Mössbauer study of the magnetic properties of Fe2-xM xP1-yAsy type compounds, M = Cr, Mn, Fe, Co, Ni

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Mössbauer study of the magnetic properties of Fe2-xM xP1-yAsy type compounds, M = Cr, Mn, Fe, Co, Ni

M. Wautelet, A. Gérard, F. Grandjean

To cite this version:

M. Wautelet, A. Gérard, F. Grandjean. Mössbauer study of the magnetic properties of Fe2-xM xP1-yAsy type compounds, M = Cr, Mn, Fe, Co, Ni. Journal de Physique, 1977, 38 (1), pp.29-37.

�10.1051/jphys:0197700380102900�. �jpa-00208557�

MÔSSBAUER STUDY OF THE MAGNETIC PROPERTIES OF Fe2-xMxP1-yAsy ^TYPE COMPOUNDS, ^M

⁼

Cr, Mn, Fe, Co,

^Ni

M. WAUTELET

(*),

^A.

^GÉRARD

^{and F. GRANDJEAN}

(**) Département

^de

Physique Atomique

^et Moléculaire

Institut de

Physique,

Université de

Liège,

^{B 4000}

Sart-Tilman, Belgium (Reçu

^le

5 janvier 1976,

^{révisé le}

18 juin ^1976, accepté

^{le 27}

septembre 1976)

Résumé. ²⁰¹⁴ Nous avons étudié _par spectrométrie Môssbauer du 57Fe la série des phosphures ^et arséniophosphures des métaux M de la série de transition avec la formule Fe2 -

xMxP1-yAsy.

Nous avons résolu le problème de l’attribution des couplages quadripolaires ^aux deux sites de

Fe2P ^en

appliquant

^la technique ^de polarisation. ^{Tous les} composés de la série Cr2P1

_yAsy

⁽¹

% 57Fe)

ainsi que

Ni2P

^{et FeNiP sont} paramagnétiques à 77 K. Les points de Néel des composés

Mn2P1-yAsy

et FeMnP ont été déterminés en accord avec les mesures précédentes. Le composé

Co2P

^se distingue

par ^sa sensibilité à la, stoechiométrie, ^en effet, 5% ^{de P en} plus ^font décroître brutalement le point ^de

Curie.

Abstract. ²⁰¹⁴ The series of phosphides ^and

arsenophosphides

of the first row transition metals with formula

Fe2-xMxP1-yAsy

^{is studied} by ^Mössbauer spectrometry.

The question of the attribution of the quadrupolar couplings ^to both sites of Fe2P

is

^solved, by applying ^the technique

of polarization.

^{All the} compounds of the series Cr2P1-

yAsy

⁽¹

% 57Fe),

^the phosphides Ni2P (1 %

57Fe)

^{and FeNiP are found} paramagnetic ^{down to} 77 K. The Néel points

of the compounds

Mn2P1-yAsy

^{and FeMnP are} determined in rather good agreement with the pre- vious measurements. The compound CO2P distinguishes ^itself by ^its sensitivity ^{to the} stoichiometry,

indeed an addition of 5 % ^of phosphorus ^does

abruptly

decrease the Curie

point.

Classification

Physics ^Abstracts

8.516

.1. Introduction. ^- The

phosphides

^{of the first-}

row of transition metals : M with formula

M2P

and their solid solutions

crystallize

^{in two main}

structures :

hexagonal [1]

and orthorhombic

[2]

represented by Fe2P(Pb2m)

^and

Co2P(Pnma).

^These

two structures have been described in great ^detail

by

other authors

(see

for instance ref.

[3]).

^{Let us}

only

^{note that}

they

^are characterized

by

^{two metallic}

sites. The first one

(I)

^{has a} tetrahedral environment of

phosphorus

^with

symmetry

^{mm for}

Fe2P

^{and m}

for

C02P.

The second one

(II)

^{has an octahedral}

environment of

phosphorus

^with symmetry ^{mm for}

Fe2P

^{and m for}

Co2P.

Numerous papers

[3

^to

29]

deal with the

magnetic properties

of this series from the

point

^{of view of the}

measurements

[3

^to

24, 28, 29]

^{as well as} of the inter-

pretation [25

^to

27].

Fe2P

^is

ferromagnetic (F)

^{below 265 K}

[23],

^but

the Curie temperature ^is very sensitive to

the

stoi-

chiometry [27, 28].

Mn2P

^{is known to be}

antiferromagnetic (AF)

^below

103 K

[9],

^{whereas the Neel}

point

^{of FeMnP has not}

been

accurately

determined _up to now.

The

magnetic

behaviour of

C02P

has been studied

by Roger et

^al.

[3, 11, 27]

^who found that it was not

magnetic

^{at 100 K.}

However,

these authors

systema- tically

^{add a few} percent ^of

phosphorus

^to

C02P,

ⁱⁿ

order to eliminate all free cobalt

remaining

^{in the}

preparation [27].

N’2P

^and

Cr2P

^{are not}

magnetic

^{down to 77 K}

[3, 11, 27].

Another

important

^{fact is} the transition from the F state to the AF state of the solid solution

Fe2P-Mn2P

at 3

% of Mn [3, 11, 27].

Thus,

^it is very

important

^to

study

^{the local} proper-

ties,

^{i.e. the} electronic

configurations

^{at metallic}

sites. For this

study,

^the

hyperfine techniques such

as M6ssbauer spectroscopy ^{are the most suitable.}

In order to

modify

the local parameters, ^{it is} very

interesting

^to

change

the interatomic

distances,

(*) Present address : Universite de 1’Etat a Mons, Faculte des Sciences, ^Avenue Maistriau, 23-B 7000 Mons, Belgium.

(**) Charge de Recherches du F.N.R.S.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197700380102900

30

without

changing

the number of electrons _per atom.

One of the means is to

apply

^a

high

pressure ^to the

compound [21] ]

but another _one, much

easier,

^{is to} substitute arsenic for

phosphorus.

^{This increases}

the

crystalline parameters

^{but does not}

modify

^the

. structure

[20, 27, 29].

In the system

Fe2p1 -YAsy ^{(y 0.5),}

^{the Curie}

temperature ^increases

with y [20, 27],

nevertheless the

magnetic

moment saturates at 1.5

p,/Fe.

In

Co1.95P 1 - yAsy,

the Curie temperature

plotted

as a function

of y

presents ^a maximum of 110

K,

near y ⁼ 0.6

[29].

No

ferromagnetism

has been observed in the _sys- tem

Mn2P 1- yAsy ^[29].

Fe2P

^{has a} metallic electrical

resistivity

^{of about}

4 ^x

10-’

Q.cm at room temperature

[5, 12]

^and

Mn2P,

^{of 3 x 10-3 K2.cm}

[17].

Up

to now, neither the mechanism

responsible

for the transition F -+ AF in the system

Fe2 -.,Mn.,P

with x ⁼

0.03,

^{nor the reasons for the}

sensitivity

of Curie

temperatures

and saturation

magnetizations

are known.

Several _papers have dealt with the Mossbauer

study

^of

phosphides [3, 7, 8, 10, 11, 14, 18, 19, 22, 23, 27]

but neither the system

Fe2p,-YAsy

^{nor the}

compounds Cr2P, Mn2P, C02P

^or

N’2P doped

^with

a few percent ^of 57 Fe have been studied

by

^this

method. This is the aim of this _paper.

2.

Experimental

^{details. -} 2.1 PREPARATION OF THE SAMPLES. ^- The

compound Fe2P

^{has been} pre-

pared by

the method of Maronneau

[30]

^{as described}

elsewhere

[23].

’

The other

binary phosphides

^{and the} ternary

ones have been

synthetized by

direct combination of the

elements,

^at

temperatures

^{near 1 100}

K,

^for

days,

under vacuum.

Furthermore,

^some

samples

^of

C02P

^and

N’2p

have been obtained from K

&

^K Laboratories Inc.

(Plainview,

^N.

Y.) (Ref.

8904 and 22201

respectively).

Samples

with iron concentration less than 10 _per- cent have been

prepared starting

from the main

component

^and

adding 17 Fe

^{and P in} stoichiometric

quantities,

and then heated at 1 150 K for a few

days.

All the

samples

^have been checked

by X-ray

diffraction in order to

verify

^that

only

^one

crystallo- graphic phase

^is present.

2.2 M6SSBAUER EQUIPMENT. ^- The M6ssbauer

spectrometer

^is of the linear

velocity

^variation type, with a

5’ Co

source diffused in _copper. The

given

isomer shifts are relative to metallic iron. The measure- ments have been

performed

^{between 77 and 300}

K, using

^{a variable} temperature ^Air

Liquide

cryostat and between 300 and 670

K, using

^{a vacuum furnace}

described elsewhere

[32].

^The temperature is measured with a chromel-alumel

thermocouple

^{with an} accuracy of + ^{0.1 K.}

2. 3 ANALYSIS OF M6SSBAUER SPECTRA. ^- The M6ssbauer spectra ^are

analysed by

^{means of a least}

squares fit program. The lines are Lorentzian. In the case of a

quadrupole doublet,

^the

height

^{and width}

of the lines are

equal.

^{In the}

magnetic region,

^the spectra ^are

analysed by

^the

Karyagin’s

^method

[36].

The resolution of the

adjustment

^is

given by MISFIT,

as described

by Ruby [33].

3.

Experimental

results. - 3.1

Fe2P.

^{- The}

M6ssbauer spectrum, ^{at room} temperature, ^{is well} known and is

represented

ⁱⁿ

figure

^{1. It is an} asymme- tric

doublet,

^the

intensity

^{of the low}

speed

line is three times that of the

high speed

^{line. It can be}

analysed by

^the presence of two

quadrupole

doublets of

equal

intensities

corresponding

^{to both}

crystallographic

sites

[7, 8, 10, 14, 22, 23, 27].

FIG. 1. ^- Mossbauer spectrum ^of Fe2P ^{at room} temperature.

Inset : Three _ways for coupling four lines of equal intensities.

In

reality,

since both sites are

equally populated,

this spectrum ^is

composed by

four lines of

equal

intensities and

widths,

and there exists three _ways of

pairing

those lines as indicated in the inset of

figure

^1.

The

previous analysis

gave

disjointed

^{doublets as}

in

(b)

and furthermore

Wdppling et

^al.

^{[14, 22]}

^deduce

that both

quadrupole

doublets have not the same

intensity,

^because of different

Debye-Waller

^factors

for both sites.

In agreement ^with

Wiippling et

^al.

[14, 22]

^and

Roger [27],

^we

get

^a

fitting

^{of our}

spectrum

^{with an}

intensity

ratio of 1.05 and with a MISFIT

[33]

^amount-

ing

^{to 0.053 3} ± ^{0.009 3}

%. However,

^a better fit is obtained when four lines of

equal

^{width and}

intensity

are

introduced,

then the MISFIT fall to : 0.023 8 ± 0.007 ⁰

% .

Now,

^the

problem

^{is to} distribute the four lines into two doublets.

For that purpose, ^{we are}

making

^{use of the}

possibi-

lities of

polarization

in M6ssbauer

spectroscopy.

^{It is} well known

[34]

^{that in a}

single crystal,

the intensities of

a

quadrupolar

^{doublet are} function of the

angle 0

between the incident _{y beam} and the

principal

^axis

of the electric field

gradient (EFG)

^{tensor. We recall}

in

figure

^2a and b the relative intensities for two

particular

^{cases 0=0 and 0 =} 900. It is to be noted that in case

(a),

the transition

1/2

^-+

3/2

^{is the most}

intense,

^whilst the situation is reversed for case b.

FIG. 2. ^- Relative intensities of a quadrupole doublet for a

1/2 ^-+ 3/2 transition in a single crystal (a) ^{for 0 =} 0°, (b) ^{for 0 = 90°.}

Examples of relative intensities of two superimposed ^{doublets as in} Fe2P (c) ^{for 0 =} 00, (d) ^{for 0 = 90°.}

In our case, ^we have two doublets which _may be

arranged

^{in different} ways,

examples

^are

given

ⁱⁿ

figure

2c and d for 0 ⁼ 0 and 0 ⁼ 900. Our

sample

of

Fe2P

^{is a}

powder, but,

^{in an external}

magnetic field,

the c-axis of

Fe2P

orients itself

parallel

^{to this}

field. The

powder

is thus mixed with a

glue

^{in an}

external

magnetic

^{field of 1 700}

Ole,

^{and dried in this} field. The

powder

^is

thereby preferentially

^ordered.

Let us note that in the

magnetic

spectrum ^{of this}

sample

^for

0=0,

^{the lines Om = 0 have an}

intensity

close to zero.

The Mossbauer spectra ^of

Fe2p

^{with c-axis} perpen- dicular and

parallel

^{to the} y rays ^are

represented

ⁱⁿ

figure

^{3a and b.}

The differences between those spectra ^and

spectrum

of

figure

^{1 are} indicated in

figure

^{3c to e.}

From

figure 3c, showing

^the difference between the spectra ^{for 0 = 0 and for 0 =}

900,

^it appears that the intensities of both

high speed

^lines vary

by

^the

same manner, thus

they

^cannot

belong

^{to the same}

doublet and

consequently,

^the attribution

of Wappling

et al.

[14, 22] corresponding

^to

disjointed

^doublets

as indicated in b of

figure 1,

^{has to} be eliminated.

In order to

make

a choice between both

remaining

combinations a and c of

figure 1,

^a solid solution

Fe2-,,,M.,P

^{has to} be studied.

FIG. 3. ^- Mossbauer spectra ^of Fe2P ^{at room} temperature : (a)

c-axis parallel ^to y ; (b) ^c-axis perpendicular ^to y ; (c) ^Difference

between spectra (a) ^and (b) ; (d) Difference between spectra (a)

and figure 1 ; (e) Difference between spectra (b) ^and figure ^1.

The

compound Fe1.sNio.sP

^{has the same}

crystal

structure as

Fe2P [27]

^with

only

^a small deviation of the lattice parameters, ^{it is not}

magnetically

^ordered

at room temperature and the iron atoms are

prefe- rentially

^{situated in site II}

[19].

This solid solution is therefore well

adapted

^{to the}

problem.

The Mossbauer spectrum ^of

Fel.5Nio.5p

is shown in

figure

^{4 at room} temperature. ^{The solid} line is the result of a least _squares fit with two doublets whose parameters ^are

given

^{in table I. The difference}

between the isomer shifts is 0.22 ± ^0.01

mm/s

^and

MISFIT amounts to 0.026 ± 0.010

%.

^{Because of the}

different

populations

^{of iron atoms on both}

sites,

the attribution is

straightforward

^and

corresponds

to included doublets

(Fig. 1;

^case

a).

32

TABLE I

Mössbauer characteristics

of phosphides

^and

arsenophosphides

Since

Fe2P

^and

Fe1.sNio.sP

^{have the same}

crystal-

line structure, the differences between isomer shifts for both

sites,

^{in both}

compounds,

^are

expected

^to

be similar.

Only

^{case c of}

figure

^{1 with}

crossing

^doublets

leads to an isomer shifts difference of 0.275

mm/s

and thus the

hyperfine

parameters ^for

Fe2P

^{are those}

given

^{in table}

I ;

the attribution of the doublets to iron sites

being immediate,

if the local environments of the metallic atoms are considered.

The evolution of the M6ssbauer spectra ^with tempe-

rature in the

magnetic

range has been

analysed

elsewhere

[23]

^{and the} parameters ^{found are summa-} rized in table I. The Curie

temperature

^of

Fe2P

determined

by

M6ssbauer and

magnetization

^measure-

ments is 265 + ^{2 K.}

Contrary

^{to the} observations

of Wäppling et

^al.

[22],

the variations in the isomer shifts ^are continuous

on

passing through

^{the Curie}

point (Fig. 5).

^{This is}

due to a different

analysis

^{of the}

paramagnetic

spec- trum.

3 . 2

Fe2P1 1 - yAsy ⁽⁰

^y

^0.5).

^- The M6ssbauer

spectra for y

^{= 0.1 and}

0.5,

^{at room}

temperature,

are similar to the

magnetic

spectrum ^of

Fe2P

^and

can be

analysed by

^the

superposition

^{of two Zeeman}

FIG. 4. ^- Mossbauer spectrum ^of Fe1.sNio.sP ^{at room} tempera-

ture. The solid line is the result of a least squares fit. The positions

of the components of both doublets are indicated by vertical lines.

FIG. 5. ^- Evolution of isomer shifts in Fe2P ^with temperature : (9) ^this work, (0) results from ref. [14].

patterns characterized

by

^the

magnetic

internal fields

reported

^{in table I.}

From the evolution of the spectra, ^Curie tempera-

tures

amounting

^to 320 ± ^{5 K} for

Fe2Po.9Aso.1

^and

460 ± ^{5 K} for

Fe2pO.5Aso.5

^{are deduced in}

good

agreement with other authors

[20, 27].

^The

magnetic

characteristics of this series are collected in table II and will be discussed later.

3. 3

Cr2P1_yAsy

DOPED WITH 1

% OF 5 7 Fe2P

(0

y

0.5).

^{- These}

compounds

^{have the ortho-}

rhombic structure of

C02P [37].

M6ssbauer spectra ^{at room}

temperature

^are qua-

drupolar

^{doublets more or} less well resolved and the

hyperfine

parameters ^are

given

^{in table}

I,

for various temperatures. It results that iron atoms are located

at site I. At 77

K,

^those

compounds

^{are not}

magneti- cally

^ordered.

The evolutions of isomer shift and

quadrupolar splitting

^{at room} temperature

with y are reported

ⁱⁿ

figure

^{6. An} increase, ^with y, in the isomer

shift, accompanied by

^{a decrease in the}

quadrupole split- ting

^is observed. This decrease may be

explained by

the

expansion

of the unit cell with arsenic substitution.

The

explanation

of the variation of the isomer shift is not so

simple.

FIG. 6. ^- Evolution of isomer shift (e) ^and quadrupole splitting (x)

in Cri.98Feo.02Pl -,As, ^withy.

The unit cell of

Cr2Po.sAso.s

^{is not much} greater , than that of

Cr2P,

since the volume ratio is

only

1.020 ± ^0.005.

However,

the atomic radii of P and As

are

respectively

1.28 and 1.39

A.

The iron atoms are

therefore more

compressed

^when

they

^{are situated}

in

Cr2Po.sAso.s

^{than when}

they

^{are in}

Cr2P.

^Conse-

quently,

^{the electrons are more} delocalized in

Cr2Po. sAso. s

and the isomer shift at the iron sites increases

[35].

The behaviour of FeCrP is similar to that of

Cr2P (1 % 5’ Fe)

^{and its}

hyperfine

parameters ^are

reported

in table I.

3.4

Mn2P1_yAsy

DOPED WITH 1

% OF s7Fe2P

(0

y

0.35).

^{- The}

crystalline

^{structure of this}

series is that of

Fe2P [29].

At room temperature,

quadrupolar

^{doublets more}

or less well resolved are also observed in this _case, the

hyperfine

parameters will be found in table I.

At 77

K,

all those

compounds

^are

magnetically ordered,

^some

examples

^are

given

ⁱⁿ

figure

^{7a and b.}

Those spectra ^{are difficult to}

fit; nevertheless,

^the Neel temperature has been deduced from the evolu- tion of the width of the

absorption peak

^with tempe-

rature

(Fig. 8).

Within the

experimental

^{errors, the}

Neel temperatures ^{are all}

equal

^to

110 ±

⁵

K,

ⁱⁿ agreement ^{with the neutron} diffraction data

[9].

3.5 FeMnP. - In the orthorhombic structure of

FeMnP,

^{iron atoms are situated}

only

^on tetrahedral sites

[19].

At room temperature, ^{the M6ssbauer} spectrum ^is

an unresolved

quadrupolar

doublet whose charac- teristics are

given

^{in table I.}

From the variation of the width at half

height

of the spectrum

(Fig. 9),

^{a Neel} temperature ^of 252 ± ^{1 K is}

deduced,

^{in marked}

disagreement

^with

the

susceptibility

measurements which

give

^{320 K}

[17].

34

FIG. 7. ^- Mossbauer spectra ^{at 77 K of} (a) Mn1.9SFeo.o2P, (b) Mni.9gFeo.o2Po.8Aso.2, (c) ^FeMnP.

FIG. 8. ^- Evolution of the width at half-height of Mossbauer spectra ^{for the} compounds Mn1.9sFeo.o2P -y,Asy ^{with (a) y = 0 (- -),}

(b) y ^{= 0.1} (-), (c) y ⁼ 0.2 ( ... ).

FIG. 9. ^- Evolution of the width at half-height of Mossbauer spectra ^{of FeMnP.}

The spectrum ^{at 77 K is}

represented

ⁱⁿ

figure

^7c

and is characterized

by

^{an internal}

magnetic

^{field of}

56 ± ² kUe. These M6ssbauer results are in very

good

agreement ^{with the}

partial

results of Suzuki

et al.

[18].

The

application

^{of the}

Karyagin’s

^method

[36]

gives

^two solutions for the

hyperfine

parameters defined as

usually :

Suzuki et al.

[18]

from the internal

magnetic

^field

measured

by

^Mossbauer spectroscopy ^{deduce a}

magnetic

^moment

amounting

^{to at most 0.5}

,uB jFe

atom and build _up a noncollinear

magnetic configura-

tion which is not in

disagreement

^{with our new}

M6ssbauer results.

3.6

Co2P

DOPED WITH

I %

^oF

57Fe2P.-Several

samples

^of

Co2P doped

^{with 1}

% of "Fe2P

^{have been}

studied. The conditions of

preparation

^are

reported

in table III.

Sample (b)

^{has been}

doped

^from

C02P

^from

K & K Laboratories.

X-rays analysis

indicates that

only

^one

crystallographic phase

^is present. ^{At room}

temperature, Co2P

^{is attracted}

by

^a magnet. ^The Mossbauer

spectrum

^of

sample (b)

is shown in

figure

^10b.

Heatings

^followed

by

^slow

coolings

^do

not alter the

spectrum

^{due to a}

magnetically

^ordered

compound, probably ferromagnetic.

The maximum internal

magnetic

^field

compatible

with the spectrum ^{is 72 + 5} kOe and the mean isomer shift amounts to 1.05 + ^0.02

mm/s

^{as for an}

^{Fe2 +}

ion. This

sample

has been studied _up to 668

K,

where it is still F. At 573

K,

the maximum internal field is still

equal

^{to 55} + ⁵ kOr,. Such a behaviour is due neither to iron diffused in

cobalt,

^{since in this}

FIG. 10. ^- Mossbauer spectra ^{of different} samples ^of Co2P doped

with 57Pe (see table III) ^{at room} temperature. ^{Bars in a and c} represent lines characteristic of 17 Fe in Co.

sample,

characteristic lines of iron in cobalt do not appear ^as in

samples (a)

^and

(c) (Fig. I Oa, c)

^{nor to the}

high

temperature

phase

^{observed at 1 100 K}

[38],

because

X-rays

^{do not} reveal this

phase

and because

heatings

^{at 1 000 K do not}

modify

^the spectra.

Roger et

^al.

[27, 29]

observe that

C02P

^{is not F}

at 77

K,

^but

they

^{add 5}

%

^of

phosphorus

^{in order to}

remove free cobalt present ^{in their}

compound.

5

%

^of

phosphorus

have been added in

sample (b),

which thus becomes

sample (d).

^It is characterized at room temperature

by

^a

paramagnetic

spectrum

as shown in

figure

10d with isomer shifts and

quadru- pole splittings given

^{in table I. One} of the observed

quadrupole couplings :

^0.89

mm/s

may be attributed to site II

by comparison

with the other

phosphides

and

arsenides,

but the other one : 2.18

mm/s

^similar

to an Fe" ion

quadrupole coupling

is very

surpris- ing.

^{It is known}

[1]

^that

Co2P

is often understoichio- metric in

cobalt,

^{site II not}

being fully occupied.

This creates in the lattice a

large

^{number of vacancies}

in which iron would have a

tendency

^{to diffuse.}

If the iron atom is surrounded

by

^{sites II}

occupied by

Co atoms, it ^is characterized

by

^{the normal}

quadru- pole coupling :

^0.89

mm/s.

^{On the} contrary, ^{if it is} surrounded

by

empty

pyramidal sites,

^it

might

^be

characterized

by

^a much greater

quadrupole coupling.

The widths of the lines of the broadest doublet

(0.6 mm/s)

may be related to the distribution of the vacancies.

In

conclusion, Co2P

^is

probably ^F,

^{but the addition}

of a few percent ^of

phosphorus abruptly

decreases its Curie

temperature

^{from more than 670 K to less than}

77 K. A more accurate

study

^{of the}

magnetic properties

of

C02P

^is necessary.

3.7

Co2Po.sAso.s

DOPED WITH

2 %

^OF

s7Fe2P’-

This

compound

^{has the same structure as}

Fe2P [29].

The

paramagnetic

spectrum ^is

composed

^{of three}

doublets whose parameters ^are

reported

^{in table I.}

The smallest ^two doublets

(0.575

^{and 0.821}

mm/s)

may be ascribed to site I and site II and the

largest

one should

correspond

^{to a site II} surrounded

by

vacancies as described for

Co2P.

This

sample

^is

magnetically

^{ordered at low tem-}

perature ^{and a Curie} temperature ^{of 116} ± ^{2 K is} deduced from the evolution of the M6ssbauer spectra, in agreement ^{with the} measurements of 109 K

by Krumbiigel-Nylund et

^al.

^[29].

^{At 77}

^K,

^{a maximum}

internal

magnetic

^field

amounting

^{to 175 ± 5 k0e}

is

estimated,

^{which is} greater than that observed in

C02P

but similar to that

reported

^for

Co2As [32].

3.8 FeCoP. - At room temperature, ^{this com-}

pound presents

^a spectrum characterized

by

^the

superposition

^{of two Zeeman patterns with}

hyper-

fine fields :

HI

^{= 74 k0e and}

Hn

^{= 146 ka} in agree- ment with the observations of

Roger et

^al.

[11]

^but

in

disagreement

with those of

Wappling et

^al.

[14].

The

magnetic

^structure

disappears

^at

385 ±

^{5 K} in

disagreement

with the determination of

Roger [27] :

440 K. The

paramagnetic

spectrum shows that the iron atoms are present ^on both sites

with, however,

^a

preference

^{for site II. The} parameters ^{at 395 K are}

given

^{in table I.}

3.9

Ni2P

DOPED WITH 1

% oF 57Fe2P.

^{- The Mbss-}

bauer spectra ^{at 77 and 292 K are similar and the}

compound

^{is not}

magnetically

^{ordered. See table I}

for parameters.

3.10 FeNiP. - As for

Ni2P, only paramagnetic

spectra ^have been observed. Their parameters ^are

given

^{in table I.}

.

4.

Synthesis

of the results. ^- It is well known that

the

internal

magnetic

^{field is the sum} of different terms

[40, 41] :

(1)

^{The Fermi} contact term

(Hr ,)

^{due to and} propor- tional to the

polarization

^{of the s electrons}

by

^{the d}

electrons.

(2)

The orbital term

(H,) arising

from the inter- action between the nucleus and the orbital moment of 3d electrons. In the

compounds

^{studied in this}

paper, ^one may ^assume that the orbital moment is

quenched.

(3)

^The

dipolar

^term

(Hd) originating

^{in the inter-}

action between the nucleus and the moments of the electronic

spins.

^{In first}

approximation,

^{this term is}

equal

^to

Hd

⁼ ,uB q,

where PB

is the Bohr magneton,

and q

^the

principal

component of the electric field

gradient.

^In

Fe2P, -,Asy compounds,

^{this term}

amounts to at maximum 10 kOe.

(4)

The contribution of transferred and _super- transferred fields

(H,)

^{which are}

generally

^small.

So,

ⁱⁿ

Fe2Pl -YAsy compounds,

^the internal field may be written :

36

TABLE II

Magnetic

characteristics

of

^{the series}

Fe2P 1 - yAsy ⁽⁰

^y

^0.5)

It appears from table

II,

that the Curie

temperature

in the

Fe2p, -YAsy

series increases with _y, but that the

mean

magnetic

^{moment at iron} atoms saturates at 1.5 PBI

whilst,

^{at sites I} the internal

magnetic

^field

increases in a continuous _way. On the other

hand,

at sites

II,

it increases until the mean

magnetic

^moment

saturates, ^and then

decreases.

That indicates that both types ^of

iron

^atoms behave

differently

^{and this}

is an

important

^{fact to take into account in order to}

understand the

magnetic properties

of the series

M2P l_yAsy.

The substitution of arsenic for

phos- phorus

^{has the same effect on} the Curie tempera-

ture than the

application

^{of a} pressure ^on

Fe2P [21].

This confirms the fact that

Fe2P

^{is an itinerant}

ferromagnet [25].

In

figure 11,

^{the internal}

magnetic

^fields

extrapolated

to 0 K have been

plotted

^{as a} function of the number of electrons _per atom

(or

^the

composition),

^{for the}

series

Fe2 -xMxP.

^{At sites}

I,

the internal fields at iron atoms are

nearly

^{constant for the}

hexagonal

structure as well as for the orthorhombic one. At sites

II,

the variations are more

important.

^Further-

more, at sites

I,

the isomer shifts _vary

only slightly

within the series

Fe - 2 ,,M. P.

The electronic structures of the iron atoms in sites I are therefore

nearly

^the

same in the

Fe2P

and in the

C02P

structures.

For the series

Cr . 9 8 Feo. 0 2p 1 - yASy,

^{accurate measu-}

rements of the

quadrupole couplings

^{at different tem-}

FIG. 11. ^- Internal magnetic ^fields extrapolated ^{to 0 K as a func-}

tion of the composition (electrons number/atom) ^{for both} crystallo- graphic structures of the series M2-xFexP. (o) hexagonal structure, (0) orthorhombic structure, (A) ^internal magnetic ^{fields mean} values, ^- sites I, - - - - sites II. Note : Fel.8CO0.2P ^{and FeNiP}

from ref. [14].

peratures ^{have been}

performed.

Variations amount-

ing

^{to 5 x}

10-4/K

^{have been}

^observed,

ⁱⁿ agreement with other measurements on metallic

compounds [39].

Experimental

^and

theoretical

studies on these

compounds

^{are in} progress in the

laboratory.

TABLE III

Preparation

conditions

of

^{the various}

samples of C02P

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