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

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

"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

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 …

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)

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)

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

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

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

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

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

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

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

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

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

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.

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.

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

The resonance frequency, Mn3+ ~ 350-435 MHz Mn4+ ~ 330 MHz

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

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) 220405 A.M.L. Lopes et. Al. J. Mag. M. Mat. 272 (2004) 1667 R. Govindraj et.al. Phys. Scripta 74 (2006) 247

Thank You!