Heterogeneous nucleation and heat flux avalanches in La(Fe, Si)13 magnetocaloric compounds near the critical point

Abstract

The phase transformation kinetics of LaFe 11.41 Mn 0.30 Si 1.29 -H 1.65 magnetocaloric compound is addressed by low rate calorimetry experiments. Scans at 1 mK/s show that its first order phase transitions are made by multiple heat flux avalanches. Getting very close to the critical point, the step-like discontinuous behaviour associated with avalanches is smoothed out and thermal hysteresis disappears. This result is confirmed by magneto-resistivity measurements and allows to measure accurate values of the zero field hysteresis (∆T hyst = 0.37 K) and of the critical field (H c = 1.19 T). The number and magnitude of heat flux avalanches change with magnetic field, showing the interplay between the intrinsic energy barrier between phases and the microstructural disorder of the sample.A strong attention is nowadays directed to room temperature refrigeration techniques based on the magnetocaloric effect (MCE) because they allow a reduced energy consumption and a lower environmental impact with respect to gas compression technologies 1,2 . A class of materials which are promising candidates for magnetic cooling, is the one based on the La(Fe,Si) 13 compound 3 . These intermetallics show a large MCE because they exploit a sharp drop in magnetization associated with a ferromagnetic (FM) to paramagnetic (PM) phase transition. Near the transition temperature, T t , magnetic fields of about 2 T can provide an adiabatic temperature variation up to ∆T ad = 7 K 3 . This giant MCE, observed in magnetic transitions of the first order type, implies thermo-magnetic hysteresis as a drawback for applications. Understanding the mechanism which underlies thermo-magnetic hysteresis is thus of great importance for the modelling of magnetic refrigeration cycles. It is known that the hysteresis width of La(Fe,Si) 13 is influenced by the strength of the magnetic field 4 , by hydrostatic pressure 5 and by substitution element at Fe sites. Moreover, as pointed out on several works 6,7 , and particularly on those regarding La(Fe,Si) 13 based materials 8 , the transformation process of first order magnetocaloric materials is due to the motion of phase boundaries between FM and PM phases. This motion takes place on a complex energy landscape influenced by several factors. For example, the strains generated by the lattice shrinking at the PM/FM transitions may influence the free energy profile at local site 9 as well as the magnetic and structural disorder which can block or favour the transition front advance 10,11 .We address this issue by investigating the in-temperature transformation process of a LaFe 11.41 Mn 0.30 Si 1.29 -H 1.65 sample.The chosen composition has a transition on the border between first and second order types and it represents the best compromise for application near room temperature due to the large ∆S iso (19 J kg −1 K −1 ) and the low zero field thermal hysteresis (0.4 K at T t ≈ 295 K) 12 . By exploiting low (1 mK/s) and fast (up to 100 mK/s) temperature rates calorimetry experiments, and by using electrical resisti...

Millisecond Dynamics of the Magnetocaloric Effect in a First‐ and Second‐Order Phase Transition Material

“…The recently developed method of modulation infrared thermometry allows the direct determination of Δ T even for low‐volume samples, thus avoiding the limitations of calorimetric measurements or indirect approaches such as isothermal magnetization measurements. Going beyond the mere determination of the magnetocaloric parameters, the dynamics of the MCE has attracted growing interest . The influence of, for example, field‐sweep rate or field‐cycling frequency on the magnetocaloric effect is not only of fundamental interest, but also relevant for the design of magnetocooling devices …”

“…[2,3] Reliable methods for the determinationo fm agnetocaloric key parameters such as the adiabatic temperature change DT are ap rerequisite for the identification of suitable materials.T he recently developed method of modulation infrared thermometry [4,5] allows the direct determinationo fDT even for low-volume samples,t hus avoiding the limitations of calorimetric measurements or indirect approaches such as isothermal magnetization measurements.G oing beyond the mere determination of the magnetocaloric parameters,t he dynamics of the MCE has attracted growingi nterest. [6][7][8][9][10] Thei nfluence of,f or example,f ield-sweep rate or field-cycling frequency on the magnetocaloric effect is not only of fundamental interest, but also relevant for the designo fm agnetocooling devices. [11,12] Here,w ei nvestigate the magnetocaloric effect in gadolinium and aL a(Fe,Si) 13 -type compound for field-cycling frequenciese xceeding1kHz, thus significantlye xtending the dynamical regimei nvestigateds of ar.…”

Hysteresis and Phase Transition Kinetics in Magnetocaloric Materials

“…This effect is probably absent in MCE materials working around room temperature because the thermal energy in this case is sufficiently large. A particularly nice approach to search for intrinsic kinetic effects is to look at the heat flux measured through temperature or field scans performed at very low rates . The heat flux reveals an avalanche‐like behavior consisting in heat flux spikes characterized by rising times and falling times .…”

“…A particularly nice approach to search for intrinsic kinetic effects is to look at the heat flux measured through temperature or field scans performed at very low rates . The heat flux reveals an avalanche‐like behavior consisting in heat flux spikes characterized by rising times and falling times . A possible route to describe this effect consists in associating a kinetic coefficient as the proportionality coefficient between the speed of the magnetostructural phase boundary motion responsible for the transition and the distance of the system from equilibrium.…”

Kinetics of heat flux avalanches at the first order transition in La(Fe-Mn-Si)13-H1.65 compounds

“…Novel aspects about this feature can be learnt by analyzing heat flux signals obtained through temperature scans at very low rates [5,6]. Indeed, the low scan rate allows to distinguish single heat flux avalanches associated to the microscopic individual processes occurring during the phase transition.…”