LABORATORY-INDUCED MAGNETIZATION

Remanent magnetism resulting from short-term exposure to strong magnetizing fields at constant temperature is referred to as isothermal remanent magnetism (IRM). In the laboratory, IRM is imparted by exposure (usually at room temperature) to a magnetizing field generated by an electromagnet. IRM is the form of remanence produced in hysteresis experiments and is acquired by ferromagnetic grains with coercive force less than the applied field. The maximum remanence that can be produced is called the saturation isothermal remanent magnetization (SIRM). The field at which saturation is reached depends on the composition and microstructure of the specimen.

Coercivity of remanence is a very useful hysteresis parameter that can be used in determining magnetic mineralogy and grain size and in helping to characterize magnetic mixtures. It is the field that reduces the saturation isothermal remanence to zero.

Principle

An impulse magnetizer is designed to produce a short-duration high field pulse for the purpose of magnetizing geological samples. The field is produced by the discharge of energy from a capacitator bank through a coil surrounding the sample cavity. The capacitator bank is first charged to the desired voltage (corresponding to the desired field). It is then discharged through the coil very quickly to magnetize the sample.

The impulse magnetizer is ideally suited for the study of acquisition of IRM and the coercivity of remanence of discrete samples. Both are characteristic rock magnetic properties that are used for a preliminary magnetic carrier identification.

ASC Model IM-10 Impulse Magnetizer

The ASC IM-10 impulse magnetizer (Fig. F23) is capable of imparting a magnetization of up to 1.5 T. The desired charging voltage can be approximately adjusted with the voltage adjustment knob and is precisely displayed on the charging voltmeter. The charging voltages corresponding to specific field levels for the unit on board are given in "Appendix D." Caution: The magnetizer produces a strong magnetic field capable of remagnetizing material within 1 m of solenoid.

Operation of the ASC Model IM-10

1. Turn the "Power" switch on. No warm-up period is necessary.
2. Place the sample in the holder in the desired position. Insert the holder into the sample cavity until it hits the back of the cavity. Make certain the bottom of the holder sits in the groove in the bottom of the cavity.
3. Set the voltage adjustment knob so that the ascending voltage displayed on the meter approaches the desired charging voltage slowly. There is a lag time between adjusting the voltage knob and charging of the capacitors to a given voltage. The set voltage is approached asymptotically over a 30- to 60-s time period. The voltage buildup can be monitored via the digital meter. The most accurate and reproducible results will be obtained if the adjusting knob is set to a point slightly above the desired voltage, so that the desired voltage is approached at a rate of 0.5 V/s.
4. If you overshoot the desired voltage, push the trim button until the displayed voltage drops below the desired value. Allow the charging voltage to ascend to the desired value and trigger.
5. Measure the induced magnetization in the cryogenic magnetometer or with the spinner magnetometer.
6. At the end of each work session, turn the voltage adjustment knob fully counterclockwise and trigger the circuit before turning off the power.

Note: after being magnetized, samples with a high concentration of magnetic minerals may become too strong to be measured in the SRM. In this case a small rock chip can be used for IRM acquisition experiments or SIRM determinations.

Anhysteretic remanent magnetization is imparted to a sample by applying a direct magnetic field in the presence of a decaying AF. This direct magnetic field will increase the applied field intensity in one direction along the axis of demagnetization, orienting mobile domains in the direction of the applied direct field. The result is a remanent magnetization of the sample over the range of coercivities below the peak applied demagnetizing field. By applying ARM, then demagnetizing with a series of increasing intensities and measuring the difference in remanent magnetization (differentiating AF demagnetization of ARM), a profile of the magnetic domains in the sample can be obtained.

PARM is imparted to a sample by magnetizing, via a direct magnetic field, over a specified range of coercivities. The sample is exposed to a large, decreasing, alternating magnetic field, and when the demagnetization level reaches a specified value, a small, direct magnetic field is applied until the AF decays to a preset lower value. If this "window" of coercivities is passed over the entire range of demagnetization and the remanent magnetization is measured at each interval, the pARM can be used for determining the distribution of remanence coercivities.

PARM, like ARM, can be used to map the grain size and preferred orientation of grain boundaries within a sample. The strength of pARM imparted at different coercivities is related to grain size distribution (Jackson et al., 1988; diagram in Tarling and Hrouda, 1993). PARM is applicable to many samples whose grain size falls above the stable SD threshold. Because only a small number of domains are magnetized at any time, pARM may reduce errors caused by grain interactions.

As previously described ("DTECH Model D-2000 Demagnetizer" in "Alternating Field and Thermal Demagnetization"), a DTech 2000 AF demagnetizer (Fig. F22) is available in the shipboard laboratory for demagnetization of specimens up to 200 mT. The D-2000 can also be used to impart an ARM, in which a DC magnetic field is produced continuously across the AF demagnetizer coil, or a pARM, in which the user selects the demagnetization interval over which the field is applied. Direct fields within the range of 0.001 to 0.2 mT may be applied to samples. Both ARM and pARM experiments are useful for conducting rock magnetic studies.