A. Rigamonti

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

Summary139La NQR spin-spin and spin-lattice relaxation measurements in La2−xSrxCuO4−y are presented, for x 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 the x and T 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 τd. The activation energy for τd is inversely proportional to the concentration of mobile defects. It is shown that the T1 data yield the x and T 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.RiassuntoSono presentate misure di rilassamento NQR spin-spin e spin-reticolo per 139La in La2−xSrxCuO4−y, per x compreso tra 0 e 0.3 e a contenuto variabile di ossigeno, nell’intervallo di temperatura T 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 di x e di T 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 τd che dipende dalla concentrazione di tali difetti ed è termicamente attivato. L’energia di attivazione per τd risulta inversamente proporzionale alla concentrazione di difetti mobili. Oltre a giustificare compiutamente le risultanze sperimentali, tale modello consente di ricavare quantitativamente la dipendenza da x e da T della lunghezza di correlazione, che risulta in accordo con quella nota a T=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 da x e la conducibilità elettrica.РезюмеПриводятся результаты измерений 139La NQR спин-спиновой и спинрешеточной релаксации в La2−xSrxCuO4−y для x в области от 0 до x=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 значений x 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 τd. Энергия активации для τd обратно пропорциональна концентрации подвижных дефектов. Показывается, что данные для T1 определяют зависимость корреляции от x и от T, 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) от x и электропроводности.

Slice-selection method for topical NMR in solids

Abstract A method to perform NMR and relaxation measurements in narrow slices or lines of a solid is described. The method is based on acquisition and averaging of free induction decays with stepped field gradients. An FID signal is recorded for a given gradient; after a time of the order of T1 another gradient is applied and the FID is coadded to the previous one, et cetera. The method has the peculiarity of requiring H1 only of the order of the intrinsic linewidth while the maximum gradient can be increased up to a value giving offsets much larger than H1.

Critical dynamics at the structural phase transitions in NaNO2 and NaNbO3 and quadrupole relaxation T1 and T1ϱ in high fields

Abstract Measurements of quadrupole spin-lattice relaxation in the laboratory and in the rotating frames for 23 Na in NaNO 2 and for 23 Na and 93 Nb in NaNbO 3 are reported and discussed. From the analysis of the temperature dependence of the critical contribution to T 1 and T 1 ϱ , information is obtained on the dimensionality and the characteristic frequencies of fluctuations. For the ferroelectric order-disorder transition in NaNO 2 , the orientational fluctuations in adjacent chains of the NO 2 − dipoles are uncorrelated and their frequencies remain greater than ω L = 130 MHz even at the transition. For the typical displacive cubic-tetragonal transition in NaNbO 3 at T c = 641°C the fluctuations of the oxygen octahedra have a quasiplanar character and a strong slowing down occurs so that at T ≅ T c + 4°C the characteristic frequency at the critical wave vector is about 270 MHz. An analysis of T 1 ϱ near the phase transition is given, taking into account the fact that a further mechanism of relaxation due to vacancy diffusion is present, as revealed by the high-temperature measurements.

Magnetic properties, hyperfine interactions, and spin dynamics in the layer compounds CsVF4 and RbVF4

The magnetic properties of the layer compounds CsVF4 and RbVF4 have been investigated by means of magnetic susceptibility measurements and by NMR and relaxation measurements of 19F, 133Cs, and 87Rb nuclei. The crystals are found to undergo an antiferromagnetic transition at about 50 K with weak ferromagnetic behavior and pronounced two‐dimensional correlations. The paramagnetic shift and the relaxation rates are analyzed in terms of the hyperfine fields generated by the magnetic vanadium ions.

31P NMR relaxation study of spin dynamics in layered transition‐metal compounds MPX3

Measurements of 31P NMR relaxation times in layered antiferromagnets FePSe3, FePS3, and NiPS3 have been carried out around the transition temperature TN to the ordered state, in the presence of different external magnetic fields. The relaxation rates have been related to the fluctuations of the hyperfine magnetic field at the P site due to the six nearest‐neighbors magnetic ions. From the temperature behavior of the relaxation rates, information on the correlation properties of the spin fluctuations and on their dependence from the external field has been extracted.

Magnetic field effect on sup 63 Cu nuclear relaxation rates in YBa sub 2 Cu sub 3 O sub 7 and in La sub 1. 85 Sr sub 0. 15 CuO sub 4 above and below T sub c

Nuclear electric quadrupole measurements of {sup 63}Cu nuclear spin-lattice relaxation rates ({ital R}{sub 1}) in zero applied magnetic field and nuclear magnetic resonance {ital R}{sub 1} in 8.2 T are reported for YBa{sub 2}Cu{sub 3}O{sub 7} and La{sub 1.85}Sr{sub 0.15}CuO{sub 4}. A direct effect of the magnetic field is seen for the first time both above and below {ital T}{sub {ital c}}. Above {ital T}{sub {ital c}} the effect is ascribed to the partial suppression of antiferromagnetic (AF) spin fluctuations. For {ital T}{lt}{ital T}{sub {ital c}} the effect goes in the opposite sense and may be related to the spin-temperature diffusion for the core of the fluxons or to the enhancement of AF spin fluctuations persisting in the superconducting phase.