Control Of Microstructure And Magnetic Properties Of Isotropic Nd₂Fe₁₄B/α-Fe-Type Nanocomposite Magnets

Nanocomposite exchange coupled magnets consisting of magnetically hard and soft phases at the nanoscale are most likely next-generation magnets because a high maximum energy product above 100 MGOe would be expected in the magnets according to micromagnetic calculation.Unfortunately,practically obtained maximum energy products in nanocomposite magnets are only in the range of 10-20 MGOe,which is far below the theoretically predicted value.This is attributed to the difficulty in achieving a simultaneous enhancement of both the magnetization and coercivity in Nd₂Fe₁₄B/α-Fe nanocomposite magnets.There is an inherent trade-off between magnetization and coercivity,a high fraction of soft-magnetic phase is usually accompanied by a considerable reduction in coercivity for the nanocomposite magnets.Hence,that is the key to overcome the trade-off between the magnetization and coercivity for achieving high-performance nanocomposite magnets.In this study,the subjects about overcoming the trade-off between the magnetization and coercivity of isotropic Nd₂Fe₁₄B/α-Fe-type nanocomposite magnets have been investigated by employing severe plastic deformation（SPD）combined with thermal annealing,and melt spinning techniques as following:Using（Nd,Pr）₁₀Fe₇₉Co₃Nb₁B₇ amorphous ribbons as the precursors,the Nd₂Fe₁₄B/α-Fe-type bulk nanocomposite magnets were prepared by a combination of SPD and thermal annealing,and the effect of strain on their phase evolution,microstructure and magnetic properties have been studied.Experimental results demonstrate that the volume fraction of metastable intermediate phases decreases substantially with increasing the strain,yielding a high volume fraction of soft phase in the bulk nanocomposite magnets.The large strain（ε=6.1）in SPD processes effectively inhibits the formation of metastable phases,e.g.Nd₂Fe_<sub>23B₃ and Fe₃ B,during subsequent thermal annealing,promoting the precipitation of α-Fe phase with a relatively high volume fraction(V_α-Fe 37%)in the magnets,and thus achieving a high saturation magnetization,4πMs = 15.2 kG,increasing by 10% as compared to the magnets prepared by directly annealing（Nd,Pr）₁₀Fe₇₉Co₃Nb₁B₇ amorphous ribbons（4πMs = 13.8 k G）.Using（Nd,Pr）₁₀Fe₇₉Co₃Nb₁B₇ partially amorphous ribbons（a small amount of nanocrystals were induced to amorphous matrix）as the precursors,combined SPD with thermal annealing,the microstructure of the Nd₂Fe₁₄B/α-Fe-type bulk nanocomposite magnets were successfully controlled.The volume fraction of α-Fe phase in the magnets increases as the strain is increased to ε = 6.2,and the grain size of nanocrystrals significantly decreases,12 nm for α-Fe phase and 22 nm for（Nd,Pr）₂Fe₁₄B phase in the magnets.As a result,the bulk nanocomposite magnets made at e = 6.2 show enhanced magnetic properties with increased by 12% in 4πMs,57% in Hci and 77% in（BH）_max as compared with that of directly annealed（Nd,Pr）₁₀Fe₇₉Co₃Nb₁B₇ amorphous ribbons,achieving a simultaneously increasing the magnetization and coercivity.The physical mechanism that governs the extraordinary increase of coercivity at a high fraction of soft-magnetic phase up to 35% has been revealed,and show that the coercivity mechanism is wall-pinning mechanism in the Nd₂Fe₁₄B/α-Fe-type bulk nanocomposite magnets.The size and distribution of nanocrystals,and the interfacial atomic structure and chemical environments will affect the strength of domain wall-pinning.The interface between the grains can act as the pinning site of magnetic reversal,which can impede the motion of the domain wall.The enhanced domain wall pinning strength can be attributed to a high interface fraction that results from a small grain size in the SPD magnets.Therefore,refining the grain size of the magnets is an effective way to increase the coercivity of the Nd₂Fe₁₄B/α-Fe-type bulk nanocomposite magnets.For a specific study of the interfacial structure and chemical environments in the bulk nanocomposite magnets,the positron annihilation measurement technique was employed.The study shows that the severe plastic deformation makes an enrichment of nonmagnetic atoms（Nb and B）in the interfaces and modifies the interfacial chemical properties,and thus increases the domain wall pinning strength.Therefore,combined the severe plastic deformation with thermal annealing techniques overcomes the inherent trade-off between magnetization and coercivity and simultaneously increases the magnetization and coercivity of Nd₂Fe₁₄B/α-Fe-type bulk nanocomposite magnets.The melt temperature of the alloy is controlled in order to have an ordered cluster structure(which may have a structure similar to that of Nd₂Fe₁₄B)in the liquid phase,which might facilitate the preferential growth of the Nd₂Fe₁₄ B crystals during the subsequent solidification process;then,α-Fe crystals could have grown between the large Nd₂Fe₁₄ B crystals,yielding a three-dimensional（3D）self-assembly of core/shell-like Nd₂Fe₁₄B/α-Fe nanocomposite magnets.The results demonstrate that a core/shell-like nanocomposite magnets with well-controlled soft-phase grain sizes（13-14 nm）,high soft-phase fraction（²8%）,and ideal soft-phase distribution（with a direct contact with the hard phase）was achieved.The resulting magnet exhibits a record high energy product,the maximum energy product（BH）_max = 25 MGOe,which is the highest than previously reported the isotropic Nd₂Fe₁₄B/α-Fe nanocomposite magnets.The unique core/shell-like architecture promotes a strong exchange coupling between the hard and soft phases at a high soft-phase fraction,and increases the strength of domain wall pinning,and thus allows both large remanent magnetization and high coercivity,overcoming the old trade-off between the magnetization and coercivity in Nd₂Fe₁₄B/α-Fe-type nanocomposite magnets.Our findings make an important first step toward the fabrication of 3D core/shell-like nanostructure for application in highperformance permanent magnets.The Nd₂Fe₁₄B/α-Fe melt-spun ribbons with different compositions were prepared by melt spinning technique,and the effect of content of Nd element on the microstructure and magnetic properties of Nd_xFe_94-xB₆ melt-spun ribbons has been discussed.With increasing the content of Nd element,the microstructure and magnetic properties can be improved,in which the magnetic properties of Nd₉Fe₈₅B₆ melt-spun ribbons are the best.The coercivity of Nd_xFe_94-xB₆ melt-spun ribbons is mainly deterimined by the wall-pinning field.A coercivity model based on the pinning effect of grain interfaces was proposed to calculate and fit the wall-pinning strength of hard-hard and soft-hard grain interfaces,describing the dependence of the pinning strength of nanocomposite magnets on microstructure parameters,providing a theoretical basis for further increasing the coercivity of nanocomposite magnets.