SPINTRONICS: Colossal Magnetoresistance Lubna SHAH Department of physics University of Delaware

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SPINTRONICSColossal Magnetoresistance
Lubna SHAHDepartment of physicsUniversity of
Delaware
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• Spintronics
• ? new branch of electronics in which electron
  spin, in addition to charge, is manipulated to
  yield a desired outcome.
• Magnetoresistance
• ? a change in electrical resistance due to the
  presence of a magnetic field.
• Colossal
• ? effect of an exceptional or astonishing degree.

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• Magnetoresistance, or MR.
• Consider an electric current running in a
  material like iron. Placed in a strong magnetic
  field, its resistance drops or increases by
  several percent, depending on orientation.
• Giant Magnetoresistance GMR.
• 1988, thinly layered materials were found that
  increased or decreased their resistivity by 20
  percent or more in relatively weak magnetic
  fields -- hence "giant" Magnetoresistance, or
  GMR. The basic effect depends on the alignment of
  electron spins at the interface of different
  kinds of magnetic materials.
• Colossal Magnetoresistance CMR.
• 1993, materials were found that could increase or
  decrease resistance not by a few percent but by
  orders of magnitude. Hence "colossal"
  Magnetoresistance

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• Discovery of huge magnetoresistance effects in
  the manganese oxide class of materials (such as
  Lal-xAxMnO3 (A Sr, Ca, Ba)) materials has
  rekindled intense interest in these systems for
  reasons.
• CMR offers potential in a number of technologies,
  such as for read/write heads in magnetic
  recording media, sensors, and spin-polarized
  electronics.
• CMR materials are interesting from a fundamental
  viewpoint. In contrast to traditional
  ferromagnets such as Fe, Co and Ni where the spin
  system is isolated from the lattice, in the
  manganites the charge, spin, and lattice degrees
  of freedom are strongly coupled together, leading
  to a delicate balance of interactions that gives
  rise to a rich variety of physical phenomena of
  current interest in condensed matter science.

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Colossal Magnetoresistance ? predominantly
discovered in manganese-based perovskite
oxides. General formula of manganese
oxides? RE(1-x)M(x)MnO3 RErare earth, MCa, Sr,
Ba, Pb. CMR effect arises because of
strong mutual coupling of following degrees of
freedoms - Spin - Charge - Lattice BUT
HOW ????????
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Phase diagram of La1-xSrxMnO3

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• How can we explain Phase Diagram
• different resistivity regions
• different magnetic regions
• ????????

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• Important physical features of CMR manganites
• Crystallographic structure
• Electronic structure
• Exchange interactions

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• Crystallographic structure
• Structure of RE(1-x)M(x)MnO3 oxides? cubic
  pervoskite
• Large sized RE trivalent ions and M divalent ions
  ? A-site with 12-fold oxygen coordination.
• Smaller Mn ions (in mixed valence state of
  Mn3-Mn4) ? B-site with 6-fold oxygen
  coordination.
• Proportions of Mn ions in 3 and 4 ? 1-x, x
  respectively.

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• Electronic structure
• The d-orbitals of transition metals  are  of five
  types

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• Crystal field ? partly lifting degeneracy (5
  d-orbitals3 t2g and 2 eg )
• Mn3 (3d4)?S2
• Mn4 (3d3)?S3/2

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• Exchange interaction
• Magnetic properties of manganites ?exchange
  interaction between Mn ion spins.
• Mn-O-Mn interaction? controlled by overlap
  between Mn d-orbital and O p-orbitals.
• Corresponding superexchange interactions depend
  on orbital configuration.
• M4-O-Mn4? AF
• M3-O-Mn3? F or AF
• Interesting case? Mn3-O-Mn4
• Exchange of valency ? jump of eg electron of
  Mn3 on the O p-orbital and from O p-orbital to
  the empty eg orbital of Mn4 ? ensure strong
  Ferromagnetic interaction.

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• What kind of ordering occurs in CMR compounds
• ???????????

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• Most of the perovskite oxides exhibit the
  following types of ordering.
• Charge ordering
• Orbital ordering
• Spin ordering

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• Charge Ordering
• The study of CMR in rare-earth manganate
  perovskites ?novel features such as
  charge-ordering.
• Charge ordering in the manganates is interesting
  because charge-ordering is resulting due to
  localization of charges therefore it is
  associated with insulating and antiferromagnetic
  (or paramagnetic) behavior.
• Double-exchange gives rise to metallicity along
  with ferromagnetism.
• Therefore a competition arises between
  ferromagnetic, metallic behavior and cooperative
  Jahn-Teller effect with charge-ordering.

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Charge ordering along ab-plane in La0.5Sr0.5MnO4

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• Orbital ordering
• Orbital-ordering gives rise to the anisotropy of
  the electron-transfer interaction.
• This favors or disfavors the double-exchange
  interaction or superexchange interaction in an
  orbital direction-dependent manner.
• Hence gives a complex spin-orbital coupled state.
  The orbital-ordering is coupled with Jahn-teller
  distortion .
• Different types of orbital ordering occuring  in
  manganese-oxides perovskites is shown above.

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• Spin Ordering
• Interactions with neighboring atoms make the spin
  of electrons align in a particular fashion.
• Ferromagnetism ?when the spins are arranged
  parallel to one another.
• Antiferromagnetism ?results when they are
  anti-parallel to one another. 
• Antiferromagnetic ordering is of three types
  particularly in perovskite-type oxides.
• A-type The intra-plane coupling is ferromagnetic
  while inter-plane coupling is antiferromagnetic.
• C-type The intra-plane coupling is
  antiferromagnetic while inter-plane coupling is
  ferromagnetic.
• G-type Both intra-plane and inter-plane coupling
  are antiferromagnetic.

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• Types of spin ordering in perovskite oxides

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• Knowing above Physics of CMR , what can you do
• ?????

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• Colossal magnetoresistance materials-based
  junctions with antiferromagnetic insulating
  barriers.
• Appl. Phys. Lett. 85(3) 437 (19 Jul 2004)
• Colossal magnetoresistance in spinel type
  Zn1xNixFe2O4.
• J. Appl. Phys. 96(2) 1273 (15 Jul 2004)
• La0.7Pr0.3MnO3 ceramic An electron-doped
  colossal magnetoresistive
• APP. Phys. Lett. 84(23) 4741 (07 Jun 2004)
• Effect of Isovalent Doping of Manganite
  (La1  xPrx)0.7Ca0.3MnO3 Films (0 x 1) on Their
  Optical, Magnetooptical, and Transport Properties
  near the MetalInsulator Transition.
• Phys. Solid State 46(7) 1241 (01 Jul 2004)

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• Colossal magnetoresistance effect of
  electron-doped manganese oxide thin film
  La1xTexMnO3 (x 0.1,0.15).
• J.Appl. Phys. 95(11) 6322 (01 Jun 2004)
• Phase separation and huge enhancement of
  magnetoresistance in Pr0.65Ca0.35xSrxMnO3.S.
• J. Appl. Phys. 95(11) 6813 (01 Jun 2004)
• Substrate and thickness effects on structure and
  transport properties of La2/3Ca1/3MnO3
• J. Appl. Phys. 95(11) 7109 (01 Jun 2004)
• Transport and magnetic properties of bulk
  La1xSbxMnO3 (x 0.05,0.1).
• J. Appl. Phys. 95(10) 5666 (15 May 2004)

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• There are many more articles available to read

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• References
• J. Physics D, 36, R127(2003)
• J. App. P, 79, 5288, (1996)
• PRB,58,3697,(1998)
• Weblinkhttp//folk.uio.no/ravi/activity/ordering/
  colossal-magnet.html