These are models for the magnetization of Fe60Pt40 hard/soft composite magnetized along the hard axis of the magnetic hard phase. Image: Copyright Science China Press |
A
magnetic force microscope (MFM) can determine the distribution of stray
fields at a level of tens of nanometers near the surface of magnetic
films, and therefore is an effective tool for observing the domain
structures in magnetic grains of submicrometer size.
At
present, the coercivity, or resistance to becoming demagnetized, of
normal MFM cantilevers is about 0.3 kOe (kilo-Oersteds, or magnetizing
field strength). Because they are affected by the magnetism of the
measured material, the stability of these cantilevers is unsatisfactory.
By applying a FePt layer, the coercivity can reach ~10 kOe. However,
the accompanying high-temperature (over 750 C) annealing spoils the
resolution owing to the growth of the grains and the subsequent increase
in tip radius. It is essential for the fabrication of a
high-performance MFM cantilever to obtain larger coercivity at lower
annealing temperature.
The
group of Professor LI Guoqing at Southwest University of China proposed
a novel method to tackle this problem. With inadequate annealing at 500
C, a kind of hard/soft composite was formed in Fe60Pt40 films. The
strong interaction of the exchange spring between the hard phase and
soft phase contributes to larger coercivity exceeding 5 kOe as the
sample is magnetized along any direction.
The
annealing temperature is lower, and the magnetic properties are
isotropic. Cantilevers coated with this kind of material have improved
stability. The research results were published in Scientia Sinica Phys, Mech & Astron, 2011, Vol 41(10), as a paper entitled ‘Structure and magnetic properties of FexPt100-x films’.
This image shows the coercivity of FexPt100-x films (x = [10, 85]). Image: Copyright Science China Press |
By
researching the properties of (001) textured Fe-Pt hard/soft composite
on MgO(001) substrate, the group found the coercivity is favorable even
if the sample is magnetized along the hard axis of the hard phase. A
so-called tri-domain model was used to schematize the mechanism in view
of the slight coherent strain near the hard–soft interface.
With
a coating of this kind of hard/soft composite, the MFM resolution is
better than 13 nm. This research has initiated the design of the coating
layer for the MFM cantilever by choosing an off-stoichiometric
composition and a matching lower annealing temperature to generate the
exchange spring. The adequate saturation magnetization of the
Fe-enriched alloy satisfies the sensitivity of the MFM cantilever. These
efforts will benefit the fabrication of an excellent MFM cantilever.
This research project was partially supported by a grant from the National Natural Science Foundation of China.
Structure and magnetic properties of FexPt100-x films (in Chinese)
