Summary of Research Project Results under JSPS FY2001
"Research for the Future Program"

1.Research Institution Tohoku University
2. University-Industry Cooperative Research Committee
 147th Committee on Amorphous and Nano−Crystalline Materials
3.Term of Project FY 1997 〜 FY 2001
4.Project Number 97R14701
5.Title of Project Fabrication and Device Technology of Extremely Small Magnetically Hard Materials

6.Project Leader
Name Institution,Department Title of Position
Yutaka, Shimada Tohoku University, Institute of Multidisciplinary Research for Advanced Materials Professor

7.Core Members

Names Institution,Department Title of Position
Kazuaki, Fukamichi Tohoku University, Graduate School of Engineering Professor
Osamu, Kitakami Tohoku University, Institute of Multidisciplinary Research for Advanced Materials Assoc. Professor
Yosichika, Otani Tohoku University, Graduate School of Engineering Assoc. Professor

8.Summary of Research Results

The superparamagnetism for ferromagnetic materials downsized to the nanometer scale is an unavoidable obstacle to development of the spin-electronics technology. The commercial magnetic recording media that consist of Co-Cr fine particles are now seeing the limitation of the recording density because for further increase of the recording density the particles are required downsizing into the few nm scale region where the superparamagnetism makes the magnetic memory unstable. Confronting this limitation our research project worked on four subjects. The subjects and the achieved results are as following.
1) The surface anisotropy , that is, concentration of the magnetic energy density on the surface of ferromagnetic material is expected to play a dominant role as the size of the material becomes smaller. We made a search for a giant surface anisotropy for a variety of surfaces and interfaces. The result is that for transition metal particles the surface anisotropy is enhanced by forming the interfaces where a stress by hydrogen absorption, increase of orbital moment by rare earth addition, or a hard layer formation by oxide are introduced. In the final stage of the research we found a giant surface anisotropy in the trilayer of MgO/FePt(L10)/Pt, that is several ten times larger than the usually observed surface anisotropy. The ordered L10 phase has also a very high crystalline anisotropy and is expected to be a good candidate as ultra-high density recording recording media
2) The ordered L10 FePt alloy in 1) has, however, been available only when annealed at high temperatures in the range 500〜 700℃. This high temperature annealing destroys the nano-size particle structure. We investigated how to lower this ordering temperature and found that by addition of insoluble metals with low melting points, by addition of interstitially stable boron, by accelerating coalescence of the particles or by making use of the intensive atomic diffusion at the interfaces of Fe and Pt multilayers, the ordering temperature was successfully reduced down to 300℃. Based on these results a process of two-dimensional assembly of ordered FePt particles was developed. This assembly exhibits a powerful feature as a new magnetic recording media.
3) Another means to overcome the superparamangetism in the recording technology is the patterned media. In the media one memory bit is recorded in a small ferromagnetic dot and, if the dot size, the material and the dimension are designed as to overcome thermal agitation, the dot pattern may have a high potential as ultra-high density recording media. In the initial stage of the project Read/Write experiments were performed and a possibility of 29Gbit/inch2 was demonstrated. In the final stage, patterning of 130Gbit/inch2 and a very high potential of FePt were confirmed.

9.Key Words

(1)surface anisotropy、(2)nanocrystal、(3)fine particles
(4)crystalline anisotropy、(5)magnetic recording media、(6)superparamagnetism
(7)FePt ordered phase、(8)patterned media