Weak ferromagnetism and spin-glass state with nanosized nickel carbide

Magnetic properties of Ni(3)C nanoparticles, about 40 nm in diameter, have been investigated. The sample exhibits weak ferromagnetic (FM), spin-glass (SG), and paramagnetic (PM) properties by alternating current susceptibility and direct current magnetization measurements. The freezing temperature of the SG phase at zero applied field is determined as T(F)(0) similar to 11.0 K. At T > T(F)(0), a very weak ferromagnetism has been observed over a strong PM background. The Curie temperature T(C) is shown to exceed 300 K, and the ferromagnetism at 300 K is determined as about 0.02 emu/g similar to 6.7 x 10(-4) mu(B) per Ni(3)C formula unit. An anomalous dip appears in the temperature dependent coercivity H(C)(T) near the freezing temperature T(F)(0). It reflects the distortedly reduced coercivity in the M(H) hysteresis loop measured at T similar to T(F)(0) with the applied sweeping field approaching H = 0. This is attributable to the interaction between the SG and the weak FM phases. The possible origin of the magnetic moments that account for the observed FM, SG, and PM properties is discussed. (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3155997]