Formation of Sm ( Co 1 – x Ni x ) 5 epitaxial thin films on Cu ( 111 ) underlayers

Sm17(Co1–xNix)83 (at. %, x = 0, 0.2, 0.8, 1) alloy thin films are prepared on Cu(111) underlayers hetero-epitaxially grown on MgO(111) substrates by molecular beam epitaxy. The effects of substrate temperature and Ni/Co composition on the growth behavior and the detailed resulting film structure are investigated. Formation of RT5 ordered phase is enhanced with increasing the substrate temperature and the Ni composition. The long-range order degrees of Sm17Co83, Sm17(Co0.8Ni0.2)83, Sm17(Co0.2Ni0.8)83, and Sm17Ni83 films deposited at 500 °C are estimated to be 0.77, 0.82, 0.89, and 0.97, respectively. The Sm17(Co1–xNix)83 films with RT5 structure consist of two types of epitaxial (0001) variant whose orientations are rotated around the film normal by 30° each other. With increasing the Ni content ratio of x from 0 to 1, the volume ratio of two variants is varied from 53:47 to 94:6. The nucleation of only one-type variant with a smaller lattice mismatch with respect to Cu underlayer is promoted with increasing the Ni content ratio.

The Co site in SmCo 5 structure can be replaced with a 3d ferromagnetic transition metal element of T = Ni [14].The crystallization temperature and the film structure are considered to vary depending on the T element.Ni (1455 °C) has a lower melting point than Co (1495 °C).Ni atoms are thus considered to diffuse more easily and to have a lower activation energy in forming the SmT 5 compound.In our previous studies [8,9], SmCo 5 epitaxial films were prepared on Cu(111) underlayers by employing a molecular beam epitaxy (MBE) system equipped with a reflection high-energy electron diffraction (RHEED) facility.In-situ observations revealed the crystallographic property during film formation.In the present study, Sm

Experimental procedure
Thin films were prepared on MgO(111) substrates at temperatures ranging from 100 to 500 °C by using an MBE system with the base pressures lower than 7×10 -9  Pa.Substrates were heated at 500 °C for 1 h before film formation to obtain clean surfaces.Co, Co 80 Ni 20 , Co 20 Ni 80 alloy, and Ni materials were evaporated by electron beam heating, whereas Sm and Cu were evaporated by using Knudsen cells.The purities of evaporation sources were higher than 99.9%.The film layer structure is Sm 17 (Co 1-x Ni x ) 83 (20 nm)/Cu(20 nm)/MgO.A 20-nmthick Cu underlayer and a 20-nm-thick Sm 17 (Co 1-x Ni x ) 83 (x = 0, 0.2, 0.8, 1) films were sequentially deposited on the substrate.The epitaxial orientation relationship of Cu underlayer with respect to substrate [8,9] was Cu(111 . The Cu underlayer consisted of two epitaxial fcc(111) variants whose atomic stacking sequences of close-packed plane along the perpendicular direction were ABCABC… and ACBACB… The Sm 17 (Co 1-x Ni x ) 83 film was deposited by co-evaporation of Sm and Co 100(1-x) Ni 100x materials.The compositions of Sm 17 (Co 1-x Ni x ) 83 films were confirmed by energy dispersive X-ray spectroscopy and the errors were less than 2 at.% from the Sm(Co 1-x Ni x ) 5 stoichiometries.

Results and discussion
The effects of Co/Ni composition on the growth behaviour and the film structure were investigated.Here, the substrate temperature was fixed at 500 °C.Figures 1(a  variants with respect to Cu underlayer are -2.5%,-2.8%, -3.4%, and -3.6%, while those of B-type variants are +12.5%,+12.3%, +11.5%, and +11.3%.Here, the lattice constants of bulk SmCo 5 (a SmCo5 = 0.4982 nm [15]), SmNi 5 (a SmNi5 = 0.4926 nm [16]), and Cu (a Cu = 0.3615 nm [17]) crystals and the lattice constants of a SmCo4Ni (= 0.8a SmCo5 +0.2a SmNi5 ) and a SmCoNi4 (= 0.2a SmCo5 +0.8a SmNi5 ), which are estimated by using the   [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 s and F f are respectively S[f Sm -{(1-x)f Co +xf Ni }] and f Sm +5{(1-x)f Co +xf Ni } [19], where f is the atomic scattering factor of Sm, Co, or Ni and the subscripts of s and f refer to the superlattice and fundamental reflections, respectively.Therefore, I s /I f is expressed as (1) By solving this equation, S is given as  data to be 0.4054 nm, 0.4026 nm, 0.4005 nm, and 0.4001 nm, respectively.The lattice constants, a type A , a type B , c, of respective films are slightly larger than those of bulk SmCo 5 (a SmCo5 = 0.4982 nm, c SmCo5 = 0.3975 nm [15]), SmCo 4 Ni (a SmCo4Ni = 0.4971 nm, c SmCo4Ni = 0.3976 nm), SmCoNi 4 (a SmCoNi4 = 0.4937 nm, c SmCoNi4 = 0.3979 nm), and SmNi 5 (a SmNi5 = 0.4926 nm, c SmNi5 = 0.3980 nm [16]) crystals and smaller than that of bulk SmCu 5 (a SmCu5 = 0.507 nm, c SmCu5 = 0.410 nm [20]) crystal.The T site in Sm(Co 1-x Ni x ) 5 structure may be partially replaced with Cu atoms with larger atomic radius.There is thus a possibility that Cu atoms of underlayer diffuse into the Sm 17 (Co 1-x Ni x ) 83 film and an alloy compound of Sm(Co 1-x Ni x ,Cu) 5 is formed, similar to the case of SmCo 5 films formed on Cu underlayers [2][3][4][5]8,9].
In order to study the effect of substrate temperature on the ordered phase formation, Sm 17 (Co 1-x Ni x ) 83 films were deposited at temperatures ranging from 100 to 500 °C.Figures 3(a content decrease from 500 °C and x = 1, the sharpness of RHEED pattern gradually decreases with overlapping diffuse contrast.The variation suggests that amorphous phase is appearing.The transformations from crystalline to amorphous phase as functions of film thickness and temperature are summarized in Figs.3(a-6)-(d-6).The crystallization temperature apparently decreases with increasing the Ni content.The result is possibly due to a lower activation energy of Ni in forming the SmT 5 compound.Figure 4 shows the substrate temperature dependences of S of Sm 17 (Co 1-x Ni x ) 83 films.The S value increases with increasing not only the substrate temperature but also the Ni content.A replacement of Co site in SmCo 5 structure with Ni atom is useful for enhancing the RT 5 ordered phase formation.
-1)-(d-1) show the RHEED patterns of Sm 17 (Co 1-x Ni x ) 83 films deposited on Cu(111) underlayers observed by making the incident electron beam parallel to MgO.Clear diffraction patterns corresponding to RT 5 (0001) texture [figure 1(e-3)] are observed for all the films.Epitaxial Sm 17 (Co 1-x Ni x ) 83 films with RT 5 ordered structure and with the c-axis perpendicular to the substrate surface are obtained.The diffraction patterns are analyzed to be an overlap of two reflections [figures 1(e-1, e-2)], as shown by the arrows A and B in the intensity profiles of figures (a-2)-(d-2 Figures 2(a-2)-(d-2) show the out-of-plane XRD patterns of Sm 17 (Co 1-x Ni x ) 83 films.RT 5 (0001) superlattice reflections are clearly observed in addition to RT 5 (0002) fundamental reflections for all the films.The out-of-plane XRD confirms the formation of RT 5 ordered phase.Longrange order degree (S) is estimated by comparing the intensity ratio of RT 5 (0001) superlattice to RT 5 fundamental reflection.The intensity (I) is proportional to structure (FF * ), Lorentz-polarization (L), and absorption (A) -3)̽(d-3) shows the in-plane XRD patterns measured by making the scattering vector parallel to MgOfrom B-type variant are recognized.The in-plane XRD confirms the epitaxial orientation relationship determined by RHEED.The lattice constants of A-and B-type variants, (a type A , a type B ) = (2din Sm 17 Co 83 , Sm 17 (Co 0.8 Ni 0.2 ) 83 , Sm 17 (Co 0.2 Ni 0.8 ) 83 , and Sm 17 Ni 83 films are calculated from the in-plane XRD data to be (0.5041 nm, 0.5021 nm), (0.4978 nm, 0.4972 nm), (0.4966 nm, 0.4968 nm), and (0.4964 nm, 0.4959 nm), respectively.The lattice constants, c type A+B = d (0001) , of Sm 17 Co 83 , Sm 17 (Co 0.8 Ni 0.2 ) 83 , Sm 17 (Co 0.2 Ni 0.8 ) 83 , and Sm 17 Ni 83 films are estimated from the out-of-plane XRD

Sm 17 (
Co 1-x Ni x ) 83 films are deposited on Cu(111) underlayers by varying the Ni content of x from 0 to 1 and by varying the substrate temperature from 100 to 500 °C.The film growth behavior and the detailed films structure are investigated by RHEED and XRD.With increasing the substrate temperature and the Ni content, formation of RT 5 ordered phase is promoted.The S values of Sm 17 Co 83 , Sm 17 (Co 0.8 Ni 0.2 ) 83 , Sm 17 (Co 0.2 Ni 0.8 ) 83 , and Sm 17 Ni 83 films are 0.77, 0.82, 0.89, and 0.97, respectively.A replacement of Co site in SmCo 5 structure with Ni atom is useful for enhancing the RT 5 ordered phase formation.The Sm 17 (Co 1-x Ni x ) 83 films consist of two types of (0001) variant.As the Ni content of x increases from 0 to 1, the volume ratio of two variants varies from 53:47 to 94:6.The epitaxial growth of only one-type variant with a smaller lattice mismatch with respect to Cu underlayer is enhanced with increasing the Ni content. 17 (Co,Ni)83 and Sm 17 Ni 83 (at.%) films are deposited on Cu(111) underlayers in addition to Sm 17 Co 83 films.The effects of Ni/Co composition and substrate temperature on the ordered phase formation are investigated.
Co 83 , Sm 17 (Co 0.8 Ni 0.2 ) 83 , Sm 17 (Co 0.2 Ni 0.8 ) 83 , and Sm 17 Ni 83 SmCo5 and a SmNi5 , are used.Although there are fairly large mismatches in the cases of B-type variants, epitaxial growth is taking place.With increasing the Ni composition, the intensity of RHEED reflection from B-type variant becomes weaker, as shown in figures 1(a-2)-(d-2).In order to investigate the volume ratio of two types of variant, pole-figure XRD was carried out.The volume ratios of A-type to B-type variant involved in Sm 17 Co 83 , Sm 17 (Co 0.8 Ni 0.2 ) 83 , Sm 17 (Co 0.2 Ni 0.8 ) 83 , and Sm 17 Ni 83 films are estimated from the integrated intensities of {1 1 _ 01} reflection of each variant to be 53:47, 57:43, 88:12, and 94:6, respectively.The nucleation of only A-type variant is apparently enhanced with increasing the Ni content.