All-Optical Switching of Magnetic Tunnel Junctions with Single Subpicosecond Laser Pulses

The magnetic tunnel junction (MTJ) is one of the most important building blocks of spintronic logic and memory components for beyond-CMOS computation and communication. Although switching of MTJs without magnetic field has been achieved by charge and spin current injection, the operation speed is limited fundamentally by the spin-precession time to many picoseconds. We report the demonstration of ultrafast all-optical switching of an MTJ using single subpicosecond infrared laser pulses. This optically switchable MTJ uses ferrimagnetic Gd(Fe,Co) as the free layer and its switching is read out by measuring its tunneling magnetoresistance with a $\mathrm{\Delta }R/R$ ratio of 0.6%. A switching repetition rate at MHz has been demonstrated, but the fundamental upper limit should be higher than tens of GHz rate. This result represents an important step toward integrated optospintronic devices that combines spintronics and photonics technologies to enable ultrafast conversion between fundamental information carriers of electron spins and photons.

Figure 1

All-optical switching of Gd(Fe,Co) films. (a) Schematic of Gd(Fe,Co) film structures. Tantalum layers are used as a buffer and capping to prevent film oxidation. (b) The MOKE image of single bubble domains created via AOS by scanning single subpicosecond laser pulses across the boundary between two large magnetic domains in the sample. (c) The coercivity $H_c$ (blue symbols) and saturated magnetization $M_s$ (red symbols) of Gd(Fe,Co) samples versus their Gd composition ($x_{\mathrm{Gd}}$). Samples with Gd composition in the purple-shadowed region (22%–26%) show AOS behavior. The solid lines are used to guide the eyes.

Figure 2

Direct magnetoelectric readout of AOS in Gd(Fe,Co) film by the anomalous Hall effect (AHE). (a) Optical microscope image of a typical Hall device with pillar diameter of $15\mu \mathrm{m}$ and transparent ITO/Cu electrodes; (b) $R_{\mathrm{AHE}}$ hysteresis loop of the device measured by sweeping a perpendicular magnetic field and constant dc bias current of $100\mu \mathrm{A}$; (c) $R_{\mathrm{AHE}}$ of the device measured during AOS by 0.4-ps single laser pulses with 0.5-Hz repetition rate. The consistent change of $R_{\mathrm{AHE}}$ in (b) and (c) confirms that the Gd(Fe,Co) pillar is completely switched by the laser pulses.

Figure 3

AOS of an MTJ with subpicosecond single laser pulses without external magnetic field. (a) Schematic of the MTJ structure used in the experiment. (b) Optical microscope image of a typical MTJ device with ITO electrode on the top for TMR measurement. (c),(d) The MOKE images of the MTJ pillar before and after AOS by a single laser pulse, showing the Gd(Fe,Co) layer is completely switched. The pillar diameter is $12\mu \mathrm{m}$. (e) The $R_{\mathrm{TMR}}(H)$ minor loop measured by sweeping a perpendicular magnetic field, which switches the Co/Pd layers. The red line is the smoothing of the raw data (open circles). (f) $R_{\mathrm{TMR}}$ of the MTJ device measured during AOS by 0.4-ps single laser pulses at 0.5-Hz repetition rate. The changes of $R_{\mathrm{TMR}}$ in (e) and (f) have the same value of $\sim 0.6\pm 0.05\mathrm{\Omega }$, indicating the Gd(Fe,Co) layer has been completely switched.

Figure 4

Repetitive AOS of the Gd(Fe,Co) Hall device. The Gd(Fe,Co) Hall device is exposed to trains of single (red), dual (blue), three (green), and four (magenta) consecutive pulses with $1\text{−}\mu \mathrm{s}$ pulse-to-pulse time spacing and the AHE resistance ($R_{\mathrm{AHE}}$) is measured. The device is switched repetitively by the pulses: the second pulse resets the switching by the first pulse and the fourth pulse resets the switching by the third pulse. Therefore, no $R_{\mathrm{AHE}}$ change is measured for trains of double and quadruple pulses. The result demonstrates 1-MHz AOS repetition rate.