Hydrostatic pressure tuned magneto-structural transition and occurrence of pressure induced exchange bias effect in Mn_0.85Fe_0.15NiGe alloy

Abstract

Magnetic and magneto-functional behavior of a Fe-doped MnNiGe alloy with nominal composition Mn0.85Fe0.15NiGe have been investigated in ambient as well as in high pressure condition. The alloy undergoes first order martensitic phase transition (MPT) around 200 K and also shows large conventional magnetocaloric effect (MCE) (∆S ∼ -21 J/kg-K for magnetic field (H) changing from 0-50 kOe) around the transition in ambient condition. Application of external hydrostatic pressure (P ) results a shift in MPT towards the lower temperature and a clear decrease in the saturation moment of the alloy at 5 K. The peak value of MCE is also found to decrease with increasing external P (∼ 18 J/kg-K decrease in ∆S has been observed for P = 12.5 kbar). The most interesting observation is the occurance of exchange bias effect (EBE) on application of external P . The competing ferromagnetic and antiferromagnetic interaction in presence of external P plays the pivotal role towards the observation of P induced EBE.

Martensitic transition assisted modification of the antiferromagnetic ordering in Ge-site doped MnNiGe alloys

“…As is evident from previous studies, transition temperatures in MnNiGe are highly sensitive to the compositional elements and their molar percentage [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Partial substitution of different sites of the parent compound with a suitable foreign element corresponds to a significant change in both structural and magnetic transition temperatures [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Interestingly, the alloy retains the AFM ordering after doping at the Mn/Ni site, as confirmed by the magnetic and neutron powder diffraction (NPD) analysis [9,10,[15][16][17][18][19][24][25][26][27][28].…”

“…Partial substitution of different sites of the parent compound with a suitable foreign element corresponds to a significant change in both structural and magnetic transition temperatures [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Interestingly, the alloy retains the AFM ordering after doping at the Mn/Ni site, as confirmed by the magnetic and neutron powder diffraction (NPD) analysis [9,10,[15][16][17][18][19][24][25][26][27][28]. On the other hand, the effect of doping at the Ge site is slightly different than Mn/Nisite doping.…”

“…Among the various members of the magnetic equiatomic family (general formula MM X , where M and M are transition metals and X is a nonmagnetic sp element), MnNiGe is widely accepted as a promising candidate for practical applications as it possess multiple functional properties, including the shape memory effect, a large magnetocaloric effect, magnetoresistance, the exchange bias effect, etc. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. In addition, the undoped MnNiGe alloy undergoes multiple phase transformations (structural and magnetic depending on the thermal state of the material), namely, (i) MPT around 470 K from the high-temperature Ni 2 In-type hexagonal austenite phase (space group P6 3 /mmc) to the TiNiSi-type orthorhombic martensite phase (space group Pnma) and (ii) a paramagnetic (PM) to spiral antiferromagnetic (AFM) transition below or around 346 K [8][9][10]16,[24][25][26][27][28].…”

Doping site induced alteration of incommensurate antiferromagnetic ordering in Fe-doped MnNiGe alloys

“…The magnetic equiatomic alloys (MEAs) with general formula MM X (where M and M are transition metals, and X is a nonmagnetic sp element) and their derivatives, identified as the new class of shape memory alloy, have gained significant interest among researchers not only for the shape memory property, but also for their diverse magnetofunctionality, which includes a sizable magnetocaloric effect, magnetoresistance, the exchange bias effect, and so on [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. MnNiGe is one of the prime members of the MEA family that experiences a martensitic-type structural transition around 470 K from high-temperature (T ) hexagonal austenite phase (Ni 2 In-type structure with space group P6 3 /mmc) to low-T orthorhombic martensite phase (TiNiSi-type structure with space group Pnma) and ordered antiferromagnetically below 346 K [1,3,4,8,[20][21][22][23][24].…”

“…In the magnetically ordered state at 80 K, the Mn-site moment * souvik@alpha.iuc.res.in; souvik@csr.res.in size is around 2.75 μ B [20]. To achieve enhanced functionality, researchers adopted various doping strategies to tune these structural and magnetic transition temperatures, including (i) doping of foreign elements at different sites of the alloy and (ii) self doping [1,[3][4][5]7,8,[13][14][15][16][17][18][19][25][26][27]. As is evident from the recent studies, Mn/Ni site doped alloys retain the antiferromagnetic (AFM) ordering of the low-T martensite phase.…”