Magnetic Properties of GdCoAsO

The magnetic properties of GdCoAsO have been investigated as a function of temperature and magnetic field. Complicated magnetism consists of antiferromagnetic, ferromagnetic, and paramagnetic has been observed. Most interestingly, magnetization measurements performed under low field, exhibit negative magnetization. Ferrimagnetic transition occurs at 137 K, which results from antiferromagnetic exchange interaction between Gd and Co magnetic moments, and then followed by transition to antiferromagnetic state. There exist three antiferromagnetic transitions at 12 K, 43 K, and 75 K, respectively. Antiferromagnetic transition at 75 K is independent of the magnetic field and at 43 K decreases with increasing field. Antiferromagnetic transition at 12 K corresponds to the antiferromagnetic ordering of Gd. A metamagnetic transition between two antiferromagnetic orders is observed just below 75 K. With increasing temperature, the metamagnetic transition field decreases. The magnetic phase diagram has been established.