<p>Ferrimagnetism I </p><p>I</p><p>Recall three types of magnetic properties of materials </p><p>I</p><p>Diamagnetism </p><p>I</p><p>Paramagnetism </p><p>I</p><p>Ferromagnetism </p><p>III</p><p>Anti-ferromagnetism Parasitic ferromagnetism Ferrimagnetism </p><p>I</p><p>Ferrimagnetism </p><p>I</p><p>Spinel structure is one of the common crystal structure of rock-forming minerals. </p><p>II</p><p>Tetrahedral and octahedral sites form two sublattices. Fe<sup style="top: -0.3174em;">2+ </sup>in 1/8 of tetrahedral sites, Fe<sup style="top: -0.3174em;">3+ </sup>in 1/2 of octahedral sites. </p><p>Ferrimagnetism II </p><p>I</p><p><a href="xmujpkc.xmu.edu.cn/jghx/source/chapter9.pdf" target="_blank">xmujpkc.xmu.edu.cn/jghx/source/chapter9.pdf </a></p><p>Ferrimagnetism III </p><p>I</p><p><a href="/goto?url=http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_2/basics/b2_1_6.html" target="_blank">www.tf.uni-kiel.de/matwis/amat/def_en/kap_2/basics/b2_1_6.html </a></p><p>I</p><p>Anti-spinel structure of the most common iron oxides </p><p>Fe<sup style="top: -0.3174em;">3+ </sup>in 1/8 tetrahedral sites, (Fe<sup style="top: -0.3174em;">3+</sup>, Fe<sup style="top: -0.3174em;">2+</sup>) in 1/2 of </p><p>I</p><p>octahedral sites. </p><p>Ferrimagnetism IV </p><p>I</p><p>Indirect exchange involves antiparallel and unequal magnetization of the sublattices, a net spontaneous magnetization appears. This phenomenon is called </p><p><strong>ferrimagnetism</strong>. </p><p>II</p><p>Ferrimagnetic materials are called <strong>ferrites</strong>. Ferrites exhibit magnetic hysteresis and retain remanent magnetization (i.e. behaves like ferromagnets.) </p><p>II</p><p>Above the Curie temperature, becomes paramagnetic. Magnetite (Fe<sub style="top: 0.1282em;">3</sub>O<sub style="top: 0.1282em;">4</sub>), maghemite, pyrrhotite and goethite (' rust). </p><p>Magnetic properties of rocks I </p><p>IIII</p><p>Matrix minerals are mainly silicates or carbonates, which are diamagnetic. </p><p>Secondary minerals (e.g., clays) have paramagnetic properties. </p><p>So, the bulk of constituent minerals have a magnetic susceptibility but not remanent magnetic properties. </p><p>Variable concentrations of ferrimagnetic and matrix minerals result in a wide range of susceptibilities in rocks. </p><p>Magnetic properties of rocks II </p><p>II</p><p>The weak and variable concentration of ferrimagnetic minerals plays a key role in determining the magnetic properties of the rock. </p><p>Magnetic properties of rocks III </p><p>II</p><p>Important factors influencing rock magnetism: </p><p>I</p><p>The type of ferrimagnetic mineral. </p><p>I</p><p>its grain size. </p><p>I</p><p>the manner in which it acquires a remanent magnetization. </p><p>We’ll learn more about each of these. </p><p>Ferrimagnetic Minerals I </p><p>I</p><p>The most important ferrimagnetic minerals: Fe-Ti oxides. </p><p>Ferrimagnetic Minerals II </p><p>I</p><p>Titanomagnetite series </p><p>I</p><p>Responsible for the magnetic properties of oceanic basalts. </p><p>I</p><p>0.5 km-thick surface basaltic layer of the oceanic crust has very fine grained titanomagnetite or titanomaghemite </p><p>I</p><p>Molecular fraction of ulvöspinel is about 0.6 in oceanic basalts. </p><p>Ferrimagnetic Minerals III </p><p>II</p><p>Magnetite (Fe<sub style="top: 0.1281em;">3</sub>O<sub style="top: 0.1281em;">4</sub>) </p><p>Has a strong spontaneous magnetization (M<sub style="top: 0.083em;">s </sub>= 4.8 × 10<sup style="top: -0.3174em;">5 </sup></p><p>I</p><p></p><ul style="display: flex;"><li style="flex:1">Am<sup style="top: -0.3174em;">−1 </sup></li><li style="flex:1">.</li></ul><p>Curie temperature of 578<sup style="top: -0.3174em;">◦</sup>C. </p><p>I</p><p>Ferrimagnetic Minerals IV </p><p>I</p><p>Susceptibility is the strongest of any naturally occurring mineral. </p><p>I</p><p>Maghemite (γ−Fe<sub style="top: 0.1281em;">2</sub>O<sub style="top: 0.1281em;">3</sub>) </p><p>II</p><p>Produced by low-temperature oxidation of magnetite. Likewise, titanomagnetite becomes titanomaghemite by low-T oxidation. </p><p>Has a strong spontaneous magnetization (M<sub style="top: 0.083em;">s </sub>= 4.5 × 10<sup style="top: -0.3174em;">5 </sup></p><p>I</p><p></p><ul style="display: flex;"><li style="flex:1">Am<sup style="top: -0.3174em;">−1 </sup></li><li style="flex:1">.</li></ul><p></p><p>I</p><p>Titanohematite series </p><p>I</p><p>The Curie temperature and cell size shows the same trend with titanomagnetite as Ti content changes. </p><p>I</p><p>Hematite (α−Fe<sub style="top: 0.1281em;">2</sub>O<sub style="top: 0.1281em;">3</sub>) </p><p>III</p><p>Parasitic-ferromagnetism. Has a relatively weak M<sub style="top: 0.083em;">s</sub>, 2.2 × 10<sup style="top: -0.3174em;">3 </sup>Am<sup style="top: -0.3174em;">−1 </sup>Important for paleomagnetics because of abundance and stability. <br>.</p><p>Grain size I </p><p>I</p><p><strong>Magnetic relaxation</strong>, i.e., decrease of magnetization with time, occurs in ferrimagnetic materials. </p><p>I</p><p>The relaxation is described as </p><p></p><ul style="display: flex;"><li style="flex:1">ꢀ</li><li style="flex:1">ꢁ</li></ul><p></p><p>t</p><p>τ</p><p>M<sub style="top: 0.1364em;">r </sub>(t) = M<sub style="top: 0.1601em;">r0 </sub>exp − </p><p>,</p><p>(1) (2) </p><p>and the <strong>relaxation time </strong>τ is given as </p><p></p><ul style="display: flex;"><li style="flex:1">ꢀ</li><li style="flex:1">ꢁ</li></ul><p></p><p>1</p><p>ν<sub style="top: 0.1601em;">0 </sub></p><p>ν K<sub style="top: 0.1363em;">u </sub>κ T τ = </p><p>exp </p><p>,</p><p>where ν is the grain volume. </p><p>II</p><p>So, the relaxation is slower in a bigger grain. Read Sec. 5.3.5 of Lowry (1997 or 2004) for details. </p><p>Remanent Magnetization I </p><p>I</p><p>The small concentration of ferrimagnetic minerals in a rock has the ability to acquire a remanent magnetization (or just remanence). </p><p>II</p><p>The untreated remanence of a rock is called it <strong>natural </strong></p><p><strong>remanent magnetization </strong>(NRM). </p><p>Remanence acuqired at known times in the rock’s history, such as rock formation and subsequent alteration, is geologically important. </p><p>II</p><p><strong>Primary magnetization</strong>: A remanence acquired at or </p><p>close to the time of formation of the rock. <strong>Secondary </strong>if acquired at a later time. </p><p>Thermoremanent magnetization of igneous rocks or depositional remanent magnetization of sedimentary rocks are primary. </p><p>Remanent Magnetization II </p><p>I</p><p>Secondary remanence may be caused by chemical change of the rock during diagenesis or weathering or by sampling and lab procedure. </p><p>I</p><p>Thermal remanent magnetization (TRM) </p><p>I</p><p>What is a blocking temperature? Read Sec. 5.3.6.1. </p><p>Remanent Magnetization III </p><p>I</p><p>Depositional and post-depositional remanent magnetization (DRM or pDRM) </p><p>Remanent Magnetization IV </p><p>I</p><p>DRM </p><p>I</p><p>Small ferrimagnetic mineral grains oriented like a compass needle. </p><p>I</p><p>Declination (due to water current) and inclination (due to grain rolling) error </p><p>I</p><p>pDRM </p><p>I</p><p>Very fine grains suspended in pore space can be oriented along the external magnetic field. </p><p>II</p><p>Occurs within the top ∼10 cm of sediments. Lock-in time delay of 100 to 10k yrs. </p><p>Remanent Magnetization V </p><p>I</p><p>Chemical remanent magnetization (CRM) </p><p>Remanent Magnetization VI </p><p>I</p><p>Isothermal Remanent Magnetism (IRM) </p><p>Remanent Magnetization VII </p><p>I</p><p>Isothermal Remanent Magnetism (IRM) cont’d </p><p>Remanent Magnetization VIII </p><p>II</p><p>Environmental magnetism Biogenic magnetite: </p><p>I</p><p>Evolutionary feature: When sedimentary layers are disrupted, magnetotactic bacteria can follow the geomagnetic field lines back down to the sediments rich in nutrition they need. </p><p>I</p><p>Submicroscopic magnetites found in the brains of dolphins and birds. </p><p>I</p><p>Also nanometer-scale magnetites found in the human brain, which may be related to neurological disorders such as epilepsy, Alzheimer’s disease and Parkinson’s disease. </p>