Microwave Assisted Magnetic Recording (MAMR)

2        Microwave Assisted Magnetic Recording

2.1       Literature

           

            The Microwave Assisted Magnetic Recording was first introduced by Professor Jimmy Zhu in 50^th Annual Conference Magnetism and Magnetic Materials (2005) [11]. The idea is to utilize the ferromagnetic resonance phenomena to help in magnetization reversal. Figure 2 shows the effective switching field with three difference writing angle. The switching field is relatively reduced for all range of external wave frequencies. For each writing angle, there has an optima ac field frequency at which minimum switching field threshold is smallest.

Cited from: J. Zhu, et al., IEEE Trans. Mag. Vol. 44, No. 1, Jan 2008

Figure 7

            The simulation do work well for MAMR, however the way to generate a localized as field in the microwave regime with amplitudes at kOe scale is needed in order to be practical. Local ac field generating scheme utilizing the SMT effect was proposed by Professor Jimmy Zhu in INTERMAG 2006 [12]. Figure 8 shows a proposed design of the ac field generator or spin-torque oscillator (STO). The generated ac field frequency is inversely proportional to the Magnetization saturation (Ms) of the field generation layer (FGL) while the strength of the generated ac field is proportional to it.

Cited from: J. Zhu, et al., IEEE Trans. Mag. Vol. 44, No. 1, Jan 2008

Figure 8

            The ferromagnetic resonance frequency is proportional to the anisotropy field of the material. For magnetic material with anisotropy field of 50 kOe, the ac field frequency generated by FGL required about 100 GHz in order able to reverse the magnetization.  Dr. Goh Chi Keong had proposed by using the square-wave external field to assist the magnetization reversal, lower frequency is required [15]. Further study by Dr. Goh found that lowered switching field can even achieved by just applying 1^st and 3^rd harmonic of the square-wave [14]. The comparison of lowered switching field between sine-wave, square-wave, and the 1^st and 3^rd harmonic microwave is shown in figure 9.

Cited from: C. K. Goh, MMM, 2008

Figure 9

2.2       Issues on MAMR

In order to increase areal densities, a corresponding increase in magnetic anisotropy is required to match the reduction of the grain size. The critical issues for the spin-torque oscillator (STO) is the Ku required for the FGL is about exponentially increase with the Hk of the media, as pictured in figure 10. Since the ac field strength (proportional to Ms of FGL) and the ac field frequency (proportional to Hk of FGL) should correspond increase with higher Hk media in use.

Figure 10