Phase separation in strained cation- and anion-deficient Nd_(0.52)Sr_(0.48)MnO_3 films

摘要

The magnetic and transport properties of anion- and cation-deficient Nd_(0. 52)Sr_(0. 48)MnO_3 films with different thicknesses, as well as of two films from this system grown on different SrTiO_3 and LaAlO_3 substrates, are studied. Below Curie temperature T_C, the films with different thicknesses exhibit phase separation: they represent magnetic clusters (drops) embedded in a nonconducting paramagnetic (at T > T_N, where T_N is the Néel temperature) or antiferromagnetic (T < T_N) matrix. The temperature dependences of the resistivity of the films are well described in terms of the polaron mechanism of conduction. In external magnetic field H = 0.01 T, the drops may reach 15 nm in size. They consist of magnetic polarons with a small radius (1-2 nm). The drops are shown to interact with each other in the films. Because of competition between drop-drop dipole interaction and the magnetic energy, the drops disintegrate into droplets with a size comparable to that of a magnetic polaron in a field of 1 T. An explanation is given for the discrepancy between our results and the frequently observed growth of the drops with a rise in the external magnetic field. As the film gets thicker, the fraction of the ferromagnetic phase grows with thickness nonlinearly. In the film grown on SrTiO_3 (compressed by 0.9%), the characteristic Néel and Curie temperatures are lower than in the film grown on LaAlO_3. The diameters of ferromagnetic drops (both maximal at H = 0.01 T and minimal at H = 1 T) turn out to be roughly the same as in the films with different thicknesses.