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Submitted on 1 Jan 1988
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HALL EFFECT OF SOME TRANSITION METAL
PNICTIDES
M. Fieber, H. Dunkel, J.-W. Schünemann, K. Bärner
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, SupplGment au no 12, Tome 49, decembre 1988
HALL EFFECT OF SOME TRANSITION METAL PNICTIDES
M. Fieber, H. Dunkel, J.-W. Schiinemann and K. Bkner
4. Phys. Institut d e r Universitat Gottingen, F B Physik, 0-3400 Gottingen, F.R.G.
Abstract. - Hall effect and magnetization data of two mixed magnetic systems are correlated for magnetic coupling due to free carriers.
Peculiar magnetic order-order transitions in mixed crystals like (Fel-,Mn,)zP and MnAsl-,P, have at- tracted attention 11, 21 and have been connected with competing magnetic couplings of different quality - for example magnetic couplings due t o bound and free d
-
type carriers-
131. From the equivalence of these systems t o FezP and MnAs under pressure one can conclude that these phenomena are not due to pecu- liarities of the anion or cation mixture but rather a consequence of the electronic configuration which is likely close t o a decomposition 3d-
state (pd and/or sd-
hybridization).As a relation between magnetic coupling strength and carrier concentration has been proposed for at least one specific combination of couplings (double ex- change/superexchange) [4], the search for an empirical correlation between carrier concentration and order- ing temperature is indicated [I, 21. Since then it has been shown that the compounds in question show a two band conduction with n
=
p, so that from Hall data (and resistivity) only weighted carrier concentra- tions nee = n (1+
b)/
(1-
b) are obtained [I] and that only the mobility ratio b = ,un/,up passes 1 (,un = ,upat T = To). For magnetically equivalent states the m e bility ratio should be weakly temperature dependent and in that case nee and Tc should show a correlation, too.
Tc and To do not necessarily coincide: the mobili- ties apparently change both a t magnetic order-disorder and magnetic order-order transition temperatures, but also ,un (T) and pp (T) might be different within a given magnetic phase [2, 51. Figure 1 shows schemat- ically three cases of b (T) and the resulting effective carrier concentrations nea (T) ; here, possible gradual changes in b are sharpened to a step function.
Since in both mixed crystal systems negative signs are only found within the ferromagnetic phases, we correlate n,a with Tc (fm order-disorder temperature) or TI (antiferromapetic-fm order-order temperature) and, for completeness, pe5 with TN (afm order-disorder temperature) (Fig. 2). For both systems, the same correlation is found for neR and TI
(T,)
but none forp d and TN. The latter suggests, that the hole con- centration does not affect the negative coupling at all
Fig. 1.
-
Effective electron and hole concentrations derived from three (Aj B, C) possible temperature dependences of the mobility ratio b = ~ , / j + ; p e s = p(1+ b*)/
(1-
b*) ;n,a = n (1
+
b)/
(1-
b) ; b* = l/b; Tc, TI fm order- disorder, afm-fm order-order transition temperature; To :Pn = Pp. . /' I I I I I 1 1 I
I
0 1 "eff - " 0Fig. 2.
-
Reduced transition temperatures versus re-duced effective electron and hole concentrations; data from (Fel-, Mn,)z P and MnAsl-,P, - mixed crystals. Tc, Ti fm-pm and afm-fm transition temperatures; (0); TN afm- pm transition temperature ( a ) ; po = no = 5 x
loz1
for (Fel-,Mn,)2 P; To = 100 K; ( o 0); po = no = 1.25 xloz0
for MnAsl-,P,; To = 140 K(a.).
C8
-
150 JOURNAL DE PHYSIQUEand that
TN
is probably determined by superexchange couplings via lower 3d-
states [6]. The former corre- lation suggests thatTI
(T,) is indeed a function of the electron concentration. If one uses reduced units, one can plot data from both systems in the same graph: this is justified by different spin moments S and trans- fer integrals t in either alloy system-
the superex- chmge/double exchange model, for example, gives a critical concentration of n, = 2.5 .Ts2t-' (J superex- change integral) [4] for the crossover from fm-pm to pm-afm-fm transitions. Further experiments, in par-ticular time resolved thermoelectric power in these and related mixed magnetic systems, are planned in or-
der to determine n and p simultaneously and indepen- dently.
[I] Fieber, M., Heinemann, K. and Bkner, K., J.
Magn. Magn. Muter. 73 (1988) 345.
[2] Krokoszinski, H. J., J. Phys.
C
17 (1984) 6829.[3] Bkner, K., J. Magn. Magn. Mater. 70 (1987) 268.
[4] de Gennes, P. G., Phys. Rev. 118 (1960) 141.
