PHY331 Magnetism. Lecture 6

Transcription

1 PHY331 Magnetism Lecture 6

2 Last week Learned how to calculate the magnetic dipole moment of an atom. Introduced the Landé g-factor. Saw that it compensates for the different contributions from the orbital motion and the electron spin. Compare calculated values with measured data

3 This week. Will derive a quantum theory of Paramagnetism. Will obtain a new expression for paramagnetic susceptibility that we will compare with the classical value. Will see that it confirms Curie s Law. Will introduce some basic concepts relating to Ferromagentism.

4 Quantum theory of paramagnetism What are the changes from Langevin s classical theory? The magnetic moment value is, ( ) µ B µ J = g J J +1 and the resolved component of the moment along the field direction is, where M J gµ B M J = J,( J 1),..., ( J 1), J the potential energy of the dipole in the field is then, M J gµ B B

5 so this gives all we need to substitute into, M = [1] Resolved component X of a dipole in field direction [2] Number of dipoles with this orientation

6 we now have a rather unusual result: the dipoles no longer have an infinite set of orientations in space,they have a finite set of orientations described by M J, this means that, the integral over dθ that was used in Langevin s treatment m m = π 0 cosθ cmb sinθ exp( mb cosθ kt) dθ π 0 cmb sinθ exp( mb cosθ kt) dθ can be replaced by a summation over the M J µ J µ J = J J M J gµ B exp( M J gµ B B /k B T ) J J exp( M J gµ B B /k B T )

7 whose solution is: µ J µ J = 2J +1 2J coth 2J +1 2J y 1 2J coth 1 2J y (1) coth x = 1 tanh x = 1 x + x 3 x This is the Brillouin function B J (y). It is plotted as a function of y = µ J B kt

8 µ J // µ B B/T ~ y

9 The magnetic susceptibility χ Just as with the classical theory, we can derive an expression for the magnetic susceptibility χ in the limit of small applied fields and high temperatures, that is y = µ J B kt << 1 Use coth x = 1 x + x 3 Substitute into equation (1) to show B J ( y) J +1 3J y +... ( ) J +1 or B J y (2) 3J µ J B kt

10 but, µ J ( ) µ J = B J y or µ J = µ J B J y ( ) (3) and M = N µ J (4) so that substituting (3) into (4) gives, M = N µ J = Nµ J B J y ( ) (5) Using (1) so that, M = Nµ J J +1 3J χ = µ J B kt = N J +1 J µ J 2 µ 0 H 3kT M H = µ 0 N J µ J J 3kT

11 Finally using, µ J = J gµ B gives, χ = µ 0 N g 2 J( J +1) 2 µ B 3kT which is Curie s Law (again). = C T So we compare with the classical result, χ = M H = µ 0 N m 2 3kT = C T which indicates that, J effective = g J J +1 ( )

12 which is consistent with the earlier definitions, µ L = L L +1 ( ) µ B µ S = 2 S S +1 and our understanding of what the Landé g factor is meant to do. Gadolinium salt 1 ( ) µ B χ Gd(C2H3SO4)3.9H2O Experimental evidence of Curie s Law behaviour 1 T χ T

13 Paramagnetism.. Calculated paramagetic susceptability via both quantum and classical approaches similar. Both confirm statistical competition between magnetic alignment and thermal effects. Now consider systems where have a strong, spontaneous alignment between dipoles even in the absence of a magnetising field.

14 Ferromagnetism The bulk properties of a ferromagnet are: The magnetisation M is large and positive It is contribution to the total B field is significant χ >> 0 and µ r >> 1 The magnetisation M is a complex function of the applied magnetic field H The magnetisation M also depends on the past history of the sample

15 A graph of M against the applied magnetic field H gives the characteristic magnetisation curve of the ferromagnet

16 The curve divides naturally into three regions: A linear region at small values of H in which the initial susceptibility can be defined χ = M H An intermediate region, in which the slope rises to a maximum dm dh A region where dm dh decreases to show saturation of the magnetisation

17 The magnetisation M is a function of the past history of the sample A permanent magnet and an iron nail can both lie in the same Earth s magnetic field but, the magnetisation M inside each is quite different The permanent magnet has has already been exposed to a much larger magnetising field On demagnetising M lags behind H. It requires a reverse field to demagnetise the sample

18 Important definitions: R M = Remanent Magnetisation H C = Coercive Field M sat = Saturation magnetisation M sat = Npµ B N: atoms m -3 pµ B : Magnetic dipole per atom M sat

19 Summary Derived a quantum theory of Paramagnetism. χ = µ 0 N g 2 2 J( J +1) µ B 3kT = C T Introduced ferromagentism. Saw magnetization was a function of previous history. Next week will look at this phenomena in more detail using the domain theory of ferromagnetism.