi level, which can only be achieved in compositions with AMPn and AMPn (A = lanthanides, M = transition metals, Pn = pnictogens P-Bi) charge distribution. While such lanthanide compounds have been thoroughly studied as candidate magnetic topological materials, their heavy element analogs with uranium and bismuth remain largely underexplored. In this report, we present the synthesis of UCuBi single crystals and study their magnetic properties. Detailed structural analysis revealed that flux-grown crystals always form as a site-deficient UCuBi composition, where varies between 0.20 and 0.64. Magnetic property measurements revealed a dependence of the magnetic coupling on the Cu site deficiency, linearly changing the Néel temperature from 51 K for UCuBi to 118 K for UCuBi. Moreover, higher Cu concentration promotes a metamagnetic transition in highly magnetically anisotropic UCuBi single crystals. We show that DFT calculations can successfully model site deficiency in the UCuSb and UCuBi systems. This work paves the way toward using the site deficiency to tune the Fermi level in more ubiquitous AMPn phases, which previously have not been considered topological candidate materials due to unfavorable electron count.