Partial substitution of Ba 2+ for Ho in the Ho 1+ yBa 2− yCu 3O χ thin films grown at low temperature

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We investigate the effect of the nonstiochiometry of Ho-Ba-Cu-O thin films, which are grown by the reactive co-evaporation at temperatures below 580°C, on the c-lattice length, surface morphology and superconducting property. Although a nonstoichiometric (Ho+Ba)/Cu ratio results in phase separation, a nonstoichiometric Ho/Ba ratio results in a solid solution, Ho 1+ y Ba 2− y Cu 3O χ, in which the excess ions substitute at other sites. The partial substitution of Ba ions at Ho sites in the Ba-excess films ( y<0) was confirmed by the dependence of the X-ray diffraction peak intensity on y. The Ba-excess composition is known to cause phase separation at thermal equilibrium because of the unstable occupation of Ho sites by Ba ions. The solid solution with Ba-excess composition is therefore thought to result from nonequilibrium thin film growth at low temperature.