INTRODUCTION TO COLOSSAL MAGNETORESISTANCE

Engineering Physics , Class of 2010

JAMES LOUREMBAM

OUTLINE

Overview of Magnetoresistance effects Structure of CMR materials Theory of CMR manganites Future Scope

Overview of Magnetoresistance effects

Ordinary magnetoresistance (OMR)

Magnetoresistance is the relative change in the electrical resistance or resistivity of a material produced on the application of a magnetic field

MR= [ /D r r(o)] = [ ( )- ( )]/ ( )r H r o r o Magnetic induction affects orbits of

electrons at Fermi surface. In clean metals, OMR typically ~ B2, and

can be ~ 10% at 10 Tesla

Pointed out experimentally as early as 1930s (though really solidly confirmed in 1960s) that FM material measured resistance depends on relative directions of M and J.

Physical origin: spin-orbit coupling leads to spin-dependent scattering of conduction electrons.

Typical size of effect: ~ 1%. Weak temperature dependence

Anisotropic magnetoresistance (AMR)

Giant Magnetoresistance (GMR)

Discovered in laboratory c. 1988. Typical size of effect: ~ 100%. The basic effect depends on the alignment

of electron spins at the interface of different kinds of magnetic materials

Not a trait of pure FM materials! Superlattice of very thin alternating layers of

FM and N metals. Effect gets larger at lower T and for cleaner

metal layers as other scattering contributions are reduced.

GMR (contd.)

Colossal magnetoresistance (CMR)

Discovered in 1993. Takes place in specific family of compounds,

perovskites of the form A1-xBxMnO3, where A = (La, Pr, Nd, Sm), B = (Ca, Sr, Ba).

Physical mechanism is completely different than any described so far.

Size of effect: ~ 100000% (!) Extremely temperature and doping

dependent - challenging to get useful, reproducible behavior at room temperature.

CMR (contd.)

Mechanism: phase transition between conductive FM ordering of Mn ions and insulating AFM ordering of Mn ions.

Still not well understood!

CMR

Jan-Teller distortion

Perovskite

Superexchange

interactionMetal-

insulator transition

Phase separation

Structure of CMR materials

Perovskite Manganites

Colossal Magnetoresistance is predominantly discovered in manganese-based perovskite oxides.

The structure of the RE1−xMxMnO3 oxides is close to that of the cubic perovskite

The large sized RE trivalent ions and M divalent ions occupy the A-site with 12-fold oxygen coordination. The smaller Mn ions in the mixed-valence state Mn3+–Mn4+ are located at the centre of an oxygen octahedron, the B-site with 6-fold coordination. The proportions of Mn ions in the valence states 3+ and 4+ are respectively, 1 − x and x.

Perovskite Manganites(contd.)

Theory of CMR manganites

Main features of CMR

Possess large room-temperature magnetoresistivity complemented by a unique type of metal- insulator transition

Also associated with a paramagnetic–ferromagnetic phase transition.

Theories

DE mechanism Jahn Teller distortion and CO Phase seperation

DE mechanism

Mn-O-Mn interaction controlled by overlap between Mn d-orbital and O p-orbitals.

Interesting case Mn+3-O-Mn+4

Exchange of valency jump of eg electron of Mn+3 on the O p-orbital and from O p-orbital to the empty eg orbital of Mn+4 ensure strong Ferromagnetic interaction, as well as conductivity

The shortcomings of this theory was that it was not able to explain the insulator transition or the AFM phase. Also, the experimentally observed resistivity values were higher than the theoretically calculated values

Jahn Teller distortion and CO

Jahn-Teller effect: A distortion of a highly symmetrical molecule, which reduces its symmetry and lowers its energy

This leads to a strong e-ph coupling manifesting CO state

The resultant localization of charges is associated with insulating and antiferromagnetic behavior

Jahn Teller distortion and CO (contd.)

Therefore a competition arises between ferromagnetic, metallic behavior and cooperative Jahn-Teller effect with charge-ordering.

However even after combining these two theories the theoretically calculated transition temperature were found to be much larger than the experimental values

Phase seperation

Both AFMI and FMM phases exist in CMR materials

The AFMI phase dominates in higher temperatures and the FMM dominates in lower temperatures

Supported by STM , transport and magnetic property studies, neutron diffraction studies.

Future Scope

Applications and promise

Discovery of huge magnetoresistance effects in the manganese oxide class of materials has rekindled intense interest in these systems

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.

The scientific challenge lies in discovering materials that manifest CMR at room temperatures and at low magnetic fields

Applications and promise (contd.)

THANK YOU