Strong In-plane Uniaxial Anisotropy Research Articles

Plotting philosophy picture of uniaxial magnetic anisotropy in arsenide and phosphide dilute ferromagnetic semiconductors

In this work, we performed a systematic investigation on the comparison of magnetic anisotropy between three III-Mn-V dilute ferromagnetic semiconductors (Ga,Mn)As, (In,Mn)As and (Ga,Mn)P prepared by ion implantation and pulsed laser melting. Compressive strain induces in-plane magnetic anisotropy in (Ga,Mn)As and (Ga,Mn)P, while out-of-plane magnetic anisotropy is present in (In,Mn)As due to tensile strain introduced by Mn substitution. Interestingly, all materials prepared herein does not present strong in-plane uniaxial anisotropy, between [110] and [11¯0] directions, which always exhibits in low temperature molecular beam epitaxy (LT-MBE) grown (Ga,Mn)As samples. The reason is ascribed to the fact that the ultra-fast recrystallization induced by pulsed laser melting weakens the formation of Mn-Mn dimers along the [11¯0] direction which presents in LT-MBE grown (Ga,Mn)As.

Dependence of exchange bias and coercive field on Cu spacer thickness in oblique deposited Co/Cu/CoO multilayers

Effect of growth-induced uniaxial anisotropy and thickness of non-magnetic Cu spacer between the ferromagnetic/antiferromagnetic Co/CoO multilayers on the exchange bias, coercive field, and magnetic anisotropy have been studied in 10–300 K temperature range. Co(8 nm)/Cu(tCu)/CoO(4 nm) (tCu = 0.6–2.6 nm) multilayers on Si (100) substrates were prepared using the oblique-angle magnetron sputtering deposition to induce growth induced magnetic anisotropy. The top Co layer was allowed to be naturally oxidized to obtain antiferromagnetic CoO. The films show a strong uniaxial in-plane anisotropy determined by ferromagnetic resonance measurements. Magnetization measurements for both easy and hard magnetization directions revealed that the exchange bias and coercive fields decrease by increasing Cu thickness. The existence of strong exchange bias across the thickest Cu layer shows the long-range character of exchange and dipolar interactions at 10 K. Dependence of Cu spacer thickness shows an oscillatory behavior that can be ascribed to the competitions between Ruderman–Kittel–Kasuya–Yosida (RKKY) and dipolar interactions.

Magnetic behaviour of $$^{57}$$Fe/Co/Al multilayers deposited on a glass substrate

Multilayers of \(^{57}\hbox {Fe/Co/Al}\) 20 and 40 trilayers (TLs) prepared on a glass substrate using ion beam sputtering deposition techniques have been studied. Magnetization curves obtained at different angles show the anisotropic nature of both the samples. Both samples show strong in-plane uniaxial anisotropy. Negative \(\delta m\) values suggest that dipolar type interactions between grains contribute to the in-plane uniaxial anisotropy. Angular dependence of coercivity indicates that the magnetization reversal mechanism is different for both samples. The variation in coercivity with azimuthal angle is well reproduced in the 20 TL sample using the modified Kondorsky model and by the Stoner–Wohlfarth model in the 40 TL sample.

Unidirectional and uniaxial anisotropies in the MnN/CoFeB exchange bias system

Abstract We report on broadband and in-plane angle dependent ferromagnetic resonance measurements of MnN (30 nm)/CoFeB ( t CoFeB ) exchange bias bilayer systems with CoFeB thicknesses ranging from 5 to 20 nm. We find that the strong unidirectional anisotropy in this system is accompanied by a strong uniaxial in-plane anisotropy and a small interfacial perpendicular anisotropy ( K ⊥ , i = 0.043 ± 0.001 erg / cm 2 ). The strength of the interfacial exchange bias coupling in this system Δ σ eb = 0.193 ± 0.005 erg / cm 2 is only 2.5 times larger than the interfacial uniaxial coupling strength Δ σ u = 0.076 ± 0.003 erg / cm 2 . The relaxation in this system also shows a strong unidirectional and uniaxial in-plane anisotropy. While the observed thickness dependence of the uniaxial anisotropy of the relaxation is consistent with an interfacial two-magnon scattering contribution, we find that this is not the case for the unidirectional relaxation.

Magnetostriction and Magnetic Microscopy Studies in Fe-Co-Si-B Thin Films

This paper reports the effect of substrate temperature on the magnetostrictive behavior of sputtered Fe-Co-Si-B thin films. Magnetostriction measurements carried out by measuring the tip deflection in thin film cantilevers were found to increase with increase in substrate temperatures. To understand the variation of magnetostriction with substrate temperature, detailed structural, microstructural, magnetization and domain imaging studies were carried out. Magnetization studies indicated presence of strong in-plane magnetic anisotropy for all the films. The change in the in-plane coercivity with increase in substrate temperature was correlated with grain size and surface roughness effects. Angular dependent magnetic hysteresis and domain imaging studies carried out using longitudinal Magneto-optic Kerr microscopy displayed presence of strong in-plane uniaxial anisotropy for the as deposited film. Upon increase in substrate temperature, in-plane anisotropic nature was found to gradually change towards isotropic nature owing to the enhancement in crystallization and relaxation of stresses. The magnetization reversal along the in-plane easy axis and hard axis of magnetization was found to be dominated by domain wall motion and through a combination of coherent rotation, domain wall motion & rotation respectively. The increase in the magnetostriction with substrate temperature was correlated with the strong interplay between the crystallization, grain size and magnetic anisotropy.

Effect of Ta Underlayer on Thickness-Dependent Magnetic Properties of Ni–Fe Films

We report the effect of Ta underlayer on the structural and the magnetic properties of Permalloy (Ni81Fe19) thin films over a wide range of film thicknesses ( t Ni–Fe = 10–200 nm). For this purpose, Ta (5 nm)/Ni–Fe ( t Ni–Fe nm)/Ta (2 nm) films were grown on thermally oxidized Si substrates under the presence and the absence of biasing magnetic field (1000 Oe). The Ni–Fe films grown under the presence and the absence of biasing magnetic fields are referred to Ta/Ni–Fe(M) and Ta/Ni–Fe(NM), respectively. X-ray diffraction studies revealed that the Ta underlayer plays an important role in the formation of fcc-(111) Ni–Fe texture in both the Ta/Ni–Fe(M) and Ta/Ni–Fe(NM) films. In addition, the values of average crystallite size depend on the film thickness, biasing field, and Ta underlayer. Thickness-dependent magnetic hysteresis ( M–H ) loops demonstrate a strong in-plane uniaxial anisotropy in both the Ta/Ni–Fe(NM) and Ta/Ni–Fe(M) films. The uniaxial anisotropy constant ( Ku ) depends not only on the film thickness, but also strongly on the biasing magnetic field. Coercivity decreases with increasing film thickness for both the Ta/Ni–Fe(NM) and Ta/Ni–Fe(M) films, and approaches to a minimum of $\sim$ 0.4 Oe even at t Ni–Fe = 200 nm. Furthermore, the observed magnetic domain patterns of Ni–Fe films show their dependence on the Ta underlayer, biasing field, and Ni–Fe thickness. The effects of the Ta underlayer, the Ni–Fe film thickness, and the biasing-field driven uniaxial anisotropy on the structural and the magnetic properties are discussed on the basis of reduction in strain at the Ta/Ni–Fe interface.

Generation of pure spin currents via spin Seebeck effect in self-biased hexagonal ferrite thin films

Light-induced generation of pure spin currents in a Pt(2.5 nm)/BaFe12O19(1.2 μm)/sapphire(0.5 mm) structure is reported. The BaFe12O19 film had strong in-plane uniaxial anisotropy and was therefore self-biased. Upon exposure to light, a temperature difference (ΔT) was established across the BaFe12O19 thickness that gave rise to a pure spin current in the Pt via the spin Seebeck effect. Via the inverse spin Hall effect, the spin current produced an electric voltage across one of the Pt lateral dimensions. The voltage varied with time in the same manner as ΔT and flipped its sign when the magnetization in BaFe12O19 was reversed.

Magnetic anisotropy in ultrathin Fe films on GaAs, ZnSe, and Ge (001) substrates

We discuss magnetic anisotropy parameters of ferromagnetic body-centered cubic (bcc) Fe films grown by molecular beam epitaxy (MBE) on (001) substrates of face-centered cubic (fcc) GaAs, ZnSe, and Ge. High-quality MBE growth of these metal/semiconductor combinations is made possible by the fortuitous atomic relationship between the bcc Fe and the underlying fcc semiconductor surfaces, resulting in excellent lattice match. Magnetization measurements by superconducting quantum interference device (SQUID) indicate that the Fe films grown on (001) GaAs surfaces are characterized by a very strong uniaxial in-plane anisotropy; those grown on (001) Ge surfaces have a fully cubic anisotropy; and Fe films grown on ZnSe represent an intermediate case between the preceding two combinations. Ferromagnetic resonance measurements carried out on these three systems provide a strikingly clear quantitative picture of the anisotropy parameters, in excellent agreement with the SQUID results. Based on these results, we propose that the observed anisotropy of cubic Fe films grown in this way results from the surface reconstruction of the specific semiconductor substrate on which the Fe film is deposited. These results suggest that, by controlling surface reconstruction of the substrate during the MBE growth, one may be able to engineer the magnetic anisotropy in Fe, and possibly also in other MBE-grown ferromagnetic films.

Spatial constraints on the source of uniaxial anisotropy in (Ga,Mn)As films

We use time-resolved Kerr microscopy to perform ferromagnetic resonance experiments on nanostructured elements to show that strain is not the origin of the strong uniaxial in-plane anisotropy observed in (Ga,Mn)As films. From local angular dependent ferromagnetic resonance measurements we are able to extract the anisotropy constants. We observe no change of the strong uniaxial anisotropy in disks as small as 200 nm.

Uniaxial anisotropy induced in 57Fe/Co/Al multilayers

The magnetic properties of 57Fe/Co/Al multilayers with 20 and 40 trilayers, deposited on Si (100) substrate using ion beam sputtering, are reported here. X-ray reflectivity and X-ray diffraction studies indicate the formation of good quality films with preferential growth along the (110) direction. Conversion electron Mössbauer spectra show considerable inter-mixing between the layers and the formation of Fe-Al/Fe-Co-Al phases. The samples are extremely soft with coercivities ≤0.48 × 103 A/m, exhibit strong in-plane uniaxial anisotropy and calculations show that they possess a high ferromagnetic resonance frequency of ∼2 GHz. The saturation magnetization value of 1.80 × 106 A/m is comparable with that obtained in multilayer samples with much higher content of Co. The combination of magnetic properties in these multilayers thus makes them ideal candidates for high frequency device applications.

Uniaxial anisotropy of two-magnon scattering in an ultrathin epitaxial Fe layer on GaAs

We report an on-chip, electrically detected ferromagnetic resonance study on microbars made from GaAs/Fe(1 nm)/GaAs layers. Our experiments, performed at several different microwave frequencies and static magnetic field directions, enable us to observe a strong in-plane uniaxial anisotropy of the linewidth. We attribute the linewidth anisotropy to the two magnon scattering process, supporting this by calculations of possible linewidth broadening mechanisms. Our findings are useful for designing future high-performance spintronic devices based on nanoscale magnetic structures.

Ferromagnetic nanoparticular films studied by optical and magneto-optical ellipsometry

Fe and Co nanoparticles embedded in SiO2 matrix were prepared using dual radiofrequency magnetron sputtering. Films were characterized by high resolution transmission electron microscopy (HRTEM). Magnetic properties were measured using longitudinal magneto-optical Kerr effect. Magneto-optical hysteresis loop measurements show differences between Fe particular film with strong in-plane uniaxial anisotropy and Co one, which is almost isotropic in the plane of the film. Nanoparticular film thicknesses and volume fractions of particles were obtained using spectroscopic ellipsometer UVISEL. Ellipsometric data were fitted to the model based on the Bruggeman effective medium approximation.

Morphological, Magnetic, and Microstructural Study of Fe/Co Multilayers Grown at Different Temperatures

Co/Fe multilayers were grown onto different substrates by means of a molecular beam epitaxy system equipped with e-guns. The microstructural and magnetic properties were studied as a function of both the growing temperature and the Fe-layer thickness. All the samples grown at room temperature show a magnetic behavior typical of systems having a strong uniaxial in-plane anisotropy. The Fe-layer thickness influences the strength of the in-plane anisotropy. By growing the samples at high temperature, a reduction of the Fe/Co intermixing occurs and the magnetic behavior becomes strongly influenced by the nature and crystallinity of the substrates. In particular, using glass substrates, the in-plane uniaxial anisotropy strengthens, whereas when using MgO monocrystalline substrates, a second in-plane easy magnetization axis establishes perpendicular to the main one. Using naturally oxidized Si substrates, a significant out-of-plane contribution to the anisotropy appears, giving rise to a magnetic behavior controlled by the competition between the perpendicular anisotropy and the demagnetizing field.

Magnetic anisotropy and exchange interactions in Fe/Co multilayers grown on different substrates

Fe/Co ultra-thin multilayers were simultaneously e-beam evaporated onto different substrates, varying both the Co and the Fe thickness as well the deposition order. For all the samples a strong uniaxial in-plane anisotropy has been detected. In particular the low intermixing at the interfaces, the adopted growing conditions and the low thickness of the elemental layers allow to obtaining magnetically textured samples also when the starting layer is constituted by Fe. The substrates influence the microstructure of the samples determining the strength of coercivity and the tilting out of the film plane of the easy magnetization axis.

Hysteresis and noise in ferromagnetic materials with parallel domain walls

We investigate dynamic hysteresis and Barkhausen noise in ferromagnetic materials with a huge number of parallel and rigid Bloch domain walls. Considering a disordered ferromagnetic system with strong in-plane uniaxial anisotropy and in-plane magnetization driven by an external magnetic field, we calculate the equations of motion for a set of coupled domain walls, considering the effects of the long-range dipolar interactions and disorder. We derive analytically an expression for the magnetic susceptivity related to the effective demagnetizing factor and show that it has a logarithmic dependence on the number of domains. Next, we simulate the equations of motion and study the effect of the external field frequency and the disorder on the hysteresis and noise properties. The dynamic hysteresis is very well explained by means of the loss separation theory.

Dipole–dipole interaction in square arrays of MnAs nanodisks on GaAs(001)

We demonstrate dipolar correlation in the orientation of magnetization in square arrays ofMnAs nanodisks fabricated from epitaxial layers on GaAs(001). The MnAs() layers possess strong in-plane uniaxial magnetocrystalline anisotropy, which enablesus to discard microvortex-type ordering from degeneracy while maintaining thedisks as circularly shaped with a small diameter. The autocorrelation functionreveals antiferromagnetic collinear arrangements of the magnetic moments in thearrays both in the single-domain and flux-closure-state regimes of the disks. Theinteraction range is deduced to be nearly identical to the period of the arrays.

Ferromagnetic resonance and spin momentum exchange in crystalline magnetic ultrathin films in noncollinear configuration

Au ∕ Fe ∕ Au ∕ Fe ∕ Ga As ( 001 ) ultrathin film structures were grown by molecular beam epitaxy on 4×6 reconstructed GaAs(001) substrates. Measurements were carried out using a network microwave analyzer in the range of frequencies from 1to12GHz allowing one to determine both the in-phase and out-of-phase components of the rf magnetic susceptibility. The ferromagnetic resonance (FMR) results indicate that the ultrathin Fe films are of high crystalline quality. The bottom Fe film [Au∕Fe∕GaAs(001)] has its magnetic hard axis along the [1−10] direction due to a strong in-plane uniaxial interface anisotropy. This leads to a unique behavior of the FMR in noncollinear configuration. Beside the 2 two FMR peaks expected an additional FMR peak was observed in a narrow microwave frequency range. This peak can be explained as a “ghost resonance” which was driven purely by magnetic losses. The FMR linewidth is mostly given by the bulk intrinsic Gilbert damping and spin pumping, and is only weakly affected by small local variations of the internal anisotropy fields. An accidental crossover of the collinear FMR branch of the top Fe layer (Au∕Fe∕Au) with the noncollinear branch of the bottom (Au∕Fe∕GaAs) layer was used to investigate nonlocal spin pumping and spin sink effects in a noncollinear configuration of magnetic moments.

Magnetic anisotropies in ultrathin iron films grown on the surface-reconstructed GaAs substrate

Magnetic anisotropies of epitaxial ultrathin iron films grown on the surface-reconstructed GaAs substrate were studied. Ferromagnetic resonance technique was exploited to determine magnetic parameters of the films in the temperature range of 4–300K. Extraordinary angular dependence of the FMR spectra was explained by the presence of fourfold and twofold in-plane anisotropies. A strong in-plane uniaxial anisotropy with magnetic hard axis along the [11¯0] crystallographic direction is present at the GaAs∕Fe(001) interface while a weak in-plane uniaxial anisotropy for the Fe grown on Au has its easy axis oriented along [11¯0]. A linear dependence of the magnetic anisotropies as a function of temperature suggests that the strength of the in-plane uniaxial anisotropy is affected by the magnetoelastic anisotropies and differential thermal expansion of contacting materials.

Anisotropy effects on the magnetic excitations of epitaxial ultrathin films below and above the Curie temperature

Brillouin light scattering is exploited to study magnetic excitations in magnetic films consisting of a few atomic layers, characterized by a strong in-plane uniaxial anisotropy, namely NiFe/Cu(1 1 0) and Fe/Cu(1 1 0). Well below the Curie temperature, a nonmonotonic field dependence of the magnon frequency is measured for magnetic field H applied along the hard in plane direction, while for field along the easy direction the frequency is found to increase almost linearly with increasing H. When temperature is increased, the measured magnon gap at zero field decreases until, for all the investigated systems, it vanishes at room temperature. The anisotropy of the magnetic excitations is found to persist even in the paramagnetic phase: in fact, the magnon frequency is found to increase almost linearly with increasing H, but with different slopes depending on the field direction. The smaller slope is found for the field along the hard axis. A finite-temperature Green’s function theory, recently developed for the study of the field-driven reorientation transition of an anisotropic monolayer, is able to account for the experimental data of the investigated ultrathin films in the whole temperature range.

Spin-wave excitations in epitaxial ultrathin FeCo with zero magnetocrystalline anisotropy

Spin-wave theory predicts reduced thermal spin-wave excitations due to a magnetic anisotropy. Recent results show that a strong uniaxial in-plane anisotropy in ultrathin Fe∕GaAs(001) films indeed stabilizes the ferromagnetic order versus thermal spin excitations [Kipferl et al., J. Appl. Phys. 97, 10B313 (2005)]. In order to study whether a fourth-order in-plane anisotropy has a similar effect epitaxial Fe70Co30∕Au(001) samples with zero magnetocrystalline anisotropy were studied and compared to results for Fe∕Au(001). The temperature dependence of the spontaneous magnetization MS for T<0.5Tc can be well described by the Bloch [Z. Phys. 61, 206 (1930)] T3∕2 law in the entire thickness range. However, for each thickness B is reduced in Fe70Co30∕Au(001) compared to Fe∕Au(001) films despite the reduced anisotropy. This means that the effect of a weak in-plane anisotropy is overcompensated by the enhanced exchange interaction.