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CRITICAL RESISTIVITY AND LOW FIELD
MAGNETORESISTANCE IN Pd2MnSn
P. Pureur, G. Fraga, J. Kunzler, W. Schreiner, D. Brandão, C. Hurd
To cite this version:
JOURNAL DE PHYSIQUE
Colloque CS, Supplkment au no 12, Tome 49, dkembre 1988
CRITICAL RESISTIVITY AND LOW FIELD MAGNETORESISTANCE IN Pd2MnSn
P. Pureur (I), G. L. Fraga ( I ) , J. V. Kunzler('1,
W. H. Schreiner (I), D. E. Brand% (I)and C. M. Hurd (2)
(I) Institute de Fisica
-
UFRGS, Campus do Vale, POB 15051, 91500 Porto Alegre, RS, Brasil(2) Solid State Chemistry, National Research Council of Canada, Ottawa KlAOR9, Canada
Abstract. - We report on the temperature derivative of the resistivity of the Heuder compound PdaMnSn near the para-ferromagnetic transition. For T
>
Tc a critical exponent a = 0.00 f 0.05 is found. Below Tc, we observe a complex behaviour which depends on the quality of the samples and can be suppressed by small magnetic fields.The Heusler compound PdzMnSn is often viewed as representative of an ideal local moment Heisenberg ferromagnet [I, 21. This system orders in the L21 cubic structure [3]. The magnetism is localized on the Mn ions which are well separated from each other and the magnetic coupling is indirect via the conduction elec- trons. As the Mn ions are expected to possess practi- cally zero orbital moment, the compound should be a pure localized spin ferromagnet. The PdzMnSn system is thus a good candidate for critical properties investi- gations a t the para-ferromagnet transition, in order to test the theoretical predictions for model Heisenberg systems. However, none of its critical point exponents was known so far. In this communication, we report on the temperature derivative of the resistivity, dp
/ dT,
in two different samples of PdaMnSn in the vicinity of T,. It is known both experimentally [4] and theoreti- cally [5] that dp/
d T close t o Tc has the same critical divergence as the magnetic specific heat.One of the samples measured by us (sample I) has a residual resistivity of 1.17 pSl.cm, which is exception- ally low for an intermetallic compound. Details in the preparation method as well as its electrical and mag- netic properties are reported by Hurd et a[. [6]. The other sample has a residual resistivity of 9.52 pSl.cm (sample 11). Its preparation follows the prescription given by Schreiner et al. [7]. X-ray analysis on sam- ple I1 shows the L21 superlattice lines with the ex- pected intensity. The accuracy for measuring both re- sistivity and temperature is high enough to numeri- cally determine the temperature derivative of the re- sistivity, dp/ dT. Measurements could also be done in the presence of small magnetic fields applied parallel to the current axis.
In figure l a , dp
/
d T results for sample I are shown. Aside from the curve for zero field, measurements for several fields up t o 20 G are also displayed. Because of the minimum in dp/
d T observed in temperatures just below Tc, we could only analyse the critical behaviour in the paramagnetic range. For T>
,T: the data may be fitted with the usual power law expression [5]:Fig. 1. - a) Temperature derivative of the resistivity as a function of the temperature for sample I in units of the resistivity at Tc. The measurements are performed in the quoted magnetic fields; b) the same as (a) but for sample 11.
where E = (T
-
Tc)/
T,, A and B are constants and a! is the critical point exponent of the specific heat for T>
Tc. Non-linear least square fits with equa- tion (1) allows the determination of a. The second derivative of the resistivity is also helpful for deter- mining a, as it does not involve the parameter B. In figure 2a is shown the second derivative of the resistiv- ity of sample I in zero field. As a criterion for deter- mining Tc, we choose the sharp minimum in dzP/
d~~ as shown in the figure. This agrees well with the best fitted Tc-parameter determined with equation (1). For H = 0, we obtain T, = 189.10 f 0.05 K which is in good agreement with the magnetic measurements by Webster [3] and Hurd [6]. In figure 2b we show a plot of In ld2p/
d~~1
as a function of In E . The slope inthe appropriate E range gives directly the a-exponent.
We obtain a = 0.00 f 0.05 for sample I. This value for a is remarkably independent of the field ant the fit holds for 0.005
<
E<
0.05. Rounding effects are ob-served close to Tc which is a well known fact in many systems [8, 91. A value a = 0 corresponds to a log-
C8
-
180 JOURNAL DE PHYSIQUEFig. 2. - a) Second derivative of the resistivity in the tem- perature range near Tc for sample I; b) logarithmic plot of item (a) for T
>
Tc.arithmic divergence at TT, which is a bahaviour of- ten observed in localized moment ferromagnets [8, 101.
The results for the higher residual resistivity sample I1 are shown in figure lb. Fields up to 80 G could be applied. For this sample, the rounding effects near Tc are much more pronounced, making it difficult to de- termine the asymptotic value of the a-exponent. For sample 11, we obtain T, = 187.5
f
0.1 K, which is well confirmed by AC susceptibility measurements.An interesting aspect of our dp/ d T results for both samples is the unusual regime observed just below Tc. The derivative first goes through a minimum, followed by a round maximum at lower temperatures. Below this maximum d p / d T takes a more usual trend. The complex structure below T, is rather unexpected for a system believed to represent a model Heisenberg fer- romagnet. Indeed, for most of the simple collinear and homogeneous ferromagnets, dp/ d T presents a cusp singularity, with a very small negative a-exponent which is the same above and below Tc [9, 111. Min- ima in dp/ d T just below Tc have been observed in some rare-earth ferromagnets and have been at- tributed to lattice distortions accompanying the mag- netic ordering [lo] or to mean field effects associated with the growth of the spontaneous magnetization [8].
In PdzMnSn one possible explanation for the mini- mum may come from the occurrence of regions in the sample where the Mn moments are stably coupled anti- ferromagnetically. Antiferromagnetic ordering is often known t o produce a minimum in the resistivity due to the overture of magnetic superzone gaps in the con- duction band. The minimum feature in our samples is obviously associated with structural disorder as it is more pronounced for sample 11, as shown in figure lb,
where d p / d T attains negative values. On the other hand, very low external magnetic fields gradatively suppress the minimum feature. For sample I, at 20 G, one gets a dp
/
d T curve typical of simple ferromag- nets. This would mean that in the antiferromagnetic coupled regions, in the temperature range just belowTc, the antiferro and ferromagnetic ground-states are quite close in energy. Higher field values are necessary to remove the minimum in sample 11, which indicate that disorder favors the antiferromagnetic couplings. A question that arises is how these antiferromagnetic regions may form. Shinohara et al. [12] discovered a remarkable reduction in the saturation magnetiza- tion of plastic deformed PdzMnSn. They ascribe this behaviour to shortening of the Mn-Mn distance by de- formation, favouring thus the antiferromagnetic cou- plings. X-ray and spin-echo studies [13] indicate that only a few Mn ions are out of their site positions even in cold worked PdzMnSn samples. However, in our well annealed samples, it may be possible that antifer- romagnetic regions nucleate around extended lattice defects as dislocations or antiphase boundaries, which are known to play an important role in the magnetic properties of the Heusler compound CuaMnAl [14].
[I] Hamzic', A., Asomoza, R. and Campbell, I. A., J. Phys. F 11 (1981) 1441.
[2] Noda, Y. and Ishikawa, Y., J. Phys, Soc. Jpn 40
(1976) 690.
[3] Webster, P. J. and Tebble, R. S., Philos. Mag.
16 (1967) 347.
[4] Simons, D. S. and Salamon, M. B., Phys. Rev. B
10 (1974) 4680.
[5] Richard, T. G. and Geldart, D. J. W., Phys. Rev.
Lett. 30 (1973) 290.
[6] Hurd, C. M., Shiozaki, I. and McAlister, S. P., Phys. Rev. B 26 (1982) 701.
[7] Schreiner, W. H., Pureur, P. and Brand%, D. E., Phys. Status Solidi
A
60 (1980) K123.[8] Moreira, J. M., Amado, M. M., Braga, M. E. and Soma, J. B., J. Phys. France 45 (1984) 779.
[9] K U b k k , O., Humble, S. G. and Malmstrom, G., Phys. Rev. B 24 (1981) 5214.
[lo]
Soma, J. B., Amado, M. M., Pinto, R. P., Mor- eira, J. M., Braga, M. E., Ausloos, M., Leburton, J. P., Van Hay, J. C., Clippe, P., Vigneron, J. P. and Morin, P., J. Phys. F 10 (1980) 933. [ l l ] Lederman, F. L., Salamon, M. B. and Shacklette,L. V., Phys. Rev. B 9 (1974) 2981.
[12] Shinohara, T., Sakasaki, K., Yamauchi, H., Watanabe, H., Sekizawa, H. and Okada, T., J. Phys. Soc. Jpn 50 (1981) 904.
[13] Schaf, J., Le Dang, K., Veillet, P. and Campbell, I. A., J. Phys. F 13 (1983) 1311.
