From Exchange Bias to Magnetic Memory:
A Speckle View on Nanomagnetism


Karine Chesnel
Department of Physics and Astronomy
Brigham Young University

The combination of synchrotron X-ray radiation with in-situ magnetic field offers tremendous possibilities to investigate the electronic and magnetic properties of complex materials, in particular via scattering techniques. We will overview here a specific approach called resonant magnetic scattering performed under coherent light in soft X ray range, which gives the possibility to probe domain morphologies at the nanometer scale in magnetic materials.

Coherent X-ray Resonant Magnetic Scattering (C-XRMS) is an original tool allowing the investigation of nanoscale structural and magnetic domain topologies. The use of a spatially-filtered coherent X-ray beam implies interference effects between the scattering paths and therefore gives information about specific local morphologies1. Furthermore the resonance effect provides a specific sensitivity to magnetic structure2. The resulting coherent scattering pattern exhibits speckle feature, which gives an indirect image of the specific magnetic topologies3. I will present here results obtained on perpendicular exchange bias thin films made by stacking ferromagnetic (FM) Co/Pt multilayers with antiferromagnetic (AFM) IrMn layers4. These systems exhibit a perpendicular magnetization leading to striped domain structures in the plane of the film. Exchange coupling effects appear at the interface between the FM and AFM layers while cooling the system down, below a blocking temperature. The C-XRMS studies evidence that the ferromagnet acquires a strong magnetic domain memory through exchange coupling effects with the antiferromagnetic layer. This phenomenon is interpreted by the formation of a reference magnetic template in the antiferromagnetic underlayer during the cooling process.

             

 

1. Chesnel, K. et al. Phys. Rev. B 70, 180402 (R) (2004)

2. Kortright, J.B. et al. J. Magn. Magn. Mater. 207, 7(1999)

3. Pierce, M. et al. Phys. Rev. Lett.  94, 017202 (2005)

4. Maat, S.  et al. Phys. Rev. Lett.  87, 087202 (2001)