Generalidades sobre el Mantenimiento basado en el riesgo

9.2.1 Peak Shape -86.0 -85.5 -85.0 -84.5 1 10 100 Angle ( o ) (300) I n t e n sit y ( Co u n t s/ S e c)

Figure 9.2: The lineshape of the fer- romagnetic (3,0,0) peak measured at 2 K in zero field. The peak was fitted with a single Gaussian (blue line) and a flat background.

Both for the powder and single crystal sam- ples, ferromagnetic peaks appear in field at positions on top of the nuclear peaks. For the single crystal, these peaks could adequately be fitted with a single Gaussian, and the (3,0,0) is shown in figure 9.2 as an example. The width of these peaks was considered to be limited by the resolution of the D10 instrument, with a typical FWHM of 0.35◦_{. This implies that the}

ferromagnetic component of the ferrimagnetic phase is fully long-range ordered.

For all the ferromagnetic peaks, the nuclear component was measured in zero field and has been subtracted to give a purely fer- romagnetic intensity. This analysis has been applied to the data shown in figures 9.3, 9.4 and 9.5.

9.2.2 Field Dependent Magnetic

Behaviour

Figure 9.3 shows the intensity of the ferromag- netic component of the (3,0,0) peak at 12 and

2 K. At 12 K, there is single step at 3.6 T marking the transition from ferrimag- netic to ferromagnetic behaviour, with little hysteresis between the measurements made with the field ramping up and the field ramping down. When cooled to 2 K a much larger hysteresis develops. This large hysteresis has previously been seen in magnetisation measurements [38]. Additional steps are distinguishable in the 2 K data, at 2.4 T and 4.8 T when the field is ramped up and steps in similar positions were also observed when the field was ramped down.

If the positions of the magnetic peaks in the (hkl) scattering plane change significantly as function of magnetic field then the method we have used of integrat-

0 40 80 120 0 40 80 0 1 2 3 4 5 (300) T=12K P e a k I n t e n si t y ( C o u n t s/ S e c) 0 H (T) (300) T=2K

Figure 9.3: The intensity of the ferromagnetic (3,0,0) peak as a function of applied magnetic field at 12 K and 2 K. The nuclear component of the ferromagnetic peak (89 Counts/Sec) has been subtracted from both data sets. The ramp rate of magnetic field was 0.1 T/min.

ing over a constant area of the detector is not valid. In this case, integrating scans through the peak is a more accurate way of assessing the overall peak intensity. Figure 9.4 shows the results of integratingω scans made as a function of magnetic field. One of the advantages of using this method is that the peak widths and peak positions as a function of magnetic field can also be measured. As the inset to fig- ure 9.4 shows, the position of the centre of the ferromagnetic/nuclear peaks moves very little, with a small drift of less than 0.01◦ _{below 2.4 T becoming fixed above}

2.4 T. Comparing to the measurements made using a constant detector area, the two methods of data collection are in good agreement with each other, justifying the use of the area detector and the sweeping method. This is important as the shape of the hysteresis curve in Ca3Co2O6 is strongly dependent on measurement

time and the sweeping method is a faster method of data collection than integrating the peak intensity at each point.

Figure 9.4 shows both the integrated ω scans made on D10 at 2 K and a magnetisation curve taken at the same temperature [156], with the positions of the steps marked by blue dashed lines. The equally-spaced steps in intensity of the ferromagnetic peak match well with those seen in magnetisation measurements, with clear steps observed at 2.4, 3.6 and 4.8 T, and a likely feature at 1.2 T. The observed increase in the intensity of the ferromagnetic peaks close to 0 T is relatively small. However, given the fact that the magnetic intensity of the ferromagnetic peaks is proportional to the magnetisation squared, equation 3.6, the agreement between the

0 5 10 15 20 25 30 0 1 2 3 4 5 0 1 2 3 4 I n t e g r a t e d I n t e n si t y ( A r b . U n i t s) (300) 0 1 2 3 4 5 -85.215 -85.210 -85.205 -85.200 P o s i t i o n o f P e a k ( o ) 0 H (T) (b) 0 H (T) (a) M a g n e t i sa t i o n ( B / f . u . )

Figure 9.4: Panel (a) shows the integrated intensity of the ferromagnetic (3,0,0) peak at a temperature of 2 K as a function of applied magnetic field. These data were taken after zero field cooling. The nuclear component of the FM peak (equating to 24.9 in the units of integrated intensity used in the figure) has been subtracted. The inset to this figure shows the position of this nuclear/magnetic peak as a function of applied field. Panel (b) shows magnetisation data as a function of applied field for a single crystal of Ca3Co2O6 aligned with the field along thecaxis. These data are

presented for comparison purposes and are not my own work [156]. The blue dashed lines indicate the positions of the magnetisation steps, and show that the positions of the steps, where they could be observed, are the same for both measurements.

neutron data and the magnetisation curve shown in figure 9.4 is quantitatively good.

9.2.3 Temperature and History Dependent Behaviour Figure 9.5: Temperature depen-

dence of the intensity of the ferromagnetic (3,0,0) peak mea- sured in three different fields, 0.6, 1.8 and 3 T. The data taken while warming the sample in field after zero field cooling (ZFCW) is shown in black and the red symbols show the data taken in field while increasing the temperature after field cool- ing (FCW). An irreversibility temperature is visible at around 10 K, where the intensities of the ZFCW and FCW measurements diverge, marked by a dashed line on the figure. The temperature ramp rate was 0.3 K/min.

0 2 4 6 0 5 10 15 20 25 30 35 0 5 10 15 0 2 4 6 FCW (a) 0.6 T ZFCW Temperature (K) (c) 3 T P e a k I n te n s it y ( C o u n ts /S e c ) (b) 1.8 T

The intensities of the ferromagnetic peaks were also measured in magnetic fields of 0.6, 1.8 and 3 T as a function of temperature. These fields were chosen as they are the mid-points on the plateaux seen in the bulk magnetisation curve. The results are shown in figure 9.5. In addition to the apparent transition to the paramagnetic state at 20 K in a magnetic field of 3 T (figure 9.5(c)) another notable feature is the presence of an irreversibility temperature around 10 K. Below this temperature there is a pronounced difference between the zero field cooled and field cooled data for the 0.6 and 3 T measurements, while in the intermediate field of 1.8 T the difference is very slight. There is also a crossover in the the zero field cooled and field cooled data at 1.8 T, as at this point the field cooled data has greater magnitude than the zero field cooled data at 2 K, whereas at 0.6 and 3 T the opposite is true. This observation should be linked to the magnetisation relaxation measurements [38], which revealed that at lower temperatures the magnetisation of Ca3Co2O6 has

a pronounced time dependence and that the magnetisation could relax up or down depending on the value of the applied field.