Spin-polarized photoelectron diffraction and valence-band photoemission from MnO(001).

Abstract

Spin-polarized photoelectron diffraction (SPPD) has previously been proposed as a technique for studying short-range magnetic order in magnetic materials, and the first experimental study of this kind was performed on the ionic antiferromagnetic ${\mathrm{KMnF}}_{3}$ [B. Sinkovic, B. Hermsmeier, and C. S. Fadley, Phys. Rev. Lett. 55, 1227 (1985)]. We present here a much more detailed study of SPPD for the antiferromagnetic transition-metal oxide MnO with a (001) surface orientation. The Mn 3s and Mn 3p multiplets have been studied using both low-energy (192.6 eV) and high-energy (1486.7 eV) x-ray excitation and the intensity ratios I${(}^{5}$S(\ensuremath{\uparrow}))/I${(}^{7}$S(\ensuremath{\downarrow})) and I${(}^{5}$P(\ensuremath{\uparrow}))/I${(}^{7}$P(\ensuremath{\downarrow})) have been measured as a function of both direction and temperature. Data obtained with the lower excitation energy and resulting in kinetic energies of 50--100 eV show an abrupt change or step in both the I${(}^{5}$S(\ensuremath{\uparrow}))/I${(}^{7}$S(\ensuremath{\downarrow})) and I${(}^{5}$P(\ensuremath{\uparrow}))/I${(}^{7}$P(\ensuremath{\downarrow})) intensity ratios at \ensuremath{\approxeq}530\ifmmode\pm\else\textpm\fi{}20 K or \ensuremath{\approxeq}4.5 times the N\'eel temperature ${\mathit{T}}_{\mathit{N}}$.This change is interpreted to be a new type of short-range-order transition occurring at what is denoted ${\mathit{T}}_{\mathrm{SR}}$. Also, these same quintet or septet intensity ratios show a weak peak at ${\mathit{T}}_{\mathit{N}}$, suggesting for the first time that SPPD has sensitivity to long-range magnetic order. Data obtained for the I${(}^{5}$S(\ensuremath{\uparrow}))/I${(}^{7}$S(\ensuremath{\downarrow})) intensity ratio with the higher excitation energy show no such effects, a result consistent with the much weaker exchange scattering expected at such energies. Additional x-ray photoelectron spectroscopy spectra and azimuthal scans of Mn and O core-level intensities are considered and these establish that (i) the sample surface had good stoichiometry and was very clean and well ordered, (ii) the SPPD effects observed at ${\mathit{T}}_{\mathrm{SR}}$ are not due to any surface structural change, and (iii) a single-scattering cluster (SSC) theoretical model is at least a qualitatively reasonable starting point for describing such effects. We also compare experimental results for the magnitudes of these steps with calculations based upon exchange scattering in the spin-polarized SSC model [B. Sinkovic and C. S. Fadley, Phys. Rev. B 31, 4665 (1985)], and conclude that there is at least qualitative agreement. A final aspect of our data concerns the temperature dependence of the Mn 3d--dominated valence-band spectra: These spectra are found to show no measurable change in crossing ${\mathit{T}}_{\mathrm{SR}}$, but by contrast exhibit a large 0.4-eV increase in width in going below ${\mathit{T}}_{\mathit{N}}$, which is in contradiction to recent theoretical predictions.