Time evolution of fields in strontium ferrite permanent magnets

Field strengths for strontium ferrite permanent magnets built for the Fermilab Recycler and the 8 GeV transfer line have been measured for the past 4 years. The ferrite magnetization exhibits a time dependence parameterized by M1/M/sub 2/ = -9 /spl times/ 10/sup -4/ /spl times/ log (t/sub 1//t/sub 2/) as determined from measurements of a gradient magnet. This parameterization has been checked against several other styles of permanent dipoles, and quadrupole magnets with good agreement.