Magnetic behaviour of nano-particles of Ni 0.5 Co 0.5 Fe 2 O 4 prepared using two different routes

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1 Indian Journal of Pure & Applied Physics Vol. 42, May 24, pp Magnetic behaviour of nano-particles of Ni. Co. Fe 2 O 4 prepared using two different routes Subhash Chander, Bipin K Srivastava & Anjali Krishnamurthy Department of Physics, University of Rajasthan, Jaipur 32 4 Received 3 February 23; revised 6 May 23; accepted 12 February 24 Literature reports on bulk sample of the ferrite Ni. Co. Fe 2 O 4 confirm a complete inverse spinel structure. On site occupancy and magnetic anisotropy on nano-particle samples of the titled ferrite prepared using two different routes a wet chemical co-precipitation process and sol-gel method have been studied. Estimated average particle sizes of the two samples (referred to as NCWET and NCSOL) are ~7Å and ~19Å respectively. Small angle neutron scattering measurements made on the fluid sample comprising the sample prepared by wet chemical process gives median size ~77Å with standard deviation σ =.6. Mössbauer studies at 3K show that the two ferrite samples maintain inverse spinel structure even in nanocrystalline state. Magnetization and Mössbauer measurements show that part of the sample in both the cases is in superparamagnetic state. Magnetic anisotropy is larger in the nano-particle state and surface spin disorder also contributes to this. [Keywords: Magnetic anisotropy, Nano-particles, Spinel ferrites] IPC Code: H 1 F 41/3 1 Introduction Magnetism of fine particles has drawn considerable interest in last two decades. When particle size goes below a critical value, domain structure changes causing drastic modifications in magnetic properties such as saturation magnetization and anisotropy 1. In the context of spinel ferrites, reduction of particle sizes to nano-size domain also causes modification in site occupancies. Thus while in bulk particle form ZnFe 2 O 4 has normal spinel structure with Zn 2+ occupying only the tetrahedral (A) site, in nano-particle state the spinel is reported to be not perfectly normal 2. In the case of NiFe 2 O 4 which is an inverse spinel (in bulk particle form), Chinnasamy et al. 3 have found that in nano-crystalline state, obtained by mechanical milling, the nature of the spinel changes to a mixed spinel one. Further, it has also been reported that site occupancies may also change with the method of preparation 4. In this paper we report our studies on cationic distribution and magnetic properties of nano-particle samples of the ferrite Ni. Co. Fe 2 O 4 prepared by two methods, viz., wet chemical process and sol-gel technique. Lee and Lee in their study of a bulk particle sample of this ferrite report a complete inverse spinel structure with cationic distribution (Fe 3+ )[Ni. 2+ Co. 2+ Fe 3+ ]O 4. It is ferri-magnetic below Curie point of 64K. For examining the site occupation and magnetic nature of the nano-particle samples, we have made 7 Fe Mössbauer measurements at 3K and dc magnetization measurements in the temperature range 8-3K. 2 Experimental Details The nano-particles of Ni. Co. Fe 2 O 4 have been prepared following chemical co-precipitation method and following sol-gel method. Starting materials in the chemical co-precipitation process were analytical grade FeCl 3.6H 2 O, Co(NO 3 ) 2.6H 2 O, Ni(NO 3 ) 2.6H 2 O and NaOH. Following chemical co-precipitation, first we prepared a ferro-fluid (FF), comprised this ferrite, using oleic acid as surfactant and kerosene as the dispersing medium. The FF was then centrifuged at 12 rpm for a period of 2 minutes for yielding a narrower size distribution. Subsequently carrier liquid has been removed by repetitive washing with acetone providing dried particles of the ferrite. This sample is hereafter referred to as NCWET. For the sol-gel autocombustion method analytical grade Fe(NO 3 ) 3.6H 2 O, Co(NO 3 ) 2.6H 2 O, Ni(NO 3 ) 2.6H 2 O and citric acid were used as the raw materials. Small amount of ammonia

2 CHANDER et al.: MAGNETIC BEHAVIOUR OF NANO-PARTICLES 367 solution was added to adjust the ph value. The reaction was carried out at ~13 C. The obtained dried gel burnt in a self-propagating combustion manner getting transformed to a loose powder. This sample is hereafter referred to as NCSOL. For characterization of the samples and for estimation of average particle sizes X-ray diffraction (XRD) patterns have been recorded on Philips make powder diffractometer PW184 using Fe K α radiation. For estimating distribution of the particle sizes, small angle neutron scattering (SANS) measurement has been made on the ferro-fluid (containing NCWET) on Dhruva reactor at Bhabha Atomic Research Centre, Mumbai. DC magnetization have been measured on vibrating sample magnetometer in the sample temperature range 8-3K and in fields up to 8. koe. 7 Fe Mössbauer spectra were recorded at room temperature in transmission geometry and in constant acceleration mode. Velocity calibration was done using the spectrum of metallic natural iron (α-fe) at room temperature. 3 Results and Discussion X-ray powder diffraction patterns confirm formation of single phase cubic spinel structures for both the samples. Obtained lattice constants are ~8.3Å. The reflection lines are quite broad which is suggestive of fine particle nature of the samples. Fig. 1 shows 311 reflections in the XRD patterns of the two samples. For the purpose of comparison, a reflection line in the XRD pattern of a bulk particle polycrystalline piece of silicon is also shown. Average particle sizes estimated using Debye- Scherrer equation and the widths of 311 reflections are ~7Å and ~19Å for NCWET and NCSOL, respectively. Fig.2 shows SANS measurements on the fluid sample containing NCWET. Assuming spherical shape and a log-normal distribution for the particle sizes, this curve yields a size distribution with median diameter D m = 77Å and standard deviation σ =.6. The particle size distribution has been shown in inset of Fig. 2. Figs 3 and 4 respectively show the room temperature Mössbauer spectra of NCWET and NCSOL samples. The spectra have been analyzed using the program developed by Jernberg and Sundqvist 6. Both the spectra could be resolved into two sextets and two doublets. Table 1 gives the 2 g Fig reflection lines in XRD patterns of NCWET and NCSOL samples (reflection of a bulk sample of Si is shown for comparison) d Σ/d Ω (cm -1 ) P(d) D (A) obs cal Q (A -1 ) D m = 77A σ =.6 Fig. 2 SANS distribution from ferro fluid of NCWET. In Inset is shown the particle size distribution Mössbauer parameters. Based on these parameters one each of the doublets and the sextets, in both the spectra, are assigned to Fe 3+ at tetrahedral (A) site and to Fe 3+ at the octahedral (B) site. Intensities of the components for the two sites are almost equal which shows that Fe 3+ occupies the two sites in equal proportion. Thus both Co 2+ and Ni 2+ go only to the octahedral B site and the spinel maintains inverse

3 368 INDIAN J PURE & APPL PHYS, VOL 42, MAY 24 Table 1 Mössbauer parameters* at 3K Sample A-site B-site IS dq B I IS dq B I NCWET NCSOL # # 3 *IS is isomer shift in mm s -1 with respect to metallic iron at 3K; dq is quadrupole splitting in mm s -1 ; B is hyperfine field in Tesla; I is relative intensity of different sub-spectra in per cent. # This has a Gaussian distribution with a width of ~1. % of B value. Velocity (mm s -1 ) Fig. 3 Mössbauer spectrum of NCWET at 3K Velocity (mm s -1 ) Fig. 4 Mössbauer spectrum of NCSOL at 3K VVel structure in the nano-crystalline state as well. This is in contrast to, for example, the case of a normal spinel ZnFe 2 O 4 and that of an inverse spinel NiFe 2 O 4, where in nano-crystalline state the cation distributions change 2,3. Further, the fact of both the spectra containing non-magnetic as well as magnetically split subpatterns implies a distribution of blocking temperatures (owing to distribution of particle sizes). Thus the spectrum of NCWET with smaller average particle size has less (~12%) magnetically split component than has the spectrum of NCSOL (~ 8%). Fig. displays M H curves for the sample NCWET at 3K and 8K. Fig.6 shows the same for the sample NCSOL. Both the samples show finite remanance (M r ) and coercivity (H c ) at both the temperatures implying that at 3 K the entire sample, in both the cases, is not in superparamagnetic state an observation in conformity with the appearance of Mössbauer spectra. It is seen that moments of both the samples do not show a saturating trend at either 3K or at 8K. This is more so at 8K implying larger values of magnetic anisotropy K at the lower temperature of 8K. Now, at 8K and 8. koe the moment for NCWET (7 Å) is ~ 12 emu/g and that for NCSOL (19 Å) is ~ 6 emu/g. As Ni. Co. Fe 2 O 4 is an inverse spinel, its saturation moment would be ~ 61 emu/g. Much smaller values for the two samples at a high field of 8. koe and at 8K are suggestive of large K values in the nano-particle state. Between the two samples K for NCSOL is larger suggesting that the fraction of single domain particles is more in NCSOL. Further, Fig. 7 displaying M H curves at 8K recorded after cooling the sample from 3K in zero field (zfc mode) and after cooling it in field (fc mode) for NCWET shows an interesting observation, viz., a large shift of fc curve with respect to the zfc curve. This suggests that a large contribution to magnetic anisotropy comes from surface spin disorder. Concerning temperature dependence of magnetization, Figs 8(a and b) show the magnetization temperature (M T) measurements recorded in zfc mode for NCWET and NCSOL respectively in a field of Oe. In both the cases, M shows a continuous increase with T right up to 3K. This shows that blocking temperature of the particles in both the

4 CHANDER et al.: MAGNETIC BEHAVIOUR OF NANO-PARTICLES fc 8K zfc - - 3K Fig. M-H curves of NCWET recorded at 8K and 3K Fig. 7 M-H curves of NCWET recorded at 8 K after zfc and fc modes (in 8. koe) 1. (a) K 8 K Moment (arb. units) (b) Temperature (K) Fig. 6 M-H curves of NCSOL recorded at 8K and 3K samples is above 3 K. Thus, this further confirms the inference drawn on the basis of M H curves and Mössbauer spectra that all the particles in the two samples are not in superparamagnetic state at 3K. We conclude by noting: (i) the substituted ferrite maintains its completely inverse spinel structure in nano-crystalline state; also this is not affected by the process of preparation, (ii) the two nano-particle samples have distributed particle sizes such that part of the sample (more in NCWET having smaller average sized particles) is in SPM state at 3K, Moment (arb. units) Temperature (K) Fig. 8 M-T curves recorded in zfc mode in a field of Oe for (a) NCWET and (b) NCSOL (iii) M r and H c are more at 8K than at 3K and (iv) magnetic anisotropy is more in the nano-particle state and surface spin disorder makes a large contribution to magnetic anisotropy.

5 37 INDIAN J PURE & APPL PHYS, VOL 42, MAY 24 Acknowledgement Authors thank Ms R. Chitra, Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai, for SANS measurements at Dhurva reactor. Financial assistance from Inter University Consortium for DAE facilities (IUC-DAEF) is gratefully acknowledged. SC thanks IUC-DAEF for research fellowship. References 1 Kodama RH & Berkowitz AE, Phys Rev B, 9 (1999) Sepelak V, Tkacova K, Boldyrev VV, Wiβmann S & Becker KD, Physica B, (1997) Chinnasamy CN, Narayanasamy A, Ponpandian N, Chattopadhyay K, et al., Phys Rev B, 63 (21) Sepelak V, Wiβmann S & Becker KD, J Mag Mag Mater, 23 (1999) 13. Choong Sub Lee & Chan Young Lee, J Appl Phys, 79 (1996) 7. 6 Jernberg P & Sundqvist T, University of Uppasala, Institute of Physics Report UUIP-9 (1983).