Magnetic disorder effects in A2MnGaO5 oxides

1. Magnetic structure, disorder effects and unconventional superexchange interactions in A2MnGaO5+ (A=Sr,Ca)layered oxides 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 V. Pomjakushin I.M.Frank Laboratory of Neutron Physics, JINR, Dubna

2. …(MnO2)n –(AO) (AO) – (MnO2)n… 3D Mn-O network MnO2 (AO) MnO2 LaSrMn3+O4/ Sr2Mn4+O4 LaMn3+O3/ SrMn4+O3 La2Sr1Mn3+2O7/ La1Sr2Mn4+2O7 Why A2MnGaO5+x (A=Sr, Ca)? Manganese oxides with possible CMR SINQ user’s meting 2002, Bonn

3. There is space for extra oxygen in GaO1+x-layers Crystal structures of A2MnGaO5+x (A=Sr, Ca) x=0, Mn3+ x=0.5, Mn4+ P4/mmm, … Pnma, Ima2, Imcm MnO2 (AO) GaO1+x 8Å (AO) MnO2 4Å Brownmillerite type of crystal structure SINQ user’s meting 2002, Bonn

4. HRPT/SINQ,  =1.5 Å, Sr2MnGaO5+x, T=10K • x=0 • Ima2 • Imcm with disorder in GaO4 tetrahedra x=0.5 P4/mmm with partially filled GaO6 octahedra Neutron diffraction. Crystal structure SINQ user’s meting 2002, Bonn

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

6. C-type Short-range G-type Sr2MnGaO5.5 C-type 40Å 5layers G C C Size effect peak broadening 10 spins 40Å C G-type short-ranged phase inside predominant C-type matrix Short-range G-type Short range ordering effects SINQ user’s meting 2002, Bonn

7. C-type G-type GaO6 AF F GaO4 Mn4+ Mn3+ AF Magnetic and crystal structures Sr2MnGaO5.5 Sr2MnGaO5 SINQ user’s meting 2002, Bonn

8. Mn3+ Mn4+ xy, yz, zx t2g Mn3+/ Mn4+ in octahedral site SINQ user’s meting 2002, Bonn

9. Antiferromagnetic MnO2 planes both for Mn3+ and Mn4+ in accord with standard SE. 180o SE of half-filled both t2g- t2g and dz2- dz2 orbitals is always AFM O2- Mn4+ Mn4+ Mn3+ Mn3+ O2- 2.4Å   1.9Å Mn3+O2 plane AF in-plane superexchange (SE) Similar to CaMnO3 Similar to RP phase (n=1) (SrCa)MnO4 SINQ user’s meting 2002, Bonn

10. Mn3+ and GaO4 Unconventional “diagonal” superexchange Mn4+ and GaO6 Vertical SE MnA –MnB weak FM weak AFM Ga AF AF Diagonal SE MnA –MnC weak FM AFM px 180o-AFM superexchange Interplane Mn-O-O-(O)-Mn superexchange SINQ user’s meting 2002, Bonn

11. 90o weak FM superexchange px, pz orbitals rotated by 45o 180o-AFM superexchange Mn-O-O-Mn diagonal 180o superexchange SINQ user’s meting 2002, Bonn

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

13. Mn3+ Mn4+ Magnetic moments Mn3+ DMC/SINQ Spin-only values Mn4+ Magnetic moment seen by neutron diffraction is appreciably reduced -- local disorder, hybridization? SINQ user’s meting 2002, Bonn

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

15. Ca2MnGaO5.0 • Sr2MnGaO5.0(G-type) • fSr=fCa, while M/M10% • (/(2f))  12% • Spin flips (concentration<<1) can explain ND-SR “contradiction” Moment from neutron diffraction Sr2MnGaO5.5 (C-type) Local spin fluctuations (spin-flips) Mn3+ Mn4+ Volume fraction decreased – second phase develops spatially separated Local magnetic disorder SINQ user’s meting 2002, Bonn

16. Summary • Novel manganese layered oxides A2MnGaO5+ (A=Sr,Ca) with adjustable Mn3+/Mn4+-valence: synthesis and structure. • The principal structure difference between the 0 (Mn3+) and 0.5 (Mn4+) is GaO1+ buffer layer, which is formed by tetrahedra or partially filled octahedra • AFM (0) --> FM (0.5) coupling between the AFM ordered MnO2-layers. Unconventional diagonal superexchange Mn4+-O-O-O-Mn4+ • 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 GaO1+ -layer. SINQ user’s meting 2002, Bonn

17. The end C-type G-type GaO6 AF F GaO4 Mn4+ Mn3+ AF Sr2MnGaO5.5 Sr2MnGaO5 SINQ user’s meting 2002, Bonn

18. 1 2 Intermediate Mn-valence in Sr2MnGaO5+. =0.13, =0.41 G C + = SINQ user’s meting 2002, Bonn

19. SINQ user’s meting 2002, Bonn

20. Configurational disorder Disorder of spin-configuration: Spin-flips with concentration c<<1 Disordered component of moment -r, -mf Disordered field from the flipped spin mf(r) Local field r, mf Bloc SR frequency f ~ m0 ND magnetic moment M ~ m0 f~ m, while M ~ m(1-2c) SINQ user’s meting 2002, Bonn

21. Why A2MnGaO5+x (A=Sr, Ca)? • Known Mn-oxides with intermediate Mn-valence 3+x possessing CMR effect are from Ruddlesden-Popper (RP) series (LaSr)n+1MnnO3n+1 …(MnO2)n –(AO) (AO) – (MnO2)n… • La1-xSrxMn3+xO3 – 3D Mn-O network (RP, n=) • La2-2xSr1+2xMn3+x2O7 – 2D Mn-O network (RP, n=2) • A2MnGaO5+x are novel manganese oxides with Mn3+/Mn4+—Onetwork allowing double exchange. New doping possibility through incomplete oxygen sublattice. …MnO2 –(AO)(GaO1+x)(AO) – MnO2… SINQ user’s meting 2002, Bonn

22. 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, Bonn

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