Polarized Neutron Reflectometry Data Research Articles

Elucidating proximity magnetism through polarized neutron reflectometry and machine learning

Polarized neutron reflectometry is a powerful technique to interrogate the structures of multilayered magnetic materials with depth sensitivity and nanometer resolution. However, reflectometry profiles often inhabit a complicated objective function landscape using traditional fitting methods, posing a significant challenge for parameter retrieval. In this work, we develop a data-driven framework to recover the sample parameters from polarized neutron reflectometry data with minimal user intervention. We train a variational autoencoder to map reflectometry profiles with moderate experimental noise to an interpretable, low-dimensional space from which sample parameters can be extracted with high resolution. We apply our method to recover the scattering length density profiles of the topological insulator–ferromagnetic insulator heterostructure Bi2Se3/EuS exhibiting proximity magnetism in good agreement with the results of conventional fitting. We further analyze a more challenging reflectometry profile of the topological insulator–antiferromagnet heterostructure (Bi,Sb)2Te3/Cr2O3 and identify possible interfacial proximity magnetism in this material. We anticipate that the framework developed here can be applied to resolve hidden interfacial phenomena in a broad range of layered systems.

Polarized neutron reflectometry characterization of interfacial magnetism in an FePt/FeRh exchange spring

We report on the depth-sensitive, temperature-dependent exchange coupling in an FePt/FeRh thin-film exchange-spring structure. The depth-dependent in-plane magnetization is measured as a function of applied magnetic field and sample temperature using polarized neutron reflectometry (PNR). The magnetization profiles are interpreted in terms of the competition between anisotropy, exchange coupling, and dipolar coupling as the FeRh undergoes the magnetic phase transition from antiferromagnetic to ferromagnetic ordering. The PNR data are combined with bulk magnetometry and x-ray characterization, allowing us to determine characteristic length scales over which the exchange-spring mechanism is effective at ambient and elevated temperatures.

Transition in a Magnetic Non-Collinear Spin-Flop State in a Fe/Pd/Gd/Pd Superlattice

The influence of a Pd spacer on the magnetic properties of Gd/Fe heterostructures has been studied. We have prepared using ultrahigh vacuum sputtering multilayered [Fe(3.5 nm)/Pd(1.2 nm)/Gd(5.0 nm)/Pd(1.2 nm)]12 structure. By using Polarized Neutron Reflectometry at temperature of 10 K we have observed strong increase in the intensity of neutron spin-flip scattering above magnetic field of 1.5 kOe. Combined analysis of magnetometry and polarized neutron reflectometry data allowed us to find out that the observed increase in spin-flip scattering is related to the spin-flop transition. Although the spin-flop transition were already seen in Fe/Gd systems, the use of Pd in our case allowed the suppression of the magnetic field of the transition making thus possible the use of Fe/Pd/Gd systems for diverse spintronic applications.

Unveiling structural, chemical and magnetic interfacial peculiarities in \u03b5-Fe2O3/GaN (0001) epitaxial films

The metastable ε-Fe2O3 is known to be the most intriguing ferrimagnetic and multiferroic iron oxide phase exhibiting a bunch of exciting physical properties both below and above room temperature. The present paper unveils the structural and magnetic peculiarities of a few nm thick interface layer discovered in these films by a number of techniques. The polarized neutron reflectometry data suggests that the interface layer resembles GaFeO3 in composition and density and is magnetically softer than the rest of the ε-Fe2O3 film. While the in-depth density variation is in agreement with the transmission electron microscopy measurements, the layer-resolved magnetization profiles are qualitatively consistent with the unusual wasp-waist magnetization curves observed by superconducting quantum interference device magnetometry. Interestingly a noticeable Ga diffusion into the ε-Fe2O3 films has been detected by secondary ion mass spectroscopy providing a clue to the mechanisms guiding the nucleation of exotic metastable epsilon ferrite phase on GaN at high growth temperature and influencing the interfacial properties of the studied films.

Self-Assembled Layering of Magnetic Nanoparticles in a Ferrofluid on Silicon Surfaces.

This article describes the three-dimensional self-assembly of monodisperse colloidal magnetite nanoparticles (NPs) from a dilute water-based ferrofluid onto a silicon surface and the dependence of the resultant magnetic structure on the applied field. The NPs assemble into close-packed layers on the surface followed by more loosely packed ones. The magnetic field-dependent magnetization of the individual NP layers depends on both the rotational freedom of the layer and the magnetization of the adjacent layers. For layers in which the NPs are more free to rotate, the easy axis of the NP can readily orient along the field direction. In more dense packing, free rotation of the NPs is hampered, and the NP ensembles likely build up quasi-domain states to minimize energy, which leads to lower magnetization in those layers. Detailed analysis of polarized neutron reflectometry data together with model calculations of the arrangement of the NPs within the layers and input from small-angle scattering measurements provide full characterization of the core/shell NP dimensions, degree of chaining, arrangement of the NPs within the different layers, and magnetization depth profile.

Spin Solid versus Magnetic Charge Ordered State in Artificial Honeycomb Lattice of Connected Elements.

The nature of magnetic correlation at low temperature in two‐dimensional artificial magnetic honeycomb lattice is a strongly debated issue. While theoretical researches suggest that the system will develop a novel zero entropy spin solid state as T → 0 K, a confirmation to this effect in artificial honeycomb lattice of connected elements is lacking. This study reports on the investigation of magnetic correlation in newly designed artificial permalloy honeycomb lattice of ultrasmall elements, with a typical length of ≈12 nm, using neutron scattering measurements and temperature‐dependent micromagnetic simulations. Numerical modeling of the polarized neutron reflectometry data elucidates the temperature‐dependent evolution of spin correlation in this system. As temperature reduces to ≈7 K, the system tends to develop novel spin solid state, manifested by the alternating distribution of magnetic vortex loops of opposite chiralities. Experimental results are complemented by temperature‐dependent micromagnetic simulations that confirm the dominance of spin solid state over local magnetic charge ordered state in the artificial honeycomb lattice with connected elements. These results enable a direct investigation of novel spin solid correlation in the connected honeycomb geometry of 2D artificial structure.

Ferromagnetic State of LaMnO<sub>3</sub> Interlayer in La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub>/LaMnO<sub>3</sub>/SrRuO<sub>3</sub> Heterostructures

Magnetic state of epitaxial heterostructures La0.7Sr0.3MnO3/LaMnO3/SrRuO3 (LSMO/LMO/SRO) was studied by SQUID magnetometer, ferromagnetic resonance (FMR) and polarized neutron reflectometry (PNR) techniques. The fit of PNR and FMR data give us the magnetization of each layer, while the SQUID gives the total magnetization of our structures. The magnetic moment of the LMO layer has the magnetization 4πMLMO=4.2kGs (2.4 mB/Mn) at T=140K according to our PNR data fitting.

Discrete helicoidal states in chiral magnetic thin films

Magnetoresistance (MR), polarized neutron reflectometry (PNR), and magnetometry measurements in MnSi thin films and rigorous analytical solutions of the micromagnetic equations show that the field-induced unwinding of confined helicoids occurs via discrete steps. A comparison between the theoretical results and the PNR and magnetometry data shows that finite-size effects confine the wavelength and lead to a quantization of the number of turns in the helicoid. We demonstrate that the magnetic state of these finite helicoids can be read by electrical means.

Depth profile of the ferromagnetic order in a YBa2Cu3O7/La2/3Ca1/3MnO3superlattice on a LSAT substrate: A polarized neutron reflectometry study

Using polarized neutron reflectometry (PNR) we have investigated a YBa2Cu3O7(10nm)/La2/3Ca1/3MnO3(9nm)]10 (YBCO/LCMO) superlattice grown by pulsed laser deposition on a La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) substrate. Due to the high structural quality of the superlattice and the substrate, the specular reflectivity signal extends with a high signal-to-background ratio beyond the fourth order superlattice Bragg peak. This allows us to obtain more detailed and reliable information about the magnetic depth profile than in previous PNR studies on similar superlattices that were partially impeded by problems related to the low temperature structural transitions of the SrTiO3 substrates. In agreement with the previous reports, our PNR data reveal a strong magnetic proximity effect showing that the depth profile of the magnetic potential differs significantly from the one of the nuclear potential that is given by the YBCO and LCMO layer thickness. We present fits of the PNR data using different simple block-like models for which either a ferromagnetic moment is induced on the YBCO side of the interfaces or the ferromagnetic order is suppressed on the LCMO side. We show that a good agreement with the PNR data and with the average magnetization as obtained from dc magnetization data can only be obtained with the latter model where a so-called depleted layer with a strongly suppressed ferromagnetic moment develops on the LCMO side of the interfaces. The models with an induced ferromagnetic moment on the YBCO side fail to reproduce the details of the higher order superlattice Bragg peaks and yield a wrong magnitude of the average magnetization. We also show that the PNR data are still consistent with the small, ferromagnetic Cu moment of 0.25muB that was previously identified with x-ray magnetic circular dichroism and x-ray resonant magnetic reflectometry measurements on the same superlattice.

Growth kinetics of intermetallic alloy phase at the interfaces of a Ni/Al multilayer using polarized neutron and x-ray reflectometry

Al-Ni-based binary alloys offer several intermetallic phases of immense technological importance. We have attempted to understand the growth of interface alloy in an Al-Ni multilayer sample as a function of annealing time using primarily x-ray reflectometry (XRR) and polarized neutron reflectometry (PNR). Powder x-ray diffraction was also used to determine various crystallographic phases in the sample. The multilayer showed remarkable stability with respect to annealing time, following an initial alloy formation at the interface. Stability of such multilayers is important for their applicability as corrosion resistant coatings as well as metallization layers in microelectronic devices. Using XRR and PNR data we have identified the interface layer as ${\text{Al}}_{3}\text{Ni}$ intermetallic phase. Magnetic depth profile obtained from PNR shows that the interface alloy layer is magnetically dead. From the Bragg peak intensities of polarized neutron reflectivity measurements, we have estimated the diffusion lengths after annealing at $160\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ for 1--8 h.

Correlation between microstructure and magnetotransport in organic semiconductor spin-valve structures

We have studied magnetotransport in organic-inorganic hybrid multilayer junctions. In these devices, the organic semiconductor tris(8-hydroxyquinoline) aluminum $({\text{Alq}}_{3})$ formed a spacer layer between ferromagnetic (FM) Co and Fe layers. The thickness of the ${\text{Alq}}_{3}$ layer was in the range of 50--150 nm. Positive magnetoresistance (MR) was observed at 4.2 K in a current perpendicular to plane geometry, and this effect persisted up to room temperature. The devices' microstructure was studied by x-ray reflectometry, Auger electron spectroscopy, and polarized neutron reflectometry (PNR). The films show well-defined layers with modest average chemical roughness (3--5 nm) at the interface between the ${\text{Alq}}_{3}$ and the surrounding FM layers. Reflectometry shows that larger MR effects are associated with smaller $\text{FM}/{\text{Alq}}_{3}$ interface width (both chemical and magnetic) and a magnetically dead layer at the ${\text{Alq}}_{3}/\text{Fe}$ interface. The PNR data also show that the Co layer, which was deposited on top of the ${\text{Alq}}_{3}$, adopts a multidomain magnetic structure at low field and a perfect antiparallel state is not obtained. The origins of the observed MR are discussed and attributed to spin-coherent transport. A lower bound for the spin-diffusion length in ${\text{Alq}}_{3}$ was estimated as $43\ifmmode\pm\else\textpm\fi{}5\text{ }\text{nm}$ at 80 K. However, the subtle correlations between microstructure and magnetotransport indicate the importance of interfacial effects in these systems.

Magnetic depth profiling of Fe/Au multilayer using neutron reflectometry

We present unpolarized and polarized neutron reflectometry data on Fe/Au multilayer sample for characterizing the layer structure and magnetic moment density profile. Fe/Au multilayer shows strong spin-dependent scattering at interfaces, making it a prospective GMR material. Fe/Au multilayer with bilayer thickness of 130 A was grown on Si substrate by RF magnetron sputtering technique. Unpolarized neutron reflectivity measurement yields nuclear scattering length density profile. The magnetic scattering length density profile has been obtained from polarized neutron reflectivity measurements.

Studies of Interlayer Magnetic Coupling in All-Semiconductor Superlattices by Means of Neutron Scattering Techniques

An overview of neutron scattering studies of ferromagnetic and antiferromagnetic all-semiconductor superlattices is presented. Diffraction experiments on MnTe/CdTe, MnTe/ZnTe and EuTe/PbTe superlattices show pronounced correlations between the MnTe and EuTe layers across the non-magnetic spacers, even though these layers are antiferromagnetic and the systems are nearly-insulating. Current theory status of these systems is discussed. Diffractometry and reflectometry data from EuS/PbS superlattices reveal pronounced antiferromagnetic coupling between the ferromagnetic EuS block. First polarized neutron reflectometry data from superlattices prepared of a novel ferromagnetic spintronics material, Ga(Mn)As, are also presented.

Magnetic interlayer exchange coupling in epitaxialFe/Si/Fe(001)studied by polarized neutron reflectometry

Polarized neutron reflectometry (PNR) has been used to investigate the magnetic interlayer coupling in a MBE-grown Fe/Si/Fe(001) sandwich at room temperature and at 10 K. Both the magnitude and orientation of the magnetic moments of the Fe layers are obtained from a rigorous analysis of the PNR data. Orthogonal configurations of the Fe magnetizations were observed, providing unambiguous evidence for the presence of a biquadratic term in the exchange coupling energy. The competition between the bilinear and biquadratic exchange couplings results in distinct orthogonal and antiparallel configurations of the Fe magnetizations at room temperature. A previously unresolved magnetic configuration in the room-temperature hysteresis curve was identified by the PNR measurements as a $180\ifmmode^\circ\else\textdegree\fi{}$ spin-flop transition. The dominant role of the biquadratic coupling at low temperatures is evident from the orthogonal configuration of the magnetizations at remanence in the measurements at $T=10\mathrm{K}.$ The magnetic configurations deduced by PNR are in good agreement with those obtained by fitting the magnetic hysteresis loops using a global energy minimum calculation.

(Anti-)ferromagnetic coupling in Fe/Si multilayers from polarized neutron reflectometry

Polarized neutron reflectometry data on Fe/Si multilayers are interpreted using strongly depth-dependent magnetization in the Fe layers. This behaviour is ascribed to a depth-dependent mixture of ferromagnetic and anti-ferromagnetic coupled regions in the sample.

Interlayer coupling in Fe/Si multilayers studied by polarized neutron reflectometry

Results of a room temperature polarized neutron reflectometry (PNR) study of the depth-dependent magnetization in (A)FM-coupled Fe/Si multilayers (MLs) are presented. Simultaneous interpretation of PNR data measured at increasing field values, keeping the nuclear structure of the MLs equal, shows strongly depth-dependent magnetization profiles. At low fields the Fe layers at the substrate side turn out to be predominantly ferromagnetically (FM) coupled, while an antiferromagnetic (AFM) alignment of adjacent Fe layers is observed towards the top of the MLs. When the field is increased the region of FM (AFM) alignment grows (diminishes) by rotation of the magnetization in the individual Fe layers. The overall AFM alignment is more perfect for MLs with 1.4 nm thick Si layers than for 1.1 nm thick Si layers. These observations are in excellent agreement with magneto-optical Kerr effect and vibrating sample magnetometry (VSM) investigations [1] and can be explained by growth-induced pinholes, formed predominantly in the beginning of the sputtering process.

Magnetic alignment in spin-valve system studied by polarized neutron reflectometry

Exchange biased spin-valve systems with the nominal structure: 5 × [Ta (3.5 nm)/Ni 80Fe 20 (8 nm)/Cu ( t)/Ni 80Fe 20 (6 nm)/Fe 50Mn 50 (10 nm)/Ta (3.5 nm)], for t = 1.0 and 1.5 nm, were dc magnetron sputtered at 5 mTorr on Si(100). Similar systems with thicker Cu layers, are of interest for magnetoresistive read heads. The maximum in magnetoresistance is attributed to antiparallel alignment of the magnetization of the two Ni 80Fe 20 layers separated by the Cu layers. The magnetic alignment in this system as a function of field is investigated by Polarized Neutron Reflectometry (PNR), as this technique is sensitive to the magnetization as a function of depth. Rijks et al. [1] calculated the optimal orientation of the magnetization in both NiFe layers as a function of applied field. Models fitted to PNR data measured at various magnetic fields support these calculations.

Unidirectionally biased Permalloy: A polarized-neutron-reflection experiment.

The depth profile of the magnetization of an exchange-coupled Permalloy-${\mathrm{Fe}}_{50}$${\mathrm{Mn}}_{50}$ bilayer is determined nondestructively from polarized-neutron reflectometry. The interfacial exchange coupling between the ferromagnetic Permalloy layer and the antiferromagnetic ${\mathrm{Fe}}_{50}$${\mathrm{Mn}}_{50}$ layer gives rise to an unusual unidirectional anisotropy of the Permalloy layer, whose magnitude is much smaller than would be expected from the bulk exchange constants of these materials. A variety of models have been proposed to account for this result, some of which require inhomogeneous magnetization distributions within the Permalloy layer or at the Permalloy-${\mathrm{Fe}}_{50}$${\mathrm{Mn}}_{50}$ interface. These experiments reveal that, for a structure of the form Si(111)/${\mathrm{Ni}}_{81}$${\mathrm{Fe}}_{19}$(400 \AA{})/${\mathrm{Fe}}_{50}$${\mathrm{Mn}}_{50}$(400 \AA{})/Ta(300 \AA{}), the depth profile of the magnetization is uniform within experimental error. In particular, for fields sufficiently large to magnetize the Permalloy layer at one or other extreme of the hysteresis loop, the layer-by-layer profile of the magnetization is identical, ruling out the possibility of planar domain-wall formation in the Permalloy layer. The hysteresis loop is derived from polarized-neutron reflectometry data as a function of in-plane magnetic field and is the same as that found using conventional magnetometry. The actual layer thicknesses of the structure were separately determined by neutron reflectometry for the structure magnetized with the magnetic moment perpendicular to the neutron-scattering plane.