1. Hyperfine interaction studies in Manganites
Kumaresa Vanji. M EXP, CBPF.

2. Plan of Talk  Introductions to manganites  Theoretical explanations
Hyperfine interaction studies

3. "If the computer industry is going to continue to miniaturize electronics beyond silicon's current limitations, it will probably be necessary to look at materials like manganites, where, for example, nanoscale structures such as coexistent metallic and insulating phases can be built within media that are otherwise homogenous,"
-Lookman
Los Alamos National Laboratory
Nature, 428 (2004) 401

4. Introduction-Manganites
 Family of Manganese oxide (Mn-O) ceramics
 The name ''manganites'' was first given by Jonker and Van Santen in 1950
 The starting material was LaMnO3
 General formula of Mn oxides
RE1-XMXMnO3
RE = Rare earth
M = Ca, Sr, Ba, Pb …

5. Interest in the study of Manganites
Coexistence of metal and insulator phases
1857 – William Thomson (MR)
1950 – Jonker and Van Santen (LaMnO3)
1951 – Zener DE theory
1955 – Wollen and Koehler (LaCaMnO3)
1955 – Millis, Polaron formation
1988 – Peter Grünberg (GMR)
1993 – Sungho Jin et al (CMR)

6. Applications Magnetic storage Sensor technology Spintronics
IBM research
Magnetic storage
Sensor technology
Spintronics
Frequency switching devices
Storage density
In 1990 – 1 Gb/in2 (before GMR)
In 2000 – 20 Gb/in2 (after GMR)

7. Structure of Manganites
MnO6 Octahedra
Perovskite (La1-xMxMnO3)
Exist mixed valence sates (Mn3+, Mn4+)
 Distorted under Crystal field effect
 Formation of polarons under Jahn-Teller distortion
Double Perovskite
La2-2xM1+2xMn2O7

8. -behavior of MnO6 Octahedra
Crystal field splitting
Jahn-Teller distortion
Cubic Orthorhombic
eg – high energy level
t2g – low energy level

9. Properties of Manganites
Magnetoresistance (MR)
Metal insulator transition (TIM)
Ferromagnetic to paramagnetic phase transition (FM-PM)
Charge Ordering (CO)
Orbital Ordering (OO)

10. Colossal Magnetoresistance (CMR)
Dramatic change in electrical resistance of a material due to application of an external magnetic field.
Magnetoresistance ratio
MRR = [(H - 0) / 0 ] ×100%
H → resistivity with magnetic field
0→ resistivity without magnetic field

11. Charge Ordering (CO) due to localization of charges
associated with insulating and AFM behavior

12. Orbital Ordering (OO)
Favors or disfavors the DE interaction  Gives a complex spin-orbital coupled state  Orbital ordering coupled with Jahn-Teller distortion

13. Metallic behaviour -Double Exchange
Exchange of electron between two magnetic atoms through another element site   Mn(+3) – O(-2) – Mn(+4) Mn(+4) Mn(+3) eg
t2g
J- exchange integral
J 0 leads to FM ordering
J 0 leads to AFM ordering

14. Hopping amplitude of the eg hole from one site to another...
hopping is
maximum ( =t0) when the magnetic moments of the
manganese ions are aligned parallel (FM)
Minimum ( =0) when they are aligned antiparallel (AFM)
DE is possible only if the spins are aligned in parallel manner

15. Insulating behaviour -Jahn Teller Polaron
Self-trapping of d-electrons
by Jahn-Teller distortion
Coupling of eg electrons to Jahn Teller (JT) phonons ( ħωo , EJT )
Electrical Conductivity
Transition temperature
Ep – polaron binding energy
EJT – JT stabilization energy
- frequency of phonon

16. Hyperfine interaction in manganites
In the magnetically ordered state,
the nuclear spin I of the Mn3+/4+ ion is coupled to the
electron spin S by the hf interaction
= IAS
Where, A is the hf tensor.
The nuclei experience a hf field
Where, is the average value of the e- spin proportional to spontaneous mag.

17. Quadrupole splitting
T = 150K, ε = 0.223(1)
T = 200K, ε = 0.143(1)
T = 240K, ε = 0.111(1)
Structural transformation
related to the J-T dis.

18. Isomer shift
δ = 0.463(1) mm/s
Fe has high spin 3+
Spectrum shows the mixture of AFM and FM phases
Narrow 6 line pattern with absorption line intensities
3:4:1 ratio indicates magnetic moments have been
oriented parellel to the H.
Hyperfine field Hhy = 458 kOe
- spin of Fe is FM coupled to its neighbor Mn

20. The resonance frequency,
Mn3+ ~ MHz
Mn4+ ~ 330 MHz

22. Conclusions
MS, NMR and PAC studies were reported for manganite materials
Hyperfine interaction studies are an effective tools to study and understand the magnetic and electronic properties of manganites

23. References C. Zener Phy. Rev. 82 (1951) 403
E.O. Wollan and W.C. Koehler, Phys. Rev. 100 (1955) 545
T. Kimura et. al. Phys. Rev. Lett. 79 (1997) 3720
Y. moritomo et. al. Nature 380 (1996) 141
G. Kallias et. al. Phys. Rev. B 59 (1999) 1272
M. Kopcewicz et. Al J. Phys. Cond. Matt. 16 (2004) 4335
R. Govindraj et. Al. Phys. Rev. B 70 (2004)
A.M.L. Lopes et. Al. J. Mag. M. Mat. 272 (2004) 1667
R. Govindraj et.al. Phys. Scripta 74 (2006) 247

24. Thank You!