A. Kratzer

A μSR Study of Magnetic Correlations in CeNiSn

The formation of magnetic correlations in CeNiSn has been studied by muon spin rotation and relaxation (μSR) spectroscopy covering the temperature range from 250 K to 33 mK. Mainly a single crystalline sample was used. At high temperatures (≥ 2 K) a behaviour similar to that of a spin fluctuator like UAl2 is observed, meaning that over the whole temperature range the paramagnetic moments fluctuate extremely fast (≥ 31014 Hz). Below 1 K the material exhibits properties typical for a paramagnet moving towards magnetic order, but no transition into long-range order could be observed down to the lowest measured temperature. The dependences of muon spin relaxation rate and muon spin precession frequency on external field (applied parallel to the a-axis) are unusual and indicate the formation of extended spin correlations up to short-range order.

Mössbauer spectroscopy of actinide intermetallics

Abstract Due to their wider radial extent the 5f electrons may form bands of different width and hybridization in metallic compounds of the light actinides. This leads to a broad spectrum of magnetic properties ranging from the localized magnetism of the lanthanides to the itinerant electron magnetism often found in transition metal compounds. Also, the influence of the crystalline electric field tends to be more pronounced than in rare earth compounds, but is usually not as dominant as in the 3d series. Magnetic structures and the question of 5f electron delocalization will be reviewed with respect to actinide Mossbauer data and new results will be presented. In particular the influence of applying external pressure will be discussed.

Magnetic behavior of CeTSn (T = Ni, Pt) from μSR and Mössbauer spectroscopy

Abstract Between 1 K and 33 mK CeNiSn behaves as a paramagnet approaching magnetic order, but no transition is observed. A strong reduction in the moment participating in the magnetic correlations is likely. The La-doped compounds give no magnetic response down to 1.5 K. CePtSn shows unusual spin freezing just above T N ≈8K and sharp transition into a more complex spin order around 5.2 K.

Magnetic properties of the Kondo metals CePtSn and CePdSn from μSR

Abstract While the transition into the antiferromagnetic state in CePdSn proves to be of simple second order, the transition in CePtSn is of an unusual nature. About 1 K above TN the Ce moments enter a quasi-static state with the paramagnetic spins frozen at random orientations. In the antiferromagnetic states, well-developed shapr spin precession patterns are seen in both compounds. This feature is not easily reconciled with the incommensurate spin structures proposed by neutron diffraction measurements. Implications on spin structure resulting from the μSR data are discussed.

μSR studies on CeAl2

AbstractμSR studies on REAl2 type compounds have so far given rather inconclusive results since no μSR frequency has been observed in the ordered magnetic states. Therefore, the results from the paramagnetic region [1,2] have been interpreted without detailed knowledge of the muon site or the mobility of the muons. In the present study of a single crystal sample of CeAl2 we investigated in some detail the paramagnetic temperature range including the transition region to magnetic ordering around 3.6 K. The ordered magnetic state is antiferromagnetic with a modulated structure [3], and the absence of a spontaneous μSR precession signal belowTN is therefore not unexpected.

Disorder-induced heavy fermion behaviour in CeNi1−xCuxSn-alloys

Abstract We have investigated the effect of alloying Cu to the Kondo-semimetal CeNiSn on its specific heat, magnetic susceptibility and μSR relaxation rate. We find a strongly non-monotonic behaviour, in that the electronic specific heat first increases very strongly (at 5% Cu), before decreasing again when magnetic order appears (at 10% and 20% Cu). The μSR data show that no spin freezing to a spin glass state occurs for concentrations less than ≈6% Cu and that the Ce3+ spins remain in a fluctuating state below this concentration. We interpret this behavior as a consequence of the destruction of the high coherence of the antiferromagnetic spin fluctuations in pure CeNiSn, when Cu is alloyed to the system.

Radio-frequency spin resonance of positive muons in α-iron at high temperatures

Resonant transitions between the Zeeman levels of positive muons implanted into α-iron foils have been observed above the Curie temperature by applying a 17.8 MHz transverse radio-frequency field and varying the longitudinal external field. Resonance signals of free and trapped muons are detected.

Spin dynamics in CeNiSn and Ce0.85La0.15 NiSn at very low temperatures

Abstract We performed muon spin rotation/relaxation (μSR) studies on polycrystalline and single crystalline samples of CeNiSn. The system appears to be a dense system of weak magnetic moments with no evidence for static magnetic order down to 11 mK, the lowest temperature studied. In contrast, the spin system retains slow dynamics; it fluctuates at a rate of ∼10−7 s at the base temperature. These remaining fluctuations may be related to the absence of long range magnetic order. Results for polycrystalline Ce0.85La0.15NiSn show even weaker magnetic effects than those in pure CeNiSn, negating the notion that a partial replacement of Ce by La leads to enhanced magnetism and even magnetic order.

A study of UAl2 and UAs by muon spin rotation and relaxation (μSR)

Abstract Transverse (up to 0.4 T) and zero field μSR was carried out between 300 and 4.2 K in the spin fluctuator UAl 2 and the fairly localized antiferromagnet UAs. In UAl 2 the fluctuation of uranium moments is too fast at any temperature to be seen by μSR. In UAs quite different μSR spectra were found for the two antiferromagnetically ordered spin structures. In zero field, the type I (single k ) phase showed no μ + spin rotation, while in the type IA (double k ) structure an overlay of three subspectra was observed, two of which have temperature dependent rotation frequencies.

μSR measurements on UAl2

The cubic Laves Phase UAl2 is known to exhibit unusual magnetic and transport properties at low temperatures which have been explained by a narrow 5f-band in connection with strong spin fluctuations. Transverse and longitudinal μSR spectra taken between 300 K and 2 K show little variation in damping rate and muonic Knight shift. The fact that the weak damping seen in zero field can fully be decoupled by a longitudinal field of less than 10 mT shows that its origin lies in the interactions between the muon and the27Al nuclear moments, rather than in an influence of the uranium magnetic moments.