Bicriticality inFexCo1−xT

Abstract

X-ray and neutron-diffraction, dc magnetic susceptibility, magnetization, and specific-heat measurements are reported for FexCo(1-x)Ta2O6 mixed oxides. X-ray refinement indicates homogeneous samples for all the reported concentrations. The neutron-diffraction measurements reveal magnetic structures with double propagation vectors (+/-1/4,1/4,1/4) for CoTa2O6, and (1/2,0,1/2) and (0,1/2,1/2) for FeTa2O6. The latter remain unchanged in the Fe-rich samples, for 0.46< or =x<1.00, while the Co-rich samples show propagation vectors (+/-1/4,1/4,0) for 0.09< or =x<0.46. The temperature vs x phase diagram exhibits a bicritical point at about T=4.9 K and x=0.46. For this concentration, and at low temperatures, the system shows coexistence of both magnetic structures. This novel bicritical behavior is interpreted as induced by competition between the different magnetic and crystallographic structures.

Magnetic phases of the quasi-two-dimensional compounds FexCo1 −xTa2O6

“…For the intermediate sample, x = 0.01, we observe the presence of both kinds of reflections, indicating coexistence of these two magnetic phases. A similar coexistence pattern was observed earlier for concentrations near x = 0.46 [13], in this case involving the AFI and AFF phases. The above result confirms that a bicritical point exists at x ≃ 0.01 in the T vs. x phase diagram.…”

“…Structure parameters were confirmed to be in agreement with those previously obtained for the whole series [13].…”

“…Both the low-dimensional character and the strong in-plane easy-axis anisotropy are very robust features of these compounds. In fact, they are also evident in the paramagnetic phase where the susceptibility data can be fitted to a high-temperature series evaluated for a two-dimensional (2D) Heisenberg model with an in-plane easy axis and competing nearest-neighbor (nn) and next-nearest-neighbor (nnn) exchange interactions [2,13].…”

Suppression of magnetic ordering in quasi-one-dimensional FexCo1−xNbSarvezuk1, Kinast2, Colin3 et al. 2011Phys. Rev. BSelf Cite

“…Our ND results and magnetic measurements show that magnetic order is preserved in the Fe-rich region (x = 0.8), while no order is established down to 1.7 K for x = 0.2, 0.4, and 0.6, in spite of the observation of short-range magnetic correlations. This is in contrast to the quasi-two-dimensional (Fe,Co)Ta 2 O 6 systems, 33,34 where Fe for Co substitution was found to preserve the magnetic order. The lower strength of magnetism in (Fe,Co)Nb 2 O 6 compounds can be traced to their different crystal structure, in particular the existence of weakly interacting magnetic chains, as well as topological frustration due to the triangular arrangement of these chains with respect to the transverse ab plane of the lattice.…”

New investigation of the magnetic structure of CoNb2O6 columbite

“…The AB 2 O 6 compounds are considered as prototype materials since they exhibit low dimensional magnetic characteristics. Whereas tetragonal ATa 2 O 6 phases are model two-dimensional magnetic systems, 9,11,12 the orthorhombic ANb 2 O 6 phases exhibit weakly interacting one-dimensional magnetic chains. [17][18][19][20] The CoNb 2 O 6 compound was found to order at 2.95 K with a transition from a paramagnetic phase to an ordered spin structure.…”