Long You

Deterministic Domain Wall Motion Orthogonal To Current Flow Due To Spin Orbit Torque

Spin-polarized electrons can move a ferromagnetic domain wall through the transfer of spin angular momentum when current flows in a magnetic nanowire. Such current induced control of a domain wall is of significant interest due to its potential application for low power ultra high-density data storage. In previous reports, it has been observed that the motion of the domain wall always happens parallel to the current flow – either in the same or opposite direction depending on the specific nature of the interaction. In contrast, here we demonstrate deterministic control of a ferromagnetic domain wall orthogonal to current flow by exploiting the spin orbit torque in a perpendicularly polarized Ta/CoFeB/MgO heterostructure in presence of an in-plane magnetic field. Reversing the polarity of either the current flow or the in-plane field is found to reverse the direction of the domain wall motion. Notably, such orthogonal motion with respect to current flow is not possible from traditional spin transfer torque driven domain wall propagation even in presence of an external magnetic field. Therefore the domain wall motion happens purely due to spin orbit torque. These results represent a completely new degree of freedom in current induced control of a ferromagnetic domain wall.

Single crystal functional oxides on silicon

Single-crystalline thin films of complex oxides show a rich variety of functional properties such as ferroelectricity, piezoelectricity, ferro and antiferromagnetism and so on that have the potential for completely new electronic applications. Direct synthesis of such oxides on silicon remains challenging because of the fundamental crystal chemistry and mechanical incompatibility of dissimilar interfaces. Here we report integration of thin (down to one unit cell) single crystalline, complex oxide films onto silicon substrates, by epitaxial transfer at room temperature. In a field-effect transistor using a transferred lead zirconate titanate layer as the gate insulator, we demonstrate direct reversible control of the semiconductor channel charge with polarization state. These results represent the realization of long pursued but yet to be demonstrated single-crystal functional oxides on-demand on silicon.

Interface Engineering of Domain Structures in BiFeO3 Thin Films

A wealth of fascinating phenomena have been discovered at the BiFeO3 domain walls, examples such as domain wall conductivity, photovoltaic effects, and magnetoelectric coupling. Thus, the ability to precisely control the domain structures and accurately study their switching behaviors is critical to realize the next generation of novel devices based on domain wall functionalities. In this work, the introduction of a dielectric layer leads to the tunability of the depolarization field both in the multilayers and superlattices, which provides a novel approach to control the domain patterns of BiFeO3 films. Moreover, we are able to study the switching behavior of the first time obtained periodic 109° stripe domains with a thick bottom electrode. Besides, the precise controlling of pure 71° and 109° periodic stripe domain walls enable us to make a clear demonstration that the exchange bias in the ferromagnet/BiFeO3 system originates from 109° domain walls. Our findings provide future directions to study the room temperature electric field control of exchange bias and open a new pathway to explore the room temperature multiferroic vortices in the BiFeO3 system.

The effects of strain relaxation on the dielectric properties of epitaxial ferroelectric Pb(Zr0.2Ti0.8)TiO3 thin films

We study the effects of strain relaxation on the dielectric properties of epitaxial 40 nm Pb(Zr0.2Ti0.8)TiO3 (PZT) films. A significant increase in the defect and dislocation density due to strain relaxation is observed in PZT films with tetragonality c/a < 1.07 grown on SrTiO3 (001) substrates, which results in significant frequency dispersion of the dielectric constant and strong Rayleigh type behavior in those samples. This combined structural-electrical study provides a framework for investigating strain relaxation in thin films and can provide useful insights into the mechanisms of fatigue in ferroelectric materials.

Dynamic Heating in Micron-and Submicron-Patterned TbFe Films

The dynamic heating of micron- and submicron-patterned TbFe films using current pulses for the thermally assisted writing scheme has been investigated. The evolution of the required power/energy density with pulse width has been studied using experimental measurements and numerical thermal simulations. Two heating regimes (adiabatic and diffusive regimes) are revealed by simulation in the investigated range of the current pulse widths (0.01 ns–1 µs). The energy density required for writing is almost independent of the pulse width and element size, and about 13 pJ/µm2 at the very short pulse width associated with the adiabatic regime. Two distinct zones are also observed in the diffusion regime, and the dependence of the heating power density on the pulse width variation (Δt) shows a 1-exp (-Δt/t1) phenomenological variation in each zone. The power/energy densities required for the writing of four patterned TbFe films with different sizes are measured in the range of the pulse width from 10 up to 1000 ns, and the experimental results are well explained by the simulation. The required power density for the writing of 0.74 µm2 patterned films using the pulse duration of 10 ns is experimentally estimated to be around P = 2 mW/µm2, which corresponds to the energy density of E = 20 pJ/µm2.

Nonvolatile MoS2 field effect transistors directly gated by single crystalline epitaxial ferroelectric

We demonstrate non-volatile, n-type, back-gated, MoS2 transistors, placed directly on an epitaxial grown, single crystalline, PbZr0.2Ti0.8O3 (PZT) ferroelectric. The transistors show decent ON current (19 μA/μm), high on-off ratio (107), and a subthreshold swing of (SS ∼ 92 mV/dec) with a 100 nm thick PZT layer as the back gate oxide. Importantly, the ferroelectric polarization can directly control the channel charge, showing a clear anti-clockwise hysteresis. We have self-consistently confirmed the switching of the ferroelectric and corresponding change in channel current from a direct time-dependent measurement. Our results demonstrate that it is possible to obtain transistor operation directly on polar surfaces, and therefore, it should be possible to integrate 2D electronics with single crystalline functional oxides.

Reconfigurable Skyrmion Logic Gates

Magnetic skyrmion, a nanosized spin texture with topological property, has become an area of significant interest due to the scientific insight that it can provide and also its potential impact on applications such as ultra-low-energy and ultra-high-density logic gates. In the quest for the reconfiguration of single logic device and the implementation of the complete logic functions, a novel reconfigurable skyrmion logic (RSL) is proposed and verified by micromagnetic simulations. Logic functions including AND, OR, NOT, NAND, NOR, XOR, and XNOR are implemented in the ferromagnetic (FM) nanotrack by virtue of various effects including spin orbit torque, skyrmion Hall effect, skyrmion-edge repulsions, and skyrmion-skyrmion collision. Different logic functions can be selected in an RSL by applying voltage to specific region(s) of the device, changing the local anisotropy energy of FM film. Material properties and geometrical scaling studies suggest RSL gates fit for energy-efficient computing as well as provide the guidelines for the design and optimization of this new logic family.

Electrically reversible cracks in an intermetallic film controlled by an electric field

Cracks in solid-state materials are typically irreversible. Here we report electrically reversible opening and closing of nanoscale cracks in an intermetallic thin film grown on a ferroelectric substrate driven by a small electric field (~0.83 kV/cm). Accordingly, a nonvolatile colossal electroresistance on–off ratio of more than 108 is measured across the cracks in the intermetallic film at room temperature. Cracks are easily formed with low-frequency voltage cycling and remain stable when the device is operated at high frequency, which offers intriguing potential for next-generation high-frequency memory applications. Moreover, endurance testing demonstrates that the opening and closing of such cracks can reach over 107 cycles under 10-μs pulses, without catastrophic failure of the film.Electric-field-induced cracks are generally detrimental to functionality of ferroelectric ceramics. Liu et al. use an intermetallic alloy and ferroelectric oxide junction to mediate the reversible formation of cracks at nanoscales, resulting in colossal electroresistance modulation for memory applications.

3D multilevel spin transfer torque devices

Spin-transfer torque magnetic tunneling junction devices capable of a multilevel three-dimensional (3D) information processing are studied in the sub-20-nm size range. The devices are built using He+ and Ne+ focused ion beam etching. It has been demonstrated that due to their extreme scalability and energy efficiency, these devices can significantly reduce the device footprint compared to the modern CMOS approaches and add advanced features in a 3D stack with a sub-20-nm size using a spin polarized current.

Highly secure physically unclonable cryptographic primitives based on interfacial magnetic anisotropy

Information security is of great importance for the approaching Internet of things (IoT) era. Physically unclonable functions (PUFs) have been intensively studied for information security. However, silicon PUFs are vulnerable to hazards such as modeling and side-channel attacks. Here we demonstrate a magnetic analogue PUF based on perpendicularly magnetized Ta/CoFeB/MgO heterostructures. The perpendicular magnetic anisotropy originates from the CoFeB/MgO interface, which is sensitive to the subnanometer variation of MgO thickness within a certain range (0.6-1.3 nm). When the MgO layer is thinned, a thickness variation resulting from ion milling nonuniformity induces unclonable random distributions of eas y-axis magnetization orientations in heterostructures. The analogue PUF can provide a much larger key size than a conventional binary-bit counterpart. Moreover, after the thinning process, the unique eas y-axis magnetization orientation in each single device was formed, which can avoid setting random states to realize low power consumption and high-density integration. This magnetic PUF is a promising innovative primitive for secret key generation and storage with high security in the IoT era.