Investigation of irreversibility and the temperature dependence of interfacial magnetic properties of Pt/Co-based systems

Local and interfacial magnetic properties of inhomogeneous finite linear chains

Magnetic skyrmions are promising candidates for computing and memory applications. The static and dynamic behaviors of skyrmions are tunable by altering the interfacial magnetic properties. These interfacial magnetic properties are alterable by modifying the interface structure of thin films. However, the relationship between the structural properties of the interface and the skyrmions properties is not straightforward, and a comprehensive insight is required to facilitate better controllability of the skyrmions’ behaviors. Here, we comprehensively understand the relationship between atomic displacements at the interface and skyrmions’ static behavior. In this study, we used ion irradiation to achieve inter-atomic displacements. We observed that the inter-atomic displacements could tailor the physical properties of skyrmions. We noticed a peculiar increase in the magnetization, Dzyaloshinskii–Moriya interaction, and exchange stiffness. The modifications in magnetic properties reduced the domain wall energy, which enhanced the skyrmion density (by six-folds) and reduced the average skyrmion diameter (by 50%). Furthermore, we compared the observed results of ion irradiation with those from the annealing process (a well-studied method for modifying magnetic properties) to better understand the effect of atomic displacements. Our study provides a route to achieve a highly-dense skyrmion state, and it can be explored further to suppress the skyrmion Hall effect for skyrmion-based applications.

Recent work has been reviewed concerning the interphase interface between two materials which have different crystal structures and chemical compositions. We focus on the structure, and magnetic and mechanical properties of the interphase interface.

The Pt-Co-Ni multi-layer system has shown promising tunability of its interfacial magnetic properties (including Dzyaloshinskii-Moriya interaction (DMI) and magnetic anisotropy) for the development of future chiral spintronic devices. Previous work has shown that the system supports stabilization of Néel Skyrmions [1] and efficient movement of Dzyaloshinskii domain walls (DW) [2]. However, the vast majority of the work has focused on the case of a Pt/Co deposition order (from substrate side) leading to predominantly left-handed chiral Neel domain walls. Moreover, it is expected that there will be interesting properties that could result from combining left and right-handed domain walls in more complicated heterostructures [3]. Here, we leverage the Pt-Co/Ni multi-layer system to compare the role of the Pt/Co interface when on the bottom vs top of the asymmetric repeat unit.The film stacks under consideration are [Pt/(Co/Ni)M] N or [Pt/(Ni/Co)M]N and were deposited by DC magnetron sputtering in an argon plasma with pressure fixed at bar (2.5 mTorr). Film stacks were grown on oxidized Si substrates with a Ta/Pt seed layer to induce FCC (111) texture. Pt/(Co/Ni)M]N multi-layers show strong perpendicular magnetic anisotropy (figure 1a) with a pinched perpendicular MH loop characteristic for bubble materials. Lorentz Transmission Electron Microscopy (LTEM) shows contrast only in the tilted orientation confirming the presence of Néel-type Skyrmions and domain walls, which suggests a significant interfacial DMI strength. This is likely due to differences in the DMI associated with Pt/Co and Ni/Pt interfaces. For these samples, Brillouin Light Scattering (BLS) spectroscopy shows a shift in the spin wave spectra corresponding to |D|= 0.21 mJ/m2, which is sufficient to stabilize the Néel configuration.For the [Pt/(Ni/Co) M] N sample with all deposition conditions otherwise identical, there is a marked drop in perpendicular magnetic anisotropy with a less clearly defined pinch in the MH loop (Figure 2 a). Lorentz TEM shows only Bloch type domain walls with no preferred chirality (Figure 2 b). Furthermore, application of a perpendicular field stabilizes achiral Bloch bubbles along with a large number of topologically trivial bubbles (or type 2 bubbles) with two vertical Bloch lines inside. It is reasonable to expect that the reduction in magnetic anisotropy is related to the Pt/Co interface quality, which would in turn impact the DMI strength explaining the absence of Neel DWs. It has previously been proposed that the origin of the deposition order effect is due to intermixing between the Pt and Co [4]. It is possible the that higher adatom energy of Pt results in a higher degree of intermixing at the interface when deposited onto Co, which will be discussed along with possible steps to mitigate the problem experimentally.As a secondary note, although it has a relatively small DMI, this system can serve as a playground for evaluating the role of topology on the annihilation field of the magnetic bubbles. In-situ Lorentz TEM images show an appreciably larger annihilation field for the non-trivial bubbles (i.e. Bloch Skyrmions) than for the trivial bubbles. Statistics on the annihilation field will be presented in detail in this paper. **

One major challenge in engineering the magnetism of oxide heterostructures is controlling orbital reconstruction by tuning the charge transfer effect. This paper investigates the forward and backward charge transfer effect and the induced magnetic properties of BiFe1−xMnxO3/La2/3Ca1/3MnO3 heterostructures. Interfacial ferromagnetism is found to be qualitatively tunable by tuning the spatial distribution of interfacial electronic states. The subtle balance of magnetic coupling between FeOMn and MnOMn changed as the electronic structures are modified by charge transfer. The ions' valence is strongly correlated with the interfacial magnetic properties, extending the concept of the charge degrees of freedom affecting the magnetic coupling properties.

In this work, we study the interface obtained by depositing a monolayer of a Blatter radical derivative on polycrystalline cobalt. By examining the occupied and unoccupied states at the interface, using soft X-ray techniques, combined with electronic structure calculations, we could simultaneously determine the electronic structure of both the molecular and ferromagnetic sides of the interface, thus obtaining a full understanding of the interfacial magnetic properties. We found that the molecule is strongly hybridized with the surface. Changes in the core level spectra reflect the modification of the molecule and the cobalt electronic structures inducing a decrease in the magnetic moment of the cobalt atoms bonded to the molecules which, in turn, lose their radical character. Our method allowed us to screen, beforehand, organic/ferromagnetic interfaces given their potential applications in spintronics.

Understanding and controlling the interfacial magnetic properties of ferromagnetic thin films are crucial for spintronic device applications. However, using conventional magnetometry, it is difficult to detect them separately from the bulk properties. Here, by utilizing tunneling anisotropic magnetoresistance in a single-barrier heterostructure composed of La0.6Sr0.4MnO3 (LSMO)/LaAlO3 (LAO)/Nb-doped SrTiO3 (001), we reveal the presence of a peculiar strong two-fold magnetic anisotropy (MA) along the [110]c direction at the LSMO/LAO interface, which is not observed in bulk LSMO. This MA shows unknown behavior that the easy magnetization axis rotates by 90° at an energy of 0.2 eV below the Fermi level in LSMO. We attribute this phenomenon to the transition between the eg and t2g bands at the LSMO interface. Our finding and approach to understanding the energy dependence of the MA demonstrate a new possibility of efficient control of the interfacial magnetic properties by controlling the band structures of oxide heterostructures.

In this work, we study the interface obtained by depositing a monolayer of a Blatter radical derivative on polycrystalline cobalt. By examining the occupied and unoccupied states at the interface, using soft X‐ray techniques, combined with electronic structure calculations, we could simultaneously determine the electronic structure of both the molecular and ferromagnetic sides of the interface, thus obtaining a full understanding of the interfacial magnetic properties. We found that the molecule is strongly hybridized with the surface. Changes in the core level spectra reflect the modification of the molecule and the cobalt electronic structures inducing a decrease in the magnetic moment of the cobalt atoms bonded to the molecules which, in turn, lose their radical character. Our method allowed us to screen, beforehand, organic/ferromagnetic interfaces given their potential applications in spintronics.

Thin buried magnetic layers ranging from thicknesses of a few atomic monolayers to several nanometers are omnipresent in the fields of magnetism and spintronics. For the functionality and fine tuning of devices build with such layers, exact knowledge of the depth dependent magnetic properties is essential. Especially the interfacial magnetic properties are important. Hence, understanding how magnetism is affected by structural variations, such as thickness or interface roughness, is mandatory. In this study, we use x-ray resonant magnetic reflectometry and magnetometry to study the high-resolution depth dependent magnetization profiles of thin magnetic transition metal layers sandwiched between an oxide and chromium layer. Compared to bulk materials, the room temperature saturation magnetization of these layers is reduced by up to 67%. These reductions are extremely sensitive to small structural variations. From the magnetic depth profiles, we disentangle different effects contributing to the magnetization reduction and the exact magnetic properties of the interface.

The origin of interfacial electronic and magnetic degradation for a ferromagnet atop organic conjugated molecules

Interfacial magnetism and metal-insulator transition at LaNiO_3-based oxide interfaces have triggered intense research efforts, because of the possible implications in future heterostructure device design and engineering. Experimental observation lack in some points a support from an atomistic view. In an effort to fill such gap, we hereby investigate the structural, electronic, and magnetic properties of (LaNiO_3)_n/(CaMnO_3)_m superlattices with varying LaNiO_3 thickness (n) using density functional theory including a Hubbard-type effective on-site Coulomb term. We successfully capture and explain the metal-insulator transition and interfacial magnetic properties, such as magnetic alignments and induced Ni magnetic moments which were recently observed experimentally in nickelate-based heterostructures. In the superlattices modeled in our study, an insulating state is found for n=1 and a metallic character for n=2, 4, with major contribution from Ni and Mn 3d states. The insulating character originates from the disorder effect induced by sudden environment change for the octahedra at the interface, and associated to localized electronic states; on the other hand, for larger n, less localized interfacial states and increased polarity of the LaNiO_3 layers contribute to metallicity. We discuss how the interplay between double and super-exchange interaction via complex structural and charge redistributions results in interfacial magnetism. While (LaNiO_3)_n/(CaMnO_3)_m superlattices are chosen as prototype and for their experimental feasibility, our approach is generally applicable to understand the intricate roles of interfacial states and exchange mechanism between magnetic ions towards the overall response of a magnetic interface or superlattice.

Magnetization induced effects in the nonlinear optical (second harmonic generation) response appear to be a very sensitive probe of magnetic interface properties. This leads to attractive applications for the study of magnetic multilayer structures and also leads to new modes of nonlinear magneto-optical imaging. PACS: 42.65.-k; 78.20.Ls; 75.70.Cn Some of the most exciting recent discoveries in magnetism, such as the giant magneto-resistance (GMR) and the oscillatory exchange coupling, are related to the properties of multilayers of alternating ferromagnetic and paramagnetic layers. For magnetic recording on the other hand, antiferromagnetic biasing of ferromagnetic films is used (spin-valve structures). The interfaces between these layers appear to play an essential role for these phenomena and consequently for the device properties based on them [1]. The well known magneto-optical Kerr effect (MOKE) is based on the changes in the linear susceptibility as a function of the magnetization. As this results in a (small) rotation of the polarization of light traveling through a magnetic material, it yields a probe for the bulk magnetization. Though very sensitive and even applicable to monolayers, MOKE is not interface specific. Magnetization induced second harmonic generation (MSHG) is a new nonlinear magneto-optical technique that combines interface sensitivity with huge magneto-optical effects [2] (up to three orders of magnitude times their linear equivalent). These effects are due to the simultaneous breaking of inversion symmetry (at interfaces) and time-reversal symmetry (by the magnetization), which lead to the appearance of even and respectively, odd tensor components in the magnetization that are of comparable magnitude. Because of the latter, the relative phase between these contributions plays an important role [3]. The higher-order nonlinear optical tensor is also responsible for the appearance of essentially new magneto-optical effects that have no equivalent in the linear case [4]. Since the initial theoretical predictions of Ru-Pin Pan et al. [5] and Hubner et al. [6], there have been a large number of experimental studies that have demonstrated the interface sensitivity as well as new and large magneto-optical effects (for example, Kerr rotations close to 90◦). An overview of these results can be found in a number of review papers [7– 9]. In this paper I will concentrate on some of the more recent developments and future challenges of this new technique. Because most magnetically ordered materials are centrosymmetric in their bulk form, MSHG is a particularly useful probe to study the magnetic properties of the interfaces in magnetic multilayer systems. A great challenge for this nonlinear magneto-optical technique is to correlate structural and electronic interface properties with their magnetic properties. Using MSHG, we have found that the spin orientation at the interface of CoNi/Pt multilayers can be different from the bulk due to specific preparation conditions. By a careful analysis of the experimental data, a correlation between increasing interface roughness and a canting spin orientation at the interface could be observed. MSHG has also been shown to be extremely sensitive to the appearance of quantum well states in ultra thin overlayer or spacer films [10]. We have studied several combinations of noble metal overlayers (Au, Ag, Cu) on a variety of magnetic materials (Co, Fe). Spectroscopic MSHG studies of these quantum well states show strong peculiarities of the oscillatory MSHG response. We have also observed QWS within thin Fe films, though here the effects on the MSHG response appear to be much weaker. These combined results reveal the complexity and the role of the interplay between electronic bandstructure, dipole matrix elements and the symmetry of the wavefunction of the quantum well states [11]. Nonlinear magneto-optics also appears to give promising new possibilities for nonlinear optical imaging [12]. We have used this to study the switching behaviour in thin CoNi films which may be used for magnetic recording. 1 Magnetization induced second harmonic generation The optical second harmonic polarization P (2ω) of a magnetic medium is generally described by a third rank polar tensor χcr ijk for the crystallographic contribution and a fourth

Entropy studies in interface science: An ageless tool

We use molecular simulation to study the wetting behavior of water near flat nonpolar surfaces. The interface potential approach is used to capture the evolution of various interfacial properties over a broad range of temperature and substrate strength. Three model substrates are considered: an atomistically detailed face-centered cubic (FCC) lattice, a graphite lattice, and a structureless wall. We first examine the evolution of the contact angle with substrate strength for conditions ranging from near drying to complete wetting at select temperatures. A notable characteristic of all systems is the presence of a surface strength at which the contact angle is independent of temperature. We also identify an analogous point in which the quantity γlv cos θ is temperature invariant. We discuss the relationship between this point and the excess entropy of the solid–liquid interface. We next consider the temperature dependence of interfacial properties. We find the contact angle to decrease with temperature at relatively strong surfaces and increase with the temperature at relatively weak surfaces. The work of adhesion is found to be a useful quantity for describing the interfacial properties of water. The enthalpic and the entropic contributions to the work of adhesion are obtained from its temperature dependence. These properties are found to be directly related to the affinity of water for a solid surface and therefore serve as useful measures of the hydrophobicity of a surface. Finally, we examine the effect of surface strength and temperature on the density depletion associated with water at hydrophobic surfaces. We study various metrics that quantify the density and compressibility of water in the vicinity of hydrophobic surfaces. Comparisons are drawn between the behavior of water and simple nonpolar fluids at solvophobic surfaces.

Effect of magnetic nanoparticles coating on their electrochemical behaviour at a polarized liquid/liquid interface