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

Macroscopic phenomenon of exchange coupling and microscopic effect on magnetization reversal in sintered Nd–Fe–B magnets

Permanent magnets of Nd13.5Fe79.18M1.52B5.8 and Nd13.5Fe79.76M0.94B5.8 were prepared, respectively, via strip casting, jet milling and sintering followed by annealing. By adding the non-ferromagnetic elements M (Al, Cu, Ga and Zr) into the magnets, it could not only modify the microstructure, but also regulate the exchange coupling effect in the sintered magnets. From the macroscopic point of view, the recoil loops exhibit spring behavior in Nd13.5Fe79.76M0.94B5.8, indicating that the energy barrier can be overcome by the intergranular exchange coupling. From the microcosmic point of view, the exchange coupling can increase the domain wall size by suppressing the nucleation of reversed domains, and so the activation volume increases with thermal activation. In Nd13.5Fe79.76M0.94B5.8 the exchange coupling effect is stronger, and both the coercivity of 15.0 kOe and the remanence of 14.3 kGs are a little higher than those of Nd13.5Fe79.18M1.52B5.8 magnets in which the content of non-ferromagnetic elements is a little higher and the exchange coupling effect is weaker. Thus, the exchange coupling does not decrease the coercivity due to the exchange coupling suppressing the nucleation of reversed domains, though the microstructure is inhomogeneous in the sintered magnets of Nd13.5Fe79.76M0.94B5.8. Reducing the defect size and decreasing the defect concentration should be a practical way to improve the coercivity in Nd–Fe–B permanent magnets.

Exchange coupling effect and magnetic properties in sintered RE-Fe-B magnets

Preparing the magnetic materials bearing both a high coercive field and a high energy product is a great challenge. In this paper the content of Nd was reduced in Nd-Fe-B for enhancing the interaction effect as well as for increasing the energy product, and Tb-Fe-B powders were mixed to enhance the coercivity. In the sintered RE-Fe-B magnets some of Tb atoms diffuse into the outer regions of Nd-Fe-B grains, and Tb-rich RE2Fe14B grains also exist with the crystal size at micrometer scale. The recoil loops bear the spring behaviors, and the reversal susceptibility implies the availability of exchange coupling effect among the micro grains. The activation volume also indicates that the exchange coupling is effective in magnetization reversal. So the magnetization reversal in Nd-Fe-B grains could be restrained by the high anisotropy phase of Tb-rich core. Therefore, besides the effects of Tb-diffusion into grain outer region and the intergranular phase, the exchange coupling effect could not be neglected in improving the coercivity in the Nd-content-reduced magnets, and both a high coercive field of 20.0 kOe and a high energy product of 50.1 MGOe are achieved in the micro-crystal composite magnets.

Fabrication and application prospects of Pr–Fe–B/Alnico nanocomposite alloys

The potentials of rare earth-based nanocomposite alloys have never been realized due to strict microstructural constraints. Owing to the easy demagnetization it is challenging to increase the soft magnetic phase content. To avoid the easy demagnetization, Pr–Fe–B/Alnico magnets were fabricated and reported in this manuscript. The content of the Alnico phase is increased from 0 to 25 wt%, while the content of Pr element is reduced to below the sub-stoichiometry of the 2:14:1 main phase. The maximum magnetic energy product, which is the figure-of-merit for permanent magnets, is increased from 122 kJ/m3 for the standard alloy to 146 kJ/m3 for the alloy with 15 wt% Alnico which shows a significant improvement considering the fact that the Curie point of the magnet is also increased by ∼66 K. The special microstructure contains distinctly and heterogeneously distributed 2:14:1 and Alnico phases. The dimensions of neither the 2:14:1 nor the Alnico phases meet the dimensional requirements of the nanocomposite magnets, but still the smooth demagnetization curves are noted for the alloys. The behavior of effective anisotropy, the performance of the magnets in applied magnetic field and the magnetic interactions among the various constituent grains were quantitatively studied by reversible susceptibility, irreversible susceptibility and recoil loop openness. This study may provide some guiding principles for the development of nanocomposite magnetic alloys with excellent magnetic properties by using much less RE elements.

Synchronous enhancement mechanism of magnetic property for dual-main-phase Ce magnets by grain boundary diffusion with Pr-Al alloy

The Pr80Al20 (at%) alloy was diffused into the dual-main-phase Ce magnets (Ce/TRE = 40 wt.%, TRE: total rare earth) with different diffusion time by grain boundary diffusion process (GBDP). The magnetic property and the microstructure of a series of magnets with different diffusion time were investigated. The coercivity gradually increased as the diffusion time increased, while the remanence remained basically constant. The improvement of coercivity originated from the synergistic contribution of modified grain boundaries with decoupling effect and Ce-rich main phase containing Pr with improved magnetocrystalline anisotropy. The percentage of the irreversible portion and the nucleation field of the reverse domain calculated from recoil loops curves further confirmed that the diffusion of Pr80Al20 alloy enhanced the coercivity. The enhanced anisotropy of the Ce-rich grains compensates for the decay of the remanence carried by the diffusion-induced increase in the nonmagnetic grain boundary phase, which ultimately results in the remanence remaining essentially constant. The above phenomena further broaden the room-temperature applications of dual-main-phase magnets with high Ce content without heavy rare-earth grain boundary diffusion.

Ion irradiation induced direction of collapsed hard-magnetization axis in thin Co films

A number of polycrystalline films feature distinct sharp peaks in the angular variations of their in-plane major-loop remanent magnetization and coercivity, centered ${90}^{\ensuremath{\circ}}$ off the easy-axis position, a property referred to as hard-axis collapse. In such systems a striking phenomenon, recoil-curve overshoot (RCO), recoil magnetization branches that lie outside the major loop, was recently reported, resulting in a significantly greater recoil loop area compared with that of the respective major loop. In the present work pieces of polycrystalline magnetron-sputtered Co films were subjected to ${\mathrm{Ne}}^{+}$ bombardment in vacuum at different ion fluences in the presence of magnetic field ${H}_{ib}$ applied along different in-plane directions. Ion irradiation at a certain fluence range results in films with easy-magnetization directions parallel to that of ${\mathbf{H}}_{\mathbf{ib}}$. Moreover, the same holds for posterior sequential irradiations. The films' hard-magnetization axes are always collapsed, accompanied by significant RCO as well. The analysis and interpretation of our data strongly indicate that both hard-axis collapse and RCO result predominantly from a domain splitting when the measurement magnetic field is nearly perpendicular to the grain's easy axis during demagnetization.

Temperature dependence of magnetization reversal mechanism in misch-metal substituted Nd-Fe-B magnets sintered by dual alloy method

The substitution of Nd by misch-metal (MM) in Nd-Fe-B permanent magnets has attracted extensive attention due to the effective reduction of production cost and rational utilization of rare earth resources. In particular, multi-main phase (MMP) magnets prepared by dual alloy method possess a higher permanent magnetic performance than that of single main phase (SMP) magnets. However, the coercivity mechanism of MMP magnets is different from the nucleation and propagation of SMP magnets, especially the interactions of different 2:14:1 main phases have not been clarified. In this study, we investigated the temperature dependence of magnetization reversal mechanism in (MM,Nd)-Fe-B magnets by using recoil loops, minor hysteresis loops, first-order and second-order reversal curves. It found that with the increase of temperature, the magnetic moments motion is mainly affected by long-range magnetostatic interaction rather than exchange coupling. The weakened coupling interaction makes the demagnetization process of softer and harder magnetic phases asynchronous. The thermal activation energy barrier index related to coercivity mechanism is 1.45, 1.18 and 1.0 at 200 K, 300 K and 380 K, respectively. It indicates that nucleation and expansion of reversed domains are the main mechanisms controlling the magnetization reversal process at low temperature. While the coercivity at high temperature is mainly determined by the domain wall pinning mechanism. In addition, we noted that both two coercivity mechanisms coexist in MMP magnets at 300 K. Micromagnetism has also been used to verify the coercivity mechanism of (MM,Nd)-Fe-B magnets sintered by dual alloy method, which is of great significance for improving the magnetic properties of high abundance rare earth magnets.

Structural and magnetic properties of SrFe12O19/CoFe2O4 composites with exchange coupling interaction

SrFe12O19 and CoFe2O4 powders obtained via a chemical coprecipitation method were used to prepare SrFe12O19/CoFe2O4 composites after sintering at 1200 °C, and the structure, the magnetic properties and the effects of aligning magnetic field during molding are studied. Only SrFe12O19 and CoFe2O4 without any other impurity phase were found in all the composites, and the exchange coupling interaction was proved to exist by using the Henkel plots and the magnetic recoil loops. The exchange coupling interaction resulted in the improvement of saturation magnetization, but the reduction of coercivity in composite specimens. It was also found that for SrFe12O19/CoFe2O4 composites, the aligning magnetic field helped to improve the saturation magnetization and the coercivity, but did not affect the phase composition, the morphology and the exchange coupling interaction obviously.

Effect of the heat treatment on the electrical resistivity and magnetization reversal behavior of MnAl alloys

The casting Mn51Al47C2 alloy were undergone two distinct heat treatment processes, consisting of the heat treatment at 1000 and 925 °C. The process followed by quenching and annealing at 500 °C for 15 min. The low electrical resistivity for the heat treatment stage at 1000 °C (165* 10-4 Ω.m) can be described by the small fraction of the L10-ordered τ-phase (40 %). But the high resistivity for heat treatment at 925 °C (290.4* 10-4 Ω.m) originates within the large fraction (>95%) of the L10-ordered τ-phase with space inversion symmetry inhibiting the transport process. The magnetic characterization revealed higher Ms, Mr and Hc along with superior (BH)max for Mn51Al47C2 alloy heat-treated at 925 °C. Additionally, the derivative hysteresis loops assign narrow maxima to Mn51Al47C2 alloy when containing less amount of the L10-ordered τ-phase. Also, tracking the magnetization reversal behavior through the Henkel plots display the increasing negative-peak dominated curve for Mn51Al47C2 alloy when moving from the as homogenized sample (100% τ-phase) to those heat-treated at 1000 (40% L10-ordered τ-phase) and 925 °C (>95% L10-ordered τ-phase), respectively. This highlights the deterioration of the exchange coupling interaction between the grains when dominated with a highly ordered L10-τ-phase. The trend is best confirmed by the quantitative results gained from recoil loops for demagnetization, reversible and irreversible magnetization curves, indicative of a low degree of exchange interactions when the L10-ordered τ-phase is dominant. Furthermore, analyzing the nucleation field through the Kondürsky model gives a low value of 0.41 kOe for the alloy in the as homogenized state. This behavior can be attributed to the boosted propagation of the anisotropy field and enhanced magnetic exchange coupling in a structure with dominant regular τ-phase for which the deleterious effects of the interfaces for the L10-ordered τ-phase can be avoided.

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.

Interaction mechanism, magnetic properties and microstructure of Ce-Fe-B/Alnico spark plasma sintered magnets

By combining rare earth-based Ce-Fe-B and rare earth free Alnico powders, composite Ce-Fe-B/Alnico spark plasma sintered (SPS) magnets have been fabricated and reported for the first time. It is observed that the two phases exit in the alloys interacting magnetostatically. The Alnico phase constitutes α1 and α2 phases as a consequence of spinodal decomposition process, while the Ce-Fe-B phase contained Ce2Fe14B and CeFe2 phases. By increasing the content of Alnico phase, the remanence magnetization and the maximum energy density of the composite alloys increased owing to the higher saturation magnetization of Alnico phase. Magnetic properties, such as remanence magnetization Js = 0.93 T, intrinsic coercivity Hcj = 202 kA/m and maximum energy density (BH)max = 19.8 kJ/m3 were obtained in spark plasma sintered composite magnets containing 15 wt% Alnico. The recoil loops demonstrated the presence of anisotropy variations in the alloys specifically at high applied magnetic field. Analysis of the Curie temperature and elemental analysis validated that the constituent phases of SPS magnets exist in the alloys. This work may shed light on the development of composite magnets with distinct phases and enhanced magnetic properties.

Optimized Microstructure and Improved Magnetic Properties of Pr-Dy-Al-Ga Diffused Sintered Nd-Fe-B Magnets.

The grain boundary diffusion process (GBDP) has become an important technique in improving the coercivity and thermal stability of Dy-free sintered Nd-Fe-B magnets. The influence of Dy70Al10Ga20 and (Pr75Dy25)70Al10Ga20 alloys by the GBDP on sintered Nd-Fe-B magnets are investigated in this paper. After diffusing Dy70Al10Ga20 and (Pr75Dy25)70Al10Ga20 alloys, the coercivity (Hcj) of the magnets increased from 13.58 kOe to 20.10 kOe and 18.11 kOe, respectively. Meanwhile, the remanence of the magnets decreased slightly. The thermal stability of the diffused magnets was improved by the GBDP. The microstructure shows continuous Rare-earth-rich (RE-rich) grain boundary phases and (Dy, Pr/Nd)2Fe14B core-shell structures which contribute to improving the coercivity. Moreover, the Dy concentration on the surface of the (Pr75Dy25)70Al10Ga20 diffused magnets decreased with the Pr substitution for the Dy element. The openness of the recoil loops for the (Pr75Dy25)70Al10Ga20 diffused magnets is smaller than that of the original magnets and Dy70Al10Ga20 diffused magnets. The results show that the (Pr75Dy25)70Al10Ga20 alloys can effectively optimize the microstructure and improve the magnetic properties and thermal stability of the sintered Nd-Fe-B magnets.

Effect of the Ga Content on the Magnetic Properties and Microstructure of the Nanocrystalline Ce-Fe-B Alloys

In this work, the Ce17Fe78-xB6Gax (x = 0~3) nanocrystalline alloys are prepared by the direct melt spinning method with the wheel speed of 18 m/s. The effect of Ga content on the hard magnetic properties and the crystal structure of Ce17Fe78-xB6Gax (x = 0~3) nanocrystalline alloys has been investigated. The results show that the Curie temperature (Tc) and the phase content hard magnetic Ce2Fe14B phase can be increased by adding appropriate amount of low melting point Ga element. The results of magnetic properties show that the enhanced magnetic properties are obtained for the alloy with x = 1: Hcj = 5.61 kOe, Br = 5.4 kG, Hk/Hcj = 0.49, and (BH)max = 5.1 MGOe. The magnetization reversal characteristics of the alloys are analyzed by the recoil loops and the δM-H curves; the sample with the Ga-doped shows a single hard magnetic behavior. The positive peak of δM-H curve of the sample (x = 1) is obviously sharper and the peak value is higher, indicating the strongest magnetic interaction between grains. The reasons of the enhanced magnetic properties for the Ce-Fe-B alloy by the Ga addition are analyzed depending on the microstructure.

Tailoring the microstructure, magnetic properties and interaction mechanisms of Alnico-Ta alloys by magnetic field treatment

Reducing the spatial dimensions of spinodally decomposed phases and enhancing their magnetization difference is a promising route for enhancing the magnetic properties of Alnico alloys. In this paper we report the effects of various heat treatments on the formation of spinodal phases and magnetic properties of Alnico cast alloys. After optimizing the heat treatment conditions, small amount of refractory element Ta is introduced into the alloys and the effect of Ta on the formation and purity of spinodal phases, magnetic properties and phase transition temperatures are investigated. The addition of Ta element improved the intrinsic coercivity (Hcj) of the alloys from 1.52 kOe for standard alloy to 1.83 kOe for 0.6% Ta, while the remanence (Jr) remained nearly unchanged. Scanning electron microscopy analysis confirmed the rod-like Fe–Co rich (α1) phase embedded in Al–Ni–Cu–Ti rich matrix phase. A volume fraction of higher than 67% is obtained for α1 phase, while an ameliorating effect of γ-phase suppression at grain boundaries and triple junctions was noted. The alloy with optimized composition possessed extraordinary thermal stability in the temperature range 300–800 K, which is attributed to the shape anisotropy and uniformly distributed nano mosaic structure. Finally, the interaction mechanisms in Alnico alloys were investigated by means of Henkel plot and the motor applicability was analyzed by recoil loops.

Nd2Fe14B hard magnetic powders: Chemical synthesis and mechanism of coercivity

Chemical synthesis of Nd2Fe14B hard magnetic powders are still hindered due to the complicated methods. Herein, we put forward a modified sol–gel strategy to prepare Nd2Fe14B magnetic powders with excellent magnetic properties. In this synthesis procedure, acetone and boric acid are simultaneously used as the coordination reagents. The formation mechanisms of the oxide precursors as well as the effect of the reduction-diffusion temperature on the composition and magnetic properties of the products have been investigated. Magnetic properties measurement shows that the sample prepared at 950 °C possess the highest coercivity value of ~4.8 kOe. The Henkel plots and recoil loops suggest that the existence of α-Fe is responsible for the stronger exchange-coupling behavior and better magnetic performance.

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.

Magnetization reversal behavior and first-order reversal curve diagrams in high-coercivity Zr-doped α-Fe/Nd2Fe14B nanocomposite alloys

In the present work, the magnetization reversal behavior for the melt spinning (Nd0.8Ce0.2)2Fe12Co2−xZrxB (x = 0, 0.5) permanent alloys with high coercivity was investigated by analyzing the hysteresis curves and the recoil loops. Compared to the Zr-free alloy, the Zr-doped sample obtains higher magnetic properties: coercivity of Hcj = 650.5 kA·m−1, squareness of Hk/Hcj = 0.76 and maximum energy product of (BH)max = 131.0 kJ·m−3. The first-order reversal curves (FORCs) analysis was taken to identify optimal conditions of exchange coupling for the Zr-free and Zr-doped alloys. The coercivity mechanism of the α-Fe/Nd2Fe14B nanocomposite alloys was analyzed by the angular dependence of the coercive field as measured for the Zr-doped sample. The results show that the magnetic reverse process of the Zr-doped sample can be explained by the pinning model.

Microstructure evolution and coercivity enhancement in Pr50Dy20Cu15Ga15-doped hot-deformed Nd-Fe-B magnets

To improve the coercivity (Hcj) and thermal stability of hot-deformed (HD) Nd-Fe-B magnets, Pr50Dy20Cu15Ga15 (at.%) alloy powders with minor heavy rare earth (HRE) are introduced. The magnetic properties, thermal stability and microstructure of the HD magnets with different amounts of Pr50Dy20Cu15Ga15 alloy powders are investigated. The Hcj of the HD magnets is enhanced from 1242 kA/m to 1595 kA/m by doping 3 wt.% Pr50Dy20Cu15Ga15 alloy powders. The thermal stability of the doped HD magnet is also improved at high temperatures. The better isolation of Nd2Fe14B grains by rare earth-rich (RE-rich) phases and the formation of local high-anisotropy (Nd, Pr, Dy)2Fe14B hard magnetic phases are the main reasons for the increase of Hcj. Notably, the non-uniform aggregation of RE-rich phases results in the inhomogeneity of magnetic anisotropy, which eventually leads to the large openness of the recoil loops. This study gives an intuitive view on the relationship between microstructure evolution and magnetic properties of the HD magnets.

Beneficial effects of Cr addition on the nanocrystalline Si and B modified Co-Zr permanent magnetic alloys

Co-Zr alloys with Si and B modifications show potential for rare earth-free permanent magnets. In this work, Cr was employed for improving the hard magnetic properties of melt-spun Co78-xZr16CrxSi3B3 (x = 0, 1, 2, 3, and 4) alloys. Their phase constitution, microstructure, and properties have been investigated. The results show that Cr addition can suppress the formation of the soft magnetic FCC-Co phase. The coercivity increased almost linearly with increasing Cr addition, accompanied by decreasing remanence. A high coercivity of 7.7 kOe was obtained in the alloy with 4 at.% Cr substitution for Co, manifesting a 67% coercivity enhancement from 4.6 kOe of the Cr-free alloy. Cr atoms were found to partially substitute Co atoms in the Co-Zr-Cr-Si-B alloys. The size of nanograin decreased with Cr addition alloys, partly due to the precipitation of the secondary Cr phase. After Cr addition, the enhanced positive peak in Henkel plots and the closed recoil loop indicate the improved exchange coupling and consistent magnetic reversal. The alloys with x = 0 and 4 exhibited much lower absolute values of coercivity temperature coefficient, 0.183%/℃ and 0.251%/℃, respectively, and one or two orders of magnitude smaller corrosion current density than that of melt-spun Nd-Fe-B alloys, demonstrating that Co-Zr based alloys have not only excellent thermal stability but also good corrosion resistance. In addition, Cr substitution can further reduce the corrosion rate of Co-Zr alloys in the 3.5 wt% NaCl solution.

On the magnetism of grain boundary phase and its contribution to the abnormal openness of recoil loops in hot-deformed magnets

The magnetism of grain boundary (GB) phase plays a critical role in the coercivity of Nd–Fe–B magnets. In this work, the aggregation of the GB phase was formed in the hot-deformed magnet due to the low-density high-velocity compaction (HVC) precursor. The ferromagnetism of GB aggregation was confirmed by the analysis of 57Fe Mössbauer spectrometry, and its correlation to the magnetic performance was investigated by micromagnetic simulation. Results show that the soft magnetic GB aggregation is responsible for the low coercivity of the hot-deformed magnet. Moreover, an abnormal large openness of the recoil loop under the reversal magnetic field HR was presented in this work, and the underlying mechanism is interpreted by introducing demagnetization energy. To compensate for the problem of low density, low-melting-point Nd70Cu30 powders were added into the HVC magnet, which makes the GB aggregation paramagnetic and enhances the coercivity.