Intrinsic Coercivity Research Articles

Influence of Alloying Elements Zr, Nb and Mo on the Microstructure and Magnetic Properties of Sintered Pr-Fe-Co-B Based Permanent Magnets

The addition of alloying elements on rare-earth permanent magnets is one of the methods used to improve the magnetic properties. This present work evaluates the influence of alloying elements such as Zr, Nb and Mo on the microstructure and magnetic properties of sintered Pr-FeCo-B based permanent magnets. The permanent magnets were produced by the conventional powder metallurgy route using powder obtained by hydrogen-decrepitation (HD) method from as cast alloys. In order to produce the magnet Pr16Fe66,9Co10,7B5,7Cu0,7 without alloying elements the mixture of alloys method was employed, mixing two compositions: Pr20Fe73B5Cu2 (33% w.t) and Pr14Fe64Co16B6 (67% w.t). With the purpose of evaluating the influence of the alloying elements, the Pr14Fe64Co16B6X0,1 (where X= Zr, Nb or Mo) (67% w.t) alloy was employed. The characterization of the alloys and the magnets was carried out using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDXS) and the magnetic properties were measured using a permeameter. The magnet without any additions presented the highest intrinsic coercivity (μ0iHc = 748 KA.m-1) while the magnet with Nb addition presented higher remanence (Br = 1,04 T). The magnet with Zr addition presented the highest maximum energy product (BHmáx = 144 KJ.m-3), and the magnet with Mo addition showed the highest squareness factor (SF = 0,73).

Magnetic properties and structures of Y-substituted Nd–Y–Fe–B melt-spun ribbons

The nanocrystalline magnetic ribbons of (Nd1−xYx)14.5FebalB6Co0.2Al1Cu0.15 (x = 0, 0.1, 0.2, 0.3, 0.4) were prepared by melt-spinning technique. The magnetic properties and microstructures were systematically investigated for the ribbons. It is found that Y substituting Nd in the R2Fe14B main phase grains gives rise to the decrease in the crystal lattice constant. When Y content increases from x = 0 to x = 0.4, the remanence and intrinsic coercivity (Hcj) decrease from 0.80 to 0.71 T and 1400 to 809 kA·m−1, respectively, with the corresponding descent rate of 11.5% and 42.5%. It is impressed that the average grain size increases with Y content increasing. And the strong intergrain exchange coupling interactions exist in all the ribbons and could be enhanced with an appropriate Y substitution, such as x = 0.4. Moreover, magnetization reversal mechanism is confirmed to be pinning type for the ribbons. The results provide valuable reference to the fabrication of Y-substituted Nd–Y–Fe–B sintered magnets.

Optimization of microstructures and magnetic properties of Sm(CobalFe0.227Cu0.07Zr0.023)7.6 magnets by sintering treatment

Magnetic properties and microstructures of Sm(CobalFe0.227Cu0.07Zr0.023)7.6 sintered magnets were optimized by sintering treatment. Results show that the knee-point magnetic field, Hknee, is twofold up and the intrinsic coercivity Hcj increases by 40%, ranging from 21.64 to 30.39 kOe at the cost of a little decrease of Br from 10.84 to 10.31 kGs with sintering temperature decreasing from 1488 to 1473 K. And the average domain width is narrower and more uniform for the specimen sintered at 1473 K than that of the specimen sintered at 1488 K. It is impressive that the density of lamellar phase increases from ∼0.050 to ∼0.058 nm−1 with the sintering temperature decreasing from 1488 to 1473 K. Moreover, the average cellular size is about ∼84 nm for the magnets sintered at 1473 K, which is 80% of that of the magnets sintered at 1488 K (∼97 nm). And the cell boundary width of the magnets sintered at 1473 K (∼7 nm) is only half average width of the magnets sintered at 1488 K (∼14 nm). It is found that the Cu content in the cell boundaries is much higher (∼17 at%) in the magnets sintered at 1473 K compared to that of the magnets sintered at 1488 K (∼10 at%). It can be concluded that smaller cells and narrower cell boundaries together with higher gradient of Cu content are key points for obtaining the optimum Hknee and Hcj.

Effect of magnetic field treatment on magnetic properties and microstructure of Fe69.5Nd7B21Nb2.5 bulk permanent magnets

In this study, the Fe69.5Nd7B21Nb2.5 permanent magnets in the form of rods were prepared by annealing the bulk amorphous precursors. The effect of magnetic field heat treatment on the magnetic properties and microstructure of the alloys was investigated. It was found that the as-cast Fe69.5Nd7B21Nb2.5 alloy showed soft magnetic properties, which became magnetically hard after normal annealing. The annealed alloys were mainly composed of soft α-Fe, hard Nd2Fe14B and nonmagnetic Nd1.1Fe4B4 phases. Furthermore, magnetic field heat treatment at the temperature close to Curie temperature (595 K) of Nd2Fe14B phase was applied to the annealed rods. The intrinsic coercivity Hci remanence Br and maximum energy product (BH)max increased from 414 kA/m to 441 kA/m, 0.60 T–0.64 T and 28.9 kJ/m3 to 35.4 kJ/m3 respectively, after the heat treatment at 573 K with 1 T magnetic field. However, the magnetic properties did not improve when the temperature was above the Curie temperature. The results of X-ray diffractively (XRD) and transmission electron microscope (TEM) indicated that the magnetic field heat treatment under the Curie temperature can not only contribute to a uniform microstructure but also be helpful in suppressing the nonmagnetic phase.

Preparation of Al3+-Co2+ co-substituted M-type SrCaNd hexaferrites and their controlled magnetic properties

Al3+-Co2+ co-substituted M-type SrCaNd hexaferrites with cation compositions Sr0.5Ca0.2Nd0.3Fe12.0-x(Al0.5Co0.5)xO19 were synthesized using the traditional ceramic process by varying AlCo content (x) (0.0 ≤ x ≤ 0.5). The microstructures, morphologies and ferromagnetic properties of the samples were investigated as a function of AlCo content (x) by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transformer infrared (FT-IR) spectroscopy and Hysteresis graph meter. The X-ray diffraction patterns show that the hexaferrites with AlCo content (x) of 0.0 ≤ x ≤ 0.2) exhibited M-type phase and α-Fe2O3 as impurity phase, while the hexaferrites with AlCo content (x) ≥ 0.3 exhibited the single M-type phase. XRD, along with FT-IR analysis confirmed the formation of M-type hexaferrites and the successful substitution of Al3+ and Co2+ ions in the hexaferrite lattice. The results of FE-SEM images proposed that all the particles with regular hexagonal platelet-like shape were homogeneous dispersed. The remanence (Br) first increased with AlCo content (x) from 0.0 to 0.3, and then decreased when AlCo content (x) ≥ 0.3. The intrinsic coercivity (Hcj) and magnetic induction coercivity (Hcb) first increased with AlCo content (x) from 0.0 to 0.2, and then decreased with AlCo content (x) from 0.2 to 0.3, and increased when AlCo content (x) ≥ 0.3. Maximum energy product [(BH)max] first increased with AlCo content (x) from 0.0 to 0.2, and then decreased at AlCo content (x) ≥ 0.2. Squareness ratio (Hk/Hcj) decreased with increasing AlCo content (x) from 0.0 to 0.5.

Study on Microstructure and Coercivity of Thermally Processed Nd-Fe-B–type HDDR Material

Variation of intrinsic coercivity of the HDDR-treated NdSUB12.5/SUBFeSUB80.8/SUBBSUB6.4/SUBGaSUB0.3/SUB alloy after heating in various modes was investigated. Influence of vacuum degree and cooling after heating on the coercivity of the HDDR material was examined. The heat-treated HDDR material had consistently higher coercivity when it was quenched after heating compared to when slow-cooled. Higher coercivity in the quenched material was attributable to the grain boundary with lower Fe content. HDDR-treated material heated in high vacuum showed consistently higher coercivity than the material heated in lower vacuum, and this was attributed to less heavily oxidized surface. Reduced coercivity of the HDDR-treated material heated at moderate temperature was noticeably recovered at higher temperature, and this was attributed to lower Fe content in the grain boundary.

Crystalline and magnetic microstructures of iron-rich Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.8 sintered magnets: Isothermal aging effect

Crystalline and magnetic microstructures of the iron-rich Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.8 anisotropic sintered magnets after isothermal aging process were investigated systematically by using transmission electron microscope (TEM) and magnetic force microscopy (MFM). As the isothermal aging time (t) increases from 0.5 to 20 h, the remanences of the specimens almost keep constant (11.4 kGs), while the intrinsic coercivity (Hcj) shows an increase from 1.13 to 7.93 kOe, which is much higher than the well-known value of the order of 1 kOe. TEM investigation indicates that the cellular microstructures are incomplete together with partially phase transformation of 1:7H to 2:7R and 5:19H when t = 0.5 h. When t reaches 5 h, the phase transformation is almost completed. With t increasing from 2.5 to 20 h the average cellular size of the specimens increases from ∼90 to ∼150 nm and the density of the lamellae phase increases from ∼0.014 to ∼0.033 1/nm. MFM investigations show that the average surface width (Ws) decreases from 0.86 to 0.67 μm initially and then increases to 0.98 μm and the main domain width (Wm) decreases from 2.17 to 1.21 μm initially and then increases to 1.51 μm. And the Wm is about two times of Ws for the specimens because of the smaller magnetostatic energy of the surface layer of the specimens with c-axis parallel to the imaging plane. It is impressive that the domain walls form in straight line through cellular structures as t is less than 5 h, and in a zig-zag shape when t is over 5 h. This is the first time to reveal systematically the domain structures for the iron-rich 2:17-type Sm-Co magnets during isothermal aging process. The evolution of the magnetic microstructures is ascribed to the variation of the effective intrinsic magnetic properties of cell and cell boundary phases originating from both the phase transformation and development of cellular structures.

Microstructure versus magnetic properties correlations in melt-spun Hf-Zr-Co-Fe-B alloys: role of thermal treatment

Effect of annealing on magnetic property-structure/microstructure correlation in nanostructured ribbons of a rare-earth-free Hf1.5Zr0.5Co10FeB alloy has been reported in the present study. Melt-spinning of the arc melted alloy was employed to obtain the ribbons. The compositional, structural and magnetic measurements demonstrate that under optimized annealing conditions, the magnetically hard phases Hf2Co11B and ZrCo5.1 appear as the dominant ones, while the magnetically soft phases Zr6Co23 and cubic Co appear as minor phase. The optimally annealed ribbons show promising values for characteristic magnetic parameters such as the saturation magnetization Ms ∼ 76.7 emu g−1, intrinsic coercivity iHc ∼ 2.4 kOe and magnetic energy product (BH)max ∼ 4 MGOe. Processing steps like magnetic alignment and magnetization would further improve the characteristic parameters.

The role of cobalt addition in magnetic and mechanical properties of high intrinsic coercivity Nd-Fe-B magnets

The role of cobalt addition in magnetic and mechanical properties of high intrinsic coercivity Nd-Fe-B magnets by adding cobalt element and blending micron cobalt powder was investigated. The results showed that the excellent magnetic properties of high intrinsic coercivity Nd-Fe-B magnets were obtained at the Co content of 1.5 wt%, and then the intrinsic coercivity and fracture toughness decline after blending 3 wt% micron Co powder. Blending Co powder increased the Co content of main phase grain and Nd-rich phase, and the phenomenon that the Co powder additive is in the presence of pure Co particles was not been found in the sample. Simultaneously, blending Co powder reduced the heavy rare earth content of main phase grain, and new phases containing heavy rare earth and Co were formed, such as soft ferromagnetic phase R(FeCo)3, the low-anisotropy phase R(FeCo)5 and rich-B phase R1.1(FeCo)4B4. These phenomena lead to a decrease in the intrinsic coercivity of high intrinsic coercivity Nd-Fe-B magnets. Further investigation on the grain and grain boundary indicated that the effect mechanism of blending Co powder on magnetic properties is not the same as that of the traditional alloying route.

Enhanced formation of 2:14:1 phase in La-based rare earth-iron-boron permanent magnetic alloys by Nd substitution

Aiming at promoting the formation of hard magnetic 2:14:1 phase in La-Fe-B alloys, the effects of Nd substitution on the melt-spun nanocrystalline (La1-xNdx)yFe94-yB6 alloys (x = 0–0.5; y = 12–16) have been investigated. No hard magnetic phase was formed in the (La1-xNdx)16Fe78B6 alloys with x < 0.2. A soft magnetic phase (La,Nd)2Fe23B3 with Curie temperature of 649 K was observed in the x = 0.2 alloy and it disappeared as x increased to 0.3. 30% and above Nd substitution can dramatically enhance the formation of 2:14:1 phase and the magnetic properties of La-based permanent magnets. Three different alloy systems (y = 16, 14 and 12) show similar behavior of magnetic properties dependences on Nd substitution. (La0.7Nd0.3)16Fe78B6 alloy with mean grain size 10–20 nm exhibits high remanent magnetization, maximum energy product and intrinsic coercivity up to 0.71 T, 10.0 MGOe and 562 kA/m, respectively, indicating it is possible to achieve excellent magnetic properties in La-based rare earth permanent magnets. The present work provides a good understanding on the fundamental magnetism of La-based magnets and the potential to develop low cost rare earth permanent magnets.

On spinodal decomposition in alnico - A transmission electron microscopy and atom probe tomography study

Alnico is a prime example of a finely tuned nanostructure whose magnetic properties are intimately connected to magnetic annealing (MA) during spinodal transformation and subsequent lower temperature annealing (draw) cycles. Using a combination of transmission electron microscopy and atom probe tomography, we show how these critical processing steps affect the local composition and nanostructure evolution with impact on magnetic properties. The nearly 2-fold increase of intrinsic coercivity (Hci) during the draw cycle is not adequately explained by chemical refinement of the spinodal phases. Instead, increased Fe-Co phase (α1) isolation, development of Cu-rich spheres/rods/blades and additional α1 rod precipitation that occurs during the MA and draw, likely play a key role in Hci enhancement. Chemical ordering of the Al-Ni-phase (α2) and formation of Ni-rich (α3) may also contribute. Unraveling of the subtle effect of these nano-scaled features is crucial to understanding on how to improve shape anisotropy in alnico magnets.

Effect of Zr on magnetic properties and electrical resistivity of Sm(CobalFe0.09Cu0.09Zrx)7.68 magnets

In this work, the effect of Zr content on magnetic properties and electrical resistivity of Sm(CobalFe0.09Cu0.09Zrx)7.68 (x = 0.020, 0.025, 0.030, 0.035) high temperature magnets has been investigated. More Zr content results in the decrease of average cell size and the increase of the density of lamellar structure. In addition, a large number of phase boundaries between lamellar phase and cellular structure have also been formed. The intrinsic coercivity of 550 °C enhances from 3.21 kOe to 6.05 kOe with the increase of Zr content. Furthermore, electrical resistivity shows obviously different variation in different directions with increasing Zr content. At room temperature and 127 °C, the electrical resistivity improves from 68.2 μΩ cm to 91.7 μΩ cm and from 87.1 μΩ cm to 103.3 μΩ cm with increasing x from 0.020 to 0.035 in the direction parallel to c-axis, respectively. However, there is only a slight increase in the electrical resistivity in the direction perpendicular to c-axis. The rise of electrical resistivity in different directions has been discussed. Research results indicate that increasing the density of the lamellar phase is an effective way to increase electrical resistivity in the 2:17-type Sm-Co magnets.

Densification of nanocrystalline NdFeB magnets processed by electro-discharge sintering – Microstructure, magnetic, and mechanical properties

This work investigates the densification process of nanocrystalline NdFeB powder by electro-discharge sintering (EDS) and the associated magnetic properties. The EDS technique is used as a fast and energy-saving compaction process for metal powders. A large current is discharged from capacitors into a pre-compacted loose powder, thus resulting in complete compaction. In this study, the microstructure, magnetic, and mechanical properties of the compacted, hard magnetic NdFeB specimens were investigated under variation of the energy EEDS and compression load pEDS. For all specimens, the intrinsic coercivity HcJ decreases on increasing the discharge energy. However, the compaction load has apparently no influence on the coercivity HcJ, whereas the residual induction Br decreases only with increasing discharge energy. An increase in the compression load pEDS causes an increase in the specimens’ density and thus promotes residual induction Br. The applied EDS parameters led to the formation of three different microstructures (insufficiently densified zone, fully densified zone, and remelted zone) along the cross-section of the EDS-densified specimens. Volume fractions of the three different microstructures that form during the EDS process determine the resulting mechanical and magnetic properties of the specimens.

UV-assisted direct write of polymer-bonded magnets

This work presents a novel fabrication technique called “UV-assisted Direct Write (UADW)”, which combines extrusion-based direct write (DW) and in situ layer-by-layer UV curing to fabricate polymer-bonded permanent magnets of arbitrary shapes at room temperature. The process requires minimal post heat treatment and therefore preserves the magnetic properties of the raw powder in the printed product. Experimentation achieved a maximum solid loading of printable ink of 60% by volume, or 91% by weight. Samples with 60 vol.% nominal particle concentration exhibited remanence of 3.80 kG (52% of raw powder value) and intrinsic coercivity of 9.50 kOe. Magnetized samples showed no reduction in intrinsic coercivity, which proves good curing of the final product and good bonding between particles and binder. Compared to other 3D-printed magnets reported in the literature, UADW magnets possess the highest intrinsic coercivity with one of the highest magnetic remanence values.

Fabrication of highly dense isotropic Nd-Fe-B nylon bonded magnets via extrusion-based additive manufacturing

Magnetically isotropic bonded magnets with a high loading fraction of 70 vol.% Nd-Fe-B are fabricated via an extrusion-based additive manufacturing, or 3D printing system that enables rapid production of large parts. The density of the printed magnet is ∼ 5.2 g/cm3. The room temperature magnetic properties are: intrinsic coercivity Hci = 8.9 kOe (708.2 kA/m), remanence Br = 5.8 kG (0.58 T), and energy product (BH)max = 7.3 MGOe (58.1 kJ/m3). The as-printed magnets are then coated with two types of polymers, both of which improve the thermal stability as revealed by flux aging loss measurements. Tensile tests performed at 25 °C and 100 °C show that the ultimate tensile stress (UTS) increases with increasing loading fraction of the magnet powder, and decreases with increasing temperature. AC magnetic susceptibility and resistivity measurements show that the 3D printed Nd-Fe-B bonded magnets exhibit extremely low eddy current loss and high resistivity. Finally, we demonstrate the performance of the 3D printed magnets in a DC motor configuration via back electromotive force measurements.

Influence on the structural and magnetic properties of pre-alloyed gas-atomized Fe49Co49V2 powders during mechanical milling

The influence on the structural and magnetic properties of pre-alloyed gas-atomized Fe49Co49V2 powders during mechanical milling was investigated. The powders were mechanically milled for up to 45 h. The powders, before milling, were principally comprised of α-phase. No noticeable phase transformation was observed during mechanical milling of the pre-alloyed powders. Mechanical milling resulted in a gradual decrease in crystallite size to as low as ~10 nm, accompanied by an increase in lattice strain to ~1.2–1.5%. Mechanical milling led to an appreciable reduction in the particle size while the morphology of the milled powders appeared similar once the steady-state was attained. The lattice parameter of the nanostructured powders at steady-state was estimated as 0.2857 ± 0.0004 nm. The saturation magnetization (MS) of the nanostructured powders milled for ~45 h was ~211 ± 1 Am2/kg, and it decreased only by ~6% compared to that of the pre-alloyed gas-atomized powders before mechanical milling. The intrinsic coercivity (HCI) of the pre-alloyed powders was estimated as ~1.4 ± 1% kA/m. The HCI peaked to ~8.4 kA/m during the initial stages of milling and subsequently remained between ~6.5 kA/m to ~6 kA/m, during the later stages of milling. As would be expected, the HCI was strongly dependent on the crystallite size. The ferromagnetic exchange length of the nanostructured powders was estimated to be ~27 nm. The magnetic properties of nanostructured Fe49Co49V2 powders obtained by mechanical milling of pre-alloyed powders had relatively superior magnetic properties compared to the nanostructured powder obtained by mechanical alloying of the constituent elemental powders.

HIGHLY ANISOTROPIC MnBi MAGNETS

The rare-earth-free MnBi magnetic material is promising for high-temperature (150 200 oC) application of permanent magnets because of its large magnetocrystalline energy and especially the positive thermal coefficient of coercivity (dHc/dT &gt; 0). Because of the moderate value of the spontaneous magnetization Ms 74 emu/g, the anisotropy of MnBi bulk magnets should be investigated to enhance the remanence Mr. With large ratio Mr/Ms and appropriate microstructure, the squareness of MnBi magnets should have high value leading the remanent coercivity bHc close to the intrinsic coercivity iHc, thus enhancing the energy product (BH)max. The paper presents an approach to loading and compacting of MnBi powders in the 18 kOe magnetic field oriented perpendicular to the pressing direction where MnBi grains can be freely rotated and oriented parallel to the field direction. Based on the energy minimization of the assembly of magnetized grains, the compacting pressure was chosen to optimize two parameters, the mass density and the coercivity iHc of magnets. The prepared MnBi bulk magnet had 8.4 g/cm3, Mr/Ms 0.92, 0.89 and (BH)max reached 8.4 MGOe.

Coercivity mechanism of rare-earth free MnBi hard magnetic alloys

In this work, we present and discuss results concerning the hard magnetic behavior of rare earth-free MnBi alloys obtained by suction casting technique. The physics of coercivity for these type of alloys is based on the nucleation process of reverse domains, which in turn is determined by the alloy microstructure features such as phase distribution, morphology, grain size and in particular, defects, which are characteristic ofreal materials. The microstructure of the as-cast alloy presented here comprises the formation of the Low Temperature Intermetallic Phase (LTIP)-MnBi, interspersed within Bi- and Mn-rich areas. A considerable intrinsic coercivity field of 238 kA/m together with a saturation magnetization of 0.04 T were observed. The nucleation controlled mechanism of this alloy was described in terms of the Kronm¨uller equation, which incorporates the detrimental effect of microstructure defects through fitting parameters associated to reduced intrinsic magnetic properties at grain size boundaries, interfaces and local demagnetizing fields. A notorious switching of coercivity mechanism associated with domain wall pinning was found to be produced upon annealing of the alloy at 583 K for 24 hrs, yielding a drastic reduction of coercivity (down to 16 kA/m). The key microstructural feature determining the switching of coercivity mechanism is the formation/suppression of Bi-rich areas, which promotes the nucleation and growth of LTIP.

Microstructural and magnetic properties of Pr–BiCo co-substituted M-type Sr–Ca hexaferrites

We have synthesized the Pr–BiCo substituted hexaferrites with compositions of Sr0.8-xCa0.2PrxFe12.0-x(Bi0.5Co0.5)xO19 (0.0 ≤ x ≤ 0.5) by the standard ceramic method. Results of X-ray diffraction analysis exhibits that the synthesized hexaferrites with x from 0.0 to 0.3 are in single magetoplumbite structure, and impurity phases are observed when x ≥ 0.4. The surface morphology of magnets shows that hexaferrite grains have a hexagonal platelet shape with clear grain boundaries. The remanence first increases with x from 0.0 to 0.1, and then decreases when x ≥ 0.1. The intrinsic coercivity decreases with x from 0.0 to 0.1, and then increases when x ≥ 0.1. With x from 0.0 to 0.4, the changing trend of magnetic induction coercivity is in agreement with that of Hcj, while at x ≥ 0.4, Hcb decreases. The maximum energy product initially increases with x from 0.0 to 0.2, and then decreases when x ≥ 0.2.

Structure and magnetic properties of melt-spun Sm–Fe–Nb ribbons and their nitrides

SmFe10−xNb x (x = 0, 0.1, 0.2, 0.3) ribbons and their nitrides were prepared by melt-spinning, followed by annealing and subsequent nitriding. The structure and magnetic properties were investigated by means of X-ray diffraction (XRD) using the Rietveld method, vibrating sample magnetometer (VSM), transmission electron microscope (TEM) and Mossbauer spectroscopy. XRD analysis shows that the addition of Nb can impede the precipitation of Sm2Fe17 and α-Fe phases and the Nb atoms occupy 2e site in the alloys. At 300 K, the mean hyperfine fields of 2e site are 29.58 T with Nb doping at x = 0.1 and the corresponding Curie temperature is 552 K. The optimal properties of remanence of Br = 0.9 T, intrinsic coercivity of Hcj = 741.5 kA·m−1, and maximum magnetic energy product of (BH)max = 124.2 kJ·m−3 are gained at x = 0.10 in the nitrides.