Fe₂Co₂Nb₂O₉: a magnetoelectric honeycomb antiferromagnet

Abstract

Fe4Nb2O9 and Co4Nb2O9 are antiferromagnetic compounds belonging to the M4M’2O9 series (M=Mn, Fe, Co ; M’=Nb, Ta), whose structure (P¯3c1) derives from the Cr2O3 corundum one. By mixing cobalt and...

Modulated non-collinear magnetic structure of (Co$_{0.97}$$^{57}$Fe$_{0.03}$)$_{4}$Nb$_{2}$O$_{9}$ as revealed by Mössbauer spectroscopy

“…The amount of the impurity 126-phase were refined to be about 1.8 wt% and 1.4 wt% for Co 4 Nb 2 O 9 and (Co 0.97 57 Fe 0.03 ) 4 Nb 2 O 9 , respectively. In agreement with earlier reports 7,19 , the determined lattice parameters are a = 5.1702(1) and c = 14.1306(4) for (Co 0.97 57 Fe 0.03 ) 4 Nb 2 O 9 , which are a little larger than the values of a = 5.1680(1) and c = 14.1268(4) for Co 4 Nb 2 O 9 due to bigger ionic radius of Fe 2+ than Co 2+ , indicating the successful doping of the 57 Fe atom into the lattice of the parent 429-phase.…”

“…2, the Néel transition temperatures were found to be T N ∼ 27 K and T N ∼ 30 K for Co 4 Nb 2 O 9 and (Co 0.97 57 Fe 0.03 ) 4 Nb 2 O 9 , respectively. The slightly enhanced T N suggests that the relatively small doping of ∼ 3% 57 Fe is not totally effect-less to the parent compound 19 but we expect that the impact should be small with such a small doping level for insulators. The upturn at low temperatures can be ascribed to a weak amount of paramagnetic impurities usually observed in polycrystalline samples that can not be seen by our XRPD and Mössbauer spectroscopy.…”

Structural, magnetic and magnetoelectric properties of Co2Fe2Ta2O9, a TN=53 K honeycomb antiferromagnet

“…In CFTO, the M(H) at T = 10 K (figure 4b) is not linear, mimicking the ΔP(H) T = 10K curve (figure 4a). A similar situation was previously encountered in CFNO [19] and explained by the local non compensation of magnetic moments of two neighbouring AF coupled Fe 2 + and Co 2 + cations. Finally, for CFTO, despite its larger magnetic susceptibility below T N (figure 3), the maximal magnetization value, at T = 10 K in a magnetic field of 9T reaching only 3.8 μ B / f.u.…”

“…(figure 4b), is consistent with the AF state observed in CFNO. [19] Indeed, a magnetic state where all magnetic moments are flipped under magnetic field application would have yielded 14 μ B /f.u..…”

“…Interestingly, the mixing of Co and Fe to get the Co 2 Fe 2 Nb 2 O 9 (CFNO) compound has been shown to be efficient to overcome the issue related to the high H sf value of FNO: [19] CFNO exhibits a T N = 58 K (i. e. higher than for CNO, T N = 27 K), a H sf similar to CNO, and a α ME = 37.7 ps/m value extracted from the 0T to 4T region of P(H) T = 10K . [19] This value, at least double as compared to that of CNO, [8] and triple compared to our value calculated from the P(H) curve of CNO [19] (α ME = 12.56 ps/m) motivated the synthesis, structural characterization and measurements of the magnetic and electric properties for the equivalent tantalate, Co 2 Fe 2 Ta 2 O 9 (CFTO). These results allow a comparison between the two solid solutions, Fe 4-x Co x Ta 2 O 9 and Fe 4-x Co x Nb 2 O 9 , to be made.…”