There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

Critical phenomena and renormalization-group theory

Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review, we outline some of the progressive milestones in this stimulating field, especially for those single-phase multiferroics where magnetism and ferroelectricity coexist. First, we highlight the physical concepts of multiferroicity and the current challenges to integrate the magnetism and ferroelectricity into a single-phase system. Subsequently, we summarize various strategies used to combine the two types of order. Special attention is paid to three novel mechanisms for multiferroicity generation: (1) the ferroelectricity induced by the spin orders such as spiral and E-phase antiferromagnetic spin orders, which break the spatial inversion symmetry; (2) the ferroelectricity originating from the charge-ordered states; and (3) the ferrotoroidic system. Then, we address the elementary excitations such as electromagnons, and the application potentials of multiferroics. Finally, open questions and future research opportunities are proposed.

Multiferroicity: the coupling between magnetic and polarization orders

Heat-assisted magnetic recording is a promising approach for enabling large increases in the storage density of hard disk drives. A laser is used to momentarily heat the recording area of the medium to reduce its coercivity below that of the applied magnetic field from the recording head. In such a system, the recording materials have a very high magnetic anisotropy, which is essential for the thermal stability of the magnetization of the extremely small grains in the medium. This technology involves new recording physics, new approaches to near field optics, a recording head that integrates optics and magnetics, new recording materials, lubricants that can withstand extremely high temperatures, and new approaches to the recording channel design. This paper surveys the challenges for this technology and the progress that has been made in addressing them.

Heat Assisted Magnetic Recording

1 Introduction to Micromagnetic Recording Physics- 2 Microstructure of Longitudinal Media- 3 Magnetization Dynamics and Thermal Fluctuations in Fine Grains and Films- 4 Measurement of Dynamic Properties in Thin-Films- 5 Thermal Effects in High-Density Recording Media- 6 Dynamic Effects in High-Density Recording Media- 7 Patterned Media- 8 Perpendicular Recording Media- 9 Self-Assembled Magnetic Nanoparticle Arrays- 10 Theory of Magnetotransport for Magnetic Recording- 11 Recording Head Design

The physics of ultra-high-density magnetic recording

We investigate through Monte Carlo simulations the critical behavior of the antiferromagnetic Heisenberg model on a hexagonal lattice. Critical exponents associated with both magnetic and chiral orderings were estimated. The transition temperatures of these two kinds of order were found to be the same within error margins, in agreement with previous results. The calculation of the Binder parameter also demonstrates that the transition is most likely continuous. The exponents obtained agree, for the most part, with those of Kawamura and thus provide further support for the existence of a new universality class.

Finite-size scaling of the frustrated Heisenberg model on a hexagonal lattice

A mean-field theory of spin-density-wave wavevector and polarisation vector rotation induced by the application of a magnetic field to MnSi in its spin-density-wave phase, and a prediction of the associated spin reorientation phase transitions, is presented. The dependences of the Neel temperature and the homogeneous magnetisation on applied magnetic field strength and orientation are studied, as is the staggered susceptibility which diverges at the spin orientation phase transition.

Wavevector and spin reorientation in MnSi

Critical properties of the axial triangular antiferromagnet ${\mathrm{CsNiCl}}_{3}$ and the planar triangular antiferromagnet ${\mathrm{CsMnBr}}_{3}$ under an applied magnetic field are studied using scaling theory. Novel types of multicritical behavior observed in these compounds are associated with n=2 (${\mathrm{CsMnBr}}_{3}$) and n=3 (${\mathrm{CsNiCl}}_{3}$) chiral universality classes recently identified by the renormalization-group analysis. Various experimentally testable predictions are derived on the basis of a scaling ansatz combined with the numerical estimates of critical exponents. In particular, all phase boundaries emanating from the multicritical points are found to be scaled by the corresponding anisotropy-crossover exponent. It is also predicted that the criticality along the high-field critical line of ${\mathrm{CsNiCl}}_{3}$ (paramagnetic to spin flop) is of n=2 chiral universality.

Scaling analysis of the new multicritical behavior of CsMnBr3 and CsNiCl3.

A phenomenological Landau-type free energy forms the basis of a theory for the low-temperature magnetic properties of CsNi${\mathrm{Cl}}_{3}$ under the influence of an applied magnetic field. Three magnetically ordered phases, as well as the paramagnetic phase, are found to coexist at a novel type of multicritical point representing the intersection of one line of first-order transitions and three lines of second-order transitions in the $H\ensuremath{-}T$ phase diagram. The temperature dependences of the critical fields are in good agreement with corresponding experimental data. Similar results are predicted for CsNi${\mathrm{Br}}_{3}$.

Martin L. Plumer

Papers

The physics of ultra-high-density magnetic recording

Finite-size scaling of the frustrated Heisenberg model on a hexagonal lattice

Wavevector and spin reorientation in MnSi

Scaling analysis of the new multicritical behavior of CsMnBr3 and CsNiCl3.

Multicritical point in the magnetic phase diagram of CsNiCl3.