I. Introduction
Hard disk drives (HDDs) have taken the central role as the information storage system. In addition, the request of capacity enlargement to HDDs knows no boundaries in a modern information society. To meet it, new technologies, such as shingled magnetic recording (SMR) [1], heat-assisted magnetic recording [2], microwave-assisted magnetic recording [3], and bit patterned media [4], are studied, and the combination with 2-D magnetic recording (TDMR) is expected to achieve higher recording densities.