Magnetic cores need to be readily magnetised by a small force and conversely loose their magnetismjust as easily - just like soft iron or Ferroxcube as we discussed last time. For the production of permanent magnets a whole different set of properties is required where they need to be magnetised readily but NEVER loose their magnetism and that's where Magnadur, another Mullard invention comes in.
Magnadur is a sintered oxide admixture of Barium and Iron and is practically impossible to be demagnetised once magnetised - unlike conventional steel and steel alloy magnets which can demagnetise with a sharp knock or with temperature change. A chief use of Magnadur was for CRT focussing magnets where their stability and light physical mass were a real boon. Here's a picture of a Magnadur focussing ring by Elac: -