Sub-nanosecond switching of spin-transfer-torque device for energy-assisted perpendicular magnetic recording

Abstract

One of the key challenges for perpendicular magnetic recording (PMR) heads in hard disk drives (HDD) is to generate high enough magnetic field to switch the magnetization of the recording media grains with high magnetic anisotropy. In a start-of-art writer design a trailing shield (TS) separated from the main magnetic pole (MP) by a narrow non-magnetic top gap (TG) is used to produce a high down-track field gradient, the narrow TG however causes flux leakage between MP and TS and reduces the maximum field amplitude seen by media. To mitigate this trade-off a new magnetic write head structure was proposed to have an additional spin-transfer torque (STT) device, with a current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) structure, inside this narrow TG, as shown in Fig 1(a) [1] . During write operation, a bias current is applied and electrons flow from the soft magnetic spin torque layer (STL) to TS. The electron flow generates a large enough torque to the magnetization of the STL and makes it overcome the large gap field and switch its direction to be antiparallel to the gap field and TS magnetization. The antiparallel STL magnetization reduces the flux leakage, improves both field amplitude and gradient seen by the media. This mechanism of STL magnetization switching direction is similar with the switching of the free layer magnetization to antiparallel state in spin-transfer-torque magnetic random-access memory (STT-MRAM), except that here the STL magnetization switching has to overcome a very large magnetic field, and that it has to switch very quickly when writing high frequency data patterns. The write time of STT-MRAM is typically a few nanoseconds, the STL switching time however needs to be in less than about 500 picoseconds, or the shortest bit length in magnetic recording. In this study we measured the switching time of the STL and found it can be 300 picoseconds or below, as predicted by micromagnetic simulation.