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Various approaches for the more effective spin-orbit torque switching
Suhyeok An,Eunchong Baek,Yeh-Ri Kim,Dongryul Kim,Jin-A Kim,Hyeongjoo Seo,Chun-Yeol You 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.2
Spin orbit torque (SOT) induced magnetization switching is promising technique for the logic-in-memory applications. SOT switching has many advantages compare to the spin transfer torque switching: separated read/write current paths, faster switching speed, independent switching behavior on the free layer damping constant, and less energy consumption. Despite many advantages, there are still unresolved issues in SOT switching. We studied various technique to improve the efficiency of SOT switching. First, we will discuss about the effect of the He+ ion irradiation on the SOT switching, He+ ion irradiation leads not only the magnetic anisotropy, but also the spin Hall angle variations. We will also introduce several approaches in order to achieve field-free SOT switching by introducing lateral in-plane symmetry breaking.
Field free switching of ferrimagnetic GdCo with artificially broken symmetry
Jisu Kim,Seyeop Jeong,Taekhyeon Lee,Suhyeok An,Kab-Jin Kim,Ki-Seung Lee,Chun-Yeol You,Soogil Lee,Byong-Guk Park,Sanghoon Kim 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.2
During the last decade, current-induced magnetization switching by spin-orbit torque (SOT) is of great importance to nanoelectronics due to energy-efficient control of spintronic memory and logic devices. However, it has a problem that an external magnetic field is required to switch magnetization of a device when magnetic layers are perpendicularly magnetized. This implies that there should be limitation of scalability and low energy efficiency because of necessity to use the external magnetic field. Therefore, field-free switching of a magnetization should be achieved for realization of the energy efficient and ultra-fast SOT-driven devices. Here, we report field-free SOT switching of ferrimagnetic GdCo thin films with a He ion microscope (HIM) technique. The Pt(5)/Gd36Co64(5)/Ta(3) structure is irradiated by He ions with the dose range from 5 to 50 ions/nm². We find that magnetic properties of the GdCo layer such as coercivity (HC) and magnetization compensation temperature (TM) strongly depend on ion dose of a He irradiation. Based on the idea that ferrimagnetic properties can be controlled using He ion irradiation method, we introduced a lateral gradient of magnetization by the HIM to form a broken mirror symmetry in the device. The mirror symmetry breaking was observed after the local modulation; TM and perpendicular magnetic anisotropy (PMA) properties were locally modulated by He ion irradiation. As the magnetic mirror symmetry is broken, we observed the field-free SOT switching behavior. The Sz component of the spin current generated by the broken mirror symmetry was also observed. We will discuss details about structural change in the ferrimagnetic GdCo by the He ion irradiation.
Dongryul Kim,Woo-Yeong Kim,Suhyeok An,Eunchong Baek,Ki-Seung Lee,Chun-Yeol You 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
Dynamics of the Domain Wall (DW) has been actively investigated because of its possibility for future memory and logic devices. Up to recently, there are several suggestions for logic and memory devices which is based on DW dynamics including the racetrack memory, especially, Spin Torque Majority Gate (STMG) for logic devices which have logic state assigned by the configuration of magnetization. Plenty of struggles have been tried to reduce the operation energy in the field of DW based devices. One of them is to modulate the geometry of the propagation line of DW. The energy decreases as the magnetic DW propagates to a narrow region of a trapezoidal shaped wire. In the point of view of the STMG operating principle, the transmit direction of logic state is not necessary to move reversibly. Then we can designate domain wall energy reduction direction as a flow direction of information. It means that STMG with a change structure take advantage of smaller operating energy. To identify DW energy is affected by shape, we choose a Pt (5 nm)/Co (1.2 nm)/Pt (4 nm) heterostructure film that has a perpendicular magnetic anisotropy with the trapezoidal shaped wire, which is designed to be 30, 40 and 50 degrees. Here we experimentally confirmed that the finite effective field which push DW to the narrow wire region exists, and the effective field increases as the trapezoidal angle increases. Ultimately, if we find the optimal condition for the trapezoidal pattern size and angle to get large enough effective field, DW even can move spontaneously without the externally driving field or current.
Jisu Kim,Seyeop Jeong,Donghyeon Lee,Taekhyeon Lee,Suhyeok An,Kab-Jin Kim,Ki-Seung Lee,Chun-Yeol You,Sanghoon Kim 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
Ferrimagnet is a material whose magnetic moments are aligned in opposite directions like antiferromagnets with non-zero net moment. In a class of ferrimagnets composed of rare earth (RE)-transition metal (TM), the magnitude of each magnetic moment can be adjusted by changing the ratio of the RE-TM or the measurement temperature. Since the temperature dependence of the magnetization of each element is different, the magnetic moments cancel each other at the magnetization compensation temperature T<sub>M</sub>. In this study, He<sup>+</sup> ions were accelerated with 20 keV energy to a 15-nm-thick Gd<sub>36</sub>Co<sub>64</sub> film with perpendicular magnetic anisotropy. We observe that the T<sub>M</sub> decreases after He<sup>+</sup> irradiation. We also succeed in a local modulation of T<sub>M</sub> using He<sup>+</sup> ion beam. In addition, we will present that magnetic switching properties by current or external field can be modulated by the He<sup>+</sup> irradiation.
Asymmetry of Spin-Orbit Torque induced Magnetization Switching by Local Helium Ion Irradiation
Suhyeok An,Jin-A Kim,Hyeong Joo Seo,Chun-Yeol You 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.2
The spin-orbit torque (SOT) induced magnetization switching based memory devices with perpendicular magnetic anisotropy (PMA) has a perspective as replacement of spin transfer torque (STT). However, magnetization switching using SOT, unlike STT, requires in-plane magnetization symmetry breaking, thus the applying an in-plane external magnetic field is essential. Therefore, several methods have been proposed to break the in-plane magnetization symmetry without the application of an external magnetic field. Among them, there is a few reports that magnetization switching caused by in-plane non-uniformity of PMA creates deterministic SOT-based magnetic field-free switching, but the discussion on how the magnetization reversal starts is still insufficient. In this report, we prepared the sample having in-plane non-uniformity of PMA by locally irradiated He<SUP>+</SUP> ion in Pt(5)/Co(0.8)/MgO(2) structure. And the asymmetry of SOT induced magnetization switching current is observed according to magnetization initial status (+z or -z direction). By local irradiation of He<SUP>+</SUP> ion, the sample is divided two (irradiated and non-irradiated) regions. The PMA energies of two regions are districted and its differences between them increase gradually with higher dose irradiation. The results show different critical switching current according to initial direction of magnetization and this asymmetry reaches ~ 12.3% at dose amount of 45 ions/nm². The domain patterns measured by magneto-optical Kerr microscopy display lower switching current when the domain starts to nucleation at PMA boundary, and it implies that SOT switching can be assisted by internally-formed effective field caused by PMA energy gradient.