This is the submitted version of the journal article: Bigiani, Lorenzo; Hassan, Mariam; Peddis, Davide; [et al.]. «High magnetic coercivity in nanostructured Mn3O4 thin films obtained by chemical vapor deposition». ACS Applied Nano Materials, Vol. 2, issue 3 (March 2019), p. 1704-1712. DOI 10.1021/acsanm.9b00141 This version is available at https://ddd.uab.cat/record/271929 under the terms of the license Page 1 of 23 ACS Applied Nano Materials 1 2 3 4 High Low-Temperature Coercivity 5 6 7 8 9 in Mn3O4 Thin Films 10 11 12 13 Obtained by Chemical Vapor Deposition 14 15 16 17 18 Lorenzo Bigiani†, Mariam Hassanb,c, Davide Peddisb, Chiara Maccatoa,*, Gaspare Varvarob,*, 19 20 Cinzia Sadad, Elza Bontempie, Sara Martí-Sánchezf, Jordi Arbiolf,g, Davide Barrecah 21 22 23 24 a Department of Chemical Sciences, Padova University and INSTM, 35131 Padova, Italy 25 26 27 b CNR-ISM, Monterotondo Scalo, 00015 Roma, Italy 28 29 c Department of Matter Sciences and Engineering, Environment and Urban Planning, Marche 30 31 32 Polytechnic University, 60121 Ancona, Italy 33 d Department of Physics and Astronomy, Padova University and INSTM, Padova, Italy 34 35 36 e Chemistry for Technologies Laboratory, Department of Mechanical and Industrial 37 38 39 Engineering, Brescia University and INSTM, Brescia, Italy 40 f Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Bellaterra, 41 42 43 08193 Barcelona, Catalonia, Spain 44 45 g ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain 46 47 h CNR-ICMATE and INSTM, Department of Chemical Sciences, Padova University, 35131 48 49 50 Padova, Italy 51 52 53 54 55 56 57 58 59 * Corresponding authors; phone: +39-0498275234; e-mail: chiara.maccato@unipd.it (C.M.); phone: +39- 60 0690672651; e-mail: gaspare.varvaro@ism.cnr.it (G.V.). 1 ACS Paragon Plus Environment ACS Applied Nano Materials Page 2 of 23 1 2 3 ABSTRACT: High-quality -Mn3O4 (haussmannite) thin films consisting of evenly 4 5 6 interconnected nanoaggregates were prepared on Si(100) substrates by chemical vapor 7 8 deposition. The use of different reaction atmospheres (dry vs. wet O2) and total pressures 9 10 enabled to tailor the system structural and morphological features. The obtained films showed 11 12 13 bulk-like magnetic properties together with an extraordinarily high low-temperature in-plane 14 15 coercivity (up to  1 T) dependent on the microstructural defect content. These issues may 16 17 foster the implementation of the obtained films in eventual technological applications. 18 19 20 21 22 KEYWORDS: Mn3O4, thin films, chemical vapor deposition, magnetic properties, 23 24 25 26 coercivity. 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 ACS Paragon Plus Environment Page 3 of 23 ACS Applied Nano Materials 1 2 3 4  INTRODUCTION 5 6 7 Nanostructured magnetic spinel oxides have been widely investigated over the last two 8 9 10 decades owing to the possibility of finely tuning their structural, chemical and physical 11 12 properties by varying the metal cations and their distribution among tetrahedral and 13 14 octahedral sites.1-4 Among the family members, -Mn3O4 (haussmannite), one of the most 15 16 17 stable manganese oxides endowed with large natural abundance, cost-effectiveness and 18 19 environmental compatibility,5-7 has been attracting an ever increasing interest. In fact, its 20 21 chemico-physical properties render it an attractive multi-functional platform for various 22 23 24 applications, including, among others, super-/pseudo-capacitors,8-13 electrodes for 25 26 batteries,7,14-15 gas sensors,16-17 heterogeneous catalysts,5,10,18 and soft magnetic materials.19 In 27 28 particular, bulk Mn3O4 is magnetically ordered at 42 K to form a collinear ferrimagnetic 29 30 array,20-21 whereas at lower temperatures a canted spin array is produced.22-24 Since the system 31 32 33 magnetic properties are directly dependent on the aggregate size and shape,2-3,25-27 as well as 34 35 on their structural features and the presence of defects,28-30 nanostructured Mn3O4 systems in 36 37 the form of nanoparticles20,24,30-32 and nanorods/wires19,21-22,29,33 fabricated by means of 38 39 40 different synthesis techniques have been widely studied over the last years. In comparison to 41 42 these cases, magnetic characteristics of Mn3O4 thin films have been scarcely investigated,20,32 43 44 and further efforts in this regard are undoubtedly of importance from both a fundamental and 45 46 47 an applicative point of view. Indeed, advances in the fabrication of these systems is a strategic 48 49 subject of ongoing investigations in order to control the resulting magnetic properties. 50 51 In the present work, chemical vapor deposition (CVD), a versatile bottom-up route for the 52 53 preparation of thin films and nanostructures with specific properties,6,34 was used to deposit 54 55 56 high-quality -Mn3O4 films. In particular, the latter were prepared under both dry O2 and 57 58 O2+H2O reaction atmospheres and at different pressures (3.0 and 10.0 mbar). The target 59 60 systems are investigated in their structure, morphology and chemical composition, and their 3 ACS Paragon Plus Environment ACS Applied Nano Materials Page 4 of 23 1 2 3 magnetic properties are characterized as a function of the adopted processing conditions. To 4 5 6 the best of our knowledge, there have been no literature reports on the magnetic properties of 7 8 CVD Mn3O4 materials up to date. 9 10 11 12 13 14  EXPERIMENTAL SECTION 15 16 17 Material preparation. A cold-wall CVD reactor was used for the synthesis of manganese 18 19 20 oxide films, using Mn(hfa)2•TMEDA35 as Mn precursor (Hhfa = 1,1,1,5,5,5-hexafluoro-2,4- 21 22 pentanedione; TMEDA = N,N,N’,N’-tetramethylethylenediamine; vaporization temperature = 23 24 65°C). Precursor vapors were transported by an O2 flow [purity = 6.0; rate = 100 standard 25 26 27 cubic centimeters per minute (sccm)] into the reaction chamber, in which an auxiliary oxygen 28 29 flow (100 sccm) was also introduced. Growth processes were performed under dry O2 and 30 31 O2+H2O atmospheres. In the latter case, the auxiliary O2 flow was passed through a water 32 33 34 reservoir maintained at 35°C before entering the reactor. Depositions were carried out for 1 h 35 36 on pre-cleaned p-type Si(100) substrates (MEMC®, Merano, Italy). Basing on preliminary 37 38 optimizations, the growth temperature was set at 500°C, whereas the total pressure was fixed 39 40 either at 3.0 or at 10.0 mbar. 41 42 43 Material characterization. Two-dimensional X-ray diffraction (XRD2) analyses were carried 44 45 out in reflection mode on a Dymax-RAPID X-ray micro-diffractometer (CuK radiation), 46 47 48 equipped with a cylindrical imaging plate detector, that allows data collection from 0 to 160° 49 50 (2) horizontally and from - 45 to + 45° (2) vertically. A collimator diameter of 300 m and 51 52 an exposure time of 40 min were used. Conventional XRD patterns were obtained by 2D 53 54 55 image integration. 56 57 Field emission-scanning electron microscopy (FE-SEM) analyses were performed by means 58 59 of a Zeiss SUPRA 40 VP instrument, using primary beam voltages between 10 and 20 kV. 60 4 ACS Paragon Plus Environment Page 5 of 23 ACS Applied Nano Materials 1 2 3 The mean thickness and nanoaggregate dimensions were evaluated by the ImageJ software 4 5 6 (https://imagej.nih.gov/ij/, accessed December 2018). 7 8 High resolution-transmission electron microscopy (HR-TEM) and high angle annular dark 9 10 field-scanning TEM (HAADF-STEM) analyses were carried out with a FEI Tecnai F20 TEM 11 12 microscope operated at 200 kV. Sample preparation was performed by means of diamond 13 14 15 wire saw cutting, mechanical thinning with diamond lapping films, and ion milling in a Gatan 16 17 precision ion polishing (PIPS) system. Oxidation state maps were obtained through scanning 18 19 TEM-electron energy loss spectroscopy (STEM-EELS) analysis by estimating the 20 21 22 MnL3/MnL2 edge intensity ratio after background subtraction and applying a Gaussian fitting 23 24 on both edges.36 25 26 X-ray photoelectron spectroscopy (XPS) measurements were performed on a PerkinElmer Φ 27 28 29 5600ci spectrometer using a standard Al K radiation (1486.6 eV). Binding energy (BE) 30 31 values (uncertainty = ± 0.2 eV) were corrected for charging effects by assigning a BE of 32 33 284.8 eV to the adventitious C1s peak.37-38 Atomic percentages (at. %) were calculated by 34 35 36 peak integration, using Φ V5.4A sensitivity factors. 37 38 Secondary ion mass spectrometry (SIMS) analysis was carried out by a Cameca IMS 4f 39 40 instrument, using a Cs+ primary ion beam (14.5 keV, 20 nA) and negative secondary ion 41 42 43 detection, using an electron gun for charge compensation. Rastering over a 150×150 μm2 area 44 45 was performed, and secondary ions were collected from a 8×8 μm2 sub-region to avoid crater 46 47 effects. Beam blanking mode and high mass resolution configuration were adopted. The 48 49 sputtering time in the profile abscissa was converted into depth using the mean film thickness 50 51 52 values measured by cross-sectional FE-SEM analyses. 53 54 Temperature-dependent magnetization measurements were performed by using a 55 56 superconducting quantum interference device (SQUID) magnetometer (MPMS 10) with the 57 58 59 external field applied either in the film-plane or along the surface normal. 60 5 ACS Paragon Plus Environment ACS Applied Nano Materials Page 6 of 23 1 2 3 4 5 6 7  RESULTS AND DISCUSSION 8 9 10 11 The system structure, with particular attention to the presence of possible orientation effects, 12 13 was investigated by the joint use of XRD and XRD2 experiments. The results shown in Figure 14 15 1(a-b) clearly indicated the presence of the tetragonal -Mn3O4 polymorph (Figure 1c; space 16 17 18 group I41/amd; a = 5.762 Å, c = 9.470 Å) with a spinel-type structure.21,28,39 No other 19 20 reflections from different Mn oxides were present, indicating the selective formation of 21 22 phase-pure -Mn3O4, at variance with previous studies.22-23,29 A closer inspection of Figure 1a 23 24 25 revealed an inhomogeneous intensity distribution along the Debye rings, suggesting the 26 27 occurrence of preferential growth orientation. This effect was particularly evident for the 28 29 specimen obtained in O2+H2O at 3.0 mbar, which was preferentially oriented along the (101) 30 31 32 direction, as also revealed by conventional XRD analyses (Figure S1). Indeed, the obtained 33 34 patterns highlighted the occurrence of different relative peak intensities with respect to the 35 36 reference haussmannite,39 as well as different intensity ratios between samples grown in dry 37 38 O2 and in O2+H2O reaction atmospheres. In particular, specimens fabricated in dry O2, 39 40 41 irrespective of the used pressure, presented two main reflections pertaining to (211) and (220) 42 43 planes, the latter exhibiting a higher relative intensity if compared with the reference pattern. 44 45 On the other hand, specimens fabricated in O2+H2O showed a (211) intensity decrease and a 46 47 48 concomitant increase of (101) and (103) signals with respect to the reference powder 49 50 spectrum. These findings, combined with the texture coefficient (TC) values (§ S-1 and Table 51 52 1), suggest an enhanced (220) or (101) orientation and/or a concomitant anisotropic growth 53 54 55 for samples obtained in O2 and in O2+H2O atmospheres, respectively. This effect was 56 57 particularly marked for the system obtained at 3.0 mbar in O2+H2O (TC101 = 2.3). 58 59 XRD data were also used to determine the dislocation density (δ) (Table 1), which resulted 60 6 ACS Paragon Plus Environment Page 7 of 23 ACS Applied Nano Materials 1 2 3 4 5 (a) (b) 10 mbar, O2 6 3 mbar, O2 7 10 mbar, O2 + H2O Intensity (a.u.) 8 3 mbar, O2 + H2O 9 10 11 12 13 14 15 16 17 18 20 40 60 80 19 2 (°) 20 (c) 21 22 23 Mn(III) 24 25 Mn(II) 26 27 O 28 29 30 31 32 33 34 (d) (e) 35 36 200 nm 200 nm 37 38 39 40 41 42 200 nm 100 nm 43 44 45 46 47 Figure 1. (a) XRD2 map for a manganese oxide thin film deposited at 3.0 mbar in O2+H2O. 48 (b) Integrated XRD patterns for specimens fabricated under different conditions. Vertical 49 50 black lines mark peak positions for tetragonal -Mn3O4 (hausmannite). (c) Representation of 51 the α-Mn3O4 crystalline structure 39. Plane-view FE-SEM micrographs of samples obtained 52 53 54 under (d) O2+H2O, 3.0 mbar; (e) O2, 10.0 mbar. The corresponding cross-sectional images are 55 reported as insets. 56 57 58 higher for samples obtained under dry O2 and, in particular, for the one fabricated at 10.0 59 60 7 ACS Paragon Plus Environment ACS Applied Nano Materials Page 8 of 23 1 2 3 mbar ( = 7.4×1014 lines/m2). These results suggested an increase in the content of 4 5 6 microstructural defects under dry O2 and, in particular, at a pressure of 10.0 mbar. 7 8 9 10 Sample TC  (lines/m2) T (K) Hc,// (T; 5 K) 11 12 3 mbar, O2+H2O 2.3 (101) 5.61014 42±2 0.88 13 14 10 mbar, O2+H2O 1.5 (101) 5.41014 42±2 0.89 15 16 3 mbar, O2 1.5 (220) 7.21014 41±2 0.91 17 18 10 mbar, O2 1.5 (220) 7.41014 42±2 0.97 19 20 21 22 Table 1. Texture coefficients (TC) and dislocation density () values, along with phase 23 24 transition temperatures (T; uncertainty =  2 K) and in-plane coercivity (Hc,//) values for the 25 26 Mn3O4 specimens analyzed in the present work. 27 28 29 A preliminary investigation of material morphology was carried out by means of FE-SEM, 30 31 which evidenced an optimal film/substrate adhesion. The system nano-organization (Figures 32 33 1d,e and S2) was dominated by the occurrence of extensively interconnected nanoaggregates, 34 35 36 typical of a three-dimensional growth mode, with a direct influence of the adopted working 37 38 pressure on film morphology and thickness. In particular, irrespective of the reaction 39 40 atmosphere, samples obtained at 10.0 mbar presented pseudo-columnar nanostructures (mean 41 42 43 diameter = 100 nm; average thickness = 350 nm), whereas, for specimens obtained at 3.0 44 45 mbar, imaging revealed the occurrence of more faceted aggregates (mean dimensions = 100 46 47 nm; average thickness = 630 nm), similarly to Mn3O4 films obtained by pulsed laser 48 49 deposition.8 50 51 52 An additional important insight into Mn3O4 nanostructure was gained by TEM analyses on 53 54 representative samples (Figures 2 and S3). In both cases, Mn3O4 formed a polycrystalline 55 56 layer on the top of the Si(100) substrate with crystalline domains that, in agreement with XRD 57 58 59 results, comprised the sole Mn3O4 tetragonal phase. The absence of epitaxial relations 60 8 ACS Paragon Plus Environment Page 9 of 23 ACS Applied Nano Materials 1 2 3 between the Si(100) substrate and Mn3O4 was due to Si oxidation at the film/substrate 4 5 6 interface, resulting in the formation of a thin SiO2 layer ( 2 nm). Further analyses carried out 7 8 by STEM-EELS enabled to perform a chemical map of manganese valence state by 9 10 examining the intensity ratios between Mn L3 and L2 edges.36 The results highlighted the even 11 12 13 coexistence of Mn(II) and Mn(III) oxidation states (Figures 2d,e), in agreement with the 14 15 above compositional results, as also confirmed by XPS data (see below). 16 17 18 19 (a) (b) 20 21 22 (12-1) 23 24 (21-1) 25 26 27 28 (1-10) 29 30 31 32 33 10 nm 10 nm [113] Mn3O4 34 35 IL3/IL2 Valence x103 (c) (d) (e) 36 OK 6 State 37 180 +2 38 39 4 120 40 Mn L32 41 42 60 +3 2 +4 43 44 0 200 nm 100 nm 45 200 nm 0 46 550 650 750 47 eV 48 49 50 Figure 2. TEM analyses on a Mn3O4 thin film deposited at 10.0 mbar under dry O2. (a) HR- 51 TEM micrograph and (b) corresponding power spectrum. (c) Low magnification cross- 52 53 sectional HAADF-STEM image. (d) O and Mn EELS signals recorded on the region marked 54 55 by a red rectangle in (c). (e) Oxidation state map obtained by the MnL3/MnL2 edge intensity 56 57 ratio (IL3/IL2). 58 59 60 The system surface and in-depth chemical composition were investigated by combining XPS 9 ACS Paragon Plus Environment ACS Applied Nano Materials Page 10 of 23 1 2 3 and SIMS analyses. Wide scan XPS spectra (Figure 3a) evidenced the presence of manganese 4 5 6 and oxygen, along with adventitious surface carbon contamination. The Mn2p3/2 signal 7 8 position [binding energy (BE) = 641.9 eV] and the corresponding spin–orbit splitting value 9 10 (11.5 eV; Figure 3b) indicated the formation of pure Mn3O4,5,9,14-15,21,28,38 as also confirmed 11 12 13 by the Mn3s multiplet splitting separation ( = 5.4 eV, Figure 3c),6,11,16-17,34 which yielded a 14 15 finger- print for the sole presence of this oxide, in agreement with structural results. 16 17 18 19 OKLL 10 mbar, O2 20 3 mbar, O2 21 Mn2p CKLL 10 mbar, O2 +H2O 22 3 mbar, O2 +H2O Mn2s 23 O1s 24 Intensity (a.u.) 25 26 Mn3p 27 C1s Mn3s 28 29 30 MnLMM 31 32 33 34 35 36 1200 1000 800 600 400 200 0 37 Binding Energy (eV) 38 (b) (c) Mn2p Mn3s (d) 39 O1s 40 Intensity (a.u) 41 42 I Mn2p1/2 Mn2p3/2 43 44 45 II 46 47 48 670 660 650 640 95 90 85 80 540 535 530 525 49 Binding Energy (eV) Binding Energy (eV) Binding Energy (eV) 50 51 52 53 Figure 3. Surface XPS spectra: (a) wide-scans (surveys) for manganese oxide specimens. 54 55 High resolution Mn2p (b), Mn3s (c) and O1s (d) photoelectron peaks for a Mn3O4 film 56 deposited at 10.0 mbar under O2+H2O atmospheres. 57 58 59 The O1s signal (Figure 3d) resulted from the concomitant contribution of lattice oxygen 60 10 ACS Paragon Plus Environment Page 11 of 23 ACS Applied Nano Materials 1 2 3 bonds; BE = 529.9 eV) and surface adsorbed oxygen/hydroxyl/carbonate species resulting 4 5 6 from atmospheric exposure (BE = 531.7 eV).11-12,28-29,34,40 As a consequence, the O/Mn 7 8 atomic ratio was slightly higher than the stoichiometric one ( 1.7). The energy difference 9 10 between the Mn2p3/2 maximum and the lowest O1s BE component yielded a value of 111.9 11 12 13 eV, in line with Mn3O4 presence.34 14 15 Complementary information on the in-depth composition were gained by SIMS analysis. 16 17 Irrespective of the preparation conditions, carbon content was lower than 100 ppm, indicating 18 19 20 an appreciable system purity. As shown in Figure S4, manganese and oxygen signals had a 21 22 parallel trend throughout the investigated depth, confirming the uniform composition of the 23 24 target films. At a depth of  350 nm, the net decrease of Mn and O ionic yields and the 25 26 27 concomitant increase of the Si one evidenced a relatively sharp film/substrate interface. 28 29 30 (a) (b) 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 Figure 4. (a) FC () and ZFC () curves of a Mn3O4 sample obtained in O2+H2O at 3.0 50 51 mbar; inset: absolute value of the derivative |dM/dT| of the FC curve. (b) FC (filled dots) and 52 ZFC (open dots) curves of the same specimen measured under different applied fields. 53 54 55 In order to investigate the interplay between the sample preparative conditions and the 56 57 58 corresponding magnetic properties, field- and temperature-dependent magnetization 59 60 measurements were performed. Figures 4a and S5 show the temperature dependence of the 11 ACS Paragon Plus Environment ACS Applied Nano Materials Page 12 of 23 1 2 3 field-cooled (FC) and zero-field-cooled (ZFC) magnetization measured under a magnetic field 4 5 6 of 20 mT applied in the film plane. In the ZFC process, the film was cooled from 70 to 5 K in 7 8 the absence of the magnetic field, and the magnetization was measured during the heating 9 10 process under a magnetic field of 20 mT. In the FC process, a field of 20 mT was applied 11 12 during cooling, and the magnetization measurement was performed upon heating under the 13 14 15 same magnetic field. 16 17 The sharp magnetization increase observed in the FC curve corresponds to the onset of the 18 19 paramagnetic-ferrimagnetic phase transition. Fitting the derivative of the FC magnetization 20 21 22 curve (inset of Figure 4a) allowed to estimate the phase transition temperature by the position 23 24 of the observed maximum. As a result, an average value of 42 K was measured for all the 25 26 samples (Table 1), in excellent agreement with the transition temperature reported for - 27 28 29 Mn3O4 bulk phase.21,33,41-42 The cusp appearing in the ZFC magnetization curve at the 30 31 transition temperature could be related to the large magneto-crystalline anisotropy value of 32 33 the -Mn3O4 phase (Kmc  1.5×105 J/m3),41 which, in turn, leads to a high magnetic coercivity 34 35 36 that increases at low temperatures. Under these conditions (starting from 5 K), the 20 mT 37 38 applied field is not large enough to overcome the high coercivity, resulting thus in a small 39 40 ZFC magnetization. Upon increasing the temperature, the magnetization rises due to the 41 42 43 gradual reduction of the coercive field and then abruptly drops due to the loss of the long- 44 45 range ferrimagnetic order when the transition temperature is approached. In order to 46 47 corroborate such conclusions, FC and ZFC curves were collected under larger external fields 48 49 50 (Figure 4b). Increasing the applied field up to 0.1 T does not affect the overall shape of the 51 52 ZFC magnetization curve, and only a magnetization enhancement around the transition 53 54 temperature is observed. When the applied field is increased up to 3 T, i.e. a value large 55 56 enough to reach saturation, the cusp disappeared, confirming thus the high magnetic 57 58 59 anisotropy of the samples and the above reported observations. 60 12 ACS Paragon Plus Environment Page 13 of 23 ACS Applied Nano Materials 1 2 3 4 (a) (b) 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Figure 5. Low temperature (5 K) in-plane () and out-of-plane () field-dependent 23 24 magnetization loops of samples Mn3O4 samples obtained: (a) in O2+H2O at 3.0 mbar; (b) in 25 dry O2 at 10.0 mbar. The vertical axis is normalized to the magnetization value at H = 5 T 26 27 (M5T). 28 29 30 Figures 5 and S6 display the low temperature (5 K) field-dependent magnetization loops of 31 32 Mn3O4 films measured with the field applied either in the film plane or along the surface 33 34 35 normal. The similarity of in-plane and out-of-plane loops suggests the lack of a well-defined 36 37 magnetic anisotropy symmetry. Only a slightly preferential in-plane anisotropy is observed, 38 39 as indicated by the weakly larger values of remanent magnetization and coercivity (the latter 40 41 42 effect being observed only for samples fabricated in dry O2) when measurements are 43 44 performed within the film plane. In -Mn3O4 films, the magneto-crystalline anisotropy, which 45 46 forces the magnetization to lie along specific crystallographic directions (called easy- 47 48 49 axis/plane), is expected to be the dominant contribution to the effective magnetic anisotropy. 50 51 Its value is indeed almost one order of magnitude higher than the shape anisotropy (which 52 53 would favor the magnetization alignment along the film plane), whose upper limit for a 54 55 56 uniformly magnetized and infinite thin film corresponds to 1⁄20Ms2 = 0.2×105 J/m3, where 0 57 58 and Ms are the vacuum permeability and the saturation magnetization, respectively. However, 59 60 the (001) plane and the <001> crystallographic direction, which represent the magneto- 13 ACS Paragon Plus Environment ACS Applied Nano Materials Page 14 of 23 1 2 3 crystalline easy-plane and hard-axis, respectively, are randomly distributed, thus giving rise to 4 5 6 a similar magnetic response along different directions. The slightly preferential in-plane 7 8 anisotropy could be due to the shape anisotropy of the films. The random distribution of the 9 10 easy-axes can also explain the linear contribution observed in the high-field region. 11 12 Notably, all the analyzed samples showed extraordinarily large in-plane coercivity values 13 14 15 (Table 1), significantly higher than that reported for bulk -Mn3O4 at 5 K ( 0.3 T).41-42 16 17 Overall, the coercivity was higher for samples obtained under dry O2, and increased for the 18 19 20 systems obtained at higher pressures. To the best of our knowledge, the measured coercivity 21 22 values are the highest among those reported in the literature for these systems in the form of 23 24 films,20,32 and similar (or slightly lower) values have been only obtained in nanoparticles23- 25 26 24,30-31 and nanowires/rods19-22,29,33 where surface and shape anisotropy, respectively, were 27 28 29 indicated as the main responsible for the observed effects. In the present case, the high 30 31 coercivity values could be traced back to the presence of microstructural defects (see the 32 33 above discussed XRD results) acting as pinning sites for domain wall motion.43-44 This 34 35 36 observation accounts for the highest coercivity being observed for the Mn3O4 specimen 37 38 synthesized under dry O2 at 10.0 mbar, which presented the larger amount of microstructural 39 40 defects, as indicated by structural analyses (see above). 41 42 43 44 45 46  CONCLUSIONS 47 48 49 50 In summary, high-quality Mn3O4 films consisting of uniformly interconnected nanoaggregates 51 52 were obtained by a CVD route. Controlled variations of the growth atmosphere and working 53 54 pressure enabled to tailor the preferential crystallographic orientation and film morphology, 55 56 57 while maintaining the same phase composition, corresponding to pure tetragonal -Mn3O4 58 59 (haussmannite) free from other manganese oxides. All the target systems showed bulk-like 60 14 ACS Paragon Plus Environment Page 15 of 23 ACS Applied Nano Materials 1 2 3 magnetic properties together with an extraordinarily high low-temperature in-plane coercivity, 4 5 6 which was traced back to the occurrence of microstructural defects. The successful fabrication 7 8 of the target Mn3O4 materials demonstrated in this work is a key requirement for their ultimate 9 10 mastering towards eventual end-uses. In fact, the present studies can trigger both fundamental 11 12 and applied advancements towards the exploitation of a-Mn3O4 film properties in various 13 14 15 tecnological applications. 16 17 18 19  ASSOCIATED CONTENT 20 21 22 23 Supporting Information. The Supporting Information is available free of charge on the ACS 24 25 Publication website at DOI: 10.1021/acsanm.XXX- 26 27 28 Details and further data on XRD, FE-SEM, TEM, EELS and magnetic measurements. 29 30 31 Author Contributions 32 33 34 The manuscript was written through contributions of all authors. All authors have given 35 36 approval to the final version of the manuscript. 37 38 39 Notes 40 41 42 The authors declare no competing financial interest. 43 44 45 46 Conflict of interests 47 48 49 The authors declare no conflict of interests. 50 51 52 53 54  ACKNOWLEDGMENTS 55 56 The authors thank Padova University DOR 2016–2018 and P-DiSC #03BIRD2016-UNIPD 57 58 projects. Funding from Generalitat de Catalunya 2017 SGR 327, the Spanish MINECO 59 60 15 ACS Paragon Plus Environment ACS Applied Nano Materials Page 16 of 23 1 2 3 ENE2017-85087-C3-3-R and Severo Ochoa Program SEV-2017-0706, as well as CERCA 4 5 6 Programme/Generalitat de Catalunya is also acknowledged. Part of the present work has been 7 8 performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD 9 10 program. 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 16 ACS Paragon Plus Environment Page 17 of 23 ACS Applied Nano Materials 1 2 3  REFERENCES 4 5 6 7 (1) Scialabba, C.; Puleio, R.; Peddis, D.; Varvaro, G.; Calandra, P.; Cassata, G.; Cicero, 8 9 L.; Licciardi, M.; Giammona, G. Folate Targeted Coated SPIONs as Efficient Tool for 10 11 MRI. Nano Res. 2017, 10, 3212-3227. 12 13 (2) Rizzuti, A.; Dassisti, M.; Mastrorilli, P.; Sportelli, M. C.; Cioffi, N.; Picca, R. A.; 14 15 16 Agostinelli, E.; Varvaro, G.; Caliandro, R. Shape-Control by Microwave-Assisted 17 18 Hydrothermal Method for the Synthesis of Magnetite Nanoparticles using Organic 19 20 Additives. J. Nanopart. Res. 2015, 17, 408. 21 22 23 (3) Aslibeiki, B.; Varvaro, G.; Peddis, D.; Kameli, P. 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