Q. Y. Jin

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-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.

[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}

Co(22 A)/Cu multilayers with Cu thicknesses varying from 6 to 40 A are studied by vibrating‐sample magnetometry and ferromagnetic‐resonance spectroscopy (FMR). The magnetization was found to increase with decreasing Cu thickness, whereas the linewidth of the uniform mode of the FMR was found to decrease. This is attributed to the spin polarization of the Cu layers due to interlayer coupling and its effect on the lowered dimensionality of the Co layers. We also observed a peak on the high‐field side of the uniform FMR mode, which we attribute to antiparallel coupling of adjacent ferromagnetic layers producing an ‘‘optical’’ mode.

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

The perpendicular anisotropy and thermal stability of [CoFeB/Pt]5 multilayers are investigated with various CoFeB thicknesses (tCoFeB) and a fixed Pt thickness of 10.0 A. Magnetic hysteresis loops exhibit a square shape for 2.2–4.0A tCoFeB and suggest that multilayers with appropriate CoFeB and Pt thicknesses display perpendicular magnetic anisotropy (PMA). After post-deposition annealing at temperatures of Ta < 300 °C, no obvious change occurs in the loop shape for tCoFeB = 2.2 A, while the perpendicular coercivity increases with increasing Ta for the sample with tCoFeB = 4.5 A due to the enhanced (111) texture. At Ta ∼ 300 °C, the original square loops for both samples start to tilt, showing that the occurrence of the PMA degradation is independent of the CoFeB thickness. The XRD results indicate that the observed decay of the PMA in CoFeB/Pt multilayers upon postannealing is associated with the interdiffusion and alloying effects at the CoFeB/Pt interfaces.

A STUDY OF INTERLAYER COUPLING IN CO/CU MULTILAYERS

Photoinduced magnetization dynamics in FePt∕CoFe bilayer is investigated by optical pump-probe measurements. The transient Kerr loops exhibit an instantaneous separated switching behavior due to exchange decoupling between FePt and CoFe layers induced by photoexcitation, which subsequently recover to the original uniform switching at a certain delay time of 10–20ps. By analyzing the temporal dependence of magnetization ratio of FePt to CoFe and coercivity of FePt phase, we infer that the decoupling and slow magnetization recovery phenomena are ascribed to the laser-created spin disordering mainly in FePt layer, while the following onset of interfacial coupling results from spin/lattice cooling process.

Thermal stability of CoFeB/Pt multilayers with perpendicular magnetic anisotropy

Two series of perpendicular exchange coupled composites (ECC) films are prepared by dc magnetron sputtering, FePt(5)/[Co(0.2)/Pt(0.3)]N (ECC-I-N) and FePt(5)/[Co(0.2)/Pt(0.6)]N (ECC-II-N), respectively. Structure analyses reveal the epitaxial growth on (001) oriented L10 FePt island-like grains of [Co/Pt]N with (200) orientation. Coercivity HC and remanent coercivity HCR of both series samples decrease sharply compared to FePt, with the increase of the thickness of [Co/Pt]N. The angular dependence of HCR shows excellent angular tolerance.

Ultrafast optical modulation of exchange coupling in FePt∕CoFe composite structure

Tetragonal L10 FePt films are deposited on the pyramid-type Si substrates fabricated by chemical etching process from (100) Si wafers. The films are composed of different easy-axis orientation FePt grains, most of which with the easy axis along substrate normal, the others with the easy axis tilted from the normal direction. When the magnetic field is applied along the perpendicular direction, the easy-axis tilted grains reverse first, and then the magnetization reversal of the whole film is promoted. Small coercivity, suitable α, and high squareness are obtained. And the coercivity increases when the applied field is tilted from the normal direction.

Microstructure and magnetization reversal of L10-FePt/[Co/Pt]N exchange coupled composite films

Micromagnetic simulation is employed to study the effect of fieldlike spin torque in a perpendicularly magnetized tunnel junction. We find that the effect of fieldlike spin torque strongly depends on the current direction, showing the torque significantly reduces the switching time for the electrons flowing from free layer to pinned layer while has less effect for the opposite current. This asymmetric behavior is consistent with the theoretical prediction on the bias dependence of the fieldlike spin torque. The observed absence of preswitching and postswitching oscillations can be attributed to the opposite precession phase of local magnetic moments within the free layer.