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Probing Interfaces in Metals Using Neutron Reflectometry metals Article Probing Interfaces in Metals Using Neutron Reflectometry Michael J. Demkowicz 1,2,* and Jaroslaw Majewski 3,4,* Received: 25 November 2015; Accepted: 21 December 2015; Published: 20 January 2016 Academic Editor: Klaus-Dieter Liss 1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 2 Materials Science and Engineering, Texas A & M University, College Station, TX 77843, USA 3 MPA-CINT/Los Alamos Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 4 Department of Chemical Engineering, University of California at Davis, Davis, CA 95616, USA * Correspondence: [email protected] (M.J.D.); [email protected] (J.M.); Tel.: +1-979-458-9845 (M.J.D.); +1-505-667-8840 (J.M.); Fax: +1-505-665-2676 (J.M.) Abstract: Solid-state interfaces play a major role in a variety of material properties. They are especially important in determining the behavior of nano-structured materials, such as metallic multilayers. However, interface structure and properties remain poorly understood, in part because the experimental toolbox for characterizing them is limited. Neutron reflectometry (NR) offers unique opportunities for studying interfaces in metals due to the high penetration depth of neutrons and the non-monotonic dependence of their scattering cross-sections on atomic numbers. We review the basic physics of NR and outline the advantages that this method offers for investigating interface behavior in metals, especially under extreme environments. We then present several example NR studies to illustrate these advantages and discuss avenues for expanding the use of NR within the metals community. Keywords: neutron reactor; spallation source; metals; extreme conditions 1. Interfaces in Metals Metals form a wide variety of interfaces, including grain and phase boundaries [1], surface-liquid interfaces [2,3], solidification fronts [4], and mechanical contacts [5]. Although they typically occupy a small fraction of the total volume, interfaces play an outsized role in determining the properties of metals [6–9]. Understanding interfaces is therefore critical to predicting and controlling the behavior of metals. Experimental investigation of interfaces presents significant challenges. Because they are often buried within the material, accessing them frequently requires destructive characterization or sample preparation methods, such as transmission electron microscopy (TEM) [10] or atom probe tomography (APT) [11]. Interfaces in metals typically have low thickness; indeed, some are atomically sharp [12]. Thus, characterizing them requires high—sometimes Å-level—spatial resolution. Moreover, certain interfaces only exist at high temperatures and pressures [13–15] or under contact with external media, such as gases or liquids [2,16]. Investigating such interfaces requires special in situ characterization methods. An expanded experimental toolbox promises to accelerate progress in understanding metal interfaces, especially in extreme environments. This paper offers a primer on neutron reflectometry (NR): a characterization method with several advantages for studying metal interfaces [17–21]. NR is a mature experimental tool. The first NR experiments were conducted by Fermi and Zinn [22] and Metals 2016, 6, 20; doi:10.3390/met6010020 www.mdpi.com/journal/metals Metals 2016, 6, 20 2 of 17 Fermi and Marshall [23]. The technique has experienced continuous improvement since then [17,24–26]. Nevertheless,Metals 2016 use, 6, of20 this method within the metals community has been relatively limited. We2 of 17 illustrate the potential benefits of NR to investigations of interfaces in metals by explaining the physics of the NR experimentNevertheless, and use by presentingof this method several within example the metals studies. community has been relatively limited. We illustrate the potential benefits of NR to investigations of interfaces in metals by explaining the 2. The Physicsphysics of of the Neutron NR experiment Reflectometry and by presenting (NR) several example studies. Figure2. The1 showsPhysics aof schematic Neutron Reflectometry of a typical (NR) NR measurement. The sample is a thin, planar film on a substrate.Figure The 1 shows experiment a schematic is usuallyof a typical conducted NR measurement. in air orThe vacuum, sample is a but thin, may planar also film be on carried a it out in othersubstrate. media The (e.g.,experiment see Section is usually 6.4 conducted)[27]. The in air neutron or vacuum, source but may may also be a be fission carried nuclear it out in reactor or a spallationother media source. (e.g., see In theSection nuclear 6.4) [27]. reactor, The neutron the sustained source may nuclear be a fission fission nuclear of 235 reactorU- or or239 aPu-rich spallation source. In the nuclear reactor, the sustained nuclear fission of 235U‐ or 239Pu‐rich fuels fuels immersed in H2O, D2O, or solid graphite produces neutrons that may be used for scattering immersed in H2O, D2O, or solid graphite produces neutrons that may be used for scattering experiments.experiments. Spallation Spallation sources sources usually usually utilize utilize pulsed pulsed high-energy high‐energy (~GeV) (~GeV) protons protons toto bombard bombard targets made oftargets heavy made elements of heavy (such elements as W, (such Hg, as U) W, to Hg, extract U) to neutronsextract neutrons [28,29 [28,29].]. FigureFigure 1. Schematic 1. Schematic of of a a neutron neutron reflectometry reflectometry measurement. measurement. In material property studies, the neutrons extracted from nuclear reactors or spallation sources In materialare moderated property to decrease studies, their the energies neutrons and therefore extracted increase from their nuclear wavelengths reactors to or Å ranges. spallation Such sources are moderatedmoderation, to depending decrease on their the final energies wavelength and thereforeof neutrons increaserequired, is their usually wavelengths achieved by passing to Å ranges. Such moderation,high‐energy neutrons depending through on H the2O, finalliquid wavelengthH2, or solid methane. of neutrons Low energy required, neutrons is are usually typically achieved detected indirectly through absorption reactions with materials of high cross‐sections for such by passing high-energy neutrons through H O, liquid H , or solid methane. Low energy neutrons reactions. Typically, 3He, 6Li, or 10B is used to2 emit high‐energy2 particles, whose ionization signatures are typicallymay be detected detected by indirectly a number through of means. absorption reactions with materials of high cross-sections 3 6 10 for such reactions.Upon interacting Typically, withHe, the sampleLi, or at Ba isparticular used to angle emit of high-energy incidence, θ (or particles, a particular whose value ionization of signaturesthe mayneutron be momentum detected by transfer a number vector, of Q means.z), the incoming neutron beam can undergo absorption, Uponreflection, interacting transmission, with the or refraction. sample atConsequently, a particular there angle is a difference of incidence, betweenθ (or the a intensity particular of the value of outgoing, specularly reflected neutron beam and that of the incident beam. This difference—measured the neutron momentum transfer vector, Qz), the incoming neutron beam can undergo absorption, as a function of Qz—encodes information about the distribution of the nuclear scattering length reflection, transmission, or refraction. Consequently, there is a difference between the intensity of the density along the direction normal to the sample surface. Moreover, the neutron is a ½‐spin fermion outgoing,and specularly possesses a reflected magnetic neutron moment beamoppositely and thatoriented of the to the incident spin. Therefore, beam. This its difference—measuredinteraction with as a functionmatter may of Q dependz—encodes on the sample’s information spin or about magnetic the field. distribution Neutrons interact of the both nuclear with nuclear scattering spins length densityand along the the magnetic direction moments normal of tounpaired the sample electrons surface. via dipole Moreover,‐dipole processes. the neutron Interactions is a ½-spin with fermion and possessesunpaired a electrons magnetic may moment be of similar oppositely magnitude oriented as nuclear to the scattering. spin. Therefore, However, its they interaction are not with matter mayinherently depend isotropic. on the Rather, sample’s they depend spin oron magneticthe orientation field. of the Neutrons sample’s interactmagnetization both vis with‐a‐vis nuclear the direction of the neutron momentum wavevector transfer Qz: only the component of sample’s spins and the magnetic moments of unpaired electrons via dipole-dipole processes. Interactions with magnetization which is perpendicular to Qz affects the neutron scattering. Therefore, the intensity of unpairedspecularly electrons reflected may be neutrons of similar measured magnitude as a asfunction nuclear of scattering. Qz also encodes However, information they are about not inherentlythe isotropic.distribution Rather, they of the depend magnetization on the in orientation the sample as of a the function sample’s of depth magnetization [30]. vis-a-vis the direction of the neutronDepending momentum on the specific wavevector NR technique, transfer NRQ canz: only take advantage the component of a range of of sample’s different neutron magnetization‐ sample interactions [31]. However, this short review focuses on elastic specular NR,
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