Single-Shot Time-Domain Studies of Spin-Torque-Driven Switching in Magnetic Tunnel Junctions

Abstract

We report single-shot measurements of resistance versus time for thermally assisted spintorque-driven switching in magnetic tunnel junctions. We achieve sufficient sensitivity to resolve the resistance signals leading up to switching, including the variations between individual switching events. Analyses of pre-switching thermal fluctuations allow detailed measurements of coherence times and variations in magnetization precession amplitude. We find that with a small in-plane hard-axis magnetic field the magnetization dynamics are more spatially coherent than for the case of zero field. Page 2 of 13Magnetization switching induced by spin transfer torque [1,2] is of interest both for probing the fundamental physics of magnetic dynamics and for applications in storage technologies [3,4]. Measurements in the time domain [5][6][7][8][9][10][11] can provide the most direct information about spin-torque-driven magnetic dynamics. However, most previous timeresolved studies [5][6][7][8][9] employed stroboscopic approaches, which average over many events so that they reveal only average behavior and hide individual variations. Magnetic tunnel junctions (MTJs) can now provide sufficiently large resistance signals (relative to metal spin valves [5][6][7][8]) for single-shot measurements. Two initial experiments have measured distributions of spin-torque switching times in MTJs using single-shot techniques [10,11] but they did not resolve the dynamics leading to switching. Here we report single-shot measurements of spin-torque switching in MTJs with sufficiently improved sensitivity to study the pre-switching resistance signals in detail. We observe the variations between switching processes caused by thermal fluctuations and can perform comprehensive analyses of the fluctuations prior to switching. We find that switching is more spatially coherent when the magnetic moments of the electrodes are initially offset (at an angle different than 0º or 180º) than when the moments are collinear.The MTJ samples that we study have the layer structure (in nm): bottom contact . Our discussion of currentdriven reversal will focus on switching from the anti-parallel (AP) state to parallel (P) state because this required approximately 30% lower voltages than P-to-AP switching.Page 3 of 13However, measurements of P-to-AP dynamics are qualitatively similar. We will examine switching both in the case of zero total field on the free layer (in which an applied field cancels H d ) and the case that a small (100 Oe) field is applied along the in-plane hard axis, to rotate the free-layer magnetization approximately 15º from the strictly AP configuration (see inset, Fig. 1(a)). We will present data from a single sample, but have studied two other devices in zero field and one other in the hard-axis field, with all showing the same differences depending on field geometry.We perform single-shot measurements using the circuit shown in Fig. 1(b). After initializing the sample in the AP state we use a pulse generator to produce an 100 ns long, 300 ps ri...

Anomalous Hall Effect Measurement on Nanostructure with Magnetic Pulse Fields

“…We also note that no quasi-ballistic switching was found after precession more than the half period within this simulation, which has been reported for the pulse eld well above H k . [9][10][11][12][13] This behavior is clearly seen in pulse rise time τ r dependence of transverse pulse elds h p required for switching shown in Fig. 10 (a).…”

“…Electrical detection of spin dependent transport properties has been also applied for various structures [8][9][10][11][12][13][14][15][16][17][18][19] . In particular, giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) in junction structures 8,9) have been intensively studied since in the structures magnetization can be manipulated by spin-polarized current. The anomalous Hall effect is another spin dependent transport measurement and has been used as a probe to detect magnetization behavior of perpendicularly magnetized lm because of its terminal geometry.…”

“…Understanding of magnetization behavior of magnetic nanostructures has become more important since behavior of those nanostructures dominate device properties. In order to study magnetization behavior of magnetic nanostructures, various measurement techniques had been developed for nanostructures [1][2][3][4][5][6][7][8][9][10] because the low sensitivity of conventional magnetometry is insuf cient to detect magnetization reversal of a single structures. Detection of magnetic ux would be the most straightforward method.…”

Time-resolved magnetic relaxation of a nanomagnet on subnanosecond time scales

“…It also has recently become possible to drive individual nanometer scale magnetic elements far from equilibrium using spin-currents and probe their dynamical response. However, most electronic transport studies focus on the excitation during the drive pulses 6,7 . Less is known about the relaxation processes following a pulse excitation.…”

Magnetization reversal in sub-100 nm magnetic tunnel junctions with ultrathin MgO barrier biased along the hard axis

“…This, however, may contribute to an enhanced influence of ST effects and of barrier inhomogenieties. With few exceptions, 10 spin torque switching in HA biased MTJs remains poorly understood.…”