Defeng Guo

Simultaneously increasing the magnetization and coercivity of bulk nanocomposite magnets via severe plastic deformation

In general, there is a trade-off between magnetization and coercivity in nanocomposite magnets. Here, we demonstrate a simultaneous enhancement of both the magnetization and coercivity in bulk α-Fe/Nd2Fe14B nanocomposite magnets prepared via a severe plastic deformation (SPD) compared with thermally annealed magnets. The enhanced magnetization results from a high fraction (>30%) of α-Fe phase induced by SPD, while the increase in coercivity from 4.6 to 7.2 kOe is attributed to an enhancement in domain wall pinning strength. This study shows an increase in energy product is possible in the nanocomposite magnets for a large inclusion of soft-magnetic phase.

Three-Dimensional Self-Assembly of Core/Shell-Like Nanostructures for High-Performance Nanocomposite Permanent Magnets

Core/shell nanostructures are fascinating for many advanced applications including strong permanent magnets, magnetic recording, and biotechnology. They are generally achieved via chemical approaches, but these techniques limit them to nanoparticles. Here, we describe a three-dimensional (3D) self-assembly of core/shell-like nanocomposite magnets, with hard-magnetic Nd2Fe14B core of ∼45 nm and soft-magnetic α-Fe shell of ∼13 nm, through a physical route. The resulting Nd2Fe14B/α-Fe core/shell-like nanostructure allows both large remanent magnetization and high coercivity, leading to a record-high energy product of 25 MGOe which reaches the theoretical limit for isotropic Nd2Fe14B/α-Fe nanocomposite magnets. Our approach is based on a sequential growth of the core and shell nanocrystals in an alloy melt. These results make an important step toward fabricating core/shell-like nanostructure in 3D materials.

Atomic-scale structural evolution in amorphous Nd9Fe85B6 subjected to severe plastic deformation at room temperature

Understanding the deformation-induced nanocrystallization in amorphous alloys at room temperature is proved to be a great challenge. In the present study, the formation of vacancy-type defects richly surrounded with Fe atoms in amorphous Nd9Fe85B6 subjected to room-temperature high-pressure torsion deformation is directly evidenced by positron lifetime measurements combined with a coincident Doppler broadening measurement of the positron-electron annihilation photons. This demonstrates a direct experimental evidence for shear inducing the enrichment of Fe atoms in the deformed amorphous alloy. These results presented here are of importance for understanding the deformation-induced nanocrystallization and for the technique development of nanocrystalline materials.

Diameter- and current-density-dependent growth orientation of hexagonal CdSe nanowire arrays via electrodeposition.

Controlling the growth orientation of semiconductor nanowire arrays is of vital importance for their applications in the fields of nanodevices. In the present work, hexagonal CdSe nanowire arrays with various preferential growth orientations have been successfully yielded by employing the electrodeposition technique using porous alumina as templates (PATs). We demonstrate by experimental and theoretical efforts that the growth orientation of the CdSe nanowires can be effectively manipulated by varying either the nanopore diameter of the PATs or the deposited current density, which has significant effects on the optical properties of the CdSe nanowires. The present study provides an alternative approach to tuning the growth direction of electrodeposited nanowires and thus is of importance for the fabrication of nanodevices with controlled functional properties.

Bulk anisotropic Nd2Fe14B/α-Fe nanocomposite magnets prepared by hot-deformation processing of amorphous alloys

We succeeded in producing bulk anisotropic Nd2Fe14B/α-Fe nanocomposite magnets by hot-deformation processing of Nd-lean amorphous Nd9Fe85B6. The bulk Nd2Fe14B/α-Fe nanocomposite magnets yielded at a uniaxial stress of 310 MPa at 700 °C for 2 min show a strong magnetic anisotropy and enhanced magnetic properties, e.g., an increase of ∼51.4% in the maximum energy product along the stress direction as compared with the magnets produced by annealing amorphous Nd9Fe85B6. A large uniaxial stress is favorable for the (00l) texture development of Nd2Fe14B nanocrystals in the amorphous matrix, which may be attributed to a preferential nucleation and growth of Nd2Fe14B crystals at the stress. The present study provides a strategy to induce the texture for R2Fe14B phase in R-lean (R=rare earth) alloys and thus is of wide interest for yielding bulk anisotropic nanocomposite magnets with a high volume fraction of α-Fe phase.

Bulk α-Fe/Nd2Fe14B nanocomposite magnets produced by severe plastic deformation combined with thermal annealing

Bulk α-Fe/Nd2Fe14B nanocomposite magnets with a maximum energy product up to 17.5 MG Oe have been successfully produced, using Nd9Fe85B6 with a few α-Fe crystallites in an amorphous matrix as precursors, by a combination of severe plastic deformation at room temperature and thermal annealing. The α-Fe and Nd2Fe14B composite nanocrystals are induced in the amorphous matrix during the room-temperature deformation, which have a significant effect on the microstructure development of a small and uniform grain size in the bulk α-Fe/Nd2Fe14B nanocomposite magnets produced from the deformed alloy by thermal annealing. Moreover, a high volume faction up to 36% for soft-magnetic phase is achieved in the deformation-processed bulk magnets. These lead to enhanced magnetic properties in the magnets as compared with those fabricated from nondeformed ribbons.

Activation volume for nanocrystal growth in amorphous Nd9Fe85B6

We have measured the pressure dependence of the growth of α-Fe and Nd2Fe14B nanocrystals in amorphous Nd9Fe85B6 up to 6 GPa at a temperature of 923 K, yielding an activation volume of ΔV∗=(0.76±0.04) and (0.57±0.05)Ω for atomic diffusion in the growth process of α-Fe and Nd2Fe14B nanocrystals, respectively, where Ω is the mean atomic volume of the alloy. This demonstrates that the growth of nanocrystals is dependent on atomic diffusion mediated by vacancy-type thermal defects. Atomic processes of the growth of nanocrystals are discussed.

Thermal-vacancy-assisted phase transition in FePt thin films

Understanding the ordering transition from A1 to L10 structure in FePt thin films is of great significance for developing L10-FePt films as ultrahigh density magnetic recording media. Here, the L10-ordering transition of FePt films has been investigated based on activation volume measurements. A large activation volume ΔV∗=10–11 A3=(0.75–0.8) Ω, where Ω is average atomic volume of FePt, is determined for atomic diffusions in the L10-ordering transition, indicating a thermal-vacancy-assisted phase transition. This transition is suggested to be predominantly dependent on the diffusion of Fe atoms. These findings have direct implications for yielding L10-FePt films at low temperatures and optimizing their microstructures.

Bulk anisotropic nanocomposite magnets prepared by the thermo-mechanical processing of Nd3.6Pr5.4Fe83Co3B5 with different microstructures

Bulk nanocomposite magnets with enhanced magnetic properties and (0 0 l) crystallographic texture for the (Nd,Pr)2Fe14B hard magnetic phase were prepared by thermo-mechanical process of Nd3.6Pr5.4Fe83Co3B5 ribbons. Ribbons with different microstructures, i.e. an amorphous state and amorphous matrix with a few α-(Fe,Co) nanocrystals, were employed in this study. Optimum magnetic properties were obtained in the magnets made by the hot deformation of the ribbons with nanocrystals in the amorphous matrix; the maximum energy product (BH)max = 118.6 kJ m−3. The magnets made from ribbons with a fully amorphous structure showed the largest magnetic anisotropy, and the values of (BH)max measured parallel (∥) and perpendicular (⊥) to the stress direction were ~102.7 kJ m−3 and 79.6 kJ m−3, respectively. This study is of importance for the development of bulk anisotropic nanocomposite magnets with enhanced magnetic properties.

Improving the interfacial structure of nanocomposite magnets on an atomic scale

We succeeded in improving the interfacial structure of α-Fe/Nd2Fe14B type nanocomposite magnets on an atomic scale using an intergranular amorphous phase. An enrichment of Fe atoms surrounding vacancy-type interfacial free volumes in the magnets is experimentally evidenced by positron lifetime measurements combined with a coincident Doppler broadening measurement of the positron–electron annihilation photons, whereas the interfacial free volumes in the common nanocomposite magnets are dominantly surrounded by non-magnetic Nd atoms. This study provides an experimental approach to enhance magnetic coupling between the hard and soft phases and thus is of considerable interest for the development of nanocomposite magnets.