Sze Ter Lim

Influence of Ta insertions on the magnetic properties of MgO/CoFeB/MgO films probed by ferromagnetic resonance

We show by vector network analyzer ferromagnetic resonance measurements that low Gilbert damping α, down to 0.006, can be achieved in perpendicularly magnetized MgO/CoFeB/MgO thin films with ultrathin insertions of Ta in the CoFeB layer. Although increasing the number of Ta insertions allows thicker CoFeB layers to remain perpendicular, the effective areal magnetic anisotropy does not improve with more insertions, which also come with an increase in α.

Perspectives of Electric Field Controlled Switching in Perpendicular Magnetic Random Access

Based on requirements for spin-transfer torque (STT)-magnetic random access memory (MRAM), fundamental challenges in current CoFeB- and MgO-based STT-MRAM with perpendicular magnetic anisotropy (PMA) are addressed when scaling down to sub-20 nm. Electric-field (EF)-induced PMA modulation has been proposed to significantly reduce the switching current. As EF alone is unable to induce the magnetization reversal but PMA reduction, additional mechanisms should be applied to realize EF switching. We discuss difficulties in the implementation of currently reported two switching approaches and propose a new approach using Oersted field guided EF switching in MRAM arrays. Our latest experimental results on the EF effect show that the spin reorientation transition can be realized at an EF of 0.8 V/nm for the top-pin CoFeB/MgO magnetic tunnel junctions (MTJs) with the thermal stability factor of as high as 57 in the absence of EF. However, the bidirectional switching of the free layer cannot be achieved at a fixed biasing field, which is attributed to small STT effect and switching uncertainty due to unipolar feature of EF-induced PMA modulation. Perspective of EF-MRAM is analyzed based on currently published data on EF efficiency. We show that the EF switching is very promising for future MRAM applications in spite of challenges in materials and MTJ stack engineering.

Electric Field Assisted Switching in Magnetic Random Access Memory

Electric field (EF)-assisted magnetization reversal is investigated in both top-pinned and bottom-pinned CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). EF modulation in coercivity (Hc) shows an increasing dependence with the thickness of the free layer (tFL) at small tFL values. This abnormal variation was attributed to possible Ta diffusion through the free layer to the interface with MgO barrier. It is found that the bidirectional switching is not achievable using unipolar EF only in our MTJ devices, which is due to small spin-transfer torque (STT) effect and switching uncertainty due to unipolar feature of EF-induced Hc modulation. A bipolar external magnetic field should be applied to realize EF-controlled magnetic random access memory (MRAM). For quasistatic magnetization reversal, the required field can be as low as 10 Oe when Hc is effectively modulated to nearly zero. Switching field as a function of switching time shows that the magnetization switching is dominated by thermal activation. Simulation results show that switching time is mainly determined by the damping constant and the EF efficiency. Different from STT-MRAM, a large damping constant is desired to achieve fast switching. Using a sweeping field method, we show that MTJs with a thermal stability factor as high as 58 can be reliably switched using an EF-modulated anisotropy scheme with a bipolar magnetic field as low as 10 Oe.

Effect of different seed layers with varying Co and Pt thicknesses on the magnetic properties of Co/Pt multilayers

Magnetic and structural properties of Co/Pt multilayers with varying Co (tCo) and Pt (tPt) thickness grown on various seed layers have been examined. It is challenging to grow Co/Pt multilayers as a top electrode with high perpendicular magnetic anisotropy (PMA) for magnetic tunnel junctions which usually comprise of CoFeB/MgO/CoFeB. We show that a thicker Pt layer of 1.2 nm is necessary to attain effective anisotropy (Keff) up to 0.14 MJ/m3. On the other hand, Co/Pt multilayers with high PMA have been achieved for proper textured seed layers of Ru and Pt. In the case of Ru seed layer, a higher Keff = 0.45 MJ/m3 can be achieved for tCo = 0.5 nm and tPt = 0.2 nm. This can be attributed to the lower lattice mismatch (∼4%) within the multilayers and hence a more coherent CoPt (111) structure. Finally, we note that the film roughness could play an important role in influencing the PMA of the multilayers. The highest Keff ∼ 0.6 MJ/m3 is achieved for Pt seed layer.

Control of offset field and pinning stability in perpendicular magnetic tunnelling junctions with synthetic antiferromagnetic coupling multilayer

In a magnetic tunnelling junction (MTJ) with perpendicular magnetic anisotropy (PMA), offset field (Ho) of the free layer is usually controlled by using a synthetic antiferromagnetic (SAF) coupling structure, which is composed of an antiferromagnetic coupling (AFC) layer sandwiched by two ferromagnetic (FM) layers. However, Ho increases significantly as the size of MTJ devices shrinks to accommodate high density. In addition, magnetostatic field in PMA SAF structure tends to destabilize the antiferromagnetic (AFM) alignment of the SAF layers, in contrast to the in-plane anisotropy SAF, where the closed flux forms stable AFM magnetic configuration. Here, we present a double SAF structure to control Ho, while maintaining high magnetic stability of the reference layer (RL). The double SAF consists of FM1/AFC/FM2/AFC/FM3 multilayer. An AFM layer like PtMn is added to further stabilize the magnetic configuration of the double SAF. As the magnetization of other FM layers (FM1 and FM2) is aligned oppositely, the...

Composition and Annealing Temperature Dependence of Magnetic Properties in MgO/Co–Fe–B/Ta/Co–Fe–B/MgO Films

We study the dependence of anisotropy and damping in MgO/Co-Fe-B/Ta/Co-Fe-B/MgO films on annealing temperature and Co-Fe-B composition by vector network analyzer ferromagnetic resonance. The effective areal magnetic anisotropy K_eff · t_eff and Gilbert damping α in samples with Co₄₀Fe₄₀B₂₀ were found to be constant for 240 °C and 300 °C annealing. As the annealing temperature is increased to 350 °C, both K_eff · t_eff and α decrease. Increasing the Fe content in the Co-Fe-B layer to 60% increases its saturation magnetization, but the resulting increase in demagnetization energy is counteracted by the improvement in interfacial anisotropy. Fe-rich layers also exhibit lower α, although the difference in α is not observed for effective Co-Fe-B thickness less than 1.5 nm-the thickness regime where the largest increase in K_eff · t_eff occurs.

Non-proportionality of magnetic anisotropy and damping in CoFeB/MgO-based systems

We study the relationship between anisotropy K and damping α in MgO/CoFeB/Ta/CoFeB/MgO/cap films using vector network analyzer ferromagnetic resonance. Capping the stack with Ta and changing the thickness of the top MgO layer allow us to create significant variations in anisotropy while keeping the thickness and process conditions of the magnetic layer constant. The change in anisotropy can be attributed to the degradation of the CoFeB/MgO interface due to Ta intermixing with MgO upon deposition. This hypothesis is supported by measurements of similar samples with the bottom MgO thickness varied instead, which exhibit no significant change in anisotropy. This method of varying K allows identifying a regime where a spin reorientation transition occurs while α remains constant, proving the non-proportionality of K and α in perpendicular CoFeB/MgO systems. The effects of changing the capping layer material, notably Ru, are also discussed.

A non-collinear double MgO based perpendicular magnetic tunnel junction

Double MgO based magnetic free layers are state-of-the-art solutions for providing high performance perpendicular spin-transfer torque-magnetic random access memory devices. We provide device measurements showing reduction of switching current in perpendicular magnetic tunnel junctions (p-MTJs) using non-collinear ferromagnets on the double MgO template. This structure is engineered by introducing an in-plane ferromagnetic cap, which produces in-plane stray field effects on the free layer. The non-collinear structure delivers ∼53% reduction in critical current density in STT switching without weakening the thermal stability of the devices. The advantages in device performance using non-collinearity in magnetization is sustained down to ∼20 nm MTJs. Micromagnetic simulations suggest inherent differences in the magnetization reversal process between our proposed non-collinear p-MTJ and a well-known double MgO based p-MTJ.Double MgO based magnetic free layers are state-of-the-art solutions for providing high performance perpendicular spin-transfer torque-magnetic random access memory devices. We provide device measurements showing reduction of switching current in perpendicular magnetic tunnel junctions (p-MTJs) using non-collinear ferromagnets on the double MgO template. This structure is engineered by introducing an in-plane ferromagnetic cap, which produces in-plane stray field effects on the free layer. The non-collinear structure delivers ∼53% reduction in critical current density in STT switching without weakening the thermal stability of the devices. The advantages in device performance using non-collinearity in magnetization is sustained down to ∼20 nm MTJs. Micromagnetic simulations suggest inherent differences in the magnetization reversal process between our proposed non-collinear p-MTJ and a well-known double MgO based p-MTJ.

A portable dynamics switching model for perpendicular magnetic tunnel junctions considering both thermal and process variations

Spin-transfer torque magnetic random access memory (STT-MRAM) has become a promising candidate for future nonvolatile and universal memory because it features non-volatility, fast access time, almost unlimited programming endurance and zero standby power [1-2]. The magnetic tunneling junction (MTJ) is the fundamental building element of STT-MRAM. An MTJ consists of two ferromagnetic layers (a free layer, FL and a reference layer, RL) separated by a thin tunneling dielectric film. The magnetization of the FL can be set as parallel or anti-parallel with the RL by a spin polarized current, which leads to low or high resistance state of the MTJ. The applications of STT-MRAM have been successfully demonstrated [3-4]. However, like all the other nanotechnologies, STT-MRAM suffers from process variations and environment fluctuations, such as thermal fluctuations, which significantly affect the performance and stability of MTJ devices. Several studies have been performed to address the impact of the process variations on the reliability of STT-MRAM and the thermal fluctuation effects on the magnetization switching [5-6]. These works either need costly Monte-Carlo simulations with complex macro-magnetic and SPICE models or do not integrate both effects of the thermal fluctuation and process variations. Wang et al [7] developed a compact MTJ switching model for MTJs derived from the macro-magnetic modeling to simulate the statistical electrical and magnetic properties of MTJ due to both thermal fluctuation and process variations; however it is restricted for in-plane MTJs.

Magnetization Switching of a Thin Ferromagnetic Layer by Spin-Orbit Torques

We conducted micromagnetic simulations to investigate magnetization switching dynamics in ferromagnet/nonmagnet bilayers driven by an in-plane current for 1) free layer with out-of-plane anisotropy, 2) free layer with in-plane anisotropy, and 3) free layer with both in-plane anisotropy and electric-field-controlled out-of-plane anisotropy. The effects of various material parameters on the switching performance are discussed. Based on the simulations, a fast and reliable switching scheme may be realized by using in-plane magnetization switching assisted by an electric-field-controlled out-of-plane magnetic anisotropy. This fast and deterministic switching does not require any external magnetic fields.