The perpendicular magnetic anisotropy (PMA) properties of Pt/Co/MgO trilayers are investigated as a function of the MgO sputtering power (PMgO) and its thickness (tMgO), both of which are important parameters affecting the degree of oxygen interpenetration into Co during sputtering. A strong PMA is achieved at small values of PMgO and tMgO, where the oxygen interpenetration into Co is expected to be small. The range of oxygen interpenetration is relatively extended in such a way that it affects both the Pt/Co and Co/MgO interfaces. The PMA properties of as-deposited samples are improved by post-annealing for temperatures up to 400 °C examined in this study, probably due to the diffusion of the interpenetrated oxygen atoms toward the Co/MgO interface. In a structure of Pt/Co (0.6 nm)/MgO (2 nm), which is fabricated at PMgO = 50 W and then annealed at 400 °C, a huge saturation field is achieved (over 40 kOe) indicating a very strong PMA. Between the two interfaces of Pt/Co and Co/MgO, the PMA is mainly due to the former in the as-deposited state, but the contribution of the latter increases with the increase in the annealing temperature.

An accurate method is developed to extract the spin-orbit (SO) effective fields by analyzing harmonic Hall voltage measurements and deriving detailed analytical equations that consider both the z component of the applied magnetic field and the secondorder perpendicular magnetic anisotropy. The method is tested by analyzing the results of a macrospin simulation. The SO effective fields that are extracted from the analysis are consistent with the input SO effective fields that are used in the macrospin simulation over the entire range of the polar magnetization angle and for a wide range (0-2) of the ratio of the planar to the anomalous Hall voltage considered in this study. The accuracy of the proposed method is demonstrated via a systematic study that involves comparing its results with those of a conventional analytical method. NPG Asia Materials (2017) 9, e449; doi:10.1038/am.2017.200; published online 17 November 2017 INTRODUCTIONIn-plane currents in a nonmagnetic (NM)/ferromagnetic (FM) bilayer nanostructure can generate a torque due to the spin-orbit (SO) coupling, which is known as the SO torque (SOT). SOT is sufficient for reversing the magnetization in the FM layer. 1 Numerous studies have been conducted to identify the principal mechanism of the SOT as either the spin Hall effect (SHE) in the NM layer 2-4 or the interfacial SO coupling-frequently referred to as the Rashba effectat the NM/FM interface. [5][6][7][8][9][10][11][12] In a system in which the NM/FM interface is perpendicular to the z axis and the in-plane currents flow along the x axis, spin currents that are polarized along the y axis are generated in the system based on the SHE induced by a bulk SO coupling in the NM layer. The spin currents are injected into the adjacent FM layer, which causes torque to be transferred to the magnetization of the FM layer. The SHE-induced SOT generates a strong damping-like torque (T DL pm × m × y) and a weak field-like torque (T FL pm × y). 13,14 Theoretically, the strength of the SHEinduced SOT is known to be independent of the magnetization direction of the FM layer. For an interfacial SO coupling-induced SOT, spins that are polarized along the y axis accumulate due to the broken inversion symmetry at the NM/FM interface. Direct exchange coupling between the magnetization of the FM layer and the accumulated spins generates a strong T FL but a weak T DL . 7,[15][16][17][18] Unlike the strength of the SHE-induced SOT, the interfacial SO coupling-induced SOT is known to depend on the magnetization direction of the FM layer. [19][20][21] In these two cases, both the SHE and the interfacial SO coupling qualitatively induce the same torque on the FM layer. To identify the dominant mechanism of the SOT, a quantitative analysis of the values of T DL and T FL over an extensive range of magnetization angles is required. [19][20][21]

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Strong interlayer exchange coupling (IEC) and high post-annealing stability are demonstrated for perpendicular synthetic ferrimagnets (p-SyFs) with [Pt/Co]6/Ru/[Co/Pt]3 structures. The observed IEC strength was 2.55 ergs/cm2 for a Ru thickness of 0.35 nm, representing the highest value achieved up to date for similar structures. The IEC remained strong even after annealing at 450oC, for the practically important Ru layer thickness of 0.85 nm. The biquadratic IEC, a parameter quantifying the pinhole effects in SyFs, was confirmed by analyzing the experimental results by using the total energy functional, and its strength increased with decreasing the temperature and Ru layer thickness.

In this study, we derive analytical expressions for the critical switching current density induced by spin Hall effect in magnetic structures with the first- and second-order perpendicular magnetic anisotropy. We confirm the validity of the expressions by comparing the analytical results with those obtained from a macrospin simulation. Moreover, we find that for a particular thermal stability parameter, the switching current density can be minimized for a slightly positive second-order perpendicular magnetic anisotropy and the minimum switching current density can further be tuned using an external magnetic field. The analytical expressions are of considerable value in designing high-density magnetic random access memory and cryogenic memory.