简介概要

Relating atomic local structures and Curie temperature of NdFeB permanent magnets:an X-ray absorption spectroscopic study

来源期刊：Rare Metals2018年第11期

论文作者：Mu-Nan Yang Hang Wang Yong-Feng Hu Liu-Yi-Mei Yang Aimee Maclennan Bin Yang

文章页码：983 - 988

摘要：Relationship between atomic local structures and Curie temperature of NdFeB permanent magnets was investigated semi-quantitatively using synchrotron radiation technique. Fe K-edge X-ray absorption spectroscopy（XAS） was employed to study the local structure of Fe atoms for samples before and after doping Dy, Tb or Gd. It is found that the bond lengths and coordination numbers are changed. Thus, the exchange interaction between Fe atoms increases with Dy, Tb or Gd doping, resulting in the improvement of Curie temperature of NdFeB permanent magnets. The doping effect is proven by experimental measurement of the magnetic properties. Microstructural characterization using scanning electron microscopy（SEM） was also used to further analyze the effect of different rare earth elements doping on Curie temperature of NdFeB permanent magnets.

稀有金属(英文版) 2018,37(11),983-988

Relating atomic local structures and Curie temperature of NdFeB permanent magnets:an X-ray absorption spectroscopic study

Mu-Nan Yang Hang Wang Yong-Feng Hu Liu-Yi-Mei Yang Aimee Maclennan Bin Yang

Institute of Engineering Research, Jiangxi University of Scienceand Technology

Canadian Light Source, University of Saskatchewan

作者简介：*Bin Yang,e-mail: yangbin65@126.com;

收稿日期：16 December 2016

基金：financially supported by State High-Tech Development Plan (No. 2011AA061901);the TechnologyLanding Project of Jiangxi Province (No.KJLD13041);the Science and Technology Plan of Ganzhou (No.[2014]131);the Research Support Plan of Jiangxi University of Science and Technology(No. jxxjbs15001);

Relating atomic local structures and Curie temperature of NdFeB permanent magnets:an X-ray absorption spectroscopic study

Mu-Nan Yang Hang Wang Yong-Feng Hu Liu-Yi-Mei Yang Aimee Maclennan Bin Yang

Institute of Engineering Research, Jiangxi University of Scienceand Technology

Canadian Light Source, University of Saskatchewan

Abstract：

Relationship between atomic local structures and Curie temperature of NdFeB permanent magnets was investigated semi-quantitatively using synchrotron radiation technique. Fe K-edge X-ray absorption spectroscopy(XAS) was employed to study the local structure of Fe atoms for samples before and after doping Dy, Tb or Gd. It is found that the bond lengths and coordination numbers are changed. Thus, the exchange interaction between Fe atoms increases with Dy, Tb or Gd doping, resulting in the improvement of Curie temperature of NdFeB permanent magnets. The doping effect is proven by experimental measurement of the magnetic properties. Microstructural characterization using scanning electron microscopy(SEM) was also used to further analyze the effect of different rare earth elements doping on Curie temperature of NdFeB permanent magnets.

Keyword：

Permanent magnet; Synchrotron radiation; X-ray absorption fine structure; Crystal structure; Curie temperature;

Received： 16 December 2016

1 Introduction

Sintering NdFeB permanent magnets are widely used in electronic technology and modern industry because of their outstanding magnetic properties.However,the thermal stability of NdFeB permanent magnets is still low,with Curie temperature being～583 K,which greatly limits its industrial application [ 1, 2, 3, 4, 5, 6, 7, 8] .

In order to increase the Curie temperature and enhance the thermal stability of NdFeB permanent magnets,a large number of experimental researches have been carried out.Alloying (elements doping) is one of the most important and effective ways to improve the thermal stability of NdFeB permanent magnets [ 9, 10, 11, 12] .Mottram et al. [ 13] found that the Curie temperature increased by 10.93℃with every 1 at%Co addition.Adding more than one element simultaneously has also been proved to be an efficient way to obtain magnets with high Curie temperature;i.e.,adding (Al,Co,B) [ 14] ,(Dy,Co,Ga) [ 15] and (Mn,Bi) [ 16, 17] .However,the effect of adding heavy rare earth elements,such as Gd,Dy and Tb,is somehow controversial [ 18, 19, 20] .Yan et al. [ 21] indicated that Tb or Dy addition can benefit the Curie temperature and thus the thermal stability,while Liu et al. [ 22] found Dy had little effect on the improvement of Curie temperature.The occurrence of this discrepancy is because of the different phases which the alloying elements dissolve in.If Dy atoms were to replace those Nd atoms in the main phase of Nd₂Fe₁₄B,the Curie temperature could increase;otherwise,no improvement could be observed.

Although it is widely accepted that alloying elements in Nd₂Fe₁₄B phase are good for Curie temperature of NdFeB permanent magnets,the mechanism of the influence is still unclear.Therefore,in the present work,synchrotron radiation technique based X-ray absorption spectroscopy(XAS) tests were conducted to investigate the relationship between electronic structure and Curie temperature of the Dy,Tb or Gd doped NdFeB permanent magnets.

2 Experimental

The NdFeB samples in rod shape were supplied by FUERTE PLC.,prepared using vacuum sintering method.Identical size for all the samples was prepared to beΦ8 mm×25 mm.The mean compositions of the four investigated samples were Nd_31.5Fe_67.5B_1.0,Nd_30.5Dy_1.0-Fe_67.5B_1.0,Nd_30.5Tb_1.0Fe_67.5B_1.0 and Nd_30.5Gd_1.0Fe_67.5B_1.0(wt%),respectively.Impurities contents were less than0.1 wt%.

Fe K-edge XAS was employed to study the local structure of Fe atoms for samples before and after doping Dy,Tb or Gd.These measurements were performed using SXRMB beam line from Canadian Light Source,Saskatoon,Canada.Total electron yield and fluorescence yield,using a 4-element Si(Li) drift detector,were used to record the Fe K-edge spectra.X-ray absorption data were processed using Athena package [ 23] .Multiple scan spectra were first averaged and the spectra were normalized to unity using a linear pre-edge subtraction and quadratic polynomial as the post edge line for background subtraction.The X-ray absorption fine structure (XAFS) signal(χ(k²)),covering a k range from 0.22 to 1.10 nm,was Fourier transformed to R space usingk-weightings of 2,without phase shift correction.Extended X-ray absorption fine structure (EXAFS) data were then subjected to theoretical calculations using Artemis V0.9.24 [ 24] with IFEFFIT package version 1.2.1 1d [ 25] .Curie temperatures(T_C) of the sintered samples were measured by vibrating sample magnetometer (VSM,Lake Shore 7400).Magnetic properties of the sintered samples were measured using high temperature permanent magnetic measuring instrument NIM-500C,i.e.,coercivity (H_cj),remanence (B_r) and maximum magnetic energy products ((BH)_max).Finally,scanning electron microscope (SEM,FEI MLA 650F) was used to examine the microstructure of the sintered magnets.

3 Results and discussion

3.1 XANES for Fe-K edge

Figure 1a shows X-ray absorption near edge structure(XANES) spectra of NdFeB alloys and pure Fe foil tested in total electron yield (TEY),where xμ/E indicates the absorptivity of photoelectron in different energy.It is clear that the shape of spectra representing NdFeB alloys is similar to that for pure Fe sample (shell at almost the same energy positions).Moreover,the first shell of all samples in the smoothed first derivative spectrum exists around7112.8 eV,as presented in Fig.1b.Figure 1 indicates that the valence state of Fe in NdFeB alloys does not change compared that in metal.Also the total area of absorption shell for spectra of NdFeB samples in Fig.la is much lower than that of pure Fe sample due to the following two reasons.One is the presence of 4f electrons brought by rare earth elements.Since the holes left by electron transition in Fe atoms will be filled by these 4f electrons,the probability of the excitation of Fe 1s electron to the empty states is reduced,leading to smaller area in XANES shell.However,no obvious trends,as a function of 4f electrons,can be observed among different rare earth elements because of the uncertainty for the movement of 4f electrons.The other reason is the degree of arrangement order of the Fe atoms.Lower degree of order will result in less electron transition and thus smaller shell area.

3.2 R-space for Fe-K edge

EXAFS spectra of NdFeB samples recorded in fluorescence yield are shown in Fig.2,with k²χ(k) results shown in insert which reflects the EXAFS oscillations.The radial structure function (in R-space) without phase correction was also calculated by performing Fourier transformation(FT) of EXAFS results in energy space,as shown in Fig.3,indicate the relationship between coordination number(|χ(R)|) and atomic distance (R).The maximum shell around 0.21 nm corresponds to Fe-Fe coordination as the interaction between Fe atoms are the greatest in NdFeB alloys.The Fe-Fe distances of different samples are listed in Table 1.The distance of each sample is at 0.212,0.210,0.210 and 2.08 nm for Gd-doped,Tb-doped,Dy-doped and undoped samples,respectively.It is obvious that when adding Dy,Tb,or Gd into NdFeB alloys,the Fe-Fe shell will shift slightly towards the direction of greater distance between neighboring Fe atoms.

In addition,a bond length of 0.25 nm is found to be a critical distance (r_a) between neighboring Fe atoms for NdFeB permanent magnets [ 26, 27] .When r_a is larger than0.25 nm,the exchange integration constant (A) is positive;while it is negative when r_a is less than 0.25 nm.The values of r_a for Dy-,Tb-or Gd-doped samples in R-spaces(0.210,0.210 and 0.212 nm in FT of EXAFS) are greater than that of NdFeB sample (0.0208 nm),thus indicating that particle negative interaction between neighboring Fe atoms decreases.Consequently,the intensity of Fe-Fe shell will be enhanced,and the total magnetic interaction of the samples will be strengthened by doping Dy,Tb or Gd.What should be noticed is that the magnetic interaction between Fe-RE atoms can be neglected as the intensity for the second shell (0.29-0.34 nm in FT of EXAFS) is much lower than that for Fe-Fe ones (～0.21 nm in FT of EXAFS).This is due to the large distance between RE and Fe,of which the interaction is weaker than that of Fe-Fe [ 28, 29] .

Fig.1 Normalized Fe K-edge XANES spectra of NdFeB alloys doped with Dy,Tb or Gd:a observed XANES spectra compared with reference metal Fe and b smoothed derivative of normalized spectra

Fig.2 EXAFS spectra of NdFeB alloy doped with Dy,Tb or Gd and normalized k²x(k) weighted EXAFS spectra,(k ranging from 0.22 to1.10 nm;pectra being shifted along y axes for better visualization)

Fig.3 FT of EXAFS spectra for NdFeB alloy doped with Dy,Tb or Gd

3.3 Fitting the first shell

The first shells for the four samples were fitted using Artemis software,as shown in Fig.4.The fitted shells are represented by solid lines and the experimental data are in dotted lines.The amplitude reduction factor ( ),energy shift parameter (ΔE₀),the mean inter-atomic distance (ΔR),variance Debye-Waller (DW,σ²) factor and quality factor(R-factor) are determined by inpidual fits,as listed in Table 2.Theσ² values are low as expected for wellordered crystal structure.Parameter of bothσ² and R-factor are also satisfactory for this fitting.

According to the fitting results,the coordination numbers of the Fe 4c site are unchanged with Dy,Tb or Gd doped,and the bond lengths of Fe on 4c site to 16k₁ site are0.2497,0.2501,0.2501,0.2503 nm for the undoped,Dydoped,Tb-doped and Gd-doped samples.This is consistent with the value of 0.2485 nm by transmission electron microscopy (TEM) [ 29] .This confirms that doping Dy,Tb or Gd could increase positive interaction between Fe atoms.

3.4 Curie temperature

The measured Curie temperatures of NdFeB samples are listed in Table 1.The Curie temperature of the sample without doping is 585 K,which is consistent with the result in Ref. [ 30] ;and the value of Curie temperature rises to586.0,588.5 and 593.0 K when doped with Dy,Tb and Gd,respectively.

The Curie temperature of the Dy doped sample is lower than that of the Tb doped one.As the distances between Fe-Fe atoms are the same for these two samples (both0.210 nm in Table 1),higher Curie temperature of Tbdoped sample is due to higher total spin quantum number S～-being 3 for Tb-doped sample and 2 for Dy-doped sample (please note that the Curie temperature constant (C)is proportionate to the total spin quantum number S;T_C∝C∝cS [ 26] ).

For undoped sample and Dy-doped sample,the total spin quantum numbers are the same,but the distance between neighboring Fe atoms of undoped sample is0.208 nm,a little less than that of Dy doped sample,0.210 nm.As stated in Sect.3.2,the magnetic interaction is weaker with smaller Fe-Fe distance.Hence,the Curie temperature of undoped sample is slightly less than that of Dy-doped sample.

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Table 1 Properties of NdFeB alloys doped with Dy,Tb,Gd

Fig.4 Fourier transformed EXAFS spectra for Dy-,Tb-or Gd-doped NdFeB at Fe K-edge

Since both the total spin quantum number and the distance between neighboring Fe atoms can influence Curie temperature,Gd-doped sample with the highest values of the two factors has the greatest Curie temperature of593 K,which is consistent with the experimental results in Table 1.

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Table 3 Measured magnetic properties of sintered NdFeB magnets

3.5 Magnetic properties

As the energy required to break down the ordered arrangement of magnetic moments increases with Dy,Tb or Gd doping,some magnetic properties of NdFeB magnets are also expected to be different.Besides Curie temperature,some other properties such as coercivity,remanences and maximum magnetic energy products were also determined for undoped,Dy-doped and Tb-doped samples,the values of which are also listed in Table 3.The coercivity of Dy-doped sample is higher than that of Tb-doped sample.This could be due to the sample treatment during the sintering process,since the sample coercivity depends on magnetocrystalline anisotropy,as well as microstructure;whereas Curie temperature is independent on micro structure.

The remanences and maximum magnetic energy products of undoped sample and Dy-doped sample are almost the same:1.518 and 1.494 T;424.9 and 412.1 kJ·m^-3,respectively.But the values of Tb-doped sample are much lower than those of the other two samples,being 1.281 T and 308 kJ·m^-3,respectively.This again shows that there should be some effect from the microstructure.

Scanning electron microscopy (SEM) was used to analyze the microstructure of the magnets in present work.Back-scattered electron (BSE) images are shown in Fig.5,where D represent grain size.In Fig.5a,it presents the microstructure of the sample without doping.The grain size varies from 4.78 to 6.66μm,and discontinuous Ndrich phase (up to 1.41μm in thickness) along grain boundary can be clearly observed.The microstructure of Dy-doped sample is shown in Fig.5b.The grain size is about 4.1μm (±0.7μm).A continuous thin layer of Ndrich phase exists along the grain boundaries.Figure 5c is BSE image of the sample doped with Tb.The grain size is much smaller than those of the other two,～3μm,and discreted Nd-rich phase can be observed along grain boundary.

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Table 2 Fitting results of the first shell EXAFS

Fig.5 BSE images of sintered NdFeB magnets:a undoped sample,b Dy-doped sample,and c Tb-doped sample

It is known that the coercivity of permanent magnets can be affected greatly by the grain size and morphology of the Nd-rich phase along the grain boundaries:(1) the smaller the grain size is,the higher the coercivity is;and (2)coercivity benefits from continuous and thin layer of Ndrich phase.It is Dy-doped sample in the present work that has the best micros true ture (Fig.5b),and thus the highest coercivity can be expected (Table 3),while Tb-doped sample with the worst boundary (Fig.5c) has the lowest coercivity (Table 3).This implies that it is equally important to control the microstructure as well as the composition (atomic local structure) in order to achieve good magnetic properties for NdFeB-based permanent magnets.

4 Conclusion

In summary,XAFS of NdFeB permanent magnets with and without doping was studied using synchrotron radiation technique.Synchrotron results show that the bond length between Fe atoms increases as represented after doping Dy,Tb or Gd.Accordingly,the Curie temperature is analyzed to increase due to two factors——total spin quantum number and bond length between the nearest neighbors of Fe atoms representing the exchange interaction.The Gd doped sample has the greatest exchange energy and thus the highest Curie temperature,which is consistent with the measured values.

Acknowledgements This work was financially supported by State High-Tech Development Plan (No.2011AA061901),the Technology Landing Project of Jiangxi Province (No.KJLD13041),the Science and Technology Plan of Ganzhou (No.[2014]131) and the Research Support Plan of Jiangxi University of Science and Technology (No.jxxjbs15001).