Anomalous thermal expansion in Nd2Fe17-δGaδ (δ = 0-2)

In Nd2Fe17-deltaGadelta (delta=0, 0.5, 1, 1.5, 2), Ga substitutes for Fe without changing the crystal structure. Neutron diffraction measurements show that Ga preferentially substitutes for Fe in the 18 h sites only. The magnetic moments in Nd2Fe17-deltaGadelta, below the Curie temperature, T-c, are found to be in the basal plane. Below T-c the unit cell constant, c, decreases with increasing temperature for Nd2Fe17-deltaGadelta (delta less than or equal to 1). For Nd2Fe17-deltaGadelta (delta>1), the thermal expansion coefficient changes sign and becomes positive for Nd2Fe17-deltaGadelta. The relative decrease in the Fe(6c) - Fe( 6c) distance is twice as large as the decrease in the c axis below T-c. Below T-c, the Fe(6c) - Fe(6c) distance decreases with an increase in the Ga concentration, while above T-c the Fe(6c) - Fe(6c) distances are found to be independent on Ga concentration indicating that the presence of Ga decreases the negative exchange coupling between Fe atoms. (C) 1999 American Institute of Physics. [S0021-8979(99)56408-6].