Metamagnetism of Siderite (FeCO3)

A uniaxial antiferromagnet with high anisotropy should show a sharp transition to a ferromagnetic configuration in fields sufficient to overcome the antiferromagnetic exchange. Examples of this restricted type of metamagnetism are rare, but are here extended to ferrous carbonate in its natural mineral form, siderite. Magnetic susceptibility measurements on powder and oriented crystals at 10 kOe between 4.2° and 300°K confirm in detail the high anisotropy and indicate a Neel point at 38°K for this mineral. Pulsed field magnetization measurements to 200 kOe at 4.2°K exhibit a metamagnetic transition toward ferromagnetism starting near 100 kOe for both the powder and c‐axis oriented crystal. The transition in this crystal is nearly complete at 200 kOe where the magnetization approaches a saturation of about 6μB per iron ion. From an elementary statistical model of the broad transition, the critical field for pure FeCO3 is estimated at about 200 kOe. These data and other factors lead to a conclusion that the predominant exchange interaction in FeCO3 is antiferromagnetic.