Isothermal and anhysteretic remanent magnetization

Isothermal remanent magnetization (IRM) can be measured after exposure to a field at ambient temperature. Depending on the magnetic coercivity of the mineral and on the artificial field intensity, all magnetic moments will be magnetized in alignment with the external field. The induced magnetization does not change when increasing the applied field. This is the saturation isothermal remanence (SIRM). The anhysteretic remanent magnetization (ARM) is a stable magnetization used as a proxy for magnetite concentration. The ARM measurement involves magnetizing a sample using a low field in the presence of an alternating magnetic field that is smoothly reduced to zero.

The ARM/SIRM ratio can be analyzed as an indicator of magnetite granulometry. Higher ratio values indicate a decrease in grain size, as opposed to low values indicating an increase in the size of the magnetic population¹⁰. For the ARM acquisition, samples were submitted to an AF demagnetization protocol, superimposed on a constant field of 50 μT. The following field values were used: 0, 25, 30, 40, 50, 60, 70, 80, 90, 100 mT. IRM acquisition was performed, inducing magnetization in the following steps: 1000 mT and backfield 300, and 100 mT. Measurements were performed in a cryogenic magnetometer SQUID type (Superconducting Quantum Interference Device), model 755U (2G-Enterprises), and the induction by a pulse magnetizer model MMPM10 (Magnetic Measurements Ltd.), located in a shielded room at the USPMag laboratory.