Doped manganite perovskite (ultra)thin films (R1-xAxMnO3 - R = rare-earth, A = Ca, Sr, Ba and Pb) show a very rich phase diagram and are considered as model system in fundamental solid state physics [1]. Moreover ferromagnetic metallic manganites are employed as prototypical spin injectors in several systems in order to reveal insights in a variety of spin related effects [2]. The La0.7Sr0.3MnO3 (LSMO) compound has stimulated an intense study since, in bulk form, it has one of the highest ferromagnetic transition temperature TC ~ 370 K. Understanding and controlling the morpho-structural and magnetic properties of LSMO films as a function of thickness is crucial for realizing applications that commonly demand for ultrathin layers. La0.7Sr0.3MnO3 thin films, with thickness t in the 4 - 16 nm range, were deposited on (001) oriented SrTiO3 substrates by means of channel spark ablation of a stoichiometric target in oxygen atmosphere [3]. The oxygen pressure in the chamber was 3.9·10-2 mbar, the substrate temperature was around 890 °C and the deposition rate was between 0.10 and 0.15 Å per pulse for a repetition rate of 6 Hz. After the deposition the films were cooled in vacuum (5.0·10-4 mbar) down to about 400 °C, where they were in situ conditioned by an annealing before removing them from the vacuum. The film thickness was estimated by XRR measurements, which also provided information on roughness and density. Both temperature (100 K £ T £ 300 K) and angular dependence of the magnetic properties were studied by a vector Vibrating Sample Magnetometer (vVSM). Ferromagnetism is observed down to 4nm (TC ~ 250 K), with TC increasing with the film thickness. For t = 16 nm, TC ~ 315 K and, at T = 100 K, the magnetization M ~ 490 emu/cm3, not far from the saturation value of bulk LSMO (560 emu/cm3). However, this gradual approach towards the ferromagnetic properties of bulk LSMO follows a non-monotonic trend. This suggests the existence of a strict correlation between the magnetic behaviour and the evolution of the microstructural and stoichiometric features during the growth process. Moreover, with increasing T from 200 K up to 300 K, a change of the magnetic anisotropy from a biaxial to an uniaxial symmetry has been clearly observed in all the films. Such behavior – probably related to a crossover from a low-temperature regime, where crystalline anisotropy dominates, to a high-temperature one, governed by magnetoelastic anisotropy – occurs progressively, with a substantial isotropic behaviour actually existing in a narrow temperature range. Magnetotransport properties have been carried out in four probes geometry, with the magnetic field in the film plane in order to complement and support the vVSM characterization. Most features are confirmed while open issues are related to the observed coercive field .

Magnetic properties of (ultra)thin LaSrMnO films

DEL BIANCO, Lucia;
2015

Abstract

Doped manganite perovskite (ultra)thin films (R1-xAxMnO3 - R = rare-earth, A = Ca, Sr, Ba and Pb) show a very rich phase diagram and are considered as model system in fundamental solid state physics [1]. Moreover ferromagnetic metallic manganites are employed as prototypical spin injectors in several systems in order to reveal insights in a variety of spin related effects [2]. The La0.7Sr0.3MnO3 (LSMO) compound has stimulated an intense study since, in bulk form, it has one of the highest ferromagnetic transition temperature TC ~ 370 K. Understanding and controlling the morpho-structural and magnetic properties of LSMO films as a function of thickness is crucial for realizing applications that commonly demand for ultrathin layers. La0.7Sr0.3MnO3 thin films, with thickness t in the 4 - 16 nm range, were deposited on (001) oriented SrTiO3 substrates by means of channel spark ablation of a stoichiometric target in oxygen atmosphere [3]. The oxygen pressure in the chamber was 3.9·10-2 mbar, the substrate temperature was around 890 °C and the deposition rate was between 0.10 and 0.15 Å per pulse for a repetition rate of 6 Hz. After the deposition the films were cooled in vacuum (5.0·10-4 mbar) down to about 400 °C, where they were in situ conditioned by an annealing before removing them from the vacuum. The film thickness was estimated by XRR measurements, which also provided information on roughness and density. Both temperature (100 K £ T £ 300 K) and angular dependence of the magnetic properties were studied by a vector Vibrating Sample Magnetometer (vVSM). Ferromagnetism is observed down to 4nm (TC ~ 250 K), with TC increasing with the film thickness. For t = 16 nm, TC ~ 315 K and, at T = 100 K, the magnetization M ~ 490 emu/cm3, not far from the saturation value of bulk LSMO (560 emu/cm3). However, this gradual approach towards the ferromagnetic properties of bulk LSMO follows a non-monotonic trend. This suggests the existence of a strict correlation between the magnetic behaviour and the evolution of the microstructural and stoichiometric features during the growth process. Moreover, with increasing T from 200 K up to 300 K, a change of the magnetic anisotropy from a biaxial to an uniaxial symmetry has been clearly observed in all the films. Such behavior – probably related to a crossover from a low-temperature regime, where crystalline anisotropy dominates, to a high-temperature one, governed by magnetoelastic anisotropy – occurs progressively, with a substantial isotropic behaviour actually existing in a narrow temperature range. Magnetotransport properties have been carried out in four probes geometry, with the magnetic field in the film plane in order to complement and support the vVSM characterization. Most features are confirmed while open issues are related to the observed coercive field .
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2339035
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