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An anomalous magnetic behavior has been observed in rare-earch-rich metallic glasses of the composition ${({R}_{75}{\mathrm{Au}}_{25})}_{100\ensuremath{-}x}{\mathrm{B}}_{x}$ where $R$ denotes Pr, Gd, Tb, Er and $x=0,10$. The observed hysteresis in all samples except Gd at liquid-helium temperatures is due to the large anisotropy which develops below the ordering temperature. High-field-magnetization measurements up to 220 kOe were used to determine the spontaneous magnetization and the magnetic anisotropy constant which was found to be of order ${10}^{8}$ erg/${\mathrm{cm}}^{3}$ for the Tb and Er glasses. These parameters were used together with the observed remanence ratio to examine the high coercive fields in terms of recently proposed models of random anisotropy.

The magnetic properties of various commercial Fe-Nd-B magnets have been measured and compared with their microstructure and magnetic domain structure. The coercivity is found to be strongly dependent on both the applied field and temperature. Remanent magnetization curves indicate two possible domain wall pinning mechanisms. The microstructure of all magnets studied is very similar showing the predominant Fe 14 Nd 2 B grain in coexistence with some Nd-rich phases and the tetragonal Fe 4 NdB 4 phase. The higher coercivities observed in Neo30H are attributed to the presence of a small amount of Dy in this magnet leading to higher anisotropy fields.

We present magnetic and electrical properties of amorphous rare-earth alloys of composition (R75Au25)100−xBx (R = Pr, Gd, Tb, Er and x = 0, 10). Low-field susceptibility, electrical resistivity and high-field magnetization measurements were made between 1.6 and 300 K. Resistivity measurements show both positive and negative temperature coefficients and in some cases maxima and minima that are associated with magnetic ordering and Kondo-like scattering respectively. For all the samples except those containing Gd, the local random anisotropy produces asperomagnetic ordering and high coercive fields. Large magnetic after-effects have been observed down to liquid helium temperatures particularly for the Tb glass. Metamagnetic effects have been found in (Er75Au25)90B10.

Fine particles of cobalt-boron (Co-B) were produced by chemical reduction of aqueous solutions of cobalt chloride with sodium borohydride. The chemical composition, particle size, magnetic properties, and crystallization behavior have been studied as a function of NaBH4 concentration. The particle size was in the range of 200–1000 A. Chemical analysis showed the B content to be in the range 32–40 at%. The initially reduced powder was amorphous having very low magnetization and negligible coercivity. The particles were gradually crystallized by varying the annealing temperature. The saturation magnetization Ms for the annealed samples was in the range of 70–135 emu/g and the maximum coercivity Hc obtained was 2000 Oe. X-ray diffraction results indicated that the reason for the drastic change in magnetic properties is the formation of crystalline phases upon heating which included mostly Co3B together with Co2B and Co.

The effect of particle size on the magnetization reversal in ferrite magnets has been examined by correlating the hard magnetic properties with the microstructure. Initial magnetization curves, field and temperature dependence of coercivity and the remanence curves were obtained and compared with the predictions of theoretical models. The coercivity of bulk samples can be explained by the nucleation model. In ultrafine particles the coercivity is very close to the value predicted for single domain particles while the larger particles show a behavior which is a mixture of both the bulk magnets and ultrafine particles behavior.

Amorphous alloys with composition Nd100-xMx where M=Co or Fe and 25

Etude de Nd 15 Fe 77 B 8 , SmCo 5 , Sm 2 (Co, Fe, Cu, Zr) 17 , des aimants a ferrite de strontium et de particules monodomaines de ferrite de Ba et de Cu-Mn-Al. Mesure des courbes d'aimantation initiale, de la variation avec le champ de la coercivite et des courbes de remanences

On presente des mesures thermomagnetiques en courant continu sur des aimants permanents Fe-Nd-B commerciaux ainsi que sur des alliages rapidement refroidis pour illustrer quelques proprietes magnetiques. Caracteristiques entre 30 et 900 C. Etudes preliminaires par spectrometrie Auger sur l'homogeneite de composition et sur les effets d'oxydation pendant le chauffage des aimants commerciaux surtout Neomax

A new magnetic phase has been observed in as-cast and melt-spun Nd-Fe and Pr-Fe alloys with a Curie temperature around 265°C. SEM studies show the presence of this phase in the form of spherical and elongated particles 5 μ m in size with a composition having a ratio of Fe/Nd=4:1. This phase is believed to be a ternary Nd-Fe-O phase stabilized with oxygen. The phase appears to have a high anisotropy leading to coercivities of about 6 kOe in as-cast samples at room temperature. The Mossbauer spectra of this phase can be fitted to four Fe sites with magnetic moments ranging from 1.7 to 2.54μ B .

The magnetic domain structure of R-Fe-B alloys has been examined in several alloys from the less anisotropic Y-Fe-B to the most anisotropic Tb-Fe-B. In samples with larger grains the domain walls are observed to be pinned at grain boundaries. When the size of the grains is very fine, some very large domains are observed to coexist together with the much finer single domains. These large interaction domains are formed because of strong interactions between spins of adjacent domains.

A.c. and d.c. magnetization measurements have been performed in the temperature range 4.2–300 K on amorphous Fe90 −xRxZr10 alloys, where R ≡ La, Pr, DyorGdandx = 0.3–6. The magnetic phase diagrams have been obtained and a double-transition behavior has been observed, from which the higher temperature transition has been defined as paramagnetic to ferromagnetic, and the lower temperature transition as ferromagnetic to a “spin-glass-like” state. The results obtained show consistently that the above alloys have a mictomagnetic behavior below the lower temperature transition.

The hard magnetic properties of Fe-R-B powders have been examined using various magnetic techniques. High coercivities were obtained in fine powders with a particle size around 1 μ m. A domain wall pinning process is suggested for the less anisotropic, Nd-containing samples where the domain walls are pinned at the particles surface. The initial magnetization curves, field dependence of coercivity and remanence curves of thermally demagnetized samples change significantly after ac and dc demagnetization indicating a more uniform domain wall pinning process in the ac and dc demagnetized samples. A magnetization rotation reversal process is suggested for the more anisotropic, heavy rare-earth containing powders.

The validity of nucleation and domain wall pinning models has been examined in R‐Fe‐B magnets by correlating the hard magnetic properties with the microstructure and magnetic domain structure. The nucleation model can be used to explain the magnetic properties of R‐Fe‐B magnets at low temperatures, but it fails at high temperatures. On the other hand, domain wall pinning can explain the magnetic properties over a much wider temperature range (up to the Curie temperature). The magnetic and structural data suggest that the coercivities (Hc) of powders and stoichiometric melt‐spun ribbons are due to ‘‘strong pinning’’ by a random distribution of pins while the Hc of sintered magnets and ribbons with off‐stoichiometric composition are due to ‘‘localized pinning’’ at grain boundaries.

The magnetic properties of rapidly quenched FeRM alloys where R=La,Y,Pr,Nd,Gd and M=B,Si,Al,Ga,Ge have been investigated over a wide range of chemical compositions. The samples are generally magnetically soft in the as‐quenched state. Magnetic hardening is produced by annealing the samples around 700 °C. The best properties have been obtained in samples containing Pr and Nd together with B and Si. An energy product of 13 MGOe and a coercive field of 15 kOe have been obtained in a Fe76Pr16B5Si3 sample. The higher Fe content samples appear to be more promising with a potential energy product of 49 MGOe. Thermomagnetic data show that a structural transformation takes place upon heating the samples to 700 °C. The Curie temperature of the as‐quenched phase is around 160 °C while that of the new phase is around 320 °C. Transmission electron microscope studies show fine precipitates (∼100 A) dispersed in a matrix of different chemical composition. X‐ray and electron diffraction data indicate that the precipitates have the Fe21R3B tetragonal structure. The high anisotropy of this phase together with its fine size and distribution give rise to the observed high coercive fields.

Results of resistivity, magnetization and susceptibility measurements are reported on U66T34 where T denotes Fe, Co, or Ni. Magnetic susceptibility data on each of the alloys displayed thermomagnetic-history effects indicating some spin-glass like character. The high-field magnetization data showed a significant non-linearity indicating a contribution to the magnetization from a cluster term. The evidence suggests that these alloys form cluster glasses in which only those T atoms whose local environment is sufficiently dense in T atoms bear a stable magnetic moment.

The magnetic and structural properties of rapidly quenched (Pr80Ga20)100-xFex, RTbFe and RTbFe(Co)M alloys are examined over a wide range of chemical compositions, where R ≡ Pr, Sm, MM and M ≡ B and Si. The Ga-containing samples show relatively high coercive fields (up to 3 kOe) in the amorphous state which subsequently disappear after crystallization. On the other hand, the high coercive fields (≈5 kOe) of melt-spun RTbFe samples decrease slightly after crystallization but their magnetic moment increases substantially. Melt-spun RTbFe(Co)M samples are generally magnetically soft in the as-quenched state. Magnetic hardening is produced by annealing the samples around 750°C leading to coercive fields which could not be measured with an ordinary electromagnet (Hc #62; 23 kOe). The best properties have been obtained on a Pr14Fe71B15 sample with a coercive field of 8 kOe and an energy product of 8.5 MGOe. Thermomagnetic data show that a structural transformation takes place upon heating the samples to 750°C. The Curie temperature of the precipitate phase is around 320°C while that of the as-quenched phase is around 160°C. Transmission electron microscope studies show a very fine precipitate structure with a precipitate size below 100 A. The precipitate phase is believed to be highly anisotropic leading to the observed hard magnetic properties.

The origin of high coercivities in as‐cast RFe2Ni2B alloys is investigated by correlating the magnetic properties with the microstructure. Hysteresis loop measurements showed high coercivities in samples containing Sm and Er. In SmFe2Ni2B the coercivities could not be measured in a field of 80 kOe at 4.2 K but they decreased to 56 kOe at 50 K. The coercivities are lower in ErFe2Ni2B, reaching the value of 38 kOe at 4.2 K. Electron diffraction studies showed a multiphase structure with a majority phase having a crystal structure of CeCo4B type. In SmFe2Ni2B, many stacking faults were observed which may explain the high coercivities.

The magnetic and structural properties of quaternary RCo4−xFexB alloys with R=Nd, Sm, Er have been examined with magnetometry and x‐ray diffraction. The CeCo4B‐type phase is found for all x in Er, for x≤3 in Sm, and for x=0, 1, and 2 in Nd. Maximum coercivities have been obtained in a crystallized SmFe2Co2B sample with Hc>17 kOe at room temperature.

The hard magnetic properties of melt-spun and sintered Nd-Fe-B with small substitutions of Al and Dy have been investigated in the temperature range of 4.2-600 K. High temperature magnetic measurements do not show any clear evidence of a secondary pinning phase with a curie temperature at around 150°C. Low temperature measurements show a shoulder in the demagnetization curve below the spin reorientation temperature which is largest for the isotropic melt-spun samples.

The hysteresis and crystallization behavior of an amorphous Nd54Co36B10 alloy have been examined over a range of temperatures. Crystallization studies show a glass transition temperature around 210 °C, two crystallization peaks at around 220 and 360 °C, and two endothermic peaks at around 530 and 600 °C, respectively. ac susceptibility measurements show an ordering temperature of 38 K for the amorphous phase. Four additional magnetic phases have been observed in the crystallized samples with ordering temperatures 17, 45, 90, and 190 K, respectively. Magnetic measurements on melt‐spun ribbons reveal a coercivity of 6 kOe at 4.2 K. The coercivity at 4.2 K of a sample heat treated at the first crystallization temperature is increased to 8 kOe but is reduced gradually upon annealing at higher temperatures and is only 2.5 kOe in a sample annealed at a temperature close to the second endothermic peak.

The magnetic properties of Fe77R15B8 powders have been measured as a function of particle size in the temperature range 77–300 K. The coercivity is found to increase first with particle size to a maximum value and then to decrease again. This behavior is explained with the single domain particle theory. The peak values of coercivity obtained in the various alloys are found to scale with the anisotropy field. The temperature coefficients of coercivity are smaller than those observed in sintered magnets.

Magnetic hysteresis and crytallization studies are reported for several (Pr80Ga20)100−xFex metallic glasses, where 30≤x≤80. Thermomagnetic data show the presence of two magnetic phases which are probably responsible for the relatively high coercive fields observed at cryogenic temperatures. As the iron content of the sample is increased, the magnetic moment increases substantially but the coercivity is drastically reduced. In (Pr80Ga20)70Fe30 the observed coercive fields are strongly temperature dependent indicating strong thermal activation effects which are consistent with the magnetic viscosity measurements. The hard magnetic properties disappear after crystallization and the samples appear to be paramagnetic at room temperature.

Large coercive fields have been observed in as-cast SmFe 4-x Ni x B alloys. The coercive fields increase drastically at lower temperatures and complete hysteresis loops cannot be measured at cryogenic temperatures. Microstructure studies show the presence of two types of grain with slightly different chemical composition (Fe-rich or Ni-rich) but with the same crystal structure. These differences in chemical composition may also occur at the microscopic level and can cause fluctuations in exchange and magnetic anisotropy that can pin the domain walls.

The magnetic and structural properties of melt‐spun Sm(FeT)12 alloys where T=Ti,V,Al,Cu,Ga,Si and with some containing small amounts of B and C have been studied. The samples are magnetically soft in the as‐quenched state. High coercive fields are developed upon heating the samples at temperatures ranging from 750 to 1050 °C. The tetragonal ThMn12‐type structure was found in all the alloys after annealing. The highest coercivities have been obtained on a Sm10Fe80Ti7.5B2.5 sample with Hc ≂7.7 kOe. These coercivities are much higher than those obtained previously and make these alloys good candidates for permanent magnet development.

Bulk Nd73Fe27 and Nd66.8Fe24.7Ti8.5 alloys exhibit relatively high coercivities with a room‐temperature value of about 5 kOe. The origin of the high coercivities has been investigated with magnetic and structural studies. Thermomagnetic data show the presence of a magnetic phase with a Curie temperature of about 270 °C which is close to that of NdFe2. This phase is metastable changing to a magnetic phase with an ordering temperature of 65 °C after heating to 700 °C. The lower Curie temperature phase is related to Nd2Fe17. The coercivity of the sample decreased to negligible values after the heat treatment. It is believed that the high coercivities are due either to the anisotropic NdFe2 phase or possibly to an oxygen‐stabilized Nd‐Fe‐O phase which is probably present in the form of fine particles.

The effects of sputtering conditions on the structure and magnetic properties of Nd2(Fe0.9Co0.1)14B films and multilayers are reported. The samples were sputtered onto temperature‐controlled mica and Ta substrates, at temperatures between 20 and 620 °C. X‐ray diffraction results indicate that films deposited directly onto 500–600 °C substrates with Fe buffer layers are nearly single‐phase materials with the c axis of the grains largely perpendicular to the film plane. Room‐temperature magnetization measurements to 17.4 kOe indicate that these films have strong perpendicular anisotropy with coercivities in excess of 10 kOe and magnetizations greater than 100 emu/g. Results for Nd2 (Fe0.9 Co0.1 )14 B/Fe multilayers show that films can be produced with either in‐plane or perpendicular anisotropy depending on the layer thicknesses. The Fe multilayers were found to have coercivities similar to those for the single layer films. Multilayers made with Ag were found to have significant interdiffusion and secondary...

The magnetic and structural properties of melt‐spun R8Fe84Ti8 with R=Nd, Sm, Dy, Gd and R8(Fe,Co)76[V(Mo)]16 with R=Nd, Sm have been studied with x‐ray diffraction, transmission electron microscopy, and magnetic measurements. The tetragonal ThMn12‐type structure was found in all alloys after annealing at about 700 °C. The Curie temperature (Tc) of the as‐spun amorphous samples was in the range of 40–100 °C. Crystallized Nd8(Fe1−xCox)84Ti8 samples with the 1:12 structure have a Tc around 250 °C for x=0, reaching the value of 660 °C for x=0.5 and then decreasing again to 490 °C for x=1. ac susceptibility studies on Co‐substituted samples showed magnetic transitions in the temperature range 100–200 K. The coercivity was found to be strongly dependent on the microstructure. The highest value of Hc (2 kOe) was obtained in Sm‐Fe‐Ti samples with a grain size about 500 A. The upper value of Hc is low because of the presence of some α‐Fe in crystallized ribbons.

Magnetic field and temperature dependences of the electrical resistivities and Hall resistivities were measured for the metallic glass ferromagnets Fe13Ni60Cr5Si10B12, Fe37Ni36Cr5Si10B12, Fe81B13.5Si3.5C2, and Fe5Co75Si15B5. Resistance minima and magnetoresistivity of the FeNiCr glasses have been found to be consistent with a modified Kondo model of low temperature scattering. The Hall resistivities are positive and large. The spontaneous Hall coefficients of the FeNiCr glasses are in good agreement with previous magnetization measurements on the same glasses.

Differential scanning calorimetry (DSC) has been employed to study the changes which occur in the powders used to prepare Fe‐R‐B permanent magnets upon heating. Three exothermic reactions are observed upon heating the powders. The correlation of DSC data with the magnetic properties of Fe‐R‐B powders shows that these reactions cause a significant deterioration of the hard magnetic properties of the resulting magnets. The most probable explanation of these exothermic reactions is the reaction between chemisorbed oxygen and rare‐earth metals to form bulk oxides. It appears that there is a critical oxygen concentration associated with a critical particle size which limits the development of hard magnetic properties.

The magnetic properties and microstructure of melt-spun iron-rich ternary alloys with the ThMn/sub 12/-type structure are reported. Rapidly quenched samples were prepared with composition Sm/sub x/Fe/sub y/T/sub z/, where T=Ti, V, Mo, Cu, Al, Si or a mixture of these, and 8 6.5 kOe. >

We report studies of high‐field magnetization and thermomagnetic effects in rapidly quenched and heat treated alloys based on Fe‐Pr and Fe‐Nd. Coercivities up to ∼40 kOe and large energy products result from the precipitation of a finely dispersed crystalline phase. Studies of varying the alloy composition and heat treatment are reported.

Thin films of Co100−xAlx with 15 30. After a heat treatment at 650 °C the films with x

The thermal and magnetic properties of amorphous alloys prepared by melt spinning and vapor deposition have been studied in the system Nd100−xCox where 25≤x≤75. It is found that the alloys around the equiatomic composition are the most stable thermally. ac susceptibility studies show that the ordering temperatures of the amorphous phase increase with cobalt content from about 30 K in Nd60Co40 to about 90 K in Nd50Co50. Upon crystallization of the samples four different phases appear with ordering temperatures 24, 105, 140, and 190 K. The phases with ordering temperatures of 24 and 105 K have been identified with Nd3Co and NdCo2, respectively.

The magnetic properties of splat‐cooled (Fe1−xNix)78–yCrySi10B12 Metglasses have been measured over the temperature range of 300–1000 K and the results correlated with measurements of sample microstructure. Thermomagnetic data indicate the presence of two magnetic phases with different Curie temperatures in the samples containing Cr. Transmission electron microscopy studies indicate that both phases are amorphous with different Fe concentrations. X‐ray studies reveal that crystallization of the amorphous samples occurs at temperatures around 720 K with the formation of a bct Fe3B and a fcc FeNi phase. The high coercive fields which exist from liquid helium temperatures to above the Curie point of the amorphous sample have been attributed to the heterogeneous magnetic structure. The increase of the coercive field (150 Oe) after crystallization has been attributed to the coexistence of two crystalline phases.

Results of magnetic susceptibility, high‐field magnetization, and Mossbauer effect are reported for several R‐Ga and R‐Ga‐Fe metallic glasses, where R = Pr and Tb. Magnetic ordering temperatures, effective moments, and coercive forces were determined. In certain cases the data indicate that the low temperature states of magnetic order are asperomagnetic or spin‐glass‐like. For (Pr80Ga20)80Fe20, several anomalies were observed including a large coercive field (45 kOe) at 100 K.

The magnetic properties of melt-spun R 2 Co 7- x Fe x B 3 with R = Sm, Nd and Pr 2 Fe 7 B 3 alloys habe been investigated. The as-spun samples are magnetically soft but they develop a high coercivity in the range of 10–30 kOe after a heat-treatment at around 700°C. The coercivity of Fe-rich samples is higher indicating a larger anisotropy of the R 2 (Co, Fe) 7 B 3 phase. The Curie temperature of this phase is in the range of 300–430°C. X-ray diffraction studies on crystallized samples showed a multiphase structure with the majority phase having the hexagonal Sm 2 Co 7B 3 -type structure.

The microstructure of two commercial 2:17 precipitation hardened rare-earth magnets has been examined and compared with the magnetic properties. Transmission electron microscope studies on Sm(Co,Fe,Cu,Hf) 7.25 , Hicorex 99C show a fine cellular microstructure with 2:17 rhombohedral cells surrounded by coherent 1:5 hexagonal cell boundaries. In Sm(Co,Fe,Cu,Zr) 7.22 , TDK REC-26 the cells are much coarser and 1:3 phase thin lamellae are superimposed on the cellular structure. X-ray energy dispersive analysis shows a higher Cu content in the cell boundaries. Lorentz electron microscopy shows "wavy" domain walls in both cases indicating that the domain walls are pinned at the cell boundaries. The higher Cu content of the cell boundaries dilutes the magnetic properties of the SmCo 5 phase and produces a large domain wall energy gradient across the cell boundary leading to the observed high coercivities.

The magnetic and structural properties of fine iron and iron‐oxide particles have been studied. The samples of iron oxide were prepared by the aerosol method and those of pure iron were prepared by the vapor deposition technique. Electron micrographs showed the size of aerosol particles to be 200 nm and that of vapor deposition particles to be 10–20 nm. The maximum coercivity at 10 K obtained for aerosol particles was approximately 700 Oe, and for vapor deposited particles it was 1540 Oe.

The magnetic and structural properties of rapidly quenched FePr(BSi) alloys are presented. The samples are usually magnetically soft in the as‐quenched state. However, large coercive fields in the range of 5–20 kOe are developed upon heating the samples to a temperature around 700 °C. The best properties have been obtained on a Fe76Pr16B5Si3 sample with a maximum energy product (BH)m≂12 MGOe. This value is much better than that of AlNiCo and makes these materials outstanding as cobalt‐free permanent magnets.

The magnetic and structural properties of melt-spun Fe-R-B alloys are reviewed. The hard magnetic properties of these alloys are attributed to the presence of a highly anisotropic tetragonal phase Fe14R2B which in the case of ribbons occurs in very fine grains. The interachoot of domain walls with this fine microstructure leads to the observed high coercivities. The effects of partial substitution of iron, rare-earth and boron on the formation of Fe14R2B and on its magnetic properties are also discussed.

The magnetic and structural properties of melt-spun Fe-R-B alloys are reviewed. The hard magnetic properties of these alloys are attributed to the presence of a highly anisotropic tetragonal phase Fe 14 R 2 B which in the case of ribbons occurs in very fine grains. The interachoot of domain walls with this fine microstructure leads to the observed high coercivities. The effects of partial substitution of iron, rare-earth and boron on the formation of Fe 14 R 2 B and on its magnetic properties are also discussed.

Gd and Tb nanoparticles were obtained via annealing of sputtered multilayer precursors. Detailed structural examination, involving energy-filtered imaging and high-resolution electron microscopy along with transmission electron microscopy and scanning electron microscopy techniques, revealed an island structure for the rare earths within the precursors, which further develops to nanoparticles upon annealing. A dramatic change in the magnetic characteristics of the nanoparticles is observed comparing to the bulk. Magnetic data are discussed to distinguish between true size effects and superparamagnetism. The obtained results give a credit to the former scenario.

Granular CoPt/C films, consisting of nanoparticles of the face-centered-tetragonal (fct) CoPt phase embedded in a carbon matrix, have been made by cosputtering from CoPt and C targets using a tandem deposition mode. The as-made films have a disordered face-centered-cubic structure and low coercivity (below 100 Oe). The granular structure is obtained after an aging heat treatment, which is followed by a drastic increase in coercivity to values up to 9 kOe due to the formation of the highly anisotropic fct phase. Transmission electron microscope studies showed the size of the CoPt particles to increase from 5 nm in the as-made state to 7-12 nm in the optimally annealed state. The measured coercivities are much lower than the value expected for noninteracting, randomly distributed single-domain particles. The large discrepancy is attributed to interparticle interactions, particle-size effects, and mainly to a lower anisotropy of the fct phase because of incomplete ordering. (C) 2000 American Institute of Physics. [S0021- 8979(00)00909-9]. Journal of Applied Physics

The magnetic and structural properties of Fe 1003− x Cr x ultrafine particles with x = 5–20 have been studied as a function of particle size. Particles with a size in the range of 80–360 A were prepared by gas evaporation under argon atmosphere. The particles with smaller diameter had a high coercivity at low temperatures and showed a stronger temperature dependence of coercivity. The x = 20 particles with a size 80 A had a coercivity about 2100 Oe at 10 K with a superparamagnetic blocking temperature about 150 K. Mossbauer spectra showed the presence of Fe-Cr, α-Fe and Fe-oxide components in the bigger particles, and Fe-Cr and Fe-oxides in the smaller particles. The coercivity at low temperatures increased with decreasing particle size and this was attributed to the higher percentage of Fe-oxide on the surface of the smaller particles. This interpretation was further supported by the temperature dependence of coercivity of Fe–Cr particles sandwiched between two Ag films.

The origin of magnetic hardening has been examined in melt-spun Fe 76 Nd 16 B 8 samples heat-treated at around 700°C. Microstructure studies show the same phases as in sintered magnets consisting of Fe 14 Nd 2 B, Fe 4 NdB 4 and two high-Nd content phases. These phases exist in both equiaxed and faceted crystallites of submicron size. Lorentz microscopy shows domain walls which end at grain boundaries indicating that they are pinned there.

Ce1-xSmxFe9Co2Ti alloys based on the ThMn12-type crystal structure have been synthesized via melt-spinning of prefabricated alloys and via mechanochemical processing of CeO2- Sm2O3- Fe2O3-TiO2- Co - Ca - CaO powder mixtures. Coercive fields up to 0.8 kOe and 2.1 kOe were obtained in annealed melt-spun alloys with x = 0 and x = 0.5, respectively. Submicron, partially anisotropic particles collected after the mechanochemical synthesis for x = 0.5 and x = 1 exhibited coercivity (energy product) of 1.8 kOe (5.4 MGOe) and 5.8 kOe (9.9 MGOe), respectively. The low magnetic anisotropy field of CeFe9Co2Ti alloy requires at least a partial Sm-substitution for Ce in order to develop a reasonably high coercivity.