AMORPHOUS COBALT-PHOSPHORUS ALLOYS : ATOMIC ARRANGEMENTS AND MAGNETIC PROPERTIES

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AMORPHOUS COBALT-PHOSPHORUS ALLOYS : ATOMIC ARRANGEMENTS AND MAGNETIC

PROPERTIES

G. Cargill, R. Cochrane

To cite this version:

G. Cargill, R. Cochrane. AMORPHOUS COBALT-PHOSPHORUS ALLOYS : ATOMIC ARRANGE-

MENTS AND MAGNETIC PROPERTIES. Journal de Physique Colloques, 1974, 35 (C4), pp.C4-

269-C4-278. �10.1051/jphyscol:1974451�. �jpa-00215642�

JOURNAL DE PHYSIQUE

Colloque C4, suppKment au no 5 , Tome 35, Mai 1974, page C4-269

AMORPHOUS COBALT-PHO SPHORUS ALLOYS :

ATOMIC ARRANGEMENTS AND MAGNETIC PROPERTIES (*)

G . S. CARGILL 111

Department of Engineering and Applied Science Yale University, New Haven, Connecticut 06520, U. S. A.

and

R. W. C O C H R A N E

Eaton Electronics Research Laboratory McGill University, Montreal 101, QuCbec, Canada

RLsum6. -

Des mesures de diffraction des rayons X, de densite physique, et de magnktisation ont etC effectuees sur une serie d'alliages Co-P amorphes tlectrodeposes de teneur en phosphore comprise entre 19 et 23,6 %. Bien que la fonction de distribution radiale des alliages a 22 % en P presente les caracteristiques du modele de Bernal (empilement dense au hasard de spheres incom- pressible~ de memes dimensions), une etude plus detaillee indique qu'un empilement au hasard binaire avec des spheres plus petites representant les atomes P, serait un modkle structural plus approprie. Pour cet alliage, la distance entre plus proches voisins (dominee par les paires Co-Co) est 2,58

&

0,5 a et le premier nombre de coordination moyen est 13,O

&

0,5. Des mesures entre 5 et 700 K a faible et intense champ de magnktisation indiquent que les alliages sont ferromagnk- tiques, avec une temperature de magnetisation qui suit, basse tempkrature, une loi en

T3/2,

ceci presque jusqu'a

Tc/4.

Ces mesures montrent aussi que les points de Curie

Tc

sont entre 550 et 750 K et que le moment magnetique par atome de Co a 0 K varie de 1,15 a 1,36 magnetons de Bohr.

Tc

et

nn

diminuent tous deux en fonction de l'augmentation de la teneur en phosphore.

Abstract. -

X-ray diffraction, physical density, and magnetization measurements have been performed on a series of electrodeposited amorphous Co-P alloys between 19.0 and 23.6 at. %

P.

Although the radial distribution function of the 22.0 at. % P alloy exhibits characteristic features of a Bernal dense random packing of equal size hard spheres, more detailed comparisons indicate that binary random packing, with smaller spheres representing P atoms, would be a more appro- priate structural model. For this alloy the nearest neighbour distance (dominated by Co-Co pairs) is 2.58

k

0 . 5 8 and the average first coordination number is 13.0

i

0.5. Low and high field magnetization measurements from

5

to 700 K indicate that the alloys are ferromagnetic with low temperature demagnetization following a

T3/z

temperature dependence almost to

Tc/4,

that the Curie, temperatures

Tc

are between 550 and 750 K, and that

n B ,

the magnetic moment per Co atom at 0 K, varies from 1.15 to 1.36 Bohr magnetons. Both

Tc

and

^{n B}

decrease with increasing P content.

1. Introduction.

-

A h m i c arrangements in amor- phous materials lack long range structural periodi- cities which characterize crystalline solids. Conside- rable experimental and theoretical attention is being devoted t o characterizing atomic arrangements in amorphous solids and t o evaluating and interpreting effects of their atomic scale disorder o n electronic and magnetic properties.

This paper describes structural and magnetic pro- perties of amorphous Co-P alloys prepared by electro- deposition. Preliminary results for these alloys have been reported earlier [I], [2]. In the present paper new

(*)

Supported

by

National Science Foundation.

results on the role of phosphorus in determining the structure of the amorphous alloys are discussed.

The alloy compositions given in our earlier paper [I], 121 were incorrect. Corrected alloy compositions are given here, a n d effects of phosphorus content on magnetic properties are reexamined. New low tempe- rature magnetic measurements, which indicate that spin wave spectra of such amorphous alloys differ significantly from those of simple, crystalline solids, are also presented here.

2. Materials.

-

This paper deals with the same amorphous Co-P alloys discussed in reference [I], which were prepared by electrodeposition. Samples

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974451

C4-270 G. S. CARGILL 111 AND R. W. COCHRANE

used for magnetic and density measurements were initially ring-shaped with inner and outer diameters 1.6 cm and 3.3 cm, 0.025 cm thick, and the sample used for quantitative X-ray scattering measurements was rectangular, 2.0 cm x 2.5 cm, 0.01 cm thick.

Chemical compositions given in reference [Il were determined by wet chemical analysis of large pieces broken from the ring-shaped samples and included material from both the inner and outer circumferential edges. X-ray fluorescence and magnetization measu- rements indicate that these edge regions are signi- ficantly richer in cobalt than the remainder of the foil.

The new magnetization measurements reported here, as well as the density and X-ray scattering measure- ments always avoided edge regions of the foils. We believe that alloy compositions reported previously [I]

are not representative of the central parts of the foils on which most of our property measurements have been made. Corrected compositions given in Table I were obtained from electron microprobe and X-ray fluorescence measurements which avoided the edge regions.

Densities of the amorphous foils were obtained by weighting 20 mg pieces of each sample in air and in toluene. These densities are given in Table I, together with atomic volumes calculated from measured densi- ties and compositions. Densities of the amorphous Co-P alloys are compared with those of amorphous Ni-P alloys

[3]

and with equilibrium crystalline phase mixtures 141-[6] in figure 1. Densities of the Co-P alloys are very similar to those of the Ni-P alloys and are within 2 % of the density of the equilibrium Co-Co2P crystalline phase mixture.

3.

Structure. -

3.1 EXPERIMENTAL.

-

All samples used for magnetic measurements were examined by X-ray scattering using a General Electric diffractome- ter with diffracted beam crystal monochromator and MoK, radiation. They produced nearly identical scattering patterns, similar to those reported for electrodeposited Ni-P alloys and for most

⁽⁽

splat- cooled

⁾⁾

amorphous metallic alloys

[3],

[7].

Quantitative step-scan scattering measurements were performed on a 22 at. % P sample. Experimental details and methods of data analysis were similar to those described in reference [3]. The interference

Sample

0 10 2 0

3 0

AT. % P

FIG. 1. - Measured densities of amorphous Co-P

(m)

and Ni-P (A) [3] alloys and calculated densities for the equilibrium crystalline phase mixtures Co-CozP and Ni-NisP, using pco = 8.84 glcm3, pco,p = 7.54 g/cm3 [5], P N i = 8.91 glcm3,

and p ~ i ~ p = 7.82 g/cm3 [6].

function Z(k) for this alloy and for a Ni-P alloy of 24 at. % P [3] are shown in figure 2. Radial distribution functions

obtained from these data are shown in figure 3. The second maximum in Z(k) for these alloys has a shoulder on its high-k side and the second maximum in RDF(v) is split. These features are common to many amor- phous metallic alloys produced by various methods [3], [7].

The distribution function p(r) obtained from expe- rimental X-ray scattering data for the Co-P alloy is a weighted sum of three partial distribution func- tions pC,,(rj, pcOp(r), and ppdr), with pij(?>

=

the number of j-type atoms per unit volume at distance r from an i-type atom, averaged over all i-type atoms in the scattering sample [8]. Weighting factors depend on the atomic fractions of the two components cco TABLE I

Compositions, densities, and atomic volumes ,for amovphous Co-P alloys Previously

reported [l]

composition (at. % P)

Average

Corrected atomic

composition Density volume

(at. % PI (g/cm3) (A3/atom)

- -

-

AMORPHOUS COBALT-PHOSPHORUS ALLOYS : ATOMIC ARRANGEMENTS C4-271

K = 4 r S I N ( e ) / x (i-')

FIG. 2.

-

Measured interference function I(k) for amorphous C O ~ ~ P ~ ~ and Ni76Pz4, and Z(k) calculated from Finney's [lo]

DRPHS distribution function with DHS = 2.46 A to five sphere diameters extended to 25 using experimental distribution function of Ni76P24, and with damping factor exp(- 0.01 kz).

FIG. 3.

-

Radial distribution functions obtained from inter- ference functions of figure 2 with km, = 17 8-1 for C078P22 and Ni77Pz3, and distribution function for DRPHS with

DHS = 2.46 A [lo].

and cp and on their X-ray atomic scattering factors fco(k) and .fp(k).

With

where it is assumed that the quotients of atomic scattering factors are independent of k. Taking

h&/<

f > 2 2

Zi Zj/< Z >' yields for the weighting factors 0.96, 1.06, and 0.08.

If the Co and P atoms had identical surroundings in the amorphous structure, then p(r) would be the true distribution function for the structure, the total number of atoms per unit volume at distance r from another atom, averaged over all atoms in the diffract- ing sample. In any case, Fourier transform termination contributes to peak broadening in RDF(r).

3 . 2 COMPARISONS

WITH STRUCTURAL

MODELS.

-

3 . 2 . 1 Single-size-hard-sphere models.

-

Models for amorphous metallic alloys based on micropolycrys- talline structures have been unable to reproduce interference functions like those of figure 2 131.

Cargill [9] pointed out that the RDF's of many amorphous alloys, particularly electrodeposited Ni-P alloys, are very similar to those obtained by Finney [lo]

and others [ I l l for dense random (Bernal [12], [13]) packing of hard spheres (DRPHS) of a single size.

The histogram in figure 3 is the RDF for such a hard sphere packing [lo]. The only adjustable parameter in the comparison with experimental RDF's is the value taken for the hard sphere diameter DHs.

In figure 2 is an interference function calculated from

the hard sphere distribution function with

DIIs

=

2.46 A up to five diameters, but extended

beyond five diameters to 25 A with the experimentally

determined distribution function [3] for amorphous

Ni,,P2,. This extension of the hard sphere data

reduced termination effects which would have been

introduced by obtaining the interference function by

Fourier transformation of only the hard sphere

distribution function to five sphere diameters, the

upper limit of Finney's [lo] hard sphere data. A

damping factor of exp(- 0.01 k2) was also included

to simulate thermal and additional structural disorder

present in the amorphous alloys. The characteristic

features of the amorphous metallic alloy interference

functions, a sharp first maximum and a weaker,

broader second maximum with a shoulder on its

high-k side, are reproduced by the model I(k), although

the shoulder is less pronounced than in most expe-

rimental data. These features are also present in the

interference function calculated from just the hard

sphere R D F to five diameters and without any

subsequent damping. Sadoc et al. [14] also found

C4-272 G. S . CARGILL I11 A ND R. W. COCHRANE

these characteristic features in interference functions calculated from much smaller, computer generated dense random packings of single sized spheres.

If atomic arrangements in the Co-P alloys were truly like those of spheres in such packings, then P atoms would have the same average surroundings as Co atoms, and interpretation of the experimental RDF's would be simplified [8]. The 12-coordinated Goldschmidt radii for Co and P, 1.25 A and 1.28 A,

are very similar, as are those for Ni and P [15]. This motivated initial comparisons of experimental Ni-P RDF's with those for dense random packings of single sized hard spheres

[9].

Several experimental observations indicate that identical surroundings for metal (Co) and metalloid (P) atoms in these alloys are unlikely. Three conflicts arise when detailed comparisons are made between experimental RDF's for Ni-P or Co-P alloys and Finney's [lo] DRP of single sized hard spheres (91

:

(1) Although the hard sphere diameter DHs may be chosen to produce excellent agreement between peak positions in experimental and DRPHS distribution functions in the region beyond the minimum following the nearest neighbour maximum, positions of the nearest neighbour maximum in the two distribution functions differ by several percent when DHs is chosen in this way. The best least-square-fit to the Ni7,P2, data [3] in the form 4 nr[p(r) - p o l , excluding the region of the nearest neighbour maximum, yields DHs

=

2.46 A. However, the first maximum in the experimental R D F occurs at R,

⁼

2.56 A, larger by 4 % than the position of the first maximum in the DRPHS distribution function. A 5 % difference between the best fit DHs and R, occurs for the Co7,Pzz alloy. A consequence of DHs being less than R, is evident when the model and experimental interference functions are compared in figure 2. The position of the first maximum in

Z(k)

is determined mainly by the large-r behavior of the distribution functions and agrees well in the model and experimental inter- ference functions. However, the large-k oscillations in I(k) arise mainly from the nearest neighbour part of the distribution functions

;

agreement between the model and experimental interference functions becomes worse as k increases.

(2) The volume per sphere has been carefully determined for physically constructed DRPHS struc- tures with single sized spheres [lo] and is given by

For DHs

=

2.46 A, VH,

^- =

12.2 A3/atom

;

for DHs

=

R,

=

2.56 A,

^V,, =

13.8 A3/atom. Both values are significantly larger than either the experi- mental values for amorphous Ni-P alloys,

or the experimental val~les for amorphous Co-P alloys given in Table I.

(3) The first maximum in RDF(r) for the Co7,Pzz alloy has a slight shoulder on its small-r ride. Similar shoulders are found for the three most phosphorus rich Ni-P alloys studied previously [3]. Dixmier and Duwez [16] have attributed such peak asymmetries in (Pd,oNi,o),oo~xPx, x

⁼

15

-

27.5, to phosphorus- to-metal distances between 2.2 A and 2.4 A. The peak asymmetries for Co-P and Ni-P alloys may be due to phosphorus-to-metal distances of approximately 2.2 A, as shown in figure 4. However, the indicated contribution is too small to account for the anticipated number of P-to-Co neighbours. The occurrence of P-to-Co and P-to-Ni distances of this value, significantly less than the sum of their Goldschmidt radii [15], is consistent with the phosphorus-to-metal distances found in the crystalline phases Co2P [5]

and Ni3P [6], as discussed below, but conflicts with assuming identical surroundings for the metal and metalloid atoms in these alloys.

FIG. 4. - First maximum of radial distribution function for

Co78P22 (heavy solid line), Gaussian curve fit to top half of RDF maximum (dashed line), and peak at 2.2 b obtained by sub- tracting Gaussian curve from RDF (light solid line). This small maximum may be due to P-to-Co near neighbour distances.

3.2.2 Binary hard sphere models.

^-

Other proposed structural models for amorphous alloys of these types involve random packings of spheres of two sizes [14], [17], [18]. The larger spheres represent metal atoms

;

the smaller spheres represent metalloid atoms. Each P atom in the crystalline phases C0,P [5], Ni2P [19], and Ni3P [6] has nine metal atom near neighbours but no phosphorus near neighbours.

The smallest P-Co distance in C0,P is 2.14 A and the average P-Co near neighbour distance is 2.33 A [S].

For Ni3P the corresponding distances are 2.21 A and

AMORPHOUS COBALT-PHOSPHORUS ALLOYS : ATOMIC ARRANGEMENTS C4-273

Sizes of idealized Bernal holes and the number of each type expectedper 100 dense I-andom packed hard spheres Minimum central position

to vertex distance in units of sphere diameter

- -

(a) Archimedian antiprism (b) Trigonal prism

(c) Tetragonal dodecahedron (d) Tetrahedron

(e) Octahedra

(often present as half octahedra)

0.82 0.76 0.62 0.61 0.71 (for full octahedron)

Occurrences per 100 spheres

(")

- 1.6 12.8 12.4 292.0

4.0 (counted as full octahedra)

(")

Calculated from Bernal's values [12] for the frequency of occurrence of each type of hole.

2.28 A [6]. Data from these crystalline phases indicate that the Goldschmidt radii of Ni, Co, and P may not yield appropriate estimates of phosphorus-to-metal distances in the amorphous alloys and suggest models involving different sizes and surroundings for metal and metalloid atoms.

Polk [17] proposed that Cargill's suggestion [9]

of a simple DRPHS model for these amorphous alloys be modified by allowing the metal atoms to have an arrangement similar to the DRPHS, but with most of the metalloid atoms occupying cr the larger holes inherent in the random packing

H.

Using Bernal's observations on the numbers and types of holes which occur in simple DRPHS structures [12], he showed that filling all of the holes of types (a)-(c) in Table I1 with metalloid atoms would yield an alloy of 79 at. % metal content, a composition about which most metal-metalloid amorphous alloys have been produced. He further showed that such structures would have densities as great (or atomic volumes -C/' as small) as those observed for amorphous Ni-P and other similar alloys.

This filling of voids by metalloid atoms was jus- tified by Polk's calculation that ((the central point of these holes is at least 0.84 times the length of a side from the nearest vertices

))

1171. More accurate values of these ratios for the idealized holes are shown in Table 11, together with the number of each type of hole expected in a simple DRPHS.

None of the holes are as large as originally believed by Polk [17], [18]. Taking the Goldschmidt diameter of Ni, 2.48 A, for the diameter of the larger hard spheres making up the simple DRPHS skeleton, i. e.

the minimum edge length, and the smallest P-Ni distance in crystalline Ni,P, 2.21 A, for the minimum metal-to-metalloid distance yields a minimum central- point-to-vertex-distance

:

length-of-side-between-the- nearest-vertices of 0.89, significantly greater than the correct ratio for holes of type (c), which occur with a large number fraction. Therefore, viewing amorphous alloys of this type as metalloid atoms strictly filling

holes in a simple DRPHS skeleton of metal atoms cannot explain either the small volume per atom of the amorphous alloys or the special stability of these alloys around 80 at. % metal content. The R , > D,, conflict also remains unexplained by this model.

Polk [18] later generalized his view of the DRPHS void-filling model to allow the metal atoms to occupy random packing structures somewhat less dense than those of Bernal [12], [13] and Finney [lo], which should provide more larger holes to accommodate the metalloid atoms. This point of view is similar to that of Sadoc, Dixmier, and Guinier [14], who propose for the structure of these alloys a binary dense random packing model in which no small spheres, which represent metalloid atoms, are allowed to be near neighbours. They used computer techniques to generate structures of 300 and 500 spheres and calculated interference functions and RDF's for various sphere diameter ratios and number fractions of small spheres.

3 . 3 SOME

EXPERIMENTALLY OBSERVABLE CONSE- QUENCES OF BINARY DENSE RANDOM PACKING. -

Binary DRPHS with smaller spheres representing metalloid atoms and with no metalloid-metalloid nearest neighbours is probably a more realistic struc- tural model for the metal-metalloid amorphous alloys than simple DRP of equal size hard spheres with metal and metalloid atoms occupying randomly selected sites. Small binary DRPHS models have been generated by Sadoc,

et

al. [14], and similar models might be produced by following Polk's suggestion [I81 of placing small spheres in the larger holes present in random packings of larger spheres.

Although published work on neither of these

approaches has been sufficiently detailed for quanti-

tative comparison with the following experimental

observations, the observations can be interpreted

easily in the general framework of binary DRPHS

structural models. They involve subtle structural

effects of size differences between metal and metalloid

C4-274 G. S. CARGILL 111 AND R. W. COCHRANE

atoms in amorphous alloys like those in the Ni-P and Co-P systems.

The nearest neighbour maximum in RDF's of amorphous Ni, ,, -,P, alloys [3] occurs at distances R, between 2.54 A and 2.57 A for x between 19 and 26

;

R, is approximately 3 % greater than the Ni Goldschmidt diameter 2.48 A. However,

R ,

decreases in these alloys with decreasing phosphorus content and extrapolates reasonably to 2.48 A for x

-+ 0,

as shown in figure 5. Likewise, for Co,,P,,, R , is 3 % greater than the Co Golaschmidt diameter 2.50 A.

0 10 2 0 30

AT. % P

FIG. 5.

-

(a) Dependence on phosphorus content of first peak position R1, from RDF's of Ni-P alloys [3]. (b) Dependence on phosphorus content of D ~ s l R l for Ni-P alloys from experimental RDF's

(m),

and composition dependence predicted using one-

dimensional binary hard sphere model (dashed line).

The position of the first maximum in X-ray RDF's of alloys like Ni-P and CO-P with

Zmetal %

2

Zmetauoid

and with approximately 20 at. % metalloid is deter- mined mainly by metal-metal nearest neighbour pairs, and the metal-metal separations ought to be determined to first order by the metal atom size.

However, the increased values of R , can be explained, within the binary DRPHS model framework, by the large spheres surrounding a smaller sphere not packing as close to one another as they would if they were contained in a simple DRPHS. Alternately viewed, introduction of small spheres requires the large spheres to form a looser random packing, to

provide enough large holes to accommodate the small spheres.

Previously mentioned differences between the values of DHs, obtained from least-squares-fitting of simple DRPHS distribution functions with those of amor- phous Ni-P and Co-P alloys but excluding the nearest neighbour region, and the values of R , for these alloys can also be interpreted in terms of binary DRPHS. The RDF's of Ni-P and Co-P alloys are dominated by metal-metal pairs, but except for the nearest neighbour region of the RDF, the distances between these pairs of metal atoms are determined by the sizes of the atoms, both metal and metalloid, in the region between the pair of metal atoms being considered and by the topological arrangement of these intermediary atoms.

It is easy to estimate effects of metalloid content on the large-r structure in RDF(r) if changes in the arrangement of intermediate atoms with changes in metalloid content are ignored and only the sizes of the intermediate atoms are considered. A simple one- dimensional example is shown in figure 6. Replacing the intermediate metal atoms by metalloid atoms reduces the distance between the endpoint metal atoms. Similar effects are to be expected for the more complicated three-dimensional configurations which give rise to the large-r structure in DRPHS models and in experimental RDF's. One dimensional models involving four and five atoms have been used to pre- dict shifts in large-r structure with changes in metalloid concentration. Results from such one-dimensional models with metal-metal and with metal-metalloid endpoint atoms were averaged using the coefficients of metal-metal and metal-metalloid partial distribution

FIG. 6.

-

One-dimensional configurations of four atoms used in predicting shifts in large-r structure with changes in metalloid

concentration.

AMORPHOUS COBALT-PHOSPHORUS ALLOYS : ATOMIC ARRANGEMENTS C4-275

functions of eq. (3) for weighting factors. Metalloid- metalloid nearest neighbour configurations were disal- lowed in all models. Input for the calculation consisted just of the experimentally observed value of R1 for the metal-to-metal distance, the 2.2 A value assumed for the metal-to-metalloid distance, the at. % of metalloid in the alloy, and the weighting factors given in eq. (3). Always having at least one metal endpoint member makes the predicted DHs value larger than the simple composition weighted average of metal and metalloid atom diameters.

Appropriate averaging of the four-atom configurations shown in figure 6 yields

A similar expression applies to five-atom configu- rations, and the average of the four- and five-atom results predicts DExs/Rl

=

0.96 for Co,,P2,, for which the observed ratio is 0.95. This type of calcu- lation also correctly predicts the magnitude and composition dependence of the DHS/Rl ratio in the amorphous Ni-P alloys, as shown in figure 6. There- fore the behavior of DHS/Rl observed for the Ni-P and Co-P alloys can be explained solely in terms of the sizes of intermediary atoms, without considering changes in the configuration of these intermediary atoms.

The small atomic volumes of the amorphous Co-P and Ni-P alloys can also be understood in terms of binary DRPHS by approximating the volume per sphere in such a binary packings with the volume per sphere in a simple DRPHS whose single sphere diameter is just the composition weighted average of the metal and metalloid diameters. Taking D,,

=

2.50 A and

with < D >

⁼

cco Dco + cp Dp for C O , ~ ~ - ~ P , and x

=

19.0

-

23.6 yields < D > between 2.39 A

and 2.36 A and VHs between 11.2 A3/atom and 10.8 A3/atom. Observed VcO-, are between 11.2 A3/atom and 11.0 A3/atom.

With DNi

=

2.49 A and

for Ni,,,-xPx and x

⁼

18.6 - 26.2, this approach

yields D between 2.38 A and 2.34 A and VHs between 11.1 A3/atom and 10.5 A3/atom. Observed

^VNi-p

are between 11.1 A3/atom and 11.0 W3/atom. Although this method correctly predicts the magnitude of 7

for the Co-P and Ni-P alloys using reasonable values of Dc,, DNi, and Dp, it over estimates the dependence of 7 on metalloid content.

Although these experimental observations can be interpreted in terms of binary DRPHS models, acceptance of particular computer generated struc- tures as appropriate models for amorphous Co-P and Ni-P alloys requires more detailed comparisons between experimental and model distribution functions and densities, with particular regard to the composi- tion dependent structural features described above.

It is also important to investigate effects of replacing hard sphere interactions by more realistic pairwise potentials [20].

4. Magnetic properties.

-

4 . 1 EXPERIMENTAL.

^-

The magnetic properties of the amorphous Co-P alloys and their temperature and composition depen- dence have been investigated using a number of different techniques. Initial measurements [I] employed ring-shaped foils to reduce demagnetizing effects.

These samples were mounted in phenolic holders and wound with primary and pickup oils. These measurements employed a conventional A. C. induc- tion hysteresis loop tracer at temperatures between 77 and 525 K with sinusoidal 60 Hz drive fields having amplitudes up to 400 Oe.

Additional magnetization measurements [2] were made on small pieces from the ring-shaped foils in another A. C. induction loop tracer with 60 Hz, 300 Oe drive fields at temperatures between 77 and 700 K, extending our previously reported measure- ments [I] beyond the crystallization temperatures of the Co-P alloys. Detailed magnetization data have also been obtained at temperatures as low as 5 K using a vibrating sample magnetometer operating in fields as high as 55 kOe provided by a NbZr superconducting solenoid.

4 . 2 HYSTERESIS

^{AND ANNEALING}

BEHAVIOUR.

-

Typical M-H curves for the amorphous Co-P alloys, obtained using the ring-shaped foils, are shown in figure 7

;

also shown is the effect of annealing on the field HK required to saturate the samples. Heating the samples to 525 K irreversibly reduced H K , but the thermal cycling produced no observable changes in saturation magnetizations or in X-ray diffraction patterns.

4 . 3 TEMPERATURE

^{AND FIELD} DEPENDENCE OF

MAGNETIZATION.

-

LOW temperature magnetization

measurements were extrapolated to T

=

0 K to

obtain M(0) values for the alloys. Measurement

of the magnetization at high temperatures was limited

by cristallization of the alloys between 550 K and

600 K. All of the alloys crystallized while still ferro-

C4-276 G. S. CARGILL I11 AND R. W. COCHRANE

FIG. 8. -

M(T)/M(O)

versus TITc for four amorphous Co-P alloys showing corresponding states behavior which contrasts

with a similar plot for crystalline nickel.

ANNEAL A N N E A L

FIG. 7. - Upper : Effect of temperature on saturating field HI< for CoglP19. Lower : Typical M-H curves for amorphous

Co-P alloys, shown for CoslPls.

magnetic

;

however, from high temperature data for sample B (22.0 at. % P) we were able to estimate T,

=

620

⁾

30 K. This value was used to calculate Tc for the remaining alloys be requiring that all of the curves of M(T)/M(O) versus TITc coincide at M(T)/M(O)

⁼

0.5. The data for all the alloys are shown in figure 8 and indicate that these alloys have similar corresponding states curves.

The region 0.8 < TIT, < 1.0 will be very sen- sitive to composition fluctuations which may give rise to a range of Curie temperatures in a single sample. M(T) for more homogeneous samples may fall more sharply in this region with increasing T than indicated by our present results. Also shown in figure 8 are data for crystalline nickel which were

obtained in our apparatus and which agree with published data. Nickel provides an apt comparison with the Co-P alloys

;

it is a cubic ferromagnet saturating at low fields with Tc

=

631 K [21], which falls into the middle of the Tc values for the Co-P alloys. These values of T, are listed in Table 111 and the composition dependence of Tc is shown in figure 9.

Tc increases with decreasing phosphorus content and extrapolates reasonably to T, of crystalline Co for 0 at. % P.

The vibrating sample magnetometer was used to extend measurements to 5 K and to investigate effects of applied fields as large as 55 kOe. The magne- tization curve for sample D (19.0 at. % P) at 5 K is shown in figure 10, where the insert is a blow-up of the low field behaviour. All samples exhibited identical characteristics

:

a rapid linear rise to satu- ration with very small hysteresis or remanence and negligible paraprocess susceptibility beyond the knee of the magnetization curve. The low field data agree with the loops recorded at higher temperatures with the A. C. loop tracer.

Curie temperatures T,, low temperature magnetizations M(0) as emu/gm and as Bohr magnetons per Co atom n,, and spin wave dispersion coeflcients for amorphous Co-P alloys and crystalline Ni

Composition Tc M(O) D

Sample (at. % PI (K) (emulgm)

^{E B}

(meV A2)

-

-

- - - -

A 23.6 + ^{1.0 550}

^$.

^{30 94} + 4 1.15 + .07 99 + ¹⁰

B 22.0 620 97 1.18 121

C 20.3 720 114 1.36 118

D 19.0 750 108 1.28 129

Ni - 63 1 57

^-

370

AMORPHOUS COBALT-PHOSPHORUS ALLOYS : ATOMIC ARRANGEMENTS C4-277

-

0 ^{\ 0}

Y

\* t* ^{1 9.0 AT. % P}

2

-.020

\

'.

^{' 0}

2

\Z

'.

0

0 10 20 3 0

^-.040

AT. % P 1

4 -

80- +M (emu /gm)-+

8a -

40

- -

O

I

100 200

/I- -

⁴⁰⁰

^50.0

FIG. 9. - Composition dependence of estimated Curie tempe- ratures of amorphous Co-P alloys. The dashed line reaches To

FIG. 10. - High field magnetization curve for CoslPlg at 5 K.

Insert present details of the low field rise to saturation.

In order to describe the low energy spin excitations in these ferromagnets we have measured the demagne- tization as a function of temperature from liquid He temperature up to

300

K in constant external fields.

Figure 11 presents data for samples B and D plotted as

AM/M(O) = [ M ( T ) - M(O)]/M(O)

versus

T3I2

at a field of

0.5

kOe. The

T3I2

dependence is indicative of spin wave excitations obeying the usual dispersion relation

E(q) = Dq2

(6)

of crystalline cobalt at 0 at. % P and illustrates the approxima- ^{1 I I 1} tely linear dependence of Tc on phosphorus content for the four 0 400 800 1200 1690

alloys studied.

T3/ 2

for wave vector

q.

The values of

D

calculated from the slopes of T~~~ plots are given in Table I11 along with the extrapolated values of

M(0).

The

T3I2

temperature dependence extends almost to

150

K

^(FZ

T J 4 ) which is well beyond the range of

FIG. 11. - AM/M(O) versus T3/2 plot for alloys B (22.0 at. % P) and D (10.9 at. % P) at. 0.5 kOe.

simple

T3I2

behaviour for crystalline nickel [22].

Also, the magnitude of the spin wave magnetization

AM/M(O)

is approximately three times that of nickel, with a corresponding decrease in the dispersion coefficient D. Thus the low temperature differences Letween crystalline nickel and amorphous Co-P shown in figure 8 arise from the relative ease with which the latter can excite spin waves. This feature correlates with available model calculations 1231, which characterize the excitations in amorphous ferromagnets by an increase in their low energy density of states over that of the comparable crystalline phase.

4 . 4 COMPOSITION

DEPENDENCE OF MAGNETIZATION.

-

The vibrating sample magnetometer measure- ments yield higher values of

M(0)

than those reported previously for the ring-shaped samples [I], perhaps because of the nonuniformity of composition already mentioned. Values of

M(0)

and of n,, the moment per Co atom in Bohr magnetons, are given in Table 111.

The values of n, are larger than those reported pre- viously [ l ] because of the higher values of

M(0)

reported here and because of the revised values for sample compositions given in Table I. More precise measurements of the composition dependence of

M(0)

will require samples which are more uniform in composition than those employed here.

The composition dependence of the moment per Co

atom is shown in figure 12, together with experimental

data of Simpson and Brambley [24] for a chemically

deposited Co-P alloy in both amorphous and crys-

C4-278 G. S. CARGILL I11 AND R. W. COCHRANE

1.8

^I I

electrodeposited Co-P alloy results agree more closely

with the calculation based on phosphorus atoms

I - contributing only two or three electrons rather

0

than five as previously reported [I], and may

'4 indicate that the phosphorus 3 s2 electrons do not

o

participate in the d-band filling.

0

W

lx 5.

Conclusions. -

Structural data presented in

a this paper indicate that the binary dense random

00

packing of hard spheres, with smaller spherds repre-

s

- senting phosphorus atoms, is a more appropriate

structural model for amorphous Co-P and Ni-P

- alloys than models in which metal and phosphorus

atoms have the same average surroundings. Several composition dependent structural features are des-

- cribed which support this conclusion and which

provide tests for proposed structural models.

- Detailed magnetic studies presented here indicate

that the amorphous Co-P alloys are strongly ferro-

0.0

^I I

magnetic. Both T, and M(0) decrease with increasing

0 10 20 30 phosphorus content so that all the alloys obey a AT.

%

P magnetic corresponding states curve. The low tem-

FIG. 12. - Magnetic moment per cobalt atom nB in Bohr magnetons for amorphous Co-P alloys from low temperature vibrating sample magnetometer data extrapolated to 0 K (O), values from Simpson and Brambley [24] for amorphous (A) and single phase crystalline

(A)

alloys, and rigid band model calculations with phosphorus atoms contributing Z = 2, 3,

and 5 electrons to the cobalt 3-d band.

talline forms. Also shown are rigid band model calculations in which each phosphorus atom contri- butes two, three, or five electrons to filling the 3-d band of cobalt [24]. With our revised composition values and M(0) measurements, the amorphous

perature demagnetization obeys a T3I2 temperature dependence, characteristic of low energy spin waves with quadratic dispersion, almost to Tc/4 and the magnitude of spin wave magnetization is several times that of common crystalline ferromagnets. The compo- sition dependence of magnetization in these alloys is consistent with each phosphorus atom contributing two or three electrons to filling the 3-d band of cobalt.

Similar results have been obtained independently by D. Pan [25]. The data presented here distinguish amorphous Co-P alloys from crystalline Ni in several ways which are characteristic of the lack of long range atomic order.

References

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CARGILL, G. S. I11 and COCHRANE, R. W., Bull. Am. Phys.

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CARGILL, G. S. 111, J. Appl. Phys. 41 (1970) 12.

HANSEN, M., Constitution of Binary Alloys, Second Edition (McGraw-Hill, New York), 1958, p. 488 and 1027.

RUNDQVIST, S., Acta Chem. Scand. 14 (1960) 1961.

RUNDQVIST, S., HASSLER, E., LUNDVIK, L., Acta Chem.

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[8] KAPLOW, R., STRONG, S. L., AVERBACH, B. L., in Local [211 MORRISH, A. H., The Physical Principles of Magnetism Atomic Arrangements Studied by X-Ray Difraction, (John Wiley and Sons, Inc., New York) 1965.

J. B. Cohen and J. E. Hilliard, ed. (Gordon and [221 ARGYLE, B. E., CHARAP, S. H., PUGH, E. W., Phys. Rev. 132 Breach, New York), 1966, p. 159. (1963) 2051.

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[12] BERNAL, J. D., PYOC. R. SOC. A 280 (1964) 299. [25] PAN, D. and TURNBULL, D., Submitted to J. Appl. Phys.