▁deriv ation ▁by ▁William ▁Full er ▁Brown , ▁Jr . ▁ ▁Né el ▁used ▁his ▁theory ▁to ▁develop ▁a ▁model ▁of ▁therm orem an ent ▁magnet ization ▁in ▁single - domain ▁fer romagnet ic ▁min er als ▁that ▁explained ▁how ▁these ▁min er als ▁could ▁reli ably ▁record ▁the ▁geom agnet ic ▁field . ▁He ▁also ▁mode led ▁frequency - dependent ▁sus cept ibility ▁and ▁altern ating ▁field ▁dem agnet ization . ▁ ▁Super param agnet ism ▁Super param agnet ism ▁occurs ▁in ▁fer romagnet ic ▁and ▁f err im agnet ic ▁nan op articles ▁which ▁are ▁single - domain , ▁i . e . ▁composed ▁of ▁a ▁single ▁magnetic ▁domain . ▁This ▁is ▁possible ▁when ▁their ▁diameter ▁is ▁below ▁ 3 – 5 0   nm , ▁depending ▁on ▁the ▁materials . ▁In ▁this ▁condition , ▁it ▁is ▁considered ▁that ▁the ▁magnet ization ▁of ▁the ▁nan op articles ▁is ▁a ▁single ▁giant ▁magnetic ▁moment , ▁sum ▁of ▁all ▁the ▁individual ▁magnetic ▁moments ▁carried ▁by ▁the ▁atoms ▁of ▁the ▁nan op article . ▁This ▁is ▁what ▁people ▁are ▁working ▁on ▁in ▁the ▁sub field ▁of ▁super param agnet ism ▁call ▁“ macro - sp in ▁approximation ”. ▁ ▁Mean ▁transition ▁time ▁Because ▁of ▁the ▁nan op article ’ s ▁magnetic ▁an is ot ropy , ▁the ▁magnetic ▁moment ▁has ▁usually ▁only ▁two ▁stable ▁orient ations ▁ant ip ar allel ▁to ▁each ▁other , ▁separated ▁by ▁an ▁energy ▁bar rier . ▁The ▁stable ▁orient ations ▁define ▁the ▁magnetic ▁easy ▁axis ▁of ▁the ▁nan op