Magnetic films on nanoperforated templates: a route towards percolated perpendicular media

We present a study on the magnetization reversal in Co/Pt multilayer films with an out-of-plane easy axis of magnetization deposited onto substrates with densely distributed perforations with an average period as small as 34?nm. Deposition of magnetic Co/Pt multilayers onto the nanoperforated surface results in an array of magnetic nanodots surrounded by a continuous magnetic film. Following the evolution of the magnetic domain pattern in the system, we suggest that domain walls are pinned on structural inhomogeneities given by the underlying nanoperforated template. Furthermore, a series of micromagnetic simulations was performed in order to understand the modification of the pinning strength of domain walls due to the magnetic interaction between nanodots and the surrounding film. The results of the simulations show that magnetic exchange coupling between the nanodots and the surrounding film strongly influences the pinning behavior of the magnetic domain walls which can be optimized to provide maximal pinning.     

Spintronics in antiferromagnets

Magnetic domains and the walls between are the subject of great interest because of the role they play in determining the electrical properties of ferromagnetic materials and as a means of manipulating electron spin in spintronic devices. However, much less attention has been paid to these effects in antiferromagnets, primarily because there is less awareness of their existence in antiferromagnets, and in addition they are hard to probe since they exhibit no net magnetic moment. In this paper, we discuss the electrical properties of chromium, which is the only elemental antiferromagnet and how they depend on the subtle arrangement of the antiferromagnetically ordered spins. X-ray measurement of the modulation wavevector Q of the incommensurate antiferromagnetic spin-density wave shows thermal hysteresis, with the corresponding wavelength being larger during cooling than during warming. The thermal hysteresis in the Q vector is accompanied with a thermal hysteresis in both the longitudinal and Hall resistivity. During cooling, we measure a larger longitudinal and Hall resistivity compared with when warming, which indicates that a larger wavelength at a given temperature corresponds to a smaller carrier density or equivalently a larger antiferromagnetic ordering parameter compared to a smaller wavelength. This shows that the arrangement of the antiferromagnetic spins directly influences the transport properties. In thin films, the sign of the thermal hysteresis for Q is the same as in thick films, but a distinct aspect is that Q is quantized.     

An all-metallic logic gate based on current-driven domain wall motion

The walls of magnetic domains can become trapped in a ferromagnetic metallic point contact when the thickness of the film and the width of the contact are less than their critical values. The discovery that domain walls can be moved from such constrictions by a sufficiently large current has attracted considerable attention from researchers working on both fundamental research and potential applications. Here we show that Invar nanocontacts fabricated on silica substrates exhibit a sharp drop in resistance with increasing bias voltage at room temperature in the absence of an applied magnetic field. Moreover, when two nanocontacts are combined in an all-metallic comparison circuit, it is possible to perform logical NOT operations. The use of electrical currents rather than applied magnetic fields to control the domain walls also reduces energy consumption and the risk of crosstalk in devices.     

Magnetic Domain Structure of U2D2 Solid 3He by Using the Magnetic Resonance Imaging

A spatial distribution of the magnetic domain structure in the U2D2 phase of solid ³ He was examined by a special Magnetic Resonance Imaging (MRI) technique. There were three kinds of magnetic domains in a single seed crystal of a few mm ³ size and each domain size was as large as the seed crystal size. We investigated by MRI the distribution and growth process of domains in the following two ways. First, we observed how the domains evolved from pre-existing domains as the crystal grew in a long vertical sample cell. When the crystal became long enough, one or two domains from the three were selected at the upper part of the cell. Our analyses of domain walls show that the (110)a or (110)b, proposed by Tsubota et al., is consisted with our observation. The pre-existing domains grow in the fixed directions of the domain walls and neither the solid-liquid interface nor the solid-cell wall boundary control the domain distribution during the crystal growth. Second, we observed how the domains were formed when a crystal made a transition from the high field phase (HFP) to the U2D2 phase by a magnetic field cycling. After the magnetic field cycling, the original domain distribution reappeared with slight modifications, particularly near the solid-liquid interface. Possible causes of this memory effect are discussed.     

Control of magnetic domain-wall polarization by means of angled Oersted field writing

We propose a method to control the polarization of the magnetic domain walls (DWs) in ferromagnetic nanowires. Two neighboring DWs with antiparallel polarization alignment rather than parallel alignment are found to exhibit better stability with a helical magnetic structure that can be hardly be detangled. To achieve such an antiparallel alignment, two co-planar current lines with an angle to the nanowire are designed, from which the Oersted field creates a domain in between the current lines while keeping the polarization of the DWs beneath the current lines, as confirmed by a micromagnetic calculation for ferromagnetic nanowires with perpendicular magnetic anisotropy.     

Atomic spin structure of antiferromagnetic domain walls

The search for uncompensated magnetic moments on antiferromagnetic surfaces is of great technological importance as they are responsible for the exchange-bias effect that is widely used in state-of-the-art magnetic storage devices. We have studied the atomic spin structure of phase domain walls in the antiferromagnetic Fe monolayer on W(001) by means of spin-polarized scanning tunnelling microscopy and Monte Carlo simulations. The domain wall width only amounts to 6-8 atomic rows. Although walls oriented along <100> directions are found to be fully compensated, detailed analysis of <110>-oriented walls reveals an uncompensated perpendicular magnetic moment. Our result represents a major advance in the field of antiferromagnetism, and may lead to a better understanding of the magnetic interaction between ferromagnetic and antiferromagnetic materials.     

Effect of easy-plane-type surface anisotropy on domain wall assisted magnetic recording

The effect of a surface anisotropy of the easy-plane-type on the magnetic data recording in a single-crystalline film in which walls of the domain band structure play the role of information bits has been studied by micromagnetics simulations. It is established that the surface anisotropy leads to the appearance of a domain structure with the walls of different (Neél and vortex) types. Under the action of an external field, the Neél walls can be transformed into vortex walls that can be used for magnetic data recording.     

Zero-field optical manipulation of magnetic ions in semiconductors

Controlling and monitoring individual spins is desirable for building spin-based devices, as well as implementing quantum information processing schemes. As with trapped ions in cold gases, magnetic ions trapped on a semiconductor lattice have uniform properties and relatively long spin lifetimes. Furthermore, diluted magnetic moments in semiconductors can be strongly coupled to the surrounding host, permitting optical or electrical spin manipulation. Here we describe the zero-field optical manipulation of a few hundred manganese ions in a single gallium arsenide quantum well. Optically created mobile electron spins dynamically generate an energy splitting of the ion spins and enable magnetic moment orientation solely by changing either photon helicity or energy. These polarized manganese spins precess in a transverse field, enabling measurements of the spin lifetimes. As the magnetic ion concentration is reduced and the manganese spin lifetime increases, coherent optical control and readout of single manganese spins in gallium arsenide should be possible.     

Twisted domain wall structure for stripe domains in bubble films

A four-parameter variational model is used to calculate the properties of stripe domains in uniaxial magnetic films. The width of the stripes and the thickness of the walls between the stripes are allowed to vary. In addition, the effects of the demagnetizing fields across the walls are included by allowing the spins within the walls to twist out of the wall plane. It is shown that the presence of neighboring walls in this model yields a substantially higher wall energy density in most cases of importance to bubble technology than was previously reported by Schlömann for a similar model of an isolated wall, the stripe wall energy density being over 50 percent higher in the case where the dimensionless anisotropy parameterq = K_{u}/2piM^{2}is 1.1 and the film thickness to material length ratioh/lis 9. A critical discussion of the significance of wall energies in such models is given, and a method is discussed by which the stripe-width information from this model may be used in interpreting stripe-width data to obtain material parameters to an accuracy hitherto unavailable.     

Unravelling the Elusive Antiferromagnetic Order in Wurtzite and Zinc Blende CoO Polymorph Nanoparticles

Although cubic rock salt‐CoO has been extensively studied, the magnetic properties of the main nanoscale CoO polymorphs (hexagonal wurtzite and cubic zinc blende structures) are rather poorly understood. Here, a detailed magnetic and neutron diffraction study on zinc blende and wurtzite CoO nanoparticles is presented. The zinc blende‐CoO phase is antiferromagnetic with a 3rd type structure in a face‐centered cubic lattice and a Néel temperature of T_N (zinc‐blende) ≈225 K. Wurtzite‐CoO also presents an antiferromagnetic order, T_N (wurtzite) ≈109 K, although much more complex, with a 2nd type order along the c‐axis but an incommensurate order along the y‐axis. Importantly, the overall magnetic properties are overwhelmed by the uncompensated spins, which confer the system a ferromagnetic‐like behavior even at room temperature.     

Anomalous Domain Wall Magnetoresistance in Ultrathin Manganite Films Near M–I Transition Boundary

Resistance and magnetoresistance in compressively strained epitaxial ultrathin films have been studied. The samples were first demagnetized in different ways so that different magnetic structures were created, such as random domain and single domain states. Very large difference in resistance in zero applied magnetic field was observed between different states. The large change of resistance between states is attributed to spin-dependent scattering at the domain walls. We have shown for the first time that large domain wall resistance can be obtained in strained ultrathin manganite films and the result cannot be explained by the double-exchange model.     

Magnetic Sensor Principle for Susceptibility Imaging of Para- and Diamagnetic Materials

In ferromagnetic materials, magnetic domain walls interact with microstructure over similar mechanisms as dislocations do. Under the absence of stress as an additional influence, the magnetic properties of these materials are often correlated with mechanical-technological characteristics such as hardness and strength. This fundamental observation is the basis of micro-magnetic materials characterization. It is not yet well known, but quite legitimate to assume that such a correlation also exists between magnetic and mechanical properties of para- and diamagnetic materials, since the interactions on the electron spin level depend on the molecular structure and determine the magnetic behavior. Similar effects are known to exist in the electrical domain. As an example, the electrical conductivity of nonmagnetic materials such as aluminum or nickel based alloys, which can be assessed non-destructively using eddy current impedance measurements, is affected by stress and dislocation density. Much less is known about the correlation between the mechanical properties and the magnetic susceptibility of non-ferromagnetic materials such as graphite, aluminum and plastics. Today, there is no commercialized non-destructive method which uses the concept of magnetic susceptibility measurements for the characterization of dia- and paramagnetic materials. This article proposes a force-based magnetic sensor principle for susceptibility imaging of such materials.     

Cross-tie domain wall ground state in thin films

Cross-tie domain wall is a complex two dimensional magnetic domain wall structure, often found experimentally in this soft ferromagnetic films with thicknesses of about several exchange lengths. The structure of such a wall can roughly be imagined as a sequence of magnetic vortices and anti-vortices, arranged along a straight line. In this work, equilibrium energies of different one-dimensional magnetic domain wall configurations, existing in this soft ferromagnetic films, are calculated (fully taking into account the exchange and long range dipolar interaction), based on the various classical Ritz models. They are compared to the energy of two-dimensional cross-tie domain wall, thus, estimating the region of the film thicknesses, where the cross-tie structure is the lowest energy domain wall configuration. The upper boundary of this region is estimated here for the first time. Convenient approximate analytical energy expressions in terms of elementary functions are given here both for classical and for cross-tie domain walls.     

Atomic-scale magnetometry of distant nuclear spin clusters via nitrogen-vacancy spin in diamond

The detection of single nuclear spins is an important goal in magnetic resonance spectroscopy. Optically detected magnetic resonance can detect single nuclear spins that are strongly coupled to an electron spin, but the detection of distant nuclear spins that are only weakly coupled to the electron spin has not been considered feasible. Here, using the nitrogen-vacancy centre in diamond as a model system, we numerically demonstrate that it is possible to detect two or more distant nuclear spins that are weakly coupled to a centre electron spin if these nuclear spins are strongly bonded to each other in a cluster. This cluster will stand out from other nuclear spins by virtue of characteristic oscillations imprinted onto the electron spin decoherence profile, which become pronounced under dynamical decoupling control. Under many-pulse dynamical decoupling, the centre electron spin coherence can be used to measure nuclear magnetic resonances of single molecules. This atomic-scale magnetometry should improve the performance of magnetic resonance spectroscopy for applications in chemical, biological, medical and materials research, and could also have applications in solid-state quantum computing.     

Models for I and T pattern permalloy bars based upon bitter pattern observation of domain wall movements

The magnetostatic fields of the I and T pattern Permalloy overlay bars are analyzed by proposing a model based on the Bitter pattern observation of the domain wall structure in Permalloy bars. The magnetic charges that appear on the 90° domain walls are assumed to be the sources of the magnetic fields of the bars. The model has a two-dimensional reaction to an applied rotating in-plane field due to its two-dimensional domain wall movement and the consequent two-dimensional change of magnetic domain pattern inside the bar. The magnetization of the bar is equal to Msthe saturation magnetization of the bar at every section of the bar except on the domain walls. The magnetization curve and the magnetic field well Bz(bubble drive field) under the overlay bars are calculated and compared to that of the previous models. A qualitative explanation of the rotation of the bubble around the bars is given by the three-dimensional plots of the field well obtained for different orientations of the in-plane field.     

Rotation of Ferromagnetically Coupled Mn Spins in (Ga,Mn)As by Hole Spins

We present experimental data obtained from ferromagnetic p-(Ga,Mn)As layers that indicate the collective rotation of ferromagnetically coupled Mn spins by optically generated spin-polarized holes through the p–d exchange interaction. The rotation occurs reversibly between the in-plane and perpendicular directions, causing a large change in perpendicular magnetization without the application of a magnetic field. Pump-and-probe experiments suggest that the rotation of Mn spins take place in the picosecond time domain.     

Cross-Tie Domain Wall Ground State in Thin Films

No Heading Cross-tie domain wall is a complex two dimensional magnetic domain wall structure, often found experimentally in thin soft ferromagnetic films with thicknesses of about several exchange lengths. The structure of such a wall can roughly be imagined as a sequence of magnetic vortices and anti-vortices, arranged along a straight line. In this work, equilibrium energies of different one-dimensional magnetic domain wall configurations, existing in thin soft ferromagnetic films, are calculated (fully taking into account the exchange and long range dipolar interaction), based on the various classical Ritz models. They are compared to the energy of two-dimensional cross-tie domain wall, thus, estimating the region of the film thicknesses, where the cross-tie structure is the lowest energy domain wall configuration. The upper boundary of this region is estimated here for the first time. Convenient approximate analytical energy expressions in terms of elementary functions are given here both for classical and for cross-tie domain walls.PACS numbers: 75.75.+a, 75.25.+z, 75.60.–d.     

Some characteristics of ion-implanted bubble chips

The relations between the position of charged walls and the bubble motion around propagation circuits are discussed. Long walls which extend between adjacent propagation loops are revealed by the Bitter technique. The examination of the domain structure in the implanted layer shows the existence of a magnetic gradient which is a function of the distance from the propagation circuits. The switching of magnetization in particular directions of the in-plane field is reported and correlated with the bubble movement. An additional easy axis is observed along the circuits due to shape anisotropy. Propagation margins are very similar to those obtained with permalloy circuits. Fabrication technology as well as design of 16 μm period circuits is discussed. Nucleation and transfer have been achieved with currents in the range of 50 mA to 200 mA. Phase margins of about a quarter of a period are found, and bias field margins fall between 10 and 15 Oe.