Quantum critical behavior of the hyperkagome magnet Mn3CoSi

?-Mn-type family alloys Mn3TX (T = Co, Rh, and Ir; X = Si and Ge) have a three-dimensional antiferromagnetic (AF) corner -shared triangular network, i.e., the hyperkagome lattice. The antiferromagnet Mn3RhSi shows magnetic short-range order over a wide temperature range of approximately 500 K above the Neel temperature TN of 190 K. In this family of compounds, as the lattice parameter decreases, the long-range magnetic ordering temperature decreases. Mn3CoSi has the smallest lattice parameter and the lowest TN in the family. The quantum critical point (QCP) from AF to the quantum paramagnetic state is expected near a cubic lattice parameter of 6.15 angstrom. Although the Neel temperature of Mn3CoSi is only 140 K, the emergence of the quantum critical behavior in Mn3CoSi is discussed. We study how the magnetic short-range order appears in Mn3CoSi by using neutron scattering, mu SR, and bulk characterization such as specific heat capacity. According to the results, the neutron scattering intensity of the magnetic short-range order in Mn3CoSi does not change much at low temperatures from that of Mn3RhSi, although the mu SR short-range order temperature of Mn3CoSi is largely suppressed to 240 K from that of Mn3RhSi. Correspondingly, the volume fraction of the magnetic short-range order regions, as shown by the initial asymmetry drop ratio of mu SR above TN, also becomes small. Instead, the electronic -specific heat coefficient gamma of Mn3CoSi is the largest in this Mn3T Si system, possibly due to the low -energy spin fluctuation near the quantum critical point.