Magnetization measurements on UAs single crystals : observation of a ferrimagnetic spin structure

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Magnetization measurements on UAs single crystals : observation of a ferrimagnetic spin structure

G. Busch, O. Vogt, H. Bartholin

To cite this version:

G. Busch, O. Vogt, H. Bartholin. Magnetization measurements on UAs single crystals : observation of a ferrimagnetic spin structure. Journal de Physique Colloques, 1979, 40 (C4), pp.C4-64-C4-65.

�10.1051/jphyscol:1979419�. �jpa-00218815�

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JOURNAL DE PHYSIQUE Colloque C4, suppliment au no 4 , Tome 40, avril 1979, page C4-64

Magnetization measurements on UAs single crystals : observation of a ferrimagnetic spin structure

G. Busch, 0. Vogt and H. Bartholin (*)

Laboratorium fiir Festkorperphysik, ETH, CH-8093 Ziirich, Switzerland

(*) Laboratoire Louis-NCel, Service National des Champs Intenses, B.P. 166, F-38042 Grenoble, France

R6surn6. - Les aimantations de monocristaux de UAs ont CtC dCtermin6es en fonction de la tempQature (2 < T < 120 K ) , du champ magnktique (0 < H < 156 kOe) et de l'orientation du cristal. Ces mesures r6vklent une trks forte anisotropie. Dans le domaine de temperature 2 < T < 66 K et juste au-dessous de la temperature de NCel un Ctat ferrimagnktique est observC sous champ.

Abstract. - Magnetizations of UAs single crystals have been measured as a function of temperature

(2 < T < 120 K), magnetic field (0 < H < 156 kOe) and crystal orientation. The measurements reveal strong

anisotropy. In the temperature range 2 < T < 66 K and just below the NBel temperature ( T N = 127 K ) a ferrimagnetic intermediate structure is observed.

Susceptibility and neutron diffraction measurements show an antiferromagnetic behaviour for UAs below its N6el temperature of about 127 K [I]. From this temperature down to 66 K type I ordering has been observed. Between 66 K and 4.2 K the spin arrangement is of type IA. The spins are confined along the fourfold axis of the rocksalt structure.

Furthermore at 66 K the magnetic moment jumps to a 10 % lower value (cf. Lander et al. [2]).

Single crystals were grown in tungsten crucibles at 2 300 "C by mineralization. Three different crystals (cubes about 4 x 4 x 4 mm) were X-ray oriented along the three main crystallographic axes. Their magnetic moments were measured by the moving- sample method in static magnetic fields up to 156 kOe (Bitter coil at the SNCI in Grenoble [3]).

Figure 1 shows magnetizations versus applied in- creasing fields along the three main axes at 10 K.

The most obvious feature is the step behaviour due

presumably to the formation of intermediate magnetic structures. At 150 kOe, the magnetization equals 0.44 p, (Bohr magnetons) per U atom along the ( 100 ) direction, 0.60 p B along the ( 1 10 ) direction and 0.49 pB along the ( 111 ) direction. Magnetiza- tion values along the ( 100 ) and ( 1 1 1 ) axes can be deduced from magnetization along the ( 110 ) axis by multiplication with the cosine of the angle between the corresponding axes. Usually, such a behaviour is due to strong uniaxial anisotropy. However, it is surprising that the threshold field (field required to induce a moment jump) is not lowest along the ( 110 ) direction as would be expected for uniaxial anisotropy. The lowest field is rather found along the ( 100 ) axis. This fact can be explained by a multiaxial structure type which must be determined in detail by neutron diffraction measurements in high fields. We cannot be sure that the observed saturation along the ( 111 ) direction corresponds to the same state as in

Fig. 1.

-

Magnetization of UAs at 10 K versus applied field as a

function of crystal orientation. Fig. 2. - Magnetic phase diagram of UAs.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979419

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MAGNETIZATION MEASUREMENTS ON UAS SINGLE CRYSTALS C4-65 the other directions. Indeed, hysteresis is observed

up to 60 K for the ( 11 1 ) direction whereas along the other directions hysteresis vanish at about 20 K. The magnetic moment determined by neutron experiments is 2.25 p B [ 2 ] . Thus, the state observed at 150 kOe with a much lower moment is certainly an intermediate state.

Figure 2 shows the magnetic phase diagram, the coordinates being the magnetization (M), the temperature (T) and the field (H), along the ( 100 ) direction. The magnetization curves remain essentially unchanged below 60 K. Observed remanence and intermediate steps of the magnetization must be attributed to domain effects. Above 66 K the transition field increases sharply and cannot be observed in our experiment. The transition is again observable

just below the NCel temperature. Strong thermal variation of transition fields has been already observed for CeBi [4] for temperatures close to the transition between the antiferromagnetic states of types I and IA.

The lack of neutron diffraction experiments ma- kes any conclusion very difficult. The threshold fields are essentially temperature-independent. The stability of the intermediate magnetic structure de- pends therefore crucially on exchange. The drop-off at 66 K must be associated to the moment jump occurring at this same temperature.

The authors are very grateful to Pr. R. Pauthenet of SNCI Grenoble for the permission of performing the measurements in static dc fields in his laborato- ries.

References

[I] For a review of results see LAM, D. I. and ALDRED, A. T . , The LOT, M., LACAZE, A., PAUREAU, J . , PAUTHENET, R., Actinides : Electronic Structure and Related Properties, PICOCHE, J. C., RUB, R . , VALLIER, J. C. and WAINTAL, ed. A. J . Freeman and J . D. Darby (Academic Press, A . , International Conference on Magnetism, Moscow

New York) 1974, p. 131. 1973 (Nauka, Moscow, 1974).

[2] LANDER, G. H., MUELLER, M. H. and REDDY, J. F . , Phys. (41 BARTHOLIN, H . , FLORENCE, D., WANG TCHENG-SI and VOGT, Rev. 36 (1972) 1880. O . , Phys. Status Solidi ( a ) 24 (1974) 631.

131 AUBERT, G . , BARTHOLIN, H., BLOCH, D., CHAUSSY, J., GUIL-