Two-dimensional Magnetic Systems Research Articles

Frustration-induced one-dimensionality in the isosceles triangular antiferromagnetic lattice of δ -(EDT-TTF- CONMe2)2AsF6

The $1/4$-filled organic compound, $\delta$-(EDT-TTF-CONMe$_{2}$)$_{2}$AsF$_6$ is a frustrated two-dimensional triangular magnetic system as shown by high-frequency (111.2 and 222.4 GHz) electron spin resonance (ESR) and structural data in the literature. The material gradually orders antiferromagnetically below 40~K but some magnetically disordered domains persist down to 4 K. We propose that in defect free regions frustration prevents true magnetic order down to at least 4 K in spite of the large first- and second-neighbor exchange interactions along chains and between chains, respectively. The antiferromagnetic (AFM) order gradually developing below 40 K nucleates around structural defects that locally cancel frustration. Two antiferromagnetic resonance modes mapped in the principal planes at 4~K are assigned to the very weakly interacting one-dimensional molecular chains in antiferromagnetic regions.

Anisotropic Magnetostriction of Nonoriented Silicon Steel Sheet and Its Frequency Dependence

Non-oriented (NO) silicon steel sheet is widely used as industrial materials in electrical machine, and its magnetostriction is a key factor of causing vibration and noise of a large-scale synchronous machine [1, 2]. There have been some foregoing researches to develop magnetostriction of silicon steel sheet under alternating and rotational magnetic flux density based on a two-dimensional (2-D) magnetic measurement system [3-5], in which the sample was 60mm square, and the three-axis strain gauge was employed to measure the in-plane magnetostriction at arbitrary direction. In this paper, an improved one-dimensional (1-D) magnetostriction tester was built to measure the principal strain along different magnetization directions, and the anisotropic magnetostriction was discussed. The model describing the vector relation between the principal strain and magnetic flux density (B) was established and applied to the finite element (FE) analysis of magnetostriction in stator core of a synchronous machine. The validation of proposed method is verified.

Quantum phase transition in a frustrated two-dimensional magnetic system: A matrix projection approach

A matrix projection formalism is developed for the description of a frustrated two-dimensional antiferromagnet. This approach keeps safe all the advantages of the spherically symmetric self-consistent treatment (it strictly meets the spin constraint and the Mermin—Wagner theorem) but does not have its disadvantages. In particular, even in its simplest implementation, this approach provides a good agreement with the numerical simulations for two best-studied reference points: the zero frustration point and the point of a transition from the antiferromagnetic ordered phase to the disordered phase.

Parallelized Stochastic Cutoff Method for Long-Range Interacting Systems

We present a method to parallelize the stochastic cutoff (SCO) method, which is a Monte-Carlo method for long-range interacting systems. After interactions are eliminated by the SCO method, we subdivide the lattice into non-interacting interpenetrating sublattices. This subdivision enables us to parallelize Monte-Carlo calculation in the SCO method. Such subdivision is found by numerically solving the vertex coloring of a graph created by the SCO method. We use an algorithm proposed by Kuhn and Wattenhofer to solve the vertex coloring by parallel computation. The present method was applied to a two-dimensional magnetic dipolar system on an $L\times L$ square lattice to examine its parallelization efficiency. The result showed that, in the case of L=2304, the speed of computation increased about 102 times by parallel computation with 288 processors.

Casimir effect mechanism of pairing between fermions in the vicinity of a magnetic quantum critical point

We consider two immobile spin $1/2$ fermions in a two-dimensional magnetic system that is close to the $ O(3)$ magnetic quantum critical point (QCP) which separates magnetically ordered and disordered phases. Focusing on the disordered phase in the vicinity of the QCP, we demonstrate that the criticality results in a strong long range attraction between the fermions, with potential $V(r) \propto -1/r^{\nu}$, $\nu \approx 0.75$, where $r$ is separation between the fermions. The mechanism of the enhanced attraction is similar to Casimir effect and corresponds to multi-magnon exchange processes between the fermions. While we consider a model system, the problem is originally motivated by recent establishment of magnetic QCP in hole doped cuprates under the superconducting dome at doping of about 10\%. We suggest the mechanism of magnetic critical enhancement of pairing in cuprates.

Real-time observation of domain fluctuations in a two-dimensional magnetic model system.

Domain patterns of perpendicularly magnetized ultra-thin ferromagnetic films are often determined by the competition of the short range but strong exchange interaction favouring ferromagnetic alignment of magnetic moments and the long range but weak antiferromagnetic dipolar interaction. Detailed phase diagrams of the resulting stripe domain patterns have been evaluated in recent years; however, the domain fluctuations in these pattern forming systems have not been studied in great detail so far. Here we show that domain fluctuations can be observed in ultra-thin two-dimensional ferromagnetic Fe/Ni/Cu(001) films with perpendicular magnetization in the stripe domain phase. Non-stroboscopic time-resolved threshold photoemission electron microscopy with high temporal resolution allows analysing the dynamic fingerprint of the topological excitations in the nematic domain phase. Furthermore, proliferation of domain ending defects in the vicinity of the spin reorientation transition is witnessed.

On the range of the attenuated magnetic ray transform for connections and Higgs fields

For a two-dimensional simple magnetic system, we study the attenuated magnetic ray transform $I_{A,\Phi}$, with attenuation given by a unitary connection $A$ and a skew-Hermitian Higgs field $\Phi$. We give a description for the range of $I_{A,\Phi}$ acting on $\mathbb{C}^n$-valued tensor fields.

Study of Two-Dimensional Magnetic Recording System Including Micromagnetic Writer

Study of Two-Dimensional Magnetic Recording System Including Micromagnetic Writer

An iterative inter-track interference mitigation method for two-dimensional magnetic recording systems

At high recording density, the readback signal of two-dimensional magnetic recording is inevitably corrupted by the two-dimensional (2D) interference consisting of inter-symbol interference and inter-track interference (ITI), which can significantly degrade the overall system performance. This paper proposes an iterative ITI mitigation method using three modified 2D soft-output Viterbi algorithm (2D-SOVA) detectors in conjunction with an iterative processing technique to combat the 2D interference. The codeword of the outer code is divided and then written on three separate tracks. For every iteration, all 2D-SOVA detectors exchange the soft information to improve the reliability of the a priori information and use it in the branch metric calculation, before feeding the refined soft information to the outer decoder. Simulation results show that the proposed method outperforms the conventional receiver and the existing partial ITI mitigation method.

Detection for Two-Dimensional Magnetic Recording Systems

Detection for Two-Dimensional Magnetic Recording Systems

Ideas for Detection in Two-Dimensional Magnetic Recording Systems

In two-dimensional magnetic recording, detection for a perfect writer and perfect reader based on an iterative threshold technique are compared to detection employing single threshold and adaptive threshold techniques. The simulation indicates that the algorithm can reach a BER of 9.3% compared to 11.2% and 10.5% for the latter cases when the bit size is 8 by 8 nm with a grain of 8.7 nm diameter. This increases data capacity by 11% to 6.6 TBits/inch2. Additionally, detection with an array of three conventional heads has been studied: the simulation shows that for bits with dimension of 12 × 12 nm, grains of 8.7 nm diameter and reader width of 18 nm, the BER drops from 11.3% for a single conventional head to 5.4% for the head array with a two-dimensional generalized partial response equalizer at the optimum offset of 10 nm. The density is increased by 42%, reaching 4.0 Tbits/Inch2 .

Finite-size scaling relation of the Curie temperature in barium hexaferrite platelets

High-temperature magnetic measurements were carried out on barium ferrite BaFe12 O19 nanoparticles coated with amorphous silica. We find that the Curie temperature of this material decreases with decreasing particle size, in agreement with the finite-size scaling theory. In contrast to what one expects, the observed particle-size dependence of the Curie temperature does not follow a finite-size scaling relation for a zero-dimensional magnetic system. Instead, the data follow a finite-size scaling relation for a two-dimensional magnetic system with the scaling exponent ν = 0.78±0.06. The validity of the two-dimensional scaling relation in this material is due to the fact that the nanoparticles have a platelet-like shape.

Measurement of Three-Dimensional Magnetostriction on Grain-Oriented Electrical Steel Sheet

Measurement of Three-Dimensional Magnetostriction on Grain-Oriented Electrical Steel Sheet This paper deals with the magnetostriction of a grain-oriented electrical steel sheet measured by using the two-dimensional vector magnetic property measurement system and a three-axial strain gage. The magnetostrictions in arbitrary direction are calculated with the measured strain tensors. The magnetostriction characteristics were changed depending on the magnetization process according to the exciting magnetic flux density and the exciting direction. In this paper, we make clear the relationship among the two-dimensional magnetostriction, the magnetic flux density vector, and the magnetic field intensity vector in the grain-oriented electrical steel sheet.

Vortices in Low-Dimensional Magnetic Systems

Vortices are objects that are important to describe several physical phenomena. There are many examples of such objects in nature as in a large variety of physical situations like in fluid dynamics, superconductivity, magnetism, and biology. Historically, the interest in magnetic vortex-like excitations begun in the 1960s. That interest was mainly associated with an unusual phase-transition phenomenon in two-dimensional magnetic systems. More recently, direct experimental evidence for the existence of magnetic vortex states in nano-disks was found. The interest in such model was renewed due to the possibility of the use of magnetic nano-disks as bit elements in nano-scale memory devices. The goal of this study is to review some key points for the understanding of the vortex behavior and the progress that have been done in the study of vortices in low-dimensional magnetic systems.

Two-Dimensional Local Vector Magnetic Property Measurements by Use of Simple Transducers

A new simple two-dimensional local magnetic property measurement system is presented. In this paper, we developed a new transducer whose two amorphous cores enables measure in a high magnetic induction range. Experimental results show very good agreements with ones of conventional 2-D measurement system.

A study of the stripe domain phase at the spin reorientation transition of two-dimensional magnetic system

Stripe domain phase in a two-dimensional magnetic system was studied using model calculation and Monte-Carlo simulation in the spin reorientation transition (SRT) region. We find that near the SRT point the stripe domains evolve into a static spin wave structure with a fractional in-plane magnetization along the stripe direction and a fractional out-of-plane magnetization in a sinusoidal form. With increase in the uniaxial perpendicular anisotropy, both the wavelength and the height of the static spin wave increase slowly until the saturation of the wave height, after which the stripe width increases exponentially with the magnetic anisotropy. Our theoretical result is in good agreement with experimental observations.

Diagram for vortex formation in quasi-two-dimensional magnetic dots

The existence of nonlinear objects of the vortex type in two-dimensional magnetic systems presents itself as one of the most promising candidates for the construction of nanodevices, useful for storing data, and for the construction of reading and writing magnetic heads. The vortex appears as the ground state of a magnetic nanodisk whose magnetic moments interact via the dipole-dipole potential {D∑[S⃗i⋅S⃗j−3(S⃗i⋅r̂ij)×(S⃗j⋅r̂ij)]/rij3} and the exchange interaction (−J∑S⃗i⋅S⃗j). In this work it is investigated the conditions for the formation of vortices in nanodisks in triangular, square, and hexagonal lattices as a function of the size of the lattice and of the strength of the dipole interaction D. Our results show that there is a “transition” line separating the vortex state from a capacitorlike state. This line has a finite size scaling form depending on the size, L, of the system as Dc=D0+1/A(1+BL2). This behavior is obeyed by the three types of lattices. Inside the vortex phase it is possible to identify two types of vortices separated by a constant, D=Dc, line: An in-plane and an out-of-plane vortex. We observed that the out-of-plane phase does not appear for the triangular lattice. In a two layer system the extra layer of dipoles works as an effective out-of-plane anisotropy inducing a large Sz component at the center of the vortex. Also, we analyzed the mechanism for switching the out-of-plane vortex component. Contrary to some reported results, we found evidences that the mechanism is not a creation-annihilation vortex anti-vortex process.

Exact calculation of entanglement in a19-site two-dimensional spin system

Using the Trace Minimization Algorithm, we carried out an exact calculation of entanglement in a 19-site two-dimensional transverse Ising model. This model consists of a set of localized spin-1/2 particles in a two dimensional triangular lattice coupled through exchange interaction J and subject to an external magnetic field of strength h. We demonstrate, for such a class of two-dimensional magnetic systems, that entanglement can be controlled and tuned by varying the parameter $\lambda$ = h/J in the Hamiltonian and by introducing impurities into the systems. Examining the derivative of the concurrence as a function of $\lambda$ shows that the system exhibits a quantum phase transition at about $\lambda_c$ = 3.01, a transition induced by quantum fluctuations at the absolute zero of temperature.

任意方向交番磁束条件下での3次元磁気歪みとベクトル磁気特性

This paper presents the measured magnetostriction of electrical steel sheets under alternating magnetic flux conditions. In the measurements, we used a two-dimensional vector magnetic property measurement system, and a three-axial strain gauge. In order to know the magnetostriction in an arbitrary direction, the strain tensor was evaluated. In addition, we try to evaluate the magnetostrictions in the thickness direction. In this paper, the three-dimensional magnetostriction in nonoriented and grain-oriented silicon steel sheets are compared. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 179(4): 1–9, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21257

Structural and magnetic properties of metal telluromolybdates M xM′ 1− xTeMoO 6 (M, M′ = Mn, Co, Cd)

Synthesis, crystal structures and magnetic properties of metal telluromolybdates M x M′ 1− x TeMoO 6 (M, M′ = Mn, Co, Cd) have been investigated. Their crystal structures have two-dimensional arrays of M and M′ atoms. From the powder X-ray diffraction measurements, Mn x Co 1− x TeMoO 6 adopt an orthorhombic structure throughout the composition range ( x = 0.0–1.0). On the other hand, Mn x Cd 1− x TeMoO 6 and Co x Cd 1− x TeMoO 6 adopt two types of structures corresponding to their end members (orthorhombic for Mn- or Co-rich solid solutions; tetragonal for Cd-rich ones). In the intermediate compositions, it was found that two phases coexist with different metal components. Magnetic properties of these solid solutions were investigated. All the Mn x Co 1− x TeMoO 6 exhibits an antiferromagnetic transition at ∼23 K. The antiferromagnetic transition was also observed in Mn x Cd 1− x TeMoO 6 and Co x Cd 1− x TeMoO 6. However, the Néel temperature rapidly decreases with increasing the concentration of Cd and disappeared below x = 0.6, which is characteristic for two-dimensional magnetic system.