Crystal structure and magnetic and transport properties of 154 Sm 0.5 Sr 0.5 MnO 3 : A-type antiferromagnetic phase and ferromagnetic polarons A. I. Kurbakov, A. V. Lazuta, V. A. Ryzhov, V. A. Trounov, and I. I. Larionov Petersburg Nuclear Physics Institute RAS, Gatchina, St. Petersburg 188300, Russia C. Martin, A. Maignan, and M. Hervieu Laboratoire CRISMAT, UMR 6508 ISMRA et Universite de Caen, Boulevard de Marechal Juin, 14050 Caen, France Received 2 June 2004; revised manuscript received 25 August 2005; published 29 November 2005 Structural investigations neutron diffraction and electron microscopy, as well as data on resistance, mag- netization, and second harmonic of magnetization, are presented for 154 Sm 0.5 Sr 0.5 MnO 3 manganite. The neu- tron diffraction studies reveal a structural phase transition at T st  135 K from a high-temperature Pbnm space group to a mixture of two Pbnm phases, which are coherently coupled by atomic positions but differ in cell parameters. Above T st , this compound is a paramagnetic insulator. At room temperature, a strained and com- plex nanostructure state is observed in the high-resolution electron microscopy images. In the Pbnm structure it is generated by the coexistence of twinning domains and a small deviation from orthorhombicity for a monoclinic structure. Below T st , both structural phases begin to exhibit coherent Jahn-Teller distortions, which are different in the two phases. The structural transition is accompanied by the development of a magnetic ordering, so that the ground state is a mixture of ferromagnetism F and A-type antiferromagnetism AF-A. The AF-A order develops in the new low-temperature structural phase II, whereas the F moment is related to the high-temperature structure I. The several variants of the magnetic ordering are considered for phase I. A more physically justified scenario is that this phase does not possess an AF-A component and exhibits F ordering with an F moment of 1.91  B /Mn at 1.5 K. In this case, the AF moment of phase II is found to be 4.11  B / Mn at 1.5 K. This unexpected result is attributed to the formation of ferromagnetic polarons in connection with a new electronic structure proposed for the AF-A state. A metalliclike behavior is only observed below 50 K and is related to the F phase. The AF-A phase is expected to be insulating. It can exhibit a short-range charge ordering that is observed by electron diffraction at 92 K. A field hysteresis of the second harmonic is found above T C  T N  T st , which is associated with AF-A regions in the paramagnetic matrix which possess a weak ferromagnetism. DOI: 10.1103/PhysRevB.72.184432 PACS numbers: 75.40.Gb, 71.30.h, 75.25.z, 64.70.Kb I. INTRODUCTION This work is the continuation of the systematic investiga- tion of the Sm 1-x Sr x MnO 3 series 1,2 that is a natural develop- ment of the initial studies. 3 These compounds reveal some unique properties especially in the region 0.4  x  0.5. They possess a low Curie temperature T C  130 K and display co- lossal magnetoresistance near T C . The x = 0.4 compound has at low temperature a phase-separated metallic ground state with the coexistence of ferromagnetism F, A-type antifer- romagnetism AF-A, and a small amount of charge-ordered CE AF spin structure. The ground state of the x = 0.45 com- pound is F metallic. Both materials exhibit the Pbnm space group and show unusual magnetic behavior above the tem- perature of the magnetic phase transition which is due to the formation of AF domains in the paramagnetic matrix. At x = 0.4 this state develops in the O-structural phase the lat- tice constants a  b  c /  2 at 160 K just below an O-O- structural transition 180 K. The same behavior starts to be observed below 300 K in the x = 0.45 compound which also belongs to the O structure in this temperature range. We report here systematic investigations of the x = 0.5 manganite which include neutron and electron diffraction ED measurements and high-resolution electron microscopy HREM investigations, as well as data on resistivity, mag- netization, and nonlinear response. It exhibits a colossal magnetoresistance CMR below T C , whereas the CMR is weak or unobservable in other x = 0.5 manganites in the se- ries L 0.5 Sr 0.5 MnO 3 L =La,Pr,Nd. 4,5 Sm 0.5 Sr 0.5 MnO 3 re- veals a phase-separated PS ground state, including two co- existing phases I and II, which can be described by the crystallographic Pbnm space groups but with different sets of unit-cell parameters. The PS state is characterized by the coexistence of the F and AF-A magnetic phases. The struc- tural transition from the high-temperature insulating para- magnetic state to the PS one occurs at T st  135 K and is accompanied by the development of magnetic orderings. At room temperature, the HREM reveals a complex and strained nanostructural state. In the Pbnm phase, it is a result of the coexistence of the twining domains and weak monoclinic distortions. An analysis of the magnetic structure of the ground state shows that the F arrangement corresponds to the high-temperature structure I. The AF-A order is identified with the low-temperature structural phase II. Phase I is found to exhibit the F ordering that can be accompanied by an admixture of AF-A components. It will be argued that the “pure” F magnetic structure of phase I is more physically justified. In this scenario, an antiferromagnetic moment  M AF  of phase II is unexpectedly established to be M AF = 4.11 B /Mn at 1.5 K. This nontrivial result can be asso- PHYSICAL REVIEW B 72, 184432 2005 1098-0121/2005/7218/18443210/$23.00 ©2005 The American Physical Society 184432-1