Crystal structure and magnetic properties of some ...

Cryst. Res. Technol.

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25

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1990

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2

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165-169

V. M. RYZHKOVSKII, N. D. ZHIGADLO, I. L. PASHKOVSKII Institute of Physics of Solids and Semiconductors, Byelorussian Academy of Sciences, Minsk, USSR

Crystal Structure and Magnetic Properties of Some Mn2Sb-based Alloys

The results of the investigation of structural and magnetic properties of the Mn,(Ni)Sb, Mn,(Cr,Cu)Sb, Mn,(Ti,Zn)Sb alloys are reported. The solubility limits of the elements substituting Mn in the Mn,Sb matrix, keeping the crystal structure of the Cu2Sb(C38)-type,are established and the lattice parameters, temperatures of magnetic phase transitions, specific magnetization of the materials are determined. The influence of thermal treatment and thermal cycling on the magnetic properties of the alloys, which is interpreted by the quantitative redistribution, depending on temperature, of the amount of the main 6-phase (C38) in the samples, with the phase composition changing, and that of an additional nickel-arsenide &-phasehas been found.

1. Introduction Characteristic of the tetragonal Cu,Sb-type crystal structure (C38, space group P4/nmm), wherein an Mn,Sb compound crystallizes, is the presence of two structural nonequivalent positions (I and 11) of the metal atoms, tetrahedrally and octahedrally surrounded by Sb atoms, respectively. The unit cell contains 6 atoms with the coordinates: I - (0, 0, 0), (1/2, 1/2,0); I1 - (0, 1/2, Zl), (1/2,0, ZJ; Sb - (1/2, 0, Zz),(0, 1/2, Zz).For Mn,Sb Z , = 0.29; 2, = 0.28 (ALPERIN). As seen from Figure 1 the Mn atoms of each kind and those of Sb form layers parallel to the crystal lattice basal plane, with the distance between the layers of the MnII and Sb atoms being only 0.06 bi, i.e. they, practically, form one layer. The layers sequence and the distances between them are such that the structure can be considered as a system of three layered complexes perpendicular to the four-fold symmetry axis: MnII(Sb)-MnI-MnII(Sb) MnII(Sb)-MnI-MnII(Sb) etc. The layered character of the Cu,Sb-type structure is confirmed by the estimates of interatomic interactions reported by VITKINAet al. The overlapping integrals of atomic wave functions, which characterize the bond strength, are maximum for the metallic bonds 1-1 in the layer and 1-11between the layers inside the three-layered complex and are much smaller in value for the bonds 11-11in the neighbouring three-layered complexes. 12 Cryst. Res. Technol., Vol. 25, No. 2

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RYZHKOVSKII et

a!.: Structure and Magnetic Properties of Mn,Sb-based Alloys

6

F

AF

Fig. 1. Crystal and magnetic structure of Mn,Sb

The layered structure of the Mn,Sb compound manifests itself by its magnetic properties. The magnetically active Mn(1) and Mn(I1) atoms are in various electronic states possessing ma wetic moments different in value. The magnetic structure is collinear with ferromagnetic ordering of magnetic moments in the layer and antiferromagnetic between the layers I and 11. Since the magnetic moments of the atoms 1 and I1 are different the three layered complex possesses a resulting magnetic moment. As shown by the estimates of the exchange interaction forces the value of the exchange interactions inside the complex is 3 -4 times higher than that of the exchange interactions between the complexes. The mutual parallel or antiparallel ordering of the resulting moments of the complexes leads to the formation of a ferrimagnetic of antiferromagnetic structure in the material respectively. Mn,Sb is a ferrimagnetic in the whole temperature range of magnetic ordering. However, in the Mn,Sb-based solid solutions the substitution of the Mn atoms by a number of elements (Cr, Cu, Zn, Co, V) brings about a magnetic phase transition from the ferrimagnetic state to the antiferromagnetic one (F-AF), which is characterized by specific features of the 1-st order phase transition (drastic change of the lattice parameters, hysteresis phenomena etc.). It should be noted that in the layered structure of the Cu,Sb-type the exchange interactions of the magnetically active atoms are strongly dependent on the structure parameters (interatomic distances and bond angles), which to a great extent accounts for the variety of the magnetic properties of this class of the materials (Cu,Sb - diamagnetic, Mn,Sb ferrimagnetic, Mn,As, Fe,As, Cr,As - antiferromagnetics with different magnetic structures) as well as for new features revealed in the solid solutions. This paper reports the results of the investigation of the influence of the substitution of the Mn atoms by the Ni atoms in the Mn,Sb matrix and substitution by atoms of two elements (Cr and Cu, Zn and Ti) simultaneously. 2. Experimental technique. Results The specimens were prepared by direct melting of the components placed in evacuated (to lo-' Pa) quartz ampoules on heating up to 1000 "C with subsequent thermal treatment (homogenizing annealing at 700 "C, slow cooling or quenching in cold water). The X-ray analysis was performed on a DRON-3 apparatus using CUKEradiation. The magnetic measurements were carried out by the Faraday method in dc magnetic field intensity of 5 kOe.

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Cryst. Res. Techno]. 25 (1990) 2

The Curie temperature was determined from the M Z ( 7 )dependence, the temperature of the phase are the temperature of the onset and transition F-AF defined as (z,, + T,,)/2, where T,, and completion of the transition, respectively. The results obtained are summarized in the Table.

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Table Composition

r

lattice parameters, A (300 K)

(300K) emu/g

TF-AF K

T, K

Note

C

a

6.55 6.49 6.41 6.34 6.30

4.08 4.10 4.11 4.12 4.125

30.0 28.0 24.0 16.0 4.0

-

550 520 480 430 375

Ni-containing samples quenched from 700°C

Mn1.9Cro.05C~o.osSb 6-52 M n ~ , ~ ~ C r o , ~ ~ C u ~6.516 ,~Sb Mnl,,Cro,o~Cuo,,sSb 6.518 Mn1.8Cr0.1C~0.1Sb 6.505

4.072 4.073 4.075 4.075

27.6 23.2 21.7 20.2

245 250 260 270

515 540 535 540

slowly cooled samples

Mnl.8Zno.,Tio.lSb Mn 1. ,Zno.*Tio.1 Sb

4.06 4.10

21.0 15.6

-

105

535 525

slowly cooled samples

Mn,Sb Mnl.9SNio.OsSb Mnl .88Ni0.1ZSb

Mnl.78Ni0.Z2Sb Mn1.73Ni0,27Sb

6.54 6.48

3. Discussion As seen from the Table the increase in the Ni concentration in forming the Mn,(Ni)Sb solid solutions is accompanied for the quenched sample by the strong contraction of the lattice along the tetragonal c axis with less appreciable broadening along the a axis. The temperature of magnetic disordering which is the measure of the resulting exchange interaction in the material, and the specific magnetization of the M alloys in this case decrease. It seems that the decrease in the value of the atomic magnetic moments in the alloys with increasing concentration of Ni as determined earlier for Cr (CLOUDet al.), is primarily responsible for such behaviour of T, and M . Following KITTEL’S theory the exchange-inversion phase transition F-AF, characteristic of a series of the Mn,Sb-based solid solutions, is connected with the critical value of the lattice parameter ccr.at which the resulting exchange interaction between three layered complexes changes the sign and which is achieved through the thermal or chemical contraction of the lattice. In this case it is considered that the chemical element substituting the atoms in the starting matrix does not play an essential role. However, in the Mn,(Ni)Sb alloys the magnetic phase transition F-AF does not occur, though the c lattice parameter values are sufficient, according to KITTEL’Stheory, for changing the sign of the resulting exchange interaction in such a structure. This implies that in the mechanism of the phase transition under investigation it is essential to know the atoms of which chemical element take part in the substitution while forming the solid solutions. Furthermore it is evident that the comprehensive account of the change of the interatomic distances and angles of indirect exchange bonds is needed for the description of the F-AF transition.

z,

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RYZHKOVSKh

et al.: Structure and Magnetic Properties of Mn,Sb-based Alloys

In the Mn,-x-yCrxCuySb alloys the limits of solubility over the sum of two elements + y 5 0.2) are much lower than those in the case of substitution by one element. The lattice parameters vary insignificantly along the axis c with the concentration of the substituting elements increasing from 6.52 to 6.50 A. However, in all the cases the magnetic phase transition F-AF takes place. It could be expected that the use of double substitution would enable one to vary the temperature of the F-AF transitjon over a wide temperature range, since in the Mn,(Cr)Sb and Mn,(Cu)Sb alloys this transition occurs in different ranges (DARNELL et al.; RYZHKOVSKII et al.). It appeared that Cr plays the key role in the change of magnetic properties of the Mn,(Cr, Cu)Sb alloys. This can be accounted for by the fact that the Cu atoms do not possess magnetic moments and therefore act as “diluent” in the Mn,Sb magnetic matrix which at small concentrations reduces their influence to minimum as compared with the Cr magnetically active atoms. While substituting the Mn atoms in the Mn,Sb matrix by the Ti and Zn atoms the solubility limits in this case are much wider, which is connected with greater solubility of Zn in Mn,Sb (JOHNSON, JEITSCHKO). It is known (KANOMATA, JDO) that the Mn,(Ti)Sb solid solution formation is accompanied by an increase in the c lattice parameter and that the magnetic phase transition F-AF has not been observed. The increase of the Zn concentration in the Mn,(Ti, Zn)Sb alloys creates the structure chemical conditions for realizing this transition. It should be noted that the properties of the materials investigated are very sensitive to thermal treatments. In particular, the quenched and slowly cooled samples differ in temperatures of magnetic transitions, values of magnetization and its temperature dependence. This is especially specific to the Ni- and Cu-containing samples. Figure 2 illustrates the temperature dependences of specific magnetization for the Mn,,,,Cr,,,,Cu,,,Sb composition in different states with subsequent thermal cycling. As evident from the literature data (ASKHEIM, GRONVOLD) the Mn,Sb compound (&phase) of stoichiometric composition exists only at high temperatures (900 K). At room temperature the composition of the &phase is, approximately, Mn,,,Sb and, in addition, the sample possesses a small amount of the nickel arsenide &-phase(MnSb). When temperature rises from 550 to 850 K the transformation process E --$ 6 occurs with the 6-phase composition changing to Mn,Sb. A qualitatively similar mechanism, related to the change of composition of the main 6-phase as a function of temperature and the quantitative redistribution of the 6 and E phases in the sample, can be assumed as a basis while interpreting the influence of the conditions of thermal treatment and thermal cycling on the magnetic properties of the investigated alloys. At 300 K the &phase with some metal excess - Mn(Cr, Cu),+, Sb is stable. In this case a small amount (5-7.%) of the nickel arsenide &-phaseis present as evidenced from the additional weak reflections in the X-ray diffraction patterns. The 6-phase of the stoichiometric composition obtained by quenching of the samples from 1000 K undergoes at T > 450 K the transformation 6 + E, with the composition changing towards the increase of A . The magnetization of saturation of the ferromagnetic &-phase(MnSb) being rather high (111.8 emu/@, an increase in its amount influences greatly the resulting picture of the magnetic state of the sample (curves 2 and 3). It is significant that upon subsequent prolonged exposure of the sample at room temperature the reverse E 4 6 transition with the restoration of the initial composition of the phase, stable at 300 K (curve 4), completes.

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I0 Fig. 2. Specific magnetization vs temperature for the Mn,,,,Cr,,,,Cu,,,Sb sample. 1.) 1-st cycle (after slow cooling from 700 "C);2.) 2-nd cycle (after quenching in water from T 700 "C); 3) 3-d cycle (after slow cooling from T 700 "C); 4.) 4-th cycle (after slow cooling from T 700 "C, within 22 days); Inset. Specific magnetization vs time at T = 470 K *

0