The planar triangular S = 3/2 magnet AgCrSe₂: magnetic frustration, short range correlations, and field tuned anisotropic cycloidal magnetic order

Our studies evidence an anisotropic magnetic order below T_N = 32~K. Susceptibility data in small fields of about 1~T reveal an antiferromagnetic (AFM) order for H ⊥ c, whereas for H || c the data are reminiscent of a field-induced ferromagnetic (FM) structure. At low temperatures and for H ⊥ c, the field-dependent magnetization and AC susceptibility data evidence a metamagnetic transition at H⁺ = 5~T, which is absent for H || c. We assign this to a transition from a planar cycloidal spin structure at low fields to a planar fan-like arrangement above H⁺. A fully FM polarized state is obtained above the saturation field of H_⊥S = 23.7~T at 2~K with a magnetization of M_s = 2.8~_μB/Cr. For H || c, M(H) monotonously increases and saturates at the same M_s value at H_IIS = 25.1~T at 4.2~K. Above T_N, the magnetic susceptibility and specific heat indicate signatures of two dimensional (2D) frustration related to the presence of planar ferromagnetic and antiferromagnetic exchange interactions. We found a pronounced nearly isotropic maximum in both properties at about T* = 45~K, which is a clear fingerprint of short-range correlations and emergent spin fluctuations. Calculations based on a planar 2D Heisenberg model support our experimental findings and suggest a predominant FM exchange among nearest and AFM exchange among third-nearest neighbors. Only a minor contribution might be assigned to the antisymmetric Dzyaloshinskii-Moriya interaction possible related to the non-centrosymmetric polar space group R3m. Due to these competing interactions, the magnetism in AgCrSe₂, in contrast to the oxygen based delafossites, can be tuned by relatively small, experimentally accessible, magnetic fields, allowing us to establish the complete anisotropic magnetic H-T phase diagram in detail.