Journal of Magnetism and Magnetic Materials 104-107 (1992) 925-926

North-Holland

Neutron diffraction studies of the ferrimagnetic-antiferromagnetic phase transition in cobalt modified Mn,Sb

M. Ohashi a, Y. Yamaguchi a, and T. Kanomata b

a Institute for Materials Research, Tohoku University, Katahira, Sendai 980, Japan b Department of Applied Physics, Faculty of Engineering, Tohoku Gakuin University, Tagajo, Miyagi 985, Japan

Neutron diffraction has been performed on a powder sample of Mn,,,Co,,,Sb which has the transition temperatures

T, = 170 K and T, = 400 K. Co atoms randomly occupy the Mn(I) sites. The Mn,Sb-type ferrimagnetic structure is transformed into the Mn,As-type antiferromagnetic ordering at T, = 170 K, where the sign of the moment coupling between

the nearest neighbors of the M&I) site is reversed and the moment of Mn(II) in the antiferromagnetic state is estimated to

be larger than that in the ferrimagnetic state.

Manganese antimonide is a ferrimagnet with a NCel temperature T, = 550 K. The crystal structure of Mn,Sb is tetragonal (space group P4/nmm) and con- tains Mn atoms at two nonequivalent sites, as shown in fig. 1, Mn(I) atoms occupy the 2a position (0 0 0) and (l/2 l/2 0) while Mn(I1) atoms whose spins are oppo- sitely directed occupy the 2c positions (0 l/2 U) and (l/2 0 U). Sb atoms are also located at the 2c posi- tions but with V parameter. The 3d electrons of Mn(1) site have been inferred to be more metallic in charac- ter than those of Mn(II), because of the small moment of Mn(1) compared to that of Mn(II). One of the authors (T.K.) [l] has reported that the first order magnetic phase transition from a ferrimagnetic state (Fr) to an antiferromagnetic state (AF) with decreasing temperature can be produced by Co substitution for

0 Mn, (site1 )

0 Mq (siten)

@I Sb

Fr

MnzAs-type Mn$b- 1

Fig. 1. Crystal structure and magnetic moment arrangement for the Mn,Sb-type and Mn,As-type structure.

L

OO 0.1 0.2 0.3 0.b x ln Mng_xCoxSb

Fig. 2. Magnetic phase diagram of Mn,_,Co,Sb.

Mn atoms in Mn,Sb. As shown in fig. 2, the solubility limit is x = 0.35 in Mn ,_,Co,Sb. The ferrimagnetic phase of Mn,_,Co,Sb seems to be more stable than that of Cr-modified Mn,Sb which is a typical material originated in Kittel’s exchange inversion model [2].

Neutron diffraction experiments of Mn,,,Co,,,Sb were carried out to investigate the site preference of the Co atoms, the magnetic structure and moment at each site.

The specimen was prepared by the usual solid reac- tion method, and then the product was quenched into ice water after annealing at 800°C for 5 days. X-ray diffraction test shows that the powder specimen is in single phase with the tetragonal structure of the C$,Sb type, and&gives the lattice parameters a = 4.087 A and c = 6.44 A at room temperature in good agreement with the previous results. The NCel temperature TN = 400 K and the ferrimagnetic to antiferromagnetic tran- sition temperature T, = 170 K are determined by mag- netic measurements.

Neutron diffraction experiments were performed at temperatures of 78, 293 and 444 K, by using the KID diffractometer installed at JRR-2 reactor in JAERI. The incident beam used was 1.00 A obtained by the pyrolitic graphite monochrometer. Nuclear reflections in the paramagnetic state, which is shown in fig. 3(a), favor to have Co atoms substituted randomly for the

0312.8853/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved

Y2h M. Ohashi et al. / Ferri~antiferromafinetir transition in Co modified Mn,Sh

Mn(I) site. The position parameters of the Mn(I1) and Sb atom site are determined to be lJ = 0.300 and l’= 0.277 with a relia)ility R = 4.0%. The lattice con- stants are a = 4.064 A and c = 6.377 A at 444 K. As shown in fig. 3(b), the magnetic reflections in the fcrrimagnctic state are superimposed on the nuclear reflections, and give the same ferrimagnetic structure as that of MnzSb illustrated in fig. 1. The magnetic moments of the Mn(1) site and the Mn(I1) site are pLMn(,) = 0.98~~ and pMn(,,) = 1.86~~ at 293 K, respec- tively. On cooling through the phase transition temper- ature T, = 170 K, the additional magnetic reflections appear at scattering angles with the index (h, k, L/21 of odd l’s while the ferromagnetic reflections superim- posed on the nuclear one fade away simultaneously. Fig. 3(c) shows the diffraction pattern in this antiferro- magnetic state. A magnetic unit cell requires the dou- bling of the chemical cell dimension along the c axis.

SCATTERING ANGLE (deg )

Fig. 3. Neutron diffraction pattern of Mn, ,Co,,,Sb for (a)

paramagnetic state, (b) ferrimagnetic state and (c) antiferro- magnetic state.

T(K)

Fig. 4. Temperature dependence of magnetic moments with

the solid line from the Brillouin function.

The magnetic pattern at 78 K is explained by the same antiferromagnetic structure as that of MnZAs, which has the opposite moment coupling between the nearest neighbors of Mn(I1) site, as illustrated in fig. 1. The magnetic form factor of Mn2’ calculated by Watson and Freeman [3] is used for both Mn sites. The mag- netic moment of Mn(1) site is an arithmetical average for the Co and Mn atom. The magnetic moment of each site at 78 K is pLMnoj = 1.37~~ for the Mn(1) site and pLMnon = 3.47~, for the Mn(II) site, respectively, and lies in the c plane as in the ferrimagnetic phase.

The magnetic moment of the Mn(I1) site at 78 K is distinguishably larger than that at 293 K. As shown in fig. 4 where the observed magnetic moments are as- sumed to be fitted to the Brillouin function, the abrupt change in the moment of Mn(I1) should take place at T, where the c-axis contraction was observed by Kanomata and Ido [ 11. On the other hand, the moment of the Mn(1) site is described well by a Brillouin function for both the ferrimagnetic state as well as the antiferromagnetic state. It should be noted that the first order phase transition of M~,,,CO,,~S~ seems to originate not only from the exchange inversion but also from an enlargement of the moment of Mn(I1) with decreasing temperature.

The authors wish to thank Mr. K. Nemoto for assistance in neutron diffraction experiments.

References

[l] T. Kanomata and H. Ido, J. Appl. Phys. 55 (1984) 2039.

[2] C. Kittel, Phys. Rev. 120 (1960) 335.

[3] R.E. Watson and A.J. Freeman, Acta Cryst. 14 (1961) 27.