At room temperature a large magnetoresistance, IsR/R(H =0), of 60% has been observed in thin magnetic films of perovskitelike La-Ba-Mn-o, The films were grown epitaxially on SrTi03 substrates by oA'-axis laser deposition. In the as-deposited state, the Curie temperature and the saturation magnetization were considerably lower compared to bulk samples, but were increased by a subsequent heat treatment. The samples show a drop in the resistivity at the magnetic transition, and the existence of magnetic polarons seems to dominate the electric transport in this region. PACS numbers: 75.70.Ak, 72. 15.Gd, 73.50.Jt Giant magnetoresistance (GMR) due to spin-dependent scattering at the interface between ferromagnetic and nonmagnetic regions has been the subject of intense research in the last years [1-4]. At room temperature, resistance changes hR/R(H=0) as high as 40% have been observed in Cu/Co multilayers [2] and up to 11% in heterogeneous Cu/Co alloys [3,4], compared to only 2%-3% for "conventional" materials such as permalloy. A much higher magnetoresistance was found near a metal/insulator and simultaneous magnetic phase transition, e.g. , in Eu~-"Gd"Se [5], but in this material the eAect is restricted to temperatures below 50 K. The anomalous transport phenomena in these Eu-chalcogenide alloys have been explained by the appearance of giant spin molecules [6] and spin polarons [7]. Similar observations have been made in the mixed valence perovskitelike Ndo 5Pbo 5Mno3 in the region around the ferromagnetic phase transition at 184 K [8]. A large drop in resistivity and a high magnetoresistance AR/ R (0) & 50% was found, which is also believed to be caused by the existence of magnetic polarons. La2/3-Ba~g3Mno3 is also well known as a mixed valence metallic ferromagnet, but with a considerably higher Curie temperature of Tc =343 K [9,10]. At the Curie temperature a cusp in the resistivity was found and therefore gives rise to the assumption of a high magnetoresistance eAect at room temperature. Targets of stoichiometric composition were prepared by standard ceramic techniques from the metallic oxides and carbonates of 99.9% purity by repeated grinding and annealing in air. The x-ray diAraction patterns showed no other reAections than those of the rhombohedral unit cell found in Ref. [10] (a =0.3910 nm, y=90. 12'). Thin films of 150~10 nm thickness were grown on SrTi03 10&&10 mm substrates in (100) and (110) orientation. A laser deposition technique in off'-axis geometry was used, which had previously turned out to be a powerful tool in preparing high-T, superconducting thin films with a smooth surface [11]. Different substrate temperatures between 600 and 900 C were used, at a deposition pressure of 0.4 mbar oxygen. Epitaxial films were obtained for a substrate temperature of Tg =600 C as confirmed 250 T= 300K 200 8 A 0 bP c5 150 100 50 0:-0.0~a nnealed O 0.5 1.0 1.5 magnetic field p, H [T] 2.0 FIG. 1. Room temperature magnetization curve for the asdeposited sample (Ts =600'C) showing paramagnetic behav-ior. The ferroma...

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Diffusionless phase transitions are at the core of the multifunctionality of (magnetic) shape memory alloys, ferroelectrics and multiferroics. Giant strain effects under external fields are obtained in low symmetric modulated martensitic phases. We outline the origin of modulated phases, their connection with tetragonal martensite and consequences for their functional properties by analysing the martensitic microstructure of epitaxial Ni-Mn-Ga films from the atomic to macroscale.Geometrical constraints at an austenite-martensite phase boundary act down to the atomic scale. Hence a martensitic microstructure of nanotwinned tetragonal martensite can form. Coarsening of twin variants can reduce twin boundary energy, a process we could observe from the atomic to the millimetre scale. Coarsening is a fractal process, proceeding in discrete steps by doubling twin periodicity. The collective defect energy results in a substantial hysteresis, which allows retaining modulated martensite as a metastable phase at room temperature.In this metastable state elastic energy is released by the formation of a 'twins within twins' microstructure which can be observed from the nanometre to millimetre scale. This hierarchical twinning results in mesoscopic twin boundaries. Our analysis indicates that mesoscopic boundaries are broad and diffuse, in contrast to the common atomically sharp twin boundaries of tetragonal martensite. We suggest that observed extraordinarily high mobility of such mesoscopic twin boundaries originates from their diffuse nature which renders pinning by atomistic point defects ineffective.

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