Study On The Structure,Magnetic Properties And Thermal Expansion Anomalies Of ThMn₁₂-type Rare-earth--Fe Compounds

In this thesis, the crystal structure and magnetic properties of R(Fe,M)₁2 compounds and their nitrides as well as carbides with the ThMn₁2-type structure are studied by x-ray diffraction, neutron diffraction and magnetic measurement. The anomalous thermal expansions of R(Fe_xM₁-x)₁2 (R=Y, Nd, Dy, M=Mo, x=1.5; R=Y, Nd, M=Si, x=2. 0) are investigated by x-ray at high temperature and neutron diffraction at room temperature and 77K. The main research contents and results are as followings:1.The Nd₁-xPr_xFe₁0.5Mo1.5(x=0.0,0.1,0.2,0.3 ...,1.0) alloys were prepared by arc-melting method, Nd₁-xPr_xFe₁0.5Mo₁.5N_y were obtained by gas-solid reaction, the structural and magnetic properties of Nd₁-xPr_xFe₁0.5Mo₁.5 and their nitrides have been studied by means of magnetic measurements and x-ray diffraction method. It is found that the parent alloys have good single phase with ThMn₁2-type structure and nitrides retain the ThMn₁2-type structure, the unit cell increases ². 18-3. 42%. The magnetocrystalline anisotropic field of the nitrides is 100-130KOe. The Curie temperature of parent alloys is about 460K, while the Curie temperature of ni trides can reach 600K on average. After nitrogenation, the saturation magnetization is improved. The substitution of Pr for partial Nd has no obvious influence on the magnetic properties, which can lower the material price, so it is very favorable for industrialization.2. The RFe₁2-xMo_xC_y, (R=Nd, x=1.5, y=0.2,0.4; R=Nd, x=2, y=0. 6; R=Y, Tb, Dy, x=l. 5, y=0. 3) carbides are prepared by arc-melting method. The structure of these carbides are studied by x-ray and neutron diffractions, the results indicate that the main phase of Nd(Fe,Mo)₁2C_x(x=0.2,0.4,0.6) is ThMn₁2-type structure,only a part of carbon atoms occupy the 2b interstitial sites. For x=0. 2 and 0.4, the amount of a^Fe impurity phase increases with the carbon content. When the content of Mo is increased from 1. 5 to 2. 0, the amount of a^Fe is restrained, but the carbon content in the 2b interstitial sites is still very low. For NdFeioMo2Cv(y=0, 0. 6), the magnetic measurements show that the effect of carbon in the interstitial sites is the same as that of nitrogen, which can improve the Curie temperature > saturation magnetization and can change the magnetocrystalline anisotropy. RFem^Mo, sd,.,,(R=Y, Tb, Dy) compounds only have single ThMn,2-type structure and almost all carbon atoms enter the 2b interstitial sites. The key problem of enhancing the content of carbon at the interstitial sites is decreasing the amount of impurity phase.3. For RFeio.sMo, S(R=Y, Dy), the weak thermal expansion anomalies of the lattice parameter a and c are observed below Tc. For NdFeu> 5Mo,.5, the spontaneous magnetostriction of the lattice parameter c is weak, while the thermal expansion of lattice parameter a is very strong. When the measurement temperature is above 425K, the lattice parameter a increases with the increasing temperature, when the temperature is below 425K, the lattice parameter a increases with the decreasing temperature. The anomalous thermal expansion of NdFe,?.sMo> 5 has a strong anisotropy. The results of neutron diffraction indicate that the lattice parameter a of RFeio.sMo, B(R:=Y, Nd, Dy) increases and lattice c decreases for Y and Nd compounds when the temperature decreases from room temperature to 77K. The distances between atoms are calculated based on the lattice parameters and atomic coordinates obtained from neutron diffraction. When the temperature decreases from room temperature to 77K, the distance between 8i⁸i increases for Y and Dy compounds, while the distance of 8i-8i decreases for NdFem.sMo, B. In the RFeio.sMo1R(R=Y, Nd, Dy) alloys, the distances of 2a-8j\ 2a-8f and 8j-8jincrease obviously. The distance between 2a⁸i increases for Nd compound, while the bond length of 2a-8i decreases for Y and Dy.4. The lattice parameter a of RFemSiaU^Y, Nd) has an evidence of spontaneous magnetostriction around Tc, the lattice parameter c has an obvious anomaly. Below Tc, the lattice parameter c increases with decreasing temperature and reaches its maximum value, and then it decreases with further decreasing temperature. The anomalous expansion of RFei?Si2(R=Y, Nd) also has a strong anisotropy. The phenomenon that the spontaneous magnetostriction of lattice parameter c is larger than that of lattice parameter a in the ThMn12 rare-earth-Fe permanent magnetic materials is observed for the first time. The neutron diffraction study shows that the lattice parameter a and c all decrease when the temperature decreases from room temperature to 77K.5. it is believed that the magnetic properties are mainly determined by the Fe-Fe interatomic distances and the number of Fe nearest neighbors in the R-Fe compounds, the magnetic energy of these compounds strongly varies with distances-positive interactions increase while negative interactions decrease when distance increases. Competition between magnetic and elastic energy provokes the observed expansion anomalies. The thermal expansion anomalies of RFe,n5Mo, 5(R=Y, Nd, Dy) and RFeH,Si2(R=Y, Nd) can not be explained by the Fe-Fe interatomic distances and the number of Fe nearest neighbors. This implies that not only the direct interaction of Fe-Fe contributes to the phenomenon, R-Fe and R-R indirect interaction also have a contribution to the thermal expansion anomalies.