Strong uniaxial pressure dependencies evidencing spin-lattice coupling and spin fluctuations in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>Cr</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>Ge</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>Te</mml:mi></mml:mrow><mml:mn>6</mml:mn></mml:msub></mml:math>
S. Spachmann, S. Selter, B. Büchner, Saicharan Aswartham, R. Klingeler
Abstract
Single crystals of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ were studied by high-resolution capacitance dilatometry to obtain in-plane ($B\ensuremath{\parallel}ab$) and out-of-plane ($B\ensuremath{\parallel}c$) thermal expansion and magnetostriction at temperatures between 2 and 300 K and in magnetic fields up to 15 T. The anomalies in both response functions lead to the ``magnetoelastic'' phase diagrams and separate the paramagnetic (PM), ferromagnetic low-temperature/low-field (LTF) and aligned ferromagnetic (FM) phases. Different signs of magnetostriction anomalies as well as the evolution of thermal expansion anomalies at small fields $B\ensuremath{\parallel}ab$ of different magnetic-field dependence clearly supports the scenario of an intermediate region separating PM and LTF phases in finite external in-plane magnetic fields and implies a triple point in the magnetic phase diagram. Simulations of the magnetostriction using the Stoner-Wohlfarth model for uniaxial anisotropy demonstrate that the observed quadratic-in-field behavior in the LTF phase is in line with a rotation of the spins from the preferred $c$ direction into the $ab$ plane. Both the LTF and the PM phase close to ${T}_{\mathrm{C}}$ exhibit very strong pressure dependencies of the magnetization, $\ensuremath{\partial}ln{M}_{\mathrm{ab}}/\ensuremath{\partial}{p}_{\mathrm{ab}}$, of several hundred %/GPa and the transition from the LTF to the FM phase strongly depends on ${p}_{\mathrm{ab}}$ ($\ensuremath{\sim}\ensuremath{-}280$%/GPa), indicating a strong decrease in the uniaxial anisotropy under applied in-plane pressure. Our data clearly demonstrate the relevance of critical fluctuations and magnetoelastic coupling in ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$.