Critical phenomena and renormalization-group theory

Static critical phenomena in ferromagnets with quenched disorder

hysteresis in magnetism (electromagnetism)

Single-crystalline Ni nanowires have been successfully fabricated with anodic aluminum oxide as template by electrodeposition. Structural characterization (X-ray diffraction, XRD, and high-resolution transmission electron microscopy, HRTEM) shows that the single-crystalline Ni nanowire has a preferred orientation along the [220] direction. The effects of electrochemical deposition conditions on the structure of Ni nanowires are systematically studied to investigate the growth mechanism. Possible reasons for the growth of the single-crystalline Ni nanowires were discussed on the basis of electrochemistry and thermodynamics. These single-crystalline Ni nanowires have exhibited excellent magnetic properties (large anisotropy, large coercivity, and high remanence). By a similar process, single-crystalline Co nanowires with hexagonal close-packed (hcp) structure were achieved, also having large anisotropy, large coercivity (1.8 kOe), and high remanence ratio (80.8%).

Growth of single-crystalline Ni and Co nanowires via electrochemical deposition and their magnetic properties.

Due to its manufacturing and size tailoring ease, porous anodic alumina (PAA) templates are an elegant physical-chemical nanopatterning approach and an emergent alternative to more sophisticated and expensive methods currently used in nanofabrication. In this review, we will describe the ground work on the fabrication methods of PAA membranes and PAA-based nanostructures. We will present the specificities of the electrochemical growth processes of multifunctional nanomaterials with diversified shapes (e.g., nanowires and nanotubes), and the fabrication techniques used to grow ordered nanohole arrays. We will then focus on the fabrication, properties and applications of magnetic nanostructures grown on PAA and illustrate their dependence on internal (diameter, interpore distance, length, composition) and external (temperature and applied magnetic field intensity and direction) parameters. Finally, the most outstanding experimental findings on PAA-grown nanostructures and their trends for technological applications (sensors, energy harvesting, metamaterials, and biotechnology) will be addressed.

Nanoporous alumina as templates for multifunctional applications

National Outstanding Youth Science Foundation of China [50825101]; National Natural Science Foundation of China [50671087]; Fujian Province of PR China

https://iopscience.iop.org/article/10.1088/0953-8984/16/45/027/pdf

Structure and magnetic properties of Fe1-xCox nanowires in self-assembled arrays

We have investigated the predictions of a mean-field effective-field model for ferromagnets with a distribution of exchange coupling strengths, concerning the region of validity of the Arott-plot criterion for ferromagnetism. We find that the range of fields over which such plots are applicable shrinks dramatically as the width of the exchange distribution increases and the spin-glass regime is approached.

Arrott-plot criterion for ferromagnetism in disordered systems.

The physical orign of peaks observed in the field-dependent susceptibility of a number of ferromagnets at temperatures above the critical temperature ${T}_{c}$ is identified. As a corollary it is shown that the field-dependent susceptibility of a paramagnet exhibits an obvious but little discussed structure at finite temperature, of similar origin. This structrure can be used to extract information on the zero-temperature transition that should occur in paramagnetic systems (systems which, formally, do not experience any interactions), in a manner similar to that used to extract information on the finite-temperature phase transition in ferromagnets. This provides strong confirmatory evidence that this type of anomaly is static rather than dynamic in origin.

Field-dependent susceptibility of a paramagnet.

Detailed measurements of the field- and temperature-dependent ac susceptibility and magnetization of LaMnO3 substituted with 5 at.% of the divalent cation Mg are presented. This system is a semiconducting ferromagnet at low temperature; in particular, analysis of these data yields Tc = 147.2±0.2 K, with critical exponents δ = 4.75±0.15 (from the field dependence of the susceptibility along the `crossover' line, and confirmed by measurements along the critical isotherm), γ + β = 1.75±0.05 (from the temperature dependence of the crossover line) and γ = 1.39±0.05 (from the temperature dependence of the susceptibility along the same line). Within experimental uncertainty these exponent values agree with those predicted by the isotropic, three-dimensional Heisenberg model. Nevertheless this system exhibits some unusual characteristics, specifically in the temperature dependence of the (low-) field-cooled and zero-field-cooled magnetization. The latter have been modelled by a Preisach-based approach, which helps to resolve a ubiquitous dilemma in the doped manganites, viz. the appearance of a technical hardness irreconcilable with low coercive field values. A possible origin for the large, reversible component in the response below Tc is presented.

http://iopscience.iop.org/article/10.1088/0953-8984/12/30/318/pdf

La0.95Mg0.05MnO3: an ideal ferromagnetic system?

The ac susceptibility of two PdMn spin glasses containing 5.0 and 5.5 at. % Mn have been measured as a function of temperature in a large number of static biasing fields between 0 and 500 Oe in the vicinity of the spin-glass (SG) temperature ${T}_{\mathrm{SG}}$. Our analysis of the field dependence of these data both above and below ${T}_{\mathrm{SG}}$ provides the first experimental observation of a nearly symmetric divergence in the coefficients of the leading terms in ${H}^{2}$ and ${H}^{4}$ in the nonlinear reversible susceptibility as T approaches ${T}_{\mathrm{SG}}$ from above and below. The exponents which characterize the temperature dependence of the divergences are in good agreement with estimates obtained above ${T}_{\mathrm{SG}}$ by other authors.

R. M. Roshko

Papers

Structure and magnetic properties of Fe1-xCox nanowires in self-assembled arrays

Arrott-plot criterion for ferromagnetism in disordered systems.

Field-dependent susceptibility of a paramagnet.

La0.95Mg0.05MnO3: an ideal ferromagnetic system?

Critical effects in the reversible, nonlinear susceptibility of PdMn spin glasses.