Magnetic behaviour of Co-Cr alloys above the critical concentration for ferromagnetism

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Magnetic behaviour of Co-Cr alloys above the critical concentration for ferromagnetism

G. Gavoille, S. Durupt, J. Hubsch

To cite this version:

G. Gavoille, S. Durupt, J. Hubsch. Magnetic behaviour of Co-Cr alloys above the crit- ical concentration for ferromagnetism. Journal de Physique, 1982, 43 (5), pp.773-777.

�10.1051/jphys:01982004305077300�. �jpa-00209450�

Magnetic behaviour of Co-Cr alloys above the critical concentration for ferromagnetism

G. Gavoille, ^S. Durupt

Laboratoire d’Electricité et d’Automatique, Université de Nancy I, C.O.140, ⁵⁴⁰³⁷ Nancy Cedex, ^France

and J. Hubsch

Laboratoire de Minéralogie ^et Cristallographie, Université de Nancy I, ^C.O. 140, ⁵⁴⁰³⁷ Nancy Cedex, ^France (Rep ^{le 25 mai 1981,} révisé les 15juillet, 23 octobre 1981, accepte ^le 12 janvier 1982)

Résumé. 2014 Nous avons étudié des alliages Co-Cr dans un domaine de concentrations en chrome légèrement supé-

rieures à la concentration critique ^de disparition ^du ferromagnétisme (25 %). Nos résultats suggèrent ^{un état} magnétique ^très inhomogène, formé d’amas présentant ^une large distribution de températures ^{de formation.}

Aux basses températures, ^{on observe une} transition superparamagnétisme-ferromagnétisme ^{et une transition}

ferro-verre de spin ^à plus ^basse température dans la gamme de concentration 25 %-29 %.

Abstract.

We have investigated ^Co-Cr alloys with chromium concentrations slightly larger ^{than the critical}

concentration for long range magnetic ^order (25 %). Our results suggest ^a strong inhomogeneous magnetic ^state consisting of clusters with a large distribution of formation températures. ^{At low} temperatures ^{one observes a} superparamagnetic ^to ferromagnetic transition followed by ^a ferromagnetic ^to spin glass transition at lower temperature in the range of concentration 25 % ^{to 29} %.

Classification

Physics ^Abstracts 75.30C - 75.40 - 75.50C201375.60

1. Introduction.

Magnetization measurements

performed ^on dilute Co-Cr alloys ^{have shown that}

the variation d7p/dc of the average magnetic ^moment

with the chromium concentration is as high ^as

-

6.6pB [1, 2]. ^This and other physical proper- ties [3, 4], ^{have been} interpreted ^{as the} occurrence of

a virtual bound state above the Fermi level. However

a recent calculation of the band structure of the Ni-Cr dilute alloy [5] shows that the density ^{of states} or the majority spin ^{electrons on the} impurity ^site

not only consists of the virtual bound state but also extends over the whole d band of the matrix which lies below the Fermi level. Consequently ^the magnetic

moment localized on the impurity ^site may be much smaller than 5 _PB and a large decrease of the _average

magnetic ^moment may also result from the decrease of the magnetization of the matrix. In fact NMR [6]

and diffuse neutron scattering [7, 8] experiments suggest that the decrease of the magnetization mainly arises from the decrease of the magnetic

moment of cobalt atoms which are nearest neighbours

of the impurities, ^the magnetic ^{moment of Cr atoms} being smaller than 1 _PB. This behaviour extends

over alloys ^more concentrated in Cr until the long

range magnetic ^order disappears ^{near 25 at.} % ^Cr.

As the chromium solubility ⁱⁿ h.c.p. cobalt is limited to 40 % [9] ^the range of concentrations that we may

investigate above the critical concentration is rather

narrow and we only ^{observed a} typical giant ^moment

behaviour without reaching ^a high enough ^concen-

tration for the magnetic moments to collapse ^{as in}

similar alloys ^{such as CuNi} [10,11], ^VFe [12], ^NiV [13],

and PdNi [14].

In the _range of concentrations where the giant

moments are observed, ^the temperature dependence

of the low field magnetization evolves from typical ferromagnetic ^to spin glass like behaviour as the concentration in chromium increases. As an inter-

mediary step ^the magnetization ^{exhibits a} plateau,

the interpretation of which needs to be clarified from both theoretical and experimental points ^{of view.}

A similar behaviour has been observed in Au-Fe concentrated alloys and different interpretations ^have

been suggested [15, 16, 17, 18].

2. Experimental.

^{2.1 ALLOY} PREPARATION.

The alloys ^were prepared from 99.99 % pure elements.

Appropriate quantities ^of each constituent were melted

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01982004305077300

774

together ^{in a water} cooled copper boat in an induction furnace under He atmosphere. ^Each ingot ^was

turned and melted five times and weight ^{losses were}

such that concentrations are known within 0.1 at jo.

After melting, ^the alloys ^were homogeneized ^{at 1000 C}

for 4 days ^under hydrogen atmosphere. Samples four magnetic investigations ^were shaped ^into spheres

and the remaining parts ^{of the} ingots ^{were cut and} polished ^for X-rays ^{and electron} microprobe analysis.

The entire samples ^{were annealed at} appropriate temperature ^under hydrogen atmosphere ^{for 48 h}

before quenching ^{into water in order to obtain the} h.c.p. ^structure.

2.2 STRUCTURAL ANALYSIS.

An electron micro-

probe analysis ^{of the} samples ^{revealed no} significant segregation after the heat treatment and X-ray

diffraction patterns only ^{indicated a well resolved} h.c.p. ^structure.

2.3 MAGNETIC MEASUREMENTS. - All measure- ments were performed using ^a vibrating sample magnetometer in fields up ^to 20 k0e and at tempe-

ratures in the range 2.5-300 K. Demagnetizing ^cor-

rections were applied.

3. Discussion of the data.

We have investigated

six specimens respectively containing ^{25, 26, 27.3,} 28.2, 29.7 and 32.3 at jo ^{Cr. We} report ⁱⁿ figure ¹

the saturation magnetization Qo obtained from the Arrott plots ^{at 4.2 K. We have} reported figure ^{2 the} temperature dependence ^{of the} magnetization ⁱⁿ

a small d.c. field applied ^{at low} temperatures ^for five specimens. ^For C07SCr2S ^the magnetization ^is

«cut off » by ^the demagnetization ^{factor at low}

Fig. ^1.

Saturation magnetization Qo ^versus alloys ^con-

centration.

Fig. ^2.

Magnetization ^versus temperature : - ^{in a} 30 Oe d.c. field for (a) 25, (b) 27.3, (c) ^29.7 % ^{at. Cr} specimens;

-

in a 40 Oe d.c. field for (d) 26, (e) ^28.2 % ^{at. Cr} specimens.

The full line corresponds ^to increasing temperatures ^after cooling ^{in zero} field, ^and the dashed line to decreasing temperatures in the field.

temperatures. ^{This is an} indication of strong ^sus-

ceptibility ^{but not} necessarily ^of long range magnetic

order [16]. The other specimens ^{exhibit a} quasi plateau ^{between a} higher temperature Tc ^{and a lower} temperature TF ^{where the} magnetization ^decreases abruptly ^to lower values. We observe figure ^{4 that} Tc ^and T F tend, ^on increasing ^the concentration,

to a unique value when a unique ^{maximum of the} susceptibility is observed. The field cooled _magne- tization (dashed ^line figure 2) ^{which is often inter-} preted ^{as the} equilibrium response departs slightly

from the zero field cooled magnetization ^at tempe-

rature much higher ^than Tc but it increases smoothly

on decreasing ^the temperature ^below Tc ^and TF.

The same field cooled data are shown (Fig. 3) ^{in a} x -1 ^{vs. T} plot. ^{We observe on} the data relative to the 28.2 % ^{and to the 29.3} % that deviations from the Curie Weiss high temperature regime ^are already

observed at a temperature TS ^much higher ^than Tic (see Fig. 4).

From the general trend of the concentration

dependence ^of Ts ^{for two} concentrations it _may be

Fig. ^3.

Reciprocal susceptibilities ^versus temperature.

Black dots : cooled in the field. Light ^{dots : cooled in a zero}

field. The dashed lines correspond ^{to the} high temperature Curie Weiss behaviour.

inferred (Fig. 4) ^{that the} high temperature para-

magnetic ^{Curie Weiss} regime ^{is out} of reach in our measurement range for the alloys ^{of lower concen-}

tration in Cr. The paramagnetic ^{Curie constant}

in the high temperature Curie Weiss regime ^does

not seem inconsistent with what would be expected

for the Cr and Co present with the usual values of their magnetic ^{moment. The average effective} spin

taken from the data is near 2. On decreasing ^{T below} TS the observed susceptibility ^variation may be inter-

preted ^as traducing the formation of large super-

paramagnetic clusters. We tentatively estimated their

magnitude and their number N by comparing ^the

776

Table I.

Bulk magnetization (Jo, Curie constant C, effective magnetic ^moment J1.eff’ number N of clusters and

superparamagnetic asymptotic ^Curie temperature 0p for four specimens.

superparamagnetic ^{Curie constant in the super-} paramagnetic regime just ^above Tc ^{with the satu-}

ration magnetization Qo measured at low tempe-

rature using ^the approximate expression :

The data are reported in table I.

If the effective magnetic ^moment of the clusters is nearly concentration independent the number of clusters sharply ^{decreases as} the chromium concen-

tration increases.

4. Interpretation of the data.

The essential results of our investigation ^are condensed in the phase diagram ^of figure ^4.

Tc ^is interpreted ^as the Curie temperature ^when the large clusters which are formed essentially ^around Ts, ferromagnetically order. The lower temperature

Fig. ^4.

Temperatures T s of formation of _superpara-

magnetic ^clusters (Stars). Temperatures Tc (light dots) ^and TF (black dots) of transition.

TF is then the ferromagnetic ^to spin glass ^transition predicted by several theoretical models [19, 20, 21, 22]

in the _presence of a large ferromagnetic component of the interaction. The existence of such ferromagnetic

correlations is attested by the fact that the _super-

paramagnetic ^Curie temperature 0p ^is positive (table I).

On increasing the concentration we have directly

a transition from superparamagnetism ^{to the} spin glass regime for concentrations larger ^{than 29 %.}

Notice that 0p changes ^from positive ^to negative

values between 29.7 % ^and 32 % Cr concentration.

This interpretation ^{is made} by analogy ^with

similar interpretations which have been made on

other systems [23, 24, 25, 26, 27, 28] ^{and in} particular

in AuFe [17, 18] which exhibit essentially ^{the same}

data.

The fact that the field cooled and the reversible

susceptibilities ^are already ^{different at} temperatures much higher ^than Tc ^{is not} necessarily ^{a flaw for}

such an interpretation ^{since the} occurrence of rema- nence can be already justified in the frame of a super-

paramagnetic picture. There is anyway ^a large ^increase

in the difference between field cooled and reversible

susceptibility occurring ^{at both} temperatures Tic

and TF which still confirms the hypothesis ^{of a} phase

transition occurring ^{at those two} points.

5. Conclusion.

We have determined the phase diagram (Fig. 4) ^{of the} magnetic transitions occurring

in CoCr in the concentration _range immediately

above the concentration c = 25 % ^{Cr where} long

range ferromagnetism disappears. ^We conclude that there is a transition from superparamagnetic ^{to ferro}

and then to spin glass order in the _range 25 ;’lo ^{c 29} jo ^{and a} direct transition from super-

paramagnetic ^to spin glass order above this concen-

tration. However the particular mechanism which governs the formation of the superparamagnetic

clusters in this system ^{is still} only partially ^understood

and probably ^more complicated in CoCr than it is in CuNi or AuFe.

Acknowledgments.

The authors would like to thank Mr J. Steinmetz, ^P. Steinmetz, and B. Malaman

(Laboratoire de Chimie Min6rale, University de

Nancy-I, Nancy, France) for structural analysis ^of

the samples. ^{The authors are indebted to Dr Souletie}

for his advices in the interpretation of the data.

..

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