Patent Application
20080044346
Currently preferred magnetite particles of the powder composition have a particle size such that at least 99.9% of the particles have a diameter of less than 1.56 μm, and the magnetite particles have an average particle size of about 0.45 μm, but other particle size distributions may also used with good results.
The dependency of magnetic properties of milled natural magnetite particles on average particle size was investigated.
Natural magnetite was milled to 8 different average particle sizes from 0.35 μm to 33.6 μm, after which the saturation magnetisation, remanence and coercivity of all respective different average particle sizes where determined for an external magnetic field of 10 kOe and 1 kOe respectively. The results are given in Table 1 below.
It is clear from the results that in order to obtain the sought combination of saturation magnetisation (75-95 emu/g), remanence (20-40 emu/g) and coercivity (250-500 Oe) at a field strength of 10 kOe, a low average particle size is needed.
Concerning the variables used, “saturation magnetisation” is the limit of magnetisation that a given material can reach i.e. a further increase of an external magnetic field will give no further magnetisation of the material, “remanence” is the magnetization left behind in the material after the external magnetic field is removed (as regards the present description, unless otherwise specified, the external magnetic field is a field of 10 kOe, which is believed sufficient to obtain saturation magnetisation), and “coercivity” is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation (as regards the present description, unless otherwise specified, a magnetic field of 10 kOe was used to obtain saturation magnetisation).
Concerning the units used, Oe stands for Oersted which is the CGS-unit for magnetic field strength and emu/g stands for the dipole moment (“electro magnetic unit”) per mass.
The “diameter” or “particle size” of a magnetite particle is defined as the smallest possible diameter of a sphere which is large enough to essentially encompass the particle.
The “average” particle size is defined as the weigh average particle diameter.
In accordance with a preferred embodiment the powder composition for use in a medium for magnetically storing information comprises at least 98% by weight of magnetite particles, wherein at least 99.9% by weight of the magnetite particles have a particle size of less than 3 μm, and wherein the magnetite particles have a polyhedral shape and essentially isotropic magnetic properties, the magnetite particles having a saturation magnetisation of 80-90 emu/g at 10 kOe, a remanence of 25-35 emu/g and a coercivity of 300-450 Oe.
As mentioned above, satisfactory results may in many cases be achieved also with powders of a slightly different composition, in respect of one or more characteristics, as indicated above and in the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0601697-6 | Aug 2006 | SE | national |
The benefit is claimed under 35 U.S.C. § 119(a)-(d) of Swedish Application No. 0601697-6, filed Aug. 16, 2006, and under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 60/838,898, filed Aug. 21, 2006.
| Number | Date | Country | |
|---|---|---|---|
| 60838898 | Aug 2006 | US |
