LOW-TEMPERATURE MAGNETIC-STRUCTURE OF YBACUFEO5 AND THE EFFECT OF PARTIAL SUBSTITUTION OF YTTRIUM BY CALCIUM

Two magnetic phase transitions at T-N1 approximate to 475 K (AF1 phase ) and T-N2 approximate to 240 K (AF2) are observed in YBaCuFeO5. The m agnetic unit cell of AFL is four times the chemical cell, a(m) = root 2a, c(m) = 2c, and the magnetic moments within one chemical cell are a ntiparallel to each other. The magnetic unit cell of the low-temperatu re phase AF2 is similar to that of AF1 with a(M) = a(m), but with a fo ur times larger c(M) axis, i.e., c(M) =4c(m) = 8c and a more complicat ed sequence along [001] than in AF1. Denoting the magnetic unit cell o f AF1 as A, the magnetic unit cell of AF2 can be considered as built u p from the sequence AA-A-A along [001]. in the Ca-doped compound, oxyg en vacancies have a detrimental effect on the long-range correlation a long [001]. As a consequence, two magnetic phases coexist: One phase w ith magnetic unit cell a(1) = root 2a and c(1) = c appears at room tem perature, and the Neel temperature, T-N1, for the other, predominant p hase decreases as compared to the undoped compound. The transition to AF2 seems to be hindered, and AF1 is observed down to 8 K in (Y0.94Ca0 .06)BaCuFeO4.95.