Magnetic properties of CrxMn1−xAs mixed crystals

Journal of Magnetism and Magnetic Materials 13 (1979) 119-l 20 0 North-Holland Publishing Company

MAGNETIC PROPERTIES OF Cr,Mn r _-xAs MIXED CRYSTALS

R. WijHL, H.J. KROKOSZINSKI

and K. BARNER *

IV. Physikalisches Institut der Universitiir Gdttingen, Fed. Rep. Germany

Received 22 March 1979

The magnetic moment of Cr,Mnr_x As mixed crystals has been measured for temperatures between 3 and 400 K and for ten different compositions between x = 0 and x = 1. Measurements of the electrical resistivity have been added in order to achieve an unambiguous identification of the various magnetic ordering temperatures. The combination of all the data enabled us to construct the complete magnetic phase diagram. It is found that the transition from antiferromagnetic CrAs to ferromagnetic MnAs is very complex. We propose to explain the transition as an interplay of two competing double exchange-superexchange coupling mechanisms.

densation phenomenon involving the double exchange electrons similar to the one suggested for MnAs,Pr_, compounds with 0.13 >x >O.OS [ll]. Fig. 2 shows the magnetization versus field curve for x = 0.4. At low fields we observe a metamagnetic behaviour (H < H,) which is followed by a linear approach towards saturation (H > Hr). The linear rise in these high fields is unlikely to be connected with a para-process. It

In this work an attempt has been made to observe the transition from antiferromagnetism (afm) to ferromagnetism (fm) for a metallic 3d-system. Since CrAs [l-4] and MnAs [5-71 are both metallic and miscible, but order afm and fm, we have choosen the quasibinary alloy series Cr,Mnr --x As [3,8,9] as an example. We have found an afm behaviour up to 50 at% Mn. The Neel point is rather well marked in the susceptibility versus temperature curve x(T) of our polycrystalline specimens and can be found also in the resistivity versus temperature curves p(T). For Mn-contents above 50 at% the magnetization versus temperature curves M(7’) exhibit a behaviour which is characteristic of fm double exchange (DE) which is superimposed on an afm superexchange (SE) coupling [lo]. As an example fig. 1 shows M(T) and X(7’) for the compound Cre4Mne.eAs. Three different characteristic temperatures can be distinguished; a N&e1point TN is followed by a ferromagnetic-like rise at Tg From the thermodynamics of double exchangesuperexchange systems this indicates afm order followed by a canted spin order below T3 [lo]. Curiously, at a lower temperature TKM(T) drops again; this might be connected with a low temperature con-

T3=1Xl K

\ I 0

1 XM

ml

300

l(K)

Fig. 1. Magnetic moment as a function of temperature M(T) at a magnetic induction of 0.3 T and reciprocal susceptibility versus temperature for the compound Cro.4Mno.eAS; TN N&e&type magnetic ordering temperature, T3 Curie-type magnetic ordering temperature, TK characteristic temperature where M falls below its maximum value when the temperature is lowered.

* Work supported in part by the Deutsche Forschungsgemeinschaft. 119

R. Wijhl et al. / Cr,Mn 1 _,As

mixed crystals

TCKI 0

1

2

3

‘

6

5 BIT1

Fig. 2. Magnetic moment as a function of applied field at 110 K; H, critical field of metamagnetic behaviour, HI onset of linear region.

rather indicates a continuous change of the cant angle with external field due to the competition of internal and Zeeman energies [lo]. Fig 3 shows the magnetic phase diagram as obtained by summarizing all the characteristic transition temperatures [ 121. In addition to the temperatures which have been characterized already, there appears a fourth characteristic temperature T2. For the compound x = 0.4 it is not clearly marked in x(T), but it shows distinctly in the p(T) curve (fig. 4). From fig. 3 it follows, that there is a complex transitional region between the afm behav-

Fig. 4. Electrical resistivity as a function of temperature T2 characteristic temperature connected with spin ordering (PO= 1.1 X 10e3 ncm). iour for x > 0.5 and the DE-SE behaviour with T, > T, for x < 0.32. From the similarity of the phase diagram between x = 0.5 and x = 0.32 with that of simple binary alloy series two different magnetic couplings which are partically miscible with each other suggest themselves. In an attempt to identify these couplings we have resorted to the triangular 3d3-3d4-3d5 resonant states which have been proposed for manganese pnictides from other evidence [13]. Calculations of the probabilities for the formation of infinite clusters of Mn-Mn-Mn and Mn-MnCr triangles [14] suggest that one of the couplings and with it the DE-SE behaviour for x < 0.32 is likely to be attributed to a (poe,)3 - (pne2)3 resonant state [131.

References [l] M. Yuzuri, J. Phys. Sot. Japan 15 (1960) 2007. [2] N. Kazama and H. Watanabe, J. Phys. Sot. Japan 31 (1971) 943. [3] N. Kazama and H. Watanabe, J. Phys. Sot. Japan 30 (1971) 1319. [4] H. Boller and A. KaIIel, Solid State Commun. 9 (1971) 1699. [5] C. GuiUaud, J. Phys. Radium 12 (1951) 492. [6] G. Fischer and W. Pearson, Can. J. Phys. 36 (1958) 1010. [7] K. BIrner, Phys. Stat. Sol. (b) 84 (1977) 385. [81 H. Ido, J. Phys. Sot. Japan 27 (1969) 318. C-X

Cr,Mn,_,As Fig. 3. Magnetic phase diagram for the quasibinary alloys Cr,Mnl_,As; T, Curie-type magnetic ordering temperatures T2 characteristic temperature as deduced from the resistivity and magnetization curves; canted spin ordering as drawn is tentative, T,, first order B81 = B31 transition temperature.

[9] K. Selte, A. Kjekshus and A. Zieba, J. Phys. Chem. ‘Solids 38 (1977) 719. [lo] De Gennes, Phys. Rev. 118 (1960) 141. [ll] H. Berg and K. BHrner, J. Magn. Magn. Mat. 4 (1977) 69. [12] R. Wijhl, Diplomarbeit, Gljttingen (1978). [ 131 K. BHrner, Phys. Stat. Sol. (b) 88 (1978) 13. [14] L. Turban, Private communication (1979).