  Startling magnetic effects on the spontaneous polarization echo in some
silicate glasses at low and ultra-low temperatures have been reported in the
last decade or so. Though some progress in search of an explanation has been
made by considering the nuclear quadrupole dephasing of tunneling particles,
here we show that the effect of a magnetic field can be understood
quantitatively by means of a special tunnel mechanism associated with
paramagnetic impurities. For the Fe-, Cr- and Nd-contaminated glasses we
provide reasonable fits to the published data as a function of applied magnetic
field and temperature.
