Magnetic ordering in bulk MnSi crystals with chemically induced negative pressure

MnSi crystals with chemically induced negative pressure (doped by less than 1% Ge) have been synthesized by the Czochralski method. X-ray powder diffraction has revealed that the samples are crystallized in the B20 structure, inherent to pure MnSi, without any impurity phases. The lattice constant a is slightly larger than that of undoped MnSi. The samples have a spiral spin structure with the wave vector |k|=0.385 nm^-1 at low temperatures. The ordering temperature is enhanced up to T_C=39 K. The critical field H_C2 shows an increase of about 25% for the doped samples. Close to the critical temperature the A phase occurs. The temperature range of the A phase in the (H-T) phase diagram for the doped compound ranges from T_A=27.5 K, characteristic for pure MnSi, to T_C=39 K in the zero-field cooled (ZFC) regime of magnetization. The magnetic features of the (H-T) phase diagram of the compounds MnSi are reminiscent of those observed for the MnSi thin films on the Si substrate.