Magnetostructural transition in Co-Mn-Ge systems tuned by valence electron concentration

Switching the magnetostructural coupling in MnCoGe-based magnetocaloric materials

“…When the T t is lowered to the temperature range T C hex < T t < T C ort , a magnetostructural FOMT between the PM hexagonal and FM orthorhombic phases can be triggered, as demonstrated in the MnCoGeB 0.01 alloy. The realization of the magnetostructural FOMT in the MnCoGe alloys via other ways (e.g., off-stoichiometry, element substitution) [15][16][17][18][19][20][21][22][23], can essentially be attributed to the same origin, i.e., via tailoring the stability of the hexagonal phase. However, the underlying mechanism for stabilizing the hexagonal structure in the MnCoGe alloys has not been well understood yet.…”

“…The introduction of vacancies [12,13] and the design of off-stoichiometric compositions [14] both enable the coincidence of the magnetic and structural transitions. Besides that, the partial replacement of the Mn or Co atoms by some 3d transition metal elements [15][16][17][18][19][20][21], as well as the substitution of the Ge by In [22] or Si [23] can also realize a magnetostructural transition in the MnCoGe alloys. Additionally, hydrostatic pressure offers an alternative approach to tailor the magnetostructural coupling in the MnCoGe-type alloys [24][25][26][27].…”

On the instability of the giant direct magnetocaloric effect in CoMn0.915Fe0.085Ge at. % metamagnetic compounds

“…CoMnGe is a unique MMSMA because, unlike NiMn-based alloys, the austenite phase takes on the Ni 2 In-type hexagonal structure 32 . Moreover, it has been well documented that the compound exhibits a volume expansion of nearly 4% 47 , 53 , 54 during the austenite to orthorhombic martensite (forward) transformation. When the CoMnGe compound has been quenched in bulk form, the first martensitic transformation almost always results in microstructural crack propagation and disintegration from bulk polycrystals to a fine grain powder 55 – 60 .…”

“…Replacing Co, Mn, or Ge with a fourth element, e.g. Ga for Ge 62 , V for Mn 53 , Fe for Co 63 , Sn for Ge 41 , and Ti for Mn 64 , almost always decreases transformation temperatures by changing the volume of the hexagonal phase. Moreover, the magnetic state of the orthorhombic phase has been posited to depend on dopant valency 53 .…”

“…It was concluded that Mn carried a much larger moment than Co; therefore, CoMnGe compounds that are rich in Co in exchange for Mn exhibit a lower, though not significantly, saturation magnetization 56 . Subsequent studies have attempted to increase saturation magnetization of the bulk compound by 1. changing the spin coupling on the Ge site 41 , 55 , 62 ; 2. increasing the spin-coupling on the Mn site 44 , 53 , 55 , 64 ; and 3. increasing the spin coupling on the Co site 55 , 57 , 63 , each of which inadvertently changed the volume of the hexagonal phase and decreased the transformation temperatures.…”