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Submitted on 1 Jan 1971
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MAGNETIZATION OF VERY DILUTE Au-Fe ALLOYS
J. Tholence, R. Tournier
To cite this version:
J. Tholence, R. Tournier. MAGNETIZATION OF VERY DILUTE Au-Fe ALLOYS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-211-C1-212. �10.1051/jphyscol:1971166�. �jpa-00214493�
JOURNAL DE PHYSIQUE Colloque C I , ~upplkment au no 2-3, Totne 32, FPvrier-Mars 1971, page C I - 2 1 1
MAGNETIZATION OF VERY DILUTE Au-Fe ALLOYS
J. L. THOLENCE and R. TOURNlER
Centre de Recherches sur les T r b Basses Temperatures, Cedex 166, 38, Grenoble-Gare
Rksurnk. - Aux concentrations superieures a 0,l O/,, le systtme Au-Fe presente des proprietes caracteristiques d'une interaction de R. K. K. Y. entre les impuretes de fer magnetiques. Aux tres faibles concentrations ( < 0,01 %) la suscep- tibilite initiale xi(T = 0,05 OK) devient proportionnelle it la concentration et xi(T) suit une loi de Curie Weiss (C/T + 0 )
avec 0 - 0,4 OK, jusqu'aux tres basses temperatures, ce qui est en faveur de l'effet Kondo. Aux faibles temperatures ( T - 8/10) et faibles concentrations ( < 0,01 "/,) le moment magnetique reapparait sous I'action d'un champ externe (quelques kOe) pour atteindre la valeur a saturation de 2,2 JLB.
Abstract. - For the concentration higher than 0.1 % the properties of the Au-Fe alloys are characteristic of a R. K. K. Y. interaction between the magnetic iron impurities. At very low concentrations ( < 0.01 O/,), the initial suscep- tibility xt(T - 0.05 OK) becomes proportional to the concentration and xl(T) follows a Curie-Weiss law (CIT + 0)
with 8 - 0.4 OK, down to very low temperatures. That is in favour of the Kondo effect. At low temperature (T = 0110) and low concentrations ( < 0.01 %) the magnetic moment reappears under the action of an external field (several kOe) to reach the saturation value : 2.2 JLR.
For concentrations c above 0.1 %, iron impurities in gold have a magnetic behaviour yielding low tempe- rature orderingcharacteristic ofaR. K. K. Y. interaction [I], [2] of the form cos (2 k , r + q)!r between these impurities : the order temperature TN is proportional to c, the specific heat linear in T and the initial sus- ceptibility are independent of c below TN [3], [4]. For concentrations below 0.1 O/, we have already shown [5] that the initial susceptibility xi (in emu/g) at T = 0.05 OK decreases with the concentration instead of remaining constant like in the Cu-Mn system ; we have ascribed this phenomenonto the Kondo effect, i. e. to the non existence of a magnetic moment on a great number of iron impurities, because the magnetic order seems to disappear when c becomes smaller than 0.1 %. We have continued this study because the Au-Fe system appears to be one of the few systems where magnetic order and one impurity effects can be, alternatively, observed when c is decrea- sed : in Cu-Fe, Au-Co, Cu-Co alloys, the Kondo temperature is high and segregation phenomenon would be present a t the high concentrations necessary to make the impurities magnetic by interaction egects.
The Cu-Mn and Au-Mn alloys have too low Kondo temperatures to beeasily studied and it is necessary, to have only several ppm of impurities, t o eliminate the interaction effects.
On figure 1, we have plotted the reciprocal initial susceptibility multiplied by c, versus th: temperature for the alloys of nominal concentrations : 0.5 "/,, 0.2 %,
0.1 %, 0.05 %, 0.02 %, 0.01 1 %, 0.004 8 0/,, 0.002 6 %.
For c > 0.1 %, the ordering due t o the interactions is characterized by : the initial susceptibility independent of the concentration a t T = 0.05 OK, and the propo- tionality to c of the order temperature TM where x i is maximum, TM E 800 x c (in OK). The paramagnetic Curie temperature O varies nearly like the concentra- tion 8 1!0.4 + 760 x c. At very low concentrations, the order disappears and the initial . - susceptibility tends to follow a Curie-Weisn law xi = -- N c - k " with
M 3 k(T + 0)
the limiting value 8 - 0.4 4 0.05 OK when the concen- tration tends to zero. The effective moment deduced
A u - F e
-
FIG. 1. -The reciprocal initial susceptibility of the &-Fe alloys is multiplied by the concentration and plotted versus
the temperature.
Au-Fe
-
FIELD 4w ~e
20 40 6 0 k a z '
FIG. 2. - Magnetization per iron atom at low temperature.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971166
C 1 - 212 J. L. THOLENCE AND R. TOURNIER from the Curie constant is equal to 3.25 + 0.2 p,. At
very low concentrations (c < 0.01 %) the initial sus- ceptibility at T = 0.05 OK is decreasing and becomes nearly proportional to the concentration :
c/xi becomes almost independent of the concentration (the small differences may be explained by the experi- mental errors and the lack of knowledge of the true concentrations) and the low concentration limit of the Curie temperature : 8 21 0.4 OK, may be considered as the characteristic temperature TK for the formation of a spin-compensated state. However, at very low tempe- ratures (< 0.4 OK) some deviations from this Curie- Weiss law appear (insert of figure I), their c2 depen- dence allows us to attribute them to magnetic or nearly magnetic pairs ofimpurities like in the Cu-Fe system [7].
Our values p,,, and 8 are smaller than those deduced from the higher temperatures data (up to 300 OK) of Hurd (6) he,, = 3.7 + 0.2 p~ and 8 = 10 OK when c varies between 0.003 9 and 0.02 %. The appa- rent Curie-Weiss law observed at low temperatures is not followed at high temperatures T $- 10 TK. On the figure IT, we have reported the magnetization curves of our alloys at T N- 0.05 OK. The saturation magneti- zation of four alloys (c < 0.01 %) is reached and is equal to : 2.2 + 0.05 & per iron impurity. Near the saturation the interaction effects between the impuri- ties become dominant and it is more difficult to obtain the saturation when the concentration increases. In low fields, the one-impurity effects are evidenced by the susceptibility per impurity being independent of the concentration.
To study the one-impurity effects, it is also possible to reduce the interactions by limiting the mean free path of the conduction electrons. This effect is obtained by introduction of Titanium impurities in the Au-Fe allovs.
Au-Fe
J Temperature
L
0 1 2 3 4 S K
FIG. 3. The reciprocal initial susceptibility of the A?-Ti-Fe alloys is multiplied by the concentration of iron and plotted
versus the temperature.
c/xi tends to the same Curie-Weiss law observed for the most dilute Au-Fe alloys yielding : 0 N 0.4 OK and pen FZ 3.25 HB.
To conclude, the disappearence of the magnetic moment of iron in gold may be studied either by decreasing the concentration of impurities, or by limi- ting the mean free path of the conduction electrons.
In these two cases, the initial susceptibility per impurity tends to follow a Curie-Weiss law
da figure 111, we have plotted clx initial versus T (with perf ZJ 3.25 p,). At low temperature (< 0.40K) forthe alloys : AuTi3%Fel %, AuTi3Fe0.,, AuTi3Feo.,, and concentration (< 0.01 %), it is possible to make AuTi,Feo.,. In each case, magnetic order occurs at the magnetic moment reappear in a field of several temperatures smaller than those of the Au-Fe alIoys thousands of Oe. We have found a saturation moment with the same concentration of iron. Furthermore which is almost equal to that of the ferromagnetic iron.
References
[I] RUDERMAN (M. A.) and KITTEL (C.), Phys. Rev., [5] DREYFUS (B.), SOULETIE (J.), THOLENCE (J. L.) and
1954, 96, 99. TOURMER (R.), J. Appl. Phys., 1968, 39, 846.
[2] YOSDA (K.), Phys. Rev., 1957, 106, 893. 161 HURD (C. M.), J. Phys. Chem. Solids, 1967, 28, 1145.
[3] LUTES (0. S.) and SCHMIT (J. L.), Phys. Rev., 1964, 171 THOLENCE (J. L.) and TOURNIER (R.), submitted to
134, 3A, A 676. publication in Phys. Rev. Letters.
[4] SOULETIE (J.), Thesis, University of Grenoble (1968).
