Magnetism & Spectra

	config.	Ground	No. of			Observed
Ln	Ln3+	State	unpaired e-	Colour		meff/mB
La	4f0	1S0	0	colourless	0	0
Ce	4f1	2F5/2	1	colourless	2.54	2.3 - 2.5
Pr	4f2	3H4	2	green	3.58	3.4 - 3.6
Nd	4f3	4I9/2	3	lilac	3.62	3.5 - 3.6
Pm	4f4	5I4	4	pink	2.68	-
Sm	4f5	6H5/2	5	yellow	0.85	1.4 - 1.7
Eu	4f6	7F0	6	pale pink	0	3.3 - 3.5
Gd	4f7	8S7/2	7	colourless	7.94	7.9 - 8.0
Tb	4f8	7F6	6	pale pink	9.72	9.5 - 9.8
Dy	4f9	6H15/2	5	yellow	10.65	10.4 - 10.6
Ho	4f10	5I8	4	yellow	10.6	10.4 - 10.7
Er	4f11	4I15/2	3	rose-pink	9.58	9.4 - 9.6
Tm	4f12	3H6	2	pale green	7.56	7.1 - 7.6
Yb	4f13	2F7/2	1	colourless	4.54	4.3 - 4.9
Lu	4f14	1S0	0	colourless	0	0

Magnetic Properties

Paramagnetism

see R.L. Carlin, Magnetochemistry, Springer, N.Y., 1986 Chapter 9 for a detailed account

• Magnetic properties have spin & orbit contributions

  (contrast "spin-only" of transition metals)

• Magnetic moments of Ln³⁺ ions are generally well-described from the coupling of spin and orbital angular momenta ~ Russell-Saunders Coupling Scheme
• spin orbit coupling constants are typically large (ca. 1000 cm^-1)
• ligand field effects are very small (ca. 100 cm^-1)

  Þ only ground J-state is populated

  Þ spin-orbit coupling >> ligand field splittings

  Þ magnetism is essentially independent of environment

• Magnetic moment of a J-state is expressed by the Landé formula

Sample Landè Calculation e.g Pr3+ [Xe]4f2 Find Ground State from Hund's Rules Maximum Multiplicity S = 1/2 + 1/2 = 1 Þ 2S + 1 = 3 Maximum Orbital Angular Momentum L = 3 + 2 = 5 Þ H state Total Angular Momentum J = (L + S), (L + S) - 1, ÁL - SÁ = 6 , 5, 4 Less than half-filled sub-shell Þ Minimum J Þ J = 4 {Greater than half-filled sub-shell Þ Maximum J}

Ln³⁺ Magnetic Moments compared with Theory

Experimental ^_____Landé Formula -•-•-Spin-Only Formula - - -

• Landé formula fits well with observed magnetic moments for all but Sm^III and Eu^III
• Moments of Sm^III and Eu^III are altered from the Landé expression by temperature-dependent population of low-lying excited J-state(s)

Uses of Ln³⁺ Magnetic Moments?

NMR Shift Reagents - paramagnetism of lanthanide ions is utilized to spread resonances in ¹H NMR of organic molecules that coordinate to lanthanides (see account of Eu(fcam)₃)

Ferromagnetism / Anti-Ferromagnetism / Ferrimagnetism

see C.N.R. Rao & J. Gopalkrishnan, New Directions in Solid State Chemistry, CUP, 1986 p. 394-398

West, Solid State Chemistry p. 565-566, 575-578

• Lanthanide metals and alloys have interesting ordered magnetism effects

• SmCo₅ permanent magnets - FERROMAGNETIC
  • light weight
  • high saturation moments,
  • high coercivity
  • high magnetocrystalline anisotropy
  • Superior performance magnets for magnetic bearings / couplings / wavetubes & d.c. synchronous motors
• Garnets
  • Complex oxides A₃B₂X₃O₁₂

    A sitedistorted cubic environment

    B /X sitesoctahedral & tetrahedral sites

    

    [unit cell of Garnet contains 128 atoms!]

  • Rare Earth Garnets e.g. Ln₃Fe₅O₁₂ and Y₃Fe₅O₁₂ (yttrium iron garnet, YIG)

    FERRIMAGNETISM shows an unusual temperature-dependence

    as T‚

    moment Ø to zero at the Condensation Temperature

    above Condensation Temperature moment rises in the opposite direction to a maximum

    moment then Ø to zero at the Curie Temperature.in the normal manner

Reason?

• the magnetic moments of the rare earth and iron ions oppose each other
• the rare earth moments dominate at low temperature
• the rare earth moments randomize at a lower temperature than the iron moments

• "Magnetic Bubble" Memories
  • magnetic stripe domains which shrink to smaller cylinders in an applied field
  • Called bubbles because they follow the same equations as soap bubbles
  • Magnetic Memory since bubble = 1, and lack of bubble = 0 (binary)
  • 'Bubbles' move towards regions of lower field bias and don't coalesce
    • Þ magnetic medium doesn't need to move (unlike disks or tapes!)
    • typically the bubbles are moved along Ni-Fe tracks
  • Best bubble memory materials are 20 mm films of rare-earth Garnets, Ln₃Fe₅O₁₂
    • Smallest bubble sizes (2-3 mm) in (Sm_0.51Lu_0.42Y_1.21Ca_0.86)(Ge_0.70Si_0.16Fe_4.14)O₁₂

How a Magnetic Bubble Memory Works

Bibliography