Anhysteretic growth of exchange fixed remanent states

Exchange coupling has been studied on fine particles of Co with 35 at.% Ni which are surface oxidized to have an antiferromagnetic Co-Ni monoxide shell. Cooling a sample in the state of remanence through the Néel-temperature of the monoxide, a hysteresis loop is observed which is shifted along the field axis. A field of loop displacement is defined with respect to remanence coercivity as\Delta H_{R}=(H_{+R}- H_{-R})/2. It is found to be quantitatively equivalent to an external field in the anhysteretic magnetization process which occurs if a superimposed alternating field is slowly decreased to zero from a large initial value. An universal curve is obtained by plottingM_{ar}(0)/M_{r}(\infin), the normalized anhysteretic remanent magnetization in zero external field, as a function of\Delta H_{R} \chi_{ar}/M_{r}(\infin), where\chi_{ar}is the anhysteretic susceptibility atM_{ar}(H)=0andM_{r}(\infin)is the saturation remanent magnetization. The curve is valid for all combinations of\Delta H_{R},\chi_{ar}andM_{r} (\infin). The consequences for recording on a tape containing the above particles are discussed.