GD CONCENTRATION-DEPENDENCE OF THE SPIN REORIENTATION CRITICAL-FIELD IN EU2-XGDXCUO4

ESR measurements of the microwave absorption signal associated with we ak ferromagnetism in single crystals of Eu(2-x)GD(x)CuO(4) are present ed for X band (9.5 GHz) and L band (1.2 GHz) as a function of the Gd c oncentration. The strong absorption observed at low magnetic fields wa s interpreted, for samples with low Gd concentration, as due to a fiel d-induced spin reorientation transition occurring at a critical field H-c, coincident with the in-plane magnetic anisotropy effective field H-eff(y). For larger x the Cu-Gd magnetic interaction needs to be cons idered leading to smaller H-c values. Our measurements show that for G d concentrations in the range 0 less than or equal to x less than or e qual to 1 the experimental data can be very well fitted with parameter s derived from previous measurements. This fact indicates that these c ompounds have nearly the same in-plane anisotropy effective field, in spite of the small changes in lattice parameters. For x = 2 lattice di stortions increase causing an H-c larger than the expected one. (C) 19 96 American Institute of Physics.