ABSENCE OF A 2ND ANTIFERROMAGNETIC TRANSITION IN PURE YBA2CU3O6+X

Magnetic ordering has been investigated by means of neutron scattering in an YBa2Cu3O6+x single crystal prepared with two different oxygen c oncentrations, x = 0.1 and x = 0.18, and in an aluminium-doped YBa2(Cu 3-yAly)O6+x crystal with x = 0.25 and y = 0.14. For the undoped crysta l, a magnetic ordering transition was observed at T(N) = 410 K and 368 K (respectively for x = 0.1 and x = 0.18), but no second magnetic tra nsition was found down to 2 K. In contrast, an antiferromagnetic reord ering was observed for the aluminium-doped sample with a transition te mperature T2 = 8 K. This implies a high sensitivity of this second pha se to impurities and indicates that the transition is not necessarily an intrinsic property of YBa2Cu3O6+x.