Exchange bias in ferromagnetic nanoparticles embedded in an antiferromagnetic matrix

Int. J. Nanotechnology, Vol. 2, Nos. 1/2, 2005 23

Copyright © 2005 Inderscience Enterprises Ltd.

Exchange bias in ferromagnetic nanoparticles embedded in an antiferromagnetic matrix

J. Nogués* Institució Catalana de Recerca i Estudis Avançats (ICREA), Departament de Física, Universitat Autònoma de Barcelona, Bellaterra 08193, Spain Fax: +34 935811350 E-mail: [email protected] *Corresponding author

J. Sort SPINTEC, Unité de Recherche Associée CEA/CNRS, DRFMC, CEA/Grenoble, 38054 Grenoble Cedex 9, France

Departament de Física, Universitat Autònoma de Barcelona, Bellaterra 08193, Spain Fax: + 34 935812155 E-mail: [email protected]

V. Langlais and S. Doppiu Departament de Física, Universitat Autònoma de Barcelona, Bellaterra 08193, Spain Fax: + 34 935812155 E-mail: [email protected] Fax: + 39 079 212069 E-mail: [email protected]

B. Dieny SPINTEC, Unité de Recherche Associée CEA/CNRS, DRFMC, CEA/Grenoble, 38054 Grenoble Cedex 9, France Fax: + 33 4 38785097 E-mail: [email protected]

J.S. Muñoz, S. Suriñach and M.D. Baró Departament de Física, Universitat Autònoma de Barcelona, Bellaterra 08193, Spain Fax: + 34 935811350 Fax: + 34 935812155 E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]

24 J. Nogués et al.

S. Stoyanov and Y. Zhang Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA Fax: +1 302 8311637 E-mail: [email protected] E-mail: [email protected]

Abstract: The fabrication process and magnetic properties of three types of system consisting of ferromagnetic (FM) particles embedded in an antiferromagnetic (AFM) matrix are discussed. The preparation techniques are ball milling, H2 partial reduction of oxides and nanoparticle gas condensation. The magnetic properties of the FM/AFM composites are shown to depend strongly on the morphology of the system (e.g., nanoparticle size), the AFM anisotropy and the AFM-FM coupling. For example, all the studied systems exhibit coercivity enhancement below the Néel temperature of the AFM. However, while Co nanoparticles embedded in CoO exhibit loop shifts of thousands of Oe, Fe nanoparticles in Cr2O3 only show a few Oe shifts. An interesting effect evidenced in all systems is the increase of remanence (MR) which, in the case of Co-CoO, ultimately leads to an improvement of the superparamagnetic blocking temperature of the nanoparticles.

Keywords: exchage bias; coercivity; squareness; ferromagnetic nanoparticles; antiferromagnets.

Reference to this paper should be made as follows: Nogués, J., Sort, J., Langlais, V., Doppiu, S., Dieny, B., Muñoz, J.S., Suriñach, S., Baró, M.D., Stoyanov, S. and Zhang, Y. (2005) ‘Exchange bias in ferromagnetic nanoparticles embedded in an antiferromagnetic matrix’, Int. J. Nanotechnology, Vol. 2, Nos. 1/2, pp.23–42.

Biographical notes: J. Nogués received his BSc in physics from the Universitat Autònoma de Barcelona in 1986. He obtained his PhD in physics from Royal Institute of Technology (Stockholm, Sweden) in 1993. He later spent three years at the University of California San Diego as a post-doctoral fellow. Since 2001, he has been a research professor of the Institució Catalana de Recerca i Estudis Avançats (ICREA) at the Universitat Autònoma de Barcelona.

J. Sort received his BSc in physics from the Universitat Autònoma de Barcelona in 1998. He obtained his PhD at the same University in 2002. Since then, he has been working as a postdoctoral fellow in SPINTEC (CEA-Grenoble). The research he is carrying out is mainly focused on the effects of exchange interactions between different types of magnetic material, particularly on the nanometre scale.

V. Langlais received her BSc in physics from the Université Aix-Marseille II in 1992. She completed her European PhD in 1996 for her work on ‘Electronic properties of fullerenes hetero-epitaxially grown on metal surfaces’ from the same university. Later, she spent two years at IBM research lab (Zurich/Switzerland) and four years at Laboratoire de Cristallographie (Grenoble/France). Since 2003, she has been a research member at the Universitat Autònoma de Barcelona.

Exchange bias in ferromagnetic nanoparticles 25

S. Doppiu received her Graduation Diploma (Laurea) in chemistry from the Department of Chemistry of the University of Sassari (Italy) in 1997. In 1998 she got a graduate fellowship at the National Consortium of Materials Science and Technology (INSTM, Italy). She obtained her PhD in chemistry from the University of Siena (Italy) in 2002. She later spent two years at the Universitat Autònoma de Barcelona as a post-doctoral fellow.

B. Dieny received his PhD in physics in 1985 at University of Grenoble. He has been working as a permanent researcher at CNRS since 1988 then at CEA since1992. In 1990–1991, he spent one and half year as a visiting scientist at IBM Almaden Research Center being primary inventor of spin-valves. In 1996, he stayed one year at the Center of Magnetic Recording Research in San Diego. In 2001, together with Dr J.P. Nozieres, he founded a new laboratory SPINTEC, specialising in spin-electronics, of which he is now deputy head.

J.S. Muñoz graduated from the University of Valencia (Spain). He later carried out research in superconductivity during four year at the University of Oxford where he got his PhD in 1968. From 1968 to 1970 he worked at the Spanish National Council for Research (CSIC). In 1975 he obtained a chair in Physics at the University Autònoma of Barcelona where is still teaching magnetism.

S. Suriñach received his BSc (1977) and PhD (1983) in physics from the Universitat Autònoma de Barcelona. He is professor of Materials Science of the Department of Physics at the UAB. He has published more than 140 scientific papers and edited two books.

M.D. Baró received her BSc (1971) and PhD (1975) in physics from the Universtitat Autònoma de Barcelona. She is professor of Applied Physics of the Department of Physics at the UAB, and head of the Materials Physics II Group. She has wide experience acting as coordinator or partner group leader in European and National projects. She has published more than 170 scientific papers and edited four books. She is a member of different national and international scientific associations, steering committees and editorial boards.

S. Stoyanov received his MSc in physics from the Sofia University (Bulgaria) in 1970. He obtained his PhD in physics (Solid State Optics) from the University of Kiev (Ukraine) in 1978. He has been assistant and associate professor at the University of Sofia for 22 years. Since 2001, he has been an associate scientist at the University of Delaware, first two years with the Department of Physics and Astronomy and later with the Bartol Research Institute.

Y. Zhang received his BSc in materials science and engineering from Tianjin University (Tianjin, China) in 1986. He got his MSc in materials science and engineering from Xi’an Jiaotong University (Xi’an, China) in 1992. He obtained his PhD in materials science and engineering from the Institute of Aeronautical Materials (Beijing, China) in 1996. Since 1997, he has been an electron microscopist/research associate in the Department of Physics and Astronomy at the University of Delaware, Newark, Delaware, USA.


1 Introduction

Today’s interest in nanoparticle magnetism is stimulated by a variety of potential applications, ranging from soft to hard magnetic materials and from ultra-high density information storage to biomedical applications [1–9]. Interactions between magnetic systems at the nanoscale give rise to novel effects, such as ultrasoft magnetic alloys [10,11] or hard magnets with improved energy product, i.e., spring magnets [12,13]. A typical system where this type of interactions are present is ferromagnetic nanoparticles embedded in a matrix. The matrix introduces an extra degree of freedom which allows tailoring the properties of the nanoparticles or vice versa. Three examples of property improvement by particle-matrix interplay are:

• giant magnetoresistance in granular systems where the effects can by tuned by the density of ferromagnetic nanoparticles in the matrix and the electronic nature of the matrix (metallic or insulator) [14–16]

• enhancement of the effective magnetic moment by local spin flop of a metamagnetic matrix [17]

• improving the superparamagnetic blocking temperature in ferromagnetic nanoparticles by coupling them to an antiferromagnetic matrix, i.e., exchange bias [18].

Exchange bias refers to the shift of the hysteresis loop along the magnetic field axis direction observed in systems comprised of antiferromagnetic (AFM) and ferromagnetic (FM) components. In order to observe exchange bias, usually the Curie temperature of the FM, TC, should be higher that the Néel temperature of the AFM, TN. Then the system needs to be cooled in the presence of a magnetic field from a temperature above TN, but below TC. The loop shift originates as a result of exchange interactions between a FM and an AFM material [19]. This loop shift is often accompanied by an enhancement of the coercivity and other unusual effects such as high field rotational hysteresis [20–23].

Exchange bias was first observed in fine particle systems. Meiklejohn and Bean found that field cooled partially oxidised Co particles exhibited hysteresis loops displaced along the field axis [19]. They attributed this phenomenon to the exchange interaction at the interface between the FM Co core and the AFM CoO shell [24]. Although there has been some research effort in exchange bias in nanoparticles in the last decades, the bulk of exchange bias research has been focused mainly on thin film systems [20–23], due, in part, to the greater control of the AFM-FM interface. However, it has been the crucial role of exchange bias in spin valve and tunnelling devices, by allowing to easily set a reference FM layer, that has generated the increasing interest in AFM – FM thin film systems. Recently, the study of exchange bias effects in fine-particle systems is recovering importance, since it has been shown that FM-AFM exchange interactions can be useful to improve the hard magnetic properties of permanent magnets [25] or to combat the superparamagnetic limit in magnetic recording media [18].

Traditionally, exchange bias in nanoparticles has been focused on chemical treatments of FM particles to obtain reacted AFM phases, creating a core (FM) – shell (AFM) structure. Consequently, the number of fine-particles systems in which exchange bias could be observed was rather restricted, since the AFM had to be obtained from a simple chemical reaction of the FM core. Thus, basically, only transition metal particles


embedded in their AFM native oxides (e.g., Co-CoO [24,26,27], Ni-NiO [28,29] or Fe-FeO [30]), oxide nanoparticles with different oxidation states (e.g., CrO2 (FM) – Cr2O3 (AFM) [31]) or transition metal nanoparticles surrounded by chemically obtained sulphides or nitrides (e.g., Fe-FeS [32] and Co-CoN [33]), could be studied. Moreover, it is generally not possible to obtain AFM phases by chemically treating many interesting FM materials, e.g., hard magnetic materials. Most of this type of systems exhibit exchange bias effects far below room temperature [24,26–33], making them unattractive for applications. It is thus interesting to study magnetic nanoparticles embedded in antiferromagnetic matrices where the properties of each component can be to some extent independently controlled.

In this paper, we review the exchange coupling properties of FM-AFM (particle – matrix) systems obtained by different techniques: ball milling, H2 reduction of oxides and gas condensation nanoparticle deposition. We show that the magnetic properties of the composites depend strongly on the microstructure created during the synthesis and the magnetic properties of the particle and matrix constituents.

2 Ball milling of AFM + FM materials

Ball milling consists in grinding one or more powder materials to either reduce their size or to mix or alloy them. In ball milling the microstructure obtained by mixing different materials depends on the mechanical properties of the constituents, e.g., ductility [34]. For example, during the milling of ductile-ductile systems ‘sandwich’-type microstructures can be developed by controlling the milling parameters. Such microstructures would be favourable to obtain a large FM – AFM interface area. However, alloying between the different components can actually occur, which could prevent the observation of exchange bias phenomena. In the case of brittle-ductile components, during the milling the ductile powders acquire laminar shapes, while the brittle counterparts are simply fractured. Consequently, the ductile lamellae intermix with the brittle matrix, forming agglomerates. An example of such microstructure for Co (ductile) – NiO (brittle) can be observed in Figure 1. Finally, in brittle-brittle systems heterogeneous agglomerates, with granular morphology, are usually formed, where the two or more counterparts are hardly welded together. Hence, useful microstructures can be difficult to form.

Figure 1 Scanning electron micrograph of a Co(1) : (1) NiO powder ball milled during 20 hours. The bright regions correspond to Co, while the gray areas are NiO. The black areas correspond to the resin used to mount the powders


In this study we discuss several cases of ductile – brittle systems (FM – AFM) (Co-NiO, Co-FeS, SmCo5-NiO, SmCo5-CoO) and one case of brittle-brittle materials (ferrimagnetic-AFM) (Strontium ferrite-FeS).

Ball milling of the FM-AFM powders was carried out for different times (0.1 hour–30 hours) using a planetary mill in agate (or stainless steel) vials and agate (or stainless steel) balls. The vials were previously sealed under argon atmosphere to prevent oxidation. The initial materials were high purity powders of the AFM and FM constituents mixed in different weight ratios, although most studies were carried out in a 1 : 1 ratio. Milling of the pure FM materials, under the same conditions, were also carried out for comparison.

2.1 Co : NiO and Co : FeS

In these systems the as-milled powders were heated to above the Néel temperature of the respective AFMs (TN (NiO) = 525 K and TN (FeS) = 610 K, respectively) and subsequently field cooled to room temperature in HFC = 5 kOe, to induce exchange coupling.

The hysteresis loops of the as-milled and field cooled materials were measured using a vibrating sample magnetometer in a maximum applied field of 11 kOe.

Both 1 : 1 Co : NiO and Co : FeS systems exhibit loop shifts, HE, of a few tens of Oe at room temperature after field cooling, indicating the presence of exchange bias coupling. Accompanying the loop shifts a coercivity enhancement with respect to the as-milled material, ∆HC, is also observed (see Figure 2).

Figure 2 Hysteresis loop before (empty symbols) and after (filled symbols) a field cooling procedure (HFC = 5 kOe) for Co : NiO 1 : 1 milled for 20 hours The lines are guides to the eye

The values of HE and ∆HC depend strongly on the milling conditions. For example, for Co : NiO both quantities exhibit a maximum with the milling time [35–38] (see Figure 3). This maximum can be understood in terms of the two main effects of the milling. First, a minimum milling energy is needed to induce a favourable microstructure, i.e., with AFM-FM interfaces. However, if the system is overmilled, then severe damage is created in the powder, which consequently deteriorates the magnetic properties.


Similarly, a maximum is also observed for different AFM : FM powder ratios [37,38] (see Figure 3). Namely, there exists an optimum FM : AFM ratio to induce exchange bias effects. This is because of both structural and magnetic reasons. The amount of ductile FM with respect to brittle AFM determines the possibility to create optimised microstructures. This can be inferred from the fact that each FM : AFM ratio needs a different milling time to obtain maximum HE and ∆HC [35–37]. Moreover, it is well known in thin films that exchange bias properties depend strongly on the FM and AFM thickness [20]. In the nanoparticle-matrix case this would be equivalent, in first approximation, to the nanoparticle diameter and inter-particle distance. Obviously, these two parameters change with the FM : AFM ratio, giving rise to complex variation of HE and ∆HC with the milling parameters.

Figure 3 Dependence of the loop shift, HE (empty symbols) and coercivity enhancement, ∆HC (filled symbols), on the milling time for NiO : Co at different ratios: (a) 3 : 2, (b) 1 : 1, (c) 2 : 3 and (d) 3 : 7. The lines are guides to the eye

Another interesting property of Co : NiO and Co : FeS is an enhancement of the squareness, i.e., the remanent magnetisation, MR to saturation magnetisation, MS, ratio MR/MS [35] (see also inset in Figure 2). This effect is usually not observed in thin films, mainly because in that case the loops, due to shape anisotropy, are already rather square. In nanostructured systems the loops tend to be more sheared, thus this effect becomes evident.


When the samples are warmed, the induced HE and ∆HC vanish at the so called exchange bias blocking temperature, TB (temperature at which the exchange bias effects go to zero [20]), which is equal to the Néel temperature of NiO, TN (see Figure 4). This indicates that the milling does not substantially deteriorate the antiferromagnetic properties of NiO.

Similar exchange bias related results have been reported for other AFM : FM ball milled systems. For example, FeNi : CoO also exhibits loop shifts and coercivity enhancements which again depend markedly on the milling conditions, although in this case TB < TN [39]. In the fully metallic Fe : FeMn system effects analogous to the ones described for Co : NiO have also been reported [40].

Figure 4 Temperature dependence of the (a) loop shift, HE and (b) coercivity enhancement, ∆HC, for a Co : NiO 1 : 1 sample. The lines are guides to the eye

2.2 SmCo5 : NiO and SmCo5 : CoO

In the case SmCo5 : NiO, since SmCo5 easily decomposes upon heating, it was not possible to field cool from above the Néel temperature of NiO. However, the room temperature hysteresis loops, carried out in an extracting magnetometer in fields up to 230 kOe, show a coercivity enhancement when compared to SmCo5 milled alone and SmCo5 : CoO [25,41–45]. Note that CoO is paramagnetic at room temperature. Coercivity enhancement is evident already from very short milling times [25,41] and persists for all milling times. Moreover, different AFM-FM powder ratios for SmCo5 : NiO and SmCo5 : CoO were studied. It was found that the coercivity enhancement of SmCo5 : NiO, with respect to SmCo5 : CoO and SmCo5, mainly persisted for all AFM : FM ratios although it was more pronounced for low SmCo5 contents (see Figure 5).


Figure 5 Dependence of the coercivity, HC, and squareness ratio, MR/MS, on the SmCo5 percentage for SmCo5 : NiO (filled symbols) and SmCo5 : CoO (empty symbols) milled for 16 hours. The lines are guides to the eye

Similarly to Co : NiO, SmCo5 : NiO exhibits an important enhancement of the squareness, when compared to SmCo5 : CoO and SmCo5. The squareness enhancement in SmCo5 : NiO actually persists for all milling times and FM : AFM ratios (see Figure 5) [44], similar to what is observed for HC in this system.

The simultaneous enhancement of HC and the squareness in SmCo5 : NiO can be used to improve the magnetic energy product, (BH)Max, of the system. The energy product of a hard magnet gives an indication of the magnetic energy that can be stored in the system and is actually the figure of merit of permanent magnets [25]. However, it has to be taken into account that although AFM : FM coupling results in HC and the MR/MS improvement, the presence of the AFM reduces the overall magnetisation, which is a key factor in energy product increase. Thus, in order to improve (BH)Max an optimisation of the milling time and AFM : FM ratio is necessary [25].

The origin of the described effects is more complex than in the Co : NiO case, since for SmCo5 : NiO no field cooling from above TN has been carried out to induce exchange bias coupling. Actually, the origin is two-fold. First the presence of NiO tends to isolate the FM nanoparticles, reducing the dipolar and exchange interactions between them, and hence improving HC. This effect is also present in SmCo5 : CoO, thus the observed differences between the systems based on NiO (antiferromagnetic at room temperature) and CoO (paramagnetic at room temperature) stem from AFM-FM coupling. This exchange coupling between SmCo5 and NiO is a result of a local heating (due to the impacts between the powder and the balls) during the milling and a local magnetic field exerted by the dipolar field of FM particles on the AFM particles, which, to some extent, can be thought of as a local field cooling procedure [25,41–45].


Note that the as-milled systems do not exhibit loop shifts at room temperature. This is expected due to the random character of the local dipolar fields of the SmCo5 particles during the confined milling induced heating, which will average out any shifted loop. However, indication of unidirectional anisotropy (closely related to exchange bias [20]) has been observed using a vectorial magnetometer [25]. Further evidence of the role of exchange coupling comes from the measurements of SmCo5 : CoO at low temperatures (i.e., below TN(CoO)). In this case a coercivity enhancement, with respect to SmCo5, is observed in SmCo5:CoO after field cooling from room temperature [25].

It is noteworthy that due to the complex mechanisms involved in ball milling it can be considered that milling SmCo5 : NiO and SmCo5 : CoO may not be equivalent. For example, it has been demonstrated that the effects of milling SmCo5 alone and SmCo5 : NiO are rather different [41]. Subtle differences in microstructures and defects generated in SmCo5 : NiO and SmCo5 : CoO could result in differences in the magnetic properties of SmCo5, such as the anisotropy. Consequently, a pure microstructural origin of the described effects can not be ruled out.

The results for SmCo5 : NiO are quite unique and have not been observed in other systems, although coercivity enhancement related to exchange bias in hard magnetic bulk metallic glasses has also been reported [45,46].

2.3 Sr-ferrite : FeS

After ball milling Sr-ferrite with FeS, Sr-ferrite loses its hard magnetic properties. Annealing at high temperatures is necessary in order to recover them. This annealing procedure induces the transformation of FeS into α-Fe2O3, which is also an antiferromagnet [47]. Actually, similar procedures have been used to obtain exchange biased related properties in other ball milled systems. Note that many of these systems involve ferrimagnets (Ferri-) either playing the role of the AFM or the FM [20]. For example, milling of Fe + α-Fe2O3 can result in FeO1–x, which after annealing becomes Fe (FM) + Fe3O4 (Ferri-) [48]. Ni + α-Fe2O3 results in NiFe (FM) + Ni-ferrite (Ferri-) after annealing [49]. Hence, in these cases the processes to induce exchange bias properties are different from the two other examples discussed above. Namely, during the milling or the posterior annealing of the sample one or both of the original constituents undergo one or more crystallographic phase transitions.

The Sr-ferrite : FeS → Sr-ferrite : α-Fe2O3 system exhibits a coercivity improvement with respect to Sr-ferrite treated in the same conditions. The coercivity enhancement is larger for higher amounts of starting FeS, which result in more α-Fe2O3. However, the overall hard properties of this material are somewhat deteriorated due to the severe loss of saturation magnetisation provoked by the appearance of the AFM [47]. No loop shifts were observed in this system. This could be ascribed to the small anisotropy of α-Fe2O3, which is known to usually induce no bias [20].

Finally note that in this case, due to the chemical transformation of the components, the control of the properties becomes increasingly complex.


3 Controlled H2 reduction of oxides

It is well known that transition metal oxides can be reduced to pure transition metals by high temperature annealing in the presence of H2, e.g., NiO + H2 → Ni + H2O. In principle, if the reduction reaction is carried out controllably only partial reduction would take place, i.e., 3NiO + 2H2 → 2Ni + NiO + 2H2O. If the oxide is an AFM (e.g., NiO) while the metal is a FM (e.g., Ni), then the early stages of this controlled reduction (i.e., when only a small amount of metal has been formed) could lead to the desired FM nanoparticles embedded in a AFM matrix microstructure. Actually, several different procedures can be devised to control the reduction reaction.

In this section we discuss two procedures for controlled reduction leading to Fe–Cr2O3 and Ni-NiO FM-AFM nanocomposites: high temperature reduction of mixed oxides and reduction by reactive ball milling.

3.1 High temperature reduction of mixed oxides: Fe–Cr2O3

A binary oxide solution of composition Cr1.8Fe0.2O3 was prepared by thermal decomposition and air calcination of mixed oxalate precursors. The calcinations, carried out at 1100°C, allowed the crystallisation of the solid solution. Subsequently, the reduction of the solid solution was performed in pure dry hydrogen at 1200°C for two hours, giving rise to Fe nanoparticles embedded in Cr2O3 matrix nanocomposite. A typical microstructure of the reduced oxide can be seen in Figure 6. The Fe nanoparticles exhibit grain sizes of about 8 nm with a rather narrow size distribution (σlog-norm = 1.4 nm) [50]. This selective high temperature reduction is favoured by the higher enthalpy of formation of Cr2O3 compared to Fe oxides (∆G(Cr2O3) ~ –745 kJ/mol O2 < ∆G(Fe2O3) ~ –545 kJ/mol O2 [51]). Thus, Fe tends to precipitate before Cr during the reduction.

Figure 6 Bright field transmission electron micrograph of a Fe : Cr2O3 nanocomposite. The dark regions correspond to metallic Fe, while the brighter areas are Cr2O3

To induce AFM : FM coupling, the as-prepared powders were field cooled in the presence of a magnetic field of variable strength (0 < HFC < 50 kOe) from above the Néel temperature of Cr2O3, TN = 307 K, to T = 10 K. Magnetic hysteresis loops were subsequently recorded at increasing temperatures from 10 K to 350 K by means of a vibrating sample magnetometer.


As can be seen in Figure 7 the loop shift induced after the field cooling is rather small. The low temperature coercivity after field cooling (HC (10 K) = 185 Oe) is found to be larger that HC above TN (HC (310 K) = 110 Oe). The values of HE and HC are independent of the cooling field for fields large enough to ensure the saturation of the FM nanoparticles during the cooling [50]. As the temperature is increased HE remains fairly constant until 200 K, smoothly decreasing for higher temperatures. Finally, HE vanishes at T = 307 K, i.e., the Néel temperature of Cr2O3. Upon heating, the coercivity decreases almost linearly from low temperatures up to TN [50].

Figure 7 Hysteresis loop of a Fe : Cr2O3 nanocomposite at 10 K after field cooling in HFC = 50 kOe. The inset is an enlargment of the central part of the loop

The small loop shift and moderate coercivity enhancement can be understood from the small anisotropy of Cr2O3. In this case it is expected that the AFM spins will mostly rotate with the FM spins during the magnetisation reversal, thus only small loop shifts should be observed [20].

The increase of HC when reducing temperature could be due to an increase of the anisotropy of the FM nanoparticles. However, when comparing the zero field cooled and field cooled hysteresis loops, the latter one exhibits an increased coercivity. In addition, a clear remanence enhancement is observed in the field cooled case. Thus, FM-AFM coupling is probably also responsible for the HC enhancement.

Cr2O3 : Fe is the first AFM-FM system obtained from high temperature reduction of mixed oxides, although the reduction of other non-stoichoimetric oxides can also result in related microstructures, involving mixtures of ferrimagnetic-AFM or ferrimagnetic-FM materials, e.g., reduction of Fe-rich Ba-ferrite → Fe + Ba-ferrite, Fe-rich NiZn-ferrite → NiZn-ferrite + Fe or α-Fe2O3 → Fe + Fe3O4 [52–55].

3.2 Reduction by reactive ball milling: Ni : NiO

High purity NiO powders were ball milled at room temperature under H2 (6 bar) atmosphere. A stainless steel vial, modified to allow milling under different gas atmospheres, together with stainless steel balls were employed. The milling intensity was


tuned either by changing the milling time for a fixed ball to powder mass ratio (BPR) or by varying the BPR, from 3 : 1 to 18 : 1 for a given time, 8 hours. Upon milling, a partial reduction of the starting NiO powders occurs, which results in the formation of Ni-NiO nanocomposites (see Figure 8). However, in contrast to what is observed in the Fe : Cr2O3 case, the grain size distribution of the FM nanoparticles is rather broad, ranging from 5 nm to 25 nm [56]. Nevertheless, the amount of metallic Ni created during the reduction process can be controlled and it depends almost linearly on the milling energy [56].

Figure 8 Bright field transmission electron micrograph of a NiO powder partially reduced in H2 for 8 hours in a ball to powder ratio of 18 : 1. The dark regions correspond to metallic Ni, while the brighter areas are NiO

The hysteresis loops were measured using a vibrating sample magnetometer after field cooling, in HFC = 6 kOe, the as-milled powders from 600 K, i.e., above TN (NiO) = 525 K, to room temperature.

Figure 9 shows the evolution of the loop shift (HE) and coercivity enhancement (∆HC, i.e., the difference of coercivity between after and before field cooling) for different milling times. The observed exchange bias effects are somewhat small, HE = 10 Oe and ∆HC = 30 Oe, although they are expected for systems containing a low anisotropy AFM such as NiO [20]. Interestingly, opposite trends are observed for HE and ∆HC. Namely, while HE decreases with milling time, ∆HC increases [57]. This effect is probably related to the progressive creation of Ni during the reactive milling. For short milling times only a reduced number of small FM nanoparticles is created. As the milling proceeds, the Ni nanoparticles grow in size and new nanoparticles are formed. Since in nanoparticle systems HE is assumed to be inversely proportional to the particle size [20], the reduction of the loop shift is, to a certain extent, expected for longer milling times. The increase of ∆HC probably arises from a combination of several effects. For example, as the milling proceeds more defects are induced in NiO, e.g., crystallite size reduction or non-stoichiometry and this probably favours partial dragging of the AFM spins during magnetisation reversal of the FM, hence increasing HC [57]. Simultaneously, the Ni nanoparticles increase their coercivity for longer milling times. This fact could also influence the coercivity enhancement, if, for example, the coercivity enhancement depended on the FM anisotropy.


Figure 9 Milling time dependence of exchange bias, HE, and coercivity enhancement, ∆HC, for a NiO powder milled with a ball to powder ratio of 12 : 1

In this system, although the loop shift and coercivity enhancement change markedly with the milling time, the induced remanence enhancement is rather small. Finally, when measuring at high temperatures, HE and ∆HC vanish at temperatures lower than TN (NiO). This agrees with the arguments that reactive milling may be inducing more structural damage to NiO than conventional AFM + FM milling.

To our knowledge, Ni-NiO is the first AFM-FM system synthesised by reduction via reactive ball milling.

4 Cluster gun deposition combined with conventional sputtering

It is well known that evaporating or sputtering materials at high pressures leads to the formation of nanoparticles [1,2,6]. This technique allows for the control of the particle size via, for example, the pressure in the deposition chamber. A cluster gun has been implemented inside conventional sputtering apparatus, allowing the combination, in situ, of thin film and nanoparticle deposition (see Figure 10).

In this section we describe two systems which were grown, in slightly different conditions, using this method: Co-CoO and SmCo-CoO.

Figure 10 Schematic representation of the sputtering chamber incorporating a cluster gun


4.1 Co-CoO

The samples were grown by sequentially depositing a layer of 10 nm of antiferromagnetic matrix, CoO, followed by a layer of nanoparticles. This process was repeated 10–20 times. The base pressure inside the sputtering chamber before deposition was better than <5 × 10–7 Torr. The matrix layers are grown by standard reactive sputtering in a 7 : 1 ratio Ar : O2 mixture on a water cooled, thermally oxidised, Si(111) substrate. The pressure during deposition was about 4 mTorr. The nanoparticles were formed inside the cluster gun (see Figure 10) by a gas condensation process from dc magnetron sputtered Co atoms in high pressure argon gas plasma (200–300 mTorr). The yield of the cluster gun was maintained low to obtain a rather disperse spatial distribution of nanoparticles, as can be seen in Figure 11. It is noteworthy that during the deposition of the nanoparticles the oxygen partial pressure in the main chamber was maintained. Consequently, the Co nanoparticles were partially oxidised, forming a Co-CoO core-shell structure. Transmission electron microscopy analysis of the particles revealed a core and a shell size of about dCore = 4 nm and tShell = 1 nm, respectively [18].

For comparison, samples were also grown, using non-magnetic Al2O3 as matrix following the same procedure.

Figure 11 Transmission electron micrograph of a Co-CoO (core-shell) nanoparticles embedded in a Al2O3 matrix. In the high magnification image (right), the core-shell structure can be clearly observed

The hysteresis loops of the system were taken in warming runs after field cooling HFC = 50 kOe, from room temperature to 5 K.

As can be seen in Figure 12, at low temperatures the nanoparticles embedded in the antiferromagnetic CoO matrix exhibit a large coercivity, HC = 7.6 kOe, and loop shift, HE = 7.4 kOe, while the ones deposited in the non-magnetic Al2O3 matrix have small coercivity and very tiny exchange bias (HC = 0.2 kOe and HE = 0.05 kOe). As the temperature is increased the nanoparticles deposited in Al2O3 quickly become superparamagnetic. Namely, their coercivity and remanence become zero at the superparamagnetic blocking temperature, TB,S = 10 K. However, the nanoparticles embedded in CoO maintain their coercivity and loop shift up to 290 K, i.e., the Néel temperature of CoO. Similarly, the remanence of the loops remains non zero up to 290 K. This implies that the Co nanoparticles remain ferromagnetic up to the Néel temperature of CoO [18]. Hence, the presence of an AFM matrix improves the superparamagnetic blocking temperature of the nanoparticles by 280 K. In other words, the AFM matrix induces an extra anisotropy to the Co nanoparticles, which enhances their thermal stability.


Figure 12 Hysteresis loop of Co : CoO (dashed line) and Co : Al2O3 (continuous line) at 5 K after field cooling from room temperature in HFC = 50 kOe

Remarkably, the exchange bias in this case is much larger than the ones described previously. This can be attributed to two effects: the large anisotropy of CoO (compared to NiO or Cr2O3) and the good coupling between the Co core and the CoO matrix through the CoO shell.

These effects have been proposed as a possible route to improve the superparamagnetic limit of magnetic recording [18]. Similar results, also in Co : CoO, have been recently reported by other groups [58].

4.2 SmCo : CoO

In this case the deposition route was basically the same as for Co : CoO. Two main differences are worth highlighting. First, the O2 was pumped out of the main chamber before each SmCo nanoparticle deposition. Hence, no surface oxidation of the nanoparticles took place. Second, the nanoparticles were heated during the time of their flight from the cluster gun to the substrate. The heating of the particles was performed by absorption of the radiation of four quartz halogen projector bulbs ~360 W each. This heating promotes the crystallisation of the hard SmCo phase [59]. Note that the heating was switched off during the matrix evaporation.

In this case the reference, non-magnetic, matrix material was C. The hysteresis loops of the system were recorded after field cooling (HFC = 30 kOe)

from room temperature to 5 K. The difference between the nanoparticles deposited in the antiferromagnetic CoO and

the non-magnetic C is not as clear as in the previous example. Nevertheless, the SmCo : CoO system exhibits a larger coercivity than the SmCo : C. Moreover, SmCo : CoO exhibits a sizable loop shift, HE = 0.6 kOe, as expected for an AFM with a large anisotropy [20]. Similarly to what is observed in Co : CoO, SmCo : CoO also shows a clear remanence enhancement [59].

The main difference between Co : CoO and SmCo : CoO is the lack of the AFM shell in the latter case. This probably implies a worse coupling between the matrix and the nanoparticles. Moreover, SmCo has a much stronger anisotropy than Co, hence even within a non-magnetic matrix the coercivity is considerably large, indicating that the SmCo nanoparticles preserve a FM ordering at low temperatures. Consequently, the large effects observed in Co : CoO which arose from low superparamagnetic blocking


temperature of the Co particles in the non-magnetic matrix, become less obvious in the SmCo case.

5 Conclusions

In conclusion exchange coupling effects in three different types of systems consisting of FM nanoparticles embedded in an AFM matrix have been presented: ball milling AFM and FM materials, controlled H2 reduction of oxides and nanoparticle gas condensation in conjunction with conventional sputtering. Although the detailed microstructures rendered by the diverse techniques are different, they all exhibit exchange bias related properties, i.e., loop shifts and/or coercivity and remanence enhancements. The properties are closely related to the evolution of the microstructure with the processing parameters. Moreover, systems with small AFM anisotropy tend to have smaller loop shifts than the ones with large AFM anisotropy. Some of the effects observed in nanoparticle (FM) – matrix (AFM) systems may have applications in permanent magnets and magnetic recording. The results highlight the virtue of FM-AFM (particle-matrix) microstructures as a possible route to enhance the magnetic properties of nanoparticles.

Acknowledgement

We would like to thank V. Skumryev, Ch. Laurent, S. de Brion, G. Chouteau, G. Hadjipanayis and D. Givord for their essential contribution to different parts of this work. Partial financial support from NEXBIAS (HPRN-CT 2002-00296), CICYT (MAT2004-01679), DGR (2001SGR00189), the Generalitat de Catalunya (ITT2000-14), Midi Pyrénées Council (CTP Grant) and Human Potential Program (HPRI-1999-CT00030) are acknowledged. J.S. and V.L. thank the NEXBIAS European Research Training Network for its financial support.

References 1 Dormann, J.L., Fiorani, D. and Tronc, E. (1997) ‘Magnetic relaxation in fine-particle

systems’, Adv. Chem. Phys., Vol. 98, pp.283–494. 2 Kodama, R.H. (1999) ‘Magnetic nanoparticles’, J. Magn. Magn. Mater., Vol. 200,

pp.359–372. 3 Hernando, A. (1999) ‘Magnetic properties and spin disorder in nanocrystalline materials’,

J. Phys.: Condens. Matter, Vol. 11, pp.9455–9482. 4 Hernando, A. and González, J.M. (2000) ‘Soft and hard nanostructured magnetic materials’,

Hyperfine Interact, Vol. 130, pp.221–240. 5 Batlle, X. and Labarta, A. (2002) ‘Finite size effects in fine particles: magnetic and trasport

properties’, J. Phys. D: Appl. Phys., Vol. 35, pp.R15–R42. 6 Leslie-Pelecky, D.L. and Rieke, R.D. (1996) ‘Magnetic properties of nanostructured

materials’, Chem. Mater., Vol. 8, pp.1770–1783. 7 Skomsky, R. (2003) ‘Nanomagnetics’, J. Phys.: Condens. Matter, Vol. 15, pp.R841–R896. 8 González, J., Chubykalo, O. and González, J.M. (2004) ‘Soft and hard magnetic

nanomaterials’, Encyclopedia of Nanoscience and Nanotechnology, Vol. 10, pp.1–26.


9 Hadjipanayis, G.C. (1999) ‘Nanophase hard magnets’, J. Magn. Magn. Mater., Vol. 200, pp.373–391.

10 Herzer, G. (1995) ‘Soft magnetic nanocrystalline materials’, Scr. Metall. Mater., Vol. 33, pp.1741–1756.

11 Suzuki, K. (1999) ‘Nanocrystalline soft magnetic materials. A decade of alloy development’, Mater. Sci. Forum, Vols. 312–314, pp.521–530.

12 Kneller, F. and Hawig, R. (1991) ‘The exchange-spring magnet: a new material principle for permanent magnets’, IEEE Trans. Magn., Vol. 27, pp.3588–3600.

13 Skomski, R. and Coey, J.M.D. (1993) ‘Giant energy product in nanostructured two-phase magnets’, Phys. Rev. B, Vol. 48, pp.15812–15816.

14 Xiao, J.Q., Jiang, J.S. and Chien, C.L. (1992) ‘Giant magnetoresistance in nonmultilayer magnetic systems’, Phys. Rev. Lett., Vol. 68, pp.3749–3752.

15 Berkowitz, A.E., Mitchell, J.R., Carey, M.J., Young, A.P., Zhang, S., Spada, F.E., Parker, F.T., Hutten, A. and Thomas, G. (1992) ‘Giant magnetoresistance in heterogeneous Cu-Co alloys’, Phys. Rev. Lett., Vol. 68, pp.3745–3748.

16 Sankar, S., Berkowitz, A.E. and Smith, D.J. (2000) ‘Spin-dependent transport of Co-SiO2 granular films approaching percolation’, Phys. Rev. B, Vol. 62, pp.14273–14278.

17 Sahoo, S., Binek, C. and Kleemann, W. (2003) ‘Giant metamagnetic moments in a granular FeCl2-Fe heterostructure’, Phys. Rev. B, Vol. 68, pp.174431, 1–4.

18 Skumryev, V., Stoyanov, S., Zhang, Y., Hadjipanayis, G., Givord, D. and Nogués, J. (2003) ‘Beating the superparamagnetic limit with exchange bias’, Nature, London, Vol. 423, pp.850–853.

19 Meiklejohn, W.H. and Bean, C.P. (1956) ‘New magnetic anisotropy’, Phys. Rev., Vol. 102, pp.1413, 1414.

20 Nogués, J. and Schuller, I.K. (1999) ‘Exchange bias’, J. Magn. Magn. Mater., Vol. 192, pp.203–232.

21 Berkowitz, A.E. and Takano, K. (1999) ‘Exchange anisotropy – a review’, J. Magn. Magn. Mater., Vol. 200, pp.552–570.

22 Stamps, R.L. (2000) ‘Mechanisms for exchange bias’, J. Phys. D: Appl. Phys., Vol. 33, pp.R247–R268.

23 Kiwi, M. (2001) ‘Exchange bias theory’, J. Magn. Magn. Mater., Vol. 234, pp.584–595. 24 Meiklejohn, W.H. and Bean, C.P. (1957) ‘New magnetic anisotropy’ Phys. Rev., Vol. 105,

pp.904–913. 25 Sort, J., Suriñach, S., Muñoz, J.S., Baró, M.D., Nogués, J., Chouteau, G., Skumryev, V. and

Hadjipanayis, G.C. (2002) ‘Improving the energy product of hard magnetic materials’, Phys Rev. B, Vol. 65, pp.174420, 1–5.

26 Peng, D.L., Sumiyama, K., Hihara, T., Yamamuro, S. and Konno, T.J. (2000) ‘Magnetic properties of monodispersed Co/CoO clusters’, Phys. Rev. B, Vol. 61, pp.3103–3109.

27 Gangopadhyay, S., Hadjipanayis, G.C., Sorensen, C.M. and Klabunde, K.J. (1993) ‘Exchange anisotropy in oxide passivated Co fine particles’, J. Appl. Phys., Vol. 73, pp.6964–6966.

28 Löffler, J.F., van Swygenhoven, H., Wagner, W., Meier, J.P., Doudin, B. and Ansermet, J.P. (1997) ‘Influence of grain size and oxidation on the magnetic properties of nanostructured Fe and Ni’, Nanostruct. Mater., Vol. 9, pp.523–526.

29 Sako, S., Ohshima, K., Sakai, M. and Bandow, S. (1997) ‘Magnetic property of NiO ultrafine particles with a small Ni core’, J. Vac. Sci. Technol. B, Vol. 15, pp.1338–1342.

30 Meiklejohn, W.H. (1958) ‘Exchange anisotropy in the iron-iron oxide system’, J. Appl. Phys., Vol. 29, pp.454, 455.

31 Zheng, R.K., Liu, H., Wang, Y. and Zhang, X.X. (2004) ‘Cr2O3 surface layer and exchange bias in an acicular CrO2 particle’, Appl. Phys. Lett., Vol. 84, pp.702–704.


32 Greiner, J.H., Croll, I.M. and Sulichm, M. (1961) ‘Unidirectional properties in the iron-iron sulfide system’, J. Appl. Phys., Suppl., Vol. 32, pp.188S, 189S.

33 Lin, H.M., Hsu, C.M., Yao, Y.D., Chen, Y.Y., Kuan, T.T., Yang, F.A. and Tung, C.Y. (1995) ‘Magnetic study of both nitrided and oxidized Co particles’, Nanostruct. Mater., Vol. 6, pp.977–980.

34 Murty, B.S. and Ranganathan, S. (1998) ‘Novel materials synthesis by mechanical alloying/milling’, Int. Mater. Rev., Vol. 43, pp.101–141.

35 Sort, J., Nogués, J., Amils, X., Suriñach, S., Muñoz, J.S. and Baró, M.D. (1999) ‘Room temperature coercivity enhancement in mechanically alloyed antiferromagnetic -ferromagnetc powders’, Appl. Phys. Lett., Vol. 75, pp.3177–3179.

36 Sort, J., Nogués, J., Amils, X., Suriñach, S., Muñoz, J.S. and Baró, M.D. (2000) ‘Correlation between the microstructure and enhanced room temperature coercivity in ball milled ferromagnetic-antiferromagnetic composites’, Mater. Sci. Forum, Vols. 343–346, pp.812–818.

37 Sort, J., Nogués, J., Amils, X., Suriñach, S., Muñoz, J.S. and Baró, M.D. (2000) ‘Magnetic hardening induced by exchange coupling in mechanically milled antiferromagnetic – ferromagnetic composites’, Mater. Res. Soc. Symp. Proc., Vol. 581, pp.641–646.

38 Sort, J., Nogués, J., Amils, X., Suriñach, S., Muñoz, J.S. and Baró, M.D. (2000) ‘Room temperature magnetic hardening in mechanically milled ferromagnetic-antiferromagnetic composites’, J. Magn. Magn. Mater., Vol. 219, pp.53–57.

39 Mishra, S.R., Dubenko, I., Losby, J., Roy, S., Ali, N. and Marasinghe, K. (2004) ‘Magnetic behavior of mechanically milled FeNi-CoO nanocomposites’, IEEE Trans. Magn., Vol. 40, pp.2716–2720.

40 Anhøj, T.A., Jacobsen, C.S. and Mørup, S. (2004) ‘Magnetic properties of Fe1–xMnx/Fe nanocomposites’, J. Appl. Phys., Vol. 95, pp.3649–3654.

41 Sort, J., Nogués, J., Suriñach, S., Muñoz, J.S., Baró, M.D., Chappel, E., Dupont, F. and Chouteau, G. (2001) ‘Coercivity and squareness enhancement in ball-milled hard magnetic – antiferromagnetic composites’, Appl. Phys. Lett., Vol. 79, pp.1142–1144.

42 Sort, J., Nogués, J., Suriñach, S., Muñoz, J.S., Chouteau, G. and Baró, M.D. (2002) ‘Effect of magnetic interactions on the magnetic properties of ball-milled SmCo5 + NiO powders’, J. Magn. Magn. Mater., Vols. 242–245, pp.1287–1289.

43 Sort, J., Nogués, J., Suriñach, S., Muñoz, J.S., Chappel, E., Dupont, F., Chouteau, G. and Baró, M.D. (2002) ‘Structural and magnetic characterization of high-coercive ball milled hard magnetic (SmCo5) + antiferromagnetic (NiO) composites’, Mater. Sci. Forum, Vols. 386–388, pp.465–472.

44 Sort, J., Suriñach, S., Muñoz, J.S., Baró, M.D., Nogués, J., de Brion, S. and Chouteau, G. (2003) ‘Magnetic interaction effects on the hard magnetic properties of ball-milled SmCo5 + NiO and SmCo5 + CoO composites: a ∆M plot study’, J. Appl. Phys., Vol. 93, pp.8140–8142.

45 Branagan, D.J., Kramer, M.J., Dennis, K.W. and McCallum, R.W. (2002) ‘Low temperature hysteresis in high anisotropy systems’, Scripta Mater., Vol. 47, pp.537–543.

46 Hong, N.M., Dan, N.H. and Phuc, N.X. (2002) ‘Large unidirectional anisotropy in Nd60Fe30Al10 bulk amorphous alloys’, J. Magn. Magn. Mater., Vols. 242–245, pp.847–849.

47 Sort, J., Nogués, J., Suriñach, S., Muñoz, J.S. and Baró, M.D. (2003) ‘Coercivity enhancement in ball-milled and heat-treated Sr-Ferrite with iron sulfide’, J. Metast. Nanocryst. Mater., Vols. 15–16, pp.599–606.

48 Ding, J., Miao, W.F., Street, R. and McCormick, P.G. (1996) ‘Fe3O4/Fe magnetic composite synthesized by mechanical alloying’, Scripta Mater., Vol. 35, pp.1307–1310.

49 Shi, Y., Ding, J., Tan, S.L.H. and Hu, Z. (2003) ‘Ni/Fe2O3 magnetic composite synthesized by mechanical alloying’, J. Magn. Magn. Mater., Vol. 256, pp.13–19.


50 Sort, J., Langlais, V., Doppiu, S., Dieny, B., Suriñach, S., Muñoz, J.S., Baró, M.D., Laurent, Ch. and Nogués, J. (2004) ‘Exchange bias effects in Fe nanoparticles embedded in an antiferromagnetic Cr2O3 matrix’, Nanotechnology, Vol. 15, pp.S111–S114.

51 Reed, T.B. (1971) Free Energy of Formation of Binary Compounds: An Atlas of Charts for High Temperature Chemical Calculations, MIT Press, Cambridge.

52 Uotani, M., Taniyama, T. and Yamazaki, Y. (2000) ‘Magnetic and magneto-transport properties of γ-Fe2O3-grain-embedded Fe3O4 thin films’, J. Appl. Phys., Vol. 87, pp.5585–5587.

53 Brahma, P., Banerjee, S., Das, D., Mukhopadhyay, P.K., Chatterjee, S., Nigam, A.K. and Chakravorty, D. (2002) ‘Properties of nanocomposites of α-Fe and Fe3O4’, J. Magn. Magn. Mater., Vol. 246, pp.162–168

54 Kitahata, S. and Kishimoto, M. (1994) ‘Magnetic properties of composite Ba-ferrite particles containing α-Fe’, IEEE Trans. Magn., Vol. 30, pp.4107–4109.

55 Pal, M., Bid, S., Pradhan, S.K., Nath, B.K., Das, D. and Chakravorty, D. (2003) ‘Synthesis of nanocomposites comprising iron and barium hexaferrites’, J. Magn. Magn. Mater., Vol. 269, pp.42–47.

56 Doppiu, S., Langlais, V., Sort, J., Suriñach, S., Baró, M.D., Zhang, Y., Hadjipanayis, G. and Nogués J. (2004) ‘Controlled reduction of NiO using reactive ball milling under hydrogen atmosphere leading to Ni-NiO nanocomposites’, Chem. Mater., Vol. 16, pp.5664–5669.

57 Langlais, V., Doppiu, S., Sort, J., Suriñach, S., Baró, M.D. and Nogués, J. (2004) ‘Magnetic propreties of Ni-NiO nanocomposites obtained by ball milling H2 reduction’, unpublished.

58 Morel, R., Brenac, A. and Portemont, C. (2004) ‘Exchange bias and coercivity in oxygen-exposed cobalt clusters’, J. Appl. Phys., Vol. 95, pp.3757–3760.

59 Stoyanov, S., Skumryev, V., Zhang, Y., Hadjipanayis, G. and Nogués, J. (2003) ‘High anisotropy Sm-Co nanoparticles: preparation by cluster gun technique and their magnetic properties’, J. Appl. Phys., Vol. 93, pp.7592–7594.

Exchange bias in ferromagnetic nanoparticles embedded in an antiferromagnetic matrix

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