Page 59 - ILLS Annual Report 2018

35 40 45 50 55 2
189.32 243.31
T (K)
3
0 -3 -6 - 9
5 4 3 2 1 0 -1 -2 - 3 0
Figure 2
Temperature change ΔT(T) under a 10 kOe magnetic field reduction estimated through ΔT = -(T/Cp) ΔS with Cp the specific heat.
Inset: Entropy change ΔS(T) = S(T,0)-S(T,H) at
10 kOe on heating after field cooling under 70 kOe (open circles) and during cooling (full circles) with no magnetic field.
The measured entropy change, ΔS, for removing a
10 kOe field and its associated adiabatic temperature change linked to the different possible martensitic transformations (‘anomalous’ forward transformation, reverse transformation on heating and forward transformation on cooling) are shown in figure 2. Field reduction implies a transformation from ferromagnetic austenite (high magnetisation) to paramagnetic martensite (low magnetisation). The ‘anomalous’ martensitic transformation produces a positive ΔS while both the reverse and the standard forward transformations show negative values. On the other hand, upon heating negative temperature changes are observed around the forward anomalous transformation while positive values appear around the standard forward and reverse transformations.
In summary, the same austenite-to-martensite transformations on heating (anomalous) and on cooling (standard) show oppositely signed entropy changes. This behaviour
can be explained as the total entropy change linked
to the transformation having mainly vibrational and magnetic contributions, ∆S = ∆Svib + ∆Smag. During the transformation from ferromagnetic austenite to paramagnetic martensite (field removing), ∆Svib < 0 and ∆Smag > 0 typically occur. In the temperature range of the standard transformation the vibrational contribution is
large and then ∆S < 0. In contrast, the vibrational entropy approaches zero at very low temperatures and therefore the vibrational contribution to the MT is expected to be very small. Furthermore, the lower the temperature, the higher the positive magnetic entropy contribution. Thus,
in the range where the retained austenite is metastable (below 50 K) the magnetic contribution to the total entropy dominates and ∆S > 0.
Interestingly, a remarkably high value of the adiabatic temperature change (~9 K) is observed around the ‘anomalous’ forward martensitic transformation under a moderate applied field of 10 kOe (figure 2). This is almost twice that obtained in the conventional forward transformation under higher applied fields (∆T ~ 6 K), and one of the largest values obtained in magnetic shape memory alloys. From an application point of view, the magnetocaloric effect at low temperatures could compete with other materials like molecular magnetic compounds for magnetic refrigeration at cryogenic temperatures; while the occurrence of direct and inverse magnetocaloric effects associated with the same
forward martensitic transformation could be useful in the design of refrigeration devices based on more complex thermodynamic processes.
Austenite (200) + Martensite
Neutron thermo-diffraction patterns measured on heating (external magnetic field set to zero at 10 K) in a sample cooled from 300 K under a 50 kOe magnetic field.
50 100 150
200
250 300
Temperature (K)
0 50 100 150 200 250 300
Temperature (K)
Martensite
8 000 6 000 4 000 2 000 0
73.4 134.17
Figure 1
SCIENTIFIC HIGHLIGHTS
56-57
∆T (K)
∆S (J/kgK)
Intensity
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