| Magstripes come in two main varieties: high-coercivity (HiC) at 4000 Oe and low-coercivity (LoC) at 300 Oe but it is not infrequent to have intermediate values at 2750 Oe. High-coercivity magstripes are harder to erase, and therefore are appropriate for cards that are frequently used or that need to have a long life. Low-coercivity magstripes require a lower amount of magnetic energy to record, and hence the card writers are much cheaper than machines which are capable of recording high-coercivity magstripes. A card reader can read either type of magstripe, and a high-coercivity card writer may write both high and low-coercivity cards (most have two settings, but writing a LoC card in HiC may sometimes work), while a low-coercivity card writer may write only low-coercivity cards. In practical terms, usually low coercivity magnetic stripes are a light brown color, and high coercivity stripes are nearly black; exceptions include a proprietary silver-colored formulation on transparent American Express cards. High coercivity stripes are resistant to damage from most magnets likely to be owned by consumers. Low coercivity stripes are easily damaged by even a brief contact with a magnetic purse strap or fastener. Because of this, virtually all bank cards today are encoded on high coercivity stripes despite a slightly higher per-unit cost.
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What is coercivity?
Measured in Oersteds, coercivity is the measure of how difficult it is to encode information on the magnetic stripe. A standard bank card has a coercivity of approximately 300 Oe (Oersteds) and is considered to be low coercivity. In Japan there is a second stripe on the credit cards with a coercivity of 600 Oe. The trend is to move towards higher coercivity with values of 2100, 2750, 3600 and 4000 Oersteds being common. High coercivity magnetic stripes bring a new collection of parameters to the magnetic stripe world and higher is not always better.
Initial coercivity is defined by the type of particles used to manufacture the stripe. Gamma Ferric Oxide will give you a low coercivity stripe, Barium Ferrite will give you a high coercivity stripe. The material alone does not define the final coercivity of the stripe as the manufacturing process will change the value usually in the downwards direction. It is possible to raise the coercivity of particles by including other agents in the slurry.
Coercivity is NOT a measure of signal amplitude. Early versions of high coercivity stripes often had high signal output. This is not a requirement of high coercivity and is not usually a good thing. Most readers available today are setup to read signal levels similar to those defined in the ISO/IEC 7811 standard. Keeping the signal output in this range makes the range of available readers much greater.
Early versions of the high coercivity magnetic stripe were marketed with the name High Energy. This name suggests high output levels and often causes confusion amongst users of the technology.
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Why would I use high coercivity?
The advantage of high coercivity is that it is harder to encode the information on the stripe. This also means that the it is more difficult to erase the information and so problems of accidental erasure are diminished. It is still possible to erase the information, but common household magnets are not usually powerful enough. This means the person who puts the transit card on the refrigerator will not usually damage the encoding on the stripe.
The disadvantage is that although the encoding can be read in a standard low coercivity reader the encoder must be designed to encode high coercivity stripes.
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Is higher coercivity better?
Although the coercivity is a factor in erasing a stripe, it is by no-means the only factor. When a stripe is declared to be a 4000 Oersted (Oe) stripe, it really means that the nominal value is 4000 Oe. There are also lots of particles in that stripe with coercivities of other values. The distribution of the coercivities will typically follow a bell shape curve. The steepness of the bell shape defines the percentage of particles at the stated value, a sharp (steep) curve shows that are a large percentage are the nominal value. A flat curve shows that there are many other coercivities present in the stripe. This is important because it is used to define something called "squareness" of the stripe.
Squareness is a parameter that can be used to help define the susceptibility of a stripe to erasure. A 2700 Oe magnetic stripe with high squareness (sharp curve) has a large number of particles at the nominal coercivity. To erase that stripe, a magnetic force greater than the coercive value will have to be applied to the stripe. Another stripe with low squareness may have a higher nominal coercivity but because there may be a large proportion of low coercivity particles it may be very easy to erase the stripe. |
