Spin-glass behavior in Sr2FeRuO6 and BaLaNiRuO6: A comparison with antiferromagnetic BaLaZnRuO6

We present a quantitative comparison between dynamic measurements on spin glasses. The case of CdIn0.3Cr1.7S4 is discussed in detail and references are made to CsNiFeF6 and Ag:Mn (2.6%). The measured quantities are the relaxation of the thermoremanent magnetization MTRM(t) in the range of observation times 1–105 s, the out‐of‐phase susceptibility χ‘(ω) in the range 10−3–5×104 Hz, and the magnetic noise power spectrum M2(ω) between 10−2 and 103 Hz. The aging of the thermoremanent magnetization relaxation is analyzed on the basis of a phenomenological theory of time scaling which allows to derive the form of the equilibrium relaxation. It is shown that the measured values of χ‘ and ∼(M2(ω)) are related according to the fluctuation dissipation theorem (FDT). The consequences of the applicability of the FDT in the frequency domain are discussed, with reference to the expected violation of the theorem in the spin‐glass phase at ω=0. An important modification of the frequency dependence of χ‘(ω) is observed at ...

Spin‐glass dynamics from magnetic noise, relaxation, and susceptibility measurements (invited)

Ac linear and nonlinear susceptibilities, χ0 and χ2, of ferrimagnetic FeCr2S4 single crystals were measured in the temperature range from 4.2 to 300 K for different driving ac and applied dc magnetic fields in the frequency range of 10−1–103 Hz. For high driving ac fields the real part of χ0 exhibits a cusp at around Tm≈60 K correlated with the onset of dc magnetization irreversibilities. The imaginary part of χ0 shows a strong increase below 100 K and nonmonotonic temperature dependence with a maximum shifted toward low temperatures with an increase in the driving field. Both real and imaginary parts of the linear susceptibility, χ0′ and χ0″, show a pronounced frequency dependence between 90 and 20 K with a maximal difference at around 60 K. Below the Curie temperature the real part of the nonlinear susceptibility, χ2, exhibits a broad negative peak which is strongly shifted towards low temperatures with an increase in the driving field. No clearly divergent behavior of χ2′ around Tm is observed. The fie...

Ac susceptibility studies of ferrimagnetic FeCr2S4 single crystals

In this introductory talk, we shall present spin glasses and review the general experimental features of their glassy dynamics [1]. Spin glasses are model magnetic systems in which the interactions are disordered and frustrated, due to a random dilution of magnetic ions. Below their glass temperature Tg , their response to a magnetic excitation is a slow relaxation of the magnetization, which strongly depends on the time spent below Tg (« aging » phenomenon). In contrast with structural and polymer glasses, aging is here hardly influenced by the cooling rate. During cooling, each further step down tends to restart some aging processes (« rejuvenation »), while the memory of previous aging periods, achieved at different temperatures during cooling, can be retrieved when re-heating [1,2]. It is possible to store independently several memories, which corresponds to imprinting the trace of various spin arrangements at well separated length scales that are selected by temperature [3,4]. Similar aging, rejuvenation and memory phenomena can be observed in various other glassy systems [1], making the spin glasses conceptually simple examples for the study of out-of-equilibrium dynamics in disordered systems.

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Ageing, Rejuvenation and Memory: The Example of Spin-Glasses

Magnetically ordered multinary semiconductors

Spin glass behaviour has been observed in a very large concentration range in both CdCr2xIn2-2xS4 and ZnCr2xAl2-24 spinel systems. It is a result of the presence in these systems of competing interactions, ferromagnetic between first-nearest neighbours and antiferromagnetic between higher-order neighbours. The ratios between the interactions are such that the non-dilute x=1 compounds are ferromagnetic for the first system and helimagnetic for the second. The entire phase diagrams of both solutions have been determined by very low field AC and DC susceptibility measurements. The results for the ferromagnetic system are compared with theoretical predictions derived from a bond-diluted Ising model using the replica method and a second-order variational approximation.

M Nogues

Papers

Phase diagrams of two dilute insulating systems with competing interactions: CdCr2xIn2-2xS4 and ZnCr2xAl2-2xS4