Ising Nanoparticles Research Articles

Theoretical investigation of compensation point phenomena in a ferrimagnetic mixed spin-3/2 and spin-1 Ising nanoparticle

ABSTRACT We have explored some magnetic characteristics of a mixed spin-3/2 and spin-1 Ising ferrimagnetic square nanoparticle by utilising the finite cluster approximation. The influences of the core and surface shell couplings (Jc, Js), as well as the crystal fields (Dc, Ds) on the critical and compensation behaviour, were investigated. In the absence of crystal fields, a compensation point only exists if Jc is higher than the threshold core coupling J*c, where the dependence of J*c on Js is linear. By introducing the crystal fields, we found a variety of topologies of the compensation temperature with the possibility of double reentrance. In particular, the presence of the core crystal field Dc, according to the Js strength, may lead to one, two, or three compensation points.

Effects of size for an assembly of core-shell nanoparticles with the cubic structure: Monte Carlo simulations

Using Monte Carlo simulations, we studied phase diagrams and magnetic properties of a ferromagnetic/antiferromagnetic Ising nanoparticles assembly with the spin-1/2 core and spin-3/2 shell. The system consists of nanocubes with equivalent exchange couplings of core and shell. However, the exchange couplings between the adjacent spins of the core and shell and between the nanocubes are assumed to vary. Thus, the effects of the exchange couplings and the system size on the phase diagrams are analyzing. Interesting behaviors are found when investigating the critical behavior of the system, the magnetizations, the magnetic susceptibility and the hysteresis loops. The increase of the core/shell size involves that of the temperature one. In addition, the increase in positive exchange interactions between the spins of the core and shell, on the one hand, and between the nanocubes, on the other hand, also increase the critical temperature, which favors the stable phase (3/2, 1/2).

The hysteretic features of ternary spins (1/2, 1, 3/2) idealized Ising nanoparticles on the core–multishell structure

The hysteretic features of ternary spins (1/2, 1, 3/2) idealized Ising nanoparticles on the core–multishell structure

On the failure of effective-field theory in predicting a spurious spontaneous ordering and phase transition of Ising nanoparticles, nanoislands, nanotubes and nanowires

The present work clarifies a failure of the effective-field theory in predicting a false spontaneous long-range order and phase transition of Ising nanoparticles, nanoislands, nanotubes and nanowires with either zero- or one-dimensional magnetic dimensionality. It is conjectured that the standard formulation of the effective-field theory due to Honmura and Kaneyoshi generally predicts for the Ising spin systems a spurious spontaneous long-range order with nonzero critical temperature regardless of their magnetic dimensionality whenever at least one Ising spin has coordination number greater than two. The failure of the effective-field theory is exemplified on a few paradigmatic exactly solved examples of zero- and one-dimensional Ising nanosystems: star, cube, decorated hexagon, star of David, branched chain, sawtooth chain, two-leg and hexagonal ladders. The presented exact solutions illustrate eligibility of a few rigorous analytical methods for exact treatment of the Ising nanosystems: exact enumeration, graph-theoretical approach, transfer-matrix method and decoration-iteration transformation. The paper also provides a substantial survey of the scientific literature, in which the effective-field theory led to a false prediction of the spontaneous long-range ordering and phase transition.

Nonequilibrium multiple transitions in the core-shell Ising nanoparticles driven by randomly varying magnetic fields

The nonequilibrium behaviour of a core-shell nanoparticle has been studied by Monte-Carlo simulation. The core consists of Ising spins of σ=1/2 and the shell contains Ising spins of S=1. The interactions within the core and in the shell are considered ferromagnetic but the interfacial interaction between core and shell is antiferromagnetic. The nanoparticle system is kept in open boundary conditions and is driven by randomly varying (in time but uniform over the space) magnetic field. Depending on the width of the randomly varying field and the temperature of the system, the core, shell and total magnetization varies in such a manner that the time averages vanish for higher magnitude of the width of random field, exhibiting a dynamical symmetry breaking transitions. The susceptibilities get peaked at two different temperatures indicating nonequilibrium multiple transitions. The phase boundaries of the nonequilibrium multiple transitions are drawn in the plane formed by the axes of temperature and the width of the randomly varying field. Furthermore, the effects of the core and shell thicknesses on the multiple transitions have been discussed.

Hysteretic behavior of quadrupolar ordering in a 2D magnetic spin˗1 Ising nanoparticle

Pair approximation technique has been used to study the quadrupolar ordering properties for the 2D magnetic spin-1 Ising nanoparticles, consisting of core and surface parts with an interface coupling. Similar to the magnetic hysteresis, it has been shown that the quadrupole order parameter (Q) as a function of single-ion anisotropy (D) has a ‘hysteresis’ character which is generally called quadrupole (or QD) hysteresis. The observed QD loops strongly depend on temperature (T), external magnetic field (H) and biquadratic exchange interaction (K). Shifted QD loops with an asymmetry are found when H and K values are increased. These behaviors are also discussed in relation to other theoretical findings.

Compensation point phenomena in a graphene-like Ising nanoparticle decorated with spin-3/2 atoms

The possibility of a graphene-like Ising nanoparticle with high critical temperature or with a compensation point below its critical temperature is examined by decorating spin-3/2 atoms on the graphene-like Ising nanoparticle. The phase diagram and the temperature dependences of magnetization in the decorated nanoparticle are investigated by using the effective-field approximation with correlations. It is clarified how the paramagnetic graphene-like nanoparticle may change to a magnetic ordering nanoparticle by changing the physical parameters of decorated atom.

Magnetism in a graphene-like Ising (spin-1/2) nanoparticle decorated with spin-1 atom

ABSTRACT The phase diagram and the thermal variations of magnetizations in a graphene-like Ising nanoparticle decorated with spin-1 atom are examined by the effective-field theory with correlations. The effects of single-ion anisotropy of spin-1 atom to the magnetic properties are discussed. The following facts are clarified, when the appropriate conditions are satisfied: The transition temperature of the nanoparticle may become larger than that of bulk honeycomb lattice. The decorated nanoparticle exhibits many characteristic phenomena observed in bulk ferrimagnetic materials.

A graphene-like Ising nanoparticle decorated with Ising trimer: High critical temperature, compensation point and characteristic magnetization curves

The decorated nanoparticle which is consisted of two paramagnetic (hexagonal and trimer) nanoparticles and tied by an exchange interaction is examined by the effective-field theory with correlations. It is discussed whether it can exhibit a critical temperature larger than that of a bulk system. The phase diagram and the thermal variations of magnetizations are investigated. When some appropriate conditions are satisfied for exchange interactions, a compensation point and characteristic thermal variations can be found in the total magnetization.

Magnetization Changes with Decoration in an Ising Nanoparticle with High Critical Temperature

The phase diagram and the thermal variations of total magnetization in Ising nanoparticles with high critical temperature are investigated by decorating the magnetic atoms to the basic nanoparticle, in order to clarify how they may change with the increase of decoration. The thermal variations of magnetizations in nanoparticles under an applied magnetic field are also studied. These are examined by using the effective field theory with correlations. A lot of characteristic phenomena are reported.

Compensation point phenomena in nanoscale Ising particles with high critical temperature

ABSTRACT The phase diagrams and magnetizations of Ising nanoparticles with high critical temperature, consisting of atoms with a negative exchange interaction between the surface and the core, are investigated by using the effective-field theory with correlations. They show a compensation point below their critical temperatures, although they have not ferrimagnetic spin configuration. The effects of an applied transverse field to the nanoparticles are also discussed. One of them has exhibited the reentrant phenomena, although other does not show the phenomena. The reason is also discussed. The decoration for one of the nanoparticles is investigated. A compensation point is also found for the decoration.

Ferrimagnetism and reentrant phenomena in a tetragonal Ising nanoparticle

ABSTRACT The phase diagrams and magnetizations in a tetragonal Ising nanoparticle have been investigated by using the effective-field theory with correlations and the core–shell model. For the spin-1/2 atoms in the shell, a transverse field is included to cant their spin direction. The results show that they are heavily dependent on the given physical parameters. The ferrimagnetic behaviours, as well as novel type behaviours, have been found in the thermal variation of magnetization. The reentrant phenomena are also obtained.

Reduced magnetization curves of Ising nanoparticles with high critical temperature

ABSTRACTThe magnetic properties (phase diagram, the thermal variations of magnetization and the reduced magnetization curve) in graphene-like Ising nanoparticles are examined by the effective-field theory with correlations. The effects of surface exchange interaction JS on the magnetic properties are discussed. When JS is larger than the critical value JSC, a nanoparticle with high critical temperature is possible. The value of JSC depends on the shape and the degree of structural packing of the nanoparticle. It is found that there exist two types of reduced magnetization curves for a nanoparticle with JS > JSC.

Decoration in a Graphene-like Ising nanoparticle for the possibility of high critical temperature

The phase diagrams and the thermal variations of magnetizations in graphene-like Ising nanoparticles and the effects of decorations on them are examined by the effective-field theory with correlations. It is clarified that the transition temperatures of such small nanoparticles may become larger than those of its corresponding bulk systems, when the appropriate decorations are properly selected.

Magnetism in two antiferromagnetic Ising nanoparticles under an applied transverse field

The phase diagrams and magnetization curves of two antiferromagnetic Ising nanoparticles (A and B) under an applied transverse field are investigated by using the effective-field theory with correlations. The nanoparticle B is a size larger than that of nanoparticle A. The effects of size on the magnetic properties are discussed. Some characteristic features, such as the reentrant phenomena, are found in these properties of both systems, when a finite transverse field h is applied. The nanoparticle B exhibits the ferrimagnetic behaviors, such as a compensation point.

Decorated Ising nanoparticles with high critical temperature

ABSTRACTThe phase transitions of two magnetic nanoparticles and their decorated nanoparticles are examined by the effective-field theory with correlations. They are consisted of two isolated paramagnetic Ising nanoparticles coupled by an exchange interaction. It is discussed whether a ferromagnetic nanoparticle with a transition temperature TC(N) larger than that (TC(B)) of a bulk ferromagnetic material is possible.

On the possibility of magnetic ordering (TC(N)) induced by a surface exchange interaction in an Ising nanoparticle with TC(N) > TC(B), where TC(B) is a transition temperature in the corresponding bulk system

The phase transitions of two nanoparticles and their corresponding bulk systems are examined by the effective-field theory with correlations. It is clarified that the transition temperatures of such small nanoparticles may become larger than those of its corresponding bulk systems, when the surface exchange interaction is selected as a large value.

Phase Transition in Mixed Spin Ising Nanoparticles

The phase transitions of two mixed-spin (1/2,1) Ising nanoparticles with n = 4.0 and n = 6.0 are investigated by using the effective-field theory with correlations (EFT). The spin-1 atom with a single-ion anisotropy is decorated at the center of regular n-polygon (or nanoparticle with a value of n) and the spin-1/2 n atoms are put at the perimeter n sites. The phase transition in the nanoparticle with n = 6.0 is rather different from that in the nanoparticle with n = 4.0. It is shown that the number of n plays just like the coordination number z in the bulk mixed-spin Ising systems: the hexagonal nanoparticle with n = 6.0 has exhibited the first-order transition and the tricritical behavior, although the tetragonal nanoparticle with n = 3.0 did not show such behaviors, depending on the values of exchange interactions.

Magnetism in an antiferromagnetic Ising nanoparticle under an applied transverse field

The phase diagrams and magnetizations of two antiferromagnetic Ising nanoparticles (A and B) under an applied transverse field are investigated by using the effective-field theory with correlations. The nanoparticle B is a size larger than that of nanoparticle A. Both nanoparticles exhibit the reentrant phenomena. The numerical results show that the magnetic properties in the nanoparticle may exhibit rather different behaviors, depending on the size of a nanoparticle.

Unique phenomena induced by an exchange interaction between two graphene-like Ising nanoparticles in an applied transverse field

The phase diagrams and the magnetizations of graphene-like nanoparticles which are constructed from two paramagnetic honeycomb units and an inter-unit exchange interaction r are investigated by using the effective-field theory with correlations. They have exhibited some characteristic phenomena, such as the reentrant phenomena, when the value of r is larger than r = 1.0 and a transverse field is applied.