MAGNETIC AND STRUCTURAL PHASE TRANSITIONS OF cY-MnSe AND Mn, _xMg,Se

(0 <x < 0.15)

H. van der HEIDE, J. P. SANCHEZ* and C. F. van BRUGGEN

Laboratory of Inorganic Chemistty, Materials Science Center, Universi@ of Groningen, Nijenborgh 16, 9747 AG Groningen, The

Netherlands

Magnetic susceptibility and X-ray diffraction of polycrystalline a-M&e and Mn, _xMg,Se (0 < x < 0.15) are reported. The

experimental data reveal large thermal hysteresis and partial transformation of the room temperature cubic phase (NaCl) to a

low temperature hexagonal phase (NiAs).

a-MOnSe with rocksalt structure (a (295 K) = 5.464 A) is known to order at low temperature as type-II antiferromagnet like MnO [l]. The reported Ntel temperatures, however, range over values

from 140 to 250 K because of thermal hysteresis and sample history effects [2]. Some light upon this unusual behaviour has been let in by Jacobson and Fender from neutron diffraction studies [3]. These authors conclude to the existence of a low tempera-

ture NiAs phase and the possibility to suppress this phase by substituting magnesium (10%) for manganese. To clarify these features magnetic sus- ceptibility and X-ray diffraction measurements of the solid solution series Mn,_,Mg,Se were per- formed as a function of temperature.

The molar susceptibility (x) of Lu-MnSe after successive first cooling and warming runs between 300 and 4.2 K is given in fig. la. This cycle shows a large thermal hysteresis and sharp anomalies. The temperature (TM) at which x peaks in the cooling process is about 160 K. The strong discontinuous change of x cannot originate from a simple AF ordering. Indeed, X-ray diffraction measurements reveal that the sharp transition at 160 K is concom- itant with a partial transformation of the original cubic phase (NaCl) to a hexagonal (NiAs) type phase. This new phase is expected to have a lower susceptibility than the cubic phase, and to behave like MnTe [4]. The lattice parameters of the low temperature NiAs phase (a = 3.827 A, c = 6.342 A at 80 K) are in close agreement with those reported previously [3]. The equilibrium between the two phases is reached only after a period of a few hours. Besides, the hexagonal phase induced some stacking faults in the cubic phase as suggested by a considerable broadening of the original cubic re-

*Permanent address: Laboratoire de Chimie Nucldaire, Centre

de Recherches Nucleaires, 67037 Strasbourg Cedex, France.

flections. By warming (fig. la) a broad maximum at 150 K and a strong increase of x at about 240 K

are observed. Last transition is accompanied by the disappearance of the NiAs phase which reverts to the original NaCl structure as evidenced by X-ray analysis; the broad maximum at 150 K is ascribed to the magnetic transition of the remaining cubic

phase. When the susceptibility measurements are re-

peated on the same cr-MnSe sample after a few temperature cycles an expansion of the hysteresis loop is observed (fig. lb). Furthermore the peak values of x at the sharp transition temperatures are

I 0 1cO 200

TIK) 0

Fig. 1. Magnetic susceptibility of a-MnSe as a function of temperature with an applied field of 8.61 kOe; (a) 1st tempera-

ture cycle, (b) 5th temperature cycle.

JournaI of Magnetism and Magnetic Materials 15- 18 (1980) 1157- 1158 BNorth Holland 1157

1158 H. oan der Heide ei al./Magnetic and structural data of Mn, _,Mg,Se

KC 2co TIKI

/

I IOC

Fig. 2. Magnetic susceptibility of Mn,_,Mg,Se as a function

of temperature with an applied field of 8.61 kOe (first cycle

measurements); (a) a-MnSe, (b) x = 0.05, (c) x = 0.10, (d)

x = 0.15.

enhanced and the broad maximum observed in the first cycle is smeared out. These observations can be understood, at least partly, by a perturbation of the exchange or superexchange interactions via in- creasing lattice imperfections with increasing amount of NiAs phase. This explanation is sup- ported by X-ray measurements which show that temperature cycling increases both the amount of hexagonal phase present at low temperature and the line width of the cubic and hexagonal reflec- tions.

The solid solution series Mn, _*Mg,Se (0 < x < 0.15) crystallize in the rocksalt structure with

about the same lattice parameters at room temper- ature as cy-MnSe. Both the low temperature struc-

tural and magnetic data are strongly influenced by substituting diamagnetic Mg for Mn. X-ray diffrac-

tion reveals that an increase of the Mg content causes a decrease of the NiAs phase appearing at the lower transition temperature. This phase is no longer detected in the x = 0.15 sample. There is a correlation with the susceptibility data (fig. 2)

which show a concomitant shift towards lower transition temperatures and a gradual shrinkage of the hysteresis loop. The gradual change in descent

of the cooling curve (below r,) with increasing dilution also reflects the increasing number of clusters of exchange-coupled Mn ions [5]. The shift of TM to lower temperatures with x is consistent with the expected dilution effect on a magnetic transition. However, the experimental variation of TM as a function of x does not follow the simple molecular field prediction, i.e. a linear relation between TN and x. Furthermore 7’, decreases more rapidly with x as observed for instance in MI1 , _,Mg,S solid solution series [5].

At this time the question remains how the ap- pearance of the low temperature NiAs phase is correlated with the magnetic ordering of the cubic phase. Answers to this question need other experi- mental investigations e.g. hyperfine interaction technics such as 57Fe Mossbauer spectroscopy of diluted iron in a-MnSe.

References

[l] C. G. Shull, W. A. Strauser and E. 0. Wollan, Phys. Rev. 83

(1951) 333.

[2] T. Ito, K. Ito and M. Oka, Japan J. Appl. Phys. 17 (1978) 371.

[3] A. J. Jacobson and B. E. F. Fender, J. Chem. Phys. 52

(1970) 4563. [4] E. Uchida, H. Kondoh and N. Fukuoka, J. Phys. Sot. Japan

11 (1956) 27.

[5] H. H. Heikens, R. S. Kuindersma, C. F. van Bruggen and C. Haas, J. Magn. Magn. Mat. 8 (1978) 130.