Effects of hydrostatic pressure on the magnetic properties of disordered monosilicide FexCo1 - xSi alloys

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Effects of hydrostatic pressure on the magnetic

properties of disordered monosilicide FexCo1 - xSi alloys

J. Beille, D. Bloch, V. Jaccarino, J.H. Wernick, G.K. Wertheim

To cite this version:

J. Beille, D. Bloch, V. Jaccarino, J.H. Wernick, G.K. Wertheim. Effects of hydrostatic pressure on

the magnetic properties of disordered monosilicide FexCo1 - xSi alloys. Journal de Physique, 1977, 38

(3), pp.339-343. �10.1051/jphys:01977003803033900�. �jpa-00208592�

EFFECTS OF HYDROSTATIC PRESSURE ON THE MAGNETIC PROPERTIES OF DISORDERED MONOSILICIDE FexCo1 - xSi ^ALLOYS

J.

BEILLE,

^{D. BLOCH}

Laboratoire de

Magnétisme, C.N.R.S., 166X,

38042 Grenoble

Cedex,

^France V. JACCARINO

Physics Dept., University

^of

California,

^Santa

Barbara,

^Calif.

93106,

^U.S.A.

and

J. H.

WERNICK,

G. K. WERTHEIM Bell

Laboratories, Murray ^Hill,

^N.

Jersey 07974,

^U.S.A.

(Reçu

^le

19 juillet

^{1976, révisé le 23 novembre 1976,}

accepté

^{le 24 novembre}

1976

Résumé. ²⁰¹⁴ Les

alliages

désordonnés FexCo1 - xSi ^sont ferromagnétiques ^{pour une} concentration

en fer x

supérieure

^{à 0,2. Nous avons} mesuré leur aimantation sous champ

magnétique

jusqu’à

40 kOe à 4,2 ^{K et sous} pression

hydrostatique

jusqu’à ⁸ kbar, de même que leur

susceptibilité

^{à la} pression ^{ordinaire sous} champ magnétique ^{de 150 kOe. Nous avons aussi} déterminé les températures

de Curie d’échantillons

ferromagnétiques

^sous

pression hydrostatique

jusqu’à ^{8 kbar à}

partir

^{de la}

variation thermique ^{de la}

susceptibilité

initiale. Nous avons déterminé une concentration critique ^x

de l’ordre de 0,95 pour l’apparition ^du ferromagnétisme ^{dans les} alliages ^{riches en FeSi.} D’après ^nos résultats, ^le ferromagnétisme ^{dans les}

alliages

^{riches en CoSi semble} apparaître ^de façon ^{assez ho-} mogène, alors que les

alliages

^{riches en FeSi} présentent ^au contraire des effets locaux.

Abstract. ²⁰¹⁴ Disordered FexCo1 - xSi

alloys

^{are known to be} ferromagnetic ^{in an} iron concentration range starting ^{at x =} 0.2. We have measured their magnetization ⁱⁿ magnetic fields up ^to 40 kOe at 4.2 K and under

hydrostatic

pressures up ^to 8 kbar, ^{as well as their zero} pressure

susceptibility

at 4.2 K in high magnetic fields of 150 kOe. We have also determined the Curie temperatures ^{of the}

ferromagnetic

samples ^under hydrostatic pressure up ^to 8 kbar from the temperature variation of the initial susceptibility. ^We determined a critical iron concentration of x ~ 0.95 for

ferromagnetism

in FeSi rich

alloys.

^{From some of our results, the} ferromagnetism in the CoSi rich alloys ^{seems to}

appear in ^a rather homogeneous way, whereas the FeSi rich alloys ^{in contrast} exhibit local effects.

Classification

Physics ^Abstracts

8.516 - 8.524

1. Introduction. ^- The

magnetic

^and

transport properties

of the 3d _group transition metal mono-

silicides have been the

subject

^{of numerous investi-}

gations.

^The

FexCo 1 - xSi alloys

^{form disordered}

solid solutions of cubic structure B20 at any ^concen- tration

[1].

^{CoSi is}

diamagnetic.

^{FeSi is}

paramagnetic.

but there exists a concentration _range where the

compounds Fe.,Col -.,Si

^are

ferromagnetic [2-5].

^From

studies of the Mossbauer effect

[6].

^a

magnetic

^moment

on Fe atoms greater ^{than 0.05} JlB for 0 x 0.2 and

0.1,uB

^for

Feo. 5 Coo. 5 Si

^{has not} been detected: The N.M.R.

experiments [4. 7]

^{have been}

interpreted using

^{a model in which}

only

^{Co atoms with an Fe}

atom among their next nearest

neighbour

^transition

atoms are

magnetic.

^It should be noticed that if the value of the

hyperfine

^{field at the atom} nucleus is taken as

proportional

^to the atomic moment, then there is a moment on the Fe atom which is of the same

order of

magnitude

^{as that on the Co. Recent nuclear}

orientation

experiments [8]

have been

interpreted

as indicative of a

hyperfine

^{field at the}

^57CO

^nucleus

of 310 ± ^{15 kOe for x =} 0.13.

leading

^{to a value of}

the

magnetic

^{moment on a Co atom which is}

large compared

^to the values deduced from

previous

N.M.R. measurements.

We decided to

perform magnetic

measurements under

high

pressure in order to

clarify

the mechanism of the appearance of

ferromagnetism

^{in these}

alloys.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01977003803033900

340

2.

Experimental.

^{- The}

experimental

^set up has been

already

^described

[9]. Hydrostatic

pressure

(up

^{to 8 kbar) is}

applied by

^{means of helium} gas ^to the

sample

located in a

copper-beryllium

pressure chamber. The temperature ^of the chamber can be

regulated

^{between 4.2 and 300 K.}

Figure

¹ represents

our

experimental

chamber with the heaters for

chang- ing

^the pressure ^at low temperature. ^The

magneti-

FIG. 1. ^- High pressure chamber. A : capillary heater; ^{B : chamber} heater; C : capillary; ^{D :} seals; ^{E :} support; ^{F :} sample; ^{G : ther-}

mometer ; H : strain _gauges.

zation is measured

by

^an extraction method. Reso- lution is 2 x

10-3

e.m.u. To determine the Curie temperature. ^{we use}

primary

^and

secondary

^coils

located outside the _pressure bomb. A two

phase/

vector lock-in

amplifier

model P.A.R. 129A

gives

the

in-phase

^or

quadrature

component ^{of the secon-}

dary signal,

from which the variation of the initial

susceptibility

^{of the}

sample

^can be deduced. The

FexCol -xSi alloys

^{have been}

prepared by

^{an induction}

melting [6].

3. Curie temperature,

spontaneous magnetization

and

high

^field

susceptibility.

^- We have determined the Curie temperature

Tc

^{of the}

ferromagnetic

alloys Feo.3Coo.7Si. Feo.SCoo.SSi. Feo.8CO0.2Si.

Feo.9Coo,lSi. measuring

the thermal variation of the initial

susceptibility (Fig.

^{2). Arrows} show the inflec- tion

points

^{of the curves, that we associate with the}

Curie temperatures. ^{In table I. we list the}

experimental

values of

T,.

^{of the} spontaneous

magnetization Mo

and

high

^field

susceptibility X

^{at 4.2 K} (150 kOe).

Mo

has been determined from H ⁼ 0

extrapolation of M 2

versus

H/M

^Arrott

plots (where

^H is the _magne- tic field and M the

magnetization)

between 20 and 40k0e.

The curves.

figure

^3,

give

the concentration

depen-

dence of the Curie temperature. ^the spontaneous

magnetization

^{and the}

high

^field

magnetic

suscep-

FIG. 3. ^- Concentration dependence of the Curie temperature Tc (a), ^the spontaneous magnetization Mo (b), the 10 k0e (O),

30 k0e (D), 150 ^k0e (V) high ^field susceptibility (c).

TABLE I

Concentration

dependence of

^{the Curie} temperature, spontaneous

magnetization

^and

susceptibility

tibility

^{at 10. 30 and 150} k0e. The critical concen-

tration for the appearance of

ferromagnetism

^{in the}

FeSi concentrated

alloys

^{is estimated at x ~ 0.95.}

where

Tc

^and

Mo

vanish. The concentration of Co in FeSi _necessary to

produce ferromagnetism

^{is four}

times smaller than that of Fe in CoSi. This fact seems to be in favour of a much narrower d-band for the Co than for the Fe. This conclusion is in agreement ^with the narrower d band observed in CoSi from XPS spectra ⁱⁿ

comparison

^to that of FeSi

(Fig.

^{4). From}

the

conductivity

measurements

[10].

the d-band of the Co atoms seems to be full in CoSi.

supported by

its

diamagnetic

behaviour.

Only

^a small number of holes created in the d-band of the Co atoms which has a

high density

^{of states would be} necessary ^to render the Co atoms

magnetic.

^{The Fe}

impurity

introduces a hole in

FexCol-xSi [10].

The hole of an

Fe

impurity

is devided between the Fe atom and the six next nearest

neighbour

^{Co atoms} which thus have

a very weak

magnetic

^{moment as indicated}

by

^the

N.M.R.

experiments [7]. Conversely.

^{when a Co}

impurity

^is introduced in FeSi, the holes associated with the six next nearest

neighbour

^Fe atoms create a

larger

^{moment on the Co}

impurity.

Besides. one can

predict

^that

ferromagnetism

^{of the}

CoSi rich

alloys

^{is due to the}

coupling

of extended

FIG. 4. ^- Electron density ^{of states as a} function of the binding

energy from XPS spectra ^for MnSi, FeSi and CoSi.

weakly ferromagnetic

groups of atoms

leading

^{to a}

rather

homogeneous

^{and weak}

ferromagnetism.

^For

the FeSi rich

alloys.

^{on the} contrary. ^a

large magnetic

moment is localized on the

impurity

^{and the ferro-}

magnetism

^{is thus}

expected

^{to be more inhomo-}

geneous. In fact.

magnetic

measurements at low temperature

[11]

^{favour a local}

description

^{for the}

appearance of

ferromagnetism

ⁱⁿ the FeSi rich

alloys.

with a weak induced moment on

polarized

^{Fe atoms.}

We observe.

figure

^{3. that}

7c

^varies

linearly

^with

Mo

which is in agreement ^{with an itinerant}

description

of the

ferromagnetism

^{in these}

alloys.

Furthermore.

the

high

^field

susceptibility

exhibits maxima near the two critical concentrations with

large

enhancement factors.

4. The

high

pressure behaviour. ^- The pressure derivative of the Curie temperature ^of the ferro-

magnetic alloys

^{with Fe} concentration x ⁼ 0.3. 0.5, 0.8 and 0.9 can be determined from the thermal variation of the initial

susceptibility

^{measured at}

constant pressures

(Fig.

^5).

FIG. 5. - Thermal variation of the initial susceptibility ^of Feo.5Coo.5Si ^{at various} pressures.

The values of the

compressibility

^{have been inter-}

polated

between those of FeSi and CoSi

[12].

^The

experimental

volume derivatives of the Curie tempe-

rature and the spontaneous

magnetization

^{as well}

as the

magnetization

^{under 30 k0e are} listed. table II.

The _pressure derivatives of the Curie temperature and of the

magnetization

under various fields are

plotted. figures

^{6 and 7. as a} function of the iron concentration.

TABLE 11

Experimental

^values

of d

^Ln

Tc/d

^Ln V, ^{d Ln}

Mo/d

^{Ln V, d Ln}

M301d

^Ln

V,

^where

M30

^{is the}

high

field magnetization

^{at 30 kOe}

(*

^from

[5])

342

FIG. 6. ^- Concentration dependence of the Curie temperature Tc (a), ^{and its} pressure variation IIT,, dTe/dp (b).

FIG. 7. ^- Concentration dependence ^{of the zero field} (A), ^the

10 kOe (0) and 30 k0e (D) magnetization (a) ^{and their} pressure derivatives 1/M dM/dp (b).

For the

alloys

^{with an Fe} concentration below 0.5.

(i)

^The pressure derivatives of

Mo

^and

T,

^exhibit

large

values which favour a

weakly ferromagnetic

model

[13].

(ii)

^In addition. their maxima are

nearly

^{of the}

same order of

magnitude

^{and occur at the concen-}

tration at which the

high

^field

susceptibility

^{is the}

largest.

This maximum is about 26 as

compared,

for instance. with 29 for the

Nio.452PtO.548 alloy

with a Curie

temperature

^{of 54 K}

[13].

(iii) Conversely

^{we note that the} pressure deri- vatives

d7c/d/?

^and

dMo/dp

^{for x = 0.3 and x = 0.5}

are

proportional

^to

T.

^and

Mo

which is characteristic of a local

magnetic

^behaviour

[14].

^The

proportiona- lity

^between

dTc/dp

^and

Tc

is clear from

figure

^8.

The

properties (i). (ii), (iii)

may

imply

that in this concentration _range the

magnetism

^{is due to the}

coupling

of clusters with a weak and wide

spread magnetization.

^{In such a model the}

magnetization

FIG. 8. ^- Variation ofd7c/d/? ^{as a} function of T,.

would exhibit weak

spatial

fluctuations and would

give

mixed characteristics of weak

homogeneous

and local

magnetic

^behaviour.

The pressure effects decrease with

increasing

^Fe

concentration above 0.5

indicating

^{a more localized}

behaviour,

ⁱⁿ line with the discussion of the last section. For Fe concentration above 0.8 we observe

an anomalous increase of the _pressure effects. For

x ⁼ 0.8 to x ⁼ 0.9, the _pressure derivative

dTc/dp

varies

roughly

^as

Tc-l (Fig.

^{7). This} may be due to a

strong variation of the

exchange integral

^between

the localized moments with _pressure as well as to

electron transfer

[15. 16].

Following

Svechkarev and Panfilov

[16].

^{we have}

analysed

the effects of the _pressure on the

shape

^of

FIG. 9. ^- Variation of d Ln M3o/d ^Ln V(0) ^{and d Ln} M3 Oldqd (broken line) ^{as a} function of the dinerential filling of the d band

(see text).

the d-band and on its

filling

characterized

by

^the y

and p

parameters

respectively

^{in the}

equation :

Figure

^8. the derivative of the

magnetization

^under

30 k0e with respect to qd., the mean d-band

filling,

d Ln

M301dqd

^is

plotted

^{as a function}

of qd-

^{In the}

same

figure,

the volume derivative of the

high

^field

magnetization

^{is also}

plotted.

^{For 0 x 0.5,}

d Ln

M3o/d

^{Ln V is}

quite

^{constant and is}

independent

of d Ln

M3o/dqa

^{in line with a} very low value of

fl

and no electron transfer. This is in agreement ^{with the}

assumption

^{that the 3d-band} of the CoSi rich

alloys

is full as

suggested by conductivity

measurements.

Conversely

^{d Ln}

M3 old

Ln V varies

linearly

^with

d Ln

M3o/dqd

and the electron transfer seems to be

effective for the FeSi rich

alloys

^where many holes exist in the 3d band.

5. Conclusion. ^- We have demonstrated that

ferromagnetism

occurs more

easily

^{when Co atoms}

are introduced in FeSi than in CoSi rich

FexCol-xSi alloys

^{when Fe atoms are} substituted for Co atoms.

This difference occurs due to a more localized beha- viour of the Co atoms in FeSi rich

alloys

than in CoSi rich

Fe.,Col -.,,Si alloys.

^The

high

pressure

magnetic properties

^{in the x = 0.05 to x = 0.5 Fe} concentration range ^are in agreement ^{with a weak} itinerant model but with some local characteristics. These become,

according

^{to our}

experiments,

^more

pronounced

for

higher

^Fe concentrations. For the most concen-

trated

alloys

electronic transfer _may

explain

^the

observed anomalous _pressure variations.

References

[1] ^See WATANABE, H., YAMAMOTO, ^{H. and} ITO, K., ^J. Phys. ^Soc.

Japan ¹⁸ (1963) ^995.

[2] SHINODA, D., Phys. ^{Stat. Sol. 11a} (1972) ^129.

[3] WERNICK, ^J. H., WERTHEIM, G. K. and SHERWOOD, ^R. C., Mater. Res. Bull. 7 (1972) ^1431.

[4] YASUOKA, H., SHERWOOD, ^R. C., WERNICK, ^J. H., ^Mater. Res.

Bull. 9 (1974) ^223.

[5] BLOCH, D., VOIRON, J., JACCARINO, ^{V. and} WERNICK, ^J. H., Phys. ^{Lett. 51A} (1975) ^362.

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[9] BEILLE, J., ALBERTS, ^H. L., BARTHOLIN, H., BLOCH, ^D. and VETTIER, C., ^{C. R.} Hebd. Séan. Acad. Sci. 275 (1972) ^719.

[10] ASANABE, S., SHINODA, ^{D. and} SASAKI, Y., Phys. ^{Rev. 134} (1964) ^{A 774.}

[11] To ^be published.

[12] ZINOVEVA, ^G. P., ANDREEVA, ^{L. P. and} GELD, ^P. V., Phys.

Stat. Sol. 23 (1974) ^711.

[13] ALBERTS, H. L., BEILLE, J., BLOCH, ^{D. and} WOHLFARTH, ^E. P., Phys. ^{Rev. B 9} (1974) ^2233.

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