Experimental evidence of a two-phase magnetic state in thin epitaxial films of Re0.6Ba0.4MnO3 (Re = La, Pr, Nd, Gd)

Thin epitaxial films of Re0.6Ba0.4MnO3 (Re = La, Pr, Nd, Gd) on (001)-oriented single crystal SrTiO3 and ZrO2 (Y2O3) substrates have been prepared and studied. All films possess a cubic perovskite structure, except for the film with Re = La, which exhibited a rhombohedral distortion of the perovskite lattice. The results show evidence for the presence of two magnetic phases, ferromagnetic (FM) and antiferromagnetic (AFM), in the films studied: (i) the magnetization isotherm M (H) appears as a superposition of a linear component ( characteristic of antiferromagnets) and a small spontaneous magnetization component; (ii) the magnetic moment per formula unit is significantly reduced as compared to the value expected for the complete FM or ferrimagnetic ordering; (iii) there is a difference between magnetizations of the samples cooled with and without an applied magnetic field, which is preserved in the entire range of magnetic fields studied (50 kOe); (iv) the temperature dependence of the magnetization M ( T) in strong magnetic fields is close to linear ( for the composition with Re = Gd, M (T) is described by a Langevin function for superparamagnets with a cluster moment of 22 mu(B)); and (v) the magnetization hysteresis loops of the field-cooled samples are shifted along the field axis. The exchange integral (characterizing the Mn-O-Mn coupling via the FM-AFM phase boundary) estimated from the latter shift is |J| = 10(-6) eV. This value is two orders of magnitude lower than the negative exchange integral between the FM layers in ReMnO3, which makes the presence of a transition layer at the FM - AFM phase boundary unlikely. The temperature dependences of electrical resistance and magnetoresistance exhibit maxima at the Curie temperature ( T C), where the magnetoresistance reaches a colossal value. This behavior indicates that the two-phase magnetic state is caused by a strong s - d exchange. (C) 2004 MAIK "Nauka/Interperiodica".