Field-induced Magnetization Research Articles

Magnetoelectric force microscopy based on magnetic force microscopy with modulated electric field.

We present the realization of a mesoscopic imaging technique, namely, the Magnetoelectric Force Microscopy (MeFM), for visualization of local magnetoelectric effect. The basic principle of MeFM is the lock-in detection of local magnetoelectric response, i.e., the electric field-induced magnetization, using magnetic force microscopy. We demonstrate MeFM capability by visualizing magnetoelectric domains on single crystals of multiferroic hexagonal manganites. Results of several control experiments exclude artifacts or extrinsic origins of the MeFM signal. The parameters are tuned to optimize the signal to noise ratio.

Synchronous precessional motion of multiple domain walls in a ferromagnetic nanowire by perpendicular field pulses

Magnetic storage and logic devices based on magnetic domain wall motion rely on the precise and synchronous displacement of multiple domain walls. The conventional approach using magnetic fields does not allow for the synchronous motion of multiple domains. As an alternative method, synchronous current-induced domain wall motion was studied, but the required high-current densities prevent widespread use in devices. Here we demonstrate a radically different approach: we use out-of-plane magnetic field pulses to move in-plane domains, thus combining field-induced magnetization dynamics with the ability to move neighbouring domain walls in the same direction. Micromagnetic simulations suggest that synchronous permanent displacement of multiple magnetic walls can be achieved by using transverse domain walls with identical chirality combined with regular pinning sites and an asymmetric pulse. By performing scanning transmission X-ray microscopy, we are able to experimentally demonstrate in-plane magnetized domain wall motion due to out-of-plane magnetic field pulses.

Spin-lattice dynamics simulation of external field effect on magnetic order of ferromagnetic iron

Modeling of field-induced magnetization in ferromagnetic materials has been an active topic in the last dozen years, yet a dynamic treatment of distance-dependent exchange integral has been lacking. In view of that, we employ spin-lattice dynamics (SLD) simulations to study the external field effect on magnetic order of ferromagnetic iron. Our results show that an external field can increase the inflection point of the temperature. Also the model provides a better description of the effect of spin correlation in response to an external field than the mean-field theory. An external field has a more prominent effect on the long range magnetic order than on the short range counterpart. Furthermore, an external field allows the magnon dispersion curves and the uniform precession modes to exhibit magnetic order variation from their temperature dependence.

Tuning the Coordination Geometries and Magnetic Dynamics of [Ln(hfac)4]− through Alkali Metal Counterions

Four lanthanide compounds with formulas [Cs{Dy(hfac)4}] (1), [Cs{Er(hfac)4}] (2), [K{Dy(hfac)4}] (3), and [K{Er(hfac)4}] (4) (hfac = hexafluoroacetylacetone) are reported. Compounds 1 and 2 crystallize in the orthorhombic Pbcn space group, while 3 and 4 are in the triclinic P1̅ space group. All display chain structures in which the mononuclear [Ln(hfac)4](-) anions are linked by alkali metal ions. However, the coordination geometries around the Ln atoms are quite different depending on the cation. They adopt a distorted dodecahedron with pseudo-D2d symmetry in the cesium compounds 1 and 2, while a distorted square-antiprism with pseudo-D4d symmetry is adopted in the potassium compounds 3 and 4. The latter compounds show distinct field-induced slow magnetization relaxation. The energy barriers are 23.95 and 20.21 K for compounds 3 and 4, respectively.

A luminescent heptanuclear DyIr6 complex showing field-induced slow magnetization relaxation.

The first example of iridium/lanthanide phosphonates, i.e. [DyIr6(ppy)12(bpp)2(bppH)4](CF3SO3)·8H2O (1) (ppy(-) = 2-phenylpyridine, bpp(2-) = 2-pyridylphosphonate) is reported. It shows dual functions with the photoluminescence and field-induced slow magnetization relaxation originating from the Ir and Dy moieties, respectively.

Electric Field-Induced Magnetization Switching in CoFeB-MgO—Static Magnetic Field Angle Dependence

We investigate switching probabilities of electric field-induced magnetization switching in CoFeB/MgO based magnetic tunnel junction with perpendicular magnetic easy axis under static magnetic field with various directions. The switching takes place through magnetization precession with the period determined by the magnitude of the in-plane component of magnetic field. The experimental results are well reproduced by macro-spin simulation. The simulation reveals that decoherence induced by thermal fluctuations and/or other effects limit the switching probabilities in the present system.

Microscopic studies of the normal and superconducting state of Ca3Ir4Sn13

We report on muon spin rotation (muSR) studies of the superconducting and magnetic properties of the ternary intermetallic stannide Ca3Ir4Sn13. This material has recently been the focus of intense research activity due to a proposed interplay of ferromagnetic spin fluctuations and superconductivity. In the temperature range T=1.6-200 K, we find that the zero-field muon relaxation rate is very low and does not provide evidence for spin fluctuations on the muSR time scale. The field-induced magnetization cannot be attributed to localized magnetic moments. In particular, our muSR data reveal that the anomaly observed in thermal and transport properties at T*~38 K is not of magnetic origin. Results for the transverse-field muon relaxation rate at T=0.02-12 K, suggest that superconductivity emerges out of a normal state that is not of a Fermi-liquid type. This is unusual for an electronic system lacking partially filled f-electron shells. The superconducting state is dominated by a nodeless order parameter with a London penetration depth of lambda=385(1) nm and the electron-phonon pairing interaction is in the strong-coupling limit.

In-plane magnetic field dependence of electric field-induced magnetization switching

Electric field-induced magnetization switching through magnetization precession is investigated as a function of in-plane component of external magnetic field for a CoFeB/MgO-based magnetic tunnel junction with perpendicular easy axis. The switching probability is an oscillatory function of the duration of voltage pulses and its magnitude and period depend on the magnitude of in-plane magnetic field. Experimental results are compared with simulated ones by using Landau-Lifshitz-Gilbert-Langevin equation, and possible factors determining the probability are discussed.

Single Domain Spin Manipulation by Electric Fields in Strain Coupled Artificial Multiferroic Nanostructures

We demonstrate in situ 90° electric field-induced uniform magnetization rotation in single domain submicron ferromagnetic islands grown on a ferroelectric single crystal using x-ray photoemission electron microscopy. The experimental findings are well correlated with micromagnetic simulations, showing that the reorientation occurs by the strain-induced magnetoelectric interaction between the ferromagnetic nanostructures and the ferroelectric crystal. Specifically, the ferroelectric domain structure plays a key role in determining the response of the structure to the applied electric field, resulting in three strain-induced regimes of magnetization behavior for the single domain islands.

Antiferromagnetic resonance in the quasi-one-dimensional magnet IPACu(Cl x Br1−x )3 (x=0.83)

We have performed multi-frequency electron spin resonance (ESR) and magnetization measurements on single crystals of the quasi one-dimensional magnet IPACu(Cl x Br1−x )3 (x=0.83) in magnetic fields H along the b-axis at temperatures below an antiferromagnetic long-range ordering temperature (T N ≅ 13 K). We observed some ESR branches for typical antiferromagnetic modes with biaxial anisotropy at 1.5 K (≪ T N). We have analyzed the magnetization curve and the ESR excitation modes for H∥b by using a two sub-lattice model and obtained good agreement between experiment and calculation. In addition, we show that the origin of a field-induced magnetization jump is a spin-flop transition. These results show that the phase at temperatures below T N in this Cl-Br mixed compound can be described as a simple two sub-lattice antiferromagnet.

In situ study of electric field‐induced magnetization in multiferroic BiFeO3 nanowires

In this work, we have studied electric field-induced magnetization effect of multiferroic BiFeO3 (BFO) nanowires in situ using magnetic force microscopy (MFM). Changes in magnetic domain contrast have been observed in the MFM phase images under applied electric potential, which indicate local magnetoelectric (ME) coupling in the nanowires. The values of saturation and magnetization at different applied electric fields were evaluated. These results suggest that one-dimensional multiferroic BFO nanowires are potential candidates for realizing multiferroic devices at nanoscale with unique functionalities.

Translational and rotational dynamics in suspensions of magnetic nanorods

Using computer simulations we investigate the translational and rotational diffusion of dilute suspensions of magnetic nanorods with and without a (homogeneous) external magnetic field. The magnetic rods are represented as spherocylinders with a longitudinal point dipole at their center and length-to-breadth ratios L/D = 3 or L/D = 9. In the absence of a field, the rods tend to form compact clusters with antiparallel ordering and thus behave very differently to dipolar spheres (L/D = 0), which tend to form head-to-tail chains. Furthermore, for rod-like particles the external field tends to destabilize rather than to support cluster formation. We show that these differences in the aggregation behavior have profound consequences not only in static material properties such as the field-induced magnetization and the zero-frequency susceptibility, but also in the dynamics. In particular, for magnetic rods the translational diffusion constant parallel to the field is larger than the perpendicular one, in contrast to the behavior observed for magnetic spheres. Moreover, the rod-like character greatly affects the shape and the density dependence of the single-particle and collective dipole–dipole time correlation functions and their counterparts in the frequency domain.

Supramolecular lanthanide metallogrids exhibiting field-induced single-ion magnetic behavior

A supramolecular strategy has been applied to construct two tetranuclear lanthanide complexes for investigating the magnetic properties of individual lanthanide ions. The Ln(III) complexes (Ln = Dy, Tb) display field-induced slow magnetization relaxation, typical of single-molecule magnet behavior. The four lanthanide ions in the molecules are well separated by distances of ca. 9 Å, and thus the slow magnetization relaxation should be assigned to single-ion magnet (SIM) behavior. Therefore, the present complexes are novel supramolecular aggregates of four isolated SIMs.

Analysis of characteristics and key influencing factors in magnetomechanical behavior for cable stress monitoring

Until recently the magnetomechanical behavior has been rather poorly understood with contradictory results and interpretations from different investigators in the study of stress monitoring by magnetic method. Based on the domain motion and field-induced magnetization process of magnetic material, the stress-induced magnetization process in two different magnetization states is analyzed in detail. Characteristics of the magnetomechanical behavior are revealed. Domain motion caused by stress always makes materials tend to stable state of free energy minimum and accordingly the magnetization will approach the anhysterestic magnetization. Moreover, it can be concluded that magnetization state is an essential factor influencing the magnetomechanical behavior. Experiments of magnetomechanical behavior for galvanized steel wire have been conducted for different magnetization state, and the results are in accord with the theoretical analyses. The magnetomechanical behavior is changed with magnetization state, which can be effectively used to explain the existing complex and diverse research results.

Resonance Magnetoelectric Effect in NaNbO3 -NiFe2 O4 Composite

Electric field-induced magnetization characteristics are demonstrated in (100−x)NaNbO3–xNiFe2O4(x = 15, 25, 35, 45), synthesized by the solid-state sintering method. The composites show well-defined ferroelectric behaviors and magnetic characteristics. The CME effect characteristics with driving frequency in a range 10 k–15 kHz were investigated at room temperature. The CME couplings increase initially with increasing the driving frequencies and then decrease with the further increase in the frequency. Large CME coefficients are obtained at electromechanical resonance frequency of 12.8 kHz. These lead-free multiferroic composites exhibiting electrostatically induced ME resonance provide great opportunities for electric field-tunable microwave devices.

Effects of spin-polarized current on pulse field-induced precessional magnetization reversal

We investigate effects of a small DC spin-polarized current on the pulse field-induced precessional magnetization reversal in a thin elliptic magnetic element by micromagnetic simulations. We find that the spin-polarized current not only broadens the time window of the pulse duration, in which a successful precessional reversal is achievable, but also significantly suppresses the magnetization ringing after the reversal. The pulse time window as well as the decay rate of the ringing increase with increasing the current density. When a spin-polarized current with 5 MA/cm2 is applied, the time window increases from 80 ps to 112 ps, and the relaxation time of the ringing decreases from 1.1 ns to 0.32 ns. Our results provide useful information to achieve magnetic nanodevices based on precessional switching.

Magneto-optical Kerr effect investigation on magnetoelectric coupling in ferromagnetic/antiferroelectric multilayer thin film structures

Magnetoelectric (ME) membranes comprising soft ferromagnetic Ni and antiferroelectric (AFE) (Pb,La)(Zr,Sn,Ti)O3 (PLZST) layers were proposed and fabricated through a bulk micro-machining process on silicon wafers. An AC-mode magneto-optical Kerr effect technique was proposed to examine the magnetoelectric coupling in the multilayer membranes, in which the electric field-induced magnetization rotation was analyzed for understanding the underlying coupling mechanisms. The AFE to ferroelectric phase transformation of PLZST induced a rotation of magnetization of about 0.5° in Ni, persuaded by strain-induced anisotropy of about −0.5 kJ/m3.

Electric field-induced magnetization reversal in a perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction

The electric field-induced ∼180° magnetization reversal is realized for a sputtered CoFeB/MgO-based magnetic tunnel junction with perpendicular magnetic easy axis in a static external magnetic field. Application of bias voltage with nanoseconds duration results in a temporal change of magnetic easy axis in the free layer CoFeB to in-plane, which induces precessional motion of magnetization in the free layer. The magnetization reversal takes place when the bias voltage pulse duration is adjusted to a half period of the precession. We show that the back and forth magnetization reversal can be observed by using successive application of half-period voltage pulses.

Polarized neutron diffraction at a spallation source for magnetic studies

The availability of high-power spallation neutron sources, along with advances in the development of coupled moderators and neutron polarizers, has made it possible to use polarized neutrons on time-of-flight diffractometers forin situstudies of phenomena contributing to field-induced magnetization of a material. Different electronic and structural phenomena that contribute to the overall magnetization of a material can be studied and clearly identified with polarized neutron diffraction measurements. This article reports the first results from polarized neutron diffraction experiments on a time-of-flight instrument at a spallation source. Magnetic field-induced rotation of electron spins in an Ni–Mn–Ga single crystal was measured with polarized neutron diffraction at the MAGICS reflectometer at the Spallation Neutron Source at Oak Ridge National Laboratory. The difference in intensities measured with spin-up and spin-down polarized neutrons is proportional to the field-induced magnetization of the crystal. The polarized neutron measurements indicate that the magnetic form factor for the 3delectrons of Mn in Ni–Mn–Ga is lower than the value reported earlier for an ideal spherical symmetry of electronic distribution. Future experiments for studying field-induced magnetization in materials following the current methodology are outlined.

Simulation of Magnetization Reversal and Domain-Wall Trapping in Submicron Permalloy Wires With Different Wire Geometries

Nonvolatile data storage in today's computers is based on magnetic materials. Hard disk drives (HDD) feature ferromagnetic thin films for recording bits as regions with different magnetizations in the film. At present, the efforts of decreasing HDD size while increasing storage capacity, however, face fundamental limits, namely, the so-called superparamagnetic effect. Research into new generations of magnetic data-storage devices, amongst others, considers as its basic working principle current-induced magnetic domain-wall motion (CIDWM) in magnetic nanowires. In this paper, we present simulation studies of external magnetic field-induced magnetization reversal and domain-wall motion in nanowires of various sizes and shapes. Our results contribute to the basic design principles applied to magnetic structures used in CIDWM experiments. Parameters, such as wire thickness and wire-pad geometries are investigated, and important threshold magnetic field values are presented. In addition, we introduce a novel method of domain-wall trapping in a magnetic wire using the fringe fields of close-by magnets.