Antiferromagnetic phase transition in garnet-type AgCa 2Co 2V 3O 12 and AgCa 2Ni 2V 3O 12

Abstract

Antiferromagnetic phase transition in two vanadium garnets AgCa 2Co 2V 3O 12 and AgCa 2Ni 2V 3O 12 has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T N=6.39 K for AgCa 2Co 2V 3O 12 and 7.21 K for AgCa 2Ni 2V 3O 12, respectively. The magnetic susceptibilities exhibit broad maximum, and these T N correspond to the inflection points of the magnetic susceptibility χ a little lower than T( χ max). The magnetic entropy changes from zero to 20 K per mol Co 2+ and Ni 2+ ions are 5.31 J K −1 mol-Co 2+-ion −1 and 6.85 J K −1 mol-Ni 2+-ion −1, indicating S=1/2 for Co 2+ ion and S=1 for Ni 2+ ion. The magnetic susceptibility of AgCa 2Ni 2V 3O 12 shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ eff=3.23 μ B Ni 2+-ion −1 and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa 2Co 2V 3O 12. 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 2E( t 2 6 e) and the first excited state is spin quartet state 4T 1( t 2 5 e 2) which locates extremely close to the ground state. The low spin state S=1/2 for Co 2+ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.