shell architecture as an efficient tool to tune DC magnetic parameters and AC losses in spinel ferrite nanoparticles

Accepted Manuscript Core/shell architecture as an efficient tool to tune DC magnetic parameters and AC losses in spinel ferrite nanoparticles S.O. Solopan, N. Nedelko, S. Lewińska, A. Ślawska-Waniewska, V.O. Zamorskyi, A.I. Tovstolytkin, A.G. Belous PII:

S0925-8388(19)30742-X

DOI:

https://doi.org/10.1016/j.jallcom.2019.02.276

Reference:

JALCOM 49708

To appear in:

Journal of Alloys and Compounds

Received Date: 14 December 2018 Revised Date:

24 January 2019

Accepted Date: 25 February 2019

Please cite this article as: S.O. Solopan, N. Nedelko, S. Lewińska, A. Ślawska-Waniewska, V.O. Zamorskyi, A.I. Tovstolytkin, A.G. Belous, Core/shell architecture as an efficient tool to tune DC magnetic parameters and AC losses in spinel ferrite nanoparticles, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.02.276. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Core/shell architecture as an efficient tool to tune DC magnetic parameters and AC losses in

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spinel ferrite nanoparticles S.O. Solopan1, N. Nedelko2, S. Lewińska2, A. Ślawska-Waniewska2, V.O. Zamorskyi3, A.I. Tovstolytkin3a, A.G. Belous1 1

V.I. Vernadsky Institute of General and Inorganic Chemistry of the NAS of Ukraine, 32/34 Palladina Ave., Kyiv 03142, Ukraine

Institute of Physics, Polish Academy of Sciences, Aleja Lotnikow 32/46, PL-02668 Warsaw, Poland 3

Institute of Magnetism of the NAS of Ukraine and MES of Ukraine, 36-b Vernadsky Blvd., Kyiv 03142, Ukraine

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Effect of shell thickness on DC magnetic parameters (saturation magnetization, coercivity,

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blocking temperature) and AC losses (specific loss power, intrinsic loss power) has been studied for Fe3O4/CoFe2O4 core/shell-like magnetic nanoparticles with a fixed diameter of core ∼6.3 nm and an

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effective thickness of shell up to 2.5 nm. Regularities of the transformation of magnetic hysteresis loop parameters upon the increase in shell thickness have been analyzed and related to the intrinsic parameters of the core, shell and core-shell interfacial region. The values of specific/intrinsic loss power have been determined from experiment and compared with those calculated from the area of magnetic hysteresis loops. It is shown that the addition of 1 nm CoFe2O4 shell to the Fe3O4 core strongly enhances heating efficiency of the obtained composite nanoparticles. It is concluded that

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employment of core/shell-like architecture paves the way to tune and optimize the parameters of spinel ferrite nanoparticles.

Keywords: spinel ferrite nanoparticles, core/shell architecture, blocking temperature,

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magnetic hyperthermia, specific loss power, magnetic hysteresis loop.

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1. Introduction

Nanosize particles of ferro- and ferrimagnetic materials have been of great scientific and

practical interest as promising materials for applications in technique and medicine [1–5]. Biomedical applications of magnetic nanoparticles (MNPs) are of increasing importance in health issues due to functional tools in labelling, tracking, gene transfection and separation of cells, imaging diagnosis, drug delivery, and therapy [6–10]. A lot of effort has been made to apply these nanometric systems to treat cancer [8,11,12]. Magnetic hyperthermia, which exploits the ability of magnetic materials to dissipate energy when exposed to an alternating magnetic field (AMF), plays an important role in this fight [13–17]. The principle of magnetic hyperthermia is based on a

Corresponding author: [email protected]

1

enhanced sensitivity of cells affected by cancer to heat: when heated to a temperature of 42 – 46 °C, ACCEPTED MANUSCRIPT the affected cells are destroyed, while healthy tissues remain essentially unharmed [18,19]. Biomedical applications usually utilize ferrite-based nanoparticles due to their good biocompatibility and biodegradability properties [20–23]. Magnetic nanoparticles concentrated in the region of a tumor are used for local heating as they can be heated efficiently in a controlled way in a variable magnetic field [24–27]. A search for the ways to optimize the properties of MNP systems is currently under way, since the nanoparticles have to meet a number of rigid

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specifications: along with biological compatibility with living organisms they have to display stability with respect to agglomeration, narrow size distribution, efficient heating in external magnetic fields, and a number of others [28–30].

Recent progress in chemical and physical synthesis routes permitted the preparation of

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binary and more complex structures [31–36]. Such structures take advantages of new adjustable parameters including stoichiometry, chemical ordering, shape and segregation [9,37–41]. A

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promising way to tune the properties of magnetic nanoparticles is the combination of soft and hard magnetic materials in core/shell architecture. By combining the high saturation magnetization of a soft material with the high coercivity of a hard material, it is possible to achieve the parameters overcoming those of the individual counterparts [32,38,42,43].

As one of the most important and widely utilized magnetic materials, the spinel ferrite system consists of both magnetically hard and soft materials, such as cobalt ferrite CoFe2O4 and

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magnetite Fe3O4. Cobalt ferrite is magnetically hard with a large magnetocrystalline anisotropy constant K > 106 erg/cm3 [37,42]. On the other hand, magnetite is a ferrite with a much smaller magnetic anisotropy constant K ∼ (104 ÷ 105) erg/cm3 [44,45] and relatively high saturation

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magnetization (Msbulk ≅ 98 emu/g) [46]. Due to the same crystallographic structure and almost negligible lattice mismatch between these spinel ferrites [47,48], it should be markedly controllable to epitaxially grow a uniformed shell over a core and, thus, to controllably affect magnetic behavior

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of nanoparticles. To date, a few works have been carried out in this and related directions [42,43,48–50], but the task to understand the relation between the features of core/shell architecture, AC magnetic field induced effects and DC magnetic parameters has remained to a large extent unaddressed.

The aim of this work is to understand the effect of shell thickness on DC magnetic parameters and AC energy losses of composite Fe3O4/CoFe2O4 core/shell-like nanoparticles and pave the way to fabricate MNPs with tunable magnetic parameters for various technological and biomedical applications.

2. Materials and Methods 2

ACCEPTED MANUSCRIPT To date, the synthesis of core/shell structures has remained a complex approach, since it should enable a high crystalline quality of both core and shell, guarantee their good epitaxy and minimize intermixing processes at the core-shell interface. For this reason, the results of earlier works, carried out in our and other groups, were taken into account while fabricating Fe3O4/CoFe2O4 core/shell-like nanoparticles [48–52]. In our previous works, combined use of Xray diffraction and

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Fe Mössbauer spectroscopy techniques made it possible to conclude that

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among a number of MNP synthesis methods, only the method of co-precipitation from diethylene glycol (DEG) solutions allowed fabrication of magnetite MNPs with the smallest amount of maghemite and goethite phases [49,51]. Therefore, it is the method of co-precipitation from DEG solutions that was chosen to fabricate the nanoparticles in the present work.

microscopy, X-ray magnetic circular dichroism and

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A comprehensive analysis of the results of X-ray diffraction, high resolution electron 57

Fe Mössbauer spectroscopy studies was

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performed in works [48,52] to understand the processes at inter-crystallite interfaces in different kinds of Fe3O4-based granular composites prepared by seed mediated growth in polyol. The authors found that air exposure of the preformed magnetite cores induces their surface oxidation into maghemite and leads to a formation of cation vacancies. This non-stoichiometry favors a diffusion of shell cations into the preformed core and results in a formation of a gradient-concentrated particles instead of exact core/shell ones. On the contrary, for the case where the preformed Fe3O4

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core is not extracted from the solution and not subjected to the air exposition, only relatively thin transitional layer is formed at the interface between the core and shell. For this reason, in the present work, the synthesis of core and shell was performed in parallel with one another, and the preformed core was not extracted from the solution until the formation of the resulting composite

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structure was completed.

For the synthesis of composite Fe3O4/CoFe2O4 MNPs, iron (III) chloride nonahydrate (97%

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FeCl3⋅9H2O, Sigma Aldrich), cobalt (II) nitrate hexahydrate (98% Co(NO3)2⋅6H2O, Sigma Aldrich), iron (II) sulfate heptahydrate (99% FeSO4⋅7H2O, Sigma Aldrich), sodium hydroxide (98% NaOH), diethylene glycol (99% DEG, Sigma Aldrich) were used as starting reagents. At the first stage of the synthesis, individual Fe3O4 MNPs were prepared, which were subsequently used as respective cores of composite Fe3O4/CoFe2O4 MNPs. All stages of synthesis were carried out in a three-neck flask in argon atmosphere according to the scheme presented in Fig. 1(a-i). Synthesis of Fe3O4 MNPs. FeSO4⋅7H2O and FeCl3⋅9H2O in molar ratio (1:2) were dissolved in diethylene glycol (DEG) (Fig. 1(a)) and mixed for 1 hour. At the same time, a solution of NaOH in DEG was prepared (Fig. 1(b)). The alkali solution was added dropwise to the mixture of FeSO4⋅7H2O and FeCl3⋅9H2O salts, and the resulting mixture was stirred for 1 h, the change in color 3

being monitored (Fig. 1(c)). The obtained reaction mixture was heated up to 200 °C with a rate of

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2−3 °C/min and maintained at this temperature for 1.5 h (Fig. 1(d)). Contrary to the procedure described in our previous work [49], oleic acid was not added to the obtained product. Instead, the synthesized Fe3O4 nanoparticles were kept in the solution to prevent their agglomeration and oxidation. Synthesis of composite Fe3O4/CoFe2O4 MNPs. At first, the starting solution for the synthesis of CoFe2O4 shell was prepared: Co(NO3)2⋅6H2O and FeCl3⋅9H2O in molar ratio (1:2) were

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dissolved in DEG, and the solution was stirred for 1 h (Fig. 1(e)). At the same time, a solution of NaOH in DEG was prepared (Fig 1(f)). The alkali solution was added dropwise to the mixture of the salts Co(NO3)2⋅6H2O and FeCl3⋅9H2O, and the resulting mixture was stirred for 1 h (Fig. 1(g)). Then the solution containing pre-synthesized core (Fe3O4) nanoparticles was added to the obtained

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reaction mixture and resulting product was mixed for 1 h under the action of ultrasound (Fig. 1(h)). The obtained reaction mixture was heated up to 200 °C with a rate of 2−3 °C/min and maintained at

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this temperature for 1.5 hour (Fig. 1(i)). Oleic acid was then added, and the mixture was further stirred for 10−20 min. The resulting precipitate after cooling was centrifuged, redispersed in ethanol and dried in the air at 30−50 °C.

Fig. 1. Schematic presentation of the of

synthesis

of

composite

Fe3O4/CoFe2O4 core/shell-like nanoparticles

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procedure

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The amount of CoFe2O4, which was necessary to obtain the shell of a preset thickness, was

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calculated according to the procedure described in Ref. [53]. First, the volume of the shell material per one core/shell particle, Vshell, was calculated by the formula: Vshell = 4π[(R2)3-(R1)3]/3, where R1 and R2 are the radii of the initial and coated spherical particle, respectively. Then the mass of the shell material per one particle, mshell, was found as mshell = ρ·Vshell, where ρ is the shell material mass density. Accordingly, the mass of the core material per one particle, mcore, was calculated. The knowledge of mshell/mcore ratio made it possible to find the mass of the shell material for any chosen

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mass of the core material. For example, to cover one gram of Fe3O4 nanoparticles with an average size of 6.3 nm with a shell of 1 nm, it requires 1.287 g of CoFe2O4.

According to the method described above, a set of composite Fe3O4/CoFe2O4 magnetic nanoparticles with a fixed diameter of core ∼6.3 nm and an effective (calculated) thickness of the

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shell of 0, 0.05, 1.0 and 2.5 nm was synthesized. In the text below, the set of the synthesized samples will be denoted as Fe/Co(tshell), where tshell is the effective thickness of the CoFe2O4 shell in

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nanometers.

Nanostructured powders were investigated by PANalytical's X-ray diffraction (XRD) system on X'Pert Powder diffractometer (Co-Kα radiation, voltage 45 kV, current 35 mA, Ni filter). Calculations of the intensity redistribution and angles of X-ray peaks for individual compounds and core/shell nanoparticles were performed by PeakFit 4.12 software using individual peaks with maximum intensity in the range of 2θ-angles from 38 to 46°, in step of 0.01° and X-ray exposure

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time per point of 30 s. The size and morphology of powder particles have been determined by means of a JEM-1230 scanning electron microscope. Magnetic measurements were performed in the 5−400 K temperature range using a

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commercial Quantum Design Physical Property Measurement System PPMS equipped with Vibrating Sample Magnetometer. Magnetic moment was measured upon heating for both zerofield-cooled (ZFC) and field-cooled (FC) conditions. Isothermal magnetic hysteresis loops were

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measured at 5 and 300 K in magnetic fields up to 90 kOe. The room temperature minor loops were measured with an electromagnet based VSM system (Oxford Instruments). For calorimetric determination of specific loss power (SLP), the ferrofluids based on

synthesized MNPs (50 mg/mL) were prepared using 0.1 % aqueous agarose solutions and placed in the middle of a coil (5 turns, 3 cm in diameter), which induced the AMF with a frequency of 300 kHz and amplitude Hmax = 100 Oe [54]. The fluid temperature (Tfluid) was measured in a real time regime with the use of a fiber optic thermometer FOTEMP1-OEM (Optocon) with a fiber optic temperature sensor TS3. All measurements and calculations were done according to the procedure described in Ref. [55]. SLP values were calculated from the dependences of fluid temperature Tfluid vs residence time in external AMF (τ) by the formula [55,56]: 5

C ⋅ m dT MANUSCRIPT SLPACCEPTED = fluid ⋅ fluid , mpowder dτ

(1)

where dTfluid/dτ is an initial slope of the Tfluid vs τ dependence, Cfluid and m are the specific heat capacity and mass of the ferrofluid, respectively, and mpowder is the mass of the magnetic material in the fluid. Measurements and calculations were performed for m = 10-3 kg and Cfluid = 4186 J/(kg⋅K)

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3. Results and Discussion The results of XRD and TEM investigations for the MNPs under study were reported in our previous work [43]. The XRD data indicate that all synthesized samples have a cubic spinel structure (ICDD 19-0629). No traces of impurity phases were revealed in the samples.

As can be estimated from the results of TEM-investigations, the average size of the bare

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Fe3O4 MNPs is ∼6.3 nm [43]. With the increase in effective shell thickness, the average size of the particles increases, but experimentally obtained shell thicknesses are ~20 % smaller than calculated

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ones. This points to the fact that not all amount of the shell material precipitated on the surface of core.

Taking into account that individual Fe3O4 and CoFe2O4 MNPs have the same crystalline structure with close lattice parameters, the shell cannot be reliably distinguished from the core by the contrast of the TEM image. Therefore, prior to this work, a number of special experiments were carried out to check if the composite Fe3O4/CoFe2O4 MNPs have core/shell-like structure [49,51].

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The experiments included a comparative analysis of the most intensive (311) XRD peak collected from separate Fe3O4 and CoFe2O4 MNPs, mechanical mixture composed of these compounds, and composite Fe3O4/CoFe2O4 particles. Besides, corresponding TEM and

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Fe Mössbauer

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investigations were carried out on the individual and composite MNPs. As described in detail in Ref. [49], the results confirmed the formation of core/shell-like structure rather than a mechanical mixture. At the same time, as follows from Refs. [48,49,52], the particles may contain quite

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noticeable transitional layer between the core and the shell, which may be composed of Co1xFe2+xO4

phases with a gradient of Co concentration from the surface to the core. This means that

the synthesized composite particles are not exact core/shell structures. For this reason, we are using the term “core/shell-like Fe3O4/CoFe2O4 nanoparticles” for the synthesized composite nanoobjects. Fig. 2 shows magnetic hysteresis loops measured at T = 5 K for Fe/Co(tshell) core/shell-like nanoparticles under study. The figure demonstrates that the addition of shell and subsequent increase in its thickness strongly affect the loop shape by modifying both saturation magnetization, Ms, and coercivity, Hc. For uncoated Fe3O4 MNPs, the value of the saturation magnetization is equal to 77 emu/g, which is less than that of the bulk counterpart (Msbulk ≅ 98 emu/g [46,57]). The reduced 6

magnetization of the MNPs is likely to result from a noticeable contribution from the near-surface

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layers, which are usually characterized by enhanced magnetic disorder due to broken exchange bonds at the surface [6,32]. Surface ferrous ions oxidation may also contribute to this phenomenon [48,51]. Initial coating of MNPs (tshell = 0.05 nm) does not exert noticeable influence on the Ms value, but the increase in shell thickness up to 2.5 nm leads to the Ms reduction down to 56 emu/g. Such behavior may be a result of an increased contribution from CoFe2O4 shell, which often display

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reduced Ms in the nanoregime. Here, one should note that although bulk CoFe2O4 has relatively high saturation magnetization (Msbulk ≅ 93 emu/g) [46], its magnetization may become strongly reduced as the nanoparticle size decreases down to a few tens of nanometers [43,58,59]. In particular, CoFe2O4 MNPs, fabricated by the same method as the one used for fabrication of

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CoFe2O4 shell in the present work, have Ms ≅ 50 emu/g at 5 K [43]. The reason for this most likely

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lies in the transformation of CoFe2O4 from inverse to mixed spinel in the nanoregime [50,58]. Fig. 2. Magnetic hysteresis loops measured at 5 K for Fe/Co(tshell) MNPs under study.

T=5K

0

tshell=0

0.05 nm 1.0 nm 2.5 nm

-60 -20

0

20 H, kOe

40

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-40

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M, emu/g

60

Rather different trends are characteristic of the evolution of coercivity. A sharp increase in Hc from ∼0.4 to ∼2.7 kOe (almost one order of magnitude) is observed upon initial coating of

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MNPs. As shell thickness rises, the coercivity increases further and reaches ∼6.8 and ∼10.1 kOe for MNPs with tshell = 1.0 and 2.5 nm, respectively. This process is accompanied by a strong growth of a closure field, i.e the field where forward and backward branches of a hysteresis closely approach each other. Closure field increases from less than 5 kOe for uncoated MNPs to more than 40 kOe for Fe/Co(tshell = 2.5 nm), which points to the rise of energy barriers associated with magnetization reversal process as well as magnetic frustration induced by the addition of shell. A reasonable explanation of these trends can be achieved assuming a simultaneous action of two factors: modification of the parameters of interfacial region between the core and shell, and contribution of magnetically hard shell to enhancement of the total coercivity. Since coercivity of single domain MNPs is related to their effective anisotropy constant Keff [60], the data of Fig. 2 7

imply that both procedures, shell addition and subsequent increase in its thickness, give rise to the

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strong increase of Keff. It is noteworthy that qualitatively similar behavior was observed in MnFe2O4/ CoFe2O4 and ZnFe2O4/ CoFe2O4 core/shell nanoparticles [42,61,62]. Temperature dependences of magnetization were obtained in two different measurement modes – zero-field cooling (ZFC) and field-cooling (FC). In the ZFC mode, a sample was cooled in zero magnetic field down to 5 K. At this temperature, an external magnetic field Hmeasur = 50 Oe was applied to the sample and MZFC was measured as a function of T in the process of sample

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heating. The FC mode differs in that the sample was cooled in magnetic field Hcooling = Hmeasur. For the NPs under study, the MFC(Т) and MZFC(Т) curves coincide with each other at high temperatures (>370 K), but start diverging as temperature is lowered (Fig. 3). Each MZFC(Т) curve displays a maximum at a certain temperature Тb, which is called a blocking temperature. Below this

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temperature the difference between MFC(Т) and MZFC(Т) becomes especially noticeable. Appearance of a maximum on MZFC(Т) curves indicates that in the vicinity of Тb, the anisotropy energy of the

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nanoparticles, KeffV, becomes comparable to the thermal energy kT (here, V is the particle volume and k is the Boltzmann constant). As a result, at Т < Тb, the orientation of the magnetic moments of the particles is governed by the anisotropy energy (blocked state), rather than the thermal energy or the energy of relatively small external magnetic field.

Fig. 3. MFC(Т) (red line) and MZFC(Т) (black

4

0

for Fe/Co(tshell) samples under investigation.

0.05 nm

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M, emu/g

3

0

3 (c)

0

line) dependences obtained in a field of 50 Oe

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(b)

tshell = 0

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(a)

(d)

1.0 nm

2.5 nm

1 0

0

200

400 T, K 8

ACCEPTED MANUSCRIPT All MNPs, except for Fe/Co(tshell = 2.5 nm), display a single blocking temperature suggesting that the spins of the core and shell are strongly coupled and respond jointly to the changes of temperature and magnetic field. For the MNPs with 2.5 nm shell, double-peak behavior is observed, which may originate from separate contributions from the core and shell due to a relatively high thickness of shell. It is also possible that the ensemble of these MNPs contains ultrasmall cobalt ferrite nanoparticles, which contribute to the observation of MZFC(Т) double-peak

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behavior.

The value of Tb grows from ∼175 to ∼277 K upon the initial coating of MNPs and displays further rise with the increase in tshell. It reaches ∼328 and ∼356 K for the samples with tshell = 1.0 and 2.5 nm, respectively. Such behavior is in compliance with the above conclusion about a strong

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increase of effective anisotropy constant Keff induced by the shell introduction and subsequent increase in its thickness. In addition, the rise in total MNP volume also contributes to the

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strengthening of this trend, as the value of Tb is related to the KeffV product [12,29]. Magnetic hysteresis loops measured at 300 K are shown in Fig. 4. One can notice that the rise in temperature from 5 to 300 K leads to a general reduction of the saturation magnetization by ∼30 %, but does not introduce qualitative changes in the Ms vs tshell behavior: Ms value remains almost unchanged upon the initial coating of MNPs, but noticeable decrease (from ∼52 down to ∼28

50

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0

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M, emu/g

T = 300 K

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emu/g) is observed as tshell increases from 0.05 to 2.5 nm.

-50 -8

-4

0

Fig. 4. Magnetic hysteresis loops measured at 300 K.

tshell=0

4 H, kOe

0.05 nm 1.0 nm 2.5 nm

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Unfortunately, the field range (8 kOe), which was used to obtain the data of Fig. 4, does not allow reliable determination of small coercive fields, but some suggestions can be made based on the analysis of the data of Fig. 3: superparamagnetic non-hysteretic behavior is expected for those particles for which Tb is lower than 300 K (i.e. for MNPs with tshell ≤ 0.05 nm) and hysteretic one (due to a blocking of MNP magnetic moments) for the rest of the samples. At the same time, one

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does not exclude the possibility of the formation of weak hysteresis due to non-negligible

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distribution of particles in sizes and/or partial MNP agglomeration in the first group of samples. To shed more light on the low-field magnetic behavior of Fe/Co(tshell) MNPs and obtain information about energy losses due to magnetic reversal, a series of hysteresis loops was measured at 300 K in magnetic fields with the amplitude of sweep field (Hmax) varying from 50 to 200 Oe. Representative M vs H dependencies obtained for Hmax = 100 Oe are shown in Fig. 5. General reduction of magnetization as well as strong transformation of the loop shape upon the increase in

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tshell are evident from the figure.

Fig. 5. Low-field magnetic hysteresis loops

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measured at 300 K.

T = 300 K

0 tshell=0 0.05 nm 1.0 nm 2.5 nm

-5 -100

0

H, Oe

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M, emu/g

5

100

For fixed parameters of the external magnetic field, the energy losses due to magnetization

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reversal are determined by the area of magnetic hysteresis loop [46,60]. Fig. 6 shows the area of magnetic hysteresis loops, Aloop, as a function of the amplitude of sweep field, Hmax, at 300 K. For Hmax exceeding 60 Oe, the initial increase in tshell results in the increase of Aloop, but as tshell crosses 1.0 nm, the area of the loop start decreasing. The largest value of Aloop is characteristic of the sample

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with tshell = 1.0 nm. At the same time, the MNPs with tshell = 2.5 nm display the smallest area of the

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loop.

Aloop, emu×Oe/g

400

tshell=0

function of the amplitude of sweep field at 300

0.05 nm 1.0 nm 2.5 nm

200

0

Fig. 6. Area of magnetic hysteresis loops as a

0

100

K.

200 Hmax, Oe

10

It is seen that for the samples with tshell > 0.05 nm, Aloop grows with the increase in Hmax for

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all values of Hmax used in experiment (up to 200 Oe). This means that the closure fields for these samples exceed 200 Oe and, thus, weaker fields are not enough to “open” the hysteresis loop to a full extent. At the same time, for the samples with tshell ≤ 0.05 nm, Aloop vs Hmax dependences tend to saturate at Hmax ∼ 100 Oe. It follows from these data that for the samples with tshell ≥ 1.0 nm, magnetic fields of the order of 100 Oe are not enough to make MNP magnetic moments be able to overcome the energy

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barriers hindering the reversal of the particle magnetic moments. Based on the data of Figs. 2 − 3, one can conclude that the increase in tshell gives rise to a general increase in coercivity. In this case, for MNPs with tshell = 2.5 nm, the coercive field is likely to become so high that external magnetic fields of the order of 100 Oe are unable to noticeably disturb the magnetic state of MNPs and, thus,

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the area of hysteresis loop for these MNPs becomes smaller than that for the other samples. To obtain the AC magnetic heating characteristics of the synthesized MNPs, the time

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dependence of heat generation was studied under AMF with fixed amplitude Hmax = 100 Oe and frequency f = 300 kHz. The plots of magnetic fluid temperature (Tfluid) versus residence time in external AMF (τ) for the fluids based on synthesized MNPs are shown in Fig. 7. Based on these data, the values of SLP were determined and compared with those calculated from the area of magnetic hysteresis loops. Fig. 8 compares both sets of SLP vs tshell dependences. Open squares show the data

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determined from experimental Tfluid vs τ dependences with the use of formula (1). Filled triangles mark SLP values calculated as

SLP = µ 0 fAloop ρ ,

(2)

where µ0 = 4π×10-7 H/m and ρ is the MNP material mass density [29,63]. It is seen that general

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trends in both sets of SLP vs tshell dependences are in compliance with each other. The discrepancies observed between experimental and calculated SLP values can originate from a non-negligible effect

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of frequency change on coercivity and hysteresis loop shape [29,60].

Tfluid, °C

45

Fig. 7. Fluid temperature Tfluid versus heating tshell= 1.0 nm

time τ dependences for magnetic fluids based on Fe/Co(tshell) MNPs under study.

0.05 nm 0

25

2.5 nm

0

100

200 τ, s

300

400

11

As seen from Fig. 8, the addition of 1.0 nm CoFe2O4 shell to the Fe3O4 core strongly

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enhances heating efficiency of the obtained composite MNPs. Thus, core/shell-like architecture can serve as an efficient tool to govern both the DC and AC behavior of magnetic nanoparticles.

Fig. 8. Experimentally determined (open Experiment Calculations

squares) and calculated (filled triangles) SLP

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values as functions of shell thickness for

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Fe/Co(tshell) MNPs.

5 0

1

2 tshell, nm

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0

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SLP, W/g

15

Due to SLP dependence on parameters, which are extrinsic to the MNPs properties, such as AMF amplitude and frequency, the comparison of reported SLP values does not give comprehensive information on the MNPs heating efficiency. More recently, an intrinsic loss power (ILP) parameter defined as

SLP 2 f ⋅ H max

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ILP =

(3)

has been proposed to compare heating efficiencies obtained under different experimental conditions [56]. ILP is quantified in nH⋅m2/kg. The ILP is more convenient to compute as [63]

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SLP [ W kg ]  nH ⋅ m 2  . ILP  = 2  2  kg  f [ kHz ] ⋅ H max [ kA m ]

(4)

The definition of ILP is believed to regard the magnetization losses as frequency-independent,

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allowing direct comparison of the particle-heating capabilities independently of the applied AC field strengths and frequencies. Although this approximation is not always satisfied by particles in suspensions [64], this parameter makes it possible to more adequately characterize the heating efficiency of the MNPs.

In our case, ILP is equal to 0.74 nH⋅m2/kg for the sample with tshell = 1.0 nm and smaller for other samples. Typical ILP values reported in literature for different spinel ferrite MNPs are within the range from 0.5 to 3 nH⋅m2/kg [65,66]. It follows from the analysis of these data that employment of core/shell-like architecture may show the way to optimize the parameters of spinel ferrite nanoparticles, but in our case, further works are needed to enhance the heating efficiency of the samples obtained. 12

ACCEPTED MANUSCRIPT 4. Conclusions A set of composite Fe3O4/CoFe2O4 core/shell-like magnetic nanoparticles with a fixed diameter of core ∼6.3 nm and an effective thickness of the shell of 0, 0.05, 1.0 and 2.5 nm have been synthesized. DC magnetic properties and AC energy loss parameters for the ensembles of these particles have been studied. Strong effect of both processes, initial coating of Fe3O4 nanoparticles with CoFe2O4 shell

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and increase in the thickness of the shell, on the saturation magnetization and coercivity of the particles has been observed for low (5 K) and high (300 K) temperatures. A reasonable explanation of the results is possible under assumption that a simultaneous action of two factors is at play: a modification of the parameters of interfacial region between the core and shell, and increasing

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contribution of the shell to nanoparticle characteristics with an increase in the shell thickness. All nanoparticles, except for the samples with the 2.5 nm shell thickness, display a single

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blocking temperature suggesting that the spins of the core and shell are strongly coupled and respond jointly to the changes of temperature and magnetic field. The fact that the value of blocking temperature grows with the increase in shell thickness is in compliance with the conclusion about a strong increase of effective anisotropy constant induced by the shell introduction and subsequent increase in its thickness.

Experiments in low magnetic fields at 300 K have shown that a general reduction of

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magnetization as well as strong transformation of the loop shape occur upon the increase in shell thickness. It is inferred from these data that the increase in shell thickness gives rise to a general increase in coercivity. It is concluded that for the nanoparticles with the 2.5 nm shell thickness, the coercive field becomes so high that external magnetic fields of the order of 100 Oe are unable to

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noticeably disturb the magnetic state of the particles. To obtain the AC magnetic heating characteristics of the synthesized MNPs, the time

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dependence of heat generation was studied under AMF with fixed amplitude Hmax = 100 Oe and frequency f = 300 kHz. Based on these data, the values of specific loss power were determined and compared with those calculated from the area of magnetic hysteresis loops. It is shown that the addition of 1.0 nm CoFe2O4 shell to the Fe3O4 core strongly enhances heating efficiency of the obtained composite MNPs. Thus, core/shell-like architecture can serve as an efficient tool to govern both the DC and AC behavior of magnetic nanoparticles. The values of intrinsic loss power were calculated from the SLP values and compared with those reported in literature for different spinel ferrite nanoparticles. It is concluded that employment of core/shell-like architecture paves the way to tune and optimize the parameters of spinel ferrite nanoparticles. 13

ACCEPTED MANUSCRIPT Acknowledgments The work is partially supported by the Ministry of Science and Education of Ukraine through the project “Micro- and nanofluidics in stray magnetic fields of artificial and biogenic magnetic particles” (state registration number 0118U003790). Magnetic measurements were performed in NanoFun laboratory funded by POIG.02.02.00-00-025/09.

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Declarations of interest: none

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Figure captions ACCEPTED MANUSCRIPT Fig. 1. Schematic presentation of the procedure of synthesis of composite Fe3O4/CoFe2O4 core/shell-like nanoparticles. Fig. 2. Magnetic hysteresis loops measured at 5 K for Fe/Co(tshell) MNPs under study. Fig. 3. MFC(Т) (red line) and MZFC(Т) (black line) dependences obtained in a field of 50 Oe for Fe/Co(tshell) samples under investigation. Fig. 4. Magnetic hysteresis loops measured at 300 K.

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Fig. 5. Low-field magnetic hysteresis loops measured at 300 K

Fig. 6. Area of magnetic hysteresis loops as a function of the amplitude of sweep field at 300 K.

Fig. 7. Fluid temperature Tfluid versus heating time τ dependences for magnetic fluids based

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on Fe/Co(tshell) MNPs under study.

Fig. 8. Experimentally determined (open squares) and calculated (filled triangles) SLP

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values as functions of shell thickness for Fe/Co(tshell) MNPs.

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Highlights

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Interfacial core-shell region contribute to transformation of magnetic parameters Addition of shell with optimal thickness strongly enhances AC heating efficiency Core/shell architecture is efficient tool to govern the behavior of nanoparticles

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