article . ▁At ▁finite ▁temperature , ▁there ▁is ▁a ▁finite ▁probability ▁for ▁the ▁magnet ization ▁to ▁f lip ▁and ▁reverse ▁its ▁direction . ▁The ▁mean ▁time ▁between ▁two ▁f li ps ▁is ▁called ▁the ▁Né el ▁relax ation ▁time ▁ ▁and ▁is ▁given ▁by ▁the ▁Né el - Arr hen ius ▁equation : ▁ ▁, ▁ ▁where ▁ ▁is ▁the ▁height ▁of ▁the ▁energy ▁bar rier , ▁a ▁product ▁of ▁the ▁magnetic ▁an is ot ropy ▁energy ▁density ▁ ▁and ▁volume ▁; ▁ ▁is ▁the ▁Bol tz mann ▁constant , ▁ ▁the ▁temperature ▁and ▁their ▁product ▁the ▁thermal ▁energy ; ▁and ▁ ▁is ▁a ▁length ▁of ▁time , ▁characteristic ▁of ▁the ▁material , ▁called ▁the ▁attempt ▁time ▁or ▁attempt ▁period ▁( its ▁reci pro cal ▁is ▁called ▁the ▁attempt ▁frequency ). ▁Typ ical ▁values ▁for ▁ ▁are ▁between ▁ 1 0 − 9 ▁and ▁ 1 0 − 1 0 ▁seconds . ▁ ▁The ▁Né el ▁relax ation ▁time ▁can ▁be ▁anywhere ▁from ▁a ▁few ▁nan ose cond s ▁to ▁years ▁or ▁much ▁longer . ▁In ▁particular , ▁it ▁is ▁an ▁exponential ▁function ▁of ▁the ▁gra in ▁volume , ▁which ▁explains ▁why ▁the ▁f li pping ▁probability ▁becomes ▁rapidly ▁neg lig ible ▁for ▁bulk ▁materials ▁or ▁large ▁nan op articles . ▁ ▁Block ing ▁temperature ▁▁ ▁Suppose ▁that ▁the ▁magnet ization ▁of ▁a ▁single ▁super param agnet ic ▁nan op article ▁is ▁measured ▁over ▁a ▁time ▁. ▁If ▁this ▁time ▁is ▁much ▁greater ▁than ▁the ▁relax ation ▁time ▁, ▁the ▁nan op article