Seung-heon Chris Baek

Large spin Hall magnetoresistance and its correlation to the spin-orbit torque in W/CoFeB/MgO structures

The phenomena based on spin-orbit interaction in heavy metal/ferromagnet/oxide structures have been investigated extensively due to their applicability to the manipulation of the magnetization direction via the in-plane current. This implies the existence of an inverse effect, in which the conductivity in such structures should depend on the magnetization orientation. In this work, we report a systematic study of the magnetoresistance (MR) of W/CoFeB/MgO structures and its correlation with the current-induced torque to the magnetization. We observe that the MR is independent of the angle between the magnetization and current direction but is determined by the relative magnetization orientation with respect to the spin direction accumulated by the spin Hall effect, for which the symmetry is identical to that of so-called the spin Hall magnetoresistance. The MR of ~1% in W/CoFeB/MgO samples is considerably larger than those in other structures of Ta/CoFeB/MgO or Pt/Co/AlOx, which indicates a larger spin Hall angle of W. Moreover, the similar W thickness dependence of the MR and the current-induced magnetization switching efficiency demonstrates that MR in a non-magnet/ferromagnet structure can be utilized to understand other closely correlated spin-orbit coupling effects such as the inverse spin Hall effect or the spin-orbit spin transfer torques.

Alleviation of fermi-level pinning effect at metal/germanium interface by the insertion of graphene layers

In this paper, we report the alleviation of the Fermi-level pinning on metal/n-germanium (Ge) contact by the insertion of multiple layers of single-layer graphene (SLG) at the metal/n-Ge interface. A decrease in the Schottky barrier height with an increase in the number of inserted SLG layers was observed, which supports the contention that Fermi-level pinning at metal/n-Ge contact originates from the metal-induced gap states at the metal/n-Ge interface. The modulation of Schottky barrier height by varying the number of inserted SLG layers (m) can bring about the use of Ge as the next-generation complementary metal-oxide-semiconductor material. Furthermore, the inserted SLG layers can be used as the tunnel barrier for spin injection into Ge substrate for spin-based transistors.

Frequency control of a spin-torque oscillator using magnetostrictive anisotropy

We report the working principle of a spin-torque oscillator, of which the frequency is efficiently controlled by manipulating the magnetostrictive anisotropy. To justify the scheme, we simulate a conventional magnetic-tunnel junction-based oscillator which is fabricated on a piezoelectric material. By applying mechanical stress to a free layer using a piezoelectric material, the oscillation frequency can be controlled to ensure a broad tuning range without a significant reduction of the dynamic resistance variation. Such controllability, which appears in the absence of an external magnetic field, will not only enable the integration of spin-torque oscillators and conventional complimentary metal-oxide semiconductor technology but will also broaden the applicability of spin-torque oscillators.

Complementary logic operation based on electric-field controlled spin–orbit torques

Spintronic devices offer low power consumption, built-in memory, high scalability and reconfigurability, and could therefore provide an alternative to traditional semiconductor-based electronic devices. However, for spintronic devices to be useful in computing, complementary logic operation using spintronic logic gates is likely to be required. Here we report a complementary spin logic device using electric-field controlled spin–orbit torque switching in a heavy metal/ferromagnet/oxide structure. We show that the critical current for spin–orbit-torque-induced switching of perpendicular magnetization can be efficiently modulated by an electric field via the voltage-controlled magnetic anisotropy effect. Moreover, the polarity of the voltage-controlled magnetic anisotropy can be tuned through modification of the oxidation state at the ferromagnet/oxide interface. This allows us to create both n-type and p-type spin logic devices and demonstrate complementary logic operation.Complementary logic devices based on spin–orbit torque can be created in which the tunable polarity of the voltage-controlled magnetic anisotropy effect is used to fabricate n-type and p-type spin logic devices.

Electric field control of magnetic anisotropy in the easy cone state of Ta/Pt/CoFeB/MgO structures

The electric-field control of magnetic anisotropy is of particular interest because it allows the manipulation of the magnetization direction in spintronic devices with high performance and low power consumption. In this work, we investigate the effect of an electric field on the magnetic anisotropy in Ta/Pt/CoFeB/MgO structures, whose easy axis of magnetization is canted from the z-axis, forming a cone state. When an electric field is applied to the sample, its anisotropy constants change, thus modulating the cone state. It is demonstrated that the cone angle is controlled between 22° and 32° by a bias field of 4 MV/cm and that it can persist even after removing the bias. Moreover, it fully recovers to the original value when a bias voltage with an opposite polarity is applied. The non-volatile and reversible control of the cone state paves the way towards the utilization of the magnetic cone state in spintronic devices.

Abnormal bias dependence of magnetoresistance in CoFeB/MgO/Si spin-injection tunnel contacts

We report a strong bias voltage dependence of magnetoresistance (MR) in CoFeB/MgO/Si spin-injection tunnel contacts using the three-terminal Hanle geometry. When a bias voltage is relatively large, the MR is composed of two characteristic signals: a conventional Hanle signal observed at a low magnetic field, which is due to the precession of injected spins, and another signal originating from the rotation of the magnetization at a larger magnetic field. In contrast, for a small bias voltage, additional signals appear at a wide range of magnetic fields, which occasionally overwhelms the conventional Hanle signals. Because the additional signals are pronounced at a low bias and are significantly reduced by annealing at moderate temperatures, they can be attributed to multi-step tunneling via defect states at the interfaces or tunnel barrier. Our results demonstrate that the spin injection signal caused by the defect states can be evaluated by its bias voltage dependence.

Novel Operation of a Multi-Bit SOT Memory Cell Addressed With a Single Write Line

Spin–orbit torque (SOT) originates from the spin–orbit interaction of non-magnetic heavy metals (HMs), allowing for an electrical manipulation of perpendicular magnetization in HM/ferromagnet (FM)/oxide structures. In this paper, we experimentally demonstrate the SOT-induced switching of two FM bits addressed with a single write line. We fabricate a device consisting of two perpendicularly magnetized Ta/CoFeB/MgO structures with a common Ta underlayer, in which the magnetization directions of the two FM bits could be concurrently controlled by injecting a single current pulse. This suggests that multiple bits in SOT-based devices can be written as either “0” or “1” at the same time. Moreover, the selective switching of a specific bit is achieved by differentiating the critical switching currents between the two FM bits, which is crucial in demonstrating multi-level cell SOT memory. Our results provide an efficient writing mechanism, enabling wider applications of SOT-based spintronic devices.

Contributions of Co and Fe orbitals to perpendicular magnetic anisotropy of MgO/CoFeB bilayers with Ta, W, IrMn, and Ti underlayers

We study the perpendicular magnetic anisotropy (PMA) of the CoFeB/MgO bilayers in contact with W, Ta, IrMn, and Ti, which have been suggested for use as the spin–orbit-torque-related underlayers. The saturation magnetization of CoFeB depends on the underlayer material used owing to the formation of a dead layer that affects the PMA strength of each film. The X-ray magnetic circular dichroism measurement reveals that interfacial intermixing suppresses only the perpendicular orbital moment of Fe, whereas it simultaneously suppresses both the perpendicular and in-plane orbital moments of Co.

Temperature dependence of spin Hall magnetoresistance in W/CoFeB bilayer

We investigate the temperature dependence of the spin Hall magnetoresistance (SMR) in a W/CoFeB bilayer. The SMR is found to increase with decreasing temperature. An analysis based on the SMR theory suggests that the spin Hall angle of W and/or the spin polarization of CoFeB can be the origin of the temperature dependence of the SMR. We also find that the spin diffusion length and the resistivity of W do not significantly vary with temperature, which indicates the necessity of further study on the electron transport mechanism in W films to reveal the origin of the spin Hall effect in W.

Strongly (001)-textured MgO/Co40Fe40B20 spin-tunnel contact on n-Ge(001) and its spin accumulation: Structural modification with ultrathin Mg insertion by sputtering

The sputter-deposited fcc-MgO (001)[100]/bcc-Co40Fe40B20 (001)[110] spin-tunnel contact (STC) was successfully prepared on n-Ge(001). We found that the interfacial modification by ultrathin (6 A) Mg insertion at the interface between n-Ge and MgO plays an important role in spin injection into Ge. The significantly amplified spin accumulation was observed in this STC as a result of the structural modification. The three-terminal Hanle signal of this STC was 2.7 times larger than that of the STC without Mg insertion. Our study confirms that a sputtering technique is indeed practical and useful to modify interfacial structures for the efficient injection of spins into semiconductors.