Magnetization and hysteresis

Magnetization and hysteresis
　　Now that we understand a little about the structure and origin of domains, let’s look at how they influence the magnetization and hysteresis curves of ferromagnetic materials. shows a schematic magnetization curve for a ferromagnetic material, with a sketch of the domain structure at each stage of the magnetization.
　　The magnetic field is applied at an angle (horizontal in the picture) which is slightly off the easy axis of magnetization. In the initial demagnetized state, the Magnetic lifter domains arearranged such that the magnetization averages to zero. When the field is applied,the domain whose magnetization is closest to the field direction starts to grow at the expense of the other domains. The growth occurs by domain-wall motion.
　　At first the domain-wall motion is reversible; if the field is removed during the reversible stage, the magnetization retraces its path and the demagnetized state is regained. In this region of the magnetization curve the sample does not show hysteresis.
　　After a while, the moving domain walls encounter imperfections such as defects or dislocations in the crystal. Crystal imperfections have an associated magnet static energy. However, when a domain boundary intersects the imperfection, this magnet static energy can be eliminated. The intersection of
　　the domain boundary with the imperfection is a local energy minimum. As a result the domain boundary will tend to stay pinned at the imperfection, and energy is required to move it past the imperfection. This energy is provided by the external magnetic field. A typical variation of Bloch wall energy with position in an imperfect crystal .
　　A schematic of the motion of a boundary past an imperfection. When the boundary moves as a result of a change in the applied field, the domains of closure cling to the imperfection forming spike-like domains, which continue to stretch as the boundary is forced to move further. Eventually the spike domains snap off and the boundary can move freely again. The field required to snap the spike domains off the imperfections corresponds to the coercive force of the material. A photograph of spike domains in single crystals of silicon iron,highlighted using the colloidal magnetite method. When the spikes snap from the domain boundary, the discontinuous jump in the boundary causes a sharp change in flux. The change in flux can be observed by winding a coil around the specimen and connecting it to an amplifier and loudspeaker. Even if the applied field is increased very smoothly, crackling noises are heard from the loudspeaker. This phenomenon is known as the Barkhausen effect. It was first observed in 1919 , and provided the first experimental evidence for the existence of domains. Figure 7.16 is a schematic enlargement of a portion of a magnetization curve, showing the sharp changes in magnetization produced by the Barkhausen mechanism.
　　Eventually the applied field is sufficient to eliminate all domain walls from the sample, leaving a single domain, with its magnetization pointing along the easy axis oriented most closely to the external magnetic field. Further increase in magnetization can only occur by rotating the magnetic dipoles from the easy axis of magnetization into the direction of the applied field. In crystals with large magnet crystalline anisotropy, large fields can be required to reach the saturation magnetization.
　　As soon as the magnetic field is removed, the dipoles rotate back to their easy axis of magnetization, and the net magnetic moment along the field direction decreases. Since the dipole rotation part of the magnetization process did not involve domain-wall http://www.999magnet.com/products/131-magnetic-lifter motion, it is entirely reversible. Next, the demagnetizing field in the sample initiates the growth of reverse magnetic domains which allow the sample to be partially demagnetized. However, the domain walls are unable to fully reverse their motion back to their original positions. This is because the demagnetization process is driven by the demagnetizing field, rather than an applied external field, and the demagnetizing field is not strong enough to overcome the energy barriers encountered when the domain walls intersect crystal imperfections. As a result, the magnetization curve shows hysteresis, and some magnetization remains in the sample even when the field is removed completely. The coercive field is defined as the additional field, applied in the reverse direction, which is needed to reduce the magnetization to zero.
　　So we see that the hysteresis properties of a sample depend in large part on its purity and quality. This means that we can engineer materials to optimize their properties for specific applications. For example, a sample with many defects or impurities will require a large field to magnetize it, but will retain much of its magnetization when the field is removed. As we mentioned in Chapter 2, materials which are characterized by high remanence and large coercive field are known as hard magnetic materials, and are important as permanent magnets.
　　High-purity materials, with few dislocations or dopants, are easily magnetized and demagnetized these are known as soft magnetic materials. Soft magnetic materials are used in electromagnets and transformer cores, where they must be able to reverse their direction of magnetization rapidly.
　　Finally, we show some real photographs of the domain structure in gadolinium–iron garnet as the field is cycled from zero to a value large enough to create a single domain oriented in one direction, back to zero, and then to a largevalue in the opposite direction [34]. The dark and light regions, obtained using the magneto-optic Faraday effect, which we will discuss in Chapter 16, indicate domains of opposite magnetization. The hysteresis can be seen by comparing the third and sixth frames, which occur at similar fields (the first while the field is increasing, and the second while it is being reduced from its maximum value), but show quite different domain structures.