Uniaxial Easy Axis Research Articles

Magnetic Anisotropy of FeNi Multilayer Films with Different Orientations of the Magnetic Anisotropy Axes in Adjacent Layers

FeNi films were prepared using the DC magnetron sputtering technique with an oblique deposition arrangement. Multilayers with different orientations of the magnetic anisotropy axes were obtained thanks to a rotary sample holder inside the vacuum chamber. Magnetic properties were studied using magneto–optical Kerr microscopy and a vibrating sample magnetometer. Single-layered FeNi films having thicknesses as high as 10 nm and 40 nm show in-plane uniaxial easy magnetization axes produced by the oblique incidence of incoming components of the beams. Magnetic anisotropy field for four-layered samples with orthogonal uniaxial magnetic anisotropy axes in the adjacent layers and the thickness of individual layers of 10 nm and 40 nm turned out to be less than in single-layered films. The magnetic properties peculiarities of the eight-layered sample FeNi (10 nm) × 8 obtained by rotation of the sample holder by 45° before deposition of each subsequent layer suggest the formation of a helix-like magnetic structure through the thickness of the multilayered sample similar to the magnetization arrangement in the Bloch-type magnetic domain wall.

Magnetic Skyrmionic Bubbles at Room Temperature and Sign Reversal of the Topological Hall Effect in a Layered Ferromagnet Cr0.87Te.

The search for materials that exhibit topologically protected spin configurations, such as magnetic skyrmions, continues to be fueled by the promise of outstanding candidate components for spin-based applications. In this study, in situ Lorentz transmission electron microscopy directly images Bloch-type magnetic skyrmionic bubbles in a layered ferromagnet Cr0.87Te single crystal. Owing to the competition between a magnetic dipole interaction and uniaxial easy axis anisotropy, nanoscale magnetic bubbles with random chirality can be observed in a wide temperature range covering room temperature when the external magnetic field is applied along the out-of-plane direction. Moreover, high-density and stable skyrmionic bubbles are successfully realized at zero magnetic field by appropriate field-cooling manipulation. Additionally, a sign reversal of the Hall effect and the derived topological Hall effect is observed and discussed. As quasi-two-dimensional materials, the binary chromium tellurides hosting magnetic skyrmions could have many applications in low-dimensional skyrmion-based spintronic devices in an ambient atmosphere.

Tailoring magnetic vortices of dipolar coupled nanoelements

We report a theoretical study of the magnetic profiles of vortex pairs of dipolar coupled Fe nanocylinders. We consider pairs of identical and coaxial 21 nm height Fe circular nanocylinders with the diameter values ranging from 81 nm to 129 nm. We show that, even though each isolated nanocylinder holds a vortex at remanence, the dipolar interaction may inhibit the formation of vortices for small values of the nanocylinders’ distance. For moderate values of the nanocylinders’ distance, the formation of opposite chirality vortex pairs is favored. We show that the vortex locations, the vortex pair magnetic pattern, and the dipolar field and local field profiles may be tailored using moderate external field values along preparation routes parallel and perpendicular to the Fe uniaxial anisotropy easy axis.

Study of interface induced anisotropic exchange coupling in amorphous FeCoB/MgO bilayer

In this work, we present a detailed magneto-optical Kerr effect (MOKE) study of amorphous FeCoB (FCB)/MgO bilayer grown on Si (100) substrate. Two regions with different boron density has been identified in single FCB layer in the as-deposited bilayer due to boron diffusion from FCB to MgO layer at the interface: (i) a boron-deficient layer near FCB/MgO interface (FCBi) and (ii) bulk FeCoB layer (FCBb) located farther away from FCB/MgO interface and having comparatively high boron concentration. As a result, a stress-induced uniaxial magnetic anisotropy is observed in the boron deficient FCBi layer, which is found to be superimposed with the magnetization of FCBb layer. Depending on the angle between the applied magnetic field and the uniaxial easy axis of the FCBi layer, magnetization reversal with single step or double step are observed. Role of interlayer exchange coupling between FCBi and FCBb layer is determined by separating out their magnetic contributions by fitting hysteresis loops with a mathematical function. The disappearance of double step behavior after in-situ annealing up to 473 K, suggests that the observed magnetic anisotropy mainly originates from the strain in the film.

Mechanical Strain Manipulation of Exchange Bias Field and Spin Dynamics in FeCo/IrMn Multilayers Grown on Flexible Substrates.

As a key effect in spintronic devices, exchange bias has attracted tremendous attention. Various approaches have been attempted for optimizing this effect, among which the application of strain in flexible exchange-biased systems is promising, but little significant improvement has been reported. Here, we demonstrate encouraging progress in this field. With a pure mechanical compressive strain of -6.26‰ applied to the flexible polyimide (PI) substrate, distinct enhancement of ∼900% in the bias field (from 20 to 200 Oe) is achieved for the exchange-biased (FeCo/IrMn)3/Ta multilayers grown on top of a flexible PI substrate, accompanied by a notable decrease in the Gilbert damping parameter from 0.02 to 0.008, signifying an improved exchange bias effect as well as a potentially reduced switching current density. The underlying mechanism is investigated by a systematic ferromagnetic resonance study, suggesting that the angle between the unidirectional and uniaxial magnetic easy axes plays an important role, which may be controlled by adjusting the layer number. This work offers an efficient strategy for tuning the exchange bias effect via applying appropriate mechanical strain on a multiperiodic exchange bias multilayered system, opening up an avenue for tailoring the magnetic properties of flexible spintronic devices.

Spin\u2013orbit torque switching in a T-type magnetic configuration with current orthogonal to easy axes

Different symmetry breaking ways determine various magnetization switching modes driven by spin–orbit torques (SOT). For instance, an applied or effective field parallel to applied current is indispensable to switch magnetization with perpendicular anisotropy by SOT. Besides of this mode, here we experimentally demonstrate a distinct field-free switching mode in a T-type magnetic system with structure of MgO/CoFeB/Ta/CoFeB/MgO where a perpendicular layer with tilted easy axis was coupled to an in-plane layer with a uniaxial easy axis. Current was applied orthogonal to both easy axes and thus also normal to an in-plane effective field experienced by the perpendicular layer. Dynamic calculation shows perpendicular layer could be switched at the same time as the in-plane layer is switched. These field-free switching modes realized in the same T-type magnetic system might expedite the birth of multi-state spin memories or spin logic devices which could be operated by all electric manners.

Anisotropic AC Magnetic Susceptibility in (Ga,Mn)As Films

Ferromagnetic semiconductor (Ga, Mn)As has emerged as the most thoroughly studied material for semiconductor spintronic applications, showing excellent magnetic properties that are usually described by the single-domain model [1]. However, certain uncertainties about magnetic phase structure and their dynamic properties of this system still remain unresolved. In this study, we systematically carry out both dc and ac magnetic measurements as a function of temperature and applied magnetic field in (Ga, Mn)As films with nominal Mn concentration of 5.0% grown by molecular beam epitaxy. The ac susceptibility vs. temperature shows similar results to those reported previously [2,3]: two peaks are observed with magnetic field H along [100], one close to T C and one close to T C /2, while one peak close to T C is observed with H along [1-10], and one peak close to T C /2 is seen for H along [110]. These results are consistent with dc magnetization measurements along these three axes. Careful analysis of hysteresis loops and of frequency-dependent ac susceptibility leads to the unambiguous conclusion that the ac susceptibility peak near T C occurs because of transition from paramagnetic to ferromagnetic phase; and the peak near T C /2 occurs because of the onset of a biaxial domain structure [4]. Specifically, ac susceptibility near T C involves a 180 degree magnetization flip along the [1-10] axis (i.e., the uniaxial easy axis), while the peak of the ac susceptibility near T C /2 originates from a magnetization flip between two biaxial easy axes with an angular separation of θ(T), arising from the competition between uniaxial and cubic anisotropy. Due to the difference between these two mechanisms, frequency dependences of ac susceptibilities in these two temperature regions are also quite different, providing additional information on both biaxial and uniaxial domain structures in (Ga, Mn)As films. In this connection, magnetic dynamic properties (such as the relaxation times) are analyzed and discussed using various dynamic models in order to allow us to get better insight into the evolution of magnetic domain transitions in (Ga, Mn)As films. The present observations on magnetic domain transitions and their dynamics provide new information relevant to spintronic applications such as memory and logic devices. This work was supported by the NSF Grant DMR14-00432.

Improvement of Stepped Magnetoimpedance Properties by Controlling the Demagnetizing Effect

Magnetoimpedance (MI) element is used for compasses in mobile phones [1], and its application expands to various industrial fields such as biomedical field and nondestructive testing because of its higher sensitivity [2]. On the other hand, MI element with a stepped impedance properties was obtained from the rectangular amorphous film with a uniaxial easy axis at an inclined angle relative to the width of the element [3] as a new feature of this sensor. This element is expected to apply for a novel magnetic field sensor for detection of a threshold field, with a memory function, etc. Additionally, a demand for a miniaturization of MI elements increases on the miniaturization of electronic devices and the progress in fast signal processing. Thus, an MI element fabricated using a thin film is a prospective candidate because this configuration is compatible with integrated electronic devices. However, when a thin-film MI sensor is miniaturized, decrease in the properties due to a demagnetizing field is unavoidable. Therefore, we clarified that the deterioration of the properties at the edge of the elements can be mainly attributed to the distribution of the demagnetizing field and suggested that only the central regions of an element should be utilized for sensing applications to eliminate the adverse effect due to the distribution of demagnetizing field [4]. However, controlling the distribution of demagnetizing field in the element contributes to the miniaturization of the sensor devices more. Thus, in this study, we prepared MI sensor elements in which the sensor shapes have a typical rectangular and a quasi-ellipsoidal shape for avoiding the demagnetizing field distribution. Then the impedance measured and domain observations were experimentally performed using prepared MI elements and we investigated the usefulness of the controlling of demagnetizing field distribution in the elements.

Interplay of uniaxial and cubic anisotropy in epitaxial Fe thin films on MgO (001) substrate

Epitaxial Fe thin films were grown on annealed MgO(001) substrates at oblique incidence by DC magnetron sputtering. Due to the oblique growth configuration, uniaxial anisotropy was found to be superimposed on the expected four-fold cubic anisotropy. A detailed study of in-plane magnetic hysteresis for Fe on MgO thin films has been performed by Magneto Optic Kerr Effect (MOKE) magnetometer. Both single step and double step loops have been observed depending on the angle between the applied field and easy axis i.e. along ⟨100⟩ direction. Domain images during magnetization reversal were captured by Kerr microscope. Domain images clearly evidence two successive and separate 90° domain wall (DW) nucleation and motion along cubic easy cum uniaxial easy axis and cubic easy cum uniaxial hard axis, respectively. However, along cubic hard axis two 180° domain wall motion dominate the magnetization reversal process. In spite of having four-fold anisotropy it is essential to explain magnetization reversal mechanism in 0°< ϕ < 90° span as uniaxial anisotropy plays a major role in this system. Also it is shown that substrate rotation can suppress the effect of uniaxial anisotropy superimposed on four-fold anisotropy.

Temperature dependent rotation of the uniaxial easy axis of magnetization in granular SiO2–(Co/Fe/CoFe2)–Si(1 1 1) multilayers

We have observed temperature dependent rotation of the uniaxial easy axis of magnetization and introduced the phenomenon of the superspin reorientation transition (SSRT) for the nanogranular thin films and multilayers. The effect is demonstrated on the nanogranular SiO2–M(x)–Si(111) multilayers, where M is the metal (Co, Fe or CoFe2) of the nominal thickness x=3, 5 and 10nm, respectively. The morphology and layer thickness have been examined using the transmission electron microscopy, X-ray reflectivity measurement and atomic force microscopy. Magnetic property measurements demonstrated that the orientation of the uniaxial in-plane easy axis of magnetization given by the film shape anisotropy was thickness independent at temperatures above 200K. Below 50K, the films shape anisotropy has been overcome and the uniaxial easy axis was rotated into different directions for each samples series, forming an angles from 40 up to 150° with respect to the film plane. Such effect of the low temperature magnetization rotation has been attributed to the specific arrangement of the nanoparticles in the individual layers and has been assigned to the SSRT phenomena.

Mixed modes of surface polaritons in a PML-type magnetoelectric multiferroic with canted spins

We present a theoretical discussion of surface polaritons on a ferroelectric-antiferromagnet with magnetoelectric coupling which allows the magnetic subsystem to be canted. Canting of the antiferromagnet results in weak ferromagnetism. The surface polaritons for a semi-infinite film are calculated for a propagation parallel to the uniaxial easy axis, leading to mixed modes. A superposition of two plane waves is needed to generate mixed surface modes. We find two branches, one near the magnon resonance frequency and the other near the phonon resonance frequency. We also find that the surface modes are non-reciprocal due to magnetoelectric interaction, such that ω(k) ≠ ω(−k), where ω is the frequency and k is the propagation vector.

Correction: Pattern Transfer and Electric‐Field‐Induced Magnetic Domain Formation in Multiferroic Heterostructures

The directions of the arrows in the ferroelectric domain image (FE) of Figure 2 in the above-mentioned article are incorrect and should be rotated by 90° so that they reflect the experimentally observed collinear alignment between the ferroelectric polarization of the BaTiO3 substrate and the strain-induced uniaxial magnetic easy axes of the CoFe film. We provide the corrected Figure 2 below. The arguments and conclusions of the original article are not affected. Magnetic hysteresis curve and polarization microscopy images of the ferroelectric domain structure (FE) and magnetic stripe pattern during several stages of the magnetization reversal process (R1, S1, R2, S2). The figure depicts experimental data for a magnetic field angle of θ = 10°. The hysteresis curve represents an average magnetic response from many a1 and a2 domains. The arrows in the images indicate the orientation of ferroelectric polarization (FE) and film magnetization in the remnant state (R1 and R2) and during abrupt magnetic switching (S1 and S2). The imaged areas are 30 × 40 μm.

Influence of deposition field on the magnetic anisotropy in epitaxialCo70Fe30films on GaAs(001)

The effect of the application of a magnetic field during deposition of epitaxial Co{sub 70}Fe{sub 30} onto GaAs(001) is shown; we find an initially counterintuitive result. For field applied along the interfacial uniaxial hard axis the relative effective uniaxial magnetic anisotropy is increased by a factor of two in comparison to both field along the uniaxial easy axis, or no field; usually, application of a deposition field results in a uniaxial easy axis parallel to this field direction. We show that the deposition field changes the maximal projection of the atomic orbital magnetic moments onto the easy axis, which corresponds to a deposition field induced shift in the Helmholtz free-energy landscape of the system.

Study of dynamic process of domain change in the stepped giant magnetoimpedance element

A giant magnetoimpedance (GMI) element with stepped impedance property was reported for rectangular amorphous soft magnetic thin film with an in-plane uniaxial easy axis in a controlled direction. This stepped change of impedance occurs simultaneously with a change in a magnetic domain, and its physical basis was explained as the intersection of the level of magnetic energy in each case of these domain structures. In this paper, an induced pulse-voltage caused by a non-linear leap of magnetization in the domain change is studied, for the purpose of making dynamics of the domain change clear. We report the results of detecting a pulse signal by using a planar coil fabricated with the stepped GMI element. Some fundamental properties of the pulse signal are also reported, such as its form, and variations in its height as a function of the frequency of the external magnetic field. As a result we found that the domain change in the stepped GMI element is a kind of Barkhausen jump.

Existence of Three Stable States of Magnetic Domain for the Stepped Giant Magnetoimpedance Element and Proposal for Sensor With Memory Function

In this paper, a giant magnetoimpedance (GMI) element with stepped impedance property was reported for rectangular amorphous soft magnetic thin film with an in-plane uniaxial easy axis in a controlled direction. This stepped change of impedance occurs simultaneously with a change in a magnetic domain, and its physical basis was explained as the intersection of the level of magnetic energy in each case of these domain structures. The variation of stepped impedance as a function of external magnetic field typically has a hysteretic property. The hysteretic property is explained based on an existence of threshold energy barrier which is needed overcoming to change the magnetic domain. In this paper, we will show experimentally both an existence of three stable magnetic domains around zero external magnetic field by using the stepped GMI element, and a possibility to detect these states by impedance measurement. We also propose an application of this phenomenon for a magnetic sensor which has a memory function.

The effect of carrier density on magnetic anisotropy of the ferromagnetic semiconductor (Ga, Mn)As

Planar Hall effect (PHE) measurements are used to investigate magnetic anisotropy in two (Ga, Mn)As samples which differ by the hole concentration, but are otherwise identical. The difference in the hole density is controlled via modulation doping by Be. Angular dependence of PHE measured at 13 K reveals that the uniaxial easy axis in the sample with a lower hole concentration lies along the [110] direction, and along [ 1 ̄ 10 ] in the sample with higher doping. This difference in the orientation of uniaxial easy axes in the two samples demonstrates that the magnetization of GaMnAs can be manipulated just by varying the carrier density.

Influence of crystal orientation on magnetic properties of hcp Co nanowire arrays

In this paper we develop a convenient way to control the direction of uniaxial magnetocrystalline easy axis in ac electrodeposited hcp Co nanowire arrays. The c-axis of hcp cobalt can be oriented parallel or perpendicular to the long axis of the wire by adjusting the temperature of the electrolyte. Magnetic characterizations suggest that the direction of the hcp c-axis plays an important role in the magnetization reversal process. A three-dimensional object oriented micromagnetic framework code was used to confirm the conclusion. It can be seen that the crystal orientation influences the magnetization and demagnetization process observably when the magnetocrystalline anisotropy energy is comparable to the shape anisotropy energy.

Magnetic properties of epitaxially grown Fe3Si/Ge(111) layers with atomically flat heterointerfaces

We study magnetic properties of epitaxial Fe3Si layers grown on Ge(111) with atomically flat interfaces. An unexpected uniaxial magnetic anisotropy is observed in the film plane for all as-grown samples, and the direction of the uniaxial easy axis is different for each of these samples. By postgrowth annealing, surprisingly, the random orientation of the uniaxial easy axis is aligned to a direction along about [01¯1], together with a reduction in the saturation magnetization. We discuss a possible mechanism of the variation in the magnetic properties after the annealing.

Effect of obliquely evaporated Au cap layer on the magnetic properties of thin Fe films on Si(111)

The influence of Au capping layers on the magnetic properties of thin Fe films grown on Si(111) has been studied by means of room temperature magneto-optic Kerr effect (MOKE). MOKE measurements show a peculiar behavior of the magnetic anisotropy of Fe films grown at normal incidence and covered with obliquely evaporated Au cap layers. The magnetic anisotropy is dependent on the thickness of the deposited Au layer as well as on the growth conditions. First, the initial Fe sixfold in-plane magnetocrystalline anisotropy is replaced by an in-plane uniaxial one for Au thicknesses below 2 ML. Then, for increasing Au film thickness we observe an in-plane switching of the magnetic direction depending of the Au evaporation direction with respect to the substrate. Consider, for example, oblique Au depositions below 2 ML along the [1¯21¯]Si direction. These peculiar deposition conditions lead to a uniaxial anisotropy with easy axis perpendicular to the incidence plane of the Au flux. For thicker Au thicknesses, the uniaxial easy axis undergoes an in-plane switching reorientation of 90° corresponding to the Au evaporation direction. Angular resolved ultraviolet spectroscopy and ion scattering spectroscopy investigations reveal that this complex anisotropy behavior can be attributed partly to Au/Fe interfacial interdiffusion.

Study of Large Barkhauzen Sensor with Application of Step-like Giant Magnetoimpedance Element

A giant magneto-impedance element with step-like impedance properties was obtained for rectangular amorphous Co85Nb12Zr3 soft magnetic thin film with an in-plane uniaxial easy axis at approximately 60 degrees relative to the width of the element. This step-like impedance phenomenon occurred simultaneously with a change in the magnetic domain. We found through experiments that the magnetic field with changes in step-like impedance occurs had an almost constant value, with a standard deviation in the micro-tesla (μT) range, when an alternating magnetic field was applied. There is a possibility of applying this phenomenon to a large Barkhausen sensor with a constant magnetic field where a pulse signal appears. This paper reports a first trial where a step-like GMI element was applied to a large Barkhausen sensor. We present the results of measuring the MH loop for the step-like GMI element with resulting properties of a non-linear leap in magnetization, and also present the results of detecting a pulse signal by using a planar coil fabricated with the step-like GMI element. Some fundamental properties of the pulse signal are also reported, such as its form, variations in its height as a function of the frequency of the externa magnetic field, and variations in the pulse interval as a function of the frequency of the external magnetic field. We found that the domain change in the step-like GMI element is a kind of Barkhausen leap, and can be applied to a large Barkhausen sensor.