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

Single crystals of the hexagonal triangular lattice compound AgCrSe2 have been grown by chemical vaportransport. The crystals have been carefully characterized and studied by magnetic susceptibility, magnetization,specific heat, and thermal expansion. In addition, we used Cr-electron spin resonance and neutron diffractionto probe the Cr 3d3 magnetism microscopically. To obtain the electronic density of states, we employed x-rayabsorption and resonant photoemission spectroscopy in combination with density functional theory calculations.Our studies evidence an anisotropic magnetic order below TN = 32 K. Susceptibility data in small fields of about1 T reveal an antiferromagnetic (AFM) type of order for H ? c, whereas for H c the data are reminiscent ofa field-induced ferromagnetic (FM) structure. At low temperatures and for H ? c, the field-dependent magnetizationand 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 fanlike arrangementabove H+. A fully ferromagnetically polarized state is obtained above the saturation field of H?S = 23.7 T at 2Kwith a magnetization of Ms = 2.8 ?B/Cr. For H c, M(H) monotonically increases and saturates at the sameMs value at HS = 25.1 T at 4.2 K. Above TN , the magnetic susceptibility and specific heat indicate signatures oftwo dimensional (2D) frustration related to the presence of planar ferromagnetic and antiferromagnetic exchangeinteractions. We found a pronounced nearly isotropic maximum in both properties at about T * = 45 K, which isa clear fingerprint of short range correlations and emergent spin fluctuations. Calculations based on a planar2D Heisenberg model support our experimental findings and suggest a predominant FM exchange amongnearest and AFM exchange among third-nearest neighbors. Only a minor contribution might be assigned tothe antisymmetric Dzyaloshinskii-Moriya interaction possibly related to the noncentrosymmetric polar spacegroup R3m. Due to these competing interactions, the magnetism in AgCrSe2, in contrast to the oxygen-baseddelafossites, can be tuned by relatively small, experimentally accessible magnetic fields, allowing us to establishthe complete anisotropic magnetic H-T phase diagram in detail.