Materials Science and Engineering B97 (2003) 59 /63
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Magnetic and structural properties of NdFeB thin film prepared bystep annealing
Leo K.E.B. Serrona a,1,, A. Sugimura a, R. Fujisaki a, T. Okuda a, N. Adachi a,
H. Ohsato a, I. Sakamoto a, A. Nakanishi b, M. Motokawa caNagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japanb
Sumitomo Special Metals Co., Shimamoto-cho, Osaka 618-0013, Japanc
Institute for Materials Research, Tohuku University, Sendai 980-8577, Japan

Received 8 May 2002; received in revised form 20 September 2002; accepted 8 October 2002

Abstract
The crystallization of the amorphous phase into the tetragonal Nd2Fe14B (F) phase and the corresponding changes in magnetic
properties have been examined by step annealing experiment using a 2 mm thick NdFeB film sample. Microstructural and magnetic
analysis indicate that the film was magnetically soft as deposited with the coercivity HciÞ B/16 kA m 1 and the remnant
magnetization 4pMrÞ B/0.02 T. Annealing at a temperature of 550 8C, a coercivity value around 784 kA m 1 was developed anddiffraction analysis showed evidence of F phase 002l peaks being aligned perpendicular to the film plane. At an optimum annealing
temperature of 575 8C, the remnant magnetization of this anisotropic thin film is around 0.60 T with intrinsic coercivity of /1340
kA m 1. Annealing the film sample at 200 8C5/Tann 5/750 8C showed variations in magnetic properties that were mostly due tothe change in the perpendicular anisotropy. Based on 4pMsÞ values plotted against Tann, a dip in 4pMs Þ values was observed as
Tann increased in the soft-to-hard magnetic characteristics transition region and rose as the hard crystalline phase started to form.
The results show that the magnetic properties of the NdFeB film were slightly influenced by the presence of NdO, film surface
roughening and the small increase in crystal size as a consequence of repeated heat treatment. At Tann /300 8C, the nominal
saturation magnetization indicated a certain degree of weak perpendicular magnetic anisotropy in the film sample considered to be
essential in the enhancement of coercivity in crystallized films.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: RF sputtering; Anisotropic NdFeB thin film; Vacuum annealing; High coercivity

1. Introduction
The magnetic and structural properties of NdFeB thinfilms represent a subject of growing scientific interest
because of the wide range of applications particularlythat of anisotropic and high coercivity permanent
magnet materials. The motivation for these investiga- Corresponding author
E-mail address: leokebs@hotmail.com (L.K.E.B. Serrona).
1
On leave from the Department of MSE, University of thePhilippines-Diliman, Diliman, Quezon City 1101, Philippines.

tions has come from the potential use of NdFeB thinfilm permanent magnet for micro-mechanical systems[1], micro-electronics and micro-magnetic devices [2].
In this paper, we will report on the crystallization
behavior of an NdFeB thin film showing the transformation of the magnetic properties in the soft-to-hardmagnetic characteristics transition region. In bulk
permanent magnet materials, the effect of heat treatments on the hard magnetic properties has been
extensively investigated particularly by post sintering
heat treatments [3]. The primary effect of heat treatment
on permanent magnet materials is on the microstructure, which in turn influences the coercivity. Previously,

0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 4 0 1 - 4

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L.K.E.B. Serrona et al. / Materials Science and Engineering B97 (2003) 59 /63

we found that using the typical method of annealing
under optimum conditions, film sample of high coercivity characterized by the growth of magnetic grains withthe c-axis oriented along the film normal is easily
produced from as-deposited films [4]. The same kindof as-deposited film sample used in the previous experiment was used in this study.
The motivation of this investigation is based on achallenge to analyze information on the behavior of themagnetic and structural properties of an NdFeB film
sample in each stage of the step annealing experiment.

a substrate-target distance of 5.0 cm. The purpose of theglass insulation between the Mo substrate and thecopper holder is to obtain weak perpendicular anisotropy in the as deposited film as reported in [4].
Diffraction patterns of the film sample were analyzed
by an X-ray diffractometer (Philips X’pert-MPD) using
Cu /Ka radiation. Minor hysteresis loops were plottedusing a vibrating sample magnetometer (VSM) with amaximum applied field of 1600 kA m 1, which is notenough to record the full hysteresis loops of crystallized
films.

2. Experimental procedure

3. Results and discussion

In this experiment, as-deposited NdFeB film sample
prepared by RF sputtering was annealed repeatedly invacuum at 200 8C 5/Tann 5/750 8C as depicted in Fig.
1. Each annealing step was done for 30 min after whichmagnetic and structural characterization was done on
the film. The concept of step annealing experiment isrelated to the idea of post sintering experiments in bulk
permanent magnet materials [3]. Improvement of magnetic properties depends on the temperature and duration of heat treatment, which are related directly to thegrain growth mechanisms. The thin film form of NdFeB
permanent magnet material may exhibit nearly similarmagnetic properties compared with the bulk form. It is
for this reason that the behavior of its magnetic
properties may respond similarly to some of the processing methods in bulk materials like thermal annealing.
The 2 mm film sample was deposited on a 0.1 mm Mo
substrate using Nd, B rich and Fe poor target withcomposition Nd20Fe64B16 compared with the stoichiometric composition Nd12Fe82B6. A thin Ti coating withthickness around 700 Å was deposited in order tominimize oxidation on the surface of the NdFeB film.
Sputtering deposition power used was 350 W with adeposition rate of around 260 Å min 1 in a constant
supply of Argon gas at a vacuum pressure of 6.7 /10 1
Pa. The substrate is thermally insulated from the watercooled substrate holder with a 1.0 mm glass and held at

3.1. X-ray diffraction patterns

Fig. 1. Schematic diagram of the step annealing procedure performedon the film sample.

Fig. 2. X-ray diffraction patterns of annealed film plotted in increasingorder of the annealing temperature.

Fig. 2 shows XRD patterns of the initial heat
treatments up to Tann /500 8C indicating that theL.K.E.B. Serrona et al. / Materials Science and Engineering B97 (2003) 59 /63

film remains in its amorphous structure. The patterns
exhibit broad halo pattern suggesting existence of anamorphous phase with a prominent Ti 00+ 2 peak fromthe surface coating material and other peaks from theMo substrate. It also shows a small broadening andslight line shift towards smaller 2u of the Ti 00+ 2 peakreflection. It can be attributed to the presence of a nonuniform strain in the amorphous film. Annealing at
Tann ]/550 8C, the film undergoes a structural transformation to F phase as the major component with
distinct preferential alignment of the magnetically easy
direction (tetragonal c axes) normal to the film plane. It
can also be observed that the diffraction patterns of thefilm annealed at higher temperatures ]/650 8C showed
the existence of some neodymium oxide. Similar observations were made by Gasgnier et al. [5].
A good estimate of the onset of crystallization of thesample in this experiment can be based on a plot ofcompositional dependence of crystallization temperature of NdFeB glasses formed by melt spinning which
may take place at around 550 8C [6]. Optimum vacuum
annealing conditions in our previous experiment give us
the best possible magnetic properties at 650 8C for 30
min [4,9]. And in fact, enhanced coercivities in sinteredand melt spun NdFeB magnets are attained after heat
treatment at around 650 8C [7].
The gradual discoloration of the reflecting surface of
the as-deposited film to a faded surface after the entire
annealing process was also observed. It can be considered to be a change in the surface roughness of thefilm, which resulted from the formation of F phaseduring crystallization wherein the c -axis is oriented
along the film normal. Usually, randomly oriented
film is characterized by a shiny flat surface.
The sharp peaks of the polycrystalline film show thegood crystalline quality composed of magnetic grains
with diameters between 433 and 454 Å estimated by Xray line broadening analysis [8] using the F phase 006
peak. Considering that some of the individual grains infilm material are under non-uniform strain, this estimation is still subject to further experiments. At present,
accurate determination of the origin of the observed
broadening needs to be confirmed. In contrast with castmaterials, the crystal size indicated a small increaseduring annealing up to Tann /750 8C. These results
may have specific influence on the magnetic property of
the film sample.
3.2. Magnetic hysteresis loops
Fig. 3 shows the magnetic hysteresis loops measured
at room temperature with applied field normal andparallel to the film plane of as deposited film in thestages of step annealing at 200 8C 5/Tann 5/750 8C.
For Tann B/550 8C, M /H loops are quite similar andindicate soft magnetic properties with coercivities less61

Fig. 3. Room temperature hysteresis loops as measured perpendicular
to plane and in-plane parallel to the maximum applied field of 1600 kAm 1.

than 16 kA m 1. No significant difference in coercivity
and magnetization values was observed. On the other
hand, magnetic properties of films annealed at Tann ]/550 8C varied significantly with the coercivity values(HciÞ )/132/1340 kA m 1 and 4pMrÞ /0.2 /0.6 T.
Minor hysteresis loops of the films measured withapplied field parallel and perpendicular to the film plane

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L.K.E.B. Serrona et al. / Materials Science and Engineering B97 (2003) 59 /63

Fig. 4. Annealing temperature dependence of the coercivity, remnant
magnetization and saturation magnetization measured with appliedfield perpendicular to the film plane. The data points were curve fitted
to aid visual analysis only.

revealed the presence of the soft and hard phase in thefilm before and after crystallization at around 550 8C,
respectively. The magnetically soft phase present in thefilm annealed at Tann 5/500 8C showed weak perpendicular anisotropy that was deduced from the saturation
magnetic field of the hysteresis curves [9].
Fig. 4 is a summary of the magnetic properties plottedin Fig. 3. It shows the dependence of coercivity (Hci),remanent magnetization ( 4pMrÞ ) and saturation magnetization (4pMsÞ) on the annealing temperature. The
data points were approximately curve fitted but theminimum and maximum peaks in the curves do notnecessarily coincide with the low and high data points ofthe magnetic properties. These Hci, 4pMrÞ and 4pMsÞ
values were obtained from the minor hysteresis loops
measured with a maximum applied field (H) of 1600 kAm 1 perpendicular to the film plane.
Dependence of 4pMsÞ on Tann is almost constant up
to 500 8C. For Tann /500 8C, 4pMsÞ decreases as aconsequence of the formation of the crystalline phasewith higher coercivity. Since the crystalline F phase isthought to be completely formed at Tann /575 8C,
4pMsÞ value is at a minimum around 0.67 T while
coercivity is at a maximum value of 1340 kA m 1
maintaining the high energy density of the film at thisannealing temperature. Increasing Tann further, a different behavior is observed where both Hci and 4pMrÞ
decrease together with the increase in 4pMsÞ. In thistransition region, a dip in the 4pMsÞ plot is evidently
indicating transformation of the soft magnetic phase to
the hard magnetic phase. It is in this transition region
that change in crystal structure occurs and brings acorresponding change in the magnetization of the film
sample. Around 550 8C of annealing temperature, soft
amorphous phase was brought to either partial or fullcrystalline state associated with the increase in Hci.
Therefore, the combination of soft and hard phases in
the film resulted to changes in the perpendicularmagnetic anisotropy. This behavior has been previously
obtained on permanent magnet materials without rareearths [10,11]. Gu et al. demonstrated similar results on
a series of sputtered NdFeB thin films annealed at
550 8C 5/Tann 5/850 8C [12].
At the onset of crystallization (500 8C 5/Tann 5/
550 8C), existence of a soft magnetic phase is indicatedby a small bend which is pointed by an arrow in Fig. 3,
in the magnetization curve of the in-plane hysteresis
loop around H/0 kA m 1. In-plane loops of films
annealed at Tann /550 8C and 575 8C, are shown
indicating presence of small amount of soft magnetic
phase. Above Tann /575 8C, no more evidence of theexistence of soft magnetic phase like an amorphous
phase or Fe is observed. Behavior of the 4pMsÞ curve atTann ]/550 8C, shows that the only ferromagnetic phasewith Curie temperature (Tc) higher than room temperature is the F phase. Saturation magnetization of as
deposited film is nearly equal to the saturation magnetization measured after annealing at Tann /700 8C.
Fig. 5 shows the dependence of the nominal saturation points (MNS) on the annealing temperature. MNS is
assumed to be a point where the rotation of themagnetic moments commences towards the directionof the applied field, which will eventually lead tosaturation [9]. Amorphous films prepared at 340 8C
by Lileev et al. [13] were found to have weak perpendicular anisotropy, which led to high coercivity afterthermal annealing. At temperatures below 340 8C,
isotropic and lower coercivity films were produced.
Deposition at temperatures above 340 8C resulted inanisotropic and low coercivity films. These amorphousfilms prepared on heated substrates at 340 8C may haveacquired the right amount of ferromagnetic substructure
characterized by weak perpendicular anisotropy, which
contributed to the enhancement of coercivity in the film
sample after thermal annealing.
The curve in Fig. 5 is plotted as a measure of weak
perpendicular anisotropy in the film in each annealing
step. MNS varied as a consequence of accumulated heat
treatment showing a peak at 300 8C.
The peak suggests that a sufficient amount offerromagnetic substructure is also present and ready to

Fig. 5. Dependence of nominal saturation field (MNS) on the annealing temperature (Tann).

L.K.E.B. Serrona et al. / Materials Science and Engineering B97 (2003) 59 /63

be transformed to F phase if annealed at optimum
conditions. Below 300 8C, the start of the formation of
the substructure can be estimated by the extent of
nominal saturation value in the figure. Step annealingat temperatures higher than 300 8C, MNS decreased
corresponding to the reduction of weak perpendicularanisotropy and less possibility of getting high coercivity
if annealed under optimum conditions. The correlationof these data and those from [13], is justifiable becauseof the fact that in previous experiments [4,9] higher MNS
value is desired for the as-deposited films. Although
there is slight difference in processing routes, the effectof temperature that focused on the internal structure ofamorphous phase turned out to be remarkably interesting based on the aforementioned findings. It is, therefore, worthy to note that amorphous films can beprepared under specific conditions for the purpose of
reproducing thermally crystallized NdFeB films withexcellent magnetic properties.
As a final point, the optimum properties of thisferromagnetic thin film depend on the extent of heat
treatment. Since the film sample possesses both the
crystallographic texture and perpendicular anisotropy, it
is reasonably preferable to simultaneously control bothduring heat treatment to obtain a perpendicularly
oriented and high energy product thin film material.

4. Conclusions
We have demonstrated the effect of successive vacuum annealing of as-deposited NdFeB film sample at
200 8C 5/Tann 5/750 8C. The transformation of thesoft amorphous phase to the hard magnetic phase iscarried out through a crystallization process characterized by a significant change in the structural andmagnetic properties of the film. The results showed the
crystallization behavior of the film with an optimum
Tann /575 8C from which Hci and 4pMsÞ are nearly of
optimal value. Magnetization varied considerably according to the material phase developed while Tann isincreased. Hci and 4pMrÞ varied significantly withrespect to the change in Tann from 550 to 750 8C,
which can be attributed to the changes in the microstructure of the film. On the other hand, 4pMsÞ appears
to be constant except for a dip during the soft-to-hardphase transition region where a change in the crystal
structure occurred. The step annealing technique pro-63

vided valuable information on the behavior of an
NdFeB thin film under different annealing temperature
conditions. Of particular interest are the properties of
the amorphous phase, which show signs of perpendicular anisotropy based on the nominal saturation
magnetization. The findings in this work are very
important in an effort to finally establish a correlation
between the magnetic properties of amorphous andcrystallized film.

Acknowledgements
Leo K.E.B. Serrona acknowledges the scholarshipgrant of the Japanese Government (Monbu-kagakusho). He is presently a graduate student of the
Department of Materials Science and Engineering at
the Nagoya Institute of Technology.

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