HIGH FIELD N. M. R. IN Cu(NO3)2 2 1/2 H2O BELOW 1°K

The proton spectrum of single crystals of Cu(NOs)2-2 ) Hz0 is studied between 0.3 and 1 OK. Fields up to 47 kOe were applied along the b axis. The field and temperature dependence of the magnetization obtained from these measurements can be described by an interpair exchange coupling zJ'/k = 1.9 OK that generates a transverse ordering below a critical temperature of 0.45 OK. Introduction. Low field susceptibility measurements [ l ] and specific heat measurements [2] at low temperatures have shown that in cupric nitrate (< trihydrate B the spins of the copper ions are associated in pairs by an isotropic Heisenberg coupling. This results in a pair ground state with total spin S = 0 separated by I J Ilk x 5 OK from an excited triplet. The lowest level of this triplet is crossing the singlet at a field of about 36 kOe. Adiabatic magnetization experiments show a decrease of temperature at the point of level crossing [2, 31. Anomalies found in these experiments when the initial temperature is lower than about 0.7 OK [4] can be explained by the assumption of a weak interpair exchange interaction [5 ] . This exchange interaction also follows from a small discrepancy between magnetization isotherms in the 1-4 OK region and theoretical curves for the isolated pair model [6]. Recently several theoretical investigations are reported concerning this intcrpair exchange. According to these theories an ordering of the spin component perpendicular to the field can occur in the vicinity of the point of level crossing at sufficiently low temperatures [5, 71. When the temperature is low compared to the intrapair exchange-energy of 5 OK, as is the case in all our experiments, the upper two triplet levels of the spin pair can be neglected. If the interpair exchange is assumed to be isotropic and of the form : X' = J' C (Sil.Sjl + Si2.Sj2)