EPR Study of Spin Anisotropies in the S = 1/2 Spatially Anisotropic Triangular Quantum Magnet Cu( <i>tn</i> )Cl ₂

Cu(<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tn</i>)Cl<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tn</i> = C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub>N<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>) represents a quasi-two-dimensional (2D) quantum magnet which preserves 2D features down to mK region despite the setting on magnetic long-range order at 0.55 K. The existence of large quantum fluctuations can be associated with the incommensurate modulated crystal structure. Previous studies of magnetic phase diagram revealed additional phase in the vicinity of a critical region. Since the phase appears in the fields perpendicular to the modulation vector, its dependence on the field orientation may have origin in the spatial and spin anisotropies. The latter can be estimated from the electron paramagnetic resonance (EPR) spectra. Present work is devoted to the X-band single crystal studies of angular and temperature dependence of EPR spectra. Angular dependence of linewidth within the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</i> planes shows cos<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>θ dependence associated with the contribution of spin anisotropies. The analysis of temperature dependence of linewidth along the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</i> axis enabled the quantitative estimation of spin anisotropies. In the field parallel to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> axis, which is perpendicular to modulation vector, the linewidth is determined by the combined contribution of exchange narrowed dipolar coupling, antisymmetric Dzyaloshinskii-Moriya (DM) interaction and symmetric spin anisotropies of dipolar origin <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i><sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dip</sup> and exchange anisotropy <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i><sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">EA</sup>. The absence of the additional phase in the magnetic phase diagram along the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</i> axis as well as symmetry requirements suggests that the DM contribution along the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</i> axis could be neglected and the linewidth is dominated by the contribution of symmetric spin anisotropies.