Competing incommensurate orders in a heavy fermion antiferromagnet CeNiGe₂

We have determined the magnetic structure and phase diagram of a heavy fermion compound CeNiGe₂ as a function of magnetic field and temperature using single crystal neutron diffraction. At zero field and the base temperature of 0.4 K, CeNiGe₂ shows a complex magnetic structure comprising two propagation vectors κ₁ = (0.23, 0, 0) and κ₂ = (0.18, 0, 0). The intensities observed at several magnetic satellites indicate that the moments are predominantly along the c-axis. At higher temperatures the order parameter corresponding to κ₁ (OP1) decreases until it is suppressed at T_N1 = 3 K. The order parameter for κ₂ (OP2) initially increases with increasing temperature and only begins to decrease above T_N1 until it vanishes at T_N2 = 4.2 K. A similar but opposite trend in order parameters is observed when a magnetic field is applied along the c-axis. OP2 is suppressed at a lower critical field of B_c2 = 0.75 T, whereas OP1 first increases until it reaches maximum at B_c2 and begins to decrease and vanishes at the critical field of B_c1 = 2.75 T. Taken together, CeNiGe₂ provides an interesting platform where competition between two nearby incommensurate orders can be systematically tuned by temperature and magnetic field.