Low-temperature NMR studies of SrB 6

We report the results of a 11 B nuclear magnetic resonance (NMR) study of SrB 6 at temperatures between 0.1 and 30 K and in a magnetic " eld of 4.74 T. Below 30 K the NMR spectrum is temperature independent but the spin } lattice relaxation rate „ ~11 exhibits di ! erent features in two di ! erent temperature regimes. At high temperatures, between 30 K and a " eld-dependent crossover temperature „ B between 0.5 and 2 K, „ ~11 is almost temperature independent. We point out that for „ in the crossover temperature range the magnitude of „ ~11 of SrB 6 is distinctly larger than for LaB 6 , a metal with a charge carrier concentration at least two orders of magnitude higher than that of SrB 6 . A possible cause for this behavior maybe the very weak itinerant ferromagnetism that has subsequently been established to occur in nominally pure SrB 6 . At low temperatures, below „ B , „ ~11 decreases substantially with decreasing temperature con " rming a cross-over or phase transition phenomenon as observed by measurements of thermal and transport properties. ( 2000 Elsevier Science B.V. All rights reserved.


Abstract
We report the results of a B nuclear magnetic resonance (NMR) study of SrB at temperatures between 0.1 and 30 K and in a magnetic "eld of 4.74 T. Below 30 K the NMR spectrum is temperature independent but the spin}lattice relaxation rate ¹\ exhibits di!erent features in two di!erent temperature regimes. At high temperatures, between 30 K and a "eld-dependent crossover temperature ¹ between 0.5 and 2 K, ¹\ is almost temperature independent. We point out that for ¹ in the crossover temperature range the magnitude of ¹\ of SrB is distinctly larger than for LaB , a metal with a charge carrier concentration at least two orders of magnitude higher than that of SrB . A possible cause for this behavior maybe the very weak itinerant ferromagnetism that has subsequently been established to occur in nominally pure SrB . At low temperatures, below ¹ , ¹\ decreases substantially with decreasing temperature con"rming a cross-over or phase transition phenomenon as observed by measurements of thermal and transport properties.
2000 Elsevier Science B.V. All rights reserved.
Keywords: SrB ; Low carrier system; Magnetism; NMR SrB is a semimetal with a very small itinerant charge carrier density, of at least two orders of magnitude smaller than that of LaB [1]. Its low-temperature properties seem to depend critically on details of the electronic structure [2]. From the results of magnetization measurements, it has recently been inferred that SrB orders ferromagnetically with an onset temperature ¹ ! of the order of 900 K and involving very small magnetic moments (unpublished results).
Our NMR studies were performed at the B sites which form a network of octahedra joined by covalent bonds. The symmetry of the B sites is 4 mm, which allows for a nonzero "eld gradient, with axial symmetry. In Fig.  1 we display an example of the B-NMR spectrum for SrB measured at a frequency of 64.81 MHz and at a temperature of 1.31 K. The shape of the spectrum is that of a characteristic powder pattern for spin nuclei, where a small quadrupolar perturbation splits the Zee-man lines. The Knight shift is very small and temperature independent.
The quadrupolar parameters and the width of the NMR lines are very similar to those of LaB [3]. However, one expects that for a powdered SrB sample at ¹¹ ! the ferromagnetic order would result in a distribution of demagnetization "elds of approximately 4 m (where m is the magnetization) and a corresponding broadening of the NMR lines. From this consideration and a comparison of our data with that of LaB we infer that the ordering below ¹ ! involves very small magnetic moments of less than 10\ per unit cell, in agreement with the results of the magnetization measurements on the same sample (unpublished results).
The ¹ measurements were performed on the narrow central line of the # ! B nuclear transition, in an applied "eld of 4.74 T (see Fig. 1). In Fig. 2 we display ¹\ (¹) for SrB . Two temperature regimes with qualitatively di!erent ¹-dependencies for the spin}lattice relaxation may be distinguished. At high temperature, above ¹ + 2 K, ¹\ is approximately ¹-independent. At temperatures below ¹ , ¹\ decreases substantially with decreasing temperatures, suggesting a cross-over  phenomenon previously indicated by speci"c heat and resistivity data.
The temperature dependence of ¹\ for SrB is not compatible at all with that expectated for a paramagnetic metal or a semiconductor. Furthermore the magnitude of ¹ at ¹+¹ is surprisingly large, even larger than that for LaB . This is not expected because LaB has a much larger charge carrier concentration. We rule out magnetic impurities as a possible source for the anomalous relaxation because these result in a characteristic temperature and "eld dependencies for ¹ [4], not observed in our experiments. In addition, in the absence of spin di!usion, which seems to be the case here, the relaxation of paramagnetic impurities also implies a distribution of ¹ s [4], again not observed here.
In view of the above one is tempted to associate the relaxation with excitations related to the &small-moment ordering'. However, in the temperature range of our experiments (¹¹ ! ) this yields a ¹\ J¹ [5], instead of the observed ¹-independent relaxation. The crossover phenomenon at ¹ only adds another puzzle to the unexpected features of this seemingly simple compound.