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Hyperfine fields and spin densities in magnetite below 20 k
Journal of Magnetism and Magnetic Materials 0 North-Holland Publishing Company
7 (1978)
230-233
HYPERFINEFIELDSANDSPIN DENSITIESINMAGNETITEBELOW 20K G.GALECZKI and A.A. HIRSCH Department
of Physics, Technion-Israel Institute of Technology,
Haifa, Israel
Mossbauer spectra of magnetite arc investigated between 7 and 17.1 K. The presence of Fe ions with electron configurations 3d4, 3d’ and 3d74s0,’ is determined. A new systematics of the spectra is given in terms of time averaged spin densities and isomer shifts.
mentioning that the presence of Fe+ ions in magnetite was also suggested by Chikazumi [4] in a recent study in order to interpret his own low temperature magnetocrystalline data. Because there was no possibility to perform measurements on single crystal slides, our efforts were directed toward a very good statistics, very stable low temperatures and reproducibility. In order to check reproducibility, we performed our measurements on three different samples: (i) a natural magnetite crystal; (ii) synthetic magnetite prepared as a fine powder; and (iii) very pure synthetic magnetite prepared from a rod of spectroscopically pure iron on the basis of the Darken and Gurry Fe-O phase diagram. In order to eliminate possible systematic errors induced by the experimental system, we have investigated (i) and (ii) by using a constant velocity spectrometer, while (iii) was investigated by means of a constant acceleration spectrometer. Special attention was paid to the temperature stability (+-0.01 K) during the long accumulation time. The representative spectra of(i) and (ii) are illustrated in fig. 1, of (iii) in fig. 2. The at and 14 show clearly the existence of two sextuplets oi and pi (i = 1, 2, .... 6), with hyperfine fields 330 + 10 kOe and 432 f 10 kOe, respectively, in addition to the yi sextuplet with the hyperfine field of 540 + 10 kOe. Assuming a priori [6] that H,,, in magnetite. is proportional to the time averaged spin density of an individual iron ion (called the effective magnetic moment p,ff), these data fit well the integer spin densities 3,4 and 5 C(n. In the same figure we see also a fourth sextuplet Si, with H, = 372 f 10 kOe, which suggests j&f = 3.5 pg. This sextuplet becomes more
Although numerous attempts have been made in the last decade to give a consistent interpretation of the Mossbauer spectra of magnetite (Fea04) below the Verwey transition region (r, = 119 K), the problem is still open to discussion [l-3]. The spectra of octahedral (B)-site ions in the low temperature phase are rather complicated. The assumption that the inequivalent B-sites occupied by ferrous (Fe2 ‘) ions may account for the large spread in the hyperfine fields (H,,) seems to us questionable; however, a monoclinic distorsion was recently reported by Chikazumi [4]. Rubinstein and Forester [2], in a careful re-examination of the Mossbauer studies of Hargrove and Kiindig [ 11, have resolved the spectra below TVinto five B-site sextuplets, in addition to an A-site sextuplet, and have found values for H,, varying from 539 to 350 kOe. These authors attribute this large spread to “a scale of ionicity” varying from Fe3+ (having purely ferric character) to Fe2+ (with purely ferrous character). The other ionic species, according to them, exhibit intermediate ionic character. The sextuplets with 350 and 367 kOe, as resolved by these investigators, show strong nonlinear (quadrupole) effects. Following the theoretical analysis of Okiji and Kanamori [5] for Fe’+ ions in trigonal crystals, they assume a priori large orbital contributions and quadrupole splitting even for the site symmetries of the iron ions in magnetite. In a previous publication [6], one of us (A.A.H.) has expressed the idea that ions responsible for the hyperfine fields of 350 kOe (or less) may be Fe+(3d7) ions. We decided in the present work to reinterpret the low temperature spectra (below 20 K). It is worth 230
G. Galeczki, A.A. Hirsch / Hyperfine fields and spin densities in magnetite
I
I
-5
-I(1
0 VELOCITY
Fig. 1. MGssbauer spectra of magnetite accumulated shifts of the or(Fe+) and p(Fe4+) sextuplets.
(mm/s)
by a constant
velocity
pronounced at higher temperature (14 K) fig. 1; at the same time OIibecomes less pronounced. The data shown in fig. 2 at 11.7 and 17.1 K reproduce this picture in
NISI
-5
I
i0
spectrometer.
= 2.55 mm/s
VELOCITY
Fig. 2. Mksbauer spectra of magnetite accumulated isomer shifts of the cu(Fe+) and p(Fe4+) sextuplets.
I
5
A(IS) indicates
the difference
in the isomer
the limit of the experimental accuracy. In table 1 the resolved low hyperfine fields, the corresponding quadrupole splittings (QS), isomer shifts (IS) with
I
1
231
by a constant
I 0 (mm/s) acceleration
Synthetic
Magnetite
I
I
5 spectrometer.
1( A(IS) indicates
the difference
in the
G. Galeczki, A.A. Hirsch / Hyperfine fields and spin densities in magnetite
232
Table 1 Miissbauer hyperfine parameters Sample
T
H,
QS *
W
(kOe)
(mm/s)
IS t exp
IS
(mm/s)
(iii)
(ii)
(iii)
7
11.7
14
17.1
WWJ
2.65
2.75
A(IS) (mm/s)
D
330 + 10 430 * 10
0.62 -0.56
1.57 -1.08
1.65 --1.10
6
375 f 10
1.68
1.20
1.20
o!
335 k 10 435 ? 10
0.61 -0.54
1.47 -0.108
1.65 ml.10
2.55
2.15
372 + 10
1.59
;
330 f 10 430 r 10
0.62 -0.56
1.57 1.08
1.65 -1.10
2.65
2.75
6
375 * 10
1.68
1.07
1.20
01
4
335 r 10 435 i 10
0.61 -0.54
1.47 1.08
1.65 --1.10
2.55
2.15
6
372 + 10
1.59
1.15
1.20
a
(0
WV cxP (mm/s)
P
* Qs = [(q - 9) - (us - ug)]/2; vl-MGssbauer line position. ?’ IS is calculated as (~1 + u2 + us + u6)/4, with respect to the stainless steel line (ref. [ 11 I). to stainless steel, as well as the difference (IS) in the isomer shifts of the sextuplets (Yiand pi are given. Taking into consideration the theoretical values of IS given by WalkerrWertheim-Jaccarino [7] and the suggested spin densities, we attribute the resolved sextuplets Cyi,fli, yi and 6i to electron configurations 3d’, 3d4, 3d5 and 3d74s0.5, respectively. As shown in table 1 the experimental values of IS are very close to those given by the theoretical WWJ model. We believe that by raising the temperature, pairs of Fe+ and Fe4+ ions transform gradually into Fe’+ and Fe3+ pairs, according to the quasichemical reaction of the form: respect
Fe+ + Fe4+ + Fe’+ t Fe3+ . This reaction suggested by us is not the only mechanism contributing to changes in the spin densities of the cations. We believe that a large multiplicity of ionic species with non-integer spin densities may be formed by dynamic hopping processes which persist even at the lowest temperatures.
The a priori assumed proportionality between the hyperfine fields and the time averaged spin densities may be justified if the orbital contributions to the hyperfine fields at low temperatures are very small. Let us examine first of all the existing data for the g-factor, this value being an indicator of the orbital contributions to the magnetization. The value cited by Freeman in his revue article on hyperfine interacyions was measured by Bickford twenty years ago [8] and is equal to 2.06, i.e. almost identical to the g-factor of the manganese ferrite that contains only 3d5 cations. As is known, the effective g-factor in inverted spinels coincides with the g-factor of the B-site ions with configurations different than 3d’. A 3d orbital singlet has no orbital contribution in the first order; only electrons in orbitally degenerate states give significant contributions to the expectation value (L). A good example, given by J. Smit and cited by Slonczewski [9], is Fe’+ in doublet states in cubic ferrites. However, the crystal field in the monoclinic [13,14] low temperature phase of magnetite removes completely the orbital degeneracy [lo], so
G. Galeczki, A.A. Hirsch / Hyperfine fields and spin densities in magnetite
that the orbital moment will indeed be quenched. It is worth mentioning that a linear relationship between the hyperfine field and the electronic magnetic moment was found in neptunium intermetallics [ 121. However, it seems to us that this situation is just opposite to ours, because the very large hyperfine fields (H,, = 5000 kOe) are almost entirely of an orbital nature. The authors would like to acknowledge the devoted technical assistance of I. Zvieli in all stages of this work.
References [l] R.S. Hargrove and W. Kiindig, Solid State Commun. 8 (1970) 303.
233
[2] M. Rubinstein and D.W. Forester, Solid State Commun. 9 (1971) 1675. [3] S. Iida et al., J. de Phys., Colloque Cl, 38 (1977) Cl-73. [4] C. Chikazumi, AIP Conf. Proc. 13 (1975) 382. [5] A. Okiji and J. Kanamori, J. Phys. Sot. Jap. 19 (1964) 908. [6] R.A. Buckwald and A.A. Hirsch, Solid State Commun. 17 (1975) 621. [7] L.R. Walker et al., Phys. Rev. Lett. 6 (1961) 98. [8] J. Smit and H.P.J. Wijn, Ferrites (John Wiley and Sons, New York, 1959). [9] J.C. Slonczewski, J. Appl. Phys..32S (1961) 2533. [lo] H. Kronmtiller, J. Magn. Magn. Mater. 4 (1977) 280. [ 111 C. Janot, Ldffet Mijssbauer et ces applications en metallurgie physique (Masson et Cie, editeurs, Paris, 1975) p. 100. (121 B.D. Dunlop and G.H. Lander, Phys. Rev. Lett. 33 (1974) 1046. [ 131 J. Yoshida and S. Iida, J. Phys. Sot. Jap. 42 (1977) 230. [14] J. Mada and S. Iida, J. Phys. Sot. Jap. 42 (1977) 1184.
7 (1978)
230-233
HYPERFINEFIELDSANDSPIN DENSITIESINMAGNETITEBELOW 20K G.GALECZKI and A.A. HIRSCH Department
of Physics, Technion-Israel Institute of Technology,
Haifa, Israel
Mossbauer spectra of magnetite arc investigated between 7 and 17.1 K. The presence of Fe ions with electron configurations 3d4, 3d’ and 3d74s0,’ is determined. A new systematics of the spectra is given in terms of time averaged spin densities and isomer shifts.
mentioning that the presence of Fe+ ions in magnetite was also suggested by Chikazumi [4] in a recent study in order to interpret his own low temperature magnetocrystalline data. Because there was no possibility to perform measurements on single crystal slides, our efforts were directed toward a very good statistics, very stable low temperatures and reproducibility. In order to check reproducibility, we performed our measurements on three different samples: (i) a natural magnetite crystal; (ii) synthetic magnetite prepared as a fine powder; and (iii) very pure synthetic magnetite prepared from a rod of spectroscopically pure iron on the basis of the Darken and Gurry Fe-O phase diagram. In order to eliminate possible systematic errors induced by the experimental system, we have investigated (i) and (ii) by using a constant velocity spectrometer, while (iii) was investigated by means of a constant acceleration spectrometer. Special attention was paid to the temperature stability (+-0.01 K) during the long accumulation time. The representative spectra of(i) and (ii) are illustrated in fig. 1, of (iii) in fig. 2. The at and 14 show clearly the existence of two sextuplets oi and pi (i = 1, 2, .... 6), with hyperfine fields 330 + 10 kOe and 432 f 10 kOe, respectively, in addition to the yi sextuplet with the hyperfine field of 540 + 10 kOe. Assuming a priori [6] that H,,, in magnetite. is proportional to the time averaged spin density of an individual iron ion (called the effective magnetic moment p,ff), these data fit well the integer spin densities 3,4 and 5 C(n. In the same figure we see also a fourth sextuplet Si, with H, = 372 f 10 kOe, which suggests j&f = 3.5 pg. This sextuplet becomes more
Although numerous attempts have been made in the last decade to give a consistent interpretation of the Mossbauer spectra of magnetite (Fea04) below the Verwey transition region (r, = 119 K), the problem is still open to discussion [l-3]. The spectra of octahedral (B)-site ions in the low temperature phase are rather complicated. The assumption that the inequivalent B-sites occupied by ferrous (Fe2 ‘) ions may account for the large spread in the hyperfine fields (H,,) seems to us questionable; however, a monoclinic distorsion was recently reported by Chikazumi [4]. Rubinstein and Forester [2], in a careful re-examination of the Mossbauer studies of Hargrove and Kiindig [ 11, have resolved the spectra below TVinto five B-site sextuplets, in addition to an A-site sextuplet, and have found values for H,, varying from 539 to 350 kOe. These authors attribute this large spread to “a scale of ionicity” varying from Fe3+ (having purely ferric character) to Fe2+ (with purely ferrous character). The other ionic species, according to them, exhibit intermediate ionic character. The sextuplets with 350 and 367 kOe, as resolved by these investigators, show strong nonlinear (quadrupole) effects. Following the theoretical analysis of Okiji and Kanamori [5] for Fe’+ ions in trigonal crystals, they assume a priori large orbital contributions and quadrupole splitting even for the site symmetries of the iron ions in magnetite. In a previous publication [6], one of us (A.A.H.) has expressed the idea that ions responsible for the hyperfine fields of 350 kOe (or less) may be Fe+(3d7) ions. We decided in the present work to reinterpret the low temperature spectra (below 20 K). It is worth 230
G. Galeczki, A.A. Hirsch / Hyperfine fields and spin densities in magnetite
I
I
-5
-I(1
0 VELOCITY
Fig. 1. MGssbauer spectra of magnetite accumulated shifts of the or(Fe+) and p(Fe4+) sextuplets.
(mm/s)
by a constant
velocity
pronounced at higher temperature (14 K) fig. 1; at the same time OIibecomes less pronounced. The data shown in fig. 2 at 11.7 and 17.1 K reproduce this picture in
NISI
-5
I
i0
spectrometer.
= 2.55 mm/s
VELOCITY
Fig. 2. Mksbauer spectra of magnetite accumulated isomer shifts of the cu(Fe+) and p(Fe4+) sextuplets.
I
5
A(IS) indicates
the difference
in the isomer
the limit of the experimental accuracy. In table 1 the resolved low hyperfine fields, the corresponding quadrupole splittings (QS), isomer shifts (IS) with
I
1
231
by a constant
I 0 (mm/s) acceleration
Synthetic
Magnetite
I
I
5 spectrometer.
1( A(IS) indicates
the difference
in the
G. Galeczki, A.A. Hirsch / Hyperfine fields and spin densities in magnetite
232
Table 1 Miissbauer hyperfine parameters Sample
T
H,
QS *
W
(kOe)
(mm/s)
IS t exp
IS
(mm/s)
(iii)
(ii)
(iii)
7
11.7
14
17.1
WWJ
2.65
2.75
A(IS) (mm/s)
D
330 + 10 430 * 10
0.62 -0.56
1.57 -1.08
1.65 --1.10
6
375 f 10
1.68
1.20
1.20
o!
335 k 10 435 ? 10
0.61 -0.54
1.47 -0.108
1.65 ml.10
2.55
2.15
372 + 10
1.59
;
330 f 10 430 r 10
0.62 -0.56
1.57 1.08
1.65 -1.10
2.65
2.75
6
375 * 10
1.68
1.07
1.20
01
4
335 r 10 435 i 10
0.61 -0.54
1.47 1.08
1.65 --1.10
2.55
2.15
6
372 + 10
1.59
1.15
1.20
a
(0
WV cxP (mm/s)
P
* Qs = [(q - 9) - (us - ug)]/2; vl-MGssbauer line position. ?’ IS is calculated as (~1 + u2 + us + u6)/4, with respect to the stainless steel line (ref. [ 11 I). to stainless steel, as well as the difference (IS) in the isomer shifts of the sextuplets (Yiand pi are given. Taking into consideration the theoretical values of IS given by WalkerrWertheim-Jaccarino [7] and the suggested spin densities, we attribute the resolved sextuplets Cyi,fli, yi and 6i to electron configurations 3d’, 3d4, 3d5 and 3d74s0.5, respectively. As shown in table 1 the experimental values of IS are very close to those given by the theoretical WWJ model. We believe that by raising the temperature, pairs of Fe+ and Fe4+ ions transform gradually into Fe’+ and Fe3+ pairs, according to the quasichemical reaction of the form: respect
Fe+ + Fe4+ + Fe’+ t Fe3+ . This reaction suggested by us is not the only mechanism contributing to changes in the spin densities of the cations. We believe that a large multiplicity of ionic species with non-integer spin densities may be formed by dynamic hopping processes which persist even at the lowest temperatures.
The a priori assumed proportionality between the hyperfine fields and the time averaged spin densities may be justified if the orbital contributions to the hyperfine fields at low temperatures are very small. Let us examine first of all the existing data for the g-factor, this value being an indicator of the orbital contributions to the magnetization. The value cited by Freeman in his revue article on hyperfine interacyions was measured by Bickford twenty years ago [8] and is equal to 2.06, i.e. almost identical to the g-factor of the manganese ferrite that contains only 3d5 cations. As is known, the effective g-factor in inverted spinels coincides with the g-factor of the B-site ions with configurations different than 3d’. A 3d orbital singlet has no orbital contribution in the first order; only electrons in orbitally degenerate states give significant contributions to the expectation value (L). A good example, given by J. Smit and cited by Slonczewski [9], is Fe’+ in doublet states in cubic ferrites. However, the crystal field in the monoclinic [13,14] low temperature phase of magnetite removes completely the orbital degeneracy [lo], so
G. Galeczki, A.A. Hirsch / Hyperfine fields and spin densities in magnetite
that the orbital moment will indeed be quenched. It is worth mentioning that a linear relationship between the hyperfine field and the electronic magnetic moment was found in neptunium intermetallics [ 121. However, it seems to us that this situation is just opposite to ours, because the very large hyperfine fields (H,, = 5000 kOe) are almost entirely of an orbital nature. The authors would like to acknowledge the devoted technical assistance of I. Zvieli in all stages of this work.
References [l] R.S. Hargrove and W. Kiindig, Solid State Commun. 8 (1970) 303.
233
[2] M. Rubinstein and D.W. Forester, Solid State Commun. 9 (1971) 1675. [3] S. Iida et al., J. de Phys., Colloque Cl, 38 (1977) Cl-73. [4] C. Chikazumi, AIP Conf. Proc. 13 (1975) 382. [5] A. Okiji and J. Kanamori, J. Phys. Sot. Jap. 19 (1964) 908. [6] R.A. Buckwald and A.A. Hirsch, Solid State Commun. 17 (1975) 621. [7] L.R. Walker et al., Phys. Rev. Lett. 6 (1961) 98. [8] J. Smit and H.P.J. Wijn, Ferrites (John Wiley and Sons, New York, 1959). [9] J.C. Slonczewski, J. Appl. Phys..32S (1961) 2533. [lo] H. Kronmtiller, J. Magn. Magn. Mater. 4 (1977) 280. [ 111 C. Janot, Ldffet Mijssbauer et ces applications en metallurgie physique (Masson et Cie, editeurs, Paris, 1975) p. 100. (121 B.D. Dunlop and G.H. Lander, Phys. Rev. Lett. 33 (1974) 1046. [ 131 J. Yoshida and S. Iida, J. Phys. Sot. Jap. 42 (1977) 230. [14] J. Mada and S. Iida, J. Phys. Sot. Jap. 42 (1977) 1184.