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磁存储 磁膜 纵向磁记录 垂直磁记录

磁 性 薄 膜 ( 数纳米至几十（一百）纳米 ). 磁存储 磁膜 纵向磁记录 垂直磁记录 磁泡 磁光 RAM 自旋磁电子学 层间反铁磁耦合和交换偏置 自旋阀 GMR (Giant magneto-resistivity (MR))

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磁存储 磁膜 纵向磁记录 垂直磁记录

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  1. 磁 性 薄 膜 (数纳米至几十（一百）纳米) 磁存储 磁膜 纵向磁记录 垂直磁记录 磁泡 磁光 RAM 自旋磁电子学 层间反铁磁耦合和交换偏置 自旋阀 GMR (Giant magneto-resistivity (MR)) TMR (Tunning MR) CMR (Colossal MR) STT (Spin torque transfer) Spin injection 理论 各向异性，层间耦合，磁阻

  2. Magnetic Thin Films • Magnetic Domains and Domain Walls • Magnetic Anisotropy, Reversal and Micro-magnetic • Magnetic Recording • Amorphous Films, Magneto-optical Films and Magnetic Semiconductor Films • Ultra-thin Films and Preparation of Magnetic Thin Films

  3. Magnetic Domain and Domain Walls 1. Domain walls Bloch wall, Neel wall, Cross-Tie wall 2. Magnetic domains Uniaxial wall spacing , Closure (塞漏) domain Stripe domains, Head to Head (Votex), Domain in antiferromagnetic films 3. Single domain and super-paramagnetism 4. Some methods for the domain observation SEMPA, MFM, Magneto-optical

  4. Relevant Energy Densities Exchange energy Magnetostatic Magnetocrystalline Magnetoelastic Zeeman

  5. In 1907, Weiss Proposed that magnetic domains that are regions inside the material that are magnetized in different direction so that the net magnetization is nearly zero. Domain walls separate one domain from another. P. Weiss, J.Phys., 6(1907)401. Modern Magnetic materials Principles and Applications by Robert C. O’handley

  6. Schematic of ferromagnetic material containing a 180o domain wall (center). Left, hypothetical wall structure if spins reverse direction over one atomic distance. Right for over N atomic distance, a. In real materials, N: 40 to 104.

  7. magnified sketch of the spin orientation within • a 180o Bloch wall in a uniaxial materials; (b) an appro- • ximation of the variation of θ with distance z through • the wall.

  8. Bloch Wall Thickness ? In the case of Bloch wall, there is significant cost in exchange energy from site i to j across the domain wall. For one pair of spins, the exchange energy is : , Surface energy density is , In the other hand, more spins are oriented in directions of higher anisotropy energy. The anisotropy energy per unit area increases with N approximately as

  9. The equilibrium wall thickness will be that which minimizes the sum with respect to N thus the wall thickness The minimized value No is of order , where A is the exchange stiffness constant. A=Js2/a ～10-11 J/m (10-6 erg/cm), thus the wall thickness will be of order 0.2 micron-meter with small aniso- tropy such as many soft magnetic materials

  10. Wall energy density ? The wall energy density is obtained by substituting into To give For uniform rotation γ=γex +γa =∫[g(θ)+A(әθ/әz)2]dz σdw = 4π(AKu)1/2

  11. Neel Wall Comparision of Bloch wall, left, with charged surface on the external surface of the sample and Neel wall, right, with charged surface internal to the sample.

  12. Energy per unit area and thickness of a Bloch wall and a Neel wall as function of the film thickness. Parameters used are A=10-11 J/m, Bs=1 T, and K=100 J/m3.

  13. In the case of Neel wall, the free energy density can be approximated as Minimization of this energy density with respect to δN gives For t/δN ≤1, the limiting forms of the energy density σN and wall thickness δN follow from above Eq.

  14. Neel wall near surface Calculated spin distribution in a thin sample containing a 180o domain wall. The wall is a Bloch wall in the interior, but it is a Neel wall near the surface. PRL 63(1998)668

  15. Cross-tie wall The charge on a Neel wall can destabilize it and cause it to degenerate into a more complex cross-tie wall Micromagnetics-dynamic simulation Cross-tie wall in thin Permalloy film: simulated (a and b) and observed (c) Nakatani et al., Japanese JAP 28(1989)2485.

  16. Wall energy as a function of thin film thickness

  17. Magnetic Domains Once domains form, the orientation of magnetization in each domain and the domain size are determined by Magnetostatic energy Crystal anisotropy Magnetoelastic energy Domain wall energy

  18. Domain formation in a saturated magnetic material is driven by the magnetostatic (MS) energy of the single domain state (a). Introduction of 180o domain walls reduces the MS energy but raises the wall energy; 90o closure domains eliminate MS energy but increase anisotropy energy in uniaxial material

  19. Uniaxial Wall Spacing The number of domains is W/d and the number of walls is (W/d)-1. The area of single wall is tL The total wall energy is . The wall energy per unit volume is

  20. Domain Size d ? The equilibrium wall spacing may be written as the quantity of do was substituted into fdw and fms The total energy density reduces to Variation of MS energy density and domain wall energy density with wall spacing d.

  21. For a macroscopic magnetic ribbon; L=0.01 m, σw= 1mJ/m2, ｕoMs= 1 T and t = 10ｕm, the wall spacing is a little over 0.1 mm.

  22. A critical thickness for single domain (The magnetostatic energy of single domain) Single domain size Variation of the critical thickness with the ratio L/W for two Ms (σdw=0.1mJ/m2)

  23. Size of MR read heads for single domain ? If using the parameters: L/W=5, σdw≈ 0.1 mJ/m2,ｕoMs= 0.625 T; tc ≈13.7 nm; Domain walls would not be expected in such a film. It is for a typical thin film magnetoresistivity (MR) read head.

  24. Closure Domains Consider σ90 =σdw /2, the wall energy fdw increases by the factor 1+0.41d/L; namely δfdw≈ 0.41σdw/L Hence the energies change to Geometry for estimation of equilibrium closure domain size in thin slab of ferro- magnetic material. If Δftot < 0, closure domain appears.

  25. Energy density of △ftot versus sample length L forｕo Ms=0.625 T, σ=0.1 mJ/m2, Kud=1mJ/m2, and td=10-14 m2.

  26. Domains in fine particles for large Ku Single domain partcle σdw πr2 =4πr2(AK)1/2 △EMS≈ (1/3)ｕo Ms2V=(4/9)ｕo Ms2πr3 The critical radius of the sphere would be that which makes these two energies equal(the creation of a domain wall spanning a spherical particle and the magnetostatic energy, respectively). rc≈ 3nm for Fe rc≈ 30nm for γFe2O3

  27. Domains in fine particles for small Ku If the anisotropy is not that strong, the magnetization will tend to follow the particle surface The spin rotate by 2π radians over that radius (a) (b) (a) A domain wall similar to that in bulk; (b) The magneti- zation conforms to the surface.

  28. The exchange energy density can be determined over the volume of a sphere by breaking the sphere into cylinders of radius r, each of which has spins with the same projection on the axis symmetry =2(R2-r2)1/2 Construction for calculating the exchange energy of a particle demagnetized by curling.

  29. If this exchange energy density cost is equated to the magnetostatic energy density for a uniformly magnetizes sphere, (1/3)ｕoMs2, the critical radius for single-domain spherical particles results: Critical radius for single-domain behavior versus saturation magnetization. For spherical particles for large Ku, 106 J/m3 and small one.

  30. Stripe Domain Spin configuration of stripe domains

  31. Spin configuration in stripe domains The slant angle of the spins is given as, θ = θo sin ( 2πx/λ ) The total magnetic energy (unit wavelength); When w >0 the stripe domain appears.

  32. Minimizingwrespect toλ (1) Using eq.(1) we can get the condition for w>0,

  33. Striple domainsin 10Fe-90Ni alloys film observed by Bitter powder (b) After switch off a strong H along the direction normal to striple domain. (a) After switch of H along horizontal direction. (c)As the same as (b), but using a very strong field.

  34. The stripe domain observed in 95Fe-5Ni alloys film with 120 nm thick by Lolentz electron microscopy.

  35. Head to head • Transverse domain structure for a head-to-head wall calculated in a 2nm thick, • 250 nm wide strip of Ni80Fe20, and • b) Vortex domain structure for head-to-head wall calculated in a 32 nm thick,250nm • wide strip of Ni80Fe20. R.D.McMichael (Gaithersburg) IEEE Trans Mag 33(1997)4167

  36. Fig. Schematic of discretization scheme for head-to-head domaim walls in an infinite strip. Landau-Lifshits-Gilberg equation Heff is a effective field including applied magnetic field, de-magnetic field and exchange field. Here anisotropy field is neglected.

  37. Domain wall energy as a function of film thickness for head-to-head wall with transverse (open symbols) and vortex (filled symbols) in strip of Ni80Fe20with widths of 75, 125, 250 and 500 nm.

  38. Partial phase diagram of head-to-head domain wall structure in thin magnetic strips. δ is the magnetostatic exchange length. The crossover critical dimensions suggest a phase boundary of the form tw=cδ2 (δ=128)

  39. Magnetic vortex core observation in circular dots in permalloy Shinjo et al., Science 289(2000) 930.

  40. Li et al., PRL 97(2006)107201

  41. Votex nucleation field

  42. Domain in AF interlayer coupling MLs Hellwig et al., PRL 91(2003)197203

  43. Superparamagnetism ProbabilityP per unit time for switching out of the metastable state into the more stable demagnetzed state: the first term in the right side is an attempt frequency factor equal approxi- mately 109 s-1. Δf is equal to ΔNµo Ms2 or Ku . For a spherical particle with Ku = 105 J/m3 the superparamagnetic radii for stability over 1 year and 1 second, respectively, are

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