Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa2Co2V3O12 and AgCa2Ni2V3O12 has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Neel temperature T-N = 6.39 K for AgCa2Co2V3O12 and 7.21 K for AgCa2Ni2V3O12, respectively. The magnetic susceptibilities exhibit broad maximum, and these TN correspond to the inflection points of the magnetic susceptibility chi a little lower than T(chi(max)). The magnetic entropy changes from zero to 20 K per mol Co2+ and Ni2+ ions are 5.31 J K-1 mol-Co2+-ion(-1) and 6.85 J K-1 mol-Ni2+-ion(-1), indicating S = 1/2 for Co2+ ion and S = 1 for Ni2+ ion. The magnetic susceptibility of AgCa2Ni2V3O12 shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment mu(eff) = 3.23 mu(B) Ni2+-ion(-1) and the Weiss constant theta = -16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa2Co2V3O12. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state E-2(t(2)(6)e) and the first excited state is spin quartet state T-4(1)(t(2)(5)e(2)) which locates extremely close to the ground state. The low spin state S = 1/2 for Co2+ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity. (c) 2005 Elsevier Ltd. All rights reserved.