Magnetic tunnel junctions (MTJs) with ferromagnetic electrodes possessing a perpendicular magnetic easy axis are of great interest as they have a potential for realizing next-generation high-density non-volatile memory and logic chips with high thermal stability and low critical current for current-induced magnetization switching. To attain perpendicular anisotropy, a number of material systems have been explored as electrodes, which include rare-earth/transition-metal alloys, L1(0)-ordered (Co, Fe)-Pt alloys and Co/(Pd, Pt) multilayers. However, none of them so far satisfy high thermal stability at reduced dimension, low-current current-induced magnetization switching and high tunnel magnetoresistance ratio all at the same time. Here, we use interfacial perpendicular anisotropy between the ferromagnetic electrodes and the tunnel barrier of the MTJ by employing the material combination of CoFeB-MgO, a system widely adopted to produce a giant tunnel magnetoresistance ratio in MTJs with in-plane anisotropy. This approach requires no material other than those used in conventional in-plane-anisotropy MTJs. The perpendicular MTJs consisting of Ta/CoFeB/MgO/CoFeB/Ta show a high tunnel magnetoresistance ratio, over 120%, high thermal stability at dimension as low as 40 nm diameter and a low switching current of 49 microA.

A perpendicular-anisotropy CoFeB–MgO magnetic tunnel junction

We have developed a nanowire nanogenerator that is driven by an ultrasonic wave to produce continuous direct-current output. The nanogenerator was fabricated with vertically aligned zinc oxide nanowire arrays that were placed beneath a zigzag metal electrode with a small gap. The wave drives the electrode up and down to bend and/or vibrate the nanowires. A piezoelectric-semiconducting coupling process converts mechanical energy into electricity. The zigzag electrode acts as an array of parallel integrated metal tips that simultaneously and continuously create, collect, and output electricity from all of the nanowires. The approach presents an adaptable, mobile, and cost-effective technology for harvesting energy from the environment, and it offers a potential solution for powering nanodevices and nanosystems.

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Direct-current nanogenerator driven by ultrasonic waves

A M was supported by the King Abdullah University of Science and Technology (KAUST) T J acknowledges support from the EU FET Open RIA Grant No 766566, the Ministry of Education of the Czech Republic Grant No LM2015087 and LNSM-LNSpin, and the Grant Agency of the Czech Republic Grant No 19-28375X J S acknowledges the Alexander von Humboldt Foundation, EU FET Open Grant No 766566, EU ERC Synergy Grant No 610115, and the Transregional Collaborative Research Center (SFB/TRR) 173 SPIN+X K G and P G acknowledge stimulating discussions with C O Avci and financial support by the Swiss National Science Foundation (Grants No 200021-153404 and No 200020-172775) and the European Commission under the Seventh Framework Program (spOt project, Grant No 318144) A T acknowledges support by the Agence Nationale de la Recherche, Project No ANR-17-CE24-0025 (TopSky) J Ž acknowledges the Grant Agency of the Czech Republic Grant No 19-18623Y and support from the Institute of Physics of the Czech Academy of Sciences and the Max Planck Society through the Max Planck Partner Group programme

/pdf/current-induced-spin-orbit-torques-in-ferromagnetic-and-3tdt106r3u.pdf

Current-induced spin-orbit torques in ferromagnetic and antiferromagnetic systems

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

https://link.aps.org/accepted/10.1103/RevModPhys.89.025006

Interface-induced phenomena in magnetism

The integration of magnetic material with semiconductors has been fertile ground for fundamental science as well as of great practical interest toward the seamless integration of information processing and storage. We create van der Waals heterostructures formed by an ultrathin ferromagnetic semiconductor CrI3 and a monolayer of WSe2. We observe unprecedented control of the spin and valley pseudospin in WSe2, where we detect a large magnetic exchange field of nearly 13 T and rapid switching of the WSe2 valley splitting and polarization via flipping of the CrI3 magnetization. The WSe2 photoluminescence intensity strongly depends on the relative alignment between photoexcited spins in WSe2 and the CrI3 magnetization, because of ultrafast spin-dependent charge hopping across the heterostructure interface. The photoluminescence detection of valley pseudospin provides a simple and sensitive method to probe the intriguing domain dynamics in the ultrathin magnet, as well as the rich spin interactions within the heterostructure.

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Van der Waals engineering of ferromagnetic semiconductor heterostructures for spin and valleytronics

https://cyberleninka.org/article/n/296274.pdf

Magnetic tunnel junctions

Magnetization switching in magnetic tunnel junctions using spin-transfer torque and spin–orbit torque is key to the development of future spintronic memories. However, both switching mechanisms suffer from intrinsic limitations. In particular, the switching current in spin-transfer torque devices needs to be lowered, whereas an external magnetic field is required for spin–orbit torque devices to achieve deterministic switching in perpendicular magnetic tunnel junctions. Here, we experimentally demonstrate field-free switching of three-terminal perpendicular-anisotropy magnetic tunnel junction devices through the interaction between spin–orbit and spin-transfer torques. We show that the threshold current density of spin–orbit torque switching can be reduced by increasing the spin-transfer torque current density, and thus an optimal point for low-power perpendicular magnetic tunnel junction switching can be found by tuning the two current densities. Furthermore, and due to this interplay, low-power switching in two-terminal perpendicular magnetic tunnel junctions without an external magnetic field is also achieved. The interplay between spin–orbit and spin-transfer torques can be used to develop a low-power route to magnetization switching of perpendicular magnetic tunnel junctions without an external magnetic field.

Field-free switching of a perpendicular magnetic tunnel junction through the interplay of spin–orbit and spin-transfer torques

A method and system for providing a magnetic structure in magnetic transducer is described. The magnetic structure includes a pinned layer, a nonmagnetic spacer layer, and a free layer. The nonmagnetic spacer layer is between the pinned layer and the free layer. The free layer includes a first magnetic layer, a second magnetic layer, and a magnetic insertion layer between the first magnetic layer and the second magnetic layer. The first magnetic layer has a first magnetostriction. The second magnetic layer has a second magnetostriction opposite to the first magnetostriction. The magnetic insertion layer provides a growth texture barrier between the first magnetic layer and the second magnetic layer.

Method and system for providing a read sensor having a low magnetostriction free layer

A tunneling magnetoresistive read head produced thereby are disclosed. A shield layer is provided. A magnetic etch-stop layer is formed over the shield layer, where the magnetic etch-stop layer comprises a nonmagnetic metal and a soft magnetic material with overall property still being magnetically soft. A sensor stack is formed over the magnetic etch-stop layer. A patterned mask layer is formed over the sensor stack. Material from a portion of the sensor stack not covered by the patterned mask is removed.

Magnetic etch-stop layer for magnetoresistive read heads

A spin tunneling element includes a pinned layer, a barrier layer, and a free layer. The free layer includes a ferromagnetic layer including a ferromagnetic material that adjoins the barrier layer at a first interface. The free layer also includes a layer of amorphous material that adjoins the ferromagnetic layer at a second interface opposite the first interface. A first crystallization temperature of the ferromagnetic material at the first interface is lower than a second crystallization temperature at the second interface.

Chando Park

Papers

Magnetic tunnel junctions

Field-free switching of a perpendicular magnetic tunnel junction through the interplay of spin–orbit and spin-transfer torques

Method and system for providing a read sensor having a low magnetostriction free layer

Magnetic etch-stop layer for magnetoresistive read heads

Spin tunneling magnetic element promoting free layer crystal growth from a barrier layer interface