Study of growing incongruent antiferromagnetic single crystals and characterization
Antiferromagnets are gaining increased attention as potential spintronic materials with faster switching rates compared to conventional ferromagnets. Recently reported studies have demonstrated partial switching of the Néel vector of antiferromagnetic CuMnAs and Mn2Au with current-induced spin-orbit torques. More intermetallic antiferromagnets share the structure types of CuMnAs and Mn2Au, or at least contain degenerated in-plane magnetic configurations, are proposed as candidates for current-induced magnetic switching. Single crystals of such materials are needed to study the orientation dependence of their magnetic dynamics, but the growth of single crystals of intermetallics is challenging, especially for incongruent candidates that cannot be obtained by conventional solid-state synthesis. Here, multiple synthesis methods for single crystals of the metallic antiferromagnets Fe2As, FeSn, and Cr2Al were investigated. The chemical vapor transport (CVT) has been used to assemble large crystalline grains below their melting/decomposition temperatures. In this thesis, the thermodynamics of these CVT reactions are investigated and presented alongside experimental efforts for single crystals of Fe2As and FeSn. Reasonably reliable predictions and analysis can be attained, but numerous approximations must be main due to unknown entropy and enthalpy for many of the phases involved in a given CVT reaction. Compared to the previously reported models of the intermetallic CVT systems, the proposed reactions provide a more comprehensive means to consider the effective parameters (including the transport agents, the possible intermetallic phases to deposit, and the partial pressures of each individual transport species) for predicting the outcomes of a CVT system. The flux method was used to grow single crystals of Cr2Al, with crystallinity and purity confirmed by X-ray diffraction. The neutron diffraction of a Cr2Al powder sample revealed the magnetic ordering of Cr2Al. The Rietveld refinement, together with thermal analysis and resistivity, indicates a likely in-plane magnetic ordering instead of a canted angle to the c-axis as published in a previous study. The magnetometry measurements carried out on an aligned crystal sample provide the first high-temperature susceptibility confirmation of the Néel temperature in Cr2Al. According to magnetometry, resistivity, and differential scanning calorimetry measurements, the Néel transition is observed at 360±2 °C. This thesis reports the first experimental characterization of a single crystal of an antiferromagnetic material with the MoSi2 structure type. It provides detailed experimental supporting information that enables further studies of magnetic dynamics of Cr2Al closely-related antiferromagnetic spintronic candidates.
antiferromagnetic, single crystals, chemical vapor transport, flux method, neutron diffraction.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign