Magnetic and Structural Transitions Tuned through Valence Electron Concentration in Magnetocaloric Mn(Co_1–xNi_x)Ge

Abstract

The structural and magnetic properties of magnetocaloric Mn(Co 1-x Ni x)Ge compounds have been studied. Two responses to the increase of valence electron concentration on substitution of Ni (3d 8 4s 2) for Co (3d 7 4s 2) in the orthorhombic phase (Pnma) are proposed: expansion of unit-cell volume and redistribution of valence electrons. We present experimental evidence for electronic redistribution associated with the competition between magnetism and bonding. This competition in turn leads to complex dependences of the reverse martensitic transformation temperature T M (orthorhombic to hexagonal (P6 3 /mmc)) and the magnetic structures on the Ni concentration. Magnetic transitions from ferromagnetic structures below x = 0.50 to noncollinear spiral antiferromagnetic structures above x = 0.55 at low temperature (e.g., 5 K) are induced by modification of the density of states at the Fermi surface due to the redistribution of valence electrons. T M is found to decrease initially with increasing Ni content and then increase. Both direct and inverse magnetocaloric effects are observed.

From Waste-Heat Recovery to Refrigeration: Compositional Tuning of Magnetocaloric Mn1+xSb

“…Since ∆S M is maximized near T C , the ability to tune T C , and thus the temperature at which a material is most effective, is valuable from an applications standpoint. Rare-earth free 4 compounds and those not containing toxic elements [5][6][7][8][9] are particularly appealing.…”

Effect of doping with rare-earth element La in MnCoGe system

“…元素化学替代: 元素替代法是最为常见、也是较容易 控制和调整相变温度的方法. 用Si [13] , Sn [14,15] , Al [16,17] , Fe [18,19] , Cr [20] , V [21] , Co [22] , Ti [23] , Cu [24][25][26] , Ni [27,28] , Zn [29] , Ga [30] , In [12] , Si [31] , Zr和Pd [32] 等元素替代过渡金属(Mn, Co)或主族元素(Ge)都被证明是有效的. Liu等人 [33] [34] 及Fe-Ga磁致伸缩材料 [35]…”

Switching the magnetostructural coupling in MnCoGe-based magnetocaloric materials

“…5(b). [20,41] reveal that the Co atoms tend to form covalent bonding with the nearest Ge atoms due to the relatively short Ge-Co interatomic distance (∼2.33 Å). The electron pairing due to the Co-Ge covalent bonding reduces the exchange splitting between the majority and minority 3d bands of the Co atoms, leading to small Co moment (see Table I).…”

“…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].…”