L. X. Liao

Formation, crystallization, and magnetic properties of Nd‐Fe‐B glasses

The glass formation, crystallization, and magnetic properties of (Fe1−xNdx)100−yBy (0.025≤x≤0.25, 5≤y≤20) have been investigated. Glass formation by melt spinning was found possible for Nd‐Fe‐B alloys with iron content less than 87.5 at. %. The thermal stability of ternary amorphous Nd‐Fe‐B alloys is proportional to boron concentrations, but is strongly dependent on Nd:Fe ratio as well. The strong increase of the crystallization temperature with additions of small amounts of neodymium to Fe‐B can be attributed to the suppression of α‐Fe formation upon crystallization. With increasing Nd:Fe ratio, the increase in Curie temperature with boron content is gradually diminished, and for Nd:Fe≥0.176 even a decrease of Curie temperature with increasing boron content is observed. Three metastable ternary structures were produced during the crystallization process.

Structure and magnetic properties of rare‐earth iron nitrides, carbides and carbonitrides (invited)

Iron‐rich rare‐earth (R) compounds, such as R2Fe17 do not show great potential for high‐performance magnet materials due primarily to their low Curie temperatures (Tc∼300–400 K). However, relatively large quantities of nitrogen or carbon atoms can be introduced into the structure, resulting in a dramatic enhancement of magnetic properties including Tc (≥700 K). The N or C atoms cause a volume expansion of a few percent of the unit cell without changing the crystal structure. The large increase in Tc can be attributed to the volume dependence of the Fe–Fe exchange interactions. A large uniaxial anisotropy field develops for R=Sm upon nitriding/carbiding with an anisotropy field that is almost double the value for Nd2Fe14B at room temperature. Problems including the precipitation of soft magnetic phases (mainly α–Fe) and the limited thermal stability of the nitrides have so far restricted the applications of these compounds. Here data are presented on combined carbide/nitride alloys prepared using a novel t...

Structure and magnetic properties of RFe11TiNx (R=Y, Sm, and Dy)

Single phase alloys of composition RFe11Ti with R=Y, Sm, and Dy were prepared by induction melting. The samples were nitrided by thermal cycling to 770 K, at a heating rate of 10 K/min, under an atmosphere of nitrogen in a thermopiezic analyzer (TPA). For YFe11TiN the x‐ray diffraction (XRD) patterns give a=0.8611 nm and c=0.4802 nm for the tetragonal structure, space group I4/mmm. This represents a 3% volume expansion of the nitrogen‐free unit cell. The amount of absorbed nitrogen corresponds to one nitrogen atom per formula unit indicating that RFe11TiN is a true nitrogen compound with the nitrogen atoms occupying the 2b site in the structure. The expansion of the unit cell is accompanied by a dramatic increase in the Curie temperature for all compounds.

Structures and magnetocrystalline anisotropies of NdFe10Mo2N0.5 and NdFe10V2N

Abstract Nitrides of the intermetallic compounds NdFe 10 M 2 M=Mo, V) have been prepared and studied. Unit cell volume expansions are observed to be 1.3% for NdFe 10 Mo 2 N 0.5 and 2.6% for NdFe 10 V 2 N. Increases in Curie temperatures are about 146 K for both nitrides. The singular point detection technique has been used to measure the anisotropy field over a temperature range from 100 to 500 K, giving values of 7.8 T for NdFe 10 Mo 2 N 0.5 and 7.6 T for NdFe 10 V 2 N at 300 K.

Formation of high pressure phases in rapidly quenched Fe‐Nd alloys

Fe100−xNdx amorphous ribbons were obtained for compositions with 25<x<50, and partially amorphous ribbons for all other compositions.The amorphous phases were magnetically ordered with Curie temperatures ranging from 421 to 493 K. During crystallization, three metastable phases (M1, M2, and M3) were formed. X‐ray structural studies together with Mossbauer and thermomagnetic measurements suggest that the M1 phase is Fe23Nd6 (Mn23Th6 structure) with lattice parameter 1.152 nm and a Curie temperature of 515 K. The M2 phase is identified as Fe2Nd(Cu2Mg structure) with a lattice parameter of 0.745 nm and a Curie temperature of 567 K. The M1 and M2 phases transform to α‐Fe and Nd2Fe17 at high temperatures (≥1000 K). The M3 phase is present in the as‐quenched ribbons with x≥60 as well as in all crystallized ribbons. Structural data show that it is γ‐Nd, an fcc form of Nd. All three nonequilibrium structures are high pressure phases which are often formed during rapid solidification and/or crystallization of amor...

Mössbauer study of intercalation modified compounds R2Fe17 (R=Y, Sm)

Mossbauer studies were carried out at 77 K in two series of R2Fe17 compounds with a magnetic (Sm) and nonmagnetic (Y) rare‐earth, intercalated with H, C, and N. The increase in hyperfine field is largest at the 12j(18f) sites for R2Fe17H3.7 and R2Fe17N2.3. While the lattice expansion in R2Fe17C2 is similar to that in the nitrides and hydrides, the small change in hyperfine fields at the 12j(18f) and 12k(18h) sites indicates that the presence of neighboring carbon largely cancels the moment increase associated with the volume increase. For R2Fe17 carbonitrides, a single, sharp magnetic transition indicates a uniform compound. However, Mossbauer spectra suggest the existence of both C‐rich and N‐rich precipitates in the carbonitrides whose average hyperfine fields and isomer shifts scale with the nitrogen to carbon ratio.

Direct determination of cobalt site preferences at infinite dilution in iron‐based intermetallic compounds (invited)

Extremely low doping levels (∼1 ppm) and unambiguous interpretation combine to make the Mossbauer‐source technique an ideal method for determining cobalt site preferences in intermetallic compounds. Data on Gd2Fe17 and Nd2Fe14B are presented and compared with earlier work using Mossbauer spectroscopy, NMR, and neutron diffraction.

Local order in amorphous pure iron

Abstract We compare the effect of metalloid and metal alloying on the local structure of amorphous iron alloys. Data are presented on the quadrupole splitting and isomer shift measured above the magnetic ordering temperature for (Fe 1−x Nd x ) 100−y B y (0 ≤x ≤ 25, 5 ≤y ≤ 20). A common limit for x, y → 0 is obtained. No evidence for polymorphism in amorphous pure iron is found.

A simple conversion electron detector for Mössbauer source experiments

A vibration‐free conversion electron detector has been constructed for Mossbauer source experiments with extremely low doping levels (∼1 ppm) of cobalt‐57. While the overall efficiency is somewhat less than that of microfoil systems, the design is greatly simplified and it still yields more than a sixfold enhancement in signal‐to‐noise ratio over standard transmission methods. The lightweight detector (∼25 g) may be mounted directly on a conventional transducer without limiting the performance. This allows low‐temperature source measurements to be made by moving the detector and fixing the sample/source in a cryostat. The overall linewidth (HWHM) of the detector is 0.16 mm/s with the broadening being due to saturation effects in the enriched foil.

Temperature dependence of magnetocrystalline anisotropy of Sm2Fe17C2

Abstract The temperature dependence of the magnetocrystalline anisotropy of Sm 2 Fe 17 C 2 , prepared by the gas-solid reaction, has been determined over the temperature range from 379 to 633 K using the singular point detection method. The anisotropy field, μ 0 H A , at room temperature is extrapolated to be (13.9±0.2) T. The high anisotropy field in combination with the high Curie temperature and large magnetization make this material a very promising candidate for permanent magnet application.