S. Rehmann

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 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.

Transport and magnetic properties of mixed valent SmB6

Abstract We present the results of transport and magnetic measurements performed on two intermediate valent SmB6 single crystals. The received results above 15 K are in agreement with the results obtained by other groups. At the lowest temperatures, however, the resistivity, magnetization, and susceptibility seem to be strongly influenced by the presence of impurities in the investigated samples.

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.

Magnetic properties of highly dilutedPdFex andPtFex-alloys. Part II. Susceptibility at micro- and milli-kelvin temperatures

We have measured the 16 Hz susceptibility of the giant magnetic moments induced by Fe impurities in highly dilutedPdFex andPtFex samples with 2.5 ppm ≤ x ≤ 75 ppm in a wide temperature range, 30 μK≤ T ≤ 300 mK, and at static magnetic fields 0,01 mT≤ B ≤ 25 mT. We find spin glass freezing at Tf(X)/X≅0,19mK/ppm Fe forPdFex and the larger value 0.26 mK/ppm Fe forPtFex. This is the first observation of spin glass freezing inPtFex. In the low-temperature range T ≲ 0.5Tf(x), the susceptibilities follow χ — χ0∽ T with small zero-temperature χ0 values forPdFeX and vanishing χ0 values forPtFex. In the paramagnetic high-temperature range, we find χ℞(T — θ)it-1 at T ≥ 10 mK independent of x forPdFex, and at T ≥ 2Tf(x) dependent of x forPtFex with vanishing θ values for both systems. The data compare well to the predictions of the Thouless-Anderson-Palmer “TAP” approach of the Sherrington-Kirkpatrick “SK” model for spin glasses.

Interplay of Nuclear Magnetism and Superconductivity

The ratio of nuclear saturation magnetization and superconducting critical field, μ0Msat/ BS0*≡∈, classifies the strength of mutual influence of nuclear magnetism and superconductivity. In order to investigate the interplay of both phenomena for the three distinct cases ∈ ≪ 1, ∈ ≍ 1, and ∈ ≫ 1 we have measured the ac susceptibility of Al, of the intermetallic compound AuIn2, and of the metal hydride TiH2.07at ultralow temperatures, 17 μK ≤ T ≤ 1 K, as function of static field 0 ≤ B ≤ 15 mT. For Al, the interplay enables an absolute measurement of the nuclear magnetization. For AuIn2, we get a steep decrease of BS(T) and a broadening of the superconducting transition in its nuclear ferromagnetic phase. Surprisingly, the nuclear ferromagnetic state coexists with type-I superconductivity in AuIn2. The metal hydride TiH2.07, which is under present investigation, is a good candidate to show reentrant superconductivity.

Magnetic impurities in glasses and gelatine

Abstract We have measured the d.c. magnetization at 4.2 K ≤ T ≤ 300 K and the a.c. susceptibility at 0.3 mK ≤ 7 ≤ 100 mK of a standard glass pipette, of Duran and Suprasil glasses, as well as of a gelatine capsule. Whereas the concentration of magnetic impurities in the first two glasses is ≈0.4 and 0.2%, and ≈0.01% for the gelatine capsule, it is below 1 ppm for the Suprasil glass (assuming μ impurity = 2 μ Bohr ). This makes the latter glass very suitable as a sample holder with a very small, constant susceptibility background in investigations of the magnetic properties of materials.

Low temperature magnetic properties of samarium hexaboride

Magnetization measurements to 2K and ac susceptibility measurements to 10mK have been performed on two intermediate valent SmB6 single crystals. The temperature dependence of susceptibility at temperatures above 50K follows the van Vleck behaviour based on the contribution of 4f6 and 4f55d1 configurations of Sm-ions, and is in agreement with the results obtained by other groups. Below 15K, however, the magnetization and susceptibility dependences seem to be strongly influenced by impurities in the samples.

Interplay between nuclear ferromagnetism and superconductivity in AuIn2

Recently we have reported on the nuclear ferromagnetic ordering transition of In in AuIn2 at Tc=35 μK [1, 2]. Here we report on a substantial decrease of the critical field of superconductivity (Ts=207 mK for B=0) of AuIn2 in its ordered nuclear ferromagnetic phase. Decreasing the temperature from T=42 μK, where the maximum of the susceptibility occurs, to 25 μK, the critical field is reduced from Bs (T>Tc)=1.45 mT in the nuclear paramagnetic phase to Bs (T<Tc)=0.87 mT in the nuclear ferromagnetic phase at T=25 μK. In addition, we observe a broadening of the superconducting transition in thenuclear ordered phase. This is the first study on the interplay ofnuclear magnetism and superconductivity.

Electronic and nuclear magnetism inPtFe at milli-, micro-, and nanokelvin temperatures

Nuclear magnetic resonance, ac susceptibility and nuclear heat capacity of two Pt samples with different concentrations of magnetic impurities (41 ppm and 11 ppm), most likely Fe, have been studied in fields of (0±0.05)mT≤B≤248mT and at temperatures of 0.3μK≤T≤100mK. No indication for a nuclear magnetic ordering toTe=2μK (NMR) in 2.5mT and to a nuclear temperatureTn=0.3μK (ac susceptibility) in (0±0.05)mT could be detected, and a maximum for the Weiss constant of 0.3μK can be estimated. The nuclear heat capacity data show enhanced values which scale with the impurity concentration. We are able to describe this latter data with a spatially varying field caused by the magnetic impurities.