Curious magnetic phase transition in polycrystalline Gd-Y alloys: ultraacoustic study

Ultraacoustic and magnetic investigations have been performed in the Gd100-xYx polycrystalline system with 0 less than or equal to x less than or equal to 12 at%. The unexpected first-order phase transition at the temperature T-h approximate to 248 K, which is slightly below the Curie temperature, is revealed in all studied alloys except pure Gd. The surprising feature of this transition is not only its presence, since it is accepted that there is no phase transformation in this temperature region in this system for x less than or equal to 10, but its weak nonmonotonic dependence on x, unlike the Curie temperature. The analysis of the temperature dependence of the ultraacoustic and magnetic properties allows one the supposition that the magnetic phase transition at T-h is of the order-to-order type from collinear ferromagnetic structure to ferromagnetic helicoid structure. (C) 1999 Elsevier Science B.V. All rights reserved.