Table-like Magnetocaloric Effect in Ho36Co48Al16 Multiphase Alloy

Magnetocaloric effect (MCE) has recently gained wide attention in both fundamental research [1, 2], and application in magnetic refrigeration (MR) systems. It is promising as an efficiently competitive technology for the replacement of the potentially environmentally risky gas compression/expansion refrigeration [3–5]. Ericsson cycle implementation of MR is one of the most suitable thermodynamic cycles for enhanced performance. However, its requirement that magnetic entropy change (–ΔS
^M
) of the magnetic refrigerant is constant (table-like MCE) over the operating temperature range severely limits the availability of appropriate magnetocaloric materials (MCMs). The search for MCMs with such property is a daunting challenge. Some proposed research approach include multilayered and powder composite mixtures but the low density of powder composites significantly deteriorates the magnitude of –ΔS
^M
. Moreover, the lifespan of sintered or glued layer refrigerants is shortened by repeated differential expansion and contraction at the interfaces. Single bulk multiphase alloys with table-like feature could be a solution to overcoming the issues associated with layered and ex-situ composite mixtures. However, the challenge of identifying such alloy systems with temperature independent –ΔS
^M
or improved refrigerant capacity (RC) can be overwhelming. Their discovery is strictly dictated by the nature of the related phase diagrams. Several Ho(Co, Al)-based binary and quasi-binary crystalline alloys are known to have good magnetocaloric properties. Therefore, we examine the Ho-Co-Al system to identify a suitable multiphase region for study in expectation of a single bulk multi-crystalline alloy that exhibits table-like MCE. In particular, HoCo
2
, HoCoAl, and Ho
2
Co
2
Al compounds have good magnetocaloric properties [6–8]. Consequently a carefully selected region of the Ho-Co-Al system, bounded by the three compounds and expected to contain all three phases, was studied. Results show that the title multiphase alloy, which comprises the HoCo
2
, HoCoAl, and Ho
2
Co
2
Al main phases, exhibits table-like MCE. The effect of interatomic substitution on the magnetic properties of relevant phases and its contributions to the attainment of the table-like MCE and RC are discussed in detail.