Layer-Selective Switching of a Double-Layer Perpendicular Magnetic Nanodot Using Microwave Assistance

Layer-selective manipulation of the magnetization direction in a multilayer magnetic structure offers a way to develop a large-capacity magnetic recording device with multiple recording layers. Here, we use a double-layer perpendicular magnetic nanodot consisting of two layers with low and high perpendicular magnetic anisotropy (PMA) and show that layer-selective magnetization switching can be accomplished by utilizing a microwave-assisted magnetization switching technique. Since the low- and high-PMA magnetic layers have different ferromagnetic resonance frequencies, their magnetization excitation that reduces the switching field can be individually induced by adjusting the frequency of the applied microwave field ($f_{\mathrm{rf}}$), and this principle allows $f_{\mathrm{rf}}$ to select the layer to be switched. In addition, by measuring thermally excited ferromagnetic-resonance signals from the nanodot, we find that the magnetization dynamics of the two PMA layers couple through interactions, and we identify the excitation modes responsible for the layer-selective magnetization switching.

Figure 1

(a) Stacking structure of the TMR film consisting of two perpendicular magnetic layers, a MgO tunnel barrier, and two in-plane magnetic layers. The order of layers shown for the $\mathrm{Pt}/\mathrm{Co}$ multilayers is from bottom to top. All thicknesses are given in angstroms except for the MgO layer where the area product value is given. (b) Perpendicular $M\text{−}H$ loop for the film sample. In the inset, the dashed line shows a minor loop of UL switching. (c) $H_z$ dependence of VNA reflection spectra for the film sample. Dashed lines show the fitting results using Eqs. (1) and (2).

Figure 2

(a) Sample structure and experimental setup. (b) $H_z$ dependence of sample resistance obtained by applying $I_{\mathrm{dc}}=+5\mu \mathrm{A}$. Schematics represent the magnetization configuration of the sample. (c) Resistance loops for all four magnetization states ($d$, $d$), ($u$, $d$), ($d$, $u$), and ($u$, $u$).

Figure 3

(a) $H_z$ dependence of sample resistance with initial state being ($d$, $u$) for several values of $f_{\mathrm{rf}}$. Current $I_{\mathrm{dc}}$ is set to $+5\mu \mathrm{A}$. Each curve is offset for clarity. (b) $f_{\mathrm{rf}}$ dependence of $H_{\mathrm{sw}}$. Circles and error bars represent the average, maximum, and minimum values among 20 repeated measurements. (c) MTJ signal spectra obtained by applying $I_{\mathrm{dc}}= +1.6\mathrm{mA}$. During spectra measurement, $H_z$ is swept in the upward direction. The discontinuity in spectra at $H_z=+3200\mathrm{Oe}$ corresponds to LL switching. Open circles represent the average $H_{\mathrm{sw}}$ shown in (b).

Figure 4

(a) $H_z$ dependence of sample resistance with initial state being ($u$, $d$) for several values of $f_{\mathrm{rf}}$. Current $I_{\mathrm{dc}}$ is set to $+5\mu \mathrm{A}$. Each curve is offset for clarity. The dashed circle indicates an intermediate resistance, which corresponds to partial switching of UL. (b) $f_{\mathrm{rf}}$ dependence of $H_{\mathrm{sw}}$. Circles and error bars represent the average, maximum, and minimum values among 20 repeated measurements. For $f_{\mathrm{rf}}=16$ to 19 GHz, filled circles and open circles represent partial switching and subsequent complete switching, respectively. (c) MTJ signal spectra obtained by applying $I_{\mathrm{dc}}= +1.6\mathrm{mA}$. The discontinuity in spectra at $H_z=+4300\mathrm{Oe}$ corresponds to UL switching. Open circles represent the average $H_{\mathrm{sw}}$ shown in (b).

Figure 5

(a) $H_z$ dependence of sample resistance with the initial state being ($d$, $d$) obtained for several values of $f_{\mathrm{rf}}$. The current $I_{\mathrm{dc}}$ is set to $+5\mu \mathrm{A}$. The curve for $f_{\mathrm{rf}}=20\mathrm{GHz}$ is multiplied by 4 and each curve is offset for clarity. (b) $f_{\mathrm{rf}}$ dependence of $H_{\mathrm{sw}}$. Circles and error bars represent the average, maximum, and minimum values among 20 repeated measurements. Lines represent $H_{\mathrm{sw}}$ of LL and UL from the ($d$, $u$) and ($u$, $d$) configurations [the same results shown in Figs. 3 and 4]. The inset shows the number of occurrences of LL, UL, and simultaneous switching. (c) MTJ signal spectra obtained by applying $I_{\mathrm{dc}}= +1.6\mathrm{mA}$. The discontinuities in spectra at $H_z=+2300$ and $+3200\mathrm{Oe}$ correspond to UL and LL switching. Open circles represent the average $H_{\mathrm{sw}}$ shown in (b).

Figure 6

Manipulation of magnetization states with and without microwave assistance.