Fernande Moussa Laboratoire Léon Brillouin, CEA / CNRS, CEA-Saclay

1. Intra- and Interlayer Exchange Tuned by Magnetic Field in the Bilayer Manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7, Probed by Inelastic Neutron Scattering. Fernande Moussa Laboratoire Léon Brillouin, CEA / CNRS, CEA-Saclay F-91191 Gif sur Yvette Cedex France

2. Martine Hennion, Arsen Gukasov, Sylvain Petit Laboratoire Léon Brillouin, Saclay France Louis-Pierre Regnault, Alexandre Ivanov Institut Laue-Langevin Grenoble France R. Suryanarayanan, Mircea Apostu, Alexandre Revcolevschi. Laboratoire de Physico-Chimie de l’Etat Solide, F-91405 Orsay France ESOS 18-22 June 2008, Cracow

3. Plan • Crystal Structure of (La0.4Pr0.6)1.2Sr1.8Mn2O7 • Long Range Ferromagnetic State Induced by the Magnetic Field • Strong Anisotropy in the Exchange Couplings • Comparison with the Parent Compound La1.2Sr1.8Mn2O7 • Discussion ESOS 18-22 June 2008, Cracow

4. eg Motivation • Among the CMR manganites, the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7 attracts a great interest because of its ferromagnetic and metallic phase induced by a magnetic field. Its remarkable properties of CMR are mainly due to the entanglement of the different degrees of freedom of lattice, charge, spin and orbital. Double exchange of Zener dx2-y2 d3z2-r2 eg t2g t2g dxy, dyz,dzx Mn4+ Mn3+ Qi,j : angle between Si and Sj ESOS 18-22 June 2008, Cracow

5. J J// STRUCTURE of(La0.4Pr0.6)1.2Sr1.8Mn2O7 Derived from the n = 2 member of the Ruddlesden-Poper series (La1-xSrx)n+1Mn2O7, (La0.4Pr0.6)1.2Sr1.8Mn2O7 has the same structure as the parentcompound La1.2Sr1.8Mn2O7I4/mmm (F. Wang et al. Phys. Rev. Lett. 91 (2003) 47204) with strongest distortions of MnO6 octahedra. It has the same hole doping rate x = 0.4. Structure of La2-2xSr1+2xMn2O7 : I4/mmm. T.G. Perring et al. Phys. Rev. Lett. 87 (2001) 217201 ESOS 18-22 June 2008, Cracow

6. c* <Sz> = 3.8 B H b* Sz (110) a* Long Range Ferromagnetic Order Induced by the Magnetic Field A Pr-substitution on La-site suppresses the ferromagnetic order observed in the parent compound La1.2Sr1.8Mn2O7. The substituted compound remains a paramagnet and an insulator at any temperature. But, under a magnetic field, it becomes ferromagnetic and metallic with a large CMR ratio ~ 106. (M. Apostu et al. Phys. Rev. B 64 (2001) 12407 and I. Gordon et al. Phys. Rev. B 64 (2001) 92408) Imes = IN + Imag Imag <Sz 2> ESOS 18-22 June 2008, Cracow

7. Determination of exchange couplings between spins inside a layer (Jab) and spins belonging to distinct layers forming the bilayer (Jc). The neutron inelastic magnetic scatteringallows the measurement of the spin wave spectrum. The cross section S(Q,w), allows the determination of Jab and Jc. For the Heisenberg model, the dispersion of the spin waves propagating in the (a,b) plane consists of two branches: an acoustic one, w(q)ac, and an optic one, w(q)opt: ESOS 18-22 June 2008, Cracow

8. Q// is the Q-component parallel to H, Dz is the thickness of the bilayer, Dz ~ a, and qx = qy. The intensities of spin wave branches are: The intensity of the acoustic branch is maximum for : The intensity of the optic branch is maximum for ESOS 18-22 June 2008, Cracow

9. Spin wave dispersion curves in (La0.4Pr0.6)1.2Sr1.8Mn2O7 ESOS 18-22 June 2008, Cracow

10. Examples of spectra allowing the determination of the dispersion curves Spin wave acoustic branches with H//c andH//a+b ESOS 18-22 June 2008, Cracow

11. H//c Zoom of the acoustic branch near q=0 to determine the spin wave gap In Ref : F. Moussa et al. PRL 63, 107202 (2004) w0 = 0.87 meV  0.04 meV and gmBH = 0.58 meV ÞD = 0.29 meV D = 146 meV  5 meV ESOS 18-22 June 2008, Cracow

12. Examples of spectra allowing the determination of the dispersion curves Spin wave optic branches with H//c andH//a+b ESOS 18-22 June 2008, Cracow

13. Comparison between (La0.4Pr0.6)1.2Sr1.8Mn2O7 and the parent compound La1.2Sr1.8Mn2O7 : [1] T. Chatterji et al. PRB 60 R6965 (1999) [2] T.G. Perring et al. PRL 87 217201 (2001) ESOS 18-22 June 2008, Cracow

14. O w Mn Mn DISCUSSION Pr ion on the La-site plays a major role. It distorts the structure and destroys the long range ferromagnetic order. The electronic band W width becomes narrower, a gap opens and the compound becomes insulating. However the energies of the different ground states are not so far each other. And a moderated magnetic field is able to restore a ferromagnetic and metallic state. In the substituted compoundwe have found an in-plane exchange coupling Jab nearly the same as in the parent compound and an interlayer exchange coupling clearly smaller than in the parent compound. ESOS 18-22 June 2008, Cracow

15. Jc is controlled by the rod-like orbital Jabiscontrolled by the electron occupancy of the planar orbital Jc Jab ESOS 18-22 June 2008, Cracow

16. In the parent compound La1.2Sr1.8Mn2O7, the evolution of exchange couplings with doping rate x shows interesting features. T.G. Perring et al. (PRL 87 217201 (2001))have shown that Jab does not depend on x while Jc does. • SJeff// (SJab) and SJeff^(SJc) for the nnHFM model derived from the spin-wave stiffness and optic gap. • (b) Jahn-Teller Distortion of the MnO6octahedra DJT, where • DJT= 1/2(dMn-O(1)+ dMn-O(2))/dMn-O(3). [10] D. N. Argyriou et al., Phys. Rev. B 59, 8695 (1999). [11] J. F. Mitchell et al., Phys. Rev. B 55, 63 (1997). [12] M. Kubota et al., J. Phys. Soc. Jpn. 69, 1606 (2000). In the Pr-substituted compound (La0.4Pr0.6)1.2Sr1.8Mn2O7, SJc= 1.7 meV as if x would be larger than 0.4. But DJT = 1.043 for H//c and 1.026 for H//a+b (ref. A. Gukasov et al. PRB 72, 92402 (2005)) as if x would be smaller than 0.4 !!!!! ESOS 18-22 June 2008, Cracow

17. A possible explanation for the more or less constant value of Jab whatever the direction of the magnetic field and whatever the bilayer system ( with or without Pr) could be that the in-plane square structure of Mn ions is very robust against any perturbation. The in-plane distance Mn-O(3) cannot vary very much. This is not the case in the direction perpendicular to the layers. Why is Jc smaller than in the parent compound? A possible explanation of that reduced value of Jccould be that the randomly distributed Pr ions induce spatial fluctuations of the distance Mn-O(2). According to the nature of the ion (La, Sr, Pr) at the La- site, the thickness of the bilayer “O(2)-Mn-O(1)-Mn-O(2)” fluctuates. This, in turn, leads likely to spatial fluctuations in the electronic density which could reduce the ferromagnetic coupling along the c axis Jc . ESOS 18-22 June 2008, Cracow

18. Les Polarons de réseau corrélés révélés par la diffusion diffuse des neutrons In the PI phase, it appears a short range ordered charge. This effect reminds the CE phase in the manganites with x = 0.5, where a long range charge order exists revealed by superstructure Bragg peaks in the (a*,b*) plane (h + 0.25, k + 0.25, l). The Mn3+ ions have one egelectron mainly on the 3d3z2-r2 orbital, while the t2g electronsof Mn4+ ions are on 3dxy,yz,zx -type orbitals. So the MnO6 octahedra are differently distorted following the type of Mn ion which is at their center. LaSr2Mn2O7 de D. N. Argyriou et al. Phys.Rev.B 61(2000) 15269 ESOS 18-22 June 2008, Cracow

19. In the PI phase of (La0.4Pr0.6)1.2Sr1.8Mn2O7with H=0, we measured neutron diffuse scattering around (220) Bragg peak. This diffuse scattering is made of a central peak at the foot of the Bragg peak and of a modulation located by respect to (220) at  (0.2,-0.2,0). The application of a magnetic field (reddots) has two effects: a large reduction of the central peak and a complete cancellation of the modulation. The purely nuclear intensity of the Bragg peak (220) does not change under the field . The FM phase appears homogeneous, with a strong spin-lattice coupling, resulting in a large magnetostriction effect. ESOS 18-22 June 2008, Cracow