Hyperfine Interactions 31 (1986) 339-343 339 /SR STUDY OF ANTIFERROMAGNETIC PHASE- TRANSITIONS IN DYSPROSIUM AND HOLMIUM S.G. BARSOV, A.L. GETALOV, G. GREBINNIK, I.I. GUREVICH, B.E KIRILLOV, A.I. KLIMOV, S.P. KRUGLOV, L.A. KUZMIN, A.B. LAZAREV, S.M. MIKIRTYCHYANTS, N.I. MOREVA, B.A. NIKOLSKY, A.V. PIROGOV, A.N. PONOMAREV, V.I. SELIVANOV, G.V. SHCHERBAKOV, S.N. SHILOV, V.A. SUETIN and V.A. ZHUKOV RP.. Konstantinov Leningrad Nuclear Physics Institute, Academy of Science of the U.S.S.R., Gatchina, Leningrad district, 188350, U.S.S.R. I.V. Kurchatow Institute of Atomic Energy, Moscow, 123182, U.S.S.R. It is shown that the temperature dependence ]i (T) of the muon spin relaxation rate in the paramagnetic state of dys- prosium and holmium does not reveal any singularity at T->TN, where TN is the Neel temperature. This result contradicts the assumption of the second-order antiferromagnetic phase-tran- sition in these metals and is in good agreement with computa- tions within the renormalization group method. Computations with the renormalization group method lead to a considerable limitation of the possible second-order phase transitions. This result stimu- lates a more detailed experimental stydy of phase transitions which were traditionally considered as second-order ones. In the present study the phase transition from the paramagnetic into the antiferromagnetic state of dyspro- sium and holmium were investigated. The Neel temperatures for dysprosium and holmium are 180 K and 130 K, respectively. The dysprosium and holmium samples used were composed from 6 disks 35 mm in diameter and 5 mm thick. The disks underwent deformation annealing which lead to the increase of the individual grains up to 3-10 mm, and their hexagonal axes become oriented predominantly perpendicular to the disk base plane. The measurements were performed in zero external magnetic field for two orienta- tions of the dysprosium sample corresponding to the angles e(= 0 and =~-- between the muon polarization direction and the perpendicular to the dis~k plane, and for one holmium orientation corresponding to the angle ~- xr The experimental asymmetry coefficient a of the positron angular d~s~ibu- tion in the u->e decay and the muon spin relaxation rate Jkwere measured. It was assumed that the muon spin relaxation occurs exponentially as P(t)=e-~t The experiment confirms such a form of P(t). Fig. 1 gives the temperature dependence a(T) for dysprosium at T=100 - 300K. The asymmetry coefficients are for Dy: a 0 = 0.229 (i) T>TN, o~ = 0; ~=~ a I = 0.086 (2) T<TN, ~=~/Z (i) a 2 = 0.058 (2) T<TN, ~-- 0 The coefficient a decreases at T<TN due to the unobservably rapid muon preces- sion in the strong local magnetic fields (B>I0 kOe) directed perpendicular to the crystallographic c-axis. The measured values of a0, aI and a 2 agree with the relation 2al/a + a2/a . . . . . =i which must work for the case were the fields Bu on the muon at T<~, are ~irected prependlcular to the c-axls. The slmllar dependence a(T) for ~olmium measured at ~= IT/~ is given in Fig. 2. The dependences 2~(T) of the muon spin relaxation rate in the paramagnetic state (T>TN) of dysprosium and holmium are given in Figs. 3 and 4. It is seen from these figures that -~ increases at T->T N. This increase of.A may be ex- plained by the correlation time growth r~of hhe fluctuating formation of the magnetically-ordered phase in the paramagnetic state of the metal since in the 9 J.C. Baltzer A.G., Scientific Publishing Company