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Local magnetic moments at X-ray spectra of 3d metals
Journal of Magnetism and Magnetic Materials 256 (2003) 396–403
Local magnetic moments at X-ray spectra of 3d metals M.V. Yablonskikha,b,*, Yu. M. Yarmoshenkob, E.G. Gerasimovb, V.S. Gavikob, M.A. Korotinb, E.Z. Kurmaevb, S. Bartkowskic, M. Neumannc b
a Physics Department, Uppsala University, Box 530, S-75121 Uppsala, Sweden Institute of Metal Physics, Russian Academy of Sciences-Ural Division, 620219 Yekaterinburg GSP-170, Russia c Department of Physics, Osnabruck Germany . University, D-49069 Osnabruck, .
Received 17 April 2002; received in revised form 7 August 2002
Abstract The Mn X-ray emission spectra and X-ray photoelectron spectra of Mn-based Heusler alloys Co2 MnAl, Co2 MnSb and La1x Smx Mn2 Si2 rare-earth compounds (x ¼ 0; 0.8) have been measured and discussed in connection with a value of the local magnetic moment at Mn site. The spectra peculiarities reflect also the localization degree of 3d valence electrons of 3d metals in the considered compounds and could be used as an indicator of the closeness of the density of states to half-metallic character. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.70.En; 75.25.+z; 75.20.Hr; 87.64.Ni Keywords: X-ray spectroscopy; Electronic structure; Half-metallic ferromagnets; Local magnetic moments
1. Introduction Magnetic properties of a material are usually bound with the peculiarities of their electronic structure and explained within the framework of these terms. It is proposed that difference in an electronic structure of atoms should affect the values of their local magnetic moments. The X-ray spectroscopy is well known to be an elementselective probe of electronic structure. It opens the principal ability to provide information about the *Corresponding author. Physics Department, Uppsala University, Box 530, S-75121 Uppsala, Sweden. Tel.: +46-18-47162-88; fax: +46-18-471-35-24. E-mail address: [email protected] (M.V. Yablonskikh).
relation between magnetic properties and X-ray spectra because it deals with short-lived states, so it is complementary to data obtained from static magnetic measurements. The principal opportunity of using the spectroscopy to detect the influence of an atom magnetic moment to X-ray spectra had been shown by Van Acker et al. [1]. The corresponding experimental data have been obtained for a series of alloys and compounds containing Fe. From more recent experiments in which polarized radiation had been used it is evident that the dispersion as well as the energetic position of the Fe 3s band varies as a function of the electron spin [2]. Crystallizing in the space groups L21 Mn-based Heusler alloys (HA) X2 MnZ [3] has been found at
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 9 7 4 - 5
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Table 1 Structural and magnetic properties of Mn-based compounds data for Heusler alloys have been taken from Ref. [35] and references therein Material
Structure
dMn2Mn (nm)
mMn (mB Þ
meff (mB )
Mn La0:2 Sm0:8 Mn2 Si2 LaMn2 Si2 Co2 MnAl Co2 MnSb
a-Mn ThCr2 Si2 ThCr2 Si2 B2=L21 L21
0.283 0.291 0.407 0.419
1.9; 1.7; 0.6;0.2 2.3 2.5 3.01 3.75
3.8 3.5 4.01 4.9
Tc (K)
305 693 600
TN (K) 95 430 470
Data for rare-earth compounds are taken from Ref. [16] and references therein.
most in a ferromagnetic ground state. They offer the unique possibility to study manganese compounds where the Mn atom has only other transition X metals as nearest neighbors and non-transition Z elements of group III–V in the second coordination sphere. Webster [4] and later on Robinson et al. [5] have investigated the crystal structure of these alloys in detail. Furthermore, it has been shown that some disorder is often appearing in Mn-based HAs and that in total up to 10% of the Mn atoms change places with elements from a different sublattice [5,6]. Both neutron scattering measurements [7–9] and theoretical predictions [10] show well-defined local moments mMn (see Table 1). The local magnetic moment at Co atoms is much smaller than at Mn (less then 0:5 mB ; [4,11]). The remarkable feature of Mn 3d valence band for these alloys is a theoretically predicted pseudo-gap for minority spin electrons, which makes alloys to be close to the class of half-metallic ferromagnets [12]. The RMn2 Si2 (R is a rare-earth metal) compounds crystallize in the tetragonal ThCr2 Si2 structure, where layers are stacking along c-axis in the –Mn–X–R–X –Mn– sequence. The distance between R(0 0 1) and Mn(1 0 0) planes is about ( which exceeds the one between R c=2 ¼ 5:2 A, ( atoms (dR2RpEa ffiffiffi ¼ 4:2 A) and Mn atoms ( (dMn2Mn ¼ a= 2E2:8 A). The compounds are assumed to exhibit an unusually strong dependence of interlayer Mn–Mn exchange interaction from the intralayer dMn2Mn distance which is defined as one between nearest neighborhood Mn atoms in a single layer [13,14]. In case of ( is a critical dMn2Mn odc ; where dc ¼ 2:85–2.87 A value, the Mn magnetic moments in neighbor
layers are ordered antiferromagnetically. In case of dMn2Mn > dc ; the ferromagnetic ordering between Mn magnetic moments in neighbor layers appears. The dc value is close to a critical Mn–Mn distance ( at which a localization–delocalization of B2:84 A Mn 3d electrons is supposed to occur in Mn-based alloys [15]. Therefore, an assumption is made that at dMn2Mn Bdc the electron band structure of the compounds changes significantly. The doping of the second rare-earth metal in RMn2 Si2 compounds allows to gradually change the Mn–Mn distance from dMn2Mn > dc for LaMn2 Si2 to dMn2Mn odc for SmMn2 Si2 [16]. We focused at the influence of localization degree of 3d electrons and the values of the local magnetic moments of 3d metals at X-ray spectra. The Mn-based Heusler alloys and rare-earth intermetallic compounds have a different degree of the localization of Mn 3d electrons; so attention was paid mostly to analysis of Mn spectra. Also we compared X-ray photoelectron (XP) spectra and X-ray emission (XE) spectra of 3d metals Co and Mn for HA’s, in order to show both the difference regarding the magnitude of local magnetic moment at an atom of 3d metal and spin polarization of its electrons. To further facilitate comparison with experiment, we have also calculated spinpolarized distribution of Mn 3d density of states (DOS) for LaMn2 Si2 : The corresponding DOS calculations for Heusler alloys had been already performed, so the used results will be only cited below. Thereafter, we compared the spectroscopic results for Co2 MnZ alloys with those for La1x Smx Mn2 Si2 compounds which are considered to be more itinerant magnetics then Heusler alloys.
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2. Experimental details The specimens were prepared from a melt in an atmosphere of purified argon, annealed at 7203 C during 24 h for Heusler alloys and at the temperature T ¼ 9001C for La1x Smx Mn2 Si2 in argon and quenched in water. According to X-ray diffraction patterns (Cu Ka) samples were in single phase. X-ray photoelectron spectra were measured using a PHI 5600ci spectrometer. The monochromatized Al Ka radiation had an FWHM of 0.3 eV which combined with the energy resolution of the analyzer (1.5% of the pass energy) provided there was an estimated energy resolution somewhat less than 0.35 for XPS measurements. The compounds were crushed in situ in vacuum 1 107 Torr in order to avoid the impact of contaminated surface. We found that during the measurement process the intensities of O-1s and C-1s peaks were stable during the measurements which were being done in vacuum 1 109 Torr. X-ray emission spectra were obtained using an X-ray spectrometer of type RSM-500. The measurements have been carried out at a voltage of U ¼ 4:5 kV, an anode current of about 0.3–0.4 mA and 1 107 Torr vacuum. The energy resolution in the XE spectra resulted in 0.7 eV. Last but not least the rest contamination with oxygen has been estimated during the heating procedure by the periodic control of the intensity ratio of the lines O Ka and Mn Ll in the corresponding XE spectra. The scan length was about 10–15 min at the one beam position on the sample. Then the position of the beam had been changed and the next scan was started. The calibration has been performed according to the spectra of pure metals.
netic spin arrangement of Mn ions. The choice of atomic sphere radius (R(La)=4.13 a.u., R(Si)=2.67 a.u., R(Mn)=2.64 a.u.) provides the overlapping of atomic spheres less than 15% and full packing of cell volume according to the rule of ASA. The states 4s, 4p, 3d for Mn, 3s, 3p, 3d for Si and 6s, 6p, 5d, 4f for La were considered as valence states. The density of states were calculated by tetrahedron method of Brillouin zone integration with 162 k points in the irreducible part. The electronic spectrum of ferromagnetic LaMn2 Si2 is found metallic with density of states on the Fermi level equal to 3.05 states/eV. The spin magnetic moment on d shell of Mn ion is 1.77 mB : The La and Si magnetic moments (0.13 and 0.10 mB ; correspondingly) are opposite to the direction of magnetic moment of Mn. This leads to a value of 3.25 mB per unit cell. The total DOS and partial DOS of Mn d states are presented in Fig. 1. The low-energy DOS region (10oeo 6 eV) is formed mainly by 3s states of Si. The remaining part of valence band is formed
3. Calculations Electronic structure of LaMn2 Si2 was determined in the local spin density approximation (LSDA) [17]. The self-consistent ab initio calculation was made in frames of linear muffin–tin orbital method in tight-binding representation using the atomic sphere approximation (TBLMTO-ASA) [18,19]. We used the experimentally determined lattice parameters and (ferro-)mag-
Fig. 1. Total and partial Mn 3d DOS of LaMn2 Si2 : Filled regions correspond to occupied states.
M.V. Yablonskikh et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 396–403
by 3p states of Si hybridized with 3d states of Mn. The region around the Fermi energy (2oeo2 eV) is formed mainly by 3d states of Mn with noticeable contribution of 5d states of La. Our calculations are in good agreement with those for RMn2 Si2 compounds [20]. First principle band structure calculations [21] for Heusler alloys have shown that in the electron system with spins directed along the magnetization axis (m) the Mn 3d states are occupied and hybridized with the 3d states of the X atoms. In the electron system with spins directed against the magnetization (k) not hybridized empty Mn d states are located at 1.0–1.5 eV above the Fermi level [21–24]. Further investigations on Mn-based HAs [12,25,26] predict the existence of a halfmetallic ferromagnetic (HMF) density of state, for example, in NiMnSb, PdMnSb, and PtMnSb. This means that an energy gap at the Fermi level exists for one spin orientation and a metallic behavior has been obtained for the other spin direction. Particularly [27], Co 3d DOS is found to be metallic while Mn 3d DOS has a close to halfmetallic character [27–29].
4. Discussion The Mn 3s photoelectron spectra of compounds and pure manganese are shown in Fig. 2. The multiplet splitting is observed in the form of two peaks at the binding energies EB ¼ 83 and 87.5 eV for pure Mn and RMn2 Si2 : No clear dependence of the spectra shape from the value of the local magnetic moment at Mn site is observed for whole series of compounds. Roughly it is possible to distinguish only small splitting of Mn 3s core level due to interaction between 3d valence electrons and 3s core hole created and the binding energy shift of Mn 3s XPS about 0.6 eV toward highenergy side. Briefly, such an effect is explained by the difference in the energy for Mn 3s1j 3d5 S final state that is to be result of the strong exchange interaction between Mn 3sj hole and strongly spinpolarized Mn 3d electrons in case of Co2 MnSb. The Mn 2p XPS have been shown in Fig. 3. The spin–orbit splitting leads to the appearance of Mn 2p1=2 peak with binding energy EB ¼ 649:7 eV and
399
Fig. 2. Mn 3s X-ray photoelectron spectra. The spectrum of pure Mn is given as a reference.
the Mn 2p3=2 peak with EB ¼ 638:5 eV. Opposite to Mn 3s photoelectron spectrum in Fig. 2, where it is very difficult to observe the changes between spectra, every Mn 2p spectrum differs both from one of the pure Mn and those for both series of compounds. The Mn 2p3=2 peak splits at two position each marked as A and B: The energy position of peak A is the same for all Mn 2p spectra. The energy position of peak B is moving toward high binding energies with intensity rising in the series La02 Sm08 Mn2 Si2 ; LaMn2 Si2 ; Co2 MnAl; Co2 MnSb: For RMn2 Si2 compounds, the value of splitting is about 0.7–0.8 eV and for Heusler alloys it is approximately 1 eV. No energy shifts of Mn 2p XPS have been observed compared to pure metal spectrum. A peak of very weak intensity at EB ¼ 641 eV is observed at the edge of Mn 2p3=2 spectra due to the small amount of oxygen at the surface. It could also mean the small disorder in Mn sublattice. The main feature of the spectra is the Mn 2p3=2 energy splitting corresponding to the strong 2p–3d exchange interaction [30]. The magnitude of the splitting correlates with
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400
Mn 2p La 0.2 Sm0.8 Mn 2 Si 2
Intensity(arb.units)
LaMn2 Si 2
Co2 MnAl
Co 2 MnSb
A B
Mn 2p3/2
2p1/2 660
655
650
645
640
635
630
Fig. 4. Co 2p X-ray photoelectron spectra of Heusler alloys.
Binding energy, eV Fig. 3. Mn 2p X-ray photoelectron spectra. The vertical lines mark the positions of 2 peaks at the Mn 2p3=2 spectrum. The spectrum of pure Mn is given as a reference.
the value of mMn : Such qualitative dependence is observed both for RMn2 Si2 and Heusler alloys with mMn ; see Table 1. The correlation between the magnitude of core level exchange splitting and that of local magnetic moment at an atom is also in agreement with no splitting observed in Co 2p and La 3d photoelectron spectra in Figs. 4 and 5, where local magnetic moment is small, compared to those for manganese in compounds. The Mn L2 ; L3 X-ray emission spectra correspond to X-ray emission transition of Mn 3d valence electron to 2p1=2 and 2p3=2 core hole, respectively. Comparing spectra between each other in (Fig. 6) one finds two characteristic features. The height of IðL2 Þ=IðL3 Þ intensity ratio correlates with mMn : The L3 peak consists of two peaks A0 and B0 that is observed in the case of Heusler alloys and again is due to high localization of strongly spin-polarized Mn 3d electrons [31]. In XE spectra of 3d metals, such an enhancement of
L3 X-ray emission line (marked here as peak B0 ) located toward high photon energies of the main peak A0 is explained by the influence of inner Coster–Kronig transition [32–34], but this is opposite to the high IðL2 Þ=IðL3 Þ intensity ratio observed. It was shown in Ref. [31] that the intensive peak B0 results due to intensive reemission which appeared because of the existence of the pseudogap for one of Mn 3d spin-projection at valence band and the trap for excited core level 2p electron in not hybridized empty Mn d states. The trap is located at 1.0–1.5 eV above the Fermi level. Both the presence of the peak B0 with energy exceeding the Mn 2p3=2 core level binding energy and the IðL2 Þ=IðL3 Þ high intensity ratio indicate the presence of strong 2p–3d exchange interaction formed at Mn atom in Heusler alloys. Cobalt L2 ;L3 emission spectra (see Fig. 7) resemble one of pure Co that correlates with a small value (less then 0.5 mB ; through Ref. [35]) of Co magnetic moment in compounds. This observation is in good agreement with results taken from the analysis of L2 ; L3 XE spectra of 3d metals like Cr, Mn and Co for other compounds [35,36].
M.V. Yablonskikh et al. / Journal of Magnetism and Magnetic Materials 256 (2003) 396–403
Fig. 5. La 3d X-ray photoelectron spectra of rare-earth compounds.
401
Fig. 6. Mn L2 ;3 X-ray emission spectra of Heusler alloys, La1x Smx Mn2 Si2 : The spectrum of pure Mn is given as a reference. The position of normal emission peak is marked as A0 ; the position of reemission peak is marked as B0 : The position of the Mn 2p3=2 core level is taken from Fig. 3.
5. Conclusion The presented analysis discussed the degree of the localization of the valence electron for different compounds with a metallic type of conductivity. The exchange splitting of the 2p3=2 core level photoelectron spectra of the 3d metals indicates the presence of the local magnetic moment with a value about more than 2:3 mB : This result is supported by both the comparison of the Co 2p with Mn 2p X-ray photoelectron spectra of Heusler alloys and by the observed dependence of the Mn 2p X-ray photoelectron spectra on the value of the mMn for all investigated compounds. The same indication of large magnetic moment at an atom of 3d metal is reflected in respective 2p- 3d X-ray emission transitions by the increase of the intensity ratio IðL2 Þ=IðL3 Þ: This comes both from the comparison of the Co 2p- 3d and Mn 2p- 3d X-ray emission spectra for Heusler alloys
and the observed dependence of Mn 2p- 3d Xray emission spectra for all investigated compounds. The growth of intensity of the peak B0 towards peak A0 maps different spin polarization degree of the 3d valence electrons in series La0:2 Sm0:8 Mn2 Si2 ; LaMn2 Si2 ; Co2 MnAl, Co2 MnSb. DOS calculations show the difference in the band structure for the minority spin electrons for these compounds. The Mn 3d DOS structure becomes close to the half-metallic state in Heusler alloys while the absence of the energy gap for 3d electrons with minority spin projection for RMn2 Si2 compounds indicates the metallic character of DOS. The presence of an energy gap for 3d valence electrons creates conditions for intensive reemission peak B0 to appear in half-metallic alloys.
402
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Fig. 7. Co L2;3 X-ray emission spectra of Heusler alloys. The spectrum of pure Co is given as a reference.
Acknowledgements The support from Russian Foundation for Basic Research (Projects 00-15-96575, 02-02-16674, 0102-17063) and DeutscheForschungsgemeinschaft (DFG) is gratefully acknowledged.
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Local magnetic moments at X-ray spectra of 3d metals M.V. Yablonskikha,b,*, Yu. M. Yarmoshenkob, E.G. Gerasimovb, V.S. Gavikob, M.A. Korotinb, E.Z. Kurmaevb, S. Bartkowskic, M. Neumannc b
a Physics Department, Uppsala University, Box 530, S-75121 Uppsala, Sweden Institute of Metal Physics, Russian Academy of Sciences-Ural Division, 620219 Yekaterinburg GSP-170, Russia c Department of Physics, Osnabruck Germany . University, D-49069 Osnabruck, .
Received 17 April 2002; received in revised form 7 August 2002
Abstract The Mn X-ray emission spectra and X-ray photoelectron spectra of Mn-based Heusler alloys Co2 MnAl, Co2 MnSb and La1x Smx Mn2 Si2 rare-earth compounds (x ¼ 0; 0.8) have been measured and discussed in connection with a value of the local magnetic moment at Mn site. The spectra peculiarities reflect also the localization degree of 3d valence electrons of 3d metals in the considered compounds and could be used as an indicator of the closeness of the density of states to half-metallic character. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.70.En; 75.25.+z; 75.20.Hr; 87.64.Ni Keywords: X-ray spectroscopy; Electronic structure; Half-metallic ferromagnets; Local magnetic moments
1. Introduction Magnetic properties of a material are usually bound with the peculiarities of their electronic structure and explained within the framework of these terms. It is proposed that difference in an electronic structure of atoms should affect the values of their local magnetic moments. The X-ray spectroscopy is well known to be an elementselective probe of electronic structure. It opens the principal ability to provide information about the *Corresponding author. Physics Department, Uppsala University, Box 530, S-75121 Uppsala, Sweden. Tel.: +46-18-47162-88; fax: +46-18-471-35-24. E-mail address: [email protected] (M.V. Yablonskikh).
relation between magnetic properties and X-ray spectra because it deals with short-lived states, so it is complementary to data obtained from static magnetic measurements. The principal opportunity of using the spectroscopy to detect the influence of an atom magnetic moment to X-ray spectra had been shown by Van Acker et al. [1]. The corresponding experimental data have been obtained for a series of alloys and compounds containing Fe. From more recent experiments in which polarized radiation had been used it is evident that the dispersion as well as the energetic position of the Fe 3s band varies as a function of the electron spin [2]. Crystallizing in the space groups L21 Mn-based Heusler alloys (HA) X2 MnZ [3] has been found at
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 9 7 4 - 5
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397
Table 1 Structural and magnetic properties of Mn-based compounds data for Heusler alloys have been taken from Ref. [35] and references therein Material
Structure
dMn2Mn (nm)
mMn (mB Þ
meff (mB )
Mn La0:2 Sm0:8 Mn2 Si2 LaMn2 Si2 Co2 MnAl Co2 MnSb
a-Mn ThCr2 Si2 ThCr2 Si2 B2=L21 L21
0.283 0.291 0.407 0.419
1.9; 1.7; 0.6;0.2 2.3 2.5 3.01 3.75
3.8 3.5 4.01 4.9
Tc (K)
305 693 600
TN (K) 95 430 470
Data for rare-earth compounds are taken from Ref. [16] and references therein.
most in a ferromagnetic ground state. They offer the unique possibility to study manganese compounds where the Mn atom has only other transition X metals as nearest neighbors and non-transition Z elements of group III–V in the second coordination sphere. Webster [4] and later on Robinson et al. [5] have investigated the crystal structure of these alloys in detail. Furthermore, it has been shown that some disorder is often appearing in Mn-based HAs and that in total up to 10% of the Mn atoms change places with elements from a different sublattice [5,6]. Both neutron scattering measurements [7–9] and theoretical predictions [10] show well-defined local moments mMn (see Table 1). The local magnetic moment at Co atoms is much smaller than at Mn (less then 0:5 mB ; [4,11]). The remarkable feature of Mn 3d valence band for these alloys is a theoretically predicted pseudo-gap for minority spin electrons, which makes alloys to be close to the class of half-metallic ferromagnets [12]. The RMn2 Si2 (R is a rare-earth metal) compounds crystallize in the tetragonal ThCr2 Si2 structure, where layers are stacking along c-axis in the –Mn–X–R–X –Mn– sequence. The distance between R(0 0 1) and Mn(1 0 0) planes is about ( which exceeds the one between R c=2 ¼ 5:2 A, ( atoms (dR2RpEa ffiffiffi ¼ 4:2 A) and Mn atoms ( (dMn2Mn ¼ a= 2E2:8 A). The compounds are assumed to exhibit an unusually strong dependence of interlayer Mn–Mn exchange interaction from the intralayer dMn2Mn distance which is defined as one between nearest neighborhood Mn atoms in a single layer [13,14]. In case of ( is a critical dMn2Mn odc ; where dc ¼ 2:85–2.87 A value, the Mn magnetic moments in neighbor
layers are ordered antiferromagnetically. In case of dMn2Mn > dc ; the ferromagnetic ordering between Mn magnetic moments in neighbor layers appears. The dc value is close to a critical Mn–Mn distance ( at which a localization–delocalization of B2:84 A Mn 3d electrons is supposed to occur in Mn-based alloys [15]. Therefore, an assumption is made that at dMn2Mn Bdc the electron band structure of the compounds changes significantly. The doping of the second rare-earth metal in RMn2 Si2 compounds allows to gradually change the Mn–Mn distance from dMn2Mn > dc for LaMn2 Si2 to dMn2Mn odc for SmMn2 Si2 [16]. We focused at the influence of localization degree of 3d electrons and the values of the local magnetic moments of 3d metals at X-ray spectra. The Mn-based Heusler alloys and rare-earth intermetallic compounds have a different degree of the localization of Mn 3d electrons; so attention was paid mostly to analysis of Mn spectra. Also we compared X-ray photoelectron (XP) spectra and X-ray emission (XE) spectra of 3d metals Co and Mn for HA’s, in order to show both the difference regarding the magnitude of local magnetic moment at an atom of 3d metal and spin polarization of its electrons. To further facilitate comparison with experiment, we have also calculated spinpolarized distribution of Mn 3d density of states (DOS) for LaMn2 Si2 : The corresponding DOS calculations for Heusler alloys had been already performed, so the used results will be only cited below. Thereafter, we compared the spectroscopic results for Co2 MnZ alloys with those for La1x Smx Mn2 Si2 compounds which are considered to be more itinerant magnetics then Heusler alloys.
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2. Experimental details The specimens were prepared from a melt in an atmosphere of purified argon, annealed at 7203 C during 24 h for Heusler alloys and at the temperature T ¼ 9001C for La1x Smx Mn2 Si2 in argon and quenched in water. According to X-ray diffraction patterns (Cu Ka) samples were in single phase. X-ray photoelectron spectra were measured using a PHI 5600ci spectrometer. The monochromatized Al Ka radiation had an FWHM of 0.3 eV which combined with the energy resolution of the analyzer (1.5% of the pass energy) provided there was an estimated energy resolution somewhat less than 0.35 for XPS measurements. The compounds were crushed in situ in vacuum 1 107 Torr in order to avoid the impact of contaminated surface. We found that during the measurement process the intensities of O-1s and C-1s peaks were stable during the measurements which were being done in vacuum 1 109 Torr. X-ray emission spectra were obtained using an X-ray spectrometer of type RSM-500. The measurements have been carried out at a voltage of U ¼ 4:5 kV, an anode current of about 0.3–0.4 mA and 1 107 Torr vacuum. The energy resolution in the XE spectra resulted in 0.7 eV. Last but not least the rest contamination with oxygen has been estimated during the heating procedure by the periodic control of the intensity ratio of the lines O Ka and Mn Ll in the corresponding XE spectra. The scan length was about 10–15 min at the one beam position on the sample. Then the position of the beam had been changed and the next scan was started. The calibration has been performed according to the spectra of pure metals.
netic spin arrangement of Mn ions. The choice of atomic sphere radius (R(La)=4.13 a.u., R(Si)=2.67 a.u., R(Mn)=2.64 a.u.) provides the overlapping of atomic spheres less than 15% and full packing of cell volume according to the rule of ASA. The states 4s, 4p, 3d for Mn, 3s, 3p, 3d for Si and 6s, 6p, 5d, 4f for La were considered as valence states. The density of states were calculated by tetrahedron method of Brillouin zone integration with 162 k points in the irreducible part. The electronic spectrum of ferromagnetic LaMn2 Si2 is found metallic with density of states on the Fermi level equal to 3.05 states/eV. The spin magnetic moment on d shell of Mn ion is 1.77 mB : The La and Si magnetic moments (0.13 and 0.10 mB ; correspondingly) are opposite to the direction of magnetic moment of Mn. This leads to a value of 3.25 mB per unit cell. The total DOS and partial DOS of Mn d states are presented in Fig. 1. The low-energy DOS region (10oeo 6 eV) is formed mainly by 3s states of Si. The remaining part of valence band is formed
3. Calculations Electronic structure of LaMn2 Si2 was determined in the local spin density approximation (LSDA) [17]. The self-consistent ab initio calculation was made in frames of linear muffin–tin orbital method in tight-binding representation using the atomic sphere approximation (TBLMTO-ASA) [18,19]. We used the experimentally determined lattice parameters and (ferro-)mag-
Fig. 1. Total and partial Mn 3d DOS of LaMn2 Si2 : Filled regions correspond to occupied states.
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by 3p states of Si hybridized with 3d states of Mn. The region around the Fermi energy (2oeo2 eV) is formed mainly by 3d states of Mn with noticeable contribution of 5d states of La. Our calculations are in good agreement with those for RMn2 Si2 compounds [20]. First principle band structure calculations [21] for Heusler alloys have shown that in the electron system with spins directed along the magnetization axis (m) the Mn 3d states are occupied and hybridized with the 3d states of the X atoms. In the electron system with spins directed against the magnetization (k) not hybridized empty Mn d states are located at 1.0–1.5 eV above the Fermi level [21–24]. Further investigations on Mn-based HAs [12,25,26] predict the existence of a halfmetallic ferromagnetic (HMF) density of state, for example, in NiMnSb, PdMnSb, and PtMnSb. This means that an energy gap at the Fermi level exists for one spin orientation and a metallic behavior has been obtained for the other spin direction. Particularly [27], Co 3d DOS is found to be metallic while Mn 3d DOS has a close to halfmetallic character [27–29].
4. Discussion The Mn 3s photoelectron spectra of compounds and pure manganese are shown in Fig. 2. The multiplet splitting is observed in the form of two peaks at the binding energies EB ¼ 83 and 87.5 eV for pure Mn and RMn2 Si2 : No clear dependence of the spectra shape from the value of the local magnetic moment at Mn site is observed for whole series of compounds. Roughly it is possible to distinguish only small splitting of Mn 3s core level due to interaction between 3d valence electrons and 3s core hole created and the binding energy shift of Mn 3s XPS about 0.6 eV toward highenergy side. Briefly, such an effect is explained by the difference in the energy for Mn 3s1j 3d5 S final state that is to be result of the strong exchange interaction between Mn 3sj hole and strongly spinpolarized Mn 3d electrons in case of Co2 MnSb. The Mn 2p XPS have been shown in Fig. 3. The spin–orbit splitting leads to the appearance of Mn 2p1=2 peak with binding energy EB ¼ 649:7 eV and
399
Fig. 2. Mn 3s X-ray photoelectron spectra. The spectrum of pure Mn is given as a reference.
the Mn 2p3=2 peak with EB ¼ 638:5 eV. Opposite to Mn 3s photoelectron spectrum in Fig. 2, where it is very difficult to observe the changes between spectra, every Mn 2p spectrum differs both from one of the pure Mn and those for both series of compounds. The Mn 2p3=2 peak splits at two position each marked as A and B: The energy position of peak A is the same for all Mn 2p spectra. The energy position of peak B is moving toward high binding energies with intensity rising in the series La02 Sm08 Mn2 Si2 ; LaMn2 Si2 ; Co2 MnAl; Co2 MnSb: For RMn2 Si2 compounds, the value of splitting is about 0.7–0.8 eV and for Heusler alloys it is approximately 1 eV. No energy shifts of Mn 2p XPS have been observed compared to pure metal spectrum. A peak of very weak intensity at EB ¼ 641 eV is observed at the edge of Mn 2p3=2 spectra due to the small amount of oxygen at the surface. It could also mean the small disorder in Mn sublattice. The main feature of the spectra is the Mn 2p3=2 energy splitting corresponding to the strong 2p–3d exchange interaction [30]. The magnitude of the splitting correlates with
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400
Mn 2p La 0.2 Sm0.8 Mn 2 Si 2
Intensity(arb.units)
LaMn2 Si 2
Co2 MnAl
Co 2 MnSb
A B
Mn 2p3/2
2p1/2 660
655
650
645
640
635
630
Fig. 4. Co 2p X-ray photoelectron spectra of Heusler alloys.
Binding energy, eV Fig. 3. Mn 2p X-ray photoelectron spectra. The vertical lines mark the positions of 2 peaks at the Mn 2p3=2 spectrum. The spectrum of pure Mn is given as a reference.
the value of mMn : Such qualitative dependence is observed both for RMn2 Si2 and Heusler alloys with mMn ; see Table 1. The correlation between the magnitude of core level exchange splitting and that of local magnetic moment at an atom is also in agreement with no splitting observed in Co 2p and La 3d photoelectron spectra in Figs. 4 and 5, where local magnetic moment is small, compared to those for manganese in compounds. The Mn L2 ; L3 X-ray emission spectra correspond to X-ray emission transition of Mn 3d valence electron to 2p1=2 and 2p3=2 core hole, respectively. Comparing spectra between each other in (Fig. 6) one finds two characteristic features. The height of IðL2 Þ=IðL3 Þ intensity ratio correlates with mMn : The L3 peak consists of two peaks A0 and B0 that is observed in the case of Heusler alloys and again is due to high localization of strongly spin-polarized Mn 3d electrons [31]. In XE spectra of 3d metals, such an enhancement of
L3 X-ray emission line (marked here as peak B0 ) located toward high photon energies of the main peak A0 is explained by the influence of inner Coster–Kronig transition [32–34], but this is opposite to the high IðL2 Þ=IðL3 Þ intensity ratio observed. It was shown in Ref. [31] that the intensive peak B0 results due to intensive reemission which appeared because of the existence of the pseudogap for one of Mn 3d spin-projection at valence band and the trap for excited core level 2p electron in not hybridized empty Mn d states. The trap is located at 1.0–1.5 eV above the Fermi level. Both the presence of the peak B0 with energy exceeding the Mn 2p3=2 core level binding energy and the IðL2 Þ=IðL3 Þ high intensity ratio indicate the presence of strong 2p–3d exchange interaction formed at Mn atom in Heusler alloys. Cobalt L2 ;L3 emission spectra (see Fig. 7) resemble one of pure Co that correlates with a small value (less then 0.5 mB ; through Ref. [35]) of Co magnetic moment in compounds. This observation is in good agreement with results taken from the analysis of L2 ; L3 XE spectra of 3d metals like Cr, Mn and Co for other compounds [35,36].
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Fig. 5. La 3d X-ray photoelectron spectra of rare-earth compounds.
401
Fig. 6. Mn L2 ;3 X-ray emission spectra of Heusler alloys, La1x Smx Mn2 Si2 : The spectrum of pure Mn is given as a reference. The position of normal emission peak is marked as A0 ; the position of reemission peak is marked as B0 : The position of the Mn 2p3=2 core level is taken from Fig. 3.
5. Conclusion The presented analysis discussed the degree of the localization of the valence electron for different compounds with a metallic type of conductivity. The exchange splitting of the 2p3=2 core level photoelectron spectra of the 3d metals indicates the presence of the local magnetic moment with a value about more than 2:3 mB : This result is supported by both the comparison of the Co 2p with Mn 2p X-ray photoelectron spectra of Heusler alloys and by the observed dependence of the Mn 2p X-ray photoelectron spectra on the value of the mMn for all investigated compounds. The same indication of large magnetic moment at an atom of 3d metal is reflected in respective 2p- 3d X-ray emission transitions by the increase of the intensity ratio IðL2 Þ=IðL3 Þ: This comes both from the comparison of the Co 2p- 3d and Mn 2p- 3d X-ray emission spectra for Heusler alloys
and the observed dependence of Mn 2p- 3d Xray emission spectra for all investigated compounds. The growth of intensity of the peak B0 towards peak A0 maps different spin polarization degree of the 3d valence electrons in series La0:2 Sm0:8 Mn2 Si2 ; LaMn2 Si2 ; Co2 MnAl, Co2 MnSb. DOS calculations show the difference in the band structure for the minority spin electrons for these compounds. The Mn 3d DOS structure becomes close to the half-metallic state in Heusler alloys while the absence of the energy gap for 3d electrons with minority spin projection for RMn2 Si2 compounds indicates the metallic character of DOS. The presence of an energy gap for 3d valence electrons creates conditions for intensive reemission peak B0 to appear in half-metallic alloys.
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Fig. 7. Co L2;3 X-ray emission spectra of Heusler alloys. The spectrum of pure Co is given as a reference.
Acknowledgements The support from Russian Foundation for Basic Research (Projects 00-15-96575, 02-02-16674, 0102-17063) and DeutscheForschungsgemeinschaft (DFG) is gratefully acknowledged.
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