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 magnetic 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 antiparallel to each other. The magnetic unit cell of the low-temperature phase AF2 is similar to that of AF1 with a(M) = a(m), but with a four times larger c(M) axis, i.e., c(M) =4c(m) = 8c and a more complicated sequence along [001] than in AF1. Denoting the magnetic unit cell of AF1 as A, the magnetic unit cell of AF2 can be considered as built up from the sequence AA-A-A along [001]. in the Ca-doped compound, oxygen vacancies have a detrimental effect on the long-range correlation along [001]. As a consequence, two magnetic phases coexist: One phase with magnetic unit cell a(1) = root 2a and c(1) = c appears at room temperature, and the Neel temperature, T-N1, for the other, predominant phase 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.