Itinerant antiferromagnetism of the Ti-Si solid solutions

218

Journal

ITINERANT ANTIFERROMAGNETISM T. PETRISOR, Cluj-Napoca Received

A. GIURGIU

OF THE Ti-Si

and Magnetic

Materials 68 (1987) 218-220 North-Holland, Amsterdam

SOLID SOLUTIONS

and I. POP

University, Physics department,

23 March

of Magnetism

Cluj-Napoca

3400, Romanta

1987

Magnetic susceptibility measurements of Ti-Si solid solutions are reported. Silicon by alloying acts on the NCel temperature of pure titanium. The NCel temperature decreases monotonously with increasing silicon concentration. The experimental and theoretical magnetic phase diagrams are in good agreement. This means that the antiferromagnetism of the Ti-Si solid solutions can be interpreted in terms of itinerant electron antiferromagnetism of the SDS type magnetic structure.

1. Introduction

2. Experimental

Titanium has two allotropic modifications [l], a-titanium up to 1155 K, which crystallizes into a hexagonal close-packed lattice and p-titanium at higher temperature, which crystallizes into a body-centered cubic lattice. The experimental results on a-titanium [2] indicate the existence of an anomaly in the temperature dependence of the magnetic susceptibility. The temperature dependence of the specific heat indicates that the observed anomaly is a secondorder phase transition. The similarity between magnetic and thermal properties of a-titanium and that of pure chromium suggests that OLtitanium is an electron itinerant antiferromagnet as chromium, and has a NCel temperature T,,, = (276 + 4) K. In this paper we have investigated the effect of Si nonmagnetic impurities on a-titanium. It is well known [3,4] that the nonmagnetic impurities have a drastic influence on the itinerant antiferromagnetic systems. This influence is generally explained by a depairing mechanism introduced in the ordered phase by the impurities and also by their effect on the density of states and on the Fermi level [5,6].

The Ti-Si solid solutions were prepared from 99.9% Ti and Si. The samples were prepared in an argon-arc furnace on a water cooled cupper hearth. The arc melted buttons were turned and remelted more than four times to ensure the reaction between the components. In order to prevent the oxidation of the samples, the furnace chamber was evacuated and purged with pure argon several times before melting. The magnetic susceptibility measurements were carried out from 100 to 900 K with a Weiss-Forrer type equipment. The results obtained are plotted in fig. 1, where the temperature dependence of the magnetic susceptibility for the investigated Ti-Si solid solutions is given, in comparison with that of pure titanium used. As one can see, the thermal variation of the magnetic susceptibility of the Ti-Si solid solutions is very similar to that of pure titanium metal. This indicates that the Ti-Si solid solutions are itinerant electron antiferromagnets just like pure a-titanium metal. So we consider that the anomaly in the temperature dependence of the magnetic susceptibility for Ti-Si solid solutions corresponds to the NCel temperature. In order to point out the NCel temperature and x0 as concentration function, the plot x versus T (fig. 1) is not relevant enough. A more suitable

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T. Petriqor et al. / Itinerant AF of the Ti - Si solid solutions

I 200

100

Fig. 1. Temperature

I

I

I

I

I

I

300

400

500

600

700

800

dependence

of the magnetic

plot for this reason is the plot (x-BT*) versus T [7]. The concentration dependence of TN and x0 determined from this plot are shown in figs. 2 and 3. One can see from these figures that TN and x0 are continuous concentration functions and for

t-2, 0.1

02

a3

0.4

C at%

susceptibility

for Ti-Si

solid solutions.

c = 0 their values coincide with that of pure titanium. This fact indicates, once again, that the silicon impurities do not change the magnetic behaviour of the Ti-Si solid solutions in comparison with that of pure titanium, but influence only

I

Si -

Fig. 2. Magnetic phase diagram for Ti-Si solid solutions. The full line represents a fit with relation (4), where the fitting constants are: a = 0.0984; p = - 0.0027; y = - 0.654.

T[Kl--

0.2 Fig. 3. Concentration

I

I

0.3

0.4

dependence tions.

C at%

Si

-

of x0 for Ti-Si

solid solu-

220

T. Petrifor et al. / Itinerant AF of the Ti- Si solid solutions

the x0 and TN values. As seen from figs. 2 and 3 the concentration dependences of x0 and TN are different. The temperature independent part of the magnetic susceptibility, x0, is proportional with the density of states at the Fermi level. In a rigid band model the modification of the density of states is caused by the shift of the Fermi level. Because TN and x0 concentration dependence have different shapes we might conclude that the effect of Si on the density of states and on the Fermi level is not so important, in order to explain the influence of silicon as a nonmagnetic impurity on the itinerant antiferromagnetism of a-titanium. The quasi-linear decreasing of TN with increasing silicon concentration suggests that the main mechanism through which the silicon impurity acts on the itinerant antiferromagnetism of cu-titanium is a depairing mechanism.

3. Conclusion and discussion The effect of nonmagnetic impurities on the NCel temperature of the itinerant electron antiferromagnet is investigated in refs. [5,6,8] using a Feders-Martin model, described by the following Hamiltonian z= .X?((+X&,,

(1)

the density states; A is a constant depending on the difference between the values of the impurity atoms and the atoms of the host; 19(x) is the step function and p(ei) is the density of states at Fermi level for pure chromium, on get for the concentration dependence of TN the following formula TN = TN,( 1 - CYC f ,!3c’/( y + c’)).

(4)

In fig. 2, the full line is obtained using formula (4), where the fitting constants are: (Y= 0.0984; /3 = -0.0027; y = -0.654. As seen from fig. 2, the experimental results are in good agreement with the theoretical relation (4). These facts confirm once again that a-titanium is an itinerant antiferromagnet with a spin density wave magnetic structure like metallic chromium. Finally, we would like to conclude the following. Ti-Si solid solutions are itinerant electron antiferromagnets just like titanium. The silicon impurities acts on the Nkel temperature of pure titanium. TN decreases monotonously with increasing silicon concentration. The main mechanism through which silicon atom acts on the antiferromagnetism of CXtitanium is a depairing mechanism. The experimental and theoretical magnetic phase diagrams are in good agreement.

where References

t11 H. and

(2) and where standard notations have been used [8]. Using a density of states of the form [8] P(E)=p(EOF)+Ae(f+~F-Zk),

(3)

where ei is the Fermi level for pure chromium, ek the energy value for which occur the singularity in

Hansen and K. Anderko, Constitution of Binary Alloys (McGraw-Hill, New York, 1958). 121I. Pop, T. Petrivor, A. Giurgiu and A. Neda, J. Phys. Chem. Solids 46 (1985) 1077. [31 S. Arajs, K. Rao, H.V.D. Aston and T. Young, Phys. Scripta 8 (1983) 109. [41 I. Pop, D. Dgd&rlat, T. Petrigor and A. Giurgiu, J. Phys. Chem. Solids IO (927) 1981. PI I. Zittartz, Phys. Rev. 164 (1967) 575. WI M. Crisan and AI. Anghel, J. Magn. Magn. Mat. 1 (1976) 267. t71 T. Petrisor, I. Pop, A. Giurgiu and N. Farbag, J. Magn. Magn. Mat. 59 (1986) 309. [8] D. D&d%rlat and Zs. Gulacsi, Phys. Stat. Sol. (b)98 105 (1980).