Strong uni-directional anisotropy in disordered NiFe2O4

Y. Shia, J. Dinga,*, Z.X. Shenb, W.X. Sunb, L. Wangb

aDepartment of Materials Science, National University of Singapore, Lower Kent Ridge Road, Singapore, Singapore 119260bDepartment of Physics, National University of Singapore, Singapore, Singapore 119260

Received 31 March 2000; accepted 18 April 2000 by T. Tsuzuki

Abstract

High-energy mechanical milling of spinel NiFe2O4 leads to the formation of a disordered wustite-like structure. Cluster glass

behavior was found in the MoÈssbauer study. The investigation suggested ferrimagnetic clusters in an antiferromagnetic matrix.

The ferrimagnetic and antiferromagnetic exchange coupling results in a strong uni-directional anisotropy and a coercivity of

over 10 kOe after magnetic cooling. q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: A. Magnetically ordered materials; B. Nanofabrications; E. Nuclear resonances

1. Introduction

Magnetic ferrites are found in many applications such as

permanent magnets, recording media, ferro¯uids and micro-

wave devices [1±4]. Recently, many publications have

reported interesting behaviors in ferrite materials, such as

change in saturation magnetization, spin glass or cluster

glass, high coercivity and shift in hysteresis loop [1,2,4±

6]. The mechanisms of these behaviors are not clear. Disor-

dered structure has been suggested [5,6]. The clari®cation of

the mechanisms is certainly of interest for research and

applications, e.g. magnetic recording [1±6].

Mechanical alloying (high-energy mechanical milling) is

a powerful method for the synthesis of amorphous and non-

equilibrium materials. Many ferrite materials prepared by

mechanical alloying have shown unique properties [3,4,7].

In this work, we have mechanically milled NiFe2O4 powder.

The structure and magnetic properties were studied.

2. Experimental

The starting powder for the mechanical milling was

NiFe2O4 powder, which was calcined at 13008C for 2 h

after chemical co-precipitation. NiFe2O4 powder together

with several steel balls was loaded in a hardened steel vial

before mechanical milling. The mechanical milling was

performed using a Spex 8000 for 32 h. The powder/steel

ball weight ratio was 1:5. After mechanical milling, the

as-milled powder was annealed at different temperatures

(400±10008C) for 1 h in air atmosphere.

The structure was examined by X-ray diffraction with

CuKa radiation, transmission electron microscopy and

Raman spectroscopy. The magnetic properties were studied

using a vibrating sample magnetometer (VSM) with a maxi-

mum ®eld of 90 kOe in the temperature range 4.2±293 K

and a 57Fe-MoÈssbauer spectrometer from room temperature

to 4.2 K.

3. Results and discussion

The calcined powder possessed a saturation magnetiza-

tion of 49 emu/g at room temperature. The MoÈssbauer spec-

trum and X-ray diffraction pattern were expected for the

spinel ferrite phase with a composition of NiFe2O4 [8].

Fig. 1 shows the X-ray diffraction patterns of the as-milled

powder and powders after annealing at different tempera-

tures. After mechanical milling, the diffraction peaks were

broad. The major peak ((311) plane) at 2u � 35:88 for the

spinel structure was absent. The broad diffraction peaks

could be well identi®ed with the wustite structure (FeO).

After annealing at 4008C, the major peak at 2u � 35:88 for

the spinel structure began to appear. After annealing at

6008C, the X-ray diffraction pattern could be described

with the spinel NiFe2O4 structure. Annealing at higher

Solid State Communications 115 (2000) 237±241

0038-1098/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.

PII: S0038-1098(00)00176-9

PERGAMONwww.elsevier.com/locate/ssc

* Corresponding author. Tel.: 165-874-4317; fax: 165-776-

3604.

E-mail address: masdingj@nus.edu.sg (J. Ding).

temperatures (800 and 10008C) lead to sharp crystalline

peaks, indicating grain growth. The as-milled powder was

studied under a transmission electron microscope. Small

grains of approximately 10 nm exhibited diffuse electron

diffraction rings, indicating a disordered structure. These

diffuse rings could be described with the wustite structure.

Fig. 2 shows the Raman spectra of the as-milled and the

subsequently annealed powders. No signi®cant difference

between the as-milled powder and the powders annealed

at 10008C is evident. The broadened peaks of the as-milled

corresponded to the nanocrystalline structure. The Raman

result indicated that the as-milled powder should have a

structure similar to that of the powder annealed at 10008C(i.e. ordered NiFe2O4 structure).

Fig. 3 shows the saturation magnetization Ms of the as-

milled and subsequently annealed powders. The as-milled

powder exhibited a much lower saturation magnetization,

which is approximately 1/3 of that expected for NiFe2O4. Ms

increased with increasing annealing temperature. After

annealing at 10008C, Ms was measured to be 48 emu/g,

which is nearly the same as the starting powder before

milling and is well expected for NiFe2O4 [8].

Y. Shi et al. / Solid State Communications 115 (2000) 237±241238

Fig. 1. X-ray diffraction patterns of the as-milled and the subsequently annealed samples.

Fig. 2. Raman spectra of the as-milled and the subsequently

annealed samples.

Fig. 3. Saturation magnetization Ms versus the annealing tempera-

ture Ta.

Fig. 4 shows the MoÈssbauer spectra of the as-milled

powder taken at different temperatures. The MoÈssbauer

spectrum of the as-milled powder at room temperature

could be well ®tted with two non-magnetic doublets, indi-

cating superparamagnetism in consideration of the magnetic

result discussed above. With decreasing temperature, the

average hyper®ne ®eld increased, while the population of

the non-magnetic component decreased. This behavior is

typical for cluster glass [6]. At 4.2 K, the average hyper®ne

®eld was close to the average hyper®ne ®eld of NiFe2O4, but

with a much broader hyper®ne ®eld distribution.

All the results discussed above (X-ray diffraction, trans-

mission electron microscopy, Raman spectroscopy,

magnetic measurements and MoÈssbauer spectroscopy)

suggest a disordered structure after the high-energy mechan-

ical milling. The structure of the as-milled powder is

Y. Shi et al. / Solid State Communications 115 (2000) 237±241 239

Fig. 4. MoÈssbauer spectra of the as-milled and the subsequently annealed samples.

probably similar to the wustite structure. A possible conver-

sion between magnetite (Fe3O4) and wustite (FeO) has been

reported in thin ferrite ®lms [5]. In this work, XPS was used

for the study of the as-milled powder. Many Ni21 ions were

converted into Ni31, while some Fe31 changed into Fe21

after mechanical milling. This result is probably associated

with a disordered structure in the A and B sites of the spinel

structure. Similar results have been reported previously

[7,9].

Fig. 5 shows the zero-®eld-cooling (ZFC) and ®eld-cool-

ing (FC) curves of the as-milled powder, when the magne-

tization was measured under the maximum ®eld of 90 kOe.

At 5 K, the difference in magnetization was approximately

15% between FC and ZFC. This is a characteristic behavior

for cluster or spin glasses [6]. Recently, similar results have

been reported in other ferrite materials [1,2,5,6] that may

possess a disordered structure [5,6].

Fig. 6 shows the hysteresis loops taken at 5 K after ZFC

and FC, respectively. The hysteresis loop after ZFC exhib-

ited a relatively low coercivity and a symmetric hysteresis

loop. The hysteresis loop after FC was shifted. A coercivity

of over 10 kOe was measured. The high coercivity and the

large shift in the hysteresis loop are attributed to a large

exchange coupling [1,2].

From a combination of our magnetic and MoÈssbauer

results, we suggest that cluster glass [6] is the control-

ling mechanism in the mechanically alloyed NiFe2O4

powder. We may propose a tentative microstructure

for the powder after mechanical milling. The high-

energy mechanical milling of spinel NiFe2O4 results in

a disordered structure. This structure is associated with

both spinel and wustite [5]. There is probably a mixture

of ferrimagnetic clusters (spinel) in an antiferromagnetic

matrix (wustite). The interaction between the ferrimag-

netic clusters and the antiferromagnetic matrix results in

a large exchange anisotropy, which leads to the shifted

hysteresis loop and large coercivity (Fig. 6). At room

temperature, the ferromagnetic clusters exhibit superpar-

amagnetism, as found in our MoÈssbauer and magnetic

measurements.

4. Summary

Mechanical milling of NiFe2O4 lead to a disordered struc-

ture. In the X-ray diffraction patterns, the as-milled powder

exhibits broadened diffraction peaks which can be identi®ed

as wustite-like, while ordered spinel was reformed after heat

Y. Shi et al. / Solid State Communications 115 (2000) 237±241240

Fig. 5. Field-cooling (FC) and zero-®eld-cooling (ZFC) of the as-

milled powder. The sample was cooled under a magnetic ®eld of

90 kOe in the ®eld cooling. The magnetization was measured at

90 kOe.

Fig. 6. Hysteresis loops of the as-milled NiFe2O4 powder taken at 5 K after zero-®eld cooling (ZFC) and after ®eld cooling (FC).

treatment at a temperature of 6008C or higher. The as-milled

powder under a transmission electron microscope

consists of small grains with a grain size of approxi-

mately 10 nm. The diffuse electron diffraction rings can

be described with the wustite structure. However,

Raman spectroscopic study suggests that the as-milled

powder should have a structure similar to that of the

ordered NiFe2O4 structure.

A cluster glass behavior [6] was observed in the

MoÈssbauer spectra of the as-milled powder taken at

different temperatures. The spectrum taken at 4.2 K

possessed the average hyper®ne ®eld of ,51 T, which

is close to that of the ordered NiFe2O4 and is signi®-

cantly higher than that of wustite [10]. Cluster glass was

con®rmed in FC and ZFC curves. Shifted hysteresis

loop and high coercivity was measured at 5 K after

magnetic cooling. These results lead to a tentative

microstructure which cab be described as ferrimagnetic

clusters (based on spinel) in an antiferromagnetic matrix

(based on wustite).

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