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Physical unclonable function (PUFs) utilize inherent random physical variations of solid-state devices and are a core ingredient of hardware security primitives. PUFs promise more robust information security than that provided by the conventional software-based approaches. While silicon- and memristor-based PUFs are advancing, their reliability and scalability require further improvements. These are currently limited by output fluctuations and associated additional peripherals. Here, highly reliable spintronic PUFs that exploit field-free spin-orbit-torque switching in IrMn/CoFeB/Ta/CoFeB structures are demonstrated. It is shown that the stochastic switching polarity of the perpendicular magnetization of the top CoFeB can be achieved by manipulating the exchange bias directions of the bottom IrMn/CoFeB. This serves as an entropy source for the spintronic PUF, which is characterized by high entropy, uniqueness, reconfigurability, and digital output. Furthermore, the device ensures a zero bit-error-rate under repetitive operations and robustness against external magnetic fields, and offers scalable and energy-efficient device implementations.

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.

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.

Spin-orbit torques arising from the spin-orbit coupling of non-magnetic heavy metals allow electrical switching of perpendicular magnetization. However, the switching is not purely electrical in laterally homogeneous structures. An extra in-plane magnetic field is indeed required to achieve deterministic switching, and this is detrimental for device applications. On the other hand, if antiferromagnets can generate spin-orbit torques, they may enable all-electrical deterministic switching because the desired magnetic field may be replaced by their exchange bias. Here we report sizeable spin-orbit torques in IrMn/CoFeB/MgO structures. The antiferromagnetic IrMn layer also supplies an in-plane exchange bias field, which enables all-electrical deterministic switching of perpendicular magnetization without any assistance from an external magnetic field. Together with sizeable spin-orbit torques, these features make antiferromagnets a promising candidate for future spintronic devices. We also show that the signs of the spin-orbit torques in various IrMn-based structures cannot be explained by existing theories and thus significant theoretical progress is required.

In this letter, we discuss the mechanism of Schottky barrier height (SBH) modulation of the TaN/Ge contact by varying the nitrogen concentration in the TaN. The Fermi level, which is strongly pinned near the valence band edge of Ge, moves to the conduction band edge of Ge with higher nitrogen concentration in the reactive sputtered TaN. This SBH modulation is attributed to the presence of an electric dipole induced by Ge–N bonds at the interface of the TaN/Ge contact. This SBH modulation due to the semiconductor-nitrogen bonds at the interface is not specific to TaN/Ge, but rather is a general feature in various transition-metal nitride systems on various semiconductors.

Magnetic torques generated through spin–orbit coupling1–8 promise energy-efficient spintronic devices. For applications, it is important that these torques switch films with perpendicular magnetizations without an external magnetic field9–14. One suggested approach 15 to enable such switching uses magnetic trilayers in which the torque on the top magnetic layer can be manipulated by changing the magnetization of the bottom layer. Spin currents generated in the bottom magnetic layer or its interfaces transit the spacer layer and exert a torque on the top magnetization. Here we demonstrate field-free switching in such structures and show that its dependence on the bottom-layer magnetization is not consistent with the anticipated bulk effects 15 . We describe a mechanism for spin-current generation16,17 at the interface between the bottom layer and the spacer layer, which gives torques that are consistent with the measured magnetization dependence. This other-layer-generated spin–orbit torque is relevant to energy-efficient control of spintronic devices. Spin–orbit torques are reported in ferromagnetic trilayers that lead to the switching of perpendicular magnetizations without an external magnetic field.

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.

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.

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.

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.

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 Å) 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.

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.

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.

The accurate characterization of highly leaky dielectrics has been a serious challenge in MOSFET and capacitor studies. We have shown that time domain reflectometry (TDR) can be used to measure the capacitance of ultrathin SiO2 MOS capacitors even at a leakage current density as high as ~3000 A/cm2, which is approximately 103 times higher than the limit of a conventional impedance analyzer. The extremely short interaction time of the TDR C–V method makes the TDR capacitance measurement more immune to the leakage current. Since the TDR C–V method does not require special high-frequency test structures other than a ground-signal pad pattern, the TDR C–V method is a promising capacitance measurement method for leaky dielectrics.