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A 57Fe Mössbauer study of Mn2P
Journal of Magnetism and Magnetic Materials 60 (1986) 171-174 North-Holland, Amsterdam
A -Fe
MGSSBAUER
L. HAGGSTRijM, Institute of Physics,
171
STUDY OF Mn,P
J. SJaSTRijM
and
T. ERICSSON
University of Uppsala, Box 530, S-751 21 Uppsala, Sweden
Received 9 October 1985; in final form 13 February 1986
It is shown that 57Fe substitutes for Mn(l) in Mn,P. The NM temperature is found to be 110 K and the magnetic moments for the iron atoms are estimated to be 0.63~~. All iron atoms have the same moment in contrast to what was found for Mn(1) atoms in an earlier neutron diffraction study by Yessik. A magnetic triangular substructure for Mn(1) is therefore proposed.
1. Introduction
Mn,P is hexagonal with space group P62m and isomorphous with FqP. The structure contains two metal positions, Mn(1) surrounded by four phosphorus atoms in a distorted tetrahedron and Mn(2) surrounded by five P atoms in a square based pyramid [l]. Yessik [2] reported Mn,P to be an antiferromagnet with a N6el temperature of 103 K. The magnetic structure, determined by neutron diffraction measurements on single crystals, was described as a spin modulation com-
mensurate with the lattice propagating along three hexagonally equivalent directions. The spin components were all perpendicular to the propagation directions and the mean values of the magnetic moments at the two manganese sites were for Mn(1): O.Ol(4)pn and for Mn(2): 0.84(3)pr,. The model of the magnetic structure in a related C face centered orthorhombic cell is show in in fig. 1. Further conclusions drawn by Yessik were: i) 2/3 of the Mn(2) atoms have significant moments; ii) l/3 of the Mn(2) atoms have no moments;
dl
x -1
eb
-
oMn(l) Z-0 lMn(2) Z=v2
A
B
Fig. 1. (A) The (001) projections of the three unit cells: double outline = chemical unit cell (hexagonal), single outline = magnetic unit cell (hexagonal), broken outline = C face entered orthorhombic cell; (B) model of the magnetic structure in the (001) projection of the orthorhombic cell (both figures from ref. [2]).
03048853/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
172
L. Hiiggstrih
et al. / ‘?Fe Miissbauer study of Mn z P
iii) 2/3 of the Mn(1) atoms might have very small moments; iv) l/3 of the Mn(1) atoms have no moments; v) all moments are strictly along the b axis. In a recent study of the solid solutions Fe, P- Mn 2P by Srivastava et al. [3], it was verified that Fe in this series only occupies the Mn(1) positions. This preferential occupation gave us a good opportunity to test the conclusions drawn by Yessik, using 57Fe Mossbauer spectroscopy.
P
(Mn0.99Fe00d2
--If--
295K
1
2. Experimental Mn ,P, 57Fe ano were mixed in appropriate proportions and heat treated at 1100 K for four days. The product was crushed and the heat treatment at 1100 K was repeated four times. The final result was checked by X-ray and there were diffraction lines only from the Mn,P structure plus some very weak lines from MnO. The unit cell parameters we!e found to be a = 6.0786 (2) A and c = 3.4595(2) A, which can be compared with the results for Mn,P by Rundqvist [l]: a = 6.081 .& c = 3.460 A. The chemical composition of the product can be written as (Mn,,,57Fe,,01) *P. The Mbssbauer spectrometer was of conventional constant acceleration type utilizing a double-ended vibrator with 57CoRh sources mounted on both ends. One source was used to simultaneously record the calibration spectra using a natural iron foil at room temperature as standard. The Miissbauer spectra of (Mn,,,,Fe,,e,),P were recorded using a flow cryostat with liquid helium or nitrogen as cooling media. The recorded spectra were folded and analyzed by a computer program written by Jernberg and Sundqvist [4].
3. Results Representative Miissbauer spectra are displayed in fig. 2. Above TN = llO(5) K the resonance is a broad single line. The broadening is caused by a small electric quadrupole splitting [3]. The spectra recorded in the magnetic region were fitted in the first stage of the data analysis by just one sextet. The results from these fittings are
I_L--L
-3.0
-2.0
-1.0
0.
1.0
2.0
3.0
mm/s Fig. 2. MBssbauer spectra recorded at 295 and 5 K. The full lines represent the sum of the fitted Lorentzian functions. The fitting of the spectrum at 295 K shows a deficiency at the center of the resonance. This is most reasonably due to the influence of the random distribution of next nearest Fe(l) atoms, which is not taken into account in the fitting model.
given in table 1. In the paramagnetic region the electric quadrupole splitting, AE& is given by: AEo’=
I eQW2
l111+12/3.
In the magnetic region, the quadrupole splitting is defined by AE;;=;[(u~-us)-(u+i)]. Using first-order perturbation theory this splitting can be approximated to AEa”=2
eQV,, 3 cos*B - 1 + 9 sin28 cos 2+ 2
Here Q is the nuclear quadrupole moment of the excited state in 57Fe, V,, is the main principal component and T) is the asymmetry parameter of the EFG tensor. The polar and azimuthal angles for the magnetic field B in the principal axes
173
L. HiiggstrCm et al. / “Fe Miissbauer study of Mn z P
Table 1 Results from the Miissbauer measurements. shift versus natural Fe at room temperature, hyperfine field in tesla, AEo is the electric ting (defined in text) and F is the absorber maximum (i.e. source line-width excluded). mm/s for 8, AEo and F, and 0.1 T for B 8
B
TKI
(mm/s)
0-I
295 110 105 92 80 5
0.33 0.43 0.43 0.44 0.44 0.45
_ 2.1 4.2 4.7 6.3
15 is the centroid B is the magnetic quadrupole sphtline-width at half Precision is 0.01
AEQ wvs)
r
0.20 0.21 -0.14 -0.14 -0.14 -0.14
0.16 0.25 0.30 0.31 0.25 0.21
@ws)
system of the EFG are denoted B and (p and ui, 02 *** u, are the peak positions with increasing velocity in the sextet.
of Mn(1). One can therefore conclude that the magnetic moments of the Mn(1) atoms are all the same. Whether the conclusions i) and ii) also are wrong we cannot say as iron does not replace Mn(2) in Mn z P. In analogy with Fe(l) in Fe,P, the principal component V,, for Fe(l) is Mn,P is in the hexagonal c-plane [6]. Further analogies are that V,, c 0 and that n = 0 for Fe(l) atoms. From symmetry considerations V,, could be along either of two directions, d, or d 2 as shown in fig. 1. Assuming the conclusion v) mentioned above to be correct, the alternative d, requires that 2/3 of the Fe atoms have 19= 60” and l/3 of the Fe atoms 8 = 0”. This means that in principle we should have two MSssbauer patterns, one doubly populated with AEC =
(eQK:,/2)(--l/8)
and one singly populated with 4. Discussion The near metal surrounding for an iron atom replacing Mn(1) in Mn,P is 2 Me(l) atoms and 6 Me(2) atoms at = 2.7 A. The next Me(l) atoms close to Fe are at a distance of = 3.5 A. According to the chemical formula of the present sample, 2% of Mn(1) atoms are replaced by Fe atoms. Under the assumption that iron substitutes for Mn(1) in a random way, the probability that an iron atom has 2 Mn(1) as nearest (metal) neighbours (nn) is calculated to be 96% whereas the corresponding probability for 1 Mn(1) + 1 Fe(l) as nn is 4%. A MSssbauer pattern of 4% intensity is normally hidden in the resonance and one cannot in the present spectra see this pattern. One can therefore consider the present spectra as representative for Fe atoms in a surrounding equivalent to Mn(1) in Mn,P. It is clear that all 57Fe atoms, replacing Mn(1) experience practically the same magnetic field. By applying the experimentally found value for Fe,P of 10 T/pn [5], the saturation magnetic moment for 57Fe is found to be = 0.63~~. In addition it is clear that the conclusion iv) mentioned above is wrong, if the Fe-Fe interaction is negligible and if 57Fe acts as a probe of the magnetic substructure
AE/j’= (eQV,,/2)1. For the alternative d, one gets the corresponding values @= 30” (doubly populated) and 8 = 90” (singly populated) and therefore A EC = (eQV,,/2)5/8 and AEG = (eQ1/,,/2)(
- l/2),
respectively. We have tried to fit our spectra with both alternatives but neither was as good as the simple fit using only one sextet. Moreover, for both the alternatives the mean value of the quadrupole splitting in the magnetic region, (AE;), should be eQV,,/8. Assuming n = 0 and V,, to be the same just above and just before the transition temperature, one finds using expression (1) that (AEC) = AEJ/4 = -0.05 mm/s. This value is to be compared with the value of - 0.141) for the quadrupole splitting, given in table 1. With B antiferromagnetically directed in the hexagonal c-plane, but not necessarily in the orthorhombic b-direction, the Mossbauer spectrum should consist of three equally intense patterns with, in general, three different values of AEC. However, the mean value should still be eQVzz/8. It is therefore
174
L. Hiiggstriim et al. / “Fe Miissbauer study of Mn 2 .’
tempting to consider conclusion v) to be incorrect for Mn(1) atoms. The most simple explanation of our result is that all Fe atoms have the same value of AE;. This means that the polar angle 8 can be calculated from: “Ee” -= AEa’
3 cos2e - 1 = -0.14(l) 2 -0.20(l).
This gives 0 = 27(5)‘. The B-directions for Fe(l) are then rotated 120” from each other having an angle = 30” from the K’,,-directions, resulting in a triangular magnetic substructure for Mn(1). Triangular magnetic structures have been reported in other hexagonal alloys, for instance Mn,Sn and Mn,Ge [7].
References [I] S. Rundqvist,
Acta Chem. Stand. 16 (1962) 992. [2] M. Yes&, Phil. Magn. 17 (1968) 623. [3] B.K. Srivastava , T. Ericsson, L. HPggstrom, H.R. Verma and Y. Andems on, to be published in J. Phys. D. [4] P. Jemberg and T. Sundqvist, Institute of Physics, Uppsala University, Box 530, S-751 21 Uppsala, S&den. UUIP-1090 (1983). L. Hlggstriim, T. Ericsson, S. Devana151 R. Wappling, rayanan, E. Karlsson, B. Carlsson and S. Rundqvist. J. Solid State Chem. 13 (1975) 258. Kl T. Ericsson, L. Haggstrom, R. WIppIing and T. Methasiri, Phys. Scripta 21 (1980) 212. 171J.S. Kouvel and J.S. Kasper, Proc. Intern. Conf. Magnetism, Nottingham, 1964, p. 169. Inst. Phys. Sot., London (1965).
A -Fe
MGSSBAUER
L. HAGGSTRijM, Institute of Physics,
171
STUDY OF Mn,P
J. SJaSTRijM
and
T. ERICSSON
University of Uppsala, Box 530, S-751 21 Uppsala, Sweden
Received 9 October 1985; in final form 13 February 1986
It is shown that 57Fe substitutes for Mn(l) in Mn,P. The NM temperature is found to be 110 K and the magnetic moments for the iron atoms are estimated to be 0.63~~. All iron atoms have the same moment in contrast to what was found for Mn(1) atoms in an earlier neutron diffraction study by Yessik. A magnetic triangular substructure for Mn(1) is therefore proposed.
1. Introduction
Mn,P is hexagonal with space group P62m and isomorphous with FqP. The structure contains two metal positions, Mn(1) surrounded by four phosphorus atoms in a distorted tetrahedron and Mn(2) surrounded by five P atoms in a square based pyramid [l]. Yessik [2] reported Mn,P to be an antiferromagnet with a N6el temperature of 103 K. The magnetic structure, determined by neutron diffraction measurements on single crystals, was described as a spin modulation com-
mensurate with the lattice propagating along three hexagonally equivalent directions. The spin components were all perpendicular to the propagation directions and the mean values of the magnetic moments at the two manganese sites were for Mn(1): O.Ol(4)pn and for Mn(2): 0.84(3)pr,. The model of the magnetic structure in a related C face centered orthorhombic cell is show in in fig. 1. Further conclusions drawn by Yessik were: i) 2/3 of the Mn(2) atoms have significant moments; ii) l/3 of the Mn(2) atoms have no moments;
dl
x -1
eb
-
oMn(l) Z-0 lMn(2) Z=v2
A
B
Fig. 1. (A) The (001) projections of the three unit cells: double outline = chemical unit cell (hexagonal), single outline = magnetic unit cell (hexagonal), broken outline = C face entered orthorhombic cell; (B) model of the magnetic structure in the (001) projection of the orthorhombic cell (both figures from ref. [2]).
03048853/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
172
L. Hiiggstrih
et al. / ‘?Fe Miissbauer study of Mn z P
iii) 2/3 of the Mn(1) atoms might have very small moments; iv) l/3 of the Mn(1) atoms have no moments; v) all moments are strictly along the b axis. In a recent study of the solid solutions Fe, P- Mn 2P by Srivastava et al. [3], it was verified that Fe in this series only occupies the Mn(1) positions. This preferential occupation gave us a good opportunity to test the conclusions drawn by Yessik, using 57Fe Mossbauer spectroscopy.
P
(Mn0.99Fe00d2
--If--
295K
1
2. Experimental Mn ,P, 57Fe ano were mixed in appropriate proportions and heat treated at 1100 K for four days. The product was crushed and the heat treatment at 1100 K was repeated four times. The final result was checked by X-ray and there were diffraction lines only from the Mn,P structure plus some very weak lines from MnO. The unit cell parameters we!e found to be a = 6.0786 (2) A and c = 3.4595(2) A, which can be compared with the results for Mn,P by Rundqvist [l]: a = 6.081 .& c = 3.460 A. The chemical composition of the product can be written as (Mn,,,57Fe,,01) *P. The Mbssbauer spectrometer was of conventional constant acceleration type utilizing a double-ended vibrator with 57CoRh sources mounted on both ends. One source was used to simultaneously record the calibration spectra using a natural iron foil at room temperature as standard. The Miissbauer spectra of (Mn,,,,Fe,,e,),P were recorded using a flow cryostat with liquid helium or nitrogen as cooling media. The recorded spectra were folded and analyzed by a computer program written by Jernberg and Sundqvist [4].
3. Results Representative Miissbauer spectra are displayed in fig. 2. Above TN = llO(5) K the resonance is a broad single line. The broadening is caused by a small electric quadrupole splitting [3]. The spectra recorded in the magnetic region were fitted in the first stage of the data analysis by just one sextet. The results from these fittings are
I_L--L
-3.0
-2.0
-1.0
0.
1.0
2.0
3.0
mm/s Fig. 2. MBssbauer spectra recorded at 295 and 5 K. The full lines represent the sum of the fitted Lorentzian functions. The fitting of the spectrum at 295 K shows a deficiency at the center of the resonance. This is most reasonably due to the influence of the random distribution of next nearest Fe(l) atoms, which is not taken into account in the fitting model.
given in table 1. In the paramagnetic region the electric quadrupole splitting, AE& is given by: AEo’=
I eQW2
l111+12/3.
In the magnetic region, the quadrupole splitting is defined by AE;;=;[(u~-us)-(u+i)]. Using first-order perturbation theory this splitting can be approximated to AEa”=2
eQV,, 3 cos*B - 1 + 9 sin28 cos 2+ 2
Here Q is the nuclear quadrupole moment of the excited state in 57Fe, V,, is the main principal component and T) is the asymmetry parameter of the EFG tensor. The polar and azimuthal angles for the magnetic field B in the principal axes
173
L. HiiggstrCm et al. / “Fe Miissbauer study of Mn z P
Table 1 Results from the Miissbauer measurements. shift versus natural Fe at room temperature, hyperfine field in tesla, AEo is the electric ting (defined in text) and F is the absorber maximum (i.e. source line-width excluded). mm/s for 8, AEo and F, and 0.1 T for B 8
B
TKI
(mm/s)
0-I
295 110 105 92 80 5
0.33 0.43 0.43 0.44 0.44 0.45
_ 2.1 4.2 4.7 6.3
15 is the centroid B is the magnetic quadrupole sphtline-width at half Precision is 0.01
AEQ wvs)
r
0.20 0.21 -0.14 -0.14 -0.14 -0.14
0.16 0.25 0.30 0.31 0.25 0.21
@ws)
system of the EFG are denoted B and (p and ui, 02 *** u, are the peak positions with increasing velocity in the sextet.
of Mn(1). One can therefore conclude that the magnetic moments of the Mn(1) atoms are all the same. Whether the conclusions i) and ii) also are wrong we cannot say as iron does not replace Mn(2) in Mn z P. In analogy with Fe(l) in Fe,P, the principal component V,, for Fe(l) is Mn,P is in the hexagonal c-plane [6]. Further analogies are that V,, c 0 and that n = 0 for Fe(l) atoms. From symmetry considerations V,, could be along either of two directions, d, or d 2 as shown in fig. 1. Assuming the conclusion v) mentioned above to be correct, the alternative d, requires that 2/3 of the Fe atoms have 19= 60” and l/3 of the Fe atoms 8 = 0”. This means that in principle we should have two MSssbauer patterns, one doubly populated with AEC =
(eQK:,/2)(--l/8)
and one singly populated with 4. Discussion The near metal surrounding for an iron atom replacing Mn(1) in Mn,P is 2 Me(l) atoms and 6 Me(2) atoms at = 2.7 A. The next Me(l) atoms close to Fe are at a distance of = 3.5 A. According to the chemical formula of the present sample, 2% of Mn(1) atoms are replaced by Fe atoms. Under the assumption that iron substitutes for Mn(1) in a random way, the probability that an iron atom has 2 Mn(1) as nearest (metal) neighbours (nn) is calculated to be 96% whereas the corresponding probability for 1 Mn(1) + 1 Fe(l) as nn is 4%. A MSssbauer pattern of 4% intensity is normally hidden in the resonance and one cannot in the present spectra see this pattern. One can therefore consider the present spectra as representative for Fe atoms in a surrounding equivalent to Mn(1) in Mn,P. It is clear that all 57Fe atoms, replacing Mn(1) experience practically the same magnetic field. By applying the experimentally found value for Fe,P of 10 T/pn [5], the saturation magnetic moment for 57Fe is found to be = 0.63~~. In addition it is clear that the conclusion iv) mentioned above is wrong, if the Fe-Fe interaction is negligible and if 57Fe acts as a probe of the magnetic substructure
AE/j’= (eQV,,/2)1. For the alternative d, one gets the corresponding values @= 30” (doubly populated) and 8 = 90” (singly populated) and therefore A EC = (eQV,,/2)5/8 and AEG = (eQ1/,,/2)(
- l/2),
respectively. We have tried to fit our spectra with both alternatives but neither was as good as the simple fit using only one sextet. Moreover, for both the alternatives the mean value of the quadrupole splitting in the magnetic region, (AE;), should be eQV,,/8. Assuming n = 0 and V,, to be the same just above and just before the transition temperature, one finds using expression (1) that (AEC) = AEJ/4 = -0.05 mm/s. This value is to be compared with the value of - 0.141) for the quadrupole splitting, given in table 1. With B antiferromagnetically directed in the hexagonal c-plane, but not necessarily in the orthorhombic b-direction, the Mossbauer spectrum should consist of three equally intense patterns with, in general, three different values of AEC. However, the mean value should still be eQVzz/8. It is therefore
174
L. Hiiggstriim et al. / “Fe Miissbauer study of Mn 2 .’
tempting to consider conclusion v) to be incorrect for Mn(1) atoms. The most simple explanation of our result is that all Fe atoms have the same value of AE;. This means that the polar angle 8 can be calculated from: “Ee” -= AEa’
3 cos2e - 1 = -0.14(l) 2 -0.20(l).
This gives 0 = 27(5)‘. The B-directions for Fe(l) are then rotated 120” from each other having an angle = 30” from the K’,,-directions, resulting in a triangular magnetic substructure for Mn(1). Triangular magnetic structures have been reported in other hexagonal alloys, for instance Mn,Sn and Mn,Ge [7].
References [I] S. Rundqvist,
Acta Chem. Stand. 16 (1962) 992. [2] M. Yes&, Phil. Magn. 17 (1968) 623. [3] B.K. Srivastava , T. Ericsson, L. HPggstrom, H.R. Verma and Y. Andems on, to be published in J. Phys. D. [4] P. Jemberg and T. Sundqvist, Institute of Physics, Uppsala University, Box 530, S-751 21 Uppsala, S&den. UUIP-1090 (1983). L. Hlggstriim, T. Ericsson, S. Devana151 R. Wappling, rayanan, E. Karlsson, B. Carlsson and S. Rundqvist. J. Solid State Chem. 13 (1975) 258. Kl T. Ericsson, L. Haggstrom, R. WIppIing and T. Methasiri, Phys. Scripta 21 (1980) 212. 171J.S. Kouvel and J.S. Kasper, Proc. Intern. Conf. Magnetism, Nottingham, 1964, p. 169. Inst. Phys. Sot., London (1965).