Transport and magnetic study of the spin reorientation transition in the Tb₅(Si_0.5Ge_0.5)₄magnetocaloric compound

Abstract

Detailed measurements of the electrical resistivity ρ(T ), thermopower S(T ) and magnetization of Tb 5 (Si 0.5 Ge 0.5 ) 4 in the vicinity of the spin reorientation transitions observed in this compound are reported. Our results indicate a complex spin reorientation process associated with three different lattice sites occupied by the Tb ions. We identify two critical transition temperatures: one at T SR 1 = 57 K, as previously reported, and a new one at T SR 2 = 40 K. A simple model based on an approximate magnetic anisotropy energy is presented; it gives a satisfactory qualitative description of the main features of the reorientation processes.

Tailoring the magnetism of Tb5Si2Ge2compounds by La substitution

“…Despite this fact, the R 2 site is the one exhibiting higher spin canting in the parent compound (x = 0) and, thus, the La occupation of this site probably favors the magnetic moments collinearity (decrease of AFM interactions between Tb atoms). 29,30 For x > 1 at RT, these compounds crystallize on the tetragonal phase (Zr 5 Si 4 structure), remaining unchanged down to 5 K. In this concentration region, a decrease of T C is observed as generally expected, 20 with a rate of dT C /dx ∼ −37.5 K/x and extrapolating to zero for the concentration x = 5 (absence of Tb). We highlight that the percolation theory of diluted magnetic systems predicts a threshold for the non-magnetic ion concentration from which the long-range magnetic order is supressed (absence of T C ).…”

“…29,30 A decrease of T SR with x is observed in this La concentration range, reaching a minimum of 48 K also for the x = 0.75 compound. Furthermore, the reciprocal susceptibility represented in the inset of Fig.…”

“…This observation reinforces the hypothesis that La substitution favors the collinearity of the Tb magnetic moments (similarly to what was observed in the binary compounds), thus probably reducing the antiferromagnetic (AFM) exchange which is responsible for the observed canting effect within the rigid slab in Tb 5 Si 2 Ge 2 . 29,30 As mentioned in the Introduction, since the ferromagnetism in these complex compounds stems from the R1-T3-T3-R1 chain, 20 La is expected to occupy the R2 or R3 sites. Thus, in order to further investigate the preferential specific site of La substitution, density functional calculations were performed using the WIEN2K code (L/APW + lo method) considering the These calculations assumed a ferromagnetic spin polarization of the Tb atoms and kept the structure fixed.…”

“…In the low-temperature region, no spin reorientation transition is detected. 29,30 Likewise, there is also no evidence of the Griffiths-like phase on the reciprocal susceptibility data for these compounds [see insets of Figs. 6(a) and 6(b)].…”

Transport and magnetic properties of the Er5Si4 compound

“…Inset: thermopower derivative near the structural transition (O: orthorhombic phase; M: monoclinic phase; PM: paramagnetic phase).experimental details on thermopower, magnetization and electrical resistivity measurements can be found in refs [7,8]…”