AVS2014 Session MI+MG-TuM: Advanced Materials Discovery 

Throughout the history of mankind, scientists and engineers have relied on the slow and serendipitous trial-and-error approach for materials discovery. In 1990s, the combinatorial approach was pioneered in the pharmaceutical industry in order to dramatically increase the rate at which new chemicals are identified. The high-throughput concept is now widely implemented in a variety of fields in materials science. We have developed combinatorial thin film synthesis and characterization techniques in order to perform rapid survey of previously unexplored materials phase space in search of new inorganic functional materials. Various thin film deposition schemes including pulsed laser deposition, electron-beam deposition, and co-sputtering are implemented for fabricating massive arrays of compositionally varying samples on individual combinatorial libraries. A suite of high-throughput characterization tools are employed to screen the combinatorial libraries and map different physical properties of materials as a function of sweeping composition changes. They include room-temperature scanning SQUID microscopy, microwave microscopy, and micromachined MEMS cantilever arrays. Advanced characterization techniques at synchrotron beam lines are used for rapid diffraction as well as x-ray magnetic circular dichroism measurements.

The electronic structures of three different spin crossover molecules have been obtained by temperature dependent X-ray absorption spectroscopy (XAS). We show compelling evidence that the electronic structure changes associated with the spin crossover transition occur at significantly lower temperature than observed for the change in the molecular spin state. The transition temperatures indicated by XAS is about 20~60 K lower than that given by the magnetic moment switching. The changes in electronic structure are in agreement with density function theory (DFT) results that shows that the molecular electronic structures are different for high spin (HS) and low spin (LS) states. The conclusion that the electronic structure changes occur at significantly lower temperature than observed for the change in the molecular spin state associated with the spin crossover transition are also supported by transport measurements and the temperature dependence of the dielectric properties of SCO molecular system: [Fe(PM-AzA)₂(NCS)₂].

Topological states in magnetism, such as chiral skyrmions, with an integer topological charge are currently a topic of intensive fundamental research [1-3]. If one was able to control their properties in a digital manner, such as switching their topological charge deliberately in storage devices, a novel path in spintronics would be opened [4]. However, so far, most of these topological spin textures have been only observed in exotic materials with low symmetry and at low temperatures, making them rather impractical for applications. Here, we offer an alternative route by designing synthetic magnetic heterostructures where specific spin textures resembling swirls, vortices or skyrmions with distinct topological charge densities can be tailored at ambient temperatures. This is achieved by vertically stacking two magnetic nanopatterns with in-plane and out-of-plane magnetization and imprinting the in-plane non-collinear spin textures into the out-of-plane magnetized material. Key mechanisms of our concept are demonstrated both by micromagnetic simulations and experimental observations with element-specific magnetic soft x-ray microscopy [6] in a common ferromagnetic thin film element stack, e.g. Co/Pd multilayers coupled to Permalloy. Utilizing the interlayer coupling strength as tuning parameter, a gradual transition in the magnetic pattern of the out-of-plane layer from the decoupled magnetized state to a strongly coupled state with a vortex spin texture is achieved. At an intermediate coupling strength, magnetic spirals with tunable opening angle and in particular donut textures form which can be referred to as skyrmion system with D_n symmetry. Applying a small magnetic field, a controlled and reliable switching between two topologically distinct donut textures is realized.

Supported by the German Science Foundation (DFG) grant MA 5144/1-1, the European Research Council under European Union's Seventh Framework program (FP7/2007-2013)/ERC grant agreement n.306277. M.-Y.I. and P.F. acknowledge support by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DEAC02-05-CH11231 and by Leading Foreign Research Institute Recruitment Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science and Technology (MEST)(2012K1A4A3053565).

[1] X.Z. Yu et al, Nature 465 901 (2010)

[2] F. Jonietz et al, Science 330 1648 (2010)

[3] S. Heinze et al, Nat. Phys. 7 713 (2011)

[4] A. Fert et al, Nat. Nano 8 152 (2013)

[5] J. Sampaio et al, Nat. Nano 8 839 (2013)

[6] P. Fischer et al, JESRP 189 196 (2013)

Magnetic skyrmion lattice, a vortex-like spin texture recently observed in chiral magnets, is of great interest to future spin-electronic data storage and other information technology applications . The origin of the magnetic skyrmion phase can be traced to the anti-symmetric Dzyaloshinski-Moriya (DM) interaction that is allowed in space groups lacking inversion symmetry. The combined effect of a large ferromagnetic exchange and a weak DM interaction is to twist the magnetization into a long-period spiral that can be tens to hundreds of nanometers in length. As these spirals are only weakly bound to the underlying lattice in cubic systems, they can be readily manipulated with modest applied fields. Prototypical materials with the skyrmion ordering are those compounds with B20 structure, like MnSi and FeGe. The skyrmion lattice in MnSi appears in a small region (known as the A phase) of the H-T phase diagram in bulk samples, but in 2D samples like thin films the skyrmion phase is much more robust. It is of great interest to determine the properties of the skyrmion phase in quasi-1D nanowires and 2D thin films. If skyrmion ordering can persist in one-dimensional MnSi nanowires and 2D films, then these systems may be very promising for spintronics applications as the magnetic domains and individual skymions could be manipulated with small currents. We have systematically explored the synthesis of single crystal MnSi nanowires via controlled oxide-assisted chemical vapor deposition and observed a characteristic signature of skyrmion magnetic ordering in MnSi nanowires. The SiO₂ layer pla ys a key role for the high yield, correct stoichiometric and crystalline growth of the B20 MnSi nanowires. A growth phase diagram was constructed. For the thin films, an unique growth receipt was developed for the growth of high quality of thin films. The structure and magnetic properties of the films at different thickness were studied.

LaSrMnO3/PbZrTiO3 (LSMO/PZT) magnetoelectric heterostructures make them attractive not only for data storage applications but also for studying strain measurements. There are many reasons why LSMO/PZT magnetoelectric heterostructures were selected for our studies such as the excellent lattice matching and high ordering temperatures. X-ray micro diffraction technique is used to observe local strain behavior at the interface of LaSrMnO3/PbZrTiO3 magnetoelectric heterostructures. Due to high spatial resolution in X-ray spot size (~1 mm) laterally, the observation of strain measurements in thickness-dependent PbZrTiO3 reveals shiftings of LSMO and PZT peaks, allowing an understanding of the behavior of strain at the interface which can be related to the mechanism of magnetoelectric coupling.