Investigation of the Collective Properties in Monolayers of Exchange

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Investigation of the Collective Properties in Monolayers of Exchange Biased Fe O@CoO Core-Shell Nanoparticles 3-#

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Yu Liu, Xiaojie Liu, Mathias Dolci, Cédric Leuvrey, Elodie Pardieu, Alain Derory, Dominique Begin, Sylvie Begin-Colin, and Benoit P. Pichon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04615 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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The Journal of Physical Chemistry

Investigation of the Collective Properties in Monolayers of Exchange Biased Fe3-δδO4@CoO CoreShell Nanoparticles

Yu Liu,1 Xiaojie Liu,1,2 Mathias Dolci,1 Cédric Leuvrey,1 Elodie Pardieu,1,2,3 Alain Derory,1 Dominique Begin,2 Sylvie Begin-Colin,1 Benoit P. Pichon*,1, 4 1

Université de Strasbourg, CNRS, Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504, F-67034 Strasbourg, France 2

Université de Strasbourg, CNRS, Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé, UMR 7515, F-67087 Strasbourg, France

3

Université de Strasbourg, CNRS, Institut Charles Sadron, UPR 22, F-67034 Strasbourg, France

4

Institut universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05

[email protected] Tel: 0033 (0)3 88 10 71 33, Fax: 0033 (0)3 88 10 72 47

Abstract Owing to its potential to enhance the effective magnetic anisotropy energy of nanoparticles, exchange bias has been extensively studied during the last few years. Although most of studies have been conducted in the powder state, the effect of the dimensionality of nanoparticle assemblies has been scarcely investigated. In this context, we report on the study of the collective properties of monolayers of exchange biased nanoparticles with different core-shell structures. Nanoparticles consist in a ferrimagnetic (FiM) Fe3-δO4 core coated with an antiferromagnetic (AFM) CoO shell. They were assembled in monolayers with local order by using the Langmuir-Blodgett (LB) technique. Exchange bias is significantly altered in 2D assemblies in comparison to powder samples which can be assimilated to 3D random assemblies. We show that exchange bias in Fe3-δO4@CoO nanoparticles is directly influenced by dipolar interactions between Fe3-δO4 cores which are enhanced by shape anisotropy of 2D assemblies. Besides the dramatic influence of the core-shell structure, the systematic variation of the core size and the shell thickness showed that above a critical value, dipolar interactions compete exchange bias.

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Introduction Exchange biased magnetic nanoparticles are gaining increasing interest because of their ability to enhance magnetic anisotropy energy while maintaining sizes in the range of few nanometers.1,2 The exchange bias phenomenon is usually observed in core-shell nanoparticles which combine a ferro(i)magnetic (F(i)M) phase and an antiferromagnetic (AFM) phase. In these systems, the higher anisotropy of the AFM phase favors interfacial pinning of F(i)M spins and induces an extra magnetic anisotropy. Interfacial exchange bias was initially discovered in Co@CoO nanoparticles by Meiklejohn and Bean in 1956.3 Recent advances in nanoparticle synthesis with regard to the fine control over size, shape and chemical composition opened new perspectives toward the design of exchange biased core-shell nanoparticles with much more complex structures.4 Further, these nanoparticles with blocked magnetization at room temperature and enhanced coercivity are envisioned to contribute to the development of advanced technological devices such as sensors (detection, position tracking,..) and magnetic recording media (extension of areal bit densities beyond 1 Tbit/in2).5 In this context, cheap and nontoxic metal oxides offer a promising alternative to noble metals and rare earth elements which have been classified as critical raw materials by the European Union due to shortage and increase of prices.6 A renewed interest on alternative materials among which, ferrites (MFe2O4) nanoparticles may bring new inputs in the production of more efficient and cost less magnetic materials.7–10 Core-shell nanoparticles based on F(i)M ferrites have been recently investigated to address this challenge. For instance, the modulation of shell thickness11–13, and core size14, in F(i)M@AFM nanoparticles significantly affect exchange bias. We have also reported on Fe3δO4@CoO nanoparticles exhibiting enhanced coercive field and blocking temperature close to room temperature when increasing the shell thickness.15,16 Although exchange biased nanoparticles have been widely investigated, they are usually considered in the powder state where they are subjected to random, strong and uncontrolled interparticle interactions. In contrast, the collective properties of F(i)M “single phase“ nanoparticles have been widely investigated as function of their spatial arrangement. The dimensionality of these nanoparticle assemblies has been repeatedly demonstrated to influence the collective properties by modulating the directionality of dipolar interactions.17,18 For instance, a 2D monolayer of iron oxide nanoparticles strengthens in-plane dipolar interactions in comparison to the powder state. Anisotropic dipolar interactions were even stronger in low dimensional nanoparticle assemblies such as a local chain structure in a monolayer19 and further, in single nanoparticle chains.20 From the experimental point of view, studies on exchange bias in nanoparticle assemblies are very scarce. It has been reported few years ago that alignments of Co@CoO nanoparticles enhance exchange field in comparison to dispersed nanoparticles, although no clear explanation was given.21 Nogues et al. experimentally showed the role of shell mediated interactions between Co@CoO nanoparticles embedded in Al2O3 matrix at different concentrations.22 Diluted nanoparticles resulted in the degradation of exchange bias while nanoparticles in direct contact for higher concentrations recovered extra anisotropy. More recently, a slight decrease of the exchange field was reported in CoxFe3−xO4@CoO nanoparticles after coating with PMMA which was correlated to the increase of interparticle distance.23 Theoretical calculations have been also conducted on the magnetic properties of FiM@AFM core-shell nanoparticles assemblies. Interfacial exchange bias was enhanced 2 ACS Paragon Plus Environment

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by interparticle dipolar and exchange interactions in 3D disordered assemblies.24 On the other hand, the suppression of interparticle exchange interactions (nanoparticles not in direct contact) resulted in the opposite behavior. The modulation of the coercive field (HC) and the exchange field (HE) were explained by the competition between dipolar and anisotropy energies. Simulations on nanoparticles assembled in 2D triangular arrays have shown that the evolution of HE and HC when increasing the dipolar interactions were also dependent on the strength of interfacial exchange coupling.25 More recently, we reported that the enhancement of exchange bias in FiM@AFM core-shell nanoparticles is favored by the interplay of weak dipolar interactions and random orientation of magnetic magnetization.26 Nevertheless, exchange bias in core-shell nanoparticles remains a very complex phenomenon. The precise understanding of exchange bias as a function of the structure of both nanoparticles and their assemblies still represents a huge challenge to address. Here, we report on the study of interfacial exchange bias in Fe3-δO4@CoO core-shell nanoparticles assembled in 2D monolayers. The Langmuir-Blodgett technique allowed the preparation of nanoparticle monolayers with local order. We investigated the effect of core size and shell thickness on exchange bias in monolayers. We have also extended our study to the effect of the dimensionality of nanoparticle assemblies. The collective magnetic properties of 2D monolayers were compared to the ones of powder samples which were assimilated to 3D random assemblies.



Results Core-shell nanoparticles were synthesized by a seed mediated growth method based on the thermal decomposition of metal complexes as we recently reported.15,16 This strategy first consisted in the synthesis of iron oxide (Fe3-δO4) nanoparticles which were subsequently covered by a cobalt oxide (CoO) layer. The adjustment of synthesis parameters allowed the fine tuning of the iron oxide core size27 and of the CoO shell thickness and crystallinity16 (Table 1). Experimental conditions are detailed in reference 16 excepted that of CS01 (see experimental section). For each nanoparticle, the refinement of XRD patterns showed the formation of cubic CoO (JCPDS file 70-2856) in addition to the partially oxidized spinelle Fe3-δO4 (see reference 16). Figure 1 shows the narrow size distribution, spherical shape and crystal structure of CS01 and CS02, while irregular shapes are observed for CS03 and CS04. Such different structures were ascribed to the kinetic of CoO growth.16 Shell thicknesses were deduced by comparing size distributions of nanoparticles before and after decomposition of the Co metal precursor. All nanoparticle sizes increased. CS01 and CS02 shell thicknesses were similar (about 1 nm) while the core diameters were different (9.6 nm and 8.2 nm, respectively). In contrast, shell thicknesses of CS01 and CS03 were different (1 nm and 1.5 nm, respectively) while their core size is similar (9.6 nm).

Figure 1. TEM micrographs of Fe3-δO4@CoO core-shell nanoparticles. a) CS01, b) CSO2, c) CS03, d) CS04. Insets show size distributions.


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Furthermore, highly stable suspensions of these nanoparticles were obtained in chloroform as shown by granulometry measurements (Figure S2). Hydrodynamic diameters were slightly larger than nanoparticle sizes measured from TEM micrographs (Table 1), which agreed with nanoparticle coated with oleic acid as evidenced by FTIR spectroscopy (Figure S2). Therefore, no aggregates were obtained which means that nanoparticles were considered as single nanobuilding blocks prior to the preparation of assemblies.

Table 1. Structural parameters of synthesized Fe3-δO4@CoO core-shell nanoparticles. Core sizes and shell thicknesses were measured from TEM micrographs.

Fe3-δδO4@CoO (nm)

CS01 11.5± 1.4

CS02 10.1± 0.8

CS03 12.6 ± 1.4

CS04 13.9 ±1.5

Fe3-δδΟ4 core (nm)

9.6± 0.9

8.2 ± 0.9

9.6 ± 0.9

8.3 ±0.7

CoO shell (nm)

0.9± 1.4

0.9

1.5

2.5

14.5

13.1

15.6

_

VCoO (nm )

187

141

330

_

Hydrodynamic diameter (nm)

12.3

11.9

13.6

14.9

Intercore dist. (nm) 3

Nanoparticle monolayers were prepared by performing the Langmuir-Blodgett (LB) technique.28,29 Colloidal suspensions in chloroform were spread onto a water subphase. After evaporation of the solvent, barriers positioned at each through extremity were brought together. Films of tightly packed nanoparticles were prepared at the air/water interface and were further transferred onto silicon wafers. Continuous and dense films of nanoparticles were prepared for each type of core-shell nanoparticles (Figure 2). Spherical nanoparticles CS01 and CS02 displayed well-defined film structures with a local order as shown by auto correlograms (Figure 2, insets). Coherence lengths of about 60 nm and similar average interparticle separating distances of 3.0 nm were calculated.28 It corresponds to center to center distances of 14 nm for CS01 and 13 nm for CS02. The CS03 film being much disordered, the value of 15 nm could not be calculated from the auto correlogram. Given the size of CS03, it was deduced from the edge-to-edge distances calculated from the auto correlograms of CS01 and CS02. In contrast, the auto correlogram of the CS04 assembly could not be solved because of contrast variations in the SEM micrograph which may be explained by a disordered structure. These features were confirmed by AFM measurements (Figure 3). CS01, CS02 and CS03 were assembled into nearly continuous monolayers with thicknesses of about 9-12 nm which were close to the nanoparticle sizes. Therefore, the slight variation in nanoparticle size does not significantly affect the structure of monolayers. In contrast, the irregular shape of CS04 nanoparticles resulted in a multilayer structure which displayed a much thicker (93 nm) and roughened profile section. Therefore, tight packed assemblies with local order which favor interactions between nanoparticles were obtained in all cases.



Figure 2. SEM micrographs of nanoparticle assemblies prepared by the Langmuir-Blodgett technique. a) CS01. b) CS02. c) CS03. d) CS04. Insets correspond to auto correlograms.


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Figure 3. AFM measurements performed on nanoparticle assemblies of a) CS01, b) CS02, c) CS03 and d) CS04. Left: 3D images. Center: 2D height images. Right: cross section profiles corresponding to the line in height images. Zero height is given by the flat and uncovered area obtained after scratching the nanoparticle assembly.

The magnetic properties of nanoparticle assemblies have been studied by SQUID magnetometer. Field dependent magnetization curves recorded at 300 K perfectly overlap for all nanoparticle assemblies which agree with unblocked magnetic moments (Figure 4a). In contrast, M(H) curves recorded after zero field cooling (ZFC) down to 5 K all show a large hysteresis which correspond to blocked magnetic moments (Figure 4b). The coercive field (HC) values depend significantly on the core-shell structure (Table 2). On one hand, increasing the shell thickness from 1 nm (CS01) to 1.5 nm (CS03) while considering a similar core size of 9.6 nm led to the slight increase in HC from 3 090 Oe to 4 270 Oe. On the other hand, the reduction of the iron oxide core size from 9.6 nm (CS01) to 8.2 nm 7 ACS Paragon Plus Environment


(CS02), while maintaining the same shell thickness of 1 nm, resulted in the dramatic increase of HC from 3 090 Oe to 10 640 Oe. Field-dependent magnetization curves recorded after 7 T field cooling (FC) down to 5 K exhibited the largest hysteresis cycles (Figure 4c). FC cycles are negatively shifted for all assemblies which agree with exchange bias at the core-shell interface. Such a shift allows quantifying the exchange bias through calculation of the exchange field (HE). HC (FC) values evolved in a similar way to the one of HC (ZFC). Furthermore, as observed for HC, HE is directly dependent on the core-shell structure. Finally, FC M(H) curves display also a slight vertical shift which may be ascribed to uncompensated spins30 or frozen spins in the shell.31

0,5 0,0 -0,5

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CS01 CS02 CS03 CS04

Magnetization (M/Ms)

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a) Magnetization (M/Ms)

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0,0 -0,5 -1,0

-1,0 -60

-40

-20

0

20

40

60

80

0,5

CS01 CS02 CS03 CS04

0,0 -0,5 -1,0

-50 -40 -30 -20 -10

-80

1,0

0

10

20

30

40

50

Applied field (KOe)

-50 -40 -30 -20 -10

0

10

20

30

40

50

Applied field (KOe)

Applied field (KOe)

Figure 4. Field dependent magnetization curves recorded for nanoparticle assemblies. a) 300 K. b) ZFC curves recorded after cooling down to 5K under zero field. c) FC curves recorded after cooling down to 5 K under a magnetic field of 7 T.

In the case of CS03, the increase of both HC and HE with respect to CS01 (40 % and 25 %, respectively) was correlated to stronger exchange bias which was favored by thicker AFM shell.12,16,32 The increase of the shell thickness from 1 to 1.5 nm roughly corresponds to an extra layer of CoO (cubic, a = 0.42 nm). In contrast, the reduction of the FiM core from CS01 to CS02 resulted in the largest increase of HC and HE (about 250 % and 200 %, respectively) which agree with enhanced exchange bias.33 The higher increase of HE results directly from the lower magnetization saturation and size reduction of the FiM core as shown by equation (1)34 and agrees with the phenomenological Meiklejohn and Bean´s model.35 Although the shell of CS04 is the thickest (up to 2.5 nm), it displays much lower values of HC and HE which is correlated to the discontinuity (lower FiM/AFM interface) and polycrystalinity of the CoO shell as observed for nanoparticles in the powder state.16 Such differences of exchange bias can be explained by considering the interface coupling energy Eint which was calculated according to:1 Eint = HEMSD/6

(1)

where D is the Fe3-δ04 core size and MS is the saturation magnetization measured for nanoparticle of similar size to the core. From our previous work,36 we used MS = 58 emu/g and 60 emu/g for Fe3-∂O4 cores of 8.2 and 9.6 nm, respectively. The interface coupling energy per surface unit is the highest for CS02 (0.17 erg/cm2) and is much larger than values reported for similar core-shell nanoparticles (0.01-0.05 erg/cm2).37,38 In contrast, the other samples display values which are twice lower (≈ 0.07 erg/cm2) and agree with weaker exchange bias. 8 ACS Paragon Plus Environment

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One may also notice that both ZFC and FC M(H) curves display a small but steep demagnetization at low applied fields. Such a jump is ascribed to the sudden reversal of magnetic moments of nanoparticles. It is reversible since it is observed in both descending and ascending branches. This behavior has been attributed to a reorientation of surface spins with the applied magnetic field,39 or to the dominant magneto crystalline anisotropy of the Fe3O4 phase of dispersed Fe/Fe3O4 core-shell nanoparticles.40 In our study, small jumps were only observed for monolayers but not for powders, whatever the core-shell structure (see ref 16). Therefore, it can be ascribed neither to a population of nanoparticles with different core-shell structures, nor to a population of nanoparticles with different spatial arrangement since SEM micrographs show that nanoparticles are tightly packed in homogeneous assemblies. An explanation may be that tight packing of nanoparticles may result in the formation of magnetic domains of strongly correlated magnetic moments which favor the fast reversal of magnetization, thus overcoming exchange bias.

Table 2. Magnetic characteristics of LB assemblies of nanoparticles.

HC ZFC (Oe) HC FC (Oe) HE FC (Oe) TMAX (K) TB (K)

CS01 3 090 3 860 1 365 194 145

CS02 10 640 13 670 4 040 252 230

CS03 4 270 5 300 1 680 224 160

CS04 3 360 5 200 1 650 164 130

Temperature-dependent magnetization curves recorded after zero field cooling (ZFC) showed maximum at temperature (TMAX) which is usually assimilated to the limit between blocked magnetization (T < TMAX) and the superparamagnetic behavior (T > TMAX) (Figure 5). Each value is larger than the ones recorded for Fe3-δO4 nanoparticles with sizes corresponding to the FiM cores.41 It is ascribed to the enhancement of effective magnetic anisotropy resulting from exchange interaction between the FiM core and the AFM shell as we have reported for these samples in the powder state.16 To get a better insight on the magnetic properties of exchange biased nanoparticles in 2D assemblies, we plotted the distribution of blocking temperature by calculating the derivatives of the difference between the ZFC and FC magnetization curves (-d(MZFC-MFC)/dT) (see SI).42 Each curve shows a maximum at temperature which can be correlated more accurately to the blocking temperature (TB) (Table 2). It is worth to notice that each value is lower than TMAX. In exchange biased nanoparticles, TB is expected to reach a maximum value which corresponds to the Néel temperature (TN) above which the AFM order vanishes, so exchange bias does.2 However, TB measured for all of our 2D samples remain below TN = 290 K of bulk CoO which can be explained by finite size effects.43 Indeed, for CoO shells thinner than 2 nm, thermal fluctuations of the uncompensated magnetic moments shrinked down the blocking temperature of CoO to about 150 K which agree with TB values of CS01 and CS03.44 Nevertheless, a thicker shell from 1 nm (CS01) to 1.5 nm (CS03) resulted in the shift of TB from 145 K to 160 K which can be correlated to a larger magnetic anisotropy energy of the AFM phase.45,46 However, such an increase is rather weak when considering the volume increase of CoO by almost twice (Table 1). It may be explained by the irregular shape and less resolved FiM/AFM interface of CS03.16 In contrast, TB increased markedly up to 230 K for CS02 which is clearly correlated 9 ACS Paragon Plus Environment


to the highest interface coupling energy; in this case, the core size reduction from CS01 to CS02 predominates over the thin CoO shell. A step further, the thickest average CoO shell (about 2.5 nm), CS04 displays the smallest TB (130 K) which can be explained by the discontinuous CoO shell which induced a smaller interface and weaker exchange bias.16

Table 3. Effective magnetic anisotropy energy of Fe3-δO4@CoO nanoparticles (KeffV), magnetic anisotropy energy of the Fe3-δO4 core (KCoreVCore), interface coupling energy (Eint), anisotropy constant of the CoO shell (KCoO)

-3

KCoO (J.m ) KeffV (J) KCoreVCore (J) Eint (J) Ed (J) Ed (K)

CS01 1.60.105 5.0.10-20 2.1.10-20 1.9.10-20 1.6.10-21 110

CS02 3.17.105 7.9.10-20 1.3.10-20 3.5.10-20 0.9.10-21 90

CS03 0.9.105 5.5.10-20 2.1.10-20 2.3.10-20 1.3.10-21 60

CS04 _ 4.5.10-20 1.4.10-20 1.5.10-20 _ _

In exchange biased nanoparticles, the effective magnetic anisotropy energy KeffV is usually reported to mainly consist in KCoOVCoO with KCoO the anisotropy constant and VCoO the shell volume.37 KeffV ≈ KCoOVCoO = 25 KBTB

(2)

Nevertheless, it is worth to note that dipolar energy between F(i)M cores is neglected although it may contribute to the effective magnetic anisotropy energy (KeffV).41,47 The fit of the distribution of blocking temperature curves gave KeffV values from to 5.0 – 7.9 10-20 J which can be assimilated to the magnetic anisotropy energy of the AFM shell. KCoO was calculated from equ. 1 and was found from 0.9 to 3.2 105 J/m3 (Table 3) which is much smaller than values reported for core-shell nanoparticles (1.6 - 3.17 106 J/m3).37,45,48 It is usually admitted that KAFM is highly dependent on structural parameters such as the interface, crystallinity, core size and shell thickness. Nevertheless, the maximum KCoO (4 105 J/m3) that could be reached for CS02 below the Néel temperature (290 K) remains one order of magnitude lower than for core-shell nanoparticles reported in the literature. Such low values can be explained for CS03 and CS04 which display shells composed of several nanograins as we reported earlier.16 Although, such nanograins were not observed for CS01 and CS02, it may arise from some defects in the CoO structure. Therefore, KCoO may be underestimated because CoO shell volumes which are considered may be too large. Indeed, KCoO values from the literature would correspond to CoO grain sizes below 2 nm which correspond to the shell thicknesses. Nevertheless, KCoOVCoO is larger than the interface coupling energy (Eint) and the magnetic anisotropy energy of the FiM core (KeffV) for all nanoparticles, which fulfil the requirements for exchange bias.48,49


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M a g ne ti za t io n (M / M s) M a g ne ti za t io n (M / M s) M a g ne ti za t io n (M / M s)

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M a g ne ti za t io n (M / M s)

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1 ,0 0 ,8 0 ,6 0 ,4 C S 01

0 ,2 0 ,0 1 .0 0 .8 0 .6 0 .4 CS 02

0 .2 0 .0 1 .0 0 .8 0 .6 0 .4 C S 03

0 .2 0 .0 1 .0 0 .8 0 .6 0 .4

C S 04

0 .2 0 .0 0

50

100

150

20 0

250

3 00

Tem pe ra tu r e (K )

Figure 5. Temperature dependent magnetization field cooled (FC) and zero field cooled (ZFC) curves recorded for nanoparticle monolayers. a) CS01, b) CS02, c) CS03 and d) CS04. Curves were normalized to the maximum magnetization of FC curves.

Discussion It is well known that the magnetic properties of F(i)M nanoparticles are also directly influenced by dipolar interactions.47,50–52 Dipolar interactions enhance significantly the collective behavior of nanoparticles under a magnetic field which can be often observed through the reduction of HC.53 The dipolar energy also contributes to the enhancement of the effective magnetic anisotropy energy of Fe3-δO4 nanoparticles which can be observed through the increase of TB.47,54,55 The dipolar energy is directly related to the magnetic moment µ of each nanoparticle and to the distance d between their center (equation 3). d includes the separating distance s (edge-to-edge) of 3 nm calculated from SEM micrographs, the shell thickness tCoO and, the Fe3-δO4 core 2r which were measured from TEM micrographs (Table 1). 11 ACS Paragon Plus Environment


Ed = µ2/d3 = (Ms x 4/3 π r3)2/(2r+s)3

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(3)

Therefore, in FiM@AFM nanoparticles, we expect dipolar interactions to modulate the magnetic properties of the Fe3-δO4 cores. In contrast, dipolar interactions between AFM shells can be neglected. On the basis of our previous study on Fe3-δO4 nanoparticles,41,56 we calculated the dipolar energy between FiM cores by using the 2-dipole approximation model (see SI). At first approximation, we assume that nanoparticles in a monolayer are arranged in a triangular lattice57 for which Ed is given by the following equation: Ed = 2.8 x 10-7 µ2/d3

(4)

CS01 displays the highest value of 1.6 10-21 J while the thicker shell of CS03 resulted in a lower dipolar energy of 1.3 10-21 J which decreased down to 0.9 10-21 J for CS02 (Table 3). As expected, a smaller core with reduced magnetization saturation MS (from CS01 to CS02) and a thicker shell with larger intercore distances (from C01 to CS03), both contributes to reduce dipolar energy between FiM cores. It is confirmed by the TB distribution curves (see SI) which is significantly the narrowest for CS02. It is worth to note that the increase of dipolar energy is correlated to the decrease of the effective magnetic anisotropy and of the interface energy. These results agree with theoretical calculations performed on ordered arrays of exchange biased nanoparticles which show the decrease of HC and HE with the increase of dipolar energy.58 Indeed, dipolar interactions contribute to the enhancement of the magnetic anisotropy energy of the Fe3-δO4 core which becomes closer to the magnetic anisotropy energy of the AFM shell. Therefore, dipolar interactions compete exchange bias at the FiM / AFM interface.1,48 It is also well known that the dimensionality of assemblies strongly influences the dipolar interactions between nanoparticles. Several studies on Fe3-δO4 nanoparticle assemblies,50,59 including ours,41,47,55 have shown that dipolar interactions are stronger in a monolayer than in a powder. It results from the two dimensionality which favors the alignment of magnetic moments in the plane of the monolayer. Such an enhancement of shape anisotropy favors collective magnetic behavior which can be observed through the increase of TB and the decrease of HC. Although most of studies on interfacial exchange bias in nanoparticles have been conducted in the powder state, the effect of the dimensionality of their assembly have been poorly investigated. Therefore, we compared the magnetic characteristics of monolayers to powders for the same nanoparticles we have reported earlier to which we added values for CS01 (Table S1).16 Each monolayer exhibits lower values of HC, HE and TB than the corresponding powder, whatever the coreshell structure (Figure 6). These results show that the dimensionality of the assembly influences the magnetic properties of exchange biased nanoparticles. In contrast to FiM nanoparticles, monolayers of FiM@AFM nanoparticles display lower effective magnetic anisotropy energies than powders. The variation of the magnetic characteristics between monolayer and powder samples allow evaluating qualitatively the effect of anisotropic dipolar interactions on exchange bias. Indeed, magnetic characteristics of monolayer and powder should be the closest when dipolar energy decreases.55 Therefore, the variations of HC and HE are the largest for CS01 (215 % and 115 %, respectively) and CS03 (182 % and 155 %, respectively) which display the strongest dipolar energy. In contrast, HC and HE are the closest for CS02 monolayer and powder (16 % and 27 %, respectively) and TMAX are very similar. It confirms that dipolar interactions are the weakest in CS02. Such an effect of 12 ACS Paragon Plus Environment

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anisotropic dipolar interactions on exchange bias is confirmed by the very small variation of HC and HE values (about 15 %) observed for CS04. Indeed, both multilayer and powder display similar local 3D structures and dipolar energy. Furthermore, the distribution of effective magnetic anisotropy energy being directly dependent on dipolar interactions, the narrowest distribution of TB observed for CS02 agree with the weakest dipolar interactions. It is also related to the narrowest size distribution calculated from TEM micrographs. These results show that the enhancement of dipolar energy resulting from shape anisotropy of monolayers alters the effective anisotropy energy and the interface coupling energy. Furthermore, some structural inhomogeneity in monolayers may favor strong dipolar interactions between FiM cores as shown by the fast magnetization reversal at zero field observed in M(H) curves recorded at 5 K. Exchange bias seems to vanish in strongly correlated nanoparticles. Therefore, it also confirms that dipolar energy competes exchange bias at FiM/AFM interface.

b)

30 25

4

20 3 15 2

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280 Blocking temperature (K)

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Coercive field (kOe)

a) Exchange field (kOe)

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260 240 220 200 180 160

CS01

CS02

CS03

CS04

CS01

CS02

CS03

CS04

Figure 6. Magnetic characteristics of CS01, CS02, CS03 and CS04 as a monolayer (closed circles) and as powder (opened circles). a) Exchange field (black) and coercive field (red). b) Blocking temperature.

It is also worth to note that the effect of dipolar interactions on the magnetic properties of exchange biased nanoparticles is usually neglected.46,60 The comparison of the magnetic properties of monolayer and powder of the same nanoparticles allows us probing the effect of dipolar interactions on exchange bias. Here, we show that dipolar energy should be considered above a “critical value”. Indeed, it is two order of magnitude lower than the effective magnetic anisotropy energy and the interface coupling energy for CS02. Therefore, dipolar energy is too low to compete exchange bias and results in the most efficient exchange bias. In contrast, it is only one order of magnitude lower for CS01 which effective magnetic anisotropy energy is low. In contrast to FiM nanoparticles, the effective magnetic anisotropy energy of FiM@AFM nanoparticles can be markedly altered by dipolar interactions. Indeed, dipolar energy contributes to the enhancement of the magnetic anisotropy of the FiM core which competes the interface coupling energy.



Conclusion To summary, we have reported on the preparation of monolayers of Fe3-δO4@CoO nanoparticles by the Langmuir-Blodgett technique in order to study the influence of dipolar interactions on exchange bias. Dense monolayers of tight packed nanoparticles with local order have been obtained in the case of well-shaped spherical nanoparticles. In contrast, the irregular shape of nanoparticles increased the roughness of monolayer and even resulted in a multilayer structure of tight packed nanoparticles. To the best of our knowledge, such an assembling strategy afforded us, for the first time, to study interfacial exchange bias in monolayers of locally ordered FiM@AFM core-shell nanoparticles. Besides, exchange bias is directly dependent on the core–shell structure, our study emphasizes the effect of dipolar interactions by introducing the dimensionality of the assembly. Each monolayer exhibited lower values of HC, HE and TMAX than the nanoparticles in the powder state, whatever the core-shell structure. On one hand, 2D assemblies which promote stronger dipolar interactions than powder samples result in lower exchange bias. On the other hand, the reduction of the core size and the increase of the shell thickness resulted in weak dipolar interactions between FiM cores which enhanced exchange bias. Therefore, we can conclude that even though dipolar energy is lower than effective magnetic anisotropy energy and interface coupling energy, it competes actively exchange bias above a critical value.

Supporting information XRD pattern, HR TEM micrograph and magnetic characterizations for CS01 in the powder state. FTIR spectra and granulometry measurements for each core-shell nanoparticle. Calculation of dipolar energy. Magnetic characteristics of powder samples. Blocking temperature distribution curves calculated for monolayers and powders.


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Experimental section Synthesis of nanoparticles. The synthesis of iron oxide nanoparticles has been performed following the exact procedures reported previously in reference. The core-shell Fe3-δO4@CoO nanoparticles have been synthesized by following the exact procedure we reported previously in reference 16. CS01 and CS03 were synthesized from 9.6 nm sized iron oxide nanoparticles. CS02 and CS04 were synthesized from 8.3 nm sized iron oxide nanoparticles. The CoO shell was grown by adjusting the molar ratio between Fe and Co precursors to 1 for CS01 and CS02, to 2 for CS03 and to 3 for CS04.

Film preparation by the Langmuir Blodgett technique Silicon wafers were soaked at room temperature in a piranha solution (3 : 1 H2SO4:H2O2) for 10 minutes and thoroughly rinsed with H2O and dried before deposition. A small volume (150–200 µl) of a 5 mg.ml-1 nanoparticle suspension in chloroform was spread onto the water subphase of the Langmuir trough (KSV 5000, 576 x 150 mm2) at room temperature. After 10 min stabilization, the surface area was reduced by moving barriers (compression rate of 5mm.min-1), and the surface pressure–area isotherm is recorded during the film compression using a Wilhelmy plate. The pressure is stabilized for 10 min at 30 mN m-1. Then, it was transferred onto a silicon wafer by the concomitant pulling out of the substrate from the air/water interface at a rate of 1 mm.min-1 and by moving barriers with the aim to maintain the surface pressure at 30 mN.M-1.

Characterization techniques Nanoparticles were characterized by transmission electron microscopy TEM with a TOPCON model 002B transmission electron microscope operating at 200 kV. The size distribution was calculated from the size measurement of more than 200 nanoparticles. Granulometry measurements were performed on the suspension of nanoparticles in chloroform using a nano-size MALVERN (nano ZS) apparatus. All the transferred films were also characterized by Scanning Electron Microscopy (SEM) by using a JEOL 6700F equipped with a field emission gun (FEG) and operating at 3 kV. The image analysis was performed using Digital Micrograph (Gatan) and Image J software. Atomic Force Microscopy (AFM) was performed using a Veeco Multimode Nanoscope V (Brucker) to characterize the topography of the core shell nanoparticle films. The images were obtained in contact mode in dry condition with silicon nitride cantilever with a spring constant of 0.6 N/m (model MSCT-AUHW, Veeco, CA). Deflection and height mode images were scanned simultaneously at a fixed scan rate (1 Hz) with a resolution of 512 × 512 pixels. The average roughness of the deposited films, corresponding to the root mean square values given by the Nanoscope software, was determined



from 5 different areas of 1 × 1 µm2. Magnetic measurements were recorded on samples in the powder state by using a Superconducting Quantum Interference Device (SQUID) magnetometer (Quantum Design MPMS-XL 5). The measurements were performed by applying the magnetic field in the direction defined by the plane of the substrate. Zero-field cooled (ZFC) and field cooled (FC) magnetization curves as a function of the temperature were recorded as follows: the sample was introduced at room temperature and cooled down to 5 K with no applied field after applying a careful degaussing procedure. A magnetic field of 7.5 mT was applied, and the ZFC magnetization curve was recorded upon heating from 5 to 300 K. The sample was then cooled down to 5 K under the same applied field, and the FC magnetization curve was recorded upon heating from 5 to 300 K. The magnetization was then measured at constant temperature by sweeping the magnetic field from +7 T to −7 T, and then from −7 T to +7 T. To evidence exchange bias eﬀect, FC M(H) curves have been further recorded after heating up at 400 K and cooling down to 5 K under a magnetic field of 7 T. The FC hysteresis loop was then measured by applying the same field sweep as for the ZFC curve. The coercive field (HC) and the MR/MS ratio were measured from ZFC M(H) curves. The exchange bias field (HE) was measured from FC M(H) curves.


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Investigation of the Collective Properties in Monolayers of Exchange

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