Fourfold Magnetic Anisotropy Research Articles

Microstructural contribution to the magnetic anisotropy in Fe 1−x Ga x thin films: correlation between crystallographic texture and magnetic response

In this study, we present a systematic analysis of the relation between the structural properties and magnetic anisotropies of as-grown and heat-treated polycrystalline Fe 1−x Ga x (0.11 <x< 0.19) thin films deposited on glass and Si(100) substrates. The results show that the crystallographic texture of the films has a significant impact on the evolution of the magnetic anisotropies. The samples grown on glass do not exhibit a preferred texture while, for samples grown on Si(100), the appearance of a weak in-plane fourfold magnetic anisotropy is reported. Such anisotropy is enhanced and better defined in the heat treated samples. Via a phenomenological model we show that this anisotropy has a microstructural origin. These findings demonstrate the possibility of tuning magnetic anisotropies by growing on different substrates and/or performing thermal treatments, which in turn reinforces the possibility of developing smart magnetic switching materials for electronic applications.

Extreme terahertz magnon multiplication induced by resonant magnetic pulse pairs

Nonlinear interactions of spin-waves and their quanta, magnons, have emerged as prominent candidates for interference-based technology, ranging from quantum transduction to antiferromagnetic spintronics. Yet magnon multiplication in the terahertz (THz) spectral region represents a major challenge. Intense, resonant magnetic fields from THz pulse-pairs with controllable phases and amplitudes enable high order THz magnon multiplication, distinct from non-resonant nonlinearities such as the high harmonic generation by below-band gap electric fields. Here, we demonstrate exceptionally high-order THz nonlinear magnonics. It manifests as 7th-order spin-wave-mixing and 6th harmonic magnon generation in an antiferromagnetic orthoferrite. We use THz two-dimensional coherent spectroscopy to achieve high-sensitivity detection of nonlinear magnon interactions up to six-magnon quanta in strongly-driven many-magnon correlated states. The high-order magnon multiplication, supported by classical and quantum spin simulations, elucidates the significance of four-fold magnetic anisotropy and Dzyaloshinskii-Moriya symmetry breaking. Moreover, our results shed light on the potential quantum fluctuation properties inherent in nonlinear magnons.

Fourfold anisotropic magnetic damping induced by anisotropic spin absorption in amorphous CoFeB/epitaxial IrMn3 bilayers

Magnetic damping rooted in various relaxation processes of spin angular momentum plays a crucial role in determining the energy consumption and the operating speed of emerging spintronic devices. Here, we reported a fourfold anisotropic magnetic damping extrinsically caused by spin current absorption in amorphous ferromagnetic/antiferromagnetic bilayers. The angular dependent broadband ferromagnetic resonance measurements were employed to investigate the effect of interfacial exchange coupling on magnetization dynamics of the CoFeB/IrMn3 bilayers. In addition to the conventional exchange bias, the amorphous CoFeB layer exhibits an induced fourfold magnetic anisotropy along the IrMn3⟨100⟩ axes because of the exchange coupling with the uncompensated IrMn3 moments along IrMn3⟨100⟩ caused by the 3Q antiferromagnetic structure. The magnetic damping of the CoFeB/IrMn3 bilayer also exhibits an obvious fourfold symmetry, which is ascribed to the anisotropic spin absorption caused by the uncompensated IrMn3 moments. The ratio of the fourfold anisotropic magnetic damping decreases dramatically with reducing the interfacial exchange coupling. When the interfacial exchange coupling is isolated by a Cu spacer layer, both the induced magnetic anisotropy and the magnetic damping exhibit a uniaxial symmetry.

Giant four-fold magnetic anisotropy in nanotwinned NiMnGa epitaxial films

A giant four-fold magnetic anisotropy (with an anisotropy field up to 4 kOe) was observed in the twinned NiMnGa epitaxial film. Its appearance is explained in terms of moderate coupling between twin variants having strong uniaxial magnetocrystalline anisotropies directed orthogonally when the intertwin exchange field is comparable with the anisotropy field. This finding paves the way to increase the order of magnetic anisotropy in a many-component system while keeping the value of the anisotropy field by tuning the intercomponent exchange strength and can be extended to exchange-coupled multilayers and arrays of nanoelements.

Twisted Integration of Complex Oxide Magnetoelectric Heterostructures via Water-Etching and Transfer Process

HighlightsThe (001)-oriented ferromagnetic La0.67Sr0.33MnO3 films are stuck onto the (011)-oriented ferroelectric single-crystal 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 substrate with 0° and 45° twist angle.By applying a 7.2 kV cm−1 electric field, the coexistence of uniaxial and fourfold in-plane magnetic anisotropy is observed in 45° Sample, while a typical uniaxial anisotropy is found in 0° Sample.Manipulating strain mode and degree that can be applied to epitaxial complex oxide thin films have been a cornerstone of strain engineering. In recent years, lift-off and transfer technology of the epitaxial oxide thin films have been developed that enabled the integration of heterostructures without the limitation of material types and crystal orientations. Moreover, twisted integration would provide a more interesting strategy in artificial magnetoelectric heterostructures. A specific twist angle between the ferroelectric and ferromagnetic oxide layers corresponds to the distinct strain regulation modes in the magnetoelectric coupling process, which could provide some insight in to the physical phenomena. In this work, the La0.67Sr0.33MnO3 (001)/0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (011) (LSMO/PMN-PT) heterostructures with 45º and 0º twist angles were assembled via water-etching and transfer process. The transferred LSMO films exhibit a fourfold magnetic anisotropy with easy axis along LSMO < 110 >. A coexistence of uniaxial and fourfold magnetic anisotropy with LSMO [110] easy axis is observed for the 45° Sample by applying a 7.2 kV cm−1 electrical field, significantly different from a uniaxial anisotropy with LSMO [100] easy axis for the 0° Sample. The fitting of the ferromagnetic resonance field reveals that the strain coupling generated by the 45° twist angle causes different lattice distortion of LSMO, thereby enhancing both the fourfold and uniaxial anisotropy. This work confirms the twisting degrees of freedom for magnetoelectric coupling and opens opportunities for fabricating artificial magnetoelectric heterostructures.

Fourfold magnetic anisotropy varied by interfacial exchange coupling in epitaxial Fe/IrMn bilayers

We report variable in-plane fourfold magnetic anisotropy in epitaxial exchange-biased Fe/IrMn bilayers with different layer thicknesses. The fourfold magnetic anisotropy of the bilayers continuously decreases and even appears negative as the thickness of the Fe layer decreases, indicating a change in the easy axes from Fe 100 to Fe 110, which can be interpreted by the competition between the intrinsic magnetocrystalline anisotropy along Fe 100 and the extrinsic fourfold magnetic anisotropy along Fe 110 induced by the interfacial exchange coupling to IrMn. As the IrMn thickness increases, the fourfold magnetic anisotropy of the bilayers decreases rapidly from a value close to the intrinsic magnetocrystalline anisotropy of Fe and then gradually reaches saturation. The extrinsic fourfold magnetic anisotropy depends on the amount of the induced uncompensated IrMn spins near the edge of antiferromagnetic domains. In contrast, the exchange bias is determined by the net difference of the opposite uncompensated IrMn spins.

Field-free spin-orbit-torque switching of a single ferromagnetic layer with fourfold in-plane magnetic anisotropy

We present the observation of field-free spin-orbit-torque (SOT) switching of magnetization in a ferromagnetic semiconductor (Ga,Mn)As film with four-fold in-plane magnetic anisotropy. Magnetization switching is demonstrated between two orthogonal in-plane easy axes through planar Hall resistance measurements as a current is scanned in the absence of a magnetic field. The chirality of the switching of the current hysteresis loop is consistent with SOT arising from spin polarization caused by Dresselhaus- and Rashba-type spin-orbit-induced effective magnetic fields in the (Ga,Mn)As film. The chirality of magnetization switching can be reversed either by changing the direction of the initial magnetization or by applying a constant external magnetic field bias. The SOT magnetization switching between two states was consistently repeated by applying sequential current pluses with alternating polarities, indicating the potential for developing a field-free SOT device.

Investigation of orthogonal spin–orbit fields in crystalline ferromagnetic films with four-fold in-plane magnetic anisotropy

We report an investigation of current-induced spin–orbit fields (SOFs) in a crystalline (Ga,Mn)As ferromagnetic film with four-fold in-plane anisotropy. By designing a Hall device with current channels along crystallographic 〈100〉 directions, we observe the effects of SOFs in all four magnetization transitions over the four 〈110〉 hard axes of the film. The SOF effects are observed in the form of opposite shifts of angles at which the magnetization of the film switches sign for opposite current polarities in anisotropic magnetoresistance (AMR) measurements carried out as the applied magnetic field was rotated. Note that the Dresselhaus- and Rashba-type SOFs generated by currents flowing along the 〈100〉 directions—as in the device designed for the present experiments—are orthogonal to one another and can thus be separated by appropriate analysis of AMR. The analysis of experimental results based on magnetic free energy, in which the effects of SOFs are included, reveals that the Dresselhaus-type SOF is much stronger than the Rashba-type SOF. The values of the two SOFs obtained independently with currents flowing either along the [100] or [010] channels are consistent with each other, showing the reliability of the above-mentioned approach for investigating SOFs in ferromagnetic films.

Modulating the magnetization dynamic properties of zigzag magnetic nanowires by the four-fold magnetic anisotropy

The magnetization dynamic properties of zigzag magnetic nanowires with varied fourfold magnetic anisotropy, which modulated novel microwave responses without any external magnetic field, were investigated. The microwave properties associated with two different remanent magnetic states were found to be highly sensitive to fourfold magnetic anisotropy. A large frequency shift between the two remanent magnetic states was observed when fourfold magnetic anisotropy was added at a particular direction. The results are valuable for the realization of bias field-free microwave applications. In addition, the microwave properties demonstrated by two different remanent magnetic states could be transformed using a nanosecond pulse magnetic field, and fast switching in the system could be achieved with the fourfold magnetic anisotropy.

Ion-Beam Synthesis of Structure-Oriented Iron Nanoparticles in Single-Crystalline Rutile TiO2

Magnetic nanoparticles embedded into semiconductors have current perspectives for use in semiconducting spintronics. In this work, 40 keV Fe+ ions were implanted in high fluences of (0.5 ÷ 1.5) × 1017 ion/cm2 into an oxide semiconductor and single-crystalline TiO2 plates of rutile structure with (100) or (001) face orientations. Microstructure, elemental-phase composition, and magnetic properties of the Fe-ion-implanted TiO2 were studied by scanning and transmission electron microscopies (SEM and TEM), X-ray photoelectron (XPS) and Rutherford backscattering (RBS) spectroscopies, as well as vibrating-sample magnetometry (VSM). The high-fluence ion implantation results in the formation of magnetic nanoparticles of metallic iron beneath the irradiated surface of rutile. The induced ferromagnetism and observed two- or four-fold magnetic anisotropy are associated with the endotaxial growth of Fe nanoparticles oriented along the crystallographic axes of TiO2.

Magnetic and structural properties of sputtered thick Co2FeSi alloy films

We investigate magnetic and structural properties of the sputtered thick Co2FeSi (CFS) alloy films. X-ray diffraction measurements show CFS alloy film changes from amorphous to disordered A2 structures after annealing, which leads to pronounced increase in saturation magnetization. For the as-deposited films in-plane uniaxial magnetic anisotropy is determined to be in the range of 103–104 erg/cm3, showing strongly dependent to film thickness. After annealing a fourfold cubic magnetic anisotropy is witnessed. Structural ordering is observed to improve with increasing film thickness. Similiar trend of the magnetic anisotropy with sputtering power is witnessed, which can be explained by equivalent annealing effect of the high-power sputtering. Our results show that magnetic anisotropy properties of the CFS alloy films are strongly dependent to film thickness and structural ordering, tuned by anealing tempeature and sputtering power. These findings may be helpful for designing Co2FeSi-based spintronic devices.

Lateral magnetic anisotropy modulated by antiphase domain boundaries in PrBaCo2O5+δ thin films

Distorted oxygen octahedral structures in complex oxides induced by deformations, tilts or rotations have emerged as a well-established approach to control the physical properties of functional oxide materials. In this work, the lateral magnetic anisotropy is modulated in highly epitaxial PrBaCo2O5+δ (PBCO) thin films. Under the same growth conditions, a high density of anti-phase domain boundary (APB) defects is present in the PBCO films grown on (001)-oriented MgO substrate while less on (001)-oriented SrTiO3 substrate. The APBs break the co-corner connection between oxygen octahedra, thereby changing the lattice symmetry and leading to different lateral magnetic anisotropy, i.e., the fourfold symmetric lateral magnetic anisotropy in PBCO/MgO while twofold symmetry in PBCO/SrTiO3. Our results demonstrate that the magnetic anisotropy can be modulated through defect engineering, which can be further extended to a broader class of perovskite transition-metal oxides and pave the way for the development of correlated oxide-based magnetic spintronic devices.

Fourfold magnetic anisotropy induced in CoFeB/IrMn bilayers by interfacial exchange coupling

Exchange bias (EB) occurring in ferromagnetic (FM)/antiferromagnetic (AFM) bilayers conventionally can lead to a unidirectional magnetic anisotropy () as well as an accompanied uniaxial magnetic anisotropy (). We observed an additional fourfold magnetic anisotropy () induced by interfacial exchange coupling in amorphous CoFeB/epitaxial IrMn bilayers with an EB. Because of the combined effect of the three kinds of magnetic anisotropies, one- and two-step magnetic switching processes were observed at different magnetic field orientations, which usually appear in single-crystal FM layer with an intrinsic magnetocrystalline anisotropy but not in amorphous FM layer. The angular dependent magnetic switching fields can be nicely fitted by a phenomenological model based on domain wall nucleation and propagation with the in-plane along <100>. The ferromagnetic resonance measurements indicate that the specific strength of for EB along [100] is larger than that for EB along [110]. The induced can be understood by considering two types of AFM domains caused by both monatomic steps and defects and their induced net uncompensated spins along the in-plane <100> axes. The different dependence of on the EB direction are because of the different effects of growth magnetic field on the presence of AFM domains.

Magnetocrystalline anisotropy of epitaxially grown FeRh/MgO(001) films

We quantitatively investigated the magnetocrystalline anisotropy (MCA) of metamagnetic FeRh films epitaxially grown on MgO(001) substrates. The in-plane fourfold magnetic anisotropy of epitaxial FeRh films is reoriented from the<100>to<110>directions and meanwhile the strength is obvious enhanced when FeRh transfers from the ferromagnetic (FM) to antiferromagnetic (AF) states. In the nominal AF state, the observed fourfold magnetic anisotropy originates from the exchange coupling between the residual FM moments caused by antisite defects and the AF matrix, which therefore indicates the orientation and strength of the MCA of AF FeRh. The steep increase of the FM resonance linewidth when crossing the FM-AF phase transition temperature reveals the enhanced effective magnetic damping and an increased fourfold anisotropic two-magnon scattering. The first-principles calculations suggest that the in-plane lattice compressive strain imposed by the MgO substrate plays an important role in determining the magnetic structures FeRh films.

Structural and magnetic properties in the Heusler compounds Co3−x Fe x Al thin films

The structural and magnetic properties are important parameters for spintronic applications of Heusler compounds. The Co2FeAl and CoFe2Al alloys are considered as the representatives of the regular and inverse Heusler structure, respectively. Here, we present a systematic study of the structural and magnetic properties of Co3−x Fe x Al (x = 1 ∼ 2) Heusler thin films grown on MgO (001) substrates. The lattice parameters and magnetic properties, such as magnetic coercivity, saturated moment and four-fold magnetic anisotropy constant, display different change trends in the Co-rich and Fe-rich regions, which suggest that magnetic properties have relevance to the Heusler structures. Our findings give a better understanding of the Heusler structures and magnetic properties, which is helpful for developing spintronic applications.

Exchange-coupling-induced fourfold magnetic anisotropy in CoFeB/FeRh bilayer grown on SrTiO3(001)

Exchange coupling across the interface between a ferromagnetic (FM) layer and an antiferromagnetic (AFM) or another FM layer may induce a unidirectional magnetic anisotropy and/or a uniaxial magnetic anisotropy, which has been extensively studied due to the important application in magnetic materials and devices. In this work, we observed a fourfold magnetic anisotropy in amorphous CoFeB layer when exchange coupling to an adjacent FeRh layer which is epitaxially grown on an SrTiO3(001) substrate. As the temperature rises from 300 K to 400 K, FeRh film undergoes a phase transition from AFM to FM phase, the induced fourfold magnetic anisotropy in the CoFeB layer switches the orientation from the FeRh〈 110〉 to FeRh〈 100 〉 directions and the strength is obviously reduced. In addition, the effective magnetic damping as well as the two-magnon scattering of the CoFeB/FeRh bilayer also remarkably increase with the occurrence of magnetic phase transition of FeRh. No exchange bias is observed in the bilayer even when FeRh is in the nominal AFM state, which is probably because the residual FM FeRh moments located at the interface can well separate the exchange coupling between the below pinned FeRh moments and the CoFeB moments.

Interfacial engineering manipulation of magnetic anisotropy evolution via orbital reconstruction in low-dimensional manganite superlattices

Control of magnetic anisotropy in low-dimensional systems is of paramount importance in terms of their fundamental and technological perspectives. La0.7Sr0.3MnO3 (LSMO) is a ferromagnetic half-metal with a high Curie temperature and many efforts have been made to control its magnetic anisotropy. However, the relationship between the evolution of the magnetic anisotropy orientation and the electronic structure of low-dimensional LSMO still remains poorly understood. Here, the high-quality superlattices comprised of LSMO and SrMnO3 (SMO) layers are synthesized with a compatible structure at the atomic scale. Their magnetic anisotropy is gradually varied from planar to perpendicular by increasing the SMO thickness, and the special fourfold magnetic anisotropy is also observed at the intermediate superlattice thickness. The evolution of the magnetic anisotropy in these systems is confirmed by the electronic transport and magnetic measurements. Moreover, X-ray linear dichroism measurements and first-principles calculations reveal the interfacial orbital reconstruction with the in-plane to out-of-plane magnetic reorientation transition. Therefore, a new microscopic method for magnetic anisotropy manipulation is developed in the present study, enabling discovery of novel phenomena as well as control of the magnetic properties.

Twofold and fourfold anisotropies in zinc ferrite thin films investigated by ferromagnetic resonance

By means of the ferromagnetic resonance (FMR) technique (at the $X$ band) we investigated the magnetic properties of ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$ (ZFO) films grown on different substrates. Angular dependence of the FMR field showed the existence of twofold and fourfold anisotropies for the same material, grown on Si(111), MgO(100), and STO(100), respectively. By using a model that takes into consideration the relevant contributions to the energy, we were able to extract the following parameters: effective magnetization ${M}_{\mathrm{eff}}$, cubic magnetocrystalline anisotropy field, in-plane and out of plane uniaxial anisotropy fields, and the $g$ factor. The samples deposited on Si showed a twofold in-plane anisotropy while a fourfold magnetic anisotropy was observed for the ZFO/MgO and ZFO/${\mathrm{SrTiO}}_{3}$ samples. All samples showed a strong perpendicular uniaxial anisotropy above 1 kOe and low in-plane uniaxial and cubic anisotropy field, on the order of tens of oersteds. The FMR linewidth ($\mathrm{\ensuremath{\Delta}}H$) fitted from FMR data shows $\mathrm{\ensuremath{\Delta}}H$ values above 190 Oe for all samples, indicating high damping of this material to be $\ensuremath{\sim}{10}^{\ensuremath{-}2}$ for the ZFO/MgO. The present analysis compared the FMR data obtained for the in-plane and out of plane measurements. Likewise, the effective magnetization and perpendicular uniaxial anisotropy field by FMR were compared with the magnetic response obtained by a vibrating sample magnetometer.

Epitaxial thin-film Pd1−xFex alloy: a tunable ferromagnet for superconducting spintronics

In the paper we present the results of extensive studies of palladium-rich Pd1-xFex alloy films epitaxially grown on MgO single-crystal substrate. In a composition range of x = 0.01-0.07 these materials are soft ferromagnets, the saturation magnetization and magnetic anisotropy of which can be tuned by its composition. Vibrating sample magnetometry was used to study temperature dependences of spontaneous magnetic moment and to establish the temperature of magnetic ordering (Curie temperature). Ferromagnetic resonance (FMR) measurements at low temperatures in the in-plane and out-of-plane geometries revealed the four-fold in-plane magnetic anisotropy with the easy directions along the <110> axes of the substrate. The modelling of the angular dependence of the field for resonance allowed to extract the cubic and tetragonal contributions to the magnetic anisotropy of the films and establish their dependence on the concentration of iron in the alloy. Experimental data are discussed in the framework of existing theories of dilute magnetic alloys. Using the anisotropy constants established from FMR, the magnetic hysteresis loops are reproduced utilizing the Stoner-Wohlfarth model thus indicating the predominant coherent magnetic moment rotation at low temperatures. The obtained results compile a database of magnetic properties of a palladium-iron alloy considered as a material for superconducting spintronics.

Magnetocrystalline anisotropy imprinting of an antiferromagnet on an amorphous ferromagnet in FeRh/CoFeB heterostructures

Magnetic anisotropy is a fundamental key parameter of magnetic materials that determines their applications. For ferromagnetic materials, the magnetic anisotropy can be easily detected by using conventional magnetic characterization techniques. However, due to the magnetic compensated structure in antiferromagnetic materials, synchrotron measurements, such as X-ray magnetic linear dichroism, are often needed to probe their magnetic properties. In this work, we observed an imprinted fourfold magnetic anisotropy in the amorphous ferromagnetic layer of FeRh/CoFeB heterostructures. The MOKE and ferromagnetic resonance measurements show that the easy magnetization axes of the CoFeB layer are along the FeRh〈110〉 and FeRh〈100〉 directions for the epitaxially grown FeRh layer in the antiferromagnetic and ferromagnetic states, respectively. The combined Monte Carlo simulation and first-principles calculation indicate that the fourfold magnetic anisotropy of the amorphous CoFeB layer is imprinted due to the interfacial exchange coupling between the CoFeB and FeRh moments from the magnetocrystalline anisotropy of the epitaxial FeRh layer. This observation of imprinting the magnetocrystalline anisotropy of antiferromagnetic materials on easily detected ferromagnetic materials may be applied to probe the magnetic structures of antiferromagnetic materials without using synchrotron methods.