Ferrimagnetism I

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Recall three types of magnetic properties of materials

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Diamagnetism

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Paramagnetism

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Ferromagnetism

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Anti-ferromagnetism Parasitic ferromagnetism Ferrimagnetism

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Ferrimagnetism

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Spinel structure is one of the common crystal structure of rock-forming minerals.

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Tetrahedral and octahedral sites form two sublattices. Fe²⁺ in 1/8 of tetrahedral sites, Fe³⁺ in 1/2 of octahedral sites.

Ferrimagnetism II

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xmujpkc.xmu.edu.cn/jghx/source/chapter9.pdf

Ferrimagnetism III

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www.tf.uni-kiel.de/matwis/amat/def_en/kap_2/basics/b2_1_6.html

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Anti-spinel structure of the most common iron oxides

Fe³⁺ in 1/8 tetrahedral sites, (Fe³⁺, Fe²⁺) in 1/2 of

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octahedral sites.

Ferrimagnetism IV

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Indirect exchange involves antiparallel and unequal magnetization of the sublattices, a net spontaneous magnetization appears. This phenomenon is called

ferrimagnetism.

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Ferrimagnetic materials are called ferrites. Ferrites exhibit magnetic hysteresis and retain remanent magnetization (i.e. behaves like ferromagnets.)

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Above the Curie temperature, becomes paramagnetic. Magnetite (Fe₃O₄), maghemite, pyrrhotite and goethite (' rust).

Magnetic properties of rocks I

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Matrix minerals are mainly silicates or carbonates, which are diamagnetic.

Secondary minerals (e.g., clays) have paramagnetic properties.

So, the bulk of constituent minerals have a magnetic susceptibility but not remanent magnetic properties.

Variable concentrations of ferrimagnetic and matrix minerals result in a wide range of susceptibilities in rocks.

Magnetic properties of rocks II

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The weak and variable concentration of ferrimagnetic minerals plays a key role in determining the magnetic properties of the rock.

Magnetic properties of rocks III

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Important factors influencing rock magnetism:

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The type of ferrimagnetic mineral.

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its grain size.

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the manner in which it acquires a remanent magnetization.

We’ll learn more about each of these.

Ferrimagnetic Minerals I

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The most important ferrimagnetic minerals: Fe-Ti oxides.

Ferrimagnetic Minerals II

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Titanomagnetite series

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Responsible for the magnetic properties of oceanic basalts.

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0.5 km-thick surface basaltic layer of the oceanic crust has very fine grained titanomagnetite or titanomaghemite

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Molecular fraction of ulvöspinel is about 0.6 in oceanic basalts.

Ferrimagnetic Minerals III

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Magnetite (Fe₃O₄)

Has a strong spontaneous magnetization (M_s = 4.8 × 10⁵

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• Am⁻¹
• .

Curie temperature of 578^◦C.

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Ferrimagnetic Minerals IV

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Susceptibility is the strongest of any naturally occurring mineral.

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Maghemite (γ−Fe₂O₃)

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Produced by low-temperature oxidation of magnetite. Likewise, titanomagnetite becomes titanomaghemite by low-T oxidation.

Has a strong spontaneous magnetization (M_s = 4.5 × 10⁵

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• Am⁻¹
• .

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Titanohematite series

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The Curie temperature and cell size shows the same trend with titanomagnetite as Ti content changes.

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Hematite (α−Fe₂O₃)

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Parasitic-ferromagnetism. Has a relatively weak M_s, 2.2 × 10³ Am⁻¹ Important for paleomagnetics because of abundance and stability.
.

Grain size I

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Magnetic relaxation, i.e., decrease of magnetization with time, occurs in ferrimagnetic materials.

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The relaxation is described as

• ꢀ
• ꢁ

t

τ

M_r (t) = M_r0 exp −

,

(1) (2)

and the relaxation time τ is given as

• ꢀ
• ꢁ

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ν₀

ν K_u κ T τ =

exp

,

where ν is the grain volume.

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So, the relaxation is slower in a bigger grain. Read Sec. 5.3.5 of Lowry (1997 or 2004) for details.

Remanent Magnetization I

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The small concentration of ferrimagnetic minerals in a rock has the ability to acquire a remanent magnetization (or just remanence).

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The untreated remanence of a rock is called it natural

remanent magnetization (NRM).

Remanence acuqired at known times in the rock’s history, such as rock formation and subsequent alteration, is geologically important.

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Primary magnetization: A remanence acquired at or

close to the time of formation of the rock. Secondary if acquired at a later time.

Thermoremanent magnetization of igneous rocks or depositional remanent magnetization of sedimentary rocks are primary.

Remanent Magnetization II

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Secondary remanence may be caused by chemical change of the rock during diagenesis or weathering or by sampling and lab procedure.

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Thermal remanent magnetization (TRM)

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What is a blocking temperature? Read Sec. 5.3.6.1.

Remanent Magnetization III

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Depositional and post-depositional remanent magnetization (DRM or pDRM)

Remanent Magnetization IV

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DRM

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Small ferrimagnetic mineral grains oriented like a compass needle.

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Declination (due to water current) and inclination (due to grain rolling) error

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pDRM

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Very fine grains suspended in pore space can be oriented along the external magnetic field.

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Occurs within the top ∼10 cm of sediments. Lock-in time delay of 100 to 10k yrs.

Remanent Magnetization V

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Chemical remanent magnetization (CRM)

Remanent Magnetization VI

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Isothermal Remanent Magnetism (IRM)

Remanent Magnetization VII

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Isothermal Remanent Magnetism (IRM) cont’d

Remanent Magnetization VIII

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Environmental magnetism Biogenic magnetite:

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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.

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Submicroscopic magnetites found in the brains of dolphins and birds.

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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.