"P.gtoreq.1. and Q.gtoreq.0;selecting magnetization states S(1), S(2), . . . , S(M) corresponding to the island groups G(1), G(2), . . . , G(M), respectively, for each island group G(m) (m=1, 2, . . . , or M) consisting of an island pair, the magnetization state S(m) isdenoted as [S1; S2](m), wherein S1 and S2 are a first and second magnetic state in a first and second magnetic island, respectively" . . . . .