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Ferromagnetism in the oxygen spinel LiZn0.5Mn1.5O4
Journal of Magnetism North-Holland
and Magnetic
Materials
FERROMAGNETISM
83 (1990) 311-312
311
IN THE OXYGEN SPINEL LiZn,,Mn
,,504
and M. SOUGI
R. PLUMIER
Service de Physique du Solide et de Resonance Magktique,
CEN-Saclay,
91191 Gif-sur- Yvette Cedex, France
which is a ferromagnet with T, = 38.5 K develops only High magnetic field measurements indicate that LiZna,s Mn,,50, experiments which also 72% of the expected moment length for Mn 4+ ion. This result is confirmed by neutron diffraction show that this spine1 is a normal one.
In
the
series
of
LiZn,,,M&O, been
reported
oxygen
occupies
an
spinels, unusual
[l] as a ferromagnet
the
compound
situation. with
It has
T, = 22 K and
on the tetrahedral (A) sites in 1: 1 order (L&s :Zn,,,) addition de 1 : 3 order (Lie., : Mn,,,) on the octahedral B sites. Although ferromagnetism has also been reported [l] in a few isomorphous compounds (CuMg,,, which also have Mn’::O,, LiMgesMn?>O,) Mn4’-Mn4’ magnetic interactions on B sites only, such a situation is uncommon in oxygen spinels which usually display [2] strong antiferromagnetic interactions between magnetic ions on octahedral sites. We have just performed in our laboratory detailed magnetization measurements up to H = 105 kOe on a powdered specimen of Liin,,,Mn,,,O, at twenty temperatures, most of them in the range 20 K < T < 50 K (fig, 1). Extrapolating to H = 0 the linear part of the magnetization curves (fig. l), the spontaneous magnetizationvalues m(T) up to T = 30 K are readily obtained (fig. 2a). At T > 30 K, we use Arrott’s plots for the determination of m(T) (fig. 3), a procedure very well suited for the determination of low spontaneous magne-
0
20
40
0
50
100
T(K) Fig. 2. (a) Temperature dependence of the spontaneous magnetization; (b) inverse of the low field susceptibility as a function of temperature.
tization values near T,. We then obtain T, = 38.5 K (fig. 2a), in contradiction to the previously reported [l] T, = 22 K. It may be observed that even at T = 1.4 K and at the highest laboratory field value, the M(T, H) curve
15K 25K 38K 40x
3’
20 Fig. 1. Magnetization
0304-8853/90/$03.50 (North-Holland)
LO
60
80
curves obtained at various up to H = 105 kOe.
100
120
0’
,'
111' M2x103 (emu.g-'I2 I 6 8I 10 I
I' 4 2 m2
temperatures
0 Elsevier Science Publishers
Fig. 3. Arrott’s B.V.
150
plots near T, = 38.5 K.
R. Plumier, M. Sougi / Ferromagnetism in LiZn, ,Mn, j04
312
0
I
II 0
a -0.19 A X=2.52A
(a)
7
30°
32O
34O
36O
38O
CO’
42’
Fig. 4. Part of the neutron diffraction spectrum (a) at T= 50 K; (b) difference spectrum I(1.5 K)- I(50 K).
displays a finite slope AM/AH (fig. 1). At the same temperature, we observe that m(1.4 K) = 0.718M, where magnetization for M, = 135 emu g -’ is the saturation spin only Mn4’ ions (d’, S = 3/2). This spin only value is derived from the Curie constant corresponding, in the paramagnetic region, to the linear part of the inverse low field susceptibility (fig. 2b) which also points out an asymptotic Curie temperature 0 = 45 K slightly higher than T,. It may also be observed that the shape of the x-‘-T curve (fig. 2b) gives strong support that we are dealing with a ferromagnet and not with a ferrimagnet since it that case it would have had the typical hyperbolic shape [3]. Preliminary neutron diffraction experiments have also been performed at H = 0 on the same powdered specimen (fig. 4). The negative scattering lengths of both Mn and Li versus the positive one of Zn (b,, = -0.214, b,, = -0.39, bz, = 0.57 in lo-l2 cm units) make elastic neutron scattering a much better tool than X-rays to study the ionic distribution of the cations among the A and B sites of the spine1 structure. From the intensities of the nuclear reflections observed at T > T, (fig. 4a), the best reliability factor (R = 6%) is reached for an ionic distribution corresponding to the normal spine1 Li[Zn,,,Mn,,,]O, ruling out the previously [l] reported distribution Li,,,Zn,,,[Li,,Mn,.,]O~
which, in addition to an observable (200) reflection would have given totally distinct nuclear intensities. The 1 : 3 order (Zn,,,Mn,,,) observed on the B sites breaks the fee symmetry and leads to the overall nuclear space group 0’. On the other hand, we observe that the magnetic reflections, which may all be indexed in the same cubic cell as the nuclear one, display a larger line width (fig. 4b). Assuming Gaussian profiles for both the experimental and natural contributions to the total line width. an average size d = 150 A of the ferromagnetic domains is obtained. At T= 1.5 K, the best fit between the observed and calculated magnetic intensities is reached for a Mn 4+ ion moment length o = 2.04 pp corresponding to m’ (1.5 K) = 91.8 emu g-’ = 0.68Ms. We attribute the reduction of the Mn4’ moment lengths observed in both magnetization and neutron diffraction experiments to the finite size of the ferromagnetic domains, the slightly larger value obtained from magnetization measurements being explained by an increase of the domain size under the application of an external magnetic field. Size effects also explain the fairly high coercivity field value (H = 40 kOe) and the hindrance to full ferromagnetic alignment observed even at low temperature in the highest laboratory field (fig. 1). From the foregoing, it may be concluded that in LiZn,,,Mn,.,O,, the 90 o Mn4’-Mn4’ magnetic interaction between first nearest neighbours is a positive one as a result of the orthogonality of the d,, orbitals [4]. Although the cell parameter of this spine1 is among the smallest reported ones [l], the direct overlap of these orbitals which is expected to be antiferromagnetic [4] is obviously not strong enough to overcome the effects of that positive interaction. On the contrary, in the extensively studied isomorphous lithium ferrite (a = 8.33 A) with a cation distribution Fe3’[Fe~,:Li,,,]04, these two effects apparently cancel each other since it has been recently reported that the magnetic properties of this spine1 may be accounted for with zero intra-sublattice interactions [5]. References [l] G. Blasse, J. Phys. Chem. Solids 27 (1966) 383. [2] P.K. Balker, P.J. Wojtowicz, M. Robbins and E. Phys. Rev. 151 (1966)
367.
[3] L. NCel, Ann. Phys. 3 (1948) [4] J.B. Goodenough, [5] C.J. Tinsley,
137.
Phys. Rev. 117 (1960)
Phil. Mag. B 56 (1987)
351.
1442.
Lopatin,
and Magnetic
Materials
FERROMAGNETISM
83 (1990) 311-312
311
IN THE OXYGEN SPINEL LiZn,,Mn
,,504
and M. SOUGI
R. PLUMIER
Service de Physique du Solide et de Resonance Magktique,
CEN-Saclay,
91191 Gif-sur- Yvette Cedex, France
which is a ferromagnet with T, = 38.5 K develops only High magnetic field measurements indicate that LiZna,s Mn,,50, experiments which also 72% of the expected moment length for Mn 4+ ion. This result is confirmed by neutron diffraction show that this spine1 is a normal one.
In
the
series
of
LiZn,,,M&O, been
reported
oxygen
occupies
an
spinels, unusual
[l] as a ferromagnet
the
compound
situation. with
It has
T, = 22 K and
on the tetrahedral (A) sites in 1: 1 order (L&s :Zn,,,) addition de 1 : 3 order (Lie., : Mn,,,) on the octahedral B sites. Although ferromagnetism has also been reported [l] in a few isomorphous compounds (CuMg,,, which also have Mn’::O,, LiMgesMn?>O,) Mn4’-Mn4’ magnetic interactions on B sites only, such a situation is uncommon in oxygen spinels which usually display [2] strong antiferromagnetic interactions between magnetic ions on octahedral sites. We have just performed in our laboratory detailed magnetization measurements up to H = 105 kOe on a powdered specimen of Liin,,,Mn,,,O, at twenty temperatures, most of them in the range 20 K < T < 50 K (fig, 1). Extrapolating to H = 0 the linear part of the magnetization curves (fig. l), the spontaneous magnetizationvalues m(T) up to T = 30 K are readily obtained (fig. 2a). At T > 30 K, we use Arrott’s plots for the determination of m(T) (fig. 3), a procedure very well suited for the determination of low spontaneous magne-
0
20
40
0
50
100
T(K) Fig. 2. (a) Temperature dependence of the spontaneous magnetization; (b) inverse of the low field susceptibility as a function of temperature.
tization values near T,. We then obtain T, = 38.5 K (fig. 2a), in contradiction to the previously reported [l] T, = 22 K. It may be observed that even at T = 1.4 K and at the highest laboratory field value, the M(T, H) curve
15K 25K 38K 40x
3’
20 Fig. 1. Magnetization
0304-8853/90/$03.50 (North-Holland)
LO
60
80
curves obtained at various up to H = 105 kOe.
100
120
0’
,'
111' M2x103 (emu.g-'I2 I 6 8I 10 I
I' 4 2 m2
temperatures
0 Elsevier Science Publishers
Fig. 3. Arrott’s B.V.
150
plots near T, = 38.5 K.
R. Plumier, M. Sougi / Ferromagnetism in LiZn, ,Mn, j04
312
0
I
II 0
a -0.19 A X=2.52A
(a)
7
30°
32O
34O
36O
38O
CO’
42’
Fig. 4. Part of the neutron diffraction spectrum (a) at T= 50 K; (b) difference spectrum I(1.5 K)- I(50 K).
displays a finite slope AM/AH (fig. 1). At the same temperature, we observe that m(1.4 K) = 0.718M, where magnetization for M, = 135 emu g -’ is the saturation spin only Mn4’ ions (d’, S = 3/2). This spin only value is derived from the Curie constant corresponding, in the paramagnetic region, to the linear part of the inverse low field susceptibility (fig. 2b) which also points out an asymptotic Curie temperature 0 = 45 K slightly higher than T,. It may also be observed that the shape of the x-‘-T curve (fig. 2b) gives strong support that we are dealing with a ferromagnet and not with a ferrimagnet since it that case it would have had the typical hyperbolic shape [3]. Preliminary neutron diffraction experiments have also been performed at H = 0 on the same powdered specimen (fig. 4). The negative scattering lengths of both Mn and Li versus the positive one of Zn (b,, = -0.214, b,, = -0.39, bz, = 0.57 in lo-l2 cm units) make elastic neutron scattering a much better tool than X-rays to study the ionic distribution of the cations among the A and B sites of the spine1 structure. From the intensities of the nuclear reflections observed at T > T, (fig. 4a), the best reliability factor (R = 6%) is reached for an ionic distribution corresponding to the normal spine1 Li[Zn,,,Mn,,,]O, ruling out the previously [l] reported distribution Li,,,Zn,,,[Li,,Mn,.,]O~
which, in addition to an observable (200) reflection would have given totally distinct nuclear intensities. The 1 : 3 order (Zn,,,Mn,,,) observed on the B sites breaks the fee symmetry and leads to the overall nuclear space group 0’. On the other hand, we observe that the magnetic reflections, which may all be indexed in the same cubic cell as the nuclear one, display a larger line width (fig. 4b). Assuming Gaussian profiles for both the experimental and natural contributions to the total line width. an average size d = 150 A of the ferromagnetic domains is obtained. At T= 1.5 K, the best fit between the observed and calculated magnetic intensities is reached for a Mn 4+ ion moment length o = 2.04 pp corresponding to m’ (1.5 K) = 91.8 emu g-’ = 0.68Ms. We attribute the reduction of the Mn4’ moment lengths observed in both magnetization and neutron diffraction experiments to the finite size of the ferromagnetic domains, the slightly larger value obtained from magnetization measurements being explained by an increase of the domain size under the application of an external magnetic field. Size effects also explain the fairly high coercivity field value (H = 40 kOe) and the hindrance to full ferromagnetic alignment observed even at low temperature in the highest laboratory field (fig. 1). From the foregoing, it may be concluded that in LiZn,,,Mn,.,O,, the 90 o Mn4’-Mn4’ magnetic interaction between first nearest neighbours is a positive one as a result of the orthogonality of the d,, orbitals [4]. Although the cell parameter of this spine1 is among the smallest reported ones [l], the direct overlap of these orbitals which is expected to be antiferromagnetic [4] is obviously not strong enough to overcome the effects of that positive interaction. On the contrary, in the extensively studied isomorphous lithium ferrite (a = 8.33 A) with a cation distribution Fe3’[Fe~,:Li,,,]04, these two effects apparently cancel each other since it has been recently reported that the magnetic properties of this spine1 may be accounted for with zero intra-sublattice interactions [5]. References [l] G. Blasse, J. Phys. Chem. Solids 27 (1966) 383. [2] P.K. Balker, P.J. Wojtowicz, M. Robbins and E. Phys. Rev. 151 (1966)
367.
[3] L. NCel, Ann. Phys. 3 (1948) [4] J.B. Goodenough, [5] C.J. Tinsley,
137.
Phys. Rev. 117 (1960)
Phil. Mag. B 56 (1987)
351.
1442.
Lopatin,