Magnetic properties of TiZr alloys

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Journal of Magnetism and Magnetic Materials 67 (1987) 118-122 North-Holland, Amsterdam

MAGNETIC P R O P E R T I E S OF Ti-Zr ALLOYS T. PETRI~OR, A. GIURGIU and I. POP Physics Department, Cluj-Napoca University, Cluj-Napoca 3400, Rumania Received 30 October 1986

Magnetic susceptibility measurements of TiZr alloys are reported. Zirconium, by alloying, acts on the N6el temperature giving rise to a complicated magnetic phase diagram. The main mechanism in the T~ shift is the change of the Fermi level. The experimental dependence between Ts and Xo is in good agreement with the theoretical dependence obtained on the Fedders-Martin model by taking into account the effect of nonmagnetic impurity on the Fermi level.

I. Introduction

ture dependence of the magnetic susceptibility of the electric resistivity and of the specific heat. The temperature dependence of the specific heat indicates that the observed anomaly is a second-order

The experimental results on a-titanium [1] indicate the existence of an anomaly in the tempera-

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Fig. 1. Temperature dependence of the magnetic susceptibility of TiZr alloys containing 2, 4, 7 and 10 at% Zr.

0304-8853/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

119

T. Petrqor et a L / Properties of Ti- Zr alloys

phase transition with critical temperature Tc = (276 + 4) K. For T > T~ the magnetic susceptibility of a-titanium has a square low temperature dependence x ( T ) = X o + B T 2, specific for the Pauli paramagnetism of itinerant electrons, and for T < Tc the magnetic susceptibility decreases with temperature under the value predicted by the paramagnetic law from the region T > Tc. These facts indicate that the anomaly observed in the temperature dependence of the magnetic susceptibility, of the electric resistivity and of the specific heat, represent a magnetic phase transition of the itinerant electron antiferromagnetPauli paramagnet type. The similarity between the temperature dependence of the magnetic susceptibility of a-titanium and of pure chromium suggests that a-titanium is an itinerant electron antiferromagnet with SDW structure just as chromium, with N6el temperature at (276 + 4) K. In order to investigate the antiferromagnetism of a-titanium, we will study in this paper the

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effect of Zr as an impurity in a-titanium. It is well known [2,3] that nonmagnetic impurities have a drastic influence on the itinerant antiferromagnetism of chromium. This influence is explained by a depairing mechanism introduced in the ordered phase by the impurities and by taking into account the effects of the nonmagnetic impurities on the density of states and on the Fermi level [4,5].

2. Experimental The T i - Z r alloys were prepared from 99.9% pure Ti and Zr up to 30 at% Zr. The alloys were prepared in an argon-arc furnace on a water cooled copper hearth. The arc melted buttons where turned and remelted more than four times. To prevent oxidation of the melts the furnace chamber was evacuated and purged with argon several times before melting.

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Fig. 2. Temperaturedependenceof the magnetic susceptibilityof TiZr alloys containing 12, 14 and 15 at% Zr.

120

T. Petrqor et al.

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Properties of Ti - Zr alloys

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Fig. 3. Temperature dependence of the magnetic susceptibility of TiZr alloys containing 20, 25 and 30 at% Zr.

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Fig. 4. Magnetic phase diagram of the TiZr alloys.

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T. Petrqor et al. / Properties of Ti - Zr alloys

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Fig. 5. Concentration dependence of X0 for TiZr alloys.

Magnetic susceptibility measurements were performed from 100 to 900 K on a Weiss-Forrer equipment. The results are plotted in figs. 1-3, together with the magnetic susceptibility of pure titanium used. As seen from figs. 1-3 the temperature dependence of the magnetic susceptibility of TiZr alloys has a similar shape with that of pure titanium. This similarity indicates that the TiZr alloys are itinerant-electron antiferromagnets just as a-titianium. For this reason we will consider that the anomaly in the temperature dependence of the magnetic susceptibility of TiZr alloys corresponds to the N6el temperature. In order to point out the N6el temperature we used the plot ( x ( T ) - B T ~) versus T [6]. The concentration dependence of T s and Xo determined from this plot are shown in figs. 4 and 5. One can see from this figures that Tr~ and X0 are continuous functions of concentration and for c = 0 their values coincide with that of pure titanium. This fact indicates once again, that the Zr impurities do not change the basic magnetic behaviour of c~-titanium acting only on the T N and X0 values.

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Fig. 6. In(TN/TNo ) versus (X~ - X o ) / X o for TiZr alloys.

122

T. Petrqor et al. / Properties of Ti - Z r alloys

T N and X0 first decrease with increasing concentration reaching a minimum at about 15 at% Zr. As seen in figs. 4 and 5, the concentration dependence of T N and that of X0 has a similar shape indicating a relation between T N and Xo.

3. Discussion and conclusions The temperature independent part of the magnetic susceptibility (X0 = 2#~P(eF) + Xorb) is proportional with the density of states at Fermi level. In a rigid band model the modification of the density of states is caused by the shift of the Fermi level. Taking into account the similarity of the Tr~ and X0 concentration dependence we might conclude that the main mechanism in the T~ shift is the change of Fermi level. In order to explain the effect of Al-impurities on the N~el temperature of a-titanium we used [6] a Fedders-Martin model [7] in wich we took into account only the influence of the nonmagnetic impurity on the Fermi level and we obtained for the concentration dependence of the N6el temperature (TN) the following relation: p

in TN _ I___L__X0-X____o,

rNo

vp(0)

(1)

Xo

where TNo, Xo and p(0) refer to pure titanium and T N and Xo to alloy. Relation (1) indicates a logarithmic relation between T N and Xo. Fig. 6 shows a plot In(TN/T~o ) versus ( X o - Xo)/Xo for TiZr alloys.

As seen from fig. 6 the plot In(TN/TNo) versus (X'o- Xo)/Xo is linear, in good agreement with relation (1). Finally, we would like to conclude the following. The experimental results on TiZr alloys indicate that the Zr impurities do not change the magnetic behaviour of a-titanium acting only on the T N and X0 values. This indicates that the TiZr alloys are itinerant-electron antiferromagnets just like a-titanium. The Zr impurities act on the N6el temperature of pure titanium giving rise to a complicatet magnetic phase diagram. The main mechanism in the T N shift is the change of the Fermi level. The experimental dependence between T N and Xo is in good agreement with the theoretical dependence obtained on the Fedders-Martin model. This indicates that the influence of nonmagnetic impurities on a-titanium can be explained by the model of itinerant-electron antiferromagnetism with SDW structure. References [1] I. Pop, T. Petri,$or, A. Giurgiu and A. N&la, J. Phys. Chem. Solids 46 (1985) 1077. [2] S. Arajs, K. Rao, H.V.D. AstiSn and T. Young, Phys. Scripta 8 (1973) 109. [3] I. Pop, D. D~d~trlat, T. Petri~,or and A. Giurgiu, J. Phys. Chem. Solids 10 (1981) 927. [4] I. Zittartz, Phys. Rev. 164 (1967) 575 [51 M. Cri§an and AI. Anghel, J. Magn. Magn. Mat. 1 (1976) 267. [6] T. Petri§or, 1. Pop, A. Giurgiu and N. Farba~, J. Magn. Magn. Mat. 59 (1986) 309. [7] P.A. Fedders and P.C. Martin, Phys. Rev. 143 (1966) 245.