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Tetragonal Like Phase in Core-Shell Iron Iron-Oxide Nanoclusters Maninder Kaur, John Stuart McCloy, Ravi K. Kukkadapu, Carolyn I. Pearce, Ji#í Tu#ek, Mark Bowden, Mark H. Engelhard, Elke Arenholz, and You Qiang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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The Journal of Physical Chemistry

Tetragonal Like Phase in Core-Shell Iron Iron-Oxide Nanoclusters Maninder Kaur1*, John S. McCloy2, Ravi Kukkadapu3, Carolyn Pearce4, Jiri Tucek5, Mark Bowden3, Mark Engelhard3, Elke Arenholz6, You Qiang1* 1) Department of Physics, University of Idaho, Moscow, ID 83844 USA 2) School of Mechanical & Materials Engineering, Washington State University, Pullman, WA 98163 USA 3) Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352 USA 4) Pacific Northwest National Laboratory, Richland, WA 99352 USA 5) Regional Centre of Advanced Technologies and Materials, Department of Experimental Physics, Palacký University, 17. listopadu 1192/12, Olomouc CZ-77146, Czech Republic 6) Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, 94720, USA

ABSTRACT Two sizes of iron/iron-oxide (Fe/Fe-oxide) nanoclusters (NCs) of 10 nm and 35 nm diameters were prepared using a cluster deposition technique. Both these NCs displayed X-ray diffraction (XRD) peaks due to body-centered cubic (BCC) Fe0 and magnetite-like phase.

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Fe

Mössbauer spectroscopy measurements: a) confirmed the core-shell nature of the NCs, b) the Feoxide shell to be nanocrystalline and partially oxidized beyond magnetite, and c) the Fe-oxide spins are significantly canted. In addition to the BCC Fe and magnetite-like phases, a phase similar to tetragonal σ-Fe-Cr (8% Cr) was clearly evident in the larger NC, based on XRD. Origin of the tetragonal-like phase in the larger NC could be due to significant distortion of the Fe0 core lattice planes; subtle peaks due to this phase were also apparent in the smaller NC. Unambiguous evidence for the presence of such a phase, however, was not clear from X-ray photoelectron spectroscopy, vibrating sample magnetometry, X-ray magnetic circular dichroism, nor transmission electron microscopy.

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Fe Mössbauer spectroscopy, however, provided

supporting for a non-BCC Fe0 phase in the strongest 6th and the 1st resonance lines of the Fe0 sextet. To our knowledge, this is the first report of tetragonal-like phase in the Fe/Fe-oxide coreshell systems.

*Authors to whom correspondence should be addressed. Electronic mail: [email protected], [email protected]

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Introduction: Due to their reduced dimensions,1,2 nanostructured materials display peculiar magnetic and

transport characteristics that are not present in the bulk materials, as a consequence of the interplay between intrinsic properties arising from finite size effects and collective effects due to different kinds of nanoparticle interactions. One type of magnetic nanomaterials, oxide passivated magnetic nanoclusters (NCs), has found interest in wide areas of science ranging from magnetic recording and quantum computing to magnetic separation in nuclear waste treatment and medical imaging.3–5 Oxide passivated Fe NCs constitute a typical example of a core-shell nanostructure of which the magnetic properties have been widely studied.6–11 The core-shell morphology gives rise to a variety of novel magnetic phenomena such as shape/size/surface anisotropy, frustrated spin structures, spin-glass-like state, reduction of the effective anisotropy etc. These are all dependent on the relative fraction of constituent phases present in NC, the magnetic coupling among the clusters across the interfaces, and the exchange interactions between the shell and the core. An oxidized surface around the metal core is the medium though which exchange interactions and conduction electrons must pass to reach the ferromagnetic (FM) cores. This thin oxide surface can easily modify the magnetic behavior of the cluster depending on the chemical composition and lattice structure of the oxide-shell. Previous studies have shown that the inclusion of impurities or dopants12,13 in the core-shell NCs and reduction of oxide shell with the increase of metal core under ion irradiation of the system exert considerable physical, structural, and chemical changes in the oxide-shell.14,15 The oxide formed on the surface of zero-valent Fe (Fe0) is of inverse spinel type, either magnetite (Fe3O4), maghemite (γ-Fe2O3), or a nonstoichiometric mixture. The exact microstructure and composition, the crystallinity, the thickness, and the stability have been observed to be governed by the oxide-shell formation conditions – such as temperature, oxygen partial pressure, and oxidation time. In other words, the synthesis method and the corresponding controlling parameters highly influence the overall oxide shell and core formation in the NC.7,16– 18

The existing literature tells that the general microstructural features (lattice phase and composition) of the Fe-oxide layer formed on the top of Fe-core depends on the distance of the layer from the innermost Fe/oxide interface such that a progression from Fe0 (zero valent Fe):


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FeO (wüstite): Fe3O4 (magnetite): Fe2O3 (maghemite) occurs.19,20 Recently, based on X-ray absorption spectroscopy (XAS) analysis, it has been suggested that the oxide shell adjacent to the Fe core is dominated by Fe3O4, while the outer portion of the shell is mainly γ-Fe2O3.15,21 The presence and detectability of various oxidation states in the oxide shell depends on layer thickness. A thicker layer (> 3 nm) is more likely to show a FeO and Fe3O4 signature compared to a thinner oxide shell (< 3 nm), which is often best represented as γ-Fe2O3. The exchange interaction at the interface produces an exchange bias and a horizontal shift in the hysteresis loop.18 The exchange bias can be tuned by the structural modification and competition of different magnetic orderings at the interface—i.e., ferrimagnetic (FI) (Fe3O4/γ-Fe2O3) or antiferromagnetic (AFM) (FeO). The presence of a specific lattice structure in the oxide shell plays a large role in magnetization dynamics such as relaxation of magnetic spins of core-shell systems, and the longer range magnetic spin interactions among the clusters across their interfaces. However, for the very thin oxide-shell formed on NCs, it is difficult to distinguish the spatial differences of the lattice structure of the oxide-shell as one transport from the external surface to the innermost Fe/oxide interface towards the core. Since the lattice constants of the spinel Fe3O4 and γ-Fe2O3 structures are quite similar, they are not easily resolved by X-ray diffraction (XRD) or electron diffraction. Mössbauer spectroscopy, on the other hand, distinguishes these two oxide phases from each other, provided the crystals are “large” in size (>50 nm).

Large-

cluster/crystalline Fe3O4/γ-Fe2O3 display sextets at room temperature, unlike their counterparts that exhibits doublet/collapsed sextets at room temperature due to superparamagnetism. Kuhn et al.7 observed large-cluster Fe3O4/γ-Fe2O3 signatures for thin layers of Fe-oxide shell. The objective of the conducted work is to study the unidentified tetragonal like structure in comparatively large core-shell Fe/Fe-oxide NCs which is less evident in smaller NCs prepared from the same Fe target through the same cluster deposition technique. A suite of characterization techniques, including XRD, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM),

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Fe

Mössbauer spectroscopy, and X-ray magnetic circular dichroism (XMCD) were used to investigate the physical, chemical, structural, and magnetic properties of two cluster sizes of core-shell Fe/Fe-oxide NCs, the larger of which shows strong evidence of an additional crystalline phase.


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Experimental section

2.1

Synthesis The core-shell Fe/Fe-oxide NCs were fabricated in a cluster deposition source, consisting of

magnetron sputtering combined with gas aggregation, described in detail in our previous papers.18,22 A Fe sputtering target with high purity (99.98%) was mounted in an aggregation chamber and a high power was applied in the presence of a combination of inert gases (Ar and He) which was delivered to the aggregation chamber. The sputtering process produced a supersaturated metal vapor consisting of energetic metal atoms. By several steps of differential pumping of the Ar/He gas mixture, the metal vapor was cooled and condensed which lead to nucleation of clusters. The system was designed such that the clusters formed a beam with low kinetic energy to prevent the rapid escape of atoms from the chamber. During their transit through the aggregation chamber, the small cluster seeds grew into larger clusters. A differential pressure across the aggregation and deposition chamber helped in the cluster transportation to the deposition chamber. The clusters were allowed to react with a small amount of oxygen in the deposition chamber. The oxygen atoms reacted with the metal surface and form a thin shell of oxide around Fe, which provided a core-shell morphology to individual NCs. The last step in the deposition chamber was the accumulation of the NCs onto a Si (100) substrate, which was kept at room temperature, resulting in a porous NC film. A cluster size range from 4 nm to 100 nm, with size distribution of typically ~1-2 nm, could be achieved by tuning the sputtering parameters such as aggregation distance, He to Ar gas ratio, power, and temperature of the aggregation chamber. For the current study, two different types of Fe/Fe-oxide NC samples were fabricated. Cluster sizes of 10 nm (sample #S_10) and 35 nm (sample #S_35) were prepared by varying the aggregation distance and keeping the other parameters constant at 200 W (power), 0.1 (He to Ar gas ratio), -10°C (temperature).

2.2

Structural and Magnetic Characterization

The NC samples were characterized for crystallographic phase information and average crystallite size by XRD. The diffraction study was performed on the as-prepared samples using a Rigaku D/MAX RAPID II micro-diffractometer (µ-XRD) with a curved imaging plate and a rotating Cr anode (Cr Kα = 2.2897 Å) operating at 35 kV and 25 mA.


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The oxidation states of the NC oxide shells were measured by XPS using a Physical Electronics Quantera Scanning X-ray Microprobe system with a focused monochromatic Al Kα X-ray (1486.7 eV) source and a spherical section analyzer. The high energy resolution photoemission spectra were collected using a pass energy of 69.0 eV. Binding energies (BEs) reported are referenced to the adventitious carbon C 1s peak at 284.6 eV BE. Magnetization measurements were performed using a VSM (Lakeshore PMC 3900). The nanocluster films were placed with the magnetic field in the plane of the samples for all measurements. Major hysteresis loops were measured using a swept magnetic field to an applied field of ±18 kOe in ~100 Oe increments with 0.25 s averaging time. Temperature-dependent measurements were taken at various increments from 15 K to 300 K after zero field cool (ZFC) conditions. Low temperature (5 K) measurements were taken after ZFC and after field cooling (FC) with an applied field of 18 kOe (HFC). Transmision zero-field

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Fe Mössbauer spectra were recorded employing a Mössbauer

spectrometer operating at a constant acceleration mode and equipped with 50 mCi

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Co(Rh)

source. For low-temperature (5 K) and in-field (50 kOe) measurement, the sample was placed inside the chamber of the cryomagnetic system (Oxford Instruments, U.K.). With the Mössbauer spectrometer attached to the system, the setup works in a parallel geometry when the external magnetic field is applied parallel to the propagation of γ-rays. For fitting the 23

spectra, the MossWinn software program was used.

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Fe Mössbauer

Prior to fitting, the acquired

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Fe

Mössbauer spectra were processed to enhance signal-to-noise ratio employing simultaneously methods built in the MossWinn software program and statistically-based algorithms developed by Prochazka et al.24 The isomer shift values are referenced to α-Fe at room temperature. XAS and XMCD measurements were performed on the larger NC sample using Beamline 4.0.2 at the Advanced Light Source (Berkeley, CA) with an eight-pole resistive magnet endstation. 25 Fe and Cr L2,3 XAS spectra of the nanocrystals were recorded at room temperature in total electron yield mode, which has an effective probing depth of 50 Å. XMCD spectra were obtained by measuring two XAS spectra with fixed circular polarization (90%) and with opposing magnetization directions by reversing the applied field of 6 kOe at each energy point. The XAS spectra were normalized to incident beam intensity and the XMCD spectrum was obtained as the difference between the two spectra. XMCD spectra were fit by means of a nonlinear least-squares analysis using the calculated spectra for each site, along with measured


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spectra for Fe metal and Fe3O4, to determine the contributions of the different iron species in the magnetization signal. In the described calculations,26,27 the 10 Dq crystal field parameters were taken as 1.2 and 0.6 eV for Fe Oh (octahedral) and Td (tetrahedral) sites, respectively. The results were convoluted by a Lorentzian of C = 0.3 (0.5) eV for the L3 (L2) edge to account for intrinsic core-hole lifetime broadening and by a Gaussian of r = 0.2 eV to account for instrumental broadening. 3

Results and Discussion

3.1

X-ray diffraction Figure 1 shows the XRD spectra of as prepared samples, S_10 and S_35. The XRD patterns

of both NC sizes show two similar phases: a metallic BCC-Fe and an oxide with inverse spinel structure that can be identified as magnetite (Fe3O4) and/or maghemite (γ-Fe2O3), which we denote as Fe/FexOy. The presence of hematite, α-Fe2O3, can be safely excluded as it has a different crystal structure. Observation of severe peak broadening indicates that the oxide-shell is composed of ultrathin crystallites with dimensions of 2-3 nm. The Fe-oxide phase is identified as Fe3O4 as measured lattice constants of 8.422 Å (S_35) and 8.419 Å (S_10) are closer to Fe3O4 (a = 8.391 Å) than to γ-Fe2O3 (a = 8.339 Å). It is likely this phase is structurally different from pure Fe3O4. Given the small crystallite size of this phase in samples, this is not unreasonable. The relatively poor fit for Fe3O4 could also arise from a number of origins. A mix of Fe3O4 and γ-Fe2O3 is one possibility, and is a special case of the presence of lattice vacancies. Other permutations of vacancies might also exist. The difference from perfect Fe3O4 could also be due to other elemental substitutions and/or shifts in atomic positions caused by strain arising from the coherence of this phase with other materials (γ-Fe2O3) in the sample. The analysis showed that the Fe components in both the samples are very similar and they originate from a BCC lattice with lattice constant of ~ 2.87 Å corresponding to bulk α-Fe. The average crystallite sizes for the Fe cores were obtained as d =18 nm and d = 6 nm from three peaks ((110), (200), and (211)) for S_35 and S_10, respectively. In addition to BCC-Fe and mixed oxide (FexOy) phases, the larger clusters (S_35) showed a sharp shoulder along the metallic Fe (110), which closely resembled a sigma-FeCr phase (tetragonal phase) with no Cr content. The absence of Cr or any other impurity was confirmed


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from elemental mapping, XPS, secondary ion mass spectrometry (SIMS), and XMCD. This phase is not well-developed or apparent in the smaller cluster sample, possibly due to less strain on the Fe-core and a relatively thicker oxide shell. The stability of the unknown phase in S_35 was verified by repeating the XRD measurement three times at intervals of six months each. The XRD patterns replicated each time with diffraction peaks falling exactly on the first XRD taken after the synthesis of the sample. No structural changes with time were observed. In general, our cluster deposited core-shell Fe/Feoxide NC were shown to be very stable over time after the formation of oxide shell (2-3 nm).19 In the present case, the XRD data reflected that no structural changes with time occur, irrespective of the presence of a tetragonal-like phase. The crystallite size of S_35 was measured using whole pattern (Rietveld) fitting. The instrumental peak shapes were obtained by fitting a standard material, and then the diffraction pattern was calculated using the crystal structures of a mixture of Fe, magnetite, and sigma-FeCr. The peak broadening (beyond the instrumental broadening) gives the crystallite size in a similar manner to the Scherrer equation, although this is applied consistently to all the peaks of a given phase rather than to a single peak. The method also has the advantage of de-convoluting overlapping peaks. The average crystallite sizes for the Fe cores were obtained d =18 nm and d = 6 nm from three peaks ((110), (200), and (211)) for S_35 and S_10, respectively, an identical result to that obtained previously by the simpler method. The lattice parameters were derived from least squares fitting of diffraction peaks, including the (hkl) peak near 2-theta where the relative precision is much greater. The unaltered lattice parameter of BCC-Fe (~ 2.868 Å) and Fe-oxide (~ 8.422 Å) of S_35 indicates that the unknown phase with lattice constants, a = 8.808 Å and c = 4.640 Å remain as an independent phase in the NC. The classification of crystallite size of BCC-Fe, Fe-oxide, and unknown Fe-phase is tabulated in Table 1. 3.2

X-ray photoelectron spectroscopy Since the XPS signal sensitively represents the surface composition of the NCs, possible

segregation of any impurities on the cluster surface could be readily identified using XPS. The spectral plots were collected at 90° and 45° emission angles. At 90° the measurement was less surface sensitive compared with the standard 45° configuration. The quantification results (relative atomic %) were similar for all the scans (both wide and high energy resolution selected


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photoemission narrow scans). Note that trace levels of Si, Cl, and possible Mn were detected in addition to the expected Fe, O, and C. The Si signal came from the substrate used for cluster deposition and Cl was considered to be a surface contamination. Figure 2 shows the representative XPS spectra of Fe 2p, O 1s, and Cr 2p (to test for the existence of Cr). XPS provided information about the chemical composition of the nanostructure and the density of states in the valence band. The Fe 2p spectra of both smaller and larger NCs are shown in Figure 2(b). The spectra were composed of two edges split by spin-orbit coupling as 2p1/2 and 2p3/2. The Fe 2p3/2 edge is composed of three peaks. The shoulder peak located at binding energy of 707 eV corresponded to zero valent Fe (Fe0) metal, and this peak was more prominent in S_35 compared to S_10. This suggested that the amount of Fe0 is higher in the larger clusters. The second peak located at binding energy of 711 eV indicated Fe3+. The Fe3+ satellite peak for tetrahedral structure was located at 719.1 eV, towards the higher binding energy side of main peak 2p3/2. By comparison with the reference spectrum, which is a fingerprint for determining oxidation states, the oxidation state of Fe in the sample was Fe3+. XPS analysis led to the notion that the environmentally exposed outer oxide layer is dominated by γ-Fe2O3 as reported earlier. The O 1s spectrum showed two significant peaks, as in Figure 2(c). The peak located at a binding energy of ~ 529.9 eV was associated with O2- whereas the other peak with a binding energy of ~ 531.5 eV corresponded to OH-. The O2- formed an oxide with Fe 2p3/2 and OHformed a hydroxide (Fe(OH)2). Since the O2- peak was dominant in both the samples, it could be seen that both the samples were mainly covered by oxide layer with some surface hydration on the very top of the cluster surface. However, the O2- spectra included minor contributions from many other possible sources, such as SiO2 and C-O bonds from surface adventitious hydrocarbon contamination. In order to support the absence of Cr, at least at the surface, the binding energy spectrum of Cr 2p was measured, as shown in Figure 2(d). The measured spectrum was similar for both samples, including an O 1s plasmon line at ~587 eV. Plasmons resulted from the interaction of photoelectrons with conduction electrons. No Cr or Cr-oxide lines were detected. 3.3

Microscopy Characterizations Energy filtered transmission electron microscopy (EFTEM) with elemental mappings were

performed on both types of samples for the verification of the elemental composition of NC. The


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map distributions of the elements gave a complete picture regarding the spatial correlation of the oxide shell and the metal core. Results showed that the core region of the NC is rich in Fe and the shell region is rich in O. In addition to mapping, Energy dispersive spectroscopy (EDX) on the NCs shows the corresponding energy peak of Fe and O. No foreign element was detected from mapping and EDX (Figure 3). 3.4

X-ray Magnetic Circular Dichroism Due to the similarities in the XRD pattern of the new phase with tetragonal sigma – FeCr, the

XA spectrum was collected at the Cr L2,3-edge to verify the existence of Cr in the sample. The spectrum did not indicate any Cr content, as shown by the lack of the Cr L3 peak at 576 eV in Figure 4(a) unlike in Cr doped (8 at.%) core-shell Fe/Fe-oxide NCs where the Cr L2,3 XA spectrum resembled Cr3+, and a small Cr L2,3 XMCD spectrum was measured that reversed its sign by switching the polarization from +0.9 to -0.9, indicating long range ferromagnetic order in the Cr. The room temperature Fe L2,3 XA spectra for S_35 sample showed superposition features characteristic for Fe metal, Fe2+ and Fe3+ (Figure 4b). The magnitude of the XMCD signal relative to the XAS indicated that the larger NC sample was strongly ferromagnetic. The fit of the XMCD spectrum (Figure 4c) showed that the major contributors to the magnetization signal were Fe0 metal and slightly non-stoichiometric, reduced magnetite, within the ~5 nm probe depth of XAS experiment. The magnetite component of the spectrum showed octahedral sites containing Fe3+ and Fe2+ and tetrahedral sites containing Fe3+ (Fe2+Oh 1.1 : Fe3+Td 0.9 : Fe3+Oh 1.0). The Fe2+/ Fe3+ ratio of 0.58 for the magnetite component is much higher than core-shell iron/iron-oxide NCs, 0.38 as reported in our previous paper.15 3.5

Magnetic Data from Vibrating Sample Magnetometer Magnetization versus field (MH or hysteresis loop) measurements performed on both the

samples from T=5 K to T=100 K indicated ferromagnetic behavior of the NCs. The larger NCs have higher saturation magnetization (MS) (108 emu/g @ 5 K, ZFC) compared to the smaller NCs (62.8 emu/g @ 5 K, ZFC). A minimal MS difference is observed between FC and ZFC condition for the same samples. The temperature dependent MS curves, shown in Figure 5, indicated that the thermal effect is very weak as the majority of the spins are aligned along the direction of applied field. According to the sizes of ferromagnetic Fe core, the MS of the S_35 was expected to be greater than the smaller cluster sample S_10. The magnetization drop for S_35 and S_10 was 12% and 5%, respectively, on warming from 5 K to 300 K. The loop at low


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temperatures exhibited a larger exchange bias shift towards negative magnetic fields for S_10 (212 Oe @ 5 K, FC) than for S_35 (113 Oe @ 5 K, FC), which was opposite to the cooling field direction. A close-up of the major loops versus temperature for the core-shell NCs are shown in Figure 6(a). It can be seen that coercive field (HC), Figure 6(b), and remanence, Figure 6(c), increased as temperature was decreased. Similarly, for the S_10 sample the major loops grew larger as the temperature decreased, as shown in Figure 7(a). Additionally, HC, Figure 7(b), and remnant magnetization, Figure 7(c), decreased with increasing temperature. Compared to the S_35 NC film, HC of the S_10 is larger at all temperatures, but the MR is much smaller due to the smaller iron core and thus smaller overall magnetic moment. Errors on determination of coercivity, remanent magnetization, and saturation magnetization were estimated at 0.5%, 1%, and 1%, respectively of the base value.

This was determined from repeated room temperature

measurements and parameter extraction. Additionally, for both samples, the 5 K remanence increased with field cooling, but also with the second loop iteration after ZFC, as shown in Figure 8. In other words, at low temperatures these NC films underwent training28,29 with successive hysteresis loops, due to a competition between exchange and magnetostatic interactions between NCs, resulting in a successively larger MR with loop iteration.18 In both cases, the field cooling resulted in the largest MR value. The measured data showed that the sample S_10 sample exhibited higher exchange bias and coercive field than the larger cluster S_35 sample. This could be a consequence of the ratio of the ferrimagnetic shell thickness to the Fe core being larger in the case of small clusters. 3.6

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Fe Mössbauer Spectroscopy

Zero-field Mössbauer spectra (Figure 9) obtained at room temperature displayed peaks due Fe0 (well-defined sextet) and nanoparticulate Fe-oxide phase(s) (doublet and broad/collapsed sextet feature). The relative spectral areas of these phases were in general agreement with XRD; the Fe0 contribution to the spectrum was higher in the larger NC. To gain further insights into the nature of these phases, in-field (50 kOe)

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Fe Mössbauer spectra were recorded at 5 K (Figure

10). Three sextet components were identified for the in-field

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Fe Mössbauer spectra, for both

samples; i.e., the two sextets belonging to iron oxide phase and one sextet reflecting the presence of Fe0. This supported the conclusions that the studied systems are composed of NCs with coreshell architecture; i.e., the core of Fe0 origin and the shell of iron oxide nature. The sextet with


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the lowest value of the effective hyperfine magnetic field (Beff) corresponded to the Fe0 as given by Table 2; the non-zero value of the quadrupole splitting (∆EQ) implied a rise of spherically non-symmetric surroundings due to both the small size of the iron core and the interface between the core and the NC shell. The two remaining sextets can be assigned to an iron oxide phase having a spinel structure. The sextet with the higher value of the isomer shift (δ) and lower value of the effective hyperfine magnetic field corresponded to the octahedral cation sites in the spinel cubic crystal structure whereas the other sextet with the smaller δ and higher Beff belonged to the tetrahedral cation sites in the spinel cubic crystal structure. Furthermore, δ values for both iron oxide spectral components were within the range expected for Fe3+ ions in a high-spin state (S = 5/2), despite evidence of some Fe2+ from XMCD. This could be likely due to persistence of electron hopping even at 50 kOe in these nanoparticulate phases. In an external magnetic field (Bext) of 5 kOe applied parallel to the propagation of γ-rays, for ferromagnetic and ferrimagnetic materials, one would expect zero intensities of the 2nd and the 5th sextet resonant lines as a result of alignment of atomic (ion) magnetic moments to the direction along or opposite to the external magnetic field. This was, however, not valid for the two γ-Fe2O3 sextets for which non-zero intensities of the 2nd and the 5th sextet lines were observed in the in-field

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Fe Mössbauer spectra recorded in the parallel (i.e., Bext || γ-rays)

measuring geometry (i.e., 1.49/1.85 as amplitude of the 2nd and 5th line for tetrahedral/octahedral site of S_10; while 1.30/1.47 for S_35). This indicated a significant degree of misalignment of surface Fe3+ magnetic moments,7 which can be explained by a rise of the surface anisotropy and formation of other easy axes of magnetization favoring orientation of magnetic moments in different directions with respect to the external magnetic field. Besides this, vacancies in the shell crystal structure could have induced local magnetic anisotropy which obstructed the alignment of the respective magnetic moments. The value of Beff for Fe0 component was lowered compared to the value of the hyperfine magnetic field of Fe0 in a zero field (~290 kOe vs. ~330 kOe) (Table 2). As the hyperfine magnetic field at the Fe0 site is given solely by the negative Fermi contact term (other contributions being neglected), the observed Beff value implies alignment of Fe0 magnetic moments towards the direction of Bext. At 50 kOe, all the Fe0 magnetic moments perfectly lie in the parallel direction to Bext as evidenced by zero intensity of the 2nd and the 5th sextet lines. A slightly higher Beff of Fe0 spectral component compared to expected value (= 281 kOe) could be


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explained by internal magnetic fields originating from the phase interface or from the shell (i.e., transferred magnetic fields), establishing magnetic exchange coupling between the core and the shell, as also witnessed from the shift in the hysteresis loop (see above).

4.0 Tetragonal phase Due to the core-shell structure of the NCs, the magnetic behavior of the samples was influenced by magnetic behavior of both the core and the shell. The NC core behaved in a ferromagnetic manner (Fe0) and the shell showed ferrimagnetic ordering (Fe/FexOy). FeO and Fe3O4 were the first oxidation product forming an exterior shell, which further oxidized and lead to ferric oxide (γ-Fe2O3).21,30 Thus, in the cluster, there is an interface between magnetically distinct phases which could induce emergence of an exchange bias phenomenon, experimentally manifested by a shift in the hysteresis loop. (However, the shift in the hysteresis loop is also caused by strong intercluster magnetic interactions of exchange origin if the clusters were in a very close contact.22) A proposed hypothesis for the formation of a tetragonal like phase structure in the core-shell Fe/Fe-oxide NC involves the strain of BCC-Fe within the core, when the cluster size crossed a minimum limit. The BCC-Fe lattice planes (at the core) get distorted to tetragonal once the strain within the core increases due to the large size of the core which is constrained by the oxide shell. As explained previously, the thickness of the oxide shell is independent of the cluster size, which implies that only the core size increases with the cluster growth. Hence, the tetragonal-like phase is highly dependent on the cluster size and the cluster growth environment, as it may not be seen in clusters prepared by other techniques. The tetragonal phase is poorly developed in the smaller cluster size material (S_10 sample) due to the smaller and hence less strained Fe-core. The lattice distortion at the core of such a NC is larger than that at the surfaces, indicating that the cluster is composed of a highly strained core encapsulated in a less-strained Fe-oxide shell that helps stabilize the strained distorted core. Similarly for other materials, Sun et al.31 reported that the internal strains in twinned silver nanoparticles distorted the cubic silver lattice to the bodycentered tetragonal phase. It is evident that a larger NC can manifest relatively more of this strained phase compared to smaller clusters, due to decreasing surface to volume ratio. Some degree of misalignment might occur in smaller clusters as well, as XRD shows minor poorly-developed peak positions similar


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to a tetragonal species around the BCC-Fe peaks. Since smaller clusters contain a relatively thicker oxide shell, the internal strain in the core is much less than in larger clusters. In other words, the thicker oxide shell compensates the core strain up to a point as the core size increases, and at one specific limit the core strain is greater than can be accommodated by the oxide shell and results in third phase. The newly developed phase was inconsistent with any crystallographic phase other than a sigma-FeCr-like phase. Our samples, however, are devoid of any impurities/contaminates, confirmed by various methods. Until now there was no relevant literature in the crystallography database which experimentally observed a tetragonal phase of Fe. There are, however, number of theoretical studies which reported the tetragonal phase of Fe.32–38 Belonoshko et al.37 theoretically investigated the stability of a body-centered-tetragonal (BCT)-Fe phase at high pressure and low/high-temperature conditions. Using first principles total energy calculations, Marcus et al.34 showed that the FM and AFM phases have tetragonal equilibrium states with c/a>1. The c/a for S_35 was measured as 0.526. The lesser value of c/a indicates that the tetragonal phase was not completely evolved, as BCC-Fe was observed together with a tetragonal phase. The oxide shell thickness (2-3 nm) indicated that the new phase lay within the core of the cluster. The crystallite size of tetragonal phase was measured as 14.7 nm and BCC-Fe as 18.8 nm. This phase has not been discovered in Fe nanoclusters before. Its existence might help to understand how the distortion of lattice structure influence the superconductivity for Fe-based nanomaterials39, and applications in the nanomagnetism in FePt nanoparticles40 and Fe-doped HfO2 nanoparticles41.

Conclusions The presented study performed on two different sizes of core-shell Fe/Fe-oxide NCs prepared using a cluster deposition technique from the same Fe target source showed an unidentified welldeveloped magnetic phase in the larger cluster size material of ~35 nm diameter, S_35. The purity of the samples was verified by mapping the elemental distribution using transmission electron microscopy. The surface measurement using X-ray photoelectron spectroscopy did not detect any elements other than Fe and O. The magnetization study indicated no immediately differentiating hysteresis characteristics of S_35. The saturation magnetization scales with the


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size of the core-shell Fe/Fe-oxide NC, indicating the dominance of the Fe core moment.

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57

Fe

Mössbauer spectroscopy and magnetic circular dichroism (XMCD) measurements investigated the properties of the tetragonal phase in S_35. Since the tetragonal phase resembled that from a previous study of Cr doped Fe, Cr L2,3 X-ray absorption and XMCD was checked, which verified the absence of Cr in S_35 but its presence in Fe8%Cr. Unlike the aforementioned Fe8%Cr samples, where Cr L2,3 XMCD reversed its sign with respect to the Fe L2,3 XMCD, S_35 did not flip the corresponding moment. The tetragonal phase nanostructure likely originates from the distortion of BCC-Fe lattices due to internal strains in the core-shell nanoclusters.

Acknowledgements This study was supported by the U.S. Department of Energy (DOE) under Contracts DE-FC0708ID14926 and DE-FG02- 07ER46386. Parts of the work were conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a DOE User Facility operated by Battelle for the DOE Office of Biological and Environmental Research, under Proposal ID 39391. Pacific Northwest National Laboratory is operated for the DOE by Battelle under Contract DE-AC05- 76RL01830. J. T. acknowledges support from the Ministry of Education, Youth and Sports of the Czech Republic under Project No. LO1305 and assistance provided by the Research Infrastructure NanoEnviCz supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project No. LM2015073. The authors thank Dr. Weilin Jiang for SIMS at EMSL and Dr. Y.Q. Wu for TEM at Center for Advanced Energy Studies, Idaho Falls. The ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences (OBES) of the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231.


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(14) McCloy, J. S.; Jiang, W.; Droubay, T. C.; Varga, T.; Kovarik, L.; Sundararajan, J. A.; Kaur, M.; Qiang, Y.; Burks, E. C.; Liu, K. Ion Irradiation of Fe-Fe Oxide Core-Shell Nanocluster Films: Effect of Interface on Stability of Magnetic Properties. J. Appl. Phys. 2013, 114, 083903–083903.9. (15) Kaur, M.; Qiang, Y.; Jiang, W.; Pearce, C.; McCloy, J. S. Magnetization Measurements and XMCD Studies on Ion Irradiated Iron Oxide and Core-Shell Iron/Iron-Oxide Nanomaterials. IEEE Trans. Magn. 2014, 50, 4800305. (16) Sun, X.; Frey Huls, N.; Sigdel, A.; Sun, S. Tuning Exchange Bias in Core/Shell FeO/Fe3O4 Nanoparticles. Nano Lett. 2012, 12, 246–251. (17) Khurshid, H.; Lampen-Kelley, P.; Iglesias, Ò.; Alonso, J.; Phan, M.-H.; Sun, C.-J.; Saboungi, M.-L.; Srikanth, H. Spin-Glass-like Freezing of Inner and Outer Surface Layers in Hollow γ-Fe2O3 Nanoparticles. Sci. Rep. 2015, 5, 15054. (18) Kaur, M.; McCloy, J. S.; Qiang, Y. Exchange Bias in Core-Shell Iron-Iron Oxide Nanoclusters. J. Appl. Phys. 2013, 113, 17D715. (19) Wang, C. M.; Baer, D. R.; Thomas, L. E.; Amonette, J. E.; Antony, J.; Qiang, Y.; Duscher, G. Void Formation during Early Stages of Passivation: Initial Oxidation of Iron Nanoparticles at Room Temperature. J. Appl. Phys. 2005, 98, 094308–094308. (20) Wang, C. M.; Baer, D. R.; Amonette, J. E.; Engelhard, M. H.; Qiang, Y.; Antony, J. Morphology and Oxide Shell Structure of Iron Nanoparticles Grown by Sputter-GasAggregation. Nanotechnology 2007, 18, 255603. (21) Signorini, L. Size-Dependent Oxidation in Iron/Iron Oxide Core-Shell Nanoparticles. Phys. Rev. B 2003, 68, 195423. (22) Kaur, M.; McCloy, J. S.; Jiang, W.; Yao, Q.; Qiang, Y. Size Dependence of Inter- and Intracluster Interactions in Core–Shell Iron–Iron Oxide Nanoclusters. J. Phys. Chem. C 2012, 116, 12875–12885. (23) Klencsár, Z.; Kuzmann, E.; Vértes, A. User-Friendly Software for Mössbauer Spectrum Analysis. J. Radioanal. Nucl. Chem. 1996, 210, 105–118. (24) Prochazka, R.; Tucek, P.; Tucek, J.; Marek, J.; Mashlan, M.; Pechousek, J. Statistical Analysis and Digital Processing of the Mössbauer Spectra. Meas. Sci. Technol. 2010, 21, 025107. (25) Stöhr, J.; Padmore, H. A.; Anders, S.; Stammler, T.; Scheinfein, M. R. Principles of XRay Magnetic Dichroism Spectromicroscopy. Surf. Rev. Lett. 1998, 05, 1297–1308. (26) van der Laan, G.; Thole, B. T. Strong Magnetic X-Ray Dichroism in 2p Absorption Spectra of 3d Transition-Metal Ions. Phys. Rev. B 1991, 43, 13401–13411. (27) van der Laan, G.; Kirkman, I. W. The 2p Absorption Spectra of 3d Transition Metal Compounds in Tetrahedral and Octahedral Symmetry. J. Phys. Condens. Matter 1992, 4, 4189–4204. (28) Vasilakaki, M.; Trohidou, K. N. Numerical Study of the Exchange-Bias Effect in Nanoparticles with Ferromagnetic Core/Ferrimagnetic Disordered Shell Morphology. Phys. Rev. B 2009, 79, 144402. (29) Hoffmann, A. Symmetry Driven Irreversibilities at Ferromagnetic-Antiferromagnetic Interfaces. Phys. Rev. Lett. 2004, 93, 097203. (30) Gilbert, B.; Katz, J. E.; Denlinger, J. D.; Yin, Y.; Falcone, R.; Waychunas, G. A. Soft XRay Spectroscopy Study of the Electronic Structure of Oxidized and Partially Oxidized Magnetite Nanoparticles. J. Phys. Chem. C 2010, 114, 21994–22001.


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(31) Sun, Y.; Ren, Y.; Liu, Y.; Wen, J.; Okasinski, J. S.; Miller, D. J. Ambient-Stable Tetragonal Phase in Silver Nanostructures. Nat. Commun. 2012, 3, 971. (32) Marcus, P. M.; Ma, H.; Qiu, S. L. On the Importance of the Free Energy for Elasticity under Pressure. J. Phys. Condens. Matter 2002, 14, L525. (33) Ma, H.; Qiu, S. L.; Marcus, P. M. Pressure Instability of Bcc Iron. Phys. Rev. B 2002, 66, 024113. (34) Marcus, P. M.; Moruzzi, V. L.; Qiu, S.-L. Tetragonal Equilibrium States of Iron. Phys. Rev. B 1999, 60, 369-372. (35) Söderlind, P.; Moriarty, J. A.; Wills, J. M. First-Principles Theory of Iron up to Earth-Core Pressures: Structural, Vibrational, and Elastic Properties. Phys. Rev. B 1996, 53, 1406314072. (36) Vočadlo, L.; Brodholt, J.; Alfè, D.; Gillan, M. J.; Price, G. D. Ab Initio Free Energy Calculations on the Polymorphs of Iron at Core Conditions. Phys. Earth Planet. Inter. 2000, 117, 123–137. (37) Belonoshko, A. B.; Isaev, E. I.; Skorodumova, N. V.; Johansson, B. Stability of the BodyCentered-Tetragonal Phase of Fe at High Pressure: Ground-State Energies, Phonon Spectra, and Molecular Dynamics Simulations. Phys. Rev. B 2006, 74, 214102. (38) Cottrell, A. H.; Bilby, B. A. Dislocation Theory of Yielding and Strain Ageing of Iron. Proc. Phys. Soc. Sect. A 1949, 62, 49-62. (39) Uhoya, W. O.; Montgomery, J. M.; Samudrala, G. K.; Tsoi, G. M.; Vohra, Y. K.; Weir, S. T.; Sefat, A. S. High-Pressure Structural Phase Transitions in Chromium-Doped BaFe 2 As 2. J. Phys. Conf. Ser. 2012, 377, 012016. (40) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Monodisperse FePt Nanoparticles and Ferromagnetic FePt Nanocrystal Superlattices. Science 2000, 287, 1989–1992. (41) Stable Tetragonal Phase and Magnetic Properties of Fe-Doped HfO2 Nanoparticles. AIP Adv. 2017, 7, 056315.


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Table 1 Quantitative analysis of S_10 and S_35 samples using X-ray diffraction. The crystallite size of samples was measured using whole pattern (Rietveld) fitting.

Phase

Sample S_10

Parameters

Magnetite

BCC-Fe

Unknown phase fitted with σ-(Fe,Cr)

Sample S_35

Wt. fraction

76%

57%

a (Å)

8.419(2)

8.422(1)

crystallite size (nm)

2.7(2)

2.7(1)

Wt. fraction

24%

21%

a (Å)

2.868(1)

2.867(1)


6.9(2)

18.8(6)

Wt. fraction

–

22%

a (Å)

–

8.808(1)

c (Å)

–

4.640(1)


–

14.7(2)

Table 2 Values of the Mössbauer hyperfine parameters, derived from the fitting of the Mössbauer spectrum of the S_10 sample and S_35 sample, where T is the temperature of measurement, Bext is the induction of external magnetic field, δ is the isomer shift, ∆EQ is the quadrupole splitting, Beff is the effective hyperfine magnetic field (i.e., a vector sum of the hyperfine magnetic field and external magnetic field), and RA is the relative spectral area of individual spectral components. Sample S_10

S_35

T Bext (K) (kOe) 5 50

5

50

δ ± 0.01 (mm/s) 0.08 0.42

∆EQ ± 0.01 (mm/s) 0.08 – 0.06

Beff ± 3 (kOe) 291 502

RA ± 1 (%) 16 41

0.52

– 0.05

457

43

0.10 0.42

– 0.04 – 0.23

293 503

34 33

0.52

– 0.20

461

33

Assignment Fe0 Non-stoichiometric γFe2O3 – tetrahedral sites Non-stoichiometric γFe2O3 – octahedral sites Fe0 Non-stoichiometric γFe2O3 – tetrahedral sites Non-stoichiometric γ-


19


Fe2O3 – octahedral sites

Fe (110)

S1 - smaller size NCs S_10 S2 - larger bigger size size NCs NCs S_35 tetragonal-like phase

Fe3O4 (511)

200

Fe3O4 (311)

Fe3O4 (220)

300

Fe3O4 (440)

400

Intensity (counts/s)

1 2 3 4 5 6 7 8 9 10 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

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Fe (211)

Fe (200)

100

0 40

60

80

100

120

140

160

2-theta Figure 1: XRD patterns of S_10 and S_35 samples. Both samples share Fe and Fe3O4 crystal structures, while tetragonal-like phase has been observed only in larger size nanoclusters S_35 sample.


20

60

S_10S1 S_35 3

Intensity (counts/s*10 )

O 2s

Si 2p Fe 3s Fe 3p

(a) 1000

800 600 400 Binding Energy (eV)

200

30 FeIII satellite 0

3


-

OH (531.5 eV)

30 15

(b)

smaller nanoclusters larger nanoclusters

740

13.0

2-

Fe (707.0 eV)

20

0

O (529.9 eV)

60 45

40

10

75 O1s

3

Si 2s

C 1s

5

1200

(711.0 eV)

Fe 2p1/2

50

10

0

Fe2p3/2

Fe 2p

S2

O 1s

Fe LMM Fe LMM Fe LMM Fe 2p1 Fe 2p3

4

Intensity (counts/sec x10 )

15


1 2 3 4 5 6 7 8 9 10 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


O KLL

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O loss

730

720 710 Binding energy (eV)

700

O1s plasmon lines

12.8 12.6 12.4

No detectable Cr lines

12.2 0

(c)

540

smaller nanocluster larger nanocluster 535

smaller nanocluster larger nanocluster

(d)

530 Binding energy (eV)

525

12.0 600

590

580 570 Binding energy (eV)

560

Figure 2: XPS spectra of as prepared S_10 and S_35 samples. (a) Wide scan survey spectra; (b) Fe 2p binding energy spectra; (c) O1s binding energy spectra; (d) Cr 2p binding energy spectra (No Cr-oxide lines were detected. O 1s plasmon line at ~587 eV resulted from the interaction of photoelectrons with conduction electrons).


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Figure 3: Transmission electron microscopy image and EDX of sample S_10 (Top) and S_35 (bottom). Both samples showing Fe and O signal.


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Figure 4: XAS and XMCD spectra of S_35 sample: (a) Cr L2, 3 XA and XMCD spectra (+ 6 kOe). No Cr present in the XAS; (b) Room temperature Fe L2, 3 XA and XMCD spectra (+ 6 kOe); (c) Fe L2, 3 XMCD spectrum fit.

Figure 5: Saturation magnetization as measured from hysteresis curve is plotted against the corresponding measurement temperature for S_10 and S_35 samples.


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Figure 6. Hysteresis of S_35 sample: (a) close up of major loops versus temperature; (b) coercive field versus temperature; (c) remnant magnetization versus temperature.

Figure 7. Hysteresis of S_10 sample NC film: (a) close up of major loops versus temperature; (b) coercive field versus temperature; (c) remnant magnetization versus temperature.

Figure 8. Hysteresis curve at 5 K after ZFC and FC of (a) S_35 sample; (b) S_10 sample.


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Normalized Counts

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Figure 9: Room temperature Mössbauer spectrum under zero applied field for S_10 and S_35 sample.


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Figure 10: 57Fe Mössbauer spectrum of (a) S_10 and (b) S_35 sample, measured at a temperature of 5 K and in an external magnetic field of 50 kOe applied in the parallel direction with respect to the propagation of γ-rays. T-sites and O-sites represent tetrahedral and octahedral sites in the γ-Fe2O3 crystal structure.


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TOC: Larger nanocluster shows Fe-core with tetragonal Fe-phase and oxide shell composed of magnetite (Fe3O4) and/or maghemite (γ-Fe2O3). Lack of Cr from XAS confirms the purity of cluster which is composed of Fe and O.


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