Understanding the magnetization dynamics induced by spin transfer torques in perpendicularly magnetized magnetic tunnel junction nanopillars and its dependence on material parameters is critical to optimizing device performance. Here we present a micromagnetic study of spin-torque switching in a disk-shaped element as a function of the free layer's exchange constant and disk diameter. The switching is shown to generally occur by 1) growth of the magnetization precession amplitude in the element center; 2) an instability in which the reversing region moves to the disk edge, forming a magnetic domain wall; and 3) the motion of the domain wall across the element. For large diameters and small exchange, step 1 leads to a droplet with a fully reversed core that experiences a drift instability (step 2). While in the opposite case (small diameters and large exchange), the central region of the disk is not fully reversed before step 2 occurs. The origin of the micromagnetic structure is shown to be the disk's non-uniform demagnetization field. Faster, more coherence and energy efficient switching occur with larger exchange and smaller disk diameters, showing routes to increase device performance.
Spin-transfer magnetic random access memory is of significant interest for cryogenic applications where a persistent, fast, low-energy consumption and high device density is needed. Here we report the low-temperature nanosecond duration spin-transfer switching characteristics of perpendicular magnetic tunnel junction (pMTJ) nanopillar devices (40 to 60 nm in diameter) and contrast them to their room temperature properties. Interestingly, at fixed pulse voltage overdrive the characteristic switching time decreases with temperature, in contrast to macrospin model predictions, with the largest re…
We fabricate devices in which a magnetic nanopillar spin valve makes contact to a Co/Pt bilayer thin film with perpendicular magnetic anisotropy, in order to achieve local control of domains in the Co/Pt bilayer underneath the nanopillar. The goal is to develop the ability to nucleate, detect, and annihilate magnetic skyrmions in the Co/Pt using spin-polarized currents from the nanopillar. We demonstrate the ability to distinguish the local behavior of the Co/Pt film beneath the nanopillar from the extended film and show that the two can switch independently of each other. This allows us to is…
Spin-transfer torque effects in the dynamic forced response of the magnetization of nanoscale ferromagnets in superimposed ac and dc bias fields in the presence of thermal agitation
通过一个通用的纳米柱模型的自旋力矩器件，包括两个铁磁层代表的自由和固定层和一个导电的间隔都夹在两个欧姆接触的静态强制响应的纳米级铁磁体的热波动和驱动的任意强度和方向的交流磁场的自旋转移力矩（STT）的影响进行了研究。STT效应通过布朗磁朗之万方程处理，推广到包括Slonczewski STT项，从而扩展了统计矩方法[Y. P. Kalmykov等人，Phys.Rev.B88，144406（2013）]对纳米柱模型的最一般版本的受迫响应。动态磁化率，非线性频率依赖的直流磁化，动态磁滞回线等，然后评估突出STT对低频热弛豫过程和高频铁磁共振等的影响，证明了这些对自旋极化电流的显著依赖性，并有助于解释STT实验。
The perpendicular shape anisotropy-spin transfer torque-magnetic random access memories (PSASTT-MRAMs) takes advantage of the nanopillar free-layer geometry for securing a good thermal stability factor from the shape anisotropy of the nanomagnet. Such a concept is particularly well-suited for small junctions down to a few nanometers. At such a volume size, the nanopillar can be effectively modeled as a Stoner-Wohlfarth (SW) particle, and the shape anisotropy scales with the spontaneous magnetization by ~ Ms^2. For almost all ferromagnets, Ms is a strong function of temperature, therefore, the …
Temperature plays an important role in spin torque switching of magnetic tunnel junctions causing magnetization fluctuations that decrease the switching voltage but also introduce switching errors. Here we present a systematic study of the temperature dependence of the spin torque switching probability of state-of-the-art perpendicular magnetic tunnel junction nanopillars (40 to 60 nm in diameter) from room temperature down to 4 K, sampling up to a million switching events. The junction temperature at the switching voltage---obtained from the thermally assisted spin torque switching model---sa…
Impact of current paths on measurement of tunneling magnetoresistance and spin torque critical current densities in GaMnAs-based magnetic tunnel junctions
We study the current-induced torques in asymmetric magnetic tunnel junctions containing a conventional ferromagnet and a magnetic Weyl semimetal contact. The Weyl semimetal hosts chiral bulk states and topologically protected Fermi arc surface states which were found to govern the voltage behavior and efficiency of current-induced torques. We report how bulk chirality dictates the sign of the non-equilibrium torques acting on the ferromagnet and discuss the existence of large field-like torques acting on the magnetic Weyl semimetal which exceeds the theoretical maximum of conventional magnetic…
Current-induced switching of magnetic tunnel junctions: Effects of field-like spin-transfer torque, pinned-layer magnetization orientation and temperature
A three-dimensional self-consistent spin transport model is developed, which includes both tunnelling transport, as well as metallic transport. Using the spin accumulation computed either side of a tunnel barrier, spin torques are obtained, and it is shown the model reproduces both damping-like and field-like spin-transfer torques, with the expected sinusoidal angular dependence, and inverse ferromagnetic layer thickness dependence. An explicit solution to the drift-diffusion model is derived, which allows analysing the effect of both the reference and free layer thickness on the spin-transfer…
We simulate the spin torque-induced reversal of the magnetization in thin disks with perpendicular anisotropy at zero temperature. Disks typically smaller than 20 nm in diameter exhibit coherent reversal. A domain wall is involved in larger disks. We derive the critical diameter of this transition. Using a proper definition of the critical voltage, a macrospin model can account perfectly for the reversal dynamics when the reversal is coherent. The same critical voltage appears to match with the micromagnetics switching voltage regardless of the switching path.
Using micromagnetic simulation, we investigate the self-sustained oscillation of magnetic skyrmion in a ferromagnetic circular nanodot, driven by spin-torque which is generated from a reference layer of a circular nanopillar device. We demonstrate, by lowering the value of uniaxial anisotropy constant ($K_u$), the velocity of the skyrmion can be increased and using this property, gyration frequency of the skyrmion oscillator can be enhanced. Annihilation of the skyrmion at higher current densities, limit the gyration frequency of the oscillator, whereas by modifying the $K_u$ value at the edge…
