Fourfold Magnetic Anisotropy Research Articles

Efficient Spin Torques in Antiferromagnetic CoO/Pt Quantified by Comparing Field- and Current-Induced Switching.

We achieve current-induced switching in collinear insulating antiferromagnetic CoO/Pt, with fourfold in-plane magnetic anisotropy. This is measured electrically by spin Hall magnetoresistance and confirmed by the magnetic field-induced spin-flop transition of the CoO layer. By applying current pulses and magnetic fields, we quantify the efficiency of the acting current-induced torques and estimate a current-field equivalence ratio of 4×10^{-11} T A^{-1} m^{2}. The Néel vector final state (n⊥j) is in line with a thermomagnetoelastic switching mechanism for a negative magnetoelastic constant of the CoO.

The influence of interface strain on the magnetization switching process in FeSi/(011)PMN-0.3PT heterostructures

The magnetization switching process of Fe80Si20/(PbMg1/3Nb2/3O3)0.7–(PbTiO3)0.3 (FeSi/PMN-0.3PT) heterostructures under electric-field-induced interface strain effects was investigated using the magneto-optical Kerr effect. In the as-deposited FeSi films with low four-fold magnetic anisotropy, the one-jump and two-jump loops corresponding to the 180° and 90° domain walls were observed and could be regularly tuned by applying saturated and converse electric fields in these crystalline regions. After 300 °C annealing treatment, the heterostructures presented enhanced crystallinity and interface strain effect. By applying an impulse electric field, we achieved a reliable and non-volatile two-jump and three-jump magnetization switching transition. These regulation results further confirm the magnetic anisotropy transition and magnetic energy superposition relationship in FeSi/PMN-0.3PT. The controllable magnetization switching process is significant for understanding the mechanism of magnetic anisotropy competition and for designing the relevant magnetoelectric devices.

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.

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.

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.

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.

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.

The evolution of in-plane magnetic anisotropy in CoFeB/GaAs(001) films annealed at different temperatures

A considerable in-plane uniaxial magnetic anisotropy (UMA) field (Hu ∼ 300 Oe) could be achieved when the amorphous CoFeB film was deposited on the GaAs(001) wafer by magnetron-sputtering after proper etch-annealed procedure. In order to get deep insights into the mechanism of the UMA, the film was annealed at different temperatures and the evolution of the in-plane magnetic anisotropy was investigated carefully. With increasing the annealing temperature (TA), the UMA could be maintained well when TA reached 250°C, but became very weak at 300°C. However, when TA was elevated to 400°C, another UMA (Hu ∼ 130 Oe) was built accompanied with a fourfold magnetic anisotropy with its strength of about 50 Oe. In terms of the magnetic anisotropy evolution along with TA, the anelastic strain, which is thought to be resulted from the interfacial interaction between CoFeB and GaAs, may play a dominant role in producing the enhanced UMA based on the ‘bond-orientational’ anisotropy (BOA) model.

Annealing and thickness effects on magnetic properties of Co2FeAl alloy films

Co2FeAl (CFA) films in a wide thickness range between 2 and 100 nm are sputtered at room temperature. Perpendicular magnetic anisotropy (PMA) is achieved in the annealed structure of Pd/CFA/MgO with CFA thickness ranging between 2.3 and 4.9 nm. PMA as high as 2 × 106 erg/cm3 is demonstrated in the structures annealed in the temperature range between 300 and 350 °C. Positive contributions to the PMA made by the interfaces of Pd/CFA and CFA/MgO are identified. For the as-deposited structure of MgO/CFA/Ta with thick CFA alloy up to 5 nm or above a high effective saturation magnetization of 983.9 ± 30.1 emu/cc is derived from the fitting and an in-plane uniaxial magnetic anisotropy of 104 erg/cm3 in magnitude is revealed by angular dependent magnetic measurements. In addition to the increase in saturation magnetization, a fourfold cubic magnetic anisotropy is found to develop with annealing, in line with the improvement of the crystalline structure confirmed by X-ray diffraction measurements. Out results provide some useful information for the design of the CFA-based magnetoelectronic devices.

Magnetic domain walls in nanostrips of single-crystalline Fe4N(001) thin films with fourfold in-plane magnetic anisotropy

We investigated head-to-head domain walls in nanostrips of epitaxial Fe4N(001) thin films, displaying a fourfold magnetic anisotropy. Magnetic force microscopy and micromagnetic simulations show that the domain walls have specific properties, compared to soft magnetic materials. In particular, strips aligned along a hard axis of magnetization are wrapped by partial flux-closure concertina domains below a critical width, while progressively transforming to zigzag walls for wider strips. Transverse walls are favored upon the initial application of a magnetic field transverse to the strip, while transformation to vortex walls is favored upon motion under a longitudinal magnetic field. In all cases, the magnetization texture of such fourfold anisotropy domain walls exhibits narrow micro-domain walls, which may give rise to peculiar spin-transfer features.

Magnetic anisotropy of Co thin films: Playing with the shadowing effect, magnetic field and substrate spinning

The shape and magneto-crystalline anisotropies of 10nm thick Co sputtered films have shown to be dependent on the oblique deposition angle (αi), the angular velocity of the substrate-holder (ωS) and the applied magnetic field (H0) during the deposition. Oblique deposition geometry is natural in our sputtering setup, being α equal to 22° at the edge of 4in. sample-holder and 32° at its central part. X-ray diffraction analysis has evidenced a (111) texturized fcc structure for all films. Ferromagnetic resonance has shown that samples prepared under H0 of 250Oe present dominantly the uniaxial HU field contribution independent of the ωS-value, however its magnitude depends on αi. For a non-magnetic holder, Co films show a mixture of twofold (uniaxial) with fourfold (cubic) in-plane magnetic anisotropies. The fourfold contribution is small and it is not influenced by αi or ωS within the experimental error, while the dominant twofold contribution, which is governed by the shadowing effect, is reduced for higher ωS and for samples positioned at the center of the sample-holder. In addition, the intrinsic isotropic Gilbert damping dominates the relaxation process, which is followed by anisotropic twofold scattering mechanism due to stripes and defects, interestingly not influenced by the substrate rotation during depositions.

Magnetic properties of epitaxial Fe/MgO structures on Si(100)

The magnetic properties of epitaxial Fe/MgO structures on Si(100) substrates are investigated over an Fe thickness range of 2–20nm. Superparamagnetic behavior is observed at an Fe thickness of 2nm, indicating that no continuous Fe thin film is formed. At Fe thicknesses of 5nm and higher, a continuous two-dimensional layered structure is formed, which has a dominant epitaxial relationship of Fe[010](100)//MgO[011¯] (100)//Si[011¯](100) and a minor portion of Fe[010](100)//MgO[010](100)//Si[011¯](100). This structural feature is echoed by a four-fold magnetic anisotropy in the film plane, and this tendency increases with increasing Fe thickness. The strength of the first-order cubic magnetocrystalline anisotropy, which can only be extracted from the structures, is in the range of 3.87×105–4.04×105erg/cm3, weaker than that of bulk Fe (4.8×105erg/cm3).

Magnetic, thermal, electrical properties and crystallization kinetics of Co60Fe20B20 alloy films

Magnetic, thermal and electrical properties of sputtered Co60Fe20B20 alloy films are investigated. An excellent linear relationship between the saturation magnetization per unit area and thickness is obtained in the as-deposited film with thickness ranging from 10 to 100 nm. For the as-deposited films a high effective saturation magnetization of 793.7 ± 42.9 emu cm−3 is derived from the fitting. An in-plane uniaxial magnetic anisotropy of ~104 erg cm−3 in magnitude is revealed by angular dependent magnetic measurements. A fourfold cubic magnetic anisotropy is found to develop with annealing, in line with the improvement of the crystalline structure in the film confirmed by X-ray diffraction measurements. Thermomagnetic measurements reveal a considerable variation of the saturation magnetization with temperature arising from the competition of the crystallization and the temperature dependent ferromagnetic behavior of the alloy film. Pronounced changes in electrical resistivity with temperature were observed in the alloy film, in accompany with the structural change. The activation barrier for crystallization was determined to be 1.43 eV from the electrical measurements with variable heating rates, showing a good thermal stability of the alloy film.

Magnetostrictive behaviors of Fe-Al(001) single-crystal films under rotating magnetic fields

Magnetostrictive behaviors of Fe100−x − Alx(x = 0 − 30 at.%)(001) single-crystal films under rotating magnetic fields are investigated along the two different crystallographic orientations, [100] and [110]. The behaviors of Fe and Fe90Al10 films show bath-tub like waveform along [100], easy magnetization axis, and triangular waveform along [110], hard magnetization axis, with respect to their four-fold magnetic anisotropy. On the other hand, the behaviors of Fe80Al20 film are different from those of Fe or Fe90Al10 film. The output of the film along [100] shows a strong magnetic field dependence. The Fe70Al30 film shows similar magnetostrictive behaviors along both [100] and [110] reflecting its magnetic properties, which are almost same for the both directions. The growth of ordered phase (B2) in Fe80Al20 and Fe70Al30 films is considered to have affected their magnetostrictive behaviors. The Al content dependence on λ100 and λ111 values shows similar tendency to that reported for the bulk samples but the values are slightly different. The Fe90Al10(001) single-crystal film shows a large magnetostriction along [100] under a very small magnetic field of 0.02 kOe, which is comparable to the saturated one, and changes the value abruptly in relation to the angle of applied magnetic field.

Investigation of magnetic properties in thick CoFeB alloy films for controllable anisotropy

CoFeB alloy material has attracted interest for its wide uses in magnetic memory devices and sensors. We investigate magnetic properties of thick Co40Fe40B20 films in the thickness range from 10 to 100 nm sandwiched by MgO and Ta layers. Strong in-plane uniaxial magnetic anisotropy is revealed in the as-deposited amorphous films by angular dependent magnetic measurements, and the growth-induced anisotropy is found to strongly depend on the film thickness. A fourfold cubic magnetic anisotropy develops with annealing, as a result of improved crystalline structure in films confirmed by X-ray diffraction measurements. The observed magnetic properties can be explained by the superposition of the uniaxial and additional cubic magnetic anisotropy, tuned by annealing temperature.

Large four-fold magnetic anisotropy in two-dimensional modulated Ni80Fe20 films

2-D modulated structures consisting of square arrays of 60 nm thick Ni80Fe20 circular dots underneath a continuous Ni80Fe20 film were fabricated using deep ultraviolet lithography. The array pitch for all the samples was fixed at 620 nm, while the dot diameter varied from 300 nm to 550 nm. Four-fold in-plane magnetic anisotropy (FFA) was detected in the samples using ferromagnetic resonance technique. It was found that the deposition of continuous film on top of the array drastically increases the FFA of the system, with a maximum anisotropy field of 120 Oe being observed for the sample with intermediate diameter. This increase is due to the appearance of a field well in the film underneath the dots with a shape that is dependent on magnetic field orientation and to the coupling of magnetization oscillations in dots and film.

Role of the substrate on the magnetic anisotropy of magnetite thin films grown by ion-assisted deposition

Magnetite (Fe3O4) thin films were deposited on MgO (001), SrTiO3 (001), LaAlO3 (001) single crystal substrates as well on as silicon and amorphous glass in order to study the effect of the substrate on their magnetic properties, mainly the magnetic anisotropy. We have performed a structural, morphological and compositional characterization by X-ray diffraction, atomic force microscopy and Rutherford backscattering ion channeling in oxygen resonance mode. The magnetic anisotropy has been investigated by vectorial magneto-optical Kerr effect. The results indicate that the magnetic anisotropy is especially influenced by the substrate-induced microstructure. In-plane isotropy and uniaxial anisotropy behavior have been observed on silicon and glass substrates, respectively. The transition between both behaviors depends on grain size. For LaAlO3 substrates, in which the lattice mismatch between the Fe3O4 films and the substrate is significant, a weak in-plane fourfold magnetic anisotropy is induced. However when magnetite is deposited on MgO (001) and SrTiO3 (001) substrates, a well-defined fourfold in-plane magnetic anisotropy is observed with easy axes along [100] and [010] directions. The magnetic properties on these two latter substrates are similar in terms of magnetic anisotropy and coercive fields.