Two-dimensional Magnetic Systems Research Articles

Read Channel Modeling for Detection in Two-Dimensional Magnetic Recording Systems

In this paper, we describe a read channel model for detector design for two-dimensional magnetic recording (TDMR) system, a novel strategy for recording at upto 10 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . We describe a scheme for (1) modeling of the recording medium, (2) modeling of the write/readback process, and (3) an experimental method for the characterization of noise in the TDMR channel, occurring due to irregularities in the bit-boundaries in the recording medium, that can be used for detection purposes.

Cluster-algorithm renormalization-group study of universal fluctuations in the two-dimensional Ising model

In this paper we propose a method to study critical systems numerically, which combines collective-mode algorithms and renormalization group on the lattice. This method is an improved version of the Monte Carlo renormalization group in the sense that it has all the advantages of cluster algorithms. As an application we considered the 2D Ising model and studied whether scale invariance or universality are possible underlying mechanisms responsible for the approximate "universal fluctuations" close to a so-called bulk temperature T(L) . "Universal fluctuations" were first proposed in the work of Bramwell, Holdsworth, and Pinton [Nature (London) 396, 552 (1998)] and stated that the probability density function of a global quantity for very dissimilar systems, such as a confined turbulent flow and a two-dimensional (2D) magnetic system, properly normalized to the first two moments, becomes similar to the "universal distribution," originally obtained for magnetization in the 2D XY model in the low-temperature region. The results for the critical exponents and the renormalization-group flow of the probability density function are very accurate and show no evidence to support that the approximate common shape of the PDF should be related to both scale invariance or universal behavior.

Modeling the Berezinskii-Kosterlitz-Thouless transition inNiGa2S4

In the two-dimensional superfluidity, the proliferation of the vortices and the antivortices results in a class of phase transition, the Berezinskii-Kosterlitz-Thouless (BKT) transition. This class of phase transition is also anticipated in the two-dimensional magnetic systems. However, its existence in the real magnetic systems still remains mysterious. Here we propose a phenomenological model to illustrate that the spin-freezing transition recently uncovered in the nuclear magnetic-resonance experiment on the ${\text{NiGa}}_{2}{\text{S}}_{4}$ compound is of BKT type. The spin-freezing state observed in the ${\text{NiGa}}_{2}{\text{S}}_{4}$ possesses the power-law decayed spin correlation.

Magneto-electrical cyclic voltammetry for visualization of current distribution in the electrolyte

Cyclic voltammetry is widely used in electrochemistry. To visualize the current distribution in the electrolyte, we developed a two-dimensional magnetic imaging system, combining the electric measurement component used in conventional cyclic voltammetry with a pair of MR (magnetic resistive) sensors to detect the x- and y-magnetic components. The resulting system allows simultaneous measurement of the electronic current of the working electrode and the magnetic field resulting from the transport of ions by migration in the bulk electrolyte. The current distribution in the electrolyte was reconstructed using the magnetic field strength data obtained at multiple points. Consequently, we could observe the change in current distribution when changing the length of the working electrode immersed in the electrolyte.

Stochastic Cutoff Method for Long-Range Interacting Systems

A new Monte-Carlo method for long-range interacting systems is presented. This method consists of eliminating interactions stochastically with the detailed balance condition satisfied. When a pairwise interaction $V_{ij}$ of a $N$-particle system decreases with the distance as $r_{ij}^{-\alpha}$, computational time per one Monte Carlo step is ${\cal O}(N)$ for $\alpha \ge d$ and ${\cal O}(N^{2-\alpha/d})$ for $\alpha < d$, where $d$ is the spatial dimension. We apply the method to a two-dimensional magnetic dipolar system. The method enables us to treat a huge system of $256^2$ spins with reasonable computational time, and reproduces a circular order originated from long-range dipolar interactions.

Persistent spin waves above the Néel temperature inYMnO3

We have investigated the spin excitations in the frustrated triangular antiferromagnet $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_{3}$ by inelastic neutron scattering in the ordered magnetic state and also above the N\'eel temperature. In the ordered phase, we observe broad spin excitations, which can be described by an anisotropic Heisenberg model. Above ${T}_{N}$, well defined signatures of persistent spin waves were found evidenced by inelastic peaks in energy spectra. Neatly separated excitations from the quasielastic fluctuations have been identified in this two-dimensional magnetic system. The clear observation might be related to the geometrical frustrated and short ranged ordered nature of this triangular antiferromagnet.

Properties of the interface in the confined Ising magnet with competing surface fields

A two-dimensional magnetic Ising system confined in an L × D geometry ( L ⪡ D ) in the presence of competing magnetic fields ( h) acting at opposite walls along the D -direction, exhibits an interface between domains of different orientation that run parallel to the walls. In the limit L → ∞ , this interface undergoes a wetting transition that occurs at the critical curve T w ( h ) , so that for T < T w ( h ) such an interface is bound to the walls, while for T w ( h ) ⩽ T < T cb the interface is freely fluctuating around the center of the film, where T cb is the bulk critical temperature. By considering both short- and long-range magnetic fields acting at the walls, we study the divergence of the (equilibrated) average position of the interface when approaching the wetting critical point. Furthermore, starting from a monodomain structure with the interface bound to one wall, we also study the dynamics of the interface unbinding.

Development and analysis of a PCB vector 2-D magnetic field sensor system for electronic compasses

A high-sensitivity vector two-dimensional (2-D) magnetic sensor system for low magnetic field measurements has been realized and tested. The system, made in PCB technology, consists of a double-axis Fluxgate magnetic sensor and the readout electronic circuitry, based on second-harmonic detection. The amorphous magnetic materials Vitrovac 6025X (25 /spl mu/m thick) and Vitrovac 6025Z (20 /spl mu/m thick) were used as the ferromagnetic core of the sensor. By applying a sinusoidal excitation current having a 450-mA peak at 10 kHz with Vitrovac 6025Z, the measured magnetic sensitivity was about 1.25 mV//spl mu/T. This value seems to be adequate for the Earth's magnetic field detection (/spl plusmn/60 /spl mu/T). The full-scale linearity error was about 1.5%. By using the thicker Vitrovac 6025X and a sinusoidal excitation current having a 600-mA peak at 10 kHz, a maximum sensitivity of approximately 1.68 mV//spl mu/T with a linearity error of about 1.55% full scale in the range of /spl plusmn/60 /spl mu/T were measured. Due to the use of commercially available ferromagnetic materials, the vector 2-D magnetic sensor system presented is characterized by a very simple fabrication process, thus allowing low-cost devices to be designed.

Study of the dynamic growth of wetting layers in the confined Ising model with competingsurface fields

A two-dimensional magnetic Ising system confined in anL × D geometry () in the presence of competing magnetic fields(h) acting at oppositewalls along the D-direction exhibits an interface between domains of different orientation thatruns parallel to the walls. In the limit of infinite film thickness () this interface undergoes a wetting transition that occurs at the critical curveTw(h), sothat for T<Tw(h) such an interface is bound to the walls, while forTw(h)≤T≤Tcb the interface is freely fluctuating around the centre of the film, whereTcb is the bulk critical temperature. Starting from a monodomain structure with the interfacebound to one wall, we study the onset of the interface unbinding by considering bothshort- and long-range magnetic fields acting at the walls. It is shown that, within the criticalwetting regime, in both cases the correlation length of interfacial fluctuations grows with time t as with z = 2, while the interfacial position follows both in the case of short-range and long-range surface fields, respectively, consistent withdynamic scaling predictions. Furthermore, considering the complete wetting regime and inthe presence of a bulk magnetic field, we find that the interface location also obeysstandard dynamic scaling behaviour for both short-range and long-range fields.

Magneto-Structural Correlations in Cu(tn)Cl2 (tn = 1,3-Diaminopropane): Two-Dimensional Spatially Anisotropic Triangular Magnet Formed by Hydrogen Bonds

A novel polymeric one-dimensional compound Cu(tn)Cl2 (tn = 1,3-diaminopropane) was prepared and structurally characterized, and its spectral, magnetic, thermodynamic, and thermal properties were studied. The unique structure shows ladderlike chains composed of Cu(II) atoms and chloro bridging ligands [Cu(-mu(3)-Cl-)Cu2] running along the crystallographic c axis. The coordination geometry about copper (4 + 2) approximates that of a strongly elongated octahedron. The equatorial plane of the coordination octahedron is formed by a chelate N-bonded tn ligand and two chloro ligands. One of the chloro ligands is terminal, and the other one, mu3-Cl-, forms two additional longer bonds to the neighboring copper atoms and thus occupies the axial octahedral positions. The electronic ground state of the Cu(II) ion is of d(z)2 symmetry and suggests the activation of intraladder and interladder Cl...H-N hydrogen bonds as exchange paths that form a two-dimensional pattern of a triangular symmetry. The interaction due to the hydrogen bonds seems to play an important role in molecular packing and magnetic coupling. The studies of magneto-structural correlations including electron paramagnetic resonance measurements and thermodynamic and magnetic properties revealed a two-dimensional character of magnetic correlations with the effective intralayer exchange coupling J/k(B) approximately -3 K. No phase transition to the ordered state has been observed down to 60 mK. Cu(tn)Cl2 with the interlayer coupling J' approximately 10(-3)J and moderate intralayer interaction represents an excellent example of a two-dimensional magnetic system.

Intrinsic mechanism of anomalous Hall effect in a twodimensional magnetic system with impurities

AbstractWe report some new results on the Anomalous Hall effect induced by the Berry phase in momentum space. Our main calculations are performed within the model of a two‐dimensional electron gas with the spin‐orbit interaction of Rashba type, taking into account the scattering from impurities. We demonstrate that such an “intrinsic” mechanism can really dominate but there is a competition between the geometric Berry‐phaseinduced term σ II xy in the Hall conductivity and the impurity‐induced term σ I xy , related to the contribution of the states in the vicinity of the Fermi surface. We also show that the contribution to the Hall conductivity from the electron states close to the Fermi surface has the intrinsic property as well, and it does not vanish in the clean limit. The main effect of the impurity‐related contribution is a possible change of sign of the off‐diagonal conductivity. The resulting magnitude and sign of the Hall conductivity strongly depend on the electron density in the system. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Rashba and Dresselhaus spin–orbit coupling effects on tunnelling through two-dimensional magnetic quantum systems

We investigate the influence of the Rashba and Dresselhaus spin–orbit coupling interactions on tunnelling through two-dimensional magnetic quantum systems. It is showed that not only Rashba spin–orbit coupling but also Dresselhaus one can affect spin tunnelling properties greatly in such a quantum system. The transmission possibility, the spin polarization and the conductance are obviously oscillated with both coupling strengths. High spin polarization, conductance and magnetic conductance of the structure can be obtained by modulating either Rashba or Dresselhaus coupling strength.

Probing two-dimensional magnetic switching in Co∕SiO2 multilayers using reversible susceptibility experiments

The magnetic switching behavior of Co∕SiO2 multilayers has been studied using reversible susceptibility experiments performed along different orientations in the sample’s plane. A sensitive method for critical curve determination of two-dimensional magnetic systems was proposed. It was shown that this method, based on reversible susceptibility’s singularities detection, is general and can be applied independent of the expression of free energy describing the magnetic system under study. It is found that as the Co∕SiO2 ratio increases in the samples, the switching mechanism is governed by a noncoherent rotation mechanism.

Readback Responses for Complex Recording Media Configurations

This paper describes a relatively simple approach to calculating the frequency response for a two-dimensional (2-D) magnetic recording system with a medium that can include an arbitrary number of layers. Each layer may have an arbitrary magnetization direction, anisotropic permeability, and exchange coupling. The approach relies on an initial transformation into the spatial frequency domain and then the use of transmission matrices to relate the fields in the different layers. The approach is general in that it provides a method for finding the field configuration for any set of 2-D magnetic sources embedded in a layered magnetic medium with a linear B--H relationship. Here, we focus on calculating the readback response for a variety of longitudinal and perpendicular recording configurations. Since the permeability may have any wavelength dependence, we can easily include the effect of exchange coupling in the underlayer.

Physical Properties of The 3dTransition Metal Intercalates of Misfit Layer Compounds (LaS)1.14[Mx(NbS2)2] (M= Mn, Fe, Co)

The pseudo two-dimensional (2D) magnetic systems (LaS) 1.14 [ M x (NbS 2 ) 2 ] ( M = Mn, Fe, Co) were synthesized for the first time, and their magnetic and transport properties were investigated. (LaS) 1.14 [ M x (NbS 2 ) 2 ] show three-dimensional (3D) magnetic transitions at 4.4, 6.2 and 15 K for M = Mn, Co and Fe, respectively. The 3D transition temperatures are lowered by the quasi-2D magnetic property brought about the insertion of the extra layer LaS to the layered magnets M x NbS 2 , and the 2D short-range order effects are observed in the heat capacity at much higher than the transition temperatures. The minimum behavior of ρ and the maximum behavior of R H appear in the low temperatures. We consider that the weak localization in the quasi-2D transport system causes the anomalous behaviors of the transport properties.

Magnetic properties of a two-dimensional mixed-spin system

Using a Langmuir–Blodgett (LB) synthesis method, novel two-dimensional (2D) mixed-spin magnetic systems, in which each magnetic layer is both structurally and magnetically isolated, have been generated. Specifically, a 2D Fe–Ni cyanide-bridged network with a face-centered square grid structure has been magnetically and structurally characterized. The results indicate the presence of ferromagnetic exchange interactions between the Fe3+(S=1/2) and Ni2+(S=1) centers.

NMR study of two-dimensional ferromagnet K 2CuF 4 below [formula omitted

Two-dimensional ferromagnet K 2CuF 4 is an ideal compound to study two-dimensional magnetic systems because the interplane exchange coupling constant is much smaller than the intraplane exchange coupling constant. We measured the temperature dependence of the NMR frequencies of the 1 2 ↔ 3 2 transition for 63 Cu and 65 Cu nuclei in K 2CuF 4 single crystal far below T c =6.25 K . The NMR measurements were made by spin-echo method at zero magnetic field. The experimental results are discussed in the framework of the two-dimensional spin wave theory at millikelvin temperatures.

Evidence for ferromagnetic ordering of [formula omitted] films on graphite

Recent experiments at USC have shown that two-dimensional films of 3 He on graphite order ferromagnetically at finite temperatures ( T>0) for densities above 20 atoms/ nm 2 . These results, obtained by measuring NMR on these films in the zero field limit, appear to contradict several studies which concluded that there is no spontaneous order by measuring magnetic susceptibility through conventional NMR in finite magnetic fields, and heat capacity in zero field. The low field limit is important in understanding these two-dimensional magnetic systems. At higher temperatures and/or fields, these films have been described in terms of the multiple spin exchange model. However, two-dimensional systems are extremely sensitive to anisotropies such as nuclear dipole interactions, even when they are several orders of magnitude less than the dominant exchange mechanism.

One-dimensional model of a vortex in an easy-plane ferromagnet

A simple one-dimensional model of a ferromagnet in a vortex configuration is proposed for qualitative description of the structure and dynamics of a vortex in a two-dimensional magnetic system. The model describes a system of four parallel spin chains with single-ion anisotropy of the easy-plane type and includes elements of both the continuum (along the chains) and discrete (perpendicular to the chains) descriptions. An analytical study is made of the static vortices and moving vortex solutions of stationary profile. The analytical results of a calculation of the static structure of the vortices are supplemented by a numerical analysis for the analogous discrete systems and are in good agreement with the numerical data. The approach is generalized to the case of the description of gyrotropic motion of vortices of stationary profile in the framework of the collective variables method for the combined continuum–discrete description. The results are analyzed and compared with the data for two-dimensional magnets.

Magnetic properties of CuSb 2− xTa xO 6 with tri-rutile structure

We report magnetic properties of CuSb 2− x Ta x O 6 with tri-rutile type structure. Solubility limit was determined to be x=1.0 by the solid-state reaction and X-ray diffraction measurements. The value of J, the magnetic interaction obtained by Bonner–Fisher fitting, was found to decrease gradually with Ta content x. The value of T N also decreases with x and seems to be zero at x=0.6. As a result of our structural analysis, this fact shows the crossover from one-dimensional to two-dimensional magnetic system with a local distortion caused by Ta substitution. Although the ground state of the compound with x>0.6 is not clear, it can be considered that the long-range antiferromagnetic order disappears and that the quantum critical regime such as a spin-singlet state is realized in this system.