The MnCoGe alloy can crystallize in either the hexagonal Ni2In- or the orthorhombic TiNiSi-type of structure. In both phases MnCoGe behaves like a typical ferromagnet with a second-order magnetic phase transition. For MnCoGeBx with B on interstitial positions, we discover a giant magnetocaloric effect associated with a single first-order magnetostructural phase transition, which can be achieved by tuning the magnetic and structural transitions to coincide. The results obtained on the MnCoGe-type alloys may be extensible to other types of magnetic materials undergoing a first-order structural transformation and can open up some possibilities for searching magnetic refrigerants for room-temperature applications.

Substitution of some Cr for Mn atoms in MnCoGe was employed to control the magnetic and structural transitions in this alloy to coincide, leading to a single first-order magnetostructural transition from the ferromagnetic to the paramagnetic state with a giant magnetocaloric effect observed near room temperature. Further increase in the Cr content in the Mn1−xCrxCoGe alloys can induce another first-order magnetoelastic transition from the antiferromagnetic to the ferromagnetic state occurring at lower temperature. The giant magnetocaloric effect as well as the simultaneous tunability of the two magnetic transitions make these materials promising for future cooling applications.

Effects of physical and chemical pressures in the Mn 1−x Cr x CoGe series of compounds are studied. Cr substitution and hydrostatic pressure play similar roles in displacing T C to lower temperatures and coupling or decoupling magnetic and crystallographic transitions. In this work the similarities and differences between the effects of chemical and physical pressures are explored, helping unveil the nature of the first-order phase transition presented by MnCoGe-based compounds. DOI: 10.1103/PhysRevB.84.020414 PACS number(s): 75.30.Cr, 75.30.Sg, 75.50.Cc, 75.80.+q Since the discovery of the giant magnetocaloric effect on FeRh 1 and the subsequent observation of a tunable giant entropy change ( S M ) on Gd 5 Ge 2 Si 2 , 2 magnetic refrigeration based on the magnetocaloric effect has gained increasing attention. It is regarded as a promising, more efficient, and environmentally friendly alternative to gas-compression-based refrigeration due to its potential applications in a wide range of temperatures.Materials presenting first-order magnetic phase transitions are of special interest since they intrinsically present high S M originating from the discontinuous character of the transition. These transitions can be observed in a series of low-cost 3d metal-based compounds such as MnAs,Ni 0.5 Mn 0.5 Sn, 5 and (Mn,Fe) 2 (P,As) 1 . 6 Such transitions are always accompanied by a discontinuous change in the lattice parameters and often in volume, but do not always result in a crystal symmetry change. Materials presenting only lattice parameters and/or volume change around the phase transition are said to undergo a magnetoelastic phase transition, e.g., all Fe 2 P-based compounds as well as La(Fe,Si) 13 , 7 while those which also show a change in crystal structure undergo a magnetostructural phase transition, e.g., FeRh, Gd 5 Ge 2 Si 2 , MnAs, and Ni 0.5 Mn 0.5 Sn.Since magnetic interactions are sensitive to interatomic distances, chemical pressure-substitutions, dopings, and interstitial elements-has been largely used to tune magnetic properties. Both magnetic and crystallographic phasetransition temperatures can be tuned using chemical pressure. This allows for first-order phase-transition temperatures to be easily tuned. But, more importantly, it allows for chemical pressure to be used to simultaneously tune separate magnetic and crystallographic transitions to coincide, giving rise to coupled first-order transitions. Thus, chemical pressure is an invaluable tool not only to tune but also to create magnetoelastic and magnetostructural couplings. A good example of both the creation and tuning of a magnetostructural coupling comes from the MnCoGe system.MnCoGe is a 3d metal-based ferromagnet with a Curie temperature (T C ) of ∼345 K and a diffusionless crystallographic phase transition from the low-temperature orthorhombic TiNiSi type to the high-temperature hexagonal Ni 2 In type of structure at ∼650 K. 8,9 In both orthorhombic and hexagonal structures it behaves as a typical ferromagnet with second-order phase trans...

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