Quantitative measurement of voltage dependence of spin-transfer torque in MgO-based magnetic tunnel junctions

Abstract

When an electric current passes from one ferromagnetic layer via a non-magnetic layer into another ferromagnetic layer, the spin polarization and subsequent rotation of this current can induce a transfer of angular momentum that exerts a torque on the second ferromagnetic layer 1-4 . This provides a potentially useful method to reverse 3,5-7 and oscillate 8 the magnetic momenta in nanoscale magnetic structures. Owing to the large current densities required to observe spin-torqueinduced magnetization switching and microwave emission (∼10 7 A cm −2 ), accurately measuring the strength, or even the direction, of the associated spin torque has proved difficult. Yet, such measurements are crucial to refining our understanding of the mechanisms responsible and the theories that describe them 9,10 . To address this, we present quantitative experimental measurements of the spin torque in MgO-based magnetic tunnel junctions 11-14 for a wide range of bias currents covering the switching currents. The results verify the occurrence of two different spin-torque regimes with different bias dependences that agree well with theoretical predictions 10 .Magnetic tunnel junctions (MTJs) consisting of a MgO insulating layer sandwiched between two ferromagnetic layers (S 1 and S 2 in Fig. 1a) were used to provide very large magnetoresistance 11,14 . Such MTJs are now useful as data storage cells in magnetic random-access memories (M-RAMs) and as magnetic-field sensors in magnetic hard disk drives [11][12][13] . The MTJs with a layer structure of Ir-Mn/Co-Fe/Ru/Co 60 Fe 20 B 20 /MgO/Co 60 Fe 20 B 20 were prepared on a MgO substrate using an ultrahigh-vacuum sputtering system (C-7100; Canon ANELVA). The 3-nm-thick bottom Co-Fe-B layer (S 1 ) acts as a spin polarizer. The top Co-Fe-B layer (S 2 ), a 2-nm-thick free layer, is excited by the spin torque. The MgO tunnel barrier is about 1 nm thick. The MTJs are rectangular with dimensions of approximately 70 nm × 250 nm (see the Methods section for preparation details).Resistance-magnetic-field (R-H ) curves measured at a small bias voltage (0.1-0.3 mV) and different in-plane field directions, that is, θ H = 0 and 45 • , are shown in Fig. 1b. θ H is the angle between the applied field direction and the easy axis of the magnetic cell along the long axis of the rectangular cell (see Fig. 1a). The magnetoresistance ratio is defined as MR = (R AP − R P )/R P , where R P and R AP respectively represent resistance in the parallel and antiparallel magnetization alignments of S 1 and S 2 . A positive bias current denotes electron flow from S 2 to S 1 . The magnetoresistance ratio and R P at a small bias voltage are, respectively, 154% and about 120 (R P × (Junction area) = 2 µm 2 ). Figure 1c shows the bias voltage, V b , dependence of the tunnelling resistance, as measured in four different fields (A-D), which are indicated by arrows in Fig. 1b. For antiparallel alignment (curves A and B), the resistance decreases with increasing V b because new tunnelling channels open at higher bias voltages 1...

Ferroelectric control of spin-transfer torque in multiferroic tunnel junctions

“…Since STT is currently the most promising method for magnetization switching in MRAMs, the bias behavior of STT in MTJs with passive barriers has been the subject of intensive research both experimentally [37][38][39] and theoretically [40][41][42][43][44][45][46] . However, exploiting the ferroelectric polarization switching to control the STT components in non-collinear MFTJs has not been investigated.…”

Influence of MgO tunnel barrier thickness on spin-transfer ferromagnetic resonance and torque in magnetic tunnel junctions

“…The spin-torque diode effect enables quantitative measurements of STT parameters. [7][8][9] In this work, we use the spin-torque diode effect to investigate the dependence of in-plane and perpendicular spin torques on MgO tunnel barrier thickness. The tunnel barrier determines the transport properties of the device, as it affects the tunneling magnetoresistance (TMR) ratio, the resistance area (RA) product, and the coupling between the FL and the reference layer (RL).…”

What Constitutes a Nanoswitch? A Perspective

“…Fig.2.3.1b shows a typical resistance versus current characteristic of an MTJ taken from Kubota et al [13] which illustrates the basic physical phenomena underlying both the R and W operations. On the other hand Fig.2.3.1b shows that at sufficiently high positive currents the free layer switches from a P to an AP configuration while at high negative currents it switches from an AP to a P configuration.…”