The effect of distributed exchange parameters on magnetocaloric refrigeration capacity in amorphous and nanocomposite materials

Abstract

The temperature dependent magnetization of nanocomposite alloys has been fit with a modified Handrich-Kobe equation with an asymmetric exchange fluctuation parameter combined with the Arrott-Noakes equation. The two equations of state are combined to calculate the entropy change in the magnetocaloric effect associated with the ferromagnetic to paramagnetic phase transformation. The complete fit for the M(T) of (Fe 70 Ni 30 ) 88 Zr 7 B 4 Cu nanocomposite powder is accomplished by combining the two theories. We investigate the broadening of the second-order transition arising from asymmetric exchange parameters and resulting from the fluctuations of interatomic spacing found in an amorphous matrix and the asymmetric dependence of exchange energy on interatomic spacing. The magnetic entropy curve revealed extra broadening with a refrigeration capacity (RC) value of 135 J/kg at 5 T, which is comparable to (Fe 76 Cr 8-x Mo x Cu 1 B 15 ) ribbons, which have a RC value of 180 J/kg for the same applied field. Broadening of the magnetic entropy can lead to larger RC values and a wider working temperature range, making nanocomposite alloys promising for magnetocaloric applications.

The magnetocaloric effect of partially crystalline Fe-B-Cr-Gd alloys

“…28 In addition, Jones et al attributed the broadened jDS pk M j of Fe-Ni-Zr-B-Cu nanocomposite MCM to its asymmetric dependence of exchange interactions as well as the fluctuations of interatomic spacing within the amorphous matrix. 29 The decrease of jDS pk M j for the Gd5 (872 K) alloy compared to that of the Gd5 melt-spun alloy is in agreement with the sum rule. 28 The crystalline phases in partially crystalline Gd5 alloys are relatively less magnetic than the major crystalline a-Fe phase in partially crystalline Gd1 alloys, resulting in dissimilar MCE behavior, e.g., the relatively constant jDS pk M j observed in the Gd5 (876 K) and Gd5 (880 K) alloys.…”

Enhancement of the magnetic refrigerant capacity in partially amorphous Fe70Zr30 powders obtained by mechanical alloying

“…as amorphous fraction increases. This behaviour is typically observed for other partially amorphous alloys [7,8]. However, for those systems, the remaining crystallites have a Curie temperature well separated from that of the amorphous phase, which has been shown to deteriorate the MCE response in terms of the RC value.…”

“…It would be very convenient if the different phases constituting the multiphase system were produced in a single step in order to minimize production cost and to achieve a more intimate contact between the phases, which would allow to optimize the behaviour through tailoring interactions between phases and the distribution exchange inside them [7]. This is the case of nanocrystalline systems, where w5e10 nm nanocrystals are embedded in a residual amorphous matrix.…”

Milling effects on magnetic properties of melt spun Fe-Nb-B alloy

“…13 However, it should be noted that a distribution in the Fe-Fe distance intrinsic in the amorphous structures would produce a distribution of the exchange coupling constant, as studied by Gallagher et al 14 This also originates a broadening of the magnetocaloric peak. 15 Nevertheless, even if this distribution should have an influence, as in our case the width of the Fe-Fe distance distribution remains constant for the different samples, the differences observed in their magnetocaloric response should not be ascribed to this effect.…”