PROCESOS DE LA LIXIVIACION
3.2 MODELO DE TRANSFERENCIA DE MASA.
The ferrimagnetic transition temperature Tc for Y0.15Sr0.85CoO3−δ is 370 K. Q-
Figure 6.31: Scans along the (1,1, l) direction across the (1,1,2) peak in the
I4/mmm space group of
Y0.15Sr0.85CoO3−δ measured at
temperatures between 100 and 390 K at an energy transfer of 12.4 meV on the triple-axis
spectrometer 2T1. 1.0 1.5 2.0 2.5 3.0 0 200 400 600 800 1000 1200 390 K 350 K 300 K 250 K 200 K 150 K 100 K I n t e n s i t y ( A r b . U n i t s ) (1,1,l) 12.4 meV
and (1,1, l) directions at up to five different energy transfers; 4.1, 12.4, 20.7, 28.9 and 37.2 meV on the triple-axis spectrometer 2T1. Some scans were made along (−h, h,6) and (−1,1, l) to avoid spurion signals. These scans are shown in figure 6.32. At the lowest energy transfer measured, 4.1 meV, there is a strong tempera- ture dependence along both axes as the transition temperature is crossed, with the scans at 410 and 440 K having comparable intensities, and the scans at all tem- peratures along both axes showing evidence of dynamic magnetic correlations. At 12.4 meV, there is still a distinct temperature dependence of the correlations across the transition along thec axis of theI4/mmm unit cell, but this is less clear along the other direction measured. These magnetic fluctuations have become negligible by an energy transfer of 28.9 meV along the (1,1, l) axis, but there is still some ev- idence of correlations at an energy transfer of 37.2 meV in the scans along (h, h,6) direction, although neither show any temperature dependence. Due to limitations on the equipment used, we were unable to probe temperatures above 440 K and establish a temperature at which these correlations disappear.
These measurements provide two key pieces of information about the mag- netic correlations in Y0.15Sr0.85CoO3−δ. The dynamic magnetic correlations in
Y0.15Sr0.85CoO3−δ persist for a substantial temperature interval above Tc. Addi-
tionally, these correlations aboveTc have a 2D character, evidenced by the fact that
the peaks along theabdirection are significantly sharper than those along thecaxis, implying longer-range fluctuations along this direction. This observation should be related to the measurements made at 390, 370 and 360 K on the polarised neutron diffractometer D7 shown in figure 6.23. Apparent magnetic diffuse scattering corre- sponding to an integrated inelastic contribution is obvious in the (h, h, l) scattering plane atTc but not in the (h, k,0) plane.
To try and understand this behaviour it should be considered in the context of similar systems. The perovskite LaMnO3 is a antiferromagnet with a N´eel tem-
perature TN = 139.5 K. The behaviour of the magnetic moments in this system,
0.0 0.5 1.0 1.5 2.0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 350 K 370 K 390 K 410 K 440 K I n t e n s i t y ( A r b i t r a r y U n i t s ) (h, h, 6) 4.1 meV 1.0 1.5 2.0 2.5 3.0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 350 K 370 K 390 K 410 K 440 K I n t e n s i t y ( A r b i t r a r y U n i t s ) (1,1, l) 4.1 meV 0.0 0.5 1.0 1.5 2.0 0 100 200 300 400 500 600 700 800 900 1000 350 K 370 K 390 K 410 K 440 K I n t e n s i t y ( A r b i t r a r y U n i t s ) (h,h,6) 12.4 meV 1.0 1.5 2.0 2.5 3.0 0 200 400 600 800 1000 350 K 370 K 390 K 410 K I n t e n s i t y ( A r b i t r a r y U n i t s ) (1,1,l) 12.4 meV -2.0 -1.5 -1.0 -0.5 0.0 0 200 400 600 800 350 K 370 K 390 K 410 K 440 K I n t e n s i t y ( A r b i t r a r y U n i t s ) (h,h,6) 20.7 meV 5.0 5.5 6.0 6.5 7.0 0 200 400 600 800 350 K 370 K 390 K 410 K 440 K I n t e n s i t y ( A r b i t r a r y U n i t s ) (-1,-1,l) 20.7 meV 0.0 0.5 1.0 1.5 2.0 0 100 200 300 400 500 350 K 370 K 390 K 410 K 440 K I n t e n s i t y ( A r b i t r a r y U n i t s ) (h, h, 6) 28.9 meV 5.0 5.5 6.0 6.5 7.0 0 100 200 300 400 500 350 K 370 K 390 K 410 K I n t e n s i t y ( A r b i t r a r y U n i t s ) (-1,-1,l) 28.9 meV 0.0 0.5 1.0 1.5 2.0 0 100 200 300 350 K 370 K 390 K 410 K 440 K I n t e n s i t y ( A r b i t r a r y U n i t s ) (h,h,6) 37.2 meV
Figure 6.32: Scans along the (h, h,6) (left) and (1,1, l) (right) directions of the (1,1,6)/(−1,−1,6) and (1,1,2)/(−1,−1,2) peaks in the I4/mmm space group of Y0.15Sr0.85CoO3−δ respectively at energy transfers of 4.1, 12.4, 20.7, 28.9 and
37.2 meV. Scans have been made between 350 and 440 K on the triple-axis spec- trometer 2T1.
magnet, but is instead a layered antiferromagnet. The correlations are ferromag- netic in theabplane and antiferromagnetic along c, and the value of the ferromag- netic exchange integral is larger than the antiferromagnetic exchange integral [122]. Measurements on the doped manganite, La1.2Sr1.8Mn2O7 have also identified long-
lived antiferromagnetic clusters above Tc, which on first glance are not dissimilar
to those described here [123]. The weak ferromagnetic signal in La1.2Sr1.8Mn2O7
is attributed to a canted ferromagnetic component rather than the ferrimagnetism in Y1−xSrxCoO3−δ [124]. However, the principle that at the critical temperature
dynamically ordered planes order against each other to achieve 3D static magnetic ordering is a plausible scenario for both materials. The presence of 2D fluctuations in Y1−xSrxCoO3−δ would be significant as it would mean it is magnetically ordered
only in the planes above Tc, before 3D magnetic ordering onsets at Tc.
Scans in energy transfer at constant positions in reciprocal space were also made to probe the correlations above Tc in Y0.15Sr0.85CoO3−δ. These are shown
in figure 6.33. Scans were made at the (−1,−1,6), (−2,−2,6) and (−1,−1,7) Bragg positions at temperatures of 350, 370, 390 and 410 K. There is an increase in quasi-elastic scattering below Tc but the dispersive component between 10 and 20 meV shown earlier in figures 6.26, 6.27, 6.29 and 6.30 is clearly observable at all temperatures. The presence of inelastic modes with energy gaps is reinforced by peaks at∼15 and ∼20 meV in the (−1,−1,6) scan. The increased intensity of the (−1,−1,7) scan compared to the (−2,−2,6) at energy transfers up to 30 meV suggests anisotropy in the magnetic correlations, as already discussed.
6.5
Discussion
The magnetic behaviour of Y0.15Sr0.85CoO3−δ powder and single crystals has been
investigated. The simple perovskite AD/OD sample was found to be ferromagnetic with a Tc of 150 K by both magnetisation and neutron diffraction. However, the
neutron diffraction measurements on the brownmillerite AD/OO and AO/OO struc- tural variants showed the susceptibility measurements do not necessarily reflect the full story of the magnetic order in the bulk of the material. The magnetisation signal measured from the AD/OD sample was found to be due to ferromagnetic or- dering of the cobalt moments. In contrast, the magnetisation signal measured from the AD/OO sample is believed to be due to a ferromagnetic impurity phase in an otherwise antiferromagnetic system.
A ferromagnetic-like magnetisation signal from single crystals of the AO/OO structural variant of Y1−xSrxCoO3−δ was also recorded below the transition tem-
perature of 370 K, and was also found to be isotropic. This has been shown to coincide with the temperature at which a structural transition was observed, and it
0 5 10 15 20 25 30 35 40 0 100 200 300 400 500 600 350 K (-1,-1,6) (-2,-2,6) (-1,-1,7) I n t e n s i t y ( A r b i t r a r y U n i t s ) Energy (meV) 0 5 10 15 20 25 30 35 40 0 100 200 300 400 500 600 370 K (-1,-1,6) (-2,-2,6) (-1,-1,7) I n t e n s i t y ( A r b i t r a r y U n i t s ) Energy (meV) 0 5 10 15 20 25 30 35 40 0 100 200 300 400 500 600 (-1,-1,6) (-2,-2,6) (-1,-1,7) I n t e n s i t y ( A r b i t r a r y U n i t s ) Energy (meV) 390 K 0 5 10 15 20 25 30 35 40 0 100 200 300 400 500 600 410 K (-1,-1,6) (-2,-2,6) (-1,-1,7) I n t e n s i t y ( A r b i t r a r y U n i t s ) Energy (meV)
Figure 6.33: Scans of the (−1,−1,6), (−2,−2,6) and (−1,−1,−7) peaks in the
I4/mmm space group of Y0.15Sr0.85CoO3−δ as a function of energy transfer made
on the triple-axis spectrometer 2T1. Tcis 370 K, and measurements have been made
at 350, 370, 390 K and 410 K.
has been suggested that the antiferrimagnetism below 370 K in Y1−xSrxCoO3−δ is
brought on by orbital ordering of the IS cobalt atoms [45]. The second change in the magnetic behaviour of Y1−xSrxCoO3−δ as a function of temperature, evident from
the susceptibility measurements, is at 280 K. This change also seems to be accom- panied by a displacive-type structural phase transition. The width of the magnetic contribution to the powder diffraction also appears to become resolution limited at this temperature, which may indicate that this change in magnetic behaviour is associated with domains. Alternatively, some change in the magnitude of a fraction of the magnetic moments, such as a spin state transition, may be associated with the change in structure and magnetic behaviour at 280 K.
Diffuse and inelastic scattering measurements on Y0.15Sr0.85CoO3−δ have
shown that the inelastic spectrum has a minimum of two magnon branches along each axis, and probably many more, as might be expected in such a complex system. There are also gapped energy modes indicative of the magnetic anisotropy of the system. Quasi-elastic magnetic scattering was observed both below and significantly aboveTc using both techniques. The results suggest that the magnetic fluctuations
are 2D becoming 3D atTc, although more detailed investigation is necessary to fully
understand this behaviour.
Studying the magnetism in Y0.15Sr0.85CoO3−δ has demonstrated the neces-
pound. The ferromagnetic signal, although isotropic from magnetisation measure- ments, is due to the unequal, and antiferromagnetically aligned, moments on dif- ferent cobalt sites, and this remnant moment is nevertheless not constrained to lie in any particular direction accounting for the isotropic ferromagnetism. For fur- ther progress to be made, an improved structural model for Y1−xSrxCoO3−δ as a
function of temperature is necessary so the magnetic structure can be accurately determined.
Part III
Ca3Co2O6
Chapter 7
Ca3Co2O6: Introduction
Ca3Co2O6 is a quasi-1D magnetic material with Ising spins on a triangular lattice.
This configuration has the potential for geometrical frustration and other interesting magnetic behaviour. Regularly-spaced steps in the magnetisation as a function of applied magnetic field have been ascribed to either a quantum tunnelling of the magnetisation or the evolution of metastable states. This literature review covers the work on Ca3Co2O6 to date and identifies the outstanding issues which led to
the experimental work described in chapters 8 and 9.
7.1
Crystal Structure
a b c
Figure 7.1: Crystal structure of Ca3Co2O6. The
chains of alternating CoO6 octahedra and CoO6
trigonal prisms are shown in blue with the oxygen atoms in red. The green spheres are the calcium atoms. In theabplane the chains are arranged in a hexagonal lattice. Figure drawn using [113]. Interest in the compound Ca3Co2O6
was ignited in 1996 when Fjellv˚ag
et al. [28] solved the structure by refining neutron and X-ray powder diffraction data. They indexed the structure with the rhombohedral space group R¯3c
(Z=6, hexagonal setting), where the lattice parameters are a = 9.0793 ˚A and c = 10.381 ˚A
at room temperature. The
cobalt atoms (Co) alternate be- tween face-sharing CoO6 octa-
hedra (Co1) and CoO6 trigo-
nal prisms (Co2), forming chains running along thec axis, shown in figure 7.1. In the ab plane these chains form a triangular
lattice, with calcium atoms separating the chains. The intrachain Co-Co separa- tion is 2.59 ˚A whereas the interchain Co-Co separation is 5.24 ˚A. This means that Ca3Co2O6 has a highly anisotropic crystal structure and can be characterised as a
quasi-1D material.
7.2
Magnetic Structure
Ca3Co2O6has been found to magnetically order below 25 K, with an effective mag-
netic moment of 5.7µB/f.u. in the paramagnetic regime [36]. The magnetic struc-
ture was refined as trivalent cobalt ions (Co3+) whose spin state alternates, with high spin (HS) for the trigonal prisms, SHS = 3.00 µB/f.u., and low spin (LS) for
the octahedra, SLS = 0.08 µB/f.u. [34, 35, 125, 126], with a large orbital contri-
bution (∼ 1.57µB) to the magnetic moment [127]. The cobalt moments on both
sites are aligned along thec axis and ferromagnetic coupling dominates within the chains. Additional Bragg peaks (such as the (1,0,0) reflection) were observed below TN indicating antiferromagnetic order, which is present between the chains in the abplane [34].
The ferromagnetic intrachain interactions (JF M ∼ 25 K) in Ca3Co2O6 are
stronger than the antiferromagnetic interchain interactions (JAF M ∼ 0.25 K). The
moments also appear to be constrained to exhibit Ising-like behaviour with an easy direction parallel to the chain direction. As the interactions between the Ising spins are antiferromagnetic on a triangular lattice in the ab plane, Ca3Co2O6 is also a
geometrically frustrated material (section 2.4.1). The geometrical frustration has led to the suggestion that Ca3Co2O6 has a partially disordered antiferromagnetic
structure, shown in figure 7.2. The model is used to describe a system where one of the three Ising spin chains in the unit cell will have spin up, one will have spin down, and the third will be incoherent [34, 39].
+
-
0
+
+
-
-
Figure 7.2: The partially dis- ordered antiferromagnetic (PDA) structure on a hexagonal lattice. The circles represent the ferromag- netic spin chains. The plus and minus signs indicate the direction of the moment on each of the spin chains. The zero (0) indicates that the spin can be either plus or mi- nus at random and is incoherent. Adapted from reference [128].