PARAMAGNETIC EFFECTS ASSOCIATED WITH Mn ADDITIONS TO LIQUID Al-Sn ALLOYS

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PARAMAGNETIC EFFECTS ASSOCIATED WITH Mn ADDITIONS TO LIQUID Al-Sn ALLOYS

E. Collings

To cite this version:

E. Collings. PARAMAGNETIC EFFECTS ASSOCIATED WITH Mn ADDITIONS TO LIQ- UID Al-Sn ALLOYS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-516-C1-518.

�10.1051/jphyscol:19711172�. �jpa-00213994�

JOURNAL DE PHYSIQUE

Colloque C 1, supplément au n° 2-3, Tome 32, Février-Mars 1971, page C l - 516

PARAMAGNETIC EFFECTS ASSOCIATED WITH Mn ADDITIONS TO LIQUID Al-Sn ALLOYS

E. W. COLLINGS

Battelle-Columbus Laboratories Columbus, Ohio 43201, U. S. A.

Résumé. — Nous avons mesuré la susceptibilité magnétique d'une série d'alliages liquides (Àl-Sn)»sMti2. Les résul- tats sont conformes à certaines prédictions basées sur le modèle de Anderson de formation de moment local. Les ions Mn ne portent pas de moment local quand ils sont dans l'aluminium liquide et la présence d'un « paramagnétisme ren- forcé» (1,0 m uem/mole de Mn à 900 °C) est clairement mise en évidence. D'autre part, quand ils sont dissous dans l'étain pur, les ions Mn portent un moment localisé de 4,34 magnétons de Bohr ; en même temps de tels alliages pos- sèdent une susceptibilité paramagnétique résiduelle de 0,36 s m uem/mole de Mn.

On décrit la transition magnétique-nonmagnétique de l'état lié virtuel de Mn lorsque la composition de l'alliage hôte varie.

Abstract. — Magnetic susceptibility measurements have been performed on a series of liquid (Al-Sn)9sMn2 alloys. The results possess several interesting features reminiscent of predictions based on the Anderson model of local moment forma- tion. Mn ions do not carry a local moment in liquid Al, and an « enhanced paramagnetism » (1.0 m e. m. u./mole of Mn at 900 °C) is clearly present. On the other hand, when dissolved in pure Sn, Mn ions carry localized moments of 4.34 Bohr magnetons ; but at the same time such alloys have residual paramagnetic susceptibilities equivalent to 0.36 s m e. m. u./mole of Mn.

A magnetic-nonmagnetic transition of the Mn virtual bound state, with variation of host-alloy composition, is des- cribed.

1. Introduction. — In studies of the magnetic properties of dilute amounts of Fe in 4 d transition metal binary alloys, Clogston and coworkers [1, 2]

demonstrated that the Fe ions acquire localized magne- tic moments when the Mo content of a Nb-Mo host alloy system is increased beyond 40 % Mo. At about the same time the theories of Wolff [3] and Ander- son [4] showed how a virtual level could be expected to magnetize as its position and width undergo change in response to continuous adjustment of the electron density of the host alloy. According to Anderson [4], a virtual bound state will magnetize critically, depend- ing on the value of UjA. If for example the condi- tions are optimal for magnetization, a second-order nonmagnetic-magnetic phase transition will take place as UjnA increases through unity.

A feature of the Anderson model that has been sustained experimentally is the enhanced paramagne- tism which accompanies the «non-magnetic» state of the virtual-bound-state alloy. This has been obser- ved and studied in both solid (Be-Ni [5]) and liquid (Al-Mn [6]) alloys. Flynn and coworkers [6] have shown that 3 d transition elements do not carry moments in liquid Al (thus confirming observations made on solid Al-base alloys by earlier workers,

e

- g-> [7])>

^{a n}

d that the polarization-paramagnetism has its maximal value in Al-Mn.

However, attempts to design a clear-cut test of the Anderson magnetic transition in solid alloys have not been completely successful, since, as is becoming increa- singly apparent, local atomic environments exert important influences on various electronic properties of alloys. In particular, the results of several magnetic studies [8, 9, 10] have suggested that the acquisition of a moment by a 3 d transition metal dissolved in a solid binary alloy depends on the constitution of the nearest neighbor shell.

The relative importance of clustering, or spacial

correlations in general, will of course vary considera- bly from one solid alloy system to another and may turn out to be negligible in some. In the context of local moments, we felt that possibly the best test of the Anderson model would be found through an extensive study of transition metals dissolved in liquid alloys, in which spacial correlations would at least be minimized. For this reason we set out to measure the paramagnetic susceptibility of Mn in liquid Al-Sn.

2. Experimental. — Magnetic susceptibility measu- rements were made using the Curie technique, with the aid of an electronic microbalance and a calibrated inhomogeneous magnetic field ( ^ 1 0 kOe). The sam- ple nested in a bed of high-purity A1

2

0

3

powder, packed into a small quartz bucket. The alloys were prepared in situ ; and by monitoring the magnetic force during several heating and cooling cycles, it was possible to determine, from stability and repro- ducibility considerations, when a homogeneous liquid alloy existed.

3. Results and Discussion. — 3.1

THE ALUMINUM

END-POINT, Al-Mn. — A concentration of 3 at. % Al was chosen so that a direct comparison with Flynn's [6]

previous work could be made. The results were in excellent agreement with those of Reference [6] and exhibited a remarkably large Anderson enhancement effect, the susceptibility at 950 °C being equivalent to 1.03 m e. m. u./mole of Mn. The enhanced para- magnetism does possess some temperature dependence (Fig. 1, curve 100/0), but appears to tend to cons- tant value at very high temperatures.

3.2 THE TIN END-POINT, Sn-Mn, — A series of experiments on three liquid Sn-Mn alloys (5, 10, and 15 at. % Mn) has been described in an earlier

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711172

PARAMAGNETIC EFFECTS ASSOCIATED WITH Mn ADDITIONS TO LIQUID C 1

-

⁵¹⁷

I I I I I I I J I I I I

6 W 7CO 800 3 W 1000 llW IZCO Temperature (deg C)

publication [Ill. The susceptibilities per mole of Mn in the three Sn-Mn alloys fell about a common line (Fig. 1, curve3/100). This was an unexpected result, considering the relatively large concentrations of Mn employed. It was also discovered that in addition to a localized magnetic moment, which had also been observed by previous workers [12], the liquid Sn-Mn alloys supported a residual paramagnetism, x,.

The origin of the latter is still unclear, although its existences in solid alloys of Mn in various transition elements have been reported by Barton and Claus [13].

3.3 SUSCEPTIBILITY

OF

Mn

IN

LIQUID Al-Sn. - Figure 1 (curves 85/15 to 20180) shows the effect of adding Sn to liquid A1-Mn. The addenda (container plus host alloy) haveof course been allowed for and the curves refer to the additional susceptibility compo- nent introduced by the presence of 2 at. % Mn. The susceptibility temperature dependences are concave downward for all host alloy compositions from pure Al (10010) to AI,,Sn,, (45155). Clearly, no Curie- type moment can be ascribed to Gurves of this shape.

Instead we may assume the existence of an enhance- ment paramagnetism, which (at 9500C, say) has increased from 1.05 to 1.86 m e. m. u./mole within the above composition range.

A critical composition seems to be (40160) beyond which a local moment (always accompanied by a residual paramagnetism) is present on the Mn ions.

Within the scatter of the data, which is more severe than for any of the other curves, x,,(T) for the alloy (40/60) is practically linear. It is not clear whether we should interpret this as representing enhancement paramagnetism only, or the first appearance (A1

+

Sn) of a localized moment. If we choose the latter approach the same data, which also fall on a reasonably linear 1/T plot, yield an effective moment, p,,,, of 2.8 Bohr magnetons. This interpretation is consistent with

those for (20180) and (0/100), and at the same time is indicative of a moderately sizeable thceshold local moment coexisting with a large residugl paramagnei tism.

Finally, the temperature dependence data can be condensed into the format of figure 2. There it can

Aluminum Concentration in Al- Sn Host Alloy (at. pctl

be seen (a) that the totalparamagnetism contributed by the Mn ions increases monotonically from Al to Sn

;

(b) the pure polarization paramagnetism exhibited by the Al-rich alloys gives way to mixed behavior for A1 concentrations equal to or less than 40 at. % A1

;

(c) on proceeding in the direction Sn

-, Al,

as p,,, for the Mn ions decreases, X, increases

;

(4 refer- ring to the direction A1

-+

Sn, the enhanced para- magnetism increases by some 80 % prior to the appea- rance of a local moment, which seems to occur critically at 40 at. % Al.

At the present time the experimental observations, namely the apparent critical appearance at a local moment on Mn, and the concurrent variation of X,

with composition, can be fitted to several alternative models. (a) although, as was mentioned earlier, clus- tering can be expected to be minimal in liquid metallic alloys, a

cr local environment ⁾⁾

effect [8, 9, 101 still remains a possibility. In this model,

X,

could be ascribed to an Anderson enhancement polarization associated with those ions which are nonmagnetic.

(b) In addition, X, itself may eventually turn out to be

an orbital paramagnetism (see also [I I]) since the Mn

d-states are never exactly half-occupied. (c) Follo-

wing Daybell and Steyert [I41 the effect observed in

Al-Sn-Mn may be expressible in terms of a host-com-

position-dependent Kondo temperature. (d) Finally it

may be possible to interpret the observed effect in terms

of the Anderson [4] magnetic phase diagram. I t is

suggested that the addition of Sn to AI-Mn, which is in

t%e nonmagnetic phase, transforms the state of the Mn

continuously across the phase boundary and into a

C 1

-

^{518 E.} W. COLLINGS

region of critical fluctuations within the magnetic Mr. R. D. Smith for expert technical assistance, and phase field, in which local moment and temperature- Drs. K. C. Brog, M. L. Glasser, and P. R. Sievert for independent paramagnetisms would coexist. helpful

and

encouraging discussions. The research was financially supported by Battelle-Columbus 4. Acknowledgements. - We wish to acknowledge Laboratories.

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