For the measurement, the sample was cooled in zero magnetic field down to 20 K, where cooling was stopped for 5000 seconds and after that the cooling process was resumed up to 10 K. After waiting for 10 seconds M-T was recorded with probe field 5 Oe and sweep rate 2K/min. Similar procedure was followed for the ZFC reference curve except there was no intermittent stop.
It depends on the relaxation mechanism. It will occur if the time you take to do the continuous zfc measurement is too fast for some relaxation to occur but the time of the intermittant stops allows the relatxation.
You would need to do measurements as a fun of 'resting' time to identify the dominant relaxation time.
Yes, if any system possess metastability(E.g. spin-glass behavior) then ZTF cooling curve with intermittent stop can be different from reference ZFC curve without any stop. This sort of behavior happens below the freezing temperature and it has been a common phenomena called aging seen in many SG systems(e.g. Zn3V3O8, Nd5Ge3 etc.)
I agree with Gilian that it looks like an issue about relaxation. The first thing to check is whether you can reproduce the effect, so it is not an artifact.
Perhaps you can also perform some very-small-amplitude AC measurements while doing the ZFC and check if susceptibility changes while you wait. Of course, you would like to be sure such AC measurements don't affect your ZFC!
Yes, waiting times have an effect. This has been studied in more details
by Eric Vincent in a series of papers (one is about spin glass : CdCr1.7In0.3S4).
I can send details if needed
Thank you all for your valuable comments.
@Jean Louis Tholence please send me the papers you mentioned.
Generally for glassy systems a dip is expected at the temperature of stop or sometimes it could be visible in difference curve (ZFC memory curve- ref curve). If your whole ZFC curve with intermittent stop deviates from that of the reference curve, it could be due to different remnant field which remains in the magnetometer when we reduced the field to zero; which happens to be different for different time of measurements (as every time we reduced the applied field to zero remnant field will be different). Since your measurement field is quite low (5 Oe) this could be the possibility. So if your system do not have ultra low filed option it can be one of the possible reason, you can check it even for a robust ferromagnetic system. If this is not the case then aging could be the another reason as already mentioned by #Gillian Gehring..But still you will find that if your system have spin glass dynamics; the temperature at which your waited during cooling the deviation will be maximum (even you will get a dip at that temperature) or a maxima in thermo remnant magnetization (TRM) measurements with intermittent wait.