Golovashkin A.I. ,     Kuzmichev N.D. ,     Slavkin V.V. ,     Levchenko I.S. The peculiarity of the temperature dependence for harmonics of magnetisation in single crystals YBa_2Cu_3O_7-x. Physica C: Superconductivity , Vol.185-189 , с. P.581-582 , 01.1991