The magnetism of a solid is originated from the contributions of the electrons constituting that solid. The quantum properties of electrons that determines the magnetic behaviour of a solid are a) the spin angular moment, s, taken from the classic analogue of a sphere rotating about it own axis, and b) their orbital angular moment l, since electrons also carry electric charge so that ‘moving around’ in quantum orbits also contributes to the magnetic moment. These electrons also determine the strength of the interaction between atoms in a solid, making the basis of the different macroscopic behaviour observed in nature. At macroscopic scales, these magnetic interactions between atoms, together with the crystalline structure of the solid, originate the magnetic response of materials. When the magnetic interactions are weak, the thermal agitation at room temperature can make the magnetic moments to flip over continuously, so that the average magnetic moment measured is very small or zero.
Figure 3 (click on it to enlarge). Different magnetic materials display dissimilar performances: a) diamagnetic atoms in solids have negligible magnetic moments (black dots); b) in paramagnetic solids magnetic atoms are not ordered because of thermal energy that shakes each atom randomly; c) In a ferromagnetic material, displacement of the domain walls (DW, schematically shown in the inset) result in open hysteresis cycles. d) For single domain particles there are no DW, so that in the SPM state the whole magnetic moment of each particle is shook in the same way as in the paramagnetic material (b).
These materials are broadly called non-magnetic, and display a linear response to the applied field, as shown in Figure 3 a) and b). For stronger magnetic interactions, the atoms within the solid can align the atomic magnetic moments parallel (ferromagnet) or antiparallel (antiferromagnet) configurations. The former configuration result in very dissimilar magnetic behaviour, shown in Figure 3 c), whereas in the antiferromagnet the antiparallel alignment can reduce the total moment to zero, yielding a behaviour similar to a paramagnet (Figure 3 b).
Although a ferromagnetic material should have all its magnetic moments pointing in the same direction, a macroscopic piece of material cannot have this configuration because the amount of magnetostatic energy stored should be huge. The way in which a solid can reduce this magnetostatic energy is to break itself up into regions (domains), within whose moments remain parallel, but each randomly oriented so that the net magnetic moment of the sample is close to zero. (See Figure 3c.). This situation generates interfaces between domains called domain walls (DWs), where adjacent magnetic moments are in a non-favorable configuration, so that these domain walls are highly energetic. Even though some energy is stored inside domain walls, the overall decrease in the total magnetic energy favors the multi-domain configuration. Being formed by a competition between magnetostatic and exchange energies, domain walls have a finite width, d, determined by the ratio between these energies. [25]
Domain walls can move in response to an applied field: creation, growth and extinction of domains can be induced by an external magnetic field, because the external field imposes a preferred direction for the magnetic moments. For the spins in a given domain to change their orientation it is required that the walls of that domain will displace. This is known as Barkhausen effect, and is an irreversible process in the sense that the pinning and displacement of DWs depends of structural imperfections of the atomic arrangements (defects, dislocations, vacancies, etc…). However, the magnetic field required to eliminate all DWs (i.e., to align all magnetic moments in the same direction) has a definite value for a given sample, and is very reproducible. The two ways of visualizing the Barkhausen process (i.e., domain wall displacement or domain growth) are equivalent and so are used.
No comments:
Post a Comment