Zongzhi Zhang

Domain nucleation mediated spin-transfer switching in magnetic nanopillars with perpendicular anisotropy

Spin-transfer-driven switching is investigated by micromagnetic simulation in perpendicular spin valve nanopillars with a free layer structure which contains two in-film-plane regions: a main perpendicularly magnetized hard region and a soft nanocore with intrinsic in-plane anisotropy. The temporal magnetization snapshots demonstrate that the current-induced magnetization rotation starts from the nanocore, followed by an incoherent switching process mediated by domain nucleation and expansion. The initial magnetization rotation of nanocore to in-plane direction generates driving force acting on the hard region via exchange coupling, together with locally enhanced spin torque, leading to considerable reduction in both critical current and switching time.

Spin-torque-driven vortex dynamics in a spin-valve pillar with a perpendicular polarizer

Spin-torque-driven vortex dynamics are studied by micromagnetic modeling in a spin-valve pillar which contains a perpendicular polarizer and a vortex free layer. Two kinds of transient oscillations mediated by the vortex-core motion are observed. The oscillations are treated as the competition among the spin torque, gyroforce, Gilbert damping, and the restoring force, governed by the generalized Thiele equation [A. A. Thiele, J. Appl. Phys. 45, 377 (1974)]. The fundamental frequency is dominated by the gyrotropic motion, while the high-frequency oscillation is triggered by the balance of the spin torque and demagnetizing field. The polarity of the vortex core can be switched through a vortex-antivortex pair creation and annihilation process.

Interfacial effect on the ferromagnetic damping of CoFeB thin films with different under-layers

Interfacial effects on magnetic properties are investigated for the as-deposited and annealed Co64Fe16B20 films with different under-layers (Cu, Ru, or Pd). The intrinsic Gilbert damping factor is inferred to be slightly lower than the obtained value of 0.007. We found that both the in-plane coercivity Hc and ferromagnetic resonance linewidth ΔHpp rely on the interfacial morphology. The Cu under-layer provides a rough surface, which offers an extra contribution to the ΔHpp. The surface roughness was greatly enhanced by post-annealing for Cu, while little affected for Ru and Pd. Resultingly, the ΔHpp and Hc of Cu/CoFeB increase significantly after annealing. However, for the annealed Ru/CoFeB sample, the ΔHpp even decreases implying Ru is a proper under-layer material for CoFeB-based spintronic devices.

Spin-torque-induced switching in a perpendicular GMR nanopillar with a soft core inside the free layer

Considerable reduction of the switching current is observed on micromagnetic simulation in a perpendicularly magnetized giant magnetoresistive (GMR) nanopillar with a soft nanocore inside the free layer. In this paper, an analytical model based on the single-domain assumption for both the hard and the soft regions is developed to deeply understand the nanocore effect. Combining the analytical solutions with the micromagnetic simulation results, we could interpret well the essential features of the spin-torque-driven magnetization switching in such GMR pillars with perpendicular anisotropy. The asymmetric critical switching current is attributed to the stray field caused by the fixed layer together with the intrinsic difference in the spin-torque efficiency associated with the current direction. However, such an asymmetric switching could be compensated partially by an asymmetric reduction in the critical current after a soft core is inserted into the free layer. In addition, a slight jump observed in the simulated antiparallel-to-parallel switching curve could also be explained by this model.

Ultrafast laser-induced magnetization precession dynamics in FePt/CoFe exchange-coupled films

Femtosecond laser-triggered magnetization precession dynamics in exchange-coupled hard FePt/soft CoFe films has been studied by the time-resolved magneto-optical Kerr effect. The precession frequency is shown to be independent of the CoFe thickness tCoFe, while linearly proportional to the external field for the sample with tCoFe=7 nm, implying that the magnetization relaxation is governed by a uniform precession mode. The result agrees well with the theoretical prediction based on the Landau–Lifshitz–Gilbert equation. The obtained damping factor α∼0.13 is quite large and is thought to be dominated by the FePt layer. Additionally, the oscillation amplitude strongly depends on the CoFe thickness with a maximum value occurred at tCoFe=7 nm.

Enhancement of perpendicular magnetic anisotropy in Co/Ni multilayers by in situ annealing the Ta/Cu under-layers

The perpendicular magnetic properties of glass/Ta/Cu/[Co/Ni]4/Ta multilayers can be efficiently tuned by in situ pre-annealing the Ta/Cu under-layers at various temperatures (TCu) before the deposition of the Co/Ni stack. As a result of the co-effect of fcc(111) texture and Cu surface roughness, the perpendicular anisotropy Ku and coercivity Hc⊥ exhibit a similar non-monotonous dependence on the TCu, showing minimum values at TCu = 100 °C and maxima at 400 °C for Ku while at 550 °C for Hc⊥. By in situ annealing the under-layers at 550 °C and then post-annealing the whole stack at 250 °C, the Hc⊥ value can be significantly enhanced from 139 Oe up to 620 Oe, which is important for spintronic applications.

High giant magnetoresistance and thermal annealing effects in perpendicular magnetic [Co/Ni]N-based spin valves

The thermal stability and giant magnetoresistance (GMR) of pseudo- and FeMn-biased spin valves with perpendicular magnetic [Co/Ni]N multilayer as free and reference layers are investigated. The observed GMR ratio for the pseudo-spin-valve is as high as 7.7%, but it rapidly decreases below 1.0% after annealing in a perpendicular field at 200 °C. Such poor temperature stability is ascribed to simultaneous switching of the free and reference multilayers caused by loss of their coercivity difference. In contrast, an FeMn-biased sample with a similar structure has a slightly lower GMR signal of 6.5% but exhibits much better thermal stability, with the GMR reduction occurring at an elevated anneal temperature of over 300 °C. This GMR reduction is due to Mn diffusion and a reduction in perpendicular anisotropy.

[Co/Ni]

The dependence of magnetic properties on layer repetition number and Ru thicknesses have been studied for perpendicularly magnetized synthetic antiferromagnet (SAF) in a structure of [Co/Ni]N/Ru/[Co/Ni]3. The optimum SAF with strong antiferromangetic coupling field and large switching field of the net magnetization have been determined and utilized as the reference layer in the pseudo spin valves. Compared with the rapid drop of GMR signal for the normal [Co/Ni]-based pseudo spin valves after annealing at low temperature (Ta) of 150°C, the spin valve with SAF reference layer exhibits much stable thermal stability due to the large switching field difference between the free and reference layers which avoids the simultaneous magnetization rotation. The GMR signal of the SAF spin valve sample is 6.0% at room temperature, it decreases very gradually with the increase of Ta. We attribute the slow GMR reduction observed in the SAF spin valve to the effects of domain formation and perpendicular anisotropy deterioration caused by high temperature anneals.

_{\rm N}

For NiFe/FeMn bilayers, the correlation among the exchange field, the coercivity, the training effect, the hysteretic effect of the angular dependence of the exchange bias, and the rotational hysteresis loss has been studied as a function of the antiferromagnet layer thickness tAFM. With increasing tAFM, all these quantities undergo nonmonotonic variations, except for the monotonic change in the exchange field. The maximal values of the coercivity, its relative change, and the rotational hysteresis loss are almost located at the same tAFM of 3.8 nm. The maximal values of the relative change in the exchange field and of the hysteretic effect of the angular dependence are located at 2.5 and 3.0 nm, respectively. The rotational hysteresis loss and the hysteretic behavior of the angular dependence of the exchange bias have different characteristics. The variations of all physical quantities with tAFM can be ascribed to the irreversible reversal of the antiferromagnet spins, which are governed by the Arrhenius...

-Based Synthetic Antiferromagnet With Perpendicular Anisotropy and Its Application in Pseudo Spin Valves

Effective manipulation of magnetization orientation driven by electric field in a perpendicularly magnetized tunnel junction introduces technologically relevant possibility for developing low power magnetic memories. However, the bipolar orientation characteristic of toggle-like magnetization switching possesses intrinsic difficulties for practical applications. By including both the in-plane (T//) and field-like (T⊥) spin-transfer torque terms in the Landau-Lifshitz-Gilbert simulation, reliable and deterministic magnetization reversal can be achieved at a significantly reduced current density of 5×109 A/m2 under the co-action of electric field and spin-polarized current, provided that the electric-field pulse duration exceeds a certain critical value τc. The required critical τc decreases with the increase of T⊥ strength because stronger T⊥ can make the finally stabilized out-of-plane component of magnetization stay in a larger negative value. The power consumption for such kind of deterministic magnetization switching is found to be two orders of magnitude lower than that of the switching driven by current only.