Remanence

Of the ( magnetic ) remanence MR, and retentive or (back) called permanent magnetism, residual magnetism or remanence, is meant that magnetization that a previously by an external magnetic field H, for example with a current-carrying coil is saturated ( ie. maximum) magnetized particles, retains after removal of the external field. In charts that plot the magnetic flux density B through H, remains for H = 0 corresponding to the remanent flux density. Although less pronounced, magnetic remanence is all ferromagnetic materials and intrinsically desirable or undesirable depending on the application.

Applications

The magnetic remanence is the basis for all of the storage method on magnetism base such as hard disks, floppy disks, magnetic tapes (tape, compact cassette, MAZ, streamer ) in which the write or Aufsprechvorgang magnetic materials such as iron (III ) oxide or chromium ( IV) oxide crystals are magnetized on the surface of the disk and its remanence even after removal of the magnetic field - ie after the write or talk head has moved again - maintained. Magnetic data storage such as hard disks, magnetic tapes, magnetic cards are unthinkable without remanence, as well as the place in many application areas of everyday life, such as speakers or bicycle dynamo to permanent magnets.

Unwanted occurrence

Disturbingly, however, is magnetic remanence eg for switching of transformers, as they no longer can be optimally activated in the peak voltage without start as it tells the ( outdated ) textbook theory.

Is also undesirable magnetic remanence in cutting tools such as scissors, knives and cutting tools. The random contact of tools with permanent magnets or their mechanical overload can lead to which they are permanently magnetic so then chips or objects in them will stick.

Magnetic heads can show changed playback properties or change the recording in case of unwanted magnetization.

The mask color picture tubes leads, if it is magnetized, errors in the color rendition.

Deleting the period magnetism (tools, data media, magnetic heads, picture tubes ) can be achieved with decaying alternating fields.

Natural remanence

In the wild, there is remanent magnetized material in the form of rocks that contain ferrimagnetic minerals ( eg magnetite). Wherein the external magnetic field that is responsible for this remanent magnetization, it is usually about the earth's magnetic field.

The magnetization may be effected in various ways. In igneous rocks, all ferrimagnetic mineral particles align when the melt cools when it reaches its Curie temperature from following the earth's magnetic field, causing the rock is permanently magnetized. This process is called Thermoremanenz or thermal remanence.

In metamorphic or otherwise (eg, by chemical weathering or during diagenesis ) converted rocks the newly formed ferrimagnetic particles are grown by the earth's magnetic field aligned. This process is called Chemoremanenz or chemical remanence. A special form of Chemoremanenz is the biogenic Chemoremanenz that only affects sedimentary rocks: some anaerobic or microaerophilic bacteria ( different groups ) have iron-rich membrane-enclosed structures in the cytoplasm, the so-called magnetosome. The magnetosomes consist of small magnetite ( FeIII2FeIIO4 ) or rare Greigitkristallen ( FeIII2FeIIS4 ), which are surrounded by a double lipid layer. The Magnetosome contains up to one hundred and magnetite (size 0.1 microns ), which each act as small permanent magnets. These magnetite crystals are usually arranged in chains that add the effect of small permanent magnets and the bacterium as a compass needle in a magnetic field to align ( Magnetotaxis ). After the death of the bacteria, the chains of magnetically aligned Magnetikristallen are maintained and document the magnetic field at a particular time in Earth's history.

Also detrital ferrimagnetic particles in a sediment are regulated by the Earth's magnetic field. One then speaks of Sedimentationsremanenz. Here, find out coarse particles relatively early after deposit fixation in grain structure, while finer particles are still mobile in the water-saturated region and may optionally re-align with a change in the prevailing geomagnetic field. Only with increasing compaction of the sediment into a sedimentary rock, the finer particles and with them the information about the ruling at this time Earth's magnetic field are fixed. This delayed fixation is called Postsedimentationsremanenz.

The time period or, rather, in which the magnetization was carried out, can be many millions or hundreds of millions of years are back. Therefore, the corresponding rocks are pieces of information with respect to the earth's magnetic field in the geologic past ( paleo- geomagnetic field ). A well-known example of this is the normal and inverse thermoremanent magnetized parallel to the mid-ocean ridges extending strip of oceanic crust ( see → seafloor spreading, → magnetostratigraphy ).