Magnetic Behavior of a-fe2o3, I. Origin of Weak Ferromagnetism and Magnetic Characteristics Masako IWATA (The Research Institute for Iron, Steel and Other Metals, Tohoku University, Katahiracho, Sendai) (Received April 19, 1965) Magnetic hysteresis loops and torque curves have been measured on natural single crystals of a-fe2o3 obtained from various localities and on synthetic polycrystalline specimens of a-fe2o3. The
weak ferromagnetism observed on Shimotokuyama and Ayumikotan natural crystals behaves as pre dicted by Dzyaloshinsky and Moriya, while Wagasennin and Elba natural crystals possess a different magnetic property. The difference seems to be related to the origin of the natural crystals. At room temperature, polycrystalline sintered specimens of pure a-fe2o3 have a hysteresis loop cha racterized by high coercivity and linearity of the first quadrant portion of its descending branch. This shape is attributed, to the nature of the weak ferromagnetism. The observed values of the coercive force and the rotational hysteresis loss of the polycrystalline specimens are interpreted in terms of a restrictive force due to exchange interaction operating across the grain boundary. Fig. 1 Schematic diagram of the pendulum magnetometer. (a) vertical-sectional view, (b) horizontal-sectional view at the elevation of the specimen, (c) block diagram of the detecting circuit, and (d) balancing coil circuit.
Table 1 Spectroscopic analysis of natural single crystal specimens of ƒ -Fe2O3. Notation used: + present in an appreciable quantity, (+) present in a small quantity, } trace, not detected. Fig. 2 Schematic diagram of the torque magnetometer. Table 2 Chemical analysis of synthetic polycrystalline specimens of a-fe2o2.
Fig. 5 Magnetic hysteresis loops of the same crystal as in Fig. 4 measured at -192 Ž. (1) along one direction in the (111) plane, and, (2) along the [111] axis. Fig. 4 Magnetic hysteresis loops of an a-fe2o3 single crystal from Shimotokuyama measured at room temperature. (1) along one direction in the (111) plane, and (2) along the [111] axis. Fig. 6 Magnetic hysteresis loops of an a-fez% single crystal from Wagasennin measured at room temperature. (1) along one direction im the (111) plane, and (2) along the [111] axis
Table 3 Magnetic properties of natural single crystals of a-fe2o3 from various localities. x specific susceptibility, cs specific saturation magnetization, ar specific remanant magne tization, and Hc coercive force. Fig. 7 Magnetic hysteresis loops of the same crystal as in Fig. 6 measured at -192 Ž. (1) along one direction in the (111) plane, and (2) along the [111] axis.
Fig. 8 Representative magnetic torque curves of an a-fe2o3 single crystal from Shimotoku yama, measured at room temperature in a magnetic field of 23.5kOe applied in the (111) plane. (1) and (2) for rotation of increasing and decreasing ƒó, respectively, and (3) the mean curve. Fig. 9 Field dependence of the torque ampli. tudes L2, L4 and the rotational hysteresis loss Q for an a-fe2o3 single crystal from Shimo tokuyama. Fig. 10 Field dependence of the torque ampli tude L2 and the rotational hysteresis loss Q for an a-fe2o3 single crystal from Wagasennin. Fig. 11 Hysteresis loops of the polycrystalline specimen (No. 1) of a-fe2o3. (1) at room temperature, and (2) at -192 Ž.
Fig. 14 Torque curves indicating the rotational hysteresis of the polycrystalline specimen (No. 1) of a-fe2o3, measured at room tem perature with an applied field of 6.80kOe. (1) and (2) for rotation of increasing and decreasing ƒó, respectively.
Fig. 17 Spin corifigiiration in the demagnetized state of int@i`acting afe2o3 crystallites A and B. Fig. 18 Angles indicating the direction of the spin axis and the weak ferromagnetic moment in calculating the spin configuration at the grain boundary.