Thomas Herrmannsdörfer

Crystal structure and paramagnetic behaviour of

The crystal structure of the lowest-temperature -phase of has been determined as a function of temperature by neutron powder diffraction. The diffraction measurements show that there are no additional phases at low temperatures. Magnetic measurements suggest that no superconductivity exists at temperatures as low as , but instead, an increase of the magnetic susceptibility according to a paramagnetic Curie law was found. This observation indicates that unpaired carriers persist in the sample after cooling to very low temperatures.

Magnetic and transport properties of TmB12, ErB12, HoB12 and DyB12

Abstract We present measurements of the magnetization and electrical resistivity of TmB 12 , ErB 12 and HoB 12 singlecrystalline samples as well as of a DyB 12 polycrystalline sample. Data were taken in the paramagnetic state as well as in the magnetically ordered regime. To our knowledge, this is the first systematic investigation of TmB 12 , ErB 12 , HoB 12 and DyB 12 on the magnetically ordered phase by measurements of magnetization and electrical resistivity. All samples investigated undergo antiferromagnetic ordering in the lower Kelvin temperature range. From the magnetization data, the Neel temperatures, the paramagnetic Curie temperatures, and the effective magnetic moments of the corresponding rare earth ions are determined. It is shown that the indirect exchange interaction of RKKY type is the dominating mechanism leading to the observed antiferromagnetic ordering. The measured temperature dependences of the electrical resistivity show a pronounced effect of superzone boundaries near Neel temperatures. The spin wave scattering of conduction electrons below T N is discussed, too.

Magnetization anomalies in the superconducting state of RuSr2GdCu2O8 and the magnetic study of Sr2GdRuO6

Abstract d.c. magnetization measurements performed on a polycrystalline sample of RuSr2GdCu2O8 (Ru-1212) showed distinct peaks of the magnetization upon entering the superconducting state. Since Sr2GdRuO6 (Sr-2116) is the precursor for the preparation of Ru-1212, a detailed investigation of the magnetic properties of Sr-2116 was carried out. Although similarities were observed in the magnetic behavior of Sr-2116 and Ru-1212 in the temperature range of the observed peaks, we can exclude, based on a quantitative comparison, that the anomalies observed for Ru-1212 are due to Sr-2116 impurities.

NMR signal averaging in 62T pulsed fields.

Nuclear Magnetic Resonance (NMR) experiments in pulsed high magnetic fields up to 62T at the Dresden High Magnetic Field Laboratory (Hochfeld-Magnetlabor Dresden) are reported. The time dependence of the magnetic field is investigated by observing various free induction decays (FIDs) in the vicinity of the maximum of the field pulse. By analyzing each FIDs phase and its evolution with time the magnetic fields time dependence can be determined with high precision. Assuming a quadratic or cubic dependence on time near the field maximum its confidence is found to be better than ± 0.03ppm at low fields and ± 0.8ppm near 62T. In turn, the thus obtained time dependence of the field can be used to demodulate and phase-correct all FIDs so that they appear phase-locked to each other. As a consequence signal averaging is possible. The increase in signal-to-noise ratio is found to be close to that expected theoretically. This shows that the intrinsic time dependence of the pulsed fields can be removed so that the NMR signals appear to be taken at rather stable static field. This opens up the possibility of performing precise shift measurements and signal averaging also of unknown, weak signals if a reference signal is measured during the same field pulse with a double-resonance probe.

Magnetic properties of highly dilutedPdFex andPtFex-alloys. Part I. Magnetization at kelvin temperatures

Localized 3 d magnetic moments polarize in palladium and platinum matrices the Pd 4d and Pt 5d conduction electrons in their neighbourhood. This leads to effective “giant magnetic moments” μgm. We have measured the magnetization M(x, B, T) of each ninePdFex andPtFex samples with 2ppm≤x≤260ppm at 1.6K≤T≤300 K and 0 ≤B ≤ 6.0 T. Our main motivation is to determine the size of the giant magnetic moments induced by highly diluted Fe impurities in both transition metals. From the data, taken in a wide polarization range, 9% ≤M/Msat≤93%, we determine the impurity concentrations x, the effective moments μgm, and the spin quantum number J of the samples by fitting to the Brillouin function. ForPdFex, we find a slight increase of μgm with concentration from (13 ± 1.5) μB at x = (2.5 ± 0.5) ppm to (16 ± 1) μB at x = (220 ± 30) ppm. ForPtFex, the moments are almost constant with μgm = (7.8 ± 1) μB at x = 2 to 14 ppm and μgm = (8.6 ± 0.7) μB at x = 75 to 95 ppm. For all samples we obtain a concentration independent very large or possibly infinite spin quantum number, J ≥ 100, which means that the localized giant moments behave as classical ones at T > 1.6 K and Tesla magnetic fields.

Superconducting RuSr2GdCu2O8 studied by SQUID magnetometry

For polycrystalline RuSr 2 GdCu 2 O 8 (Ru-1212), distinct peaks have been reported in dc magnetization in the superconducting state of the sample. Sr 2 GdRuO 6 (Sr-2116), the precursor for the preparation of Ru-1212, shows similar peaks in the same temperature regime. Based on measurements performed on both bulk and powdered samples of Ru-1212 and Sr-2116, we exclude the possibility that the observed behavior of the magnetization of Ru-1212 is due to Sr-2116 impurities. The effect is related to the superconductivity of Ru-1212, but it is not an intrinsic property of this compound. We provide evidence that the observation of magnetization peaks in the superconducting state of Ru-1212 is due to flux motion generated by the movement of the sample in an inhomogeneous field, during the measurement in the superconducting quantum interference device (SQUID) magnetometer. We propose several tests that help to decide whether the features observed in a SQUID magnetization measurement of Ru-1212 represent a property of the compound or not.

Spontaneous nuclear ferromagnetic ordering of in nuclei in AuIn2 Part I. Nuclear specific heat and nuclear susceptibility

We have measured the nuclear specific heat Cnand nuclear susceptibility χnof In nuclei (I=9/2, μ=5.5μn) in the cubic intermetallic compound AuIn2 (Korringa constant κ=0.11 Ksec) in the normal conducting state at 30μK⩽10mK and 2mT⩽B⩽115 mT. Our data show a positive nuclear Weiss temperature θ=+ 43 μK and that the In nuclei undergo a nuclear ferromagnetic transition at Tc=35 μK. The In nuclei experience an internal field of about 10 mT (obtained from Cnat T>Tc). The nuclear ordering temperature Tcand the internal field increase with applied magnetic field. From the data we deduce exchange constants for the investigated system. The critical entropy reduction ΔS(Tc)/Smax=8.6% and critical enthalpy ΔE=0.28 RTcare in reasonable agreement with the measured ordering temperature Tc,applying the Heisenberg model for a simple cubic I=9/2 spin system. The nuclear spin relaxation time τ calculated from the real and imaginary parts of χn is 10 msec at T>50 μK, but drops to τ<1msec at Tc.This is the first observation of a spontaneous nuclear magnetic ordering transition in a not-hyperfine-enhanced metal at thermal equilibrium, i.e. at Tnuclear=Telectron.

Superconductivity at 20 mK in compacted submicrometer platinum powders

Abstract The superconducting transition temperature and critical magnetic field observed in compacted Pt powder samples with sub micrometer grain size are as high as 20 mK and 18 mT and thus more than one order of magnitude larger than for those with larger (micrometer) grain size. The submicrometer samples exhibit strong lattice strain and significantly smaller Debye temperatures compared to bulk platinum. Moreover, the ferromagnetic spin fluctuations of the conduction electrons are partially quenched in these compacts.

The interplay of electronic and nuclear magnetism in PtFex at milli-, micro-, and nanokelvin temperatures

We have measured ac susceptibility, nuclear magnetic resonance, and nuclear heat capacity of two PtFexsamples with concentrations of magnetic impurities x = 11 ppm and 41 ppm at magnetic fields (0 ± 0.05) mT≤B≤248 mT. The susceptibility data have been measured at temperatures of 0.3 μK≤T≤100 mK, no hint for nuclear magnetic ordering could be detected to a temperature of 0.3 μK. The nuclear heat capacity data taken at 1.4 μK≤T≤10 mK show enhanced values which scale with x at low polarization. This effect is described by a model assuming an internal magnetic field caused by the impurities. No indication for nuclear magnetic ordering could be detected to 1.4 μK. The nuclear magnetic resonance experiments have been performed on these samples at 0.8 μK≤T≤0.5 mK and 2.5 mT≤B≤22.8 mT as well as on three other samples with x = 5, 10, 31 ppm in a different setup at 40 μK≤T≤0.5 mK and at 5.4 mT≤B≤200 mT. Spin-lattice and effective spin-spin relaxation times τ1and τ2* of 195Pt strongly depend on x and on the external magnetic field. No temperature dependence of τ1and τ2*could be detected and the NMR data, too, give no hint for nuclear magnetic ordering to 0.8 μK.

Magnetization of superconducting RuSr2GdCu2O8 and of concurrent phases

Abstract The appearance of impurity phases belonging to the quaternary oxide system SrO–GdO 1.5 –CuO–RuO 2 in Ru-1212 samples is difficult to avoid. While the formation of the ferromagnetic compound SrRuO 3 may be suppressed by a precursor route involving Sr 2 GdRuO 6 , this latter compound shows magnetization anomalies below 30 K. We present evidence that magnetization anomalies observed for some Ru-1212 samples in this temperature range are not due to trace amounts of Sr 2 GdRuO 6 . Rather, the effect appears to be related to the establishment of inter-granular superconductivity and to the vortex dynamics in Ru-1212.