Electronically smectic-like phase in a nearly half-doped manganite

Electronically smectic-like phase in a nearly half-doped manganite J. A. Fernandez-Baca Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA INTERNATIONAL WORKSHOP ON SELF-ORGANIZED STRONGLY CORRELATED ELECTRON SYSTEMS Seillac, France May 29 -31, 2006 OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Collaborators • F. Ye (ORNL) • P. Dai (Univ. of Tennessee, ORNL) • J. W. Lynn (NIST) • H. Kawano-Furukawa (Ochanomizu University, Japan) • H. Yoshizawa (ISSP-University of Tokyo, Japan) • Y. Tomioka (CERC, Japan) • Y. Tokura (U. of Tokyo, CERC, Japan OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Outline · Motivation: Charge and orbital ordering in the CMR manganites are associated with the insulating state in the manganite perovskites. · “CE-type” CO/OO and AF ordering is prevalent and not so far energetically from the metal state · Need to better understand CE-type CO/OO and AF order · Neutron studies in Pr 1 -x(Ca 1 -y. Sry)x. Mn. O 3 near x=0. 5 Evidence of weakly coupled 1 D spin chains (smetic-like) above TN · Discussion OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY Ye et al Phys. Rev. B 72, 212404 (2005)

CMR perovskite manganites: Substitution of Ln 3+ for Ca 2+ results in mixed valence Mn Ln 1 -x. Ax. Mn. O 3 : perovskite structure • Ln 3+=Lanthanide (La, Pr, Nd…) • A 2+=Ca, Sr, Ba… OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY A site Mn: B site

Rich phase diagram: Metallic state is associated with ferromagnetism due to Double exchange P. Schiffer et al, PRL (1995) Insulator-Metal transition coupled with the Paramagnetic. Ferromagnetic (PM-FM) transition OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Rich phase diagram: Insulating state is associated with Charge/orbital Ordering (CO-OO) Pr 0. 5 Sr 0. 5 Mn. O 3 50% Mn 3+ with 50% Mn 4+ CE-type CO-OO Tomioka et al, PRL 74 CO-OO promotes AF and insulating behavior OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Rich phase diagram: Insulating state is associated with Charge/orbital Ordering (CO-OO) 50% Mn 3+ with 50% Mn 4+ CE-type CO-OO The prevalent CO-OO and AF order is of the so-called CE-type originally proposed by Goodenough (Phys. Rev. 1955) Tomioka et al, PRL 74 CO-OO promotes AF and insulating behavior OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Neutron scattering of CE-type CO-OO peak at [1/4, 1/4] OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Charge/orbital ordering CO-OO can be melted by an external magnetic field Pr 1 -x(Ca 1 -y. Sry)x. Mn. O 3 y=0. 20, x=0. 45 Tomioka and Tokura PRB 66, 104416 (2002) Ye et al (unpublished) OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Short range CO/OO fluctuations in LCMO 30 above T C CO-OO peak at [1/4, 1/4] CMR seems to result from the competition of the tendency to CE CO/OO and DE Dai et al. , PRL (2000) OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Trying to understand the CE-type CO-OO Goodenough (1955) • charge ordering at high temperature ( ~ covalent bond energy) • magnetic ordering at much lower temperature (exchange interaction). • van den Brink et al. PRL (1999) • • • Anisotropy of short range DE Competition between Kinetic Energy Magnetic Exchange Energy Orbital degeneracy OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Trying to understand the CE-type CO-OO Zener polarons (Daoud. Aladine) Trapping of electrons in Mn pairs (Zener polarons) Mn 3+/Mn 4+ picture challenged • Popović et al. , PRL (2002) • • • Jahn-Teller Distortion Onsite Coulomb repulsion Mn 3+/Mn 4+ picture ? ? ? OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY • Resonant x-ray experiments suggets magnetic moments at Mn 3+ and Mn 4+ not so different • Magnetic powder diffraction, small difference in the magnetic moments

Trying to understand the CE-type CO-OO Important questions • Origin of CE-type structure • Role of magnetism • What about Mn 3+ and Mn 4+, are they really different? (in magnitude, in character) • Are there really zigzag chains of Mn moments? OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Case study: CO-OO Pr 0. 55(Ca 1 -y. Sry)0. 45 Mn. O 3 (PCSMO 45) (Pr, Ca)Mn. O 3 (Pr, Sr)Mn. O 3 Tomioka et al. (2002) OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

CE type structure in real and reciprocal space Mn 4+ AFM (½, 0, 0) Mn 3+ AFM (¼, ¼, 0) Mn 3+ CO-OO (¼, ¼, 0)+(2, 0, 0) OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY Magnetic form factor

Charge/Orbital Ordering of PCSMO 45 (Synchrotron X-ray) CO-OO peak at large equivalent [1/4, 1/4] T=240 K T=160 K T=130 K OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Resistivity, CO-OO and AF Ordering in PCSMO 45 OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Anisotropic AFM fluctuation in PCSMO 45 • Highly anisotropic • Suggest low-dimensional behavior (1 D, smectic) OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Comparison of Mn 3+ and Mn 4+ in PCSMO 45 Mn 3+ Mn 4+ Scattering from Mn 3+ and Mn 4+ have distinct orbital character F. Ye, et al. , PRB (2005) OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Comparison of AFM (Mn 3+ and Mn 4+) in PCSMO 45 • AFM from Mn 3+ shows strong hysteresis • AFM from Mn 4+ nearly no hysteresis OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Evolution of CE-type CO-OO and magnetic ordering TN < TCO T < TN First observation of quasi long-range 1 D spin chains OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

Discussion - Quasi long range 1 D zigzag spin changes are the “building blocks” of the CE-structure - Consistent with the new “soft phase” proposed by Milward et al (Nature 2005), Dagotto et al (Science, 2005) OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY

General feature in “strongly correlated electron” system S. A. Kivelson, Nature (1998) charge/spin order in cuprates (Stripes) Tranquada, Nature (2004) OAK RIDGE NATIONAL LABORATORY U. S. DEPARTMENT OF ENERGY