Threshold Properties of Partially Switched Ferrite Cores
A switching threshold relaxation effect has been observed which is detrimental to high-speed selection of partially switched (time-limited switching) ferrite memory cores. This paper discusses this threshold effect as determined by measurements on cores of 1- to 5-oe coercivity. Two aspects of switching threshold are observed: A static threshold (H f ) which is effective at a time long after switching, and a relaxation effect which is a reduction in threshold immediately after switching. The reduction is considerable and may limit the amplitude of the digit-disturb field that a core may withstand to a small fraction of the coercive force. A first approximation to the switching threshold dependence on time may be given as: H t = H f (1−e −3t/τ) for t>o. This representation is reasonably accurate for most ferrite cores, although a sum of exponentials is necessary for an improved description. The ratio of the static threshold to the coercivity (H f /H c ) is influenced by the duration and polarity (relative to the switching field) of the disturb field, the amplitude of the switching field, the flux state, the ambient temperature, and the ferrite composition. The static threshold of a core switched to the 50% flux state by a 10-nsec pulse may be only half the value of the threshold after switching to the 50% state with a 2-μsec pulse. The relaxation time also depends on the above mentioned parameters (except relative polarity) and, in addition, is influenced by the coercivity of the sample, lower coercivity samples having longer relaxation times. Measurements at ambient temperatures ranging from −195 °C to near the Curie temperature (+250°C) show the product of coercivity and relaxation time to be nearly constant for a given core. The relaxation time is a maximum at the 40–50% flux state and approaches zero at the major loop remanent flux states.