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

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

IN two previous notes1, Prof. Max Born and I have shown that one can obtain a theory of superconductivity by taking account of the fact that the interaction of the electrons with the ionic lattice is appreciable only near the boundaries of Brillouin zones, and particularly strong near the corners of these. This leads to the criterion that the metal should be superconductive if a set of corners of a Brillouin zone is lying very near the Fermi surface, considered as a sphere, which limits the region in the momentum space completely filled with electrons.

On the theory of superconductivity

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

Exchange bias in nanostructures

Using the full-potential screened Korringa-Kohn-Rostoker method we study the full-Heusler alloys based on Co, Fe, Rh, and Ru. We show that many of these compounds show a half-metallic behavior; however, in contrast to the half-Heusler alloys the energy gap in the minority band is extremely small due to states localized only at the Co (Fe, Rh, or Ru) sites which are not present in the half-Heusler compounds. The full-Heusler alloys show a Slater-Pauling behavior and the total spin magnetic moment per unit cell ${(M}_{t})$ scales with the total number of valence electrons ${(Z}_{t})$ following the rule ${M}_{t}{=Z}_{t}\ensuremath{-}24.$ We explain why the spin-down band contains exactly 12 electrons using arguments based on group theory and show that this rule holds also for compounds with less than 24 valence electrons. Finally we discuss the deviations from this rule and the differences compared to the half-Heusler alloys.

/pdf/slater-pauling-behavior-and-origin-of-the-half-metallicity-1vlzvqk1h0.pdf

Slater-Pauling behavior and origin of the half-metallicity of the full-Heusler alloys

We have carried out detailed experimental studies of the exchange bias effect of a series of CoO/Co(111) textured bilayers with different Co layer thickness, using the magneto-optical Kerr effect, superconducting quantum interference device (SQUID) magnetometry, polarized neutron reflectivity, x-ray diffraction, and atomic force microscopy. All samples exhibit a pronounced asymmetry of the magnetic hysteresis at the first magnetization reversal as compared to the second reversal. Polarized neutron reflectivity measurements show that the first reversal occurs via nucleation and domain wall motion, while the second reversal is characterized by magnetization rotation. Off-specular diffuse spin-flip scattering indicates the existence of interfacial magnetic domains. All samples feature a small positive exchange bias just below the blocking temperature, followed by a dominating negative exchange bias field with decreasing temperature.

Interfacial domain formation during magnetization reversal in exchange-biased CoO/Co bilayers

Interfacial domain formation during magnetization reversal in exchange biased CoO/Co bilayers

We have studied superconducting and magnetic properties of sputtered Fe/Nb/Fe trilayers. For a fixed Nb thickness and with changing Fe thickness, ${d}_{\mathrm{Fe}}$, a nonmonotonic behavior of the superconducting transition temperature ${T}_{c}$ was observed with a maximum at ${d}_{\mathrm{Fe}}\ensuremath{\approx}10$ \AA{}. The analysis of the magnetization data revealed that for ${d}_{\mathrm{Fe}}\ensuremath{\le}7$ \AA{} the Fe layer is nonmagnetic. The interpretation of the observed ${T}_{c}$ behavior is attributed to the existence of this magnetically "dead" layer and the change of the interaction of the Cooper paris with this layer at the onset of ferromagnetism for ${d}_{\mathrm{Fe}}\ensuremath{\ge}7$ \AA{}.

Possible Origin for Oscillatory Superconducting Transition Temperature in Superconductor/Ferromagnet Multilayers.

We report on measurements of structural, superconducting, and magnetic properties of trilayer and bilayer systems combined of superconducting Pb and ferromagnetic Fe. The Pb/Fe layers can be grown on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ with reasonably flat interfaces, there is no alloying of the components at the interface and Fe is found to be ferromagnetic down to the monolayer range. This is a favorable situation for an $S/F$ proximity system, since it corresponds closely to the situation treated in theoretical models. We find an oscillation of the superconducting transition temperature when plotted versus the thickness of the ferromagnetic layer, which we regard as a clear indication of an unconventional, propagating superconducting pair wave function in the Pb/Fe system. We fit our results using recent theoretical model calculations and find evidence for a strongly reduced transparency of the Pb/Fe interface. We regard this as an essential feature of the proximity effect in Pb/Fe and discuss its physical origin.

Kurt Westerholt

Papers

Interfacial domain formation during magnetization reversal in exchange-biased CoO/Co bilayers

Interfacial domain formation during magnetization reversal in exchange biased CoO/Co bilayers

Possible Origin for Oscillatory Superconducting Transition Temperature in Superconductor/Ferromagnet Multilayers.

Superconductor'ferromagnet proximity effect in Fe'Pb'Fe trilayers

Structural, magnetic and magnetotransport properties of thin films of the Heusler alloys Cu2MnAl, Co2MnSi, Co2MnGe and Co2MnSn