Chun-Li Yang

57Fe high pressure Mössbauer and X-ray studies of Gd2Fe17C x interstitial compounds

The unit cell volume dependence of the Curie temperature of Gd2Fe17 is studied by means of high-pressure Mössbauer spectroscopy. From the pressure dependence of the average isomer shift of Ge2Fe17 at room temperature, the contribution of the pure magnetovolume effect to the change of the average isomer shift is derived. By combining the Mössbauer measurements of Gd2Fe17Cx with different C concentrations at room temperature, the change ofIS in Gd2Fe17Cx is dominated by the magnetovolume effect. The contribution of the chemical bonding effect is about 24% of that of the magnetovolume effect, but with negative sign.

EFFECT OF HEAT TREATMENT ON THE PHASES OF MELT-SPUN NDFEB ALLOYS

The phase composition of crystallized NdxFe81.5−xB18.5 (x=3, 4, 4.5, 6, 12, and 15) alloys annealed under optimal condition are investigated by means of zero‐field spin‐echo nuclear magnetic resonance (NMR) and Mossbauer effect (ME), together with x‐ray diffraction (XRD). It is found that the phase composition depends on the Nd concentration. The samples with 12–16 at. %‐Nd contain Nd2Fe14B magnetically hard phase and Nd1.1Fe4B4 paramagnetic phase. Their large coercive fields are due to the very fine size of Nd2Fe14B crystallite below the critical size of single domain particles, and some minor phases precipitated at grain boundaries, which have a pinning domain wall effect. The samples with a lower Nd concentration (x=3–6 at. %) consist of body‐centered‐tetragonal (bct)‐Fe3B and a small amount of α‐Fe, and no Nd2Fe14B and Nd1.1Fe4B4. Furthermore, the NMR results show that the spectra corresponding to bct‐Fe3B in the samples annealed at 670 °C distort and broaden more seriously than that of pure bct‐Fe3B ...

Mössbauer and magnetic properties study of Nd2Fe14C-type compounds

The dependence of the Curie temperature Tc and saturation magnetization σs on composition was studied in the alloys Nd14Fe78 − xTxC8 (T ≡ Co, Ni, Mn, Cr). The effects on Tc and σs of the substitution of these 3d elements for iron were found to be similar to the effects on Tc and σs of the corresponding borides. Mossbauer spectra of the tetragonal Nd2Fe14C compound were taken at various temperatures and fitted with a set of six subspectra owing to six non-equivalent iron sites. It was found that the average hyperfine fields at 295 K and 4.2 K are 28.04 T and 32.36 T, the corresponding average iron moments being about 1.89 μB and 2.19 μB, respectively.

THE ORIGIN OF HARD MAGNETIC-PROPERTIES IN RAPIDLY QUENCHED ND4FE77.5B18.5 ALLOY

Until now, the origin of hard magnetic properties of rapidly quenched Nd-Fe-B alloys with lower Nd concentration is not clear. In this paper, the phase compositions of rapidly quenched Nd4Fe77.5B18.5 alloys annealed under different conditions have been studied by using zero-field spin-echo nuclear magnetic resonance (NMR) and Mossbauer effect (ME) techniques. It is found that there exists a certain amount of Nd2Fe14B phase in the samples annealed at 960-degrees-C and ingot alloy, which have poor hard magnetic properties; while, the sample annealed under optimal condition consists only of bct-Fe3B as the main phase and a small amount of a-Fe. However, the ME result indicates that about 5 at. % Fe atoms in Fe(III) (8 g) site of bct-Fe3B have been replaced by Nd atoms; the NMR result demonstrates that B-11 NMR spectrum is the characteristic peak of bct-Fe3B, but it broadens asymmetrically to the high frequency side, which is due to the bct-Fe3B influenced by Nd atoms. The amplitude of radio frequency (rf) excitation field required to get the maximum B-11 spin-echo signal from bct-Fe3B in the sample annealed at 839-degrees-C is only about one third as much as that required to excite the B-11 in the bct-Fe3B influenced by Nd atoms in the sample annealed at 670-degrees-C for a short time, which implies the latter has a larger coercivity field than the former. It is concluded that the origin of hard magnetic properties of Nd4Fe77.5B18.5 alloy is not related to the 2:14:1 phase, but to the change of bct-Fe3B itself

Magnetism and hyperfine fields in melt-spun R-Fe-B alloys with a low R concentration (R≡Y, Pr, Nd, Gd or Dy)

The hyperfine fields and magnetic properties of amorphous and crystallized R(x)Nd(4-x)Fe(77.5)B(18.5) (R = Y, Pr, Gd or Dy; 0 less than or equal to x less than or equal to 4) alloys have been investigated by means of magnetization measurements, zero-field spin-echo nuclear magnetic resonance (NMR) and the Mossbauer effect (ME). It is found that the Curie temperatures of amorphous alloys change slightly on the addition of R, but the coercive fields of crystallized alloys decrease monotonically with increasing x for R = Y, Pr and Gd. A small addition of Dy increases the coercivity; full substitution of Nd by Dy leads to low coercivity owing to the formation of the magnetically soft Dy3Fe62B14. NMR, ME and x-ray diffraction indicate that the samples with R = Pr and Gd for 0 less than or equal to x less than or equal to 4 and R = Y and Dy for 0 less than or equal to x less than or equal to 2 consist of Fe3B with a body-centred tetragonal (BCT) structure and a small amount of alpha-Fe. Furthermore, the NMR results indicate that the B-11 hyperfine fields of sn Fe3B increase linearly from 25.3 kOe (34.7 MHz) to 26.3 kOe (36.0 MHz) on the addition of Dy or Gd, but that of alpha-Fe does not change with increasing R concentration; the Mossbauer spectra show that the relative intensity of the subspectrum corresponding to Fe-57 at Fe-III(8g) sites in BCT Fe3B is about 5% weaker than those of the other two, implying that about 5 at.% Fe atoms in this site are substituted by other atoms. According to this, it may be reasonable to assume that R atoms enter into BCT Fe3B.

MAGNETISM AND HYPERFINE FIELDS IN YFE10V2 - A COMBINED NUCLEAR-MAGNETIC-RESONANCE AND MOSSBAUER STUDY

A combined nuclear magnetic resonance (NMR) and Mossbauer effect study of YFe10V2 has been carried out. Based on the 57Fe hyperfine fields (HFS) obtained from the Mossbauer results, the resonance lines of the NMR spectrum are assigned and the site preference of V atoms is determined. On the other hand, according to the site preference of vanadium atoms determined by NMR, together with neutron diffraction results, the 57Fe Mossbauer spectrum is fitted. It can be deduced from the NMR results that the existence of one nearest-neighbour Fe atom leads to an increase in the V HF of about 0.35, and the core electron polarization contribution Hcp, the 4s conduction electron polarization contribution Hs and the effect of the second-nearest-neighbour Fe atoms on the HF sites cannot be negligible.

THEORETICAL-STUDY ON PRESSURE-DEPENDENCE OF ISOMER-SHIFT AND HYPERFINE FIELD

The pressure dependence of the isomer shift and the hyperfine field are theoretically investigated. By expanding the Gibbs free energy of the system under proper circumstances, the explicit form of the pressure dependence of the isomer shift is obtained. At low temperatures, by applying some well-known relations, the pressure dependence of the hyperfine field is approximately derived. Finally, the theoretical results are compared with the experimental data.

The effect of Gd substitution on the magnetic properties and hyperfine fields of melt-spun Nd4Fe77.5B18.5 alloys

The hyperfine fields (HFS) and magnetic properties of amorphous and crystallized GdxNd4-xFe77.5B18.5 (0<or=x<or=4) alloys have been investigated by means of magnetization measurements, zero-field spin-echo nuclear magnetic resonance (NMR) and Mossbauer effect (ME). For comparison, the x-ray diffraction (XRD) pattern of Nd4Fe77.5B18.5 annealed at 670 degrees C for a short time is also presented. It is found that the Curie temperatures of amorphous alloys increase slightly on addition of Gd, but the coercive fields of crystallized alloys decrease monotonically with increasing x. NMR, ME and XRD indicate that these samples consist of Fe3B with a body-centred tetragonal structure (BCT Fe3B) and a small amount of alpha -Fe, with no Nd2Fe14B and Nd1.1Fe4B4 phases. Furthermore, the NMR results indicate that the 11B HFS of BCT Fe3B increase linearly from 25.3 kOe (34.7 MHz) for x=0 to 26.3 kOe (36.0 MHz) for x=4, but that of alpha -Fe does not change with increasing Gd concentration; the Mossbauer spectra (MSs) show that the relative intensity of the subspectrum corresponding to 57Fe at FeIII(8g) sites in BCT Fe3B is about 5% weaker than those of the other two, implying that about 5 at.% Fe atoms in this site are substituted by other atoms. According to this, it may be reasonable to assume that the R atoms enter into the BCT Fe3B.

A 57Fe high-pressure Mossbauer study of the ferromagnetic intermetallic compound Gd2Fe17

The effect of pressure up to 5.5 GPa on the magnetic behaviour of Gd2Fe17 has been investigated using the Fe-57 high-pressure Mossbauer effect technique. We show that the decrease of the average magnetic hyperfine field at 300 K can be well explained by the decrease of Curie temperature with pressure, while the Fe local magnetic moment remains almost stable in this pressure range. The decrease of the isomer shift with increasing pressure indicates a corresponding increase of the selectron density at the Fe-57 nucleus, which is mainly caused by the volume compression of the 4s conduction electrons.

Structure and Mössbauer effect studies of Y2Sn2-xFexO7-δ pyrochlores

Y2Sn2-xFexO7-δ pyrochlores were studied by means of X-ray diffraction,119Sn and57Fe Mössbauer spectroscopy. The single phase Y2Sn2-xFexO7-δY pyrochlores up tox=0.8 were produced by the chemical coprecipitation method. The quadrupole splittings of119Sn decrease linearly with increasing iron content. The57Fe Mössbauer spectra forx=0.6 and 0.8 both are composed of a sextet and two doublets, indicating that some Fe3+ ion environments in the pyrochlores are magnetically ordered. The magnetic hyperfine splitting disappears in the sample withx= 1.0, which contains a second phase.