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S pintronics A. Kellou and H. Aourag

S pintronics A. Kellou and H. Aourag. Metallic Thin Films Revisited: Fe, Co, Ni Multilayers. S pintronics. Metallic Thin Films Revisited: Fe, Co, Ni Multilayers. Spintronics: To Control a Spin of Electrons, not a Charge. Magnetic Nanostructures for Spintronics Magnetic Multilayers

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S pintronics A. Kellou and H. Aourag

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  1. SpintronicsA. Kellou and H. Aourag Metallic Thin Films Revisited: Fe, Co, Ni Multilayers

  2. Spintronics Metallic Thin Films Revisited: Fe, Co, Ni Multilayers Spintronics: To Control a Spin of Electrons, not a Charge Magnetic Nanostructures for Spintronics • Magnetic Multilayers • Magnetic Wires • Magnetic Quantum Dots Applications of Magnetic Nanostructures • Reading Heads, Magnetic Field Sensors, MRAM • Field Effect Transistor, Spin-Valve Transistor • Quantum Computer

  3. Basic Structure The prototype device that is already in use in industry as a read head memory-storage cell is the giant-magnetoresistive (GMR) sandwich structure which consists of alternating ferromagnetic and nonmagnetic metal layers.

  4. Basic Structure Depending on the relative orientation of the magnetizations in the magnetic layers, the device resistance changes from small (parallel magnetizations) to large (antiparallel magnetizations). This change in resistance (also called magnetoresistance) is used to sense changes in magnetic fields

  5. Basic Structure

  6. Basic Structure • two different approaches: • existing GMR-based technology • developing new materials with • larger spin polarization of electrons • making improvements or variations in the existing device • that allow for better spin filtering. • finding novel ways of both generation and • utilization of spin-polarized currents.

  7. Basic Structure Problems: existing metal-based devices do not amplify signals (although they are successful switches or valves), whereas semiconductor based spintronic devices could in principle provide amplification and serve, in general, as multi-functional devices. spin polarizers and spin valves

  8. Magnetic Random Access Memory (MRAM) Reversible Low Resistance High Resistance

  9. Issuesin Magnetic Multilayers • Fabrication of Ordered Nanostructures on a Surface • A detailed understanding of the various atomic processes that occur during the formation of nanosized islands on surfaces • Surfaces are not simply a static media onto which the deposited atoms and diffuse Deposition and nucleation on a surface is important

  10. 29 III. Applications: ii) binary alloys FeCr, CoCr, and NiCr: Structural and magnetic properties

  11. 30 III. Applications: iii) Ternary alloys Semi-Heusler alloys • Half-metallic materials possess 100% electron polarization at the Fermi energy. • New class of magnetic materials displaying metallic character for one electron spin population and insulating character for the other. • Technological interest as potential pure spin sources for use in spintronic devices, data storage applications, and magnetic sensors. • Difficult to confirm experimentally the half-metallicity charcter (clean stoichiometric surfac). • To known if the intermettallic alloys based on a ferromagnet -Ti -Cr can lead to a half-metallicity behavior.

  12. 31 III. Applications: iii) Ternary alloys Semi-Heusler alloys Ground states from total energy calculations • FeCoTi, CoTiCr, NiTiCr, and FeCoNi are predicted ferromagnetic. • FeNiTi, FeNiCr, FeTiCr, and FeCoCr and are predicted antiferromagnetic. • FeCoCr and FeNiCr are nonmagnetic.

  13. 32 III. Applications: iii) Ternary alloys Semi-Heusler alloys Total DOS • All alloys are polarized except FeNiCr and CoTiCr. • FeCoTi, FeNiTi, and NiTiCr have a majority spin in a deep minimum right the Fermi level, leading to a pseudo-gap which is responsible for 100% electron polarization.

  14. 33 III. Applications: iii) Ternary alloys Heusler alloys • Stoichiometric composition X2YZ • Electronic structure can range from metallic to semi-metallic or semiconducting behavior. • Half-metallic ferromagnetism, in which the bandstructure for majority electrons is metallic while the bandstructure for minority electrons is insulating. • Anomalous peak in the yield stress and high temperature strength and excellent oxidation and corrosion resistance.

  15. 34 III. Applications: iii) Ternary alloys Heusler alloys • All alloys are ferromagnetic, except Co2AlTi and Ni2AlTi (paramagnetic). • Large magnetization in Cr alloys .

  16. 35 III. Applications: iii) Ternary alloys Heusler alloys Lattice parameters and bulk modulii • Cr has induced a volume contraction although Z(Ti) < Z(Cr). • This fact is due to changes in bonding. • Cr has allso induced large bulk modulii except ofr Ni2AlCr (large magnitzation, hgh volume)

  17. 36 III. Applications: iii) Ternary alloys Heusler alloys Total DOS • Cr has induced Fermi displacement to the right (anti-bonding states) with a prounounced half-metallicity character in Fe2AlCr and to the left in Co2AlCr and Ni2AlCr.

  18. 37 III. Applications i) Transition element family ii) Binary systems iii) Ternary systems iv) Layered structures • Clean V(001), Cr(001) and Fe (100) surfaces • TM/5Cr(001) (TM = Ti, V, Cr, Mn, Fe, Co, Ni) • Fe/Cr(001) systems

  19. 38 Z The unit cell in film calculations. III. Applications: iv) Layered structures • Interesting properties (GMR, MAE, high local moments …) when ferromagnetic and antiferromagnetic transition elements are layered. • Determination of interlayer exchange coupling (IEC). • Effect of magnetism in surface, interface, and superlattices phenomena • Ferromagnetic substrates are well studied: Cu(001), Ag(001), Au(001), Fe(001), Co(001) …but not antiferromagnetic Cr !!! Vacuum Vacuum

  20. 39 III. Applications: iv) Layered structures Clean V(001), Cr(001), and Fe(001) surfaces • Surface magnetism in the (001) direction: nonmagnetic V, antiferromagnetic Cr, and ferromagnetic Fe. 5-layers of V(001), Fe(001) and Cr(001) in repeated slab structure. Magnetism occurs in V and is enhanced in Cr and Fe (001) surfaces because of the lying bonds (coordination number). M3 (Surface) M3 M2 (Sub-surface) Z=0 M1 (Central)

  21. 40 TM Cr(1) Cr(2) Cr(3) (a) 3-Cr(001) (b) 5-Cr(001) (c) TM/5-Cr(001) III. Applications: iv) Layered structures TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni) • Several theoretical and experimental studies were devoted to the surface properties of the magnetic 3d transition metal grown on noble metal (Cu, Ag, and Au) and ferromagnetic (Fe, Co, and Ni) but not Cr(001). • Study of total and surface energies of Cr(001) films, magnetic, and electronic properties of 3d transition-metal (Ti, V, Cr, Mn, Fe, Co, Ni) monolayer on Cr(001), with two opposite spin orientations leading to ferromagnetic and antiferromagnetic configurations.

  22. 41 III. Applications: iv) Layered structures TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni) Difference in total energy Ti, V, Cr ferromagnetic coupled Fe, Co, and Niantiferromagnetic coupled TM Cr (S) Nothing about Mn (ferrimagnetic coupled ???!)

  23. 42 III. Applications: iv) Layered structures TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni) Transition metal and total magnetic moment • TM’ s magnetic moment increases from Ti to Mn and decrease from Mn to Ni, in both ferromagnetic and antiferromagnetic configurations. • Mn deposition induces the highest value, followed by Fe, Co, and Ni. • Total magnetic moment has the same behavior as TM magnetic moment.

  24. 43 III. Applications: iv) Layered structures TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni) Spin Density Waves in Cr thin films The periodic nature the oscillations in 7-Cr(001) is strongly related to the itinerant linear Spin-Density Waves (observed in Cr multilayers, bulk Cr and its alloys. Cr thin films need SDW to have antiferromagnetic ground state.

  25. 44 TM Cr(1) Cr(2) Cr(3) (a) 3-Cr(001) (b) 5-Cr(001) (c) TM/5-Cr(001) III. Applications: iv) Layered structures TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni) • Several theoretical and experimental studies were devoted to the surface properties of the magnetic 3d transition metal grown on noble metal (Cu, Ag, and Au) and ferromagnetic (Fe, Co, and Ni) but not Cr(001). • Study of total and surface energies of Cr(001) films, magnetic, and electronic properties of 3d transition-metal (Ti, V, Cr, Mn, Fe, Co, Ni) monolayer on Cr(001), with two opposite spin orientations leading to ferromagnetic and antiferromagnetic configurations.

  26. 45 Cr Fe I-3 I-2 I-1 I (a) 4Cr(001) (b) 1Fe/3Cr(001) (c) 2Fe/2Cr(001) (d) Fe50Cr50/3Cr(001) (e) 1Fe/Fe50Cr50/2Cr(001) Fig. 5.24 Upper half-slab of the unit cell in: (a) 4Cr(001), (b) 1Fe/3Cr(001), (c) 2Fe/2Cr(001), (d) Fe50Cr50/3Cr(001), and (e) 1Fe/Fe50Cr50/2Cr(001). The first layer (I) corresponds to central layers. III. Applications: iv) Layered structures Fe/Cr(001) systems • Study of the diffusion, the surface alloy formation, and the magnetic properties in Fe/Cr(001) systems and magnetic properties of Fen/Crn(001) superlattices. • Fe/Cr multilayer exhibit interlayer exchange coupling (IEC), giant magneto-resistance (GMR), …etc. • Experimental results, obtained by similar techniques, often contradict each another and theoretical calculations also demonstrated a very complex behavior and solutions with close energies.

  27. 46 III. Applications: iv) Layered structures Fe/Cr(001) systems Total energies and total and partial magnetic moments

  28. 47 III. Applications: iv) Layered structures Fe/Cr(001) systems Bilayer formation against the monolayer formation • This energy is positive (+0.54 mRy/unitcell) in the ferromagnetic state and negative (-8.10 mRy/atom) in the nonmagnetic state. • This means that magnetic moments allow BL formation (2Fe/2Cr(001)), whereas nonmagnetic state favorsML formation (1Fe/3Cr(001)). • This result contradicts the description which was discussed for Cr (ML) on Fe(001) substrate, where ML formation is preferred for the ferromagnetic configuration.

  29. 48 III. Applications: iv) Layered structures Fe/Cr(001) systems Diffusion and surface alloy formation against phase separation • Fe do not diffuse to Cr bulk layers. • No magnetism favors phase separation or clustering, whereas magnetism favors formation of Fe50Cr50/3Cr(001) followed by Fe/Fe50Cr50/3Cr(001) ordered surface alloys (confirmed in recent experimental study).

  30. 50 III. Applications: iv) Layered structures Fe/Cr(001) systems Fen/Crn(001) superlattices • The formation energy is stabilized after n = 4. • The total magnetic moment is growing with the number of Fe and Cr layers. • Total energies favor the following spin alignments: +/+, ++/--, +++/+-+, ++++/-+-+, +++++/-+-+-.

  31. 51 V. Conclusion • We have given additional results to structural, electronic, and magnetic properties the selected transition materials (Ti, V, Cr, Mn, Fe, Co, and Ni) and their related systems; binary alloys, ternary alloys in Half-Heusler and Heusler structures, thin films and superlattices. • We have shown the importance of d-states in the ground state properties in these systems. • We have also studied the equilibrium parameters and the stability mechanism from the different formation energies and from the position of the Fermi level in the density of states. • The new form of the GGA approximation is adequate for transition metals and their related alloys. • The obtained structural properties are in good agreement with experimental data and more efficient than LDA ones.

  32. 52 V Conclusion Binary alloys • In the binary systems XTi and XCr (X=Fe, Co, Ni), effects of magnetism is studied and related to the structural and electronic structures. • The martensitic transformation (MT) phenomena of NiTi have been studied and optimized lattice parameters for B19’ were given. • The different roles of d-states were highlighted and are totally responsible for unexpected and controversial behaviors.

  33. 53 V Conclusion Ternary alloys • Structural parameters, formation energies, magnetic moments, and electronic properties of XYZ Half-heusler and X2AlX’ Heusler alloys (X=Fe, Co, Ni; X’=Ti, Cr) were presented. • The obtained results of lattice parameters and local magnetic moments agree very well with the experimental results. • Cr sites carry large magnetic moments and the moments at the X sites are usually small, when compared to Ti substitution. • All the densities of states are marked by a pseudogap left the Fermi level, except for Fe2AlTi where the pseudogap is right EF. • Among the selected materials, the Fe2AlCr and Co2AlCr alloys present a pronounced half-metallicity character.

  34. 54 V Conclusion Layered structures • The existence of itinerant linear Spin-Density Wave (SDW) is responsible for antiferromagnetic coupling between two adjacent Cr layers in Cr(001). • Mn overlayer induces the highest magnetic moments and relies between two opposite spin alignments in TM/Cr(001). Ferrimagnetic (FI) coupling can occur. Further investigations within the c(2x2) unit cell are necessary. • Ti, V, and Cr overlayers are antiferromagnetically coupled to the Cr sub-surface layer; Mn, Fe, Co and Ni are ferromagnetically coupled. • Fe layers are always antiferromagnetically coupled to Cr layers in Fe/Cr systems. • Fe atoms prefer to be deposited as an overlayer rather than being diffused in the Cr layers with formation of an ordered surface alloy. • Magnetism is responsible for the BL formation and ordered surface alloying in Fe/Cr (GMR, Colossal RM)

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