Vacancy-tuned magnetism in LaMnxSb2

The layered AMP2 (A = alkali-earth or rare-earth atom, M = transition metal, P = Sb, Bi) compounds are widely studied for their rich magnetism and electronic structure topology. We provide a detailed characterization of the magnetic and transport properties of LaMnxSb2, an understudied member of the AMP2 family. LaMnxSb2 forms with intrinsic Mn vacancies, and we demonstrate that by varying the starting ratio of La, Mn, and Sb, we can synthetically control the Mn occupancy and produce single crystals with x=0.74-0.97. Magnetization and transport measurements indicate LaMnxSb2 has a rich temperature-composition (T-x) magnetic phase diagram with physical properties strongly influenced by the Mn occupancy. LaMnxSb2 orders antiferromagnetically at T1=130-180 K, where T1 increases with x. Below T1, the T-x phase diagram is complicated. At high x, there is a second transition T2 that decreases in temperature as x is lowered, vanishing below x ≤0.85. A third, first-order, transition T3 is detected at x ≈0.92, and the transition temperature increases as x is lowered, crossing above T2 near x ≈0.9. On moving below x <0.79, we find the crystal structure changes from the P4/nmm arrangement to an I4¯2m structure with partially ordered Mn vacancies. The change in crystal structure results in the sudden appearance of two new low-temperature phases and a crossover between regimes of negative and positive magnetoresistance when x ≤0.78. Finally, we provide powder neutron diffraction for x=0.93, and find that the high-x compositions first adopt a G-type antiferromagnetic structure with the Mn moments aligned within the ab plane, which is followed upon further cooling by a second transition to a different, noncollinear structure where the moments are rotated within the basal plane. Our results demonstrate that LaMnxSb2 is a highly tunable material with six unique magnetically ordered phases, depending on T and x.