Electronic Structure And Magnetic Properties Of Nd₂Fe₁₄B And Its Rare-earth Substituted Compounds

Nd₂Fe₁₄B-type compounds are the most widely used hard magnetic materials due to high saturation magnetization and high uniaxial anisotropy field.It has been confirmed that the excellent magnetic properties of the Nd₂Fe₁₄B-type compounds are intimately related to their complex electronic structures,crystal structures and chemical compositions.However,there is still significant controversy about the electronic structures of the Nd₂Fe₁₄B-type compounds and their substituted compounds.In particular,the physical nature of the rare-earth 4f electronic states and their independent contribution to magnetism have not been elucidated yet.Moreover,the intrinsic relationship between the stable tetragonal structures of the Nd₂Fe₁₄B-type compounds and their excellent uniaxial anisotropies remains ambiguous to date.Therefore,it is necessary to renew and establish the intrinsic relationship among magnetic properties,electronic structures,crystal structures and chemical compositions of the Nd₂Fe₁₄B-type compounds,leading to a comprehensive understanding of the origin of magnetism.In view of this,we focused on the rare-earth substituted?Nd,R?₂Fe₁₄B-type?R=Dy and Ce?pesudo-ternary compounds and studied the intrinsic relationship among their electronic structures,crystal structures,microscopic magnetic properties and macroscopic magnetic properties with high-quality single-crystal samples via advanced analytical tools.According to X-ray photoelectron spectroscopy?XPS?study,the valence of Nd in ternary compound is slightly lower than+3;the valence of Dy is just+3;and the valence of Ce is approximately+3.06.Besides,the valences of both Fe and B in the ternary compounds remain as 0 and-3,respectively,regardless of the rare-earth elements.In addition,in the pesudo-ternary?Nd,R?₂Fe₁₄B?R=Dy and Ce?systems,it is found that the valences of Fe,B and Dy are stable,i.e.,their valences do not change with the content of the substitution element.However,the valences of Nd and Ce are unstable,i.e.,if there is only a small amount of Nd and Ce in the?Nd,R?₂Fe₁₄B-type?R=Dy and Ce?pesudo-ternary compounds,the valence of minor element tends to be the same as the dominant element in the compounds.The 4f electronic states of both Nd and Ce exhibit a single-peak structure,locating at⁵.1 eV and⁵.2 eV,respectively,in the R₂Fe₁₄B?R=Nd and Ce?compounds.The physical nature of Nd 4f state and Ce 4f state is both highly localized and strongly hybridized with the Fe 3d electronic states.The 4f electronic state of Dy in the Dy₂Fe₁₄B compound exhibits a multimodal structure with three similar intensity peaks,locating at⁴.4 eV,⁷.7 eV and⁹.8 eV,respectively,whose physical nature is highly localized.For the?Nd,Dy?₂Fe₁₄B system,it is found that the interaction between the Nd4f electronic states and the Fe 3d electronic state is sensitive to the chemical composition,whereas the interaction between the Dy 4f electronic states and the Fe3d electronic state is insensitive to the chemical composition.Moreover,the 4f electronic states of the?Nd,Dy?₂Fe₁₄B pesudo-ternary compounds also exhibit a multi-peak structure and are composition sensitive.The dominant contribution of the4f electronic state besides the Fe 3d electronic state transforms gradually from Nd to Dy.In the?Nd,Ce?₂Fe₁₄B system,it is found that when trace of rare-earth element Ce?Nd?replace the other rare-earth element Nd?Ce?,the interaction between the 4f electronic state of the primary rare-earth element Nd?Ce?and the Fe 3d electronic state is almost unaffected.However,the interaction between the 4f electronic state of the trace rare-earth element Ce?Nd?and Fe 3d electronic states is completely different from that in the ternary compounds.Moreover,the 4f electronic states of?Nd,Ce?₂Fe₁₄B-type quaternary compounds with only a small fraction of rare-earth element replacement are similar to those of the ternary compounds.X-ray magnetic circular dichroism?XMCD?study resolves element specific orbital and spin magnetic moments of the R₂Fe₁₄B?R=Nd,Dy and Ce?compounds.The spin magnetic moment of Fe?i.e.,1.621¹.761m_B?is found to be much larger than its orbital magnetic moment?i.e.,0.043⁰.047m_B?,and both of which are parallelly coupled.For the rare earth elements on the contrary,the orbital moment is larger than the spin part.The orbital and spin moments of Nd are¹.854m_B and^-0.959m_B,respectively,which are antiparallelly coupled.The orbital and spin moments of Dy are^-2.278m_B and^-3.435m_B,respectively,which are parallelly coupled.The orbital and spin moments of Ce are⁰.124m_B and^-0.024m_B,respectively,which are antiparallelly coupled.In addition,the magnetic moments of all elements in the ternary compounds decrease with increasing temperature.Based on the study of the intrinsic physical properties of the?Nd,R?₂Fe₁₄B?R=Dy and Ce?pessudo-ternary systems,it is found that at room temperature a small amount of Dy substitution?<20%?in the Nd₂Fe₁₄B compound slightly decreases the spin reorientation temperature and significantly increases the anisotropy field.However,both the saturation magnetization and the Curie temperature are hardly affected.In addition,the temperature dependence of the saturation magnetization after about 10%and 19%of Dy substitution is similar to the situation in the Nd₂Fe₁₄B compound,while the temperature dependence of the anisotropy field is different.Moreover,the 10%of Dy substitution significantly improves the lattice anisotropy in the entire measured temperature range,but the 19%of Dy substitution increases the lattice anisotropy to some extent only when the temperature exceeds²00?.For Ce substitution,a small amount of substitution in the?Nd,Ce?₂Fe₁₄B system significantly improves both the saturation magnetization and the lattice anisotropy,but reduces both the Curie temperature and the anisotropy field to some extent and hardly affects the spin reorientation temperature.Nd substitution?⁸%?in the?Nd,Ce?₂Fe₁₄B system results in a minor increase in the saturation magnetization,a significant increase in both the anisotropic field and the Curie temperature,almost no variation in the lattice anisotropy,and does not bring out a spin reorientation phenomenon.Furthermore,the temperature dependence of the saturation magnetization for a small amount of substitution is consistent with the situations in the corresponding ternary compounds before substitution,while their temperature dependence of the anisotropy field resembles both the synthetical effect from the Nd₂Fe₁₄B and Ce₂Fe₁₄B compounds.In addition,the temperature dependence of the lattice anisotropy for a small amount of Ce and Nd substitution is similar to the situation in the Nd₂Fe₁₄B compound.