The rapid-quenched (R/Q) and subsequently optimally annealed Nd-Fe-B alloy with 12 wt% Nd was characterized using the X-Ray diffractometry (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HREM) and superconducting quantum interference device (SQUID) magnetometer. It was found that microstructure of the investigated alloy mainly consist of Fe3B, Nd2Fe14B phases and minor content of α-Fe phase, with mean crystal grain sizes being below 30 nm. The ferromagnetic exchange coupling between the grains of identified hard and soft magnetic phases has direct influence on the magnetic properties as it is illustrated by SQUID hysteresis loop. Correlation of the measured magnetic properties with results of microstructure analysis indicates that the investigated rapid-quenched Nd-Fe-B alloy has the nanocomposite structure in optimal magnetic state

Grujić, Aleksandar

Lee, Z.

Radmilović, V.

Stajić-Trošić, Jasna

Talijan, Nadežda M.

Ćosović, Vladan

Živković, Dragana

Metalurgija-Journal of Metallurgy

English

 CER - Central Repository of the Institute of Chemistry;  Technology and Metallurgy

Association of Metallurgical Engineers of Serbia 
AMES 
Scientific paper 
UDC: 669.15'781-198 
 
STUDY OF MICROSTRUCTURE AND MAGNETIC PROPERTIES OF 
OPTIMALLY ANNEALED R/Q Nd4.5Fe77B18.5 ALLOY 
V. Ćosović1, N. Talijan1, A. Grujić1, J. Stajić-Trošić1, D. Živković2, Z. Lee3,  
V. Radmilović3 
1Institute of Chemistry, Technology and Metallurgy, Njegoševa 12, 11000 
Belgrade, Serbia, vlada@tmf.bg.ac.rs 
2Technical Faculty Bor, Vojske Jugoslavije 12, 19210 Bor, Serbia 
3National Center for Electron Microscopy, Lawrence Berkeley Laboratory, 
Berkeley, USA 
 
Accepted 14.02.2009. 
Abstract 
The rapid-quenched (R/Q) and subsequently optimally annealed Nd-Fe-B alloy 
with 12 wt% Nd was characterized using the X-Ray diffractometry (XRD), transmission 
electron microscopy (TEM), high resolution transmission electron microscopy (HREM) 
and superconducting quantum interference device (SQUID) magnetometer. It was found 
that microstructure of the investigated alloy mainly consist of Fe3B, Nd2Fe14B phases 
and minor content of α-Fe phase, with mean crystal grain sizes being below 30 nm. The 
ferromagnetic exchange coupling between the grains of identified hard and soft 
magnetic phases has direct influence on the magnetic properties as it is illustrated by 
SQUID hysteresis loop. Correlation of the measured magnetic properties with results of 
microstructure analysis indicates that the investigated rapid-quenched Nd-Fe-B alloy 
has the nanocomposite structure in optimal magnetic state. 
Key words: nanocomposite Nd-Fe-B alloys, exchange coupling, phase composition, 
crystal grain size, magnetic properties 
 
1. Introduction 
 
The nanocomposite Nd-Fe-B permanent magnetic materials are of great 
commercial interest as no magnetic alignment is needed and less of the costly 
neodymium is required [1]. These high energy density magnets are essentially 
assemblies of hard magnetic and soft magnetic phases in which the former provides 
high coercivity and the latter provide high magnetization [1-3]. Most common way for 
production of these nanocrystalline materials is rapid-quenching with subsequent 
controlled nucleation or crystallization. Magnetic properties of the nanocomposite Nd-
40 MJoM Vol 15 (1) 2009 p. 39-44 
Fe-B materials are under significant influence of the interaction of ferromagnetic 
exchange coupling between the crystal grains of hard and soft magnetic phases [3-5]. 
This coupling causes deviation of the magnetization around the boundaries of the 
crystallites, whose easy axes are unfavorably aligned with respect to the original 
magnetizing field, thus providing the remanence enhancement. It was found that this 
intergranular interaction becomes more pronounced at the nanoscale. Given that the 
degree of remanence enhancement depends on the effectiveness of the exchange 
coupling the significant research efforts are put into optimization of the microstructure. 
The main condition for obtaining nanocomposite structure is uniform distribution of soft 
and hard phase in a magnetic matrix where size of the crystal grains should be less than 
40 nm [5]. Depending on the starting chemical composition the Nd-low Nd-Fe-B alloys 
in optimal magnetic state can have Fe3B/Nd2Fe14B and/or α-Fe/Nd2Fe14B 
nanocomposite structure. 
Experimental results obtained in the course of presented study were used for 
analysis and discussion of structure and magnetic properties of the investigated R/Q Nd-
Fe-B alloy with 12 wt.% Nd after optimal heat treatment. 
 
2. Experimental 
 
The rapid-quenched Nd-low Nd-Fe-B alloy (Nd4.5Fe77B18.5) with 12 wt.% Nd 
was selected for the realized study. The investigated alloy was synthesized using the 
centrifugal atomization method and then annealed at 660°C for 5 minutes. The applied 
heat treatment regime was optimized in previous investigations [6-10]. Basic magnetic 
characteristics of the investigated alloy after annealing to the optimized magnetic state 
(jHc = 2.8 kOe, Br = 10.9 kG, (BH)max = 10.7 MGOe) were measured on a vibrating 
sample magnetometer (VSM) under external magnetic field of 50 kOe. Phase 
composition of the investigated alloy after applied optimal heat treatment regime was 
determined by the X-ray analysis (XRD). X-ray diffraction measurements were 
performed on an X’Pert PRO MPD multi-purpose X-ray diffraction system from 
PANanalytical using Co Kα radiation. Mean grain size and quantitative composition of 
the identified phases were calculated from XRD data by the FullProf computer program 
[11]. The X-Ray line broadenings were analyzed through refinement of the Thompson 
Cox Hastings-pseudo Voight (TCH-pV) function parameters, which in this case was the 
most reliable peak-shape function. The microstructure of the investigated alloy in 
optimal magnetic state was analyzed using transmission electron microscopy (TEM) 
and high resolution transmission electron microscopy (HREM) on JEOL JEM 200CX 
and PHILIPS CM200 microscopes, respectively. The samples for TEM and HREM 
analysis were prepared using focused ion beam microscopy (FIB). Hysteresis loops of 
the samples of investigated Nd-Fe-B alloy were obtained on the temperature of the 
ambient, on a Quantum Design MPMS 5XL Superconducting Quantum Interference 
Device magnetometer (SQUID) with magnetic field strength of 50 kOe. 
 
 
 
 
Ćosović et al- Study of microstructure and magnetic properties... 41 
3. Results and discussion 
 
The results of XRD phase analysis of the investigated Nd4.5Fe77B18.5 alloy after 
optimal heat treatment at 660oC for 5 min (Fig.1 and Table 1) show presence of hard 
magnetic phase Nd2Fe14B, soft magnetic phases with high saturation magnetization, 
predominantly Fe3B and partially α-Fe, as well as minor quantities of soft magnetic 
phases of FeB type. Since the intensities of obtained diffraction peaks of FeB phases are 
very low (Fig.1), it can be assumed that these phases are present in minor quantities and 
that their influence on magnetic properties is therefore insignificant, which is why their 
content was not determined. The contents and mean crystal grain sizes of main magnetic 
phases, determined using FullProf software, are presented in Table 1. The calculated 
mean grain size of analyzed phases confirms the nanocrystalline structure of the 
investigated alloy, with grain sizes below 30 nm. Together with the obtained phase 
composition, this nanocrystalline structure provides necessary conditions for more 
effective interaction of ferromagnetic exchange coupling between the grains of hard and 
soft magnetic Fe-rich phases, giving the raise to the remanence enhancement. 
 
40 60 80
Position [°2θ]
8000
12000
16000
20000
24000
C
ou
nt
s
Nd2Fe14B
FeB
Fe3B
Fe
 
Fig. 1. X-Ray diffractogram of the investigated rapid quenched Nd4.5Fe77B18.5 alloy in 
the optimal magnetic state 
 
Table 1. Phase composition and mean crystal grain size of the investigated 
Nd4.5Fe77B18.5 alloy in the optimal magnetic state 
Phases Content [wt. %] Grain size [nm] 
Nd2Fe14B 43.63 12.4 
α - Fe 16.54 5 
Fe3B 39.83 24 
FeB identified - 
42 MJoM Vol 15 (1) 2009 p. 39-44 
 
1 5 2 0 2 5 3 0 3 5 4 0 4 5
0
1 0
2 0
3 0
 G ra in  d ia m e te r  (n m )
 R
el
at
iv
e 
fre
qu
en
cy
 (%
)
Average = 27.0522 nm; 
St. Dev. = 5.283 
Fig 2. Bright field TEM micrograph of Nd4.5Fe77B18.5 alloy in optimal magnetic state 
with grain size distribution 
 
 
Fig.3 HREM nanograph of the investigated Nd-low Nd-Fe-B alloy in optimal magnetic 
state 
Ćosović et al- Study of microstructure and magnetic properties... 43 
The results of TEM analysis validate the mean grain size determined by XRD 
analysis, as it can be seen on presented bright field TEM micrograph (Fig. 2) showing 
the microstructure of the analyzed alloy in the optimal magnetic state. The average grain 
size determined by TEM analysis is also below 30 nm, while the corresponding grain 
size distribution indicates that the majority of grains have sizes in the range 20-30 nm. 
Taken together, the obtained results of XRD and TEM analysis imply that the 
investigated alloy in the optimal magnetic state has a nanocomposite structure of 
Fe3B/Nd2Fe14B and partly of α-Fe/Nd2Fe14B type. 
 
The HREM analysis (Fig.3) of the investigated R/Q Nd4.5Fe77B18.5 alloy in 
the optimal magnetic state has confirmed that in the microstructure of the alloy there are 
crystal grains with the sizes about 10 nm and less, as determined by the XRD analysis. 
The shape of the hysteresis loop obtained by magnetic measurements on SQUID 
magnetometer is in correspondence with the verified microstructure of the investigated 
alloy in the optimal magnetic state. 
 
-40 -20 0 20 40
-100
0
100
M
 [e
m
u/
g]
H [kOe]  
Fig. 4. Hysteresis loop of the investigated rapid quenched Nd4.5Fe77B18.5 alloy in the 
optimal magnetic state 
 
The hysteresis loop of the Nd-low alloy presented on Fig. 4 indicates the 
presence of the interaction of ferromagnetic exchange coupling between the grains of 
hard and soft magnetic phases, suggesting the nanocomposite structure of the 
investigated alloy in the optimized magnetic state. This assumption is supported by the 
obtained high value of remanence and calculated remanence ratio (Jr/Js = 0.6) that is 
higher than the theoretical limit given by the Stoner-Wohlfarth theory [5] for an 
assembly of isotropic non-interacting single domain particles for which remanence Jr is 
half of the saturation polarization Js. 
 
44 MJoM Vol 15 (1) 2009 p. 39-44 
4. Conclusion 
 
Characterization of the rapid-quenched Nd4.5Fe77B18.5 alloy in the optimal 
magnetic state using XRD, TEM and HREM analysis has confirmed the nanocomposite 
structure of the investigated alloy, with dominant presence of Fe3B/Nd2Fe14B and partly 
of α-Fe/Nd2Fe14B nanocomposites. The mean crystal grain size of the constituent phases 
was found to be below 30 nm. Furthermore, experimentally determined value of 
remanence ratio Jr/Js from the obtained SQUID hysteresis loops is higher than the 
theoretical limit, which confirms the presence of the interaction of ferromagnetic 
exchange coupling between the grains of hard magnetic phase Nd2Fe14B and identified 
soft magnetic Fe-rich phases with high saturation magnetization. Formed 
nanocomposites, via the intergranular interaction of ferromagnetic exchange coupling, 
are responsible for high value of remanence and obtained magnetic energy despite the 
reduced Nd content compared to Nd-Fe-B alloys with stoichiometric and 
overstoichiometric composition. 
 
Acknowledgement 
The presented work has been supported by the Ministry of Science of the 
Republic of Serbia under Project OI 142035B. The authors acknowledge support of the 
National Center for Electron Microscopy, Lawrence Berkeley Lab, which is supported 
by the U.S. Department of Energy under Contract # DE-AC02-05CH11231. 
 
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Study of microstructure and magnetic properties of optimally annealed R/Q Nd4.5Fe77B18.5 alloy

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Study of microstructure and magnetic properties of optimally annealed R/Q Nd4.5Fe77B18.5 alloy

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