Magnetic properties of Ba- and Sr-hexaferrite prepared by mechanical alloying

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Journal of Magnetism and Magnetic Materials 164 (1996) 385-389

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Magnetic properties of Ba- and Sr-hexaferrite prepared by mechanical alloying J. Ding a.*, R. Street a, H. Nishio b a Research Centre for AdL'anced Mineral and Materials Processing, The University of Western Australia, Nedlands, 6907. Australia b Materials Research Centre, TDK Corporation, Narita 286, Japan

Received 19 September 1995; revised 22 May 1996

Abstract Samples of Ba- and Sr-hexaferrite were prepared by mechanical alloying and subsequent heat treatment were found to consist of single domain particles of the single hexaferrite phase. The particles had a wide distribution of anisotropy fields. Study of irreversible magnetisation suggested, that the demagnetisation process is mainly controlled by the Wohlfarth rotation. It was deduced from the results of measurements of magnetic viscosity, that the activation volume was of same order of magnitude as the cube of the domain wall thickness. Keywords: Ferrite; Irreversible magnetisation; Activation volume: Magnetic viscosity

1. Introduction Mechanical alloying is a powerful and convenient method for the production of fine- and nano-crystalline materials [1]. Recently, mechanical alloying has been introduced to the preparation of magnetic ferrite materials [2,3]. Ba and Sr M-type hexaferrite powders (referred to as BaM and SrM below) prepared by this method can have a particle size as small as 0.1 Ixm [2-4]. These fine particles have excellent magnetic properties, with coercivity values in the range of 6 - 7 kOe and high remanence values after hot-pressing [2]. As is well known, high coercivities of M-type ferrite materials require small particle size, which should be less than the single domain particle size

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(about 1 txm [6]). Fine particles prepared by other methods [5-8] have also shown high coercivity values, and other magnetic properties, such as distribution of anisotropy fields, rotational hysteresis loss and aftereffect (magnetic viscosity), are sensitively dependent on their microstructure. In this work, we report the magnetic properties of fine BaM and SrM particles prepared by mechanical alloying. The result of the study of irreversible magnetisation and timedependent behaviour will be discussed.

2. Experimental method The starting materials for the mechanical alloying were BaCO 3 or SrCO 3 (99% purity) and Fe203 (99%) powders mixed together in a composition of BaCO 3 (or SrCO 3) + 6Fe203. A small excess about 5% of BaCO 3 or SrCO 3 was added to ensure the

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J. Ding et al. / Journal of Magnetism and Magnetic Materials 164 (1996) 385-389

formation of the single hexaferrite phase [4]. Mechanical alloying was carried out in a hardened steel vial together with ten 12 m m steel balls for 24 h using a Spex 8000 mixer/mill. A ball to powder mass charge ratio of 1:8 was chosen. Hexaferrite BaM and SrM particles were formed during heat treatment of the milled powders at 1000°C for one hour. All powder handling, milling and subsequent heat treatment and pressing were performed in air [2,4]. The BaM and SrM particles were studied by C u K ~ X-ray diffraction (Siemens D5000) and by 57Fe-Mtissbauer spectroscopy (Canberra Packard). The particle size was determined using scanning electron microscopy. The coercivity, H c, saturation magnetisation, M s, and remanence, M r, were measured on cold pressed cylinder samples of 5 m m in diameter and 3 - 4 m m in height using a vibrating sample magnetometer (Oxford, 3001VSM) with a maximum applied field of 50 kOe. Irreversible magnetisation and magnetic time-dependent behaviour were studied on a resin-epoxy bonded BaM magnet. The magnet was of cylindrical form with a diameter 5 m m and length 3.5 mm. The density was 5.1 g / c m 3. The demagnetisation field was calculated from the demagnetisation factor for cylindrical samples [9]. The rotational hysteresis loss and distribution of anisotropy fields were measured on packed particles. The field dependence of rotational hysteresis loss, Wr, was measured using a torque magnetometer [7] with an applied field of up to 20 kOe at room temperature. The rotational hysteresis integral was calculate from the formula of R h = f ( W r / M ~ ) d ( 1 / H ) [7,10]. The measurements of the anisotropy field H a was carried out using a fully-automatic vibrating sample magnetometer (VSM) as follows: (1) The packed

particles were magnetised to saturation with an applied field of 20 kOe at 0 = 0 °. (2) The magnetic field H was applied at 0 = 5 ° and then reduced to zero, where the field H was increased from 0 to 20 kOe in a step of 0.5 kOe. (3) The remanent magnetisation M i, between the two direction of 0 and 5 ° was measured at 0 = 90 °. More detail of experimental process has been given in Refs. [5,11].

3. Results and discussion The both BaM and SrM samples after mechanical alloying and the subsequent heat treatment at 1000°C were shown by X-ray diffraction and MiSssbauer spectroscopy to be single phase of hexaferrite structure. Scanning electron microscopy showed that the both samples consisted of single crystal particles with an average particle size of ~ 0.3 p~m [4], which is less than the single domain particle size (about 1

~m). The magnetic properties of the two BaM and SrM samples are summarised in Table 1. The Curie temperatures, Tc, and the values of saturation magnetisation, M~, were very close to those reported for bulk Ba- and Sr-hexaferrite materials [6]. The coercivity was measured to be 4.7 kOe and 5.8 kOe for BaM and SrM respectively, indicating that SrM had a higher coercivity than that of BaM having the same particle size. This result is consistent with the observation that the anisotropy field of SrM is greater than that of BaM [6]. The coercivity values for BaM and SrM shown in Table 1 are lower than those of mechanically alloyed samples annealed at lower temperatures, e.g., coercivities of 6.0 and 7.1 kOe have been measured for BaM and SrM samples mechanically alloyed and annealed at 900°C with a particle size of about 0.1 txm [2,4]. Similar results showing

Table 1 Magnetic properties of SrM and BaM particles prepared by mechanical alloying and subsequently annealed at 1000°C (D is the particle size. T~ the Curie temperature, Ms the saturation magnetisation measured with the maximum applied field of 50 kOe, Mr the remanence, Hc the coercivity, H~ave.the average anisotropy field and R h the rotational hysteresis integral) BaM SrM

D (Ixm) ~ 0.3 ~ 0.3

Tc (°C) 456 463

Ms (emu/g) 69.8 74.2

Mr (emu/g) 36. l 37.8

HC(kOe) 4.7 5.8

Ha....
Rh 0.89 0.75

J. Ding et aL / Journal of Magnetism and Magnetic Materials 164 (1996) 385-389

the decrease of coercivity as particle size increases have also been reported in Ba- or Sr-ferrite materials prepared by other methods [6,8,12]. The distribution of anisotropy fields was determined by the Berkowitz-Flanders method [5,11]. The fraction of A M i / M ~ is plotted as function of the magnetic field H in Fig. 1, showing the distribution of anisotropy field. Both samples had a wide distribution of anisotropy field. The average anisotropy fields, Ha.av e, for BaM and SrM are 10.6 and 11.7 kOe, respectively. The two values are significantly lower than those reported for bulk BaM and SrM materials (H a = 16-17 and 18.5 kOe, respectively [6]). It can be seen from Fig. 1, that A M~ was detectable in fields around 5 kOe, indicating that a small fraction of particles had very low anisotropy values. Similar low values of H a have been reported for fine Ba-hexaferrite particles having a coercivity below 2 kOe [5]. In this case large decrease of anisotropy field has been attributed to defects and superparamagnetism [5]. Recently, Taguchi et al. [18] have found crystal distortion of submiron SrM particles after mechanical milling. Such crystal distortion can result in change of magnetic anisotropy [18]. A M i / M s almost disappeared at field of 16-17 kOe and about 18 kOe for BaM and SrM, respectively (Fig. 1). These values are almost the same as those reported for bulk materials [6]. The results shown in Fig. 1 indicate, that most BaM and SrM particles have a reduced anisotropy field. Defects, e.g., crystal distortion, are likely reasons [5,7,18], since mechanical alloyed materials with a nano- or fine-crystalline structure may have a high density of 12 ----~-- BaM - SrM

f ~

4 0

5

10

15

20

H (kOe) Fig. 1. Anisotropy field distribution of BaM and SrM prepared by mechanical alloying.

387

12. °~o

10

•

BaM

8 6 ~

,

4

\

2

4

8 H (kOe)

12

16

Fig. 2. Rotational hysteresis loss, W r / M s, versus the applied field, H.

defects, and also stresses and strains may be present [18]. In addition, the Ba- and Sr-hexaferrite particles prefer to have platelet shape, where the c-axis (the magnetically easy direction) is perpendicular to the platelet plane [4,12], so that the anisotropy field may be reduced further by the shape anisotropy. Both samples had a broad distribution of anisotropy field (Fig. 1). This is probably associated with structural non-uniformity of mechanically alloyed Ba- and Sr-hexaferrite powders. Mechanically alloyed materials can have a wide range of particle sizes, irregularity in particle shapes and an inhomogeneous distribution of defects [1-4]. Broadened distribution of anisotropy fields has been reported previously, due to non-uniformity in particle size and particle shape [11]. Similar broad distribution of anisotropy fields has been reported for Ba- and Sr-hexaferrite powders prepared by other methods [5,7]. The rotational hysteresis loss, W~, as function of the field H is plotted in Fig. 2. No rotational hysteresis was observed if the field was below 4 and 5 kOe for BaM and SrM, respectively, showing that reversible magnetisation processes were dominant in this field range. The peak of W J M s was found at a field of 5.7 and 7.0 kOe for BaM and SrM, respectively, approximately half of the average anisotropy field, predicted for the Wohlfarth coherent rotation [10]. The shape of the two W r / M S versus H curves and the maximum values of W r / M ~ were similar as those reported for high-coercivity Sr-hexaferrite particles [7]. The rotational hysteresis integral, R h, was

J. Ding et al. / Journal of Magnetism and Magnetic Materials 164 (1996) 385-389

388

0.1 • 0.5

•• • •

•

•

•

initial demagn.

005

o

/%~

7

\

-0.5

-1

n

0

I

0.2

0.4

0.6

v

0

0.8

w

~

0

5 H (kOe)

gr/Mr(~) Fig. 3. The demagnetisation mmanence M d/Mr(oc) versus the mmanence M r/Mr(OC) for the bonded BaM sample, where Mr(:¢) is the maximum remanence measured after saturation with a field of 50 kOe. The diagonal line follows the predicted Wohlfarth relation Md(H) = Mr(OC)- 2Mr(H).

calculated to be 0.89 and 0.75 for BaM and SrM, respectively (Table 1). These values are between those predicted by Wohlfarth rotation (R h = 0.380.42 [10]) and those calculated from the model of chain-of-spheres with fanning (R h = 1.0-1.5), indicating presence of interactions [14]. Henkel plots [15] may be used to reveal the effects of particle interactions. Values of Mr(H) and Md(H), remanences associated with field H on the initial magnetisation and demagnetisation curves, are plotted in Fig. 3. For a system of randomly oriented non-interacting single domain particles with uniaxial anisotropy, Wohlfarth [ 13] predicted a linear relation of the form Mo(H) = Mr(~) - 2Mr(H). It may be seen from Fig. 3, that the data do not depend markedly from the predicted linear relation, indicating that the effects are interaction between particles is relatively small. The coercivity values for both samples (Table 1) were very close to half of the values of Ha.ave, as expected by Wohlfarth rotation [13]. Values of the irreversible susceptibility, Xirr, derived from the initial and demagnetisation curves are shown in Fig. 4 for the resin-bonded BaM magnet. Irreversible magnetisation occurred over the field range 3-10 kOe. Maximum values were observed at 5-6 kOe close to coercivity. At a given field Xirr taken on the demagnetisation curve was approximately twice the value of Xirr derived from the initial curve. This result is to be expected for materials, in which the processes responsible for the initial

10

Fig. 4. The irreversible susceptibility,/tVirr , as function of magnetic field H.

magnetisation are the same as those involved in demagnetisation [16]. The magnetic viscosity coefficient, S, defined in the equation M(t) = M(O) + S ln(t + t 0) [17] is plotted as function of the magnetic field in Fig. 5. The field dependences of S were similar to those for the irreversible susceptibility shown in Fig. 4. Again, at a given value of field the value of S on the demagnetisation curve is approximately twice the corresponding value of S on the initial curve. The magnetic viscosity parameter, A, can be derived from the formula of A = d H / d ( l n Mir r) [17]. A was found to be 1 4 _ + 0 . 5 0 e and was nearly independent on the field for both of initial and demagnetisation curves. The activation volume v defined as v = kT/MsA was found to be 8 × 10 -18 cm 3. Values of activation volume in the range (820) × 10 -18 cm 3 have been measured using specimens containing fine BaM and SrM particles [5,7,8] having coercivities in the range 0.7 to 6 kOe. An 1 o/ ~

~

o - S (initial)

0.5

0

~ 0

-5

Hi (kOe)

10

15

Fig. 5. The viscosity coefficient, S, as function of magnetic field H.

J. Ding et al. / Journal of Magnetism and Magnetic Materials 164 (1996) 385-389

activation volume of 8 × 10-is cm 3 corresponds to a sphere of a diameter of 25 nm, comparable to the domain wall thickness of 17 nm reported for bulk materials [8]. The domain wall thickness calculated from an anisotropy field Ha.ave = 10.6 kOe for BaM (Table 1) is 22 nm, a value more closely approximating the sphere diameter calculated above. Low coercivity samples of BaM and SrM are associated with smaller anisotropy fields [5,7]. The increase of activation volume with decreasing coercivity [5,7,8] may thus be related to the increase of domain wall thickness as derived from the reduced anisotropy field.

4. Conclusion Mechanical alloying and subsequent heat treatment of BaCO 3 or SrCO 3 and 6F%O 3 with a small excess of BaCO 3 or SrCO 3 produces fine particles of the single hexaferrite phase, having high coercivities. These fine BaM and SrM particles had a wide distribution of anisotropy fields. The average anisotropy field was 60-70% of those reported for these compounds in bulk [6]. The reduction was probably due to defects and superparamagnetism, since fine particles prepared by mechanical alloying will have a high density of defects and a wide distribution of particle sizes. The coercivity values were close to half the average anisotropy field. Interaction effects were demonstrated in measurements of rotational hysteresis and by small deviations from straight line behaviour in Henkel plots. The variations with applied field of the irreversible susceptibility and the magnetic viscosity coefficient were similar. Values measured on the demagnetisation curve were nearly twice those taken on the initial curve, indicating that the demagnetisa-

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tion is mainly controlled by Stoner-Wohlfarth rotation. The viscosity parameter, A, appeared to be almost independent of the magnetic field and had a value of 13-14 Oe, corresponding to an activation volume equal to the volume of a sphere of 25 nm in diameter, which is approximately equal to the width of a domain boundary wall. References [1] C.C. Koch, Ann. Rev. Mater. Sci. 19 (1989) 2954. [2] J. Ding, D. Maurice, W.F. Miao, P.G. McCormick and R. Street, J. Magn. Magn. Mater. 150 (1995) 417. [3] S.J. Campbell, W.A. Kaszmarak and G.M. Wang, J. Magn. Magn. Mater. 140-144 (1995) 959. [4] J. Ding, Y. Hang, W.F. Miao, P.G. McCormick and R. Street, J. Alloys Comp. 221 (1995) 70. [5] H. Nishio, J. Magn. Soc. Japan 18 (1994) 249. [6] H, Kojima, in Ferromagnetic Materials, Ed. E.P. Wohlfarth, Vol. 3 (1992) p. 305. [7] H. Nishio, H. Taguchi, F. Hirata and T. Takeishi, IEEE Trans. Magn. 29 (1993) 2637. [8] F. Carmona, A. Martin and C. Alemany, J. Magn. Magn. Mater. 92 (1991) 417. [9] D.X. Chen, J.A. Brug and R.B. Goldfarb, IEEE Trans. Magn. 27 (1991) 3601. [10] L,S. Jacobs and F.E. Luborsky, J. Appl. Phys. 28 (1957) 467. [11] P.J. Flanders and S. Shtrikman, J. Appl. Phys. 33 (1962) 216. [12] H. St~iblein, in Ferromagnetic Materials, Ed. E.P. Wohlfarth, Vol. 3 (1982) p. 441. [13] E.P. Wohlfarth, J. Appl. Phys. 29 (1958) 595. [14] D.M. Paige, S.R. Hoon, B.K, Tanner and K. O'Grady, IEEE Trans. Magn. 20 (1984) 1852. [15] P.I. Mayo, K. O'Grady, P.E. Kelly, J. Cambridge, I.L. Sanders, T. Yogi and R.W. Chantrell, J. Appl. Phys. 68 (1991) 4733. [16] J. Ding, R. Street and P.G. McCormick, J. Magn. Magn. Mater. 115 (1992) 211. [17] Y. Estrin, P.G, McCormick and R. Street, J. Phys.: Cond. Met. 1 (1989)4845. [18] H. Taguchi, H. Nishio, F. Hirata, T. Takeishi and T. Mori, J. Magn. Soc. Japan, in press.