Behaviour below T c

The nuclear and magnetic components of the scattering from the (h, k,0) plane of theI4/mmm unit cell at 370 K, 260 K and 15 K are shown in figure 6.19. The top

370 K

370 K

260 K

260 K

15 K

15 K

Figure 6.19: The (h, k,0) scattering plane in the I4/mmm space group of Y0.15Sr0.85CoO3−δ measured at temperatures of 370, 260 and 15 K respectively as

recorded on the polarised neutron diffractometer D7. The left and right hand panels at each temperature show the nuclear and magnetic components of the scattering respectively.

left panel shows the nuclear scattering at 370 K, which as stated above is Tc from

the magnetisation data. The largest peaks in the diffraction pattern, at qx = 3.25

qy = 0 and qx = 0 qy = 3.25 are the (4,0,0) and (0,4,0) and these are intersected

by the aluminium powder lines from the sample holder. There is also clear evidence of the complex superstructure discussed in section 5.3. There are well-defined peaks at positions qx = 0.81 qy = 0.41, qx = 0.41 qy = 2.43 and qx = 0.41 qy = 0.81, with

(h, k, l) indices (1,0.5,0), (0.5,3,0) and (0.5,1,0) respectively in theI4/mmmunit cell. This implies a unit cell at least double the 7.7 x 7.7 x 15.4 ˚A unit cell of the

I4/mmm space group in the abplane, and there maybe additional superstructure peaks obscured by the aluminium powder lines.

The magnetic component of the scattering at 370 K is shown in the top right hand panel of figure 6.19. There are very few features in this figure, which is to be expected as 370 K is Tc for the compound. The strongest peak visible

is the (4,0,0), which is a large structural peak, and its presence, as well as the presence of the aluminium powder lines, implies the beam is slightly depolarised at this temperature, suggesting the presence of a ferromagnetic component to the magnetic order. The other peaks visible in this diffraction pattern are the (1,1,0) and its equivalent positions, which are known to be magnetic peaks. As they are magnetic peaks, they will have more intensity in the magnetic channel than the nuclear one which is why they appear stronger in the right hand panel than the left hand one.

The degree of depolarisation increases when the temperature is reduced be- cause the ferromagnetic contribution to the measured signal becomes greater. This means the polarisation analysis is incomplete and the separation into nuclear and magnetic components must be treated with some care. In particular, the strongest nuclear peaks will partially ‘bleed through’ into the calculated magnetic pattern. However, by comparing the intensity of particular peaks in both channels it is gen- erally possible to determine if a particular peak is purely nuclear or has a magnetic component. In the patterns recorded at 260 and 15 K, additional peaks not obvious at 370 K are visible at q-positions qx = 0.41 qy = 0.41 and equivalents. At 15 K,

these peaks are stronger in the magnetic than the nuclear channel, suggesting they are antiferromagnetic in origin. These peaks are also present at 260 K, but are much weaker than at 15 K.

On cooling below 370 K, peaks also appear surrounding the (4,0,0) and (0,4,0) structural peaks at positions corresponding to (3.75,0.25,0) and equivalents in the I4/mmm unit cell. These peaks are more intense in the structural than the magnetic channel, implying they are structural rather than magnetic in origin. They are in fact the same (0.25,0.25,0) (and equivalent) peaks observed using single crystal X-ray diffraction shown in figures 5.14 and 5.16. These (0.25,0.25,0) peaks

Figure 6.20: The in- tegrated intensities of the (3.75,0.25,0) and (1.5,1.5,0) Bragg peaks in the I4/mmm space group of Y0.15Sr0.85CoO3−δ as a

function of temperature as measured on the single crys- tal neutron diffractometer D10. Tc is marked with a red dashed line. 0 5 10 15 20 25 30 35 0 100 200 300 400 0 5 10 15 Magnetic (3.75,0.25,0) I n t e g r a t e d I n t e n s i t y ( A r b . U n i t s ) Nuclear (1.5,1.5,0) Temperature (K)

also appear to be present but weaker at 260 K. There is some small suggestion of diffuse scattering in the nuclear channel at all three temperatures along the (h,0,0) and (0, k, l) directions.

It was not practical to measure the temperature dependence of the peaks appearing below Tc on D7 because of the lack of q-resolution, however the tem- perature dependence of the peaks could be measured on the single crystal neutron diffractometer D10. Figure 6.20 shows the integrated intensity of the (3.75,0.25,0) and (1.5,1.5,0) peaks as a function of temperature between 400 K and the 2 K base temperature of the cryostat (the (1.5,1.5,0) peak is an equivalent to the (0.5,0.5,0) peak). Both peaks were fitted with Gaussian profiles, as they were too weak to be fitted with more complex functions. Although neither peak has a large integrated intensity, the measurement clearly shows both peaks appear at a temperature of 370 K. The polarisation analysis available on D7 described above showed that the (3.75,0.25,0) and (1.5,1.5,0) peaks are structural and magnetic respectively. These measurements therefore show that both a structural and a magnetic transition occur close to 370 K in this compound, and the transitions are most likely to coincide.

Figure 6.21: The integrated intensity of the (4,0,0) peak in theI4/mmm space group of Y0.15Sr0.85CoO3−δ as a

function of temperature as measured on the single crys- tal neutron diffractometer D10. The peak was fit- ted with two components, a Gaussian and a Lorentzian with widths of 0.13◦ _and

0.25◦ _{respectively.} 0 100 200 300 400 0 1000 2000 3000 4000 (4,0,0) Total Gaussian Lorentzian I n t e g r a t e d I n t e n s i t y ( A r b i t r a r y U n i t s ) Temperature (K)

The influence of the two transitions on each other cannot be determined without further study of similar compounds with different doping levels and transition tem- peratures. Resonant X-ray scattering has suggested the ferrimagnetism is brought on by the orbital ordering of the IS cobalt ions, which would fit with our observation of coincident structural and magnetic transitions [45].

The temperature dependence of the strong nuclear peak (4,0,0) in theI4/mmm

unit cell was also measured on D10. The peak was fitted with two components, a close to resolution limited Gaussian and a Lorentzian with a width of 0.25◦_{, imply-}

ing some finite length correlations in the material. The centres of the two peaks were fixed to coincide. Fitting the peak in this way also accounts for any change in the peak position as a function of temperature due to lattice expansion. The total integrated intensity of the peak would be expected to persist or slightly increase as the temperature was decreased, but as figure 6.21 shows, it decreases in intensity with temperature. In fact, between 300 and 100 K the total integrated intensity re- duces by over a third. We take this to be further evidence of a structural transition at 280 K in Y1−xSrxCoO3−δ. The presence of a structural transition at 280 K was

discussed in section 5.3.5, as a feature was also observed in the lattice parameter data along theband c axes at this temperature.

The temperature dependence of the integrated intensity of several other peaks was measured on D10, and the results for the (0,0,8) and (1,1,2) peaks in theI4/mmmunit cell are shown in figure 6.22. The (0,0,8) is a large structural peak akin to the (4,0,0). The same drop in intensity observed for the (4,0,0) peak is observed for this peak. The temperature at which this feature occurs is judged to be 280 K, the same temperature a currently unexplained feature occurs in the magnetisation. More information is required to identify the source of this change in intensity, which most likely requires a better structural model for the compound than the one currently available to fully resolve. As the (0,0,8) and (4,0,0) are nuclear peaks it might be expected that the change is due to some structural mod- ulation, and the powder diffraction measurements discussed in section 5.3.5 suggest there is a displacive-type structural transition at this temperature. On the other hand, any ferromagnetic component to the magnetic order will contribute to all the Bragg peaks, so may also be responsible, but the magnitude of the change suggests it is due to a structural transition.

The largest magnetic peak for the AO/OO form of Y0.15Sr0.85CoO3−δ is the

(1,1,2) in the I4/mmm unit cell. This peak was also measured using the single crystal neutron diffractometer D10, shown in figure 6.22. Like the nuclear peaks, it was also fitted with two components, a Gaussian with width 0.30◦ _{and a Lorentzian}

with width 0.69◦_{. Both components follow the expected trend for a magnetic peak}

0 100 200 300 400 16000 17000 18000 19000 20000 (0,0,8) I n t e g r a t e d I n t e n s i t y ( A r b i t r a r y U n i t s ) Temperature (K) Nuclear 0 100 200 300 400 0 200 400 600 800 1000 0 100 200 300 400 0 200 400 600 800 1000 Gaussian Lorentzian I n t e g r a t e d I n t e n s i t y ( A r b i t r a r y U n i t s ) Temperature (K) (1,1,2) Magnetic Total Temperature (K)

Figure 6.22: The integrated intensities of the nuclear (0,0,8) and magnetic (1,1,2) peaks in theI4/mmmspace group of Y0.15Sr0.85CoO3−δas a function of temperature

as measured on the single crystal neutron diffractometer D10.

figure 6.15, the width was not found to change with temperature. This may be due to the direction or resolution of the scan, or it may be the variation in width seen in powder is due to some strain or domain effect not found in single crystals.