(536h) Interplay Among Shape and Magnetic Properties of Core-Shell FePt-MgO Nanomagnets for Spin-Torque Transfer Memory Devices

Colloidal assemblies of FePt nanostructures surrounded by crystalline MgO provide a solution-phase, highly-scalable method to create individual atom-sized magnetic bits for spintronics devices such as Magnetic Random Access Memories (MRAMs) or Spin Transfer Torque Random Access Memory (STTRAM). FePt alloys are particular promising candidates for ultrahigh density recording media owing to their good chemical stability and high magnetocrystalline anisotropy, leading to a superparamagnetic limit as small as 3 nm, such that they are thermally stable over typical data storage periods of 10 years.^[1] Conformal deposition of crystalline MgO tunnel dielectrics on FePt nanostructures has been achieved. Low resistance-area values obtained with MgO-based magnetic tunnel junctions (MTJs) allow switching of the magnetization of the electrodes not only by external magnetic fields but also by spin-polarized tunnel currents. Here, FePt nanocrystals were synthesized by simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl in properly chosen conditions of solvent/surfactant proportions and temperature for rational design of their shape and magnetic properties.^[2] Distinctive FePt morphologies (particles, obloids and rods) were sequentially coated with MgO by vigorously refluxing a benzyl ether solution containing portions of the fabricated nanostructures in the presence of magnesium acetylacetonate, which acted as precursor for MgO. High Resolution Transmission Electron Microscopy (HRTEM) analysis revealed 2 nm coating of crystalline MgO without the need of annealing. Spatially-resolved energy dispersive X-ray (EDX) spectroscopy confirmed the stoichiometry of the oxide shell. A heat treatment was needed to transform the crystal structure of the core from the disordered fcc FePt phase to the L10 phase with high uniaxial magnetocrystalline anisotropy, which is desirable for STTRAM applications. SQUID magnetometry revealed higher coercive fields for obloids and rods, when compared with spherical particles in the core-shell FePt-MgO system. The magnetization and switching behavior were found to be dominated by shape anisotropy according to our experimental results and corresponding theoretical verification.^[3,4]

  References

[1] Plummer van Ek Weller (ed.), The Physics of Ultra-High-Density Magnetic Recording, Springer, Berlin, 2001 

[2] D. A. Ferrer, S. Guchhait, C. Corbet and S.K. Banerjee, Origin of Shape Anisotropy on Wet-chemistry Synthesized Nanostructures, under review

[3] http://math.nist.gov/oommf

[4] S. K. Banerjee, S. Ganguly, D. A. Ferrer, “Templated self-assembly for nanomagnetic memory”, submitted to Office of Technology Commercialization (OTC) of The University of Texas at Austin