- Email: [email protected]
The magnetic map of thin Mn overlayers on Ni(0 0 1)
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1215–1216
The magnetic map of thin Mn overlayers on Ni(0 0 1) S. Meza-Aguilara,*, B. M’Passi-Mabialab, C. Demangeatc a
Escuela de Ciencias Fisico-Matematicas, Universidad Autonoma de Sinaloa Av., Bldv. de las Americas y Universitarios, CU, ! CP 80010, Mexico Sinaloa, Culiacan b Departement de Physique, Universit!e Marien NGouabi, LME, Brazzaville BP 69, Congo c IPCMS, 23 rue du Loess F-63034 Strasbourg Cedex 2, France
Abstract Following experimental works of Wuttig et al. (Phys. Rev. Lett. 70 (1993) 3619) and O’Brien and Tonner (Phys. Rev. B 51 (1995) 617) very Mn thin films overlayers on Ni(0 0 1) have been investigated through ab initio density functional theory with generalized gradient corrections. A relaxed two-dimensional Mn0:5 Ni0:5 ordered surface alloy shows evidence of ferromagnetic metastable solution in reasonable agreement with experimental results. r 2003 Published by Elsevier B.V. PACS: 71.15.Mb; 75.70.i; 75.70.Cn; 75.Rf Keywords: Nickel; Manganese; Density functional calculations; Magnetic surfaces; Metal-metal interfaces
Since the works of Wuttig et al. [1] and Blugel . [2] showing evidence for a new class of magnetic materials, namely two-dimensional ordered surface alloys, not present in the bulk form, are currently the object of many investigations [2,3]. Low energy electron diffraction (LEED) structure determination of compositionally ordered MnNi films epitaxially grown on Ni(1 0 0) by deposition of 3–4 monolayers (ML) Mn above 550 K has been made by Wuttig et al. [4] showing that the resulting film structure is indicative of the formation of tetragonal MnNi films. ( was obA pronounced corrugation of 0:3070:02 A served at the MnNi film surface. Later O’Brien and Tonner [5] using a combination of soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD), have compared the Mn films grown on Cu(0 0 1) and Ni(1 0 0) crystals. They observed a large buckling on both substrates, while the ferromagnetic (FM) alignment in the Mn film was found only on Ni, indicating that the large structural change cannot be attributed to the FM alignment unlike the antiferromagnetic (AF) ordering found in the bulk alloy. *Corresponding author. Tel./fax: +52-667-7-15-64-12. E-mail address: [email protected] (S. Meza-Aguilar). 0304-8853/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.jmmm.2003.12.468
Spisak and Hafner [6] have performed ab initio calculations with local spin density approximation (LSDA) including generalized gradient correction (GGC) of Mn–Ni ordered surface alloys film (1–4 ML) on Ni(0 0 1). Only Mn0:5 Ni0:5 alloy has been considered. For the coupling between the Ni substrate and the first MnNi layer a FM alignment was assumed. The 1 ML Mn–Ni ordered alloy shows high-spin for Mn ðmMn ¼ 3:9 mB Þ and slightly reduced moment for Ni ðmNi ¼ 0:46 mB Þ compared to bulk FCC Ni. For higher thickness the AF layer sequence is energetically more favorable in contrast to what has been deduced from XMCD measurements [5]. In this work we explore the magnetic map of Mn overlayers on Ni(0 0 1). Due to the discrepancy between experimental results showing FM and the theoretical ones where the AF layer sequence seems to be more favorable, different values of interlayer distance of the surface planes are used in agreement with LEED measurements. The calculations are performed within a scalar relativistic version of the k space tight binding linear muffin tin orbitals (TB-LMTO) method with atomic sphere approximation. We have used three functionals: LSDA of von Barth and Hedin (vBH), generalized
ARTICLE IN PRESS 1216
S. Meza-Aguilar et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1215–1216
Table 1 Surface alloy formation energy (in mRy per cell) in the nonmagnetic and magnetic cases with the different exchange correlation functionals (EXF), for MnNi surface alloy on Ni(0 0 1) FCC substrate EXF
Non-mag.
Mag.
vBH LMH PW91
7.89 23.44 19.04
66:40 62:50 68:36
gradient approximation of Langreth–Mehl–Hu (GGALMH) and Perdew–Wang-91 (PW91). We have determined the surface alloy formation energy following the Blugel . formula [2] E ¼ EMnNi=Ni 12 ðENi þ EMn=Ni Þ; where EMnNi=Ni ; ENi and EMn=Ni are the total energies for MnNi/Ni(0 0 1), Ni(0 0 1) and Mn/Ni(0 0 1), respectively. The surface alloy formation energies obtained in the nonmagnetic as well as in the magnetic cases are shown for different exchange functionals in Table 1. As in the case of Spisak and Hafner [6] calculations, our results confirm that magnetism clearly stabilizes the surface alloy formation and the coupling between Mn and Ni atoms is always FM, whereas, the Mn ML is always ferrimagnetic cð2 2Þ: This trend follows the one obtained by M’Passi-Mabiala et al. [7] for Mn ML and MnCo surface on Co(0 0 1) in agreement with experimental results. However, for all functionals used here, FM ordering in the surface alloy remains the ground state contrary to the case of MnCo surface on Co(0 0 1) where only GGA functional leads to this FM ordering ground state. The layered AF sequence for the bilayer surface alloy is clearly the ground state. Following the calculations of Spisak and Hafner [6] the apparent contradiction with the experimental results [5] showing evidence of a FM coupling is resolved by considering the morphology of the films. GGA-LMH functional-calculations with an ( obtained from LEED outward relaxation of 0:25 A studies [4] show evidence of FM metastable state by only 15 meV per atom. The difference in total energies decreases with the relaxation as shown in Fig. 1 so that all configurations become degenerate. Calculations done with four inequivalent atoms per plane show, that the gap between the FM metastable state and the ground
Fig. 1. Difference of total energy per atom (in meV) for MnNi ( bilayer alloy on Ni(0 0 1) FCC as function of relaxation (in A) in each magnetic configuration.
state is reduced by an amount of 9 meV: As the energy difference lies below the thermal energy at room temperature and possibly due to structural imperfections, comparison with experimental results may be difficult. Surface-alloy formation as well as FM coupling between Mn atoms and Ni atoms for the ordered MnNi surface alloy have been confirmed in all functional used in agreement with experimental results. Taking into ( from LEED account the outward relaxation of 0:25 A studies [4], FM metastable state has been obtained for the bilayer surface alloy MnNi in qualitative agreement with experimental results [5].
References [1] M. Wuttig, Y. Gauthier, S. Blugel, . Phys. Rev. Lett. 70 (1993) 3619. . [2] S. Blugel, Appl. Phys. A: Matter Sci. Process. 63 (1996) 595. [3] C. Demangeat, J.C. Parlebas, Rep. Prog. Phys. 65 (2002) 1679. [4] M. Wuttig, C.C. Knight, Phys. Rev. B 48 (1993) 12130. [5] W.L. O’Brien, B.P. Tonner, Phys. Rev. B 51 (1995) 617. [6] D. Spisak, J. Hafner, J. Phys.: Condens. Matter 11 (1999) 6359. [7] B. M’Passi-Mabiala, et al., Phys. Rev. B 65 (2002) 12414; B. M’Passi-Mabiala, et al., Surf. Sci. 518 (2002) 104.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1215–1216
The magnetic map of thin Mn overlayers on Ni(0 0 1) S. Meza-Aguilara,*, B. M’Passi-Mabialab, C. Demangeatc a
Escuela de Ciencias Fisico-Matematicas, Universidad Autonoma de Sinaloa Av., Bldv. de las Americas y Universitarios, CU, ! CP 80010, Mexico Sinaloa, Culiacan b Departement de Physique, Universit!e Marien NGouabi, LME, Brazzaville BP 69, Congo c IPCMS, 23 rue du Loess F-63034 Strasbourg Cedex 2, France
Abstract Following experimental works of Wuttig et al. (Phys. Rev. Lett. 70 (1993) 3619) and O’Brien and Tonner (Phys. Rev. B 51 (1995) 617) very Mn thin films overlayers on Ni(0 0 1) have been investigated through ab initio density functional theory with generalized gradient corrections. A relaxed two-dimensional Mn0:5 Ni0:5 ordered surface alloy shows evidence of ferromagnetic metastable solution in reasonable agreement with experimental results. r 2003 Published by Elsevier B.V. PACS: 71.15.Mb; 75.70.i; 75.70.Cn; 75.Rf Keywords: Nickel; Manganese; Density functional calculations; Magnetic surfaces; Metal-metal interfaces
Since the works of Wuttig et al. [1] and Blugel . [2] showing evidence for a new class of magnetic materials, namely two-dimensional ordered surface alloys, not present in the bulk form, are currently the object of many investigations [2,3]. Low energy electron diffraction (LEED) structure determination of compositionally ordered MnNi films epitaxially grown on Ni(1 0 0) by deposition of 3–4 monolayers (ML) Mn above 550 K has been made by Wuttig et al. [4] showing that the resulting film structure is indicative of the formation of tetragonal MnNi films. ( was obA pronounced corrugation of 0:3070:02 A served at the MnNi film surface. Later O’Brien and Tonner [5] using a combination of soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD), have compared the Mn films grown on Cu(0 0 1) and Ni(1 0 0) crystals. They observed a large buckling on both substrates, while the ferromagnetic (FM) alignment in the Mn film was found only on Ni, indicating that the large structural change cannot be attributed to the FM alignment unlike the antiferromagnetic (AF) ordering found in the bulk alloy. *Corresponding author. Tel./fax: +52-667-7-15-64-12. E-mail address: [email protected] (S. Meza-Aguilar). 0304-8853/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.jmmm.2003.12.468
Spisak and Hafner [6] have performed ab initio calculations with local spin density approximation (LSDA) including generalized gradient correction (GGC) of Mn–Ni ordered surface alloys film (1–4 ML) on Ni(0 0 1). Only Mn0:5 Ni0:5 alloy has been considered. For the coupling between the Ni substrate and the first MnNi layer a FM alignment was assumed. The 1 ML Mn–Ni ordered alloy shows high-spin for Mn ðmMn ¼ 3:9 mB Þ and slightly reduced moment for Ni ðmNi ¼ 0:46 mB Þ compared to bulk FCC Ni. For higher thickness the AF layer sequence is energetically more favorable in contrast to what has been deduced from XMCD measurements [5]. In this work we explore the magnetic map of Mn overlayers on Ni(0 0 1). Due to the discrepancy between experimental results showing FM and the theoretical ones where the AF layer sequence seems to be more favorable, different values of interlayer distance of the surface planes are used in agreement with LEED measurements. The calculations are performed within a scalar relativistic version of the k space tight binding linear muffin tin orbitals (TB-LMTO) method with atomic sphere approximation. We have used three functionals: LSDA of von Barth and Hedin (vBH), generalized
ARTICLE IN PRESS 1216
S. Meza-Aguilar et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1215–1216
Table 1 Surface alloy formation energy (in mRy per cell) in the nonmagnetic and magnetic cases with the different exchange correlation functionals (EXF), for MnNi surface alloy on Ni(0 0 1) FCC substrate EXF
Non-mag.
Mag.
vBH LMH PW91
7.89 23.44 19.04
66:40 62:50 68:36
gradient approximation of Langreth–Mehl–Hu (GGALMH) and Perdew–Wang-91 (PW91). We have determined the surface alloy formation energy following the Blugel . formula [2] E ¼ EMnNi=Ni 12 ðENi þ EMn=Ni Þ; where EMnNi=Ni ; ENi and EMn=Ni are the total energies for MnNi/Ni(0 0 1), Ni(0 0 1) and Mn/Ni(0 0 1), respectively. The surface alloy formation energies obtained in the nonmagnetic as well as in the magnetic cases are shown for different exchange functionals in Table 1. As in the case of Spisak and Hafner [6] calculations, our results confirm that magnetism clearly stabilizes the surface alloy formation and the coupling between Mn and Ni atoms is always FM, whereas, the Mn ML is always ferrimagnetic cð2 2Þ: This trend follows the one obtained by M’Passi-Mabiala et al. [7] for Mn ML and MnCo surface on Co(0 0 1) in agreement with experimental results. However, for all functionals used here, FM ordering in the surface alloy remains the ground state contrary to the case of MnCo surface on Co(0 0 1) where only GGA functional leads to this FM ordering ground state. The layered AF sequence for the bilayer surface alloy is clearly the ground state. Following the calculations of Spisak and Hafner [6] the apparent contradiction with the experimental results [5] showing evidence of a FM coupling is resolved by considering the morphology of the films. GGA-LMH functional-calculations with an ( obtained from LEED outward relaxation of 0:25 A studies [4] show evidence of FM metastable state by only 15 meV per atom. The difference in total energies decreases with the relaxation as shown in Fig. 1 so that all configurations become degenerate. Calculations done with four inequivalent atoms per plane show, that the gap between the FM metastable state and the ground
Fig. 1. Difference of total energy per atom (in meV) for MnNi ( bilayer alloy on Ni(0 0 1) FCC as function of relaxation (in A) in each magnetic configuration.
state is reduced by an amount of 9 meV: As the energy difference lies below the thermal energy at room temperature and possibly due to structural imperfections, comparison with experimental results may be difficult. Surface-alloy formation as well as FM coupling between Mn atoms and Ni atoms for the ordered MnNi surface alloy have been confirmed in all functional used in agreement with experimental results. Taking into ( from LEED account the outward relaxation of 0:25 A studies [4], FM metastable state has been obtained for the bilayer surface alloy MnNi in qualitative agreement with experimental results [5].
References [1] M. Wuttig, Y. Gauthier, S. Blugel, . Phys. Rev. Lett. 70 (1993) 3619. . [2] S. Blugel, Appl. Phys. A: Matter Sci. Process. 63 (1996) 595. [3] C. Demangeat, J.C. Parlebas, Rep. Prog. Phys. 65 (2002) 1679. [4] M. Wuttig, C.C. Knight, Phys. Rev. B 48 (1993) 12130. [5] W.L. O’Brien, B.P. Tonner, Phys. Rev. B 51 (1995) 617. [6] D. Spisak, J. Hafner, J. Phys.: Condens. Matter 11 (1999) 6359. [7] B. M’Passi-Mabiala, et al., Phys. Rev. B 65 (2002) 12414; B. M’Passi-Mabiala, et al., Surf. Sci. 518 (2002) 104.