Magnetic phase transitions in SmCoAsO

Magnetization, x-ray diffraction, and specific-heat measurements reveal that SmCoAsO undergoes three magnetic phase transitions. A ferromagnetic transition attributed to the Co ions, emerges at $T_C=57\text{K}$ with a small saturation moment of $\sim 0.15{\mu }_B/\text{Co}$. Reorientation of the Co moment to an antiferromagnetic state is obtained at $T_{N2}=45\text{K}$. The relative high paramagnetic effective moment $P_{\text{eff}}=1.57{\mu }_B/\text{Co}$ indicates an itinerant ferromagnetic state of the Co sublattice. The third magnetic transition at $T_{N1}=5\text{K}$ is observed clearly in the specific-heat study only. Both magnetic and ${}^{57}\text{F}\text{e}$ Mossbauer studies show that substitution of small quantities of Fe for Co was unsuccessful.

Figure 1

(Color online) A room temperature x-ray diffraction pattern of SmCoAsO.

Figure 2

(Color online) ZFC and FC plots of SmCoAsO measured at 20 Oe.

Figure 3

(Color online) Selected isothermal magnetization curves of SmCoAsO at typical temperatures. Note the FM like curve measured at 50 K.

Figure 4

(Color online) Zero-field specific heat curves of SmCoAsO and SmFeAsO samples.

Figure 5

(Color online) ZFC and FC branches of ${\text{SmCo}}_{0.8}{\text{Fe}}_{0.2}\text{AsO}$ measured at 14 Oe.

Figure 6

(Color online) ${}^{57}\text{F}\text{e}$ Mossbauer spectrum of ${\text{SmCo}}_{0.8}{\text{Fe}}_{0.2}\text{AsO}$ at 90 K. Note the extra sextet originated from an extra magnetic phase.