High Pressure Synthesis and Magnetic Properties of Cubic B20 MnGe and CoGe

Abstract

Samples of MnGe and CoGe having cubic B20 structure were prepared at high pressure. Magnetic measurements showed that CoGe is a Pauli paramagnet, but MnGe exhibits antiferromagnetic-type ordering below TN = 175 K. The magnetic susceptibility of MnGe follows a Curie-Weiss law above 300 K with an effective magnetic moment µeff = 3.68 µB/f.u. and θp = + 231 K. The large positive value of θp indicates a strong ferromagnetic exchange. The value of the effective moment is close to the free Mn4+ ion (4F3/2) value 3.87 µB/f.u. From measurements of the magnetization of MnGe in fields up to 60 kOe and in the temperature range 2-350 K, we constructed its T-H magnetic phase diagram which is similar to that proposed for MnSi-type compounds with helical magnetic order. Specific heat measurements reveal that the total magnetic entropy of MnGe relative to nonmagnetic CoGe is equal to 3.55 J/mole-K (0.62Rln2).

Possible quadrupole-order-driven commensurate-incommensurate phase transition in B20 CoGe

“…1(b). While the data are consistent with the previous results reported in the literature [28,37], we detected a weak but very sharp peak at T Q = 13.7 K, as shown in the inset of Fig. 1(b), which has been overlooked so far.…”

“…In this paper, we carried out an experimental and theoretical investigation in B20 CoGe, one of the least studied B20 compounds partly due to the difficulty in synthesizing B20-type CoGe which requires the application of high pressure and temperature [35,36]. So far, B20 CoGe has been known as a Pauli paramagnet with a low carrier density without a phase transition [28,37,38], similar to the silicide counterpart CoSi, while being distinguished from other close germanides FeGe and MnGe that are helimagnets. In fact, the band crossings in the Γ and R points of the Brillouin zone in CoGe [28,38] bear remarkable resemblance to those in CoSi [22], suggesting that CoGe could host unconventional chiral fermions.…”

Two-step pressure-induced collapse of magnetic order in the MnGe chiral magnet

“…At 5.64 GPa the peak intensities and positions of both peaks decrease and the upper transition becomes ill defined. The transition at T C coincides with a saturation of the helix pitch (see the SM) and with irreversibilities of the magnetization depending on cooling conditions [21]. Both transitions are revealed by broad peak line shapes, suggesting crossover regions in the phase diagram.…”

Stress-induced magnetic textures and fluctuating chiral phase in MnGe chiral magnet

“…Comparison between the susceptibility curves in samples 1 and 2 shows that their differences are not related to the Fe doping. These curves also differ in two pure MnGe samples [21]. As for the anomaly at T c , it is strongly enhanced under pressure for a given sample [17].…”