314 Journal of Magnetism and Magnetic Materials 40 (1984) 314-318 North-Holland, Amsterdam

THE MAGNETIC A N I S O T R O P Y OF S A M A R I U M - A L N I C O PSEUDOBINARY ALLOYS

Y.H. C H A N G , C.I. W E N G

Department of Mechanical Engineerin~ National Cheng Kung University, Tainan, Taiwan

and

T.K. HSU

Material Research Laboratory, I.T.R.I., Nantzu, Kaoshiung, Taiwan

Received 27 June 1983

The anisotropy properties of samar ium-Alnico V pseudobinary alloys have been investigated. With alloys containing less than 12.0 tool% samarium, the K 1 values are negative at 77 K and increase with increasing temperature to approximately zero at room temperature. The K 2 values remain positive at all temperatures. We do not find the easy cone that has long been thought to be existed in those alloys with K 1 < 0 and K 2 > 0. In alloys with samarium contents between 13.3 and 19.0 mol%, the K l and K 2 values are positive at all temperatures. The anisotropy fields are not changed monotonically in the whole range of 10.1 to 19.0 mol% of samarium. It is concluded that the alloys are characteristic in thermodynamically of first-order transition. We have found that the "hard cone" exists in each of those alloys with samarium content more than 16.0 mol% and at temperatures above 77 K. The alloys with samarium less that 13.4 mol% also have "ha rd cone" under 77 K. However, the observed "ha rd cone" is different from the well known one in the first-order magnetization process, and it will collapse to the easy axis when the measuring field and temperature increase while under room temperature.

1. Introduction

Since the magnetic anisotropy properties of the alloys of rare-earth metal RE and transition metals T M can be expressed in terms of the single-ion model [1], the anisotropy of the crystal as a whole is the sum of the anisotropies of individual ions. Some investigators at tempted to improve the mag- netic properties of R E - T M alloy systems by sub- stituting a part of Co atoms with other metals [2-4]. As a result, the lattice sites of Co atoms are replaced in the preferred sequence of 6c, 9d and 18h, 18f in 2 : 17 alloys [5]. The atoms that replace the 6c sites sometimes enhance the magnetic mo- ment but always decrease the anisotropy proper- ties.

It is well known that the alloy SmC% has a strong uniaxial anisotropy with K 1 = 1.05 x 108 e r g / c m 3 and H a = 270 kOe. However, the substitu- tion of each one-third of Sm atoms by two Co

atoms in 6c sites to form the S m 2 f o t 7 alloy will make the anisotropy field H a drop rapidly to about 81 kOe but the magnetic moment to be increased abruptly due to a higher Co concentration. Also, the anisotropy constant K 1 of the Sm2Co17 alloy is reduced to about the same order as H a because the number of strong R E - T M sublattice exchange interactions decreases in the lattices.

In the case of alloying with iron, the increase in the magnetic moment is explained as the donation of 3d subband electrons of Fe at the rate of 1 e lect ron/atom. However, the magnetic moment deteriorates in Y2( fO l_xRux )17 compounds con- taining aluminum. The above phenomenon is in- terpreted as the occupation of the 3d holes of the Co subband with a part of the A1 valency electrons [6]. It was shown that when the elements iron and aluminum are used as constituents in R E - C o al- loys, the anisotropic property will be changed in C e E C O l T _ x T x systems [7], as well as in the

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Y.H. Chang et al. / Anisotropy of samarium-Alnico alloys 315

Y 2 ( C O l _ x M x ) 1 6 s y s t e m [8]. It was concluded that the magnetic moment of

Sm-Alnico V alloys is far less than that of Sm2Co17 alloy, and the coercivity is always less than 1 kOe [9]. The purpose of this study is to investigate the anisotropic properties of Sm-Alnico V alloys and to explain the mentioned behavior observed.

2. Experimental procedure

In this study, alloys of xSm-(100- x)Alnico were prepared by induction melting under high purity argon protection, where x = 10.1, 10.7, 12.0, 13.3, 13.7, 16.0, 17.0, 17.6, 18.4 and 19.0 mol%. The composition of Alnico is 24% Co, 8% A1, 14% Ni, 4% Cu by weight and balanced Fe, the ele- ments are all above 3-nine purity. The alloy ingots were crushed and ground under ether medium to prevent from oxidation and thus the powder of average 5 ~tm particle size will be produced. According to a standard powder metallurgy method, the powder was then consolidated through pressing and sintering to produce a sample of excellent homogeneity in composition. The sam- ples for anisotropy measurements were manufac- tured by crushing the sintered pieces to a particle size less than 100 mesh. Then they were blended with epoxy resin and aligned with a magntic field of 17 kOe. Finally, the cylindrical specimens 2.5 mm in diameter and 2.5 mm in height were cut from the resin-bond bulk.

In the experiments, the preferred orientation was examined by the X-ray powder method, and the magnetization was measured with a vibrating sample magnetometer at temperature of 77-300 K. At every particular temperature, the field was initially made parallel to the easy axis; then the sample was rotated to measure the magnetization per ten degrees untill one full cycle. After turning off the field, the sample was then brought to the initial position, and the measurements were repeated with a stronger fidd. Thus, we could obtain in the data for the field parallel to both easy and hard directions.

3. Results and discussion

A series of experiments was performed as men- tioned above. Table 1 illustrates the magnetic ani- sotropies of samples at liquid nitrogen tempera- ture. The composition and phases of these sample are also listed in this table [9]. It is clear that there are four different structure types in the whole range of samarium contents. From the table, it is shown that the change in the anisotropy constants K 1, K 2 and the anisotropy field Ha does not change in a monotonical way as the samarium content increases. The alloy with 16.0 mol% Sm has the maximum amount of 1 : 5 phase (CaCu 5 type) and we can expect that it possesses a larger anisotropic field H a as evidenced by the experimental data.

Tab le 1 The magne t ic an iso t ropies of s a m a r i u m - A l n i c o pseudob ina ry s in tered al loys (measured at 77 K)

A l loy Sm conten ts Phases a) K 1 × 105 K 2 X 105 H a H a measured

n u m b e r ( m o l ~ ) ( e r g / c m 2 ) ( e r g / c m 3) (kOe) at R T

(kOe)

1 10.1 R 1 + U 1 - 25.8 47.6 - 8.5 27.0

2 10.7 R 1 + U 1 - 4 9 . 7 203.8 - 13.0 30.2

3 12.0 R 1 + U 1 - 35.3 76.2 - 11.3 32.5 4 13.3 2 : 17 + 1 : 5 4.5 68.0 1.6 34.0 5 13.7 2 : 17 + 1 : 5 12.4 77.4 4.3 35.5

6 16.0 R 2 + 1 : 5 19.0 54.3 8.0 64.5

7 17.0 R 2 + 1 : 5 15.7 92.4 5.6 35.5 8 17.6 R 2 + K + 1 : 5 20.0 75.6 7.7 46.5 9 18.4 R 2 + K + 1 : 5 21.2 103.2 7.5 62.0

10 19.0 R 2 + K + 1 : 5 33.3 36.7 13.4 44.5

~) Phases are de te rmined at RT. U 1 is an u n k n o w n phase, R 1 and R z are supposed as the supper la t t ice of 2 : 17 (Th2Zn17) s t ruc ture [16], the K phase has a la t t ice pa ramete r of a = 8.631, c ~ 24.816, c /a ~ 2.875.

316 Y.H. Chang et aL / Anisotropy of samarium-Alnico alloys

The values of H, at room temperature as indicated is determined from the intersection of the magneti- zation curves obtained from the measurements with the field parallel and perpendicular to the aligned direction of the aligned samples. Those values are comparable to the calculated results from the equation Ha = 2(K 1 + 2 K E ) / J s [10]. For those alloys numbered 4 to 7 containing Sm con- tents from 13.3 to 17 mol%, K 1 and H a decrease with decreasing Sin, and they turn to be negative as Sm decreases to 12.0 mol% and below.

As a result of X-ray examinations at room temperature, the crystalline c axis is found to be the easy direction for every phase in these magnet- ically aligned samples. We obtain the curve of magnetization versus field in the direction normal to the c axis. As reported by Sucksmith and Thompson [11], the magnetization versus field curve could be converted into an H / J vs. J 2 line, were J is the magnetic moment per unit volume. Since the intersection of the H / J - J 2 line with the H / J axis represents the value of 2K]/Js 2 and the slope of the line is the value of 4 K 2 / J 4, the values of K 1 and K 2 can therefore be obtained. The anisotropy field Ha can also be calculated from 2 K 1 / J s, where Js is the saturation magnetic mo- ment per unit volume.

1,° t

x°° lo 500 0 " ~ 3)

q-

- (1) 0 I I I i I i i i I I I

70 110 150 190 230 270 T (K)

Fig. 1. Relationship between the anisotropy field H a and tem- perature. Curve (1) - sample no. 4, 13.3 mol% Sm; curve (2) - sample no. 8, 17.6 mol% Sm; curve (3) - sample no. 6, 16.0 tool% Sm.

Table 2 The temperature at the maximum anisotropy field and intrinsic coercivity

Sample T T at max H a at max Hci (K) (K)

1 - - - -

2 - - - -

3 - - - -

4 175 164 5 162 166 6 140 147 7 175 169 8 145 150 9 160 130

10 145 147

Fig. 1 shows the typical curves of relationship between the anisotropy field and temperature for alloys nos. 4 to 10. Each curve possesses a maxi- mum H a at the temperature which is also close the the temperature for its maximum intrinsic coerciv- ity [9]. Table 2 lists the temperatures of the max- ima in both anisotropy field and intrinsic coerciv- ity in each sample. In alloys nos. 1 to 3, no maximum anisotropy field is observed. At 77 K, none of the H a values in our experiments exceeds 15 kOe and is far less than that of Sm2Co]7(n a = 81 kOe). This is explained by that the larger

100 - - ~ K1 0

o - - - - - K 2

.~ 50 "~aT.-,, x---- .,.~ g.. (1) -- ,at - - @ - - ~ " " @ ~ " ' 3 ( .

i i i I i i i I i I

-SO70 110 1SO lcJO 230 270 T (K)

Fig. 2. Thermal variation of the anisotropy constants K 1 and g 2. The solid curve is for K 1 and the dashed curve for K 2.

Y.H. Chang et a L / Anisotropy of samarium-Alnico alloys 317

amount of Fe and non-magnetic ions take a haphazard replacement of Co atoms either in 1 : 5 phase or in 2 :17 phase, so that the T M - T M exchange interactions are weakened or screened.

The dependence of K 1 and K 2 on temperature is demonstrated in fig. 2. Curves designed as (1) are the K1 and K 2 values of the alloy number 3. The K 1 value is negative and varies gradually to nearly zero as temperature increases. The K 2 has the same trend of variation but is positive in value. Both alloys I and 2 have the similar phenomena as alloy 3 except that K~ of alloy 2 changes sign at a temperature around 200 K or above. In the single- ion model, the exchange interaction between tran- sition metals plays a predominant role on the anisotropy properties at higher temperatures. It is known that K~ of the Co-Fe alloy in double hexagonal close packed structure is negative at room temperature and is - 7 . 0 × 106 [12], so it is believed the increases of Fe and Co atoms in the alloys will explain the reason why the K~ is nega- tive. According to a theoretical analysis on the magnetic phase of a uniaxial ferromagnetic crystal, it has been shown that the crystal has an easy cone when K 1 < 0, K 2 > 0 and IKd < 2K2 [13,14]. The half apex angle of the cone, 0', equals to arcsin~/- K 1 / 2 K 2 . By calculation, the 20' values of alloys 1, 2 and 3 are 62 °, 40 ° and 56 °, respec- tively; this means that there should be an easy cone. However, according to the results to be presented below, there exists no easy cone in our experiments of alloys 1, 2 and 3, so it reveals that aforesaid theory is not rigorous because of its insufficient consideration of other parameters to cause the existence of an easy cone. The curves for the sample other than alloys 1, 2 and 3 have the similar feature, we have plotted the curve which is designated as (2) for a typical alloy 5. It is seen that K 1 and K 2 values are all positive in the whole range of temperature, and there is a minimum in the K 2 curve corresponding to the peak of the K 1 curve. Below the temperature at maximum Ha, the thermal variation of K 2 is larger than that of K1.

Typical results of magnetization at various orientations for sample 6 at 162 K are shown in fig. 3. It is obvious that only a "hard cone" is observed at low fields. As the field increases, the "ha rd cone" tends to disappear and eventually

1.5

t o

~S , ¢C

.5

i

0 ' 6~ ' 1~0 ' 1~0 ' 2~0 ' 360 3 s 0 Degree

Fig. 3. Magnetization versus the angle between the direction of applied field and the easy axis of the aligned epoxy-bond powder. The magnetic fields are 3, 5, 10, 12 and 15 kDe for curve (1) through (6), respectively.

recovers to the easy axis at the orientation of 180 ° . Therefore, it is clear that there exists no easy cone in the whole range of field and temperature. The angle 0 between the hard direction and the easy axis is also tending to close to 90 ° with the field increased stepwise. This phenomenon is similar to the relationship between the angle 0 and the tem- perature as displayed in fig. 4. For the samples 1 to 5, the characteristic feature, as indicated in the

== t _

~ u ¢ - t

120

180 ' ~ . . . . . . . . .

= :: AHoy No: 6

150 A = ,, No:7

No:8

No:9

90 a t i i i , , i i i i

70 110 150 190 230 270 T IK)

Fig. 4. Dependence of the angle between the hard direction and the easy axis, 0, on temperature.

318 Y.H. Chang et a L / Anisotropy of samarium-Alnico alloys

curve (1) of fig. 3, is not observed, but they still have a O larger than 90 ° at a field lower than 10 kOe and we may conclude that they will still have a "hard cone" at a temperature below 77 K. The transition from "hard cone" to easy axis is depen- dent on both the measuring field and temperature, and no critical transition point is found.

Now, we can conclude that we find the thermo- dynamic first-order transition in our alloys as those found by Melville et al. [10] in Pr2(COl_xFex)17 alloys in which the transition between the easy axis and easy plane is thought to occur in the magnetizing process. In this work, the transition from "hard cone" to easy axis is observed in the magnetized sample, and it differs from that in the FOMP (first-order magnetization process) [15]. so far, by what mechanism which makes the "ha rd cone" existing in this work is not known yet.

Acknowledgements

The authors wish to express their gratitude to Dr. T.S. Chin for his valuable discussions, and Material Research Laboratory, ITRI, Taiwan, for the support of this project.

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