Frustrated magnetism and the effects of titanium substitution in the mixed-valence oxide BaV_(10-x)Ti_xO_(15)

摘要

The low temperature magnetic phase transitions in BaV_(10)O_(15) at ～40 K and ～25 K are investigated in detail. The topology of the V sublattice, which consists of linked V_(10) clusters formed by four tetrahedra sharing edges and corners, leads to geometric magnetic frustration and a depression of the ordering temperature characterized by the frustration index, f=|θ|/T_c～30. Specific heat data show the coexistence of both long range and short range magnetic order at low temperature. The position of a λ peak, identified with T_c for long range antiferromagnetic order, depends on the V~(2+)/V~(3+) ratio and varies from 43.0 K to 41.6 K for samples with the highest V~(2+) content but is suppressed to zero for highly oxidized samples. A broad maximum in the specific heat occurs at ～25 K and is present in all samples regardless of the level of oxidation. For samples with a well defined T_c, estimates of the entropy removal over the range 5 K to 55 K are only 11% of that expected for 8 V~(3+)(S=1) and 2 V~(2+)(S=3/2) ions per formula unit, suggesting the presence of considerable short-range order. Neutron diffraction data support the presence of long range antiferromagnetic (AF) order with T_c ～ 40 K. The magnetic structure below T_c is complex: there are five crystallographically independent vanadium ions in the unit cell, leading to 40 magnetic ions in the crystal unit cell and 80 spins per magnetic cell. Furthermore, the magnetic structure is of the multi-k type involving the two unrelated wave vectors k = (0 0 0) and k=(1/2 0 0). Zero field muon spin relaxation data show spin freezing below 30 K. ac susceptibility results are anomalous. Two peaks, at ～0 K and ～25 K are seen in χ′ but the maxima are frequency independent as is the prominent maximum in χ″ near 25 K, behavior atypical of spin glasses. Ti-doped samples, BaV_(10-x)Ti_xO_(15), show remarkable results as only 5% Ti doping is sufficient to destroy the AF long range order and induce conventional spin glass behavior in the dc and ac susceptibility, with freezing temperatures ranging from 15 K-5 K for doping levels from 5% to 50%. The frustration indices reach enormous values to f=130 for x=3. The spin glass state is quenched for 70% Ti doping and only paramagnetic behavior is seen.