Enhanced electron-electron correlations in nanometric SrRuO 3 epitaxial films G. Herranz,* B. Martı ´ nez, and J. Fontcuberta Institut de Cie `ncia de Materials de Barcelona, CSIC, Campus UAB, Bellaterra 08193, Catalunya, Spain F. Sa ´ nchez, ² C. Ferrater, M. V. Garcı ´ a-Cuenca, and M. Varela Departament de Fı ´sica Aplicada i O ` ptica, Universitat de Barcelona, Diagonal 647, Barcelona 08028, Catalunya, Spain ~Received 27 November 2002; published 29 May 2003! Epitaxial and fully strained SrRuO 3 thin films have been grown on SrTiO 3 (100). At initial stages the growth mode is three-dimensional- ~3D-!like, leading to a finger-shaped structure aligned with the substrate steps and that eventually evolves into a 2D step-flow growth. We study the impact that the defect structure associated with this unique growth mode transition has on the electronic properties of the films. Detailed analysis of the transport properties of nanometric films reveals that microstructural disorder promotes a shortening of the carrier mean free path. Remarkably enough, at low temperatures, this results in a reinforcement of quantum corrections to the conductivity as predicted by recent models of disordered, strongly correlated electronic systems. This finding may provide a simple explanation for the commonly observed—in conducting oxides- resistivity minima at low temperature. Simultaneously, the ferromagnetic transition occurring at about 140 K, becomes broader as film thickness decreases down to nanometric range. The relevance of these results for the understanding of the electronic properties of disordered electronic systems and for the technological applica- tions of SrRuO 3 —and other ferromagnetic and metallic oxides—is stressed. DOI: 10.1103/PhysRevB.67.174423 PACS number~s!: 73.20.Fz, 73.63.Bd, 73.50.Gr I. INTRODUCTION The development of devices based on thin films is an active area of research. In particular, magnetic tunneling junctions with ferromagnetic oxide electrodes are interesting due to their promising technological applications. Successful fabrication of heterostructures requires to have a deep under- standing of the effects related with the interfaces between layers. These effects may manifest itself in a variety of phe- nomena, including vacancies, structural disorder or phase segregation, and may be relevant to the transport and mag- netic properties and thus, have a straightforward impact on the performance of the devices. Indeed, it has been recently demonstrated that the interface microstructure limits the spin-injection efficiency across heteroepitaxial interfaces in ZnMnSe/AlGaAs-GaAs spin-polarized light emitting diodes. 1 Manganite-based tunnel junctions such as La 2/3 Sr 1/3 MnO 3 /SrTiO 3 /La 2/3 Sr 1/3 MnO 3 , also display a dra- matic suppression of their performance at temperatures well below the Curie temperature. It has been proposed that this effect originates from complex electronic inhomogeneities existing at the interfaces of these heterostructures. 2 The ex- treme tendency of manganites towards electronic phase sepa- ration is intimately related to the double exchange mecha- nism of ferromagnetic coupling and the existence of strong electron-phonon coupling. On the other hand, understanding of the impact of disor- der in the electronic properties of strongly correlated systems is an important issue, since metallic oxides involving 3 d or 4 d metal transition ions are commonly used in heterostruc- tures and, as will be shown below, microstructural disorder close to interfaces plays a major role. In this paper we will focus on these important issues by exploring in detail the magnetotransport properties of epitax- ial SrRuO 3 thin films as a function of thickness (4 nm,t ,320 nm). SrRuO 3 is a promising material for using in technological applications. 3 It is a metallic ferromagnet, whose ferromagnetism is tied to the conduction electrons lying in a p * narrow band formed by the overlap of Ru t 2 g orbitals with the O 2 p orbitals. 4,5 The itinerant character of magnetism in SrRuO 3 is well founded both theoretically and experimentally, 4–6 and it has been proposed that the nature of the magnetic coupling in this compound should be more ro- bust against interface effects when compared to double- exchange ferromagnets such as manganites. 7 Small distor- sions induced in the lattice reverts in a narrowing of the p * band that affect in a significant way its magnetic properties. In fact, substitution of Sr cations by other cations leads to the formation of non magnetic compounds such as CaRuO 3 or BaRuO 3 , with crystalline structures differing slightly from that of SrRuO 3 , due to slight tilts and rotations of the RuO 6 octahedra. 8 This fact indicates that strain-induced effects arising from the film—substrate interactions could be rel- evant for the magnetotransport properties of SrRuO 3 films. On the other hand, the existence of strong electronic corre- lations in SrRuO 3 has been experimentally confirmed either from specific heat measurements, 4 low-temperature transport behavior, 9 or photoemission spectra, 10 among others; it has also been found that the Fermi-liquid state is remarkably sensitive to disorder and breaks down at a very low tempera- ture ( T ;10 K), further confirming the significance of the electronic correlations. 11 Therefore, SrRuO 3 appears as an ideal candidate to explore the role of the growth-induced disorder on the transport properties of strongly correlated ferromagnetic oxides, a key issue in the field of spintronics. Epitaxial thin films and heterostructures grown on a vari- ety of substrates by different deposition techniques have been studied and their magnetic and transport properties analyzed. 12–23 In addition, there is some information about PHYSICAL REVIEW B 67, 174423 ~2003! 0163-1829/2003/67~17!/174423~8!/$20.00 ©2003 The American Physical Society 67 174423-1