Tiến trình từ hóa theo  thời gian nghiền được thảo luận và đánh giá bởi sự sắp xếp lại các cation trong cấu trúc spinen do quá trình nghiền gây ra…

Magnetic properties of zinc ferrite synthesized by high-energy ball milling

 TRAN QUOC LAP, PHAM THAO, LE HONG THANG NGUYEN HOANG VIET and NGUYEN THI  HOANG OANH*
Hanoi University of Science and  Technology

*E-mail: oanh.nguyenthihoang@hust.edu.vn

Ngày nhận bài: 25/9/2015, Ngày duyệt đăng: 28/11/2015

 ABSTRACT

The nanocrystalline zinc ferrite (ZnFe2O4) was obtained by high-energy ball milling in planetary  mill from a mix- ture of Fe3O4 and Zn. The formation of zinc ferrite was controlled by X-ray diffraction and magnetic measurements. The chemical homogeneity and  morphology of powders were studied by X-ray microanalysis and  scanning electron microscopy. The evolution of the  magnetization versus milling time was  discussed and  evaluated in terms  of milling induced cations  reorganization in spinel structure.
Keywords: high-energy ball milling, ZnFe2O4, magnetic, spinel

TÓM TẮT

Ferit kẽm ( ZnFe2O4) dạng nano  tinh thể thu được khi nghiền năng lượng cao hỗn  hợp bột Fe3O4 và Zn trong máy nghiền hành tinh. Sự hình thành của ferit kẽm được nghiên cứu bởi nhiễu xạ Rơnghen và các phép đo từ tính. Tính đồng nhất hóa  học  và hình thái của các loại  bột  đã được nghiên cứu bằng  vi phân tích X-ray và kính hiển vi điện tử quét. Tiến trình từ hóa theo  thời gian nghiền được thảo luận và đánh giá bởi sự sắp xếp lại các cation trong cấu trúc spinen do quá trình nghiền gây ra.
Từ khoá: nghiền năng lượng cao,  ZnFe2O4, từ tính, spinen

1. Introduction

Soft magnetic ferrites  with spinel structure are materials with a large variety of applications in electronic and telecommunication industries [1]. These ferrites have  a  structural formula of MFe2O4,  where  M is divalent  metal  ion   from 3d transition  elements such  as Mn, Cu, Ni, Co, Zn or their combinations. Among spinel  ferrites,  zinc fer- rite has  long been the subject of study  because of its unique  properties such  as chemical  and  ther- mal stability and  the particle  size  dependent mag- netic  properties.  ZnFe2O4  is  characterized by  a normal  spinel structure in which Zn2+  cations with no  magnetic  moment  occupy   tetrahedral  sites, while all the Fe3+  cations are  in octahedral sites of close  packed cubic  lattice.  No exchange interac- tion of each lattice  site  of ZnFe2O4  configuration takes place.  The  zinc  ferrite  is  paramagnetic  at room  temperature. ZnFe2O4 is of particular  inter- est in changes of magnetic behavior for nanoscale particles prepared by milling treatments or other means [2]. Several  methods have  been used to synthesize zinc  ferrite nanoparticles such  as ball milling, sol-gel, co-precipitation, hydrothermal technique  auto-combustion route,  ultrasonic cavi- tation, etc. Among  above techniques, high-energy ball-milling or mechanochemical activation  is the technique most  used to prepare nanocrystalline powders of various  materials due  to its simplicity, low cost  and  good  reproduction [3,4].

This paper presents the synthesis of nanocrys- talline  zinc  ferrite  by high-energy ball milling and the characterization of some structural, morpho- logical and  magnetic properties.

2. Experimental

Magnetite and  zinc powders were  processed in a planetary ball mill with harden steel ball and  jar. Raw   materials  were   commercial  Fe3O4   (98% pure)  and  Zn (99%  pure)  powders. The  composi- tion  of  starting   mixture  was  stoichiometric. The material/balls ratio was 1/10.  Milling time was  set to 10, 20, 30 and  50 h. The phase composition of  samples was analyzed by X-ray diffractometer (XRD) with CuKu radiation. XRD patterns were recorded in range of 28 = 25+70  degree. Microstructure analysis was carried  out using scanning electron  microscope (SEM)  and  energy dispersive  X-ray  spectroscopy. Magnetic  proper- ties of milled powders were  measured at room temperature in field  up  15  kOe  using a  Vibrating Sample Magnetometer (VSM).

3. Results and discussion

Fig. 1 shows X-ray diffraction patterns of start- ing powder  mixture (Fe3O4 and Zn). The main dif- fraction  peaks are  belong to Fe3O4 and  Zn phas- es. After milling for 50 h, the Bragg  peaks of mag- netic  and  zinc  phases disappeared and  full  zinc ferrite phase was obtained. During milling process, ZnO and  Fe2O3 intermediate phases may  appear after  2+10  h  of  milling fig. 1(b).  It can   be  also noticed  a small contamination of powders with iron from  vial  and  balls.  The  powders contamination with  elements from  milling  bodies  is  difficult  to avoid completely and  frequently encountered.

Fig. 1. XRD patterns of Zn - Fe3O4 mixture  milling at different  times

Fig. 1. XRD patterns of Zn – Fe3O4 mixture  milling at different  times

Fig. 2. SEM-EDX images of the mixtures milled for 50 h

Fig. 2. SEM-EDX images of the mixtures milled for 50 h

An element mapping image  showing  the spatial distribution  of  Zn,  Fe  and   oxygen   in  a  sample milled for 50  h can  be  seen in fig. 2. These ele- ments distribute  rather  uniform in the  sample vol- ume.  Particle  sizes of milled powders are  ranged in submicrometer scale.

Fig. 3 shows the  VSM measurements for mix- tured powders after different milling time at applied field ranged from -15 to 15 kOe.  All the  magneti- zation curves have  hysteresis loops and saturation magnetization corresponding to ferromagnetic behavior [5].

Fig. 3.The M-H curves at room temperature of mixtures, milled at different  times

Fig. 3.The M-H curves at room temperature of mixtures, milled at different  times

A continuous decrease of the  saturation mag- netization  and   coercivity  with  increasing  milling time from 10 to 50 h, reflects a structure change of  the   milled   powders.  The   powders  tend   to become superparamagnetic materials. Which has no loop, coercivity (Hc) and  remanence. For 50 h milled, powders the  saturation magnetization value  is 6.3 (emu/g)  and  Mr is 0.95  (tab.  1). The zinc  ferrite  phase  obtained by  ball  milling  pos- sesses a configuration change of cations sites in spinel   structure leading   to  change of  magnetic properties.  In  a   normal   spinel   structure  Fe3+ cations   are   ordered  in   octahedral   sites   and cations Zn2+  in tetrahedral ones.

But due to milling there  is a partial redistribution of Fe3+  and  Zn2+  cations between octahedral and tetrahedral sites in the spinel structure. Consequently these cations have  non-equilibrium distribution and a change in magnetic properties occurs.

Tablet  1. Magnetic  parameters of Fe3O4 and Zn powders milled at different  times

 Milling time (h)  HC (Oe)  M(emu/g) M(emu/g)
10 233.46 17.75 4.25
20 201.67 14.55 3.50
30 184.35 13.40 2.75
40 155.04 12.05 2.10
50 140.08 6.30 0.95

 

Fig.4. Saturation magnetization, Ms and coercivi- ty, Hc, as a function of activation time

Fig.4. Saturation magnetization, Ms and coercivi- ty, Hc, as a function of activation time

The drop  of Ms from sample milling at 10 h to 50  h  (fig.4  and  tab.  1)  is  due   to  accumulated structural damage of magnetite (which is the only magnetic  phase in  the  system), as it  was observed by XRD (Fig. 1). For prolonged milling time to 50 h, a superparamagnetic behavior can be   seen for  as-milled  ZnFe2O4  powders.  It  is explained that  the  cations inversion  and  in addition, a spin  canted effect and  ionic disorder were attributed to the  effect  of milling  [6]. The  satura- tion magnetization value  of sample milled  for 50 h is about  6.3 (emu/g). By comparison to values obtained by other  research groups for nanocrys- talline ZnFe2O4, there  is a broad  range of the reported values for Ms (12 ÷ 88 emu/g)  showing that  Ms value  may  strongly depend  on  various factors including the  synthesis route  and  its con- ditions, type of the precursors, subsequent annealing treatments,  etc  [7]. The  evolution  of coercivity, Hc, with activation  time represented in fig. 4. Hc values increase with prolonging  milling time due  to the  damage of Fe3O4 phase and  the formation  of ZnFe2O4 phase. Fig.4.  Saturation magnetization,  Ms and  coercivity,  Hc, as a func- tion of activation  time.

4. Conclusion

Nanocrystalline zinc ferrite was obtained as single phase after 50 hours of milling from a sto- ichiometric  mixture  of the  commercial magnetite and  zinc. The  magnetization studies suggest an inversion  of the  Fe3+   and  Zn2+  cations  between the  tetrahedral and  octahedral sites in the spinel structure. The  magnetization was saturated under  a  magnetic  field  of 15  T. This  behaviour could be explained by a superparamagnetic component that  can  be  given  by very small  par- ticles and  by a spin canted effect. Ball milling method is suitable to obtain  nanocrystalline zinc ferrite with good magnetic properties. Further investigations in order to clarify the cations dis- tribution are  in progress.

References

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  2. H. Ehrhardt, S. J. Campbell, M. Hofmann; Structural evolution of ball-milled ZnFe2O4; Journal of Alloys and Compounds 339,  (2002),  pp. 255–260.
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