V. Pomjakushin I.M.Frank Laboratory of Neutron Physics, JINR, Dubna In collaboration with: A. Balagurov Frank Laboratory of Neutron Physics, JINR, Dubna P. Fischer, D. Sheptyakov Laboratory for Neutron Scattering, ETH Zurich and PSI, Villigen D. Khomskii Solid State Physics Laboratory, Materials Science Centre, University of Groningen V. Yushankhai Bogolyubov Laboratory of Theoretical Physics, JINR, Dubna A. Abakumov, E. Antipov, M. Lobanov, M. Rozova Chemistry Department, Moscow State University, Moscow Magnetic structure, disorder effects and unconventional superexchange interactions in A 2 MnGaO 5+  (A=Sr,Ca) layered oxides

SINQ user’s meting 2002, Bonn2 Why A 2 MnGaO 5+x (A=Sr, Ca)? Manganese oxides with possible CMR LaMn 3+ O 3 / SrMn 4+ O 3 La 2 Sr 1 Mn 3+ 2 O 7 / La 1 Sr 2 Mn 4+ 2 O 7 LaSrMn 3+ O 4 / Sr 2 Mn 4+ O 4 (AO) MnO 2 …(MnO 2 ) n –(AO) (AO) – (MnO 2 ) n … 3D Mn-O network

SINQ user’s meting 2002, Bonn3 Crystal structures of A 2 MnGaO 5+x (A=Sr, Ca) x=0, Mn 3+ Pnma, Ima2, Imcm P4/mmm, … x=0.5, Mn 4+ MnO 2 (AO) MnO 2 GaO 1+x 8Å8Å 4Å4Å Brownmillerite type of crystal structure There is space for extra oxygen in GaO 1+x -layers

SINQ user’s meting 2002, Bonn4 Neutron diffraction. Crystal structure HRPT/SINQ, =1.5 Å, Sr 2 MnGaO 5+x, T=10K x=0 Ima2 Imcm with disorder in GaO 4 tetrahedra x=0.5 P4/mmm with partially filled GaO 6 octahedra

SINQ user’s meting 2002, Bonn5 Neutron diffraction. Magnetic structure DMC/SINQ, =2.56 Å, T=2K G-structure h=2n+1 C-structure h=2n Short-range G-type G-type C-type

SINQ user’s meting 2002, Bonn6 Short range ordering effects G 40Å 5 layers 10 spins C C C Size effect peak broadening 40Å G-type short-ranged phase inside predominant C-type matrix Short-range G-type C-type Sr 2 MnGaO 5.5 Short-range G-type C-type

SINQ user’s meting 2002, Bonn7 Magnetic and crystal structures G-type GaO 4 Mn 3+ C-type GaO 6 Mn 4+ AFF Sr 2 MnGaO 5 Sr 2 MnGaO 5.5 AF

SINQ user’s meting 2002, Bonn8 Mn 3+ / Mn 4+ in octahedral site Mn 3+ t 2g xy, yz, zx Mn 4+

SINQ user’s meting 2002, Bonn9 AF in-plane superexchange (SE) Similar to CaMnO 3 Similar to RP phase (n=1) (SrCa)MnO 4 Mn 3+ O 2 plane 2.4Å 1.9Å Mn 3+ O 2- Mn 3+ Antiferromagnetic MnO 2 planes both for Mn 3+ and Mn 4+ in accord with standard SE. 180 o SE of half-filled both t 2g - t 2g and d z2 - d z2 orbitals is always AFM Mn 4+ O 2-  

SINQ user’s meting 2002, Bonn10 Interplane Mn-O-O-(O)-Mn superexchange Mn 3+ and GaO 4 AF Ga 180 o -AFM superexchange Vertical SE Mn A –Mn B weak FM weak AFM Diagonal SE Mn A –Mn C weak FM AFM Unconventional “diagonal” superexchange pxpx Mn 4+ and GaO 6 AF

SINQ user’s meting 2002, Bonn11 Mn-O-O-Mn diagonal 180 o superexchange 180 o -AFM superexchange 90 o weak FM superexchange p x, p z orbitals rotated by 45 o

SINQ user’s meting 2002, Bonn12 M eff accessed by neutron diffraction Effective magnetic moment effective occupancy real moment volume fraction Spin vacancy Spin flip  M/M=-2c

SINQ user’s meting 2002, Bonn13 Magnetic moments Magnetic moment seen by neutron diffraction is appreciably reduced -- local disorder, hybridization? Mn 3+ Mn 4+ Spin-only values Mn 3+ Mn 4+ DMC/SINQ

SINQ user’s meting 2002, Bonn14 Muon spin polarization P(t) below T N Local magnetic field distribution seen by  SR Coherent precession – long range ordering of Mn-spins Muon spin relaxation – disorder of Mn-spin configuration/value/direction ordered fraction

SINQ user’s meting 2002, Bonn15 2 types of disorder I. Disordered component of moment  SR frequency f ~ m 0 ND magnetic moment M ~ m 0 Local field B loc  SR frequency f~ m, while M ~ m(1-2c) II. Disorder of spin-configuration: Spin-flips with concentration c<<1 Disordered field from the flipped spin m f (r) -r, -m f r, m f

SINQ user’s meting 2002, Bonn16 Local magnetic disorder Ca 2 MnGaO 5.0 Sr 2 MnGaO 5.0 (G-type)  f Sr =f Ca, while  M/M  10%   (  /(2  f))  12% Spin flips (concentration<<1) can explain ND-  SR “contradiction” Sr 2 MnGaO 5.5 (C-type) Local spin fluctuations (spin-flips) Volume fraction decreased – second phase develops spatially separated Mn 3+ Mn 4+ Moment from neutron diffraction

SINQ user’s meting 2002, Bonn17 Summary  Novel manganese layered oxides A 2 MnGaO 5+  (A=Sr,Ca) with adjustable Mn 3+ /Mn 4+ -valence: synthesis and structure.  The principal structure difference between the  0 (Mn 3+ ) and  0.5 (Mn 4+ ) is GaO 1+  buffer layer, which is formed by tetrahedra or partially filled octahedra  AFM (  0) --> FM (  0.5) coupling between the AFM ordered MnO 2 - layers. Unconventional diagonal superexchange Mn 4+ -O-O-O-Mn 4+  Disorder effects in magnetic ordering - spin flops and short range phase separation. The magnetic disorder can be caused by the disorder in oxygen positions in GaO 1+  -layer.

SINQ user’s meting 2002, Bonn18 The end G-type GaO 4 Mn 3+ C-type GaO 6 Mn 4+ AFF Sr 2 MnGaO 5 Sr 2 MnGaO 5.5 AF

SINQ user’s meting 2002, Bonn19 Intermediate Mn-valence in Sr 2 MnGaO 5+ .  =0.13,  =0.41 GG + CC = 11 22

SINQ user’s meting 2002, Bonn20

SINQ user’s meting 2002, Bonn21 Configurational disorder Disordered component of moment B loc Disorder of spin-configuration: Spin-flips with concentration c<<1  SR frequency f ~ m 0 ND magnetic moment M ~ m 0 Local field f~ m, while M ~ m(1-2c) Disordered field from the flipped spin m f (r) -r, -m f r, m f

SINQ user’s meting 2002, Bonn22 Why A 2 MnGaO 5+x (A=Sr, Ca)?  Known Mn-oxides with intermediate Mn-valence 3+x possessing CMR effect are from Ruddlesden-Popper (RP) series (LaSr) n+1 Mn n O 3n+1 …(MnO 2 ) n –(AO) (AO) – (MnO 2 ) n …  La 1-x Sr x Mn 3+x O 3 – 3D Mn-O network (RP, n=  )  La 2-2x Sr 1+2x Mn 3+x 2 O 7 – 2D Mn-O network (RP, n=2)  A 2 MnGaO 5+x are novel manganese oxides with Mn 3+ /Mn 4+ —O network allowing double exchange. New doping possibility through incomplete oxygen sublattice. …MnO 2 –(AO)(GaO 1+x )(AO) – MnO 2 …

SINQ user’s meting 2002, Bonn23 Experiment Sample synthesis, X-ray at Inorg. Chem. Lab., MSU, Moscow Crystal and magnetic structures: powder neutron diffraction at HRPT and DMC, SINQ/PSI, Villigen Local disorder:  SR at PSI muon facilities

SINQ user’s meting 2002, Bonn24 M eff accessed by neutron diffraction Magnetic structure factor Effective magnetic moment effective occupancy real moment volume fraction Spin vacancy Spin flip  M/M=-2c