ANTIFERROMAGNETISM OF THE NEW NONSUPERCONDUCTING 2201 STACKING POLYTYPE BIPBSR2FE1-XMXO6+Z WITH M = CO, NI AND MORIN-LIKE SPIN-FLOP-TRANSITION IN BIPBSR2FEO6+Z

The magnetic properties and the spin structure of the BiPbSr2Fe(1-x)M(x)O6 + z compounds with M = Co, Ni, isotypical with the members of the Bi2(Tl2)Sr2(Ba2)Ca(n - 1)Cu(n)O2n + 4 compositional series with n = 1, were studied using magnetic susceptibility measurements and Fe-57 Mossbauer spectroscopy. All the compounds investigated (with x = 0, x(Co) = 0.5 and x(Ni) = 0.25) are basically antiferromagnetic. The highest Neel temperature of 212 K is reached for BiPbSr2FeO6 + z, where the hyperfine field at the Fe site has with 48.9 T a value close to the expectation for Fe3+ in a high spin state. A comparison between the values of the quadrupole splitting in the paramagnetic and in the magnetically ordered state shows that a spin-flop transition from the basal plane to the c-axis occurs in the temperature range 120-77 K in BiPbSr2FeO6.1. Such a behaviour is reminiscent of the Morin transition in alpha-Fe2O3, where the spin direction of the antiferromagnetic sublattices rotates from the plane to the trigonal axis. No spin-flop transition takes place in BiPbSr2Fe0.5Co0.5O6.01.