We have modeled the magnetic Bragg scattering in two antiferromagnetic materials, NiO and LaMnAsO, using transmission electron microscopy. Experimentally, Loudon detected these weak magnetic phenomena in NiO. As a more difficult situation with a lower concentration of magnetic elements and higher concentration of heavier non-magnetic elements that significantly scatter, we did computations for the LaMnAsO compound in order to compare our computational replication of Loudon's experimental data. Additionally, we have discussed the antiferromagnetic Bragg spot's thickness and voltage dependency for both compounds. We used two computational methods, one assuming a static lattice with smeared Debye-Waller potentials and the other explicitly taking into account the atomic vibrations within the quantum excitations of phonons model (thermal diffuse scattering). According to the structural study, the antiferromagnetic Bragg spot in NiO is located between the (111) and (000) reflections. However, in LaMnAsO, it is located at the site of the (110) reflection in the diffraction pattern, which is a forbidden reflection of the crystal structure. According to calculations, the magnetic Bragg spot in NiO has an intensity that is much greater than thermal diffuse scattering at room temperature. The magnetic Bragg spot for LaMnAsO is weaker than the thermal diffuse scattering at room temperature, but its identification can be made easier at lower temperatures.