Low Temperature Magnetization Studies of ... - IEEE Xplore

IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013

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Low Temperature Magnetization Studies of Nanocrystalline Zn-Ferrite Thin Films Murtaza Bohra

, Shiva Prasad , N. Venkataramani , S. C. Sahoo

, Naresh Kumar

, and R. Krishnan

Indian Institute of Technology Bombay Mumbai 400076 India Okinawa Institute of Science and Technology Graduate University (OIST), Okinawa, Japan Department of Physics, Central University of Kerala, Kasaragod India Motilal Nehru National Institute of Technology, Allahabad 211004 India Groupe d’Etude de la Matière Condensée,CNRS/Universite de Versailles-St-Quentin, Versailles Cedex, 78035 France A study of temperature dependance of magnetization was carried out on nanocrystaline Zn-ferrite thin films deposited using Pulsed Laser Deposition (PLD) at three different substrate temperatures. The temperature dependence of the films deposited at ambient temperature, 350 and 850 C showed marked difference in the temperature dependence of their magnetization. While the sample deposited at 850 C showed predominantly antiferromagntic grains, the one deposited at 350 C showed a ferrimagnetic behavior. The ambient temperature deposited film showed mainly superparamagnetic grains. Index Terms—Grain size dependent magnetism, low temperature magnetic properties, nano-crystalline Zn-ferrite.

I. INTRODUCTION

W

ITH the development of spintronic and microwave-frequency devices, sensors and multiferroic based composites, the ferrite thin films have emerged as potential contenders for the next generation devices [1]–[4]. Among ferrites, Zn-ferrite (ZnFe O ) thin films have attracted interest of researchers not only for novel technological applications (due to their low processing temperature, high sensing capability, tunable conductivity and narrow FMR line widths) but also for their grain size dependent intriguing magnetic properties [5]–[7]. Bulk Zn-ferrite is a normal ferrite and shows antiferromagnetism with very low Ne’el temperature, of 10 K. At room temperature, therefore, it is paramagnetic. Nanosized Zn-ferrite material, on the other hand, exhibits magnetic ordering even at room temperature [8]. This behavior suggests that there is a change of cation distribution and the nanosized ferrite is partially inverted. This has prompted extensive studies on this material by researchers in recent years both in nano sized powder and the nanocrystalline thin films. Recently, we have carried out a study on laser ablated Zn-ferrite thin films deposited at various substrate temperatures ranging from room temperature (RT) to 850 C and shown that the grain size can play a significant role on the room temperature magnetic properties of these films [9]. It was shown by us that when the grain size of the film is very small or very large, the magnetization is very low. However, at intermediate values of the grain size, the magnetization is high. Normally the magnetization of ferrite thin film increases with grain size. Hence the observation of increase of magnetization with the grain size is normal. The abnormal decrease of magnetization for the larger grain sample was explained in terms of the cation distribution returning Manuscript received November 05, 2012; accepted January 02, 2013. Date of current version July 15, 2013. Corresponding author: M. Bohra (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2013.2239969

to normal bulk value. In this paper we present a study of the temperature dependence of the magnetic properties of these films. II. EXPERIMENT Zn-ferrite thin films were grown on to amorphous fused quartz substrate by pulsed laser ablation with laser energy density of 2.5 J/cm . The third harmonic (355 nm) of Nd:YAG laser, with 10 Hz repetition rate and 5–6 ns pulse width was used to ablate Zn-ferrite target. The bulk Zn-ferrite target was prepared by double sintering ceramic method. Stoichiometric ratio of high purity (99.99%) ZnO and -Fe O powders were thoroughly mixed and ground to fine particles and presintered at 900 C for 4 hrs, and were final sintered at 1300 C for 4 hrs in air. Thin films were deposited under oxygen pressure of 0.16 mbar at of RT, 350 and 850 C. These films have been designated as ZRT, Z350 and Z850 respectively throughout the paper. To estimate grain sizes and crystal phases, transmission electron microscopy (TEM) micrographs and selected area electron diffraction (SAED) pattern were collected from Philips CM 200 transmission electron microscope. Grain sizes distribution was obtained by analyzing TEM bright field micrographs using ImageJ 1.46 analysis software. Magnetic properties were measured with in-plane film configuration by using a Quantum design PPMS. Temperature dependence of magnetization (M-T) was measured in the following way. The sample is first cooled in zero fields from the room temperature to 5 K. Next the measurement field of 5 kOe is applied and the temperature is then increased and the zero-field cooled (ZFC) curve is recorded. The temperature is next reduced without changing the measurement field, then increased again and the field cooled (FC) curve is recorded again in the same field. III. RESULTS AND DISCUSSION Fig. 1 shows the TEM results for the three films. It is clear from this figure that we observed diffraction rings (right Insets) in all the samples including ZRT film, which did not show any XRD peak [9]. All the diffraction rings in the selected area electron diffraction (SAED) pattern have been identified to

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TABLE I AVERAGE GRAIN SIZES FOR THREE FILMS

Fig. 2. In-plane M-H loops taken at 100 K for three samples.

Fig. 1. TEM images for the three films. The right insets show SAED patterns while left insets show grain size distribution of the corresponding films.

Zn-ferrite phase. Film Z850 shows discontinuous rings with spots which indicate that this film consists of larger grain

sized crystallites. The left insets show respective grain size distributions. For ZRT film, we can see that maximum number of grains are in the 10–20 nm size range. However, the grains as large as 50 nm can also be seen in this film. In case of Z350 film, we note that the distribution of grains is shifted to higher values, peaking around 20–30 nm. We also see grains 65–70 nm as well as a few of 10 nm sizes in this film. On the other hand, Z850 film has maximum number of grains are in 50–70 nm size range. One can also note that the grain size distribution for the Z850 film has become broader. The average grain sizes estimated from TEM micrograph are consistent with those obtained from Scherrer’s formula and given in Table I. The TEM results thus show that all the films contain nanocrystallites and their grain sizes increase with . The M-H loops of three films at 100 K, plotted in Fig. 2, show that the loop of Z350 film is very different from the other two. This film shows a well-defined ferrimagnetic M-H loop. Even though the film does not saturate even at the highest field of 6 kOe, still it shows much better saturation than the other two films. The other two films show loops that are qualitatively similar to each other in the sense that they show a large nonsaturation. Temperature dependences of ZFC and FC magnetization curves measured at 5 kOe for all three films are plotted in Fig. 3. The magnetizations data of bulk Zn-ferrite target is also shown on the top of this figure for comparison on a blown up scale. In case of bulk, the ZFC and FC curves were identical. The magnetization at 300 K for Zn-ferrite films is quite high in comparison to bulk Zn-ferrite (0.02 kG). This is expected because bulk Zn-ferrite is anti-ferromagnetic at low

BOHRA et al.: LOW TEMPERATURE MAGNETIZATION STUDIES OF NANOCRYSTALLINE ZN-FERRITE THIN FILMS

Fig. 4.

Fig. 3. ZFC and FC magnetization curves for Zn-ferrite thin films and bulk Zn-ferrite target.

temperature and expected to show only paramagnetic behavior at 300 K. The higher value of magnetization in films is due to ferrimagnetic order in nano-crystalline films, which arises due to changed cation distribution [8]–[10]. We also note that the film Z350 has a maximum value of magnetization of 2.3 kG. The same for the ZRT is 0.23 kG and for Z850, is 0.12 kG. From Fig. 3, we note that not only the absolute value of M but also the temperature dependence of magnetization (M) is dependent on the substrate temperature. ZFC magnetization of ZRT film increases with decreasing temperature and show broad maxima around 50 K. While the FC magnetization keeps on gradually increasing and starts deviating from the ZFC one below 30 K. For Z350 film we see largest magnetization, which also monotonically increases as the temperature is lowered. Here also ZFC and FC magnetizations start deviating from each other at very low temperature. ZFC magnetization behavior of Z850 film does not differ much from FC, somewhat similar to bulk zinc ferrite target. Both Z850 film and bulk show a maximum magnetization around 10 K, even though the maximum value of magnetization of the film is an order of magnitude larger than the same for bulk Zn ferrite. Fig. 4 shows reciprocal molar susceptibility vs. temperature curve for bulk Zn-ferrite target. A minimum around 10 K in the low temperature region of vs. curve can be seen, which reminisces of antiferromagnetic Ne’el temperature of Zn-ferrite. According to molecular field theory, in antiferromagnetic systems, the inverse susceptibility above Neel temperature should show a linear behavior with temperature, which when extrapolated should intercept with negative temperature-axis at a temperature equal to Neel temperature. In our case when we try to fit vs. data from around room temperature below, we see a deviation from linearity close to 150 K. The intercept of the straight line is found on positive

vs.

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curve for bulk Zn-ferrite target.

x-axis, in place of the expected negative a temperature close to 14 K. Similar type of deviation of vs. data from the linear fit was reported by Kamazawa et al. [11] in a study on single crystal Zn-ferrite. Their data starts deviating from straight line even below 280 K and shows positive intercept at high temperature around 120 K. They attributed this abnormal result to the ferromagnetic spin correlation dominance at high temperatures. A relatively old work on polycrystalline Zn-ferrite [12] showed that the vs. curves do not deviate much from straight line and straight line show a zero intercept on T-axis. They also semi-quantitatively described that the intercept depends strongly on the amount of Fe ions at A-site (Fe(A)) and it can reach up to 100 K for 6% of Fe(A). In spite of the differences at the intercept value of vs. curve, the value of temperature at which minimum in is seen is reasonably close to each other in all the three results. The minimum for our bulk Zn-ferrite target and reported polycrystalline and single crystal Zn-ferrites are 10 K, 13 K and 15 K respectively. The slope of the fitted line for our bulk sample turns out to be 0.14 mole Oe/emu K. The room temperature susceptibility calculated from the slope is 0.023 emu/Oe mole. Fig. 5 shows vs. curve of Z850 film. In the low temperature region of vs. curve, a minimum around 10 K can be seen, which is located at similar temperature as the bulk sample. A linear fit to the vs. data of Z850 film deviates well above 150 K, and the intercept is found on positive x-axis at a temperature close to 78 K, which is higher than our bulk Zn-ferrite (14 K) target but lower than reported single crystal Zn-ferrite (120 K) [11]. The slope of the fitted line for this sample is 0.05 mole Oe/emu K, which yields room temperature susceptibility value 0.067 emu/Oe mole. This value of susceptibility is almost three times higher than the one seen in bulk Zn-ferrite target. We, thus note that the M-T behavior of Z850 sample is very similar to the bulk ferrite. Both show minimum in at nearly the same temperature and for both the ZFC and FC are identical. However, the difference is that the for this sample is much higher than bulk. This fact can be understood if assume that this sample contains some ferrimagnetic grains also in addition to the antiferromagnetic ones. From TEM micrograph [Fig. 1(c)]

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the other hand, is mainly superparamagnetic at room temperature. IV. CONCLUSION

Fig. 5. vs. curve for Z850 film. Molar susceptibility was calculated using theoretical density (5.33 g/cm ) of Zn-ferrite.

we see that the there is a reasonable overlap of the grain sizes between this and the Z350 samples and from Fig. 2 we find clear ferrimagnetic behavior for Z350 sample. Hence if we conclude that it is around the 40 nm grain sizes that the cation distribution of Zn ferrite sample starts leading it towards antiferromagnetism, then we can understand that the Z850 sample would have both ferrimagnetic and antiferromagnetic grains. Observation of largest difference between ZFC and FC magnetization at low temperatures (5–30 K) region of ZRT sample makes us infer that this sample may predominantly contain super-paramagnetic grains. Therefore, the maximum around 50 K in ZFC magnetization is likely to be due to blocking of magnetic moments into ferromagnetic state for very small grain sizes. The temperature at which the maximum occurs is, therefore, blocking temperature, . This result is expected because maximum numbers of grain in this sample are of sizes 10–20 nm [Fig. 1(a)]. The M-H loop on this sample did . This could show a small coercivity at temperatures above be because of the presence of a few intermediate sizes (20–30 nm) grains, which are ferrimagnetic. The magnetization behavior of Z350 sample, with an intermediate nano-crystalline grain sizes, is consistent with ferrimagnetic behavior. This film, therefore, also has the largest value of magnetization. These films show well defined M-H loops and M-T behavior similar to the conventional ferrimagnetic system. The TEM micrograph [Fig. 1(b)] of this film shows majority of intermediate sizes of ferrimagnetic grains range 20–30 nm. Because of presence of fewer smallest size super-paramagnetic grains ( 10–20 nm), a small difference between ZFC and FC magnetizations can be seen. Thus, we see that even though the ZRT and Z850 samples showed similar M-H loops, their temperature dependences are quite different from each other. This study confirms that Z850 sample, which has the highest grain size behaves closer to bulk and is predominantly antiferromagnetic. The ZRT sample, on

Our study shows that broadly the Zn-ferrite nanocrystalline films contain three types of nano-grains. The first one is smallest in size and are superparamagnetic at room temperature. The second are intermediate sized ones, which are ferrimagnetic, due to changed cation distribution from bulk. The third types of grains are larger in size and possess properties closest to bulk and thus are paramagnetic at room temperature and antiferromagnetic at low temperatures. The RT-deposited film contains a very large number of first type of grains and a fewer of second type of grains. In the film deposited at 350 C, we have the grains mainly of second type with presumably small fractions of first and the third types. In the 850 C film we have grains largely of third type with a small fraction of second type. Our TEM results seem to corroborate this conclusion. REFERENCES [1] C. N. Chinnasamy, S. D. Yoon, A. Yang, A. Baraskar, C. Vittoria, and V. G. Harris, “Effect of growth temperature on the magnetic, microwave, and cation inversion properties on NiFe O thin films deposited by pulsed laser ablation deposition,” J. Appl. Phys., vol. 101, p. 09M517-3, 2007. [2] Y.-Y. Song, M. S. Grinolds, P. Krivosik, and C. E. Patton, “Pulsed laser-deposited single-crystal LiZn-ferrite films with low microwave loss,” J. Appl. Phys., vol. 97, p. 103516-5, 2005. [3] M. Bohra, N. Venkataramani, S. Prasad, N. Kumar, D. S. Misra, S. C. Sahoo, and R. Krishnan, “RF sputter deposited nanocrystalline (110) magnetite thin film from-Fe O target,” J. Nanosci. Nanotechnol., vol. 7, no. 6, pp. 2055–2057, 2007. [4] M. Liu, O. Obi, J. Lou, S. Stoute, Z. Cai, K. Ziemer, and N. X. Sun, “Strong magnetoelectric coupling in ferrite/ferroelectric multiferroic heterostructures derived by low temperature spin-spray deposition,” J. Phys. D: Appl. Phys., vol. 42, p. 045007, 2009. [5] Z. Jiao, M. Wu, J. Gu, and Z. Qin, “Preparation and gas-sensing characteristics of nanocrystalline spinel zinc ferrite thin films,” IEEE Sens. J., vol. 3, p. 435, 2003. [6] M. Bohra, S. Prasad, N. Venkataramani, N. Kumar, S. C. Sahoo, and R. Krishnan, “Narrow ferromagnetic resonance linewidth polycrystalline Zn-ferrite thin films,” IEEE Trans. Magn., vol. 47, no. 2, pp. 345–348, Feb. 2011. [7] M. Lorenz, M. Brandt, K. Mexner, K. Brachwitz, M. Ziese, P. Esquinazi, H. Hochmuth, and M. Grundmann, “Ferrimagnetic ZnFe O thin films on SrTiO single crystals with highly tunable electrical conductivity,” Phys. Status Solidi (RRL)—Rapid Res. Lett., vol. 5, no. 12, pp. 38–440, 2011. [8] C. Chinnasamy, A. Narayanasamy, N. Ponpandian, K. Chattopadhyay, H. Guérault, and J.-M. Greneche, “Magnetic properties of nanostructured ferrimagnetic zinc ferrite,” J. Phys.: Condens. Matter, vol. 12, p. 7795, 2000. [9] M. Bohra, S. Prasad, N. Kumar, D. S. Misra, S. C. Sahoo, N. Venkataramani, and R. Krishnan, “Large room temperature magnetization in nanocrystalline zinc ferrite thin films,” Appl. Phys. Lett., vol. 88, p. 262506-3, 2006. [10] A. Yang, C. N. Chinnasamy, J. M. Greneche, Y. Chen, S. D. Yoon, Z. Chen, K. Hsu, Z. Cai, K. Ziemer, C. Vittoria, and V. G. Harris, “Enhanced Néel temperature in Mn ferrite nanoparticles linked to growthrate-induced cation inversion,” Nanotechnol., vol. 20, p. 185704, 2009. [11] K. Kamazawa, Y. Tsunoda, H. Kadowaki, and K. Kohn, “Magnetic neutron scattering measurements on a single crystal of frustrated ZnFe O ,” Phys. Rev. B, vol. 68, p. 024412, 2003. [12] F. Lotgering, “The influence of Fe ions at tetrahedral sites on the magnetic properties of ZnFe O ,” J. Phys. Chem. Solids, vol. 27, no. 1, pp. 139–145, 1966.