J. Strzeszewski

A new hard magnetic phase in binary Nd-Fe and Pr-Fe alloys

Abstract 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 .

Magnetic and structural properties of melt-spun rare-earth transition-metal intermetallics with ThMn12 structure

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.

High coercivities in Sm(FeT)/sub 12/ type magnets

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. >

High coercivities in as‐cast Nd‐Fe and Nd‐Fe‐Ti alloys

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.

High coercivity in rapidly quenched Sm(Fe, T)12-type magnets

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.

Electron microscopy studies of new phases in Nd‐Fe alloys

The magnetic properties, structure, microstructure, and chemical composition of new phases found in melt‐spun Nd‐Fe alloys are reported. High coercivities were obtained in ribbons after a heat treatment for 10–15 min at temperatures in the range of 450–550 °C. Thermomagnetic data showed the existence of two new phases with Curie temperatures at around 250 and 340 °C, respectively. Electron microscopy observations confirmed the existence of two different iron‐rich phases. One of them had 70 at. % Fe and a tetragonal structure with a=8.0 A and c=11.9 A, similar to Nd2Fe14B. The other one had 85 at. % Fe and a hexagonal structure with a=20.5 A and c=11.5 A. These phases are probably ternary Nd‐Fe‐O stabilized by oxygen. The hexagonal phase is believed to correspond to the phase with a Curie temperature of 250 °C.

Magnetic and structural studies in Co‐Al thin films

Thin films of Co100−xAlx with 15 30. After a heat treatment at 650 °C the films with x<25 changed microstructure to a fine mixture of hcp Co and an ordered fcc structure which may be interpreted as Co3Al. Films with 25≤x≤50 were transformed to ordered fcc Co3Al and bcc CoAl structures after the heat treatment. The magnetization of the films was greater than for bulk alloys with the same composition and it decreased when the films were annealed at 650 °C. The finely distributed two‐phase microstructure is probably responsible for the observed coercivity (50–150 Oe).

The effect of Al substitution on the coercivity of Nd‐Fe‐B magnets

The effect of Al on the coercivity of Nd‐Dy‐Fe(Al)B magnets has been investigated with magnetic measurements, differential scanning calorimetry, and microstructure studies. The Al containing samples were found to have a much higher coercivity, a relatively steeper initial magnetization, a different Hc(Hef) curve, and a larger temperature coefficient of coercivity than the samples without Al. Morphology studies made with a scanning transmission electron microscope did not show any significant differences between the two samples. The only evident difference, found in x‐ray and electron diffraction data, was in the c/a ratio of the Nd2Fe14B tetragonal phase which was higher for the sample with Al. Another difference was observed in the magnetization reversal mechanism. In the Al‐containing samples the magnetization is reversed by changing the magnetization of entire grains, while for the samples without Al the magnetization is reversed by dividing into domains inside the grains.

Magnetic properties of rapidly quenched and annealed Fe10RTi and related alloys

Magnetic measurements are reported for iron‐rich ternary alloys with the following compositions: Fe10NdxDy1−xTi (0≤x≤1), Fe10SmM (M=Ti, V, and Mo), and Fe10RV (R=Y, Gd, and Dy). The samples were prepared by splat cooling or melt spinning and selected samples were heat treated. The results of x‐ray diffraction, electron microscopy, and thermomagnetic measurements on the Fe10NdxDy1−xTi series are presented and indicate that the rapidly quenched alloys are nearly single phase and become multiphase upon heating. Magnetic measurements on the other two series show that the substitution of V for Ti increases the saturation magnetization.

Microstructure studies in Nd2(Fe0.9Co0.1)14B thin films

Abstract Large coercive fields (above 10 kOe) have been produced in thin sputtered films of Nd(FeCo)B. The films were either deposited at room temperature and heat treated or deposited onto high-temperature substrates (to about 730 °C). Transmission electron microscopy was used to study the microstructure and magnetic domain structure of the films. Correlations were observed between sputtering and growth parameters, microstructure and magnetic properties.