ИСТИНА

The dugganites being a Te-based subgroup of langasites represent quite versatile family of compounds. The dugganite itself, Pb3TeZn3As2O14, is non-magnetic, but its Co-based counterparts contain magnetically active ions in C-sites. There are three members in this subgroup of dugganites, namely Pb3TeCo3P2O14 (PTCP), Pb3TeCo3V2O14 (PTCV) and Pb3TeCo3As2O14 (PTCA). The crystal structure of V-based and P-based compounds is monoclinically distorted with P121 space group. In the crystal structure of dugganites, both large CoO4 and small (V, P, or As)O4 tetrahedra constitute slightly corrugated layers with a triangular motif in the arrangement of magnetically active Co2+ ions. Another layer is organized by PbO10 decahedra and TeO6 octahedra. The magnetism in Co-based dugganites is due to Co solely since the V ions in D-sites are in pentavalent state V5+, S = 0. In PTCV an antiferromagnetic ordering takes place through succession of second-order phase transition at TN1 = 8.9 K and first-order phase transition at TN2 = 6.3 K. The specific magnetic phase diagram of PTCV was discussed recently. PTCP orders antiferromagnetically at TN = 12.9 K. The magnetic structure of this compound was indexed with propagation vector q = (1/3, 1/3, 1/2) indicating that there are ferromagnetic triangular cobalt clusters coupled antiferromagnetically both in the ab-plane and along the c-axis. To establish the magnetic phase diagrams of PTCP and PTCA dugganites was the primary goal of the present work. It was realized in measurements of thermodynamic properties over wide ranges of magnetic field and temperature on polycrystalline samples. The magnetization M(B,T) and specific heat C(B,T) measurements were done using “Quantum Design” PPMS-9T. We also perform the M(B,T) measurements up to 25 T in the pulse magnetic field regime. The parameters of magnetic subsystem are very close for PTCP and PTCA samples. These parameters are listed in the Table I and compared with parameters of PTCV. The values of effective magnetic moment are in good agreement with theoretical estimation for the Co2+ in high-spin state S = 3/2. Low-frequency electron spin resonance (ESR) studies were carried out using an X-band ESR spectrometer CMS 8400 (ADANI) (f ≈ 9.4 GHz, B ≤ 0.7 T) equipped with a low temperature mount, operating in the range T = 5 – 300 K. The effective g-factors of our samples have been calculated with respect to a BDPA reference.