The vortex state, characterized by a curling magnetization, is one of the equilibrium configurations of soft magnetic materials(1-4) and occurs in thin ferromagnetic square and disk-shaped elements of micrometre size and below. The interplay between the magneto-static and the exchange energy favours an in-plane, closed flux domain structure. This curling magnetization turns out of the plane at the centre of the vortex structure, in an area with a radius of about 10 nanometres-the vortex core(5-7). The vortex state has a specific excitation mode: the in-plane gyration of the vortex structure about its equilibrium position(8-10). The sense of gyration is determined by the vortex core polarization(11). Here we report on the controlled manipulation of the vortex core polarization by excitation with small bursts of an alternating magnetic field. The vortex motion was imaged by time-resolved scanning transmission X-ray microscopy(12). We demonstrate that the sense of gyration of the vortex structure can be reversed by applying short bursts of the sinusoidal excitation field with amplitude of about 1.5 mT. This reversal unambiguously indicates a switching of the out-of-plane core polarization. The observed switching mechanism, which can be understood in the framework of micromagnetic theory, gives insights into basic magnetization dynamics and their possible application in data storage.

Magnetic vortex core reversal by excitation with short bursts of an alternating field

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

https://link.aps.org/accepted/10.1103/RevModPhys.89.025006

Interface-induced phenomena in magnetism

https://macul.ciencias.uchile.cl/~mkiwi/CV86.pdf

Exchange bias theory

Modern applications for thin film magnets involve unique requirements for the control and design of specific magnetic properties. The exchange bias effect in ferromagnet/antiferromagnet bilayers appears to be a useful feature for controlling one of the most important characteristics of a ferromagnet: coercivity. Prospects for control and enhancement of desirable effects depend upon a clear understanding of mechanisms governing exchange bias. The processes underlying the existence and properties of exchange bias are reviewed, with particular emphasis on the roles of interface structure and temperature. Results from numerical simulations are used to illustrate how exchange bias is modified by geometric structures at the interface and randomly placed defects. A general theoretical formulation of the bias problem is proposed, and an expression for the interface energy is derived. A key result is the existence of higher-order coupling terms when more than one sublattice of the antiferromagnet is present at the interface. Results from calculations of finite temperature effects on bias and coercivity are described, and the concept of viscosity in the antiferromagnet is discussed. A brief discussion is also included of how a dynamic linear response, such as ferromagnetic resonance or light scattering, can be used to determine relevant anisotropy and exchange parameters.

https://iopscience.iop.org/article/10.1088/0022-3727/33/23/201/pdf

Mechanisms for exchange bias

This paper reviews the experimental studies of displacive phase transitions in solids. Primary emphasis is upon inelastic light scattering and neutron scattering; related infrared reflectivity measurements, as well as x-ray and EPR analyses are also summarized. Several prototype structures are considered in detail: (1) the rocksalt IV-VI semiconductors PbTe, SnTe, and GeTe; (2) the ferroelectric perovskites exemplified by PbTi${\mathrm{O}}_{3}$ and BaTi${\mathrm{O}}_{3}$; (3) perovskites which exhibit cell-doubling transitions, such as LaAl${\mathrm{O}}_{3}$, SrTi${\mathrm{O}}_{3}$, and KMn${\mathrm{F}}_{3}$; (4) crystals having the $\ensuremath{\alpha}$-quartz structure, including Ge${\mathrm{O}}_{2}$, Si${\mathrm{O}}_{2}$, and AlP${\mathrm{O}}_{4}$; (5) the "improper ferroelectrics" ${\mathrm{Gd}}_{2}$${(\mathrm{M}\mathrm{o}{\mathrm{O}}_{4})}_{3}$ and ${\mathrm{Tb}}_{2}$${(\mathrm{M}\mathrm{o}{\mathrm{O}}_{4})}_{3}$; (6) the V-VI-VII semiconductors typified by SbSI; (7) the hydrogen-bonded ferroelectrics of the K${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$ family; (8) Jahn-Teller systems such as DyV${\mathrm{O}}_{4}$ and RbCo${\mathrm{F}}_{3}$, in which structural distortions occur as secondary effects; (9) order-disorder systems such as NaN${\mathrm{O}}_{2}$ and the ammonium halides (N${\mathrm{H}}_{4}$Cl, N${\mathrm{H}}_{4}$Br), in which no "soft mode" occurs in the spectral region ($\ensuremath{\omega}g{10}^{11}$ Hz) probed by ir, Raman, and neutron spectroscopy; (10) $\ensuremath{\beta}$-tungsten ($A\ensuremath{-}15$) structures such as ${\mathrm{V}}_{3}$Si and ${\mathrm{Nb}}_{3}$Sn, which exhibit high-temperature superconductivity. These crystal categories are used to illustrate several phenomena of current physical interest: Specifically, we discuss harmonic and anharmonic mode coupling; "critical exponents" $\ensuremath{\beta}$ differing from one-half in the temperature dependences of the order parameter $\ensuremath{\phi}(T)={\ensuremath{\phi}}_{0}{(T\ensuremath{-}{T}_{0})}^{\ensuremath{\beta}}$ and of the soft-mode frequency $\ensuremath{\omega}(T)={\ensuremath{\omega}}_{0}{(T\ensuremath{-}{T}_{0})}^{\ensuremath{\beta}}$; and the recently discovered "central" modes centered at zero frequency, which grow in intensity as the transition temperature ${T}_{0}$ is approached from above or below. The review covers the period 1940-1972. A few 1973 works are mentioned for which the author had preprints in 1972 or very early 1973. This review is in no sense a comprehensive survey of ferroelectricity. Readers are referred to the following earlier reviews on that subject: Silverman (1966, 1969), Cochran and Cowley (1967), Blinc (1968), Murzin et al. (1968), Nettleton (1970), and Blinc and Zaks (1972).

Soft-mode spectroscopy: Experimental studies of structural phase transitions

We show how spatial fluctuations of chromium thickness amounting to one atomic monolayer account phenomenologically for the biquadratic magnetic coupling recently reported in epitaxial Fe/Cr/Fe trilayers. Corresponding fluctuations of the short-period oscillatory term of conventional exchange coupling induce static waves of magnetization whose energy has the observed biquadratic form. An existing calculation of conventional exchange and its recent direct measurements in highly ordered trilayers support our interpretation.

Fluctuation mechanism for biquadratic exchange coupling in magnetic multilayers.

EPR spectra of $\mathrm{F}\mathrm{e}\ensuremath{-}{V}_{\mathrm{O}}$ pairs were used to study how the order parameter of SrTi${\mathrm{O}}_{3}$ varies with uniaxial stress applied to a (111) face. A second-order cubic-trigonal phase boundary appears above the stress-free transition temperature ${T}_{a}$. A first-order tetra-gonal-trigonal phase boundary is found below ${T}_{a}$. An independently determined Landau potential describes the results.

Order Parameter and Phase Transitions of Stressed SrTi O 3

The first use of linear magnetic birefringence (optical Cotton-Mouton effect) to observe a magnetic resonance is reported. This allows us to make the first direct observation of resonant motion of a ferromagnetic vortex. This vortex is present in an easy-plane garnet film at the intersection of two 90\ifmmode^\circ\else\textdegree\fi{} N\'eel walls. Agreement between the resonant frequencies and our theory confirms a previously proposed equation of vortex motion.

Magnetic Vortex Dynamics Using the Optical Cotton-Mouton Effect

Effect of Bloch Lines on Magnetic Domain-Wall Mobility

We show that steady translation of a magnetic bubble possessing Bloch lines requires the application of both longitudinal and transverse field gradients. The predicted longitudinal gradient obeys the usual relation valid for a normal bubble. The transverse gradient is proportional to the product of velocity and number of Bloch lines. Experiments of Tabor et al. support the theory.

J. C. Slonczewski

Papers

Fluctuation mechanism for biquadratic exchange coupling in magnetic multilayers.

Order Parameter and Phase Transitions of Stressed SrTi O 3

Magnetic Vortex Dynamics Using the Optical Cotton-Mouton Effect

Effect of Bloch Lines on Magnetic Domain-Wall Mobility

Translational Mobility of Hard Ferromagnetic Bubbles