Investigation of thermomagnetic effects in monocrystalline cobalt near the martensitic phase transition

Abstract

The initial susceptibility and the magnetic after‐effect (MAE) of [0001]‐oriented, prismatic cobalt single crystals are investigated in the temperature range 100 K < T < 800 K using ac techniques. The amplitudes of both the susceptibility and the MAE are characteristically influenced by the temperature‐dependent variations of the crystal anisotropy constants, K1 and K2, and by magnetostrictive interactions with dislocations and point defects. It is shown that the concentration of these intrinsic defects may be increased considerably by continued annealing in the vicinity of the allotropic α → β phase transformation.

Environment-Dependent Catalytic Performance and Phase Stability of Co3O4in the Preferential Oxidation of Carbon Monoxide StudiedIn Situ

“…We note that the formation of pure hcp Co particles that are smaller than 20 nm is predicted to be thermodynamically unfavorable. 62,67,68 Therefore, at this stage, we are unable to provide further details on how these very small particles may have formed. On the other hand, the presence of some large particles indicates that sintering took place under realistic CO-PrOx conditions (i.e., involving H 2 , H 2 O, and CO 2 ).…”

“…Metallic hcp Co crystallites are thermodynamically stable above 20 nm. ,, However, below 20 nm, the presence of hcp Co is mostly believed to be possible when it is intergrown with fcc Co. − The calculated crystallite size of hcp Co during the reduction in a 50:50 H 2 /N 2 mixture is much smaller than 20 nm at all temperatures (Figure c). Therefore, it is also possible that (partially) intergrown particles and those with pure hcp or fcc Co are present in the reduced sample.…”

“…Therefore, the sintering of the fcc Co may have caused some of the loss in methanation activity. As the temperature is increased from 275 to 450 °C, the fcc Co concentration increases up to 21.8 ± 0.8 wt % at the expense of hcp Co, which is a thermodynamically favored process. ,,, Metallic hcp Co is known to be more active than fcc Co in terms of CO activation and hydrogenation, ,, therefore, the slight loss in hcp Co may have also negatively affected the progress of the methanation reaction. However, a decrease of about 79.6% in the CH 4 yield may also imply other effects, such as changes to the surface structure/composition of the metallic Co crystallites/nanoparticles, especially in the event of (partial) intergrowth of hcp and fcc Co. − Sławiński et al and du Plessis et al have pointed out the importance of PXRD in determining the probability of intergrowth in reduced cobalt samples.…”

On Temperature Dependence of Domain Structure in Cobalt

“…The dislocations, due to their strong magnetostrictive interactions with the domain walls, act as effective obstacles to domain wall mobility. A growth of lattice imperfections with each subsequent heating cycle was also observed [8]. On the basis of Table we can see that the domain behaviour during the heating cycle was reversible as the specimen was not carried througl the hcp-tofcc phase transition.…”

On the Correlation between the High-Temperature Initial Susceptibility and the Magnetic After-Effect in Single-Crystal Cobalt

“…Secondly, in region 11, Kronmiiller et al calculated the susceptibility due to domain wall displacements only; these results differ from our equation (6). However, in our ge-ometry, and important contribution from rotational processes is also expected [5].…”