Significant Coercivity Enhancement Research Articles

Effects of Ti on the solidification and crystallization behavior of nanocomposite permanent material under different cooling rates

Effects of Ti on the crystallization behavior, phase composition, microstructure and magnetic properties of Nd2Fe14B/α-Fe dual-phase nanocomposite permanent magnet materials in different states were studied, respectively. The results show that Ti could affect the solidification and crystallization behavior of dual-phase nanocomposite materials by regulating the cooling rate during the rapid solidification process. When the cooling rate is slow, part of Ti atoms enters the soft magnetic phase α-Fe, and part forms the TiFe2 phase around the hard magnetic phase. Exchange coupling is enhanced. The maximum energy product and the remanence of the alloy are 10.54 MGOe and 8.34 kGs, respectively. When the cooling rate faster, Ti can stabilize another ferromagnetic ThMn12-type phase with a low coercivity and maximum energy product, which should be avoided. When the cooling rate is fast enough to form amorphous, Ti and Fe preferentially combine to form the TiFe2 phase during crystallization, which selectively inhibits the growth of the hard magnetic phase and the precipitation of the soft magnetic phase, resulting in a significant enhancement of coercivity to 12.82 kOe. By regulating the mechanism of Ti in the solidification and crystallization process, nanocomposite permanent magnet materials with varying magnetic characteristics can be obtained, offering a flexible approach to tailoring their performance according to specific application requirements.

Significant coercivity enhancement of Ga-doped sintered Nd-Fe-B magnet by grain boundary diffusion perpendicular to the c-axis

Grain boundary diffusion (GBD) process is the most effective approach for enhancing coercivity of Nd-Fe-B magnets with minimal consumption of heavy rare earth (HRE). However, understanding of GBD mechanism is still limited, especially on the anisotropic diffusion behavior within sintered magnets. Grain boundary (GB) phase is the key factor in determining the diffusion process, which acts as diffusion channel for HRE atoms. The effects of doping Ga on the grain boundary microstructure and diffusion behavior of Tb in the sintered Nd-Fe-B magnet were investigated. Sintered magnets with different Ga content (0.15 wt%, 0.45 wt%) were prepared. It should be noted that Tb was diffused perpendicular to the easy axis (c-axis) of the magnets. The coercivity increased more significantly to 2.802 T (1.006 T higher than that of the post-sinter annealed state) in magnets with high Ga content than the low Ga content magnets (2.291 T, 0.669 T higher than the post-sintered annealed state). Such a huge coercivity increment almost reached the same value by diffusing Tb along the easy axis reported by other researchers. Chemical compositions and microstructure of the GB phase were found to be key factors in influencing the HRE diffusion process. A considerable amount of Nd6Fe13Ga phase was observed near triple junctions after post-sinter annealing in the high Ga content magnets, which markedly decreased Fe concentration in the intergranular phase. In addition, amorphous intergranular metallic Cu-rich phases provided smooth diffusion channels for HRE atoms. (Nd1-x, Tbx)2Fe14B shell was much more obvious in the high Ga content magnets. Ga dopping induced novel modifications in diffusion behavior of HRE atoms and strongly influenced the magnetic properties of permanent magnets. The present approach provide new path for the manufacture of high performance magnets, which is interesting both for technical applications and fundamental studies.

High-efficiency utilization of Tb in enhancing the coercivity of Nd-Fe-B magnets by multicomponent Tb70−xPrxCu10Al10Zn10 (x = 0–30) film diffusion

The effect of multicomponent Tb70−xPrxCu10Al10Zn10 (x = 0–30) film diffusion on microstructure and magnetic properties of sintered Nd-Fe-B magnets was investigated in this paper through magnetron sputtering system. The coercivity increased significantly from 11.5 kOe to 17.5 kOe, 18.8 kOe, 19.8 kOe, and 17.8 kOe for the processed samples by Tb40Pr30Cu10Al10Zn10, Tb60Pr10Cu10Al10Zn10, Tb70Cu10Al10Zn10 and Tb, respectively. The remanence and the maximum magnetic energy product of multicomponent film diffused magnets were all higher than those of Tb diffused magnet. Particularly, compared with Tb diffused magnet, the ΔHcj/wt%Tb (the coercivity enhancement per wt% Tb usage) of Tb40Pr30Cu10Al10Zn10, Tb60Pr10Cu10Al10Zn10 and Tb70Cu10Al10Zn10 diffused samples increased by 105%, 63.4% and 57.5%, respectively. It is obviously that the Tb utilization in multicomponent diffusion sources has a higher efficiency in coercivity enhancement. The quantitative analysis of Tb distribution was characterized for comparison. The extra addition of Cu/Al/Zn elements in Tb70Cu10Al10Zn10 diffusion source would enter into the rare earth-rich (RE-rich) grain boundary phases, and an easy diffusion channel was formed to promote the infiltration of Tb element. The results show that improved distribution uniformity of the Tb-rich shell, deeper shell-core structure and the increased local grain boundary phases are the key factors for the significant enhancement of coercivity in multicomponent diffusion magnet. In addition, on one hand, the easily combination of Tb and Cu/Al/Zn elements effectively prevented Tb from entering the main phases. On the other hand, the thinner shell-core structure was observed due to the uniform distribution of Tb in Tb70Cu10Al10Zn10 diffused magnet. These two factors jointly lead to the increase of remanence. This work puts forward that using multicomponent alloy as diffusion sources provides a composition diversity and cost-effective way to enhance the magnetic properties of sintered Nd-Fe-B magnets, while reducing the consumption of Tb.

Significant coercivity enhancement of hot-deformed bulk magnets by two-step diffusion process using a minimal amount of Dy

This work has demonstrated that a high coercivity of 2.5 T can be achieved in a 5.6-mm-thick Nd-Fe-B anisotropic magnet by applying the two-step diffusion process using Nd50Dy30Cu20 and Nd80Cu20 eutectic alloys to a Dy-free Nd-Fe-B hot-deformed magnet. This is in contrast to the fact that about 5 wt.% Dy was needed to reach this coercivity level in sintered magnets. The overall Dy concentration in the sample was determined to be only 0.45 wt.% Dy, which is about 10 % of that used in the conventional Dy-alloyed sintered magnets with a comparable coercivity. Due to the low Dy concentration, a relatively high remanence of 1.32 T is retained even after the two-step diffusion process with an excellent temperature coefficient of coercivity of –0.41 %/°C.

Coercivity enhancement of hot-deformed NdFeB magnet by doping R80Al20 (R = La, Ce, Dy, Tb) alloy powders

Significant enhancement in coercivity for hot deformed (HD) NdFeB magnets is achieved by doping R80Al20 (R= La, Ce, Dy, Tb) alloy powders. All studied samples exhibit fine microstructure with good (00L) texture. By doping R80Al20 powders, the coercivity of HD NdFeB magnets is significantly increased from 16.0 kOe to 19.4 kOe, 20.0 kOe, 21.8 kOe, and 23.8 kOe for R = La, Ce, Dy, and Tb, respectively, while (BH)max is decreased from 41.5 MGOe to 39.3-40.8 MGOe slightly. Microstructure analysis shows that Ce or La prefers to distribute mostly at grain boundary, while Dy or Tb and Al locate at grain boundary and surface. The magnetic isolation due to R-rich phase containing Nd, Ce or La, Al at the grain boundary contributes to coercivity enhancement with the samples with R = La and Ce. In additional to magnetic isolation, the strengthened magnetic anisotropy field of 2:14:1 phase due to Tb or Dy, and slight Al substitutions gives rise to the significant enhancement of coercivity for those with R80Al20 (R = Dy and Tb).

PrAl and PrDyAl diffusion into Nd-La-Ce-Fe-B sintered magnets: Critical role of surface microstructure in the magnetic performance

To solve the challenging hurdle of low coercivity, Pr80Al20 and Pr40Dy40Al20 diffusion is applied to the Nd-La-Ce-Fe-B sintered magnets with high La-Ce substitution (La-Ce/TRE = 40 wt%, TRE: total rare earth). Coercivity approaching 15 kOe with approximately 200% increment level is acquired for PrAl diffusion. Anomalously, PrDyAl diffusion cannot further enhance the coercivity whilst lowers the remanence drastically from 12.60 to 12.16 kG. This anomaly is correlated with the critical role of surface microstructure that consumes large amounts of expensive Dy, i.e. thick Dy/Fe-rich top layer, and below that Dy/Al diffusion into the interior of 2:14:1 grain. Such predominant Dy agglomeration at the surface region can well interpret that the negative role of Dy in deteriorating remanence is exacerbated, while its positive role in strengthening the local shell of 2:14:1 phase and enhancing the coercivity is weakened. Comparably, PrAl diffusion generates the Pr-rich magnetically hardening shell along the continuous grain boundary layer, paving it a versatile route for significant coercivity enhancement of Nd-La-Ce-Fe-B without sacrificing remanence.

Ce3+-doped nanocrystalline cobalt–zinc spinel ferrite: microstructural, magnetic, and optical characterizations

A series of Ce3+ ions-doped cobalt–zinc spinel cubic ferrite nanoparticles with the general formula Co0.5Zn0.5CexFe2–xO4 (x = 0.00, 0.03, 0.06 and 0.09) were synthesized using soft chemical co-precipitation route. The synthesized nanoferrites were in single-phase as ensured by the X-ray diffractograms. Average crystallite sizes were 16 nm, 12 nm, 10 nm, and 07 nm with increasing Ce content as determined using Scherrer’s formula. A significant enhancement in coercivity (HC) and decrement in saturation magnetization (MS) was observed in field-cooled (9 T) low-temperature (3 K) hysteresis loops with increasing Ce content. A correlation between the mean crystallite size of the synthesized nanoparticles and disordered surface spins has been established. The increment in Ce3+ ions substitution also favored the onset of superparamagnetic behavior as confirmed by the room temperature hysteresis loops. The blocking temperature was reduced with the increase of Ce3+ ions content which makes the nanoparticles a suitable candidate for various biomedical applications. The indirect optical bandgaps were observed to increase gradually with the increasing Ce content due to the nanosize effect.

Significant coercivity enhancement at low temperatures in magnetically oriented cobalt ferrite nanoparticles

The present work describes a synthesis and characterization strategy employed to study the magnetic anisotropic properties of a diluted nanoparticulate system. The system under analysis is composed of monodisperse and highly crystalline 16 nm Co0.5Fe2.5O4 nanoparticles (NPs), homogenously dispersed in 1-octadecene. Owing to the liquid nature of the matrix at room temperature, the relative orientation of the nanoparticle easy axis can be controlled by an external magnetic field, enabling us to measure how the magnetic properties are modified by the alignment of the particles within the sample. In turn, by employing this strategy, we have found a significant hardness and squareness enhancement of the hysteresis loop in the magnetically oriented system, with the coercive field reaching a value as high as 30.2 kOe at low temperatures. In addition, the magnetic behavior associated with the system under study was supported by additional magnetic measurements, which were ascribed to different events expected to take place throughout the sample characterization, such as the melting process of the 1-octadecene matrix or the NP relaxation under the Brownian mechanism at high temperatures.

Comparison on the coercivity enhancement of sintered NdFeB magnets by grain boundary diffusion with low-melting (Tb, R)75Cu25 alloys (R= None, Y, La, and Ce)

A significant coercivity enhancement of the commercial NdFeB magnets with the magnetic properties of (BH)max = 48.4 MGOe and iHc = 17.5 kOe through grain boundary diffusion (GBD) with low-melting Tb55R20Cu25 alloys is demonstrated. Adopting Tb55R20Cu25 alloys as GBD sources is effective in increasing coercivity to 29.0 kOe for R = None, 23.8 kOe for R = Y, 25.6 kOe for R = La, 28.0 kOe for R = Ce, respectively. Yet, (BH)max is slightly reduced to 46.2-48.2 MGOe. The preferential appearance of Cu at grain boundary and triple junction of the grains, and the core-shell structure occurred due to Tb at grain surface remarkably enhance the coercivity. Interestingly, higher coercivity enhancement per wt% Tb usage (ΔiHc/wt%Tb) of 7.2 kOe/wt% for the magnet with Tb55Ce25Cu25 than 5.9 kOe/wt% for that with Tb75Cu25 has been found due to the magnetic isolation effect caused by the preferential appearance of Ce at grain boundary, though a slight lower coercivity enhancement was found for the samples with R = Y and La. Lower melting point (637 °C) for Tb55Ce20Cu25 than Tb75Cu25 (743 °C) leads to larger diffusion depth of Tb into the magnet and therefore contributes to higher efficiency of coercivity enhancement for the magnet with R=Ce.

A facile synthesis of directly gas-phase ordered high anisotropic Sm[sbnd]Co based non-segregated nanoalloys by cluster beam deposition method

Synthesis of nanoalloys containing three or more elements with controllable size and morphology and desired phase is very challenging. Rare earth (RE)-transition metal (TM) nanoalloys have great potential applications, however, their controllable synthesis is even a daunting task. Here we find that the direct gas-phase ordering of Sm-Co-based nanoalloys (around 5–15 nm) into 1:5 phase without size and shape change can be achieved by in-flight annealing through reducing their crystallization-ordering temperature (COT) with additive Cu or Ni. The homogenous element-distribution and desired phase in these trimetallic nanoalloys are evidenced by high-resolution-transmission-electron-microscopy analysis despite a preferred surface segregation from thermodynamic calculation and analysis. Our results demonstrate a promising way to synthesize trimetallic nanoalloys especially RE-TM with independently controlled size, composition and phase as well as their building-blocks with novel functionalities. The reduction in COT of Sm-Co-Cu nanoalloys is further confirmed by post-annealing the deposited nanocluster film. A significant coercivity-enhancement is achieved at a lower annealing-temperature. Compared to FePt, the higher anisotropy and much lower COT of Sm-Co-based nanoalloy make it emerge as a promising next-generation recording media with density beyond FePt. Our study also opens a door for fabricating trimetallic or even multicomponent nanoalloys with controllable size, composition and phase.

Significant coercivity enhancement of Ti doping in Nd–Ce–Fe–B melt spun alloys

Abstract The influences of doping Ti on the phases, microstructures and magnetic properties are systematically investigated in (Nd0.8Ce0.2)12Fe82−xTixB6 melt spun alloys. It is found that Ti could promote the glass forming ability of the studied alloys and increase the stability of the amorphous phase. Significant coercivity enhancement due to Ti doping is observed. The coercivity increases from 9.5 kOe for Ti-free sample to 18.5 kOe with Ti addition of 5 at%, and further reaches to 19.5 kOe when Ti addition rises to 7 at%. The Fe2Ti phase is found to precipitate in the 2-14-1 phase grain boundaries investigated by transmission electron microscopy (TEM) and high angle annular dark field (HAADF). With the Ti contents increasing, the precipitated phase becomes larger and more continuous, which is responsible for the huge improvement of the coercivity.

Structural, Magnetic, and Electrical Properties of RE Doped Sr<sub>0.82</sub>RE<sub>0.18</sub>Fe<sub>12-x</sub>Al<sub>x</sub>O<sub>19</sub> (RE = Gd, Pr, Sm) Compound

Among the family of ferrites, M-type hexaferrites has many industrial applications ranging from simple magnets to microwave devices. Improvement in magnetic and dielectric properties of ferrites is of continuous interest. In this present work details study is done to observe the effect of co-doping of rare-earth (RE3+: Pr3+, Sm3+, and Gd3+) and aluminum in Sr0.82RE0.18Fe12-xAlxO19 (x = 0.0, 0.5, 1.0, 1.5, 2.0). The adopted samples were synthesized via autocombustion technique. Detailed synthesis, structural, magnetic, and electrical measurements of samples were performed to understand structural-magnetic-electrical property relationship. The Al3+ substitution for Fe3+ brings in a significant enhancement in coercivity but reduces magnetization due to the magnetic dilution effect. Additional coercivity enhancement was possible with RE3+ doping without affecting the magnetization of samples. Among all RE3+ doped samples, Pr3+ doped samples showed the highest Curie temperature, (Tc ~ 465℃), while Gd3+ doped samples showed little variation in dielectric properties in GHz frequency range. This makes RE3+ doped samples as an ideal candidate for high-frequency microwave applications. Pr3+ with oblate charge distribution (negative Stevens constant) was observed to substitute well into the lattice consequently bringing in desired improvements in physical properties of Sr0.82RE0.18Fe12-xAlxO19 ferrite.

Impact of thermal oxidation on chemical composition and magnetic properties of iron nanoparticles

The main objective of this work is to study the influence of thermal oxidation on the chemical composition and magnetic properties of iron nanoparticles which were manufactured in a simple chemical reduction of Fe3+ ions coming from iron salt with sodium borohydride. The annealing processing was performed in an argon atmosphere containing the traces of oxygen to avoid spontaneous oxidation of iron at temperatures ranging from 200 °C to 800 °C. The chemical composition and magnetic properties of as-prepared and thermally-treated nanoparticles were determined by means of X-ray diffractometry, Raman spectroscopy, Mössbauer spectroscopy and vibrating sample magnetometry. Due to the magnetic interactions, the investigated iron nanoparticles tended to create the dense aggregates which were difficult to split even at low temperatures. This caused that there was no empty space between them, which led to their partial sintering at elevated temperatures. These features hindered their precise morphological observations using the electron microscopy techniques. The obtained results show that the annealing process up to 800 °C resulted in a progressive change in the chemical composition of as-prepared iron nanoparticles which was associated with their oxidation. As a consequence, their magnetic properties also depended on the annealing temperature. For instance, considering the values of saturation magnetization, its highest value was recorded for the as-prepared nanoparticles at 1 T and it equals 149 emu/g, while the saturation point for nanoparticles treated at 600 °C and higher temperatures was not reached even at the magnetic field of about 5 T. Moreover, a significant enhancement of coercivity was observed for the iron nanoparticles annealed over 600 °C.

Coercivity enhancement in Nd-Fe-B magnetic powders by Nd-Cu-Al grain boundary diffusion

A significant enhancement in coercivity of Nd7.92Pr2.20Fe83.91ZrxB5.57Al0.4−x powders was achieved by Nd80Cu10Al10 grain boundary diffusion. The room temperature coercivity was drastically increased from 7.4 kOe to 23.14 kOe by the diffusion process with the content 30% of Nd80Cu10Al10 alloy. Nd-Cu-Al alloy diffusion process resulted in grain growth and formed continuous grain boundary phases. The boundary layers were very effective in pinning the motion of domain walls, leading to an increased coercivity. In addition, an increased coercivity was also obtained from 3.1 kOe to 9.8 kOe at 175 °C in the bonded magnets with the diffusion of 30% content NdCuAl alloy. So the thermal stability of the bonded magnets was improved.

Significant coercivity enhancement of hot deformed NdFeB magnets by doping Ce-containing (PrNdCe)70Cu30 alloys powders

Coercivity enhancement of hot-deformed NdFeB magnets made from commercially available NdFeB powders doped with Ce-containing alloys is demonstrated. By doping (Pr71Nd27Ce2)70Cu30 powders, where Pr71Nd27Ce2 is a commercial mischmetal, the hot-deformed magnets exhibit significant enhancement of coercivity from 15.0kOe to 19.0kOe, and it can reach 20.1kOe by doping higher Ce-content (Pr71Nd9Ce20)70Cu30 powders, yet sustains high energy product. The magnetic isolation effect with Ce-containing phase in the grain boundary and the microstructure refinement lead to coercivity enhancement. This study provides an economic way to enhance coercivity of hot deformed NdFeB magnets without using heavy rare earth.

Effect of Rare-Earth Dopant on Coercivity of Hot-Pressed Nd–Fe–B Magnet Doped With RF3

An RF3 (R = La, Ce, Pr, Nd, and Dy)-doped Nd13.6Fe73.6Co6.6Ga0.6B5.6 hot-pressed magnet was prepared using melt-spun flakes. The substitution of Nd in Nd–Fe–B flakes by rare-earth dopant in the hot-pressed magnets doped with various RF3 and its effect on coercivity were investigated. In the RF3-doped Nd13.6Fe73.6Co6.6Ga0.6B5.6 magnet hot pressed at the modest temperature (735 °C), the dopants Ce, Pr, Nd, and Dy actively substituted Nd in the region near the flake surface to form the thin shell in the flake. RF3 (R = Dy, Nd, and Pr) doping achieved significant coercivity enhancement in the hot-pressed magnet, and it was attributed to the enhanced anisotropy field of the Nd2Fe14B-type grains (for Dy and Pr doping) and grain boundary restructuring (for Nd doping) in the shell. The most profound coercivity enhancement as high as 5 kOe was achieved in the DyF3-doped magnet. Doping with the light rare-earth elements using RF3 (R = La and Ce) led to poor coercivity enhancement with respect to the RF3 (R = Dy, Nd, and Pr) doping.

High coercivity in rare-earth lean nanocomposite magnets by grain boundary infiltration

A significant enhancement in coercivity was achieved by grain boundary modification through low temperature infiltration of Pr75(Cu0.25Co0.75)25 eutectic alloy in rare-earth lean (Pr/Nd)–Fe–B/α-Fe nanocomposite magnets. The infiltration procedure was carried out on ribbons and hot-deformed magnets at 600–650°C for different time durations. In Nd2Fe14B/α-Fe ribbons, the coercivity increased from 5.3 to 23.8kOe on infiltration for 4h. The Pr2Fe14B/α-Fe hot-deformed magnet shows an increase in coercivity from 5.4 to 22kOe on infiltration for 6h. The increase in the coercivity comes at the expense of remnant magnetization. X-ray diffraction studies confirm the presence of both the hard Nd2Fe14B and soft α-Fe phases. A decrease in the soft α-Fe phase content was observed after infiltration.

Origins of the significant improvement in nanocrystalline Samarium–Cobalt’s magnetic properties when doping with Niobium

SmCo7-based compounds are prototypical rare-earth based hard magnetic materials that are the backbone of numerous devices and applications. We have found that by doping the SmCo7 phase with Niobium (e.g. SmCo6.7Nb0.3), the enhanced coercivity being measured. Using a combination of microstructural and compositional characterization techniques with ab initio calculations, we find that the Nb-doping stabilizes the nanocrystalline nature of the material by residing at the 2e site of the SmCo7 structure, decreases the average grain size, and substantially increases the magnetocrystalline anisotropy constant (K) (e.g. from 1.25×107erg/cm3 for SmCo7 to 1.40×107erg/cm3 for SmCo6.7Nb0.3). A significant enhancement in coercivity with Nb doping is attributed to the increased K and the reduced grain size.

Complex exchange anisotropy behavior in Co3O4–Ni0.6Zn0.4Fe2O4 composite with different Co3O4 content

Co3O4–Ni0.6Zn0.4Fe2O4 composites with different Co3O4 content have been synthesized by the hydrothermal combining with sol–gel methods. Exchange bias and significant coercivity enhancement have been observed around and below Néel temperature, TN, of Co3O4 at certain Co3O4 content. Interestingly, the exchange anisotropy does not vanish at a much higher temperature of 200K above TN. The exchange anisotropy behaves complexly with Co3O4 content at different temperatures, which suggest that exchange anisotropy are affected by not only the coupling between ferrimagnetic and antiferromagnetic phases but also the interfacial magnetic states.

Giant exchange bias and exchange enhancement observed in CoFe2O4-based composites

Precursor powders, containing mainly CoFe2O4 and Co(OH)2, have been hydrothermally synthesized and then annealed in either air to obtain a ferrimagnetic CoFe2O4/antiferromagnetic Co3O4 composite, or H2 ambient to reduce the precursors to Co/Fe alloy. Giant exchange bias (HE=2233Oe) and significant coercivity enhancement (HC=10153Oe) were observed below the Néel temperature of Co3O4 in samples annealed in air, resulting from the exchange coupling between ferrimagnetic CoFe2O4 and antiferromagnetic Co3O4 phases. By reducing precursor powders in H2 ambient, a solid solution of Fe and Co (Co3Fe7) was obtained; the average atomic moment for Co3Fe7 is 2.4μB, larger than that of separated Fe or Co. We also show that the magnetic parameters can be sensitively tuned by varying processing conditions, which may be useful for obtaining the desired magnetic performance.