APPLIED PHYSICS LETTERSVOLUME 82, NUMBER 1117 MARCH 2003

Structure and magnetic properties of high coercive NdFeB films
with a perpendicular anisotropy
L. K. E. B. Serrona,a) A. Sugimura, N. Adachi, T. Okuda,
H. Ohsato, and I. Sakamoto
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, 466-8555 Japan

A. Nakanishib)
Sumitomo Special Metals Co., Shimamoto-cho, Osaka, 618-0013, Japan

M. Motokawa
Institute for Materials Research, Tohuku University, Sendai, 980-8577, JapanD. H. Ping and K. Hono
National Institute for Materials Science, Sengen, Tsukuba 305-0047, Japan

共Received 16 November 2002; accepted 23 January 2003兲
Relatively good hard magnetic properties obtained from Nd2 Fe14B films prepared by the rf
sputtering technique were investigated in terms of the microstructural development. Althoughas-deposited films on Mo substrates deposited at substrate temperatures (T s ) of 365 °C were
amorphous with the dispersion of nanocrystalline NdO particles, columnar grains of Nd2 Fe14B
phase with the c-axis perpendicular to the film plane developed after annealing at 650 °C at anoptimized heating rate. Nd2 Fe14B grain size was about 400 nm in average and NdO particles ofabout 10 nm were dispersed within the grains and along the grain boundaries. These films
exhibited good perpendicular hard magnetic properties of i H c ⫽1356 kA/m and (BH) MAX⬃216 kJ/m3 . © 2003 American Institute of Physics. 关DOI: 10.1063/1.1561576兴

Anisotropic high-energy-product Nd2 Fe14B magnetic
thin films are the focus of intense interest because of theiroutstanding
potentials
formicromagnetic
deviceapplications.1,2 Since the sputter-controlled crystallization
process of Nd2 Fe14B thin films led to high coercivity and caxis crystalline texture,3 several studies were made toachieve excellent anisotropic behavior, higher coercivity, andbetter saturation polarization.4,5 In the postdeposition annealing process, it has been necessary to optimize both the sputtering and thermal annealing conditions separately in orderto optimize the transformation of the amorphous Nd–Fe–B
phase to a magnetically anisotropic crystalline Nd2 Fe14B
phase 共⌽ phase兲. One of the key factors revealed by a few
previous investigations is the effect of substrate deposition
temperature on the weak uniaxial magnetic anisotropy in theamorphous films.5,6 The existence of weak out-of-plane anisotropy in the soft magnetic phase was reported indicated by
the increase in both local and macroscopic anisotropy causedby the predominant location of easy axis magnetization perpendicular to the film plane. Earlier investigations further
reported that the appropriate substrate deposition temperature to attain excellent hard magnetic properties is limited
within a small range below the crystallization temperature(T cryst). In our previous investigation, we also found that theweak out-of plane anisotropy gave rise to enhanced magnetic
properties in annealed films even without any epitaxial rela-tionship between the 共200兲 oriented Mo substrate and theas-deposited amorphous Nd–Fe–B film.7 Recently, Shimaet al.8 reported on highly anisotropic thin films with the caxis columnar growth of the Nd2 Fe14B phase occurred by thecrystallization of amorphous Nd–Fe–B phase triggered by
the Cr cap layer and these columnar grains did not have any
epitaxial relationship with the substrate.
In this letter, we present magnetic and nanostructuralcharacteristics of 2 ␮m Nd–Fe–B as-deposited film deposited at substrate temperatures of 300 °C 关low temperaturedeposition 共LTD兲兴 and 365 °C 关high temperature deposition
共HTD兲兴 and correlate them with the corresponding magnetic
properties in the crystallized films. We will demonstrate thatweak perpendicular anisotropy in as-deposited film is a potential origin of good crystalline texture and enhanced perpendicular magnetic properties in these crystallized films.
The sputtering and annealing conditions are summarized
TABLE I. Sputtering and annealing conditions.
rf sputtering
TargetAr pressure
Substrate temperature
Deposition time
Deposition rate
Oxidation protection coating

Nd20Fe64B16⫹(1.5⫻1.5) cm2 Fe Sheet
6.7⫻10⫺1 Pa
300 °C 共LTD兲, 365 °C 共HTD兲
1h2 ␮m/hTi⬃700 ÅAnnealinga兲

On leave from the Department of MSE, University of the PhilippinesDiliman, Diliman, Quezon City, 1101 Philippines; electronic mail:leokebs@hotmail.comb兲
Present address: Takuma National College of Technology, Takuma-cho,
Mitoyo-gun, Kagawa 769-1192, Japan.

Vacuum pressure
Ramp rateTemperatureTime

6.7⫻10⫺4 Pa
50 °C/min
650 °C
30 min0003-6951/2003/82(11)/1751/3/$20.00
1751© 2003 American Institute of Physics
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Appl. Phys. Lett., Vol. 82, No. 11, 17 March 2003

Serrona et al.

FIG. 1. X-ray diffraction patterns and the corresponding low field part magnetic hysteresis loops for films deposited at 共a兲 300 °C 共LTD mode兲 and 共b兲
365 °C 共HTD mode兲.

in Table I. Magnetic and microstructural characterizations
were carried out by superconducting quantum interference
device 共SQUID兲 and x-ray diffraction 共XRD兲 analysis.
Cross-sectional and planar transmission electron microscopy
共TEM兲 observations were also made.
Figures 1共a兲 and 1共b兲 show the XRD patterns of asdeposited LTD and HTD films. Both exhibit broad halo pattern, which is the characteristics of the existence of an amorphous structure.
It can be understood that the Ti protective coating crystallized during deposition indicated by the 共100兲 and 共00•2兲
texture both in the LTD and HTD modes. No significant data
yet have been established whether this texture has any influence on the microstructure of the as-deposited and annealedfilms.
Figure 1共c兲 shows the corresponding low-field part magnetic hysteresis curves of as-deposited films, measured withthe magnetic field applied parallel to the film plane. Weak
perpendicular anisotropy (K A ) in the as-deposited films wasestimated by K A ⫽M s H s /2, where M s is the saturation magnetization value at a point H s where the hysteresis curve start
to saturate under an applied field.9 K A values estimated forthe films deposited at 300 and 365 °C were 0.014 and0.059 J/cm3 , respectively. It was reported by Kapitanov
et al.5 that the inclination in the loops with respect to the4 ␲ M -axis suggests the formation of uniaxial magnetic anisotropy in the magnetically soft state provided that deposition is made within a certain temperature range below thecrystallization point of Nd2 Fe14 B. Figures 2共a兲 and 2共b兲 are
TEM planar view images and the corresponding diffraction
patterns of the as-deposited films. In both diffraction patterns, no traces of the reflections from the Nd2 Fe14B phaseare observed. Figure 2共a兲 shows that the film is a homogeneous and featureless continuum suggesting a fully amorphous structure. Figure 2共b兲 is an image for HTD film having
small dark spots and long fibrous patterns of crystallized

FIG. 3. X-ray diffraction patterns of annealed 共a兲 LTD and 共b兲 HTD films.

particles observed in the amorphous matrix of the film.
Shown in Fig. 2共c兲 is one of these particles identified to be
NdO with a fcc structure (a⬃5 Å) by microbeam diffractionand energy-dispersive x-ray spectroscopy 共EDXS兲. These Nd
oxides are believed to have formed during the film deposition. The black and white contrasts of the particles are due tothe diffraction contrast from different crystallographic orientation. Observing the cross section TEM image of Fig. 2共b兲,NdO particles form a thin columnar stacking pattern separating the amorphous phase in a columnar morphology. The
tendency to form columnar-like morphology in the asdeposited film is probably associated with the shape anisotropy that caused weak out-of-plane anisotropy. The difference in the K A values and the distinction observed in thenanostructure of the as-deposited films may be related to thecolumnar grain structure formation after crystallization.
The x-ray diffraction patterns of the annealed films arepresented in Figs. 3共a兲 and 3共b兲. The diffraction pattern of
the annealed LTD film reveals the formation of randomly
oriented ⌽ phase. For the annealed HTD film, a pronounced
texture with the c axis of the tetragonal ⌽ phase aligned withfilm normal is observed. The appearance of strong (002l)
indexes indicates that the crystals of ⌽ phase grew with the caxis along the film normal. The existence of NdO 共111兲 and共220兲 is a significant concern because it may cause decreasein the volume fraction of ⌽ phase.
Magnetic hysteresis loops measured parallel and perpendicular to the film plane of the LTD and HTD films areshown in Fig. 4. The LTD film was found to be magneticallyisotropic as shown by the nearly similar hysteresis loops inboth field directions. Magnetic properties for this isotopicfilm are i H c⬜ ⫽1427 kA/m, 4 ␲ M r⬜ ⫽0.52 T and obviously
low energy-product which cannot be exactly estimated because the loop is not fully saturated even at high fields. In theHTD case, magnetic properties are i H c⬜ ⫽1356 kA/m,4 ␲ M r⬜ ⫽1.06 T, (BH) MAX⫽216 kJ/m3 and M r /M s ⬃1.
The HTD loop was corrected for demagnetizing field with afactor of 0.75. The perpendicular anisotropic properties areconsistent with the perpendicular orientation of the c axis of⌽ phase.
Figure 5共a兲 represents a bright-field planar view of acrystallized HTD film. The diffraction pattern obtained fromone of the grains is characterized as the 关001兴 zone axis ofthe Nd2 Fe14B phase. A weak, spotty ring-like pattern in themiddle part of the diffraction pattern corresponds to fcc NdO

FIG. 2. Planar view bright-field TEM images and corresponding diffraction
patterns for films deposited at 共a兲 300 and 共b兲 365 °C. 共c兲 High-resolution
electron microscope image and the corresponding Fourier transformed electron diffraction pattern of one of the small black NdO particles in Fig. 2共b兲.
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Serrona et al.

Appl. Phys. Lett., Vol. 82, No. 11, 17 March 2003

1753

FIG. 5. TEM images of HTD film with corresponding e-beam diffraction
patterns: 共a兲 a planar view grain size and 共b兲 cross-sectional view.

FIG. 4. Magnetic hysteresis loops of annealed LTD and HTD films, formagnetic field directions parallel 共储兲 and perpendicular 共⬜兲 to the film plane.
The nucleation type initial magnetization curve of the film (H⬜plane) isshown in an inset figure. (BH) MAX for the HTD film is around 216 kJ/m3 .

(a⬃5 Å), which are embedded in the matrix grains. AsXRD analysis results indicates 共Fig. 3兲, the variation in thevolume fraction of NdO against ⌽ phase may also affect themagnetic properties of the films. Figure 5共b兲 is a bright-fieldcross-section TEM image of the annealed HTD film. As it isclearly seen, a strongly textured columnar grain with an approximate diameter of around 400 nm stands vertically on
the substrate surface. The diffraction pattern for the columnar
grain represents the 关310兴 zone axis, suggesting that the caxis is normal the film plane.
The annealed HTD film resulted in a highly c-axis textured and high-energy-product thin film magnet. Lileev
et al.6 pointed out the existence of an isotropy-anisotropytransition at 340 °C⬍T s ⬍395 °C. The result of the present
study is consistent with their work, that is HTD sample resulted in high perpendicular coercivity, while LTD sample
was rather isotropic. The remarkable magnetic properties of
the annealed HTD film can be attributed to the completely
crystallized ⌽ phase grains that dominate the entire structure
together with a considerable amount of nonmagnetic NdOparticles. Although the origin of high coercivity and its
mechanism is not yet fully understood, more TEM work is
still underway to examine closely the possible cause of theformation of large and columnar grains that is considered to
be a major source of perpendicular anisotropy. Planar andcross sectional TEM images in Fig. 5 reveal the presence ofa lot of NdO particles both within the grains and along thegrain boundaries. Grain boundaries are considerably sharp
without any indication of a grain boundary phase. The
Nd20Fe64B16 共target composition兲 phase is Nd and B richwith respect to the stoichiometric composition of theNd2 Fe14B phase, thus the formation of NdO is possible. It
will remove excess Nd and increase the resultant volume
fraction of the Nd2 Fe14B phase. The excess Nd is transformed into a nanometer-scale distribution of Nd oxides bothin the amorphous and annealed state as confirmed by microdiffraction and EDXS investigation. The occurrence of NdO
represents oxidation that took place during the HTD deposition and annealing processes. The volume fraction of theoxide phase in the film is considerable, however, these NdO
particles seem to play no significant role for magnetic do-main wall pinning. Pinning can correspond to high coercivity
in the films but initial magnetization curve of the annealed
films is of nucleation type as shown in an inset in Fig. 4.
The Mo substrate used has a surface roughness 共rms兲 of⬃104 nm and also, due to the nonepitaxial depositiongrowth of the film, defects and vacancies in the substratefilm interface may occur resulting in a substantial interface
stress. Figure 5共b兲 also shows a thin layer of nanocrystalline
particles separating the Mo grains from the homogenous
Nd2 Fe14B grains. During rapid crystallization grain growth,
nucleation of the ⌽ phase may have taken place in Nd richareas enclosed by clustered NdO particles previously deposited from the Nd–B rich target. Nucleation seed particles
started to grow at the expense of very fine crystallites toreduce interfacial energy of the system. At the interface, particles remained unaffected due to the difficulty of overcoming defects and internal stresses.
We showed that the presence of weak out-of-plane anisotropy and existence of NdO particles in as-deposited
NdFeB thin films depend sensitively on T s . The as-deposited
film grown at 365 °C containing clustered NdO particles
yielded a higher effective anisotropy constant of 0.059 J/cm3
possibly caused by the shape anisotropy from the columnarlike formation at the cross section of the film. The weakanisotropy resulted to the realization of highly anisotropic
and high-energy product thin film after crystallization. TEM
observations confirmed the existence of fcc NdO and columnar ⌽ phase grains having nearly single domain size around
400 nm. A sharp interface exists between the 关001兴 oriented
Nd2 Fe14B columnar grain structures.

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