Nonhysteretic Vortex Magnetic Tunnel Junction Sensor with High Dynamic Reserve

Multiple geometrical conditions for different magnetic vortex states to appear in micrometer-sized magnetic tunnel junctions (MTJs) are investigated in experiment and simulation. Both results match up well, providing clear images of vortex behaviors. We pattern a compact array of single-vortex MTJ elements and perform magnetotransport and noise measurements. This sensor features nonhysteretic behavior, a small size of 0.5 × 0.5 ${\mathrm{mm}}^2$, a low normalized noise of ${10}^{-13}{\mathrm{Hz}}^{-1}$, a detectability of $18nT/\sqrt{Hz}$ at 1 Hz, and a large dynamic range of 100 Oe. Its 115 dB broad dynamic reserve and superior stability against temperature and environmental stray fields are ideally suited for magnetic sensing applications.

Figure 1

Magnetic conductance transfer curve of MTJs in (a) single-vortex (SV) state (X = 5 µm, Y = 5 µm), (b) double-vortex (DV) state (X = 14 µm, Y = 7 µm), and (c) antivortex (AV) state (X = 6 µm, Y = 10 µm). Spin maps on the right side of each curve show the simulated magnetization at different positions of the transfer curve. In (b), an enlarged view shows a small hysteresis. (d) Summary of all simulation results, showing the vortex-state diagram over a wide range of X and Y dimensions.

Figure 2

Micrograph of the MTJ array sensor with an enlarged view of local MTJ circular elements.

Figure 3

(a) Transfer curve of the MTJ array sensor shown in Fig. 2. Inset shows the linear hysteresis-free response of the sensor near zero field. (b) Sensitivity of the sensor versus field. Inset shows the peak sensitivity versus temperature. (c) Voltage noise at 1 Hz versus field. (d) Magnetic field detectability at 1 Hz versus field. Results are shown at T = 250, 300, and 350 K.

Figure 4

(a) Normalized noise spectra at position 1 and position 2 in Fig. 3, with 1/f noise as the dashed line. (b) Scaling relation between voltage noise and sensitivity at position 1 and position 2 measured at 300 K. Linear dashed line is a guide to the eye.