Análisis del mantenimiento en el centro

dc-magnetisation measurements as function of applied magnetic field were made in a 7 T Quantum Design SQuID MPMS. The sample was initially aligned with the field parallel to thec axis of the I4/mmm unit cell and mounted on a plastic PEEK sample holder using GE varnish and put in a non-magnetic sample straw. The sample was then rotated so that the field was parallel to the ab plane and remeasured.

Figure 6.7 shows the dc-magnetisation as a function of applied magnetic field for the O2-annealed crystal measured across the 370 K transition temperature.

TheseM(H) curves clearly reveal the ferromagnetic character of the magnetic tran- sition seen at 370 K in theM(T) data. As the temperature is reduced from 400 K to 270 K there is a rapid increase in the magnitude of magnetisation which is accompa- nied by the appearance of hysteresis in theM(H) loops below 370 K, shown in figure 6.8. The coercive field is around 0.2 T at 270 K and the non-saturating component at high fields suggests an antiferromagnetic component to the magnetisation.

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 300 K M a g n e t i s a t i o n ( / C o ) 0 H (T) 400 K 370 K H || (001) 270 K

Figure 6.7: The field depen- dence of the dc-magnetisation of an O2-annealed crystal of

Y0.15Sr0.85CoO3−δ. The sample

was warmed above Tc between

runs.

Magnetisation measurements made on annealed single crystal samples with the magnetic field applied parallel and perpendicular to thec axis, shown in figure 6.8, reveal that the magnitude of the magnetisation at 7 T is_∼5% higher forH_||c, indicating a possible small net magnetic moment aligned along thecaxis, although this may also be due to a sample shape effect. On the other hand, given the highly anisotropic crystal structure, the lack of anisotropy in the magnetic response of the materials again suggests that Y0.15Sr0.85CoO3−δ is not a bulk ferromagnet.

The observation of an isotropic magnetic response should be considered in the context of the underlying mechanism for the measured ferromagnetic signal. The TGA measurements for the O2-annealed crystal suggest the presence of a small

amount of Co4+_{, which would be expected if clustering were the mechanism be-}

-6 -4 -2 0 2 4 6 -0.4 -0.2 0.0 0.2 0.4 H||C HC M a g n e t i s a t i o n ( B / C o ) 0 H (T) 300 K -6 -4 -2 0 2 4 6 -0.4 -0.2 0.0 0.2 0.4 250 K H||C H C M a g n e t i s a t i o n ( B / C o ) 0 H (T) -6 -4 -2 0 2 4 6 -0.4 -0.2 0.0 0.2 0.4 200 K H||C H C M a g n e t i s a t i o n ( B / C o ) 0 H (T) -6 -4 -2 0 2 4 6 -0.4 -0.2 0.0 0.2 0.4 100 K H||C HC M a g n e t i s a t i o n ( B / C o ) 0 H (T) -6 -4 -2 0 2 4 6 -0.4 -0.2 0.0 0.2 0.4 10 K H||C H C M a g n e t i s a t i o n ( B / C o ) 0 H (T)

Figure 6.8: The field dependence of the dc-magnetisation of an O2-annealed crystal

of Y0.15Sr0.85CoO3−δ at 300, 250, 200, 100 and 10 K. Measurements made parallel

thecaxis are shown in black and measurements made perpendicular tocare shown in red. The sample was warmed aboveTc between runs.

be unlikely if spin canting was occurring in this compound, as the moments would generally be constrained to lie close to the direction of the overall antiferromagnetic order. More recent works [24, 45] have suggested the observed magnetisation signal is due to an overall ferrimagnetic ordering of the magnetic moments. On first con- sideration these ferrimagnetic moments might be expected to be constrained to lie close to the direction of the underlying antiferromagnetic order. However, in some cases such a constraint may not exist, and the remnant ferromagnetic signal due to spin canting or ferrimagnetism may be free to orientate with the applied field, which is likely to be the case here, explaining the isotropic magnetisation measurements.

6.1.6 Specific Heat

Specific heat measurements were performed on the O2-annealed crystal using a

two-tau relaxation method in a Quantum Design Physical Properties Measurement System (PPMS). The heat capacity of the empty sample stage, together with the Apiezon H grease used to attached the sample to the stage, was subtracted from the measured signal to give the sample heat capacity.

Figure 6.9 shows the temperature dependence of the specific heat capac- ity,C(T), of the O2-annealed Y0.15Sr0.85CoO3−δ crystal and the non-magnetic per-

ovskite LaGaO3 that was used to estimate the lattice contribution to the heat ca-

pacity. A mass correction was found to give no improvement to the data. The heat capacity of the Y0.15Sr0.85CoO3−δ contains only one significant feature correspond-

ing to a bulk transition atTc= 370 K. This means that the entropy associated with

the features seen at 150 K and 280 K in theM(T) data is small and may be linked with either spin reorientations within an ordered state, or to some magnetic ordering that occurs in a very small volume fraction of material. The extreme sensitivity of the magnetic properties of Y0.15Sr0.85CoO3−δ to both oxygen disorder and oxygen

concentration has led previous researchers to conclude the latter is more likely [92]. The total magnetic entropy released between 3 and 420 K [114] is 11.8 J mol−1_K−2

which is _∼ 88% of the R ln(2S+ 1) = 13.4 J mol−1_K−2 _{expected for a Co}3+ _ion

with a fully quenched orbital moment,S = 2 (R is the gas constant).

The low temperature (T _≤ 12 K) data, shown in figure 6.9, can be fitted using the Debye model, as described in equation 2.13. This gives a γ = 0.90 mJ mol−1_K−2_{, a rather low value that is consistent with previous reports on transition}

metal oxides that are close to the boundary between an insulating antiferromagnetic and a metallic ferromagnet state [115, 116, 117]. Using β we calculate the Debye temperatureθD =

12

5 π4pR/β

1/3

wherepis the number of atoms in each molecule givingθD = 391 K, a result that is compatible with the data at higher temperatures.

Adding contributions that vary asTnwithn= 3/2 or 2 does not improve the quality of the fits suggesting that any magnetic spin-wave contribution to C varies as T3_,

0 100 200 300 400 0 20 40 60 80 100 120 140 Y 0.15 Sr 0.85 CoO 3- LaGaO 3 C ( J / m o l K ) Temperature (K) 3R 0 20 40 60 80 100 120 140 0 5 10 15 20 25 C / T ( m J / m o l K 2 ) T 2 (K 2 ) C(T) = T + T 3 = 0.9(3) mJ/ mol K 2 = 0.150(6) mJ /mol K 4

Figure 6.9: Upper panel: Temperature dependence of the specific heat of a single crystal of O2-annealed Y0.15Sr0.85CoO3−δ. LaGaO3, a non-magnetic perovskite,

was also measured and is shown to indicate the lattice contribution to the recorded signal. The horizontal line indicates the value of 3R per atom for Y0.15Sr0.85CoO3−δ.

Lower panel: C/T vs T2 _{for the low temperature specific heat (2< T <10 K) of a}

single crystal of O2-annealed Y0.15Sr0.85CoO3−δ. The fit is to a model consisting of

the electronic (γT) and the lattice (βT3) contributions to the specific heat, which appears to be a good description of this low temperature data.

a behaviour that is typical for a bulk antiferrromagnet and consistent with the arguments presented earlier.