news & views is the tunability of both carrier type and density by the incline angle of the wall11. In this regard, a charged domain wall is a means to gate electron transport through the bulk of the ferroelectric crystal, similarly to electrostatic gating in a transistor geometry (Fig. 1c), but with the incline angle rather than the gate voltage as the control parameter (Fig. 1d). Variable carrier density hints not only to the existence of metallic and insulating domain walls, but also to phase transitions within the conducting sheet surrounding the domain wall owing to carrier-density effects, electron–phonon interactions or even electron–electron correlations. Carrier-density-controlled phase transitions in electrostatically

gated perovskite oxides1 as well as superconductivity of doped twin walls in WO3–x (ref. 12) yield further plausibility to this prospect and will stimulate further studies of electronic phenomena associated with ferroelectric domain walls. From a fundamental perspective, the domain wall conductance can be exploited as a sensitive probe of domain wall dynamics, wall–defect interactions and dynamics of ionic degrees of freedom subject to strong large intrinsic electric fields at the domain walls. ❐ Petro Maksymovych is at the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. e-mail: [email protected]


1. Ohtomo, A. & Hwang, H. Y. Nature 427, 423–426 (2004). 2. Zubko, P., Gariglio, S., Gabay, M., Ghosez, P. & Triscone, J.‑M. Annu. Rev. Condens. Matter Phys. 2, 141–165 (2011). 3. Crassous, A., Sluka, T., Tagantsev, A. K. & Setter, N. Nature Nanotech. 10, 614–618 (2015). 4. Seidel, J. et al. Nature Mater. 8, 229–234 (2009). 5. Vasudevan, R. K. et al. Adv. Funct. Mater. 23, 2592–2616 (2013). 6. Meier, D. et al. Nature Mater. 11, 284–288 (2012). 7. Oh, Y. S., Luo, X., Huang, F.‑T., Wang, Y. & Cheong, S.‑W. Nature Mater. 14, 407–413 (2015). 8. Maksymovych, P. et al. Nano Lett. 12, 209–213 (2012). 9. Sluka, T., Tagantsev, A. K., Bednyakov, P. & Setter, N. Nature Commun. 4, 1808 (2013). 10. Balke, N. et al. Nature Nanotech. 4, 868–875 (2009). 11. Eliseev, E. A., Morozovska, A. N., Svechnikov, G. S., Gopalan, V. & Shur, V. Y. Phys. Rev. B 83, 235313 (2011). 12. Aird, A. & Salje, E. K. H. J. Phys. Condens. Matter 10, L377–L380 (1998).

Published online: 15 June 2015 Corrected online: 19 June 2015


Skyrmions under strain

The high sensitivity of magnetic skyrmions to mechanical deformation of the underlying crystal lattice provides a new tuning parameter for potential applications of these nanosized spin whirls.

Robert Ritz


kyrmions — nanosized spin whirls found in certain materials — are one of the hottest topics in magnetism, as they are promising in the development of novel spintronics devices (Fig. 1a). The basic property of skyrmions is a non-trivial topology, which means that they cannot be continuously transformed in a topologically trivial state (such as a ferromagnetic one) and are hence relatively stable against perturbations. A regular lattice of skyrmions was first discovered in the cubic chiral magnet MnSi (ref. 1), whose crystal structure without inversion symmetry gives rise to an exchange interaction known as the Dzyaloshinskii–Moriya interaction (DMI). The DMI favours an orthogonal alignment of neighbouring spins, and in combination with a strong ferromagnetic exchange, it leads to long-wavelength helimagnetism. The skyrmion lattice forms in a phase pocket in finite fields just below the magnetic ordering temperature, where it is stabilized by thermal fluctuations. As well as in other bulk cubic chiral magnets such as FeGe (ref. 2), skyrmion lattices have been observed in many other compounds3,4. Current efforts are aimed at the manipulation of skyrmions in thinned and/or nanopatterned samples, where

skyrmions are typically observed in larger parts of the magnetic phase diagram. In addition, skyrmions are intensively investigated in thin films of ferromagnets, where they form due to the breaking of inversion symmetry at the sample surface5. Furthermore, single skyrmions in thin films can be written and deleted with the tip of a scanning tunnelling microscope6. The nanometre-scale size of skyrmions, the fact that they can be moved by electrical currents with ultralow densities7, and their observation near room temperature8, triggered suggestions of novel skyrmionbased data-storage devices with ultrahigh bit densities9. Beside currents, additional a


mechanisms to manipulate skyrmions and skyrmion lattices include for example hydrostatic pressure10, compositional doping 11 and electromagnetic fields12–14. However, so far, the mechanical control of skyrmions through strain — potentially useful for applications — has not been explored. Writing in Nature Nanotechnology, Kiyou Shibata and co-workers at the University of Tokyo, RIKEN Center for Emergent Matter Science, Hitachi Central Research Laboratory, ROHM, and Tohoku University now report the effects on the magnetic skyrmion lattice of anisotropic strain in the crystal lattice of the host material15. c

Figure 1 | A skyrmion lattice under strain. a, Schematic of spin arrangement in a magnetic skyrmion. b, Schematic of a unit cell of a skyrmion lattice in a thin sample. c, Tensile strain on the crystal lattice leads to a large anisotropic deformation of the skyrmion lattice as well as of the individual skyrmions along the direction of strain. The deformation of the skyrmion lattice is two orders of magnitude larger than the deformation of the underlying crystal lattice. This effect may be used to tune the response of the skyrmion lattice to microwaves (black arrow). Panel a adapted from ref. 17, Nature Publishing Group.


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news & views Shibata and colleagues report in situ Lorentz transmission electron microscopy measurements on a thin plate of FeGe with a thickness of about 150 nm. They show by analysis in real- and reciprocal-space that a uniaxial tensile strain on the FeGe sample leads to large deformations of both the skyrmion lattice and individual skyrmions (Fig. 1b,c). Anisotropic strain on the crystal lattice as small as 0.3% induces a distortion of the skyrmion lattice along the strain direction of up to 20%. The researchers propose that the crystal lattice deformation results in an anisotropic change of the DMI; furthermore, the change in the strength of the DMI is approximately 100 times larger than the lattice strain. The proposal is supported by theoretical analysis that explores a strain-induced anisotropic DMI. The fact that the DMI may be efficiently tuned by strain in the crystal lattice provides a mechanism to control magnetic structures, such as skyrmions, that are based on this interaction. The role of strain in the properties and control of magnetic skyrmions is particularly important in thin

films, which are, in general, prone to strain effects (for example due to interactions with the substrate) and are relevant for most potential applications. In particular, the work of Shibata and co-workers is relevant for applications based on skyrmion lattices, such as those involving microwave excitations, rather than for applications based on the manipulation of single skyrmions, such as concepts proposed for data storage9. When exposed to microwaves, skyrmion lattices exhibit collective spin excitations that are typically in the gigahertz range12,13. The microwave response can be tailored by changing the shape and material of the sample16. In combination with the control of the spin texture using magnetic fields or uniaxial strain, new microwave devices may be designed. For instance, one might think of a microwave diode that uses the large directional dichroism present in chiral magnets with magnetoelectric coupling 14, where the absorption may be fine-tuned via mechanical strain. The strain-induced properties of skyrmions provide an efficient control

parameter and furthermore represent another step towards the development of a new generation of spintronics devices based on skyrmions. ❐ Robert Ritz is in the Department of Physics, Technische Universität München, 85748 Garching, Germany. e-mail: [email protected] References

1. Mühlbauer, S. et al. Science 323, 915–919 (2009). 2. Yu, X. Z. et al. Nature Mater. 10, 106–109 (2010). 3. Tokunaga, Y. et al. Preprint at (2015). 4. Kézsmárki, I. et al. Preprint at (2015). 5. Heinze, S. et al. Nature Phys. 7, 713–718 (2011). 6. Romming, N. et al. Science 341, 636–639 (2013). 7. Jonietz, F. et al. Science 330, 1648–1651 (2010). 8. Nagaosa, N. & Tokura, Y. Nature Nanotech. 8, 899–911 (2013). 9. Fert, A. et al. Nature Nanotech. 8, 152–156 (2013). 10. Ritz, R. et al. Nature 497, 231–234 (2013). 11. Bauer, A. et al. Phys. Rev. B 82, 064404 (2010). 12. Mochizuki, M. et al. Phys. Rev. Lett. 108, 017601 (2012). 13. Onose, Y. et al. Phys. Rev. Lett. 109, 037603 (2012). 14. Okamura, Y. et al. Nature Commun. 4, 2391 (2013). 15. Shibata, K. et al. Nature Nanotech. 10, 589–592 (2015). 16. Schwarze, T. et al. Nature Mater. 14, 478–483 (2015). 17. Yu, X. Z. et al. Nature 465, 901–904 (2010).


Signs of stability

Studies on a perovskite photovoltaic device suggest that improved stability, one of the hurdles to large-scale applicability of perovskites in solar cells, can be achieved.

Karl Leo


hotovoltaic devices based on the inorganic–organic methylammonium lead halide perovskite absorbers (CH3NH3PbX3, X = Br, Cl, I)1 are attracting considerable attention. There are three main reasons why: the materials are inexpensive to produce; the fabrication methods are relatively simple; and the devices have a high power conversion efficiency 2–6. Indeed, improvements in the efficiency have been remarkable in the last 2–3 years, faster than that seen for any other photovoltaic material before. As Fig. 1 shows, in terms of efficiency, these materials are already better than other low-cost, thin-film technologies such as amorphous silicon, dye-sensitized solar cells, and organic solar cells. Efficiency is not the only factor that determines the suitability of a device for realistic applications however. The operational stability, which is determined by the time it takes before the device degrades, also needs to be taken into account. So far, 574

the issue of operational stability has not been addressed satisfactorily for perovskites. Unfortunately, the poor thermal stability of the basic perovskite material, which starts to decompose at temperatures around 70 °C, suggests that device stability might be a major issue (see ref. 7 for a recent review). Now, writing in Energy Technology, Michael Grätzel and colleagues, from EPF Lausanne, Switzerland, and King Abdulaziz University, Jeddah, Saudi Arabia, present a cell structure that displays encouraging stability data and that maintains a reasonably high efficiency close to 13%8. The solar cells do not contain the standard organic hole-transport layer and evaporated metal contact, but instead a porous zirconium oxide layer and a carbon back electrode. Small-area individual cells were tested outdoors under the harsh late summer conditions of Jeddah for one week. The parameters show fluctuations with time, which the authors attribute to

changes in solar illumination, but they remain almost constant overall. The devices were also subject to two indoor tests. The first was at 85 °C in the dark for 90 days to assess how the cells reacted to prolonged heat stress. Here, the cells lost about 10% of their initial efficiency, assuming a linear decay, this would correspond to a lifetime (usually defined as 20% efficiency loss) of 180 days. To pass the standard International Electrotechnical Commission damp heat test (85 °C, 85% relative humidity), photovoltaic modules would require a maximum 10% efficiency loss in 1,000 hours (just over 40 days), that is, the perovskite cells presented by Li et al. would pass at least the temperature part of the test. Furthermore, the authors performed a test under illumination at 45 °C for 44 days in an argon atmosphere. Under these conditions, the cells remained remarkably stable. The results show that the perovskites can withstand high temperatures and are


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Spintronics: Skyrmions under strain.

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