THE MAGNETIC MOMENTS OF HEXAGONAL COBALT-BASE DILUTE ALLOYS WITH 3 d-TRANSITION METAL IMPURITIES

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THE MAGNETIC MOMENTS OF HEXAGONAL COBALT-BASE DILUTE ALLOYS WITH 3

d-TRANSITION METAL IMPURITIES

T. Wakiyama

To cite this version:

T. Wakiyama. THE MAGNETIC MOMENTS OF HEXAGONAL COBALT-BASE DILUTE AL-

LOYS WITH 3 d-TRANSITION METAL IMPURITIES. Journal de Physique Colloques, 1971, 32

(C1), pp.C1-340-C1-341. �10.1051/jphyscol:19711114�. �jpa-00213930�

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JOURNAL DE PHYSIQUE Colloque C 1, supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1

-

340

THE MAGNETIC MOMENTS OF HEXAGONAL COBALT-BASE DILUTE ALLOYS WITH 3 d-TRAN SITION METAL IMPURITIES

T. WAKIYAMA

Department of Electrical Engineering, Tohoku University, Sendai, Japan

Resom6. - On a mesure l'aimantation a saturation pour de nombreux monocristaux hexagonaux d'alliages dilues de metaux de transition 3d, comme Cr, Mn, Fe, Ni et Cu, dans le cobalt. Les mesures ont BtB faites avec un magneto- m6tre d'induction 300 OK et 77 OK dans des champs jusqu'a 21 kOe suivant l'axe de facile aimantation. On a estime les grandeurs de la variation du moment magnetique moyen par atome d'impuretk. Les rksultats sont discutes sur Ia base du modele de 1'6tat lie de Friedel et le rnodele de la bande rigide.

Abstract. - Saturation magnetization was measured for a number of single crystals of hcp Co-base dilute alloys containing 3d-transition metal impurities such as Cr, Mn, Fe, Ni and Cu. The measurements were made with an induction magnetometer at 300 OK and 77 OK in fields up to 21 kOe in the directions of easy magnetization. The magnitudes of the change in average magnetic moment per impurity atom were estimated. The results are discussed on the basis of the Friedel bound state model and the rigid band model.

I. Introduction.

-

The origin of magnetic behaviour in iron-group transition metals and alloys conti- nues to be a controversial problem which has been studied extensively. One powerful method often used is to study the effect of various impurities on the saturation magnetization. Some data have been reported for hcp Co-base alloys, but all of the experiments were only for polycrystalline specimens [I-41. Because of the large magnetocrystalline anisotropy in such materials [ 5 ] , it is very difficult to obtain meaningful values of the saturation magnetization for polycrystals.

In this investigation, the saturation magnetization was measured for a number of single crystals of hcp Co-base dilute alloys containing 3d-transition metal impurities such as Cr, Mn, Fe, Ni and Cu. The case of Fe impurities is of particular importance. No defini- tive data has ever been presented on the effect of Fe impurities on the magnetic moment of hcp Co-base alloys. The results are discussed on the basis of the Friedel bound state model [ 6 ] and the rigid band model.

11. Experimental.

-

The specimens used in this study were the same single crystals used in an earlier investigation of the uniaxial magnetocrystalline anisotropy [5]. Two additional single crystals of Co-Fe alloys were also prepared. Compositions were determined by chemical analysis. Spherical samples 3- 4 mm in diameter were used for the magnetization measurements.

Magnetizations were measured with an induction magnetometer at 3000K and 77OK in fields up to 21 kOe. Tn this method, the specimen is moved bet- ween the centers of two pick-up coils in series opposi- tion. The time integral of the induced voltage was evaluated with an integrating digital voltmeter. Cali- bration was done by performing the same experiment on a pure Co standard sample. The resolution of the instrument is 1.496 x e. m. u./pV.

111. Results and discussion.

-

The high field portions of the magnetization curves at 300 OK for several specimens are shown in figure 1. For these data, the saturation magnetization was determined from the measured points at 21 kOe. At 77OK, however, the

3

- A 1 0 4 a t % Fe H I C

I

_{1.04 at}_'%_{Fe H IIC}

I _I

Y ,

- >

^- ^{2.05ot %}^Cu ^{H IIC}

--

1187at % NI H l l C

--- 074 al 96 Mn H l l C

1 . :

^{4.69 at.}^{4b NI} ^H^{II C}

924 ' Î / I Î Î Î Î

I3 14 I5 16 17 18 19 20 21

Hex ( kOe1

FIG. 1. -High field portions of the magnetization curves at 300 ^O^Kfor several single crystals of hcp Co-base alloys. H

11

c : the external field parallel to the c axis, H I c : the external field

perpendicular to the c axis.

values were slightly larger, and some increase was observed even near 21 kOe. For this reason, the saturation magnetization at 77 OK was determined from a 1 1-+ ~0 extrapolation. The data in figure ~ 1 unders- core the high accuracy of the present method.

The saturation moments a t 300 OK and 77 OK are

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711114

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THE MAGNETIC MOMENTS OF HEXAGONAL COBALT-BASE DILUTE ALLOYS C 1

-

³⁴¹

plotted as a function of impurity content in figure 2a and 6, respectively. These data were used to estimate the values of d,ii/dc, where ji is the average atomic

0 Fe-Co

14

_,a

^.

^.I-coCu-Co ( a ) 300°K ^-c m , , , , , , , I / , , ,

0 I 2 3 4 5

IMPURITY ATOMS ( a t . % )

0 Fe-Co N i - C o A Cu-Co ( b ) 77'K

0 I 2 3 4 5

IMPURITY ATOMS ( at. X )

FIG. 2.

-

Saturation magnetic moment as a function of impu- rity content for various hcp Co-base alloys, at (a) 300°K

and (b) 77 O K .

moment in a given alloy and c is the concentration of impurity, which can be directly compared with theore- tical predictions [6]. These values are listed in Table I.

Admittedly, such estimations are dubious, especially

The observed change in magnetization per impurity atom in hcp Co-base dilute alloys with other transition elements.

Solute Cr Mn Fe Ni Cu

-

- -

djildc

(300 OK) - 3.0 - 2.7

-

1.7 - 1.2

-

1.1 (&atom) djildc

(77 OK)

-

3.5

-

1.5 - 2.5

-

1.0 -0.98 (p~/atom)

for Cr and Cu impurities, where only one sample of each was available. An irregular behaviour in Co-Fe occurs at low concentrations, and it might be well that similar stacking faulting problems, as discussed later, occur in the case of other impurities.

The most interesting case is Fe in Co. At low impurity concentrations (< 1 at.

%),

the magnetic moment shows a very peculiar composition dependence (see Fig. 2a and b). This anomalous behaviour may be related to possible change in hcp stacking sequence because of the impurities. In fact, the specimen with 1.04 at.

%

Fe is known to have a stacking sequence of the ABAC type, while 0.43 at.

%

Fe sample has the ABAB type [5]. The estimates from the initial slope of the curves in figure 2a and b, although admittedly very crude, give a cc decrease ⁾⁾in magnetization which results in a deviation from the Slater-Pauling curve. Such a decrease may be explained by the Friedel bound state model 161. The rigid band model predicts an increase in magnetization with Fe content. This model probably becomes applicable above 1 at.

%.

In this region the magnetic moment increases linearly with a slope of about 1 pB/Fe atom as expected from the theory [6], and the direction of easy magnetization was observed to be in the crystallographic c-plane, as found for 1.04 at.

%

Fe sample in the earlier study [51.

The anomalous behaviour observed in Co-Fe alloys may be due to changes in the electronic state with increasing Fe content. The abrupt change may be related to the theoretical fact that the Fermi level of hcp Co exists just at the top of the up spin band [7, 81, in contrast to fcc Ni [9].

In the cases of Cr and Mn in Co, the present values are about one half of the values of Crangle [I]. The non-linear behaviour of the Co-Mn alloys may also be due to a change in the stacking sequence with Mn impurities. The saturation magnetization of the fcc phase is slightly higher than that of the hcp phase in both Co-Cr and Co-Mn alloys [I]. Consequently djildc should be very sensitive to crystal structure.

As in the case of Cr in Ni [lo], the Friedel bound state model is probably applicable to Cr in Co.

For Ni in Co, the present results are in good agreement with the early data of Weiss et al. [2], and can be explained by the rigid band model (dpldc =

-

1 p,/Ni atom [6]). For Cu in Co, dji/dc is somewhat smaller than reported by Meyer et al. [3,4].

Both determinations, however, are definitively smaller in magnitudes than the predicted value,

-

2 ,uB/Cu atom, of the rigid band model [6].

References

[I] CRANGLE (J.), Phil. Mag., 1957, 2, 659. Proc. of the Inter. Conf. on Magnetism, Not [2] WEIS (P.) and FORRER (R.), Ann. Phys., 1929, 12, tingham, 1964, 756.

279. [6] FRIEDEL (J.), NUOVO Cimento Suppl., 1958, 7, 287.

[3] MEYER (A. J. P.) and TAGLANG (P.), C. R. Acad. [7] WONG (K. C.), WOHLFARTH (E. P.) and HUM (D. M.), Sci., Paris, 1950, 231, 956. Phys. Letters, 1969, 29A, 542.

[8] WAKOH (S.) and YAMASHITA (J.), J. Phys. Soc. Japan, [4] BESNUS (M. J.) and MEYER (A. J. P.), Phys. Lettevs, 1970, 28, 1151.

1969, 28 A, 516. [9] CONNOLLY (J. W. D.), Phys. Rev., 1967, 159, 415.

[5] CHIKAZUMI (S.), WAKIYAMA (T.) and YOSIDA (K.), [lo] KANAMORI (J.), J. Appl. Phys., 1965, 36, 929.