Magnetic-field induced gap and staggered susceptibility in the S = 1/2 chain [PM·Cu(NO3)2·(H2O)2]n(PM = pyrimidine)

Abstract

Single-crystal magnetic susceptibility and specific heat studies of theone-dimensional copper complex[PM·Cu(NO3)2·(H2O)2]n (PM = pyrimidine) show that it behaveslike a uniform S = 1/2 antiferromagnetic Heisenberg chain, characterized bythe exchange parameter J/kB = 36 K. Specific heat measurements in theapplied magnetic field, however, reveal the formation of a field-induced spinexcitation gap, whose magnitude depends on the magnitude and direction of thefield. This behaviour is inconsistent with the ideal S = 1/2 Heisenberg chain.In the low-temperature region, a contribution to the susceptibility,approximately proportional to 1/T, is observed which varies strongly withthe varying direction of the magnetic field. The field-induced gap and the1/T contribution are largest for the same field direction. Previousobservations of a field-induced gap in the related compounds copper benzoateand Yb4As3 have been explained by the alternating g tensor andalternating Dzyaloshinkii-Moriya interaction, producing an effectivestaggered magnetic field at the Cu and Yb ions. We apply this model to [PM·Cu(NO3)2·(H2O)2]n and obtain aconsistent quantitative explanation of the low-temperature susceptibility, thefield-induced gap and their dependence on the magnetic-field direction.