- Email: [email protected]
A study of some properties for Li–Mn nanoparticles ferrite using positron annihilation lifetime technique
Accepted Manuscript A study of some properties for Li–Mn nanoparticles ferrite using Positron annihilation lifetime technique E.Hassan Aly, A.M. Samy PII:
S0925-8388(15)30169-9
DOI:
10.1016/j.jallcom.2015.06.052
Reference:
JALCOM 34391
To appear in:
Journal of Alloys and Compounds
Received Date: 18 May 2015 Accepted Date: 6 June 2015
Please cite this article as: E.H. Aly, A.M. Samy, A study of some properties for Li–Mn nanoparticles ferrite using Positron annihilation lifetime technique, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.06.052. 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.
ACCEPTED MANUSCRIPT
A study of some properties for Li–Mn nanoparticles ferrite using Positron annihilation lifetime technique E. Hassan Alya and A. M. Samya a
Physics Department, Faculty of Science, Ain Shams University, Abbassia, Cairo 11566,
RI PT
Egypt
___________________________________________________________________________
TE D
M AN U
SC
ABSTRACT ____________________________________________________________ Nanoparticle samples of the composition Li(0.5 – 0.5x)MnxFe(2.5-0.5x)O4, (x = 0, 0.25, 0.5 and 0.75) are prepared with a sol-gel auto combustion method. The nano grain size is decreased with increasing the manganese content. The analysis of PAL spectrum indicated the existence of cluster defects. There are different correlations between the lifetime parameters with the manganese content. The relative intensity I2 and the trapping rate қ are increased for samples with x = 0.25 & 0.75, and are decreased for the sample with x = 0.5 relative to the pure nano Li-ferrite. The electrical resistivity took the same behavior as the G.S. by increasing Mn content. The size of domain walls is decreased with Mn content for mono- grain Li ferrite samples.
AC C
EP
Keywords: Nano Li-Mn ferrite; Positron annihilation; grain size; electric resistivity
T/F: +(202)24665630 , F: +(202)2684-2123 E-mail: [email protected] E-mail: [email protected]
ACCEPTED MANUSCRIPT
1- Introduction
AC C
EP
TE D
M AN U
SC
RI PT
Fabrication of microwave devices and many other applications depend mainly on Li-bulk ferrites due to their high electrical resistivity and Curie temperature [1]. Their low coasts attracted the scientists to the importance of these ferrites for technological applications [2]. In the last few years, another study is reported on the nanoparticles ferrites because the conventional solidstate method for preparing bulk ferrites has certain disadvantages. The nonconventional methods such as sol-gel and co-precipitation attracted the researchers to prepare nanoparticles ferrites to enhance the qualities of ferrites [3,4]. For an ideal spinel, manganese must Mn2+ ion. For nanoparticles structure, small Mn2+ ions occupy B-sites [5]. The smaller the particle size, the larger would be this effect. The citrate-nitrate precursor auto combustion method produces ultrafine powders with high chemically homogeneous ferrite and uniform grains [6,7]. But, in the conventional method some Mn3+ ions are formed at final sintering occupying octahedral sites. It is reported that the single domain critical size for Li-ferrite ≈ 74 nm, which corresponds to a maximum value of coercivity [8]. Below this value, the coercive field Hc is directly proportional with the grain size in the single domain region. In this region, a nanoparticle assembly does not have a superparamagnetic behavior in an external magnetic field [8,9]. But, above Hc critical value the coercive field is inversely proportional with the grain size for multi-domain ferrites and it reaches to zero for superparamagnetic rang (It has a bulk-like behavior) [10]. In this work, we aimed to investigate the effect of Mn substituted Li-nano-single domain ferrite using positron lifetime spectroscopy (PALS). This analysis is important to study the open volume defects like vacancies, vacancy-clusters and voids (or microvoids) because they are sensitive towards electron density changes. From the decomposition of the lifetime spectra, different positron annihilation states can be obtained which are correlated with structural open volume defects in metals, semiconductors and alloys [11, 12]. In the bulk of substance, the positron will have free annihilation with an electron. But, in the
2
ACCEPTED MANUSCRIPT
presence of defects the electron density is reduced leading to an increase in its lifetime at grain boundaries.
RI PT
2. Experimental techniques:
AC C
EP
TE D
M AN U
SC
Mono-domains of nanograins ferrite with the chemical formula Li(0.5 – 0.5x)MnxFe(2.5-0.5x)O4, (x = 0, 0.25, 0.5 and 0.75) are prepared with a sol-gel auto combustion method. The stoichiometric metal nitrates of LiNO3 (anhydrous), manganese nitrate (Mn(NO3)2.4H2O) and ferric nitrate (Fe(NO3)3.9H2O) are dissolved in de-ionized water with subsequent addition of citric acid (C6O7H8) and then ammonia (NH3) is added until the pH of the solution reaches 8.3. By heating (70 – 85 oC) and after evaporation, the gel is formed. Mono-domains LiMn ferrite samples are formed after completing the chemical reaction samples. All prepared samples, annealed at temperature (Ta = 673K), at heating rate of 10 o C/min for 1 h at atmospheric pressure. The flow chart of the preparation method is reported in ref. [8]. At room temperature using x-ray diffraction with CuKα radiation of λ = 1.54056 Å, the average lattice parameter is calculated for all investigated samples using the relation a = dhkl (h2 + k2 + l2)(1/2) , where dhkl is the inter-planer spacing for a cubic crystal and h, k, l are the Miller indices of each plane. Using DebyeSherrer formula, D = (Kλ)/(β cosθ), the crystal size is calculated at each (hkl) for each sample. K is a constant (0.9), β is the full width at each half maximum and θ is the corresponding diffraction angle [13]. The porosity percentage is calculated using Archimed's principle [8]. All samples are investigated using positron annihilation lifetime spectroscopy (PALS). Positron annhilation lifetime measurements were carried out at room temperature using a standard fast-fast coincident lifetime spectrometer. 11 µCi 22 Na source sealed between two kapton foils (thickness less than 1 mg/cm2) with a small active diameter of l-2 mm in sandwich geometry with the pellets. Two identical plastic scintillator detectors fitted with Hamamatsu photomultiplier
3
ACCEPTED MANUSCRIPT
TE D
M AN U
SC
RI PT
tubes [H3378-50] NO. BA0828 with a prompt resolution of about 250 ps (full width at half-maximum, FWHM) was used in the present study. With a channel constant of 6.5 ps. Lifetime spectra were recorded for each sample with about 5 X 106 counts accumulated under the peak. After source correction was determined using a properly defect free Silicon sample, the lifetime spectra were analyzed in two components using the computer program LT [14] with the best fit χ2 < 1.1. The two lifetime components 382 ps/11 % and 1500 ps/0.7%, which were attributed to annihilation in kapton and at the surfaces, were kept fixed during the analysis. The measured PAL spectrum analyzed as two lifetime components τ1 and τ2, order of a few hundred picoseconds, which have relative intensities I1 and I2. The respective intensities indicate the relative number of positrons that annihilate with different annihilation processes. On the basis of the two-state trapping model [15], the shorter-lived component τ1 and I1 belonged to positrons annihilating in the matrix and dislocation loops , τ2 and I2 characterizing the annihilation of positrons trapped in vacancy clusters and three-dimensional mono vacancies defects. 3. Results and discussion
AC C
EP
Fig. 1 shows the variation of the relative intensity I2 and the nano grain size (G.S.) for the composition Li(0.5 – 0.5x)MnxFe(2.5-0.5x)O4, (x = 0, 0.25, 0.5 and 0.75) as a function of manganese concentration (x). The relative intensity I2 represents the trapping defects concentrations at the grain boundaries. The figure shows that I2 increases for the sample with x = 0.25 and then decreases for x = 0.5 relative to the pure Li-ferrite sample (x = 0.0). Meanwhile, the figure shows that the grain size decreases abruptly and mostly becomes constant for all values of x relative to the pure nano grain Li- ferrite sample. This decrease accounts on the relative increase of the defects concentration (I2) at the grain boundaries for the samples with x = 0.25 and x = 0.75. Obviously, the increase of I2 pointed out to the increase of the grain boundary thickness relative to the pure sample. To account on the small variation in the grain size for samples
4
ACCEPTED MANUSCRIPT
RI PT
with x > 0.25, it is reported that the total porosity for ferrite is the sum of inter and intra granular pores [ 16-17]. Accordingly, the decrease in I2 for the sample with x = 0.5 relative to that with x = 0.25 attributed to the decrease of the intergranular pores. However, the increase of I2 for the sample with x = 0.75 relative to x = 0.5 attributes to the increase of the inter-granular pores, Fig. 1.
AC C
EP
TE D
M AN U
SC
Fig. 2 shows that lifetime,τ2, of positrons annihilated in defects at the grain boundaries which almost takes a reverse behavior of porosity. Then, one can consider the increase of τ2 is due to the decrease of the inter-granular pores. A similar result was reported for bulk and nano ferrite samples [19-22]. The decrease in τ2 for the sample with manganese x = 0.75 attributes to the increase of the inter pores for this sample relative to x = 0.5 one. It is important to consider the existence of cluster defects at the grain walls. This consideration comes from the long values of τ2 [23]. This is a novel point result using the PALS for investigating the properties of Li-ferrite substituted with manganese. Furthermore, fig. 2 shows the decrease of the total porosity Pt (%) with increasing Mn-concentration except for the sample with x = 0.75. This decrease in the values of Pt (%) attributes to the increase of the average lattice parameter with increasing Mn content except for the sample with x = 0.75, it becomes nearly unchanged, Fig. 3. Then more stress at the grain boundaries as a defect can be result and account on the nearly constancy of the lattice parameter for this sample, as shown in Fig. 3. But, the increase in the average lattice parameter ȃ (Ǻ) for all other samples with x < 0.75 attributes to larger ionic radius of Mn2+ (r = 0.8 Ǻ) than the average ionic radii of Fe 3+ and Li1+ (r = 0.64Ǻ & r = 0.6 Ǻ respectively). A similar increase of lattice parameter with decreasing the size of nano-crystalline Li- ferrite single domains was reported [18]. Fig. 4 shows the behavior of the trapping rate қ and I2 with the manganese concentration. It is clear that as the concentration of defects at the grain boundaries increase (I2) the trapping rate, (κ) increases. In other words, there is
5
ACCEPTED MANUSCRIPT
a correlation between G.S. and the positron annihilation lifetime parameters (I2, τ2 and қ )for Li-Mn nano-ferrite samples.
EP
TE D
M AN U
SC
RI PT
The variation of the short lifetime τ1 and its intensity I1 with the concentration of Mn is shown in Fig. 5. The Figure shows that the relative intensity I1, which is a measure of the concentration of defects insides the grains decreases for lithium nano ferrite sample with x = 0.25. This means that the decrease in the intra- pores inside the domain (grain) is due to the decrease of its mono G.S. and the increase in lattice parameter. But, for nano-Li ferrite with x = 0.5, the G.S. is not changed relative to Li-ferrite with x = 0.25. Then, the increase in the intensity I1 attributes to the increase of the intra granular pores as a result of increasing Mn-content in Li-ferrite sample. This accounts on the increase of the lifetime τ1 with increasing the manganese content. Furthermore τ1 increases due to the increase in lattice parameter with increasing Mn content, Fig. 3 as another factor. However, the hopping of the electrons at B – sites decreases with increasing Mn content in nano grains Li ferrite, because the ionic radius of Mn2+ ions is greater than the average sum of the ionic radii that for Li1+ and Fe3+ ions. As the lattice parameter is unchanged for the sample with x = 0.75, τ1 decreased as well as the decreases of the intra granular pores relative to the sample with x = 0.5, Fig. 5. This accounts on the decrease of the intensity I1 for the sample with x = 0.75 relative to that with x = 0.5.
AC C
Fig. 6 shows the variation of G.S and the electrical resistivity ρ with Mn content. Obviously, the behavior of the electrical resistivity is almost the same as the variation of the G.S. with increasing Mn content. The figure indicates the abrupt decrease of the electrical resistivity for the sample with x = 0.25, then decreases slowly and then increases again at x = 0.75. Accordingly, the dominant factor for decreasing ρ is the decrease of the mono domain size (grain). For the nano-grain single domain, the magneto crystalline anisotropy plays an important role. However, the domain size decreases abruptly at x = 0.25 the magnetic energy decreases and consequently the anisotropy energy.
6
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
This result accounts on the decreased electrical resistivity at x = 0.25. Although, the grain boundaries increase their thicknesses decrease because these represent domain walls. These domain walls size decrease due to the decrease of the anisotropy energy [24]. Meanwhile, the increase of lattice parameter leads to the decreasing of the exchange energy. This is considered as a second factor that accounts on the decrease on the domain walls size although the grain size decreases. Accordingly, the decrease of the grain size for mono domain Liferrite means decreasing the size of their domain walls. This is considered as a new novel point for nano grain mono domain Li-Mn ferrite samples. From another point of view, a reverse behavior is reported for the bulk multi – domain ferrite [25, 26], in general. Meanwhile, the increase in the porosity plays a dominant role for the increase of the electrical resistivity for bulk ferrites as well as the increase in lattice parameters or the decrease of the grain size [2728]. Also, the decrease of the inter granular pores with increasing the Mn content except for the sample with x = 0.75 affects and accounts on the decrease of ρ, Fig. 6. The small increase in ρ for the nano sample with x = 0.75 pointed to the increase of the intra granular pores and the unchanged lattice parameter relative to the sample with x = 0.5. Furthermore, one must take into consideration the hopping of electrons at B-sites. But, this factor is not dominant. As it was mentioned before, this hopping decreases with increasing the concentration on Mn-content. This leads to increase the lifetime τ1 of the hopping of the electrons, Fig. 5 and leads to increase the electrical resistivity. 4. Conclusions
• The nano grain size decreases for single domains Li-ferrite substituted with manganese x = 0.25, and mostly becomes unchanged for all values of x. • The average lattice parameter is increased with increasing Mn content, and nearly becames unchanged for the sample with x = 0.75.
7
ACCEPTED MANUSCRIPT
M AN U
SC
RI PT
• The highest values of I2 and κ for Li ferrite with x = 0.25 due to the abrupt increase of the grain boundary thickness. • The lifetime τ2 increases due to the decrease of inter granular pores. • For sample with x = 0.25 and x = 0.75 the relative intensity I1is decreased due to the decrease of intra pores. • The lifetime τ1 increases due to the increase of the average lattice parameter with increasing Mn content. • The electrical resistivity for x = 0.25 decreases due to the dominance of decreasing the anisotropy energy for nano grain. But, the dominant factor for increasing the electrical resistivity for x = 0.75 is the increase of intra pores.
EP
TE D
The novel points for Li- nano ferrite substituted with manganese are: 1- The values of lifetime τ2 confirm the existence of cluster defects at the domain walls. 2- The decrease of the domain walls thickness with increasing manganese content is due to the decrease of the anisotropy energy and the exchange energy. 3- The decrease of the electrical resistivity is due to the decrease of the grain size and the increase of lattice parameter. It is a reverse behavior to the bulk ferrite.
AC C
Acknowledgements
The authors would like to express their deepest thanks to Prof. Dr. A.A. Sattar, head of Magnetism Lab. Physics Dep. Faculty of Science, Ain Shams University. They are also much indebted to Prof. Reinhard Krause-Rehberg for providing the positron experiments in the Martin-Luther-University HalleWittenberg, Germany.
References
8
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[1] A.A.Sattar, W.R.Agami, Journal of Alloys and Comp. 496 (2010) 341-344. [2] Yen-Pei Fe, Shen Tsao, Chen-Ti Hu, Yeong-Der Yao, Journal of Alloys and Comp. 395 (2005) 272-276. [3] I.Z. Rahman and T.T. Ahmed, J.Magn. Magn. Mater. 290-291(2005) 1576-1579. [4] S. Komarneni, E. Fregeau, E. Breval and R.Roy, J.Am.Ceram.Soc. 71(1)(1988), C26-C28. [5] Amarendra K. Singh, T.C.Goel and R.G. Mendiratta, Phys. Stat. Sol. (a) (2004) 1-5. [6] A.R.Bueno, M.L. Gregori and M.C.S. Nobrega, Mater.Chem. Phys. 105(2007) 229-233. [7] A. Verma and R. Chatterjee, J. Magn. Magn. Mater. 306(2006) 313-320. [8] W.R. Agami, M.A. Ashmawy and A.A. Sattar, J.of Materi Eng and Perform. 23(2) (2014) 604. [9] B.D.Cullity, Introduction to Magnetic Materials, John Wiley & Sons Inc., Hoboken, NJ, 2009 [10] C.Caizer and M.Stefanescu, J.Phys. D: Appl. Phys. 35(2002) 3035-3040. [11] M. George, S.S. Nair, A.M. John, P.A. Joy, M.R.Anantharaman, J. Phys. D: Appl. Phys. 39 (2006)900-910. [12] G.F.Dionne, J. Appl. Phys. 81(1997) 5064-5069. [13] B.D. Cullity and S.R. Stock, Elements of X-ray Diffraction, 3rd ed., Prentice-Hall Inc, New York, 2001, p 167-171 [14] J. Kansy, Nucl. Instrum. Methods Phys. Res., Sect. A 374 (1996) 235. [15] P. Hautojarvi, Positrons in Solids, Springer, Berlin, 1979. [16] A.M. Sattar, H.M. El-Sayed and W.R. Agami, J of Materials Engineering and Perform. 16 (5) (2007) 573. [17] A.M. Samy, Joural of Materials Science and Engineering with Advanced technology 4(2) (2011) 133. [18] H. Yang, Z. Wang, L. Song, M. Zhao, J. Wang and H. Luo, J. Phys. D: Appl. Phys. 29(1996) 2574-2578.
9
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[19] A.M. Samy , N. Mostafa, E. Gomaa, Journal Applied Surface Science 252 (2006) 3323. [20] A.M. Samy, E. Gomaa, N. Mostafa, The Open Ceramic Science Journal 1 (2010) 1. [21] E. Hassan Aly, A. M. Samy , Journal Results in Physics 5 (2015) 80-84. [22] A. M. Samy and E. Hassan Aly, Materials Sciences and Applications vol.6 No.5, May (2015) [23] T.E.M. Staab, R.Krause-Rehberg, B.Kieback, Journal of Materials Science 34 (1999) 3833-3851. [24] Charles Kittel, Elementary Solid State Physics, John Wiley & Sons, Inc., New York- London 1962 [25] A.A. Sattar and W.R. Agami, Journal of alloys and comp. 496 (2010) 341-344. [26] Y. Dong, L.Y. Xiong, C.W. Lung, J. Phys. Condens. Matter 3 (1991) 3155. [27] A.M. Samy, Journal of Materials Engineering and Perform. 20(7)(2011) 1315-1318. [28] A.M. Samy, Journal of Materials Engineering and Perform. 12(5) (2003) 569. [29] A.A. Sattar, J. Mater. Sci.39( 2004) 451. [30] A.M. Samy, A.A. Sattar and Ibrahim Hassan Afify, J. Alloy and Comp. 505 (2010) 297
10
RI PT
ACCEPTED MANUSCRIPT
TE D
M AN U
SC
Figures:
AC C
EP
Fig. 1. The variation of the grain size (G.S.) and relative intensity I2 with manganese content.
11
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
Fig. 2. Variation of the total porosity and τ2 with manganese content.
Fig. 3. Dependence of the average lattice parameter on manganese content.
12
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig.4. Variation of the trapping rate (қ) and relative intensity I2 with manganese content.
Fig. 5. Dependence of the relative intensity I1 and lifetime τ1 on manganese content.
13
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig. 6. Variation of the grain size τ2 and the electrical resistivity ρ with manganese content.
14
ACCEPTED MANUSCRIPT
Highlights • Lifetime values τ2 confirm the existence of cluster defects at the
RI PT
domain walls • The decrease of the domain walls thickness with increasing manganese content
SC
• The decrease of the anisotropy energy and the exchange energy. • The decrease of the electrical resistivity is due to the decrease of the grain size
AC C
EP
TE D
M AN U
• It is a reverse behavior to the bulk ferrite.
S0925-8388(15)30169-9
DOI:
10.1016/j.jallcom.2015.06.052
Reference:
JALCOM 34391
To appear in:
Journal of Alloys and Compounds
Received Date: 18 May 2015 Accepted Date: 6 June 2015
Please cite this article as: E.H. Aly, A.M. Samy, A study of some properties for Li–Mn nanoparticles ferrite using Positron annihilation lifetime technique, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.06.052. 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.
ACCEPTED MANUSCRIPT
A study of some properties for Li–Mn nanoparticles ferrite using Positron annihilation lifetime technique E. Hassan Alya and A. M. Samya a
Physics Department, Faculty of Science, Ain Shams University, Abbassia, Cairo 11566,
RI PT
Egypt
___________________________________________________________________________
TE D
M AN U
SC
ABSTRACT ____________________________________________________________ Nanoparticle samples of the composition Li(0.5 – 0.5x)MnxFe(2.5-0.5x)O4, (x = 0, 0.25, 0.5 and 0.75) are prepared with a sol-gel auto combustion method. The nano grain size is decreased with increasing the manganese content. The analysis of PAL spectrum indicated the existence of cluster defects. There are different correlations between the lifetime parameters with the manganese content. The relative intensity I2 and the trapping rate қ are increased for samples with x = 0.25 & 0.75, and are decreased for the sample with x = 0.5 relative to the pure nano Li-ferrite. The electrical resistivity took the same behavior as the G.S. by increasing Mn content. The size of domain walls is decreased with Mn content for mono- grain Li ferrite samples.
AC C
EP
Keywords: Nano Li-Mn ferrite; Positron annihilation; grain size; electric resistivity
T/F: +(202)24665630 , F: +(202)2684-2123 E-mail: [email protected] E-mail: [email protected]
ACCEPTED MANUSCRIPT
1- Introduction
AC C
EP
TE D
M AN U
SC
RI PT
Fabrication of microwave devices and many other applications depend mainly on Li-bulk ferrites due to their high electrical resistivity and Curie temperature [1]. Their low coasts attracted the scientists to the importance of these ferrites for technological applications [2]. In the last few years, another study is reported on the nanoparticles ferrites because the conventional solidstate method for preparing bulk ferrites has certain disadvantages. The nonconventional methods such as sol-gel and co-precipitation attracted the researchers to prepare nanoparticles ferrites to enhance the qualities of ferrites [3,4]. For an ideal spinel, manganese must Mn2+ ion. For nanoparticles structure, small Mn2+ ions occupy B-sites [5]. The smaller the particle size, the larger would be this effect. The citrate-nitrate precursor auto combustion method produces ultrafine powders with high chemically homogeneous ferrite and uniform grains [6,7]. But, in the conventional method some Mn3+ ions are formed at final sintering occupying octahedral sites. It is reported that the single domain critical size for Li-ferrite ≈ 74 nm, which corresponds to a maximum value of coercivity [8]. Below this value, the coercive field Hc is directly proportional with the grain size in the single domain region. In this region, a nanoparticle assembly does not have a superparamagnetic behavior in an external magnetic field [8,9]. But, above Hc critical value the coercive field is inversely proportional with the grain size for multi-domain ferrites and it reaches to zero for superparamagnetic rang (It has a bulk-like behavior) [10]. In this work, we aimed to investigate the effect of Mn substituted Li-nano-single domain ferrite using positron lifetime spectroscopy (PALS). This analysis is important to study the open volume defects like vacancies, vacancy-clusters and voids (or microvoids) because they are sensitive towards electron density changes. From the decomposition of the lifetime spectra, different positron annihilation states can be obtained which are correlated with structural open volume defects in metals, semiconductors and alloys [11, 12]. In the bulk of substance, the positron will have free annihilation with an electron. But, in the
2
ACCEPTED MANUSCRIPT
presence of defects the electron density is reduced leading to an increase in its lifetime at grain boundaries.
RI PT
2. Experimental techniques:
AC C
EP
TE D
M AN U
SC
Mono-domains of nanograins ferrite with the chemical formula Li(0.5 – 0.5x)MnxFe(2.5-0.5x)O4, (x = 0, 0.25, 0.5 and 0.75) are prepared with a sol-gel auto combustion method. The stoichiometric metal nitrates of LiNO3 (anhydrous), manganese nitrate (Mn(NO3)2.4H2O) and ferric nitrate (Fe(NO3)3.9H2O) are dissolved in de-ionized water with subsequent addition of citric acid (C6O7H8) and then ammonia (NH3) is added until the pH of the solution reaches 8.3. By heating (70 – 85 oC) and after evaporation, the gel is formed. Mono-domains LiMn ferrite samples are formed after completing the chemical reaction samples. All prepared samples, annealed at temperature (Ta = 673K), at heating rate of 10 o C/min for 1 h at atmospheric pressure. The flow chart of the preparation method is reported in ref. [8]. At room temperature using x-ray diffraction with CuKα radiation of λ = 1.54056 Å, the average lattice parameter is calculated for all investigated samples using the relation a = dhkl (h2 + k2 + l2)(1/2) , where dhkl is the inter-planer spacing for a cubic crystal and h, k, l are the Miller indices of each plane. Using DebyeSherrer formula, D = (Kλ)/(β cosθ), the crystal size is calculated at each (hkl) for each sample. K is a constant (0.9), β is the full width at each half maximum and θ is the corresponding diffraction angle [13]. The porosity percentage is calculated using Archimed's principle [8]. All samples are investigated using positron annihilation lifetime spectroscopy (PALS). Positron annhilation lifetime measurements were carried out at room temperature using a standard fast-fast coincident lifetime spectrometer. 11 µCi 22 Na source sealed between two kapton foils (thickness less than 1 mg/cm2) with a small active diameter of l-2 mm in sandwich geometry with the pellets. Two identical plastic scintillator detectors fitted with Hamamatsu photomultiplier
3
ACCEPTED MANUSCRIPT
TE D
M AN U
SC
RI PT
tubes [H3378-50] NO. BA0828 with a prompt resolution of about 250 ps (full width at half-maximum, FWHM) was used in the present study. With a channel constant of 6.5 ps. Lifetime spectra were recorded for each sample with about 5 X 106 counts accumulated under the peak. After source correction was determined using a properly defect free Silicon sample, the lifetime spectra were analyzed in two components using the computer program LT [14] with the best fit χ2 < 1.1. The two lifetime components 382 ps/11 % and 1500 ps/0.7%, which were attributed to annihilation in kapton and at the surfaces, were kept fixed during the analysis. The measured PAL spectrum analyzed as two lifetime components τ1 and τ2, order of a few hundred picoseconds, which have relative intensities I1 and I2. The respective intensities indicate the relative number of positrons that annihilate with different annihilation processes. On the basis of the two-state trapping model [15], the shorter-lived component τ1 and I1 belonged to positrons annihilating in the matrix and dislocation loops , τ2 and I2 characterizing the annihilation of positrons trapped in vacancy clusters and three-dimensional mono vacancies defects. 3. Results and discussion
AC C
EP
Fig. 1 shows the variation of the relative intensity I2 and the nano grain size (G.S.) for the composition Li(0.5 – 0.5x)MnxFe(2.5-0.5x)O4, (x = 0, 0.25, 0.5 and 0.75) as a function of manganese concentration (x). The relative intensity I2 represents the trapping defects concentrations at the grain boundaries. The figure shows that I2 increases for the sample with x = 0.25 and then decreases for x = 0.5 relative to the pure Li-ferrite sample (x = 0.0). Meanwhile, the figure shows that the grain size decreases abruptly and mostly becomes constant for all values of x relative to the pure nano grain Li- ferrite sample. This decrease accounts on the relative increase of the defects concentration (I2) at the grain boundaries for the samples with x = 0.25 and x = 0.75. Obviously, the increase of I2 pointed out to the increase of the grain boundary thickness relative to the pure sample. To account on the small variation in the grain size for samples
4
ACCEPTED MANUSCRIPT
RI PT
with x > 0.25, it is reported that the total porosity for ferrite is the sum of inter and intra granular pores [ 16-17]. Accordingly, the decrease in I2 for the sample with x = 0.5 relative to that with x = 0.25 attributed to the decrease of the intergranular pores. However, the increase of I2 for the sample with x = 0.75 relative to x = 0.5 attributes to the increase of the inter-granular pores, Fig. 1.
AC C
EP
TE D
M AN U
SC
Fig. 2 shows that lifetime,τ2, of positrons annihilated in defects at the grain boundaries which almost takes a reverse behavior of porosity. Then, one can consider the increase of τ2 is due to the decrease of the inter-granular pores. A similar result was reported for bulk and nano ferrite samples [19-22]. The decrease in τ2 for the sample with manganese x = 0.75 attributes to the increase of the inter pores for this sample relative to x = 0.5 one. It is important to consider the existence of cluster defects at the grain walls. This consideration comes from the long values of τ2 [23]. This is a novel point result using the PALS for investigating the properties of Li-ferrite substituted with manganese. Furthermore, fig. 2 shows the decrease of the total porosity Pt (%) with increasing Mn-concentration except for the sample with x = 0.75. This decrease in the values of Pt (%) attributes to the increase of the average lattice parameter with increasing Mn content except for the sample with x = 0.75, it becomes nearly unchanged, Fig. 3. Then more stress at the grain boundaries as a defect can be result and account on the nearly constancy of the lattice parameter for this sample, as shown in Fig. 3. But, the increase in the average lattice parameter ȃ (Ǻ) for all other samples with x < 0.75 attributes to larger ionic radius of Mn2+ (r = 0.8 Ǻ) than the average ionic radii of Fe 3+ and Li1+ (r = 0.64Ǻ & r = 0.6 Ǻ respectively). A similar increase of lattice parameter with decreasing the size of nano-crystalline Li- ferrite single domains was reported [18]. Fig. 4 shows the behavior of the trapping rate қ and I2 with the manganese concentration. It is clear that as the concentration of defects at the grain boundaries increase (I2) the trapping rate, (κ) increases. In other words, there is
5
ACCEPTED MANUSCRIPT
a correlation between G.S. and the positron annihilation lifetime parameters (I2, τ2 and қ )for Li-Mn nano-ferrite samples.
EP
TE D
M AN U
SC
RI PT
The variation of the short lifetime τ1 and its intensity I1 with the concentration of Mn is shown in Fig. 5. The Figure shows that the relative intensity I1, which is a measure of the concentration of defects insides the grains decreases for lithium nano ferrite sample with x = 0.25. This means that the decrease in the intra- pores inside the domain (grain) is due to the decrease of its mono G.S. and the increase in lattice parameter. But, for nano-Li ferrite with x = 0.5, the G.S. is not changed relative to Li-ferrite with x = 0.25. Then, the increase in the intensity I1 attributes to the increase of the intra granular pores as a result of increasing Mn-content in Li-ferrite sample. This accounts on the increase of the lifetime τ1 with increasing the manganese content. Furthermore τ1 increases due to the increase in lattice parameter with increasing Mn content, Fig. 3 as another factor. However, the hopping of the electrons at B – sites decreases with increasing Mn content in nano grains Li ferrite, because the ionic radius of Mn2+ ions is greater than the average sum of the ionic radii that for Li1+ and Fe3+ ions. As the lattice parameter is unchanged for the sample with x = 0.75, τ1 decreased as well as the decreases of the intra granular pores relative to the sample with x = 0.5, Fig. 5. This accounts on the decrease of the intensity I1 for the sample with x = 0.75 relative to that with x = 0.5.
AC C
Fig. 6 shows the variation of G.S and the electrical resistivity ρ with Mn content. Obviously, the behavior of the electrical resistivity is almost the same as the variation of the G.S. with increasing Mn content. The figure indicates the abrupt decrease of the electrical resistivity for the sample with x = 0.25, then decreases slowly and then increases again at x = 0.75. Accordingly, the dominant factor for decreasing ρ is the decrease of the mono domain size (grain). For the nano-grain single domain, the magneto crystalline anisotropy plays an important role. However, the domain size decreases abruptly at x = 0.25 the magnetic energy decreases and consequently the anisotropy energy.
6
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
This result accounts on the decreased electrical resistivity at x = 0.25. Although, the grain boundaries increase their thicknesses decrease because these represent domain walls. These domain walls size decrease due to the decrease of the anisotropy energy [24]. Meanwhile, the increase of lattice parameter leads to the decreasing of the exchange energy. This is considered as a second factor that accounts on the decrease on the domain walls size although the grain size decreases. Accordingly, the decrease of the grain size for mono domain Liferrite means decreasing the size of their domain walls. This is considered as a new novel point for nano grain mono domain Li-Mn ferrite samples. From another point of view, a reverse behavior is reported for the bulk multi – domain ferrite [25, 26], in general. Meanwhile, the increase in the porosity plays a dominant role for the increase of the electrical resistivity for bulk ferrites as well as the increase in lattice parameters or the decrease of the grain size [2728]. Also, the decrease of the inter granular pores with increasing the Mn content except for the sample with x = 0.75 affects and accounts on the decrease of ρ, Fig. 6. The small increase in ρ for the nano sample with x = 0.75 pointed to the increase of the intra granular pores and the unchanged lattice parameter relative to the sample with x = 0.5. Furthermore, one must take into consideration the hopping of electrons at B-sites. But, this factor is not dominant. As it was mentioned before, this hopping decreases with increasing the concentration on Mn-content. This leads to increase the lifetime τ1 of the hopping of the electrons, Fig. 5 and leads to increase the electrical resistivity. 4. Conclusions
• The nano grain size decreases for single domains Li-ferrite substituted with manganese x = 0.25, and mostly becomes unchanged for all values of x. • The average lattice parameter is increased with increasing Mn content, and nearly becames unchanged for the sample with x = 0.75.
7
ACCEPTED MANUSCRIPT
M AN U
SC
RI PT
• The highest values of I2 and κ for Li ferrite with x = 0.25 due to the abrupt increase of the grain boundary thickness. • The lifetime τ2 increases due to the decrease of inter granular pores. • For sample with x = 0.25 and x = 0.75 the relative intensity I1is decreased due to the decrease of intra pores. • The lifetime τ1 increases due to the increase of the average lattice parameter with increasing Mn content. • The electrical resistivity for x = 0.25 decreases due to the dominance of decreasing the anisotropy energy for nano grain. But, the dominant factor for increasing the electrical resistivity for x = 0.75 is the increase of intra pores.
EP
TE D
The novel points for Li- nano ferrite substituted with manganese are: 1- The values of lifetime τ2 confirm the existence of cluster defects at the domain walls. 2- The decrease of the domain walls thickness with increasing manganese content is due to the decrease of the anisotropy energy and the exchange energy. 3- The decrease of the electrical resistivity is due to the decrease of the grain size and the increase of lattice parameter. It is a reverse behavior to the bulk ferrite.
AC C
Acknowledgements
The authors would like to express their deepest thanks to Prof. Dr. A.A. Sattar, head of Magnetism Lab. Physics Dep. Faculty of Science, Ain Shams University. They are also much indebted to Prof. Reinhard Krause-Rehberg for providing the positron experiments in the Martin-Luther-University HalleWittenberg, Germany.
References
8
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[1] A.A.Sattar, W.R.Agami, Journal of Alloys and Comp. 496 (2010) 341-344. [2] Yen-Pei Fe, Shen Tsao, Chen-Ti Hu, Yeong-Der Yao, Journal of Alloys and Comp. 395 (2005) 272-276. [3] I.Z. Rahman and T.T. Ahmed, J.Magn. Magn. Mater. 290-291(2005) 1576-1579. [4] S. Komarneni, E. Fregeau, E. Breval and R.Roy, J.Am.Ceram.Soc. 71(1)(1988), C26-C28. [5] Amarendra K. Singh, T.C.Goel and R.G. Mendiratta, Phys. Stat. Sol. (a) (2004) 1-5. [6] A.R.Bueno, M.L. Gregori and M.C.S. Nobrega, Mater.Chem. Phys. 105(2007) 229-233. [7] A. Verma and R. Chatterjee, J. Magn. Magn. Mater. 306(2006) 313-320. [8] W.R. Agami, M.A. Ashmawy and A.A. Sattar, J.of Materi Eng and Perform. 23(2) (2014) 604. [9] B.D.Cullity, Introduction to Magnetic Materials, John Wiley & Sons Inc., Hoboken, NJ, 2009 [10] C.Caizer and M.Stefanescu, J.Phys. D: Appl. Phys. 35(2002) 3035-3040. [11] M. George, S.S. Nair, A.M. John, P.A. Joy, M.R.Anantharaman, J. Phys. D: Appl. Phys. 39 (2006)900-910. [12] G.F.Dionne, J. Appl. Phys. 81(1997) 5064-5069. [13] B.D. Cullity and S.R. Stock, Elements of X-ray Diffraction, 3rd ed., Prentice-Hall Inc, New York, 2001, p 167-171 [14] J. Kansy, Nucl. Instrum. Methods Phys. Res., Sect. A 374 (1996) 235. [15] P. Hautojarvi, Positrons in Solids, Springer, Berlin, 1979. [16] A.M. Sattar, H.M. El-Sayed and W.R. Agami, J of Materials Engineering and Perform. 16 (5) (2007) 573. [17] A.M. Samy, Joural of Materials Science and Engineering with Advanced technology 4(2) (2011) 133. [18] H. Yang, Z. Wang, L. Song, M. Zhao, J. Wang and H. Luo, J. Phys. D: Appl. Phys. 29(1996) 2574-2578.
9
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[19] A.M. Samy , N. Mostafa, E. Gomaa, Journal Applied Surface Science 252 (2006) 3323. [20] A.M. Samy, E. Gomaa, N. Mostafa, The Open Ceramic Science Journal 1 (2010) 1. [21] E. Hassan Aly, A. M. Samy , Journal Results in Physics 5 (2015) 80-84. [22] A. M. Samy and E. Hassan Aly, Materials Sciences and Applications vol.6 No.5, May (2015) [23] T.E.M. Staab, R.Krause-Rehberg, B.Kieback, Journal of Materials Science 34 (1999) 3833-3851. [24] Charles Kittel, Elementary Solid State Physics, John Wiley & Sons, Inc., New York- London 1962 [25] A.A. Sattar and W.R. Agami, Journal of alloys and comp. 496 (2010) 341-344. [26] Y. Dong, L.Y. Xiong, C.W. Lung, J. Phys. Condens. Matter 3 (1991) 3155. [27] A.M. Samy, Journal of Materials Engineering and Perform. 20(7)(2011) 1315-1318. [28] A.M. Samy, Journal of Materials Engineering and Perform. 12(5) (2003) 569. [29] A.A. Sattar, J. Mater. Sci.39( 2004) 451. [30] A.M. Samy, A.A. Sattar and Ibrahim Hassan Afify, J. Alloy and Comp. 505 (2010) 297
10
RI PT
ACCEPTED MANUSCRIPT
TE D
M AN U
SC
Figures:
AC C
EP
Fig. 1. The variation of the grain size (G.S.) and relative intensity I2 with manganese content.
11
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
Fig. 2. Variation of the total porosity and τ2 with manganese content.
Fig. 3. Dependence of the average lattice parameter on manganese content.
12
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig.4. Variation of the trapping rate (қ) and relative intensity I2 with manganese content.
Fig. 5. Dependence of the relative intensity I1 and lifetime τ1 on manganese content.
13
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Fig. 6. Variation of the grain size τ2 and the electrical resistivity ρ with manganese content.
14
ACCEPTED MANUSCRIPT
Highlights • Lifetime values τ2 confirm the existence of cluster defects at the
RI PT
domain walls • The decrease of the domain walls thickness with increasing manganese content
SC
• The decrease of the anisotropy energy and the exchange energy. • The decrease of the electrical resistivity is due to the decrease of the grain size
AC C
EP
TE D
M AN U
• It is a reverse behavior to the bulk ferrite.