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Mössbauer study of 119Sn in chromium and manganese dioxides
M O S S B A U E R S T U D Y O F 119Sn I N C H R O M I U M A N D M A N G A N E S E D I O X I D E S T. O K A D A and H. S E K I Z A W A , The Institute of Physical and Chemical Research, Wako-shi Saitama, 351, Japan
The hyperfine magnetic fields at 119Sn ions in the system CrI _xMnxO 2 (0 < X < 1) is investigated by means of l l9Sn M6ssbauer spectroscopy. A large positive hypeffine magnetic field amounting to 380 kOe is observed in CrO2 at 5 K. The mechanisms of this hyperfine magnetic field are discussed in detail.
ll9Sn M6ssbauer studies of the supertransferred hyperfine magnetic field Hhf in magnetic oxides such as MnFe204 [1], C a M n O 3 [2], NiFe204 [3] and VO 2 [4] have been made to investigate the mechanisms of the spin transfer in weakly covalent materials. We report new results on/-/he at ll9Sn in the system Crl_xMn,,O 2 (0 < x < 1) with the futile structure. Since CrO 2 is known to be ferromagnetic with the simplest spin structure, M6ssbauer measurements of//he in Sn-doped CrO 2 gives us a valuable clue to the mechanisms of/-/hr. The magnetic properties of the system Crl_xMnxO 2 have been measured by several authors [5-7]. The present samples were prepared by thermal decomposition of the powdered mixture of CrO 3, M n O 2 and SnC12 (85% enriched with 119Sn) at 420-550°C under an oxygen pressure of 150 k g / c m 2 [5, 6]. Each sample contains 0.1-0.5 at% of Sn ions. The M6ssbauer spectrum of 119Sn in CrO 2 at 80 K is shown in fig. 1. The spectrum consists of well resolved six lines due to l l9Sn in CrO 2 (as drawn with the solid lines) and others. The latter are assigned to ll9Sn in SnO 2 and Cr203. The sign of Hhf is determined by applying an external longitudinal magnetic field of 35 kOe. The obtained values of Hhf, the isomer shift IS relative to CaSnO 3, the magnetic m o m e n t o 0 and the Curie temperature Tc are listed in table 1. As can be seen, the value of IS in Cr~_~Mn~O 2 decreases with the M n content x; in CrO 2 the IS is +0.18 m m / s and nearly zero in M n O 2. This result shows that the Sn ions are in tetravalent state, and somewhat covalent in CrO 2 compared with very ionic in MnO 2. The observed electric quadrupole splitting is negligibly small. In fig. 2, t h e reduced hyperfine field H h f ( T ) / H h f ( O ) and the reduced magnetization a ( T ) / a ( O ) for CrO 2 are plotted against the reduced temperature T I T c. T h e reduced hyperfine field curve differs slightly from the reduced magnetization curve.
The observed value of Hhf at ll9Sn in CrO 2 ( + 3 8 0 kOe) is positive and very large compared with those in other magnetically ordered oxides, for instance, 250 kOe in N i F e 2 0 4. In general, the mechanisms giving rise to Hhf in various magnetic oxides have much similarity to those for the superexchange interaction between the magnetic ions [8]. In CrO2, the superexchange between the unit cell corner and the body-center Cr 4+ ions via oxygen ion is thought to be the dominant interaction for the ferromagnetism [9]. Therefore, we examined various paths of spin transfer from the chromium ion at the body-center to the Sn ion at the corner via the oxygen ion. However, we found that these paths could give negative Hhf only. We can understand the mechanism qualitatively as follows. The spin transfer to the Sn ion at the corner comes directly from the Cr ions at the neighbouring corners of the unit cell. We take the coordinate system for the nearly octahedral site for Cr ion at the corner as shown in fig. 3. The nearly degenerate three dc states would split into two states, the lower dyz, dzx states and the slightly higher dxy state, and the former states are occupied by the two d-electrons of Cr 4+ with parallel spins
z o io.
.: *
o t/) m
Cr02
Jr,
at 8 0
,t
I
I
-40
0
I
MM/S
+40
VELOCITY
Fig. 1. Mossbauer spectrum of 'lgSn in CrO2 containing 0.1 at% of Sn at 80 K.
Journal of Magnetism and Magnetic Materials 15-18 (1980) 649-650 ©North Holland
.649
650
T. Okada, H. Sekizawa/ Mossbauer study of tlgSn in Cr 1_xMnxO2
TABLE 1
X~
Sn
Hyperfine field (Hhf) and isomer shift (IS) at 5 K at ll9Sn in Crl_xMnxO 2, the magnetic moment o0~ B at 0 K and the Curie temperature Tc Hhf
IS
x
(kOe)
(ram/s)
Oo#B
Tc
0 0.25 0.50
+380 250 120
+0.18 +0.14 +0.07
1.83 1.32 0.60
392 384 380
0.0
-
TN = 90
1.0
50
[9]. Fairly strong mixing is expected between the empty dxy and the 5s of Sn 4+ because of the relatively short distance between the cations and the loose packing of the oxygen ions. The mixing of the empty LCAO dxy + X5s of the relatively low energy excited state into the occupied dy, and d ~ of the ground state results in the spin transfer giving rise to the observed large positive Hhf at ll9Sn in CrO 2. 1.0
-
Fig. 3. The empty orbitals of the Cr ions and the Sn ion in the rutile structure.
An alternative band model mechanism is also able to account for the Hhf correctly. The dc orbitals of Cr ions make up a narrow band which is split by ferromagnetic exchange interaction. In this model, the dxy orbital is partly occupied by ferromagnetic electrons which can easily be transferred to the Sn 5s orbital giving rise to the observed large positive Hhf at the ll9Sn nucleus.
1.0
References o v
o.5~
'-0.5
U
z
0 0
A 0.5
0 I / Tc
.0
Fig. 2. Temperature dependence of the reduced hyperfine field
Hhf(T)/Hhf(O ) and of the reduced magnetization o(T)/o(O) in CrO 2 containing O.1 at% of Sn.
[1] I. S, Lyubutin, T. Ohya, T. V. Dmitrieva and K. Ono, J. Phys. So¢. Japan 36 (1974) 1006. [2] M. Takano, Y. Takeda, M. Shimada, T. Matsuzawa, T. Shinjo and T. Takada, J. Phys. Soc. Japan 39 (1975) 656. [3] G. V. Novikov, V. A. Trukhtanov and V. I. Goldanskii, Soviet Phys. JETP 29 (1969) 403. [4] P. B. Fabritchnyi, M. Bayard, M. Pouchard and P. Hagenmuller, Solid State Commun. 14 (1974) 603. [5] K. Siratori and S. Iida, J. Phys. Soc. Japan 15 (1960) 2362. [6] G. Villers, P. Gibart and R. Drulllae, J. Appl. Phys. 39 (1968) 590. [7] B. L. Chamberland, W. H. Cloud and C. G. Frederick, J. Solid State Chem. 8 (1973) 238. [8] B. J. Evanee and L. J. Swarlzendruber, Phys. Rev. B6 (1972) 223. [9] J. B. Goodenough, Phys. Rev. 117 (1960) 1442.
The hyperfine magnetic fields at 119Sn ions in the system CrI _xMnxO 2 (0 < X < 1) is investigated by means of l l9Sn M6ssbauer spectroscopy. A large positive hypeffine magnetic field amounting to 380 kOe is observed in CrO2 at 5 K. The mechanisms of this hyperfine magnetic field are discussed in detail.
ll9Sn M6ssbauer studies of the supertransferred hyperfine magnetic field Hhf in magnetic oxides such as MnFe204 [1], C a M n O 3 [2], NiFe204 [3] and VO 2 [4] have been made to investigate the mechanisms of the spin transfer in weakly covalent materials. We report new results on/-/he at ll9Sn in the system Crl_xMn,,O 2 (0 < x < 1) with the futile structure. Since CrO 2 is known to be ferromagnetic with the simplest spin structure, M6ssbauer measurements of//he in Sn-doped CrO 2 gives us a valuable clue to the mechanisms of/-/hr. The magnetic properties of the system Crl_xMnxO 2 have been measured by several authors [5-7]. The present samples were prepared by thermal decomposition of the powdered mixture of CrO 3, M n O 2 and SnC12 (85% enriched with 119Sn) at 420-550°C under an oxygen pressure of 150 k g / c m 2 [5, 6]. Each sample contains 0.1-0.5 at% of Sn ions. The M6ssbauer spectrum of 119Sn in CrO 2 at 80 K is shown in fig. 1. The spectrum consists of well resolved six lines due to l l9Sn in CrO 2 (as drawn with the solid lines) and others. The latter are assigned to ll9Sn in SnO 2 and Cr203. The sign of Hhf is determined by applying an external longitudinal magnetic field of 35 kOe. The obtained values of Hhf, the isomer shift IS relative to CaSnO 3, the magnetic m o m e n t o 0 and the Curie temperature Tc are listed in table 1. As can be seen, the value of IS in Cr~_~Mn~O 2 decreases with the M n content x; in CrO 2 the IS is +0.18 m m / s and nearly zero in M n O 2. This result shows that the Sn ions are in tetravalent state, and somewhat covalent in CrO 2 compared with very ionic in MnO 2. The observed electric quadrupole splitting is negligibly small. In fig. 2, t h e reduced hyperfine field H h f ( T ) / H h f ( O ) and the reduced magnetization a ( T ) / a ( O ) for CrO 2 are plotted against the reduced temperature T I T c. T h e reduced hyperfine field curve differs slightly from the reduced magnetization curve.
The observed value of Hhf at ll9Sn in CrO 2 ( + 3 8 0 kOe) is positive and very large compared with those in other magnetically ordered oxides, for instance, 250 kOe in N i F e 2 0 4. In general, the mechanisms giving rise to Hhf in various magnetic oxides have much similarity to those for the superexchange interaction between the magnetic ions [8]. In CrO2, the superexchange between the unit cell corner and the body-center Cr 4+ ions via oxygen ion is thought to be the dominant interaction for the ferromagnetism [9]. Therefore, we examined various paths of spin transfer from the chromium ion at the body-center to the Sn ion at the corner via the oxygen ion. However, we found that these paths could give negative Hhf only. We can understand the mechanism qualitatively as follows. The spin transfer to the Sn ion at the corner comes directly from the Cr ions at the neighbouring corners of the unit cell. We take the coordinate system for the nearly octahedral site for Cr ion at the corner as shown in fig. 3. The nearly degenerate three dc states would split into two states, the lower dyz, dzx states and the slightly higher dxy state, and the former states are occupied by the two d-electrons of Cr 4+ with parallel spins
z o io.
.: *
o t/) m
Cr02
Jr,
at 8 0
,t
I
I
-40
0
I
MM/S
+40
VELOCITY
Fig. 1. Mossbauer spectrum of 'lgSn in CrO2 containing 0.1 at% of Sn at 80 K.
Journal of Magnetism and Magnetic Materials 15-18 (1980) 649-650 ©North Holland
.649
650
T. Okada, H. Sekizawa/ Mossbauer study of tlgSn in Cr 1_xMnxO2
TABLE 1
X~
Sn
Hyperfine field (Hhf) and isomer shift (IS) at 5 K at ll9Sn in Crl_xMnxO 2, the magnetic moment o0~ B at 0 K and the Curie temperature Tc Hhf
IS
x
(kOe)
(ram/s)
Oo#B
Tc
0 0.25 0.50
+380 250 120
+0.18 +0.14 +0.07
1.83 1.32 0.60
392 384 380
0.0
-
TN = 90
1.0
50
[9]. Fairly strong mixing is expected between the empty dxy and the 5s of Sn 4+ because of the relatively short distance between the cations and the loose packing of the oxygen ions. The mixing of the empty LCAO dxy + X5s of the relatively low energy excited state into the occupied dy, and d ~ of the ground state results in the spin transfer giving rise to the observed large positive Hhf at ll9Sn in CrO 2. 1.0
-
Fig. 3. The empty orbitals of the Cr ions and the Sn ion in the rutile structure.
An alternative band model mechanism is also able to account for the Hhf correctly. The dc orbitals of Cr ions make up a narrow band which is split by ferromagnetic exchange interaction. In this model, the dxy orbital is partly occupied by ferromagnetic electrons which can easily be transferred to the Sn 5s orbital giving rise to the observed large positive Hhf at the ll9Sn nucleus.
1.0
References o v
o.5~
'-0.5
U
z
0 0
A 0.5
0 I / Tc
.0
Fig. 2. Temperature dependence of the reduced hyperfine field
Hhf(T)/Hhf(O ) and of the reduced magnetization o(T)/o(O) in CrO 2 containing O.1 at% of Sn.
[1] I. S, Lyubutin, T. Ohya, T. V. Dmitrieva and K. Ono, J. Phys. So¢. Japan 36 (1974) 1006. [2] M. Takano, Y. Takeda, M. Shimada, T. Matsuzawa, T. Shinjo and T. Takada, J. Phys. Soc. Japan 39 (1975) 656. [3] G. V. Novikov, V. A. Trukhtanov and V. I. Goldanskii, Soviet Phys. JETP 29 (1969) 403. [4] P. B. Fabritchnyi, M. Bayard, M. Pouchard and P. Hagenmuller, Solid State Commun. 14 (1974) 603. [5] K. Siratori and S. Iida, J. Phys. Soc. Japan 15 (1960) 2362. [6] G. Villers, P. Gibart and R. Drulllae, J. Appl. Phys. 39 (1968) 590. [7] B. L. Chamberland, W. H. Cloud and C. G. Frederick, J. Solid State Chem. 8 (1973) 238. [8] B. J. Evanee and L. J. Swarlzendruber, Phys. Rev. B6 (1972) 223. [9] J. B. Goodenough, Phys. Rev. 117 (1960) 1442.