FePt for magnetic data storage beyond 10 Tbit/inch2

In order to achieve ultra high recording densities for future magnetic data storage media with magnetic films consisting of particles a few nanometer in size prepared from magnetic high anisotropy material are required. These nanomagnets with a well defined magnetization di-rection will represent one bit. Therefore a regular arrangement and a perpendicular magnetic texture is required. 
For future magnetic storage media these materials have to meet special criteria with respect to their extrinsic and intrinsic properties. 

Microstructural requirements (extrinsic properties)

In order to achieve ultra high storage densities the media has to consist of small, monodis-pers, magnetically decoupled permanent magnets. For read/write reasons these nanomag-nets have to be periodically arranged on a substrate. This can be realized either by a peri-odical arrangement of the magnetic particles ("particular media") or by a nanopatterning of magnetic thin films ("patterned media"). The lateral length scale of these magnets must be well below 10 nm. In addition the easy magnetization direction of all storage cells is oriented perpendicular to the substrate leading to a magnetic texture ("perpendicular recording").

Magnetic requirements (intrinsic properties)

In order to circumvent data loss due to thermal fluctuations ("superparamagnetic limit") the magnetic material should exhibit the largest possible magnetic anisotropy. For this purpose rare earth transition metal alloys (unfavorable due to easy oxidation of these materials) and L10-phases of binary intermetallic alloys with Pt or Pd are the materials with the largest mag-netocrystalline anisotropy energy densities known so far. Nowadays L10-ordered FePt is in-ternationally favored.

FePt nanoparticles have to meet the following criteria simultaneously:

  • diameter approximately. 5 nm
  • monodispers
  • L10-ordered (chemical order of the Fe and Pt layers)
  • magnetic textured (c-axis oriented)
  • periodically arranged



Figure 1: Bragg-Brentano (left) and Grazing-incidence-diffraction (GID, right) spectra for a room temperature sputter deposited FePt film (film thickness: 148 nm, substrate: SiO2/Si). The spectra are offset for clarity. After deposition (blue) the chemically disordered fcc phase is found. Upon annealing (30 min, 400°C) a phase transformation A1 -  L10 (red) is ob-served. The GID investigations have been performed at the Rossendorf Beamline in Greno-ble. Figure 2: TEM images of two icosahedral FePt nanoparticles before (a) and after (b) 5 keV He ion irradiation at room temperature (fluence: 3x1017 ions/cm2). The Inset shows the Fourier transform of the left image. After ion irradiation the multiple twinned structures are trans-formed into fcc single crystals.


Up to now these material requirements are only fulfilled partially. In the division "Nanofunc-tional Films" investigations on the preparation of the chemically ordered L10-phase and its magnetic texturing are performed. In contrast to the well known annealing procedures here the potential of He ion irradiation on the ordering process is evaluated. An enhanced bulk diffusion essential for the phase transformation is achieved at rather low process temperatures which exhibit a large chemical driving force for the disorder/order transition. Film preparation is usually done by means of magnetron sputtering. The structural quality is determined mainly by x-ray diffraction techniques and transmission electron microscopy. The investigations are performed partially with in-situ x-ray diffraction techniques at the Rossendorf Beam-line in Grenoble. Figure 1 shows exemplarily the determination of the phase transformation A1 - L10. The magnetic properties are evaluated by SQUID magnetometry.

In Fig. 2 the effect of He ion irradiation on the phase transformation of gas phase prepared FePt nanoparticles is shown. After preparation the FePt nanoparticles exhibit an icosahedral shape. This structure is extremely stable against temperature treatment. By means of He ion irradiation the icosahedral structure is transformed into the fcc structure. However, the chemically ordered L10-phase could not be observed so far. This is attributed to the fact that the as-prepared particles were already Pt-rich which is even promoted due to sputtering effects upon ion irradiation.

All these investigations are performed in close collaboration with the divisions "Structure Di-agnostics" and "Theory" of the Institute for Ion Beam Physics and Materials Research and the Institute of Metallic Materials of the Leibniz Instute of Solid State and Materials Research IFW Dresden.


1. Ion Beam Induced Destabilization of Icosahedral Structures in Gas Phase Prepared FePt Nanoparticles
O. Dmitrieva, B. Rellinghaus, J. Kästner, M. O. Liedke, J. Fassbender
J. Appl. Phys. 97, 10N112 (2005).


1. Ion Beam Induced Destabilization of Icosahedral Structures in Gas Phase Prepared FePt Nanoparticles
O. Dmitrieva, M. Acet, M. O. Liedke, B. Rellinghaus, J. Fassbender
Magnetism and Magnetic Materials Conference, Jacksonville, 2004