The layered ferromagnet Cs₂AgF₄: Antiferromagnetic inter-layer coupling driven by magnetic dipole-dipole interactions

Abstract

The layered fluoride Cs₂AgF₄ has been found to undergo a ferromagnetic-like phase transition below T_C ≈ 14.5 K leading to magnetic saturation already at very low magnetic fields (H ≥ 10 kOe). We have explored the nature of this phase transition by studying the magnetization and specific heat under magnetic field and by evaluating the spin exchange interactions of Cs₂AgF₄ on the basis of density functional calculations as well as the magnetic dipole-dipole interaction energies for several ordered spin arrangements of Cs₂AgF₄. The magnetization exhibits an almost rectangular behavior with a small hysteresis indicating Cs₂AgF₄ to be a very soft ferromagnet with a coercitive field of H ≤ 20 Oe. The magnetization saturates at a value of ∼0.9 μ_B, in good agreement with the 4d⁹ configuration and a S = ½ ground state of Ag²⁺. Without applied external magnetic field (H = 0 Oe) the specific heat C_p exhibits an anomaly at T_C that deviates considerably from a λ-type anomaly. When H is increased from 0, the C_p anomaly is smeared out without shifting its position, and eventually disappears for H > 200 Oe. This observation is ascribed to the presence of weak magnetic dipole-dipole inter-layer interactions which favor a weak antiferromagnetic coupling and just outweigh weak ferromagnetic inter-layer spin exchange interaction found from the density functional calculations.