Polarized Neutron Reflectivity Research Articles

Chromium agglomeration induced by Fe+ ion irradiation of Fe-10at%Cr

Fe-Cr alloys serve as model alloys for the investigation of radiation induced effects in ferritic-martensitic steels which are candidate structural materials for future fusion reactors. In this work the effect of Cr segregation and/or agglomeration in 490 keV Fe+ ion irradiated Fe-10at%Cr alloys in the form of thin films is investigated. The irradiations took place at 300 °C at doses ranging from 0.5 to 20 displacements per atom (dpa). Polarized Neutron Reflectivity (PNR) measurements was used for the determination of the solute Cr concentration in the Fe-Cr matrix. Cr depletion from the Fe-Cr matrix up to 2.4 at% was found. This is related to solute Cr decrement as the accumulated dose increases. After the damage of 4 dpa, solute Cr reaches the asymptotic value of 8.4 at%, close to that of the thermodynamic equilibrium in Fe-Cr. Atom Probe Tomography (APT) measurements showed that after irradiation Cr accumulates into clusters the majority of which is co-located with oxygen.

Temperature- and magnetic field-induced magnetic structural changes in the Fe3Si/FeSi2 superlattice

Artificial lattices with semiconductor spacers are expected to exhibit changes in their magnetic structure owing to the control of their electronic states. The temperature (T) and magnetic-field (H ext) dependence of the in-plane magnetic structure of an [Fe3Si/FeSi2]20 superlattice with a nonmagnetic and semiconducting FeSi2 spacer layer is investigated using magnetization and polarized neutron reflectivity measurements. When H ext = 5 mT, nearly collinear antiferromagnetic (AF) structures are observed from 4 to 298 K. When H ext = 1 T, field-induced fan-like, noncollinear AF structures showing ferromagnetic components along H ext and transverse AF components are observed at low T.

The noncollinear interlayer coupling in NiFe/NiO/NiFe trilayers

The interlayer coupling and magnetization reversal behaviors in NiFe/NiO/NiFe trilayers were investigated using polarized neutron reflectivity and Monte Carlo (MC) simulations. Our results reveal that the shape of NiFe loops transitions from square to tilted as the NiO thicknesses decrease, indicating changes in the direction of NiFe layer’s easy axis. This phenomenon can be attributed to variations occurring at NiO/NiFe interfaces for different NiO layer thicknesses. With thin NiO layer, interdiffusion at the NiO/NiFe interfaces leads to frustrated coupling, resulting in a noncollinear interlayer coupling. This observation is supported by MC simulations. Conversely, hardly any coupling frustration is observed for the sample with a thick NiO layer. Our findings propose a novel way to tailor the interlayer coupling through interface engineering.

Large exchange bias effect and interfacial magnetic structure in FePt3 (partially ordered)/FePt3 (disordered) multilayer film

Stoichiometric FePt3 thin film with modulated chemical order is a good system for the study of the magnetic exchange coupling due to distinctive structural and electronic compatibility at the interface. In this study, FePt3(partially ordered)/FePt3 (disordered) multilayer films are fabricated using magnetron sputtering. The interface, magnetic depth profile of each layer, and the magnetic properties of the film are investigated via polarized neutron reflectivity and AC susceptibility measurements. At low temperature, an exchange bias field up to 430 Oe is observed in the film. Exchange bias and coercivity of the film are subjected to a fast increase as temperature goes below 100 K suggesting that Q2 antiferromagnetic (AF) order may be induced in the film. First-principles calculations indicate, in partially ordered stoichiometric FePt3, there may be competitions among Q1, Q2 and ferromagnetic (FM) phases at low temperature due to some extent of chemical disorder. The large exchange bias in the sample may be attributed to the appearance of a small amount of FM phase and AF Q2 phase in AF layers and at the interfaces between FM and AF layers.

Controllable Spin-Orbit Torque Induced by Interfacial Ion Absorption in Ta/CoFeB/MgO Multilayers with Canted Magnetizations.

Electrically generated spin-orbit torque (SOT) has emerged as a powerful pathway to control magnetization for spintronic applications including memory, logic, and neurocomputing. However, the requirement of external magnetic fields, together with the ultrahigh current density, is the main obstacle for practical SOT devices. In this paper, we report that the field-free SOT-driven magnetization switching can be successfully realized by interfacial ion absorption in perpendicular Ta/CoFeB/MgO multilayers. Besides, the tunable SOT efficiency exhibits a strong dependence on interfacial Ti insertion thicknesses. Polarized neutron reflection measurements demonstrate the existence of canted magnetization with Ti inserted, which leads to deterministic magnetization switching. In addition, interfacial characterization and first-principles calculations reveal that B absorption by the Ti layer is the main cause behind the enhanced interfacial transparency, which determines the tunable SOT efficiency. Our findings highlight an attractive scheme to a purely electric control spin configuration, enabling innovative designs for SOT-based spintronics via interfacial engineering.

Measurement of the Induced Magnetic Polarisation of Rotated-Domain Graphene Grown on Co Film with Polarised Neutron Reflectivity.

In this paper, we determine the magnetic moment induced in graphene when grown on a cobalt film using polarised neutron reflectivity (PNR). A magnetic signal in the graphene was detected by X-ray magnetic circular dichroism (XMCD) spectra at the C K-edge. From the XMCD sum rules an estimated magnetic moment of 0.3 μB/C atom, while a more accurate estimation of 0.49 μB/C atom was obtained by carrying out a PNR measurement at 300 K. The results indicate that the higher magnetic moment in Co is counterbalanced by the larger lattice mismatch between the Co-C (1.6%) and the slightly longer bond length, inducing a magnetic moment in graphene that is similar to that reported in Ni/graphene heterostructures.

Deterministic Magnetization Switching via Tunable Noncollinear Spin Configurations in Canted Magnets.

Spin obit torque (SOT) driven magnetization switching has been used widely for encoding consumption-efficient memory and logic. However, symmetry breaking under a magnetic field is required to realize the deterministic switching in synthetic antiferromagnets with perpendicular magnetic anisotropy (PMA), which limits their potential applications. Herein, we report all electric-controlled magnetization switching in the antiferromagnetic Co/Ir/Co trilayers with vertical magnetic imbalance. Besides, the switching polarity could be reversed by optimizing the Ir thickness. By using the polarized neutron reflection (PNR) measurements, the canted noncollinear spin configuration was observed in Co/Ir/Co trilayers, which results from the competition of magnetic inhomogeneity. In addition, the asymmetric domain walls demonstrated by micromagnetic simulations result from introducing imbalance magnetism, leading to the deterministic magnetization switching in Co/Ir/Co trilayers. Our findings highlight a promising route to electric-controlled magnetism via tunable spin configuration, improve our understanding of physical mechanisms, and significantly promote industrial applications in spintronic devices.

Negligible magnetic losses at low temperatures in liquid phase epitaxy grown Y3Fe5O12 films

Yttrium iron garnet (${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$; YIG) has a unique combination of low magnetic damping, high spin-wave conductivity, and insulating properties that make it a highly attractive material for a variety of applications in the fields of magnetics and spintronics. While the room-temperature magnetization dynamics of YIG have been extensively studied, there are limited reports correlating the low-temperature magnetization dynamics to the material structure or growth method. Here we investigate liquid phase epitaxy grown YIG films and their magnetization dynamics at temperatures down to 10 K. We show there is a negligible increase in the ferromagnetic resonance linewidth down to 10 K, which is unique when compared with YIG films grown by other deposition methods. From the broadband ferromagnetic resonance measurements, polarized neutron reflectivity, and scanning transmission electron microscopy, we conclude that these liquid phase epitaxy grown films have negligible rare-earth impurities present, specifically the suppression of Gd diffusion from the ${\mathrm{Gd}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$ (GGG) substrate into the ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ film, and therefore negligible magnetic losses attributed to the slow-relaxation mechanism. Overall, liquid phase epitaxy YIG films have a YIG/GGG interface that is five times sharper and have ten times lower ferromagnetic resonance linewidths below 50 K than comparable YIG films by other deposition methods. Thus, liquid phase epitaxy grown YIG films are ideal for low-temperature experiments/applications that require low magnetic losses, such as quantum transduction and manipulation via magnon coupling.

Determining the Proximity Effect-Induced Magnetic Moment in Graphene by Polarized Neutron Reflectivity and X-ray Magnetic Circular Dichroism.

We report the magnitude of the induced magnetic moment in CVD-grown epitaxial and rotated-domain graphene in proximity with a ferromagnetic Ni film, using polarized neutron reflectivity (PNR) and X-ray magnetic circular dichroism (XMCD). The XMCD spectra at the C K-edge confirm the presence of a magnetic signal in the graphene layer, and the sum rules give a magnetic moment of up to ∼0.47 μB/C atom induced in the graphene layer. For a more precise estimation, we conducted PNR measurements. The PNR results indicate an induced magnetic moment of ∼0.41 μB/C atom at 10 K for epitaxial and rotated-domain graphene. Additional PNR measurements on graphene grown on a nonmagnetic Ni9Mo1 substrate, where no magnetic moment in graphene is measured, suggest that the origin of the induced magnetic moment is due to the opening of the graphene's Dirac cone as a result of the strong C pz-Ni 3d hybridization.

Atomic‐Scale Modulation of Synthetic Magnetic Order in Oxide Superlattices (Small Methods 2/2023)

Polarized Neutron Reflectivity In article number 2201386, Choi and co-workers showed that polarized neutron reflectivity (PNR) provides a small probe for spin orientation in quantum materials. The spin vectors in each atomic layer can be resolved with PNR. Combined with atomic-scale precision epitaxy and controllable spin textures, the customized synthetic magnetic order can be visualized.

Boron migration during amorphous to crystalline transformation in CoFeB/MgO multilayers: A reflectivity study

Migration of boron in CoFeB/MgO multilayer during amorphous to crystalline transformation in CoFeB has been studied using Polarized Neutron Reflectivity (PNR) and anomalous Soft X-ray Reflectivity (SXR). PNR gives information about the possible redistribution of B in the multilayer during thermal annealing, as well as about the variation in the average magnetic moment. Sensitivity of the soft X-ray reflectivity to the movement of boron is enhanced by doing a series of measurements with X-ray energy spanning across the K absorption edge of B. Both the PNR and anomalous SXR results clearly suggest that upon thermal annealing, boron prefers to move away from the MgO/CoFeB interfaces, and forms a layer of boride in the middle of the magnetic layer. These results are important in view of the existing ambiguity regarding the role of boron in CoFeB/MgO based magnetic tunnel junctions.

Polarized neutron reflectometry study on the modulation of resistance and magnetism in resistive switching cobalt ferrite thin films

In this work, the resistive switching and electrical-control of magnetization in Pt/CoFe2O4/Nb:SrTiO3 heterostructures have been investigated. The films exhibit a classic bipolar resistive switching effect with a maximum switch ratio of about 5 × 103 and good anti-fatigue performance. Associated with resistive switching, the saturated magnetization of the thin film at high resistance state is found to be larger than that at low resistance state. Meanwhile, polarized neutron reflectivity of the thin film under different resistance states was in situ measured. The results reveal that the interfacial migration of oxygen vacancies driven by an applied electric field plays an important role in the modulation of resistive and magnetism of CoFe2O4 resistive switching devices.

Fe+ ion irradiation effects in Fe-10at%Cr films irradiated at 300 °C

Fe-Cr alloys constitute the model systems for the investigation of radiation damage effects in ferritic-martensitic steels which are candidate structural materials for fusion reactors. In the current study Fe-10at%Cr alloy films of 70 nm thickness were irradiated by 490 keV Fe + ions at 300 °C at doses ranging from 0.5 up to 20 displacements per atom (dpa). The Fe + ion energy chosen corresponds to the energy of primary Fe(Cr) knock-on atoms from 14 MeV neutrons. The irradiation effects were investigated employing X-ray diffraction and X-ray and polarized neutron reflectivity. The irradiation produced dose dependent: a) lattice constant increase, b) grain size growth and c) Cr depletion in the matrix. These changes occur largely up to 4 dpa and afterwards the system attains a dynamic equilibrium.

Magnetotransport and magnetic textures in Ho/FeCoGd/ β -W multilayers

The enhancement of interfacial Dzyaloshinskii-Moriya Interaction (DMI) in magnetic multilayers results in the stabilization of topological spin textures such as chiral domain walls and skyrmions. Here we report on the evaluation of interface-driven magnetic interactions in a uniquely designed multilayer where each magnetic layer of two antiferromagnetically coupled sublattices of $3d$ and $4f$ moments is sandwiched between the layers of \ensuremath{\beta}-tungsten and holmium whose spin Hall angles are large but opposite in sign. The atomic and magnetic periodicity of these multilayers is established by polarized neutron reflectivity measurements and the presence of a labyrinth domain spin texture of zero remanence with x-ray photoelectron microscopy. Measurements of the Hall resistivity $[{\ensuremath{\rho}}_{xy}(T,H)]$ together with static magnetization [$M$($T, H$)] over a broad range of temperature ($T$) and magnetic field ($H$) indicate impending compensation between $3d$ and $4f$ sublattices at $T>350\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. These multilayers are characterized by a small (0.04%) but positive magnetoresistance indicative of interface enhanced scattering, and a large (40 n\ensuremath{\Omega} m) and negative anomalous ${\ensuremath{\rho}}_{xy}(T,H)$ which results from a parallel alignment of $4f$ moments with the external magnetic field. No distinct scaling is seen between ${\ensuremath{\rho}}_{xy}(T,H), {\ensuremath{\rho}}_{xx}(T,H)$, and $M$($T, H$) at temperatures above 200 K where the magnetization develops out of plane anisotropy. The field scans of ${\ensuremath{\rho}}_{xy}$ at $T>200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ show a distinct cusp in the vicinity of magnetic saturation. These Hall data have been analyzed in the framework of a model where a distinct topological contribution to ${\ensuremath{\rho}}_{xy}$ rides over the anomalous Hall resistivities of the $3d$ and $4f$ magnetic sublattices. It is suggested that this apparent topological effect results from an interfacial DMI and dominates ${\ensuremath{\rho}}_{xy}(T,H)$ in the temperature regime where the $3d$ and $4f$ lattices are nearly compensated.

Thickness dependent structural and magnetic properties investigation of Co film interfaced with Hf

Thickness dependent structural and magnetic properties of Co films interfaced with Hf have been studied. X-ray reflectivity (XRR) and Grazing Incidence X-ray Diffraction (GIXRD) provides the structural properties of as-deposited samples. Film thicknesses as observed from XRR analysis are further implemented to study the magnetic properties of samples as function of Co film thickness, using magneto optical Kerr effect (MOKE) and polarized neutron reflectivity (PNR). MOKE results show the variation of magnetic coercivity with Co film thickness which exhibits a maximum value around 6 nm. Effective magnetic moment of samples as obtained from PNR fitting displays a linear variation with film thickness. Extrapolation of such variation on thickness axis show presence of magnetically dead layer (MDL) at interface, of thickness below 1 nm, in agreement with the reported values in literature for magnetic films interfaced with 5d heavy metals.

Stabilizing effects of Ag doping on structure and thermal stability of FeN thin films

In this work, we investigated the effect of Ag doping (2–20 at.%) on the phase formation of iron mononitride (FeN) thin films. Together with deposition of FeN using reactive dc magnetron sputtering, Ag was also co-sputtered at various doping levels between 2–20 at.%. We found that doping of Ag around 5 at.% is optimum to not only improve the thermal stability of FeN but also to reduce intrinsic defects that are invariably present in (even in epitaxial) FeN. Conversion electron Mössbauer spectroscopy and N K-edge x-ray near edge absorption measurements clearly reveal a reduction of defects in Ag doped FeN samples. Moreover, Fe self-diffusion measurements carried out using secondary ion mass spectroscopy depth-profiling and polarized neutron reflectivity in 57Fe enriched samples exhibit an appreciable reduction in Fe self-diffusion in Ag doped FeN samples. Ag being immiscible with Fe and non-reactive with N, occupies grain-boundary positions as nanoparticles and prohibits the fast Fe self-diffusion in FeN.

Polarized neutron reflectometry characterization of perpendicular magnetized Ho3Fe5O12 films with efficient spin-orbit torque induced switching

Rare-earth iron garnet films with perpendicular magnetic anisotropy (PMA) are important to develop magnon-based spintronic devices. In this study, high quality epitaxial Ho3Fe5O12 (HoIG) films with PMA are fabricated on (111) Y3(Sc2Ga3)O12 substrates by sputtering. The magnetization compensation temperature of HoIG films is determined to be 130 K, highly consistent with its bulk value. The magnetic dead layer at the interface is quantified as thin as about 0.6 nm through polarized neutron reflectivity. Furthermore, spin–orbit torque switching in HoIG/Pt films is realized with the threshold current density of 1.4 × 1011 A/m2 and assisting field as small as 10 Oe. These results provide one more alternative for spintronic materials using compensated ferrimagnetic insulators.

Thermal stability of interfacial mixed layers in c-Ni/a-Zr multilayer during annealing: Structural and magnetic properties

Annealing of crystalline multilayers composed of two miscible elements usually causes interfacial mixing of the constituent atoms, possibly leading to the formation of binary alloys at the interfaces. Magnetron sputtered c-Ni/a-Zr multilayers deposited at room temperature were vacuum annealed isochronally at 200 °C and 400 °C to observe thermal stability of the interfaces constituting the crystalline/amorphous multilayers. Control over the interfacial behaviour can aid the formation of materials with novel properties. The resultant changes in structural and magnetic properties of the multilayers were investigated in detail through X-ray Reflectivity, Polarized Neutron Reflectivity, SIMS and SQUID-based magnetization measurement techniques. The compositional variations in the amorphous mixed layers formed at the Ni-on-Zr and Zr-on-Ni interfaces due to atomic transport, were carefully observed as a function of annealing. Interface widths proceeded to increase with annealing at the expense of the Ni and Zr layers. The Ni-on-Zr interface was seen to be unstable at both temperatures; but the overall response of the interfaces to atomic diffusion was more pronounced when the multilayers were annealed at 400 °C. Under conditions of supersaturation of atoms within restricted spaces of the interfacial layers and the limited availability of components within the multilayer, unexpected demixing effects were observed at both the interfaces. A large increase in magnetic moment obtained after annealing at 400 °C, was attributed to the high densification of the Ni layers as well as to the incorporation of Ni crystallites into the Zr-on-Ni interface layers.

Interface-driven static and dynamic magnetic properties of ultrathin Fe/Ge multilayers

Ferromagnet-semiconductor (FM-SC) alloy phase with room temperature ferromagnetic properties at the interfaces of FM/SC heterostructures are believed to improve the electron spin injection from the FM to the SC for the realization of spintronic devices. Here we report the formation of a crystalline alloy phase (Fe3Ge) at the interfaces on growing ultrathin Fe/Ge multilayers at an elevated substrate temperature of ∼ 523 K as well as on annealing the multilayer at a temperature > 523 K. The alloy phase formation at interfaces is achieved at a much lower temperature than the Fe3Ge phase formation in bulk material (∼973 K), mainly due to rapid interdiffusion of Fe and Ge at the interfaces which is about 4 (∼2) order higher in magnitude than the diffusion of Ge (Fe) in the bulk crystalline phase. We find a large magnetic anisotropy constant (K ∼ 3 × 106 to 7 × 107 erg/cm3) on the formation of the nanocrystalline alloy phase at interfaces. Ultrathin Fe/Ge multilayers were characterized using x-ray scattering, macroscopic magnetization measurement, and polarized neutron reflectivity (PNR). The temperature-induced diffusion process indicates that the alloy phase is structurally stable up to higher temperature. PNR provided a detailed depth profile of the magnetization in the system and suggested that the alloy phase is ferromagnetic at room temperature. We have also studied the dynamic magnetic properties of these systems by employing ferromagnetic resonance measurements.

Tuneable exchange-spring stiffness in amorphous magnetic trilayer structures

We investigate the magnetic properties of amorphous Sm10Co90/Co60(Al70Zr30)40/Co85(Al70Zr30)15 exchange-spring magnet trilayers. The magnetically soft Co85(Al70Zr30)15 layer is coupled to the magnetically hard Sm10Co90 layer through the weakly magnetic low-T c Co60(Al70Zr30)40 spacer layer. The strength of the coupling can be controlled with temperature and the coupling persists above the intrinsic T c of the spacer layer due to a long-range magnetic proximity effect. Polarized neutron reflectivity is used to examine the magnetic profile of the trilayers during magnetization reversal. A two-step switching occurs, with the switching angle of the soft layer strongly dependent on the strength of the coupling. In the strong coupling regime a magnetic state can be achieved where the soft layer magnetization is perpendicular to the hard layer whereas in the weak coupling regime the soft layer reverses fully.