Open Recoil Loops Research Articles

Asymmetric behavior of irreversible weak pinning at the soft/hard magnetically interface revealed by the open recoil loops

Asymmetric behavior of irreversible weak pinning at the soft/hard magnetically interface revealed by the open recoil loops

Simulation on the openness in recoil loop of nanocomposite magnet

In this paper, the open recoil loop is simulated by establishing the simulation model of hard-phase grains (Sm2Co7) dispersed in soft-phase (α-Fe) matrix. The influence of inter-phase exchange coupling effect on the openness is simulated by adjusting the soft/hard phase interface integral exchange constant (Aint). The results show that the openness does not depend on the inter-phase exchange coupling effect, since the open recoil loops can be acquired even the Aint decrease from 20 to 0 pJ/m. The interaction energy density analysis also does not provide strong evidence to support the correlation between openness and exchange coupling effect. The magnetization configuration evolution shows the irreversible reversal of the hard-phase grains is root cause of the openness, and the adjacent soft phase enlarge the openness degree for the recoil loop of the nanocompiste magnet. Our results are helpful to deepen the understanding of the openness in recoil loops for nanostructural permanent magnet materials.

Understanding the composition effects on the hot-deformed Nd-Fe-B magnets based on two different melt spun powders

The properties of RE-rich phase have important effects on the process and magnetic properties of NdFeB magnets. In this work, two MQ powders with different compositions were employed to fabricate hot pressed and hot-deformed magnets. The hot-deformed magnets produced from MQP-01 powders with Pr containing and Zr/Nb-free composition exhibit lower coercivity and less pinning effect compared to those from MQP-02 powders with Pr-free and Zr/Nb-containing composition, though the former has high rare earth (RE) content. These differences are attributed to the different mobilities of the RE-rich phase in the grain boundaries due to the composition difference. The RE-rich phase in MQP-01 powders has relatively good mobility, which leads to the aggregation of RE-rich phase and the open recoil loops for deformed magnets. On the contrary, relatively poor mobility of RE-rich phase in MQP-02 powders results in the thick and continuously distributed grain boundary phase in the hot-deformed magnets, which contributes to the strong pining effect and the high coercivity. The present results provide reference for optimizing the magnetic properties of hot deformed NdFeB type magnets.

Microstructure Characterization and Magnetic Characteristics of Ce–Fe–B Based Spark Plasma Sintered Magnets

Nanocrystalline Ce–Fe–B based rare earth permanent magnets are prepared by spark plasma sintering (SPS) method using grounded ribbons as raw material. The phase compositions, microstructure, magnetic properties, intergranular magnetic interactions, recoil loops, and corrosion resistance of the magnets are investigated systemically. The relationship among sintering processes, microstructures, and magnetic characteristics are discussed in depth by mainly using transmission electron microscope (TEM) and Physical Property Measurement System (PPMS) techniques. It is demonstrated that the sintering temperature has a significant influence on the average grain size and width of coarse grain areas. The optimized magnetic properties of remanent magnetization $J_{r}= 0.37$ T, coercivity $H_{\mathrm {ci}} = 227$ kA/m, and maximum energy product $(BH)_{\mathrm {max}}= 16$ kJ/m3 are obtained at 650 °C under 50 MPa for 2 min. Their exchanging coupling interactions are quite weak compared with those of the counterpart alloys. The exchanged decoupled of soft magnetic materials and inhomogeneous magnetic anisotropy result in open and steep recoil loops. The electrochemical experiment results display that the corrosion resistance of the magnets is improved by sintering at high temperature. This paper plays a role in guiding the future research and development of nanocrystalline Ce–Fe–B based permanent magnetic materials.

Micromagnetic simulation of the effect of grain boundaries and secondary phases on the magnetic properties and recoil loops of hot-deformed NdFeB magnets

Abstract Hot-deformed NdFeB magnets have attracted considerable attention due to their high thermal stability and exceptional corrosion resistance. In this study, the dependence of magnetic properties and recoil loops on the presence of a grain boundary phase (GBP) and Nd-rich secondary phases in hot-deformed NdFeB magnets was explored using micromagnetic simulations. By adjusting the parameters of the simulation model, the influence of several factors (thickness and spontaneous magnetization (Ms) of the GBP, Ms, total volume fraction and spatial distribution of the secondary phase), on the main and recoil loops was investigated. A change in thickness and in Ms of the GBP affects the coercivity (Hc) and remanence of the main loop. In particular, an increase of Ms leads to a decrease in the magnet’s Hc. The influence of the secondary phase depends on its spatial distribution. Large volume grains result in lower Hc, but the role in decreasing Hc is weaker in contrast to a random distribution of smaller grains of the same overall volume content. Open recoil loops were interpreted by analyzing the distribution of calculated magnetic moments and energy terms. Loop opening is not directly related to Hc of the main loop. Rather it is mainly due to small irreversibilities in the dipolar energy of different magnetic configurations that appear along the recoil loop.

Microstructure, improved magnetic properties, and recoil loops characteristics for MnBi alloys

Abstract MnBi is a potential candidate for permanent magnets that can be used in high-temperature applications due to the large magnetocrystalline anisotropy and unique positive temperature coefficient of coercivity. In this work, MnBi magnets with a high content of low temperature phase (LTP) were prepared by arc melting followed by heat treatment. The high coercivity of 7.11 kOe at 300 K and 21.97 kOe at 500 K can be obtained for MnBi ingots, respectively. Two-step thermal annealing can promote the transformation from the Mn and Bi phases to form LTP MnBi, allowing a maximum remanence of 38.3 emu/g to be acquired. Open recoil loops for MnBi ingots have never been reported and are worthy of investigation. It was found that the non-uniform distribution of the Mn phase, Bi phase, LTP MnBi, and even the amorphous phase, as well as their large variation in size, could produce a non-negligible local demagnetization field with non-uniform distribution and resulted in open recoil loops. Nearly closed recoil loops can be achieved by improving the non-uniform distribution of the microstructure. The dependence of the magnetic properties and the recoil loops on the microstructure was investigated in detail.

A nanocomposite structure in directly cast NdFeB based alloy with low Nd content for potential anisotropic permanent magnets

The magnetic properties of NdFeB magnets can be tailored with different kinds of microstructures. In this work, a nanocomposite structure with amorphous phase remaining inside the micro-sized Nd2Fe14B type grains was observed by Scanning and Transmission Electron Microscopy on the suction cast Nd11Fe60Co10Ti2.5Nb0.5C0.5B15.5 alloy, and then confirmed by various characterization techniques. The worm-like soft magnetic amorphous phases show strong exchange coupling with Nd2Fe14B phase. As a result, the open recoil loops, which are typical characteristics for nanocomposite alloys consisting of hard and soft magnetic phases, have been clearly evidenced. Thus, the formation of this structure may provide an alternative approach to design and fabricate anisotropic nanocomposite NdFeB-based permanent magnets with enhanced magnetic properties.

Effects of Nd-rich phase on the improved properties and recoil loops for hot deformed Nd-Fe-B magnets

Nd-rich phase plays a critical role in wetting grain boundary and facilitating texture formation for hot deformed (HD) Nd-Fe-B magnets. In this study, a non-uniform distribution of Nd-rich phase with dimension up to a few micrometers was observed in nanocrystalline HD magnets. The aggregation of the Nd-rich phase is confirmed to result from the low density precursor prepared by spark plasma sintering (SPS). The large local demagnetizing fields induced by Nd-rich phase aggregation led to the open recoil loops and reduced coercivity. Upon reducing recoil loop openness by eliminating Nd-rich phase aggregation, the coercivity of the HD magnet was significantly improved from 226 kA/m to 995 kA/m, and a high maximum energy product of 293 kJ/m3 was obtained. The dependences of microstructure and coercivity on the recoil loop characteristics suggest an essential approach for improving the magnetic properties of nanocrystalline HD Nd-Fe-B magnets.

The magnetization behavior and open recoil loops of hot-deformed Nd-Fe-B magnets infiltrated by low melting point PrNd-Cu alloys

Ultrahigh coercivity of hot-deformed Nd-Fe-B magnets was obtained by low melting point PrNd-Cu alloys diffusion process. The coercivity was largely increased from 15.35 to 27.30 kOe. Magnetic isolation by nonmagnetic phases should be the primary explanation for the enhancement of coercivity. High susceptibility of the recoil loops at low field indicated that magnetic domain wall motion within grains was easy, while a near-zero reversible susceptibility of recoil loops implied that domain wall strong pinning at grain boundaries was dominant at higher field in initial magnetization process. Open recoil loops were also obtained in the infiltrated magnets and disappeared when the external field exceeded about 16 kOe lightly higher than the coercivity of the magnet without diffusion process. The inhomogeneous distribution of infiltrated phases that led to the grains aggregation and single grains with softer and harder magnetic characteristics, respectively, may be the primary reason for the phenomenon. The stray field produced by isolated single grains plays a critical role in the magnetization behavior. A phenomenological model for the magnetization reversal was built to interpret the open recoil loops in a recoil loop process.

Properties enhancement and recoil loop characteristics for hot deformed nanocrystalline NdFeB permanent magnets

Nanocrystalline NdFeB magnets were prepared by spark plasma sintering (SPS) and SPS followed by HD using melt spun ribbons as the starting materials. The microstructure of SPSed and HDed magnets were analyzed. The effects of process including temperature and compression ratio on the microstructure and properties were investigated. High magnetic properties were obtained in anisotropic HDed magnets. The combination of Zn and Dy additions was successfully employed to improve the coercivity and thermal stability of the SPSed magnets. Open recoil loops were found in these magnets with Nd-rich composition and without soft magnetic phase for the first time. The relationship between the recoil loops and microstructure for SPS and HD NdFeB magnets were investigated. The investigations showed that the magnetic properties of SPS+HDed magnets are related to the extent of the aggregation of Nd-rich phase, which was formed during HD due to existence of porosity in SPSed precursor. Large local demagnetization fields induced by the Nd-rich phase aggregation leads to the open loops and significantly reduced the coercivity. By reducing the recoil loop openness, the magnetic properties of HDed NdFeB magnets were successfully improved.

Grain boundary contribution to recoil loop openness of exchange-coupled nanocrystalline magnets

It has been well known that recoil loop openness is related to soft-phase presence in exchange-coupled hard-soft nanocomposite magnets. Our study on recoil loop openness of exchange-coupled nanocrystalline magnets (both single-phase and composite) using a micromagnetic finite-element method has revealed that the recoil loop openness is also due to decreased grain size. Open recoil loops exist in single-phase magnets as well. Simulation of magnetization distribution in both nanocrystalline single-phase magnets and nanocomposite magnets shows that the openness of the recoil loops is correlated with unstable magnetization behavior in grain boundary and soft-phase regions, which is attributed to high energy state caused by exchange coupling in these regions. The simulation results are supported by experimental data.

The physical origin of open recoil loops in nanocrystalline permanent magnets

The numerical simulation of the open recoil loops has been carried out using micromagnetic finite element method. By giving an example for this issue, the magnetization behaviors during the recoil processes of nanocomposite Pr2Fe14B∕α-Fe magnets have been analyzed. It is the strong intergrain exchange coupling that results in the magnetization reversal in some hard grains during the recoil processes, which leads to the large opening of recoil loops. The magnetization reversal of α-Fe grain follows that of its neighboring hard grains. Consequently, the opening of recoil loops is enhanced due to the presence of α-Fe grains.

Element-specific recoil loops in Sm–Co∕Fe exchange-spring magnets

In two-phase nanocomposite magnets, open recoil loops have shown to be sensitive to interphase interfacial conditions and have been often used to characterize the interphase exchange coupling. Typically, the open recoil loops are attributed to the soft phase volume that is decoupled from the hard phase. Our element-specific magnetic measurements on bilayer Sm–Co∕Fe exchange-spring magnets reveal that open recoil loops are present not only in the soft Fe layer but also in the hard Sm–Co layer and that the Fe- and Sm-specific remanence curves are similar to each other. The experimental results and micromagnetic modeling reveal that the observed open recoil loops can originate from the anisotropy variations in the hard Sm–Co layer.

The intergrain exchange coupling in Sm(Co,Fe,Cu,Zr)z magnets with z = 7.0 and 7.5

The magnetic behaviour of Sm(Co,Fe,Cu,Zr)z ribbons with z = 7.0 and 7.5 is investigated. The Henkel plots indicate that there is a strong intergrain exchange coupling (IGEC) effect in these ribbons. It is the IGEC that results in the phenomena of the interaction domains and remanence enhancement in the ribbons at room temperature. The contributions of IGEC and dipolar interaction to irreversible magnetization have been analysed to explain the phenomenon of open recoil loops. Due to the difference of the intensity of the IGEC, the magnetization changes with the component of the Sm(Co,Fe,Cu,Zr)z ribbons.

Origin of recoil hysteresis loops in Sm–Co∕Fe exchange-spring magnets

Open recoil loops are often interpreted as a consequence of a breakdown in exchange coupling and attributed to the decoupled soft phase in exchange-coupled permanent magnets. However, in element-specific recoil loop measurements on Sm–Co∕Fe exchange spring magnets, the authors found that the open recoil loops were present not only in the soft (Fe) layer but also in the hard (Sm–Co) layer, and were not a consequence of exchange coupling breakdown between the soft and hard layers. Comparison between the experimental results and micromagnetic calculations reveals that the observed open recoil loops originate from the anisotropy variations in the Sm–Co layer.

Evidence for the presence of a soft magnetic layer on the surface of die upset melt quenched Nd-Fe-B

Magnetic properties on the surface of the anisotropic melt quenched Nd-Fe-B material were studied and compared with the properties of the material in bulk. The coercive field is 10.3 kOe on the surface and 14.9 kOe in the bulk. The reversible susceptibility is much higher and the irreversible susceptibility is much lower on the surface than in the bulk. Open recoil loops on the surface are observed. The lower coercive field and higher reversible susceptibility are due to the presence of a soft magnetic phase on the surface. Open recoil loops are due to exchange coupling between the soft magnetic phase on the surface and the hard magnetic phase in the bulk material.