L.Z. Zhao

Influences of intergranular structure on the magnetic properties of directly cast nanocrystalline NdFeCoTiNbBC alloys

The millimeter-sized Nd9.5Fe61.5Co10Ti2.5Nb0.5B16−x C x (x = 0–1.25) alloy rods with various compositions were fabricated by direct casting. Nano-sized hard phase Nd2(FeCo)14B, soft phase ɑ-FeCo, and amorphous phase were observed in all alloys. An optimized amount of carbon additions improved the magnetic properties by enhancing the glass forming ability and forming near single domain-sized Nd2(FeCo)14B grains around the rod surface. Various intergranular structures were observed in the alloys with x = 0.25–1. Micromagnetic simulation using the images obtained from the magnetic force microscope and transition electron microscope indicates that the distribution and magnetism of the intergranular phase have an important influence on the magnetic properties and demagnetization process of the alloys. A uniformly distributed nonmagnetic intergranular amorphous phase may enhance the magnetic properties, but the coercivity decreases when the amorphous phase is magnetic. It is important to modify the structure and distribution of the inter-grain amorphous phase in order to achieve high hard magnetic properties in these alloys.

An Investigation on Nanocrystalline TbCu 7 -Type SmCo 6.4 Si 0.3 Zr 0.3 C 0.2 Alloys With Sm Partially Substituted by Various Light and Heavy Rare Earth Elements

Conventional Sm-Co-based alloys have low glass forming ability, and thus are difficult to quench to nanocrystalline structure by rapid solidification. In this paper, nanocrystalline [Sm, rare earth (RE)] SmCo_6.4Si_0.3Zr_0.3C_0.2 (RE = Ce, Sm, Gd, Ho, and Er) alloys have been obtained by melt spinning after compositional modification, as prepared alloys have a microstructure consisting of TbCu₇-type SmCo₇-based nanocrystals with the mean size of approximately 20 nm surrounded by a soft amorphous phase and a cobalt phase. After annealing at 600 °C for 1 h, the alloys were well crystallized and the grains grew up to ~100 nm. Both the coercivity and the remanence have been enhanced significantly by annealing. The effects of the substituted RE elements on the magnetic properties and thermal stability were investigated. An annealed Sm_0.8Er_0.2Co_6.4Si_0.3Zr_0.3C_0.2 alloy showed the highest remanence of 0.59 T and (BH)max of 58.8 kJ/m³. Excellent thermal stability with the remanence temperature coefficient α of -0.0178%/°C and -0.0778%/°C at temperature range of 27 °C~200 °C and 27 °C~400 °C, respectively, was also obtained.

Micromagnetic Simulation of Magnetization Reversal Process Using Magnetic Force Microscope Image

The Nd9.5Fe61.5Co10Nb20.5Ti0.5B15.5C0.5 nanocomposite magnets have been prepared by the copper-mold casting technique. The magnetic properties were calculated by the micromagnetic simulation using the magnetic force microscopy image. The influence of different saturation magnetization of amorphous grain boundary phase on the magnetization reversal process was investigated. The simulation results indicate that the coercivity increases with the reducing saturation magnetization of amorphous phase when the grain size varied from 300 to 1000 nm. The magnetization reversal process demonstrates that the magnetic moments of NdFeB phase show the prior inversion at the grain boundaries. Furthermore, the comparison between the anisotropic and isotropic model implies the importance of preparing anisotropic magnets.

Coercivity and thermal stability enhancement for spark plasma sintered nanocrystalline Nd-Fe-B magnets with Dy 2 O 3 and Zn additions

The main problems with the sintered NdFeB magnets are their bad formability, low oxidation resistance, and poor temperature stability. Relatively large grain size, typically more than 3 mm, is also not beneficial to the high coercivity. Current efforts are directed to improve the magnetic properties and reduce the cost of NdFeB magnets by compositional modification, microstructure optimization, and new process employment. Spark plasma sintering (SPS) is one of the novel sintering techniques for preparing NdFeB magnets in a laboratory scale. Low sintering temperature and short holding time make it possible to sinter nanocrystalline NdFeB powders into fully dense bulk magnet. However, it is still not possible to completely avoid the grain coarsening, which leads to inevitable decrease of coercivity. To improve coercivity, the magnetic powders can be mixed with some metal powders such as Cu and Zn, which are beneficial for densification during SPS. Another effective way to enhance the coercivity is to add the powdered Dy compounds. In this paper, we report the effects of separated and combined Dy2O3 and Zn additions on the magnetic properties, microstructure and thermal stability of SPSed nanocrystalline NdFeB magnets.

Microstructure and magnetic properties of FePt-MgO granular thin films fabricated by Co-sputtering method on the single crystal MgO substrate

L10-FePt film has the high magnetocystalline anisotropy energy (Ku), large coercivity (HC) and it is an ideal high density magnetic recording material. However, L10-FePt ordered phase is obtained by high temperature annealing, which is prone to increase the grain size of FePt. The large size grains cause the surface roughness increase, which is the source of the noise during the recording process. MgO is chosen for its widely application in the tunnel junction devices and it has a suitable lattice mismatch with FePt (4%) [1]. The FePt granular thin film with co-deposited MgO non-magnetic matrix is beneficial to suppress the FePt grain growth during annealing and decrease the exchange coupling effects between the FePt grains [2]. The utilization of MgO single crystal substrate induces the deposition of MgO matrix because of the homogeneous structure, which has the advantages of the formation of the inter-granular phase between FePt grains. In this paper, the FePt-MgO films were prepared by co-sputtering on the single crystal <;100> MgO substrate in magnetron sputtering system. The microstructure and magnetic properties are investigated. The targets are Fe55Pt45 and ceramic MgO. The deposition temperature was varied from room temperature to 300. The As-deposited FePt-MgO films were annealed at 800 for 1hour. The composition of FePt and MgO was configured by the power of the targets (both RF and DC power). All the films were characterized by XRD, AFM/MFM, and VSM.

Mössbauer study on nanocrystalline (Ce 1−x Nd x ) 16 Fe 78 B 6 alloys

To reduce the cost of the Nd-Fe-B magnets, Ce which is one of the most abundant rare earth metals in the world, is suggested to be used as a replacement of Nd element.

Micromagnetic simulation of magnetization reversal process using MFM image

This research aims to study the hysteresis loop and magnetization reversal process in Nd2Fe14B samples with grain size varying from 300 to 1000 nm using magnetic force microscope image modelling. It is shown that the remanence enhances as the saturation magnetization of amorphous grain boundary phase increases. The magnetization, however, drop down rapidly because of the bad exchange coupling interaction between two phases which lead to coercivity reduction. Thus, the non-magnetic grain boundary phase is beneficial to increase the coercivity. Distribution of magnetic moment is also discussed.

The effects of thermal gradient and magnetic field on the anisotropy and microstructure of direct cast Nd 24 Co 20 Fe 41 B 11 Al 4 magnets

The NdFeB magnets with millimeter sizes, typically between 0.5 and 12 mm, have been widely used in electronics, acoustic products, and micro-motor. Preparing these magnets by the traditional sintering and bonding techniques is associated with complicated process and sacrificed density and machinability of the magnet. Aiming at the huge market requirements and facing those technical problems, the direct casting is considered to be a simple and low cost alternative technique and has been introduced to make fully dense small size NdFeB magnets. However, it remains a big challenge to achieve high anisotropy in these cast magnets, as well as to design a high magnetic field along the casting direction. In this work, the authors proposed using thermal gradient and magnetic field during the injection casting processes to induce the anisotropy in the 2 mm Nd24Co20Fe41B11Al4 alloy rod.

Preparation of Isotropic and Anisotropic Nanocrystalline NdFeB Magnets by High-Velocity Compaction and Hot Deformation

NdFeB powders are consolidated into nanocrystalline bulk magnets by a near-net-shape process of high-velocity compaction (HVC) at room temperature with no binder employed. The magnets prepared under various conditions, including impact energy, filling weight, mold dimension, and plasticity and size of the starting powders, were investigated. The results showed that the density of as-compacted NdFeB magnets increased with the increasing impact energy and decreasing filling weight. The as-compacted magnets with relatively high density can inherit the coercivity and microstructure of the starting powders. The small flake powders with good plasticity and/or large mold diameter are beneficial to obtain high density. The relative green densities for the samples with low-Nd composition and high-Nd composition reach 92% and 87.5%, respectively. Using the HVCed magnet as the precursor, the anisotropic NdFeB magnets with enhanced magnetic properties have been prepared by hot deformation. This paper provides an alternative technique for preparing nanocrystalline NdFeB magnets.