AMORE will develop a new generation of ferromagnetic half-metallic oxides and demonstrate their potential in patented nanoscale magneto resistance. The key idea is to exploit the high degree of spin polarisation of these chemically stable oxides to achieve large magneto resistive effects in the practical temperature range up to 120) C. For this, an understanding of spin transport at grain boundaries and interfaces in these materials is required. Having selected the most suitable of the new double perovskite oxides, the aim is to evaluate the technical and economic feasibility of magneto-resistive elements fabricated from them by two routes
1) Screen printing - low-cost technique suitable for making contact-less ^potentiometers for the auto industry among others, and
2) Epitaxial thin-film growth for planar tunnel junctions suitable as magneto electronic elements.
A new generic materials technology for sensitive, robust, low-cost components will be launched, which will boost European competitivity in a fast-changing field.
AMORE aimed to develop a new generation of half-metallic oxides with Curie temperatures greater than 180 °C for use in sensors and spin-polarized tunnel junction devices. New materials in the ordered double perovskite family such as Sr2FeMoO6 (SFMO) were synthesized and their crystal structures, magnetic and electronic properties were fully characterized. Technology for compounding the powders into inks for screen-printing was created, and planar thin-film tunnel junctions with oxide barriers were developed. Emphasis was on the room-temperature magnetoresistive response. The research was designed to prove the concept of two devices: — a low cost integrated contactless potentiometer for the automotive industry and a radiation-hard memory or logic element for aerospace applications. The project was a step towards the longer-term goal of achieving 100% low-field magnetoresistive response at room temperature with no ancillary electronics.AMORE was originally a 36-month project, associating academic partners in France, Spain and Ireland with a specialized ceramics development centre (Inocermic, D), a manufacturer of components for the automotive industry (Nacesa, E) and a specialized electronics and defence company (Thales, F). The project was coordinated by Trinity College, Dublin. In month 30, two Polish partners were associated with AMORE under the NAS Program, and the project was extended for six months.
The double perovskites were intensively studied by the academic partners throughout the project, as part of the materials work package. More than 70 different compositions were synthesized in ceramic form and the Curie temperatures and intergranular magnetoresistance were measured. A series of detailed studies of the crystal structure, defects, magnetic and electronic structure were conducted using special techniques such as neutron diffraction, X-ray magnetic circular dichroism, photoemission NMR and Mossbauer spectroscopy. Results included a demonstration of a ferromagnetic spin structure in SFMO with a small Mo 4d moment of about 0.4 Bohr magnetons, and an intermediate valence state for iron. Antisite defects had an important influence on the magnetic and electronic properties; their concentration was determined by X-ray diffraction and Mossbauer spectroscopy. It was typically 1 – 10 %. Processing methods were developed which reduced the concentration of these defects. This part of the work was state-of-the-art materials science, and it led to numerous publications in refereed journals. The technical aspects of materials preparation and characterization were aimed at improving two key features of these half-metallic oxides, their Curie temperature (TC) and magnetoresistance ratio (MR), so that they could be used in practical sensors which have an operating temperature range from –40 to 120 °C. It was therefore necessary to increase both TC and MR; targets were TC > 180°C and MR > 3 % in a magnetic field of 100 kA m-1. Successful strategies foe increasing TC were electron doping (substitution of La for Sr or substitution of W or Re for Mo). The magnetoresistance was increased by careful control of the thermal processing of the ceramic, and by the development of a new combustion synthesis route. In this way the target values were exceeded, although it proved difficult to meet them simultaneously for the same material. However, results were sufficiently close to target to continue the project. The techniques for producing SFMO and the best of substituted materials were successfully scaled-up from the gram scale to the 0.2 kg scale, which was sufficient for industrial scale production of thick films. In order to produce contactless potentiometers from the new oxides, the inks including the magnetoresistive powders had to be screen printed on ceramic substrates, and processed to create stable magnetoresistive tracks. This proved more difficult than expected. The alumina substrates had to be buffered with YSZ in a separate process to achieve adhesion. However, thick films with good adhesion showed poor magnetoresistance, and vice versa. Another snag was the moisture sensitivity of SFMO, which required a protective barrier layer. Meanwhile the design work on contactless potentiometers proceeded apace.
Three potential design configurations were optimised and evaluated by ATIPIC, and Nacesa chose the most suitable for prototype manufacture. Three different types of permanent magnet were evaluated (ferrite, SmCo, NdFeB). Integrated temperature compensation and offset electronics were developed for incorporation into the device At the end of the project the temperature compensation is not included due to the lack of stability of the output signal, but the electronic circuit is ready. Prototype evaluation at room temperature was done using tracks of another half-metallic oxide (La-.7Sr0.3) MnO3, (LSMO) which has a lower Curie temperature than SFMO, but provided the necessary 4 V signal at room temperature. Cost analysis was based on this prototype potentiometer. However, production is not feasible with the present state of the screen-printed magnetoresistive tracks. In the later stages of the project, emphasis was switched to another half-metallic oxide, Fe3O4 (magnetite) that has a high Curie temperature (560°C). It offers superior temperature stability, but relatively low MR (< 1 %).
With further development, it is likely that it can be successfully adopted for contactless potentiometers with an extended operating temperature range.The other side of the work in AMORE involved thin film devices, for which the potential industrial end user was Thales. Here the research was more long-term, the ultimate goal being to achieve 100 % MR in all-oxide tunnel junctions operating at room temperature, thereby dispensing with the switching electronics, needed for example for MRAM. The films of SFMO with appropriate mechanical, magnetic and electrical properties were first grown epitaxially by pulsed laser deposition on single-crystal substrates. A problem with parasitic iron-rich phases was overcome by careful control of the process variables throughout the deposition cycle, in a three-step process. An SrTiO3 (STO) tunnel barrier layer was added, but it was impossible to produce a top SFMO electrode by the same process so it was decided at midterm to concentrate on SFMO/STO/Co structures. To overcome a problem of surface roughness, Thales developed a novel nanoindentation technology using a conducting AFM tip. In this way tunnel junctions were produced which exhibited 50 % MR at low temperature, indicating a spin polarization of SFMO in excess of 80 %.
The newly-developed oxide tunnel junction technology was applied to LSMO/STO/LSMO tunnel junctions where an absolute record MR for any tunnel junction of 1900 % was observed at low temperature. This observation dramatically confirms the principle of using oxide half-metals in these devices, which was the basis of the AMORE project. The Polish partners contributed to various aspects of the basic science and related technology in the last year of AMORE, providing NMR characterization of several oxide series, growing lattice-matched substrates for pulsed laser deposition, exploring room temperature ferromagnetic semiconductors and developing the technology for making ohmic contacts to these materials.
Funding SchemeCSC - Cost-sharing contracts
08290 Cerdanyola Del Valles
02 668 Warszawa
02 668 Warszawa