Structure and magnetic properties for FePt thin films
prepared on MgAl 2 O 4 and MgO substrates
H. Iwama, M. Doi and T. Shima
Graduate School of Engineering, Tohoku Gakuin University, Tagajo, 985-8537, Japan

In order to investigate the effect of lattice mismatch between FePt thin films and single crystal substrates on thetetragonality and the magnetization process, FePt thin films have been fabricated on MgAl2O4 (MAO) (100) and MgO(100) single crystalline substrates at a substrate temperature of 700 °C. The Fe concentration in the FePt films wasvaried from 45.0 to 50.8 at. %. In addition to the fundamental (002) peak, (001) and (003) superlattice peaks haveclearly been observed in the X-ray diffraction patterns for all the samples, indicating the formation of L10 orderedstructure. The magnetization measurements show that all the samples are perpendicularly magnetized. Large
coercivity (Hc) of 57.8 kOe and 54.4 kOe was observed for the films with Fe49.3Pt50.7 and Fe48.3Pt51.7 (at. %) depositedon MgO and MAO substrates, respectively. It was confirmed that good hard magnetic properties can be obtained for
the Fe49Pt51 (at. %) thin film.
Key words: L10 ordered FePt, hard magnetic material, thin film, MgO substrate, MgAl2O4 substrate
1. Introduction
The magnetization process of assemblies offerromagnetic nanoparticles with a large uniaxial
magnetocrystalline anisotropy is of great scientific andtechnological interest, since they are expected to be
applied in forthcoming magnetic devices such as highdensity magnetic recording media and high performancebiasing nano-magnets. It is well known that themagnetization process and the coercivity depend
strongly on the characteristic size and the morphologyof the assemblies of ferromagnetic particles. Since FePt
alloy shows high uniaxial magnetocrystalline anisotropy(Ku = 7.0×107 erg/cm3) [1], moderate saturation
magnetization and high corrosion resistance, several
studies have recently been done on L10 ordered [CuAu(I) type] FePt thin films [2–14] and nanoparticles [15,16] using conventional thin film preparation methods
and chemical syntheses. It is also thought to overcome
the instability of magnetization vectors caused bythermal fluctuation even in the nano-meter scaled
particles. However, magnetization process of FePt thinfilms has not been fully elucidated. The latticemismatch plays a crucial role in the growth of epitaxial
thin films. Generally, MgO single crystalline substrate
has been commonly used to induce the L10 ordered (001)texture of FePt films. Recently, low temperature
fabrication of epitaxial FePt thin films below 400 ºC
have been reported for MgAl2O4 [17] single crystalline
substrates. However, systematic investigations of theeffects of lattice mismatch on the structure andmagnetic properties of FePt films have rarely beenreported. Therefore, it would be worthwhile to study the
structure and magnetic properties of FePt films
prepared on different substrates. For this purpose, two
single crystal substrates MgO (NaCl-type) and MgAl2O4(Spinel-type) were chosen. The values of unstrained
lattice mismatch between a-axis of L10 FePt ordered

alloy and these substrates are 8.4 % for MgO [18] and3.8 % for MgAl2O4 [19].
In this study, in order to investigate the effect oflattice mismatch between FePt thin films and single
crystal substrates on the tetragonality and themagnetization process, FePt thin films have been
fabricated on MgAl2O4 (MAO) (100) and MgO (100)single crystalline substrates, their structure andmagnetic properties have been investigated.
2. Experimental procedure
All the samples were prepared using an ultrahighvacuum magnetron sputtering system (ULVAC, QAM4)
with co-deposition of Fe and Pt directly onto polishedsingle crystalline MgO (100) or MAO (100) substrates.
The lattice parameters of L10 ordered FePt alloy, MAO(100) substrate and MgO (100) substrate are aFePt =0.385 nm, cFePt = 0.371 nm (L10-Fe50Pt50 (at. %)) [20],aMAO = 0.404 nm [19] and aMgO = 0.420 nm [18]. The
targets were commercial products with purities higher
than 99.99 at% for Fe and 99.9 at% for Pt. The basepressure was under 8.5×10–7 Pa. High-purity argon of
0.2 Pa was introduced during sputtering. The substrates
were heated to Ts = 700 ºC during deposition. The
nominal thickness of FePt layer was fixed at 10 nm. The
compositions of the films were determined by anelectron probe X-ray microanalysis (EPMA) and X =45.0, 47.2, 48.3, 48.8, 49.3, 49.8 50.3 and 50.8 for
FeXPt100–X (at. %) are confirmed. The structural analysis
was performed by X-ray diffraction (XRD) with Cu Kα
radiation. The lattice constant of c-axis was obtained by
the out-of-plane XRD measurement, while the value ofa-axis was obtained by the in-plane XRD measurement.
The film morphology was observed by atomic force
microscopy (AFM, in tapping mode). The magnetic
properties were measured by a superconducting
quantum interference device (SQUID) magnetometer inthe field up to 70 kOe at room temperature (R. T.).

Fig. 1.

XRD patterns for FePt thin films preparedon MgO (100) substrate. Inset show thein-plane XRD patterns for FePt thin films.
The Fe content X for FeXPt100–X films are
45.0 (a), 47.2 (b), 48.3 (c), 48.8 (d), 49.3 (e),49.8 (f), 50.3 (g) and 50.8 (at. %) (h).

3. Results and discussion
XRD patterns for FeXPt100–X films with various Fe
content prepared on MgO (100) substrate are shown in
Fig. 1. The Fe content was varied as follows: X = 45.0 (a),47.2 (b), 48.3 (c), 48.8 (d), 49.3 (e), 49.8 (f), 50.3 (g) and
50.8 (at. %) (h). In addition to a fundamental (002) peak,(001) and (003) superlattice peaks of the L10-FePt phasewere clearly observed for all the samples. The peak fromthe other plane of the L10 structure was not observed.
Therefore, it is confirmed that the c-axis of FePt layers
was aligned perpendicular to the film plane for the FePt
films. With increasing X, the peak position of the (001)plane shifts to a higher angle, suggesting that thelattice parameter of c-axis decreases due to the changeof the tetragonality of L10 ordered FePt phase. From theintegrated intensities of fundamental and superlattice
peaks extracted from numerical fitting, the degree of
long-range chemical order parameter S was evaluated.
The detailed procedure for the evaluation of S was
described in a previous letter [21]. The S shows the
maximum of 0.80±0.05 for Fe49.3Pt50.7 (at. %) (e).
Magnetization curves for the FeXPt100–X films
prepared on MgO (100) substrate are shown in Fig. 2.
All the measurements were preformed at R.T.. The solidand broken lines denote the magnetic fields applied in
the perpendicular direction to the film plane andin-plane directions, respectively. The magnetic easy
axes are perpendicular to the film plane for all thesamples, since the [001] axis of the tetragonal L10
ordered structure was perpendicular to the film plane as

Fig. 2.

Magnetization curves for FePt thin films
prepared on MgO (100) substrate. The Fecontent X for FeXPt100–X films are 45.0 (a),47.2 (b), 48.3 (c), 48.8 (d), 49.3 (e), 49.8 (f),50.3 (g) and 50.8 (at. %) (h).

demonstrated in Fig. 1. Coercivity (Hc) of 38.4 kOe wasobtained for the film with X = 45.0 (at. %) (a). The
maximum Hc of 57.8 kOe was obtained for the film with
X = 49.3 (at. %) (e). With further increasing of Fe
content, Hc decreases slowly, but still keeps a quite
large value of 48 kOe for the film with X = 49.8 (at. %)(f). However, Hc decreased to 27.7 kOe for the film with
X = 50.8 (at. %) (h). The in-plane magnetization does not
saturate even at a magnetic field of 70 kOe, which is the
maximum magnetic field of the SQUID magnetometer,
indicating that they possess high uniaxial magnetic
anisotropy. A remarkable step in the initial
magnetization curve was observed for all samples. A
steep increase of the magnetization at low magnetic
field was observed. The fractional magnetization at low
magnetic field corresponds to the magnetic domain wall
displacement; in other words, particles larger than 200
nm (single domain size [22]) with multiple domain
structure are magnetized at low magnetic field.
However, it is to be magnetized at applied magnetic
field more then 10 kOe, because they contain single
domain particles. Hence, the magnetization process
governed by the magnetic domain wall displacement in
the multiple domain particles and the rotation of themagnetization in the single domain particles.
XRD patterns for FeXPt100–X films prepared on MAO(100) substrate are shown in Fig. 3. The Fe content wasvaried as follows: X = 45.0 (a), 47.2 (b), 48.3 (c), 48.8 (d),49.3 (e), 49.8 (f), 50.3 (g) and 50.8 (at. %) (h). Bothfundamental and superlattice peaks of the L10-FePt
phase were clearly observed for all the samples. The
unlabeled peaks were due to the MAO (100) substrate.

Fig. 3.

XRD patterns for FePt thin films preparedon MgAl2O4 (100) substrate. Inset show thein-plane XRD patterns for FePt thin films.
The Fe content X for FeXPt100–X films are
45.0 (a), 47.2 (b), 48.3 (c), 48.8 (d), 49.3 (e),49.8 (f), 50.3 (g) and 50.8 (at. %) (h).

The peak from the other plane of the L10 structure was
not observed. It is also confirmed that c-axis of FePt
layers was successfully aligned perpendicular to thefilm plane for the films deposited on the MAO (100)substrate. This indicates that the highly (001) oriented
L10 structure was achieved for the FePt films, even withdifferent Fe content of the FePt layer. From theintegrated intensities of fundamental and superlattice
peaks extracted from numerical fitting, the degree of
long-range chemical order parameter S was evaluated.
The S shows the maximum of 0.74±0.05 for Fe48.3Pt51.7(at. %) (c). The clear four fold symmetry was also
confirmed in both of FePt thin films on MgO (100) andMAO (100) substrates by 2θχ/φ scan of XRD. As a result,the epitaxial growth of FePt thin films on MgO (100)and MAO (100) substrates were confirmed. From theAFM observation, island growth with the island size of
about 150 nm was observed in both the FePt thin films
on the MgO (100) and the MAO (100) substrates.
Magnetization curves for FePt films with various
Fe content prepared on MAO (100) substrate are shownin Fig. 4. The magnetic field was applied in theperpendicular (solid line) and in-plane (broken line)
directions to the film. The easy magnetization axis isaligned perpendicular to the film plane for all thesamples. The [001] axis of the tetragonal L10 orderedstructure was perpendicular to the film plane asdemonstrated in Fig. 3. Hc of 44.1 kOe was obtained forthe film with X = 45.0 (at. %) (a). The maximum Hc of
54.4 kOe was obtained for the film with X = 48.3 (at. %)(c). With further increasing Fe content, Hc decreases
slowly, but still keeps a quite large value of 52.1 kOe forFig. 4.

Magnetization curves for FePt thin films
prepared on MgAl2O4 (100) substrate. The Fecontent X for FeXPt100–X films are 45.0 (a),47.2 (b), 48.3 (c), 48.8 (d), 49.3 (e), 49.8 (f),50.3 (g) and 50.8 (at. %) (h).

the film with X = 49.8 (at. %) (f). However, Hc wasdecreased to 44.8 kOe and 30.8 kOe for the films with X= 50.3 (g) and 50.8 (at. %) (h), respectively. Similar to
the result observed in the initial magnetization curvesof the films deposited on MgO (100) substrate, themagnetization process for the FePt films deposited on
MAO (100) substrate is thought to a mixture of amagnetic domain displacement and a rotation of themagnetism.
The a-axis, c-axis, c/a and Hc for FePt thin films
deposited on MgO (100) and MAO (100) substrates withdifferent Fe content are summarized in Fig. 5. Solidcircles and solid triangles denote the data for the FePt
film deposited on MgO (100) substrate, while open
marks are denote the data for the films deposited on
MAO (100) substrate. Maximum value of a-axis was
obtained for both substrates at X = 47.1 (at. %).
However, a-axis was decreased with further increased of
Fe content. While monotonical increase was obtained forc/a more than X = 48 (at. %) and maximum. Hc was
obtained at X = 49 (at. %) for both films. From theseresults, it is confirmed that lattice spacing of a-axis isstrongly dependent on the composition rather than theone of c-axis. The reason why the FePt films depositedon MAO (100) substrates show large Hc at wide
composition region is thought to arise from a smalllattice mismatch between the substrate and the FePt
film. The more detail investigation of morphologicalanalysis will be necessary for the exact discussion of Hc.
4. Summary
The structure and magnetic properties of FePt films

University.
References
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Fig. 5.

Effect of Fe content X on the lattice constantof a and c-axis, the tetragonality represented
by c/a and coercivity Hc for FeXPt100–X thinfilms prepared on MgO (100) and MgAl2O4(100) single crystalline substrates.

11)
12)
13)

prepared on MgO (100) and MAO (100) substrates havebeen investigated. The FePt layer was epitaxially grownon MgO (100) and MAO (100) substrates. Large Hc of
57.8 kOe and 54.4 kOe was obtained for the films with
Fe49.3Pt50.7 and Fe48.3Pt51.7 (at. %) deposited on MgO(100) and MAO (100) substrates, respectively. It was
confirmed that good hard magnetic properties can beobtained for the Fe49Pt51 (at. %) thin film. Although,similar behavior of Hc dependence on the composition
was observed in FePt thin films on MgO (100) and MAO(100) substrates, large Hc can be obtained at wide
composition range of FeXPt100–X thin films, even at low
Fe content between 45.0 and 48.3 (at. %).

14)

Acknowledgements
This work was partially
supported by the Research Institute for Engineeringand Technology at Tohoku Gakuin University. This
work is a cooperative program of the Cooperative
Research and Development Center for Advanced
Materials, Institute for Materials Research, Tohoku

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Received Oct. 19, 2015; Accepted Mar. 24, 2016