Yi-De Zhang

HYPERFINE FIELDS AND MAGNETISM IN THE ND2FE14B COMPOUND - A COMBINED MOSSBAUER AND NMR-STUDY

A combined nuclear magnetic resonance (NMR) and Mossbauer effect (ME) study of Nd2Fe14B has been carried out, which uses the Fe hyperfine field (HF) values obtained by NMR experiments in the ME fitting process. Taking the advantages of both NMR in determining the HF values at different Fe sites and ME in determining the intensity ratios of subspectra, the determination of the Fe HF values and their assignment can be made more precisely and more reliably. It can be deduced from the present result that the existence of a nearest Fe neighbor leads to an increase in the Fe HF of about 15 kOe/Fe, while a nearest B neighbor atom reduces it at a larger rate. The nearest Nd neighbor atom seems to have no significant effect on the Fe HF. The analysis concerning the various contributions to the HF and their correlation with the atomic magnetic moment demonstrates that the hyperfine interaction study for the case of Fe provides a simple means to measure Fe sublattice magnetization and relevant properties of the R2Fe14B system.

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 ...

On the short-range order in the Fe84B10C6 amorphous alloy

Abstract Differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) and Mossbauer effect (ME) experiments have been carried out in order to study the short-range order (SRO) in the Fe 84 B 10 C 6 amorphous alloy. NMR provides clear evidence for the existence of both body-centered tetragonal Fe 3 B-like and orthorhombic Fe 3 B-like SRO in the as-quenched alloy. The NMR results are used as a reference to determine the optimal heat treatment conditions under which the SRO existing in the as-quenched alloy nucleates and grows without undergoing a phase transition. It was found that when the alloy is annealed at the onset temperature for the second stage of the crystallization process, which corresponds to the crystallization initiation point for the SRO in the amorphous alloys, the crystallized precipitates have structures similar to that of the SRO originally existing in the as-quenched alloy.

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

NUCLEAR-MAGNETIC-RESONANCE AND MOSSBAUER STUDIES OF RAPIDLY QUENCHED AND HEAT-TREATED ND4FE77.5B18.5 ALLOYS

Using the Mossbauer effect and NMR, and phase compositions of rapidly quenched Nd4Fe77.5B18.5 alloy under different heat treatment conditions were studied. It was shown that the sample which was annealed at 670 degrees C for a short time and has the optimal magnetic properties has a main BCT Fe3B phase containing Nd atoms and a small amount of alpha -Fe (6%). It is concluded that the hard magnetic properties of crystallized melt-quenched Nd-Fe-B with a low Nd concentration do not originate from the presence of the hard magnetic Nd2Fe14B phase but from the BCT Fe3B phase containing Nd atoms.

Preparation and Mössbauer study of Gd2Fe17Cx (x≤2) with higher carbon concentration by melt quenching

Carbides Gd2Fe17Cx with composition up to x=2 were successfully prepared. The single phase Gd2Fe17Cx (x=1.5, 2.0) carbides with higher carbon concentration were first obtained by the melt quenching technique. 57Fe Mossbauer spectra of all compounds were measured and the results show that the average hyperfine field and isomer shift increase with increasing carbon concentration in the studied composition range. The effects of the interstitial carbon atoms on the hyperfine parameters are discussed. According to the result of the quadrupole splitting, the easy magnetization direction at room temperature for all carbides was determined.

Magnetic and Mössbauer studies of the intermetallic compounds Dy(Fe1−xBx)2 (0⩽x⩽0.3)

Abstract The magnetic properties and the 57 Fe Mossbauer spectra of the pseudobinary cubic Laves phase Dy(Fe 1- x B x ) 2 ( x ⩽ 0.3) are investigated. It is observed in the system that with increasing B concentration, the Curie temperature T c decreases, the low temperature coercivity H c increases. In certain cases, an anomalous behaviour of the temperature dependence of magnetization σ( T ) is found as well. The magnetic moment of the Fe ions μ Fe which is calculated from the Mossbauer results, and the average magnetic moment μ Dy of Dy ions decreases with increasing x . The decrease of μ Fe can be explained in terms of the model in which the 2p electrons of B are transferred into the 3d band of iron ions. The decrease of μ Dy can be understood with the deviation of local easy magnetization axis. The anomaly of the temperature dependence of magnetization σ( T ) is caused by the increase of low temperature magnetic hardness.

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.

MOSSBAUER STUDY OF ND2FE14B AND ND2(FE1-XCOX)14B COMPOUNDS

The Mössbauer spectra of Nd2Fe14B and Nd2(Fe1−xCox)14B compounds (x=0.0.05, 0.10 and 0.16) have been investigated at room temperature and at 77 K. Taking advantage of combined ME and NMR investigations of the Nd2Fe14B compund, the hyperfine field values and their assignment to the six Fe sites have been determined to be the following sequence: 378(j2), 346(k2), 334(j1), 325(k1), 322(c), 306(e) kOe. The substitution of Co for Fe decreases the hyperfine fields at all Fe sites. The intensity variations of the subspectra with Co content show that Co atoms have a strong preference to occupy the k2 site, but have a rather less tendency to enter the j2 site, which is preferred by Fe atoms.