Ordered phase formation in Sm-Ni thin film deposited on Cr ( 100 ) single-crystal underlayer

Sm17Ni83 (at. %) alloy thin films are prepared on Cr(100) underlayers hetero-epitaxially grown on MgO(100) single-crystal substrates by employing an ultra-high vacuum molecular beam epitaxy system. The effect of substrate temperature on the ordered phase formation is investigated. The films deposited below 300 °C consist of amorphous phase, whereas formation of SmNi5 ordered phase is recognized for the films deposited above 400 °C. The SmNi5 films with ordered phase consist of two types of (112 _ 0) variant whose c-axes are lying in the film plane and rotated around the film normal by 90° each other. With increasing the temperature from 400 to 500 °C, the long-range order degree increases from 0.65 to 0.80. The ordered film formed at 500 °C shows an in-plane magnetic anisotropy reflecting the magnetocrystalline anisotropy of SmNi5 crystal.


Introduction
Magnetic thin films with the uniaxial magnetocrystalline anisotropy energy (K u ) greater than 10 7 erg/cm 3 and with the easy magnetization axis parallel to the substrate surface have been investigated for nanocomposite magnets.In order to control the easy magnetization direction, well-defined epitaxial films are useful, since the crystallographic orientation of film can be controlled by that of single-crystal substrate.SmCo 5 alloy with RT 5 ordered structure shows K u of 1.1×10 8 erg/cm 3 along the c-axis [1].SmCo 5 epitaxial films with the c-axis lying in in-plane have been prepared on MgO(100) [2][3][4][5] and MgO(110) [3][4][5][6] substrates by employing W [4], Cr [2,3], and Fe [5,6] underlayers.In our previous studies [7,8], SmCo 5 epitaxial films with the c-axes parallel and perpendicular to the substrate surface were respectively prepared on Cr and Cu underlayers by employing a molecular beam epitaxy (MBE) system equipped with a reflection high-energy electron diffraction (RHEED) facility.In-situ RHEED revealed the crystallographic property during film formation, which varied depending not only the substrate temperature but also on the film thickness.
The Sm and Co sites in SmCo 5 structure can be replaced with other rare earth metal (R) and transition metal (T) elements, respectively.The structural and magnetic properties are considered to vary depending on the combination of T and R elements.Epitaxial RCo 5 (R = Pr, Nd, etc.) ordered epitaxial films have been prepared [9,10], whereas there are few reports on the formation of SmT 5 (T = Fe, Ni) ordered films [11][12][13].

Experimental procedure
Thin films were prepared on polished MgO(100) substrates by using an MBE system.The base pressures were lower than 7×10 -9 Pa.Before film formation, substrates were heated at 500 °C for 1 h to obtain clean surfaces.Figure 1  single-crystal underlayer is formed.A 20-nm-thick Sm-Ni film with the Sm composition of 17 at.% which is almost the SmNi 5 stoichiometry was deposited on the Cr(100) underlayer at a substrate temperature in a range between 100 and 500 °C.The Sm-Ni film compositions were confirmed by energy dispersive X-ray spectroscopy and the errors were less than 1 at.% from the SmNi 5 stoichiometry.
The surface structure during film deposition was observed by RHEED.The resulting film structure was investigated by 2ș/Ȧ-scan out-of-plane and 2șȤ/ĳ-scan in-plane X-ray diffrections (XRDs) with Cu-KĮ radiation (Ȝ = 0.15418 nm).The magnetization curves were measured by using a vibrating sample magnetometer.B).The film consists of two types of (112 _ 0) variant whose c-axes are lying in the film plane and rotated around the film normal by 90° each other.In this configuration, the lattice mismatch differs depending on the in-plane direction.The mismatches at the SmNi 5 /Cr interface along SmNi 5 [0001] and SmNi 5 [11 _ 00] are respectively calculated to be -2.4% and +5.0%,where the lattice constants of bulk SmNi 5 (a = 0.4926 nm, c = 0.3980 nm [15]) and Cr (a = 0.2884 nm [17]) crystals are used.Although there exists such a large misfit of -5.0% along SmNi 5 [11 _ 00], epitaxial growth of SmNi 5 crystal on Cr(100) underlayer is taking place.With increasing the Sm-Ni film thickness from 5 to 20 nm, the sharpness of RHEED pattern gradually increases.The result suggests that the film is strained around the SmNi 5 /Cr interface due to the lattice mismatch and the strain decreases with increasing the thickness.Figure 2(e) shows the RHEED patterns observed for an Sm-Ni film deposited at 500 °C.A clear diffraction pattern corresponding to RT 5 (112 _ 0) texture is recognized through the course of deposition.The diffraction spots are sharper when compared with those observed for the film deposited at 400 °C.The film strain is decreased by employing a higher substrate temperature.
Long-range order degree (S) of SmNi 5 phase is evaluated by comparing the intensity ratio of SmNi 5 (0001) to SmNi 5 (0002) reflection.The intensity (I) is proportional to structure (FF * ), Lorentz-polarization (L), and absorption (A) factors [18].Here, I is a product of integrated intensity multiplied by the full with at half maximum of rocking curve measured for the reflection.In the present paper, an influence of temperature factor, which is often omitted when comparing two reflection intensities, is not considered.F (0001) and F (0002) are respectively S(f Sm -f Ni ) and f Sm +5f Ni [10], where f is the atomic scattering factor of Sm or Ni.Therefore, I (0001) /I (0002) is expressed as The S of films deposited at 400 and 500 °C are estimated to be 0.65 and 0.80, respectively.The S value improves with increasing the substrate temperature.
Figure 5 shows the magnetization curves measured for the film deposited at 500 °C.The film is easily magnetized when the magnetic field is applied along Cr[001] which is the direction between the c-axes of two SmNi 5 (112 _ 0) variants.On the other hand, the magnetization curve measured along Cr[011] saturates at a higher applied field of 10 kOe.The magnetization behavior is similar to the case of hcp-Co(11 2 _ 0) film epitaxially grown on Cr(100) underlayer [19].The in-plane anisotropy is thus considered to be reflecting the magnetic property of SmNi 5 crystal with the easy magnetization axis parallel to the c-axis.

Conclusion
Sm 17 Ni 83 alloy films are deposited on Cr(100) underlayers by varying the substrate temperature in a range between 100 and 500 °C.The film growth behavior and the detailed film structure are investigated by RHEED and XRD.The films deposited above 400 °C consist of epitaxial SmNi 5 (112 _ 0) crystal, whereas the films deposited below the temperature consist of amorphous phase.As the substrate temperature increases from 400 to 500 °C, the order degree (S) increases from 0.65 to 0.80.Ordering of SmNi 5 phase is promoted with increasing the temperature.The ordered film shows an in-plane magnetic anisotropy reflecting the magnetic property of SmNi 5 crystal.

Figures 2 (Fig. 4 .
Figures 2(a)-(c) show the RHEED patterns observed during Sm-Ni deposition on Cr(100) underlayers at temperatures ranging between 100 and 300 °C.Diffuse diffraction patterns are recognized.The result shows that amorphous Sm-Ni films are formed.Figure 2(d) shows the RHEED patterns observed for an Sm-Ni film deposited at 400 °C.The RHEED patterns correspond to a diffraction pattern from RT 5 (11 2 _ 0) surface [figure 3(b)] and do not correspond to formations of (112 _ 0) crystals with hexagonal 1)-(e-1) show the out-of-plane XRD patterns.No reflections from Sm-Ni crystals are observed for the films deposited below 300 °C [figures 4(a-1)-(c-1)] due to that the films consist of amorphous phase.SmNi 5 reflections are recognized for the films deposited above 400 °C [figures 4(d-1, e-1)].The values of full width at half maximum of Ȧ-scan rocking curves of films deposited at 400 and 500 °C measured by fixing the diffraction angle of 2ș at the peak angle of SmNi 5 (112 _ 0) reflection are 2.84° and 2.67°, respectively.With increasing the substrate temperature from 400 to 500 °C, the film strain decreases.Figures 4(a-2)-(e-2) show the in-plane patterns measured by making the scattering vector parallel to MgO[001].For the films deposited above 400 °C, SmNi 5 (0001) and SmNi 5 (0003) superlattice reflections are observed in addition to SmNi 5 (0002), SmNi 5 reflections.The in-plane XRD confirms the formation of SmNi 5 ordered phase and the epitaxial orientation relationship which are determined by RHEED.The in-plane lattice spacings of (dSmNi5(0001), dSmNi5(11 _ 00)) of films deposited at 400 and 500 °C are (0.4017, 0.4247) and (0.4033, 0.4187), respectively.The dSmNi5(0001) and the dSmNi5(