Polarized neutron powder diffraction studies of antiferromagnetic order in bulk and nanoparticle NiO

In many materials it remains a challenge to reveal the nature of magnetic correlations, including antiferromagnetism and spin disorder. Revealing the spin structure in magnetic nanoparticles is further complicated by the large incoherent neutron scattering cross section from water adsorbed at the particle surfaces and by the broadening of diffraction peaks due to the finite crystallite size. Moreover, the spin structure in magnetic nanoparticles may deviate significantly from that of the corresponding bulk material because of the low-symmetry surroundings of surface atoms and the large relative surface contribution to the magnetic anisotropy. Here we explore the potential use of polarized neutron diffraction to reveal the magnetic structure in NiO bulk and nanoparticle powders by applying the $XYZ$-polarization analysis method. Our investigations address in particular the spin orientation in bulk NiO and platelet-shaped NiO nanoparticles with thickness from greater than 200 nm down to 2.0 nm. The advantage of the applied method is that it is able to clearly separate the structural, the magnetic, and the spin-incoherent scattering signals for all particle sizes. For platelet-shaped particles with thickness from greater than 200 nm down to 2.2 nm we find that the spin orientation deviates about ${16}^{\circ }$ from the primary (111) plane of the platelet-shaped particles. In the smallest particles (2.0 nm thick) we find the spins are oriented with a ${30}^{\circ }$ average angle to the primary (111) plane of the particles. The results show that polarization analyzed neutron powder diffraction is a viable method to investigate magnetic order in powders of antiferromagnetic nanoparticles.

Figure 1

TEM images of the samples (a) NiO250, (b) NiO300, and (c) NiO350. Particles seen edge on, like the ones highlighted in images (a)–(c), were used to measure the average particle thickness $t$. (d) Schematic drawing of a particle with the [111] direction normal to its face.

Figure 2

Unpolarized neutron powder diffraction data from platelet-shaped NiO nanoparticles measured at DMC. The measurements were performed at a temperature of 2 K. The measurement times were about 12 h for the smallest particles (NiO250) and a few hours for the largest (NiO600). The scattering at low $q$ is small angle scattering from the surface of the particles. A constant value of 0.001 counts/monitor has been added to the NiO300 data to avoid overlap with other data.

Figure 3

Polarization analyzed neutron powder diffraction patterns of (a) the bulk NiO-Alfa sample and (b)–(e) the four samples of platelet-shaped NiO nanoparticles. The separated magnetic (), nuclear (), and spin-incoherent $(\times )$ cross sections are shown. The full lines are the fits explained in the text. (f) Fits to the $\left\{\frac{1}{2}\frac{1}{2}\frac{1}{2}\right\}$ and $\left\{\frac{3}{2}\frac{1}{2}\frac{1}{2}\right\}$ magnetic peaks shown for all samples [notice that the intensity scale is changed with respect to (a)–(e)]. The blue and red lines below the data in (a)–(e) show the difference between model and data for the magnetic and nuclear scattering.

Figure 4

Spin angle with respect to the [111] direction normal to the particle plane calculated from the measured intensities of the magnetic reflections using Eq. (9). The calculated value of $\alpha$ is plotted against the x-ray particle size, except for the bulk sample, which has a crystallite size larger than 200 nm and is arbitrarily placed on the axis. The black and red crosses are the results from the polarized experiment at D7 using $\left\{\frac{3}{2}\frac{1}{2}\frac{1}{2}\right\}$ and $\left\{\frac{3}{2}\frac{3}{2}\frac{1}{2}\right\}$ for normalization, respectively, and the blue circles are the results from the unpolarized experiment at DMC. The error bars on the DMC data points are considerably smaller than the diameter of the circles. The dashed line is the average spin angle of ${74}^{\circ }$ for NiO300, NiO350, NiO600, and the bulk samples determined from normalizing with respect to $\left\{\frac{3}{2}\frac{1}{2}\frac{1}{2}\right\}$.