Formation of new transient walls by a constant magnetic field at the Freedericsz critical value has been observed. They are oriented along the initial alignment of the nematic phase of phenyl benzoates and appeared only in relatively thick samples with a thickness between 50 and 100 mu m of the cells. The excellent planarity of the liquid crystal orientation is considered to be the most important condition for their presence These magnetic walls are transient as they disappear either after a few seconds for 100 mu m thick nematic cells or after parts of a second for thinner (50 mu m) nematic cells. Nonregular stable magnetic walls, incorporating disclinations with core, appear immediately after the relaxation of the transient walls, when the planarity of the nematic orientation is not perfect. In thinner nematic cells of 20 mu m or less, a Freedericksz transition has only been observed. The formation of transient magnetic walls can be described by a model taking into account alignment-inversion, twisted along Y regions. The transient walls accompanied by a system of Becke lines relax by going through three-dimensional twist-splay-bend deformations.