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 this paper, recent developments in magnetic tunnel junctions (MTJs) are reported with their potential impacts on integrated circuits. MTJs consist of two metal ferromagnets separated by a thin insulator and exhibit two resistances, low (Rp) or high (Rap) depending on the relative direction of ferromagnet magnetizations, parallel (P) or antiparallel (AP), respectively. Tunnel magnetoresistance (TMR) ratios, defined as (Rap $Rp)/Rp as high as 361%, have been obtained in MTJs with Co40Fe40B20 fixed and free layers made by sputtering with an industry-standard exchange-bias structure and post deposition annealing at Ta = 400 degC. The corresponding output voltage swing DeltaV is over 500 mV, which is five times greater than that of the conventional amorphous Al-O-barrier MTJs. The highest TMR ratio obtained so far is 500% in a pseudospin-valve MTJ annealed at Ta = 475 degC, showing a high potential of the current material system. In addition to this high-output voltage swing, current-induced magnetization switching (CIMS) takes place at the critical current densities (JCO) on the order of 106 A/cm2 in these MgO-barrier MTJs. Furthermore, high antiferromagnetic coupling between the two CoFeB layers in a synthetic ferrimagnetic free layer has been shown to result in a high thermal-stability factor with a reduced JCO compared to single free-layer MTJs. The high TMR ratio enabled by the MgO-barrier MTJs, together with the demonstration of CIMS at a low JCO, allows development of not only scalable magnetoresistive random-access memory with feature sizes below 90 nm but also new memory-in-logic CMOS circuits that can overcome a number of bottlenecks in the current integrated-circuit architecture

/pdf/magnetic-tunnel-junctions-for-spintronic-memories-and-beyond-2tm3rrxd78.pdf

Magnetic Tunnel Junctions for Spintronic Memories and Beyond

Interlayer Exchange Coupling

Dynamic relaxations and relaxation-property relationships in metallic glasses

Manganese oxides (R, A)MnO3 (R = La, Y, Bi or rare earth metal, A = non-trivalent doping element) have been one focus of research for two outstanding properties: (i) conduction electrons (if any) are highly spin-polarized and (ii) competing interactions of several electronic and lattice degrees of freedom lead to extremely rich phase diagrams and complex physics. A better understanding of the latter might result in future technologies using manganites in spin-electronic devices. This review attempts to systematically outline in a phenomenological approach some fundamentals and key experiments on ferromagnetic manganites, partially with respect to thin film structures. Macroscopic magnetic properties (of ferromagnets and glassy manganites) and intrinsic electron transport are described. Preparation and basic properties of thin films, followed by experiments on spin-polarized tunnelling are reviewed. Finally, multiferroic heteroepitaxial film systems of manganites and ferroelectric titanates are addressed, which are promising candidates for significant magnetoelectric phenomena at ambient temperatures.

http://iopscience.iop.org/article/10.1088/0022-3727/39/7/R01/pdf

Ferromagnetic manganites: spin-polarized conduction versus competing interactions

A new metallic glass, which was created by vapour deposition at an appropriately high substrate temperature, shows exceptional thermal stability, and enhanced glass transition temperature and elastic modulus. Comparing this new material with other organic glasses prepared by similar routes and known as ultrastable glasses demonstrates the formation of ultrastable glassy materials correlates to the important concept of fragility.

Ultrastable Metallic Glass

Stimuli-responsive structural reorganizations play an important role in biological processes, often in combination with kinetic control scenarios. In supramolecular mimics of such systems, light has been established as the perfect external trigger. Here, we report on the light-driven structural rearrangement of a small, self-assembled Pd3L6 ring based on photochromic dithienylethene (DTE) ligands into a rhombicuboctahedral Pd24L48 sphere measuring about 6.4 nm across. When the wavelength is changed, this interconversion can be fully reversed, as confirmed by NMR and UV/Vis spectroscopy as well as mass spectrometry. The sphere was visualized by AFM, TEM, and GISAXS measurements. Due to dissimilarities in the photoswitch conformations, the interconversion rates between the two assemblies are drastically different in the two directions.

Light-Controlled Interconversion between a Self-Assembled Triangle and a Rhombicuboctahedral Sphere

Thin films of ${\mathrm{L}\mathrm{a}}_{0.7}{\mathrm{S}\mathrm{r}}_{0.3}{\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ on MgO show a metal insulator transition and colossal magnetoresistance. The shape of this transition can be explained by intrinsic spatial inhomogeneities, which give rise to a domain structure of conducting and insulating domains at the submicrometer scale. These domains then undergo a percolation transition. The tunneling conductance and tunneling gap measured by scanning tunneling spectroscopy were used to distinguish and visualize these domains.

Intrinsic inhomogeneities in manganite thin films investigated with scanning tunneling spectroscopy.

As ultrastable metallic glasses (UMGs) are promising candidates to solve the stability issues of conventional metallic glasses, their study is of exceptional interest. By means of x-ray photon correlation spectroscopy, we have investigated the stability of UMGs at the atomic level. We find a clear signature of ultrastability at the atomic level that results in slower relaxation dynamics of UMGs with respect to conventional (rapidly quenched) metallic glasses, and in a peculiar acceleration of the dynamics by near T_{g} annealing. This surprising phenomenon, called here anti-aging, can be understood in the framework of the potential energy landscape. For all samples, the structural relaxation process can be described with a highly compressed shape of the density fluctuations, unaffected by thermal treatments and regardless of the ultrastability of the glass.

/pdf/anti-aging-in-ultrastable-metallic-glasses-4zi0x12mo0.pdf

Anti-Aging in Ultrastable Metallic Glasses.

El mateix article esta publicat en angles a l'edicio anglesa d' 'Angewandte Chemie International Edition' (ISSN 1433-7851, EISSN 1521-3773), 2016, vol. 55, num. 1, p.455-449. DOI http://dx.doi.org/10.1002/anie.201508307

Y. Luo

Papers

Ultrastable Metallic Glass

Light-Controlled Interconversion between a Self-Assembled Triangle and a Rhombicuboctahedral Sphere

Intrinsic inhomogeneities in manganite thin films investigated with scanning tunneling spectroscopy.

Anti-Aging in Ultrastable Metallic Glasses.

Lichtgesteuerte Umwandlung zwischen einem selbstassemblierten Dreieck und einer rhombenkuboktaedrischen Sphäre