United States

Exchange springs are versatile systems in which domain walls can be tailored by the different magnetic properties of exchange coupled layers such as in ferrimagnetic GdCo/GdCo’ multilayers [1]. We present here an experimental study by soft X-ray magnetic vector tomography [2] of Gd&lt;sub&gt;12&lt;/sub&gt;Co&lt;sub&gt;88&lt;/sub&gt;/Nd&lt;sub&gt;17&lt;/sub&gt;Co&lt;sub&gt;83&lt;/sub&gt;/Gd&lt;sub&gt;24&lt;/sub&gt;Co&lt;sub&gt;76&lt;/sub&gt; multilayers with weak stripe domains (induced by NdCo) and in which the competition between exchange and magnetostatic coupling can be tuned by the composition of the ferrimagnetic GdCo layers (as sketched in Fig. 1) [3]. The 3D analysis of the vector magnetic configuration of the Gd sublattice m(Gd) in a demagnetized state shows a complex configuration that includes stripe domains with a distorted closure domain structure across the thickness, exchange spring walls with out-of-plane magnetization reversal and modulated ripple patterns (Fig. 2). An analytical model, proposed to describe global coupling across the exchange spring boundary between NdCo and GdCo layers, shows that the strength of effective interlayer coupling can be finely tuned playing with the averaging of lateral and in-depth competing interactions so that it can be externally adjusted by magnetic history.&lt;br/&gt;&lt;br/&gt;Work supported by Spanish MICINN under grant PID2019-104604RB/AEI/10.13039/501100011033.

![](https://assets.underline.io/uploads/markdown_image/1/image/6887055ecf6e425f078f1db90e0ff8e4.jpg)

Fig. 1 Sketch of GdCo/NdCo/GdCo&#39; ferrimagnetic multilayer with competing interactions. Exchange at the interface favors parallel alignment of M(Co) in all the layers. However, at top layer Gd24Co76 layer, net magnetization is aligned with M(Gd) and antiparallel with M(Co). Magnetostatic interactions, represented by an effective stray field εHd, favor an antiparallel alignment of M(Co) at both sides of the interface. These competing interactions create an exchange spring near the top of the multilayer.

![](https://assets.underline.io/uploads/markdown_image/1/image/ee173d4a2dd2f9bc12ca1560245e6d9e.jpg)

Fig 2. Tomographic reconstruction of m(Gd) (my contrast): At top Gd24Co76, there is a ripple pattern around easy anisotropy axis; At bottom Gd12Co88, there is a distorted closure domain structure. Across the thickness, closure vortex cores are arranged in a zig-zag. Sketch shows the exchange spring between stripe domains at NdCo/GdCo layers and in-plane domain with ripple at top Gd24Co76 layer.

2022 Joint MMM-INTERMAG

Adjustable exchange springs in ferrimagnetic multilayers with stripe domain patterns visualized by X-ray vector tomography

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

For tickets to the event, please register at the following link: https://magnetism.org/register

Please register for the Joint MMM-Intermag conference to join the event.

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

Exchange springs are versatile systems in which domain walls can be tailored by the different magnetic properties of exchange coupled layers such as in ferrimagnetic GdCo/GdCo’ multilayers [1]. We present here an experimental study by soft X-ray magnetic vector tomography [2] of Gd12Co88/Nd17Co83/Gd24Co76 multilayers with weak stripe domains (induced by NdCo) and in which the competition between exchange and magnetostatic coupling can be tuned by the composition of the ferrimagnetic GdCo layers (as sketched in Fig. 1) [3]. The 3D analysis of the vector magnetic configuration of the Gd sublattice m(Gd) in a demagnetized state shows a complex configuration that includes stripe domains with a distorted closure domain structure across the thickness, exchange spring walls with out-of-plane magnetization reversal and modulated ripple patterns (Fig. 2). An analytical model, proposed to describe global coupling across the exchange spring boundary between NdCo and GdCo layers, shows that the strength of effective interlayer coupling can be finely tuned playing with the averaging of lateral and in-depth competing interactions so that it can be externally adjusted by magnetic history. Work supported by Spanish MICINN under grant PID2019-104604RB/AEI/10.13039/501100011033.

![](https://assets.underline.io/uploads/markdown_image/1/image/6887055ecf6e425f078f1db90e0ff8e4.jpg)

Fig. 1 Sketch of GdCo/NdCo/GdCo' ferrimagnetic multilayer with competing interactions. Exchange at the interface favors parallel alignment of M(Co) in all the layers. However, at top layer Gd24Co76 layer, net magnetization is aligned with M(Gd) and antiparallel with M(Co). Magnetostatic interactions, represented by an effective stray field εHd, favor an antiparallel alignment of M(Co) at both sides of the interface. These competing interactions create an exchange spring near the top of the multilayer.

![](https://assets.underline.io/uploads/markdown_image/1/image/ee173d4a2dd2f9bc12ca1560245e6d9e.jpg)

Fig 2. Tomographic reconstruction of m(Gd) (my contrast): At top Gd24Co76, there is a ripple pattern around easy anisotropy axis; At bottom Gd12Co88, there is a distorted closure domain structure. Across the thickness, closure vortex cores are arranged in a zig-zag. Sketch shows the exchange spring between stripe domains at NdCo/GdCo layers and in-plane domain with ripple at top Gd24Co76 layer.

technical paper

Cobalt ferrite nanoparticles (NP) are competitive systems in technological and biomedical applications due to a suitable combination of excellent chemical and magnetic characteristics, such as a high chemical stability, surface active sites, and ease of synthesis and functionalization, together with a high anisotropy constant, a high coercivity, and a moderate saturation magnetization. However, in order to control the functional response of Co-ferrite NP, one of the biggest challenges is to quantitatively disentangle the dependence of the overall magnetic response on the composition, structure, or surface chemistry. In this work, we address the key role of the structural defects on the magnetic properties of Co-ferrite NP investigated by element- and site-specific X-ray magnetic circular dichroism (XMCD) combined with high-resolution transmission electron microscopy of individual NP [1]. We studied a series of samples of monodisperse 8 nm NP, prepared by chemical routes [2,3], with similar stoichiometry and cation distribution but with a tunable number of structural defects. Element- specific XAS and XMCD spectra and hysteresis loops reveal that the collinear alignment of Co2+ (Oh) cations is much more affected by the structural disorder than that of the Fe3+ cations. This is because structural defects give rise to effective local anisotropy axes, causing a prevalent Co2+ spin canting through the spin−orbit coupling due to the relatively large value of the partially unquenched moment of these cations. As the structural disorder is further increased, the rest of the cations in the two sublattices are progressively dragged off the ferromagnetic alignment, being Fe3+(Td) cations the least sensitive to defects due to the smaller number of next-nearest neighbors in the Td-sublattice. Our work highlights the importance of combining advanced synthesis with the use of complementary local probes with element, valence, site, and structural sensitivity to enhance the performance of nanoscale magnetic materials.

![](https://assets.underline.io/uploads/markdown_image/1/image/a5aa5eb0e8a24261183ee2e55ee59892.jpg)

HRTEM images for Co-ferriteNP with increasing crystalline quality. Bottom panel: XMCD hysteresis loops measured at 2 K for Co2+ cations in Oh sites for highly-defective (left) and highly-crystalline (right) NP.

Disentangling Atom- and Site- Selective Spin Canting in Co-ferrite Nanoparticles with Tunable Structural Disorder

Developments in the design and fabrication of the high saturation magnetization nanoparticles in the single-domain size regime are highly desirable due to their extensive applications in biomedicine, magnetic resonance imaging, microelectromechanical systems, radio frequency electronics, and soft robotics. In this work, we present for the first time an in-depth investigation of different-sized iron cobalt (Fe65Co35) nanoparticles covering magnetization configurations from flower to vortex to multi-domain states. A finite element micromagnetic model was used to predict magnetic properties of Fe65Co35 nanoparticles ranging in size from 10 to 100 nm. The simulation results reveal that the flower-to-vortex transition of the Fe65Co35 nanoparticles occurs at around 20 nm, corresponding to the particle size with the highest coercivity. A decrease in remanence is expected for larger particle sizes, where simulations showed the most dramatic drop for particles between 40 and 60 nm. Furthermore, the simulated behaviors were verified experimentally. By systematically varying the reaction parameters, Fe65Co35 nanocubes with tunable edge lengths were synthesized. Scanning electron microscope (SEM) images of as-synthesized samples indicate a narrow size distribution. The measured hysteresis loops confirmed the maximum coercive field for cubes of size 17 nm and the sudden decrease in remanence for samples within the range of 40–60 nm. By combining chemical synthesis and micromagnetic calculations, we have experimentally unveiled the size-dependent spin configurations in Fe65Co35 nanoparticles. These results open the door to a more accurate design and control of spin texture, remanence, and coercivity in high magnetization nanoparticles which are vital building blocks of new biomedical and electronic materials.

![](https://assets.underline.io/uploads/markdown_image/1/image/53e8432506b4365fa494f1cd89ca89bd.jpg)

Fig. 1 SEM images of Fe65Co35 nanocubes with various edge lengths, (a) 17 nm, (b) 34 nm, (c) 40 nm, (d) 62 nm, (e) 77 nm, (f) 92 nm, and (g) 97 nm. The scale bars are 200 nm.

![](https://assets.underline.io/uploads/markdown_image/1/image/fe1f235133cb8a4f69fbfa4a62a75798.jpg)

Fig. 2 (a) The simulated spin configuration for a cube of each respective size. (b) Simulated hysteresis loops of a 5×5×5 sphere array with different sizes. (c) Measured hysteresis loops of as-synthesized Fe65Co35 nanocubes at 300 K.

Exploring the Single Domain Limit of High Magnetization Nanoparticles: A Combined Theoretical and Experimental Study

Anthrax is a fatal disease caused by bacteria called bacillus anthracis. The bacteria can be used as a bioterror agent due to its high toxicity and resistance to the external environment of its spore. Dipicolinic acid (DPA) is released when the spore germinates; therefore, sensitively detecting DPA as a biomarker of the spores is focused on by many researchers [1]. Among them, nano-sensors including trivalent lanthanide ions have been used to detect DPA sensitively and promptly because the trivalent lanthanide ion such as europium (Eu3+) and terbium (Tb3+) shows unique fluorescent properties originated from their energy levels, and the intensity can be enhanced by chelating with DPA called antenna effect [2]. However, nanomaterials such as polymers or inorganic nanoparticles containing fluorescent substances usually quench the fluorescence by absorbing the excitation and emission light concurrently [3]. Therefore it is necessary to separate the fluorescent signal from the nanomaterials to improve its sensitivity. Herein, we develop terbium ion-coordinated iron oxide (Fe3O4-Tb) nanoparticles and detect DPA via measuring fluorescent signals from the Tb-DPA complexes [4]. The Fe3O4-Tb nanoparticles have a magnetite (Fe3O4) core and the surface is enveloped by Tb3+ that is coordinated with polyacrylate. Because of the small crystallite size of the Fe3O4-Tb nanoparticle, it has a trivial coercivity; therefore, Fe3O4-Tb nanoparticles do not agglomerate each other and reacts DPA with high reproducibility [5]. When the Fe3O4-Tb nanoparticles contact with DPA, Tb3+ is disassociated from the nanoparticle surface and chelated with DPA. The formed Tb-DPA complexes have a green fluorescence under certain ultraviolet (UV) wavelengths. We separate the iron oxide core via a permanent magnet to prevent quenching of the nanoparticles, then the clear signal shows a lower detection limit (5.1 nM) than the infectious dosage of anthrax (60 μM) [6]. The detecting method also has a high selectivity when compared with other aromatic carboxylic acids, which have a molecular structure similar to DPA.

![](https://assets.underline.io/uploads/markdown_image/1/image/14a2f16e4463e6542107f235846a8f38.jpg)

Fig.1. Detection mechanism for DPA with Fe3O4-Tb nanoparticles and the fluorescent intensity dependent on the concentration of DPA.

Fluorescent detection of dipicolinic acid as a biomarker for anthrax using Tb ion-coordinated Fe3O4 nanoparticles

Ensemble magnetization of magnetic nanoparticles (NPs) has a sensitive dependence on the structural arrangement of particles. To better understand the correlation between spatial positions of NPs and ensemble magnetization, an experimental system is needed where particle position can be adjusted while simultaneously probing nanoscale magnetization. Here, we describe experiments using polarization analyzed small angle neutron scattering (PASANS) to study the change in magnetic correlations of ~7 nm iron oxide (Fe3O4) NPs embedded in stretched silicone rubber. PASANS is a powerful tool for mapping both the inter- and intra- particle magnetization of NPs on length scales ranging from 0.1 – 100 nm. The magnetic NP polymer gel, or magnetogel, can be stretched on the neutron beamline during an experiment. The macroscopic stretching is accompanied by nanoscale deformations, where polymer chains move to accommodate a new equilibrium configuration. NPs embedded in the gel change their configuration as a function of applied stress, providing a method to adjust the average center-to-center spacing between particles. The scattering data from the unstretched gel in a 1 T applied magnetic field can be modelled by 7.4 nm iron oxide NPs assembled in a hard sphere packing with 30% volume fraction. When the sample is stretched 3x its original size in a direction parallel to the applied field, the morphology changes dramatically. Specifically, the data can be described using a cluster model where NP dimers have emerged (Figures 1a and 1b). Stretching causes the particles to morph from semi-close packed, but individual, 7.4 nm diameter NPs into dimer-like clusters that are 15 nm long, as viewed perpendicular to the applied field (Figures 2a and 2b). These PASANS experiments clearly demonstrate that stretching the magnetogel modulates the structural characteristics of magnetic NPs in a pronounced, controllable manner.

![](https://assets.underline.io/uploads/markdown_image/1/image/39e948b29d4a9faea0cd8a27867ee2bb.jpg)

Fig. 1: Magnetic scattering from (a) unstretched and (b) 3x stretched gel in a 1 T field. The data set in (a) is fit to a hard sphere-like packing and (b) is fit to a model containing dimers

![](https://assets.underline.io/uploads/markdown_image/1/image/5b350fa63e4727cecd3c135d0fb252f5.jpg)

Fig. 2: Schematic showing rearrangement of NPs in (a) unstretched and (b) 3x stretched configurations.

Magnetic Correlations of Iron Oxide Nanoparticles as Probed by PASANS in Stretched Magnetogel Composites

The utilization of magnetic torques at the nano and microscale induced via an externally applied magnetic field is of particular interest for applications such as micromixing [1] and mechanical cancer cell destruction [2]. Over the past several years, synthetic antiferromagnetic (SAF) nanoplatelets with perpendicular magnetic anisotropy (PMA) have been identified as one of the most promising vehicles due to their high tunability, easy-axis/hard-plane PMA efficiently transferring the full magnetic torque to their environment [3]. However, the fabrication of these nanoplatelets, especially in the submicron regime with a high yield is still a challenge and efficient characterization methods of the magnetic properties and mechanical behavior in a fluid environment are still lacking. In this work, we present a novel fabrication method based on substrate conformal imprint lithography (SCIL) [4] and discuss the size effect on the magnetic properties. The basic thin film stack is shown in the insert of Fig. 1. Platelets with 1.89 ± 0.02 µm dimeter (Fig. 1) and 102 ± 1 nm diameter were successfully produced with a yield of ~108 and ~ 1010 particles per 2 inch wafer. MOKE and SQUID were used to measure the hysteresis loop. A dramatically increase of the coercivity (Hc) after patterning is observed (see Fig. 2) rendering the particles ferromagnetic at zero applied field destroying their favorable properties. To define where the increase comes from, a systematic size study is conducted on PMA-SAF nanoplatelets fabricated through electron beam lithography (EBL). We find that Hc and its distribution increase with decreasing size (see Fig. 2). A model based on local anisotropy distribution and thermal activation was established to fit the data [5]. The fit can accurately describe the Hc behavior giving insight on how to engineer the particles. In this talk we will discuss the physics, our current methods to circumvent the Hc issue and give an outlook on single-platelet mechanical behavior characterization methods [6].

![](https://assets.underline.io/uploads/markdown_image/1/image/1f533b6fc3b9014d316dd90edc92ea86.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/cd16737989cc4310aba5cad6a49c1556.jpg)

Substrate Conformal Imprint Lithography Defined Synthetic Antiferromagnetic Nanoplatelets for Torque Related Applications

Substrate Conformal Imprint Lithography Defined Synthetic Antiferromagnetic Nanoplatelets for Torque Related Applications

The structural inversion symmetry plays an important role in low-dimensional nanomagnets, due to its strong influence on magnetic and electrical properties. It can lead to the appearance of chiral effects, such as the topological Hall effect [1], or to the formation of chiral noncollinear magnetic textures, as skyrmions [2] and chiral domain walls (DWs) [3]. These chiral structures are envisioned to be the key components for realizing novel concepts for magnonics [4], antiferromagnetic spintronics [5], spin-orbitronics [6], and oxitronics [7]. The main magnetic interaction being responsible for the stabilization of chiral magnetic textures is the intrinsic Dzyaloshinskii-Moriya interaction (DMI) [8,9]. It originate in certain magnetic crystals in which the unit cell lacks inversion symmetry, such as the gyrotropic magnetic crystals, or appear typically in ultrathin films or bilayers due to the inversion symmetry breaking on the film interface [3]. At present, tailoring of DMI is done by optimizing materials, either doping a bulk single crystal or adjusting interface properties of thin films and multilayers. A viable alternative to the conventional material screening approach can be the exploration of the interplay between geometry and topology. This interplay is of fundamental interest throughout many disciplines in condensed matter physics, including thin layers of superconductors [10] and superfluids [11], nematic liquid crystals [12], cell membranes [13], semiconductors [14]. In the emergent field of curvilinear magnetism chiral effects are associated to the geometrically broken inversion symmetries [15]. Those appear in curvilinear architectures of even conventional materials. There are numerous exciting theoretical predictions of exchange- and magnetostatically-driven curvature effects, which do not rely on any specific modification of the intrinsic magnetic properties, but allow to create non-collinear magnetic textures in a controlled manner by tailoring local curvatures and shapes [16,17]. Until now the predicted chiral effects due to curvatures remained a neat theoretical abstraction. Recently, we provided the very first experimental confirmation of the existence of the curvature-induced chiral interaction with exchange origin in a conventional soft ferromagnetic material. It is experimentally explored the theoretical predictions, that the magnetisation reversal of flat parabolic stripes shows a two step process [18,19]. At the first switching event, a domain wall pinned by the curvature induced exchange-driven DMI is expelled leading to a magnetisation state homogeneous along the parabola's long axis. Measuring the depinning field enables to quantify the effective exchange-driven DMI interaction constant. The magnitude of the effect can be tuned by the parabola's curvature. It is found that the strength of the exchange-induced DMI interaction for the experimentally realised geometries is remarkably strong, namely ~0.4 mJ/m2, compared the surface induced DMI. The presented study legitimates the predictive power of full-scale micromagnetic simulations to design the properties of ferromagnets through their geometry, thus stabilising chiral textures. We explore these curvilinear magnetic thin films for the realization of novel artificial magnetoelectric materials based on curvilinear helimagnets embedded in piezoelectric matrix [20], to enable the geometrical tuning of the magnetochirality in curvilinear 1D architectures [21], tailoring of magnetic states in flat nanospirals [22] and as components of shapeable magnetoelectronics for interactive wearables [23].

Local and Non-local Curvature-induced Chiral Effects in Nanomagnetism

In-depth investigation of structural and magnetic property correlation in technologically important heavy metal/ferromagnet/oxide heterostructures are crucial for the development of spintronics based devices [1]. In recent times, in Ta/CoFeB/TaOx heterostructures, zero-field magnetization switching, reasonably large Dzyaloshinskii-Moriya interaction, and stabilization of skyrmions have been reported [2-4]. Here, we study the change in magnetic and structural properties of sputter deposited (1 nm) Ta/(1.5 nm) CoFeB/(0.5 nm) TaOx heterostructures induced by post-deposition annealing at 300 °C. Both as-deposited and annealed film stacks possess in-plane magnetic anisotropy and we observe nearly two times increase in the saturation magnetization (Ms) of annealed film stack (~ 800 emu/cc) in comparison to the as-deposited one (~ 400 emu/cc). The surface topography study suggests reduced surface roughness for the annealed film stack as compared to the as-deposited stack. Using, x-ray photoelectron spectroscopy we analyze the B 1s, Ta 4f and Fe 2p spectra for both as-deposited and annealed heterostructures. Interestingly, the analysis of Ta spectra revealed that B has intermixed with Ta layers in the annealed stack, thus forming new chemical states such as TaB and TaOB (cf. Fig.1). Boron 1s peak analysis suggests that the annealing causes B diffusion out of the CoFeB layer and subsequent exposure to the atmosphere results in the formation of BOx. Analysis of Fe 2p spectra in the annealed stack shows an additional signature of metallic Fe0 2p1/2 peak, which likely plays a critical role in enhancing the Ms (cf. Fig.2). These XPS results are crucial in thorough understanding of the mechanism that governs Ms and ultimately, spin-orbit coupling induced effects in ferromagnetic thin film heterostructures.

![](https://assets.underline.io/uploads/markdown_image/1/image/a5c66219d056c0e02fc86687536ed4ca.jpg)

Fig. 1. XPS spectra of Ta 4f core levels of as-deposited and annealed films. The binding energy shift in the annealed spectra are due to formation of new chemical state such as TaB.


![](https://assets.underline.io/uploads/markdown_image/1/image/92c1c60f482d4ee564ad597e655c00b7.jpg)

Fig. 2. XPS spectra of Fe 2p core levels of as-deposited and annealed films. An additional peak Fe0 2p1/2 is observed in the annealed film stack.

Study of As-deposited and Annealed Ta/CoFeB/TaOx Heterostructures Using X-Ray Photoelectron Spectroscopy

The meta-magnetic phase transition of equiatomic FeRh allows the 3 temperature model of phase transitions to be probed as it undergoes: (i) a lattice expansion, (ii) an antiferromagnetic (AF) to ferromagnetic (FM) transition [1], and (iii) a change in electronic structure. A spin canting model has been used to describe a spin re-orientation pathway [2], where a feedback loop with induced Rh moments promotes FM order. To investigate such behaviour, a probe of the AF spins is required [3]. In general, the zero-stray field of anti-ferromagnetic materials means that it is hard to detect and perform such measurements. Here, we demonstrate a probe of long-range AF order using non-resonant x-ray scattering. The G2-type order of FeRh in the AF state provides a probe of AF order using ‘magnetic’ Bragg peaks [4]. We demonstrate that these peaks can be measured using x-rays at energies below the Fe K edge. Due to the low efficiency of the magnetic scattering in this material, we describe a grazing incidence geometry to optimise the intensity (see Fig. 1). Based on Scherrer analysis we estimate a coherence length of 40-80 nm, smaller than previous reports in XMLD experiments [5], indicating we are limited by structural defects. The temperature dependent behaviour of the AF order shows an inverse correlation to the emergence of FM moment as seen in Fig. 2. The AF-FM transition as determined by the ordering appears later indicating a mixed phase [6]. Long-range AF order appears to persist when FM moment is already established. Time resolved experiments based on this technique may determine the existence of a meta-stable AF state on time scales where the FM band structure has already formed [7]. It has been shown that the lattice expansion occurs in ≈ 30 ps [8], but it is not known how the long range AF order evolves on such timescales. X-ray diffraction can therefore be used to ascertain if how and if the AF structure is related to lattice expansion occurring at the phase transition.

![](https://assets.underline.io/uploads/markdown_image/1/image/a76ec58a6231ba98b20b00b66541e5c6.jpg)

Dependence of the (3/2,1/2,1/2) peak intensity upon grazing incidence angle (E=4.95 keV)

![](https://assets.underline.io/uploads/markdown_image/1/image/7d4ef6a766865d9d74ecbbf6cb3bfec2.jpg)

The inverse correlation between the intensity of the FeRh (1/2, 3/2, 1/2) magnetic peak, and the saturation moment of the FeRh film

X-ray investigation of long-range antiferromagnetic ordering in FeRh

Flexibility for interface engineering, and access to all-optical switching of the magnetization, make synthetic ferrimagnets (SFiMs) an interesting candidate for advanced spintronic devices like racetrack memory [1,2]. However, the absence of compensation at room temperature (RT) in Co/Gd bilayers has hampered the efficiency with which their magnetic order can be manipulated, as the highest efficiency of spin-orbit torques and current induced domain wall motion (CIDWM) are generally achieved under compensation [3,4]. In order to combine the favourable properties of SFiMs with compensation at RT, Co/Gd/Co/Gd quadlayer structures are investigated. We present a systematic study of the conditions for magnetization compensation and perpendicular magnetic anisotropy (PMA) in Co/Gd/Co/Gd quadlayers. These conditions were investigated on a sample with the middle Co and Gd films wedged orthogonally with respect to each other, as shown in Fig. 1a. By scanning the polar magneto-optical Kerr effect (MOKE), a magnetostatic phase diagram like in Fig. 1b can be obtained. Three regimes are distinguished: In-plane magnetization (teal), and out-of-plane (OOP) Co (red) and Gd (blue) dominated. The boundaries between these regimes provide valuable insight into the balance between shape anisotropy and PMA, and the conditions for compensation. A simple model based on partial intermixing at Co/Gd interfaces has been developed which explains the experiments well, and provides a tool to qualitatively and quantitatively understand these phase diagrams. This work provides a simple framework to experimentally characterise, and describe basic magnetostatic properties in these promising heterostructures. The compensated SFiMs investigated in this research, carry the promise of breaking the elusive 1000 m/s barrier for CIDWM, and exhibiting ultrafast exchange-driven dynamics, both relevant properties for future spintronic devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/97d2d2073b964b56a371f90f9d4c02dc.jpg)

Characterization of double wedge Co(0.65)/Gd(0-3)/Co(0-2)/Gd(1.5) SFiMs. a): Schematic of wedge orientations in the multilayer. b): Magnetostatic phase diagram measured with polar MOKE. Yellow dashed line indicates compensation at RT. Green lines indicate a toy model fit to the boundaries between the three regimes.

Tuning Room Temperature Compensation in Synthetic Co/Gd Ferrimagnetic Quadlayers for Advanced Spintronics Devices

Metallic ferrimagnets consisting of rare earth (RE) / transition metal (TM) alloys exhibit an intriguing magnetic compensation of the RE/TM magnetic sublattices as a function of composition and temperature. We are developing compositionally modulated RE/TM alloy films that exploit the large change in TMagComp as the RE content is varied. We grow Fe1-xGdx films where the Gd content x is modulated throughout the depth of the film and key question in such structures is the interface length between two Fe1-xGdx layers with different Gd content; this will establish the minimum modulation length of RE/TM structures with varying composition. We examine a series of bilayer structures [Si/ SiO2(3nm)/Ta(10nm)/Fe1-xGdx(50nm)/ Fe1-yGdy(50nm)/ Ta(8nm)] where the Gd content (x or y) is varied. To investigate the magnetic profile of these films, we utilize polarized neutron reflectometry (PNR) as a function of temperature. PNR is an ideal technique to examine these structures as it probes the magnetization parallel to the field as a function of depth and the magnetic scattering length density (mSLD) is modified significantly as the temperature is tuned through TMagComp of a particular layer. We examine samples where TMagComp of a particular layer is &gt; 300 K, ~100 K and ~50 K. A sample that has TMagComp &gt;300K and ~100K for the bottom and top layers respectively exhibits strong modifications to the spin asymmetry [SA: (↑↑- ↓↓)/(↑↑+↓↓)] of the PNR. The SA develops a pronounced dip at Q~0.045 Å-1 and reduced broadening of the peak at Q~0.28 Å-1 as the temperature decreases (Fig 1). Similar variations to the SA are observed in samples with different compensation temperatures. To extract the magnetic behavior of the samples while emphasizing the interfacial region, we fit the mSLD for each temperature while keeping the structural properties the same. The fitting suggests that the evolution of the magnetic interface is nontrivial.

![](https://assets.underline.io/uploads/markdown_image/1/image/44fb4812d3585f892c0995ff14de1a9a.jpg)

1-A: High field (1 T) magnetometry vs. temperature for a magnetic bilayer. PNR measurements performed at selected temperatures, denoted by stars, across the complicated temperature profile. 1-B: Spin Asymmetry for the bi-layer samples performed at the 5 temperatures from 1-a.

Adjustable exchange springs in ferrimagnetic multilayers with stripe domain patterns visualized by X-ray vector tomography

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