DC magnetization processes in bistable glass-coated ferromagnetic microwires

Interaction of a single magnetic domain wall with an inhomogeneous magnetic field and distribution of local nucleation fields along glass-coated Fe_77.5−xNi_xB₁₅Si_7.5 (x=0, 27.9, 34.9) microwires were experimentally studied. It was shown that the wall separating two axial domains and moving along the wire can be stopped by an inhomogeneous magnetic field. The wall remains stable and trapped in a local potential minimum after external fields are switched off. Wall coercivity increases with Ni content. For all samples the minimum of critical field for axial magnetization reversal was observed near the end of the wire. For samples with non-zero Ni content a distribution of nucleation fields lower than 950 A/m was observed in regions far enough from the wire ends. In Ni-free samples the nucleation fields were higher than 950 A/m.     

Some characteristic properties of initial susceptibility in a Ising nanotube

Initial susceptibility and magnetization of a cylindrical nanotube described by the Ising model are investigated by the use of the effective field theory with correlations (EFT), since the phase diagrams of the system have been examined in the previous work of Kaneyoshi (2010) [8] using the two theoretical frameworks of the mean field theory and the EFT. The effects of the two exchange couplings at the surface shell and in the core to the initial susceptibility are clarified. Some characteristic phenomena are found in the thermal variations, depending on the ratios of the physical parameters in the surface shell and the core. It is also discussed whether the Neel hyperbola in the paramagnetic region is valid for a nano-scaled ferrimagnetic system.     

An explicit derivation of a relation between magnetization M and saturation magnetization Ms

An explicit relationship between a magnetization vector M and its saturation magnetization M_s is derived using the definition of M along with assumptions of the continuum exchange theory. The obtained expression is found to be an extension of the commonly used fixed-length constraint and represents the continuous analog of it for the elementary moment per unit volume. The derivation of this relation is carried out in detail and important potential implications relating to equations for M are also highlighted.     

Clear distinctions between ferromagnetic and ferrimagnetic behaviors in a cylindrical Ising nanowire (or nanotube)

The initial susceptibility and reduced total magnetization of a cylindrical Ising nanowire (or nanotube) are investigated by the use of the effective field theory with correlations, in order to clarify their distinctions between the ferromagnetic and ferrimagnetic behaviors when the core-shell exchange coupling takes a different sign. Some common phenomena are found in the thermal variations between the two nanosystems. Whether the Neel hyperbola, valid in the paramagnetic region for a bulk ferrimagnetic material with a spinel structure, is valid for the nano-scaled ferrimagnetic systems is discussed.     

Structure and magnetization studies of Nd0.5−xPrxSr0.5MnO3 system

The structural and magnetic properties of Nd_0.5−xPr_xSr_0.5MnO₃ (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) system have been investigated. With the substitution of Pr in Nd_0.5Sr_0.5MnO₃, it shows a gradual structure transformation from the Imma orthorhombic symmetry to the tetragonal I4/mcm phase, and the crystallographic transition remains incomplete, even in Pr_0.5Sr_0.5MnO₃. A large bifurcation between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility has been observed below Curie temperature (T_C), which is characteristic of coexistence of ferromagnetism (FM) and antiferromagnetism (AFM) at low temperature region. The magnetization of Pr_0.5Sr_0.5MnO₃ is larger than that of Nd_0.5Sr_0.5MnO₃, while Nd_0.5Sr_0.5MnO₃ with more CE-type AFM shows larger magnetization than Nd_0.3Pr_0.2Sr_0.5MnO_3, which mixed with CE-type (majority) and A-type (minority) AFM at low temperature, indicating that the magnetization of Nd_0.5−xPr_xSr_0.5MnO₃ system is affected by A-site disorder combined with orbital ordering of A-type AFM and CE-type AFM.     

Minor magnetization loops in two-dimensional dipolar Ising model

The two-dimensional dipolar Ising model is investigated for the relaxation and dynamics of minor magnetization loops. Monte Carlo simulations show that in a stripe phase an exponential decrease can be found for the magnetization maxima of the loops, M∼exp(−αN_l) where N_l is the number of loops. We discuss the limits of this behavior and its relation to the equilibrium phase diagram of the model.     

Isothermal magnetic entropy behavior in Tb5Si3 : Sign reversal and non-monotonic variation with temperature, and implications

The magnetic entropy change (), a measure of the magnetocaloric effect, in Tb₅Si₃, a compound exhibiting unusual positive magnetoresistance following a magnetic-field-induced transition below the magnetic transition temperature (∼69 K), has been investigated. We found that is negative in the paramagnetic state. At the magnetic transition temperature, shows a sign reversal from a negative value (in the paramagnetic state) to a positive value (in the magnetically ordered state). The high-field state, which is interestingly the high resistive state, is found to be associated with higher entropy, i.e., large positive , behaving like a paramagnet. On the basis of this observation, we conclude that the magnetic field induces magnetic fluctuations in the system resulting in positive magnetoresistance, thereby rendering support to the idea of inverse metamagnetism in this compound. In addition, we note that the Arrott plots present an interesting scenario.     

Magnon gas and deviation from the Bloch law in a nanoscale Heisenberg ferromagnet

It is known that saturation magnetization of ferromagnetic nanoparticles and nanoclusters deviates strongly from the Bloch T^3/2 law. To describe the deviation one usually uses a modified power law T^α with a size dependent exponent α, which is motivated simply by its flexibility in fitting the observed behavior. By considering the Heisenberg spin model, we derive an explicit expression in terms of a magnon gas which generalizes the Bloch formula to a finite size system. Comparison to the experimental data shows a good agreement with the observed behavior and gives a better understanding of its physics.     

First quantized electrodynamics

The parametrized Dirac wave equation represents position and time as operators, and can be formulated for many particles. It thus provides, unlike field-theoretic Quantum Electrodynamics (QED), an elementary and unrestricted representation of electrons entangled in space or time. The parametrized formalism leads directly and without further conjecture to the Bethe–Salpeter equation for bound states. The formalism also yields the Uehling shift of the hydrogenic spectrum, the anomalous magnetic moment of the electron to leading order in the fine structure constant, the Lamb shift and the axial anomaly of QED.     

Controllable synthesis and property of graphene-based magnetic metal nanostructures

A facile and effective solution phase reduction method was developed to synthesize graphene-based magnetic metal nanocomposites. Metals (Co, and Ni) or alloys (Fe₅₁Co₄₉, Fe₄₈Ni₅₂, Ni₄₉Co₅₁, Co₅₁Cu₄₉, and Ni₅₂Cu₄₈)/reduced graphene oxide (RGO) nanocomposites were successfully prepared by reduction of the corresponding aqueous metal ions and ethylenediamine (EDA)–graphene oxide (GO) with hydrazine hydrate at 353 K for 1 h under N₂ atmosphere. The effects of synthetic parameters such as metal ions concentration, adding sequence of NaOH and N₂H₄·H₂O, linkage agent and reaction time on the formation of nanocomposites were investigated. The experimental results showed that using ethylenediamine and adding sequence played critical roles in the formation of metals or alloys/RGO nanocomposites. Magnetic hysteresis measurements revealed that the as-synthesized metals or alloys in nanocomposites showed excellent soft magnetic behavior with enhanced saturation magnetization, and could have promising applications in biotechnology, catalysis, and magnetic storage devices.