This review summarizes recent developments in the theory of spin glasses, as well as pertinent experimental data. The most characteristic properties of spin glass systems are described, and related phenomena in other glassy systems (dielectric and orientational glasses) are mentioned. The Edwards-Anderson model of spin glasses and its treatment within the replica method and mean-field theory are outlined, and concepts such as "frustration," "broken replica symmetry," "broken ergodicity," etc., are discussed. The dynamic approach to describing the spin glass transition is emphasized. Monte Carlo simulations of spin glasses and the insight gained by them are described. Other topics discussed include site-disorder models, phenomenological theories for the frozen phase and its excitations, phase diagrams in which spin glass order and ferromagnetism or antiferromagnetism compete, the Ne\'el model of superparamagnetism and related approaches, and possible connections between spin glasses and other topics in the theory of disordered condensed-matter systems.

Spin glasses: Experimental facts, theoretical concepts, and open questions

This is a tutorial review on the Potts model aimed at bringing out in an organized fashion the essential and important properties of the standard Potts model. Emphasis is placed on exact and rigorous results, but other aspects of the problem are also described to achieve a unified perspective. Topics reviewed include the mean-field theory, duality relations, series expansions, critical properties, experimental realizations, and the relationship of the Potts model with other lattice-statistical problems.

The Potts model

As established in the very early work of Edwin Hall, ferromagnetic conductors have an anomalous Hall conductivity contribution that cannot be attributed to Lorentz forces and therefore survives in the absence of a magnetic field. These anomalous Hall conductivities are normally assumed to be proportional to magnetization. We use symmetry arguments and first-principles electronic structure calculations to counter this assumption and to predict that Mn3Ir, a high-temperature antiferromagnet that is commonly employed in spin-valve devices, has a large anomalous Hall conductivity.

Anomalous Hall effect arising from noncollinear antiferromagnetism.

Research efforts addressing spin waves (magnons) in micro- and nanostructured ferromagnetic materials have increased tremendously in recent years. Corresponding experimental and theoretical work in magnonics faces significant challenges in that spin-wave dispersion relations are highly anisotropic and different magnetic states might be realized via, for example, the magnetic field history. At the same time, these features offer novel opportunities for wave control in solids going beyond photonics and plasmonics. In this topical review we address materials with a periodic modulation of magnetic parameters that give rise to artificially tailored band structures and allow unprecedented control of spin waves. In particular, we discuss recent achievements and perspectives of reconfigurable magnonic devices for which band structures can be reprogrammed during operation. Such characteristics might be useful for multifunctional microwave and logic devices operating over a broad frequency regime on either the macro- or nanoscale.

https://iopscience.iop.org/article/10.1088/0953-8984/26/12/123202/pdf

Review and prospects of magnonic crystals and devices with reprogrammable band structure

Nanostructured magnetic materials have a variety of promising applications spreading from nano-scale electronic devices, sensors and high-density data storage media to controlled drug delivery and cancer diagnostics/treatment systems. Magnetic nanoparticles offer the most natural and elegant way for fabrication of such (multi-) functional materials. In this review, we briefly summarize the recent progress in the synthesis of magnetic nanoparticles (which now can be done with precise control over the size and surface chemistry), and nanoscale interactions leading to their self-assembly into 1D, 2D or 3D aggregates. Various approaches to self-organization, directed-, or template-assisted assembly of these nanostructures are discussed with the special emphasis on magnetic-field enabled interactions. We also discuss new physical phenomena associated with magnetic coupling between nanoparticles and their interaction with a substrate and the characterization of the physical properties at the nanoscale using various experimental techniques (including scanning quantum interferometry (SQUID) and magnetic force microscopy). Applications of magnetic nanoparticle assemblies in data storage, spintronics, drug delivery, cancer therapy, and prospective applications such as adaptive materials and multifunctional reconfigurable materials are also highlighted.

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Magnetic nanoparticles: recent advances in synthesis, self-assembly and applications

The nanomagnetism of monodisperse 7 nm γ-Fe2O3 nanoparticles exhibits unique features due to a significant amount of surface spin disorder. To correctly characterize the superparamagnetism of a dilute dispersion requires including the effects of the magnetic anisotropy and a shell of disordered spins surrounding the ordered core. The nanoparticle shell's disordered spin structure is exchange coupled to that of the ordered core. This enables an exchange bias loop shift, Hex, when the nanoparticle dispersion is field cooled. The surface spin disorder also leads to an unusual exponential-like decrease of the nanoparticle's total saturation magnetization with increasing temperature.

The effect of surface spin disorder on the magnetism of γ-Fe2O3 nanoparticle dispersions

The Edwards and Anderson model (1975) for spin glasses is investigated by a real space rescaling transformation on an S=1/2 Ising Hamiltonian with nearest-neighbour-only interactions in three dimensions. A transition to a spin glass ordered phase is found at about half the mean transition temperature, in contrast to earlier work on a two-dimensional model where no transition was found. The results suggest that the lower critical dimensionality, dc, for spin glass order is slightly greater than 2, although possibly dc=2 exactly. The specific heat is calculated and found to exhibit a broad maximum above Tc, with no discernable singularity at the transition since the exponent alpha is large and negative.

http://iopscience.iop.org/article/10.1088/0022-3719/10/12/023/pdf

Real space rescaling study of spin glass behaviour in three dimensions

An unusual enhancement of the antiferromagnetic ordering (N\'eel) temperature is observed in a ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕{\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ thin film. This significant increase of the N\'eel temperature indicates an exchange energy of interface spins between the ferromagnet (FM) and antiferromagnet (AF) that is significantly greater than that of spins in the bulk of the film. Exchange bias is established even when the thin film is zero field cooled. Field cooling the film sets a significantly stronger exchange coupling between the FM and AF spins at the interface with a concomitant increase in the exchange-bias field.

Proximity effects in an exchange-biased Ni 80 Fe 20 ∕ Co 3 O 4 thin film

We report magnetic measurements on a macroscopic three-dimensional fcc array of iron-oxide nanoparticles. We observe typical nanomagnetism for the randomly packed configuration of nanoparticles, including dynamical freezing and superparamagnetism. By contrast, the nanoparticle "atoms" in the fcc configuration that form the crystal exhibit a low coercivity that is weakly temperature dependent with no superparamagnetism up to 400 K.

Novel aspects of magnetic interactions in a macroscopic 3D nanoparticle-based crystal.

We report room temperature electrical detection of spin injection from a ferromagnetic insulator (YIG) into a ferromagnetic metal (Permalloy, Py). Non-equilibrium spins with both static and precessional spin polarizations are dynamically generated by the ferromagnetic resonance of YIG magnetization, and electrically detected by Py as dc and ac spin currents, respectively. The dc spin current is electrically detected via the inverse spin Hall effect of Py, while the ac spin current is converted to a dc voltage via the spin rectification effect of Py which is resonantly enhanced by dynamic exchange interaction between the ac spin current and the Py magnetization. Our results reveal a new path for developing insulator spintronics, which is distinct from the prevalent but controversial approach of using Pt as the spin current detector.

B. W. Southern

Papers

The effect of surface spin disorder on the magnetism of γ-Fe2O3 nanoparticle dispersions

Real space rescaling study of spin glass behaviour in three dimensions

Proximity effects in an exchange-biased Ni 80 Fe 20 ∕ Co 3 O 4 thin film

Novel aspects of magnetic interactions in a macroscopic 3D nanoparticle-based crystal.

Electrical Detection of Direct and Alternating Spin Current Injected from a Ferromagnetic Insulator into a Ferromagnetic Metal