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Bimodal distributions of blocking temperature and their impact in exchange bias spintronic devices of reduced dimensions

Vincent Baltz

V. Baltz, G. Gaudin, S. Auffret, B. Rodmacq, B. Dieny

SPINTEC (UMR CNRS/CEA/Universités), Grenoble

The ability to pin the magnetization of a ferromagnetic layer in a fixed direction in order to define a reference direction for the spin of conduction electrons is a prerequisite to most spintronic devices. Exchange bias which refers to the exchange coupling between a ferromagnet (F) and an antiferromagnet (AF) is most often used for that particular purpose of shifting the hysteresis loop along the magnetic field axis.

Once patterned into arrays of cells, for example in magnetic random access memories MRAM, detrimental distributions of the magnetic properties from cell to cell appear. These distributions notably concern the blocking temperature (TB) i.e. the temperature (T) over which AF grains are no longer stable when cycling the F magnetization. Practical application of exchange bias in spintronic devices requires the reduction of the width of the distributions both at the film level and after patterning in the form of nanopillars. Fulfilling this requirement relies in the understanding of the underlying physical origin(s) of the above mentioned effect.

To start this talk, I will come back on some of the prerequisite to the understanding of exchange bias (simplified granular model, field cooling procedures, how to measure blocking temperature distributions, links with TA-MRAM technological issues). This will drive us to the second part of the talk for which I will deal with bimodal distributions of blocking temperature, which we recently measured. I will insist on the importance of the low-T contribution ascribed to interfacial spin-glass like regions, which are key to spintronics devices even if these latter are operated at higher temperatures. I will show attempts and success in eradicating these low freezing temperature regions by controlling the degree of interfacial frustrations via various ways. In the last part of the talk, I will discuss the effects of patterning sheet films in the form of arrays of nanopillars. We observed two-fold consequences when considering the down-size scalability of spintronic devices, each of them affecting one particular part of the distribution of blocking temperature and thus calling for distinct technological solutions.

- Some reviews on exchange bias :

J. Nogués and I. K. Schuller, J. Magn. Magn. Mater. 192, 203 (1999)

A. E. Berkowitz and K. Takano, ibid. 200, 552 (1999)

- Our latest papers on the subject :

Phys. Rev. B 81, 052404 (2010)

Appl. Phys. Lett. 96, 262505 (2010)