Magnetic correlations and 139La NQR relaxation in La2−x Sr x CuO4−y

Summary ¹³⁹La NQR spin-spin and spin-lattice relaxation measurements in La_2−x Sr _x CuO_4−y are presented, forx ranging from zero to 0.3 and for variable oxygen content, in the temperature range (1.6÷450) K. The data are analysed in terms of the different possible relaxation mechanisms. The dominant relaxation mechanism over most of thex andT range is associated with the Cu⁺⁺−Cu⁺⁺ magnetic correlations and spin dynamics. To explain the data we introduce a heuristic model whereby the degree of magnetic correlations, that would result from the strength of the Cu−Cu exchange coupling, is reduced by mobile charge defects (solitons or holons) causing a liquidlike thermal bath of magnetic excitations described by a concentration-dependent and thermally activated correlation time τ_d. The activation energy for τ_d is inversely proportional to the concentration of mobile defects. It is shown that theT ₁ data yield thex andT dependence of the correlation length, with quantitative estimate in agreement with neutron scattering results. Some implications of the model on other quantities, like magnetic susceptibility,x dependence of the Neel temperature and electric conductivity, are briefly discussed.

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Riassunto Sono presentate misure di rilassamento NQR spin-spin e spin-reticolo per¹³⁹La in La_2−x Sr _x CuO_4−y , perx compreso tra 0 e 0.3 e a contenuto variabile di ossigeno, nell'intervallo di temperaturaT tra 1.6 e 450 K. I risultati delle misure vengono dapprima analizzati alla luce dei possibili meccanismi di rilassamento e si mostra come in un ampio intervallo di valori dix e diT il meccanismo dominante è costituito dalla dinamica e dalle correlazioni degli spin di Cu⁺⁺. Allo scopo di spiegare i risultati ottenuti, viene introdotto un modello euristico in cui il grado di correlazione magnetica che conseguirebbe dall'interazione di superscambio Cu−Cu viene ridotto da difetti mobili (solitoni o holoni) che insediano un bagno termico di eccitazioni magnetiche simile a un liquido, descritto da un tempo di correlazione τ_d che dipende dalla concentrazione di tali difetti ed è termicamente attivato. L'energia di attivazione per τ_d risulta inversamente proporzionale alla concentrazione di difetti mobili. Oltre a giustificare compiutamente le risultanze sperimentali, tale modello consente di ricavare quantitativamente la dipendenza dax e daT della lunghezza di correlazione, che risulta in accordo con quella nota aT=77K dallo scattering di neutroni. Sono anche brevemente discusse le implicazioni del modello su altre grandezze come la suscettività magnetica, la dipendenza della temperatura di Neel dax e la conducibilità elettrica.

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Резюме Приводятся результаты измерений¹³⁹La NQR спин-спиновой и спинрешеточной релаксации в La_2−x Sr _x CuO_4−y дляx в области от 0 доx=0.3 и для различного содержания кислорода, в области температур (1.6÷450)K. Полученные данные анализируются в терминах различных механизмов релаксации. Доминиркющий механизм релаксации для большенства значенийx и в рассматриваемой области температур связан с магнитными корреляциями Cu⁺⁺−Cu⁺⁺ и спиновой динамикой. Для объяснения полученных данных мы вводим эвристическую модель, с помощью которой степень магнитных корреляций, которые зависят от силы Cu−Cu обменной связи, сводится к подвижным заряженным дефектам (солитовы или голоны), обусловленным термостатом магнитных возбужений, описываемых зависящим от концентрации и термически активированным временем корреляции τ_d. Энергия активации для τ_d обратно пропорциональна концентрации подвижных дефектов. Показывается, что данные дляT ₁ определяют зависимость корреляции отx и отT, причем количественная оценка согласуется с данными по рассеянию нейтронов. Вкратце обсуждаются возможные применения предложенной модели к другим величинам: магнитной восприимчивости, зависимости температуры Нееля (точки Кюри) отx и электропроводности.

     

Mn42Al50-xFe8+x合金的磁性和磁热效应

研究了Mn₄₂Al_50-xFe_8+x合金的结构、磁性和磁热效应. 通过成分调节, 居里温度T_C在室温附近一宽温区连续可调, 分别为270 K (Mn₄₂Al₄₂Fe₁₆), 341 K (Mn₄₂Al₄₀Fe₁₈)和370 K(Mn₄₂Al₃₈Fe₂₀). 磁化强度在相变温度处发生一陡降, 热磁曲线和等温磁化曲线均未观察到热和磁的滞后, 表明发生一可逆的二级相变. 在各自居里温度附近, 0-5 T的外磁场变化下磁熵变峰值分别为2.48, 2.52和2.40 J·kg^-1·K^-1. Mn_50-xAl_50-yFe_x+y合金的磁熵变峰值虽然与许多优良的磁制冷材料相比并不大, 但是制备该化合物的原材料价格非常低廉, 制备工艺简单, 加工成型也较容易, 化合物本身耐腐蚀性、延展性较好, 且在居里温度附近发生的是可逆的二级相变, 无晶格或结构的变化, 有利于制冷剂的多次循环使用. 关键词： 磁性 磁热效应 二级相变     

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)     

51V nuclear spin-lattice relaxation in V0.75-xGa0.25+x

Nuclear spin-lattice relaxation-time measurements of ⁵¹V in A-15-phase V_0.75-xGa_0.25+x have been made in both well-annealed and neutron-irradiated samples. A tight-binding analysis of the results indicates a strong correlation of T_c with the density of states at the Fermi level in the δ-subbands.     

Glassy spin dynamics in non-fermi-liquid UCu5-xPdx, x = 1.0 and 1.5

Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu5-xPdx, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function G(t) indicates strongly inhomogeneous spin fluctuations and exhibits the scaling G(t,H) = G(t/H(gamma)) expected from glassy dynamics. At 0.05 K gamma(x = 1.0) = 0.35+/-0.1, but gamma(x = 1.5) = 0.7+/-0.1. This is in contrast to inelastic neutron scattering results, which yield gamma = 0.33 for both concentrations. There is no sign of static magnetism approximately greater than 10(-3)(B)/U ion in either material above 0.05 K. Our results strongly suggest that both alloys are quantum spin glasses.     

Magnetic structure of L{i_{1 - x}}N{i_{1 + x}}{o_2}

We study the magnetic structure of layered Li_1-xNi_1+xO₂ and propose a new scheme: the AF interaction between the excess Ni²⁺ in the Li layers and the Ni³⁺ ions in the Ni planes, gives rise to the formation of ferrimagnetic clusters, which control the physics of these systems. The values of the different interactions are estimated from a mean field calculation in the high temperature limit. For the small x samples studied here the method does not yield an accurate value of , but it is very sensitive to the intralayer interactions, allowing to conclude that they are ferromagnetic. The recent proposal of a quantum spin-orbital liquid in this system is discussed and the comparison with Jahn-Teller distorted NaNiO₂ is made. Received 8 December 1999     

Orbital-selective Mott-insulator transition in Ca 2 - x Sr x RuO 4

The electronic structures of the metallic and insulating phases of the alloy series Ca_2-xSr_xRuO₄ ( 0 ? x ? 2) are calculated using LDA, LDA+U and Dynamical Mean-Field Approximation methods. In the end members the groundstate respectively is an orbitally non-degenerate antiferromagnetic insulator (x = 0) and a good metal (x = 2). For x > 0.5 the observed Curie-Weiss paramagnetic metallic state which possesses a local moment with the unexpected spin S = 1/2, is explained by the coexistence of localized and itinerant Ru-4d-orbitals. For 0.2 < x < 0.5 we propose a state with partial orbital and spin ordering. An effective model for the localized orbital and spin degrees of freedom is discussed. The metal-insulator transition at x = 0.2 is attributed to a switch in the orbital occupation associated with a structural change of the crystal. Received 27 July 2001     

Rotational excitations of NH 4 + in (NH4) x K1−x I

We propose a gaussian model for the distribution of rotational tunneling lines which constitute the experimentally observed inelastic neutron scattering spectra in the range 0 to 0.6 meV from the mixed system (NH₄) _x K_1−x I. The intensity profiles generated by this model are in remarkable agreement with the experimentally observed ones.     

Zn掺杂n型笼合物Ba8Ga16-2xZnxGe30+x的热电传输特性

用高温熔融结合放电等离子烧结法制备了Zn掺杂单相n型Ba8Ga16-2xZnxGe30 x笼合物,探索了Zn对Ga的取代对其热电传输特性的影响规律.研究结果表明,n型Ba8Ga16-2xZnxGe30 x化合物的电导率随着x的增加逐渐增大,Seebeck系数随着x的增加而逐渐减小.当Zn完全取代Ga时,Ba8Zn8Ge38化合物的电导率反而急剧下降,Seebeck系数显著增大.Ba8Ga16-2xZnxGe30 x化合物的载流子迁移率随着温度的升高而降低,当Zn掺杂后,化合物的载流子迁移率有一定的增加,随着x的增加而逐渐增大.Ba8Ga16-2xZnxGe30 x化合物的热导率和晶格热导率变化规律类似,随着x的增加先减小后增大.在所有n型Ba8Ga16-2xZnxGe30 x笼合物中,Ba8Ga8Zn4Ge34化合物的ZT值最大,在1000 K时其最大ZT值达0.85.     

Magnetic relaxation in high-temperature superconductors

The magnetic relaxation rate was calculated for the vortex-glass or collective-creep model and the thermal activated model considering backward hopping. By comparing the theoretical results with experimental data it was found that the former models were in good agreement with the experiment at relatively low temperature regions, while the latter were at higher temperatures near T_c. These results are discussed in detail.     

Magnetic relaxation in nanocrystalline iron-oxides

Nanocrystalline Fe oxides are compacted solids of fine particles. By compacting interfaces are formed between adjacent particles. Since collective magnetic relaxation and superparamagnetism depend on the anisotropic energy, it is expected that the existence of interfaces and related interface anisotropy will change the relaxation behavior of nanocrystalline Fe oxides. Measurements on nanocrystalline α-Fe₂O₃ and γ-Fe₂O₃ have shown that the exchange interaction in the interfaces is more effective in suppressing superparamagnetic relaxation upon compacting than the dipole interaction.     

Percolative phase separation in La2CuO4+δ and La2−x Sr x CuO4

We report on the formation of conducting phases in slightly doped La₂CuO₄ samples by the existence of a percolative phase separation. Phase separation can be quenched by rapid cooling and can be restored by the application of a 3 T magnetic field. Magnetically polarizable quasiparticles are shown to be formed by hole doping which fuse to form percolative conducting and below 37 K superconducting phases.     

Magnetic behavior of the oxide spinels: Li0.5Fe2.5−2x Al x Cr x O4

In order to study the effect of substitution of Fe³⁺ by Al³⁺ and Cr³⁺ in Li_0.5Fe_2.5O₄ on its structural and magnetic properties, the spinel system Li_0.5Al _x Cr _x Fe_2.5?2x O₄ (x=0.0, 0.2, 0.4, 0.5, 0.6 and 0.8) has been characterized by X-ray diffraction, high field magnetization, low field ac susceptibility and ⁵⁷Fe Mossbauer spectroscopy. Contrary to the earlier reports, about 50% of Al³⁺ is found to occupy the tetrahedral sites. The system exhibits canted spin structure and a central paramagnetic doublet was found superimposed on magnetic sextet in the Mössbauer spectra (x>0.5).     

Magnetic phase present in YBa2(Cu1−x Fe x )3O6+δ oxides

The Mössbauer study of⁵⁷Fe: YBa₂Cu₃O_6+δ oxides was very important to establish the preferential occupation of Cu(1) site by Fe at very low concentrations. Recent determination of antiferromagnetic ordering for Cu moments in Cu(2) sites (T≈450 K) and our early observation of a small proportion of a magnetic phase at room temperature for Fe:YBa₂Cu₃O₆ lead us to perform systematic studies of YBa₂(Cu_1?x Fe _x )₃O₆ withx=0.005, 0.03, 0.05, 0.10 and 0.15 in order to obtain information about the iron occupation of Cu(2) sites.     

Magnetic properties of the mixed spinel Mg1+xMnxFe2-2xO4

The structural and magnetic properties of the mixed spinel Mg_1+xMn_xFe_2-2xO₄ system for 0.1<= x <= 0.9 have been studied by means of X-ray diffraction, magnetization, a.c. susceptibility and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Mn⁴⁺ ions occupy only octahedral (B) sites replacing Fe³⁺ ions and the added Mg²⁺ ions substitute for A-site Fe³⁺ ions. All samples are magnetic at 12 K displaying Mössbauer spectra that have magnetic sextets coexisting with a central doublet that increases in population with increasing Mn concentration, indicating the presence of short range ordering (clustering). The Mössbauer intensity data show that Mn possesses a preference for the B-site of the spinel over the whole range of concentration. As expected, the hyperfine field and Curie temperature determined from a.c.susceptibility data decrease with increasing Mn content. Magnetization results indicate that on increasing dilution x, the collinear ferrimagnetic phase breaks down at x = 0.3 before reaching the ferrimagnetic percolation limit (x=0.6), as a result of the presence of competing exchange interactions, which is well supported by Mössbauer results. From all the above results, it is proposed that with increasing Mn content from x=0.6 to 0.9, the frustration and disorder increase in the system suppressing the ferrimagnetic ordering, and the system approaches to a cluster spin glass type of ordering at x=0.8 as reflected in the a.c.susceptibility and Mössbauer spectrum.     

Magnetic relaxation in pulse-magnetized high-temperature superconductors

A magnetic filed relaxation at the center of a pulse-magnetized single-domain Y–Ba–Cu–O superconductor at 78 K has been studied. In case of a weak magnetization, the magnetic flux density increases logarithmically and normalized relaxation rate defined as S = −d(lnB)/d(lnt) is negative (S = −0.037). When an external magnetic field magnitude increases, the relaxation rate first decreases in absolute value, then changes sign (becomes positive, S > 0) and after reaching some maximum finally reduces to a very small value. Non-monotonous dependence of S vs. H_a is explained by a non-homogeneous local temperature distribution during a pulse magnetization.