Structure And Magnetic Properties Of Chromium Oxide And Manganese Nitride Prepared Under High Gas-pressure

Chromium dioxide?CrO₂?has been extensively studied for its high-spin polarization and its excellent ferromagnetic performance.However,it is difficult to prepare CrO₂ at atmosphere pressure due to its metastable behavior.it is interesting to find a simple and an efficient methods to prepare CrO₂.Manganese nitrides have been extensively studied due to their abundances in phase diversity and magnetic configurations.Manganese nitrides is usually produced by nitriding metallic Mn in nitrogen or in ammonia under ambient gas pressures.However,experimental data on the effect of N2 Pressure on the preparation of manganese is less.We developed a high gas-pressure heat-treatment device with a working pressure is up to 100 atm.the structure and magnetic properties of CrO₂?CrO₂/Cr₂O₃ and Mn-N compound are studied.The main conclusions are as follows:1.The CrO₂ micro-powder were synthesized by decomposing CrO₃ at 683 K under 40 MPa O₂ for 7h.The as-prepared CrO₂ microparticles were rectangular in shape with sizes ranging from 5?m to 7?m.The samples show good ferromagnetic behaviors at room temperature.The saturation magnetization,the coercivity and the curie-temperature of samples are 98.7Am2/kg,62.5Oe and 383 K,respectively.The CrO₂ microparticles shows second order phase transition and the maximum magnetic entropy change and the relative refrigeration capacity was 2.83 J.kg-1.K-1 and 86.1J/kg under the magnetic field of 1.5T,which show the good magnetocaloric effect at high temperature.2.Cr₂O₃ nanoparticles were first prepared via one-step reactive step reactive laser ablation of Cr in Oxygen.The as-prepared Cr₂O₃ nanoparticles were subsequently annealed in air and under O₂ with gas pressure of up to 40 MPa.The as-prepared Cr₂O₃ nanoparticles are spherical or rectangular in shape with range from 20 to 50 nm,and exhibit a weak ferromagnetic behavior with an exchange bias of up to 11 mT.High oxygen pressure annealing is effective in producing meta-stableCrO₂ from Cr₂O₃ nanoparticles.The Cr₂O₃/CrO₂ nanoparticles exhibit an enhanced saturation magnetization and a reduced exchange bias with an increasing fraction of CrO₂.3.We studied the structure and magnetic properties of manganase nitrides,which were prepared by gas-solid reaction under different high N2 pessures.With the increase of nitrogen pressure,the as-prepared Mn-N compound contains more nitrogen.The morphology of the Manganase nitrides show no difference under various high N2 pressure.The magnetic measurement show that N-rich e-M4 N have a large coercivity and the antiferomagnetic nitrides?z-Mn6N2.58,?-Mn3N2?exhibits ferromagnetic-like behaviors.