Co75Fe25 (at. %) alloy films of 100 nm thickness are prepared on MgO(001) and (110) single-crystal substrates. A Co75Fe25(001)bcc single-crystal film is epitaxially grown on the MgO(001) substrate, whereas a Co75Fe25(211)bcc bi-crystalline film is formed on the MgO(110) substrate. The out-of-plane and in-plane lattice spacings of Co75Fe25 films are in agreement with the bulk values within small differences of less than 1%. The out-of-plane and in-plane orientation dispersions are smaller than 2°. These results show that well-defined Co75Fe25 epitaxial films consisting of single bcc phase are successfully obtained on both the MgO(001) and (110) substrates. The in-plane magnetic anisotropies of single- and bi-crystalline films are confirmed to be reflecting the magnetocrystalline anisotropy of Co75Fe25 crystal with the easy <111> and hard <100> axes and the demagnetization field. The magnetostriction coefficients, (λ100, λ111), of single- and bi-crystalline films are determined to be (+60×10–6, +150×10–6) and (+55×10–6, +185×10–6), respectively. The present study has shown that a Co-Fe alloy with Co-rich composition has positive moderately-large λ100 and fairly-large λ111 values.
Co-Fe alloys have recently attracted much attention as one of magnetostrictive materials and have been investigated for applications such as sensors, actuators, vibration energy harvesting devices, etc. Large magnetostriction of +150×10–6 – +250×10–6 is observed for Co-Fe alloys with Co-rich compositions,1–4 where bcc lattice is locally deformed by inclusion of fcc nanocrystals.2–5 In order to clarify the intrinsic magnetostriction property, it is important to understand the magnetostriction coefficients along <100> and <111>, λ100 and λ111. However for Co-rich compositions, these values have not yet been made clear. In our previous study,6 the magnetostrictive properties of Co-Fe alloys with Fe-rich compositions were investigated by employing epitaxial films. The purpose of the present study is to prepare well-defined Co-Fe alloy epitaxial films with Co-rich composition (Co75Fe25), where bcc single phase is expected to be formed, and to investigate the magnetostrictive properties.
II. EXPERIMENTAL PROCEDURE
Co-Fe films of 100 nm thickness were prepared on MgO(001) and (110) single-crystalline substrates of 20×20×0.3 mm3 at 300 °C by using a radio-frequency (RF) magnetron sputtering apparatus. A Co70Fe30 (at. %) alloy target of 3 inch diameter was used. The details of deposition condition are similar to those of our previous study.6 The film thickness and the composition were confirmed by x-ray reflectivity and energy dispersive x-ray spectroscopy (EDS) to be 100±1 nm and Co - 25±2 at. % Fe, respectively.
The structural properties were investigated by reflection high-energy electron diffraction (RHEED) and x-ray diffraction (XRD) with Cu-Kα radiation (wave length: 0.15418 nm). The magnetization curves were measured by vibrating sample magnetometry. The magnetostriction was observed by employing a cantilever method under in-plane rotating magnetic field up to 1.2 kOe. The details of our measurement system are reported in our previous papers.6,7
III. RESULTS AND DISCUSSION
Figure 1(a-1) shows the RHEED pattern observed for a Co75Fe25 film formed on MgO(001) substrate. A diffraction pattern from bcc(001) single-crystalline surface is observed. The arrows show formation of reconstructed surface of c(2×2). A Co75Fe25(001) single-crystalline film with bcc structure is epitaxially grown in the orientation relationship of Co75Fe25(001)bcc||MgO(001). Figure 1(b-1) shows the RHEED pattern observed for a Co75Fe25 film formed on MgO(110) substrate. The pattern is analyzed to be an overlap of reflections from two bcc(211) variants, as shown by the spots indexed with the subscripts of A and B. An epitaxial Co75Fe25(211) bi-crystalline film with bcc structure is formed in the orientation relationships of Co75Fe25(211) bcc||MgO(110) (type A) and Co75Fe25(211) bcc ||MgO(110) (type B). In this configuration, there exists a large mismatch of –17% along MgO at the Co75Fe25(211)/MgO(110) interface. It is reported that periodical misfit dislocations are preferentially introduced at the interfaces, for example, Co50Fe50(211)/MgO(110)8 and Cr(211)/MgO(110)9 etc. The presence of such periodical dislocations reduces the effective lattice mismatch from –17% to nearly 0%. Similar phenomenon is considered to be occurred in the Co75Fe25/MgO interface.
Figures 1(a-2) and (a-3) respectively show the 2θ/ω-scan out-of-plane and 2θχ/φ-scan in-plane XRD patterns of Co75Fe25(001) single-crystalline film. Out-of-plane Co75Fe25(002) and in-plane Co75Fe25(200) reflections are observed. The out-of-plane and in-plane lattice spacings are calculated from the peak angles to be d(002) = 0.1428 nm and d(200) = 0.1416 nm, which agree with the values of bulk Co75Fe25 crystal (a = 0.2837 nm10) within small differences of less than ±0.7%. The values of full width at half maximum of the ω-scan out-of-plane and φ-scan in-plane rocking curves measured by fixing the diffraction angles, 2θ and 2θχ, at the peak angles of Co75Fe25(002) and (200) reflections were Δθ50(002) = 0.53° and Δθχ50(200) = 0.58°, respectively (not shown here). Figures 1(b-2) and (b-3) show the XRD patterns of Co75Fe25 bi-crystalline . It is considered that Co75Fe25(211) reflections from the two variants are overlapped in the out-of-plane pattern and that the Co75Fe25 reflection from A-type variant and the Co75Fe25 reflection from B-type variant are overlapped in the in-plane pattern. The out-of-plane and in-plane lattice spacings are calculated to be d(211)A,B = 0.1159 nm and = 0.2012 nm, where the differences with respect to the bulk values are found to be less than +0.3%. The Δθ50(211)A,B and the were respectively 0.94° and 1.96°. These results show that the bcc-Co75Fe25 films formed on both the MgO(001) and (110) substrates are high quality epitaxial films with small strain.
B. Magnetic properties
Figure 2(a-1) shows the magnetization curves measured for the Co75Fe25(001) single-crystalline film. The easy magnetization direction is parallel to , which is obtained by projecting  on the (001) surface as shown in Fig. 2(a-2). On the other hand, the hard magnetization direction is observed along . Therefore, the in-plane magnetic anisotropy is reflecting the magnetocrystalline anisotropy of Co75Fe25 bulk alloy material11 with the easy <111> and hard <100> axes and the demagnetization field. Figure 2(b-1) shows the magnetization curves measured for the Co75Fe25(211) bi-crystalline film. The easy magnetization direction is not observed along but along . is parallel to the direction obtained by projecting A and B on the (211) surface as shown in Fig. 2(b-2). The in-plane magnetic anisotropy is influenced not only by the magnetocrystalline anisotropy but also by the demagnetization field, as shown in the case of Co75Fe25(001) single-crystalline film. Therefore, the film is considered not to be easily magnetized along due to contributions from both easy () and hard (A and B) axes. On the contrary, the hard magnetization direction is observed along , which is expected by projecting A and B. These magnetization behaviors also show that Co75Fe25(001) single- and (211) bi-crystalline films with good crystallographic quality are obtained in the present study.
C. Magnetostrictive properties
When magnetization coherently rotates in a (001) single-crystalline film under an in-plane rotating magnetic field, the relative length variations along  and , Δl/l and Δl/l, are respectively given as6
where ψ is the angle of magnetic field direction with respect to . Figures 3(a-1) and (a-2) show the Δl/l and Δl/l measured for the Co75Fe25(001) single-crystalline film under rotating magnetic field of 1.2 kOe. Figure 3(b) summarizes the magnetic field dependences of the amplitudes of Δl/l and Δl/l. The λ100 and the λ111 values are estimated from the saturated amplitudes to be +60×10–6 and +150×10–6, respectively.
The relative length variations measured for a (211) bi-crystalline film along and , and , are respectively given as6
where ξ is the angle of magnetic field direction with respect to . Figures 4(a-1) and (a-2) show the and measured for the Co75Fe25(211) bi-crystal film under rotating magnetic field of 1.2 kOe. Figure 4(b) shows the amplitudes of and as a function of magnetic field. The λ100 and the λ111 values are calculated from the saturated values to be +55×10–6 and +185×10–6, respectively.
Figure 5 summarizes the compositional dependences of λ100 and λ111 of (001) single- and (211) bi-crystalline films. Here the values measured for Fe, Co30Fe70, and Co50Fe50 epitaxial films6 are shown for comparison. The single- and the bi-crystalline films show similar λ100 and λ111 values for the respective compositions. λ100 value increases with increasing the Co composition up to 50 at. %, and then drastically decreases with further increasing the Co composition. On the contrary, λ111 value monotonously increases as the Co composition increases from 0 to 75 at. %. The Co75Fe25 films have positive moderately-large λ100 and fairly-large λ111 values.
Co75Fe25(001)bcc single- and (211)bcc bi-crystalline films are obtained on MgO(001) and (110) substrates, respectively. The lattice spacings of Co75Fe25 films agree with the bulk values within small differences of less than 1%. The orientation dispersions are less than 2°. The magnetic and the magnetostrictive properties are investigated by using the well-defined epitaxial films. The in-plane magnetic anisotropies of single- and bi-crystalline films are reflecting the magnetocrystalline anisotropy and the demagnetization field. The (λ100, λ111) values of single- and bi-crystalline films are determined to be (+60×10–6, +150×10–6) and (+55×10–6, +185×10–6), respectively. The magnetostrictive properties revealed by using well-defined epitaxial films are believed to enhance the future studies and applications of Co-Fe alloy materials.
This paper is based on results obtained from a project, 20002152-0, subsidized by the New Energy and Industrial Technology Development Organization. Authors thank Mr. Naoki Yoshihara of Instrumental Analysis Center at Yokohama National University for his excellent technical supports for XRD and EDS measurements.
Conflict of Interest
The authors have no conflicts to disclose.
The data that support the findings of this study are available from the corresponding author upon reasonable request.