B. Martínez

Enhanced field sensitivity close to percolation in magnetoresistive La2/3Sr1/3MnO3/CeO2 composites

The magnetoresistance of La2/3Sr1/3MnO3/CeO2 composites is explored as a function of the metal/insulator composition, temperature, and magnetic field. An important enhancement of the low-field magnetoresistance is observed for samples close to metallic percolation threshold. The improved field sensitivity is still fairly large at room temperature. Manganese perovskites composites may constitute a new alternative for the development of magnetoresistive devices.

Electronic transfer in Sr2FeMoO6 perovskites

The magnetic properties of Sr2FeMoO6 oxides are investigated over a broad temperature range extending from 10 K to 800 K. It is found that the magnetization fully saturates at low temperatures at a value of 3.75 µB. In the paramagnetic regime, above the Curie temperature (TC≈420 K), the magnetic susceptibility can be well described by a Curie-Weiss law. It is found that the experimental effective moment µ*eff is gradually reduced under a field, an effect that can be attributed to some non-intrinsic behaviour. By means of detailed data analysis, it is shown that the paramagnetic effective moment µeff is close to that expected for a 3d6:4d0 atomic configuration. This result is interpreted as due to some electronic charge transfer from the itinerant 4d0(Mo) towards the 3d5(Fe) orbitals in the paramagnetic phase. We have also observed that the paramagnetic Curie temperature θP is positive and that θP≈TC, thus suggesting that the nearest-neighbour magnetic interactions are ferromagnetic in nature. These observations strongly support a double-exchange picture for the ferromagnetic coupling in these oxides.

Surface behavior of La2∕3Ca1∕3MnO3 epitaxial thin films

The role of the surface layers in La2∕3Ca1∕3MnO3 magnetic oxide epitaxial thin films is analyzed. We show that the topmost layers do play a very relevant role on the transport properties acting as an insulating barrier. Atomic force microscopy (AFM) measurements in the current sensing mode exhibit typical features of tunneling conduction. The analysis of the I-V curves by using the Simmons model give values of barrier thickness in good agreement with nonmagnetic layer thickness estimates from magnetic measurements. Ex situ annealing in air at high temperature clearly improve the magnetotransport properties of the films reducing the surface insulating barrier.

Room temperature magnetoresistive sensor based on thick films manganese perovskite

Abstract La 2/3 Sr 1/3 MnO 3 perovskite (LSMO), which shows a ferromagnetic transition above room temperature, is a good candidate to be used as a magnetoresistive sensor. Magnetic and transport properties of granular LSMO have been studied. Different grain size and the use of metal/insulator composites have been explored in order to optimize the magnetoresistive response. LSMO ceramic powder have been used to prepare thick films on polycrystalline Al 2 O 3 substrates by screen printing. The magnetoresistive response of the grown thick films is analyzed in the range of technical fields. It is shown, that under appropriate conditions, materials having field sensitivity high enough for some applications at room temperature can be obtained. A magnetic sensor has been developed in order to show the feasibility of this material as a low-cost magnetic sensor.

Growth kinetics engineered magnetoresistance response in La2/3Sr1/3MnO3 thin films

A route to engineer the intrinsic colossal magnetoresistance (CMR) response in manganite thin films through an accurate control of the growth kinetics is presented. It is shown that under specific growth conditions, a particular strained state, substantially different from that of bulk-like materials and standard films, can be quenched up to film thicknesses around 60 nm. This strained state exhibits the same structural fingerprints of the interfacial dead layer in standard films and promotes surface morphology instabilities, which end up with the formation of self-organized nanopits array. At the same time, it has profound effects on the intrinsic magnetoelectronic properties of the films that exhibit an enhanced intrinsic CMR response.

Isothermal anisotropic magnetoresistance in antiferromagnetic metallic IrMn.

Antiferromagnetic spintronics is an emerging field; antiferromagnets can improve the functionalities of ferromagnets with higher response times, and having the information shielded against external magnetic field. Moreover, a large list of aniferromagnetic semiconductors and metals with Néel temperatures above room temperature exists. In the present manuscript, we persevere in the quest for the limits of how large can anisotropic magnetoresistance be in antiferromagnetic materials with very large spin-orbit coupling. We selected IrMn as a prime example of first-class moment (Mn) and spin-orbit (Ir) combination. Isothermal magnetotransport measurements in an antiferromagnetic-metal(IrMn)/ferromagnetic-insulator thin film bilayer have been performed. The metal/insulator structure with magnetic coupling between both layers allows the measurement of the modulation of the transport properties exclusively in the antiferromagnetic layer. Anisotropic magnetoresistance as large as 0.15% has been found, which is much larger than that for a bare IrMn layer. Interestingly, it has been observed that anisotropic magnetoresistance is strongly influenced by the field cooling conditions, signaling the dependence of the found response on the formation of domains at the magnetic ordering temperature.

Electroresistance and Joule heating effects in manganite thin films

Electroresistance phenomena have been investigated in La2/3Sr1/3MnO3 (LSMO) microbridges through the analysis of I-V characteristic curves. I-V curves are nonlinear in all the range of temperatures explored, but are almost reversible and exhibit origin symmetry which suggest the existence of Joule self-heating effects. However, comparing the resistance determined from the I-V curves (R = V/I) at the actual sample temperature, measured through a Pt thermometer microfabricated on top of the LSMO microbridges, with the resistance measured in thermal equilibrium, i.e., in the low current regime, it is evident that Joule self-heating effects alone cannot account for the experimental results. We demonstrate that oxygen depletion is the cause for the observed increase of resistance and the reduction of TC.

Mn valence instability in La2/3Ca1/3Mn03 thin films

A Mn valence instability on La2/3Ca1/3MnO3 thin films, grown on LaAlO3 (001)substrates is observed by x-ray absorption spectroscopy at the Mn L-edge and O K-edge. As-grown samples, in situ annealed at 800 C in oxygen, exhibit a Curie temperature well below that of the bulk material. Upon air exposure a reduction of the saturation magnetization, MS, of the films is detected. Simultaneously a Mn2+ spectral signature develops, in addition to the expected Mn3+ and Mn4+ contributions, which increases with time. The similarity of the spectral results obtained by total electron yield and fluorescence yield spectroscopy indicates that the location of the Mn valence anomalies is not confined to a narrow surface region of the film, but can extend throughout the whole thickness of the sample. High temperature annealing at 1000 C in air, immediately after growth, improves the magnetic and transport properties of such films towards the bulk values and the Mn2+ signature in the spectra does not appear. The Mn valence is then stable even to prolonged air exposure. We propose a mechanism for the Mn2+ ions formation and discuss the importance of these observations with respect to previous findings and production of thin films devices.

Transport properties across the La2/3Ca1/3MnO3/SrTiO3 heterointerface

The transport properties across La2∕3Ca1∕3MnO3∕SrTiO3 (LCMO/STO) heterostructures with different thicknesses of the STO insulating barrier have been studied by using atomic force microscopy measurements in the current sensing (CS) mode. To avoid intrinsic problems of the CS method we have developed a nanostructured contact geometry of Au dots. The conduction process across the LCMO/STO interface exhibits the typical features of a tunneling process. The analysis of I(V) curves by using the Simmons model allows us to determine the barrier height (φ0≈0.6eV) of STO barriers.

Surface resistance of La2/3Ca1/3MnO3 epitaxial thin films grown on top of LaAlO3

In this work we present results of contact resistance between La2/3Ca1/3MnO3 (LCMO) and Pt system. LCMO epitaxial thin films have been grown by rf sputtering on top of LaAlO3 (LAO) substrate. The contact between the manganite film and the metal have been prepared by exsitu deposition of a 30 nm thick Pt layer on top of the manganite film by e-beam evaporation at room temperature. Different nanostructured contact geometries have been defined by using a focus ion beam system and then transport properties have been tested by means of AFM system working on the current sensing mode with a doped diamond tip. We show that the top-most LCMO layers do play a very relevant role on the transport properties acting as an insulating barrier. AFM measurements in the current sensing mode exhibit typical features of tunneling conduction. Ex-situ annealing in air at high temperature clearly improve the magnetotransport properties of the films reducing the surface insulating barrier. X-ray absorption spectroscopy measurement at the Mn Ledge has been performed to gain a deeper insight into the properties of the top-most LCMO layers.