The N¿el temperature (TN) of cation-substituted goethites is controlled by the degree of isomorphous substitution and crystal defects. In particular, the exact relationship between the TN of the antiferromagnetism (AFM) and the unblocking temperature (TB) of the parasitic remanence is undetermined. We found that TB is systematically higher than TN for a set of well-characterized aluminous goethite samples. In addition, the difference between TB and TN increases from ~8--9 K for a pure goethite (that contains vacancies) to >20 K for Al-substituted goethites. This indicates that TB and TN change independently with the diamagnetic substitutions, suggesting a fundamental difference of magnetization process between parasitic remanence and AFM. A zonal distribution of the diamagnetic substitutions has been incorporated to account for the observed magnetic properties of the AFM and the parasitic remanence. Nonetheless, both parasitic remanence and AFM could exist along the goethite c axis.