Magnetic and electric properties of EuB6

Short Notes K133 phys. stat. sol. (b) E, a 3 3 (1979) Subject classification: 14 and 18.2; 19; 21. 7 Instituto Venezolano de Investigaciones Cientdicas, Centro de Fisica, Caracasl) (a) and Institute for Pure and Applied Physical Sciences, University of California, San Diego21 (b) Magnetic and Electric Properties of EuB, BY S. OSEROFF (a), R. CALVO (a), J. STANKIEWICZ (a), Z. FISK (b), and D C JOHNS TON (b) The purpose of this note is to report on what, we believe, a r e the first results of electron spin resonance in a pure single crystal of EuB6 and to compare them with magnetic and electric data previously published for the same sample /l/. Previous ESR measurements were performed on polycrystalline samples by several authors /2, 3/. Trivalent r a r e earth ions form hexaborides with me- tallic behaviour. The Same compounds of divalent ions were supposed to be semiconductors. EuB 6 is a cubic material crystallizing in a structure which may be regarded as CsC1-type a r r a y of Eu atoms and B6 octahedra. It has been believed that EuB6 o r d e r s ferromagnetically and contains only Eu /4/; but recently Isikawa et al. /5/ have reported magnetic and electric data on this material. Their results indicate that EuB6 is semi-metallic, contains some Eu3+, and becomes antiferromagnetic at TN 6 K. However, a more recent investigation of carbon substituted EuBs /6/ indicates that the para- magnetic Curie temperature decreases smoothly with increasing carbon con- tent, that the order changes from ferromagnetic to antiferromagnetic, and that the Eu is divalent. Kasaya et al. /6/ suggest that the data reported in /5/ may be due to carbon contamination. We felt it would be important to measure the electrical and magnetic properties on the same single crystal of pure EuB6. The sample preparation technique has been described previously /I/. ESR measurements were performed at 9 and 35 GHz in the temperature range of 1.4 to 300 K. In the ESR spectrum, we can clearly separate between two temperature regions below and above -30 K. In the high temperature range we observed 1) Apartado 1827, Caracas, Venezuela. 2) San Diego, La Jolla (California), USA.

The purpose of this note is to report on what, we believe, a r e the first results of electron spin resonance in a pure single crystal of EuB6 and to compare them with magnetic and electric data previously published for the same sample /l/.
Previous ESR measurements were performed on polycrystalline samples by several authors /2, 3/.Trivalent r a r e earth ions form hexaborides with metallic behaviour.The Same compounds of divalent ions were supposed to be semiconductors.EuB is a cubic material crystallizing in a structure which may be regarded as CsC1-type array of Eu atoms and B6 octahedra.It has been believed that EuB6 orders ferromagnetically and contains only Eu but recently Isikawa et al. /5/ have reported magnetic and electric data on this material.Their results indicate that EuB6 is semi-metallic, contains some Eu3+, and becomes antiferromagnetic at TN -6 K.However, a more recent investigation of carbon substituted EuBs /6/ indicates that the paramagnetic Curie temperature decreases smoothly with increasing carbon content, that the order changes from ferromagnetic to antiferromagnetic, and that the Eu is divalent.Kasaya et al. /6/ suggest that the data reported in /5/ may be due to carbon contamination.We felt it would be important to measure the electrical and magnetic properties on the same single crystal of pure EuB6.The sample preparation technique has been described previously /I/.ESR measurements were performed at 9 and 35 GHz in the temperature range of 1.4 to 300 K.In the ESR spectrum, we can clearly separate between two temperature regions below and above -30 K.In the high temperature range we observed 1) Apartado 1827, Caracas, Venezuela.
2) San Diego, La Jolla (California), USA.), defined as the half-power 1/2 linewidth of the absorption part of the resonance line can be fitted by the following expression: where a at 9 GHz was 750 Oe, at 35 GHz 650 Oe, and b = (0.3 + 0.1) Oe/K for both frequencies.A value of g = 1.995 -+ 0.010 was measured in both cases.
As we decreased the temperature below * 30 K, a clear distortion from a Dysonian line occurred as it is shown in Fig. 1.At the lowest temperature a "resolved" multiple line spectrum was found.The spectrum depended on orientation and strength of the magnetic field as well as on the shape of the sample.Unfortunately we have not been able to separate the different contributions to the Hamiltonian, like dipole-dipole, hyperfine, and crystal field interactions.However, if we compared the ESR data with the electrical resistivity results, shown in Fig. 2, for the same crystal, a minimum in resistivity is clearly seen at w 30 K (the same temperature at which the ESR line started to distort).This fact can be interpreted as a short range magnetic order.
At 14 K, where our magnetic susceptibility results showed that the material orders ferromagnetically / l / , one can see an almost "resolved" spectrum of ESR, together with a drastic minimum in the resistiv- Hall effect measurements performed for this sample in the range between 77 and 300 K, gave an almost temperature independent negative Hall constant and a net c a r r i e r concentration of about 5x101' electrons/cm3.Cohen has performed Mossbauer measurements on our crystals /7/ and found an extremely clean Eu2' spectrum, with less than 0.5% Eu3+ possible.To explain the effective moment of 7.76 p Eu vacancy concentration for our single crystals.
per Eu ion obtained from susceptibility, we estimated % 13% B The Hall effect indicates a larger contribution of electrons as carriers, whereas band structure considerations /8, 91 point to hole conduction for Eu vacancies.We concluded that EuB is a semi-metal with some band overlap; where the higher mobility of the electrons related to the mobility of the holes takes account of the negative Hall constant.The ESR, together with the resistivity data, show us that the order is more complicated than one would expect from the susceptibility data alone.Hall experiments a s a function of the magnetic field in the range of 1.4 to 77 K a r e in progress.

Fig. 1 .
Fig. 1 .ESR derivate spectrum of EuB single crystal as a function of temperature.v = 9 GI&

Fig. 2 .
Fig. 2. Temperature dependence of the electrical resistivity of EuBs single crystal