Journal of Magnetism and Magnetic Materials 54-57 (1986) 165-166

MAGNETIC ORDERING IN PdAuMn ALLOYS

Girish CHANDRA and J. RAY

Tutu lnstitutr of Fundonwntol Revurch. Horn; Bhnhhu Road, Bomhq- 400 (I0.S. lrtdrrr

Resiativity and longitudinal magnetoresistivity studies have been carried out on several (Pd, ~, Au, ) q4 ,,Mn,, 15 alloys where Au concentration is varied as I = 0. 0.008. 0.012 and 0.031. These results indicate long range ferromagnetic ordering for Au concentration upto 1.2 at%, whereas for the higher Au concentration alloys. complex short ranged magnetic ordering is observed. From the observance of resistivity maximum for 3.1 at% Au alloy and magnetoresistivity studies. spin glass type ordering is speculated in these higher Au concentration alloys.

1. Introduction

One of the interesting features of PdMn alloys is the presence of different magnetic processes depending on the concentration of the Mn impurities present. For Mn concentration less than 3 at%, giant moment ferromag- netism previals [1,2] whereas for Mn concentration larger than 5 at% spin glass type behaviour is found to occur [3]. We have studied the magnetic behaviour of PdMn alloys in the ferromagnetic regime and have seen the effect of dilution of the Pd host by Au in such ferro- magnetic alloys. The Mn concentration in all our ternary alloys have been kept fixed at 0.35 at%. Data from both resistivity and longitudinal magnetoresistivity are used to analyse the magnetic interactions present in these systems.

2. Experimental details

The ternary alloys were all prepared from high purity materials from Johnson Mathey. UK. The Au con- centration was varied upto 3.1 at% approximately. The alloys were prepared by dilution of PdMn master alloys melted in an arc furnace in a water cooled hearth under flowing argon condition. The buttons were remelted several times to ensure homogeneity and compositions were analysed using atomic absorption spectroscopy. The buttons were cold rolled and etched and finally annealed in vacuum for four days at 800°C. The sizes of the samples were about 7 X 0.15 x 0.01 cm3. Measure- ments were carried out in a 3He-4He dilution refrigera- tor (Oxford Instruments. UK) using a standard four probe technique. Voltages were measured using a Keith- ley dc nanovoltmeter (Model 181) with a precision of 1 part in 10’. A constant current source of the same order of precision (Model No. 5752 of Tinsley Instruments UK) was used. Temperatures below 50 mK were mea- sured by a 60CoCo nuclear orientation thermometer and those above by a calibrated Ge sensor from Lake Shore Cryotronics. USA.

0304~8853/86/$03.50 0 Elsevier Science Publishers B.V.

3. Results and discussion

Fig. 1 shows the plot of resistivlty for the series

(Pd, ,Au&,,Mn,,, where .Y = 0. 0.008, 0.012 and 0.031. The incremental magnetic resistivity is calculated by substracting the resistivity pPdAu of the host alloy PdAu from the total resistivity pPdAuMn of the ternary alloy using the following relation

Corrections arising from deviation from Matthiessen’s rule are not accounted for, mainly because. the variation of the pure PdAu resistivity is found to be very small below 1 K. The curves in fig. 1 reveal characteristic features of these alloys. For ternary alloys upto .X s 0.012. there occurs a clear change of slope below 0.9 K and this is indicative of transition of ferromagnetic nature. The transition temperature of these system is obtained by obtaining the maximum in d( Ap)/dT plots near the region of change of slope. We find that the width of transition is quite broad because of random distribution of Mn atoms in our solid solutions. How- ever. such broad transition widths have also been earlier reported [l] in dilute PdMn alloys at similar Mn con- centrations chosen by us. As the Au concentration in our alloys are fairly small and the slopes of our resistiv- ity curves remain similar to the pure PdMn alloy which have already been found to be ferromagnetic [I]. this leads to strongly suggest that our ternary alloys are also undergoing inhomogeneous ferromagnetic transition. upto .Y = 0.012. This ferromagnetic transition tempera- ture drops from r, = 0.805 K for x = 0 to T, = 0.315 K for x = 0.012. Further evidence for this ferromagnetic transition lies from the fact that there is a TJi2 depen- dence of Ap below 7; similar to that observed in pure dilute PdMn alloys [4].

When the Au concentration is increased beyond 1.2 at%, resistivity curves begin to show characteristically different features as seen for the 3.1 at% alloy. The occurrence of a broad resisitivity maximum around T,,,, = 0.245 K strongly indicates onset of short range

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Fig. 1. Jp arms T plots for PdAuMn alloys. The vertul arrow in the .I = 0.031 curve shows the position of the resistiv- ity maximum at T,,,,, = 0.245 K.

ordered magnetic interactions in contrast to the .Y i 0.012 Au alloys, which are found from our analyses to behave ferromagnetically. To investigate this feature. we have also measured below 1 K longitudinal magnetore- aistivity in these systems in external fields upto 25 kOe. These curves are shown in fig. 2. (LIP),, = Ap(T. H) - i\p( T.0) denotes the change in the magnetic resistivity with and without the external field H. (LIP)" is there- fore a measure of the spin alignment factor and would reflect the nature of magnetic interactions between the magnetic ions. For s < 0.012. where the magnetic spins are coupled in a long ranged manner through the media- tion of the Pd 4d band, (dp),, changes by about 2%’ and found negative as to be expected in magnetically ordered systems. However, this change is only about 0.5 at%* for .Y = 0.031 alloy which is again suggestive of the fact that there could be a departure from long range ferromagnetic ordering. There is no saturation in (Ap),, at the highest field studied for x = 0.031 alloy (though there appears to show tendency of saturation for the curves for x < 0.031) which seems to suggest that there

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Fig. 2. (Ap),, versus H plots at T i 0.5 K for PdAuMn alloy\. For the sake of comparison, the plots are taken at temperatures T. which IS about 0.6 times the ordering temperature T or 7,,,,,, of the alloys, as showjn. (3~) ,, in negative for all the CBSCS.

is a likelihood of occurrence of short ranged magnetic clusters. These clusters would inhibit easy alignment of the spins which could account for the small change in (Jp),, observed by us and also by other authors [5]. We speculate that if such short ranged magnetic clusters are found for the s = 0.031 alloy they may eventually give rise to spin glass type freezing at temperatures below T,,,,. We also note that the occurrence of resistivity maximum is one of the distinctive features of metallic spin glasses observed earlier in many canonical systems [6]. However. on the basis of our resisitivity studies alone. it is not possible to fully confirm the occurrence of this spin glass state. Magnetic susceptibility and neutron studies on these alloy systems are necessary to arrive at a definite picture regarding the complex mag- netic order observed.

[l] G.J. Nieuwenhuys, Adv. Phys. 24 (1975) 515. [2] R.N. Kleiman and G. Williams. J. Phq. F12 (1982) 169. [3] B.R. C‘oles. H. Jamieson, R.H. Taylor and A. Tari. .I. Phys.

FS (1975) 565. [4] G. Williams and J.W. Loram. Solid Starr C‘ommun. 7

(1969) 1261. [5] S. Senoussi. A. Hamzw and I.A. Campbell. .I. Phys. f-10

(1980) 1223. [6] P.J. Ford and 5.4. Mydosh. Phy,. Rr\. 814 (1976) 2057