Spintronics A. Kellou and H. Aourag

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SpintronicsA. Kellou and H. Aourag

• Metallic Thin Films Revisited Fe, Co, Ni
  Multilayers

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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

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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.

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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

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Basic Structure

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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.

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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

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Magnetic Random Access Memory (MRAM)
Reversible
Low Resistance
High Resistance
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Issues in 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
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III. Applications ii) binary alloys
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FeCr, CoCr, and NiCr Structural and magnetic
properties
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III. Applications iii) Ternary alloys
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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.

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III. Applications iii) Ternary alloys
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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.

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III. Applications iii) Ternary alloys
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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.

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III. Applications iii) Ternary alloys
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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.

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III. Applications iii) Ternary alloys
Heusler alloys

• All alloys are ferromagnetic, except Co2AlTi and
  Ni2AlTi (paramagnetic).
• Large magnetization in Cr alloys .

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III. Applications iii) Ternary alloys
Heusler alloys
Lattice parameters and bulk modulii

• Cr has induced a volume contraction although
  Z(Ti) lt Z(Cr).
• This fact is due to changes in bonding.
• Cr has allso induced large bulk modulii except
  ofr Ni2AlCr (large magnitzation, hgh volume)

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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.

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III. Applications
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• 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

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III. Applications iv) Layered
structures
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• 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
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III. Applications iv) Layered
structures
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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)
M1 (Central)
Z0
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III. Applications iv) Layered
structures
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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.

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III. Applications iv) Layered
structures
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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 Ni antiferromagnetic coupled
TM
Cr (S)
Nothing about Mn (ferrimagnetic coupled ???!)
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III. Applications iv) Layered
structures
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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.

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III. Applications iv) Layered
structures
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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.
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III. Applications iv) Layered
structures
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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.

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III. Applications iv) Layered
structures
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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.

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III. Applications iv) Layered
structures
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Fe/Cr(001) systems
Total energies and total and partial magnetic
moments
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III. Applications iv) Layered
structures
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Fe/Cr(001) systems
Bilayer formation against the monolayer formation

• This energy is positive (0.54 mRy/unit cell) 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 favors ML 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.

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III. Applications iv) Layered
structures
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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).

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III. Applications iv) Layered
structures
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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 /, /--, /-, /--,
  /---.

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V. Conclusion
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• 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.

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V Conclusion
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Binary alloys

• In the binary systems XTi and XCr (XFe, 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.

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V Conclusion
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Ternary alloys

• Structural parameters, formation energies,
  magnetic moments, and electronic properties of
  XYZ Half-heusler and X2AlX Heusler alloys (XFe,
  Co, Ni XTi, 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.

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V Conclusion
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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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