Abstract
Highly oriented polycrystalline Sm0.55Sr0.45MnO3 thin films (thickness ∼100 nm) deposited on LaAlO3 (LAO, (001)), SrTiO3 (STO, (001)) and (La0.18Sr0.82) (Al0.59Ta0.41)O3 (LSAT, (001)) single crystal substrates by ultrasonic nebulized spray pyrolysis have been studied. The out of plane lattice parameter (OPLP) of the film on LAO is slightly larger than that of the corresponding bulk. In contrast, the OPLP of the films on STO and LSAT are slightly smaller than the corresponding bulk value. This suggests that the film on LAO is under compressive strain while LSAT and STO are under tensile strain. The films on LAO and LSAT show simultaneous paramagnetic-ferromagnetic (PM–FM) and insulator-metal transition (IMT) temperature at TC/TIM ∼ 165 K and 130 K, respectively. The PM–FM and IM transition occur at TC ∼ 120 K and TIM ∼ 105 K, respectively in the film on STO substrate. At T < TC, the zero field cooled–field cooled (ZFC–FC) magnetization of all the films shows strong bifurcation. This suggests the presence of a metamagnetic state akin to cluster glass formed due to coexisting FM and antiferromagnetic–charge order (AFM–CO) clusters. All the films show colossal magnetoresistance but its temperature and magnetic field dependence are drastically different. The films on LAO and STO show peak CMR around TC/TIM, while the film on LSAT shows MR > 99 % over a very wide temperature range of ∼40 K centred on TC/TIM. In the lower temperature region the magnetic field dependent isothermal resistivity also shows signature of metamagnetic transitions. The observed results have been explained in terms of the variation of the relative fractions of the coexisting FM and AFM–CO phases as a function of the substrate induced strain and oxygen vacancy induced quenched disorder.
Highlights
In doped rare earth manganites of the type RE1-xAExMnO3 (RE: rare earth cations; La3+, Nd3+, Sm3+ etc., AE: alkaline earth cations; Ca2+, Sr2+ etc.) the lowering of the average RE/AE-site cationic radius decreases the eg electron bandwidth (W) that in turn results in increased carrier localization through the Jahn–Teller (JT) distortion of the MnO6 octahedra
The OPLP of films on STO and LSAT are cSTO = 3.822 Å and cLSAT = 3.826 Å, respectively. These estimations suggest that the films grown on LAO are compressively strained and slightly smaller lattice constants of SSMO on STO and LSAT substrates could be attributed to the small tensile strain
At the lattice level the compressive strain results in an elongation of the MnO6 octahedra in the OP direction with a concomitant compression in the basal plane that causes a reduction in the degree of JT distortion, and weakens the spin-lattice coupling
Summary
Several studies on narrow band manganites have shown that the first-order nature of phase transition can be preserved even in presence of quenched disorder arising due to the size mismatch between RE and AE ions.[1] Like other low bandwidth manganites, Sm1-xSrxMnO3 has a natural tendency towards phase separation/phase coexistence (PS/PE) that causes evolution of a strong metamagnetic component around half doping (x ∼ 0.50). This metamagnetic makes the composition-temperature (x–T) phase diagram extremely fragile to external perturbations. Our results clearly demonstrate that despite the polycrystalline nature of these films the impact of substrate is dramatic and unambiguously manifested in the magnetic and magnetotransport properties
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