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Ab initio investigations of surface magnetism in V–Mo
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1198–1200
Ab initio investigations of surface magnetism in V–Mo A.V. Ponomarevaa, E.I. Isaeva,*, L.V. Pourovskiib,c, Yu.Kh. Vekilova, B. Johanssonb,d, I.A. Abrikosovb,e a
Department of Theoretical Physics, Moscow State Institute of Steel and Alloys, Leninskii prospect 4, Moscow 119991, Russia b Department of Physics, Uppsala University, Uppsala SE-751 21, Sweden c Department of Theoretical Physics, University of Nijmegen, Nijmegen 6500 GL, The Netherlands d Department of Materials Science and Engineering, Royal Institute of Technology (KTH), Stockholm SE-100 44, Sweden e Department Physics and Measurement Technology, Linkoping University, Linkoping SE-581 83, Sweden . .
Abstract Magnetic properties of the (0 0 1) surface of pure vanadium and disordered binary Mo–V alloys have been investigated from the first-principles by means of the LMTO-GF-CPA technique, the fixed spin moment method, and the direct exchange Monte-Carlo statistical mechanics simulations. The binary alloys, as well as pure constituent metals, are nonmagnetic in the bulk. However, we have shown that the (1 0 0) surface of uniformly random and segregated Mo–V alloys is magnetic. The magnetism of the V monolayer on the Mo(0 0 1) surface has also been studied. In particular, the surface relaxation effect has resulted in a reduction of the magnetic moments for V atoms, but the surface magnetism is still present in the system. Including of semi-core states as valence ones does not alter this conclusion. r 2004 Elsevier B.V. All rights reserved. Keywords: Magnetism; Random alloy; Segregation
During the last two decades, the onset of surface magnetism in systems that are nonmagnetic in the bulk became a subject of intensive debates. Often results of experimental and theoretical studies seem to be contradictory. For example, Rau et al. [1] using the electroncapture spectroscopy reported that the p(1 1) V(0 0 1) surface is magnetically ordered, while the theoretical calculations using the FLAPW method within the LSDA led to a conclusion that there is no surface magnetism for the V(0 0 1) film [2]. At the same time, calculations based on the ultrasoft pseudopotentials within GGA predicted that the magnetic moments on V atoms at both unrelaxed and relaxed surfaces are 1.70 mB and 1.45 mB, respectively [3]. Besides, Kresse and Joubert [4] have shown that including the semi-core states as valence states in pseudopotential calculations is desirable. The same conclusion was made in Ref. [5], where it *Corresponding author. Tel.: +7-95-230-45-06; fax: +7-95236-21-05. E-mail address: eyvaz [email protected] (E.I. Isaev).
was also shown that the TB-LMTO calculations predict nonmagnetic V(0 0 1) surface even within GGA. Our previous papers [6,7] were devoted to an investigation of surface magnetism of vanadium-based disordered binary alloys (MoV and PdV). It has been shown that the (0 0 1) surface of uniformly random alloys is magnetic and magnetic moment exists even in segregated V–Mo alloys. The onset of surface magnetism was explained using the Stoner criterion which states that a paramagnetic solution is not stable when the so-called Stoner criterion IN(EF)>1 is fulfilled. In this case the ferromagnetic alignment of magnetic moments makes a system more stable. In this contribution we present results of our firstprinciples calculations of magnetic properties of vanadium monolayer on the Mo(0 0 1) surface, as well as for the (0 0 1) surface of uniformly random and segregated Mo75V25 alloys. We treat surfaces in the semi-infinite geometry. Calculations are done by means of the LMTO-GF-CPA technique and the fixed spin moment method in conjunction with statistical Monte-Carlo
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.045
ARTICLE IN PRESS A.V. Ponomareva et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1198–1200
method. Surface relaxation effects are studied by means of the VASP package [8] using all-electron PAWpotentials which are generated using the GGA to the exchange-correlation functional. First, we present the densities of states (DOS) for the ideal V(0 0 1) surface (Fig. 1) and for the surface of Mo75V25(0 0 1) alloy (Fig. 2). In the latter case figures (a) and (b) correspond to the uniformly random and segregated surfaces, respectively. We have found that (0 0 1) surface of pure V is nonmagnetic which is in agreement with results of Refs. [2,5]. For Mo75V25 alloy calculated magnetic moments at the surface layers are 1.51 mB and 1.20 mB for the uniformly random and segregated surfaces, respectively. It is interesting to note that the minority spin is pinned at the Fermi level for the
1199
Table 1 Magnetic moment and relaxation of V-monolayer on the Mo(0 0 1) substrate is dependent on whether P- or S-semicore (SC) states are included into valence configuration. The first two rows refer to the ideal V(0 0 1) surface simulated by means of slab geometry V
Mo
Magnetic moment of Vlayer (mB)
Relaxation of surface layer (%)
9 layers 11 layers No SC P-SC S-SC P-SC S-SC
— — No SC No SC No-SC P-SC P-SC
0 0 1.12 1.16 1.15 1.67 1.66
Unrelaxed Unrelaxed 6.7 6 6 Unrelaxed Unrelaxed
40
V(100)
n(E),states/Ry*atom
majority spin 20
0
20
minority spin
40 -0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
E-EF,Ry
Fig. 1. Local density of states of V(0 0 1) surface layer.
40
(a) majority spin
20
n(E), states/Ry*atom
0
minority spin
20
(b) majority spin
20
0
minority spin
20
40 -0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
E-EF, Ry
Fig. 2. DOS for the surface layer of Mo75V25 alloy; (a) unsegregated surface; (b) segregated surface.
both V(0 0 1) and segregated Mo75V25(0 0 1) surfaces, while the majority spin channel is shifting gradually towards the higher band filling. Note that at the surface of pure V the Fermi level is located in a pseudogap of the DOS for the majority and minority spin channels. In the nonmagnetic alloys the Fermi level should move towards the peak of the DOS due to the increasing band filling, and this leads to magnetic splitting of the d-band, seen in Fig. 2a. But in the Mo75V25 alloy segregations of V-atoms toward the surface layers take place (segregation profile is given in Fig. 3 of Ref. [6]), and the surface layer of the alloy is mainly occupied by V-atoms. Segregation of V-atoms towards the surface layers leads to some electron ‘‘depletion’’ near the surface and, consequently, majority spin component of the DOS for the segregated surface shifts somewhat back towards higher energies. The V(0 0 1) surface was also simulated by means of a slab with 9 and 11 atomic layers. Calculations were done using all-electron PAW potentials without semi-core states. In this case we have found that both ideal and relaxed V(0 0 1) surfaces are practically nonmagnetic and thus, we confirm conclusion of earlier all-electron calculations [2,5]. But we also find that a monolayer of V deposited on the Mo(0 0 1) surface is magnetic for both relaxed and unrelaxed surfaces. This conclusion does not depend significantly on different atomic configurations, i.e. on different combinations of semi-core states included as valence states. Results of our investigations are summarized in Table 1. This work is supported by the Russian Foundation for Basic Research (RFBR grant #01-02-16156), the Royal Swedish Academy of Sciences, the Swedish Research Council (VR), and the Swedish Foundation for Strategic Research (SSF).
ARTICLE IN PRESS 1200
A.V. Ponomareva et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1198–1200
References [1] C. Rau, et al., Phys. Rev. Lett. 57 (1986) 2311. [2] S. Ohnishi, et al., J. Magn. Magn. Mater. 50 (1985) 161. [3] T. Bryk, et al., Phys. Rev. B 61 (2000) R3780. [4] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758.
[5] R. Robles, et al., Phys. Rev. B 63 (2001) 172406. [6] A.V. Ponomareva, et al., J. Magn. Magn. Mater. 258–259 (2003) 128. [7] A.V. Ponomareva, et al., Phys. Low-Dim. Str. 1–2 (2002) 337. [8] G. Kresse, J. Furthm.uller, Comp. Mater. Sci. 6 (1996) 15; G. Kresse, J. Furthm.uller, Phys. Rev. B 54 (1996) 11169.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1198–1200
Ab initio investigations of surface magnetism in V–Mo A.V. Ponomarevaa, E.I. Isaeva,*, L.V. Pourovskiib,c, Yu.Kh. Vekilova, B. Johanssonb,d, I.A. Abrikosovb,e a
Department of Theoretical Physics, Moscow State Institute of Steel and Alloys, Leninskii prospect 4, Moscow 119991, Russia b Department of Physics, Uppsala University, Uppsala SE-751 21, Sweden c Department of Theoretical Physics, University of Nijmegen, Nijmegen 6500 GL, The Netherlands d Department of Materials Science and Engineering, Royal Institute of Technology (KTH), Stockholm SE-100 44, Sweden e Department Physics and Measurement Technology, Linkoping University, Linkoping SE-581 83, Sweden . .
Abstract Magnetic properties of the (0 0 1) surface of pure vanadium and disordered binary Mo–V alloys have been investigated from the first-principles by means of the LMTO-GF-CPA technique, the fixed spin moment method, and the direct exchange Monte-Carlo statistical mechanics simulations. The binary alloys, as well as pure constituent metals, are nonmagnetic in the bulk. However, we have shown that the (1 0 0) surface of uniformly random and segregated Mo–V alloys is magnetic. The magnetism of the V monolayer on the Mo(0 0 1) surface has also been studied. In particular, the surface relaxation effect has resulted in a reduction of the magnetic moments for V atoms, but the surface magnetism is still present in the system. Including of semi-core states as valence ones does not alter this conclusion. r 2004 Elsevier B.V. All rights reserved. Keywords: Magnetism; Random alloy; Segregation
During the last two decades, the onset of surface magnetism in systems that are nonmagnetic in the bulk became a subject of intensive debates. Often results of experimental and theoretical studies seem to be contradictory. For example, Rau et al. [1] using the electroncapture spectroscopy reported that the p(1 1) V(0 0 1) surface is magnetically ordered, while the theoretical calculations using the FLAPW method within the LSDA led to a conclusion that there is no surface magnetism for the V(0 0 1) film [2]. At the same time, calculations based on the ultrasoft pseudopotentials within GGA predicted that the magnetic moments on V atoms at both unrelaxed and relaxed surfaces are 1.70 mB and 1.45 mB, respectively [3]. Besides, Kresse and Joubert [4] have shown that including the semi-core states as valence states in pseudopotential calculations is desirable. The same conclusion was made in Ref. [5], where it *Corresponding author. Tel.: +7-95-230-45-06; fax: +7-95236-21-05. E-mail address: eyvaz [email protected] (E.I. Isaev).
was also shown that the TB-LMTO calculations predict nonmagnetic V(0 0 1) surface even within GGA. Our previous papers [6,7] were devoted to an investigation of surface magnetism of vanadium-based disordered binary alloys (MoV and PdV). It has been shown that the (0 0 1) surface of uniformly random alloys is magnetic and magnetic moment exists even in segregated V–Mo alloys. The onset of surface magnetism was explained using the Stoner criterion which states that a paramagnetic solution is not stable when the so-called Stoner criterion IN(EF)>1 is fulfilled. In this case the ferromagnetic alignment of magnetic moments makes a system more stable. In this contribution we present results of our firstprinciples calculations of magnetic properties of vanadium monolayer on the Mo(0 0 1) surface, as well as for the (0 0 1) surface of uniformly random and segregated Mo75V25 alloys. We treat surfaces in the semi-infinite geometry. Calculations are done by means of the LMTO-GF-CPA technique and the fixed spin moment method in conjunction with statistical Monte-Carlo
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.045
ARTICLE IN PRESS A.V. Ponomareva et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1198–1200
method. Surface relaxation effects are studied by means of the VASP package [8] using all-electron PAWpotentials which are generated using the GGA to the exchange-correlation functional. First, we present the densities of states (DOS) for the ideal V(0 0 1) surface (Fig. 1) and for the surface of Mo75V25(0 0 1) alloy (Fig. 2). In the latter case figures (a) and (b) correspond to the uniformly random and segregated surfaces, respectively. We have found that (0 0 1) surface of pure V is nonmagnetic which is in agreement with results of Refs. [2,5]. For Mo75V25 alloy calculated magnetic moments at the surface layers are 1.51 mB and 1.20 mB for the uniformly random and segregated surfaces, respectively. It is interesting to note that the minority spin is pinned at the Fermi level for the
1199
Table 1 Magnetic moment and relaxation of V-monolayer on the Mo(0 0 1) substrate is dependent on whether P- or S-semicore (SC) states are included into valence configuration. The first two rows refer to the ideal V(0 0 1) surface simulated by means of slab geometry V
Mo
Magnetic moment of Vlayer (mB)
Relaxation of surface layer (%)
9 layers 11 layers No SC P-SC S-SC P-SC S-SC
— — No SC No SC No-SC P-SC P-SC
0 0 1.12 1.16 1.15 1.67 1.66
Unrelaxed Unrelaxed 6.7 6 6 Unrelaxed Unrelaxed
40
V(100)
n(E),states/Ry*atom
majority spin 20
0
20
minority spin
40 -0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
E-EF,Ry
Fig. 1. Local density of states of V(0 0 1) surface layer.
40
(a) majority spin
20
n(E), states/Ry*atom
0
minority spin
20
(b) majority spin
20
0
minority spin
20
40 -0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
E-EF, Ry
Fig. 2. DOS for the surface layer of Mo75V25 alloy; (a) unsegregated surface; (b) segregated surface.
both V(0 0 1) and segregated Mo75V25(0 0 1) surfaces, while the majority spin channel is shifting gradually towards the higher band filling. Note that at the surface of pure V the Fermi level is located in a pseudogap of the DOS for the majority and minority spin channels. In the nonmagnetic alloys the Fermi level should move towards the peak of the DOS due to the increasing band filling, and this leads to magnetic splitting of the d-band, seen in Fig. 2a. But in the Mo75V25 alloy segregations of V-atoms toward the surface layers take place (segregation profile is given in Fig. 3 of Ref. [6]), and the surface layer of the alloy is mainly occupied by V-atoms. Segregation of V-atoms towards the surface layers leads to some electron ‘‘depletion’’ near the surface and, consequently, majority spin component of the DOS for the segregated surface shifts somewhat back towards higher energies. The V(0 0 1) surface was also simulated by means of a slab with 9 and 11 atomic layers. Calculations were done using all-electron PAW potentials without semi-core states. In this case we have found that both ideal and relaxed V(0 0 1) surfaces are practically nonmagnetic and thus, we confirm conclusion of earlier all-electron calculations [2,5]. But we also find that a monolayer of V deposited on the Mo(0 0 1) surface is magnetic for both relaxed and unrelaxed surfaces. This conclusion does not depend significantly on different atomic configurations, i.e. on different combinations of semi-core states included as valence states. Results of our investigations are summarized in Table 1. This work is supported by the Russian Foundation for Basic Research (RFBR grant #01-02-16156), the Royal Swedish Academy of Sciences, the Swedish Research Council (VR), and the Swedish Foundation for Strategic Research (SSF).
ARTICLE IN PRESS 1200
A.V. Ponomareva et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1198–1200
References [1] C. Rau, et al., Phys. Rev. Lett. 57 (1986) 2311. [2] S. Ohnishi, et al., J. Magn. Magn. Mater. 50 (1985) 161. [3] T. Bryk, et al., Phys. Rev. B 61 (2000) R3780. [4] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758.
[5] R. Robles, et al., Phys. Rev. B 63 (2001) 172406. [6] A.V. Ponomareva, et al., J. Magn. Magn. Mater. 258–259 (2003) 128. [7] A.V. Ponomareva, et al., Phys. Low-Dim. Str. 1–2 (2002) 337. [8] G. Kresse, J. Furthm.uller, Comp. Mater. Sci. 6 (1996) 15; G. Kresse, J. Furthm.uller, Phys. Rev. B 54 (1996) 11169.