Doping is a widely applied technological process in materials science that involves incorporating atoms or ions of appropriate elements into host lattices to yield hybrid materials with desirable properties and functions. For nanocrystalline materials, doping is of fundamental importance in stabilizing a specific crystallographic phase, modifying electronic properties, modulating magnetism as well as tuning emission properties. Here we describe a material system in which doping influences the growth process to give simultaneous control over the crystallographic phase, size and optical emission properties of the resulting nanocrystals. We show that NaYF(4) nanocrystals can be rationally tuned in size (down to ten nanometres), phase (cubic or hexagonal) and upconversion emission colour (green to blue) through use of trivalent lanthanide dopant ions introduced at precisely defined concentrations. We use first-principles calculations to confirm that the influence of lanthanide doping on crystal phase and size arises from a strong dependence on the size and dipole polarizability of the substitutional dopant ion. Our results suggest that the doping-induced structural and size transition, demonstrated here in NaYF(4) upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.

Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping

In current longitudinal magnetic recording media, high areal density and low noise are achieved by statistical averaging over several hundred weakly coupled ferromagnetic grains per bit cell. Continued scaling to smaller bit and grain sizes, however, may prompt spontaneous magnetization reversal processes when the stored energy per particle starts competing with thermal energy, thereby limiting the achievable areal density. Charap et al. have predicted this to occur at about 40 Gbits/in/sup 2/. This paper discusses thermal effects in the framework of basic Arrhenius-Neel statistical switching models. It is emphasized that magnetization decay is intimately related to high-speed-switching phenomena. Thickness-, temperature- and bit-density dependent recording experiments reveal the onset of thermal decay at "stability ratios" (K/sub u/V/K/sub B/T)/sub 0//spl sime/35 /spl plusmn/ 2. The stability requirement is grain size dispersion dependent and shifts to about 60 for projected 40 Gbits/in/sup 2/ conditions and ten-year storage times. Higher anisotropy and coercivity media with reduced grain sizes are logical extensions of the current technology until write field limitations are reached. Future advancements will rely on deviations from traditional scaling. Squarer bits may reduce destabilizing stray fields inside the bit transitions. Perpendicular recording may shift the onset of thermal effects to higher bit densities. Enhanced signal processing may allow signal retrieval with fewer grains per bit. Finally, single grain per bit recording may be envisioned in patterned media, with lithographically defined bits.

Thermal effect limits in ultrahigh-density magnetic recording

An experimental method based on the Landau-Zener model was developed to measure very small tunnel splittings in molecular clusters of eight iron atoms, which at low temperature behave like a nanomagnet with a spin ground state of S = 10. The observed oscillations of the tunnel splittings as a function of the magnetic field applied along the hard anisotropy axis are due to topological quantum interference of two tunnel paths of opposite windings. Transitions between quantum numbers M = -S and (S - n), with n even or odd, revealed a parity effect that is analogous to the suppression of tunneling predicted for half-integer spins. This observation is direct evidence of the topological part of the quantum spin phase (Berry phase) in a magnetic system.

https://science.sciencemag.org/content/sci/284/5411/133.full.pdf

Quantum Phase Interference and Parity Effects in Magnetic Molecular Clusters

In 1935, Schrodinger attempted to demonstrate the limitations of quantum mechanics using a thought experiment in which a cat is put in a quantum superposition of alive and dead states. The idea remained an academic curiosity until the 1980s when it was proposed that, under suitable conditions, a macroscopic object with many microscopic degrees of freedom could behave quantum mechanically, provided that it was sufficiently decoupled from its environment. Although much progress has been made in demonstrating the macroscopic quantum behaviour of various systems such as superconductors, nanoscale magnets, laser-cooled trapped ions, photons in a microwave cavity and C60 molecules, there has been no experimental demonstration of a quantum superposition of truly macroscopically distinct states. Here we present experimental evidence that a superconducting quantum interference device (SQUID) can be put into a superposition of two magnetic-flux states: one corresponding to a few microamperes of current flowing clockwise, the other corresponding to the same amount of current flowing anticlockwise.

Quantum superposition of distinct macroscopic states

Ordered magnetic nanostructures: fabrication and properties

Presented are the first magnetization measurements of individual ferromagnetic nanoparticles (15--30 nm) at very low temperatures (0.1--6 K). The angular dependence of the hysteresis loop evidenced the single domain character of the particles. Waiting time, switching field, and telegraph noise measurements showed for the first time that the magnetization reversal of a well prepared ferromagnetic nanoparticle can be described by thermal activation over a single-energy barrier as originally proposed by N\'eel and Brown. The ``activation volume'' estimated by these measurements was close to the particle volume.

/pdf/experimental-evidence-of-the-neel-brown-model-of-1cyxc9cjvk.pdf

Experimental Evidence of the Néel-Brown Model of Magnetization Reversal

Presented are switching field measurements of individual ferrimagnetic and insulating BaFeCoTiO nanoparticles containing about ${10}^{5}{\ensuremath{\mu}}_{B}$ at very low temperatures (0.1--6 K). For temperatures higher than 0.4 K, the quantitative agreement with the N\'eel-Brown theory of thermal activated magnetization reversal allowed us to identify unambiguously the magnetization reversal of uniform rotation. Below 0.4 K, strong deviations from this model are evidenced which are quantitatively in agreement with the predictions of the theory of macroscopic quantum tunneling of magnetization in the low dissipation regime.

/pdf/macroscopic-quantum-tunneling-of-magnetization-of-single-1ty9k45zb2.pdf

Macroscopic Quantum Tunneling of Magnetization of Single Ferrimagnetic Nanoparticles of Barium Ferrite

Low temperatures magnetization measurements of individual ferromagnetic particles and wires are presented (0.1<T(K)<6). The detector was a Nb micro-bridge-dc-SQUID, fabricated using electron-beam lithography. The angular dependence of the magnetization reversal could be explained approximately by simple classical micromagnetic concepts. However, our measurement evidenced nucleation and propagation of domain walls except for the smallest particles of about 20 nm. The switching field distributions as a function of temperature and field sweeping rate and the probabilities of switching showed that the magnetization reversal was thermally activated. These measurements allowed us to estimate the “activation volume” which triggered the magnetization reversal. Our measurements showed for the first time that the magnetization reversal of a ferromagnetic nanoparticle of good quality can be described by thermal activation over a single-energy barrier as originally proposed by Neel and Brown.

Mesoscopic effects in magnetism: Submicron to nanometer size single particle measurements

We show in this article the first full three dimensional measurements of switching fields of a nanometer-scale magnetic particle. The field direction dependence of the switching fields can be successfully described by the uniform rotation model. Nevertheless, we could see that the Stoner–Wohlfarth form for the anisotropy function, which is often assumed together with the uniform rotation model, is an oversimplification. In order to reproduce the measured fields, one has to take into account higher order anisotropy terms. The complete form of the anisotropy function seems to be accessible only by magnetic measurements on each individual nanoparticle.

Uniform rotation of magnetization measured in single nanometer-sized particles

Recent progress in experiment on quantum tunnelling of the magnetic moment in mesoscopic systems will be reviewed. The emphasis will be made on measurements of individual nanoparticles. These nanomagnets allow one to test the border between classical and quantum behaviour. Using the micro-SQUID magnetometer, waiting time, switching field and telegraph noise measurements show unambiguously that the magnetisation reversal of small enough single-crystalline nanoparticles is described by a model of thermal activation over a single-energy barrier. Results on insulating BaFeO nanoparticles show strong deviations from this model below 0.4 K which agree with the theory of macroscopic quantum tunnelling in the low dissipation regime.

/pdf/classical-and-quantum-magnetisation-reversal-studied-in-2wbzyhcao7.pdf

E. Bonet Orozco

Papers

Experimental Evidence of the Néel-Brown Model of Magnetization Reversal

Macroscopic Quantum Tunneling of Magnetization of Single Ferrimagnetic Nanoparticles of Barium Ferrite

Mesoscopic effects in magnetism: Submicron to nanometer size single particle measurements

Uniform rotation of magnetization measured in single nanometer-sized particles

Classical and quantum magnetisation reversal studied in single nanometer-sized particles and clusters using micro-SQUIDs