High field level crossing studies on spin dimers in the low dimensional quantum spin system Na$_2$T$_2$(C$_2$O$_4$)$_3$(H$_2$O)$_2$ with T=Ni,Co,Fe,Mn

摘要

In this paper we demonstrate the application of high magnetic fields to studythe magnetic properties of low dimensional spin systems. We present a casestudy on the series of 2-leg spin-ladder compoundsNa$_2$T$_2$(C$_2$O$_4$)$_3$(H$_2$O)$_2$ with T = Ni, Co, Fe and Mn. In allcompounds the transition metal is in the $T^{2+}$ high spin configuation. Thelocalized spin varies from S=1 to 3/2, 2 and 5/2 within this series. Themagnetic properties were examined experimentally by magnetic susceptibility,pulsed high field magnetization and specific heat measurements. The data areanalysed using a spin hamiltonian description. Although the transition metalions form structurally a 2-leg ladder, an isolated dimer model consistentlydescribes the observations very well. This behaviour can be understood in termsof the different coordination and superexchange angles of the oxalate ligandsalong the rungs and legs of the 2-leg spin ladder. All compounds exhibitmagnetic field driven ground state changes which at very low temperatures leadto a multistep behaviour in the magnetization curves. In the Co and Fecompounds a strong axial anisotropy induced by the orbital magnetism leads to anearly degenerate ground state and a strongly reduced critical field. We find amonotonous decrease of the intradimer magnetic exchange if the spin quantumnumber is increased.