Correlation Between Ferromagnetism and Defects in Mgo Nanocrystals Studied by Positron Annihilation

Physica B 407 (2012) 2665–2669

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Physica B

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Correlation between ferromagnetism and defects in MgO nanocrystals studied by positron annihilation

D.D. Wang a, Z.Q. Chen a,n, C.Y. Li a, X.F. Li a, C.Y. Cao b, Z. Tang b a Hubei Nuclear Solid Physics Key Laboratory, Department of Physics, Wuhan University, Wuhan 430072, PR China b Department of Electronic and Engineering, East China Normal University, Shanghai 200241, PR China article info abstract

Available online 5 January 2012 High purity MgO nanopowders were pressed into pellets and annealed in air from 100 to 1400 1C. Keywords: Variation of the microstructures was investigated by X-ray diffraction and positron annihilation MgO nanocrystals spectroscopy. Annealing induces an increase in the MgO grain size from 27 to 60 nm with temperature Interfacial defect increasing up to 1400 1C. Positron annihilation measurements reveal vacancy defects including Mg Ferromagnetism vacancies, vacancy clusters, microvoids and large pores in the grain boundary region. Rapid recovery of Positron annihilation Mg monovacancies and vacancy clusters was observed after annealing above 1200 1C. Room temperature ferromagnetism was observed for MgO nanocrystals annealed at 100, 700, and 1000 1C. However, after 1400 1C annealing, MgO nanocrystals turn into diamagnetic. Our results suggest that the room temperature ferromagnetism in MgO nanocrystals might originate from the interfacial defects. & 2012 Elsevier B.V. All rights reserved.

1. Introduction studies have confirmed the defect induced ferromagnetism in various materials. However, strong experimental evidence of the Diluted magnetic semiconductors have attracted significant correlation between vacancy defects and the ferromagnetism is interest in recent years because of their potential applications in still unclear. This could be due to the lack of an appropriate spintronic devices [1]. Over the past decade, extensive efforts method to characterize the vacancy-type defects. have been put on introducing ferromagnetism in the otherwise Positron annihilation spectroscopy has been proven to be a non-magnetic semiconductors by doping with magnetic impu- superb method for the study of vacancy defects in solids [17].Due rities such as transition metal impurities. Room temperature to the Coulomb repulsion between positron and positive ion cores, ferromagnetism in ZnO, GaN and AlN has been successfully vacancy-type defects are effective trapping centers for positrons. The produced in this way [2–5]. However, up to now some funda- annihilation characteristics of positrons such as lifetime and Doppler mental issues related with the ferromagnetism have not been broadening of the annihilation radiation will be different from that solved which are of vital importance. For example, the origin of in the defect-free bulk state. This makes the identification of defects magnetism still remains controversial. rather straightforward and sensitive. Using this method, we have There have been several explanations for the observed ferro- successfully found the clear correlation between vacancy defects magnetism in semiconductors. For example, the transition metal and the ferromagnetism in ZnO and BaTiO3 nanocrystals [18,19]. atom substitution for the host lattice, formation of metal clusters MgO displays a rocksalt structure with Mg and O atoms or secondary phases. Recently, ferromagnetism has been observed octahedrally coordinated, making pure MgO a diamagnetic material. in some undoped semiconductors [6–8]. Systematic studies show It has been the subject of considerable experimental and theoretical that ferromagnetism also appears in many nanoparticles of the attentions over the last decades. Room temperature ferromagnetism otherwise non-magnetic oxides such as CeO2, MgO, ZnO, In2O3 has been observed in pure MgO films and nanocrystals [20,21]. The and TiO2, etc. [9–11]. It is assumed that the observed ferromag- defect induced ferromagnetism in MgO has been studied theoreti- netism might be related with microstructural features, such as cally and experimentally [14,22–25]. In this work, we studied the the cation or anion vacancies at the surfaces of nanoparticles. recovery process of vacancy defects in MgO nanocrystals during Investigation of the unconventional defect-induced magnetism high temperature annealing by using positron annihilation measure- has become a subject of intense research [12–16]. Theoretical ments. Variation of the magnetic behavior of MgO nanocrystals was also studied. A good correlation between the ferromagnetism and vacancy defects in the MgO grain boundary region was observed, n Corresponding author. Tel./fax: þ86 27 68753880. suggesting that the ferromagnetism might originate from these E-mail address: [email protected] (Z.Q. Chen). defects.

2. Experiment annealing, XRD patterns do not exhibit phase transition. The peaks grow higher and become narrower with the increasing Pure MgO nanopowders were purchased from Beijing Nachen annealing temperature. This implies that the crystallinity of the S&T Ltd with purity Z99:9% and grain size of about 30 nm. The sample becomes better after annealing. MgO nanopowders were handmilled in agate mortar with pestle The average grain size of the annealed MgO nanocrystals can for 2 h, then they were pressed under a static pressure of about be calculated by the following Scherrer’s formula: 6 MPa for about 1 min at room temperature to get plane-faced pellets. The pellet samples were in disk shape having a diameter Dhkl ¼ Kl=b cos y, ð1Þ of 15 mm and a thickness of 2 mm. They were subsequently annealed in open air at elevated temperatures ranging from 100 where Dhkl is the average diameter of the grain perpendicular to to 1400 1C for 2 h. the (hkl) plane, K is the shape factor (usually taken as 0.89), l is Phase identification and crystal quality of MgO nanocrystals the X-ray wavelength of Cu Ka1 radiation, b is the FWHM of the were studied by the X-ray diffraction (XRD) technique (German, XRD peak, and y is the Bragg angle. In Fig. 1 each observable peak Bruker Axs D8 Focus), using Cu Ka (l ¼ 0:1540598 A)˚ and a Ni in the spectra was fitted with a Gaussian function. The calculated filter. The increment is 0.011 and the speed of scan is 0.21/s. The grain sizes of MgO as a function of annealing temperature are 2y diffraction range is from 351 to 851. Positron lifetime measure- shown in Fig. 2. The average grain size of MgO annealed below ments were performed using a conventional fast–fast coincidence 600 1C was estimated to be around 27 nm in diameter, which system with time resolution of about 210 ps. The 22Na positron keeps no change with increasing annealing temperature. Above source with the activity of 5 mCi was sandwiched between two 600 1C, the grains begin to grow rapidly. After annealing up to identical sample pellets for measurements. A well-annealed 1400 1C, the grain size grows to more than 60 nm. The grain aluminum single crystal with purity of 5N was measured to growth during annealing is commonly observed in nanopowders. determine the source correction. Doppler broadening of the Comparing with ZnO nanocrystals [18], MgO nanograins grow annihilation radiation was measured in coincidence mode using more slowly. This means that fabrication of MgO ceramic by two high purity Ge detectors. The Doppler broadening spectrum sintering MgO nanopowders requires very high temperature for was characterized by S and W parameters, which are defined as full densification of MgO [26]. This might be due to the particu- the ratio of low momentum (51170.68 keV) region and high larly high melting point of MgO which is as high as 2850 1C. momentum (51172.86–51175.73 keV) region to the total area Microstructural defects in MgO nanocrystals were studied by of the annihilation peak, respectively. The positron lifetime and positron annihilation measurements. The typical positron lifetime Doppler broadening spectra were measured simultaneously. spectra for MgO nanocrystals annealed at 100 and 1400 1C are Magnetism measurement for MgO nanocrystals was conducted shown in Fig. 3 as an example. For MgO nanocrystals annealed at using a physical property measurement system (PPMS). All the 100 1C, the positron lifetime spectrum shows a very long lifetime measurements were performed at room temperatures of about component. A detailed analysis of the lifetime spectrum by 300 K. PATFIT program [27] reveals four lifetime components. t1 and t2 are two short lifetime components. t1 is about 170 ps with intensity of 45%, which corresponds to the average lifetime of free

positrons and some trapped positrons at monovacancies like VMg 3. Results and discussion or V O, since this component is higher than the positron bulk lifetime of 166 ps in MgO [28]. t2 is about 380 ps with intensity of Fig. 1 shows the XRD patterns of MgO nanocrystals annealed at 45%, which is attributed to the annihilation lifetime of positrons different temperatures. The characteristic peaks can be indexed as trapped by vacancy clusters. These monovacancies and vacancy cubic MgO (JCPDS card no. 77-2364). During the process of clusters are most probably located at the grain boundary region.

t3 and t4 are two long lifetime components with values of 224ns (intensity 3%) and 52 ns (intensity 7%), respectively. These two long lifetime components suggest the formation of positronium. In porous materials, positron can easily catch one electron to form

Fig. 1. XRD patterns for the 100, 400, 700, 1000, and 1400 1C annealed MgO Fig. 2. Variation in the average grain size as a function of annealing temperature nanocrystals. for MgO nanocrystals. D.D. Wang et al. / Physica B 407 (2012) 2665–2669 2667

Fig. 3. Area normalized positron lifetime spectra of MgO nanocrystals annealed at 100 1C and 1400 1C. a metastable atom called positronium. According to the spin of the electron and positron, positronium has two states. One is the long-lived ortho-positronium (o-Ps) with self-annihilation lifetime of 142 ns, and another is the short-lived para-positronium (p-Ps) with self-annihilation lifetime of 125 ps. In condensed matter the lifetime of o-Ps can be greatly reduced by pick-up annihilation, with lifetime closely related to the size of the pore. Fig. 4. t4 and I4 as a function of annealing temperature for MgO nanocrystals.

Therefore t3 and t4 can be attributed to o-Ps annihilation in some microvoids and large pores, respectively, and the p-Ps annihilation also adds its contribution to the ﬁrst lifetime t1. The average size of the large pore can be estimated by a modiﬁed Tao–Eldrup model [29,30], which is about 6 nm in diameter. Considering that the measurements were performed in air, which might reduce the o-Ps lifetime, the actual pore diameter may be larger than this value. Therefore these large pores are most probably the unoccu- pied space between nanograins. After annealing at a high temperature of 1400 1C, the short lifetime part becomes shorter, while the long lifetime tail still remains. This suggests the recovery of some vacancy defects, but the large pores are rather stable.

Fig. 4 shows the variation of o-Ps lifetime t4 and its intensity I4 as a function of annealing temperature. After annealing above

200 1C, t4 remains to be about 68 ns up to 1400 1C. Its intensity first shows an increase to about 11% after annealing at 400 1C, then decreases to about 5%. This reveals that most of the large pores tend to be stable. In ZnO nanocrystals we also observed such large pores through positronium formation. But those large pores disappeared after annealing above 900 1C. This indicates again that the densification of MgO nanocrystals is more difficult than ZnO, which is in agreement with XRD measurements.

Fig. 5 shows the shorter o-Ps lifetime component t3 and its intensity I3 as a function of annealing temperature. The lifetime becomes longer after annealing. This might be due to the agglomeration of microvoids to larger pores. The intensity t3 decreases from 3% to 0.3% after annealing at 1400 1C, indicating a decrease of the number of microvoids. Most of the microvoids were recovered and some others agglomerate into larger pores.

The change of vacancy defects in MgO nanocrystals during Fig. 5. t3 and I3 as a function of annealing temperature for MgO nanocrystals. annealing is reﬂected by the two shorter lifetime components.

Fig. 6 shows t1, t2, I2 and the average lifetime of t1 and t2 as a vacancy clusters start to collapse after annealing above 1200 1C. function of annealing temperature. t1 shows slight change with This is a general trend for the recovery of vacancies. Larger annealing temperature up to 1000 1C, and then it decreases to vacancy clusters are always more stable. The average lifetime of about 152 ps, while t2 shows continuous but slight decrease up to t1 and t2 reﬂects the overall information about the vacancy 334 ps after annealing at 1400 1C. For the intensity I2, it does not defects. It shows slight decrease below 1200 1C and fast decrease show signiﬁcant change after annealing below 1200 1C. However, above 1200 1C. We can conclude that the vacancy defects in the above 1200 1C, I2 shows a rapid decrease to about 15%. This shows grain boundary region begin to recover rapidly after annealing that monovacancies begin to recover at 1000 1C, while the above 1000–1200 1C. However, after annealing at the highest 2668 D.D. Wang et al. / Physica B 407 (2012) 2665–2669

Fig. 6. t1, t2, I2 and the average lifetime of t1 and t2 as a function of annealing temperature for MgO nanocrystals.

temperature of 1400 1C, the average lifetime of t1 and t2 is around 180 ps, which is still more than 10 ps higher than the positron bulk lifetime in MgO. This indicates that some residual vacancies still remain after annealing. It should be noted that, during annealing of the MgO nanocrystals, there is an increase of the grain size. This might also lead to the decrease of positron lifetime, because the fraction of positrons diffused to the grain boundary region will be reduced with increasing grain size, which results in the decrease of the trapping fraction by grain boundary defects. However, the MgO nanograin has a diameter of only about 60 nm after annealing at 1400 1C. Its radius is still smaller than the positron diffusion length in MgO (about 50 nm estimated by van Huis et al. [31]). On the other hand, the MgO grain size shows rapid increase between 600 and 1100 1C, and has no signiﬁcant increase above 1100 1C, while the rapid decrease of positron lifetime commences at 1200 1C. Therefore the decrease of positron annihilation lifetime is less likely to be due to the increase of grain size. Fig. 7(a) shows the Doppler broadening S parameter as a function of annealing temperature. S parameter ﬁrst has a slight increase, then it decreases continuously up to 1400 1C. This shows continuous recovery of vacancy defects, which is a little different from the lifetime result. The reason might be that S parameter contains the information of not only the short lifetime components, but also the positronium annihilation. Nevertheless, the change of S parameter versus W parameter during annealing provides us other complimentary information, which is shown in

Fig. 7(b). The data in Fig. 7(b) can be fitted by a straight line, Fig. 7. (a) S parameter as a function of annealing temperature for MgO nanocrys- indicating that the defect species are the same during annealing. tals. (b) Variation of S parameter versus W parameter during annealing of MgO Fig. 8 shows the magnetic properties for MgO nanocrystals nanocrystals. annealed at 100 1C, 700 1C, 1000 1C and 1400 1C. MgO nanocrystals annealed at 100 1C show clear hysteresis loops after subtract- ferromagnetic wiggle which is superimposed on the diamagnetic ing the diamagnetic background, with a saturation magnetization line. The magnetization is around 0:03 103 emu=g, which of approximately 0:15 103 emu=g and coercive field of about might originate from the magnetic impurities such as Fe distrib- 200 Oe. This indicates room-temperature ferromagnetism in MgO uted homogeneously in the sample. nanocrystals. After annealing at 700 1C, the saturation magnetiza- The disappearance of ferromagnetism in MgO nanocrystals after tion and coercive field show nearly no change. The hysteresis loop annealing is in good agreement with the recovery process of the still remains after annealing at 1000 1C. However, it becomes vacancy defects revealed by positron annihilation study. In MgO nearly invisible after annealing at 1400 1C. There is only a tiny nanocrystals, there are large numbers of vacancy defects on the D.D. Wang et al. / Physica B 407 (2012) 2665–2669 2669

Fig. 8. Magnetism for 100 1C annealed, 700 1C annealed, 1000 1C annealed and 1400 1C annealed MgO nanocrystals.

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