High-field magnetoresistance and Hall effect in Bi2Sr2CuO x single crystals

We investigated the in-plane magnetoresistance and the Hall effect of high-quality Bi₂Sr₂CuO_x single crystals with T _c (midpoint) = 3.7–9.6 K in dc magnetic fields up to 23 T. For T < 10 K, the crystals show the classical positive magnetoresistance. Starting at T ≈ 14 K, an anomalous negative magnetoresistance appears at low magnetic fields; for T ≥ 40 K, the magnetoresistance is negative in the whole studied range of magnetic fields. Temperature and magnetic field dependences of the negative-magnetoresistance single crystals are qualitatively consistent with the electron interaction theory developed for simple semiconductors and disordered metals. As is observed in other cuprate superconductors, the Hall resistivity is negative in the mixed state and changes its sign with increasing field. The linear T-dependence of cotθ_H for the Hall angle in the normal state closely resembles that of the normal-state resistivity as expected for a Fermi liquid picture.     

Critical current behavior of superconducting granular NbN films

The temperature dependence of the critical current I_c(T) below the paracoherent-coherent phase transition T_cj in granular NbN films of high sheet resistance has been measured. Current-voltage (I–V) determinations of I_c revealed two distinctive temperature regions. Above T1, (T1≈ 0.6T_cj), nonlinear, but continuous I–V curves were obtained for I>I_c. Below T1, the voltage discontinously jumped to a finite value at I_c and then increased linearly with current. At T1, a sudden increase in I_c is observed. Small magnetic fields (< 1 mT) suppressed the excess critical current but fields up to 40 mT had minimal further effect. Data suggest that T1 is a characteristic onset temperature associated with an additional ordering in the samples.     

Disappearance of the heat capacity peak of Sc3In around the curie temperature in high magnetic fields

The low temperature (1.3–20.0 K) heat capacity of the weak itinerant electron ferromagnet Sc₃In was measured in magnetic fields up to ～ 10 T. The heat capacity peak observed around T_c = 6.0 K in zero field becomes smaller with increasing fields and at 9.98 T its magnetic entropy is ≈ 18% of the zero field value. Above T_c, the spin fluctuation contribution to the heat capacity, which is enhanced by the magnetic field at lower fields (?5 T), is quenched at higher fields (?5 T). This depression of the spin fluctuation contibution to the heat capacity by the high magnetic fields occurs at lower magnetic fields than had been considered possible heretofore. Our results suggest that the itinerant ferromagnetism is Sc₃In is completely quenched at 12 T.     

Ultrasonic studies of the high Tc superconductor (La1−xSrx)2CuO4−δ

We synthesized (La_1−xSr_x)₂CuO_4−δ crystals with 0 ≦ x ≦ 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature T_c ≲ 38 K and narrow transition width ΔT_c. For the sample with the maximum T_c among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔV_s(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T ∼ 100 K and a small peak located at T ∼ 200 K. These results show the existence of an optical mode of the energy ℏω_op/k_B ∼ 100 K and, probably, also of that of ∼ 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high T_c of this material.     

Magnetic phases in UNi<Subscript>2</Subscript>Si<Subscript>2</Subscript>

The tetragonal compound UNi₂Si₂ exhibits in zero magnetic field three different antiferromagnetic phases belowT _N =124 K. They are formed by ferromagnetic basal planes, which are antiferromagnetically coupled along thec-axis with the propagation vectorq=(0, 0, q _z ). Two additional order-order magnetic phase transitions are observed below T _N , namely atT ₁=108 K and T ₂=40 K in zero magnetic field. All three phases exhibit strong uniaxial anisotropy confining the U moments to a direction parallel to the c-axis. UNi₂Si₂ single crystals were studied in detail by measuring bulk thermodynamic properties, such as thermal expansion, resistivity, susceptibility, and specific heat. A microscopic study using neutron diffraction was performed in magnetic fields up to 14.5 T parallel to the c-axis, and a complex magnetic phase diagram has been determined. Here, we present the analysis of specific-heat data measured in magnetic fields up to 14 T compared with the results of the neutron-diffraction study and with other thermodynamic properties of UNi₂Si₂.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Mössbauer studies of hyperfine fields in disordered Fe2CrAl

Heusler-like alloy Fe₂CrAl was prepared and studied. Structure determination was done by X-ray. The structure was found to conform to the B₂ type. Magnetic hyperfine fields in this sample were studied by the Mössbauer effect. The Mössbauer spectra were recorded over a range of temperature from 40 to 296 K. The Mössbauer spectra showed the co-existence of a paramagnetic part with a magnetic hyperfine portion at all recorded temperatures. Even with the distribution in the magnetic hyperfine field, the average hyperfine field follows the (T/T _c)^3/2 law. The paramagnetic part of the hyperfine field is explained in terms of the clustering of Cr atoms.     

Magnetoresistance of Sc3In

The magnetoresistance of the weak itinerant-electron ferromagnet Sc₃In was measured from 1.5 to 40 K in magnetic fields up to 14 T. The measurements were made on three samples containing 24.1, 24.3 and 24.4 at% In with Curie temperatures at H = 0 of T_c(0) = 5.5, 6.0 and 6.3 K, respectively. By assuming that the characteristic spin-fluctuation temperature is equal to T_c(0), the negative magnetoresistance in Sc₃In was systematically and reasonably analyzed in terms of the quenching of spin fluctuations by magnetic fields.     

Bl结构MoNx薄膜的制备及物理性能研究

我们对以反应性溅射法制备的MoN_x薄膜测量了超导转变温度T_c,电阻率ρ(T)(从T_c起始到300K)。用X射线衍射技术、卢瑟福背散射(RBS)、俄歇谱仪和X射线光电子能谱(XPS)技术对这些样品进行了分析。实验结果表明T_c和ρ(T)随N含量改变而变化。当样品是B1结构时,T_c小于4.2K,而且样品内还有过量的N存在。俄歇分析表明,样品内有O,C杂质存在。这些因素都可能导致T_c很低,ρ(T)呈负的温度系数。 关键词：     

Upper critical field of disordered NbNi alloys

The critical fields for disordered Nb-Ni alloys are measured in the concentration region from 40 to 60 at% Nb down to temperatures below 0.1T_c. For these alloys T_c varies by a factor of 8 while the density of states, calculated from the slope of the critical field close to T_c, is about 1 (eV atom)⁻¹ independent of concentration. The critical fields are enhanced over the maximum value consistent with the Werthamer-Helfand-Hohenberg theory. It is suggested that weak localization effects contribute to the enhanced critical field of Nb₄₀Ni₆₀.     

Spin correlations and muon mobility in dilute Pd-Ni alloys

The onset of ferromagnetism in Pd_1-cNi_c alloys occurs at c=2.45 %. Dilute Pd-Ni alloys, with c=3.3% and 5.8% (T_c=20.5K and 90K) have been studied withSR. For transverse field geometry the data show a rapid increase in the relaxation rate as T_c is approached. However when the same alloys were studied in zero field, with longitudinal geometry, there was no sign of significant depolarisation at any temperature down to 12K. This result is attributed to the high mobility of muons in these alloys, resulting in extreme motional narrowing, as the muons sample magnetic domains of many different orientations.     

Magnetoelectric effect and magnetostriction in the paramagnetic piezoelectric NiSO4 · 6H2O

The direct magnetoelectric effect, the magnetostriction, and the magnetic moment in piezoelectric paramagnetic NiSO₄ · 6H₂O single crystals is comprehensively studied over a wide temperature range in magnetic fields up to 14 T for several magnetic field directions with respect to crystallographic axes. At temperatures above 20 K, the magnetoelectric effect is rigorously quadratic in magnetic field over the entire magnetic field range. As the temperature decreases, the region of quadratic behavior shifts toward low fields. To explain the magnetoelectric effect, the contribution of the magnetostriction-induced piezoelectric effect is considered. It is shown that the piezoelectric effect may be neglected at the helium temperature, where the magnetoelectric effect is high, since its value does not exceed 1%. However, the relative contribution of the piezoelectric effect increases to 10% at T = 40 K, where the magnetoelectric polarization decreases strongly. In contrast to the earlier assumption that the magnetoelectric effect is rapidly and fully saturated in fields higher than 3 T, complete saturation is actually not detected even at 14 T.     

利用高温超导直流量子干涉器件进行10$lt;sup$gt;-6$lt;/sup$gt; T量级磁场下核磁共振的研究

研究了水样品在10^-6 T量级磁场下的核磁共振谱.核磁共振信号由一个工作在液氮温度的高温超导直流量子干涉仪记录,测量在一个简易磁屏蔽室中进行.在7—70 μT的磁场范围内都观察到了15 ml水样品的核磁共振信号.相应的^1H的核磁共振频率为300—3000 Hz.在实验中获取的单次测量信噪比约为4,通过对信号的100次平均,信噪比可达到约40.进一步讨论了剩余磁场、预极化时间和采样时间对结果的影响.最后用数字滤波之后平均的方法初步得到了时域的自由感应衰减信号. 关键词： 超导量子干涉仪 核磁共振     

Magnetic phase transitions in Nd1 ? xDyxFe3(BO3)4 ferroborates

The magnetic properties of ferroborate single crystals with substituted compositions Nd_{1 − x} Dy _x Fe₃(BO₃)₄ (x = 0.15, 0.25) with competing exchange Nd-Fe and Dy-Fe interactions are investigated. For each composition, we observed a spontaneous spin-reorientation transition from the easy-axis to the easy-plane state and step anomalies on the magnetization curves for the spin-flop transition induced by a magnetic field B | c. The measured parameters and effects are interpreted using a unified theoretical approach based on the molecular field approximation and on calculations performed in the crystal-field model for the rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility from T = 2 K to T = 300 K, anomalies on the magnetization curves for B | c in fields up to 1.8 T, and their evolution with temperature, as well as temperature and field dependences of magnetization in fields up to 9 T are described. In the interpretation of experimental data, the crystal-field parameters in trigonal symmetry for the rare-earth subsystem are determined, as well as the parameters of Nd-Fe and Dy-Fe exchange interactions.     

Upper critical field of cellular magnesium diboride

A cellular superconducting material consisting of thin (1–20 μm) MgB_2?x layers and magnesium granules of about 100 μm has been produced. The critical temperature T _c of this superconductor decreases with the thickness of the MgB_2?x layers. In unalloyed magnesium diboride, the curvature of the temperature dependence of the upper critical field H _c2(T) changes gradually from downward to pronounced upward as the temperature T _c decreases from 38 to 36 K.     

Sign reversal of the magnetoplastic effect in C60 single crystals during the sc-fcc phase transition

It is found that the magnetoplastic effect in C₆₀ single crystals in a pulsed magnetic field with induction larger than 10 T changes its sign in the vicinity of the phase transition at T _c =250–260 K: crystal strengthening is observed for T<T _c , and softening occurs for T>T _c . This indicates a change in the crystal lattice structure in the magnetic field.     

The low field magnetisation of Y4Co3

The onset of magnetic order at 6 K and a superconducting transition at 2 K was previously observed in Y₄Co₃. In this paper we investigate the magnetisation in very low fields, in the vicinity of the magnetic and superconducting transition (T_c). Below T_c the M-H curves are characterised by broad hysteresis loops. This type of the behaviour indicates that the sample consists of different regions. Some of these regions are superconducting below T_c and others are magnetically ordered below 6 K.     

Magnetic properties of the Nd0.95Dy0.05Fe3(BO3)4 ferroborate with small substitution in the rare-earth element subsystem

The magnetic properties of a substituted Nd_0.95Dy_0.05Fe₃(BO₃)₄ ferroborate single crystal with competing Nd-Fe and Dy-Fe exchange interactions are studied experimentally and theoretically. A spontaneous spin-reorientation transition is detected near T = 4.3 K, and anomalies are observed in the low-temperature magnetization curves along trigonal axis c and in basal plane ab. The measured properties and the detected effects are interpreted in terms of a general theoretical approach, which is based on the molecular field approximation and crystal field calculations for a rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility in the range 2–300 K, the anomalies in the magnetization curves for B ‖ c and B ⊥ c in fields up to 1.5 T, and the field and temperature dependences of magnetization in fields up to 9 T are described. The effect of small substitution in the rare-earth subsystem on the magnetic properties is analyzed. The crystal field parameters and the parameters of the R-Fe and Fe-Fe exchange interactions are determined from the experimental data.     

Zeeman and orbital limiting magnetic fields in cuprates: The pseudogap connection

In cuprates, in a view where pairing correlations set in at the pseudogap energy scale T* and acquire global coherence at a lower temperature T_c, the regionT _c⪯ T ⪯ T* is a vast fluctuation regime.T _c andT* vary differently with doping and the question remains about the doping trends of the relevant magnetic field scales: the field H_c2 bounding the superconducting response and the pseudogap closing field H_pg. In-plane thermal (Nernst) and our interlayer (tunneling) transport experiments in Bi₂Sr₂CaCu₂O_8+y report hugely different limiting magnetic fields. Here, based on pairing (and the uncertainty principle) combined with the definitions of the Zeeman energy and the magnetic length, we show that both fields convert to the same pseudogap scaleT* upon transformation as orbital and Zeeman critical fields, respectively. The region of superconducting coherence is confined to the ‘dome’ that coincides with the usual unique upper critical field H_c2 on the strongly overdoped side. We argue that the distinctly different orbital and the Zeeman limiting fields can co-exist owing to charge and spin degrees of freedom separated to different parts of the strongly anisotropic Fermi surface.     

Magnetic properties of PrCo2 and its ternary hydride PrCo2H4

Magnetization and susceptibility data on PrCo₂ and PrCo₂H₄ are presented. The ac susceptibility of PrCo₂ measured in zero dc field displays a sharp and high peak at T_c = (39.9 ± 0.2) K. The magnetization versus temperature curves show ferromagnetic behaviour for B >1 T, but display a maximum at lower values of the applied field. These results, together with the behaviour of the hysteresis loops at different temperatures below T_c, indicate that PrCo₂ orders ferromagnetically, the magnetic hardness increasing strongly for T → 0. The saturation moment at 4.2 K equals 3.9 μ_B per formula unit, as found from the magnetization curve measured in a pulsed-field magnet up to B = 30 T.Similar experiments on PrCo₂H₄ provide evidence that the introduction of hydrogen in PrCo₂ not only destroys the long-range atomic order, but also considerably reduces the ferromagnetic interactions. Such an effect of the hydrogen is commonly observed in cobalt intermetallics. Part of the PrCo₂H₄ is found to have decomposed into PrH₂ and free Co. The clusters of free Co atoms give rise to a maximum in the zero-field ac susceptibility versus temperature curves, similar as observed in spin glasses or magnetic glasses. By increasing the ac frequency, the maximum shifts to higher temperatures. The behavior can be explained in terms of the Néel model for superparamagnetic particles with randomly oriented local anisotropy axes.