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Preparation and characterization of LaMnO3 thin films grown by pulsed laser deposition

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Preparation and characterization of LaMnO3 thin films grown by pulsed laser deposition
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  Preparation and characterization of LaMnO 3  thin films grownby pulsed laser deposition C. Aruta, a  M. Angeloni, G. Balestrino, N. G. Boggio, P. G. Medaglia, and A. Tebano Coherentia CNR-INFM and Dipartimento di Ingegneria Meccanica, Università di Roma “Tor Vergata,”Via del Politecnico 1, I-00133 Roma, Italy B. Davidson TASC CNR-INFM National Laboratory, Area Science Park, Basovizza, 34012 Trieste, Italy M. Baldini, D. Di Castro, P. Postorino, and P. Dore Coherentia CNR-INFM and Dipartimento di Fisica, Università di Roma “La Sapienza,” Piazzale Aldo Moro 5, I-00185 Roma, Italy A. Sidorenko, G. Allodi, and R. De Renzi  Dipartimento di Fisica and Unità CNISM, Universita di Parma, Parco Area delle Scienze 7A, I-43100Parma, Italy  Received 21 September 2006; accepted 9 May 2006; published online 28 July 2006  We have grown LaMnO 3  thin films on   001   LaAlO 3  substrates by pulsed laser deposition. X-raydiffraction confirms that the films are only slightly relaxed and are oriented “square on square”relative to the substrate. The measured Raman spectra closely resemble that observed in bulkLaMnO 3 , which indicates no relevant distortions of the MnO 6  octahedra induced by the epitaxialstrain. Therefore, no detectable changes in the lattice dynamics occurred in our LaMnO 3  strainedfilms relative to the bulk case.  55 Mn nuclear magnetic resonance identifies the presence of localizedMn 4+ states. Superconducting quantum interference device magnetization measures  T   N  =131  3   Kand a saturation moment    =1.09    B  /Mn, revealing a small concentration of Mn 4+ and placing ourfilms within the antiferromagnetic insulating phase. ©  2006 American Institute of Physics .  DOI: 10.1063/1.2217983  INTRODUCTION The discovery of the colossal magnetoresistance   CMR  effect in manganite compounds 1 has renewed the interest inthis class of materials. The chemical formula of CMR man-ganites can be written as RE 1−  x   A  x  MnO 3 , where RE stands fora   trivalent   rare earth,  A  for a   divalent   alkaline earth   Ca,Sr, or Ba  , and  x   ranges from 0 to 1. For 0.2   x   0.5, dopedmanganites show a number of extraordinary magnetotrans-port properties related to the fully spin polarized character of the electrical conduction. These properties, in view of pos-sible applications, have triggered an increasing effort towardfilm growth and optimization. Beside technical problems re-lated to film deposition, lattice distortions induced by epitax-ial strain have been shown to play a relevant role in connec-tion with physical properties of thin manganite films—epitaxial strain influences the local symmetry of theoctahedral Mn ions that is crucial in determining their mag-netotransport properties. 2 An exhaustive overview of manga-nite properties is given in Ref. 3.In studying manganite physics, of particular importanceis the investigation of the parent compound LaMnO 3   LMO  .Such a compound is an  A -type antiferromagnetic insulatorwhere the orbital ordering is established due to the coopera-tive Jahn-Teller   JT   interaction. LMO was shown to havethe O  -type orthorhombic structure 4  space group  Pbnm  with  a =0.554 nm,  b =0.572 nm, and  c =0.770 nm, at roomtemperature. The MnO 6  octahedron is Jahn-Teller distortedwith the distortion axis lying in the  a - b  plane: longer andshorter Mn–O bonds in the  a - b  plane are 0.218 and0.191 nm, respectively, along two orthogonal in-plane direc-tions, while the Mn–O bond length is 0.196 nm along the  c axis. Therefore, the Mn–Mn distances in plane and out of plane are 0.398 and 0.385 nm, respectively.The interest in this specific manganite compound hasbeen renewed by a recent paper by Yamada  et al. 5 In thispaper it was shown that a thin   few unit cell   LMO bufferlayer inserted at a SrTiO 3  /La 0.6 Sr 0.4 MnO 3  interface can im-prove the interfacial magnetic properties near the mangan-ite’s Curie temperature. Such a result could open importantperspectives in the field of manganite spin valves operatingat room temperature.In bulk LMO samples defects associated with excessoxygen, because of the difficulty of accommodating intersti-tial excess oxygen ions in the close-packed perovskite struc-ture, take the form of La- and Mn-cation vacancies. 6–8 LMOcan be either lanthanum or manganese deficient, or both. 9 The chemical formula for samples having the correct sto-ichiometric ratio between Mn and La   namely, La:Mn=1:1  , assuming that the oxygen is O 2− , can be written as  La 1−  x  3+    x   Mn 1−7  x  3+ Mn 6  x  4+    x   O 32− . In these conditions, thefraction of Mn 4+ over the total is 6  x   /   1−  x   , so that even sucha small value of   x  =0.05 corresponds to about 1/3 Mn 4+ .Samples having the correct oxygen stoichiometry can be pre-pared at low oxygen pressure    10 −3 Pa   Ref. 10   or in a  Author to whom correspondence should be addressed; electronic mail:carmela.aruta@uniroma2.it JOURNAL OF APPLIED PHYSICS  100 , 023910   2006  0021-8979/2006/100  2   /023910/6/$23.00 © 2006 American Institute of Physics 100 , 023910-1 Downloaded 30 Jul 2006 to 192.135.11.132. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp  reducing atmosphere. 11,12 La deficiency also converts Mn 3+ into Mn 4+ according to the formula   La 1−  z 3+    z    Mn 1−3  z 3+ Mn 3  z 4+  O 32− so that the Mn 4+ content is three timesthe La deficiency. Cation-deficient samples tend to be ferro-magnetic and metallic. 13–15 Because of such critical stoichi-ometry issues, the growth of stoichiometric LMO thin filmsis not a trivial achievement.Owing to the strong interplay between magnetic, trans-port, and structural properties, different experimental tech-niques must be employed in studying the manganite proper-ties. In particular, improved experimental microscopic dataare necessary in order to understand the role of distortionsinduced by the substrate in thin manganite films. In thisframework micro-Raman spectroscopy can be an importantinvestigation tool for these systems. Namely, the most in-tense peaks of the Raman-active phonon spectrum are as-cribed to vibrational modes of the MnO 6  octahedra, whichare crucial in determining transport properties of manganites.A number of Raman papers dealing with bulk and thick filmsamples witnesse the relevance and sensibility of thistechnique. 16 For instance, in a recent paper it was shown thata strong hardening of the phonon frequencies of the bendingand stretching modes is apparent in ultra-thin films   d   100 Å  . This behavior, strongly connected with the mea-sured  d   dependence of the insulator-to-metal transition tem-perature, is ascribed to co-operative effects of MnO 6  octahe-dra rotation and charge localization. 17 From the magnetic point of view pure LMO is an  A -typeantiferromagnet with  T   N  x  =0 =139.5 K   where  x   is the Mn 4+ concentration   and a small spin canting due to the anti-symmetric Dzialoshinski-Moriya interaction   less than 2°from collinearity between the two sublattices  , giving rise toa weak ferromagnetic behavior 18 with a saturation momentof     =0.16    B  /Mn. The behavior of lightly dopedLa 1−   Mn 1−   O 3  is coarsely similar to that of other lightlydoped pseudocubic manganites, 19 such as La 1−  x  Sr  x  MnO 3  Ref. 20   and La 1−  x  Ca  x  MnO 3 , 21 where small angle neutronscattering has demonstrated the presence of nanoscopic fer-romagnetic clusters embedded in an antiferromagnetic bulkbackground at low temperatures. 22 These clusters diffuse athigher temperatures, 23 giving rise to a complex magnetic be-havior characterized by a large ferromagnetic macroscopicstatic susceptibility. For an effective concentration of Mn 4+  x   0.07 the antiferromagnetic interactions dominateand the Neel temperature slightly decreases from  T   N  x  =0 downto 115 K   Ref. 19  . Above  x   0.07 both the saturation mo-ment      x    and the ordering temperature  T  C    x    increase rap-idly with  x   reaching, e.g.,  T  C  =220 K and    =3.8    B  /Mn forLa 0.8 Ca 0.2 MnO 3 , and the phase is often identified as insulat-ing ferromagnetic   FI  . 24 In the present work, we prepared thin films of the LMOcompound by pulsed laser deposition   PLD  . Films werecharacterized through different techniques, such as Ruther-ford back scattering   RBS  , x-ray diffraction, electrical resis-tivity as a function of temperature, and superconductingquantum interference device   SQUID   magnetometry. Wealso utilized Raman spectroscopy to study the LMO phononspectrum. In principle, in this compound, all Mn ions havethe same 3+valence and no structural disorder, deriving fromrandom chemical substitution on La sites, is expected. Ra-man phonon peaks much sharper than those observed in sub-stituted manganites can then be observed. 16,25 In the case of thin films, this should allow a precise measurement of smallfrequency shifts of these peaks related to substrate inducedcrystallographic distortions which affect the MnO 6  octahe-dral structure.  55 Mn zero field nuclear magnetic resonance  NMR   was also employed: nuclear spin echoes can be gen-erated in the presence of the hyperfine field, due to the or-dered electron moments. Distinct spectral contributions maybe easily assigned to the three possible electronic environ-ments: localized Mn 4+ , localized Mn 3+ , and mixed valenceband Mn DE , in the double exchange   DE   metallic phase. 26–28 Therefore NMR can directly confirm the presence of local-ized Mn 4+ ions. EXPERIMENT The PLD of the LMO films was carried out using anexcimer laser charged with KrF   wavelength of 248 nm,pulse width of 25 ns, and repetition rate of 3 Hz  . The laserbeam, with an energy of 150 mJ per pulse, was focused ontoa target in a vacuum chamber. LaAlO 3   LAO   substrates  001   oriented were placed at a distance of about 50 mmfrom the target on a heated holder. LAO was selected assubstrate since its low intensity and simply structured Ramansignal allows to extract the phonon spectrum of LMO evenfor thin films. 17 La:Mn stoichiometric targets were preparedin air by solid state reaction starting from MnO and La 2 O 3 high purity powders. The final sintering treatments were car-ried out in air at 1500 °C for 24 h. LMO films were grownin oxygen atmosphere. Optimal molecular oxygen pressurefor compensated LMO films resulted to be about 10 −2 mbarwith an initial base pressure of 10 −5 mbar. Film growth tem-perature  T  g  was about 700 °C. Film growth rate was about0.1 nm per laser shot. After the growth, samples were cooledto room temperature, in about 10 min, in the growthatmosphere.In order to determine the cation stoichiometry, LMOfilms were grown on MgO substrates in the identical growthconditions and investigated by RBS. Lattice parameters weremeasured by x-ray diffraction. Electrical resistivity as a func-tion of temperature was measured by the standard four-probetechnique. SQUID measurements were performed on a com-mercial Quantum MPMS apparatus. Raman spectra weremeasured in backscattering geometry, using a micro-Ramanspectrometer   LabRam by Jobin-Yvon   equipped with acharge coupled device detector and a notch filter to reject theelastic contribution. The sample was excited by the 632.8 nmline of a He–Ne laser. The confocal microscope wasequipped with a 50   objective, which allows obtaining alaser spot about 2    m 2 wide at the sample surface. Spectrahave been collected within the 200–1100 cm −1 frequencyrange with a spectral resolution of about 3 cm −1 . In order toreduce as much as possible the signal from the substrate, avery small confocal diaphragm   50    m   was used to limitthe scattering volume. The  55 Mn NMR spectrum was col-lected with the homebuilt phase coherent spectrometer Hy-ReSpect   Ref. 29   by a solid spin echo sequence on a tuned 023910-2 Aruta  et al.  J. Appl. Phys.  100 , 023910   2006  Downloaded 30 Jul 2006 to 192.135.11.132. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp  probe circuit. The amplitude is recorded at each frequency toobtain the zero frequency component of the fast Fourier of the echo. RESULTS AND DISCUSSION The crucial point in obtaining stoichiometric LMO filmis to optimize the oxygen content. Therefore, several filmswere grown at the same temperature  T  g  of 700 °C varyingthe pressure of the oxygen background gas between 5  10 −3 and 0.2 mbar. The oxygen content in the film wasmonitored indirectly measuring the film resistivity: filmshaving higher resistivity were assumed to have a better oxy-gen stoichiometry. Following this approach, a growth pres-sure of 10 −2 mbar was chosen. In Fig. 1 we show the resis-tivity of a LMO film, 73 nm thick   determined by RBSmeasurements  , grown at an oxygen pressure of 10 −2 mbar.Resistivity is reported in an Arrhenius plot   ln     vs 1/  T    inthe temperature range between 360 and 530 K. A semicon-ducting behavior is evident with an activation energy of about 0.32 eV, in reasonable agreement with values reportedin literature for bulk LMO compounds, 30,31 showing thatfilms grown at an oxygen pressure of 10 −2 mbar are quitewell oxygen compensated. In the inset of Fig. 1 we report thefit of the resistivity data with Holstein’s model 32  ln     /  T   1/  T   . Within such a model, the conduction takes placethrough thermal-activated polaron hopping. In our opinionexperimental data do not allow to discriminate between thetwo models. However, a fit of the experimental data accord-ing to Holstein’s model resulted in a hopping energy of   0.355 eV, which corresponds to about 5% of Mn 4+ . 33 Asshown below by RBS, magnetization, and NMR measure-ments, a small but measurable concentration of Mn 4+ hasbeen also estimated. In spite of the general trend that thepresence of Mn 4+ favors metallic conductivity, the resistivityof these films is notably higher   by a factor of 5–50 at360 K   than typical values for high-quality bulk sampleswith small Mn 4+ concentration 4  less than a few percent   andnominally stoichiometric thin films. 34,35 The Raman andNMR results discussed below demonstrate that significantstructural disorder, at levels high enough to cause the ob-served resistivities, is not present in these films.The cation stoichiometry of a film grown at such anoxygen pressure was investigated by RBS measurements,yielding a La:Mn ratio of 0.95:1, with a relative error of 3%on the estimate of both La and Mn. This result suggests,within the experimental error, a slight La deficiency withrespect to the stoichiometric target, in agreement with themagnetization and NMR results discussed below.A description of the crystallographic structure of the filmon the basis of the structure of orthorhombic bulk LMO isnot straightforward. The pseudoperovskite lattice parametersof the orthorhombic unit cell of bulk, stoichiometric LMOare  a 1 = a 2 =0.398 nm and  a 3 =0.385. In strained films, thein-plane lattice parameters are both compressed to match thesubstrate, yielding a tetragonal unit cell. The structural prop-erties of the LMO films were investigated by x-ray diffrac-tion at Cu  K     wavelength with a Bragg-Brentano diffracto-meter. In Fig. 2 a    -2    diffraction spectrum of a film grownin the optimized conditions is shown. Peaks in the spectrumbelong either to the LAO   001   oriented substrates or to theLMO film. The LMO film is   001   oriented. No spuriousphases can be detected from x-ray diffraction. From the sym-metric diffraction spectra it can be noticed that the 00 l  peaksare quite broad and a perpendicular lattice parameter  c  of 0.399±0.001 nm can be estimated. In the inset of the figurethe rocking curve, taken from the   002   peak of the film, isshown. The full width at half maximum of the rocking curveis about 0.5°, showing a satisfactory structural order in theperpendicular direction. The in-plane order was investigatedby asymmetric diffraction measurements.     scan at   103  Bragg angles reported in the inset of Fig. 3 shows that LMOfilms result to be oriented “square on square” relative to theLAO substrates. In Fig. 3 a reciprocal space map around the  103   reflection is reported. The data are given in reciprocallattice units   r.l.u.   and normalized relative to the lattice pa-rameters of LAO substrate   a = b = c =0.379 nm  . The typicaltwin structure of the LAO substrate and the presence of avery broad peak from the LMO film can be observed in themap. The major diffraction contribution of the   103   reflec-tion of the film occurs at the same  H  =1 value of the sub- FIG. 1. Arrhenius plot of an optimized LMO film between 360 and 530 K,together with the best linear fit. In the inset, the fit of the resistivity data withHolstein’s model has been reported.FIG. 2.    -2    diffraction spectrum of an optimized LMO film across the  001   and the   002   reflections of the LMO film and the LAO substrate. Inthe inset, the rocking curve      scan   across the   002   peak of the film isshown. 023910-3 Aruta  et al.  J. Appl. Phys.  100 , 023910   2006  Downloaded 30 Jul 2006 to 192.135.11.132. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp  strate, thus indicating that the film is mostly in-planematched with the substrate. The weak tail at lower  H   valuesis a consequence of the partial relaxation of the lattice pa-rameter toward the bulk values, possibly in the topmost layerof the film, together with the presence of different domainsmainly influenced by the twin structure of the substrate. Byrotating the sample of 90°, the reciprocal space map does notsignificantly change, in agreement with the in-plane squaresymmetry of our LMO film. These results indicate that thestrained unit cell has a volume that is much less   about 5%  than the pseudocubic unit cell of bulk stoichiometric LMO.The typical Raman spectrum of a LMO film is shown inFig. 4. Owing to the film thickness   about 75 nm   and to thesmall confocal diaphragm, no Raman signal from the LAOsubstrate   which, within the investigated spectral range,mainly consists of a sharp peak at 485 cm −1  Ref. 36   isdetectable. The measured spectrum closely resembles thatobserved in bulk LMO samples 36 and mainly consists of twocomponents around 500 and 600 cm −1 , usually ascribed tobending    B   and stretching   S    modes of the MnO 6  octahe-dron, respectively. 36,37 In order to distinguish the differentcomponents, we have applied a fitting procedure by employ-ing a standard model function  S      , given by the linear com-bination of damped harmonic oscillators. 37,38 Figure 4 showsthat a very good description of the measured spectrum can beobtained by including in the model  S       four components  1–4   besides the  B   at 490 cm −1   and  S    at 614 cm −1   ones.The uncertainties on the peak frequency values reported inFig. 4, as estimated on the basis of reproducibility of fitresults for the different LMO films we investigated, are ±3and ±6 cm −1 for the sharper   1,  B , and  S    and broader   2, 3,and 4   components, respectively. The peak frequency valueswe determine for LMO films are in very good agreementwith those obtained for LMO bulk samples. 36,39 Figure 5 shows the field dependence of the magnetiza-tion measured by SQUID on a 75 nm thick film with theexternal field oriented in the film plane. The background con-tribution from the substrate and the sample holder has beensubtracted. The measured saturation magnetization is  M  s =1.59  10 5 A/m, which corresponds to    =1.09    B  /Mn, tobe compared with the values of     =4    B  /Mn expected forMn 3+ , of     =3.8    B  /Mn in FI compositions, and finally with   =0.16    B  on pure LMO. The temperature dependence of the magnetization is shown in Fig. 6, obtained both by fieldcooling   FC   the film in a magnetic field of   H  =1 kOe and by FIG. 3. Isointensity contour plot on a logarithmic scale of    103   reciprocalspace map of a LMO film grown on LAO substrate. The range of thelogarithmic scale is from 4 to 30 000. In the inset the    -scan measurementaround the   103   reflection is reported.FIG. 4. Typical Raman spectrum of a LMO film. Best fit profile and com-ponents are shown separately. The low frequency region of the spectrum isshown in expanded scale in the inset.FIG. 5. Magnetization vs field at  T  =2 K of a LMO film, with the externalfield oriented in the film plane.FIG. 6. Temperature dependence of FC and ZFC magnetizations of a LMOfilm with an external field of   H  =1 kOe. Inset: Blowup of the ZFC data. 023910-4 Aruta  et al.  J. Appl. Phys.  100 , 023910   2006  Downloaded 30 Jul 2006 to 192.135.11.132. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp  zero field cooling   ZFC  , applying the same field at  T  =2 Kand measuring while heating. The difference between thetwo curves is typical of very lightly doped manganites, dueto the presence of ferromagnetic contributions from localizedMn 4+ moments in a canted antiferromagnetic bulk back-ground. The two data sets merge at  T   N  =131  3   K, where theordered moments vanish both in the antiferromagnetic bulkand at localized Mn 4+ . A peak in the ZFC curve around  T   N   isevident from the inset of Fig. 6. The ordering temperature isvery close to that of the pure LMO, but the saturation valueof the average ferromagnetic moment indicates that thesamples must lie closer to the lower edge of the FI phase.Assuming that substrate induced strain does not alterstrongly the phase diagram, the transition temperature itself,which is lower than that of pure LMO, indicates that thefilms are probably not yet within the FI phase. This is alsoconsistent with the minute value of the ZFC magnetization in  H  =1 kOe, which is instead a sizable fraction of the FC valuefor samples within the FI phase. The number of Bohr mag-netons per Mn versus Mn 4+ concentration  x   in bulkLa 1−   Mn 1−   O 3   Ref. 40   Fig. 6 therein   gives a rough esti-mate of   x  =0.05 for our sample. Such an estimate, togetherwith the RBS results indicating a slight lanthanum defi-ciency, leads to the following formula   within 0.005 atoms/ cell   unit for our film   La 0.983+  Mn 0.953+ Mn 0.054+  O 32− .The presence of a sizable amount of localized Mn 4+ ionsis directly witnessed by the NMR spectrum of Fig. 7   on thesame film  . The most prominent feature is the peak around320 MHz, corresponding to the nuclei of localized Mn 4+ ions. The Mn 3+ is mostly escaping observation, although itprobably contributes with very poor signal/noise ratio to thehigher frequency portion of the NMR spectrum. This is com-monly seen 41–43 in nanoscopically confined manganites andit is due to a combined effect of the faster relaxation rates,larger broadening, and lower enhancement of the nuclei of Mn 3+ .Relative to the pseudoperovskite bulk lattice parametersfor stoichiometric, antiferromagnetic LMO, our films showsignificant decrease of the unit cell volume that is not readilyascribed to the slight off-stoichiometry   Mn 4+  5%   dis-cussed above. However, a detailed neutron diffraction studyof the structure and magnetic ordering in LMO samples pre-pared under different conditions 7 shows that the low concen-trations of Mn 4+   2%   are typically achieved only whenannealing in a nonoxidizing environment; the presence of asmall amount of oxygen during preparation can produce anumber of different unit cells, all associated with slight non-stoichiometries. In particular, when annealing in air a secondorthorhombic   Pnma   phase results that has a unit cell vol-ume about 4% less than the stoichiometric orthorhombicphase. This second orthorhombic phase is slightly rich inMn 4+ and is ferromagnetic with  T  C  =131 K and about1.54    B  /Mn. One possible explanation for the structural  5% smaller unit cell   and magnetic properties   AF,  T   N  =131 K, 1.09    B  /Mn   of these films is that, in the oxygenenvironment during PLD, the epitaxial strain stabilizes thissecond orthorhombic phase with slightly less Mn 4+ than re-ported in Ref. 7, leaving the film insulating but on the AFside of the AF/F boundary. The epitaxial stabilization of slightly different unit cell structures under different growthconditions has been seen, for example, in cuprate thinfilms. 44 Another possible explanation can be found in thepaper of Ritter  et al. , 4 where a decrease of the unit cell vol-ume of LaMnO 3+    is observed with increasing the lanthanumdeficiency and the Mn 4+ content. CONCLUSIONS We have shown that LMO films grown by PLD at anoxygen pressure of 10 −2 mbar are nearly stoichiometric andquite well oxygen compensated. The structural parametershave been obtained by x-ray diffraction measurements: filmsresulted epitaxial, with both in-plane lattice parameters equalto the in-plane lattice parameter of the LAO substrate. Onlya partial relaxation in the topmost layer is suggested. More-over, the LMO in-plane cell orientation was “square onsquare” relative to the substrate. The perpendicular-to-the-surface lattice parameter resulted to be 0.399 nm. Ramanmeasurements did not show any sizable shift or broadeningof the peaks relative to the typical bulk spectra. Therefore wecan conclude that the epitaxial strain does not induce rel-evant tilting or rotation of the MnO 6  octahedra enough toinfluence the Raman spectra. A significant effect on the Ra-man spectra would also be expected in the case of heavycation or oxygen disorder that can be ruled out in our LMOfilms 16 as evidenced by the structural, electrical transport,and magnetic measurements. ACKNOWLEDGMENT Partial support from “THIOX” ESF network isacknowledged. 1 S. Jin, T. H. Tiefel, M. McCormack, R. A. Fastnacht, R. Ramesh, and L.H. Chen, Science  264 , 413   1994  . 2 Y. Tokura and N. Nagaosa, Science  288 , 462   2000  . 3 J. M. D. Coey, M. Viret, and S. von Molnar, Adv. Phys.  48 , 167   1999  . 4 C. Ritter  et al. , Phys. Rev. B  56 , 8902   1997  . 5 H. Yamada, Y. Ogawa, Y. Ishii, H. Sato, M. Kawasaki, H. Akoh, and Y.Tokura, Science  305 , 646   2004  . 6 M. Hervieu, R. Mahesh, N. Rangavittal, and C. N. R. Rao, Eur. J. SolidState Inorg. Chem.  32 , 79   1995  . 7 Q. A. Huang, A. Santoro, J. W. Lynn, R. W. Erwin, J. A. Borchers, J. L.FIG. 7.  55 Mn NMR spectrum of a LMO film between 310 and 410 MHz at T  =1.6 K. 023910-5 Aruta  et al.  J. Appl. Phys.  100 , 023910   2006  Downloaded 30 Jul 2006 to 192.135.11.132. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
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