Orbital and spin susceptibilities of the plane copper sites in YBa ₂Cu₃O_x system

The tensor components of the magnetic susceptibility (χ) and the Knight shift (K) at the plane Cu(2) sites have been precisely measured in YBa ₂Cu₃O_x with x=6.98, 6.48 and 6.0. Using the temperature dependent χ and K observed in YBa₂Cu ₃O_6.48, we have carried out the analysis of K-χ diagram to make a separation of the observed susceptibility into orbital and spin contributions. Assuming the orbital contribution from the Cu(2) ions is independent of x for 6.0≤x≤7.0, we have obtained the x dependent spin susceptibility of the Cu(2) ions. The obtained spin susceptibility is discussed in comparison with the theoretical predictions of the Heisenberg model, the Stoner model and a band structure calculation. A complete set of the parameter values used in the crystal field model of a Cu²⁺ ion (the crystal field splittings, the orbital reduction factor, the spin-orbit coupling and the g-factors) has been determined from the K-χ analysis.