NO ANTIFERROMAGNETIC REORDERING AT LOW-TEMPERATURE IN PURE YBA2CU3O6+X

Magnetic ordering has been investigated by neutron scattering on an YBa2Cu3O6+x single crystal with x=0.1 and x=0.18, and an Al doped YBa2(CU2.86Al0.14)O-6.25 crystal. For the undoped crystal an antiferromagnetic ordering transition (AFI) was observed at T-N=410 K and 368 K (respectively for x=0.1 and x=0.18), but no second antiferromagnetic transition was found down to 2 K. In contrast, an antiferromagnetic reordering (AFII) was observed for the aluminum doped sample below T-2=8 K. This shows the high sensitivity of this second phase to impurities and indicates that this reordering is not necessarily an intrinsic property of YBa2Cu3O6+x.