Search for magnetic order in undoped and doped spin‐gap systems by μSR

We introduce our μSR investigations of spin‐gap systems, such as, (1) a 2‐leg spin‐ladder material SrCu₂O₃, (2) a Haldane material (S=1 spin‐chain) Y₂BaNiO₅, (3) a spin‐Peierls material CuGeO₃, (4) a spin‐chain# material Sr₆Ca₈Cu₂₄O₄₁. All of these antiferromagnetic spin systems are characterized by a spin‐gap between the singlet ground‐state and the triplet first excited states. In the above‐mentioned materials, we confirmed the absence of magnetic order down to milli‐Kelvin regime, supporting the non‐magnetic feature of the ground‐state. If a spin‐gap system is doped with charges and/or vacancies at the spin site, unpaired spins are induced out of the singlet ground‐state. In some materials, doping completely destroys the singlet ground‐state and induces a bulk magnetic order. We report μSR investigations of doped materials as well, which clarifies the existence/absence of a magnetic order upon doping. This revised version was published online in July 2006 with corrections to the Cover Date.     

A muon‐spin‐rotation study of the superconducting condensate density in high temperature superconductors

Muon‐spin‐rotation studies on a variety of polycrystalline cuprate high‐T_ c superconductors reveal a generic dependence of the superconducting transition temperature T_ c upon condensate density throughout the entire range of hole doping. Upon full oxygenation the CuO chains in YBa₂Cu₃O_7-\delta become metallic and superconducting and the condensate density is dramatically enhanced. The very rapid suppression of the condensate density n_ s upon Zn substitution in YBa₂(Cu_1-xZn_x)₃O_6+\delta is inconsistent with s‐wave pairing and magnetic scattering but points towards a d‐wave pairing state with non‐magnetic scattering in the unitarity limit. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nuclear quadrupole interaction at133Ba (EC)133Cs in Eu1Ba2Cu3O7−°

We studied the nuclear quadrupole interaction (NQI) at¹³³Ba(EC)¹³³Cs in Eu₁Ba₂Cu₃O_7–6 by time differential perturbed angular correlation between 11 K and 300 K. No anomalies in the NQI parameters around T_c were detected. All Ba sites appear to be identical and the precession frequency is ω=80(2) Mrad/s (assuming axial symmetry), independent of temperature.     

Muon behaviour in diamond grown by chemical vapour deposition

Transverse‐field μSR spectroscopy was used to study the behaviour of positive muons implanted in polycrystalline chemical‐vapour‐deposited (CVD) diamond. Measurements were made at sample temperatures of 10 K, 100 K, and 300 K at a magnetic field of 7.5 mT to study the behaviour of the “normal” (isotropic) muonium state (Mu_T) and the diamagnetic states (μ^d), and at 10 K and 300 K at the so‐called “magic field” of 407.25 mT to study the anomalous (bond‐centred) muonium state (Mu_BC) and μ^d. The absolute fractions of the muonium states in the CVD diamond are observed to be close to those in high‐quality natural type‐IIa single crystal diamond. This revised version was published online in August 2006 with corrections to the Cover Date.     

Investigation of magnetic ordered state in YBa2Cu3O6+x (0.06<X<0.32) bY μSR

The magnetic state properties have been studied in YBa₂Cu₃O_6+x with (0.06SR-studies two precession signals with the frequencies close to 4MHz was observed at T=20÷80K. Possible reasons of the phenomena at T<100K are discussed.     

Study of the magnetic phase diagram of Y1-x Cax Ba2 Cu3 O6

We have studied the magnetic phase diagram for the bilayer compound Y_1-x Ca_x Ba₂ Cu₃ O₆ in the regime of low doping (hole concentration within a CuO₂ plane, p_sh < 0.1). For p_sh < 0.03, the data demonstrate the freezing of the spin degrees of freedom associated with the doped holes into a spin‐glass‐like state which is superimposed on the preexisting long range order of the Cu²⁺ spins. Only a single spin‐glass like transition at a temperature T_g is observed for samples in the hole concentration range 0.03 < p_sh < 0.1. While the threshold for the occurrence of superconductivity is almost the same (p_sh > 0.06) for the Y_1-x Ca_x Ba₂ Cu₃ O₆ and the La_2-x Sr_x CuO₄ systems, the magnetic correlations that coexist with superconductivity are considerably stronger in the bilayer Y_1-x Ca_x Ba₂ Cu₃ O₆ system as indicated by significantly increased transition temperatures T_g. This revised version was published online in August 2006 with corrections to the Cover Date.     

High‐pressure μSR studies of ferro‐ and antiferromagnetic metals

High‐pressure μSR experiments on ferromagnetic nickel and \alpha‐iron and antiferromagnetic chromium are reported. In Ni above 260 K B_Fermi was found to be proportional to the saturation magnetization, whereas at lower temperatures it is temperature independent apart from a small anomaly below 30 K which is presumably caused by a magnetoelastic interaction. There was no evidence for an occupation of metastable sites by the μ⁺ below the Curie temperature. By contrast, in \alpha‐Fe the temperature dependence of \curpartialB_μ/\curpartialp shows a structure which might be attributed to the occupation of excited muon states at elevated temperatures. High‐pressure zero‐field experiments on Cr performed in the temperature regime between 4.5 K and 8 K revealed a pressure dependence of B_μ as large as \curpartialB_μ/\curpartialp=-(89.15\pm 0.06)\times 10^-12 T/Pa. In terms of volume dependence a very large negative Grüneisen parameter \gamma =-27 was obtained. This revised version was published online in July 2006 with corrections to the Cover Date.     

μSR studies of Li‐doped La2CuO4

Magnetic properties of the Li‐doped cuprates La₂Cu_1-xLi_xO₄ (where x=0.01,\ 0.05,\ 0.10,\ 0.45, and 0.50) have been studied by μSR. For low Li concentrations (x\Eleq 0.10) we find a rapid suppression of T_N as x increases, but little change in the magnitude and temperature dependence of the AFM order parameter. This indicates that Li doping effectively destroys AFM (similar to Sr doping, but different from Zn doping) without strongly affecting the onsite Cu moments and the shape of the spinwave excitation spectrum. For high Li concentrations we find magnetic clusters in about 15% of the sample volume; the remaining volume is non‐magnetic, suggesting possible singlet‐state formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Epitaxial combination of NdBa2Cu3O7−δ /SrTiO3: growth characteristics, structure, and parameters

We have studied the growth characteristics, structure, and parameters of the epitaxial heterostructures (001)NdBa₂Cu₃O_7−δ /(100)SrTiO₃/(001)NdBa₂Cu₃O_7−δ grown by laser ablation on a (100)LaAlO₃ substrate with a thin (∼2 nm) YBa₂Cu₃O_7−δ intermediate layer. The use of an YBa₂Cu₃O_7−δ intermediate layer promotes layered growth of the (200 nm) NdBa₂Cu₃O_7−δ layer, whose free-surface roughness is 4–5 nm. The resistance of the NdBa₂Cu₃O_7−δ layers began to fall off abruptly at T=92 K, and at T≈87 K it vanished completely. The critical current density in the NdBa₂Cu₃O_7−δ layers at T=76 K exceeded 10⁶ cm² A/cm². The dielectric constant of the (400 nm) SrTiO₃ layer sandwiched between the NdBa₂Cu₃O_7−δ epitaxial layers grew by roughly threefold as the temperature was lowered in the interval 300–4.2 K. When a bias voltage of ±2.5V was applied to the NdBa₂Cu₃O_7−δ electrodes, the relative dielectric constant of the (400 nm) SrTiO₃ intermediate layer fell from 1150 to 400 (T=32 K, f=100 kHz). The conductivity of the SrTiO₃ intermediate layer in the direction perpendicular to the substrate plane increased with temperature and the electric field strength. Fiz. Tverd. Tela (St. Petersburg) 41, 395–403 (March 1999)     

The μSR study of the relaxation of Ho and Er magnetic moments in high-Tc 1-2-3 compounds

High temperature superconductors HoBa₂Cu₃O_7−δ (T _c ≅93 K), Ho_0.5Y_0.5Ba₂Cu₃O_7−δ (T _c ≅93 K) and ErBa₂Cu₃O_7−δ (T _c ≅95 K) were investigated by the zero-field μSR-technique. The muon spin depolarisation rate connected with the fluctuation frequency of rare-earth ion magnetic moments was measured. It was found that the samples with holmium show a fast increase of the muon spin depolarisation rate at temperatures below 20 K, while in ErBa₂Cu₃O_7−δ the depolarisation rate remains low in the whole temperature region studied (4.2 K-270 K). The sharp difference between the behaviours of the muon spin depolarisation rate may be explained by the difference between the ground state of Ho³⁺ and Er³⁺ ions in the crystalline field of the lattice.     

Low temperature μSR studies in NiO and CoO

Recent measurements on NiO and CoO show a complex temperature dependence. In this work, the local μ⁺ fields are examined over an extended temperature range. Only one precession frequency of 61.3 MHz was observed in the μ⁺ spin precession in zero field in NiO, similar to the case in MnO. The signal broadens at 200 K and is difficult to observe above 250 K. In contrast, in CoO, at least three lines are observed: two sharp lines at 54.2 MHz, and at 78 MHz, with an additional small peak at 162 MHz. Above 40 K, the 54.2 MHz signal (CoO) vanishes, but the 78 MHz signal survives to 110 K. However, at 270–280 K, a new signal at 13 MHz is also observed. Dipole field calculations of these 3d‐oxides (MnO, NiO and CoO) for various lattice sites (symmetric sites and O‐bonded positions) are examined and compared with the experimental results, with considerations toward the dynamics of the muons. This revised version was published online in July 2006 with corrections to the Cover Date.     

Zero‐field μ+SR study of the colossal magnetoresistance material La0.67Ca0.33MnO3

Zero‐field μ⁺SR and resistivity experiments on La_0.67Ca_0.33MnO₃ powder show that the ferromagnetic transition temperature (T_C=274\ K) and resistivity peak temperature coincide to within 1 K, about 10 K higher than T_C determined from the bulk magnetization. The sublattice magnetization \nu_μ(T) is well described for T ≤ T_C by (1-T/T_C)^β , where β =0.345 ± 0.015. Unusual relaxation dynamics suggest a wide distribution of Mn‐ion correlation times. These results are discussed in terms of the effects of polarons on the spin and charge dynamics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Temperature and magnetic field dependences of the London penetration depth and coherence length in the vortex state of YBa2Cu3O7-\delta

We report TF‐μ⁺SR measurements of the London penetration depth in the high‐T_c superconductor YBa₂Cu₃O_7-\delta as a function of temperature T and magnetic field H. The magnetic field distribution in high quality single crystals of YBa₂Cu₃O_6.95 was fit assuming a triangular vortex lattice with an average in‐plane penetration depth \lambda_ab=1155(3) Å, extrapolated to zero temperature and magnetic field. We find that \lambda_ab(T,H) increases linearly both as a function of magnetic field and temperature in the field range studied. This unusual behaviour is attributed to an unconventional pairing scheme in which there are line nodes in the superconducting energy gap function. Measurements have also been made on a deoxygenated sample, YBa_2Cu_3O_6.60. The lineshapes for YBa₂Cu₃O_6.60 clearly show the high‐field cutoff due to the finite coherence length \xi_ab. A preliminary analysis indicates that \xi_ab(T,H) increases linearly as a function of temperature and decreases with increasing magnetic field.     

Temperature dependence of the muon and proton hyperfine constants of an \alpha‐muonium‐substituted methyl radical

Muon hyperfine constants A_μ have been measured by transverse field μSR for (CH₃)₃Si\mbox\.CHMu in hexane from 167 K to 332 K. In addition, avoided level‐crossing resonance was used to determine \alpha‐proton coupling constants A_p over a similar range of temperatures. The two hyperfine constants can be described by a common temperature dependence, d|A_i|/ dT=1.4\times 10^-3 MHz\,K^-1, where A_i represents A_p or the reduced muon constant A^\prime_μ=0.3141A_μ. There is a small isotope effect (A^\prime_μ is 2.2 % larger than A_p) consistent with zero‐point motion in the anharmonic C–H bond stretch. The common temperature dependence is tentatively attributed to a coupled deviation of the C–H and C–Mu bonds out of the nodal plane of the p orbital containing the unpaired electron. This revised version was published online in August 2006 with corrections to the Cover Date.     

Low-temperature specific heat of Er2Ba4Cu7O15-δ

The magnetic ordering of Er ions in Er₂Ba₄Cu₇O_15-δ (δ = 0:7 and δ = 0:08) has been studied by low temperature heat capacity measurements. The Néel temperature T _N of Er₂Ba₄Cu₇O_15-δ is almost independent of the oxygen concentration (TN = 0:54K for δ = 0:08 and T _N = 0:50K for δ = 0:7). While an anisotropic two-dimensional (2D) Ising model describes the experimental data very well for Er₂Ba₄Cu₇O_14:92, this model cannot be applied to the oxygen reduced sample Er₂Ba₄Cu₇O_14:3. By using a model consisting of 1D-Ising chains and of 2D-Ising clusters of Er³⁺ ions we could fit the specific heat data of Er₂Ba₄Cu₇O_14:3 accurately. Our measurements show clearly that with the reduction of the oxygen content the in-plane anisotropy of the magnetic exchange coupling increases.     

Magnetization scaling in the ruthenate-cuprate RuSr2Eu1.4Ce 0.6Cu2O10-δ (Ru-1222)

The magnetic properties of the superconducting ruthenate-cuprate RuSr₂Eu _1.4Ce_0.6Cu₂O_10-δ (Ru-1222) have been studied by a scaling analysis of the dc magnetization and ac susceptibility measurements. Non-linear M(H) curves reveal the presence of nano-size clusters with a net magnetic moment of ∼10² μ_B at 180 K, near the deviation from a Curie-Weiss behavior. On cooling, no scaling was observed down to 90 K, discarding the possibility of magnetic phase separation of collinear ferromagnetic particles. We explain this result in terms of a variable number of contributing particles, with a temperature dependent net magnetic moment. For 70 K ≤T≤ 90 K the scaling plots evidence the emergence of a system of non-interacting particles, which couple on further cooling. The observed cluster-glass features are preserved down to the lowest measured temperature (10 K); no signature of long-range order was detected. The frequency shift of the peak in the real part of the ac susceptibility does not follow the Vogel-Fulcher type dependence, as previously reported. The puzzling temperature dependence of the coercive field, H_C(T), is correlated with the changes in the scaling factors.     

Effect of oxygen content on the specific heat and magnetic susceptibility of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript>

The specific heat and magnetic susceptibility of a sample of YBa₂Cu₃O_7?δ (YBCO) was measured for 0≤δ≤0.85, 0.8≤T≤120K, and H=0, 3, 5, 7, 9T. The data show the existence of both S=1/2 and S=2 paramagnetic centers, consistent with EPR results. The δ dependences of their concentrations and of other parameters are reported.     

Anomalies of the electronic spin-lattice relaxation of Gd3+ in YBa2Cu4O8 and YBa2Cu3O6+x near 200 K

A sharp kink in the temperature dependence of the electronic spin-lattice relaxation rate near 200 K was found in the high-temperature superconductors YBa₂Cu₄O₈ and YBa₂Cu₃O_6+x . The effect is correlated with opening of a spin gap and microscopic phase separation. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 567–572 (25 April 1999)     

Magnetic ordering below 10K in the high-Tc superconductor YBa2(Cu0.985Fe0.015)3O7−σ

The high-temperature superconductor with the nominal composition YBa₂Cu₃O_7?σ has been doped with iron 93% enriched in⁵⁷Fe. When 1.5 at % Fe substitutes for Cu, the transition temperature is reduced by about 30° K with full ac shielding (X′=?1/4π) achieved by ≈40 K. At room temperature the Mössbauer spectrum consists of three overlapping doublets. Below 10 K, hyperfine-field splitting is observed; relaxation still takes place at T=1.8 K.     

Mössbauer effect study of Ni1−xCuxMnyFe2−yO4 system

The Mössbauer effect technique has been employed for the study of magnetic properties of spinel series Ni_1?xCu_xMn_yFe_2?yO₄ with 0.0≤x≤1.0, and y=0.6. The substitution of Mn³⁺ and Cu²⁺ ions results in a slight decrease of the hyperfine field at B‐ as well as A‐sites. The area ratio of Fe³⁺ ions at the A‐ and B‐site at 77 K indicates that Cu²⁺, Ni²⁺ and Mn³⁺ ions occupy the octahedral sites in an evidence for complete inverse spinel in this system. The temperature dependence of the hyperfine parameters has been studied for composition with x=0.5 where Nèel point T_N and Debye temperature θ_D are found to be 650 and 679 K, respectively. The temperature dependence of the sublattice magnetization σ(T) obeys a one‐third‐power law in the range 0.5N<0.99.