Switching Behaviors of Co/Pt Nanodots and Its Dynamics under the Assistance of Rf Field

Microwave assisted switching (MAS) of magnetization has been expected as a key technology for future ultra-high density magnetic recording [1,2]. In this switching mode, large amplitude precessional motion of magnetization excited by applying an rf field with GHz frequency range promotes the magnetization switching even in a much smaller field than the coercivity. We have systematically studied the MAS effect on perpendicular magnetic Co/Pt multilayer nanodots [3-5]. From our elaborated experiments, it is found that the non-uniform magnetization precession such as spin wave significantly enhances the MAS effect.

  The Co/Pt dots with diameter ranging from 50 nm to 330 nm were lithographically fabricated on a cross-shaped Pt electrode for anomalous Hall effect (AHE) measurement. The rf field was generated from a Cu strip line with 2 μm in width fabricated just above the dot with insertion of an insulating layer. Fig. 1(a) shows the switching field H_sw of a single Co/Pt dot of 120 nm in diameter as a function of rf frequency f_rf for various rf field amplitude h_rf. For h_rf = 450 Oe, the H_sw significantly decreases from 5.5 kOe to 2.1 kOe with increasing the f_rf until the critical frequency. In the range above the critical frequency, the H_sw steeply increases and coincides with the value obtained in the absence of rf field. It should be addressed that this significant reduction of switching field is much more profound than the theoretical prediction based on the single macrospin model. To elucidate the switching mechanism, dot size dependent switching behaviors are studied as shown in Fig. 1(b). H_sw gradually decreases with increase in the dot diameter. In this figure, simulation results based on two different models, that is, single macrospin and finite cell models, are also plotted. It is very interesting that H_sw for larger and smaller dots agree with the finite cell and single macrospin simulation results, respectively. The finite cell simulation reveals that the this largely enhanced MAS effect for larger dot is resulted from the excitation of non-uniform magnetization precession. This size dependent switching behaviors are well explained by the spin wave dispersion theory for a magnetic disk.

  This work was partially supported by Grant-in-Aid for Sciencetific Research from MEXT, the Management Expense s Grants for National Universities Corporations from MEXT, Strategic Promotion of Innovative Research and Development from JST, and the Storage Research Consortium in Japan.

[1] C. Thirion et al., Nature Mater. 2, 524 (2003), [2] S. Okamoto et al., APL 93, 102506 (2008), [3]  S. Okamoto et al., APEX 5, 093005 (2012), [4] S. Okamoto et al., PRL 109, 237209 (2012), [5] M. Furuta et al., APEX 6 (2013) 053006.