Diamond Annual Review 2019/20

40 41 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 A new slant onmagnetoelectrics Related publication: Ghidini M., Mansell R., Maccherozzi F., Moya X., Phillips L. C.,YanW., Pesquera D., Barnes C. H.W., Cowburn R. P., Hu J. -M., Dhesi S. S. &Mathur N. D. Shear-strain-mediatedmagnetoelectric effects revealed by imaging. Nat. Mater. 18 , 840–845 (2019). DOI: 10.1038/s41563-019-0374-8 Publication keywords: Piezoelectric effect; X-rayMagnetic Circular Dichrosim (XMCD) ; PhotoEmission ElectronMicroscopy (PEEM); Magnetoelectric effect W hen an international teamof researchers usedDiamond Light Source’s Nanoscience beamline (I06) to investigate the piezoelectric properties of a ceramic material, PMN-PT, they made an unexpected discovery. Some solids develop electrical charge in response to an appliedmechanical stress, which is called the piezoelectric effect. Piezoelectric crystals can convert mechanical energy into electricity or vice-versa. One example of an application is the automatic focusing of mobile phone cameras. In this application, the strain varies continuously with applied voltage. However, cycling the applied voltage can lead to discontinuous changes of strain that can be used to drive magnetic switching in a thin ferromagnet film, meaning that data can be written electrically, and storedmagnetically. The researchers were using a ferromagnetic filmof nickel as a sensitive strain gauge for single PMN-PT crystal. By combining PhotoEmission Electron Microscopy (PEEM) with X-ray Magnetic Circular Dichroism (XMCD) while varying the voltage across the crystal, they were able to produce a magnetic vector map. The initial measurements showedmagnetic domains that seemingly rotated by 90° due to ferroelectric domain switching, which was what the teamwere expecting to see. However, the high-resolution data from I06 offered the opportunity to dig a little deeper, and to carry out a pixel-by-pixel comparison of the XMCD-PEEM images. This revealed the magnetic switching angles typically fell well short of 90°. Although unexpected, this result could be explained by considering a shear component. This discovery shouldbe applicable to othermaterials andwill informthe development andminiaturisation of devices based onmagnetoelectric materials. Controlling magnetic materials with a voltage could lead to low-power spintronic logic and memory devices. Recent research on beamline I06 has revealed a new twist on the mechanismbehind this control. Using a voltage to control magnetic materials could be key to developing the next-generation of spintronic devices, where information can be processed and stored by flipping spins instead of moving charge. It is widely believed that the impact of spintronics on information technology could be huge, as reducing the movement of charge would reduce Joule heat, thus leading to more energy efficient computers that work faster. Voltage-controlled magnetism is known as the converse magnetoelectric effect (CME), whereasmagnetic control of electrical polarisation is known as the direct magnetoelectric effect. The CME was first described in 1958 by Landau and Lifshitz 1 to explain the curious observation that certain crystals develop a magnetic moment when subjected to an applied electric field. An elegant theoretical paper by Dzialozinski 2 identified antiferromagnetic Cr 2 O 3 (the most thermodynamically stable oxide of chromium) as a suitable candidate for CMEs, and the first measurements were performed on thismaterial by Astrov in 1962 3 . However, the CME was so small that magnetoelectrics were almost completely forgotten until the beginning of this century, when they went mainstream by throwing together people with expertise in magnetism and people with expertise in ferroelectricity. The renewed interest in magnetoelectrics was driven by the discovery of new materials, which unlike Cr 2 O 3 show long-range charge order as well as MagneticMaterials Group Beamline I06 long-range spin order. These ‘multiferroic’ materials tend to show intrinsic magnetoelectric coupling between the two types of order, but they are not suitable for applications. This is because they only show large CMEs at low temperature. By contrast, large CMEs at room temperature can be found in multiferroic systems in which ferromagnetic films are addressed via strain from an underlying piezoelectric material. This piezoelectric material is typically drawn from the subset of piezoelectric materials that are ferroelectric, such that long-range charge order is manifested as an electrical polarisation that is spontaneous, stable and electrically switchable. The converse piezoelectric effect permits a material to convert a voltage into a mechanical deformation, e.g. to focus cameras in mobile telephones. By contrast, the direct piezoelectric effect permits a material to convert mechanical deformations into a change of electrical polarisation and thus electrical current, e.g. for microphones and energy harvesting. The piezoelectric material of choice is typically a ferroelectric oxide with the perovskite crystal structure, e.g. PZT [Pb(Zr x Ti 1 x )O 3 ] and PMN-PT [(1- x )Pb(Mg 1/3 Nb 2/3 )O 3 – x PbTiO 3 ]. The transduction is linear under normal service conditions, but a sufficiently large voltage can switch the ferroelectric domains to produce large changes of strain that are discontinuous and nonvolatile. In an overlying thin film, the resulting magnetic changes are also large, discontinuous and nonvolatile, as required for magnetoelectric memory elements. Work recently published in Nature Materials 4 exploited beamline I06 at Diamond to reveal a novel type of magnetoelectric effect in a multiferroic system. This system comprised a thin polycrystalline film of ferromagnetic nickel on a ferroelectric substrate of PMN-PT with an (011) surface. CMEs of record magnitude were initially recorded in a vibrating sample magnetometer, whose probe was wired for electrical access to the sample. From these data, it appeared that electrically switching ferroelectric domains in the PMN-PT substrate produced 90° rotations of Ni magnetisation via strain, as expected. However, PhotoEmission Electron Microscopy (PEEM) data obtained on I06, with magnetic contrast from X-ray Magnetic Circular Dichroism (XMCD), told a different story about the magnetoelectric effects on a shorter lengthscale. For different voltages across the substrate, XMCD-PEEM images were obtained for orthogonal azimuthal sample orientations, and combined to form magnetic vector maps of in-plane magnetisation (as done previously by the same team when reporting extrinsic magnetocaloric effects in Nature Materials 5 ). Visual inspection of the vector maps permits one to believe that the expected 90° magnetic rotations took place when an electric field switched the underlying ferroelectric domains (Fig. 1). However, the apparent resolution of the colour wheel is only accurate to a few tens of degrees. Instead, a pixel- by-pixel comparison revealed that the magnetic rotations typically fell short of 90° (Fig. 2). The shortfall arose due to a shear strain that accompanies the well-known normal strain associated with ferroelectric domain switching in PMN-PT. This shear strain had not been hitherto considered, even though it follows directly fromknowlege of the PMN-PT unit cell. (The predictedmagnetic rotation of 62.6° is not modal due to long-range strain between different ferroelectric domains.) Moreover, the effect of this shear strain was missed in previous magnetoelectric measurements because clockwise and anticlockwise magnetic rotations serve to cancel its signature. The high-resolution vector maps of magnetisationwere key to the discovery of the shear strain, as was the pixel-by-pixel comparison. Although high- resolution vector maps of magnetisation are rarely found in the literature, they have become standard for the team with PEEM available on beamline I06, as will pixel-by-pixel comparisons. The observation of sub-90° magnetic switching has immediate implications for the performance of proposed magnetoelectric random access memory devices. The shear strain tends to compromise the magnetoresistive read-out of the magnetic information 6,7 , but only slightly, and it offers an exciting opportunity to write data both electrically and magnetically in the same bit. More generally, the magnetically soft nickel film acted as a sensitive strain gauge, and it revealed an unexpected twist in a well-known ferroelectric material. This method of studying local strain could now become a standard technique for mapping strain, and it could, moreover, yield yet more surprises in the future. References: 1. L. D. Landau and E. M. Lifshitz, Electrodynamics of Continuous Media, Pergamon Press 1 st ed , (1958). 2. Dzyaloshinskii I. E. On the magneto-electrical effect in antiferromagnets. J. Exptl. Theoret. Phys. (U.S.S.R.) 37 , 881-882 (1959). 3. Astrov D. N. Magnetoelectric effect in chromium oxide. J. Exptl. Theoret. Phys. (U.S.S.R.) 13 (4) , 729 (1961). 4. Ghidini M. et al. Shear-strain-mediated magnetoelectric effects revealed by imaging. Nat. Mater. 18 , 840–845 (2019). DOI: 10.1038/s41563-019-0374-8 5. Moya X. et al. Giant and reversible extrinsic magnetocaloric effects in La 0.7 Ca 0.3 MnO 3 films due to strain. Nat. Mater. 12 , 52-58 (2013). DOI: 10.1038/nmat3463 6. Bibes M. et al. Towards a magnetoelectric memory. Nat. Mater. 7 , 425-426 (2008). DOI: 10.1038/nmat2189 7. Hu J.-M. et al. High-density magnetoresistive random access memory operating at ultralow voltage at room temperature . Nat. Commun. 2 , 553 (2011). DOI: 10.1038/ncomms1564 Funding acknowledgement: This work was funded by Isaac NewtonTrust grants 10.26(u) and 11.35(u), UK EPSRC grant EP/G031509/1, the Royal Society (X.M.) and a start-up fund from the University of Wisconsin-Madison (J.-M.H.). D.P. acknowledges funding from the Agència de Gestió d’Ajuts Universitaris i de Recerca – Generalitat de Catalunya (grant 2014 BP-A 00079). We thank Diamond Light Source for time on beamline I06 (proposal SI-8876), and we thank S. Zhang for discussions. Corresponding author: Massimo Ghidini, University of Parma, massimo.ghidini@fis.unipr.it Figure 1: Magnetic vector maps of a ferromagnetic Ni film 4 , obtained (a) before and (b) after applying an electric field across the ferroelectric substrate of PMN-PT. Visual inspection suggests that the local magnetisation rotates by the hitherto expected value of 90°, but quantitative analysis (Fig. 2) reveals that this is not the case. Any resemblance between (a) and a map of the world is purely coincidental. Figure 2: Detailed comparison of the two magnetic vector maps in Fig. 1 4 . (a) Map of the magnetic changes between Fig. 1a and Fig. 1b; (b) Histogram showing these magnetic changes for pixels that started green in Fig. 1a; (c) Histogram showing these magnetic changes for pixels that started purple in Fig. 1a. (Data are filtered by colour to identify peaks.) Pixel magnetisations typically switched by angles that fall short of 90 ° .