High field magnetization studies in amorphous Co-Er-B alloys

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Highfield magnetization behavior in random anisotropy amorphous CoEr alloys

H. Lassri, L. Driouch, and R. Krishnan

Citation: Journal of Applied Physics 75, 6309 (1994); doi: 10.1063/1.355382 View online: http://dx.doi.org/10.1063/1.355382

View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/75/10?ver=pdfcov Published by the AIP Publishing

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High-field magnetization behavior in random anisotropy amorphous Co-Er alloys

H. Lassri, L. Driouch, and R. Krishnan

Laboratoire de Mag&tisttte; C.N. R.S. 9219.~ Meudon, France

Amorphous Car-,rl$ ribbons with x= 55 and 65 were prepared by the melt-spinning technique.

Magnetization measurements were carried out in the temperature range 4-100 K under high magnetic fields up to 20 T. Even at 20 T the saturation is not fully attained. Assuming that Co has no moment in the alloy with .r=65 the Er moment is found to be 7.0~~ which indicates a speromagnetic spin structure. The Co moment in the alloy with x = 5.5 is then found to be 0.1 pFLB, which is ncgligihly small. By analyzing the approach to saturation using Chudnovsky’s theory we have extracted some fundamental parameters.

I. INTRODUCTlON

Amorphous alloys based on rare-earth metals are interesting materials to study the various fundamental parameters, such as ferromagnetic, antiferromagnetic interactions, random anisotropy, etc. ‘*’ We have reported on the amorphous Co-Er-B alloyF3 where we have shown that Er has a conical spin structure resultin, ‘7 from strong random anisotropy and that under sufficiently high magnetic fields the antiferromagnetic interaction between Er-Co breaks down. It has also been shown that for rare-earth metal concentrations higher than about 60 at $6, the transition metal moment would vanish.” Therefore we wanted to study the binary Co-& system rich in Er, in order to get more information on the magnetic state of Er. The magnetization of this system was stud- ied earlier’ hut under an external field of only 1.4 T which, as it will be shown here, is far too small for saturation. We report here our magnetization studies on amorphous Co4sEr,, and COATED,, alloys under applied fields up to 20 T. We have also analyzed the approach to saturation using Chudnovsky’s theory and obtained some ,magnetic parameters of interest.

II. EXPERIMENTAL DETAILS

Amorphous Co, ._,Er-x alloys with s=55 and 65 were prepared by the usual melt spinning technique under inert atmosphere. Linear velocity of the copper wheel was in the range 30-40 m/s. The diameter of the orifice of the quartz crucible \vas 0.5 mm.

The amorphous state was verified by x-ray diffraction.

The exact composition was determined by electron probe microanalysis. The magnetization was measured in the temperature range 4-100 K. and under applied fields up to 20 T.

III. RESULTS AND DISCUSSION

The ribbons were about 1 mm wide and 30 pm thick and were all amorphous as shown by the characteristic broad x-ray diffraction peak.

The field dependence of the magnetization at different temperatures for the two alloys was similar and Fig. 1 shows the result for the alloy witlh Er=65 at %. It can be seen that even at 20 T, the saturation is not yet complete. So the saturation magnetic moment Al,, was calculated at H,, using N-” dependence, which will be justified in the next section.

The magnetization of the alloy with s = 65 can be considered

to arise only from the Er atoms as we pointed out earlier.

Under this assumption, from the alloy moment of 4.56~~

we find the Er moment pn,=7.0&atom, which is smaller than 9.0~~) theoretically expected for the ground state. This indicates that the Er spin structure is not collinear. This spin structure arises from the random anisotropy which is larger than the ferromagnetic JEr.nr interaction. It is interesting to note that this situation is different from what we had ob- served earlier for Co-Er-B alloys,” where the Co atoms bear a strong moment and therefore a fairly strong antiferromagnetic interaction Jc.+ tends to maintain the Er spins in one hemisphere.” It is to be noted that our value of pu, is much higher than that reported by Xingbo and MiyazakL4 as is expected, because their measurements were carried out under fairly low fields. Now taking our experimental value of pnr=7.0pLB for the alloy with x=55, and from the alloy moment of 3.8~~ , the Co moment is calculated to be 0.1 pD which is indeed negligibly small.

The Curie temperature (?“c) of the alloys was obtained from the Arrott’s plots and the result for x=65 is shown in Fig. 2. The Curie temperature T, was found to be 30 and 40 K for x = 65 and 55, respectively. These relatively low val- ues indicate the weak ferromagnetic Er-Er interactions.

t $ f Q 0

I 8 : 1:,*+

1 1

- 1611 - & ‘2 . 1

)h - /1 : 0”

or! . ‘; . - a- * - R cl

._ .” . ,,* . ? .:

1 - .,, * LI i

5 F

.<: . A I. n

c .,7

E,20- *.’ - ij . . u”i-

.i .

A . ” P

. . . I E ~ _{.z 10K}^{. 4.2K}

80 . 20K

n 30K . 40K II 60 K

40 - 1 ” * Y 60K

* .% a

.-I ”

L I f

0 5 IO I5 20

ri ( T )

FIG. 1. The field dependence of the magnetization at various temperatures for the alloy CO,,E~,~.

J. Appl. Phys. 75 (IO), 15 May 1994 0021-8979/94/75( 10)/6309/3/$6.00 0 1994 American Institute of Physics 6309 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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*4.2K - 3?-

$0 10

“nn $ .30

g 28-

,b

c%’ : -* -40

P .:.” ’ 0

- ?4- .+ *me

52 . . .: ” . 0 .80

.

20 - l O m 0 1

l 0 .

.

. 0 0 .

16- ^.

. ”

0

’ m

ii?- o - 0

. . r

a- ”

u .

n -

4- c/

. s

. -

0 0.05 0, IO 0.15 0.20

tl!M(T.gi’emu)

O tt 1 :o 15 I

²⁰

I

H(T!

FIG. 2. The Arrott plot for the alloy CoJSErbS. The inset shows M ’ vs T. FIG. 3. The H dependence of the quantity [(fift,-M)IM,,]-‘r’ for the sample Co,Er,,~ at 4 K.

Chudnovsky 2,’ has proposed a theory to explain the approach to saturation in terms of various fundame.ntal parameters, which has been applied by several authors to analyze their results.7’8 Two cases are to be distinguished; first when the applied field is smaller than the exchange field (Happ<Hexch) and the second when (H,PP%~,xct,). In our prese.nt case considering the low Curie temperatures of the Er-Co samples, the second case is more appropriate. In this high-field regime one can write

(M’j-iGff/M()=f%fIM(,= 1/15[H,/(H+H,,j]“, 0)

where M O is the magnetization extrapolated value to H, and H, is the random local anisotropy field and is related to the local anisotropy energy K, by the relation

X,/M = H, . i2j

In some cases H,, has been neglected since it is small com- pared to the external field and H, has been calcu1ated.s How- ever, with some algebraic manipulations, as we describe below, it is also possible to consider H,, and obtain not only H, but also H,, .

Equation (1) can be rewritten as

iS11P/M,)-“,5=B(H+H,,i, (3)

where l3 = J 15 X (H,.)- I. So by plotting ( SMIM,,)-u.s as a function of H, one can obtain H, from the slope B and the exchange field H,, from the intercept. Figure 3 show such a plot for Er=SS. The data point align well in the high-field regime H.&==H, . But one observes a deviation from the linear dependence in the intermediate regime where the random anisotropy and the exchange field influence the magnetization behavior. This field region is known as the crossover field H,, and will be discussed later. The value of H, was obtained by linear regression. Table I shows the various magnetic parameters calculated from the Fig. 3. It must be cau- tione.d? however, that the value of H,, should be taken with some caution and considering the relatively low value ~7i.s n vis H app it could vary by a factor of 2 or 3. Nevertheless, this gives an order of magnitude of the parameter. For x = 65, we

find K,= 1.8 X 10” erg crne3 (which has been calculated from H,) and this agrees well with the value obtained by Hadjipanayis et a[.” for an amorphous Er-Au-B alloy, where Er are the only magnetic atoms.

From the crossover field H, mentioned above, it is possible to calculate the important structural parameter R,, which is the distance over which the local anisotropy axes are correlated. From Chudnovsky’s theory’ we can write

HLx,=2a4/Mi~(~Ra)2, (4)

where A is the exchange constant. However, generally this crossover field is difficult to observe experimentally in crys- talline and amorphous materials where T, and hence the exchange field is high.’ Fortunately the present alloys offer a possibility to observe H,. As we had mentioned when dis- cussing the M dependence on H”, one observes a deviation from the linear behavior (Fig. 3), which precisely corresponds the crossover field where the regime changes to the low field one. From Fig. 3 H,, is found to be close to 11 T.

In Eq. (3) all parameters except A are known. It is possible to calculate by combining the models proposed by Hasegawa,“’

Heiman et ~1.” as described below. From the mean-field model the exchange constant A can be writtenre (considering only Er since Co has no moment) as

A = nErmErJErvErigEr- 1 W2/aEr-Erl is) where x is the Er concentration, nEr-nr indicates the number of maximum permissible pairs of Er atoms per volume ex- tending to the first nearest neighbors, c~n~.n~ the interatomic distance between Er atoms, x the Er concentration, and the

TABLE I. Some magnetic properties of Col-,Er, alloys at 4 K.

hfll Tc A Hex ffr

iemu g-‘11 (K) (IO-” erg c m -‘j iTj 6

s (.Tj (LO” erg cm-7

55 179 40 1.9 0.4 19 1.5

65 197 30 1.7 0.4 20 1.X

6310 J. Appl. Phys., Vol. 75, No. 10, 15 M a y 1994 Lassri, Driouch, and Krkhnan

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rest of the symbols have the usual meaning. Several author8 have assumed that 1zn~-n~=2 and a.,+-ur=3.S A.

One can express the Curie temperature, after Heiman et ld’1 as

3k”I;~=2XZlir-ErJ,-Ed,(g,-l!i(JE,-t l), ia where ZEr-rr denotes the number of Er neighbors around Er atom which is generally taken as 12 in such amorphous alloys and the other symbols have their usual meaning.

g,=1.2, JE,=7.S.

Combining Eqs. f.5) and (6) we can eliminate the term JEraEr and get

A =xJt&T~~l[4(J~r+ 1 )a&~‘+]. i9

Knowing all the parameters in the above Eq. (7) A can be calculated. For example, for s= 65, A is found to be 1.,7X 10-s erg cm. Using this value of A in Eq. (4), we calculate i?, = 1.5 A. This rather small value indicates that there is practically no correlation in the direction of the easy axis from one rare-earth site to another. This is in agreement with the fact that this alloy is a. speromagnet. One finds in transition metal-rich alloys, generally, that R, is about 5 A, which corresponds to a few atomic sites.‘”

J. Appl. Phys., Vol. 75, No. 10, 15 May 1994

This work was performed under the European collabora- tion programme CEAM 3, which is gratefully acknowledged.

‘K. Moorjani and J. M. D. Coey, &gnetic Glnsscs (North Holland, Am- sterdam, 19841, Chap. VI.

‘D. J. Sellmyer and S. Nafis, J. Appl. Phys. 57, 3584 (198.5).

a R. Krishnan and H. Lassri, Solid State Ormmun. 69, X03 (1989).

‘Yang Singbo and T. Miyazaki, J. Magn. Magn, Mater. 73, 39 (1988).

‘E. M. Chudnovsky and R. A. Serota, Phys. Rev. B 26, 2697 (19%).

‘E. M. Chudnovsky, W. M. Saslow, and R. A. Serota, Phys. Rev. B 33, 251 (IYW.

7H. Lassri and R. Krishnan, J. Magn. Magn. Mater. 104-107, 157 (1992).

‘G. Hadjipanayis, D. J. Sellmyer, and B. Brandt, Phys. Rev. B 23, 3349 (1981).

‘E. M. Chudnovsky, J. Magn. Magn. Mater. 79, 127 (1989).

K’ R. Hasegawa, J. Appl. Phys. 45, 3109 (1974).

‘tN. Heiman, K. Lee, R. Potter, and S. Kirkpatrick, J. Appl. Phys. 47, 2634 i1976).

*zY. Mimura, N. Imamura, T. Kobayashi, A. Okada, and Y. Kushiro, J. Appi.

Phys. 49, 1208 (1978).

‘“See, for example, E. M. Chudnovsky, J. Appl. Phys. 64, 5770 (198s).

Lassri, Driouch, and Krishnan 6311 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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