Structure and magnetic properties of Fe-Co-B alloy thin films prepared on cubic (001) single-crystal substrates

Magnetic tunnel junctions consisting of tunnel barrier and ferromagnetic electrode layers have been studied for applications to tunnel magnetoresistance (TMR) sensors and magnetoresistive random access memory devices. In order to achieve high TMR ratios, (001)-oriented oxide layers of MgO,^1,2 SrTiO₃,^3,4 and MgAl₂O₄⁵ have been useful as the barrier layer. Fe-Co and Fe-Co-B alloys with Fe-rich compositions are typical soft magnetic materials with high saturation magnetization values and have been frequently used as the electrode material. However, the structural properties of Fe-Co and Fe-Co-B layers are affected by the B content,^6,7 the deposition temperature,⁷ the annealing temperature,^8–11 etc. When there exists a lattice mismatch of Fe-Co or Fe-Co-B layer with respect to oxide layer, misfit dislocations are considered to be introduced around the interface,¹² which influences the TMR effect.¹³ 

In order to investigate the basic structural and magnetic properties, it is useful to employ epitaxial single-crystal films, since the film uniformity and the magnetic anisotropy are well controlled. However, there are few reports on the preparation of Fe-Co and Fe-Co-B films on oxide single-crystal substrates other than MgO. In the present study, Fe₇₀Co₃₀ and (Fe₇₀Co₃₀)_0.95B₅ (at. %) alloy films are prepared on three kinds of cubic (001) oxide single-crystal substrate, MgO, MgAl₂O₄, and SrTiO₃. The mismatches of Fe₇₀Co₃₀(001)_bcc lattice with respect to (001) lattices of MgO, MgAl₂O₄, and SrTiO₃ are respectively [(2^1/2a_Fe70Co30 – a_substrate)/a_substrate] × 100 = –4.2%, 0%, and +3.5%, which are calculated by using the lattice constants of bulk Fe₇₀Co₃₀ (a_Fe70Co30 = 0.2858 nm¹⁴), MgO (a_MgO = 0.4217nm¹⁵), MgAl₂O₄ (a_MgAl2O4/2 = 0.4041 nm¹⁶), and SrTiO₃ (a_SrTiO3 = 0.3905 nm¹⁷). The film growth and the structure are investigated.

A radio-frequency (RF) magnetron sputtering system equipped with a reflection high-energy electron diffraction facility was used for film formation. The base pressure was lower than 4 × 10^–7 Pa. Polished (001) substrates of MgO, MgAl₂O₄, and SrTiO₃ were heated at 600 °C for 1 hour to obtain clean surfaces. Fe₇₀Co₃₀ and (Fe₇₀Co₃₀)_0.95B₅ alloy targets of 3 inch diameter were employed. The distance between target and substrate was fixed at 150 mm. The Ar gas pressure was kept constant at 0.67 Pa. The RF powers for Fe₇₀Co₃₀ and (Fe₇₀Co₃₀)_0.95B₅ targets were respectively adjusted to 57 and 65 W, where the deposition rate was 0.02 nm/s for both materials. Fe₇₀Co₃₀ and (Fe₇₀Co₃₀)_0.95B₅ films of 5 nm thickness were deposited on the substrates at 200 °C. The film compositions were confirmed by energy dispersive x-ray spectroscopy (EDX) and the compositional ratio of Fe : Co was within (70 : 30) ± 3 at. % for both Fe-Co and Fe-Co-B films. The B contents in (Fe_0.7Co_0.3)-B films were not determined, since B is a light element and not detectable by EDX. The compositions of (Fe_0.7Co_0.3)-B films were regarded to be similar to that of (Fe_0.7Co_0.3)₉₅B₅ target.

The crystallographic structure and orientation relationship were studied by RHEED. The structure was investigated by 2θ/ω-scan out-of-plane and 2θχ/φ-scan in-plane X-ray diffractions (XRDs) with Cu-Kα radiation (λ = 0.15418 nm). The surface morphology was observed by atomic force microscopy (AFM). The magnetization curves were measured by vibrating sample magnetometry.

Figures 1(a)–(c) show the RHEED patterns observed for Fe₇₀Co₃₀ and (Fe₇₀Co₃₀)_0.95B₅ films deposited on MgO, MgAl₂O₄, and SrTiO₃ substrates of (001) orientation by making the incident electron beam parallel to [100]_substrate. Diffraction patterns from bcc(001) single-crystal surface are recognized, as shown in the simulated pattern of Fig. 1(g). Fe₇₀Co₃₀ and (Fe₇₀Co₃₀)_0.95B₅ single-crystal films of bcc(001) orientation grow epitaxially on all the substrates. The crystallographic orientation relationships are determined by RHEED as

The (001) lattices of Fe₇₀Co₃₀ and (Fe₇₀Co₃₀)_0.95B₅ films are rotated around the film normal by 45° with respect to those of substrates, as shown in Fig. 2.

Figure 3 shows the out-of-plane XRD patterns of Fe₇₀Co₃₀ and (Fe₇₀Co₃₀)_0.95B₅ single-crystal films. bcc(002) reflections are observed in addition to reflections from the substrates. Figure 4 shows the in-plane XRD patterns measured by making the scattering vector parallel to [110]_substrate. bcc(200) reflections are recognized for the films deposited on MgO and SrTiO₃ substrates. On the other hand, bcc(200) and MgAl₂O₄(440) reflections are considered to be overlapped for the films deposited on MgAl₂O₄ substrates, since the lattice misfit value is 0%. Figure 5 shows the in-plane and the out-of-plane lattice constants, a and c, which are respectively calculated from the peak angles of bcc(200) and bcc(002) reflections by using the relations of a = 2[λ/2sin(θχ)_bcc(200)] and c = 2[λ/2sin(θ)_bcc(002)]. The a and the c values are in agreement within small differences ranging between +1.1% and –0.9% with the value of bulk bcc-Fe₇₀Co₃₀ crystal (a_Fe70Co30 = 0.2858 nm¹⁴). The result shows that the strains in the films are very small, even though there exist the lattice mismatches of –4.2% (MgO) and +3.5% (SrTiO₃). It is known that misfit dislocations are easily introduced around a film/substrate interface to reduce the strain caused by mismatch between immiscible elements when a weak binding force works between the deposited atoms and the substrate atoms. This type of epitaxial growth is reported for Cr(001)_bcc/MgO(001)¹⁸ and Fe-Co(001)_bcc/MgO(001)¹² systems. The growth of Fe₇₀Co₃₀ and the (Fe₇₀Co₃₀)_0.95B₅ films on MgO and SrTiO₃ substrates is also considered to follow the growth mode.

Figure 6 shows the AFM images. Flat surfaces with the arithmetical mean roughness (R_a) values less than 0.14 nm are formed for all films. Figure 7 shows the in-plane magnetization curves measured by applying the magnetic field along bcc[100] or bcc[110]. The films are easily magnetized when the magnetic field is applied along bcc[100], whereas the magnetization curves measured along bcc[110] saturate at higher magnetic fields. There were no clear differences in the hysteresis loops measured along bcc[100] and bcc[010] and measured along bcc[110] and bcc[1$1¯$0] (not shown here). Therefore, the films show four-fold symmetries in in-plane magnetic anisotropies, which are reflecting the magnetocrystalline anisotropy of bulk Fe₇₀Co₃₀ crystal with the easy magnetization axes of bcc<100>.

Fe₇₀Co₃₀ and (Fe₇₀Co₃₀)_0.95B₅ films are prepared on (001) substrates of MgO, MgAl₂O₄, and SrTiO₃, where the lattice misfit values are –4.2%, 0%, and +3.5%, respectively. The film growth and the structure are investigated by RHEED and XRD. Fe₇₀Co₃₀ and (Fe₇₀Co₃₀)_0.95B₅ single-crystal films of bcc(001) orientation are formed on all the substrates. The crystallographic orientation relationships are determined as Fe₇₀Co₃₀, (Fe₇₀Co₃₀)_0.95B₅(001)[110] $$$$ MgO, MgAl₂O₄, SrTiO₃(001)[100]. The (001) lattices of films are rotated around the film normal by 45° with respect to those of substrates. The in-plane and out-of-plane lattice constants, a and c, are in agreement within small differences ranging between +1.1% and –0.9% with the value of bulk bcc-Fe₇₀Co₃₀ crystal, even though there exist lattice mismatches of –4.2% and +3.5%. The result indicates that misfit dislocations are introduced at the film/substrate interface to reduce the strain caused by lattice mismatch when films are deposited on MgO and SrTiO₃ substrates. The single-crystal films show in-plane magnetic anisotropies reflecting the magnetocrystalline anisotropy of bulk Fe₇₀Co₃₀ crystal.

A part of this work was supported by Chuo University Grant for Special Research.