X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble X-Ray magnetic circular dichroism spectroscopy of Fe II – Nb IV molecular magnet Dominik Czernia 1. Molecular magnets The phenomenon of magnetism fascinate mankind for thousands of years. Although magnetic attraction has been observed in antiquity, process of understanding its origin has lasted for a very long time. A sudden progression in this field of both knowledge and technology came out just in the last century. Nowadays, most common conventional magnets, made of transition metals and rare-earths (like iron and neodymium) can be found everywhere: in cars, telephones, computers, scientific or medical instruments. Futhermore, since the 80s of XX th century a new, rapidly expanding magnetism branch - molecular magnetism - has introduced a vast variety of novel materials with unique promising for future applications properties. They offer multifunctionality: lightness, transparency, solubility, biocompatibility, magneto-optical and electrical properties. Nevertheless, the biggest advantage of molecular magnets over conventional magnets is that their properties can be tuned. By chosing appropriate molecules and by manipulation of synthesis parameters (e.g. temperature, pressure or electromagnetic radiation) one can remarkably modify the final outcome. The vast majority of molecular magnetic materials in the literature contain first row transition metal ions (3d) as the main magnetic moment carriers. The reason of their popularity is that they are easier to handle than their 4d and 5d congener. Nonetheless, the use of the heavier transition metal ions offers significant benefits which are not present in lighter ions. They are characterized by [1]: more radially extended valence orbitals (5d > 4d >> 3d) which translate into stronger exchange interactions, large spin – orbit coupling parameters which often lead to highly anisotropic g factors, variety of oxidation states with redox events which can be triggered photochemically or electronically. 1
X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble Molecular 4d and 5d magnets exhibit anisotropic long-range order, magnetic bistability and slow magnetic relaxation. Due to these inherently unusual quantum magnetic properties, they are of great interest in potential applications in spintronics and quantum computing [1]. The easy changes in oxidation states and the ability to promote these changes using external stimuli (e.g. light) allowed to produce photomagnetic compounds where magnetism of a system changes after absorption of a photon [2]. 2. Studied sample Examined compound {[Fe II (H 2 O) 2 ] 2 [Nb IV (CN) 8 ⸱ 4H 2 O} 4 , in the further part of this work indicated by an abbreviation Fe II – Nb IV , was first time synthesized and initially characterized by Dawid Pinkowicz et al. from Jagiellonian University in Cracow, Poland [3]. Fe II – Nb IV crystallizes in the tetragonal system with space group I4/m which was resolved using single crystal X-Ray diffraction (fig. 1a). Architecture comprises iron and niobium ions connected with each other through cyano-bridges (CN), caged in coordination spheres (fig. 1b). Figure 1. Crystal structure of Fe II – Nb IV (a), connectivity and geometry of the coordination spheres (b). Water molecules are omitted for clarity [3]. Magnetic properties of Fe II – Nb IV derive from eight high-spin iron (s = 2, g = 2.2) and 1 four niobium (s = 2 , g = 2) ions per mol. For temperatures above 100 K magnetic susceptibility 𝜓 𝑒𝑑 (𝑈) (fig. 2) measured in a bias magnetic field 𝐼 𝑒𝑑 = 2 𝑙𝑃𝑓 obeys Curie – Weiss law θ = 47 ± 1 𝐿 (eqn. 1) with the positive Weiss temperature and the Curie constant 𝐷 = 7.7 ± 0.2 𝑑𝑛3𝐿 𝑛𝑝𝑚 [3]. 𝐷 𝜓 𝑒𝑑 (𝑈) = 𝑈−θ (1) 2
X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble The transition to the long-range ordered magnetic state occurs at critical temperature 𝑑 = 43 𝐿 what can be estimated from sharp increase of DC susceptibility 𝜓 𝑒𝑑 (𝑈) curve about 𝑈 and from the strong peak in the AC susceptibility 𝜓 𝑏𝑑 (𝑈) signal (fig. 2). Figure 2. Magnetic 𝜓 𝑒𝑑 𝑈(𝑈) and 𝜓 𝑏𝑑 𝑈(𝑈) (inset) dependances of Fe II – Nb IV [3]. The virgin magnetization and magnetization loop of Fe II – Nb IV are presented in figure 3. The compound is a soft magnet with the coercive field 𝐼 𝑑 = 130 𝑃𝑓 and the remanence 𝑁 𝑆 = 1.24 𝑂𝛾 [3]. Authors of [3] performed calculations to estimate the value of the exchange coupling constant 𝐾 between Fe II and Nb IV assuming that the exchange interaction mediated through CN bridge is ferromagnetic and obtain 𝐾 = +8.1 1 𝑑𝑛3 . However, they also emphasized that the shape of the magnetization curve suggest non-collinear ordering of magnetic moments and therefore Fe II – Nb IV might be not ferro- but ferrimagnetic. Figure 3. Virgin magnetization and hysteresis loop (inset) of Fe II – Nb IV [3]. 3
X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble 3. Synchrotron X-rays measurment techniques Molecular magnet Fe II – Nb IV represent the foundation in the enginnering of FeNb cyano-bridged materials with diverse topologies and required multifunctionality [3]. Taking into account a high scientific and technological value of above group of materials to complement previously done measurments on this compound, X-ray Magnetic Circular Dichroism spectrocopy (XMCD) was performed at ID12 beamline at European Synchrotron Radiation Facility (ESRF) in Grenoble. 3.1. X-ray absorption spectroscopy (XAS) Among many reasearch techniques which benefit from X-rays, an absorption spectroscopy was strongly developed due to arise of synchrotron radiation. Possibility to use monochromatic beam and fast change of photons wavelengths significantly eased application of that technique. XAS measure the ability of the sample to absorb photon of a particular wavelength. Probability of this process possess three main characteristic properties: tendency to decrease with increasing energies of photons as a consequence of fotoelectric effect - cross section 𝜏 depends on energy 𝐹 and atomic numer Z [4]: 𝑎 5 𝜏 ∝ 𝐹 7/2 (2). occurrence of the sharp peaks for the specific photon energies - so called absorption edges. These discontinuities arise when the energy of an absorber photon is high enough to excite electron from one energy level to a higher energy level above Fermi level. The nomenclature used in XAS is as follows: K, L, M, N edges correspond to the principal quantum number of core electron 1, 2, 3 and 4 respectively. The subscripts 2, 3, 4 and 5 refer to 𝑞 1/2 , 𝑞 3/2 , 𝑒 3/2 , 𝑒 5/2 initial core states respictively [5]. Assuming dipole approximation: absorption on K and L 1 edges always lead to electronic transition to unfilled p shells and absorption on L 2,3 edges to s (2% cases) or d (98% cases) shells [4]. Analysis of shape and position of absorption edge is an entity of X-ray Absorption Near Edge Structure (XANES) which is the mostly performed for photons energies from about 10 eV below edge to about 30 eV above it. This technique is element selective and allows to determine electronic density of unfilled states. 4
X-Ray and neutron science - The International Student Summer Programme 2017 at ILL/ESRF Grenoble oscillations of absorption above absorption edges. From an amplitude (which is usually of an order of few percentage of jump on absorption edge) and from frequency of oscillations one can obtain information about local atomic structure around absorbing atom. Analysis is conducted in the range starting from about 20 to 1600 eV above absorption edge and is so called Extended X-ray Absorption Fine Structure (EXAFS). Figure 4. shows schematic example of absorption spectrum with marked area of applicability of XANES and EXAFS. Figure 4. XAS spectrum of a molecule in solution illustrating two regions: the low energy XANES and the high energy EXAFS [6]. 3.2. X-ray Magnetic Circular Dichroism (XMCD) In most cases the influence of magnetic moments on XAS is negligible because theoretical cross section of that process is around 10 -5 -10 -6 times smaller than other absorption processes [4]. However, near absorption edges interaction between magnetic moments and X-rays raises and can be observed by using sensitive instruments and high intensity radiation. Moreover, by using circularly polarized X-rays one can observe difference in absorption in magnetized material between right and left polarized photons. This property is used in X-ray Magnetic Circular Dichroism (XMCD). The basic principle of XMCD can be explained in two steps. Circularly polarized X-ray photon carries an angular momentum: +ℏ for a left polarized photon and −ℏ for a 5
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