View metadata,Downloaded citation and from similar orbit.dtu.dk papers on:at core.ac.uk Dec 17, 2017 brought to you by CORE provided by Online Research Database In Technology Ferromagnetic domain structures and spin configurations measured in doped manganite He, J.Q.; Volkov, V.V.; Beleggia, Marco; Asaka, T.; Tao, J.; Schofield, M.A.; Zhu, Y. Published in: Physical Review B (Condensed Matter and Materials Physics) Link to article, DOI: 10.1103/PhysRevB.81.094427 Publication date: 2010 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): He, J. Q., Volkov, V. V., Beleggia, M., Asaka, T., Tao, J., Schofield, M. A., & Zhu, Y. (2010). Ferromagnetic domain structures and spin configurations measured in doped manganite. Physical Review B (Condensed Matter and Materials Physics), 81(9), 094427. DOI: 10.1103/PhysRevB.81.094427 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. PHYSICAL REVIEW B 81, 094427 ͑2010͒ Ferromagnetic domain structures and spin configurations measured in doped manganite J. Q. He,1,2,* V. V. Volkov,1 M. Beleggia,1,3 T. Asaka,1 J. Tao,1 M. A. Schofield,1 and Y. Zhu1 1Condensed Matter Physics and Material Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA 2Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA 3Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark ͑Received 24 August 2009; revised manuscript received 5 November 2009; published 25 March 2010͒ We report on measurements of the spin configuration across ferromagnetic domains in La0.325Pr0.3Ca0.375MnO3 films obtained by means of low-temperature Lorentz electron microscopy with in situ magnetizing capabilities. Due to the particular crystal symmetry of the material, we observe two sets of independent ferromagnetic twin variants, one of which was pinned by crystallographic twinning. The spin orientation and configuration of the domains were measured quantitatively using electron diffraction and electron holography, and verified with structural modeling. The observed deviation of spin orientations from the expected 90° head-to-tail configuration across the domain walls was attributed to the variation in the domains’ aspect ratios as a result of the demagnetization field. Our study provides important insight on how the spin configuration coupled with the magnetic structure and the crystal symmetry might affect the magnetore- sistivity under an applied magnetic field in a strongly correlated electron system. DOI: 10.1103/PhysRevB.81.094427 PACS number͑s͒: 75.47.Lx, 75.70.Kw, 61.72.Ff, 75.47.Gk I. INTRODUCTION II. EXPERIMENTAL PROCEDURES Understanding the structure-property relationship of Thin LPCMO films of ϳ200 nm thickness were epitaxi- ͑ ͒ strongly correlated electron systems, including manganites, ally grown on 010 -oriented single crystals of NdGaO3 is one of the major challenges faced today in condensed- ͑NGO͒ using the pulsed laser deposition technique with 1 ͓ ͔ ʈ͓ ͔ ͓ ͔ ʈ͓ ͔ matter physics and materials science. For example, the co- 010 LPCMO 010 NGO, and 001 LPCMO 100 NGO or ͓ ͔ 4 lossal magnetoresistance ͑CMR͒ effect in manganites usually 001 NGO. A KrF excimer laser operated at 10 Hz with an accompanies transition from an insulator to metallic state, areal energy density of 2 J/cm2 per pulse on the surface of a which is directly related to the charge-ordered antiferromag- stoichiometrically sintered target of LPCMO was used for netic ͑AFM͒ and metallic ferromagnetic ͑FM͒ phases.2–5 Ac- ablation. The oxygen pressure and the substrate temperature cordingly, magnetic domains and domain walls in the FM during the growth were held at ϳ300 mTorr and 750 °C, phase of these oxides may strongly affect not only their mag- respectively. Thin films were investigated by our JEM2100F- netic properties but also their electron-transport properties.6–9 LM, a 200 kV field-emission electron microscope with a However, the electrical resistance of the magnetic domain custom-made field-free objective lens ͑residual field walls critically rests upon the domain’s configuration10–12 ϳ4Oe͒.17 In situ cooling experiments were conducted using because the reversal of magnetization at a domain wall in a Gatan cooling stage filled with liquid nitrogen. TEM the interrupted perovskite crystal significantly scatters the samples were prepared via standard mechanical thinning and charge carriers. A precise analysis and understanding of the polishing techniques to remove the substrate. The final thin- magnetic microstructure is then vitally important for the ning of the films was done using a Fischione ion miller with fundamental understanding of advanced functional magnetic a single low-voltage Argon source at glancing angles and at materials. liquid-nitrogen temperature. In this work, we focus on the orthorhombic ͑ ͒ ͑ ͒ La0.325Pr0.3Ca0.375MnO3 LPCMO pervoskite Pnma with III. OBSERVATIONS OF DOMAIN STRUCTURES a=0.5441 nm, b=0.7676 nm, and c=0.5439 nm lattice parameters.5 LPCMO is a compound that exhibits a rich va- The LPCMO samples were examined by Lorentz electron riety of physical phenomena, such as phase transitions and microscopy over a large range of temperature. At 90 K, the electronic-phase separation at low temperatures. Upon cool- FM phase dominated this sample and two sets of domain ing, the material first undergoes a transition at T=220 K walls ͑denoted as “zigzag” and “twin” domain walls in Fig. from the room-temperature paramagnetic phase to a charge- 1͒ were clearly visible, slowly disappearing as the tempera- ordered AFM phase, followed at T=160 K by a state where ture increased approaching the FM to AFM transition ferromagnetism and charge ordering appear to coexist.4 The ͑around T=170 K͒. The widths of twin domains did not magnetic domain configuration of these materials, and re- change as a function of temperature, as might be expected, lated ones, have been directly investigated by various because the magnetic domain walls are pinned to the crys- methods.13–16 Here, we employ high-spatial-resolution Lor- tallographic ͑101͒ twin domain walls. Figures 1͑a͒–1͑c͒ are entz transmission electron microscopy ͑LTEM͒ to observe Lorentz images of the LPCMO sample at 90 K showing the behavior of the magnetic domain configuration. Our ob- the typical ferromagnetic domains viewed along the ͓010͔ servations shed light on the relationship between the mag- direction taken, respectively, with nominal defocus of netic structure and the effects of demagnetization fields aris- ϳ−100 ␮m, +100 ␮m, and zero. The characteristic straight ing from the finiteness of the domains. dark lines and bright lines in Figs. 1͑a͒ and 1͑b͒ represent the 1098-0121/2010/81͑9͒/094427͑7͒ 094427-1 ©2010 The American Physical Society HE et al. PHYSICAL REVIEW B 81, 094427 ͑2010͒ FIG. 1. Lorentz TEM images of LPCMO at 90 K. ͑a͒ Under- focus image, ͑b͒ over-focus image, and ͑c͒ in-focus image. The twin domain walls follow the crystallographic ͑101͒ twinning plane. ͑d͒ Schematic drawing of the local magnetic structure and crystal struc- ture ͑enlarged part͒. The thick and thin black lines represent zigzag ͑180°͒ and twin ͑90°͒ domain walls, respectively. The twin ferro- magnetic domains coincide with the crystal twinning lamellar struc- ture of the sample. divergent and the convergent magnetic domain-wall images, respectively. These contrast features have almost disappeared in Fig. 1͑c͒; however, careful observation reveals some weak, wide, and straight contrast lines ͑marked by the white arrowheads͒ most likely due to diffraction contrast from the ͑101͒ crystallographic twin domains of the material. These magnetic domain walls coincide with the ͑101͒-type twin boundaries. Also, as evident in Figs. 1͑a͒–1͑c͒, additional sets of mag- netic domain walls were visible, with a characteristic zigzag pattern, labeled A, B, A in Fig. 1͑d͒. These magnetic domain walls appear to separate 180° magnetic domains with anti- FIG. 2. ͑Color online͒͑a͒ The ED pattern of LPCMO at 90 K. parallel magnetization across the domain wall. The magnetic Two boxed insets show the split intensity of transmitted beam and ͑ ͒ ͑ ͒ domains confined between the boundaries of twin lamellae 101 Bragg reflection. b The magnetically split fine structure of are approximately 50–150 nm wide and the superposition of transmitted beam, recorded using a large camera length of 15 m, these two types of domain walls suggests that magnetization shows two diffused lines of spotty intensities along the qy direction. Two related Foucault images ͑c͒ and ͑d͒ were taken with the dif- of individual magnetic domains should be aligned crystallo- fraction aperture at positions A and B in ͑b͒, respectively. ͑e͒ Two graphically with either ͓100͔ or ͓001͔ axes, the easy magne- nanobeam diffraction scans across the twin domain structure. The tization directions in LPCMO. Accurate measurement of the ͑ ͒ ␪ ͓ ͑ ͔͒ deviation of qx ,qy coordinates for diffraction spots versus scan- 2 twist angles see Fig. 1 a along the zigzag configuration ning nanobeam position along ͑x,y͒ coordinates, respectively, par- ͓ ͔ revealed that the angles varied in the range 90° ,120° sug- allel ͑open squares͒ and perpendicular ͑solid squares͒ to the twin gesting that the magnetization does not align exactly with the domain walls in LPCMO.