Change in Mn3dorbital state related to a metal-insulator transition in a bilayer manganite studied by magnetic Compton profile measurement

Abstract

Temperature dependence of magnetic Compton profile ͑MCP͒ has been measured on a single crystal of La 2Ϫ2x Sr 1ϩ2x Mn 2 O 7 at xϭ0.42 along the c-axis. The spin magnetic moments of Mn 3d electrons in t 2g and e g type orbitals are evaluated from the line-shape fitting analysis of MCP's using theoretical profiles derived from the (MnO 6 ) 8Ϫ ab initio calculations. The experimental ratios of e g spins to t 2g ones show higher values above the metal-insulator transition temperature T c in comparison with the value expected from the electronically homogeneous state. This is understood in terms of the phase separation between electron-rich ferromagnetic and electron-poor antiferromagnetic regions. The ratios indicate that the e g electrons are highly segregated in the ferromagnetic region. In addition, the fitting result just above T c suggests that the e g -orbital state involved with the colossal magnetoresistance effect is optimized by an external magnetic field.Perovskite Mn oxides have provided a fascinating subject such as colossal magnetoresistance ͑CMR͒ observed just around a metal-insulator transition temperature T c , 1-4 where the charge, spin, and orbital degrees of freedom are arguably connected with each other. The ferromagnetism and metallic conductivity concurrently appeared below T c have been understood on the basis of the double-exchange ͑DE͒ mechanism, where e g electrons hop around Mn sites through hybridization with O-2p orbitals, and align the localized t 2g spins by the strong Hund's coupling. 5-7 However, the transport properties including CMR and the complicated magnetic phase diagrams of manganites cannot be explained by the simple DE mechanism, and some additional ingredient is required. It is currently pointed out that the lattice degree of freedom, which is strongly coupled with the orbital degree of freedom, plays an important role in understanding the physical properties of manganites. 8-12 Recent x-ray and neutronscattering experiments have observed a short-range charge ordering ͑SRCO͒ within MnO 2 layers, suggesting the localization of e g electrons at Mn sites accompanied by local lattice distortions. [13][14][15][16] Kubota et al. have reported that in bilayer manganites, SRCO appears in the wide range of hole concentration and is characterized by a bistripe order running along the ͓100͔ direction in a MnO 2 layer which forms streaks of two large and one small lattice distortions around Mn 3ϩ and Mn 4ϩ ions, respectively. 15,16 The temperature dependence of SRCO shows a notable change in the vicinity of T c , and well corresponds to that of resistivity. 15 Therefore, determination of the electronic and orbital states around T c , which are sensitive to the local structural change, will provide a clue to elucidate the mechanism of CMR.Our previous paper demonstrated that magnetic Comptonprofile ͑MCP͒ measurement is capable of determining the population of Mn-3d orbitals which hybridize with O-2p orbitals. 17 In the present study, we have measured the temperature dependence of MCP on ...

Hole concentration dependence of Mn eg orbital state in bilayer manganites studied by magnetic Compton profile measurement

“…1 fail to reproduce the experimental MCP's, because the hybridization between Mn-3d and O-2p orbitals affects the profiles, and is not included in the atomic profiles. Then, we prepared theoretical profiles suitable for the line-shape analysis by using an ab initio molecular orbital calculation for the (MnO 6 ) 8K cluster [15,16]. In the calculation, the cluster was embedded in 913-point charges in total that describe the Madelung potential.…”

“…Okamoto et al have theoretically studied the correlation between orbital structure and magnetism based on the model with the e g orbitals under strong electron correlation, and found that there exists an optimal mixing between the 3z 2 Kr 2 and x 2 Ky 2 orbitals for ferromagnetic ordering [14]. In the previous papers, we showed the capability of magnetic Compton profile (MCP) measurement to investigate the Mn-3d orbital state in bilayer manganites from the viewpoint of orbital occupation [15], and also reported the temperature dependence of e g and t 2g orbital populations [16]. In the presnt study, the anisotropy of MCP has been measured in the FM-II, FM-I and canted AFM phases of bilayer manganites, and the results are examined by comparison with theoretical and experimental works obtained by different methods.…”

“…In the case of manganites, the magnetic moment originates in the spins in t 2g and e g orbitals aligned by the strong Hund's coupling. It is, therefore, reasonable to consider that the occupation number in each Mn-3d orbital is proportional to the spin magnetic moment in that orbital, and is analyzed by fitting theoretical profiles of respective orbitals to MCP [15,16].…”

Orbital bi-stripes in highly doped bilayer manganites

“…The discovery of colossal magneto-resistance in the x = 0.4 bilayer 1 triggered a flurry of interest in this compound 2 . Following this, numerous studies have been conducted on the x = 0.5 doping level, showing strong charge and orbital order order.…”

Magnetic Compton scattering studies of magneto-dielectric Ba(Co0.85Mn0.15)O3−δ

“…It should be noted, here, that magnetic Compton scattering is a unique probe for spin-polarized electron moment distribution ferromagnetic materials, and its cross section is proportional only to spin component of magnetization. [7][8][9][10][11] In the present studies, we have tried to evaluate the magnetic Compton profiles (MCPs) of the BCMO ceramic samples using the magnetic Compton scattering technique, in order to clarify their ferromagnetic origin. The valence state of the B-site ions was also checked by a soft X-ray photoelectron spectroscopy (XPS).…”