High Intrinsic Coercivity Research Articles

High coercivity nanocrystalline YCo 5 powders produced by mechanical milling

High coercivity nanostructured YCo 5 powders were successfully prepared by mechanical milling of as-cast alloys and subsequent vacuum annealing. Almost single phase YCo 5 alloys, obtained by arc melting, were processed by high energy mechanical milling using a SPEX 8000 mill. After 4 h of milling, powders become nearly amorphous. DSC scans revealed the existence of an irreversible broad exothermic transition with a maximum at 516 °C associated with the crystallization process. Annealing in high vacuum at 800 °C during 2.5 min led to the formation of YCo 5 nanoparticles with an average particle size of 12 nm. A high intrinsic coercivity of 7.23 kOe together with a σ r/ σ s ratio of 0.75 were obtained.

Plasma sprayed Nd–Fe–B permanent magnets

This study demonstrated that the plasma spray deposition method is an alternative process for producing Nd–Fe–B magnets in addition to the two existing principal processes: the powder metallurgy process for producing sintered Nd–Fe–B magnets and the melt spinning process for bonded Nd–Fe–B magnets. Plasma spray is a potentially better process for producing magnetic parts with complicated shape, large area, thin thickness, small dimension, or unusual geometry. High intrinsic coercivity greater than 15 kOe was readily obtained for Nd16Dy1Fe76B7 even in the as-deposited condition when the substrate was preheated. The plasma spray process contains only three steps: melting, crushing, and plasma spray, which is much simpler than the powder metallurgy and melt spinning processes. Without preheating the substrate, the coercivity was usually very low (∼0.1 kOe) in the as-deposited condition and it increased to 10 to >15 kOe after anneal. Evidence of magnetocrystalline anisotropy was observed in plasma sprayed Nd15Dy1Fe77B7 magnets when the substrate was not preheated. It is believed that a crystal texture was developed during the plasma spray as a result of the existence of a temperature gradient in the solidifying melt.

Magnetization study of a UIrAl single crystal

The field and temperature dependence of the magnetization has been studied for the first time on a single crystal of the hexagonal intermetallic UIrAl. The compound is a ferromagnet below the Curie temperature TC=62 K with a ground-state spontaneous magnetic moment of Ms=0.96 μB. The huge uniaxial anisotropy (the anisotropy field Ba exceeds 200 T) leads to characteristic hysteresis properties which obey the model of high intrinsic coercivity of narrow domain walls

Magnetic properties of Sm- and Cu-doped oriented SmCo5 ribbons prepared by melt spinning

Oriented melt-spun ribbons with composition Sm1+Y(Co1−XCux)5 (X=0–0.3 and Y=0–0.2) were fabricated with a wheel speed of 5 m/s, followed by annealing in the temperature range of 600–1000 °C for 30 min. Our results show that a high degree of texture, in which the c axis is parallel to the longitudinal direction of the ribbons, is obtained in all above ribbons. The room temperature intrinsic coercivity can be significantly enhanced by both Sm additions and Cu substitution and coercivities of 21 and 12.4 kOe an achieved in Sm1.15Co5 and Sm(Co0.8Cu0.2)5 ribbons, respectively. Compared with the Sm-doped ribbons, Cu-doped ribbons exhibit a better thermal stability of coercivity and a high intrinsic coercivity of 5.2 kOe can be maintained at 300 °C in Sm(Co0.8Cu0.2)5 ribbons. Further analysis indicates that the coercivities of Sm- and Cu-doped ribbons are determined by different demagnetization mechanisms.

Structure and magnetic properties of the Sm(Co0.68−xFe0.25Cu0.07Zrx)7.7 melt-spun alloys with Zr additions

The effects of Zr additions on the Sm(Co,Fe,Cu,Zr)z compound have been studied. Melt-spun ribbons with a composition of Sm(Co0.68−xFe0.25Cu0.07Zrx)7.7 (x=0, 0.02, 0.04, 0.07, 0.10, and 0.15) were prepared with a wheel speed of 45 m/s, followed by a vacuum heat treatment in the temperature range of 650–800 °C for 5 min. X-ray diffraction (XRD) results showed that the as-spun ribbons of Sm(Co0.68−xFe0.25Cu0.07Zrx)7.7 have a primary TbCu7-type phase. When the annealing temperature was increased, grain sizes in the ribbons were increased according to reduced XRD peak widths of the TbCu7 phase. A Co23Zr6 compound was observed in the heat-treated ribbons when the Zr content exceeded 0.04. Saturation magnetization of the ribbons was linearly decreased from 95 to 45 emu/g (under a magnetic field H=18 kOe), while the Zr content was increased from 0 to 0.15. However, intrinsic coercivity of the ribbons was significantly dependent on grain size which was associated with both the Zr contents and heat-treatment temperatures. The highest intrinsic coercivity, Hci=13 kOe, was obtained in the ribbons with Zr content of 0.10 and heat treatment at 650 °C. With a maximum energy products up to 9.5 MGOe amd Br of 7.4 kG in the heat-treated ribbons with a suitable Zr addition is considered as promising magnets for high temperature applications.

Magnetic pinning strength for the new Sm-TM magnetic materials for use up to 550 °C

A new series of Sm(CowFevCuxZry)z alloys has been developed to produce magnets with high intrinsic coercivity Hci at high temperatures for use up to 550 °C. The maximum use temperature is defined as TM, which is the maximum temperature at which the extrinsic demagnetization curve is a straight line. An important feature of the alloys with high TM is a lower temperature coefficient of Hci, β. These magnets have a higher resistance to demagnetization from increasing thermal agitation which occurs with increasing temperature. A study on magnetic pinning field Hp, which will be explained in the text, vs TM was conducted by measuring the initial magnetization curves of the magnets at 25 and 300 °C. The study shows that as TM increases, Hp increase at high temperatures. At 25 °C, all magnets with TM from 250 to 550 °C have Hp higher than 24 kOe, which is too high to be determined using a hysteresigraph. At 300 °C, the magnet with TM=250 °C has Hp=7.0 kOe, and the magnet with TM=550 °C has Hp=15.1 kOe. Higher pinning field at high temperatures (⩾300 °C) provides a greater resistance to thermal demagnetization, which leads to better performance at high temperatures.

Magnetic and microstructure studies of boron-enriched (Nd/sub 0.95/La/sub 0.05/)/sub 11/Fe/sub 76.5$/ -/sub x/Co/sub x/Ti/sub 2/B/sub 10.5/ (x=0-15) melt-spun ribbons

The magnetic properties and microstructures of rare earth lean and boron-rich (Nd/sub 0.95/La/sub 0.05/)/sub 11/ Fe/sub 76.5-x/CoxTi/sub 2/B/sub 10.5/ (x=0-15) melt-spun ribbons have been investigated. Two magnetic phases, namely /spl alpha/-Fe and R/sub 2/Fe/sub 14/B, were found, by thermal magnetic analysis (TMA), in all the ribbons studied. High intrinsic coercivity (iHc) and maximum energy product [(BH)max] values in the range of 14-18.7 kOe and 13.9-18.3 MGOe, respectively, have been achieved in these nanocomposites. Among compositions studied, the Co-substitutions, x/spl les/10, were found to be effective in increasing the Br and (BH)max, while keeping a high value of iHc. It was evidenced from transmission electron microscopy that the homogeneity, fine grain size, and high volume fraction of the R/sub 2/Fe/sub 14/B phase led to an increase in the iHc and maximum energy product of nanocomposites studied. The optimum magnetic properties of Br=9.4 kG, iHc=16.1 kOe and (BH)max=18.3 MGOe have been obtained on (Nd/sub 0.95/La/sub 0.05/)/sub 11/Fe/sub bal./Co/sub 10/Ti/sub 2/B/sub 10.5/ ribbons after an optimum treatment.

New rare-earth permanent magnets with an intrinsic coercivity of 10 kOe at 500 °C

Sm ( Co bal Fe 0.1 Cu x Zr 0.033 ) z magnets with higher Cu content were found to have higher intrinsic coercivity Hci at both room temperature and higher temperatures up to 500 °C. For the magnets with a ratio z=8.5, the intrinsic coercivity reaches a maximum of about 40 kOe at room temperature when the Cu content x reaches 0.088. But for the magnets with a ratio z=7.0, the intrinsic coercivity continues to increase with increasing Cu content x up to 0.128. Microstructure analysis showed that the cell size decreases while the density of lamella phase increases with increasing Cu content. A record high intrinsic coercivity of 10.8 kOe at 500 °C was obtained on a Sm(CobalFe0.1Cu0.128Zr0.033)z magnet.

Effect of iron on the high temperature magnetic properties and microstructure of Sm(Co, Fe, Cu, Zr)z permanent magnets

We have fabricated a new magnet with nominal composition of Sm(CobalFe0.1Cu0.078Zr0.033)8.3, which has a room temperature intrinsic coercivity Hci of 40 kOe and a saturation magnetization Ms of 92 emu/g. The new magnet has a temperature coefficient of intrinsic coercivity β(RT−773 K)=−0.15%/K and a high intrinsic coercivity of 10.7 kOe at 773 K.

Preparation of Nanoparticles of Barium Ferrite from Precipitation in Microemulsions

Magnetic nanoparticles of barium ferrite (BaFe12O19) have been synthesized using a microemulsion mediated process. The aqueous cores of water-in-oil microemulsions were used as constrained microreactors for the precipitation of precursor carbonate and hydroxide particles. These precursors were then calcined at 925°C for 12 h, during which time they were transformed to the hexagonal ferrite. The pH of reaction was varied between 5 and 12, and it was found that the fraction of non-magnetic hematite (α-Fe2O3) in the particles varied with the pH of reaction, thus affecting the magnetic properties of the particles. The same precursor particles were also prepared by bulk co-precipitation reaction for comparison. It was found that the microemulsion derived nanoparticles of barium ferrite had both higher intrinsic coercivity (Hc) and saturation magnetization (σs) than the particles derived from bulk co-precipitation. Particles were analyzed by electron microscopy, X-ray diffraction, differential thermal analysis (DTA), thermogravimetric analysis (TGA) and vibrating sample magnetometry (VSM). The best barium ferrite particles produced by the microemulsion synthesis method yielded an intrinsic coercivity of 4310 Oe and a saturation magnetization of 60.48 emu/g.

Treatment of Strontium Hexaferrite Powder Synthesized Conventionally to Produce High Coercivity

Strontium hexaferrite powder, produced conventionally from strontium carbonate and iron oxide has been treated in nitrogen and hydrogen atmospheres and then calcined in air. Magnetic measurements after the gas treatment stage indicated a marked decrease of intrinsic coercivity and an increase in saturation magnetisation. During the calcination stage there was a recovery of the magnetic properties. The material now exhibited a remanence and saturation magnetisation close to that of the starting hexaferrite, but a very much higher intrinsic coercivity. The highest coercivity obtained was around 400 kA/m, which is very high for a hexaferrite powder, particularly one synthesized conventionally, without extensive milling. The high coercivity was attributed to a much finer grain size.

Synthesis, magnetic properties and formalism of magnetic properties of high-quality refined Nd2Fe14B powders for permanent magnet devices

Stable Nd2Fe14B powders of refined grain size of 0.1–1.0 μm were prepared using a combination of the rapid quenching (of the melt into thin ribbons), mechanical attrition and grain-surface passivation (or surface hardening) and coating by a thermally rigid, adhesive and corrosion-proof material in air. The ribbons (of 15–30 μm thickness) were cut, crushed and milled under H2 gas at approximately 1 bar and room temperature to give hydrided Nd2Fe14BHx, x≲5, flakes of 1–5 μm sizes, which are brittle and easily obtained in powder form by high-energy ball milling. The interstitial H atoms in the hydride sample were desorbed by slowly heating (5°C min-1) the sample between 25 and 600°C in N2 gas (which helps the desorption of the H atoms without decomposition of the sample) in a reactor and then pumping off the total gas at 600°C. The H-desorbed specimen, when annealed at 600–800°C under a dynamic vacuum, results in a refined powder, showing a characteristically high remanence, Jr of 9–12 kG, together with a high intrinsic coercivity, Hci, of 10–28.3 kOe, depending on the size and surface structure of the grains. This powder is highly pyrophoric and catches fire in open air but can be stabilized by passivating and coating the grain surfaces with a mixture of carbon, AIN and Nd2O3 by milling the mixture in a suitable organic liquid (to allow the additives to adhere the sample without excess oxidation) followed by annealing at an elevated temperature in N2 gas at approximately 1 bar. In this process, the separated Nd2Fe14B grains acquire a thin nitride–carbide (probably amorphous) stabilized surface passivation layer which prevents further oxidation of the sample in air at room temperature. The passivation layer, in combination with a thin film of the Nd-rich intergranular phases, if any, peculiarly appears to be non-magnetic compared with the main ferromagnetic Nd2Fe14B phase. It keeps the ferromagnetic Nd2Fe14B grains separated and thus inhibits mixing between the local magnetic lines of forces confined to them. As a result, they behave like ideal single-domain particles and therefore exhibit a reasonably improved Hci value, without a significant decrease in the high Jr or the high saturation magnetization Ms which are useful for the high-energy-density magnets and related devices and components. The results are modelled and discussed with microstructures, magnetic properties, thermal stability and loss, if any, in the mass of the specimens during exposure to ambient atmosphere.

Synthesis of high-coercivity cobalt ferrite particles using water-in-oil microemulsions

Magnetic nanoparticles of cobalt ferrite (CoFe 2O 4) have been synthesized using water-in-oil microemulsions consisting of water, cetyl trimethyl ammonium bromide (surfactant), n-butanol (cosurfactant), and n-octane (oil). Precursor hydroxides were precipitated in the aqueous cores of water-in-oil microemulsions and these were then separated and calcined to give the magnetic oxide. X-ray diffraction confirmed the formation of phase pure cobalt ferrite. These nanoparticles were less than 50 nm in size and had a high intrinsic coercivity (1440 Oe) and saturation magnetization (65 emu/g).

Shock compaction of SmCo5 particles

Fine ferromagnetic particles (1–10 μm) of SmCo5 were compacted using shock pressures of 1.7–27 GPa (17–270 kbar). One-piece disks were produced with pressures in the range 2–9 GPa. X-ray-diffraction measurements revealed no changes in the composition of the compacted SmCo5 for shock pressures below 15 GPa. Magnetic properties of the shocked samples were measured with a superconducting quantum interference magnetometer. In the pressure range 2–9 GPa, the shocked specimens exhibit high intrinsic coercivity Hci. Below 2 GPa, lower values of coercivity were observed, presumably caused by incomplete bonding of particles. Above 10 GPa the intrinsic coercivity drops suddenly and levels off at higher pressures. The remnant magnetization Br, exhibits a broad peak at 6 GPa and then falls off rapidly at higher pressures. The saturation magnetization Ms, has a peak near 10 GPa that correlates with the drop in coercivity. The hardness coefficient a, derived from a fit of the phenomenological ‘‘law of approach’’ formula to the hysteresis curve, shows a linear increase with pressure near 8 GPa. This suggests an increase of inclusions in the samples and correlates with the drop in Hci and the peak in Ms. There is a non-negligible linear contribution to the magnetization given by κH. The nonzero value of κ suggests the possibility of incomplete saturation or paramagnetic impurities. Maximum values in coercivity Hci, remnant magnetization Br, and saturation magnetization Ms are found at about 9 GPa.

Magnetic hardening of rapidly solidified SmFe 7+ xM x (0.8 ≤ x ≤ 1.5, M = Mo, V, Ti) compounds

Rapidly solidified SmFe 7 + x M x (M = Mo, V, Ti) compounds were found to crystallize in the Sm(Fe,M) 7 based magnetic phase up to x = 1.5. The Sm(Fe,M) 7 phases exhibit a high Curie temperature ( T c = 355 ° C) with a quite high intrinsic coercivity ( i H c = 3–7 kOe). Once formed during rapid soidification the Sm(Fe,M) 7 phase remains stable even after annealing. The maximum energy product of 5 MG Oe was obtained from the as-spun SmFe 8.5V 1.5 compound. A remanence ( B r) of 8.0 kG for the as-spun SmFe 7.8Ti 0.3 and the highest value of intrinsic coercivity ( i H c) of 7.0 kOe could be achieved for SmFe 8.5Ti 1.5 after heat treating. The enhanced Curie temperature is attributed to an extended solid solubility of the additive transition element in the Sm(Fe,M) 7 cell causing a volume expansion of about 4.7%–6.7%.

Mössbauer spectrometric study of ternary Sm2Fe17Nx(x>3)

Among the ternary rare earth-iron-nitrogen compounds, Sm2Fe17Nx, invented and refined by Asahi Chemical Industry Co., Ltd., shows high magnetic anisotropy, intrinsic coercivity and high saturation magnetization.

CoSm-based high-coercivity thin films for longitudinal recording

In order to produce thin media with high intrinsic coercivity, we studied a number of CoSm films produced by rf-diode sputtering. We determined an optimized set of sputtering parameters for films sputtered both on glass and NiP-coated Al substrates with and without Cr underlayers. While the coercivity of the films depended strongly on argon pressure, the 〈110〉 texture of the Cr underlayer remained unaffected by it. The 〈110〉 texture of Cr changed to 〈200〉 texture at temperatures ≥300 °C, and the coercivity of the film was significantly reduced. Under optimal conditions we obtained intrinsic coercivities ≳2400 Oe for films with thickness <20 nm.

Microstructure and Coercivity of Rapidly Quenched ND-FE-B Magnets

ABSTRACTRapidly quenched Nd2Fe14B-based permanent magnets owe their high intrinsic coercivity Hci to their unique submicron-sized grain structure. While no clear consensus has yet emerged regarding the microscopic origin of coercivity in these fascinating materials, magnetic and structural data suggest pinning of magnetic domain walls at or in an intergranular phase between Nd2Fe14B grains as the most likely mechanism. The small, randomly oriented grains in isotropic melt-spun ribbons produce a magnetic structure wherein each Nd2Fe14B grain is a single magnetic domain, with the domain walls expelled to the grain boundaries. The magnetic interaction between neighboring grains plays an important role in the magnetization reversal of individual grains, as revealed, for example, by detailed examination of the initial magnetization and demagnetization processes. The formation of large domain structures via these interactions is frustrated by the random grain orientation in ribbons, but short range magnetic correlations between grains (“interaction domains”) are observed locally.The oriented grains in aligned die upset magnets produce a very different magnetic structure having large scale coherent domains which extend through many grains. A variety of experiments, including studies of the temperature dependence of Hci, suggest that coercivity arises from pinning of the extended domain walls at Nd2Fe14B grain edges.

Mechanism of intrinsic coercivity of melt-spun Nd-Fe-B bonded magnet

The microstructures of melt-spun Nd13.5Fe81.74B4.76 ribbons have been investigated by means of X-ray diffraction, Mössbauer spectroscopy and HREM. Our experiments show that optimally magnetic properties of the bonded magnet can be obtained by using melt-spun Nd13.5Fe81.74B4.76 ribbons at a single phased Nd2Fe14B crystallites, and the second phases (such as rich-Nd, rich-B or α-Fe phase) are absent. We consider that the high intrinsic coercivity iHc can be attributed to the single-domain mechanism of Nd2Fe14B crystallite phase.

Temperature dependence of anisotropy field in Co-Ti substituted Ba-ferrite particles

Co-Ti substituted Ba-ferrite particles were prepared, and the crystalline and shape anisotropy energies were measured in the temperature range of 78 K to 400 K to determine the crystalline anisotropy field of the particles. The crystalline anisotropy energy and the crystalline anisotropy field at room temperature were about 0.67*10/sup 6/ erg/cm/sup 3/ and about 4700 Oe, respectively. A similar temperature dependence was observed between the effective anisotropy field and the coercivity. This and a high intrinsic coercivity indicate coherent rotation of the magnetic moment in single-domain particles. >