Nanostructured Copper Achieves Simultaneous High Strength and Ductility
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RESEARCH/RESEARCHERS Simultaneous Spin and Structure Maps by Spin-Polarized STM One of the current challenges for materials science is to create materials that effectively utilize electron spin. An important characterization step for such materials would be to correlate the electron spin and physical structure. In a recent report, Arthur R. Smith and co-workers from Ohio University, Athens, and Case Western Reserve University, Cleveland, obtained both the chemical and spin structures of a magnetic material from a single measurement. The researchers used spinpolarized scanning tunneling microscopy (SP-STM) to map out the atom locations and spin orientations on the (010) face of Mn 3N 2 and compared their results to models based on first-principles calculations. According to Smith, “the development of this technique brings us one step closer to being able to utilize electron spin in science and technology.” As described in the November 25 issue of Physical Review Letters, the researchers first grew atomically smooth layers of Mn3N2 on a MgO substrate by molecularbeam epitaxy (MBE). In situ STM characterization of the surface was carried out in an ultrahigh vacuum chamber attached to the MBE chamber. STM images were obtained at 300 K in the constant-current mode using both nonmagnetic (W) and magnetic (W coated with Mn or Fe) STM tips. Use of very sharp tips allowed the resolution of individual Mn atoms on the surface. To ensure the reproducibility of the results, SP-STM measurements were carried out on multiple substrates using many tips. The success rate with magnetic tips was ~50%. STM images obtained using nonmagnetic tips show that the Mn3N2(010) surface consists of a series of identical rows of Mn atoms. The row heights, however, varied periodically if magnetic tips were used. This row-height modulation was modeled as resulting from the interaction of surface electron spin with the magnetic tip. Therefore, SP-STM images of magnetic materials contain chemical structure and electron spin information. Because the magnetic and nonmagnetic components have different periodicities, they can be separated to yield maps of physical structure and spin behavior. The individual height profiles of both surface maps are in excellent agreement with the simulated ones. Smith is currently applying SP-STM to new magnetic material surfaces, as well as investigating the effects of imaging parameters on magnetic contrast in order to understand spinpolarized tunneling in greater detail. GREG KHITROV 4
Nanostructured Copper Achieves Simultaneous High Strength and Ductility During the last two decades, there has been a burgeoning of interest in nanostructured metals due to their extremely small grain size and strengths far exceeding those of coarse-grained and even alloyed metals, which can potentially lead to many applications. These materials are generally considered to be nanocrystalline if the average grain or crystallite size is less than ~100 nm. However, such materials often exhibit low tensile ductility at room temperature, whi
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