Time Evolution Of Magnetization Research Articles

Influence of spin glass-like magnetic relaxation on the zero-field-cooled exchange bias effect

The zero-field-cooled exchange bias (ZEB) effect is a remarkable phenomenon recently reported for some reentrant spin glass-like compounds. In this work, the time-evolution of magnetization is thoroughly investigated for two ZEB materials in order to figure out the role played by the spin glass-like phase on such effect. La$_{1.5}$Sr$_{0.5}$CoMnO$_{6}$ and La$_{1.5}$Ca$_{0.5}$CoMnO$_{6}$ were chosen as representative samples of ZEB systems, since the former compound presents the largest ZEB reported so far, while the second has a much smaller effect, despite being structurally/chemically similar. Comprehensive magnetic measurements were carried on both samples, and the results are discussed in terms of the amount and time-evolution of the spin glass-like phase under the influence of a varying field. We also propose a phenomenological model, based on the pinning of spin glass-like moments and on the dynamics of their magnetic relaxation, to explain the asymmetry observed in the hysteresis loops. The good agreement between the simulated and experimental results confirms our hypothesis that the spin glass-like phase is key to the ZEB effect.

Asymmetry in Time Evolution of Magnetization in Magnetic Nanostructures.

Strong interest in nanomagnetism stems from the promise of high storage densities of information through control of ever smaller and smaller ensembles of spins. There is a broad consensus that the Landau-Lifshitz-Gilbert equation reliably describes the magnetization dynamics on classical phenomenological level. On the other hand, it is not so evident that the magnetization dynamics governed by this equation contains built-in asymmetry in the case of broad topology sets of symmetric total energy functional surfaces. The magnetization dynamics in such cases shows preference for one particular state from many energetically equivalent available minima. We demonstrate this behavior on a simple one-spin model which can be treated analytically. Depending on the ferromagnet geometry and material parameters, this asymmetric behavior can be robust enough to survive even at high temperatures opening simplified venues for controlling magnetic states of nanodevices in practical applications. Using micromagnetic simulations we demonstrate the asymmetry in magnetization dynamics in a real system with reduced symmetry such as Pacman-like nanodot. Exploiting the built-in asymmetry in the dynamics could lead to practical methods of preparing desired spin configurations on nanoscale.

Magnetization reversal in ferromagnetic thin films induced by spin-orbit interaction with Slonczewski-like spin transfer torque

We theoretically investigate the dynamics of magnetization in ferromagnetic thin films induced by spin-orbit interaction with Slonczewski-like spin transfer torque. We reproduce the experimental results of perpendicular magnetic anisotropy films by micromagnetic simulation. Due to the spin-orbit interaction, the magnetization can be switched by changing the direction of the current with the assistant of magnetic field. By increasing the current amplitude, wider range of switching events can be achieved. Time evolution of magnetization has provided us a clear view of the process, and explained the role of minimum external field. Slonczewski-like spin transfer torque modifies the magnetization when current is present. The magnitude of the minimum external field is determined by the strength of the Slonczewski-like spin transfer torque. The investigations may provide potential applications in magnetic memories.

Effect of substitution of Cr on magnetic behavior and electrical transport in bi-layered manganite La 1.2Sr 1.5Ca 0.3Mn 2O 7

DC magnetization and electrical resistance measurements have been made on four samples in the series La 1.2Sr 1.5Ca 0.3Mn 2-xCr xO 7 corresponding to x = 0, 0.01, 0.03 and 0.05. The systems show onset of 3D ferromagnetic (FM) state in the temperature range ∼75–90 K Magnetization–temperature measurements in zero field cooling and field cooling modes, made under different external fields show long time evolution of magnetization with time suggestive of FM disorder and high magnetic anisotropy. At two temperatures above the Curie point of ∼75–90 K, broad discontinuities are seen in the range ∼260–270 K and ∼200–220 K. These might be signatures of transitions to FM order in ab plane and to an AFM state, respectively. An alternative explanation for the two higher temperature discontinuities could be in terms of a possible intergrowth in the ab plane reported in bi-layered systems. Insulator–metal transition temperature lies close to T c but the system with 2.5 atomic percent Cr ( x = 0.05) does not attain metallic state down to 20 K. Estimated values of density of states at Fermi energy and Debye temperature, for the studied compositions, are in reasonably good agreement with those reported in other perovskites.

Photo-Induced Relaxation of Magnetization in Molecular Magnet

The experimental study of photo-induced magnetization of hybrid molecular magnet containing cobalt(II) and tungsten(V) magnetic centers bridged by 4,4′-bpy and CN− is presented. The observed increase in magnetization rate is attributed to the defects due to inter-valence transfer between CoW ⇀↽ CoW. The time evolution of magnetization is parameterized by the power law rather than exponential function.

Numerical Analysis of Thermally Assisted Magnetization Reversal in Rectangular MRAM Cells Consisted of Exchange Coupled Bilayer

Time evolution of magnetization during a cooling process in thermally assisted magnetization reversal of rectangular MRAM cell has been numerically investigated by taking account of not only temperature variations of material parameters but also a thermal fluctuation effect. The rectangular MRAM cell with 100×150×20 nm3 in size is assumed, where the shape anisotropy dominates the switching property. The threshold amplitude of magnetic field required for a successful thermally assisted switching is decreased as the exchange stiffness constant is changed from 1×10-6 to 1×10-7 erg/cm. The time for the magnetization to relax into the bias field direction can be reduced by increasing the bias field amplitude. To switch the magnetization within 1 ns, it is found that the bias field should be larger than 165 Oe. An exchange coupling field in a ferromagnetic bilayer system is considered to be available for producing such a large bias field required for a faster switching in thermally assisted MRAMs.

Micromagnetics modeling of single pole head for doublelayered perpendicular media

As recording density and data rate are going higher, the characteristics of writing field such as switching time and magnitude are receiving more attention. The head field rise is a function of magnetic dynamics of single pole head, thus it is meaningful to survey the effect of magneto-motive force rise time and eddy current. The magnetic dynamics of a single pole tip (SPT) and a soft underlayer (SUL) were obtained by micromagnetics modeling for total magnetic recording system. By analyzing the time evolution of magnetization for the SPT and the SUL, the delay of magnetization switching of the SPT for quick MMF rise was understood. The effect of eddy current on the writing field generation was also investigated. The contribution from the SPT and SUL on the writing field was analyzed during the initial reversal.

Critical dynamics of the Gaussian model with multispin transitions.

In this paper, we present a multispin transition mechanism, which is an extension of the Glauber one, to investigate critical dynamics. By exactly solving the master equation, the influence of the multispin transition mechanism on the dynamic critical behavior is studied for the Gaussian model with nearest-neighbor interactions on d-dimensional lattices (d=1, 2, and 3). The time evolution of magnetization is exactly calculated, and the exact results of relaxation time and dynamic critical exponent are obtained. Our models are divided into two kinds: one is the spin-cluster transition and the other is the arbitrary multispin transition. It is found that there are different relaxation times, but the same dynamical critical exponent for different kinds of multispin transitions. The results show that the dynamical critical exponents are independent of spatial dimensions and configurations of transitional spins, and that the dynamical critical exponent is the same as that of the Glauber dynamics, and thus give a strong support to the simple single-spin-transition dynamics. Finally, we give a brief discussion on the results.

Time evolution of magnetization in the FeRh system near antiferromagnetic-ferromagnetic transition temperature (abstract)

The body-centered cubic FeRh is known to exhibit a first-order phase transition from antiferromagnetic to ferromagnetic at about 400 K, accompanied by a volume expansion of about 1%. The time evolution of the magnetization M of ordered Fe0.5Rh0.5 and FeRh0.958Pt0.042 just below and above the transition temperature (T0) was measured as functions of temperature and magnetic field. Around T0, the magnetization increased logarithmically with time up to M*(=41 emu/g), the value of which indicates that the ferromagnetic grains occupy one third of the antiferromagnetic matrix. When M=M*, there appeared a jump in M. After that, M varies as M(t) =M∞[1−exp(−t/τ)], where τ is a relaxation time. It should be noted that the value of M* is independent of both temperatures and external magnetic fields. Microscopic observation and x-ray diffraction measurement showed that the ferromagnetic grains nucleated in the antiferromagnetic matrix began to grow with time just below T0. In this work, the time evolution of the phase transition was analyzed by considering the magnetic free energy, the elastic energy, and the strain energy at the surface of the grains. The above mentioned magnetic relaxation process is deeply related to the time evolution of the grain size.

Magnetization evolution in connected pore systems. II. Pulsed-field-gradient NMR and pore-space geometry.

The time evolution of nuclear magnetization in a porous medium, e.g., proton dipole moments on water saturating a sandstone, can be described in terms of relaxation-operator eigenstates (modes of the magnetization). Decay of the magnetization in a pulsed-field-gradient NMR (PFGNMR) or stimulated-echo experiment is described by the intermediate scattering function (ISF). The ISF is a sensitive probe of the spatial structure of the modes of the magnetization. We have studied the time evolution of the magnetization and the behavior of the ISF by solving the relaxation-operator problem. A suitable set of PFGNMR measurements provides important information about the pore-space geometry, for example, the gradient-independent part of the spectrum of decay rates is contributed by large or isolated pores.

Ordering process in the kinetic Ising model on the honeycomb lattice.

The ordering process after quenching from infinite temperature is studied in the kinetic Ising model on the honeycomb lattice below the critical temperature by means of Monte Carlo simulations. Because of the presence of metastable droplets on the honeycomb lattice, the time scale of ordering becomes very long at low temperatures. Due to the metastability, the time evolution of magnetization per site, m(t), seems to be scaled by a characteristic time scale exp(2K)${\mathit{L}}^{\mathrm{\ensuremath{-}}2}$t for large L. Here, K, L, and t denote the nearest-neighbor coupling divided by the temperature, the linear dimension of the system, and the time after the quenching, respectively.

Dynamic specific heat of spin glasses: Studies of a spin cluster.

An internal energy response to small temperature oscillations is used to define the dynamic specific heat c(\ensuremath{\omega}). The properties of c(\ensuremath{\omega}) are analyzed for a six-site cluster of Ising spins within the framework of the Glauber model. The cluster has three local energy minima and thus three diverging relaxation times in the time evolution of magnetization and two such times in the evolution of energy. The dynamic specific heat allows the study of the relaxational spectrum of the energy but, as opposed to the dynamic spin susceptibility, it is not affected by the longest relaxing mode. For a uniform ferromagnet c(\ensuremath{\omega}) is sensitive only to rapidly relaxing processes.

Panel discussion: Spin glasses

After a brief review of recent low field experiments and the time evolution of magnetisation, new data on CuMn alloys is presented. The new data shows quite clearly that the spin glass temperature Tf does not mark the abrupt onset of irreversible behavior. Rather there is clear evidence for remanent magnetisation above Tf. Comparison with earlier a.c. susceptibility data suggests that there is a continuous freezing or blocking process occurring in the range of temperature centered on Tf.