Petra Jönsson

Aging and memory effects in superparamagnets and superspin glasses

Many dense magnetic nanoparticle systems exhibit slow dynamics which is qualitatively indistinguishable from that observed in atomic spin glasses and its origin is attributed to dipole interactions among particle moments (or superspins). However, even in dilute nanoparticle systems where the dipole interactions are vanishingly small, slow dynamics is observed and is attributed solely to a broad distribution of relaxation times which in turn comes from that of the anisotropy energy barriers. To clarify characteristic differences between the two types of slow dynamics, we study a simple model of a noninteracting nanoparticle system (a superparamagnet) analytically as well as ferritin (a superparamagnet) and a dense

Memory and superposition in a spin glass

{\mathrm{Fe}}_{3}\mathrm{N}

The ambipolar diffusion coefficient in silicon: Dependence on excess‐carrier concentration and temperature

nanoparticle system (a superspin glass) experimentally. It is found that superparamagnets in fact show aging (a waiting time dependence) of the thermoremanent magnetization as well as various memory effects. We also find some dynamical phenomena peculiar only to superspin glasses such as the flatness of the field-cooled magnetization below the critical temperature and memory effects in the zero-field-cooled magnetization. These dynamical phenomena are qualitatively reproduced by the random energy model, and are well interpreted by the so-called droplet theory in the field of spin-glass study.

Critical dynamics of an interacting magnetic nanoparticle system

Magnetic properties of nanoparticle systems and spin glasses have been investigated theoretically, and experimentally by squid magnetometry.Two model three-dimensional spin glasses have been studied: a long-range Ag(11 at% Mn) Heisenberg spin glass and a short-range Fe0.5Mn0.5TiO3 Ising spin glass. Experimental protocols revealing ageing, memory and rejuvenation phenomena are used. Quantitative analyses of the glassy dynamics within the droplet model give evidences of significantly different exponents describing the nonequilibrium dynamics of the two samples. In particular, non-accumulative ageing related to temperature-chaos is much stronger in Ag(11 at% Mn) than in Fe0.5Mn0.5TiO3.The physical properties of magnetic nanoparticles have been investigated with focus on the influence of dipolar interparticle interaction. For weakly coupled nanoparticles, thermodynamic perturbation theory is employed to derive analytical expressions for the linear equilibrium susceptibility, the zero-field specific heat and averages of the local dipolar fields. By introducing the averages of the dipolar fields in an expression for the relaxation rate of a single particle, a non trivial dependence of the superparamagnetic blocking on the damping coefficient is evidenced. This damping dependence is interpreted in terms of the nonaxially symmetric potential created by the transverse component of the dipolar field.Strongly interacting nanoparticle systems are investigated experimentally in terms of spin-glass behaviour. Disorder and frustration arise in samples consisting of frozen ferrofluids from the randomness in particle position and anisotropy axes orientation. A strongly interacting system is shown to exhibit critical dynamics characteristic of a spin-glass phase transition. Ageing, memory and rejuvenation phenomena similar to those of conventional spin glasses are observed, albeit with weak temperature-chaos effects.

Comment on "Memory Effects in an Interacting Magnetic Nanoparticle System"

Recent articles have been published claiming that the ambipolar diffusion coefficient of semiconductors, Da, is basically independent of the excess‐carrier concentration, thus contradicting the conventional result of, for example, Fletcher. However, only a few experimentally verified papers on this subject are published. In the present work, a detailed experimental analysis of the ambipolar diffusion coefficient is presented regarding both temperature and injection‐level dependence. The ambipolar diffusion coefficient was measured in the low‐doped n base of a p‐i‐n type diode at different excess‐carrier concentrations and temperatures using an open‐circuit carrier decay (OCCD) method based on the free‐carrier absorption (FCA) technique. This investigation was performed in the carrier‐concentration range of 1015–2×1017 cm−3 and in the temperature range of 300–420 K, respectively. The ambipolar diffusion coefficient is experimentally found to behave in reasonable agreement with Fletcher’s theory, thus decre...

Nonequilibrium dynamics of spin glasses: Examination of the ghost domain scenario

Effects of dipole-dipole interactions on the magnetic relaxation have been investigated for three Fe-C nanoparticle samples with volume concentrations of 0.06, 5 and 17 vol%. While both the 5 and 17 vol% samples exhibit collective behaviour due to dipolar interactions, only the 17 vol% sample displays critical behaviour close to its transition temperature. The behaviour of the 5 vol% sample can be attributed to a mixture of collective and single-particle dynamics.

Nonequilibrium dynamics in an interacting Fe-C nanoparticle system

In Phys. Rev. Lett. 91 167206 (2003), Sun et al. study memory effects in an interacting nanoparticle system with specific temperature and field protocols. The authors claim that the observed memory effects originate from spin-glass dynamics and that the results are consistent with the hierarchical picture of the spin-glass phase. In this comment, we argue their claims premature by demonstrating that all their experimental curves can be reproduced qualitatively using only a simplified model of isolated nanoparticles with a temperature dependent distribution of relaxation times.

Absence of strong rejuvenation in a superspin glass

Extensive experimental and numerical studies of the non-equilibrium dynamics of spin glasses subjected to temperature or bond perturbations have been performed to investigate chaos and memory effects in selected spin glass systems. Temperature shift and cycling experiments were performed on the strongly anisotropic Ising-like system {\ising} and the weakly anisotropic Heisenberg-like system {\AgMn}, while bond shift and cycling simulations were carried out on a 4 dimensional Ising Edwards-Anderson spin glass. These spin glass systems display qualitatively the same characteristic features and the observed memory phenomena are found to be consistent with predictions from the ghost domain scenario of the droplet scaling model.

Thermodynamic perturbation theory for dipolar superparamagnets

Magnetic properties of nanoparticle systems and spin glasses have been investigated theoretically, and experimentally by squid magnetometry.Two model three-dimensional spin glasses have been studied: a long-range Ag(11 at% Mn) Heisenberg spin glass and a short-range Fe0.5Mn0.5TiO3 Ising spin glass. Experimental protocols revealing ageing, memory and rejuvenation phenomena are used. Quantitative analyses of the glassy dynamics within the droplet model give evidences of significantly different exponents describing the nonequilibrium dynamics of the two samples. In particular, non-accumulative ageing related to temperature-chaos is much stronger in Ag(11 at% Mn) than in Fe0.5Mn0.5TiO3.The physical properties of magnetic nanoparticles have been investigated with focus on the influence of dipolar interparticle interaction. For weakly coupled nanoparticles, thermodynamic perturbation theory is employed to derive analytical expressions for the linear equilibrium susceptibility, the zero-field specific heat and averages of the local dipolar fields. By introducing the averages of the dipolar fields in an expression for the relaxation rate of a single particle, a non trivial dependence of the superparamagnetic blocking on the damping coefficient is evidenced. This damping dependence is interpreted in terms of the nonaxially symmetric potential created by the transverse component of the dipolar field.Strongly interacting nanoparticle systems are investigated experimentally in terms of spin-glass behaviour. Disorder and frustration arise in samples consisting of frozen ferrofluids from the randomness in particle position and anisotropy axes orientation. A strongly interacting system is shown to exhibit critical dynamics characteristic of a spin-glass phase transition. Ageing, memory and rejuvenation phenomena similar to those of conventional spin glasses are observed, albeit with weak temperature-chaos effects.

Nonequilibrium dynamics in a three-dimensional spin glass

Effects of temperature changes on the nonequilibrium spin-glass dynamics of a strongly interacting ferromagnetic nanoparticle system (superspin glass) are studied. In contrary to atomic spin glasses, strong cooling rate effects are observed, and no evidence for temperature-chaos is found. The flip time of a magnetic moment is much longer than that of an atomic spin and hence much shorter time scales are probed within the experimental time window for a superspin glass than for an atomic spin glass. Within a real space picture the cumulative aging observed for the superspin glass can be explained considering that all investigated length scales are shorter than the temperature-chaos overlap length. The transient relaxation, observed in experiments after temperature changes, can be understood as the adjustment of thermally active droplets, which is mutatis mutandis the Kovacs effect observed in most glassy systems.