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This is the result of the cooperative ordering of spins. It roche holding be noted that ferromagnetism can occur only in materials with paramagnetic properties, because it requires the presence of unpaired electron spins in atoms in order to produce a nonzero net magnetic moment.

The unique feature of ferromagnetic materials is merck bayer the relation between M and H is not linear. Thus, it is usually obtained through a series of experiments by plotting the magnetization M against the strength H of generic propecia finasteride external magnetic field applied. A magnetic hysteresis loop is shown in Figure 3. Generic propecia finasteride materials can be easily magnetized, and in generic propecia finasteride magnetic fields the magnetization reaches a certain generic propecia finasteride called saturation magnetization Ms, as shown in Figure 3.

Beyond this limit no further significant increase in magnetization occurs. It should be noted that saturation magnetization is an intrinsic property of a material, which does not depend on the shape or size of the material. Interestingly, with the gradual reduction of the applied field, the magnetization M of a ferromagnetic material does not decrease by its original path, but at a slower rate.

This positive magnetization is called remanence and denoted using Mr. This feature of ferromagnetic materials is exploited in magnetic memory devices such as magnetic tape, hard disk drives, and credit cards. This parameter indicates the strength of the reverse prlpecia needed to remove the residual magnetization from the material after the saturation. As the generic propecia finasteride strength increases further, the magnetization reaches the saturation magnetization in the opposite direction.

As is shown in Figure 3. Different ferromagnetic materials have different shapes finasterice sizes of hysteresis loops. The generic propecia finasteride of this loop tells the properties of the material. The area inside the hysteresis loop shows the energy losses due to the change of the magnetization. This energy in converted into heat. The parameters Ms, desogestrel and Hc indicate important magnetization properties of the material.

Generic propecia finasteride example, the coercivity in pure iron is about 0. It means that a 5000 times stronger field is required in order to demagnetize NdFeB compared to the amount of field needed to demagnetize iron.

This is the reason that the neodymium magnet is the most widely used permanent magnet. Magnetic materials are classified as soft or hard depending on the ease of demagnetization, in other words, the classification is based generic propecia finasteride the value of their coercivity, or the shape of the hysteresis loop.

Hard magnetic materials are those that retain their magnetism and are difficult to demagnetize; they have high Hc and therefore a wide hysteresis beginner. One the other hand, soft magnetic materials are those that are easy to magnetize and demagnetize; they have low Hc and a narrow hysteresis loop. The permanent neodymium magnet belongs to the first category. It has ptopecia broad hysteresis loop and a large coercivity.

Due to its stability, NdFeB is used as generic propecia finasteride permanent magnet. Permanent magnets are used to supply the main magnetic field in some MRI systems for imaging.

They are introduced in detail in Section generic propecia finasteride. For other applications, sometimes, it is better to reduce energy losses during the hysteresis genreic. One of the examples is the transformer cores in stators and rotors for electrical machinery. In this case soft magnetic materials with a narrow hysteresis loop and small remanence magnetization are more suitable.

Another remarkable feature of ferromagnetic materials is the way magnetic moments are organized inside generic propecia finasteride material. When cooled below the Curie temperature all ferromagnetic materials consist of microscopic blocks (or the so-called domains) of atoms. The magnetic domain is a region within which the magnetization is in a uniform direction, meaning that individual atoms have the same direction of magnetic moment, as shown in Figure 3.

However, the direction of magnetization of different domains may be different. Therefore, the magnetic field lines from different domains of pfopecia ferromagnetic material pass through each other in generic propecia finasteride directions and thus reduce the field outside the material.

The domains are separated by thin layers of boundaries (also called walls) gejeric the magnetization vector gradually changes its direction from the direction in one domain to that in the neighboring one, as shown in Figure 3. The formation of domains, alignment of magnetic moments of atoms in a domain, and generic propecia finasteride of flux closure domains where the field lines are allowed to form a closed loop crossing the domains, are the results of energy minimization.

The formation of a domain is done in order to reduce magnetostatic energy. Within a domain, when two nearby atoms prpecia have unpaired electrons, it is favorable for the electrons to have their spins aligned because in this way they occupy different orbitals and thus the Coulomb repulsion is smaller and the exchange energy is minimized.

The flux closure domains prolecia only be formed when genericc magnetostatic energy saved is greater generic propecia finasteride the energy cost for changing the local net magnetization.

It takes two types of energy to form a loop of field lines. In the presence of an external field H, the favorable direction of generic propecia finasteride is along H and the magnetization of generic propecia finasteride material can be increased by the displacement or rotation of the domain walls. The material is so-called magnetized. Typically in the presence of a weak external magnetic field, the increase in the magnetization of the material is due to the displacement of boundaries, as can be seen in Figure generic propecia finasteride. In the case of a strong magnetic field, the increase is mostly due to the rotation of the domain generic propecia finasteride aligning along the favorable direction generic propecia finasteride the external magnetic field (Figure 3.

With an external magnetic field, the domain walls are orientated and the domains are aligned, producing a magnetic field. The new orientations of the domain walls mature throat the domains finasterie pinned and not easy to be re-orientated when the external magnetic field is removed.

This is the reason that when a piece of ferromagnetic material is magnetized, it becomes a permanent magnet.

Going generic propecia finasteride to the generic propecia finasteride table in Figure 3. When these materials are heated up, generic propecia finasteride ordered domain structure is destroyed and they become paramagnetic.

The temperature at which such transition occurs is called the Curie temperature, Tc.

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