TOPICAL REVIEW: Supermagnetism

A Guide to Monte Carlo Simulations in Statistical Physics: Some necessary background

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

Aging and memory effects in superparamagnets and superspin glasses

Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with high-speed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr(2)O(3) has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr(2)O(3) single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Néel temperature.

http://absimage.aps.org/image/MAR11/MWS_MAR11-2010-000594.pdf

Robust isothermal electric control of exchange bias at room temperature

Strong disorder RG approach of random systems

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.

Comment on "memory effects in an interacting magnetic nanoparticle system".

Domain-wall free energy sigmaF, entropy sigmaS, and the correlation function C(temp) of sigmaF are measured independently in the four-dimensional +/-J Edwards-Anderson (EA) Ising spin glass. The stiffness exponent theta, the fractal dimension of domain walls d(s), and the chaos exponent zeta are extracted from the finite-size scaling analysis of sigmaF, sigmaS, and C(temp), respectively, well inside the spin-glass phase. The three exponents are confirmed to satisfy the scaling relation zeta = d(s)/2 - theta derived by the droplet theory within our numerical accuracy. We also study bond chaos induced by random variation of bonds, and find that the bond and temperature perturbations yield the universal chaos effects described by a common scaling function and the chaos exponent. These results strongly support the appropriateness of the droplet theory for the description of chaos effect in the EA Ising spin glasses.

https://arxiv.org/pdf/cond-mat/0411138.pdf

Temperature Chaos and Bond Chaos in Edwards-Anderson Ising Spin Glasses: Domain-Wall Free-Energy Measurements

The effect of strain on Neel temperature in corundum-type Cr2O3 and Fe2O3 was theoretically studied. We calculated the exchange coupling constants up to the fifth-nearest neighbors using the first-principles density functional method and evaluated the Neel temperature using Monte Carlo simulation of the classical Heisenberg model. We showed that the Neel temperature is enhanced (decreased) by tensile (compressive) strain along the c-axis. The Neel temperatures of Cr2O3 and Fe2O3 are enhanced more than 20 and 7% by 5% strain of the crystal lattice, respectively.

http://iopscience.iop.org/article/10.7567/APEX.6.113007/pdf

Strain-Induced Néel Temperature Enhancement in Corundum-Type Cr2O3 and Fe2O3

Recently, Sun et al reported that striking memory effects had been clearly observed in their new experiments on an interacting nanoparticle system [1]. They claimed that the phenomena evidenced the existence of a spin-glass-like phase and supported the hierarchical model. No doubt that a particle system may display spin-glass-like behaviors [2]. However, in our opinion, the experiments in Ref. [1] cannot evidence the existence of spin-glass-like phase at all. We will demonstrate below that all the phenomena in Ref. [1] can be observed in a non-interacting particle system with a size distribution. Numerical simulations of our experiments also display the same features.

Munetaka Sasaki

Papers

Aging and memory effects in superparamagnets and superspin glasses

Comment on "memory effects in an interacting magnetic nanoparticle system".

Temperature Chaos and Bond Chaos in Edwards-Anderson Ising Spin Glasses: Domain-Wall Free-Energy Measurements

Strain-Induced Néel Temperature Enhancement in Corundum-Type Cr2O3 and Fe2O3

Comment on: Memory effects in an interacting magnetic nanoparticle system. Authors' reply