Magnets and Recordable MDs


Sony claims in their Magneto Optical Data Storage FAQ that "Data may be stored with magneto-optical technology for more than thirty years without loss or degradation. Once written to the disk, data are safe from the magnetic fields and heat found in normal environments". This document attempts to look more closely at this claim and investigate what sorts of magnetic fields are sufficient to cause data corruption on Minidisc.

Basic Facts

  1. The MD recording layer is a magnetic rare-earth compound of terbium, iron, and cobalt (and chromium in some cases) with a high (~450kA/m = ~5600 Oe) magnetic coercivity at room temperature [ref 2, Fig 10.32]. Coercivity (measured in Oersteds, Oe) is a property of magnetic material and is defined as the strength 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. Example coercivities: 5.25" 360k floppy: 300 Oe, VHS tape: 750 Oe, DAT tape: 1500 Oe, 1990's hard disk: 1400-2200 Oe [ref 1]).

  2. At the recording layer's Curie temperature (~180°C) [the temperature at which ferromagnetic properties in ferromagnetic materials disappear], coercivity drops to less than 100 Oe (actually about 70 Oe) [ref 2].

  3. Strong magnets can be found (e.g. Neodymium Iron Boron alloys, see that are over 10 kiloGauss (== 10kiloOersted), these will change the magnetization of the MD recording layer (see experiments that demonstrate this effect below).

  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.
  1. Secure Deletion of Data from Magnetic and Solid-State Memory
  2. Magneto-optical Recording Materials, Gambino and Suzuki, Ed. pp 400-414
  3. "Faraday effect" Encyclopdia Britannica Online.
  4. "Magnet" Encyclopdia Britannica Online.


A few MD users conducted experiments with recordable MDs and magnets, verifying the preceding finding, namely that MDs can be affected by strong magnets.

Date: Tue, 16 Dec 1997 17:16:30 -0500
From: "Renard A. Dellafave" 
Subject: MD: Magnetic field WILL erase Minidisc (experiment)

>---"Renard A. Dellafave"  wrote:
>> I'm curious how strong a magnetic field it
>> takes to screw with a recorded MD.

and Walter Thompson replied:

>I strongly suspect that a magnetic field alone would not
>screw up the data on a recorded MD. I know for a fact that
>the magnets you are likely to have around the house have
>no effect on recorded MDs.

But, Renard, being hard headed and loathe to live with mere suspicion, decided to do an MD experiment and find out.

The magnet I used was a Radio Shack 64-1985, B1 of 10800 Gauss. A small button magnet (one, indeed, more powerful than usual, but by no means herculean). The MiniDisc was a Memorex recordable MD with a music recording already on it. The MD unit was a Sony MZ-E40 portable player.

I took some non-magnetic tweezers, opened the MD recording and gently sat the magnet near the inner circumference, gently slid it outwards, and then "spotted" it down a few more places.


Magnetic fields of this strength will damage MD recordings! Warning! Trying to play the disc, the MD unit read the TOC, and then I could feel it reseeking trying to read it better. The disc name never came up. All the track names seemed intact, as did their start locations, and they played, but with regular, periodic, interruptions in the sound. This makes sense, as I only messed with the part of the MD exposed when the shutter was opened. Throughout the tests the unit did not report a disc error.

Well, I did want to know, and $4 is a reasonable price for the knowledge.

Date: Tue, 16 Dec 1997 16:49:27 -0600
From: Shawn Lin 
Subject: Re: MD: Magnetic field WILL erase Minidisc (experiment)

Renard A. Dellafave wrote:
> Magnetic fields of this strength WILL damage MD recordings!

Definitely true! I never thought to try it before, but I was very curious (and very bored... been studying for final exams and I've found ANYTHING is more interesting than that).

Anyhow, for fun I passed a couple large ceramic magnets over a Sony MDW-74 which had only one track of 4m 54s recorded on it. (I did not bother opening the shutter) The magnets are not gigantic, but they're two ceramic magnets I pulled out of a bad magnetron over a decade ago (a part out of our microwave oven). The magnets were strong enough to pick up the MD, so I guess they are of reasonable strength.

After that, I tried to play the MD and found the TOC reads fine, and the song actually played, but at 0m 28s the song started skipping and the counter wouldn't advance. After the MD player struggled for awhile, it skipped to 1m 34s and played fine from that point on.

I also tried passing a large degaussing coil (salvaged from an old TV set) over the MD, but that didn't seem to hurt it as much as the magnet did.

- -- 
Shawn Lin                        SMSU student from Springfield, MO USA

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