Surface Effects in Ultrathin Iron Oxide Hollow Nanoparticles: Exploring

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Surface Effects in Ultrathin Iron Oxide Hollow Nanoparticles: Exploring Magnetic Disorder at the Nanoscale F. Sayed,†,‡ N. Yaacoub,*,† Y. Labaye,† R. Sayed Hassan,†,‡ G. Singh,∥ P. Anil Kumar,# J. M. Greneche,† R. Mathieu,⊥ G. C. Hadjipanayis,∇ E. Agostinelli,§ and D. Peddis*,§ †

Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, Le Mans Université, 72085 Le Mans Cedex 9, France MPLAB, Faculté des Sciences Section I, Université Libanaise, Beyrouth, 1100, Lebanon § ISM-CNR, Istituto di Struttura della Materia, Via Salaria km 29.300 CP 10, Monterotondo Scalo, 00015 Rome, Italy ∥ Department of Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim 7491, Norway ⊥ Department of Engineering Sciences, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden # Faculty of Physics, CENIDE, University of Duisburg-Essen, Lotharstr. 1, 47048 Duisburg, Germany ∇ Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, United States ‡

S Supporting Information *

ABSTRACT: A detailed study of the structural and magnetic properties of polycrystalline hollow γ-Fe2O3 nanoparticles of ∼9.4 nm size was performed. High-resolution transmission electron microscopy images confirmed the crystalline structure and the presence of a ultrathin shell thickness of ∼1.4 nm, implying a very high surface/volume ratio. These hollow nanoparticles were investigated using zero-field and in-field 57Fe Mössbauer spectrometry. The zero-field hyperfine structure suggests some topological disorder, whereas the infield one shows the presence of a comp magnetic structure that can be fairly described as two opposite pseudosperomagnetic sublattices attributed to octahedral and tetrahedral iron sites. Such an unusual feature is consistent with the presence of noncollinear spin structure originated from the increased surface due to the hollow morphology. Such a complex local spin structure evidenced from Mössbauer experiments was correlated with exchange bias coupling showing at low temperature by magnetization measurements. Monte Carlo simulations on a ferrimagnetic hollow nanoparticle unambiguously corroborate the critical role of the surface anisotropy on the noncollinearity of spin structure in our samples. ferrimagnetic iron oxides with spinel structure (maghemite, γFe2O3, and magnetite, Fe3O4) showed that even in a very large magnetic field some atomic moments do not align with the external magnetic field. This feature was ascribed to magnetic disorder at the particle surface due to competing interactions between sublattices.8,9 In this context, we should mention the Monte Carlo modeling approach developed to study the effect of surface anisotropy in ferrimagnetic nanoparticles, where the competition of Ks with Kv led to three different spin structures: two of them can be developed in response to surface anisotropy. When the value of the anisotropy energy is negligible in comparison to the exchange energy J, the nanoparticle adopts a collinear ferrimagnetic configuration. As this energy increases, a “throttled” or “flower” structure is obtained. However, beyond a critical value where the energy is almost equal to J, an abrupt transition to the “hedgehog” or

1. INTRODUCTION Nanoscience became in the last two decades one of the most important research area in modern science generating new knowledge frontiers. In particular, magnetic nanoparticles (MNPs) show many interesting phenomena due to their unusual physical properties strongly correlated with their size and morphology.1−5 When dealing with assembly of MNPs, magnetic behavior can also be strongly affected by interparticle interactions. The magnetic interactions result from dipolar and exchange coupling among nanoparticle surface atoms, and they also play a fundamental role in the physics of these systems. In this general framework, among the relevant features of the size reduction of magnetic particles, the occurrence of noncollinear spin structures (spin canting) deserves a special attention as they strongly modify the global magnetic properties. In the case of small nanoparticles ( 1 (R ≈ 1.3)12,29 (Table 1). Sample under investigation shows R ≈ 1.5, with very thin shell (∼1.4 nm). Starting from this framework, 57Fe Mössbauer spectrometry in presence of external magnetic field was used for the first time to investigate the magnetic structure of hollow nanoparticles with such a high S/V ratio.29 To explain the results of Mössbauer experiments, a numerical study was conducted using atomistic Monte Carlo algorithm to better understand the effect of surface anisotropy on the spin behavior in these hollow structures. In addition, direct current (DC)/alternating current (AC) magnetization measurements were carried out to correlate magnetic properties with magnetic structure in this system.

2. EXPERIMENTAL SECTION 2.1. Synthesis. The synthesis of HNPs was carried out using commercially available chemicals. In a typical synthesis, oleylamine (0.003 mol) was dissolved into octadecene (10 mL) at room temperature using a triple-neck flask and under vigorous magnetic stirring; it was then degassed under Ar atmosphere at 120 °C for 30 min. The temperature of the reaction mixture was raised up to 220 °C, Fe(CO)5 was injected, and the mixture was kept at the same temperature for 20 min. The heating source was then removed, and the reaction mixture was allowed to cool down.20,30,31 To obtain hollow particles with ultrathin shell, several experiments were carried out, finding the optimum conditions for tO2 = 95 min.12,31 2.2. Characterization and Data Treatment. DC and AC magnetization measurements were carried out using a Quantum Design superconducting quantum interference device magnetometer, equipped with a superconducting magnet (Hmax ±5 T). To avoid any displacement and/or rotation of the nanoparticles during the measurements, the samples, in form of

(1)

where SO‑shell and SI‑shell correspond to the outer and inner surface, respectively. It is extremely important to emphasize that higher R values mean high influence of surface and as a consequence, larger topological disorder and larger the magnetic frustration. For this reason, going to high values of R opens new perspective both to explore “the no man’s land between molecular nanomagnets and magnetic nanoparticles”23 and to investigate effect of magnetic disorder in the physics of magnetic nanoparticles. Among magnetic-disordered structures, one does consider speromagnetism, sperimagnetism, and asperomagnetism,24,25 B

DOI: 10.1021/acs.jpcc.8b00300 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) TEM and high resolution TEM images showing hollow nanoparticles. (b) Distribution of the total (outer) and inner diameter (Do and Di, inset of (b)) of the hollow nanoparticles obtained by statistical analysis of TEM images for about 200 nanoparticles.

Figure 2. ZFC (empty symbols) and FC (full symbols) recorded at 20 Oe and 20 kOe are reported in (a) and (b), respectively; inset of (a) M vs H/ T in the temperature range of 77−300 K. In-phase and out-of-phase components of the AC susceptibility at several frequencies are reported in (c) and (d), respectively. Inset of (c) χ′ subtracting a Curie−Weiss-like C/(T − θ) curve (estimated from the fit of the data between Tmax_1 and Tirr).

powders, were immobilized in an epoxy resin. Hysteresis

For the magnetization versus temperature measurements, the zero-field cooled (ZFC), field cooled (FC), and thermoremanent magnetization (TRM) protocols were used. To perform ZFC measurements, the sample was first cooled from room temperature down to 5 K in zero-field, then the

measurements up to 9 T were carried out using Quantum Design physical property measurement system equipped with vibrating sample magnetometer option. C


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Figure 3. (a) 57Fe Mössbauer spectra recorded at 300 K (a) and 77 K (b); probability distribution as a function of quadrupolar splitting for each spectrum recorded at 300 K (c) and 77 K (d).

Section,12,31 based on the “Kirkendall effect”. Transmission electron microscopy (TEM) investigations clearly indicate the formation of hollow nanostructures (Figure 1a). A detailed statistical analysis was done on TEM images (Figure 1b) allowing to measure the outer diameter (Do, inset Figure 1b) and the inner diameter (Di, inset Figure 1b) of the hollow nanoparticles. Both Do and Di are log−normal distributed, and average values of 9.4(1) and 6.6(2) nm were obtained corresponding to a mean shell thickness around 1.4 nm. Assuming spherical particles and using eq 1, a value of R ≅ 1.5 is obtained. This value is quite larger compared to that in previous studies31 highlighting a more important role of the surface in our system. 3.1. Magnetic Properties. Thermal dependence of magnetization was investigated by means of ZFC/FC protocols under an applied field of 20 Oe (Figure 2a). ZFC and FC curves show irreversibility above 45(3) K suggesting a superparamagnetic behavior of the magnetic entities. Irreversibility temperature (Tirr) was determined as the temperature where the difference between MFC and MZFC, normalized to its maximum value at the minimum temperature (5 K), becomes smaller than 3%. M vs H curves were measured at different

magnetization (MZFC) was recorded warming up from 5 to 300 K, with a static applied magnetic field, whereas the MFC was recorded during the subsequent cooling from 300 to 5 K. In TRM measurements, the sample was cooled from room temperature to 5 K in an external magnetic field, then the field was turned off, and the magnetization was measured warming up from 5 to 300 K. 57 Fe Mössbauer spectra were obtained using a 57Co/Rh γ-ray source mounted on an electromagnetic transducer with a triangular velocity form. Mössbauer spectra were recorded at 300 and 77 K without external magnetic field and at 11 K in an 8 T field oriented parallel to the γ-beam. The samples were consisted in a thin layer containing 5 mg/cm2 of the powdered compound mounted on a sample holder. The hyperfine structure was modeled by a least-square fitting procedure involving Zeeman sextets composed of Lorentzian or Gaussian lines using program “MOSFIT”. The isomer shift (IS) values are referred to that of α-Fe at 300 K.

3. RESULTS AND DISCUSSION Maghemite HNPs were synthesized following a previously reported procedure, briefly described in the Experimental D


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indicate that the HNPs sample can be described neither amorphous nor crystalline. The zero-field hyperfine structure at 11 K (Figure 4a, upper spectrum) consists of a pure sextet typical of a blocked

temperatures in the range of 77−300 K (inset of Figure 2a), showing zero coercive field and remanence magnetization. In addition, a very good superposition was observed plotting magnetization versus H/T, confirming the superparamagnetic behavior of HNPs in the investigated temperature range.32,33 Below Tirr, MFC increases with decreasing temperature, suggesting the presence of very weak interparticle interactions.33,34 The ZFC curve presents two maxima: a broad one at 32 ± 2 (Tmax_1) and a narrow one at lower temperature, (7 ± 0.5, Tmax_2). In ZFC/FC recorded under higher magnetic field (20 kOe, Figure 2b), both Tmax_1 and Tirr show a shift to lower temperature, as expected for superparamagnetic blocking. On the other hand, at low temperature (between Tmax_1 and Tmax_2), both ZFC/FC curves show a 1/T-like behavior. The temperature dependence of the in-phase χ′(T,f) and out-ofphase χ″(T,f) components of the AC susceptibility (Figure 2c,d) confirms the landscape drawn by ZFC/FC curves, displaying two peaks around 35 and 7 K. The broader peak at high temperature of χ′ shows a frequency dependence resembling that of superparamagnetic blocking. As seen in the inset, subtracting a Curie−Weiss-like C/(T − θ) curve (estimated from the fit of the data between Tmax_2 and Tirr) yields curves similar to those observed of thicker hollow nanoparticles.19 As expected, two peaks may be observed in the χ″−T curves. In agreement with the above observations, the magnitude of the maximum of χ″ near the 35 K peak is rather frequency independent, as for superparamagnetic systems. The χ″( f,T) curves collapsed onto each other by plotting them again as χ″ vs T log(t/τ0), with t ∼ 1/ω = 1/2πf and τ0 ∼ 10−13 s (see Figure S4 reported in Supporting Information (SI)).35 On the other hand, the sharpest peak observed at low temperature reveals significant frequency dependence, suggesting the freezing of a more anisotropic phase. 3.2. Mössbauer Spectrometry. To get relevant information about the local cationic environment, 57Fe Mössbauer spectra were recorded at 300 and 77 K. Both spectra (Figure 3a,b, respectively) result from quadrupolar doublets with broadened and non-Lorentzian lines of similar intensities. The spectra were fitted by means of a discrete distribution of quadrupolar splitting composed of Lorentzian lines. The corresponding histograms show somehow a symmetrical form associated with pure quadrupolar splitting distributions. At a first observation this symmetrical shape seems to indicate that our sample acquires a behavior typical of an amorphous structure. A more detailed analysis based on the mean isomer shift values, the quadrupolar splitting distributions, and the ratio q = ⟨Δ2⟩/⟨Δ⟩2 (parameter quantifying the topological disorder) is presented in Table 2, where ⟨Δ⟩ and ⟨Δ2⟩ correspond to the mean and mean quadratic values of the quadrupolar separation, respectively. Yet, the q ratio obtained for the analysis of our spectra is somehow far from the value obtained in case of amorphous structure.36 These results

Figure 4. (a) 57Fe Mössbauer spectra measured at 11 K without external field (upper spectrum), with a magnetic field of 8 T fitted using a Gaussian distribution (middle spectrum), fitted using a Lorentzian distribution (lower spectrum); black line stands for the total fit, red one for tetrahedral component, and blue one for octahedral component. (b, c) Distribution of hyperfine field, P (Hhf), and angular distribution P (β) obtained by the fitting procedure using Lorentzian lines; (A) and (B) represent tetrahedral and octahedral sites, respectively.

magnetic state, whereas the slight asymmetry indicates the coexistence of different Fe species with different isomer shift values, which could be a priori attributed to octahedral and tetrahedral environments. The decomposition of the total spectrum into two independent sextets does not provide unambiguous data that could allow to accurately estimate their respective proportions but provides the mean hyperfine parameters values (isomer shift ⟨IS⟩ = 0.45 mm/s, quadrupolar shift ⟨2ε⟩ = 0 mm/s, and hyperfine field ⟨Hhf⟩ = 47.7 T at 11 K). Thus, 57Fe Mössbauer spectrometry in presence of a high external magnetic field has been carried out, to more reliably distinguish and quantify between tetrahedral and octahedral Fe species in typical ferrimagnets.6,39

Table 2. Summary of the Mean Values of the Different Hyperfine Parameters Obtained at 300 and 77 Ka 300 K 77 K

⟨δ⟩

⟨Δ⟩

⟨Δ2⟩

q = ⟨Δ2⟩/⟨Δ⟩2

0.35 0.43

1.0 1.523

1.22 2.98

1.22 1.28

⟨δ⟩, mean isomer shift; ⟨Δ⟩, mean quadrupolar separation; ⟨Δ2⟩, mean quadratic quadrupolar separation; and q, a ratio identifying the topological disorder. a

E


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Table 3. Summary of Obtained Values of Hyperfine Parametersa Obtained at 11 K under an External Field of 8 T for Gaussian and Lorentzian Distributions Gaussian 11 K Lorentzian 11 K Moshex a

1st component 2nd component 1st component 2nd component A B

⟨δ⟩ mm/s (±0.01)

2ε mm/s (±0.01)

Heff T (±0.5)

Hhf T (±0.5)

β (±10°)

% (±5%)

0.47 0.5 0.45 0.47 0.43 0.48

0.02 −0.01 0 0 −0.05 0.02

55.7 46.9 56.2 46.5

50.1 51.7 50 52.2 −48.2 50.6

42 57 37 58

22 78 24 76 18 82

Isomer shift, quadrupolar shift, effective and hyperfine fields (Heff and Hhf), angle β, and weight.

Different fitting procedures have been considered to model the in-field Mössbauer spectrum and are illustrated in Figure 4a. The corresponding refined values of hyperfine parameters are listed in Table 3. The first model (lower spectrum in Figure 4a) consists of two independent distributions of effective fields, the isomer shift, quadrupolar shift, and angle β as defined by the directions of the effective field and the γ-radiation, being commonly refined for each distribution. The second one results from two independent Gaussian distributions of Lorentzian lines sextets, the quadrupolar shift, and angle β values being commonly refined for each distribution (middle spectrum in Figure 4a). At this stage, these two models give rise to excellent description of the experimental spectrum and rather comparable mean hyperfine data. It is important to emphasize that the refined values of isomer shift are rather equal and suggest Fe3+ species located in tetrahedral and octahedral sites with respective proportions favoring a large majority of the latter sites. The next step consisted in correlating the distribution of the effective (Heff) field and that if the β to reach a reasonable fit. The probability distributions, of the angle β and the hyperfine field, are presented in Figure 4b,c, respectively. Then, this distribution allows the hyperfine field distribution for both components to be calculated. Contrary to the collinear ferrimagnetic structures expected in spinel iron oxides, such an approach provides evidence of a distribution of β angles, consistently with a noncollinear magnetic structure. Indeed, the hollow morphology of the present nanoparticles enhances the surface contribution and consequently the magnetically disordered structure, as discussed in the previous section. The last fitting procedure of the in-field Mössbauer spectrum consists in the superimposition of two different ideal magnetic subnetworks, i.e., two speromagnetic models.23−25 In this model, the isomer shift and quadrupolar shift values are consistent with the previous ones, whereas the two types of Fe moments are frozen in perfectly random orientations but opposite, according to the positive and negative refined values of hyperfine fields, as reported in Table 3 (note that the asymmetry of external lines differs for the two components). The refinement of hyperfine parameters was derived by means of unpublished program “Moshex” [J. Teillet and F. Varret, unpublished MOSHEX program, Le Mans Université France] taking into account also the external magnetic field, which can be considered as a free parameter as well as any leak magnetic field acting as polarizer on the source. This description allows a rather good fit to be achieved, as illustrated in Figure 5, but a small disagreement between the experimental and the theoretical spectra, contrarily to the first models, can be noted. We finally conclude that the Fe magnetic lattice does result from two opposite noncollinear structures close to speromagnets, corresponding to octahedral and tetrahedral networks, antiferromagnetically coupled. The idea of fitting

Figure 5. Mössbauer spectra obtained at 11 K under a magnetic field of 8 T fitted using Moshex. Black line stands for the total fit, red one for tetrahedral component, and blue one for octahedral component.

using more than one model, which ensures the complexity of such in-field spectrum, is the first time to be encountered for such hollow systems that are characterized by high magnetic disorder. In addition, the amount of octahedral Fe3+ species, which is significantly larger than that usually observed in stoichiometric maghemite nanoparticles (37.5 and 62.5% for tetrahedral and octahedral Fe sites, respectively), can be probably attributed to a higher percentage of octahedral sites occurring both at inner and outer surfaces, acting as closing units at the surface. It is important to emphasize that this feature has been previously suggested but not unambiguously demonstrated.40,41 It should be pointed out that, despite the complexity of such spectrum characterized by high spin disorder, presented results supporting our description are confirmed by three different and independent fitting approaches. To better understand the spin structure of the hollow nanoparticles established from Mössbauer experiments, Monte Carlo simulations were carried out, taking into account the surface magnetic anisotropy (Ks), for an individual hollow maghemite nanoparticle (more details about Monte Carlo modeling and simulation are reported in SI and ref 42). Increasing gradually the value of Ks, the structure gets far away from collinearity until a spike structure is attained at Ks ∼ (100 K to 25 mJ/m2), which is somehow a small value compared to that needed in the case of full nanoparticles.43 The different spin structures are illustrated in Figure S1 in SI, and the behavior of magnetization as Ks changes is also reported in Figure S2 in SI. The above spin structures can be described using the angular distribution probability to study the angle that each spin makes with the unitary direction of magnetization (Figure S3 in SI). Such a structure appears to be compatible with the results obtained from in-field Mössbauer experiments confirming that the experimental results must be explained by F


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Figure 6. (a) Thermal dependence of magnetization recorded at 5 T (b) and coercive field (continuous line is guideline for eyes); hysteresis loops recorded after ZFC (full symbols) and FC (empty symbol) for cooling field of 1 T at 4 K (c) and 20 K (d).

temperature indicated by magnetic susceptibility, interface exchange coupling disappears and ZFC and FC loops are perfectly superimposed. It is worth to underline that these novel experimental features, well described by means of two speromagnetic sublattices, can be essentially ascribed to the high R value (i.e., very thin shell) observed in the sample under investigation with respect to what reported in the literature.12,21,44 To confirm this idea, a comparison with a reference sample (RS) of hollow nanoparticles has been done (see SI for some details on synthesis and morphostructural characterization). RS shows an R value (∼1) quite lower with respect to HN samples (∼1.5) and, as a consequence, a thicker shell (tRS ≅ 2.4 vs tHN ≅ 1.4). The Mössbauer spectra under intense magnetic field are compared in Figure 7 for the sample under investigation (upper spectrum) and for RS (lower spectrum), with similar outer diameter but a thicker shell. RS is characterized by a splitting of external lines and a decrease in the intensities of intermediate lines: such an in-field hyperfine structure, which is expected in the case of a weakly canted ferrimagnetic structure, is rather similar to that observed in the case of full nanoparticles where the contribution of surface remains less important. On decreasing thickness, there could exist an intermediate regime where the inner layer (approximately a couple of atoms thick) is still distinguishable from the outer layers. In this inner layer, a ferrimagnetic-like structure could be established. These results induce to believe the presence of a trilayer-like structure where some magnetic moments of the outer and inner surface layers interact with the ferrimagnetic ultrathin central layer: the local structural distortions give rise to some short superexchange paths, with antiferromagnetic interactions, originating the exchange bias coupling with the inner ferrimagnetic layer. It is important to mention that a model involving two opposite speromagnetic components and two sextets to describe the surfaces and the thin ferrimagnetic layer,

the effects of surface anisotropy originating from the symmetry breaking and the local atomic disorder induced by the peculiar structure of the surface. One has to mention that this numerical study was recently performed and extended to other HNPs with different sizes and thicknesses.42 3.3. Correlation between Magnetic Structure and Magnetic Properties. Thermal dependence of AC and DC magnetization reveals the “canonical” blocking of superspins below 35 K and the freezing of a more anisotropic phase below 15 K, as also confirmed by thermal dependence of coercivity (Figure 6b). The Mössbauer spectra recorded at 11 K in absence and under intense magnetic field indicate the presence of a quite complex noncollinear magnetic structure. In particular, two subspectra attributed to sites A and B suggest two opposite speromagnetic-like behaviors. This description has been confirmed by Monte Carlo simulation, suggesting the presence of a very high surface anisotropy that dominates the magnetic behavior of this particle at low temperature. To better understand this behavior, field dependence of magnetization was investigated at different temperatures. Thermal dependence of magnetization measured at 5 T (Figure 6a) shows an increase with decreasing temperature, with a clear drop below 15 K. This can be ascribed to the freezing of disordered spin in speromagnetic phase with high anisotropy, leading to a decrease in magnetization. This is also confirmed by thermal dependence of coercive field (Figure 6b): a nonzero value of coercive field was measured below 35 K due to the blocking of superspins; a further increase in coercive field is observed below 10 K, compatible with the presence of a more anisotropic phase. Hysteresis loops after zero-field cooling and after field cooling (Hcooling = 20 kOe) were recorded at 4 K (Figure 6c) and 20 K (Figure 6d) using a maximum field of 90 kOe. The FC loop at 4 K displays an evident horizontal shift, indicating the presence of interface exchange coupling (i.e., exchange bias coupling). At 20 K, above the freezing G



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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/acs.jpcc.8b00300. Monte Carlo modeling and simulation for hollow nanoparticles with big surface-to-volume ratio: study of effect of surface anisotropy on spin structure; representation of simulation results by spin configuration diagrams (collinear, throttled, and spike spin structures) in addition to angular distributions; presentation of reference sample (RS) to be compared with ours (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.Y.). *E-mail: [email protected] (D.P.). ORCID

D. Peddis: 0000-0003-0810-8860 Notes

The authors declare no competing financial interest. D.P. and N.Y. lead the research. D.P., E.A., and G.S. synthesized the nanoparticles and G.C.H. was responsible for the supervision. R.M., and D.P. elaborated the model. G.S. and G.C.H. performed and analyzed TEM measurements. J.M.G., N.Y., and F.S. performed and analyzed Mössbauer spectra. Y.L., F.S., and R.S.H. elaborated the model and performed Monte Carlo simulation. R.M., P.A.K., and D.P. performed and elaborated the magnetization measurements. E.A. revised the manuscript.

Figure 7. Mössbauer spectra recorded at 10 K under intense external magnetic field (8 T) for hollow nanoparticles with R ≅ 1.5 (upper spectra) and hollow nanoparticles with R = 0.98 (lower spectra).

respectively, could be proposed to fit the in-field Mössbauer spectrum but the complexity of this model including the high number of parameters prevents to estimate accurately the thickness of this layer. When the thickness becomes lower than ≈1 nm, it does consist of only “two surfaces” with speromagnetic behaviors or a tiling of thin nanoplatelets linked to each other by grain boundaries, thus favoring noncollinear magnetic structures. But it is clear that such ultrathin objects remain a priori challenging to be obtained because the smaller the thickness is, the lower the stability.

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ACKNOWLEDGMENTS This study has been supported by bilateral agreement CNRSCNR PICS (6808) between French and Italian research groups, which financed the research stay in Roma of F.S. The Research Council of Norway is acknowledged for the support to the Norwegian Micro- and Nanofabrication Facility, NorFab. P.A.K. and R.M. thank the Swedish Research Council (VR) and the Göran Gustafsson Foundation for financial support. D.P. thanks T.a.P for inspiring discussions.

4. CONCLUSIONS In summary, the effect of the large surface area of HNPs originated from inner and outer surface contributions strongly influences the magnetic structure. Indeed, hollow nanoparticles of ultrathin thickness (R > 1.4) do exhibit a large spin disorder. The spin noncollinearity was clearly established from the shape of the in-field Mössbauer spectrum that can be described by two opposite speromagnetic structures, thus involving a large distribution of spin canting angles. This is the first time to obtain such kind of magnetic behavior in case of hollow maghemite nanoparticles, making this nanostructure completely new and original. The results of Monte Carlo simulation highlighted the role of surface anisotropy and confirmed the picture described by Mössbauer results that show the large spin distribution. From magnetic measurements, a freezing of disordered spins was observed at low temperatures. Such spins can be described in the framework of a speromagnetic-like phase with high anisotropy. In addition, the FC loop at low temperature shows an evident horizontal shift, indicating the presence of exchange coupling contribution in our samples. The origin of the strong anisotropy energy and the exchange bias effect can be associated to the presence of large portion of disordered spins on the outer and inner surfaces of HNPs interacting with a ferrimagnetic ultrathin central layer. All these features could be correlated and attributed to the large surfaceto-volume ratio R, which is strongly enhanced in the case of hollow nanoparticles morphology.

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REFERENCES

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