B y M. M. A b r a h a m *)*, J-M. B a k e r , B. B l e a n e y , A. A. J e n k i n s , P. M. M a r t i n e a u a n d J . Z. P f e f f e r Clarendon L a b o r a t o r y , Parks M easurem ents of electron m agnetic resonance are discussed for im p u rity ions in the antiferrom agnetic host com pound D y P 0 4 (Neel tem p eratu re TN = 3.39 K). This is a simple tw o sublattice an tife rro m a g n e t; the im p u rity ions are Y b 3+ and E r 3+, each w ith a K ram ers doublet as th e ground state.

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... esonance is observed a t frequencies of ca. 24 and 34 GHz, and th e angular dependence of th e m agnetic field is fitted to simple theoretical equations. Since th e m agnetic m om ents in the host lattice are ordered, th e resonance lines are relatively narrow , m aking it possible to determ ine th e value of th e intern al field acting on each im p u rity ion. From this, the exchange field is obtained by su b tractin g the value of th e dipolar field generated by the ordered m om ents of th e host ions. Some m easurem ents have been m ade in th e spin flop phase. The values of the exchange field are found n o t to conform to a simple model based on isotropic exchange interaction between th e real electron spins of the lanthanide ions. H yperfine splitting is detected from the odd isotope m Yb, n atu ra l abundance 14% , / = t Permanent address: Oak