D. Mienert

Enhanced magnetisation in nanocrystalline high-energy milled MgFe2O4

The changes in magnesium ferrite (MgFe2O4) caused by high-energy milling are investigated by means of Mossbauer spectroscopy, magnetisation measurements, and electron microscopy. The observed enhancement of the magnetisation in nanoscale milled MgFe2O4 is discussed with respect to the mechanically induced cation redistribution and spin canting.

Magnetic stripe order in La1.8−xEu0.2SrxCuO4

Abstract The magnetic order in La1.8−xEu0.2SrxCuO4 (x⩽0.2) with low-temperature tetragonal (LTT) structure has been investigated with muon spin relaxation. The magnetic phase diagram of the LTT phase resembles the generic phase diagram of the cuprates where superconductivity is replaced by a second antiferromagnetic phase. A crossover from magnetic order to superconductivity is observed at a charge doping of x=0.2. Changing the degree of the LTT lattice distortion via a variation of the Eu content the ground state at x=0.2 can be tuned from magnetism to superconductivity.

Spin dynamics in the quantum spin system KCu5V3O13

The complex Oxo-Cu-vanadate KCu 5 V 3 O 13 with 5 independent Cu-sites (s=) per unit cell has an exchange topology described as a ladder of spin tetrahedra or triangles. Magnetic susceptibility measurements on single crystals show a steplike anomaly at 213 K and an antiferromagnetic transition at 7.5 K. ZF pSR reveals only nuclear relaxation above 10 K and two spontaneous precession signals in the long range ordered regime. Below 3.7 K, a spin reorientation leads to a broad frequency distribution typical for an incommensurate spin structure.

The interplay of charge order and magnetism in the one-dimensional quantum spin system Sr14Cu24O41

Abstract We report muon Knight shift measurements on single crystals of Sr14Cu24O41 and Ca9La5Cu24O41 to determine the muon stopping site in the isostructural compounds of the (Sr,Ca,La)14Cu24O41 series. A site close to the ladder oxygen (0, 0.29, 0) is found. In addition, the magnetic correlations in the system Sr14Cu24O41 have been studied by zero field μ+SR between room temperature and 2 K . Quasi-static muon spin relaxation is found below 10 K , whereas thermodynamic and neutron scattering reveal a singlet ground state.

Stripe Order and Spin Dynamics in Nickelates

We report zero field μSR experiments on the lanthanum nickelate La2−xSrxNiO4+δ system with strontium doping level x between 0 and 0.6. In a single crystal with x=0.33 the internal field distribution shows two separated signals reflecting the antiferromagnetic domains and the non magnetic antiphase domain walls in the spin and charge ordered state below 190 K. Below 70 K a broadening of the low field line is observed. A similar behavior is observed in polycrystalline samples with 0.2≤x≤0.5.

Inhomogeneous spin order in the magnetic phase of electron-doped high-Tc superconductors

Abstract Zero-field μ+SR experiments have been performed on polycrystalline samples Nd2−yCeyCuO4 and Pr2−yCeyCuO4 with y=0.10 and 0.125. An inhomogeneous distribution of internal fields is found in the magnetically ordered phases below T≈100 K . A distinct two-peak structure in the Fourier spectra is observed for all compounds below T≈80 K . Our findings are traced back to inhomogeneous Cu2+ spin order, which can be understood in terms of an electronic phase separation. The results are discussed in comparison with the one-dimensionally modulated charge and spin order found in the hole-doped cuprate La2−x−yRxSryCuO4 (R=Nd, Eu) at charge carrier concentrations around y=0.125.

High-pressure μSR studies on La1.65Eu0.20Sr0.15CuO4

Abstract The magnetic order in La1.65Eu0.20Sr0.15CuO4 has been studied under external pressure p⩽8.3 kbar by means of zero field and transversal field muon spin relaxation. At ambient pressure this system is in the LTT structure and exhibits static spin stripe order below 25 K as evidenced by a spontaneous muon spin precession. Applying hydrostatic pressure the magnetic transition temperature as well as the sublattice magnetization and the magnetic volume fraction are reduced. Since external pressure also reduces the LTT tilt angle these experiments prove that the stability of the static stripe phase is strongly coupled to the LTT structure in this system.

The magnetic phases and structures of Ce(Ru1-xFex)2Ge2 studied by μ-SR

We report on the structures of the magnetic phases of CeRu2Ge2 and their evolution in the series Ce(Ru1-xFex)2Ge2. The crossover behavior between the ferromagnetic ground state of CeRu2Ge2 to the paramagnetic ground state of CeFe2Ge2 is compared to the behavior in the series CeRu2(Ge1-xSix)2. A profound difference is observed between these two series, which is in contradiction to the uniform behavior predicted by the Doniach-necklace model.

The magnetic phases of Ce(Cu1-xNix)2Ge2

Abstract The magnetic phase diagram of the system Ce(Cu1−xNix)2Ge2 with 0⩽x⩽0.9 is studied by muon spin relaxation (μSR). Static magnetic order has been found up to x=0.5 whereas for x=0.8 and 0.9, no magnetic relaxation has been detected for temperatures down to 20 mK .

Magnetic correlations in the spin chains of Sr2.5Ca11.5Cu24O41 studied by μ+SR

Abstract The magnetic correlations in the strongly hole-doped spin chains of Sr 2.5 Ca 11.5 Cu 24 O 41 have been studied with μ + SR. Zero field and longitudinal field μ + SR data prove the development of static magnetic order at temperatures below T N ≃2.3. However, the measured spectra markedly differ from the findings in other magnetically ordered compounds. In particular, in the ordered state we observe a peculiar nearly constant muon polarization at short times.