Article pubs.acs.org/IC
Tailoring Magnetic Behavior in the Tb-Au-Si Quasicrystal Approximant System Girma H. Gebresenbut,*,† Mikael S. Andersson,‡ Per Nordblad,‡ Martin Sahlberg,† and Cesar Pay Gómez† †
Department of Chemistry and ‡Department of Engineering Sciences, Ångström Laboratory, Uppsala University, 751 21 Uppsala, Sweden S Supporting Information *
ABSTRACT: A novel synthesis method, “arc-melting-self-flux”, has been developed and a series of five Tsai-type 1/1 approximant crystals in the Tb-Au-Si system have been synthesized. The synthesis method, by employing a temperature program which oscillates near the melting and nucleation points of the approximants, has provided high-quality and large single crystals in comparison to those obtained from the standard arc-melting-annealing and self-flux methods. The atomic structures of the approximants have been determined from single-crystal X-ray diffraction data and described using concentric atomic clusters with icosahedral symmetry. The compounds are nearly isostructural with subtle variations; two types of atomic clusters which mainly vary at their cluster centers are observed. One type contains a Tb site at the center, and the other contains a disordered tetrahedron decorated with Au/Si mixed sites. Both cluster types can be found coexisting in the approximants. The compounds have different average weighted ratios of central Tb to disordered tetrahedron in the bulk material. Furthermore, a strategy for chemically tuning magnetic behavior is presented. Magnetic property measurements on the approximants revealed that the magnetic transition temperature (Tc) decreases as the occupancy of the central Tb site increases. Tc decreased from 11.5 K for 0% occupancy of the central Tb to 8 K for 100% occupancy. Enhanced magneto crystalline anisotropy is observed for the approximants with higher central Tb occupancy in comparison to their low central Tb occupancy counterparts. Hence, the previously reported “ferrimagnetic-like” magnetic structure model remains valid.
1. INTRODUCTION The atomic structure of materials is mostly the inherent and inevitable cause for their magnetic properties. A subtle change in structure could bring original magnetic behavior.1 Hence, it is of supreme importance to understand and be able to modify atomic structures in order to obtain intended magnetic properties. Practically, it has been challenging to synthesize magnetic materials with anticipated atomic structures and apt morphology.2 This highlights the need for a complete study as presented in this work that combines novel synthesis, detailed structure analysis, and magnetic properties. High-temperature synthesis methods have direct scientific and commercial applications. The metallurgy and semiconductor industry would not be at its current technological state without high-temperature synthesis techniques such as Czochralski,3,4 Bridgeman,5−8 and casting9−11 to mention but a few.12−15 However, in cases where these techniques are not applicable, other cost-effective synthesis methods that fulfill the intended purposes are essential. A new high-temperature synthesis method that combines the desirable attributes of the present standard methods is required.16,17 In this study, an advanced high-temperature synthesis method that allows precise control at the atomic level and provides appropriate samples for diffraction and physical property measurements on © XXXX American Chemical Society
a new class of magnetic materials, quasicrystal approximants (ACs), has been developed. Basically, ACs are conventional 3D periodic materials. They often have local atomic structures, chemical compositions, and physical properties similar to those of their corresponding QCs.18,19 For example, ACs of icosahedral quasicrystals (iQCs) have similar icosahedral cluster units present in the related QCs.20 The lack of 3D periodicity while possessing positional long-range order made quasicrystals (QCs) challenge a prevalent paradigm for crystals21 and convinced the scientific community to redefine the concept of crystals.22 With the intent to observe unique magnetic behavior as a consequence of their unique crystal structure, the magnetic behavior of QCs has been investigated for nearly three decades.23−25 However, there has been no experimental report on a QC with long-range magnetic order to date.26 Rather, spin-glass behavior has been frequently observed and it has even been considered an intrinsic property of i-QCs and ACs.27 This argument has now been disproved, as long-range magnetic order has been observed in ACs of i-QCs.28 Received: October 5, 2015
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DOI: 10.1021/acs.inorgchem.5b02286 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
crucibles and sealed inside stainless steel tubes under an inert Ar atmosphere (
