In-plane Magnetization Configuration Research Articles

Chern-Insulator Phase in Antiferromagnets.

The long-sought Chern insulators that manifest a quantum anomalous Hall effect are typically considered to occur in ferromagnets. Here, we theoretically predict the realizabilities of Chern insulators in antiferromagnets, in which the magnetic sublattices are connected by symmetry operators enforcing zero net magnetic moment. Our symmetry analysis provides comprehensive magnetic layer point groups that allow antiferromagnetic (AFM) Chern insulators, revealing that an in-plane magnetic configuration is required. Followed by first-principles calculations, such design principles naturally lead to two categories of material candidates, exemplified by monolayer RbCr4S8 and bilayer Mn3Sn with collinear and noncollinear AFM orders, respectively. We further show that the Chern number could be tuned by slight ferromagnetic canting as an effective pivot. Our work elucidates the nature of the Chern-insulator phase in AFM systems, paving a new avenue for designing quantum anomalous Hall insulators with the integration of nondissipative transport and the promising advantages of the AFM order.

Rashba Field Contribution and Electric Field Control of the Magnetic Anisotropy

"The anatomy of the Perpendicular Magnetic Anisotropy (PMA) in magnetic multilayered thin film heterostructures and the possibility to efficiently manipulate it by external electric fields represent major issues for technological applications in magnetic data storage devices. Solving a standard quantum model based on a Stoner-Rashba Hamiltonian, we illustrate that the magnetic properties in ultrathin magnetic films arise from the competing components identified in the magnetic energy: the Rashba correction to the Stoner splitting, a pseudo-dipolar contribution to the anisotropy energy proportional to the electric field at the interface that would favor in-plane magnetization configuration and a uniaxial-like perpendicular anisotropy term. This last term is responsible on the perpendicular magnetization configuration in ultrathin films and depends on the square of the electric field at the surface of the film. Investigating the time evolution of the magnetic system, we described the macrospin magnetization dynamics in terms of a Rashba field induced magnetization precession. Despite its simplicity, the quantum approach underlines the basic issues related to the physical origin and the mechanisms of the perpendicular magnetization in ultrathin magnetic films and illustrates the capability of manipulation by external gating electric field, in experiments similarly to Nuclear Magnetic Resonance. Keywords: perpendicular magnetic anisotropy, electric field control of anisotropy, magnetic tunnel junctions, magnetic multilayer heterostructures. "

Miniature electron beam separator based on three stacked dipoles

We designed and built a compact bi-axial electron beam separator. This separator is an indispensable electron optical element in the development of MEMS-mirror-based miniaturized concepts for quantum electron microscopy (QEM) and aberration-corrected low-voltage scanning electron microscopy (AC-SEM). The separator provides the essential cycling of the electron beam between the two parallel optical axes that are part of these systems. This requires crossed electric and magnetic fields perpendicular to the beam path, as can be found in Wien-filter type beam separators. In our miniaturized QEM or AC-SEM concepts, the parallel axes are separated by only 1 mm. Conventional Wien-filter-based beam separator concepts rely on in-plane electric and magnetic multipole electrode configurations that are larger than the restricted available volume in these miniaturized QEM/AC-SEM systems. Our compact beam separator design introduces three stacked dipole electrode layers, which enables simultaneous beam separation at two parallel axes that are in close proximity. The outer layer electrodes maintain an electric field for which the direction on the one axis is opposed to that on the other axis. The middle layer generates a perpendicularly oriented magnetic field that spans both axes. The total field configuration enables the deflection of the beam on one axis and simultaneously provides a straight passage on the other axis. The deflection strength and distortion fields of the beam separator are experimentally obtained with a 2 keV electron beam energy. The results validate the use of the beam separator for electron energies up to 5 keV and deflection angles up to 100 mrad.

Magnetic Structure of Ion-Beam Imprinted Stripe Domains Determined by Neutron Scattering.

We present a detailed analysis of the in-plane magnetic vector configuration in head-to-head/tail-to-tail stripe domain patterns of nominal 5 μm width. The patterns have been created by He-ion bombardment induced magnetic patterning of a CoFe/IrMn3 exchange bias thin-film system. Quantitative information about the chemical and magnetic structure is obtained from polarized neutron reflectometry (PNR) and off-specular scattering (OSS). The technique provides information on the magnetic vector orientation and magnitude along the lateral coordinate of the sample, as well as the chemical and magnetic layer structure as a function of depth. Additional sensitivity to magnetic features is obtained through a neutron wave field resonance, which is fully accounted for in the presented analysis. The scattering reveals a domain width imbalance of 5.3 to 3.7 μm of virgin and bombarded stripes, respectively. Further, we report that the magnetization in the bombarded stripe significantly deviates from the head-to-head arrangement. A domain wall of 0.6 μm with homogeneous magnetization direction is found to separate the two neighboring domains. The results contain detailed information on length scales and magnetization vectors provided by PNR and OSS in absolute units. We illustrate the complementarity of the technique to microscopy techniques for obtaining a quantitative description of imprinted magnetic domain patterns and illustrate its applicability to different sample systems.

Resonant Faraday effect using high-order harmonics for the investigation of ultrafast demagnetization

During the past few years high-order harmonic generation (HHG) has opened up the field of ultrafast spectroscopy to an ever larger community by providing a table-top and affordable femtosecond extreme ultraviolet (EUV) and soft-x-ray source. In particular, the field of femtomagnetism has largely benefited from the development of these sources. However, the use of x-ray magnetic circular dichroism (XMCD) as a probe of magnetization, the most versatile and reliable one, has been constrained by the lack of polarization control at HHG sources, so studies have relied on more specific magneto-optical effects. Even the recent developments on the generation of elliptically polarized harmonics have only resulted in a few time-resolved experiments relying on this powerful technique since they add complexity to already-difficult measurements. In this article we show how to easily probe magnetization dynamics with linearly polarized EUV or soft-x-ray light with a versatility similar to XMCD by exploiting the Faraday effect. Static and time-resolved measurements of the Faraday effect are presented around the Co $M$ edges. Using simple theoretical considerations, we show how to retrieve the samples magnetization dynamics from the Faraday rotation and ellipticity transients. Ultrafast demagnetization dynamics of a few nanometers in Co-based samples are measured with this method in out-of-plane as well as in-plane magnetization configurations, showing its great potential for the study of femtomagnetism.

Mapping magnetization states in ultrathin films with Dzyaloshinskii–Moriya interaction

To succeed in the next generation of magnetic storages, nanomagnetic structures must be engineered to allow modulation of magnetization amplitude and spatial period. We demonstrate computationally how magnetic nanostructures states (domains with narrow wall, skyrmions, spin spirals, conical spin spirals, and in-plane magnetization configuration) can be designed in ultrathin films with a Dzyaloshinskii–Moriya interaction (DMI) by adjusting two material parameters: perpendicular magnetic anisotropy characterized by the quality factor Q, and reduced DMI constant . For a broad range of Q and parameters, the magnetization states are mapped in (Q, ) diagrams and characterized by the periodicity (p) of spatial distribution of magnetization and the mean value of the square of an out-of-plane normalized magnetization component . We explain the transitions between different magnetization states and describe phase transition curves in terms of vanishing of the domain wall energy for both and Q > 1 cases. We show that by changing and/or Q and approaching the phase transition curves, the discontinuous transitions accompanied by jumps of both period and amplitude take place.

Iron doping influence on interlayer magnetic coupling and magnetoresistance in Co/MoFe/Co films

Abstract The interlayer exchange coupling and magnetotransport properties have been investigated in trilayer structures where two coupled symmetric Co layers were separated by a spacer. The trilayers have the following general structure: Co(dCo)/X(dspacer)/Co(dCo), where X stands for used spacer (pure Mo or codeposition Mo and Fe with chemical composition Mo0.8Fe0.2), dCo and dspacer are the thicknesses of the cobalt and spacer layers. Two samples sets were fabricated by molecular beam epitaxy: (i) wedge-like: the both Co wedges are similar (0–3 nm) and directed along one the side of the substrate while the spacer wedge (also 0–3 nm) was grown along perpendicular direction; (ii) uniform in thickness Co(3 nm)/X(0.7 nm)/Co(3 nm), exhibiting in-plane magnetization configuration and the strongest antiferromagnetic coupling. Magnetization processes were investigated using polar and longitudinal magneto-optical Kerr effect (MOKE) for studies of out-of-plane and in-plane magnetization configurations, respectively. With increase of dMo, the interlayer exchange coupling alternates between ferromagnetic and antiferromagnetic exhibiting as high as 0.3 T coupling field HIEC of antiparallel alignment for dMo = 0.7 nm. Iron doping results in about 3.5 times decrease of the HIEC, measured at first antiferromagnetic coupling range for both perpendicular and in-plane magnetization configurations. The oscillating dependence of HIEC on dspacer is analyzed in the frame of a Ruderman–Kittel–Kasuya–Yosida model. The giant magnetoresistance (GMR) effect has been investigated in the current in plane geometry. Relation between MOKE and GMR results is discussed.

Magnetic domains characterization of crystalline Fe3O4 under DC and AC magnetic field.

Fe3O4 nanoparticles with crystallite sizes around 10 nm were synthesized by an emulsion method. X-ray diffractometer (XRD) shows that nanocrystalline Fe3O4 possesses face center cubic structure. The magnetic characteristics are investigated by magnetic force microscopy (MFM). Magnetic field directions were applied parallel and perpendicular to the Fe3O4 sample surface for magnetic measurements. Under the perpendicular magnetic field, the phase images of most magnetic nanoparticles exhibit bright or dark MFM contrast. In comparison, the parallel field phase images display a bright-dark dipole MFM contrast, with in-plane magnetic domain configurations. Furthermore, the investigation of strip domains inside Fe3O4 particles under altering magnetic fields indicates the existence of magnetic anisotropy energies, dipole energies as well as inter-grain coupling energies inside the clusters. This approach for probing magnetic responses on nanoscale magnetic domains can be further extended to the analysis of local physical features.

Topologically protected superconducting ratchet effect generated by spin-ice nanomagnets

We have designed, fabricated and tested a robust superconducting ratchet device based on topologically frustrated spin ice nanomagnets. The device is made of a magnetic Co honeycomb array embedded in a superconducting Nb film. This device is based on three simple mechanisms: (i) the topology of the Co honeycomb array frustrates in-plane magnetic configurations in the array yielding a distribution of magnetic charges which can be ordered or disordered with in-plane magnetic fields, following spin ice rules; (ii) the local vertex magnetization, which consists of a magnetic half vortex with two charged magnetic Néel walls; (iii) the interaction between superconducting vortices and the asymmetric potentials provided by the Néel walls. The combination of these elements leads to a superconducting ratchet effect. Thus, superconducting vortices driven by alternating forces and moving on magnetic half vortices generate a unidirectional net vortex flow. This ratchet effect is independent of the distribution of magnetic charges in the array.

Interface-induced anomalous Nernst effect in Fe3O4/Pt-based heterostructures

We have studied the anomalous Nernst effect (ANE) in [Fe3O4/Pt]-based heterostructures grown at high temperature, by measuring the ANE-induced electric field with a magnetic field applied normal to the sample surface, in the perpendicular magnetized configuration, where only the ANEs from the ferromagnetic layers or magnetic proximity effects can be detected. An ANE voltage is observed for [Fe3O4/Pt]n multilayers, and we further investigated its origin by performing measurements in [Fe3O4/Pt/Fe3O4] trilayers as a function of the Pt thickness. Our results suggest the presence of an interface-induced ANE in the metallic layer, possibly driven by a heat induced subnanometer interdiffusion which affects the nature of the Fe3O4/Pt interface. Despite this ANE, the spin Seebeck effect is the dominant mechanism for the transverse thermoelectric voltage in the in-plane magnetized configuration, accounting for about 70% of the voltage in the multilayers.

Interlayer coupling-driven magnetic ordering and magnetization processes in ultrathin Au/Co/Mo/Co/Au film

Abstract Interlayer magnetic coupling in an Au/Co/Mo/Co/Au system fabricated by molecular beam epitaxy was studied as a function of the thicknesses of the cobalt and molybdenum spacer layers (denoted as dCo and dMo, respectively). Magnetization processes and magnetic domain structures were investigated using magneto-optical techniques. In the investigated thickness ranges, various magnetization configurations of the Co layers were observed: (A) both perpendicular, (B) perpendicular and in-plane, and (C) both in-plane. The interlayer exchange coupling field HIEC was determined as a function of dMo. With increasing dMo, this coupling changed from antiferromagnetic with a huge coupling field of ≈0.27 T to ferromagnetic and again to antiferromagnetic. A coupling oscillation period of around 1.4 nm was deduced by comparing the experimental data with the Ruderman–Kittel–Kasuya–Yosida model. For configurations (B) and (C), oscillations of the saturation magnetic field HSAT(dMo) perpendicular to the sample plane were found. Simplified and precise analytical expressions for HSAT were proposed by considering HIEC as well as the uniaxial magnetic anisotropy of one and both magnetic layers, respectively. Magnetic ordering was numerically modeled taking into account the dependences of layer anisotropy and exchange coupling on dCo and dMo, respectively. This modeling well described the remanent magnetization map (dCo, dMo) determined using the polar Kerr effect. It is also shown that HIEC(dMo) induced magnetization reorientation from in-plane to out-of-plane or from out-of-plane to in-plane magnetization configurations and canted magnetization of the Co layers.

Thickness dependence of transverse thermoelectric voltage in Co40Fe60/YIG magnetic junctions

Measurements of transverse thermoelectric voltage were carried out in Co40Fe60 (CoFe)/Yttrium-Iron-Garnet (YIG) magnetic junctions, using the CoFe film as the spin detector. An unusual dependence of the voltage on the CoFe thickness was observed in the in-plane magnetized (IM) configuration; the junction with a relatively thick CoFe layer (40 nm) exhibits positive signals, whereas the junctions with a thinner CoFe layer (7–10 nm) exhibit negative signals. To find the origin of the behavior, we compare the voltage signals in the CoFe/YIG and CoFe/GGG systems in the IM configuration as well as perpendicularly magnetized (PM) configuration. Furthermore, the anomalous Hall effect was also measured in the Hall-bar shaped CoFe films. The experimental results suggest that the observed thickness dependence of the voltage is attributed to the combination of the inverse spin Hall effect (ISHE) and the anomalous Nernst effect (ANE) in the CoFe layers; the former shows a negative voltage and its contribution gradually increases with decreasing the CoFe thickness, whereas the latter shows a positive and mostly thickness independent voltage. The competition between the ISHE and ANE contributions results in the observed peculiar CoFe-thickness dependence of the transverse thermoelectric voltage.

Edge magnetism in triangular silicene quantum dots

The interplay among electron-electron and intrinsic spin-orbit interactions (ISOI) at the zigzag edge of triangular quantum dots (ZTQDs) was analyzed by means of the mean-field Kane-Mele-Hubbard model. It was shown that the magnetic anisotropy (in-plane and out-of-plane magnetic configurations) significantly modifies electronic and magnetic properties of ZTQDs. The in-plane configuration is more energetically stable than the other one. However, its magnetization becomes easier depolarized as a continuous function as strengths of electric field E and ISOI increase. This is in contrast to the out-of-plane magnetization, which is more robust and becomes depolarized in a step-like way. Moreover, the energy gap only vanishes in the out-of-plane case by adjusting E and ISOI strengths, and thus we cannot expect a topological insulator phase in the in-plane magnetic configuration.

Investigation of the spin Seebeck effect and anomalous Nernst effect in a bulk carbon material

Abstract Since the discovery of the spin Seebeck effect (SSE) in 2008, it has become one of the most active topics in the spin caloritronics research field. It opened up a new way to create the spin current by a combination of magnetic fields and heat. The SSE was observed in many kinds of materials including metallic, semiconductor, or insulating magnets, as well as non-magnetic materials. On the other hand, carbon-based materials have become one of the most exciting research areas recently due to its low cost, abundance and some exceptional functionalities. In this work, we have investigated the possibility of the SSE in bulk carbon materials for the first time. Thin platinum film (Pt), coated on the smoothened surface of the bulk carbon, was used as the spin detector via the inverse spin Hall effect (ISHE). The experiment for observing longitudinal SSE in the bulk carbon was set up by applying a magnetic field up to 30 kOe to the sample with the direction perpendicular to the applied temperature gradient. The induced voltage from the SSE was extracted. However, for conductive materials, e.g. carbon, the voltage signal under this set up could be a combination of the SSE and the anomalous Nernst effect (ANE). Therefore, two measurement configurations were carried out, i.e. the in-plane magnetization (IM), and the perpendicular-to-plane magnetization (PM). For the IM configuration, the SSE + ANE signals were detected where as the only ANE signal existed in the PM configuration. The results showed that there were the differences between the voltage signals from the IM and PM configurations implying the possibility of the SSE in the bulk carbon material. Moreover, it was found that the difference in the IM and PM signals was a function of the magnetic field strength, temperature difference, and measurement temperature. Although the magnitude of the possible SSE voltage in this experiment was rather low (less than 0.5 μV at 50 K), this research showed that potential of using low cost and abundant bulk carbon as spin current supplier or thermoelectric power generators.

Current-Nonlinear Hall Effect and Spin-Orbit Torque Magnetization Switching in a Magnetic Topological Insulator.

The current-nonlinear Hall effect or second harmonic Hall voltage is widely used as one of the methods for estimating charge-spin conversion efficiency, which is attributed to the magnetization oscillation by spin-orbit torque (SOT). Here, we argue the second harmonic Hall voltage under a large in-plane magnetic field with an in-plane magnetization configuration in magnetic-nonmagnetic topological insulator (TI) heterostructures, Cr_{x}(Bi_{1-y}Sb_{y})_{2-x}Te_{3}/(Bi_{1-y}Sb_{y})_{2}Te_{3}, where it is clearly shown that the large second harmonic voltage is governed not by SOT but mainly by asymmetric magnon scattering without macroscopic magnetization oscillation. Thus, this method does not allow an accurate estimation of charge-spin conversion efficiency in TI. Instead, the SOT contribution is exemplified by current pulse induced nonvolatile magnetization switching, which is realized with a current density of 2.5×10^{10} A m^{-2}, showing its potential as a spintronic material.

High power and low critical current density spin transfer torque nano-oscillators using MgO barriers with intermediate thickness

Reported steady-state microwave emission in magnetic tunnel junction (MTJ)-based spin transfer torque nano-oscillators (STNOs) relies mostly on very thin insulating barriers [resulting in a resistance × area product (R × A) of ~1 Ωμm2] that can sustain large current densities and thus trigger large orbit magnetic dynamics. Apart from the low R × A requirement, the role of the tunnel barrier in the dynamics has so far been largely overlooked, in comparison to the magnetic configuration of STNOs. In this report, STNOs with an in-plane magnetized homogeneous free layer configuration are used to probe the role of the tunnel barrier in the dynamics. In this type of STNOs, the RF modes are in the GHz region with integrated matched output powers (Pout) in the range of 1–40 nW. Here, Pout values up to 200 nW are reported using thicker insulating barriers for junctions with R × A values ranging from 7.5 to 12.5 Ωμm2, without compromising the ability to trigger self-sustained oscillations and without any noticeable degradation of the signal linewidth (Γ). Furthermore, a decrease of two orders of magnitude in the critical current density for spin transfer torque induced dynamics (JSTT) was observed, without any further change in the magnetic configuration.

Edge magnetism of finite graphene-like nanoribbons in the presence of intrinsic spin–orbit interaction and perpendicular electric field

This paper elucidates the combined effect of intrinsic spin–orbit interaction (ISOI) and perpendicular electric field on edge states in finite graphene-like nanoribbons. It is shown that the ISOI generates magnetic anisotropy which makes the in-plane edge magnetization configuration more energetically stable than the commonly studied out-of-plane one. The anisotropy less severely suppresses the former configuration than the latter. As concerns the Ez effect, the following evolution of electric transport properties is predicted: magnetic insulator, non-magnetic narrow-band semiconductor, and finally non-magnetic band insulator.

FerrimagneticDyCo5Nanostructures for Bits in Heat-Assisted Magnetic Recording

Increasing the magnetic data recording density requires reducing the size of the individual memory elements of a recording layer as well as employing magnetic materials with temperature-dependent functionalities. Therefore, it is predicted that the near future of magnetic data storage technology involves a combination of energy-assisted recording on nanometer-scale magnetic media. We present the potential of heat-assisted magnetic recording on a patterned sample; a ferrimagnetic alloy composed of a rare earth and a transition metal, DyCo$_5$, which is grown on a hexagonal-ordered nanohole array membrane. The magnetization of the antidot array sample is out-of-plane oriented at room temperature and rotates towards in-plane upon heating above its spin-reorientation temperature (T$_R$) of ~350 K, just above room temperature. Upon cooling back to room temperature (below T$_R$), we observe a well-defined and unexpected in-plane magnetic domain configuration modulating with ~45 nm. We discuss the underlying mechanisms giving rise to this behavior by comparing the magnetic properties of the patterned sample with the ones of its extended thin film counterpart. Our results pave the way for novel applications of ferrimagnetic antidot arrays of superior functionality in magnetic nano-devices near room temperature.

Enhanced modulation rates via field modulation in spin torque nano-oscillators

Spin Transfer Nano-Oscillators (STNOs) are promising candidates for telecommunications applications due to their frequency tuning capabilities via either a dc current or an applied field. This frequency tuning is of interest for Frequency Shift Keying concepts to be used in wireless communication schemes or in read head applications. For these technological applications, one important parameter is the characterization of the maximum achievable rate at which an STNO can respond to a modulating signal, such as current or field. Previous studies of in-plane magnetized STNOs on frequency modulation via an rf current revealed that the maximum achievable rate is limited by the amplitude relaxation rate Γp, which gives the time scale over which amplitude fluctuations are damped out. This might be a limitation for applications. Here, we demonstrate via numerical simulation that application of an additional rf field is an alternative way for modulation of the in-plane magnetized STNO configuration, which has the advantage that frequency modulation is not limited by the amplitude relaxation rate, so that higher modulation rates above GHz are achievable. This occurs when the modulating rf field is oriented along the easy axis (longitudinal rf field). Tilting the direction of the modulating rf field in-plane and perpendicularly with respect to the easy axis (transverse rf field), the modulation is again limited by the amplitude relaxation rate similar to the response observed in current modulation.

Effective exchange fields in spin-torque resonance of magnetic insulators

We report additional results on the spin-torque ferromagnetic resonance (ST-FMR) of a bilayer system made from a magnetic insulator such as Y3Fe5O12 (YIG) and a heavy normal metal such as Pt in terms of the interface spin-mixing conductance and including spin pumping. We analyze experimental ST-FMR spectra for out-of-plane and in-plane magnetization configurations in terms of an anisotropic imaginary part Gi of the mixing conductance (or interface effective field). The estimated ratio between imaginary and real parts Gi/Gr≲0.3 is sensitive to an (unknown) phase shift between microwave current bias and associated Oersted field.