Current-Induced magnetization switching by the high spin Hall conductivity α\alpha-W

The spin Hall effect originating from 5d heavy transition metal thin films such as Pt, Ta, and W is able to generate efficient spin-orbit torques that can switch adjacent magnetic layers. This mechanism can serve as an alternative to conventional spin-transfer torque for controlling next-generation magnetic memories. Among all 5d transition metals, W in its resistive amorphous phase typically shows the largest spin-orbit torque efficiency ~ 0.20-0.50. In contrast, its conductive and crystalline $\alpha$ phase possesses a significantly smaller efficiency ~ 0.03 and no spin-orbit torque switching has yet been realized using $\alpha$-W thin films as the spin Hall source. In this work, through a comprehensive study of high quality W/CoFeB/MgO and the reversed MgO/CoFeB/W magnetic heterostructures, we show that although amorphous-W has a greater spin-orbit torque efficiency, the spin Hall conductivity of $\alpha$-W ($|\sigma_{\operatorname{SH}}^{\alpha\operatorname{-W}}|=3.71\times10^{5}\operatorname{\Omega}^{-1}\operatorname{m}^{-1}$) is ~3.5 times larger than that of amorphous-W ($|\sigma_{\operatorname{SH}}^{\operatorname{amorphous-W}}|=1.05\times10^{5}\operatorname{\Omega}^{-1}\operatorname{m}^{-1}$). Moreover, we demonstrate spin-orbit torque driven magnetization switching using a MgO/CoFeB/$\alpha$-W heterostructure. Our findings suggest that the conductive and high spin Hall conductivity $\alpha$-W can be a potential candidate for future low power consumption spin-orbit torque memory applications

Ferrimagnetic Heusler tunnel junctions with fast spin-transfer torque switching enabled by low magnetization

Magnetic random access memory that uses magnetic tunnel junction memory cells is a high performance, non-volatile memory technology that goes beyond traditional charge-based memories. Today its speed is limited by the high magnetization of the memory storage layer. Here we show that fast and highly reliable switching is possible using a very low magnetization ferrimagnetic Heusler alloy, Mn3Ge. Moreover, the tunneling magnetoresistance is the highest yet achieved for a ferrimagnetic material at ambient temperature. Furthermore, the devices were prepared on technologically relevant amorphous substrates using a novel combination of a nitride seed layer and a chemical templating layer. These results show a clear path to the lowering of switching currents using ferrimagnetic Heusler materials and, therefore, to the scaling of high performance magnetic random access memories beyond those nodes possible with ferromagnetic devices.Comment: main manuscript 14 pages, 4 main figures and supplementary information. Submitted to Nature naotechnolog

Unforeseen plant phenotypic diversity in a dry and grazed world

23 páginas..- 4 figuras y 7 figuras.- 50 referencias y 90 referenciasEarth harbours an extraordinary plant phenotypic diversity1 that is at risk from ongoing global changes2,3. However, it remains unknown how increasing aridity and livestock grazing pressure—two major drivers of global change4,5,6—shape the trait covariation that underlies plant phenotypic diversity1,7. Here we assessed how covariation among 20 chemical and morphological traits responds to aridity and grazing pressure within global drylands. Our analysis involved 133,769 trait measurements spanning 1,347 observations of 301 perennial plant species surveyed across 326 plots from 6 continents. Crossing an aridity threshold of approximately 0.7 (close to the transition between semi-arid and arid zones) led to an unexpected 88% increase in trait diversity. This threshold appeared in the presence of grazers, and moved toward lower aridity levels with increasing grazing pressure. Moreover, 57% of observed trait diversity occurred only in the most arid and grazed drylands, highlighting the phenotypic uniqueness of these extreme environments. Our work indicates that drylands act as a global reservoir of plant phenotypic diversity and challenge the pervasive view that harsh environmental conditions reduce plant trait diversity8,9,10. They also highlight that many alternative strategies may enable plants to cope with increases in environmental stress induced by climate change and land-use intensification.This research was funded by the European Research Council (ERC Grant agreement 647038 1004 [BIODESERT]) and Generalitat Valenciana (CIDEGENT/2018/041). N.G. was supported by CAP 20–25 (16-IDEX-0001) and the AgreenSkills+ fellowship programme which has received funding from the European Union’s Seventh Framework Programme under grant agreement FP7-609398 (AgreenSkills+ contract). F.T.M. acknowledges support from the King Abdullah University of Science and Technology (KAUST), the KAUST Climate and Livability Initiative, the University of Alicante (UADIF22-74 and VIGROB22-350), the Spanish Ministry of Science and Innovation (PID2020-116578RB-I00), and the Synthesis Center (sDiv) of the German Centre for Integrative Biodiversity Research Halle–Jena–Leipzig (iDiv). Y.L.B.-P. was supported by a Marie Sklodowska-Curie Actions Individual Fellowship (MSCA-1018 IF) within the European Program Horizon 2020 (DRYFUN Project 656035). H.S. is supported by a María Zambrano fellowship funded by the Ministry of Universities and European Union-Next Generation plan. L.W. acknowledges support from the US National Science Foundation (EAR 1554894). G.M.W. acknowledges support from the Australian Research Council (DP210102593) and TERN. M.B is supported by a Ramón y Cajal grant from Spanish Ministry of Science (RYC2021-031797-I). L.v.d.B. and K.T. were supported by the German Research Foundation (DFG) Priority Program SPP-1803 (TI388/14-1). A.F. acknowledges the financial support from ANID PIA/BASAL FB210006 and Millenium Science Initiative Program NCN2021-050. A.J. was supported by the Bavarian Research Alliance for travel and field work (BayIntAn UBT 2017 61). A.L. and L.K. acknowledge support from the German Research Foundation, DFG (grant CRC TRR228) and German Federal Government for Science and Education, BMBF (grants 01LL1802C and 01LC1821A). B.B. and S.U. were supported by the Taylor Family-Asia Foundation Endowed Chair in Ecology and Conservation Biology. P.J.R. and A.J.M. acknowledge support from Fondo Europeo de Desarrollo Regional through the FEDER Andalucía operative programme, FEDER-UJA 1261180 project. E.M.-J. and C.P. acknowledge support from the Spanish Ministry of Science and Innovation (PID2020-116578RB-I00). D.J.E. was supported by the Hermon Slade Foundation. J.D. and A.Rodríguez acknowledge support from the FCT (2020.03670.CEECIND and SFRH/BDP/108913/2015, respectively), as well as from the MCTES, FSE, UE and the CFE (UIDB/04004/2021) research unit financed by FCT/MCTES through national funds (PIDDAC). S.C.R. acknowledges support from the US Department of Energy (DE-SC-0008168), US Department of Defense (RC18-1322), and the US Geological Survey Ecosystems Mission Area. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US government. E.H.-S. acknowledges support from Mexican National Science and Technology Council (CONACYT PN 5036 and 319059). A.N. and C. Branquinho. acknowledge the support from FCT—Fundação para a Ciência e a Tecnologia (CEECIND/02453/2018/CP1534/CT0001, PTDC/ASP-SIL/7743/ 2020, UIDB/00329/2020), from AdaptForGrazing project (PRR-C05-i03-I-000035) and from LTsER Montado platform (LTER_EU_PT_001). Field work of G.P. and J.M.Z. was supported by UNRN (PI 40-C-873).Peer reviewe

Development of tetragonal Heusler compounds with perpendicular anisotropy for Magnetic Random Access Memory applications

Diese Arbeit konzentriert sich hauptsächlich auf die Untersuchung der Eigenschaften von tetragonalen Mn3Ge Heusler Dünnfilmen für Spin-Transfer-Torque-Magnet-Direktzugriffsspeicher (STT-MRAM) Anwendungen. Ein Verfahren zur Entwicklung hochtexturierter, polykristalliner und (001)-orientierter Mn3Ge-Heusler-Filme auf amorphen Substraten wird vorgestellt. Diese Filme zeigten ein niedriges magnetisches Moment und eine große senkrechte magnetische Anisotropie mit Koerzitivfeldern von bis zu 6 T bei Zimmertemperatur. Diese Eigenschaften sind technologisch relevant. Der kleine und negative Tunnelmagnetwiderstand (TMR) in senkrechten magnetischen Tunnelübergängen (p-MTJs) wird einem empfindlichen Gleichgewicht zweier konkurrierender Effekte zugeschrieben: der negativen Spinpolarisation von Mn3Ge, die einen negativen TMR begünstigt, und der Brillouin-Zonen Filterwirkung an der Mn3Ge/ MgO Grenzfläche, die einen positiven TMR begünstigt. Anhand von theoretischen Berechnungen wird gezeigt, dass der letztere Effekt durch Dotieren von Mn3Ge mit einer kleinen Menge an Co unter Beibehaltung der tetragonalen Struktur und des niedrigen magnetischen Moments, d.h. unter Wahrung der Schlüsselanforderungen für STT-MRAM, eliminiert werden kann.This work has primarily focused on the study of the properties of tetragonal Mn3Ge Heusler thin films for spin-transfer torque magnetic random access memory (STT-MRAM) applications. A method to develop highly textured, polycrystalline and (001)-oriented Mn3Ge Heusler films on amorphous substrates was shown. These filmsdisplayed low magnetic moment and giant perpendicular magnetic anisotropy, with coercive fields up to 6 T,at room temperature, making them technologically relevant. However, the small and negative tunneling magnetoresistance (TMR)found in perpendicular magnetic tunnel junctions (p-MTJs) is attributed toa delicate balance of two competing effects: the negative spin polarization of Mn3Ge, that favors a negative TMR, and the Brillouin zone filtering effect at the Mn3Ge/MgO interface, that favors a positive TMR. It is shown from theoretical calculationsthat the latter effect could be eliminated by doping Mn3Ge with a small amount of Co, while maintaining the tetragonal structure and low magnetic moment, i.e. key requirements for STT-MRAM.vorgelegt von Yari Ferrant

Chiral domain wall motion in unit-cell thick perpendicularly magnetized Heusler films prepared by chemical templating

Heusler compounds are of great interest for spintronic applications. Here the authors report current driven domain wall motion in unit cell thick perpendicularly magnetized Heusler films with low current densities and show the velocity is dominated by the bulk chiral Dzyaloshinskii–Moriya exchange interaction

Tetragonal Mn3Sn Heusler films with large perpendicular magnetic anisotropy deposited on metallic MnN underlayers using amorphous substrates

Tetragonal Heusler compounds that exhibit large perpendicular magnetic anisotropy are promising materials for advanced spintronic devices. A prerequisite are thin films whose tetragonal axis is oriented perpendicular to the plane of the films. Here we show that highly textured, (001) oriented, tetragonal Mn3Sn layers can be prepared using metallic zinc-blende (ZB) MnN as underlayers. Moreover, we show that these layers can be deposited on amorphous substrates using reactive magnetron sputtering. The ferrimagnetic Mn3Sn layers exhibit perpendicularly magnetized hysteresis loops with coercive fields of ∼2 T. Stoichiometric ZB-MnN underlayers share an “equivalent” Mn-Mn layer at the interface with Mn3Sn, thus promoting their oriented growth. Other nitride underlayers are not effective due to their rock-salt (RS) crystal structure and the absence of Mn. Density functional theory calculations confirm that tetragonal Mn3Sn Heusler films are energetically stable when interfaced with ZB-MnN underlayers and not with any of the other RS nitride underlayers considered here. Such Heusler compounds have much promise as electrodes for magnetic tunnel junction memory elements for deeply scaled magnetic random access memories

Unforeseen plant phenotypic diversity in a dry and grazed world

Abstract Earth harbours an extraordinary plant phenotypic diversity1 that is at risk from ongoing global changes2,3. However, it remains unknown how increasing aridity and livestock grazing pressure—two major drivers of global change4–6—shape the trait covariation that underlies plant phenotypic diversity1,7. Here we assessed how covariation among 20 chemical and morphological traits responds to aridity and grazing pressure within global drylands. Our analysis involved 133,769 trait measurements spanning 1,347 observations of 301 perennial plant species surveyed across 326 plots from 6 continents. Crossing an aridity threshold of approximately 0.7 (close to the transition between semi-arid and arid zones) led to an unexpected 88% increase in trait diversity. This threshold appeared in the presence of grazers, and moved toward lower aridity levels with increasing grazing pressure. Moreover, 57% of observed trait diversity occurred only in the most arid and grazed drylands, highlighting the phenotypic uniqueness of these extreme environments. Our work indicates that drylands act as a global reservoir of plant phenotypic diversity and challenge the pervasive view that harsh environmental conditions reduce plant trait diversity8–10. They also highlight that many alternative strategies may enable plants to cope with increases in environmental stress induced by climate change and land-use intensification