Fourfold Anisotropy Research Articles

Magnetic Anisotropy in Thin Layers of (Mn,Zn)Fe2O4 on SrTiO3 (001)

Herein, a ferrimagnetic manganese zinc ferrite (Mn0.5Zn0.5Fe2O4) film with a thickness of 200 nm is prepared without a buffer layer on strontium titanate (001) (SrTiO3) using pulsed laser deposition. Its magnetic properties are investigated using superconducting quantum interference device (SQUID), X‐ray absorption spectroscopy with subsequent X‐ray magnetic circular dichroism (XMCD) and magneto‐optic Kerr effect (MOKE). Hysteresis loops derived from SQUID exhibits bulk‐like properties. This can further be confirmed by bulk‐like XMCD spectra. In remanent magnetization, an in‐plane magnetization with basically no out‐of‐plane component is found. The magnetic moments derived by the sum rule formalism from the XMCD data are in good agreement to the magnetization observed by SQUID and MOKE. XMCD as well as MOKE reveal an in‐plane angular fourfold magnetic anisotropy with the easy direction along [110] for (Mn0.5Zn0.5)Fe2O4 on SrTiO3. The element‐specific magnetic moments from XMCD show a stronger contribution of Fe to the anisotropy than of Mn and distinct contributions of the orbital moments.

Spin inhomogeneities at the interface and inverted hysteresis loop in La0.7Sr0.3MnO3/SrTiO3

An inverted hysteresis loop (IHL) exhibits negative coercivity and remanence, and has been ascribed to various mechanisms ranging from exchange coupling to competing anisotropies and even experimental artifacts. Here, we investigate the IHL behavior of monolithic ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ (LSMO) film using magnetization and ferromagnetic resonance (FMR) studies at 300 K. FMR measurements reveal the presence of fourfold (${K}_{4}=5.7\ifmmode\times\else\texttimes\fi{}{10}^{2}\phantom{\rule{0.28em}{0ex}}\mathrm{erg}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$), uniaxial (${K}_{u}=1.9\ifmmode\times\else\texttimes\fi{}{10}^{2}\phantom{\rule{0.28em}{0ex}}\mathrm{erg}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$), and exchange (${K}_{A/F}=\ensuremath{-}1.15\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.28em}{0ex}}\mathrm{erg}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$) anisotropy. It is known that the competition of fourfold and uniaxial anisotropy can lead to an IHL. Furthermore, magnetic measurements exhibit positive exchange bias (EB), which could also lead to an IHL. The appearance of positive EB in monolithic LSMO film indicates the presence of secondary phases and an antiferromagnetic (AFM) coupling between the phases. We show unambiguously that the observed IHL is a result of interfacial exchange coupling in the LSMO film. Using FMR, we verify the AFM coupling at the interface and, in conjunction with the results of an earlier report, we develop an intuitive picture for the spin arrangement at the interface. Our study reveals a complex interface physics in LSMO/${\mathrm{SrTiO}}_{3}$ (STO), whose understanding may generate new pathways for the development of novel functionalities in LSMO.

Helical structures in layered magnetic superconductors due to indirect exchange interactions mediated by interlayer tunneling

Motivated by the recent discovery of helical magnetic structure in RbEuFe$_{4}$As$_{4}$, we investigate interlayer ordering of magnetic moments in materials composed of spatially-separated superconducting and ferromagnetically-aligned layers. We consider the interplay between the normal and superconducting indirect exchange interaction mediated by tunneling between the conducting layers. We elaborate a recipe to evaluate the normal interlayer interaction via two-dimensional density of states of an isolated layer and demonstrate that for bands with small fillings, such interaction is typically ferromagnetic and short-range. The nearest-layer interaction is proportional to the ratio of the interlayer hopping and in-plane band width squared. On the other hand, the superconducting contribution always gives antiferromagnetic interaction and may extend over several layers when the interlayer hopping energy exceeds the superconducting gap. The frustration caused by the interplay between the normal and superconducting parts may lead to spiral ground-state magnetic configuration. The four-fold in-plane anisotropy may lock the rotation angle between the moments in the neighboring layers to $90^\circ$, as it was observed in RbEuFe$_{4}$As$_{4}$

Chirality switching of an antiferromagnetic spiral wall and its effect on magnetic anisotropy

An antiferromagnetic NiO spiral wall in $\mathrm{Fe}/\mathrm{NiO}/\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}/\mathrm{vicinal}$ Ag(001) was created by rotating Fe magnetization and investigated using x-ray magnetic linear dichroism (XMLD). Different from the Mauri's 180\ifmmode^\circ\else\textdegree\fi{} spiral wall, we find that the NiO spiral wall always switches its chirality at $\ensuremath{\sim}{90}^{\ensuremath{\circ}}$ rotation of the Fe magnetization, and unwinds the spiral wall back to a single domain with a further rotation of the Fe magnetization from 90\ifmmode^\circ\else\textdegree\fi{} to 180\ifmmode^\circ\else\textdegree\fi{}. The effect of this chirality switching on the magnetic anisotropy was studied using rotational magneto-optic Kerr effect (ROTMOKE) on $\mathrm{Py}/\mathrm{NiO}/\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{N}{\mathrm{i}}_{0.5}\mathrm{O}/\mathrm{vicinal}$ Ag(001). We find that the original Mauri's model has to be corrected by an energy folding due to the chirality switching, which consequently converts the exchange bias from the Mauri's 180\ifmmode^\circ\else\textdegree\fi{} spiral wall into a uniaxial anisotropy and a negative fourfold anisotropy.

Thermal simulation of magnetization reversals for a size-distributed assembly of nanoparticles with uniaxial and cubic anisotropies

A temperature-dependent coherent magnetization reversal model is proposed for size-distributed assemblies of ferromagnetic nanoparticles (NPs). NPs are assumed to be of uniaxial and cubic anisotropies. The thermal dependence is included by considering thermal fluctuations, implemented through the Néel-Arrhenius theory. Theoretically calculated thermal and angular dependences of magnetization reversal loops are obtained. There are similar behaviors for a single NP and an assembly of NPs. In particular, it is shown that the fourfold anisotropy results in double slope loops along the hard axis in both cases. Also, the azimuthal dependence of coercive fields is similar in both cases and with or without the presence of a fourfold anisotropy. There are also important differences in the behaviors observed for a single NP and that of an assembly of NPs. Firstly, the blocking temperature is barely enhanced by the presence of a fourfold anisotropy but is greatly enhanced in the assembly of NPs relative to a single NP. Secondly, along the easy axis, for a single size particle, the shape of the M-H loops is neither temperature dependent nor fourfold anisotropy dependent, as it is always rectangular. However, the shape of the M-H loops for an assembly of NPs is temperature dependent, but this shape is weakly dependent on the anisotropy ratio. Simulations of M-H loops using the model presented here would allow the quantitative determination of the anisotropy constants for either single sized NPs or an assembly with different sizes.

Anisotropic spin waves in two-dimensional triangular shaped bi-component magnonic crystal

Abstract Bi-component magnonic crystals with a strong dipole-exchange interaction across the interface of the constituent magnetic elements have shown promising potentials in magnonics and magnon-spintronics. Here, we have reported an all-optical investigation of spin wave dynamics in an array of periodically arranged bi-component magnonic crystal in the form of triangular-shaped Ni 80 Fe 20 nanoelements embedded in Co 50 Fe 50 matrix using time-resolved magneto-optical Kerr effect magnetometry. The spin wave spectra obtained from the sample reveal a broad band of spin wave modes where they possess a strong and systematic bias magnetic field tunability which is crucial for active control over such system in device applications. Further, the spin wave modes show a six-fold and four-fold rotational anisotropy with the bias field orientation due to combined effects of element shape and lattice symmetry. Micromagnetic simulations reproduce the experimental results qualitatively where the simulated mode profiles unravel the spatial distribution of spin wave frequencies inside both constituent elements while the internal magnetic fields play a crucial role for the observed tunability of spin wave dynamics. Development of such magnetically coupled embedded magnetic nanostructures can pave a new pathway in designing the future magnonic devices and faster microwave communication systems.

Thickness-dependent magnetic anisotropy in obliquely deposited Fe(001)/Pd thin film bilayers probed by VNA-FMR**Project supported by the National Basic Research Program of China (Grant Nos. 2015CB921403 and 2016YFA0300701), the National Natural Science Foundation of China (Grant Nos. 51427801, 11374350, and 51671212), and the Chinese Government Scholarship (Grant No. 2015GXYG37).

The thickness-dependent magnetic anisotropy of obliquely deposited Fe(001)/Pd thin films on Mg(001) is investigated by fitting the field-dependent resonant field curve using the Kittel equation. In this study, three Fe film samples with thicknesses of 50 monolayers (ML), 45 ML, and 32 ML deposited at 0°, 45°, and 55°, respectively, are used. The magnetic anisotropy constant obtained from ferromagnetic resonance (FMR) spectra exhibits a dominant fourfold magnetocrystalline anisotropy (MCA) at the normal deposition angle with larger Fe thickness. However, the in-plane uniaxial magnetic anisotropy (UMA) is induced by a higher oblique deposition angle and a smaller thickness. Its hard axis lies between the [100] and [010] directions. The FMR data-fitting analysis yields a precise measurement of smaller contributions to the magnetic anisotropy, such as in-plane UMA. Due to MCA, when the magnetic field is weaker than the saturated field, the magnetization direction does not always align with the external field. The squared frequency-dependent resonant field measurement gives an isotropic Landé g-factor of 2.07. Our results are consistent with previous experiments conducted on the magneto-optical Kerr effect (MOKE) and anisotropic magnetoresistance (AMR) systems. Thus, a vector network analyzer ferromagnetic resonance (VNA-FMR) test-method for finding UMA in obliquely deposited Fe(001)/Pd bilayer ferromagnetic thin films, and determining the magnetic anisotropy constants with respect to the film normal deposition, is proposed.

Thickness dependence of dynamic magnetic properties of soft (FeCo)-Si alloy thin films

${\mathrm{Fe}}_{69}{\mathrm{Co}}_{26}{\mathrm{Si}}_{5}$ alloy thin films of various thicknesses were deposited on MgO(100) single crystal substrates by magnetron sputtering at $230{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ substrate temperature. The thickness dependence of the dynamic magnetic properties was investigated using broadband ferromagnetic resonance (FMR). The x-ray diffraction results indicate that all films are of the bcc structure with an in-plane epitaxial alignment of ${[100]}_{\mathrm{FeCoSi}}//{[110]}_{\mathrm{MgO}}$. The exchange constant was determined from the field shift between the uniform precession FMR mode and the first order perpendicular standing spin wave resonance mode in the FMR spectra. The effective damping parameter decreases dramatically with increasing film thickness up to 16 nm due to the decrease of the spin pumping contribution and then remains relatively constant as the film thickness increases. In-plane angle dependent FMR measurements reveal that the in-plane anisotropy of these films is dominated by a fourfold magnetic anisotropy, which increases sharply with increasing film thickness up to 16 nm and then shows a slightly decreasing trend as the film thickness increases. In-plane angular dependence of the FMR linewidth shows a strong two-magnon scattering contribution.

Finite-size Kosterlitz-Thouless transition in 2DXY Fe/W(001) ultrathin films

Magnetic susceptibility measurements of 3-4 ML Fe/W(001) ferromagnetic films demonstrate that this is a 2DXY system in which a finite-size Kosterlitz-Thouless (KT) transition occurs. The films are grown in ultrahigh vacuum and their magnetic response is measured using the magneto-optic Kerr effect (MOKE). The analysis of many independently grown films shows that the paramagnetic tail of the susceptibility is described by $\chi(T) \sim \exp \bigr{(}B/(T/T_{KT}-1)^a\bigl{)}$, where $a=0.50\pm0.03$ and $B=3.48\pm0.16$, in quantitative agreement with KT theory. Below the finite-size transition temperature $T_C(L)$, the behavior is complicated by dissipation (likely related to the re-emergence of fourfold anisotropy and magnetic domains). A subset of measurements with very small dissipation most closely represents the idealized system treated by theory. In these, the temperature interval between the fitted Kosterlitz-Thouless transition temperature and the finite-size transition temperature is $T_C(L)/T_{KT} -1=0.065\pm0.016$. This yeids an estimate of the finite size $L$ affecting the film of order micrometers. This gives experimental support to the idea that even a mesoscopic limitation of the vortex-antivortex gas results in a substantial finite-size effect at the KT transition. In contrast, fitting the paramagnetic tail to a power law, appropriate to a second order critical transition, does not give reasonable results. The effective critical exponent $\gamma_{eff} \approx 3.7 \pm 0.7$ does not correspond to a known universality class, and the fitted transition temperature is much further below the peak in the susceptibility than is reasonable.

Asymmetry in magnetic behavior caused by superposition of unidirectional and four-fold magnetic anisotropies in CoPt/FeMn bilayers

We investigated magnetic anisotropy and magnetization reversal process in CoPt/FeMn bilayers by four-probe measurements of anisotropic magnetoresistance (AMR) effect. Here, a single crystal-CoPt/polycrystal-FeMn interface has been established, where A1-CoPt layer with four-fold magnetic anisotropy was epitaxially grown on MgO(100) substrate, while the top FeMn layer was 111 textured showing in-plane polycrystalline feature. Due to the noncollinear 3Q AFM spin structure and existence of uncompensated spins in ultrathin FeMn, the magnetic anisotropy of the bilayer can be tuned through interfacial exchange coupling, controlled by field cooling treatment. As a result, it gives rise to asymmetric AMR curves. We show that this asymmetry strongly relies on the direction of induced unidirectional magnetic anisotropy with respect to the easy axis of CoPt layer, leading to a modulated magnetization reversal process.

Effect of ferro-elastic twin domains of LaAlO3 substrate on the magnetic anisotropy and the magnetization reversal process of cobalt thin film

Abstract Present work reports the effects of ferroelastic domains (FE) on the magnetic properties of top magnetic layer. Cobalt film of 15 nm is deposited using electron beam evaporation technique on single crystalline LaAlO 3 (0 0 1) (LAO) substrate which consists FE structural twin domains. LAO substrate is in-situ heated above its FE transition temperature and cooled to room temperature before depositing cobalt film of 15 nm. Textured growth of cobalt film is observed in-situ from reflection high energy electron diffraction (RHEED) patterns. The presence of FE structural twins in LAO substrate is confirmed from the X-ray reciprocal space mapping measurements. From the optical polarization microscopy, it is observed that the different regions consist different modulations viz., lamellar like and square-like structures in LAO substrate. The magnetic anisotropy and magnetization reversal is studied ex-situ using magneto-optical Kerr effect (MOKE) microscopy with different spatial resolutions. The analysis of azimuthal variation of coercivity ( H C ) shows the superposition of two-, four- and six- fold anisotropy contribution at different regions of the sample. Twofold anisotropy and weak sixfold anisotropy may due to magneto-crystalline anisotropy, whereas the fourfold anisotropy might be coming because of square-like modulations of the substrate. Moreover, MOKE microscopy reveals the domain wall pinning along the twin boundaries (FE) due to which magnetization reversal is proceed by three distinct mechanisms when field is applied away from easy axis of magnetization and lamellar pattern. As a consequence of this, magnetization reversal proceeds by ‘labyrinth switching’ and abrupt magnetic domain wall switching within individual FE domains.

Component manipulated magnetic anisotropy and damping in Heusler-like compound Co2+xFe1−xAl

The component dependence of the magnetocrystalline anisotropy and the damping has been investigated in epitaxial Heusler-like compound Co2+xFe1−xAl films grown by molecular beam epitaxy (MBE) with x = −0.4, −0.2, 0, 0.2, and 0.4. All the films show a component tunable four-fold magnetocrystalline anisotropy with the easy axis along [1 1 0] orientation. The time resolved magneto-optic Kerr effect measurements reveal that the damping constant can be tuned in a range of 0.0065–0.0156 with a minimum value of 0.0065 at x = −0.2. This work provides a new approach to manipulate the magnetic dynamic properties of Heusler alloy Co2FeAl by adjusting the proportion of Co and Fe.

Evidence of anomalous switching of the in-plane magnetic easy axis with temperature in Fe3O4 film on SrTiO3:Nb by v-MOKE and ferromagnetic resonance.

The evolution of the magnetic anisotropy directions has been studied in a magnetite (Fe3O4) thin film grown by infrared pulsed-laser deposition on SrTiO3(100):Nb substrate. The magnetic easy axes at room temperature are found along the in-plane 〈100〉 film directions, which means a rotation of the easy axis by 45° with respect to the directions typically reported for bulk magnetite and films grown on single-crystal substrates. Moreover, when undergoing the Verwey transition temperature, TV, the easy axis orientation evolves to the 〈110〉 film directions. This anomalous behavior has been demonstrated by measuring first the angular dependence of coercivity and remanence well above and below TV by high-resolution vectorial magneto-optical Kerr effect (v-MOKE). Ferromagnetic resonance (FMR) measurements have additionally proven a well-defined fourfold magnetic anisotropy induced during growth with confirmed easy axis directions along 〈100〉 for T > TV and 〈110〉 for T < TV. These results provide a clear proof of the possibility of tuning magnetic anisotropy in Fe3O4 thin films by proper control on the growth parameters and substrate choice.

Influence of crystal orientation on the magnetostriction behavior of Fe films formed on MgO single-crystal substrates

Abstract Fe thin films are prepared on MgO substrates of (001), (110), and (111) orientations by ultra-high vacuum magnetron sputtering. The magnetostriction behaviors under rotating magnetic fields are observed by using a cantilever method. The relationship between in-plane magnetic anisotropy and magnetostriction behavior is investigated. An Fe(001)bcc single-crystal film is obtained on MgO(001) substrate. A four-fold symmetry in in-plane magnetic anisotropy is observed for the Fe(001)bcc film, where the easy magnetization directions are parallel to bcc[100] and bcc[010]. An Fe(211)bcc bi-crystalline film is formed on MgO(110) substrate and the film also shows an almost four-fold symmetric magnetic anisotropy. The reason is interpreted to be due to an influence of overlap of magnetic anisotropies of two bcc(211) variants with two effective easy magnetization directions which are obtained by projecting bcc[010] and bcc[001] on the bcc(211) plane. An Fe(110)bcc film is epitaxially grown on MgO(111) substrate with two types of variant with the crystallographic orientation relationships similar to Nishiyama-Wasserman and Kurdjumov-Sachs. The Fe(110) film shows an isotropic in-plane magnetic property due to an influence of the variant structure. Usual sinusoidal waveforms of magnetostriction are observed for the Fe(110) film. On the other hand, waveforms measured for the Fe(001) and the Fe(211) films are deformed from sinusoidal shape under low in-plane rotating magnetic fields and the magnetostriction behaviors are similar between the two films. Rectangular waveforms are observed for the bending parallel to easy magnetization directions, whereas triangular waveforms are recognized for the bending parallel to hard directions. The phenomenon is related to the direction difference between applied magnetic field and magnetization in magnetically unsaturated films with in-plane anisotropies. The magnetostriction behavior is influenced by the symmetry of in-plane magnetic anisotropy.

Domain-wall-assisted asymmetric magnetoimpedance in ferromagnetic nanostripes

With micromagnetic simulations, we study the dynamical magnetic response of a ferromagnetic nanostripe deposited on an AC-current-carrying nonmagnetic layer (that induces an AC Oersted field inside the nanostripe), in the presence of a static axial field. Included into a circuit, the structure induces the magnetoimpedance (MI) whose value is proportional to the time derivative of the transverse magnetization of the nanostripe, hence, it is directly related to the dynamical magnetic response of the stripe. Paying especial attention to the regime of low values of the axial field, we study in detail the mechanism of the dynamical response, thus, MI which is based on the oscillations of the positions of 90° (head-to-tail) domain walls (DWs). Such DWs are present in nanostripes of uniaxial-transverse or four-fold (in-plane) magnetic anisotropies of a small (up to 10 nm) thickness, whose combination of the structural and shape anisotropies stabilizes the structure of domains of diagonal (to stripe axes) magnetization, with a non-zero axial component of the overall magnetization of the nanostripe. For the nanostripes of specific materials (uniaxial Co, cubic Fe3O4), that ordering results in a significant asymmetry of the DW-based dynamical response with respect to the reversal of the axial field. For the nanostripes of other materials (cubic: Fe, Fe3Pt), the DW-based magnetic response is almost insensitive to the axial field, thus, the low-field MI is inefficient. We explain different dynamical behaviors of the nanostripes within the analytical (Walker-like) model of the DW motion combined with a phenomenological model of the DW-based MI, including the DW interactions. The maximum relative shift of the DW-based impedance of the nanostripes ranges from about 50% for Fe3O4 to above 200% for Co nanostripes, whereas the (low-)field regime of the DW-based response (the anisotropic MI regime) is as wide as 30 kA m−1.

Epitaxial growth of Pd1−xFex films on MgO single-crystal substrate

In the paper we report on growth conditions, morphology, crystallographic structure and magnetic anisotropy of 20 nm thick, palladium-rich Pd1−xFex (0.01 < x < 0.1) alloy films grown on (001)-oriented MgO single-crystal substrate. Molecular beam deposition at ultra-high vacuum conditions has been utilized along with the three-step procedure to achieve the epitaxy conditions for the synthesized films. The scanning electron microscopy and atomic force microscopy have shown flat and smooth morphology of the films studied in a wide range of lateral scales. Insitu low energy electron diffraction and exsitu X-ray diffraction measurements confirmed that our Pd1−xFex alloy films are epitaxial. Ferromagnetic resonance investigations have shown the in-plane four-fold magnetocrystalline anisotropy characteristic for single-crystalline films with the cubic structure. The whole set of our measurements testifies the epitaxial cube-on-cube growth and excellent magnetic homogeneity of Pd1−xFex films on MgO substrate obtained within the three-step deposition process.

Thickness dependent manipulation of uniaxial magnetic anisotropy in Fe-thin films by oblique deposition**Project supported by the National Basic Research Program of China (Grant Nos. 2015CB921403 and 2016YFA0300701), the National Natural Science Foundation of China (Grant Nos. 51427801, 11374350, and 51671212), and the Chinese Government Scholarship (Grant No. 2015GXYG37).

The uniaxial magnetic anisotropy of obliquely deposited Fe(001)/Pd film on MgO(001) substrate is investigated as a function of deposition angle and film thickness. The values of incidence angle of Fe flux relative to surface normal of the substrate are 0°, 45°, 55°, and 70°, respectively. In-situ low energy electron diffraction is employed to investigate the surface structures of the samples. The Fe film thicknesses are determined to be 50 ML, 45 ML, 32 ML, and 24 ML (1 ML = 0.14 nm) by performing x-ray reflectivity on the grown samples, respectively. The normalized remanent magnetic saturation ratio and coercivity are obtained by the longitudinal surface magneto-optical Kerr effect. Here, the magnetic anisotropy constants are quantitatively determined by fitting the anisotropic magnetoresistance curves under different fields. These measurements show four-fold cubic anisotropy in a large Fe film thickness (50 ML) sample, but highly in-plane uniaxial magnetic anisotropies in thin films (24 ML and 32 ML) samples. In the obliquely deposited Fe films, the coercive fields and the uniaxial magnetic anisotropies (UMAs) increase as the deposition angle becomes more and more tilted. In addition, the UMA decreases with the increase of the Fe film thickness. Our work provides the possibility of manipulating uniaxial magnetic anisotropy, and paves the way to inducing UMA by oblique deposition with smaller film thickness.

In-Plane Anisotropy Effect to the Spin-Wave Gap in Ultrathin Ferromagnetic Films at Finite Temperatures

In this paper, we investigate the calculated thickness- and temperature-dependent magnetization given in terms of the spin-wave gap alone for two-dimensional ultrathin ferromagnetic films with anisotropy and Zeeman energy sufficiently large to dominate over the dipolar interaction. The spin-wave gap was calculated for a magnetic field which is perpendicular to the plane. The calculated equations present a nonzero spin-wave gap at zero magnetic field which is strongly affected by anisotropies. The temperature-dependent magnetization strongly increases with decreasing thickness of the insulating spacer layer at the saturation field and at room temperature. We reported that the in-plane anisotropy strongly depends on the insulating spacer layer thickness and saturation field and overcomes the fourfold anisotropy in the spacer layers with thickness of below 0.8 nm. The results were also discussed in connection with experimental data given in Co0.9Fe0.1/MgO/C0.9Fe0.1 films.

Impact of Strain and Morphology on Magnetic Properties of Fe₃O₄/NiO Bilayers Grown on Nb:SrTiO₃(001) and MgO(001).

We present a comparative study of the morphology and structural as well as magnetic properties of crystalline Fe3O4/NiO bilayers grown on both MgO(001) and SrTiO3(001) substrates by reactive molecular beam epitaxy. These structures were investigated by means of X-ray photoelectron spectroscopy, low-energy electron diffraction, X-ray reflectivity and diffraction, as well as vibrating sample magnetometry. While the lattice mismatch of NiO grown on MgO(001) was only 0.8%, it was exposed to a lateral lattice mismatch of −6.9% if grown on SrTiO3. In the case of Fe3O4, the misfit strain on MgO(001) and SrTiO3(001) amounted to 0.3% and −7.5%, respectively. To clarify the relaxation process of the bilayer system, the film thicknesses of the magnetite and nickel oxide films were varied between 5 and 20 nm. While NiO films were well ordered on both substrates, Fe3O4 films grown on NiO/SrTiO3 exhibited a higher surface roughness as well as lower structural ordering compared to films grown on NiO/MgO. Further, NiO films grew pseudomorphic in the investigated thickness range on MgO substrates without any indication of relaxation, whereas on SrTiO3 the NiO films showed strong strain relaxation. Fe3O4 films also exhibited strong relaxation, even for films of 5 nm thickness on both NiO/MgO and NiO/SrTiO3. The magnetite layers on both substrates showed a fourfold magnetic in-plane anisotropy with magnetic easy axes pointing in directions. The coercive field was strongly enhanced for magnetite grown on NiO/SrTiO3 due to the higher density of structural defects, compared to magnetite grown on NiO/MgO.

Research Update: Focused ion beam direct writing of magnetic patterns with controlled structural and magnetic properties

Focused ion beam irradiation of metastable Fe78Ni22 thin films grown on Cu(100) substrates is used to create ferromagnetic, body-centered cubic patterns embedded into paramagnetic, face-centered-cubic surrounding. The structural and magnetic phase transformation can be controlled by varying parameters of the transforming gallium ion beam. The focused ion beam parameters such as the ion dose, number of scans, and scanning direction can be used not only to control a degree of transformation but also to change the otherwise four-fold in-plane magnetic anisotropy into the uniaxial anisotropy along a specific crystallographic direction. This change is associated with a preferred growth of specific crystallographic domains. The possibility to create magnetic patterns with continuous magnetization transitions and at the same time to create patterns with periodical changes in magnetic anisotropy makes this system an ideal candidate for rapid prototyping of a large variety of nanostructured samples. Namely, spin-wave waveguides and magnonic crystals can be easily combined into complex devices in a single fabrication step.