antiferromagnetic nanocomposites

Accepted Manuscript Unsual features of ferromagnetic/ antiferromagnetic nanocomposites A.A. Azab, S.I. El-Dek, S. Solyman PII:

S0925-8388(15)31303-7

DOI:

10.1016/j.jallcom.2015.10.048

Reference:

JALCOM 35603

To appear in:

Journal of Alloys and Compounds

Received Date: 8 August 2015 Revised Date:

19 September 2015

Accepted Date: 6 October 2015

Please cite this article as: A.A. Azab, S.I. El-Dek, S. Solyman, Unsual features of ferromagnetic/ antiferromagnetic nanocomposites, Journal of Alloys and Compounds (2015), doi: 10.1016/ j.jallcom.2015.10.048. 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.

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Unsual features of ferromagnetic/ antiferromagnetic nanocomposites A.A.Azab1, S.I. El-Dek2, S. Solyman3 1

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Solid State Electronics Laboratory, Solid State Physics Department, Physics Division, National Research Centre, Dokki, Giza, P.O. 12622, Egypt 2 Materials Science and Nanotechnology Dept., Faculty of Post graduate studies for Advanced Sciences, Beni-Suef University, Beni-Suef, Egypt. 3 Physics Department, Faculty of Science, Zagazig University, Zagazig, Egypt Physics Department, Faculty of Science and Art, Bisha University, KSA

Abstract

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Nanocomposites based on the general formula ((1-x) CFO + (x) LCFO) were successfully synthesized using citrate-nitrate autocombustion technique. Properties of the nanocomposites were characterized by X-ray diffraction and transmission electron microscope. Magnetic properties revealed a colossal compositional change in the coercivity for the nanocomposites despite its small values for the two components. The saturation magnetization and remanence magnetization were found to decrease linearly

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with the antiferromagnetic phase (AFM) content rose. The dielectric constant was found to increase with increasing (AFM) content. Potential applications of the nanocomposites in magnetic random access memories (MRAMS) and spintronics are recommended.

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Keywords: Nanocomposites; FM/AFM exchange bias; TEM; Magnetic properties; Dielectric properties.

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1. Introduction

Recently, nanocomposites have special interest because composites on the

nanoscale can exhibit improved overall magnetic performance and exhibit other physical phenomena, such as the exchange bias effect [1-4]. In nanocomposites of ferromagnetic Corresponding author. Tel.: +20 1142928039; fax: +20 235676742. E-mail address: [email protected] (A.A. Azab).

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with antiferromagnetic (AFM) materials, one can observe the exchange bias effect, which has been widely investigated because of its applications in magnetoresistive sensors and magnetic storage technology [5]. Exchange bias was first discovered by Meiklejohn and Bean [6], and its characteristic signature is the shift of the center of

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magnetic hysteresis loop from its normal position at H = 0. Exchange bias is one of the phenomena associated with the exchange anisotropy created at the interface between an AFM and a FM material. Some applications of these effects include permanent magnets, magnetic recording media [7, 8] or domain stabilizers in recording heads based on

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anisotropic magnetoresistance [9]. However, it was the reduction of the saturation fields to observe giant magnetoresistance (GMR) in exchange biased systems [10], as compared

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to standard GMR multilayer systems [11], which triggered a renewed interest in these phenomena [12]. LaFeO3 is a well-known high Neel temperature antiferomagnetic with G-type, where a weak ferromagnetic component is thought to originate from slight canting of the Fe moments [13]. Ca doping on the A site cation resulted in a clear improvement of the magnetization as previously reported [14]. CoFe2O4 is a ferrimagnetic spinel with hard like character possessing +ve magnetocrystalline

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anisotropy [15, 16]. One aimed to take advantages of merging the characteristic features of the two magnetic structures together via the synthesis of nanocomposites with different weight ratios of the two parents. One also expected new cross phenomena to appear such as exchange bias. Effort will be highlighted to fully understand the physical properties of

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such nanocomposites.

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2. Experimental techniques

Parent compounds of the nanocomposites namely CoFe2O4 (CFO) and

La0.7Ca0.3FeO3 (LCFO) were prepared the by citrate–nitrate auto-combustion method [17]. La0.7Ca0.3FeO3 was prepared by mixing stoichiometric ratios of metal nitrates La(NO3)3.6H2O, Ca(NO3)2.H2O and Fe(NO3)3.9H2O with citric acid in equimolar ratio at PH 7–8. The mixture was then heated on magnetic stirrer at the 120 oC until dry and then self-ignition took place. The powder obtained was then annealed at 600oC with a heating/cooling rate of 5oC/min for 5 hrs in a Lenton furnace UAF16/5. A similar procedure has been followed to prepare CoFe2O4 nanoparticles using Co(NO3)2.3H2O 2

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and Fe(NO3)3.9H2O as precursors. Four nanocomposites were prepared from the as synthesized CoFe2O4 and La0.7Ca0.3FeO3 in four different weight ratios according to formula (1-x)CFO + (x)LCFO, where x= 0, 0.1, 0.3, 0.5, 0.7, 1. To prepare the nanocomposites, the assigned weight per gram of the two components were mixed

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together and grinded for 2 hrs to allow good mechanical mixing and finally annealed at 350 oC for a further 2 hrs with a heating/cooling rate of 5oC/min.

X-ray diffraction (XRD) analyses were carried out using a Proker D8 advance Xray diffractometer with CuK radiation (=1.5418 Å) to check the preparation of

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CoFe2O4, La0.7Ca0.3FeO3 and their nanocomposites. The microstructure of the nanoparticles was examined with a JEOL – 2100 high resolution transmission electron

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microscope (HRTEM). The dependence of magnetization on magnetic field was investigated at room temperature using a vibrating sample magnetometer (VSM; Lake Shore -7410-USA) for the two parents of nanocomposites as well as for each one of the four nanocomposites. The powders were pressed into pellets using a uniaxial press of pressure 1.9 x108 Nm-2. The samples were well polished to obtain uniform parallel surfaces. Ohmic contacts on the sample surfaces were made by silver paste and checked

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for good conduction. The real part of the dielectric constant (ε\) and the ac conductivity () were measured using the two probe method using LCR meter model Hioki type 3532 (Japan) as a function of temperature from 300 to 750 K at different frequencies ranging from 100 kHz to 5MHz. The temperature of the samples was measured using a K-type

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thermocouple connected to a Digi-Sense thermometer with junction just in contact with the sample. The error in measuring temperature was ± 1 oC .

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3. Results and discussion 3.1 X-ray diffraction

Fig. 1 presents the XRD patterns of the parents and the nanocomposites of

La0.7Ca0.3FeO3 and CoFe2O4 with different concentrations. The observed broadening in the peaks indicates that the prepared samples crystallize in a nanosized scale. The data show single phase cubic spinel structure of CoFe2O4 as compared and indexed with ICDD card no (04-006-4148) and single phase orthorhombic structure as compared with ICDD card no. (49-1885) for La0.7Ca0.3FeO3. It can be seen that in all nanocomposites there is no detected third phase beside the 3

phases

of

CoFe2O4 and

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La0.7Ca0.3FeO3, indicating that the two phases can exist individually with good physical homogeneity and display very good chemical compatibility. As expected, with increasing La0.7Ca0.3FeO3 concentration, the diffraction peaks of this phase become strengthened gradually. It is noted that the intensities of characteristic diffraction

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peaks of the two constituents match with the variation of their respective weight ratios in the nanocomposites. The crystallite size was calculated from the corrected FWHM of the most intense peak corresponding to (311) plane of the CoFe2O4 and the (002) plane of the La0.7Ca0.3FeO3 using Scherrer’s formula and found to be 45.8 nm to CoFe2O4 ferrite and

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30.4 nm for La0.7Ca0.3FeO3. 3.2 TEM Micrographs

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Fig. 2.a-d illustrates high resolution transmission electron micrographs of CoFe2O4, La0.7Ca0.3FeO3 and nanocomposite with x=0.5. The CoFe2O4; Fig.2-a; nanoparticles appears to have good crystallinity and well defined geometric shape with clear orientations. Cubic-like crystals with diameters less than 50 nm can be observed, which agree well with that calculated from XRD. La0.7Ca0.3FeO3; Fig. 2-b; nanoparticles appears to be well dispersed with small size, spherical shape and narrow distribution.

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Some agglomeration appeared due to the absence of surfactant during preparation. The nanocomposite with x= 0.5 in Fig. 2 c-d reveals the co-existence of the two separate phases as no chemical interaction occures between them.

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3.3 Magnetic properties

The room temperature (M–H) hysteresis loops for nanocomposites with (1-x)

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CFO + (x) LCFO where 0 ≤ x ≤ 1 are shown in Fig. 3. All nano-composites exhibit typical ferromagnetic hysteresis loops. The loops reveal large area, clear saturation at relatively large magnetic field values and large coercive field. The loops represent typical hard behavior where the squareness ratio (SQR= Mr/Ms) ≥ 0.5 for all concentrations except at x=1. This fruitful result assures that, despite the soft magnetic nature of the LCFO, the character of CoFe2O4 nanoparticles predominates. The pure LCFO has typical antiferromagnetic character associated with weak ferromagnetization, lack of saturation and small area [18]. The values of saturation magnetization ( (

r ),

coercive field (

c ),

s ),

remnant magnetization

exchange bias and the squareness ratio of the nanocomposites 4

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are summarized in Table 1. Both saturation and remanent magnetization of the nanocomposites increase linearly with increasing (CFO) content as shown in Fig.4-a,b where each ferrite particle in the sample acts as the center of ferrimagnetism surrounded by AFM (LCFO) [19] and the long range order is transformed gradually to short range

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one monotonically. The linear variation of both Ms and Mr with LCFO content recommends the use of these nanocomposites in linear devices. In addition, the number of magnetic sublattices increased on the expense the AFM ones with increasing CFO content. The values of the coercive field were found to increase with increasing the CFO

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nanoferrite content in the nanocomposite as shown in Fig. 4-c. The direct reason is the positive magnetocrystalline anisotropy associated with spin orbit coupling at the Co ions.

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Maximum coercivity was obtained at x=0.1 which is a surprising result and could be related to shape anisotropy and/or exchange anisotropy. The samples here display another interesting feature which is large coercivity for all

nanocomposites despite lower

coercivity for both parents Hc=1643 Oe at x=0 and 192.7 Oe at x=1. Large coercivity was attributed to +ve magnetocrystalline anisotropy as mentioned before for CFO, this agreed well with the reported literature [20]. One of the still surprise is the obtained

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phenomenon of colossal compositional change of coercivity Hc for the sample with x= 0.1 where it is 1.4 times that of x=0. On the other hand, Hc at x=0.1 is 12 times that at x=1. One could interpret these results as the contribution of exchange anisotropy in addition to the magnetocrystalline one. The former leads to highlighted enhancement

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(colossal compositional variation) of the coercivity. We calculated the percentage increment of HC as ∆Hc % = [Hc(x=0.1) - Hc(x=0.0)]/ Hc(x=0.0) for the sample with

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x=0.1, it is 40 % while if we calculated ∆Hc % =[Hc(x=0.1) - Hc(x=1)]/ Hc(x=1) it reaches 1100% for x=1. These fantastic fruitful findings led to open a new era of applications in the field of spintronics. These nanocomposites can be useful in magnetic recording, MRAMS and spin valves. From the (M-H) loop illustrated in Fig. 3, a small shift in the loop was observed in which its value depends strongly on the ratio of the ferrimagnetic phase to the antiferromagnetic one. When two magnetic materials with different spin structures are in close proximity to each other, the interface effects play a significant role. In nanostructured systems, the exchange bias and magnetic proximity are more pronounced. Less attention is paid to fully understand this phenomena. The 5

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antiferromagnetic sublattice as in our case (LCFO) moment was thought to be orthogonal to the ferromagnetic lattice [21]. Thus, it is stressed that at the interface, there is a competition between parallel alignments and perpendicular, spin flop like alignment. The value of exchange bias field (HE) was calculated and reported in table 1. HE was strongly

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affected by the weight ratios of the two phases in the nanocomposite. The sample LCFO (x=1) displays significantly large exchange, while the one with (x=0.5) among all nanocomposites gives also a considerable shift in the loop. The exchange bias here is highlighted at room temperature. The value of the exchange bias is thought to be a

3.4 Electrical properties

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HE α 1/tF

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function of the thickness of the ferromagnetic layer [22]

Fig. 5 illustrates the compositional dependence of the real part of the dielectric constant for the investigated nanocomposites at selected frequencies and temperatures. The dielectric behavior here is attributed to the well-known value exchange between in the nanoferrite, while in the orthoferrite phase

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dominates due to the existence of Ca2+ ions as previously reported in our work [14]. The figure shows that the dielectric constant (\) of the nanocomposites is temperature and frequency dependent, where \ increases with temperature and decreases with frequency. The orientational and Maxwell - Wagner polarization are expected to

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play a role with increasing temperature. The thermal energy is quite sufficient to free more charge carriers and the field accompanied with the applied frequency orient them in

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its direction, though increases the polarization as well as the dielectric constant. Generally, the behavior of \ can be explained based on Koops model [23] for the inhomogeneous double-layer dielectric structure [24]. According to Koops’s model the dielectric constant at low frequency comes from the grain boundaries which have a high dielectric constant. At high frequencies, \ arises from the grains which have a small dielectric constant. The grain boundaries here play the hero role where the charges reside on. With increasing frequency, the dipoles could not follow up the field variation and \ decreased. Maxwell Wagner polarization is accentuated at high temperatures while interfacial and space charge are the main contributors in our scenario. Fig. 5 shows that 6

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the general trend is the increase in ε with increasing LCFO content up to x=0.7 and then decreases slightly for the pure LCFO. It is obvious that LCFO has \ higher than that of pure CFO. The interface effects here take the upper hand [25] in controlling the polarization in the nanocomposites. Minimum dielectric constant is obtained at x=0.1 and

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coincides with conductivity data. Herein, the situation is the grain/ grain boundaries resistivity and how are the LCFO particles, distributed among CFO magnetic ones. The sample with x=0.1 can be suitable in microwave applications owing to the relatively low \ as well as neglected losses. In our case the volume fraction of LCFO could serve as a

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tune to adjust the value of \. About the conductivity, it has nearly the same trend as that of \ except the dispersion that appears at room temperature. The conductivity, Fig. 6

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increases with increasing (LCFO) content at all frequencies and at the selected temperatures. Here is the conductivity could be explained easily based on the well-known Verwey conduction mechanism in the ferrite nanoparticles and the hole exchange between

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o

. For LCFO, the valence exchange

pair contributes to

the conductivity. For all nanocomposites, new conduction channels are initiated across grain boundaries at the interface in addition to the individual conductivities of the two

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parents. Accordingly, it is obviously expected that all nanocomposites exhibit conductivity higher than the parents. This situation is analogous to that of the coercivity

Conclusion

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as mentioned above in the magnetic part.

Citrate-nitrate autocombustion was successfully used for the preparation of

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nanocomposites. Exchange bias was observed and explained on the basis of interface effects. Colossal compositional variation of coercivity was remarked. The linear variation of both Ms and Mr with LCFO content recommends the use of these nanocomposites in linear devices. The results assure that these nanocomposites possess interesting features such as large coercivity, low dielectric constant and room temperature exchange bias. All these findings recommend the use of the samples in many technological applications.

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[18] A.A. Azab, N. Helmy, Sukrat Albaaj, Mater. Res. Bull. 66 (2015) 249. [19] Jyoti Rani, K.L. Yadav, Satya Prakash, Mater. Res. Bull. 60 (2014) 367. [20] M.A. Ahmed, N. Okasha, S.I. El-Dek, Ceram. Int 36 (2010) 1529. [21] Miguel Kiwi, J. Magn. Magn. Mater. 234 (2001) 584.

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Table. 1 Values for the saturation magnetization (Ms), remanence magnetization (Mr), coercivity, squareness ratio (SQR) and exchange bias (HE) of the nanocomposites (1-x) CFO + (x) LCFO: 0 ≤ x ≤ 1.

x

SQR

HE

(emu/g)

(G)

0

60.6

31.32

1643

0.516

-3

0.1

54.495

29.987

2306.7

0.550

+5

0.3

43.495

23.875

2271

0.5

40.63

22.1

0.7

27.745

1

1.97

-1.8

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17.9

15.05

2265.7

0.542

-6.8

0.316

192.77

0.160

39.4

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2189

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(emu/g)

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Fig. 1 XRD patterns of the nanocomposites (1-x) CFO + (x) LCFO: 0 ≤ x ≤ 1.

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(a)

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(b)

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(c)

Fig. 2: a-d TEM micrographs of the nanocomposites (1-x) CFO + (x) LCFO with x=0, 0.5 and 1.

(d)

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X=1

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Fig. 3 Room temperature M-H loop for the nanocomposites (1-x) CFO + (x) LCFO: 0 ≤ x ≤ 1.

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Fig. 4 Dependence of the Ms and Mr and Hc on the LCFO content.

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Fig. 5 Compositional dependence of the real part of dielectric constant at selected temperatures for the nanocomposites (1-x) CFO + (x) LCFO: 0 ≤ x ≤ 1.

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Fig. 6 Compositional dependence of the conductivity at selected temperatures for the nanocomposites (1-x) CFO + (x) LCFO: 0 ≤ x ≤ 1.

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Highlights - FM/AFM Nanocomposites were synthesized by citrate autocombustion technique.

- Colossal compositional change in coercivity was observed.

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- Ms and Mr were found to increase linearly with the ferromagnetic content.

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- Dielectric constant increases with the antiferromagnetic phase (AFM) content .