Shock Demagnetization Research Articles

UXO Detection by Multiscale Potential Field Methods

The choice of an effective interpretative methodology in terms of management costs, timing, reliability, safety and efficiency is a key issue for the identification of unexploded ordnance (UXO). Among geophysical methods, magnetometry plays an important role in detection and interpretation of UXO objects. We present a study aimed at the identification and characterization of buried UXO in Rohoznik, Slovakia, by a multiscale analysis on high-resolution magnetic data. The area is located close to a former army shooting range and is characterized by several buried UXO. When a UXO hits the ground but does not explode, the impact causes a re-organization of the magnetic domains within the armature (shock demagnetization). Due to shock demagnetization the object gets a magnetization oriented in the direction of the actual Earth magnetic field. Furthermore, the elongated shape of the UXO of Rohoznik causes their anomalies to be oriented along the longitudinal axis of the projectiles. Our multiscale analysis was performed through methods such as depth from extreme points (DEXP) and the multi-ridge method. The upward continuation employed by these two methods allowed reducing the influence of the high-frequency noise related to non-UXO buried iron objects. Also, the use of high-order derivatives in the multiscale approach decreased interference effects related to close-by sources and avoided any pre-filtering. Our approach was first validated on a test site located close to Rohoznik, where unexplored projectiles were buried with different orientations at a known depth. Then we focused on the real data of the former army shooting range. For all our analyses we employed a structural index value of 2.5, which was estimated through the scaling function analysis and the derivative euler deconvolution algorithm. Both the (DEXP) and multi-ridge depth-estimation methods proved to be effective and rapid with respect to more expensive and time-consuming methods such as inversion. Our analysis revealed average depths for the analysed UXOs matching rather well with what was found by subsequent excavations, with a maximum discrepancy of a few decimetres.

Magnetic characterization of non-ideal single-domain monoclinic pyrrhotite and its demagnetization under hydrostatic pressure up to 2 GPa with implications for impact demagnetization

Here we present a comprehensive magnetic characterization of synthesized non-ideal single-domain (SD) monoclinic pyrrhotite (Fe7S8). The samples were in the form of a powder and a powder dispersed in epoxy. “Non-ideal” refers to a powder fraction of predominantly SD size with a minor contribution of small pseudo-single-domain grains; such non-ideal SD pyrrhotite was found to be a remanence carrier in several types of meteorites (carbonaceous chondrites, SNC…), which justifies the usage of synthetic compositions as analogous to natural samples. Data were collected from 5 to 633K and include low-field magnetic susceptibility (χ0), thermomagnetic curves, major hysteresis loops, back-field remanence demagnetization curves, first-order reversal curves (FORCs), alternating field and pressure demagnetization of saturation isothermal remanent magnetization (SIRM), low temperature data (such as zero-field-cooled and field-cooled remanence datasets together with room temperature SIRM cooling–warming cycles) as well as XRD and Mössbauer spectra. The characteristic Besnus transition is observed at ∼33K. FORC diagrams indicate interacting SD grains. The application of hydrostatic pressure up to 2GPa using nonmagnetic high-pressure cells resulted in the demagnetization of the sample by 32–38%. Repeated cycling from 1.8GPa to atmospheric pressure and back resulted in a total remanence decrease of 44% (after 3 cycles). Pressure demagnetization experiments have important implications for meteorite paleomagnetism and suggest that some published paleointensities of meteorites with non-ideal SD monoclinic pyrrhotite as remanence carrier may be lower limits because shock demagnetization was not accounted for.

Experimental measurements of shock induced changes to the magnetization of unexploded ordnance

Abstract Millions of acres of land around the world are potentially contaminated by unexploded ordnance (UXO). Magnetometry is a technique widely used to both detect and characterize buried UXO. It has been hypothesized that ordnance suffer a large shock on firing and impact that erases any preexisting remanent magnetization. If such demagnetization occurs, an apparent remanence metric has been shown to be effective at distinguishing hazardous ordnance from non-hazardous metallic debris. To test the shock demagnetization hypothesis, an experiment was conducted at a firing range to measure the magnetic remanence of sixty-five inert 81 mm mortars before firing and after impact. As delivered, 64 of the 65 rounds had very low remanent magnetization and a magnetizer had to be used to impose various amounts of remanence on the mortars. Three different categories of initial remanent magnetization were created (low, medium and high remanence) and these were fired at three different initial velocities. The mortars that initially had low remanent magnetization acquired a magnetization in the direction of the Earth's inducing field after impact, with the amount of re-magnetization decreasing with an increasing impact velocity. This effect is known as shock magnetization . The mortars with medium and high initial magnetization all lost some of their magnetic remanence, with the amount of demagnetization increasing with an increasing impact velocity. However, even at the highest impact velocity, shock demagnetization of initially highly magnetized mortars was insufficient to guarantee effective discrimination using apparent remanence.

Low-magnetic crust underlying South Province of Mars

The lack of distinct magnetic signatures observed by Mars Global Surveyor (MGS) over the impact craters and impact-related Quasi-Circular-Depressions (QCDs) with diameters greater than 200 km located on South Province, south of 30S and from almost the west of Hellas to Argyre basins, implies a weakly magnetized crust. Using MOLA topography and the recent JPL gravity model of Mars we determine the structure of the crust beneath the craters and impact-related QCDs, and show that the impacts that have created these features were capable of strongly disturbing the crust directly beneath. On the basis of theoretical magnetic anomaly modeling and shock demagnetization models, we demonstrate that the impacts are capable of demagnetizing the entire crust beneath and creating distinct magnetic anomalies at the satellite altitude of 400 km in case the crust was appreciably magnetized prior to the impacts. We derive the magnetic anomalies of these features using the radial component of the high-altitude nighttime MGS data. An upper limit of <2 × 10 4 A for the bulk magnetization of the crust beneath South Province is estimated, which is about 30 times less than that underlying Terra Cimmeria and Terra Sirenum. Similar weak bulk magnetization is obtained for part of the crust surrounding Hellas, Isidis, and Argyre basins.

Study of impact demagnetization at Mars using Monte Carlo modeling and multiple altitude data

The magnetic field signatures of large demagnetized impact basins on Mars offer a unique opportunity to study the magnetic properties of the crust and the processes of basin formation and impact shock demagnetization. We present a framework for determining the effects on such signatures due to the dominant direction, strength, thickness, and vertical and horizontal coherence wavelengths of the surrounding crustal magnetization, as well as the demagnetization radius and the width of the demagnetization gradient zone caused by impact shock. By comparing model results with observed magnetic field profiles at 185 km and 400 km over the five largest apparently demagnetized impact structures, we find that (1) the dominant lateral size of coherently magnetized regions of crust falls in the range ∼325 km to 600 km, (2) the magnetic field observed over a circular demagnetized region is such that clear demagnetization signatures should only be visible in magnetic field maps at 185 km and 400 km altitude for demagnetization diameters larger than ∼600 km and ∼1000 km, respectively, (3) demagnetization radii can be meaningfully constrained despite relatively poor constraints on associated demagnetization gradient zone widths, (4) the ratio of demagnetization diameter to the outer topographic ring diameter is close to 0.8 for the Isidis, Hellas, Argyre, and Utopia basins, suggesting that similar basin‐forming and shock demagnetization processes occurred in each of these four ancient impacts, and (5) if used in conjunction with impact simulations, such modeling may lead to improved constraints on peak pressure contours and impact energies for these basins.

Unraveling the simultaneous shock magnetization and demagnetization of rocks

In the natural case of a hypervelocity impact on a planetary or asteroidal surface, two competing phenomena occur: partial or complete shock demagnetization of pre-existing remanence and acquisition of shock remanent magnetization (SRM). In this paper, to better understand the effects of shock on the magnetic history of rocks, we simulate this natural case through laser shock experiments in controlled magnetic field. As previously shown, SRM is strictly proportional to the ambient field at the time of impact and parallel to the ambient field. Moreover, there is no directional or intensity heterogeneity of the SRM down to the scale of ∼0.2 mm 3. We also show that the intensity of SRM is independent of the initial remanence state of the rock. Shock demagnetization and magnetization appear to be distinct phenomena that do not necessarily affect identical populations of grains. As such, shock demagnetization is not a limiting case of shock magnetization in zero field. As a consequence, when it can be recognized in a rock, SRM must be considered as a reliable record of the direction and intensity of the ambient magnetic field at the time of impact. The natural process of hypervelocity impact where a rock carrying a remanent magnetization is shocked in the presence of an ambient field can be studied as the simple superimposition of shock demagnetization and shock magnetization. For this there are now a variety of techniques that allow experimental study of both phenomena separately or simultaneously as in this study. These results have potential implications for the paleomagnetic study of meteorites, and lunar rocks, and for the understanding of the magnetic signature (as studied through paleomagnetism and/or magnetic anomalies) of terrestrial, lunar and Martian impact craters.

Field Measurements of Induced and Remanent Moments of Unexploded Ordnance and Shrapnel

Magnetometry is a widely used technique for clearance of areas contaminated with unexploded ordnance. The discrimination of hazardous ordnance from nonhazardous items is possible using apparent remanence if ordnance is shock demagnetized on impact with the ground. We developed a mobile device, the Magnetic Remanence Interrogation Platform (MRIP), for measuring the induced and remanent moments of steel samples. The MRIP comprises six three-component fluxgate magnetometers symmetrically distributed around a rotating sample holder. Samples are placed on the holder and are slowly spun through two complete rotations. The measurement is repeated after the sample is physically rotated by 90 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> so that the previous up direction points east. The MRIP platform was deployed to two field sites. At the first site, 76- and 90-mm projectiles were measured. Most had relatively low remanent magnetization relative to the induced, suggesting that shock demagnetization occurs. Variations in the remanent moments of different specimens of 90-mm projectiles were attributed to variations in the type of steel used. There was a strong correlation of the direction of remanent and inducing field during burial, indicating evidence of shock magnetization and/or magnetic viscosity. At the second site, intact and partial 4.2-in mortars as well as shrapnel, base plates, and cultural debris were measured. Most of the base plates and intact mortars had low remanent magnetization, whereas the shrapnel and partial mortars tended to have higher remanent magnetization. The results indicate that there is an inherent risk in using apparent remanence for discrimination as not all ordnance is demagnetized on impact.

Paleomagnetism of Lonar impact crater, India

Lonar crater, India, is the best preserved terrestrial impact crater formed in basalt and is a unique terrestrial analogue for small, simple craters on terrestrial planets and the Moon. We investigated the paleomagnetic and rock-magnetic properties of the 1.88 km diameter crater in order to understand the effect of impacts on magnetization in target rocks. The magnetization in the Lonar basalts consists of an original 65 Ma Deccan magnetization and a recent overprint. We constrained the timing of magnetization acquisition at Lonar using a combination of conglomerate tests on ejecta deposit clasts and fold tests on the overturned and jumbled rim fold. In some areas, the recumbent rim fold is preserved and can be approximated as a horizontal cylindrical fold. In other areas, substantial vertical axis rotation may have occurred where tear zones developed during folding. We observed only subtle effects from the impact on the rock-magnetic properties of Lonar materials, which include a slightly elevated coercivity in shocked ejecta blocks. We show that paleomagnetism can provide a constraint on shock heating in the absence of petrographic evidence of shock (in this case, < 187 ± 15 °C). At Lonar, viscous (and/or chemical) remanent magnetization acquired in the < 50 kyr subsequent to crater formation has obscured any evidence of shock remanent magnetization. We also find no evidence of shock demagnetization or the presence of intense impact-induced or impact-amplified transient magnetic fields that have been proposed around larger impact structures.

The effects of explosive-driven shocks on the natural remanent magnetization and the magnetic properties of rocks

The effects of shock waves on the natural remanent magnetization and the intrinsic magnetic properties of geological materials remain poorly known. Still, hypervelocity impact phenomena are of primary importance in the evolution of many extraterrestrial bodies and of Earth. We present new experiments in which four rocks with different lithology and magnetic mineralogy (titanomagnetite, magnetite, monoclinic pyrrhotite, titanohematite) were shocked using a high-order explosive (penthrite) that provided maximum pressure of about 30 GPa. The shock wave was modelled numerically and we studied the effects on the natural remanent magnetization as well as on the intrinsic magnetic properties of the shocked rocks as a function of the distance to the explosion. The intrinsic magnetic properties of the rock are permanently modified by the shock wave. The shock wave was able to superimpose a new fabric (with a minimum susceptibility axis parallel to the direction of shock) to the original magnetic fabric of the rock. Magnetite-, titanomagnetite- and pyrrhotite-bearing rocks show a noticeable increase of coercivity for pressure above 10 GPa. These changes are attributed to fracturing and/or dislocations of the ferromagnetic grains. These results show that the magnetic properties of meteorites, which are commonly shocked to pressures well above 10 GPa (e.g. Martian meteorites), may not be representative of the magnetic properties of their parent body. Natural remanent magnetization appears to be much more resistant to shock than isothermal remanent magnetization, probably because it is carried by grains with higher coercivity. For titanomagnetite-bearing rocks, we observe both a partial shock demagnetization of the original thermoremanent magnetization and the appearance of a shock magnetization. The demagnetizing effect of the shock wave depends closely on the coercivity spectrum of the grains carrying the original remanence, and the shock-remagnetizing effect depends on the presence of low-coercivity magnetic grains. With an impact occurring in an ambient magnetic field of similar intensity to the original magnetizing field, the post-shock magnetization may be higher or lower than the pre-shock magnetization depending on these two factors. The magnetic anomalies observed above impact basins are often used as a proxy to the presence or absence of an active dynamo at the time of impact. Our results show that the magnetic anomalies associated to shocked rocks can be misleading, so that the only decisive clue to the presence or absence of a dynamo at the time of impact is the possible thermoremanence carried by the volume of rock heated above blocking temperatures during the impact.

Effect of shock on the magnetic properties of pyrrhotite, the Martian crust, and meteorites

We performed planar shock recovery experiments on natural pyrrhotite at pressures up to 6.9 GPa. We find that high‐field isothermal remanent magnetization in pyrrhotite is demagnetized up to 90% by shock due to preferential removal of low coercivity components of magnetization. Contrary to static experiments, we do not observe complete demagnetization. Post shock permanent changes in magnetic properties include increasing saturation isothermal remanent magnetization, bulk coercivity and low‐temperature memory, and changes in squareness of hysteresis. These changes are consistent with an increase in the volume fraction of single domain grains. The lack of magnetic anomalies over large Martian impact basins is not expected to be solely due to shock demagnetization of the crust. We find that pyrrhotite‐bearing rocks and meteorites can retain records of Martian magnetic fields even if shocked to pressures approaching 7 GPa. However, some paleointensity techniques may underestimate this field.

Magnetic fields of lunar multi‐ring impact basins

Abstract— We survey the magnetic fields of lunar multi‐ring impact basins using data from the electron reflectometer instrument on the Lunar Prospector spacecraft. As for smaller lunar craters, the primary signature is a magnetic low that extends to ˜1.5–2 basin radii, suggesting shock demagnetization of relatively soft crustal magnetization. A secondary signature, as for large terrestrial basins, is the presence of central magnetic anomalies, which may be due to thermal remanence in impact melt rocks and/or shock remanence in the central uplift. The radial extent of the anomalies may argue for the former possibility, but the latter or a combination of the two are also possible. Central anomaly fields are absent for the oldest pre‐Nectarian basins, increase to a peak in early Nectarian basins, and decrease to a low level for Imbrian basins. If basin‐associated anomalies provide a good indication of ambient magnetic fields when the basins formed, this suggests the existence of a “magnetic era” (possibly due to a lunar core dynamo) similar to that implied by paleointensity results from returned lunar samples. However, the central basin anomalies suggest that the fields peaked in early Nectarian times and were low in Imbrian times, while samples provide evidence for high fields in Nectarian and early Imbrian times.

Distribution of crustal magnetic fields on Mars: Shock effects of basin‐forming impacts

Crustal magnetic fields on Mars are inhomogeneously distributed with the strongest fields occurring over the southern highlands in a longitude sector between approximately 130°E and 240°E. Using analytic approximations and empirical scaling laws, it is estimated that much of the weakly magnetized southern highlands (i.e., that between 110°W and 130°E) was shocked to pressures exceeding 1–2 GPa during the Hellas and Argyre impacts. Possible primary remanence carriers in the martian crust include iron oxides and iron sulfides (pyrrhotite). If pyrrhotite is the main remanence carrier, extensive demagnetization of crustal regions (∼90%) may occur at shock pressures of 2 GPa or more. Thus, at least for this remanence carrier, impact shock demagnetization can potentially explain the distribution of crustal fields in the southern highlands.

Demagnetization signatures of lunar impact craters

We investigate the crustal magnetic signatures of lunar craters using Lunar Prospector (LP) electron reflectometer data. Craters of all ages often have associated magnetic lows, showing that crustal fields were present even in pre‐Nectarian times (≳3.9 Ga). Magnetic lows extend to ∼2–4 crater radii, suggesting shock rather than thermal demagnetization. Younger craters are more likely to have clear and complete demagnetization signatures, suggesting that many older magnetic lows have been subsequently obscured. No size dependence is found for craters larger than 50 km in diameter, suggesting that demagnetization effects for all craters in this size range completely penetrate the magnetized layer. If shock demagnetization is responsible, this suggests an upper limit of ∼50 km for the depth of magnetization. Evidence of edge effects due to magnetization contrasts may show that strong far‐side crustal fields are coherent at scales of ∼25 km.

Selection of the magnetizing device and type of magnetic tape for magnetographic inspection of welded joints in large structures: 37153 Kulikov, V.P.; Belyagov, A.M.; Shalamovskaya, N.I. Soviet Journal of Nondestructive Testing, Vol. 22, No. 10, pp. 676–681 (Jun. 1987)

The effects of shock waves on the natural remanent magnetization and the intrinsic magnetic properties of geological materials remain poorly known. Still, hypervelocity impact phenomena are of primary importance in the evolution of many extraterrestrial bodies and of Earth. We present new experiments in which four rocks with different lithology and magnetic mineralogy (titanomagnetite, magnetite, monoclinic pyrrhotite, titanohematite) were shocked using a high-order explosive (penthrite) that provided maximum pressure of about 30 GPa. The shock wave was modelled numerically and we studied the effects on the natural remanent magnetization as well as on the intrinsic magnetic properties of the shocked rocks as a function of the distance to the explosion.The intrinsic magnetic properties of the rock are permanently modified by the shock wave. The shock wave was able to superimpose a new fabric (with a minimum susceptibility axis parallel to the direction of shock) to the original magnetic fabric of the rock. Magnetite-, titanomagnetite- and pyrrhotite-bearing rocks show a noticeable increase of coercivity for pressure above 10 GPa. These changes are attributed to fracturing and/or dislocations of the ferromagnetic grains. These results show that the magnetic properties of meteorites, which are commonly shocked to pressures well above 10 GPa (e.g. Martian meteorites), may not be representative of the magnetic properties of their parent body.Natural remanent magnetization appears to be much more resistant to shock than isothermal remanent magnetization, probably because it is carried by grains with higher coercivity. For titanomagnetite-bearing rocks, we observe both a partial shock demagnetization of the original thermoremanent magnetization and the appearance of a shock magnetization. The demagnetizing effect of the shock wave depends closely on the coercivity spectrum of the grains carrying the original remanence, and the shock-remagnetizing effect depends on the presence of low-coercivity magnetic grains. With an impact occurring in an ambient magnetic field of similar intensity to the original magnetizing field, the post-shock magnetization may be higher or lower than the pre-shock magnetization depending on these two factors.The magnetic anomalies observed above impact basins are often used as a proxy to the presence or absence of an active dynamo at the time of impact. Our results show that the magnetic anomalies associated to shocked rocks can be misleading, so that the only decisive clue to the presence or absence of a dynamo at the time of impact is the possible thermoremanence carried by the volume of rock heated above blocking temperatures during the impact.

Magnetic effects during shock loading of magnetized ferro- and ferrimagnets

Data in the literature about the singularities of phase transitions, as well as of magnetoelastic effects accompanying the shock compression of a magnetized substance is analyzed. This data is of interest for the development of the kinetics of the phase transformations under consideration during shock loading of the substance. The magnetic effect for a phase transition of the first kind, and in the second kind are studied. The shock demagnetization of ferrites is also studied.

The role of impact magnetization in the solar system

In order to assess the role of impact magnetization on extra-terrestrial materials, shock magnetization laboratory experiments, and paleomagnetic studies of terrestrial impact sites, are reviewed. Both impact generated and impact modified materials are found to faithfully record the direction of the ambient field at the time of the shock event. Shock demagnetization and the acquisition of thermal remanence by cooling ejecta appear to be the primary impact-related mechanisms affecting the magnetization of lunar samples. The varied magnetization levels of shock metamorphosed meteorites probably reflect the diverse magnetic environments in which they evolved.

Electrical response of a bimetallic junction to shock compression

The electrical response of a copper-Constantan junction to shock compression has been studied over a pressure range 14.5–36.0 GPa. Four possible sources of anomalous response were found: electrical noise due to circuit closure at impact, shock demagnetization of a ferromagnetic material, high local temperature at the junction interface due to shock compression of a surface damage layer, and two-dimensional flow in the pressurized region due to pressure relief from the edges. Using a diffusion-welded junction in uniaxial strain the emf measured is within 20% of the predicted emf based on normal thermocouple response to shock-compression temperatures. In a geometry in which a guard ring was used to prevent lateral flow behind the shock front, the observed voltage-time profile was a step with constant plateau. In a similar configuration where radial pressure relief was allowed behind the shock front, the initial step (which compares directly in amplitude to the above results) is followed by a marked positive ramping. This ramping was directly correlated with the two-dimensional flow due to radial pressure relief. The results indicate that any fundamental anomaly which exists for one-dimensional strain is less than 10–20% of the observed signals in the pressure range studied.

The effect of shock on the magnetism of terrestrial rocks

Experimental projectile impacts into solid intrusive rocks confirm that shock pressures on the order of tens of kilobars are sufficient to affect the natural remanent magnetism (NRM) of terrestrial materials. The principal effect is to demagnetize the preexisting remanence. A secondary effect is the acquisition of an additional component of remanence whose direction is related to the ambient field direction at the time of impact. Although longer and more intense shock pulses may affect the higher‐coercivity fractions, the low‐coercivity fraction is the most susceptible to magnetic resetting by shock. Thus igneous rocks with abundant low‐coercivity carriers (low remanent coercive force, HRC) are more disposed toward magnetic recording of low‐pressure shock events. Evidence is presented that these low‐HRC rocks are also apparently capable of recording the remanent field of a magnetized projectile at the time of impact. Analysis of the NRM of samples from a road cut in the Palisades Sill, New Jersey, demonstrated that the explosions associated with road cutting are sufficient to reset partially the magnetization in the direction of the present field. This result illustrates possible hazards in the interpretation of paleomagnetism for such samples. The variability of remanence in the Deccan basalts from the Lonar Crater, India, can be explained in terms of their observed differing response to shock, as a function of HRC. However, the lack of oriented samples precludes a conclusive test that the secondary component observed in nature is a shock remanent magnetization. The magnetism of Lonar microbreccias seems to be related to the temperatures they reached during the impact event. Despite widespread remagnetization of the Kaibab formation since the impact event, certain oriented dolomite samples from Meteor Crater, Arizona, retain evidence of partial magnetic resetting at the time of the crater‐forming event, which may well be a shock remanence. The Coconino sandstone from Meteor Crater shows changes in remanence and hysteresis properties with increasing degrees of shock metamorphism. An important application of the effects of shock upon magnetization lies in the interpretation of lunar magnetism, since all lunar samples were collected from an impact‐generated regolith. The ‘hard’ NRM curves of many mare basalts may in part be due to preferential shock demagnetization of ‘soft’ NRM. In the soil breccias the observed NRM is likely to be directly associated with the lithification process. The mechanism of shock‐associated magnetization is not well understood. In the low‐shock range of a few kilobars it appears to be similar to static‐stress‐related magnetization. In extreme shock of hundreds of kilobars the transient and residual temperatures are high enough so that the acquired remanence is transitional to thermoremanent magnetization.

Review of effects of shock (&lt;60 kbar; &lt;6x109 Pa) on magnetism of lunar samples

Experimental shock studies of highland and mare soils in the range of a few to 50 kbar (5 x 109 Pa) have given the following results: (1) Shock, if less than 20 kbar, does not change the magnetic characteristics of the soil substantially and only weak and unstable shock remanence is generated in a field of 0.5 Oe. (2) Shock of between 20 and 50 kbar lithifies the soil and gives rise to stable shock remanence. Acquisition is approximately linear in field for a given shock level. At 30 kbar the acquisition parameter for the highland soil was 10-5 G cm3 g-1 Oe-1. In this range of 20-50 kbar the products of shock are petrologically and magnetically similar to certain regolith breccias. (3) Shock demagnetization preferentially demagnetizes the softer part of thermoremanent magnetization (t.r.m.) and hence makes it relatively harder. The significance of these results is that shock remanence is likely to be the cause of the natural remanent magnetization (n.r.m.) of certain regolith breccias and shock may modify the primary remanence of other samples.

Shock-Induced Anisotropy in Ferromagnetic Material. II. Polycrystalline Behavior and Experimental Results for YIG

The theory of shock-induced demagnetization produced by strain-induced magnetic anisotropy is considered in cubic polycrystalline ferromagnetic material. Analysis of the averaging procedure required to predict the polycrystalline behavior reveals the importance of magnetic grain interaction. Magnetization curves for extreme assumptions of interacting grains and independent grains are determined. Experimental shock demagnetization data are obtained for polycrystalline yttrium iron garnet in the region of large elastic strain (approximately ⅓ and ⅔ of the Hugoniot elastic limit). Results support the independent grain assumption.