High-Resolution Synchrotron Studies and Magnetic Properties of Frustrated Antiferromagnets MAl₂S₄ (M = Mn²⁺, Fe²⁺, Co²⁺)

Abstract

Single crystals of MAl2S4 (M = Mn2+, Fe2+, Co2+) were synthesized using the chemical vapor transport method and characterized using high-resolution synchrotron X-ray diffraction to determine the structure. All the compounds have a layered slab structure separated by a van der Waals gap, which results in highly two-dimensional magnetism. Magnetic susceptibility measurements indicate spin-glass behavior below T* values of ∼2 K, ∼ 10.5 K, and ∼5 K for MAl2S4 (M = Mn2+, Fe2+, Co2+), respectively, and heat capacity measurements indicate no long-range magnetic ordering observed down to 0.4 K. These results can be attributed primarily to site disorder due to mixing of M2+ and Al3+ ions (M = Mn2+, Fe2+, Co2+) and low dimensionality with geometrical frustration also playing a role. Interestingly, MnAl2S4, with almost isotropic S = 5/2 spin, has the highest frustration parameter among the series of compounds and is a possible candidate for a two-dimensional Heisenberg spin glass system.