Perpendicular Magnetic Random Access Memory Research Articles

A perpendicular graphene/ferromagnet electrode for spintronics

We report on the large-scale integration of graphene layers over a FePd perpendicular magnetic anisotropy (PMA) platform, targeting further downscaling of spin circuits. An L10 FePd ordered alloy showing both high magneto-crystalline anisotropy and a low magnetic damping constant, is deposited by magnetron sputtering. The graphene layer is then grown on top of it by large-scale chemical vapor deposition. A step-by-step study, including structural and magnetic analyses by x-ray diffraction and Kerr microscopy, shows that the measured FePd properties are preserved after the graphene deposition process. This scheme provides a graphene protected perpendicular spin electrode showing resistance to oxidation, atomic flatness, stable crystallinity, and perpendicular magnetic properties. This, in turn, opens the way to the generalization of hybrid 2D-materials on optimized PMA platforms, sustaining the development of spintronics circuits based on perpendicular spin-sources as required, for instance, for perpendicular-magnetic random-access memory schemes.

Origin of the Resistance-Area-Product Dependence of Spin-Transfer-Torque Switching in Perpendicular Magnetic Random-Access Memory Cells

We report on an experimental study of current-induced switching in perpendicular magnetic random-access memory (MRAM) cells with variable resistance-area products (RAs). Our results show that in addition to spin-transfer torque (STT), current-induced self-heating and voltage-controlled magnetic anisotropy also contribute to switching and can explain the RA dependencies of the switching-current density and STT efficiency. Our findings suggest that thermal optimization of perpendicular MRAM cells can result in significant reduction of switching currents.

Reversibly controlled magnetic domains of Co film via electric field driven oxygen migration at nanoscale

Electric field control of perpendicular magnetic anisotropy (PMA) can enable low power consumption for perpendicular magnetic random access memory devices. However, the tuning of PMA by the electric field in ferromagnetic metal thin films is less efficient and limited to the interface due to the screening effect. Alternatively, the magnetoionic effect can control PMA efficiently, which utilizes ion migrations over the surface of the ferromagnetic metal by interfacing it with the charge reservoir of oxygen ions like GdOx. In this paper, we report the reversibly controlled magnetic domains of PMA Co in the Pt/Co/GdOx trilayer via the electric field at the nanoscale using conductive atomic force microscopy and magnetic force microscopy (MFM). The magnetic domain phase values determined by MFM decrease and increase when negative and positive bias voltages of magnitude 4 V are applied to the surface of GdOx, respectively. These results suggest a path toward control of PMA materials at the nanoscale by the electric field for information storage devices.

Low current writing perpendicular magnetic random access memory with high thermal stability

Spin reorientation temperature (TSR) is investigated in both layers, including Co/Ni multilayer and composite layer consisting of Co/Pt multilayer with CoFeB layer. The TSR of both layers can be adjusted in a wide range, and the low TSR is obtained in both layers. It is also suggested a writing method based on spin transfer torque (STT) with spin reorientation, for low current writing perpendicular magnetic random memory with high thermal stability. This method is to reorient the magnetization from a perpendicular to an in-plane state, which can be caused by Joule heating the MTJ with thermal barriers above TSR by the injected current pulse. This polarized current can also produce the STT to push the magnetization toward a deterministic perpendicular direction from an in-plane state, when decreasing the magnitude of current pulse. Since the perpendicular anisotropy dominates magnetization switching during cooling, the critical current density is determined by the Joule heating for realization of spin reorientation in this writing method. Macrospin modeling shows the critical current density is 2×106A/cm2, which is two orders magnitude lower than that of conventional STT switching. This writing method can be applied to the perpendicular magnetic tunnel junction comprised of Co/Ni multilayer or composite layer.

Spin-transfer-torque efficiency enhanced by edge-damage of perpendicular magnetic random access memories

We numerically investigate the effect of magnetic and electrical damages at the edge of a perpendicular magnetic random access memory (MRAM) cell on the spin-transfer-torque (STT) efficiency that is defined by the ratio of thermal stability factor to switching current. We find that the switching mode of an edge-damaged cell is different from that of an undamaged cell, which results in a sizable reduction in the switching current. Together with a marginal reduction of the thermal stability factor of an edge-damaged cell, this feature makes the STT efficiency large. Our results suggest that a precise edge control is viable for the optimization of STT-MRAM.

차세대 자기저항메모리 MRAM 기술의 특허동향 분석

차세대 메모리 소자 중 MRAM(Magnetic Random Access Memory)은 자기저항효과를 이용한 비휘발성 메모리로 기존 메모리를 대체할 것으로 주목을 받고 있다. MRAM은 자기터널 접합 소자를 이용해 구동할 수 있는데, 현재 고집적, 저 전력 소모 등의 장점을 극대화하기 위해 수직자화 특성을 갖는 자기터널 접합 개발과 스핀전달토크를 이용해 구동하는 STT-MRAM(Spin Transfer Torque-MRAM) 개발이 활발히 이루어지고 있다. 따라서 미국, 일본 등 MRAM 강국에서 고집적, 스위칭 전류 감소, 열적 안정성 등의 문제를 해결하기 위한 기술 특허 출원이 증가하고 있으며, 국내의 MRAM 연구기관에서의 특허 출원도 꾸준히 이루어지고 있다. 본고에서는 기존 국내외 특허 출원 및 등록 경향을 분석하고 향후 MRAM 개발방향을 제시하였다. Among the next generation memory, MRAM (Magnetic Random Access Memory) is worthy of notice for substituting the preexisting memory thanks to its non-volatile property and other advantages. Recently perpendicular MRAM and spin transfer torque MRAM techniques are under active investigation to realize a high density and low power consumption. As a result, there are increasing of patents applications for high density, low current density for magnetization switching and high thermal stability. In this paper, we analyze the trend of patent applications and registrations about MRAM and propose a direction of future investigation.

Thermomagnetic writing on deep submicron-patterned TbFe films by nanosecond current pulse

This work studies the heating process for deep submicron-patterned TbFe films to be used in a thermally assisted perpendicular magnetic random access memory's writing scheme. The dependence of the heating power density with the current pulse width required for the successful writing was measured in the investigated range of 5–100 ns. In the case of long current pulse, the heat diffuses dominantly into substrate, which resulted in large variation of the required power/energy density with the patterned size. The power/energy densities required for writing increased as the junction area is reduced. While for the short current pulse width, the power/energy densities became rather independent on the size. The required power density for writing 0.38×0.28 μm 2 patterned films using the pulse width of 5 ns is experimentally estimated to be around P=4.7 mW/μm 2, corresponding to the energy density of E=23 pJ/μm 2, under an external field of 100 Oe.

Magnetization Reversal Technique of DyTbFeCo Films at Magnetic Field as Low as 1/6 of Coercivity Using a Stress-Induced Magnetic Anisotropy

A method to cause magnetization reversal in rare earth-transition metal (RE-TM) films at low magnetic field for perpendicular magnetic random access memory (MRAM) was demonstrated. The magnetic anisotropy was changed from perpendicular to in-plane in DyFeCo and TbDyFeCo films while the mechanical tensile stress is applied to the films. It is succeeded to cause magnetization reversal at low magnetic field as low as 1/6 of original coercivity of the film

Perpendicular giant magnetoresistance composed of [Co∕Pt] multilayer and CoFe∕TbCo

Perpendicular spin valves composed of [Co∕Pt] multilayer and CoFe∕TbCo bilayer were fabricated. The perpendicular giant magnetoresistance ratio of 4.8% was obtained with current-in-plane measurements. The interlayer coupling increased with increasing the net magnetization of CoFe∕TbCo. We proposed a perpendicular pinning structure which possessed a high perpendicular coercivity (7000 Oe) but zero net moment. Consequently, the interlayer coupling between free and pinned layers can be significantly suppressed. This perpendicular pinning structure can be potentially used for high-density perpendicular magnetic random access memory cells.