Triangular magnetic structures have gained considerable interest due to their rich magnetic behavior and structural simplicity. These structures contain the motif of a triangle as the main structural feature, leading to geometric frustration and implicitly to degenerate magnetic ground states. Most of the previous work on triangular lattice structures was performed on simple transition metal halides or oxides. Therefore, it presents an interesting challenge for materials scientists to synthesize new class of materials that preserve the quasi-two dimensionality of the structures. This talk will feature two class of materials (1) triangular materials synthesized using high-pressure hydrothermal method (2) AREQ2 (A = Alkali metal, RE= rare earth, Q = O, S, Se) triangular magnetic materials.
First, I will focus on synthesis, magnetism and use of neutron diffraction to characterize the magnetic phase diagram of several classes of hydrothermally synthesized oxy-anions based transition metal compound series (EOyx-, E = As, Mo, Se). These linking groups can lead to an enormous array of new structure types with great potential for exploring and characterizing new emergent phenomena. Here, I will focus the role of vanadate building blocks (VO43-) in magnetically interesting transition metal layered materials. The vanadates display a rich diversity of structural behavior including multiple bridging modes such as corner and edge sharing. In addition, the presence of vacant d-orbitals in the bridging center can have a significant effect on the magnetic coupling behavior. As the first example, SrM(VO4)(OH) (Mn, Co, Ni) possess on-dimensional magnetic lattice with totally different magnetic properties depending on the transition metal cation even though they crystallizes in same space group. Further, Na2BaM(VO4)2 (M = Mn2+, Fe2+, Co2+) series all have similar chemical structures and are members of the glaserite family, but each one displays dramatically different magnetic behavior between room temperature and 2 K. Another interesting system for discussion is the mixed vanadate carbonate material A2M3(VO4)2(CO3) where A = K, Rb and M = Mn2+, Co2+. The chemical structure is quite complex and has two unique layers, one built of corner sharing vanadates and one with the trigonal planar carbonates. The material also has a complex magnetic behavior and undergoes three magnetic phase transitions between 300-2 K.
In the second part of my talk, I will focus on the synthesis of ARESe2 single crystal growth, magnetic properties and elastic and inelastic neutron scattering. These crystals crystallize in either trigonal (R-3m) or hexagonal (P63/mmc) crystal systems and associate with an ideal triangular RE3+ layers. The magnetic properties and heat capacity of these compounds were characterized down to 0.4 K. The Yb-compounds exhibit a broader peak in heat capacity ~10 K suggesting short range ordering.