Recently a friend asked me how important it was to keep her hotel room key (a high coercivity magstripe card) away from magnets. Realizing that I don't have data on this but do have a magstripe encoder, a LoCo (low coercivity) card, and a HiCo card, I figured I would test each of the cards against a variety of things that could be encountered in daily use and storage.

Background

The magnetic stripe on a magstripe card or ticket is made using one of two magnetic mixtures: one with high coercivity and one with low coercivity. Coercivity is a property of magnetic materials: it is the field strength required to demagnetize a magnetic material that has been previously magnetized.

Cards with high coercivity (resp. low coercivity) stripes are often referred to as "HiCo" (resp. "LoCo") cards. Often, the magstripe on HiCo cards is black and that of LoCo cards is brown. Conventional wisdom is that HiCo magstripes are used on cards intended for permanent use (like payment cards and identity/access control credentials) while LoCo magstripes are used on paper tickets and disposable plastic cards (like those that would be found in a hotel's access control system). That said, I have found hotel room keys and even paper tickets (used in a French parking garage) with HiCo magstripes. To my knowledge, HiCo cards are more widely available than LoCo cards and the price differential per card is probably not incredibly significant.

Results

for LoCo cards

• Storing atop an inactive (i.e. powered on but screen off, not playing audio) non-MagSafe iPhone for about 8 hours: no effect
• Storing in a large stack of cards including HiCo cards, LoCo cards, and RFID cards for 6 days: no effect
• Wiping against a refrigerator magnet: data corrupted
• Wiping against the cap over the speaker of a Western Electric G3 telephone handset while playing milliwatt tone (a very loud 1000 Hz tone): data corrupted
• Wiping against the cap over the speaker of a Western Electric G3 handset while playing nothing: no effect
• Wiping against the cap over the speaker of a Western Electric G3 handset while playing reorder tone: no effect

for HiCo cards

Everything discussed above does not affect data on a HiCo card.