Effect of polarizer dynamics on current-induced behaviors in magnetic nanopillars

Magnetoelectronic nanodevices such as magnetic random access memory include two essential magnetic layers: a polarizing reference layer and a free layer whose configuration can be changed by spin-polarized current via spin-transfer effect. We present measurements of current-induced behaviors in nanodevices with fixed dimensions of the free layer, and varied dimensions of the polarizer. Current-induced precession occurs only at positive current for sufficiently thick polarizer, and only at negative current for thin polarizer, with an abrupt transition between these regimes. Bipolar current-induced precession was observed for a small range of extended polarizer thickness but the amplitude of precession in these devices is reduced. These behaviors are interpreted in terms of the coupling between magnetic layers caused by spin transfer. We suggest a device architecture utilizing the coupling for improved efficiency.

Figure 1

(Color online) Data for devices with a patterned reference layer: $P8$ [(a), (d), and (g)], $P5$ [(b), (e), and (h)], and $P2$ [(c), (f), and (i)]. (a)–(c) $dV/dI$ vs $I$ at the labeled values of $H$. Curves are offset for clarity. (d)–(f) PSD at $H=750\text{Oe}$, $H=700\text{Oe}$, and $H=450\text{Oe}$, respectively. (g)–(i) cross sections of (d)–(f), at the labeled values of $I$. The peaks are harmonically related.

Figure 2

(Color online) Data for devices with extended reference layer: $E4$ [(a), (d), and (g)], $E3$ [(b), (e), and (h)], and $E2$ [(c), (f), and (i)]. (a)–(c) $dV/dI$ vs $I$ at the labeled values of $H$. Curves are offset for clarity. (d)–(f) PSD at $H=400\text{Oe}$. (g)–(i) cross sections of (d)–(f), at the labeled values of $I$. The peaks are harmonically related.

Figure 3

Total microwave power emitted at 1–13.5 GHz normalized by the largest possible emitted power $P_{\text{max}}$, calculated as described in the text. (a) $P2$ (squares), $P5$ (crosses), and $P8$ (triangles). (b) $E2$ (squares), $E3$ (crosses), and $E4$ (triangles).

Figure 4

(Color online) PSD vs $I$ for sample $E2$ acquired at the labeled values of $H$ rotated by $55°$ with respect to the nanopillar easy axis.

Figure 5

(Color online) (a) PSD for $E2$ at $H=600\text{Oe}$, rotated by $55°$ with respect to the nanopillar easy axis. (b) Integrated power $P_1$ in the first precession harmonic vs its peak frequency $f_1$ at negative current (solid symbols) and positive current (open symbols). Horizontal bars show FWHM. Arrow on top shows the direction of increasing $\left|I\right|$.

Figure 6

(Color online) Numerical simulations of current-induced dynamics at 295 K. (a) PSD vs $I$ for $P8$ at $H=750\text{Oe}$ rotated by $40°$ with respect to the nanopillar easy axis. (b) PSD vs $I$ for $P2$ at $H=450\text{Oe}$. (c) Time trace of in-plane hard-axis component of $s_F$ (line) and $s_R$ (dots) for $P8$ at $I=2\text{mA}$. (d) same as (c) for $P2$ at $-1\text{mA}$. (e) same as (c) at 4 mA and (f) same as (c) for $P2$ at $-2\text{mA}$.

Figure 7

(a) Structure of a device utilizing a secondary FeCr reference layer coupled by STT to the free layer to enhance current-induced switching. (b) Simulated precession onset current $I_C$ vs the FeCr layer thickness $t_R$ at $H=1\text{kOe}$ (solid line), and FWHM of the first precession harmonic at $H=1\text{kOe}$, $I=1.2\text{mA}$ (dots). All calculations performed at 295 K.