Magnetic transition in orbitally ordered KCuF3, K2CuF4 and heterovalently substituted compounds

We report μSR data from pseudo-cubic and layered copper fluorides, KCuF3, K2CuF4, and its substitutions K1.9Sr0.1CuF4, K2CuF3.9O0.1, K2Cu1.8Zn0.2F4. In KCuF3, the extinction of the F-Mu-F fraction and its progressive replacement by a magnetic fraction between 40 and 35 K demonstrate a first-order magnetic transition at TN, probably driven by orbital degrees of freedom. The layered compounds exhibit on the contrary conventional second-order transitions at Tc=6.25 K, independent of electron or hole doping up to at least x=0.1. However, 20% substitution of Cu with non-magnetic Zn completely suppresses long-range magnetic order above 1.3 K