Mendeleev Commun. 2004 133 Thin-film heterostructures containing La–Sr manganite and soft ferromagnets: metallorganic chemical vapour deposition, characterization and tunneling magnetoresistance Anton A. Kamenev, a Olga V. Boytsova, a Sergey V. Antonov, b Oleg Yu. Gorbenko b and Andrei R. Kaul* b a Department of Materials Science, M. V. Lomonosov Moscow State University, 119992 Moscow, Russian Federation b Department of Chemistry, M. V. Lomonosov Moscow State University, 119992 Moscow, Russian Federation. Fax: +7 095 939 1492; e-mail: kaul@inorg.chem.msu.ru DOI: 10.1070/MC2004v014n04ABEH001944 A new approach (variant-structure and heterostructures with soft ferrite) to TMR effect of manganites was demonstrated. Interest in doped manganites La 1– x Sr x MnO 3 of a perovskite structure arises from the discovery of their colossal magneto- resistance in certain ranges of doping. The main obstacle for the use of these compounds for magnetic sensoring is a high magnetic field (~1 T) required for a notable CMR effect. Magnetic field concentration with a superconducting lens and the pattering of different tunnelling structures were shown to increase the magnetic sensitivity of CMR materials. Another way is to amplify an external magnetic field using a layer of soft magnets. The latter must meet the following requirements: low coercitivity (Hc < 10 Oe), high magnetic moment, Curie temperature exceeding T c of CMR material and chemical inert- ness of ferrite to CMR – material and substrate. In this work, we used spinels CoFe 2 O 4 , MnFe 2 O 4 and garnets (La x Nd 1– x ) 3 Fe 5 O 12 (x = 0, 0.3) as magnetic layers for magnetic field enhancement because they meet all these requirements; in particular, they possess high magnetic moments. All the films were grown by a single source MOCVD pro- cess on single crystal substrates including (110) ZrO 2 (Y 2 O 3 ), (111) ZrO 2 (Y 2 O 3 ), (001) MgO, (102) Gd 3 Ga 5 O 12 and (111) Gd 3 Ga 5 O 12 . La(thd) 3 , Nd(thd) 3 , Mn(thd) 3 , Co(thd) 2 , Fe(thd) 3 , Ca(thd) 2 , Sr(thd)·2Phen, where thd = 2,2,6,6-tetramethylheptane- 3,5-dionate and Phen = o-phenanthroline, were used as pre- cursors. 1,2 The deposition temperatures 750–850 °C were used for different compositions, the precursor evaporation temperature was 250 °C, the oxygen partial pressure was 2–5 mbar, and the total gas pressure was 5–10 mbar. The manganite compositions prepared included perovskites La 0.7 Ca 0.3 MnO 3 , La 0.7 Sr 0.3 MnO 3 spinel CoFe 2 O 4 and MnFe 2 O 4 . Garnet (La x Nd 1– x ) 3 Fe 5 O 12 (x = 0, 0.3) films were deposited as ferromagnetic layers in the heterostructures with the manganites. X-ray and HREM characterization revealed the dependence of the microstructure of the manganite films prepared on the substrate materials. Films on (110) ZrO 2 (Y 2 O 3 ) and (001) MgO were epitaxial, as well with the orientations (110) and (001), respectively, but possessed a block structure with small angle boundaries between the blocks. A difference between these two sets of samples can be explained by switching from 2D to 3D nucleation mode during film growth with the increase of the lattice mismatch and lattice dissimilarity. Manganite films on (111) ZrO 2 (Y 2 O 3 ) had out-of-plane orientation (110) with the in-plane variant structure. The orientation relations were found from pole figure XRD measurements and electron diffraction patterns in HREM and can be written as follows (Figure 2): Orthogonal NCSL is formed for (111) ZrO 2 (Y 2 O 3 ) by [1 01] and 3·[12 1], for (110) perovskite by [11 1] and 4·[11 2 ], respectively. Lattice mismatch is about 7 or 0%, for the directions. There are totally six variants of the type. We supposed the growth of variant structures CoFe 2 O 4 and MnFe 2 O 4 on SrTiO 3 , but we found that the spinel layer grows epitaxially on (001) SrTiO 3 , which excludes the possibility of significant tunnel magnetoresistance because of the absence of high angle boundaries (Figure 1). However, La 0.7 Sr 0.3 MnO 3 could be grown only in the polycrystalline random state on (001) CoFe 2 O 4 /(001) SrTiO 3 . Garnet Nd 3 Fe 5 O 12 films were grown epitaxially on (102) Gd 3 Ga 5 O 12 but only random man- ganites were deposited on top of both. Nd 3 Fe 5 O 12 films were also random on (001) SrTiO 3 and (001) La 0.7 Sr 0.3 MnO 3 / (001) SrTiO 3 . All the heterostructures were used to measure the effect of the crystallinity and magnetic coupling on the magnetoresistance of the manganites. There are two important low-field phenomena related to the tunnel magnetoresistance in the manganites. One of them is the magnetic reversal transition related to the magnetization hysteresis loop and resulting in the positive magnetoresistance peaks. At a somewhat higher field, the resistance drops sharply to a value lower than that in a zero field (negative tunnel mag- netoresistance). Finally, in a much higher field, the resistance switches to the rather weak linear dependence on the field. 3 The latter corresponds to the suppression of the spin fluctuations of scattered or trapped carriers considered to rest in a kind of the paramagnetic state. It demands a rather high field and is not interesting for applications. We consider only negative tunnel magnetoresistance (TMR). The TMR data for the manganite films and heterostructures are summarised in Figure 3. Three groups of samples can be lg I 20 24 28 32 36 40 44 48 2q/° K β SrTiO 3 K β SrTiO 3 (001) SrTiO 3 (004) MnFe 2 O 4 (004) CoFe 2 O 4 (002) SrTiO 3 MnFe 2 O 4 /SrTiO 3 CoFe 2 O 4 /SrTiO 3 Figure 1 XRD patterns for CoFe 2 O 4 and MnFe 2 O 4 thin films on (001) SrTiO 3 . (111) [11 0] ZrO 2 (Y 2 O 3 ) // (110) [11 1] perovskite. 121 101 Figure 2 Orientation variants in the manganite film on (111) ZrO 2 (Y 2 O 3 ). The crystallographic directions in the perovskite film are shown. , 2004, 14(4), 133–134