08/11/2005 GKMR lecture WS2005/06 Denninger magresintro.ppt 1 1.a Magnetic Resonance in Solids The methods of M agnetic R esonance (MR) measure with high sensitivity the magnetic dipole moments and their couplings to the environment. Since the coupling between the magnetic dipoles µ i and the electromagnetic field is rather small, MR uses very specialised and highly sensitive measurement techniques. Carriers of magnetic moments: 1) electrons: spin magnetic moment orbital magnetic moment (bound electrons) 2) nuclei: spin magnetic moment for all nuclei with I ≠ 0 3) muons, positrons, neutrons: elementary magnetic moments E lectron S pin R esonance (ESR,EPR) Electrons in atoms, molecules and in the solid state have a very strong exchange interaction leading to spin-pairing (Pauli principle). Thus ESR depends on the existence of unpaired electrons.
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1.a Magnetic Resonance in Solids - Uni Stuttgart · 2005. 11. 8. · 2) nuclei: spin magnetic moment for all nuclei with I ≠0 3) muons, positrons, neutrons: elementary magnetic
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1.a Magnetic Resonance in SolidsThe methods of Magnetic Resonance (MR) measure with high sensitivity the magnetic dipole moments and their couplings to the environment.
Since the coupling between the magnetic dipoles µi and the electromagnetic field is rather small, MR uses very specialised and highly sensitive measurement techniques.
Carriers of magnetic moments:
1) electrons: spin magnetic moment
orbital magnetic moment (bound electrons)
2) nuclei: spin magnetic moment for all nuclei with I ≠ 0
3) muons, positrons, neutrons: elementary magnetic moments
Electron Spin Resonance (ESR,EPR)
Electrons in atoms, molecules and in the solid state have a very strong exchange interaction leading to spin-pairing (Pauli principle). Thus ESR depends on the existence of unpaired electrons.
Important sources of unpaired electrons in solids are:
1. Radicals
C
HH
H H
C
H
H H
CH4, methan *CH3, methyl radical
Unpaired electron
Molecules (or molecular fragments) with broken bonds. The chemical bonds are not completely saturated and some “dangling” bonds carry magnetic moments.
Radiation damage: In solids, such dangling bonds result e.g. from radiation damage associated with UV, X-ray radiation, γ-ray , neutron, electron, proton bombardment.
Si
Si
Si
Si
Si
Si
Si
Si Si
Si
Si
Si
SiSi
Si Si Si
H H
Defects:in semiconductors broken bonds (dangling bonds) are partially saturated by e.g. hydrogen.
The remaining dangling bonds are paramagnetic.
Such dangling bonds are known e.g. in silicon and in SiO2.
Donors , acceptors: Donors and acceptors in semiconductors usually have unpaired electrons.
Defects: Defects in semiconductors and in insulators are often paramagnetic.
Intermediate products: Intermediate products of chemical reactions in the solid state are often paramagnetic. Examples: Intermediate products of polymerisation reactions; Oligomers with radical character.
ESR is a very sensitive spectroscopic technique. In favourable cases, one can detect 108 to 1010 spins. Optically detected MR can detect 107 spins. In very special systems, single spin detection is possible ( time averaging).
10-1410
123
1010 spins 1.6 10 mol6 10 mol −
= ⋅⋅
High field spectrometers can detect 108 spins ≈ 10-16 mol.
1.1 Experimental foundations of ESRAll systems with angular momentum S, J, L, I, posses a corresponding magnetic dipole moment. This magnetic moment µ is fundamentally coupled to the angular momentum.
Electrons: s Bˆ
s g Sµ µ= ⋅ ⋅
Bˆ
ll g Lµ µ= ⋅ ⋅
Spin magnetic moment
Orbital magnetic momentJ B
ˆJ g Jµ µ= ⋅ ⋅
Nuclei: Kˆ
I Ig Iµ µ= ⋅ ⋅
µB : Bohr magneton24
Be
9.27410154 102e J
Tmµ −= = ⋅
µK : nuclear magneton 27K 5.0507866 102 p
e JTMµ −= = ⋅
gS, gl, gJ : g factor, spectroscopic splitting factor, for atoms: Landé factor.
gI : nuclear g factor
In atoms, central potential, l·s coupling, the total momentum is: j = l + s
jls( 1) ( 1) ( 1)1
2 ( 1)j j s s l lg
j j+ + + − +
= +⋅ ⋅ +
This Landé factor is only valid for free atoms with a central potential.
2 zs B Sµ µ= − ⋅ ⋅ The spin moment S is connected to a magnetic moment µs which is twice as large as the orbital moment. This factor of 2 is explained by the Dirac-theory.
The general formulation introduces the g-factor. This spectroscopic splitting factor describes the relation between the magnetic moment and the angular momentum. The negative sign of the equation is chosen for particles with negative charge like the electron. The g-factor is than a positive quantity.
Bg Jµ µ= − ⋅ ⋅ g = 1: orbital magnetic moment
g = 2: spin magnetic moment
The g-factor can usually be measured with very high precision in ESR-experiments.
The basic equation for the determination of the g-factor for S=1/2 is:
µw B 0h f g Bµ⋅ = ⋅ ⋅
microwave frequency
magnetic field
Frequencies can be determined and controlled to a precision of at least 10-6. Thus the precision of the g-factor determination is usually dominated by the determination of the magnetic field B0 (at the sample).
Digression: Free electron, quantum-electrodynamics corrections to the g-factor.
The electron is the lightest elementary particle with a magnetic moment. The electron couples to the zero-point fluctuations of the electromagnetic fields in vacuum. This coupling leads to consequences like the Lamb-shift of the bound electron states and to the corrections of the g-factor.
The free electron in vacuum has two eigenstates with respect to its spin: |+1/2> and |-1/2>
As a consequence of the magnetic coupling to the electromagnetic field fluctuations, the electron can temporarily “flip” into the other spin state and back again. The lowest order quantum-electrodynamic process of this kind is the emission and re-absorption of a photon.
|+1/2>
|+1/2>|-1/2>
Photon, S = 1
Feynman-diagram, first order.
There are two coupling vertices with the electromagnetic field.
The coupling strength of this first order process is ∝ α, the Sommerfeld fine structure constant. α ≅ 1/137.
The energy splitting between |+1/2> an |+1/2> is increased by this process. The g-factor in this order is:
In a magnetic field B0 the spin of the electron can be flipped by the absorption/emission of virtual magnetic photons.
|↓> Spin down
|↑> Spin up
These „fluctuations“ of the electron‘s spin state lead to measurable corrections of the spin splitting of free electrons (and bound electrons, too) in a magnetic field .
By incorporating higher order processes and calculating Feynman-diagrams up to 8th order, one can calculate the theoretical value of the g-factor to 10 decimal places.
Comparison between the experimental value and the theoretical predictions by QED:
gexperiment = 2.00231930442 (08) The (08) is the error of the last two digits.
gtheory = 2.00231930492 (40) The (40) is the error of the last two digits.
Source:
Richard Feynman, QED
..00231931.2)328.021(2 2
2=
πα⋅−π
α+⋅≈sg
This value, the g-factor of the free electron, can be measured experimentally with very high precision. Such experiments are called g – 2 experiments.
First measurement: Polykarp Kusch, 1947 Nobelprize 1955
Size of the spin splitting, experimental realisation of the magnetic fields.
B 0h f g Bµ⋅ = ⋅ ⋅ B
0
f ghBµ⋅=
For free electrons (g = 2) :0
28 GHzT
fB ≅
Which magnetic fields (with sufficient homogeneity) can be reached by which magnet systems? Small distance d for sufficient
homogeneity.a) Permanent magnet systems:
B0 up to 1 T
Problems: temperature dependence
low homogeneity
small sweep range
Permanent magnet systems are employed for small (and relatively cheap) ESR systems. These systems usually are only employed for spin systems near g = 2 and for special purposes, where small and transportable systems have to be used.
Up to the highest frequencies: Sample extension « wave length λThis is completely different to the case of e.g. optical spectroscopy. ESR is practically always concerned with electromagnetic fields in the near-field limit.
A further major difference: The magnetic field component is decisive.
Coupling of the magnetic moment µ= -gµBS to the field B0 and to the alternating magnetic field B1(t)
In the standard configuration for magnetic resonance, one applies a large and nearly static magnetic field B0 along the z-direction. A much smaller alternating magnetic field
is applied along a perpendicular direction, e.g. the x-direction. 011( ) cos(2 )B t B f tπ= ⋅ ⋅
z
x
y
011( ) cos(2 )B t B f tπ= ⋅ ⋅
B0
Sample tube
In the quantum mechanical description, the magnetic moment µ couples to the magnetic fields B0 and B1 via operators derived from the energy of µ in a magnetic field:
This is one of the simplest spin Hamilton operators encountered in magnetic resonance. We further assume for simplicity, that the g-factor is a scalar, and that all electronic interactions of the electron are just included in the g-factor g.
If this is the only spin-interaction, the direction of quantisation is clearly the direction of the magnetic field B0. By convention, this static field is applied along the z-axis:
0
0
0
0
B
B⎛ ⎞⎜ ⎟= ⎜ ⎟⎜ ⎟⎝ ⎠
zB 0ˆH g B Sµ= + ⋅ ⋅ We need the eigenstates and the
eigenvalues of the operator Sz.^
A spin with spin quantum number S has 2·S+1 different eigenstates denoted by the magnetic quantum number mS. We denote states by the Dirac notation as: |S mS>
Examples: S =1/2 : |1/2 +1/2> |1/2 -1/2>
S =5/2 : |5/2 +3/2> |5/2 -1/2> etc.
There are 2·S+1 eigenstates. These states span a Hilbert space of dimension 2·S+1.
For a given S, one very often denotes the states by mS only: |mS>
e.g. S = 5/2: |-5/2> , |-3/2> , |-1/2> , |+1/2> , |+3/2> , |+5/2>
The Hilbert space of spins is a “playground” for quantum mechanical calculations. The number of states is finite (even very small), and the energy spectrum is limited.
There are two operators for a spin S, which have simultaneous eigenvalues and can be
measured independently: and2S ˆzS 2ˆ ( 1)S SS Sm S S Sm= + ⋅
ˆz S SSS Sm m Sm= ⋅
zB 0ˆH g B Sµ= + ⋅ ⋅Thus the operator has the energy eigenvalues E for the stationary
states in a magnetic field B0
0 SBE g B mµ= ⋅ ⋅ ⋅ mS = -S, -S+1, …, S-1, S
B0
|3/2>
|1/2>
|-1/2>
|-3/2>
Energy level diagram for S =3/2 in a magnetic field
The frequency distribution p(f) of the perturbation can have (and generally has) two contributions:
1) The perturbation H1(t) is not monochromatic, and the Fourier-transform of the time dependence leads to a distribution p(f). This is particularly valid for a stochastic perturbation. Processes leading to relaxation are of this kind.
2) The energy difference ∆E = h·f is distributed. The ultimate limit is e.g. the finite lifetime of the excited state |n>. A lifetime τ of the transition leads to a frequency distribution p(f) which is a Lorentzian function with width ∆f ∝ 1/τ.
In general both contributions are operative and have to be considered. Then p(f) is the convolution of the two contributions. Very often, one contribution dominates. This is e.g. the case if the perturbation is monochromatic. For a homogeneous spin system, the lifetime of the state dominates.
The transition rate by the magnetic dipole transitions is nearly identical to the relaxation transition rates responsible for the life time. This situation leads to the partial saturation of the transition. Saturation effects are very important in ESR (and in NMR). These effects will be considered in more detail later in later lectures.