High Magnetic Coercivity in Nanostructured Mn3O4 Thin Films

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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High Magnetic Coercivity in Nanostructured Mn3O4 Thin Films Obtained by Chemical Vapor Deposition Lorenzo Bigiani,‡ Mariam Hassan,†,∥ Davide Peddis,† Chiara Maccato,*,‡ Gaspare Varvaro,*,† Cinzia Sada,§ Elza Bontempi,# Sara Martí-Sań chez,⊥ Jordi Arbiol,⊥,¶ and Davide Barreca∇ ‡

Department of Chemical Sciences, Padova University and INSTM, 35131 Padova, Italy CNR-ISM, Monterotondo Scalo, 00015 Roma, Italy ∥ Department of Matter Sciences and Engineering, Environment and Urban Planning, Marche Polytechnic University, 60121 Ancona, Italy § Department of Physics and Astronomy, Padova University and INSTM, 35131 Padova, Italy # Chemistry for Technologies Laboratory, Department of Mechanical and Industrial Engineering, Brescia University and INSTM, 25123 Brescia, Italy ⊥ Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Bellaterra, 08193 Barcelona, Catalonia Spain ¶ ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia Spain ∇ CNR-ICMATE and INSTM, Department of Chemical Sciences, Padova University, 35131 Padova, Italy

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†

S Supporting Information *

ABSTRACT: Nanostructured α-Mn3O4 (haussmannite) thin films consisting of evenly interconnected nanoaggregates were prepared on Si(100) substrates by chemical vapor deposition from a Mn(II) diketonate-diamine precursor under different reaction atmospheres (dry vs wet O2) and total operating pressures. The combination of chemico-physical results obtained by the joint use of complementary techniques enabled us to demonstrate the obtainment of high-purity Mn3O4 materials free from other manganese oxide phases, characterized by controllable structural and morphological characteristics as a function of the adopted processing conditions. Magnetic properties were investigated by analyzing temperature dependence (i.e., field-cooled and zero-field-cooled measurements) and field-dependence of the magnetization behavior. The obtained films show bulk-like magnetic properties, together with extraordinarily high low-temperature in-plane coercivities (up to ∼1 T). The possibility to tailor these values by varying the content of microstructural defects may foster the implementation of the obtained films in eventual technological applications. KEYWORDS: Mn3O4, thin films, chemical vapor deposition, magnetic properties, coercivity

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INTRODUCTION Nanostructured spinel oxides have been widely investigated over the last two decades for both fundamental studies and technological applications (such as energy, catalysis, biomedicine, sensing, and information storage) because of the large variety and tuneability of their chemico-physical properties.1−4 Among the family members, α-Mn3O4 (haussmannite), one of the most stable manganese oxides endowed with large natural abundance, cost-effectiveness, and environmental compatibility,5−7 is an attractive multifunctional platform for various end-uses, including, among others, super/pseudocapacitors,8−14 electrodes for batteries,7,15,16 gas sensors,17−21 and heterogeneous catalysts.5,10,22 Beside these applications, haussmannite has attracted attention because of its peculiar magnetic properties.23 In fact, bulk Mn3O4 is magnetically ordered at 42 K to form a collinear ferrimagnetic array,24,25 © XXXX American Chemical Society

whereas at lower temperatures a canted spin array is produced.26−28 Because the magnetic properties of nanomaterials are directly dependent on the aggregate size and shape,2,3,29,30 as well as on their structural features and the presence of defects,31−33 nanostructured Mn3O4 systems in the form of nanoparticles 23,24,27,28,33−35 and nanorods/ wires25,26,32,36,37 fabricated by means of different synthesis techniques have been widely studied in recent years. In comparison to these cases, the magnetic behavior of Mn3O4 thin films has been only scarcely investigated,23,24,38 and further efforts in this regard are undoubtedly of significant importance, taking also into account that the use of supported Received: January 23, 2019 Accepted: March 5, 2019 Published: March 5, 2019 A

DOI: 10.1021/acsanm.9b00141 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Scheme 1. Representation of the CVD Synthetic Strategy Adopted in the Present Work for the Fabrication of Nanostructured Mn3O4 Thin Films

thin films offers significant advantages for the direct integration into technological devices. Indeed, advances in the fabrication of the target systems are a strategic subject of ongoing investigation in order to finely control the resulting material features. In the present work, chemical vapor deposition (CVD), a versatile bottom-up route for the preparation of thin films and nanostructures with specific properties,6,39 was used to deposit high-quality α-Mn3O4 films. In general, CVD is a material fabrication process in which one or more molecular precursors, transported in the vapor phase, undergo chemical reactions on a heated substrate, yielding the formation of supported thin films/nanosystems.40 In the present case, the proposed synthetic strategy involves the direct adsorption of the manganese molecular precursor, Mn(hfa)2·TMEDA41 (Hhfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; TMEDA = N,N,N′,N′-tetramethylethylenediamine), on Si(100) substrates, followed by its reaction with oxygen to yield the target oxide materials. In particular, the latter were prepared under both dry O2 and O2+H2O reaction atmospheres and at different pressures (3.0 and 10.0 mbar; see Scheme 1) to examine the interplay between the processing conditions and the resulting systemproperties (Scheme 1). The system chemico-physical properties were investigated by means of various complementary techniques, namely, twodimensional X-ray diffraction (XRD2), conventional XRD, field emission-scanning electron microscopy (FE-SEM), highresolution-transmission electron microscopy (HR-TEM), and high-angle annular dark field-scanning TEM (HAADF-STEM), accompanied by electron energy loss spectroscopy (STEMEELS), as well as by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) analyses. Finally, magnetic properties were characterized as a function of the adopted processing conditions by analyzing field-cooled (FC) and zero-field-cooled (ZFC) magnetization measurements and field-dependent magnetization behavior, revealing very high coercivity values at low temperature, increasing with the content of microstructural defects in the prepared samples. To the best of our knowledge, there have been no literature reports on the magnetic properties of nanostructured Mn3O4 films obtained by CVD to date.

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ascertain the reproducibility of the obtained results and ensure a proper tailoring of the obtained system characteristics. To this aim, the precursor vaporization temperature and gas flow rates were suitably optimized, in order to enable an optimal precursor supply into the reactor chamber and a good homogeneity of the corresponding nanodeposits, avoiding at the same time detrimental gas-phase side reactions. Precursor powders, contained in an external glass vaporizer, were heated at 65 °C by means of an external oil bath and delivered by means of an O2 flow [purity = 6.0; rate = 100 standard cubic centimeters per minute (sccm)] into the reaction chamber, in which an auxiliary oxygen flow (100 sccm) was also separately introduced. On the basis of preliminary results obtained in our group regarding the preparation of Mn3O4 gas sensors,17,18 in the present work growth processes were performed under dry O2 and O2+H2O atmospheres. In the latter case, the auxiliary O2 flow was passed through a glass water reservoir maintained at a constant temperature of 35 °C before entering the reactor. Depositions were carried out for 1 h on precleaned p-type Si(100) substrates (MEMC, Merano, Italy). The growth temperature was set at 500 °C, whereas the total pressure was fixed either at 3.0 or at 10.0 mbar. Material Characterization. XRD patterns were collected adopting a conventional Bragg−Brentano geometry by means of a Bruker D8 Advance diffractometer equipped with a Göbel mirror, using a Cu Kα X-ray source (40 kV, 40 mA). XRD2 analyses were carried out in reflection mode on a DymaxRAPID X-ray microdiffractometer (Cu Kα radiation), equipped with a cylindrical imaging plate detector, which allows data collection from 0 to 160° (2θ) horizontally and from −45 to +45° (2θ) vertically. A collimator diameter of 300 μm and an exposure time of 40 min were used. Conventional XRD patterns were obtained by 2D image integration. FE-SEM analyses were performed by means of a Zeiss SUPRA 40 VP instrument, using primary beam voltages between 10 and 20 kV. The mean thickness and nanoaggregate dimensions were evaluated by the ImageJ software (https://imagej.nih.gov/ij/, accessed December 2018). HR-TEM and HAADF-STEM analyses were carried out with a FEI Tecnai F20 TEM microscope operated at 200 kV. Sample preparation was performed by means of diamond wire saw cutting, mechanical thinning with diamond lapping films, and ion milling in a Gatan precision ion polishing (PIPS) system. Oxidation state maps were obtained through STEM-EELS analysis by estimating the MnL3/ MnL2 edge intensity ratio after background subtraction and applying a Gaussian fitting on both edges.42 XPS measurements were performed on a Perkin Elmer Φ 5600ci spectrometer using a standard Al Kα radiation (1486.6 eV). No sign of sample degradation was observed upon X-ray irradiation. Binding energy (BE) values (uncertainty = ± 0.2 eV) were corrected for charging effects by assigning a BE of 284.8 eV to the adventitious C 1s peak.43,44 Atomic percentages (atom %) were calculated by peak integration, using Φ V5.4A sensitivity factors. SIMS analysis was carried out by a Cameca IMS 4f instrument, using a Cs+ primary ion beam (14.5 keV, 20 nA) and negative secondary ion detection, using an electron gun for charge

EXPERIMENTAL SECTION

Material Preparation. A cold-wall CVD reactor was used for the synthesis of manganese oxide films, using Mn(hfa)2·TMEDA as the precursor compound. The synthesis procedure and a joint theoretical−experimental characterization of the target complex in view of its eventual CVD applications have already been described.41 Preliminary CVD optimization experiments were carried out to B


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ACS Applied Nano Materials compensation. Rastering over a 150 × 150 μm2 area was performed, and secondary ions were collected from a 8 × 8 μm2 subregion to avoid crater effects. Beam blanking mode and high mass resolution configuration were adopted. The sputtering time in the profile abscissa was converted into depth using thickness values measured by cross-sectional FE-SEM analyses. Field and temperature-dependent magnetization measurements were performed by using a superconducting quantum interference device (SQUID) magnetometer (MPMS 10) with the external field applied either in the film plane or along the surface normal. In particular, attention was devoted to FC and ZFC magnetization measurements, as well as to the analyses of field-dependent magnetization loops.

importance, taking into account that previous works on manganese oxide nanowires obtained by solution routes26 and solvothermal techniques32 and on Mn3O4 powders fabricated by microwave-assisted process,27 liquid phase synthesis,35 autocombustion,34 and plasma-assisted CVD46 have reported difficult control of phase composition, often resulting in the copresence of other crystalline manganese oxide phases along with Mn3O4. A closer inspection of the 2D diffraction images (compare Figure 1a) evidenced the presence of discontinuous Debye rings (i.e., variations in the diffracted intensity along each ring), suggesting the occurrence of preferential growth orientations.47 This effect was particularly marked for the sample obtained in O2+H2O atmospheres at a pressure of 3.0 mbar, which turned out to be preferentially oriented along the (101) direction, as also revealed by conventional XRD analyses (Supporting Information section S-1 and Figure S1). In fact, the recorded patterns highlighted the occurrence of different relative peak intensities with respect to those reported for haussmannite.45 Whereas the experimental intensity ratios were found to be scarcely affected by the total operating pressure, they were directly influenced by the used reaction atmosphere (O2 or O2+H2O). In particular, specimens fabricated under dry O2 presented two main reflections pertaining to (211) and (220) α-Mn3O4 crystallographic planes, the latter exhibiting a higher relative intensity if compared with the reference pattern. In a different way, specimens fabricated in O2+H2O reaction environment showed an intensity decrease of the (211) signal and a concomitant increase of (101) and (103) signals with respect to the powder spectrum.45 These results, along with the calculated texture coefficient (TC) values (Supporting Information section S-1 and Table 1), suggested an enhanced

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RESULTS AND DISCUSSION In this work, attention was preliminarily devoted to the structural, morphological, and compositional characterization of manganese oxide specimens prepared in O2 and O2+H2O reaction atmospheres, at different total pressures. The system structure, with particular regard to the presence of possible orientation effects, was investigated by the joint use of XRD and XRD2 experiments. The results displayed in Figure 1a,b

Table 1. Texture Coefficients (TC) and Dislocation Density (δ) Values, along with Phase Transition Temperatures (T; uncertainty = ± 2 K) and In-Plane Coercivity (Hc,//) Values for the Mn3O4 Specimens Analyzed in the Present Work sample 3 mbar, O2+H2O 10 mbar, O2+H2O 3 mbar, O2 10 mbar, O2

TC 2.3 1.5 1.5 1.5

(101) (101) (220) (220)

δ (lines/ m2) 5.6 5.4 7.2 7.4

× × × ×

1014 1014 1014 1014

T (K)

Hc,// (T; 5 K)

± ± ± ±

0.88 0.89 0.91 0.97

42 42 41 42

2 2 2 2

(220) or (101) orientation and/or a concomitant anisotropic growth for samples obtained in O 2 and in O2+H2O atmospheres, respectively. Such an effect was particularly relevant for the specimen fabricated at a pressure of 3.0 mbar in O2+H2O reaction atmospheres (TC101 = 2.3). The geometric mismatches between film and substrate can cause stress phenomena and affect the material structural properties.48 Dislocation density (δ) resulting from these mismatches, calculated by XRD data (Supporting Information section S-1and Table 1), yielded values slightly inferior to those obtained for Mn3O4 films prepared by the successive ionic layer adsorption and reaction (SILAR) method14,49 and comparable50,51 or slightly higher20 than those of Mn3O4 films prepared by spray pyrolysis. As can be observed in Table 1, the dislocation density was greater for samples obtained under dry O2 and, in particular, for the one fabricated at 10.0 mbar (δ = 7.4 × 1014 lines/m2), indicating a parallel increase in the content of microstructural

Figure 1. (a) XRD2 map for a manganese oxide thin film deposited at 3.0 mbar in O2+H2O. (b) Integrated XRD patterns for specimens fabricated under different conditions. Vertical black lines mark peak positions for tetragonal α-Mn3O4 (haussmannite). (c) Representation of the α-Mn3O4 crystalline structure.45 Plane-view FE-SEM micrographs of samples obtained under (d) O2+H2O, 3.0 mbar and (e) O2, 10.0 mbar. The corresponding cross-sectional images are reported as insets.

clearly indicated the selective formation of tetragonal α-Mn3O4 (Figure 1c; space group I41/amd; a = 5.762 Å, c = 9.470 Å) with a spinel-type structure,25,31,45 because all the observed signals could be unambiguously assigned to this polymorph. No other reflections from other manganese oxides could be detected, evidencing thus the selective formation of phasepure α-Mn3O4. This result is not straightforward and is of key C


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Figure 2. TEM analyses on a Mn3O4 thin film deposited at 10.0 mbar under dry O2. (a) HR-TEM micrograph and (b) corresponding power spectrum. (c) Low-magnification cross-sectional HAADF-STEM image. (d) O and Mn EELS signals recorded on the region marked by a red rectangle in panel c. (e) Oxidation state map obtained by the MnL3/MnL2 edge intensity ratio (IL3/IL2).

Figure 3. Surface XPS spectra: (a) wide-scans (surveys) for manganese oxide specimens. High-resolution Mn 2p (b), Mn 3s (c), and O 1s (d) photoelectron peaks for a Mn3O4 film deposited at 3.0 mbar under O2+H2O atmospheres.

S2) was dominated by the occurrence of extensively interconnected nanoaggregates, typical of a three-dimensional growth mode. In line with previous results reported for Mn3O4 thin films obtained by pulsed laser deposition (PLD),8 the recorded images revealed a direct influence of the adopted working pressure on both film thickness and the resulting

defects. As discussed below, such a phenomenon has a direct influence on the resulting material magnetic properties. A preliminary investigation of material morphology was carried out by means of FE-SEM, which evidenced an optimal film/substrate adhesion, whatever the adopted preparative conditions. The system nano-organization (Figures 1d,e and D


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Figure 4. (a) FC (−●−) and ZFC (−○−) curves of a Mn3O4 sample obtained in O2+H2O at 3.0 mbar; inset: absolute value of the derivative |dM/ dT| of the FC curve. (b) FC (filled symbols) and ZFC (open symbols) curves of the same specimen measured under different applied fields.

manganese and oxygen peaks, along with carbon signals due to adventitious surface contamination arising from air exposure. The Mn 2p3/2 energy position [binding energy (BE) = 641.9 eV] and the corresponding spin−orbit splitting value (11.5 eV; Figure 3b) indicated the selective formation of pure Mn3O4,5,9,15,16,25,31,44,49 as also confirmed by the Mn 3s multiplet splitting separation. In fact, the evaluation of the latter parameter (Δ = 5.4 eV, Figure 3c) yielded a fingerprint for the sole Mn3O4 presence6,11,17,18,39 and enabled us to discard the occurrence of other Mn oxides, in agreement with the above-discussed XRD and TEM results. For all the obtained specimens, the O 1s photopeak (Figure 3d) resulted from the main contribution of lattice oxygen bonds (BE = 529.9 eV), accompanied by the copresence of surface adsorbed oxygen/hydroxyl groups/carbonate species resulting from atmospheric exposure (BE = 531.7 eV).11,12,31,32,39,58 As a consequence, the O/Mn atomic percentage ratio (∼1.7) was slightly higher than that expected in the case of stoichiometric Mn3O4. The selective formation of the latter was further confirmed by the energy difference between the Mn 2p3/2 maximum and the lowest O 1s BE component, yielding a value of 111.9 eV, which is consistent with previous data.39,54 Complementary information on the film in-depth composition was gained by SIMS analysis. Irrespective of the used reaction atmosphere and total pressure, carbon content was lower than 100 ppm, indicating an appreciable system purity and ruling out the presence of an appreciable material contamination due to undecomposed precursor residuals. As can be observed in Figure S4, manganese and oxygen signals had a parallel trend throughout the investigated depth, confirming their common chemical origin and highlighting a uniform composition of the target films. At a depth of ∼350 nm, the net decrease undergone by manganese and oxygen ionic yields and the concomitant increase of the silicon one evidenced a relatively sharp film/substrate interface, in line with the indications provided by cross-sectional FE-SEM imaging (see above). In order to investigate the interplay between the sample preparative conditions and the corresponding magnetic properties, field- and temperature-dependent magnetization measurements were performed. Figures 4a and S5 show the temperature dependence of FC and ZFC magnetization measured under a magnetic field of 20 mT applied in the film plane. In the ZFC process, the film was cooled from 70 to 5 K in the absence of the magnetic field, and the magnetization

morphological characteristics. In particular, regardless of the adopted reaction atmosphere, specimens fabricated at a pressure of 10.0 mbar were characterized by the presence of pseudo-columnar nanostructures (mean diameter = 100 nm; average deposit thickness = 350 nm), whereas for specimens obtained at 3.0 mbar, imaging revealed the occurrence of more faceted aggregates (mean dimensions = 100 nm; average deposit thickness = 630 nm). Such features were relatively similar to those previously reported for Mn3O4 films obtained by PLD8 and chemical bath deposition.52 Conversely, spraydeposited Mn3O4 films were composed by densely packed flakes13 or characterized by a poor morphology control.19 Mn3O4 films obtained by SILAR, spray pyrolysis, soft templating with block copolymers, microwave calcination, or atomic layer deposition were characterized by the presence of interconnected nanosheets, 49 by a granular morphology,14,20,21,48,50,51,53−55 or by interconnected spherical beads.56,57 An additional important insight into Mn3O4 nanostructure was gained by TEM analyses on representative samples (Figures 2 and S3). In both cases, Mn3O4 formed a polycrystalline layer on the top of the Si(100) substrate with crystalline domains that, in agreement with XRD results, comprised the sole α-Mn3O4 tetragonal polymorph and no other manganese oxide. The absence of epitaxial relations between the Si(100) substrate and Mn3O4 was due to Si oxidation at the film/substrate interface, resulting in the formation of a thin SiO2 layer (∼2 nm). The obtained HRTEM results were in good agreement with those obtained by XRD. In fact, by means of HR-TEM we could find that most of the observed grains, despite their polycrystallinity, mainly showed {220} and {211} orientations along the growth direction, explaining why XRD patterns presented two main reflections pertaining to (211) and (220) planes, with the latter exhibiting a higher relative intensity if compared with the reference XRD pattern.45 Further analyses carried out by STEM-EELS enabled us to perform a chemical mapping of manganese valence state by examining the intensity ratios between MnL3 and L2 edges.42 The obtained results (Figure 2d,e) highlight the uniform coexistence of Mn(II) and Mn(III) oxidation states, in good agreement with the above data, as also confirmed by compositional results (see below). The system surface and in-depth chemical composition were investigated by combining XPS and SIMS analyses. Surface wide-scan XPS spectra (Figure 3a) evidenced the presence of E


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Mn3O4 films, the magneto-crystalline anisotropy, which forces the magnetization to lie along specific crystallographic directions (called easy-axis/plane), is expected to be the dominant contribution to the effective magnetic anisotropy. Its value is indeed almost 1 order of magnitude higher than the shape anisotropy (which would favor the magnetization alignment along the film plane), whose upper limit for a uniformly magnetized and infinite thin film corresponds to 1/2 μ0Ms2 ≈ 0.2 × 105 J/m3, where μ0 and Ms (0.185 MA/m)27 are the vacuum permeability and the saturation magnetization in bulk systems, respectively. However, the (001) plane and the ⟨001⟩ crystallographic direction, which represent the magnetocrystalline easy-plane and hard-axis, respectively, are randomly distributed, thus giving rise to a similar magnetic response along different directions. The slightly preferential in-plane anisotropy could be due to the shape anisotropy contribution of the films. The random distribution of the easy-axes can also explain the linear contribution observed in the high-field region. Remarkably, all the analyzed samples yielded extraordinarily large in-plane coercivity values (Table 1), significantly higher than those reported for bulk α-Mn3O4 at 5 K (∼0.3 T).59,60 Overall, the coercivity was higher for samples obtained under dry O2 and increased for the systems obtained at higher pressures. To the best of our knowledge, the measured coercivity values are the highest among those previously reported for Mn3O4 films,23,24 and similar (or slightly lower/ higher) values have been obtained only in nanoparticles27,28,33,34,61,62 and nanowires/rods24−26,32,36,37 where surface/interface and shape anisotropy, respectively, were indicated as the main factors responsible for the observed effects. In the present case, the high coercivity values could be traced back to the presence of microstructural defects (see the above-discussed XRD results) acting as pinning sites for domain wall motion.63−65 This observation accounts for the highest coercivity being observed for the Mn3O4 specimen synthesized under dry O2 at 10.0 mbar, which presented the larger amount of microstructural defects, as indicated by structural analyses (see above).

was measured during the heating process under a magnetic field of 20 mT. In the FC process, a field of 20 mT was applied during cooling, and the magnetization measurement was performed upon heating under the same magnetic field. The sharp magnetization increase observed in the FC curve corresponds to the onset of the paramagnetic−ferrimagnetic phase transition. Fitting the derivative of the FC magnetization curve (inset of Figure 4a) allowed us to estimate the phase transition temperature by the position of the observed maximum. As a result, an average value of 42 K was measured for all the samples (Table 1), in excellent agreement with the transition temperature reported for the α-Mn3O4 bulk phase.25,27,35,36,59,60 The cusp appearing in the ZFC magnetization curve at the transition temperature could be related to the large magneto-crystalline anisotropy value of the α-Mn3O4 phase (Kmc ≈ 1.5 × 105 J/m3),59 which, in turn, leads to a high magnetic coercivity undergoing an increase at low temperatures. Under these conditions (starting from 5 K), the 20 mT applied field is not large enough to overcome the high coercivity, resulting thus in a small ZFC magnetization. Upon increasing the temperature, the magnetization rises because of the gradual reduction of the coercive field and then abruptly drops because of the loss of the long-range ferrimagnetic order when the transition temperature is approached. In order to corroborate such conclusions, FC and ZFC curves were collected under larger external fields (Figure 4b). Increasing the applied field up to 0.1 T does not affect the overall shape of the ZFC magnetization curve, and only a magnetization enhancement around the transition temperature is observed. When the applied field is increased up to 3 T, i.e. a value large enough to reach saturation, the cusp disappears, confirming thus the high magnetic anisotropy of the samples and the above-reported observations. Figures 5 and S6 display the low-temperature (5 K) fielddependent magnetization loops of Mn3O4 films measured with

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CONCLUSIONS In summary, high-quality nanostructured Mn3O4 films were obtained by a CVD route on Si(100) substrates. Controlled variations of the growth atmosphere (O2 vs O2+H2O) enabled us to tailor the preferential crystallographic orientations, whereas the system morphology, dominated by an even interconnection of low-sized aggregates, was mainly influenced by the total pressure. Irrespective of the specific operating conditions, the target materials maintained the same composition, corresponding to pure tetragonal α-Mn3O4 (haussmannite) free from other manganese oxide phases. For the first time, the magnetic properties of nanostructured Mn3O4 thin films obtained by CVD were characterized in detail. All the target systems showed bulk-like magnetic properties together with an extraordinarily high in-plane coercivity at low temperature, especially for specimens prepared under dry O2 and at higher total pressures. This result was traced back to the occurrence of microstructural defects involved in a domain wall pinning-controlled mechanism, whose content was mainly influenced by the used reaction atmosphere (O2 vs. O2+H2O). The successful fabrication of the target Mn3O4 materials demonstrated in this work is a key requirement for their

Figure 5. Low-temperature (5 K) in-plane (−●−) and out-of-plane (−○−) field-dependent magnetization loops of Mn3O4 samples obtained in (a) O2+H2O at 3.0 mbar and (b) dry O2 at 10.0 mbar. The vertical axis is normalized to the magnetization value at H = 5 T (M5T).

the field applied either in the film plane or along the surface normal. The similarity of in-plane and out-of-plane loops suggests the lack of a well-defined magnetic anisotropy symmetry. Only a slightly preferential in-plane anisotropy is observed, as indicated by the weakly larger values of reduced remanent magnetization (i.e., Mr/M5T, where M5T is the magnetization value at H = 5 T) and coercivity (the latter effect being observed only for samples fabricated in dry O2) when measurements are performed within the film plane. In αF


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ultimate mastery toward eventual end-uses. In fact, the present studies can trigger additional advancements toward the exploitation of a-Mn3O4 film properties in various technological applications. On the basis of the obtained results, additional attractive perspectives for the future application of the present study concern a deeper insight into the relations between the system nanostructure/morphology and the resulting magnetic properties, which might stimulate further fundamental and applied developments.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00141. Details and further data on XRD, FE-SEM, TEM, EELS, and magnetic measurements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +39-0498275234. E-mail: [email protected] (C.M.). *Phone: +39-0690672651. E-mail: [email protected] (G.V.). ORCID

Lorenzo Bigiani: 0000-0001-7404-2340 Mariam Hassan: 0000-0002-8592-5672 Davide Peddis: 0000-0003-0810-8860 Chiara Maccato: 0000-0001-6368-5754 Gaspare Varvaro: 0000-0001-7313-7268 Cinzia Sada: 0000-0002-7607-0481 Elza Bontempi: 0000-0003-1656-7506 Sara Martí-Sánchez: 0000-0003-4283-1489 Jordi Arbiol: 0000-0002-0695-1726 Davide Barreca: 0000-0002-8779-3386 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Padova University DOR 2016−2018 and PDiSC #03BIRD2016-UNIPD projects. Funding from Generalitat de Catalunya 2017 SGR 327, the Spanish MINECO ENE2017-85087-C3-3-R, and Severo Ochoa Program SEV2017-0706, as well as CERCA Programme/Generalitat de Catalunya is also acknowledged. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science Ph.D. program.

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REFERENCES

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