Hi Guys, Stainless Steel Rat <[EMAIL PROTECTED]> writes: > * "Black_Angel" <[EMAIL PROTECTED]> on Tue, 18 Apr 2000 > | Teorically it's impossible to damage a MD only with a magnetic field. > > No, it is impossible to _erase_ an MD with a magnetic field. But a > sufficiently powerful EM field could warp or crack the metallic layer, > rendering the disc unusable. > [...] > Magnetio-optical media like MD is composed of nominally non-magnetic > metallic layer bonded to an inert substrate. When recording, the MD laser > (the optical part of MO) heats a spot of the metallic layer to the curie > point -- the temperature at which a non-magnetic metal acquires magnetic > properties. Then the read/write (the magnetic part of MO) head hits that > spot with a magnetic field aligned in one direction for "0" and another for > "1". When the spot cools it loses its magnetic properties. Your comments are not quite right. 1) The MD recording layer is a *magnetic* rare-earth compound with a high (~5000 Oe) magnetic coercivity at room temperature. [ref 1] (Coercivity (measured in Oersteds, Oe) is a property of magnetic material and is defined as the amount of magnetic field necessary to reduce the magnetic induction in the material to zero - the higher the coercivity, the harder it is to erase data from a medium.) 2) At the recording layer's Curie temperature (~180c) [the temperature at which ferromagnetic properties in ferromagnetic materials disappear], coercivity drops to less than 100 Oe (actually about 70 Oe) [ref 1]. (cf. 5.25" 360k floppy @ 300 Oe, VHS tape @ 750 Oe, DAT tape @ 1500 Oe [ref 2]). 3) Strong magnets can be found (e.g. Neodymium Iron Boron alloys, see http://www.indigo.com/magnets/mgntneod.html) that are over 10 kiloGauss (== 10kiloOersted), these will change the magnetization of the MD recording layer (see experiments that demonstrate this effect http://www.minidisc.org/md_magnet.html). Ralph Smeets <[EMAIL PROTECTED]> writes: > Atoms reflect more or less light depending on the direction they > have. Thus you can read the direction (thus the information) using a > laser. 4) The MO readout technique relies on a (very slight!) rotation of the polarization of light passing through a magnetic field (known as the Faraday effect [ref 3]). The MiniDisc optical pickup is able to detect these polarization changes in reflected laser light and thereby read the orientation of the magnetized regions of the disc. The Encyclopdia Britannica Online section on "Magnet" [ref 4] is very informative. Notes: 1 kA/m == 12.5 Oe, See chart: www.eclipse-magnetics.co.uk/ferrite.html 1 Gauss == 1 Oersted, See: members.eb.com/bol/topic?idxref=53337 1) Magneto-optical Recording Materials, Gambino and Suzuki, Ed. 2) Secure Deletion of Data from Magnetic and Solid-State Memory http://www.cs.auckland.ac.nz/~pgut001/secure_del.html 3) "Kerr electro-optic effect" Encyclopdia Britannica Online. <http://members.eb.com/bol/topic?eu=46227&sctn=1> 4) "Magnet" Encyclopdia Britannica Online. <http://members.eb.com/bol/topic?eu=51232&sctn=2> Rick p.s. I apologize to those of you who are not Encyclopedia Britannica Online members. It's the best $5/month I've ever spent though. ----------------------------------------------------------------- To stop getting this list send a message containing just the word "unsubscribe" to [EMAIL PROTECTED]
