Influence of Preparation Conditions on Long-Term Stability of Magnetoresistive Properties of Nanostructured La-Sr-Mn-Co-O Films Grown by PI MOCVD

Abstract

It has been demonstrated that perovskite oxides with the general formula ABO3 (A - lanthanides, alkaline-earth metals, B - transition metal cations) exhibit the colossal magnetoresistance (CMR) phenomenon and can be used for various spintronics applications [1]. Many efforts were made in structural and chemical engineering of such materials in order to tune their functional properties along a wide range of temperatures and magnetic fields. It was found that epitaxial films exhibit high magnetoresistance (MR) values only in the vicinity of phase transition temperature, while polycrystalline films exhibit significant MR in a wide range of temperatures. Engineering of lanthanum manganites with special nanocrystalline (polycrystalline with nanosize grains) structure enabled to develop so-called CMR-B-scalar sensors, which are capable to measure the magnitude of pulsed magnetic fields in very small volumes independently on field direction [2]. Such sensors have been used in advanced scientific equipment to measure magnetic field dynamics [3-4]. It is worth to note, that in the past decades the most extensive research was performed on A-site doped manganites. This doping results in a mixed valance state of the manganese ions (Mn3+ and Mn4+) giving rise to the ferromagnetic (FM) double exchange (DE) interaction, which determines transport and magnetic properties of these materials. On the other hand, it was proposed that the B-site doping with 3d ions (Co, etc.) would destroy the long-range FM ordering of the Mn3+-O2--Mn4+ network resulting in a complex glassy state which arises from the competition of FM and antiferromagnetic (AFM) interactions. Therefore, significant change of the magnetic and electrical properties of manganites is expected. It was demonstrated, that substitution of Co for Mn in La1-xSrxMnO3 results in increased room temperature MR [5]. A variety of competing magnetic interactions in Co-doped manganites [6] still remains unclear, especially in nanostructured films, therefore, their investigations are of great interest. In addition, the long-term stability [7] of magnetoresistive properties of such films is very important for magnetic field sensors applications.In this study, the resistivity ρ and MR of nanostructured Co-doped manganite films deposited at different growth rate was investigated in magnetic fields up to 20 T at room and cryogenic temperatures. The change of these parameters were studied during the long-term storage of the films and accelerated ageing by annealing the films at O2 and Ar atmosphere.The La0.81Sr0.19Mn1.09Co0.06O3 films (LSMCO) with a thickness of 360 nm were deposited by using a Pulsed-Injection Metal-Organic Chemical Vapor Deposition (PI MOCVD ) technique onto a polycrystalline Al2O3 substrate at 600 °C temperature keeping different growth rates (9 nm/min, 13 nm/min, and 27 nm/min). The Ag electrodes of the samples were thermally deposited on a Cr sublayer and post annealed at 450°C for 1 h. The ρ dependence on temperature was measured in the temperature range of (5–310) K. The MR measurements were performed in permanent (0-0.8) T magnetic field at (25–290) K and pulsed fields up to 20 T at (80–290) K temperatures. The ρ and MR of films deposited at different growth rates was investigated after such preparation steps: 1) formation of electrodes and post annealing at 450°C for 1 h in Ar or O2 atmosphere; 2) long-term storage at room temperature for 6 months; 3) accelerated ageing procedure: additional annealing at 100 °C for 8 h in Ar or O2.It was found that electrode formation conditions and the long-term storage significantly changes the properties of the films (see Fig.1). After 6 months the resistivity maximum ρm of all films significantly increased and the metal-insulator transition temperature Tm shifted to lower temperatures. The annealing at 100 °C for 8 h in O2 decreased the ρ due to oxygen saturation of the films. The resistivity of nanostructured films mostly depends on the quality of grain boundary material, therefore, the largest changes were obtained for LSMCO film grown at the highest rate (27 nm/min). The annealing in Ar only slightly affected the ρ values. The low-field (0.7 T) MR (Fig.2) changed insignificantly in all temperature range (25-290) K. This is probably related with the change of a number of charge carriers’ hopping centers Mn3+-O2--Mn4+, which depends on the oxygen saturation level. Therefore, despite the resistivity changes, the MR=100%×[ρ(B)/ρ(0)-1] was only slightly affected. At 20 T the MR also insignificantly depended on preparation conditions and was ~(65-70)% at 80 K and (55-60)% at 290 K.It was concluded, that for magnetic field sensors application the LSMCO films grown at lower rate (9 nm/min) and annealed at Ar atmosphere are preferable due more stable parameters. **