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

Magnetic skyrmions are particle-like nanometre-sized spin textures of topological origin found in several magnetic materials, and are characterized by a long lifetime. Skyrmions have been observed both by means of neutron scattering in momentum space and microscopy techniques in real space, and their properties include novel Hall effects, current-driven motion with ultralow current density and multiferroic behaviour. These properties can be understood from a unified viewpoint, namely the emergent electromagnetism associated with the non-coplanar spin structure of skyrmions. From this description, potential applications of skyrmions as information carriers in magnetic information storage and processing devices are envisaged.

Topological properties and dynamics of magnetic skyrmions

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

“…. For the truth of the conclusions of physical science, observation is the supreme Court of Appeal….” (Sir Arthur Eddington, The Philosophy of Physical Science.) In this paper we shall review the theoretical and experimental results obtained on simple magnetic model systems. We shall consider the Heisenberg, XY and Ising type of interaction (ferro and antiferromagnetic), on magnetic lattices of dimensionality 1, 2 and 3. Particular attention will be paid to the approximation of these model systems in real crystals, viz. how they can be realized or be expected to exist in nature. A large number of magnetic compounds which, according to the available experimental information, meet the requirements set by one or the other of the various models are considered and their properties discussed. Many examples will be given that demonstrate to what extent experiments on simple magnetic systems support theoretical descriptions of magnetic ordering phenomena and contribute to their understanding. It will a...

Experiments on simple magnetic model systems

Antiferromagnetic materials could represent the future of spintronic applications thanks to the numerous interesting features they combine: they are robust against perturbation due to magnetic fields, produce no stray fields, display ultrafast dynamics and are capable of generating large magneto-transport effects Intense research efforts over the past decade have been invested in unraveling spin transport properties in antiferromagnetic materials Whether spin transport can be used to drive the antiferromagnetic order and how subsequent variations can be detected are some of the thrilling challenges currently being addressed Antiferromagnetic spintronics started out with studies on spin transfer, and has undergone a definite revival in the last few years with the publication of pioneering articles on the use of spin-orbit interactions in antiferromagnets This paradigm shift offers possibilities for radically new concepts for spin manipulation in electronics Central to these endeavors are the need for predictive models, relevant disruptive materials and new experimental designs This paper reviews the most prominent spintronic effects described based on theoretical and experimental analysis of antiferromagnetic materials It also details some of the remaining bottlenecks and suggests possible avenues for future research

Antiferromagnetic spintronics

Orientation of erythrocytes in a strong static magnetic field.

Magnetization measurements in strong pulsed magnetic fields on single-crystal samples of the S=1 linear-chain Heisenberg antiferromagnet Ni(${\mathrm{C}}_{2}$${\mathrm{H}}_{8}$${\mathrm{H}}_{2}$${)}_{2}$${\mathrm{NO}}_{2}$(${\mathrm{ClO}}_{4}$) are reported. The magnetization is very small at the low-field region and begins to increase sharply at a finite field for all of the three principal axes. This is evidence for the existence of the Haldane gap in this compound.

Magnetization process of an S=1 linear-chain Heisenberg antiferromagnet

A new model, which is able to explain systematically for both the natures of athermal and isothermal martensitic transformations, has been proposed, referring to the experimental results obtained on the effect of magnetic fields on those martensitic transformations in Fe-Ni-Mn alloys. The model has been derived by making three assumptions, in which a new condition is introduced that martensitic transformation may start when some numbers of particles make a cluster and are simultaneously excited in any place in the austenite. According to the model, the following characteristics were well explained: No C-curve exists for the athermal transformation and a C-curve does for the isothermal one, depending on a difference in their temperature dependence of the Gibbs chemical free energy difference between the austenitic and martensitic states

A New Model Explainable for Both the Athermal and Isothermal Natures of Martensitic Transformations in Fe–Ni–Mn Alloys

Magnetization and magnetoresistance of itinerant electron helical magnet MnSi has been studied down to 0.45 K. A large negative magnetoresistance is observed especially around Neel temperature T N in accord with the theoretical expectation. Below 1 K, however, a positive magnetoresistance is found and it is attributed to the normal magnetoresistance due to impurity scattering. Anomalous peaks in the magnetization and magnetoresistance are observed just below T N between about 1.0 and 2.2 kOe and the existence of two new phases are suggested.

Magnetization and Magnetoresistance of MnSi. I

Magnetic properties of a layered compound MnPS 3 were investigated by means of magnetic susceptibility, high field magnetization and electron spin resonance. Temperature and angular dependence of EPR line width are characteristic of 2D-antiferromagnet. The main exchange interaction in the a b -plane is estimated to be J / k =- 9.5±0.2 K. High field magnetization process and AFMR measurements at low temperatures below T N =78 K showed a spin-flop transition at H c =38.0 kOe. They are satisfactorily explained by assuming the uniaxial anisotropy energy K =1.52±0.10×10 5 erg/cm 3 perpendicular to the a b -plane.

Muneyuki Date

Papers

Orientation of erythrocytes in a strong static magnetic field.

Magnetization process of an S=1 linear-chain Heisenberg antiferromagnet

A New Model Explainable for Both the Athermal and Isothermal Natures of Martensitic Transformations in Fe–Ni–Mn Alloys

Magnetization and Magnetoresistance of MnSi. I

Magnetic Properties of Layered Compound MnPS3