Surface and Bulk Spin Ordering of Antiferromagnetic Materials: NiO(111)

We present a study of the prototypical NiO(111) antiferromagnet by nonresonant surface x-ray magnetic scattering. Direct access to the antiferromagnetic surface and bulk spin ordering is demonstrated. Our data support a first order antiferromagnetic to paramagnetic transition. A quantitative determination of the magnetization profile is proposed. It is shown that the NiO(111) surface spins remain ordered at higher temperatures than in the bulk and that the blocking temperature in exchange coupled ferromagnetic-NiO interfaces is most likely related to an $S$-domain structure loss occurring 25 K below the Néel temperature.

Figure 1

Principle of the GIXMS technique. (a) Experimental geometry of the magnetic surface diffraction experiment with a vertical sample surface and a horizontal polarization of the incident x-ray beam. The angles are defined as follows: $\alpha$ is the angle of the incident beam with the surface, $\omega$ is the azimuthal sample rotation, $2\theta$ (twice the Bragg angle) is the deviation of the diffracted beam, $\gamma$ and $\mu$ are the in and out-of-plane angles defining the detector position, and $\eta$ the orientation of the crystal analyzer and detector with respect to the diffracted beam in a plane perpendicular to the diffracted beam. (b) Drawing of the magnetic and structural unit lattices for NiO(111), $A$, $B$, and $C$ correspond to the face centered cubic lattice successive plane stacking, horizontal arrows indicate Ni atom spin orientations. (c) Comparison of the signals obtained using both channels during a linear scan along the (11L) reciprocal space direction taken at ${\alpha }_i=0.35°$.

Figure 2

Refraction effect on AF peaks with respect to ${\alpha }_i$ at $T=300\mathrm{K}$. (inset) $L$ scans across the ($11\frac{3}{2}$) NiO AF Bragg peak using the rotated channel at fixed ${\alpha }_i$ (from bottom to top ${\alpha }_i=0.1°$, $0.2°$, $0.25°$, $0.3°$, $0.35°$, $0.4°$, $0.5°$, $1°$, and $3°$—gray curves) compared to a scan taken in the symmetric geometry (${\alpha }_i\sim 9.45°$—black curve). For the smallest angles fitted Lorentzian lines are included. (main panel) (Left ordinate) Integrated intensity of the ($11\frac{3}{2}$) peak after active surface area correction (○) and the best fit (solid line) surface refraction law; magnetic and charge scattering exhibit identical ${\alpha }_c$. (Right ordinate) ($11\frac{3}{2}$) peak $L$ widths (●) and an exponential decay law (dashed line) included as a guide for the eye.

Figure 3

Magnetic intensity and magnetization with respect to $T$. The magnetic intensities were obtained through rocking scans corrected for the $L$ width, surface active area, electrical field strength, incident beam intensity, and corresponding charge scattering (constant internal normalization). (Left ordinate) Bulk sensitive (${\alpha }_i=3°$) diffracted intensities of the $(\overline{1}\overline{1}\frac{3}{2})$ (▽), $(1\overline{2}\frac{3}{2})$ (△), and (2$\overline{1}\frac{3}{2}$) (◇) AF Bragg peaks with respect to temperature. The individual peak intensity modulation is related to the in-surface-plane AF $S$ domain structure and vanishes at $T_S$. The overall evolution for ${\alpha }_i=0.3°$ is very similar and the individual intensities collapse again $\sim 25\mathrm{K}$ before the surface Néel transition temperature is reached. (Right ordinate) Experimental surface (${\alpha }_i=0.3°$, ○) and bulk (${\alpha }_i=3°$, ■) magnetizations derived from the ($11\frac{3}{2}$) peak family. Calculated magnetizations for first order Néel transition ($\kappa =31.6$, $ϵ=100$, $\zeta =0.1$, $T_N-T_0=25\mathrm{K}$) for bulk (solid line, $T_N=523.6\mathrm{K}$) and surface (dotted line, $T_N=529.5\mathrm{K}$), and calculated magnetization for a second order surface transition (dashed line, $T_N=530.3\mathrm{K}$, $\beta =0.22$). Abscissa axis breaks enlarge the important Néel transition temperature range.

Figure 4

Magnetization profiles in NiO(111) with respect to $z$ and $T$ for $D=15\pm 5$ and $\Delta =0.32\pm 0.02$. For each $T$ the asymptotic horizontal line is the bulk magnetization.