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Hard or soft - ferromagnetic materials

Saturation, coercivity and remanence

At the previous chapters we have seen, that the magnetization of materials depends on their design. Besides the atoms, the material consists of, we recognized the influence of the arrangement of the magnetic field to the crystal structure. In which way alter different magnetic properties the look of the hysteresis loop?.
We got to know the magnetic saturation, the coercivity and the remanence as the three basic values of the hysteresis loop. Decisive for the value of the flux density at the point of magnetic saturation is the strength and the concentration of the microscopic magnets inside the material. The more microscopic magnets are located in a volume and the stronger those magnets are, the higher the addition of their magnetic fields will become as soon as all of them point into the same direction, which happens at the point of magnetic saturation.
In addition to the strength and the concentration of the microscopic magnets, the remanence is dependent on the arrangement of the external magnetic field and the axis of the crystal structure. Essential is the fact, how many spins remain in parallel or in a small angle to the (now switched off) magnetic field. A high number of microscopic magnets keep their position while the external field was arranged in parallel to a axis of the crystal structure. If the single magnets are very strong, the remanence of the material will be very high. If the axes of the single grains, the material consists of, are arranged in a preferred direction (ideally it consists of a single crystal), the value of the remanence is highly dependent on the angle between external magnetic field and the crystal axis.
The coercivity depends significantly on the structure of the material. The harder it is to turn a magnet out of it's position, the higher the opposing magnetic field has to become, to cause exactly this. Like explained some times before, the angle between the field lines of the external field and the axis of the crystal lattice are an important criterion for the torque acting on the microscopic magnets. It is essential for the coercivity as well as for the value of the remanence, if the material consists of many grains or of just a single crystal. In contrast to the remanence, the coercivity is very low if the material consists out of one or just a few grains! Inside a single crystal, a turning magnet carries with it's neighbors and like a falling domino it can trigger a whole avalanche. A chain reaction like this is stopped at boundary areas of a grain, acting like barriers. If the material consists out of many small grains, the chain reaction is limited to the single grain. Defects (small grains of foreign material) inside the crystal structure act like barriers, too. Carbon inside steel alloys acts like such barriers. Materials with high values for the coercive field strength are called magnetically hard, those with low values magnetically soft.

Curve progression of different ferromagnetic materials

Hysteresis loops of three different ferromagnetic materials
Figure 1:
Hysteresis loops of three different ferromagnetic materials with nearly identical values for the magnetic saturation:
Let's take the already discussed blue curve as the standard material. We can see that the values of remanence and coercivity of the green loop are higher. The magnetization of this material is higher as soon as the external field is switched off and a stronger opposing field is needed to degauss the material. This material is magnetically harder.
Remanence and coercivity are significantly lower at the red loop, the material is magnetically softer. In addition, the gradient of the curve is significantly lower at the points representing the coercive magnetic field strength (intersections with the x-axis). The number of microscopic magnets, changing their alignment while the field strength is altered by a value of Δx around the coercive field strength, is lower than it is at the two other materials. At the blue and green loop, many magnets alter their alignment snowballing at the points of coercive field strength.
Energy is needed to turn microscopic magnets out of their resting position. To initialize those processes, an external magnetic field has to be created. The energy used to run the hysteresis loop once, meaning to change the magnetization twice, can be seen a the size of the inside area of the loops. More energy is needed from red to blue and finally the green loop, even though the field strength at the point of magnetic saturation doesn't differ for the three materials.

What kind of material is the best for a special purpose?

Ferromagnetic materials are used at different utilizations. Permanent magnets are made of ferromagnetic materials with a high value for the remanence. After removing the external magnetic field, many spins remain in their position resulting in a magnetization of the material. One example for a permanent magnet is the iron nail, being magnetized by a permanent magnet. Admittedly this nail isn't a very good permanent magnet itself. As soon as we move the south pole of a permanent magnet close to the south pole of the magnetized nail, it is not repelled by the magnet, but the polarity of the nail changes, meaning it's microscopic magnets start tilting. The south pole of the nail becomes the north pole and the nail is attracted by the magnet once again. Permanent magnets should be made out of material with an other important property: a high value for the coercive field strength. High coercivity means the microscopic magnets don't start turning around while being exposed to low field strength. The magnetized iron nail is factually repelled by the permanent magnet as long as it is far away from the magnet's pole and the accordant force between both objects is veeeery small. As soon as the distance is reduced, the coercive field strength of iron is reached, the polarization changes and the nail is attracted by the permanent magnet. The coercive field strength of permanent magnets should always be higher than the field strength of external fields it is exposed to. According to this, the microscopic magnets of the material remain in their position and two permanent magnets are repelled while approached with their south poles pointing to each other. Like mentioned before, defects and impurities inhibit the snowballing like tilting of many spins and therefore increase the coercivity. In contrast to soft iron, many inclusions of carbon are inside hardened iron (=steel). The coercivity of steel is therefore much higher than those of normal, soft iron. Steel was a formerly used material to produce permanent magnets and the terminology "magnetically hard" is based on the hardened steel in contrast to the "magnetically soft" properties of soft iron. Today Samarium-Cobalt or Neodymium-Iron-Boron alloys are used to form permanent magnets. The base material is a powder consisting of the designated alloy. The powder is compression-mold while being exposed to a strong external magnetic field. Thereby the atomic crystals of the single grains get arranged in parallel to the magnetic field, leading to a high remanence. The many grains, the material consists of, inhibit the snowball effect of tilting spins and lead to a high coercivity.
Electromagnets consist of a wire wound on a core of ferromagnetic material. If a current is passing the wire, a magnetic field is generated and the microscopic magnets of the core material start pointing in the direction of the field. The core material concentrates the field lines leading to a high gradient of the magnetic field at the ends of the coil. If the current through the wire is turned off, the magnetization of the core material should be as low as possible. The remanence of the core material should be low, making an electromagnet become a switchable magnet.
It was mentioned that the area the hysteresis loop defines is proportional to the amount of energy needed to change the polarity of a ferromagnetic material twice. We will see some later, that electric motors mostly consist of coils around a ferromagnetic core and that the polarity of the core material is altered periodically. To keep the loss of energy during the process of changing the magnetic polarity as low as possible, the area formed by the hysteresis loop should be preferably small. We will see some later, that a high value for the magnetic saturation and low values for the remanence and the coercivity are very helpful, too. The red loop shows the material being best qualified for use in electric motors.

Curie temperature

Until now we observed the microscopic magnets exclusively exposed to magnetic fields. One more important criterion is the temperature. Temperature is the measure of the average kinetic energy of the particles (atoms, molecules, ions) the substance consists of. We got to know kinetic energy at the chapter mechanics as the energy of moving objects. The particles of a solid material oscillate permanently around their resting position. The higher the temperature, the stronger the oscillation. Without the influence of temperature ( =0K ), the microscopic magnets of a ferromagnetic material form a static arrangement, according to their magnetic fields. We have observed those behavior at the Java-application. If one of the magnets is tilted slightly and released again, it will return to it's original position. Influenced by the particle movement of low temperature, the magnets are permanently tilted slightly, but always return to their resting position. If the temperature is increased more and more, the tilting of the magnets becomes too high and they can't return to their rest position! One reason is the fact that the time between two kicks is too short, so that the magnets can't return to their original position, The second reason is, that the neighbor magnets, being responsible for the rest position of the magnet rotate, too, leading to a completely different effect of magnetic forces! No magnetic arrangement of the spins is possible at temperatures higher than that. The spins are always randomly arranged and no magnetization can be recognized outside the material. The temperature this effect occurs, is called Curie temperature, named after the French physicist Pierre Curie.
You can observe this effect with the help of the Java application: Adjust the value for the "External field" to 100% and click at "Start". Wait until all magnets are oriented parallel to the field lines and set the external field to 0%. You can see the state of a magnetized ferromagnetic material at a temperature of 0 Kelvin. Set the temperature up to 20% and observe the oscillation of the magnets. They always return to their rest position. While reducing the temperature to 0% again, all magnets return to their initial position. Increase the temperature up to 50, 80 and finally 100% and you will see that not all of the magnets return to a position being parallel to those of the (now absent) magnetic field after "cooling down". The material gets more and more degaussed.
You can degauss magnetized materials by heating them up to values above the Curie temperature. Vice versa you should avoid to heat up permanent magnets above or even near this temperature while operating.
Most of the paramagnetic materials are ferromagnetic materials in principle. Their Curie temperature is just below the ambient temperature, whereby no magnetic arrangement can be formed. When cooling down paramagnetic materials, their ferromagnetic properties can be observed, too.

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