Magnetic phase diagram of the quantum spin chain compound SrCo₂V₂O₈: a single-crystal neutron diffraction study

Abstract

Abstract We explore magnetic order in the quantum spin chain compound SrCo 2 V 2 O 8 up to 14.9 T and down to 50 mK, using single-crystal neutron diffraction. Upon cooling in zero-field, commensurate antiferromagnetic (C-AFM) order with modulation vector <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">k</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> </mml:msub> </mml:math> = (0, 0, 1) develops below T N ≃ 5.0 K. Applying an external magnetic field ( H ∥ c axis) destabilizes this C-AFM order, leading to an order-disorder transition between T N and ∼1.5 K. Below 1.5 K, a commensurate to incommensurate (IC-AFM) transition occurs at 3.9 T, above which the magnetic reflections can be indexed by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">k</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">IC</mml:mi> </mml:mrow> </mml:msub> </mml:math> = (0, 0, 1 ± δl ). The incommensurability δl scales monotonically with H until the IC-AFM order disappears around 7.0 T. Magnetic reflections modulated by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">k</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> </mml:msub> </mml:math> emerge again at higher fields. While the characters of the C-AFM, IC-AFM and the emergent AFM order in SrCo 2 V 2 O 8 appear to fit the descriptions of the Néel, longitudinal spin density wave and transverse AFM order observed in the related compound BaCo 2 V 2 O 8 , our results also reveal several unique signatures that are not present in the latter, highlighting the inadequacy of mean-field theory in addressing the complex magnetic order in systems of this class.