Magnetic interactions and high-field properties of Ag2VOP2O7: frustrated alternating chain close to the dimer limit

We report on high-field magnetic properties of the silver vanadium phosphate Ag2VOP2O7. This compound has a layered crystal structure, but the specific topology of the V–P–O framework gives rise to a one-dimensional spin system, a frustrated alternating chain. Low-field magnetization measurements and band structure calculations show that Ag2VOP2O7 is close to the dimer limit with the largest nearest-neighbor interaction of about 30 K. Highfield magnetization data reveal the critical fields μ0Hc1 ≃ 23 T (closing of the spin gap) and μ0Hc2 ≃ 30 T (saturation by full alignment of the magnetic moments). From Hc1 to Hc2 the magnetization increases sharply similar to the system of isolated dimers. Thus, the magnetic frustration in Ag2VOP2O7 bears little influence on the high-field properties of this compound. Low-dimensional and, possibly, frustrated spin systems present numerous quantum phenomena dealing with both the ground state and finite-temperature properties. An external magnetic field alters the physics of such systems and induces a number of additional features. The field causes spin excitations which are naturally treated as bosons. These bosons can undergo Bose-Einstein condensation or form ordered structures (Mott-insulating phases) [1]. The latter scenario gives rise to plateaus in the magnetization curves. The observation and interpretation of such plateaus remains one of the attractive problems related to quantum phase transitions in low-dimensional spin systems [2–4]. The formation of bosonic Mott-insulating phases (and the respective magnetization plateaus) has been predicted for a wide range of spin models including Heisenberg models for different lattice types. One of the latter models is a frustrated alternating chain, i.e., a chain with the alternating nearest-neighbour interactions J ′ 1, J ′′ 1 , and the next-nearest-neighbour interaction J2 (see the right panel of Figure 1) [5, 6]. Despite considerable theoretical effort in studying this model (see Ref. [5] and references therein), experimental results are scarce due to the lack of appropriate model compounds. To the best of our knowledge, only two materials have been suggested as frustrated alternating chain systems: the low-temperature, spin-Peierls phase of CuGeO3 [7] and Cu2(C5H12N2)Cl4 (copper 1,4-diazacycloheptane chloride, abbreviated as CuHpCl) [8,9]. However, both the compounds do not show the desired high-field physics of the Heisenberg model for the frustrated alternating chain. In case of CuGeO3, spin-lattice coupling is essential and produces the high-field soliton phase with incommensurate modulation of the ñ a ñ J 1 ’’ J 1 ’ J 1 ’