Journal ofScience <& Technology 99 (2014) 054-058
Structure And Magnetic Properties Of Nanosized Nii-;cZatFe204 Ferrite Particles Prepared By Co-Precipitation Method
Luong Ngoc Anh*, Nguyen Phuc Duong, Dao Thi Thuy Nguyet, Do Hoang Tu, To Thanh Loan, Than Due Hien
Hanoi University of Science and Technology, No.l Dai Co Viet Str., HaNoi, Viet Nam Received. December 05, 2013, accepted: April 22, 2014
Abstract
Nii-xZn,Fe204 (x = 0- 0.8) ferrite nanoparticles were prepared by co-precipitation method annealed in air at 1100'C for 5h. The samples annealed at 1100'C crystallize solely in spinel structure according to XRD measurements of which the lattice constant increased with increasing Zn content The average particle sizes are in submicron range as revealed by SEM measurements. Magnetization curves were studied by vibrating sample magnetometer (VSM) in the temperature from 80 K to above magnetic ordenng temperature from that spontaneuos magnetization Ms and Curie temperature Tc of the samples were detennined. Blocking temperature Te was determined by zero-field-cooled (FC) and field-cooled (FC) measurements which showed that the pariicles are superparamagnetic at temperatures above room temperature and TB decreases with increasing Zn content Magnetic properiies of the samples were compared with those ofthe bulks and are discussed based on the cation distribution and the concentration of zinc ions in the crystal lattice.
Keywords: Ni-Zn fernte, Co-precipitalion, Spontaneuos magnetization Ms, Tc, TB,
1. Introduction
Ni-Zn errites belong to the family of soft magnetic materials which have vast applications in both low and high frequency devices [1]. They play an important role in many technological applications because of their high resistivity, low dielectric loss, mechanical hardness, high Cune temperature and chemical stability [2,3]. However, the recent technological advances in electronics industry demand even more compact cores for work at higher frequencies [4]. One way to solve this problem is by synthesizing the feirite particles in nanomettic scales before compacting them for sintering. When the size of the magnetic particle is smaller than the critical size for multidomain formation, the particle is in a smgle domain state. As a results, domain wall resonance is avoided, and the material can work at higher frequencies. On the other hand, the remarkable improvement in the properties of the ferrite nanoparticles finds applications in electronics, bioprocessing, magnetic resonance image enhancement and ferrofluids [5-8].
A surprising fact about mixed ferrites containing zinc is that the addition of the nonmagnetic Zn2+ ion increases the saturation magnetization. The Zn2+ ions of zero moment go to the tetrahedral A sites as in pure Zn fernte, thus
" Corresponding Author' Tel: (+84) 904.147.046 Email: ngocanh@itims edu.vn
weakenmg the tetrahedral A site moment, and the Fe3+ ions from the Zn ferrite now have parallel moments in the octahedral B sites, because ofthe strong A-B interaction. The expected net moment therefore increases from 2.0 nB, for pure Ni ferrite, to 2,8 pB for the mixed one [9]. The distribution of cations in spinel ferrites can be generally represented by crystal chemical formula (M2+1-
?„Fe3+?.)[ M2+X.Fe3+2-?.]04 where parenflieses and square brackets denote cation sites of tetrahedral (A) and octahedral [B] coordmation, respectively.
The inverse spinel structure, in which the divalent ions are on B sites, and the trivalent ions are equally divided between A and B sites. The divalent and trivalent ions normally occupy the B sites in a random fashion, i.e., they are disordered. It is knovra that nickel ferrites have the inverse spinel structure in which the divalent ions Ni2+ are on the octahedral site B and the ttivalent ions Fe3+ are equally divided between the tetrahedral A and octahedral B sites. In substituted samples, the Zn2+
ions prefere the A sites.
In the present work, the dependence of microstructure and magnetic properties of N i l - xZnxFe204 ferrite nanoparticles on Ni content and aimealing tempreture have been investigated. The spontaneous magnetization and Curie temperature were investigated and are related to the cation distribution in light ofthe mean-field theory
Joumal ofScience & Technology 101 (2014) 054-058 2. Experiments
The Nil-xZnxFe204 nanoparticles in the present study were synthesized a modified co- precipitation. A mixture of NiC12, ZnC12 and FeC13 with their molar ratios determined according to the stoichiometric formulas with x = 0 - 0.8 were mixed at 80 "C and then added to die boiling of NaOH solution with pH around 10. The concenttation of input nickel chloride solution was kept at 0.2 M.
The reactions were maintained at 95 *'C for 1 hour.
In each reaction, precipitation takes place by the conversion of metal salts into hydroxides, which occiu^ immediately, and followed by partial transformation of hydroxides into ferrites. The precipitates were annealed at 1100 '^C for 5 hours in air. This process was necessary to ensure the entire transformation of hydroxides into spinel ferrite. The fine particles were washed several times with distilled water followed by acetone rinse in a magnetic field and dry at a temperature of 80°C for 24h.
The crystal structure and particle size of the samples were detennined by X-ray diffraction (XRD) and Scanning election microscopy (SEM) (Model JSM -7600F). Magnetic properties of the samples were studied at temperatures from 88 K to
^zi^JL
900 K and m applied fields up to 10 kOe using a vibrating sample magnetometer (VSM).
J. Results and discussion
Fig. 1 shows the XRD pattems for N i l - xZnxFe204 samples. The data indicate the crystallization ofthe samples in the spinel structure.
The lattice parameter a is determined from the XRD pattems (Table 1). The broad XRD lines indicate that the particles are in nanoscale. The peaks of (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 0 0) have been deconvoluted to Lorentzian curves for the determination of the crystallite size using full-width at half-maximum value. The crystallite size of the nanocryslalline samples were measured from XRD line broadening analysis applying Scherrer's formula [9]:
D (1) where D is the dimension of the crystallites, X
thewavelength of the X-ray radiation, 9 the Bragg angle, k a shape factor taken to be 0.94 and p the peak width measured at half of the maximum intensity. The calculation showed that the mean crystallite size values <DXRD> of the samples are approximately 40-150 nm.
20 (degree)
Fig. 1. XRD pattems of Nii-xZnxFe204 samples Fig. 2. Lattice parameter "a" of Nii ,ZnxFe204 samples and bulks.
S~*^^'r
•1)
Fig, 3, SEM image of NlFeiOt and Nio6Zno4Fe204 samples 55
Journal ofScience & Technology 99 (2014) 054-058 The shape, size and morphology ofthe singie-
phase particles were examined by direct observation via scanning electron microscopy. As example, the SEM micrograph of the sample x = 0 and x = 0,4 is given in Fig, 3 which reveals that the particles are truncated polygonal and agglomerated. The particle size is distnbuted in a range from several tens to several hundreds of nanometers. It is seen that the average crystallite size <Z>XRD> determined via XRD pattem falls in the particle size range obsevered by SEM.
The lattice parameter a as a function of Zn content x is presented in Fig. 2. It can be seen that a increases almost linearly with increasing x from 0.8349 nm (J: = 0) to 0.8436 (x = 0.8). This behavior is m agreement with the experiment results of bulk Ni-Zn ferrite [10]. As is well known, both Zn ionic radius (0.082 nm) and Ni ionic radius (0.078 nm) are larger than the interstices of A site (0.058 nm) and B site (0.073 nm) in spinel cubic structure. This leads to an expansion of lattice with doping Zn ions or Ni ions into the interstices. Thus, the lattice expansion is weakened if Zn ions are replaced by Ni ions with a smaller radius than Zn ions.
n .=0 -T^=B60K
• x-O 1-T^-03SK
S^-i^Sfc ' " " ° ^
.:^:lU. :S:
T^=r^aK
VZIZ 77^
" " •^ °^9^
r 1 . 1 ? W | - . . - |,^ , A.^-.SK-.^'-.^., _
The magnetic isotherms were made at different temperattu-es from 88 K to 900 K. The spontaneous magnetization value Ms is determined by extrapolating the high-field linear part of the magnetization curve to zero field.
Temperature dependence of spontaneous magnetization of the samples is shown in Fig. 4a from that the Curie temperatures Tc were determmed using the method of intersecting tangent to the M - 7"
curves. The Curie temperature decreases with increasing non-magnetic Zn substitution level. The spontaneous magnetization at zero Kelvin Ms(0) was estimated by extrapolation of the graphical plot of Ms against 7^ to T = 0 according to a modified Bloch law [11].
M,{T)^M,(0)[l-Br] (2) where B is the Bloch's constant and a the Bloch exponent. The fits were performed in temperature range well below the Curie temperatvtre for Ni|.
iZniFe204 samples. The values of Tc extrapolated Ms(0) are listed in Table 1.
Fig, 4, Spontaneous magnetization M . as a fiinction of temperature for the nanosized Nii-xZnxFe204 samples (a) and the defendence of Ms on Zn content (x) at 0 K, 87 K, 300 K and bulk samples (b).
Table 1. Lattice constant a. crystallite size DXRD. M^(300 K. 87 K. OK). 7c ofthe Nir-xZnJ^e^O^ samples.
Nii-,Zn.Fe20,
M
0.0 0.1 0.2 0.3 ft.4 0.5 0.6 0,7 0 8
1(A) DxKT, (nm)
349 354 363 373 383 399 411 433 436
63 134 154 80 48 63 86 76 60
Tc(K)
860 835 803 752 734 682 525 425 375
Ms (emu/g) T^OK
45.2 67.3 79 89 90 108.8
114
45.2
Ms (emu/g) T=87K
45.1 65 75 84.5 87.5 99.5 102.1 80.8 42.3
Ms (emu/g) T=300K.
42.5 55.3 61.2 67.1 69.6 67.2 62.1 29.6 9.3
Journal ofScience & Technology 101 (2014) 054-058
Fig. 5. Tc curves of the Nii- (Zn,Fe204 samples and bulk samples.
Fig. 6. The difference (AT) Tc of the Nii.KZnxFe204 samples and bulk samples.
Fig. 7. ZFC-FC curves for the Nii-xZn,Fe204 samples in applied field H=100Oe.
The dependence of the spontaneous magnetization in zinc concentration in the temperature 0 K, 87 K and 300 K are shown in Figure 4b. The plot of M - ^ show in Fig. 4b indicates that Ms firstly increases gradually and attains maximum values of 69.6 emu/g with JC - 0.4 at room temprature, 114 emu/g and 102 emu/g w i t h x = 0.6 respectively at 0 K (extrapolated by using Bloch equation) and 87 K and then decreases with fiirther increasing x. The behavior of the Ms - x plot is similar to the bulk at room temperature [12]. The increase of Ms at low substitution level is a result of the lowering of magnetization of the A sublattice due to non- magnetic Zn ions. The results showed that the pure composition NiFe:04 has the coUinear (Neel) type of ordering at all temperatures while all the mixed ferrite samples with J: = O.I - 0.8 exhibit a non- collinear, Yafet-iCittel (YK) type of magnetic ordering at low temperatures [13]. The canting ofthe moments in B sites appears due to the competition between the mttasublattice B-O-B interaction and the strongly reduced A-O-B intersublattice interaction when the A sublattice is diluted by non-magnetic Zn=^
The distribution of cations in the spinel stmcture decides the magnitude of the exchange interactions between the magnetic moments at A and B sites, and therefore decide the values of Curie temperature. For the sample .x: = 0, Tc value equals to the value ofthe bulk samples (860 K) and 7c of other samples decreases when Zn concentration increases (Table I). The increase in Zn concenttations decreases A - B interaction which is responsible for the magnitude of Curie temperature. Fig. 5 shows the Tc values of particle samples were compared with the previously published values for bulk samples fabricated by ceramic methods [14], The temperature difference (AT) between the Curie temperature of the particle samples and the bulks are also shown in the Fig. 6.
The difference m Tc may be due to the difference in distribution of zinc ions in the samples. Previous studies have shown that Zn^*
ions can occupy a significant portion of the position B m the samples prepared by soft chemical methods [14], so that with the same concentration of zinc x, the percentages of zinc atoms at the positions A and B in samples prepared by different methods can be different and this leads to the difference in the Tc values .
Fig. 7 shows the zero-field-cooling (ZFC) and field-coolmg (FC) magnetization curves for samples J: = 0, 0.1 and 0.2 in an applied magnetic field//= 100 Oe. The blocking temperature (TB) is determined as the temperature at which the ZFC and FC curves are diverged. Above the blocking temperature, the samples are in superparamgnetic state. The TB value increases from 635 K to 735 K when the Zn^"^ ion concenttation increased from :*: = 0 to JC = 0 2. For the samples with JC > 0.2, no divergence of ZFC and FC curves takes place showing their ferrimagnetic behavior in the whole investigated temperature range to above Curie temperature.
For a non-interacting single domain particle, blocking temperature can be calculated by the formula: K^^V 25A;^T^ where ^eff is the effective anisotropy coefficient, V the particle volume, ke the Boitzmann constant. Because the replacement of zinc leads to the reduction of magnetocrystaline anisottopy, the increase in TB can be explained by the increased in particle sizes when increasing concenttations of zinc as revealed by SEM. In addition, TB can increase due to the effects of interparticle interactions in the agglomerated state and to the conttibutions of surface and shape anisotropics.
Journal ofScience & Technology 101 (2014) 054-058
t . Conclusions
Nii-iZnj(Fe204 nanoparticles ( jc = 0 to 0.8 ) single phase with submicron size were synthesized by co-precipitation method followed by annealing at 1100°C for 5 h. The structure, size, shape and morphology of samples were investigated. The influence of Zn substitution for Ni on the magnetic parameters ofthe samples including the spontaneous magnetization (Ms), Curie temperature (Tc) and blocking temperature ( T B ) were studied and compared with bulk materials. The changes in magnetic properties of the samples with varied zinc concenttations are explained via the change of interaction between the magnetic moments of magnetic cations in octahedral sites and tettahedaral sites. The magnetic measurements suggest remarkable influences of the particle size, interparticle interaction and cation distribution effects on their magnetic behavior.
Acknowledgment
The work was supported by Vietnam's National Foundation for Science and Technology Development (NAFOSTED) Grand No 103.02- 2012.07
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