Field-induced phase transitions and anisotropic magnetic properties of the Kitaev-Heisenberg compound Na2Co2TeO6

Spin systems with honeycomb structures have recently attracted a great deal of attention in connection with the Kitaev quantum spin liquid state (QSL) predicted theoretically. One possible Kitaev QSL candidate is Na2Co2TeO6 realizing a honeycomb lattice of pseudospin 1/2. Field-dependent single-crystal neutron diffraction technique allows us to determine the microscopic spin-spin correlations across the field-induced phase transitions for H II a and H II a* in plane field directions. Our results reveal phase transitions, initially to a canted zigzag antiferromagnetic state at approximately 60 kOe, followed by a possible transition to a partially polarized state over the range 90-120 kOe, and finally to a field-induced fully polarized state above 120 kOe. We observe distinct field dependencies of the magnetic peak intensities for H II a and H II a*. In addition, low-temperature electron spin resonance in magnetic fields H II c yields a complete softening for one of the antiferromagnetic resonances at similar to 40 kOe, revealing a field-induced phase transition. The present work thus provides insights into the field evolution of the important Kitaev-Heisenberg spin system Na2Co2TeO6.