Neutron Scattering

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Orbital magnetic spin angular ,,- momentum momentum field distribution Fit. 3.12: Schematic illustration of the magnetic field distribution due to spin - (S) and orbital- (L) angular momentum in the case ofa Bohr orbit The magnetic field due to the spin moment is much more spread out than the one due to the orbital angular momentum. 1 .0 ô 0 .8 E ô 0 .6 a> . 0 .4 ß O C 0 .2 0 .0 = Chromium 0 .0 0 .2 0 .4 0 .6 0 .8 1 .0 sin(O/~ ) Fig. 3.12 : Formfactor ofCr [7, 81 . Due to the different distribution of the magneticfeldfor S and L according tofigure 3 .11, a more rapid decrease ofthe scattering amplitude as a function of momentum transfer resultsfor the spin momentum. For the x-ray form factor, the innen electrons play an important yole, too. Therefore, the x-ray formfactor drops sloiver as compared to the magnetic formfactor. Finally, on the Â length scale of the thermal neutron ivave length, the nucleus is point-like. Therefore, nuclear scattering is independent ofthe momentum transfer. 3-25
Since the scattering amplitude is proportional to a Fourier transform of the scattering power density in direct space, the scattering amplitude decreases faster with momentum transfer if the scattering occurs from a larger object in direct space . Since the unpaired magnetic electrons are located in the outermost electronic shells, the magnetic form factor drops faster than the x-ray form factor. Compared to the natural length scale of the neutron wave length, the nucleus is point-like, which results in a scattering amplitude being independent of momentum transfer . Finally, we want to mention that the magnetic form factor can in general be anisotropic, if the magnetisation density distribution is anisotropic . How the form factor comes about is most easily understood in the simple case of pure spin scattering, i . e . for atoms wich spherical symmetric (L = 0) ground state, such as Mn2+ or Fei+ . Moreover, the derivation is simplified for ionic crystals, where the electrons are located around an atom . In figure 3 .13 we defme the relevant quantities for a derivation. FilDefinition ofthe relevant quantitiesfor a derivation ofthe spin-onlyformfactor. We denote the spin operators ofthe electrons of atour i with sik. The spatial co-ordinates ofthe electron number k in atom i are lk = R + t k, where R denotes the position vector to the nucleus of atom i. Now we proceed to separate the intra-atomic quantities . We can write the operator for the magnetisation density as : MS (Z:)= -211BY-6(!:-!:ik)»!iik ik (3 .52) The Fourier transform of this magnetisation density is calculated to: 3-26
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Forschungszentrum Jülich in der He
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Forschungszentrum Jülich GmbH Inst
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Contents . . 1 Neutron Sources H .
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1 . Neutron Sources Harald Conrad 1
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d) f, = v a, , q - ti = P a, - 0 .3
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Period Example Flux (D [1013 1950-6
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In stage 1 the primary proton knock
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get as heat, the rest is transporte
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down power and stronger absorption
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Appendix Neutron Sources - an overv
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led remotely. It is rauch more conv
Page 28 and 29: 2 . Properties of the neutron, elem
Page 30 and 31: In the 60's the ferst high flux rea
Page 32 and 33: Hot neutrons in reactors are obtain
Page 34 and 35: elated values for the FRJ-2 reactor
Page 36 and 37: 2 .5 Scattering amplitude and cross
Page 38 and 39: (A+k2 ) yr = u(Z:) yr V( u~r)= z "
Page 40 and 41: _du _ mn \2 A2-~2z r2~ I(k' IVI k)I
Page 42 and 43: E ô ô x z w 0 z _w U N Figure 2 .
Page 44 and 45: scattering is observed with the ave
Page 46 and 47: For a spherical electron distributi
Page 48: A more thorough introduction to neu
Page 52 and 53: 3 . Elastie Seattering from Many-Bo
Page 54 and 55: Q = Q = k2 + k2 - 2kk' cos2B (3 .3)
Page 56 and 57: 3 .2 Fundamental Scattering Theory
Page 58 and 59: The meaning of (3 .l4) is immediate
Page 60 and 61: 1 . e. in the second approximation,
Page 62 and 63: The saure problem will bc dealt wit
Page 64 and 65: ALQ) = Y-bfe'Q'-rB(r-(n .a+m .b+p»
Page 66 and 67: du LQ) = ALQJ2 = e'Q .A' . * -i e-
Page 68 and 69: Tab . 3.1 : The scattering lengths
Page 70 and 71: We note immediately that we should
Page 72 and 73: These magnetic structures can be un
Page 74 and 75: M1LQ)=Q-MQ)xQ M(Q)= IM (r)e i Q " d
Page 78 and 79: aQ . r 3 àQ . N . QR . aQ .t k _ML
Page 80: Schweika Analysis Polarization W .
Page 83 and 84: ducing a complex component and were
Page 85 and 86: ir v s ftft 3956 2 s 2 0.81[X] 80 c
Page 87 and 88: Some polarizers using Bragg reflect
Page 89 and 90: Different to the coherent nuclear s
Page 91 and 92: where QBG denotes the background .
Page 93 and 94: 4 .4 Applications We now consider e
Page 95 and 96: Fig. 4 .3 .4 probably represents th
Page 97 and 98: . occur only in the non-spin flip c
Page 99 and 100: The results for the magnetization d
Page 101 and 102: a large solid angle with polarizers
Page 103 and 104: scattering to spin-flip scattering
Page 105 and 106: An experimental verification of thi
Page 108: Correlation Functions Measured by S
Page 111 and 112: In real liquids however usually an
Page 113 and 114: The term in big parentheses of equa
Page 115 and 116: 4 0 -5 0 5 10 15 20 ho) [meV] Figur
Page 117 and 118: Figure 5 .5 : The pair correlation
Page 119 and 120: Figure 5 .6 : Partial differential
Page 121 and 122: Then equation (5 .21) can be conver
Page 123 and 124: The route from expression (5 .27) t
Page 125 and 126: In addition it is often useful to d
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2 . The pair correlation function h
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G(r, t) G(r, t) i(Q, t) r Q i(Q, t)
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Insertion of this result into (5 .6
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With this example we can also demon
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6. Continuum description : Grazing
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Starting point is the exact atomic
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For specular reflectivity measureme
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v v z l 2r V(z) = 2 -2erfC~ with er
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F{dV(z)/dz ) to calculate the refle
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Equation (6 .13) also shows that th
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ut is completely reflected . The cr
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0.4° . The full dynamical theory c
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1) If no magnetic induction B (whic
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Diffractometer Gernot Heger
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select a special wavelengths band (
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" The direction of the reciprocal l
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Bragg-intensities of single crystal
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monochromator, on the mosaic spread
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complex sample environments, such a
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the diffraction plane) two further
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Small-angle Scattering and Reflecto
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L= (1) ( k ) a e -(k/k T ) 2 Ak (8
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possible neutron wave lengths betwe
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figure and with neutrons of about 1
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1 .0 Danvinkurve 0 .8 0 .6 0 .4 0 .
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tails. This reflection curve leads
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For the Nickel film one gets a thic
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I The rotor of a velocity selector
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multiplier are arranged in a quadra
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9 . Crystal spectrometer : triple-a
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machine which will be followed by a
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10 .3 Beaur shaping Due to the fact
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G.k=zGz . (11) This case is fulfill
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tribution of bisecting planes (ntos
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directions which is exploited for t
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Fig . 14) Dispersion surfaces for B
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whereby E and v,, denote energy and
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[1] B . N. Brockhouse, in Inelastic
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10 . Time-of-flight spectrometers M
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Tnloderator /K V7 2/m/s A/nm Aiw/eV
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10.2 .1 Interpretation of spectra A
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----~ Probe ' (Chopper 2) Zeit I Ch
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the similar intensity distribution
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is performed without Jacobian due t
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10.2 .6 Crystal monochromators The
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larger energy transfers . However i
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10.3 Inverted TOF-spectrometer In t
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constant offset to the TOF . To be
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sample size of cm and a detection p
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Contents 10 .1 Introduction . . . .
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11 . Neutron spin-echo spectrometer
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2 0 L Tr -0 L 2 ArDet Figure 11 .1
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initial velocity-reassemble at the
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distribution (current sheets) that
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esults, where il is an irrelevant c
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10000 - 8000 - co 6000 - C ô 4000-
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e0 oX n .5 1 .0 1 .5 so 0 .0 0 .0 1
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nel" coils) the required homogeneit
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Figure 11 .9 : S(Q, t)IS(Q) of a 2
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Contents 11 .1 Introduction . . . .
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12 . Structure determination G . He
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Important: Thc measured Bragg inten
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determination of accurate atomic pa
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The knowledge of the overall occupa
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(even anharmonic) effects. This com
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Fig . 4 . Coordination ofthe D20 mo
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fenoelectric phase of orthorhombic
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(c) and (d) calculated densities at
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13 Inelastic Neutron Scattering Pho
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Figurel Schematical drawing of a ge
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equilibrium position the atom is in
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s - ' f N .mwn .I )oo K L .~um..un
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esults from the quantum mechanics o
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y absorption . 13 .1 .3 Magnetic in
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Figure 7 The plane in reciprocal sp
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5 100s1 [OSl ] IOSSI [SSS y i 4 0 r
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dynamics of ionic compounds with th
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16 14 12 F.. . 10 ô 8 q 6 cr 4 G.
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21 [ooç 1 " - 19 18 17 16 "" 298 K
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n = v - cap[i(pga . - wt)] (13 .22)
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Y c 3 c Cc E Figure 16 Magnon dispe
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. . 2 H . Bilz and W . Kress, Phono
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14 . Soft Matter : Structure Dietma
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length density of a sample against
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2 P(Q) = 1 ~exp(-Ii - j'6 Q2 ) (14
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fraction. In the region of large Q
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with the partial structure factor S
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and which lead to the scattering cr
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For the intramolecular and intermol
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500 450 -400 U 0 ;, 350 42, 300 s~.
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scaling behavior of the susceptibil
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15 Polymer Dynamics Dieter Richter
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constraints due to the mutual inter
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(15 .3) T ß ( E)=T ô exp E k,T -
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This incoherent approximation does
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10 an 0 2 0 Figure 15.4 : Relaxatio
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In order to test Eq .[15 .9] the re
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Arrhenius temperature dependence wi
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1 3kBT ~ z 2 72 2 p2 a = f2 N2 P =W
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S (Q,t) = 12 fdu exp ~ -u_ (S2Rt) v
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We now use these data, in order to
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c 0 .8 0 .6 00 .055 x 0 .1 110 .14
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small but systematic deviations fro
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S(Q,t) is predicted . Such a platea
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only one single parameter, the tube
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confinement of the relaxation of la
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16 Magnetism Thomas Brückel
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the ground state of metallic magnet
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c~ ~ o 0 `,~i) g',, b o 0 Q! b o 0
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only "sec" this component and not t
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200 ¬ 400 500 600 700 E 3 QI J (h0
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Fie. 16.4: Scattering geometry for
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0 .8 ' D3 ° Stassis 3d spin - - 3d
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Here, J;j denotes the exchange cons
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Fig. 16.8 : Contour plot of the mag
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0.0001 0.001 0 .01 0.1 2 Fig. 16 .1
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17 Translation and Rotation M . Pra
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17.1 Introduction Atomic and molecu
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17 .2.2 Diffusion, microscopic appr
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0.6 H20 2 x2tÄ z ) Figure 17 .2 :
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The scattering function of a polycr
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e (2) 1 (j 1, 0 > - 0,1 >), respect
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leads to the eigenvalue problem ~q
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allow a good determination of the E
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m Eu W Figure 17 .7 : Eigenenergies
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-so o so Energy transfer (NeV) Ener
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Bibliography [1] T. Springer, Quasi
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18 . Texture in Materials and Earth
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Further important structure related
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The axes of the cartesian Kp coordi
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4.0 Experimental Texture Analysis T
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Fig. 18.14: Variation of reflection
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measurements can be performed in tr
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Apart from the global description o
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In the following, two experimental
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The neutron diffraction pole figure
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dependent pole densities, and (4) g
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Schriften des Forschungszentrums J
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Forschungszentrum Jülich in der He
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