Device geometry dependent deterministic skyrmion generation from a skyrmionium.

A magnetic skyrmionium can be perceived as an association of two magnetic skyrmions with opposite topological charges. In this work, we have investigated the transformation of skyrmionium into multi-skyrmionic states via domain wall pairs in three different devices with variable geometric configurations. The same device geometries are considered for single ferromagnetic layer and synthetic antiferromagnetic system. It is observed that by tuning the current density, deterministic generation of skyrmions is possible via the spin transfer torque. The proposed device is efficiently adjustable to change the number of skyrmions also at room temperature. The results may lead to development of skyrmion-based devices for neuromorphic and unconventional computing.

Spin dynamics and inverse spin Hall effect study in the metallic Pt/NiMn/CoFeB system.

Generation and manipulation of pure spin current is the governing tool to develop spintronic devices. Spin pumping and the Inverse spin Hall effect (ISHE) are the frontier mechanisms to study the spin current in a system. Ferromagnets (FMs)/heavy metals (HMs) are heavily investigated in such studies. Recently it was found that antiferromagnetic (AFM) materials are a good replacement of HMs in this field. In this context, we have studied the ISHE in Ta (3 nm)/Pt (2.5 nm)/Ni50Mn50 (t nm)/Co40Fe40B20 (3 nm)/Ta (3 nm) samples where the 't' value varies from 0-40 nm. We could observe a finite spin pumping in all the samples. With the introduction of the AFM NiMn layer, the spin pumping voltage reduces to ∼20% in comparison to the reference sample with no NiMn layer. The prominent spin pumping voltage with a 40 nm NiMn layer reveals the finite spin current propagation between Pt and CoFeB. The Gilbert damping (α) decreases by ∼50% with the introduction of NiMn.

Dependence of Exchange Bias on Interparticle Interactions in Co/CoO Core/Shell Nanostructures.

This article reports the dependence of exchange bias (EB) effect on interparticle interactions in nanocrystalline Co/CoO core/shell structures, synthesized using the conventional sol-gel technique. Analysis via powder X-Ray diffraction (PXRD) studies and transmission electron microscope (TEM) images confirm the presence of crystalline phases of core/shell Co/CoO with average particle size ≈ 18 nm. Volume fraction (φ) is varied (from 20% to 1%) by the introduction of a stoichiometric amount of non-magnetic amorphous silica matrix (SiO2) which leads to a change in interparticle interaction (separation). The influence of exchange and dipolar interactions on the EB effect, caused by the variation in interparticle interaction (separation) is studied for a series of Co/CoO core/shell nanoparticle systems. Studies of thermal variation of magnetization (M-T) and magnetic hysteresis loops (M-H) for the series point towards strong dependence of magnetic properties on dipolar interaction in concentrated assemblies whereas individual nanoparticle response is dominant in isolated nanoparticle systems. The analysis of the EB effect reveals a monotonic increase of coercivity (HC) and EB field (HE) with increasing volume fraction. When the nanoparticles are close enough and the interparticle interaction is significant, collective behavior leads to an increase in the effective antiferromagnetic (AFM) CoO shell thickness which results in high HC and HE. Moreover, in concentrated assemblies, the dipolar field superposes to the local exchange field and enhances the EB effect contributing as an additional source of unidirectional anisotropy.

Magnetic properties in soft (CoFeB)/hard (Co) bilayers deposited under different Ar gas pressure.

We have studied the effect of deposition pressure on the magnetization reversal, domains, anisotropy and Gilbert damping constant in the ferromagnetic (CoFeB and Co) single and bilayer samples. Hysteresis measured by magneto-optic Kerr microscopy for the single layer films prepared at higher deposition pressure indicate no change of loop shape i.e. isotropic behaviour. An enhancement of anisotropy has been observed in the bilayer samples than the single layer samples prepared at a particular deposition condition. However, increasing the deposition pressure to 50 sccm for the bilayer samples, anisotropy gets reduced. For single layer Co film deposited at 10 sccm exhibits branch and patch like domains for different angle between the easy axis and the external magnetic field. However, the Co film deposited at 50 sccm exhibits ripple like domains. In the case of single layer CoFeB, branch and patch like domains are observed deposited at 10 sccm. Patch like domains are found for the CoFeB films deposited at 50 sccm. Pinned labyrinth and ripple kind of magnetic domains along with the big branch domains are found in the bilayer samples. The pinned domains may be due to the interfacial exchange coupling. Similar values of damping constants have been observed for different thin films prepared at different deposition pressure.

Degenerate skyrmionic states in synthetic antiferromagnets.

Topological magnetic textures, characterized by integer topological chargeS, are potential candidates in future magnetic logic and memory devices, due to their smaller size and expected low threshold current density for their motion. An essential requirement to stabilize them is the Dzyaloshinskii-Moriya interaction (DMI) which promotes a particular chirality, leading to a unique value ofSin a given material. However, recently coexistence of skyrmions and antiskyrmions, with opposite topological charge, in frustrated ferromagnets has been predicted usingJ1-J2-J3classical Heisenberg model, which opens new perspectives, to use the topological charge as an additional degree of freedom. In this work, we propose another approach of using a synthetic antiferromagnetic system, where one of the ferromagnetic (FM) layer has isotropic and the other FM layer has anisotropic DMI to promote the existence of skyrmions and antiskyrmions, respectively. A frustrated interaction arises due to the coupling between the magnetic textures in the FM layers, which enables the stabilization and coexistence of 6 novel elliptical topological textures.

Magnetism at the interface of non-magnetic Cu and C60.

The signature of magnetism without a ferromagnet in a non-magnetic heterostructure is novel as well as fascinating from a fundamental research point of view. It has been shown by Al'Mari et al. that magnetism can be induced at the interface of Cu/C60 due to a change in the density of states. However, the quantification of such an interfacial magnetic moment has not been performed yet. In order to quantify the induced magnetic moment in Cu, we have performed X-ray magnetic circular dichroism (XMCD) measurements on Cu/C60 multilayers. We have observed room temperature ferromagnetism in the Cu/C60 stack. Further XMCD measurements show that a ∼0.01 µB per atom magnetic moment has been induced in Cu at the Cu/C60 interface.

Effect of spin glass frustration on exchange bias in NiMn/CoFeB bilayers.

Exchange bias in ferromagnetic/antiferromagnetic systems can be explained in terms of various interfacial phenomena. Among these, spin glass frustration can affect the magnetic properties in exchange bias systems. Here we have studied a NiMn/CoFeB exchange bias system in which spin glass frustration seems to play a crucial role. In order to account for the effect of spin glass frustration on magnetic properties, we have investigated the temperature and cooling field dependence of exchange bias. We have observed the decrease of exchange bias field (µ0HEB) with cooling field (µ0HFC) whereas there is a negligible effect on coercive field (µ0HC). Exponential decay of µ0HEB and µ0HC is found in this exchange bias system. Furthermore, training effect measurements have been performed to study the spin relaxation mechanism. We have fitted the training effect data with a frozen and rotatable spin relaxation model. We have determined the ratio of relaxation rates of interfacial rotatable and frozen spins in this study. The training effect data are also fitted with various other models. Furthermore, we have observed the shifting of the peak temperature towards higher temperature with frequency from the ac susceptibility data. The peak temperature vs. frequency data can be described by the Vogel-Fulcher law, which indicates the spin glass like state in the bilayer system.

Simultaneous observation of anti-damping and the inverse spin Hall effect in the La0.67Sr0.33MnO3/Pt bilayer system.

Manganites have shown potential in spintronics because they exhibit high spin polarization. Here, by ferromagnetic resonance we have studied the damping properties of La0.67Sr0.33MnO3/Pt bilayers which are prepared by oxide molecular beam epitaxy. The damping coefficient (α) of a La0.67Sr0.33MnO3 (LSMO) single layer is found to be 0.0104. However the LSMO/Pt bilayers exhibit a decrease in α with an increase in Pt thickness. This decrease in the value of α is probably due to high anti-damping like torque. Furthermore, we have investigated the angle dependent inverse spin Hall effect (ISHE) to quantify the spin pumping voltage from other spin rectification effects such as the anomalous Hall effect and anisotropic magnetoresistance. We have observed a high spin pumping voltage (∼20 µV). The results indicate that both anti-damping and spin pumping phenomena occur simultaneously.

High Spin to Charge Conversion Efficiency in Electron Beam-Evaporated Topological Insulator Bi2Se3.

Bi2Se3 is a well-established topological insulator (TI) having spin momentum locked Dirac surface states at room temperature and predicted to exhibit high spin to charge conversion efficiency (SCCE) for spintronics applications. The SCCE in TIs is characterized by an inverse Edelstein effect length (λIREE). We report an λIREE of ∼0.36 nm, which is the highest ever observed in Bi2Se3. Here, we performed spin pumping and inverse spin Hall effect (ISHE) in an electron beam-evaporated Bi2Se3/CoFeB bilayer. The Bi2Se3 thickness dependence of λIREE, perpendicular surface anisotropy (KS), spin mixing conductance, and spin Hall angle confirmed that spin to charge conversion is due to spin momentum locked Dirac surface states. We propose that the role of surface states in SCCE can be understood by the evaluation of KS. The SCCE is found to be high when the value of KS is small.

Study of the magnetic interface and its effect in Fe/NiFe bilayers of alternating order.

We present a comprehensive study on the magnetization reversal in the Fe/NiFe bilayer system by alternating the order of the magnetic layers. All the samples show growth-induced uniaxial magnetic anisotropy due to the oblique angle deposition technique. Strong interfacial exchange coupling between the Fe and NiFe layers leads to single-phase hysteresis loops in the bilayer system. The strength of coupling being dependent on the interface changes upon alternating the order of magnetic layers. The magnetic parameters such as coercivity H C, and anisotropy field H K become almost doubled when a NiFe layer is grown over the Fe layers. This enhancement in the magnetic parameters is primarily dependent on the increase of the thickness and magnetic moment of the Fe-NiFe interfacial layer as revealed from the polarized neutron reflectivity (PNR) data of the bilayer samples. The difference in the thickness and magnetization of the Fe-NiFe interfacial layer indicates the modification of the microstructure by alternating the order of the magnetic layers of the bilayers. The interfacial magnetic moment increased by almost 18% when the NiFe layer was grown over the Fe layer. In spite of the different values of anisotropy fields and modified interfacial exchange coupling, the Gilbert damping constant values of the ferromagnetic bilayers remain similar to the single NiFe layer.

Tuning spinterface properties in iron/fullerene thin films.

In ferromagnetic (FM) metal/organic semiconductor (OSC) heterostructures charge transfer can occur which leads to induction of magnetism in the non-magnetic OSC. This phenomenon has been described by the change in the density of states in the OSC which leads to a finite magnetic moment at the OSC interface and it is called the 'spinterface'. One of the main motivations in this field of organic spintronics is how to control the magnetic moment in the spinterface. In this regard, there are several open questions such as (i) which combination of FM and OSC can lead to more moment at the spinterface? (ii) Is the thickness of OSC also important? (iii) How does the spinterface moment vary with the FM thickness? (iv) Does the crystalline quality of the FM matter? (v) What is the effect of spinterface on magnetization reversal, domain structure and anisotropy? In this context, we have tried to answer the last four issues in this paper by studying Fe/C60 bilayers of variable Fe thickness deposited on Si substrates. We find that both the induced moment and thickness of the spinterface vary proportionally with the Fe thickness. Such behavior is explained in terms of the growth quality of the Fe layer on the native oxide of the Si (100) substrate. The magnetization reversal, domain structure and anisotropy of these bilayer samples were studied and compared with their respective reference samples without the C60 layer. It is observed that the formation of spinterface leads to a reduction in uniaxial anisotropy in Fe/C60 on Si (100) in comparison to their reference samples.

Relaxation dynamics in magnetic antidot lattice arrays of Co/Pt with perpendicular anisotropy.

The topic of magnetic antidot lattice (MAL) arrays has drawn attention from both fundamental research as well as from application point of view. MAL arrays are promising candidates for making domain engineering in thin films. For various applications it is necessary to understand the magnetization reversal mechanism as well as the relaxation dynamics. In this context we have studied magnetic antidot lattice (MAL) arrays of Co/Pt with perpendicular anisotropy fabricated by combination of photolithography and sputtering deposition. Kerr microscopy domain imaging for the continuous thin film reveals the formation of typical bubble domains of perpendicular media with high anisotropy. However, presence of periodic holes in the MAL arrays lead to nucleation of localised smaller bubbles. We have performed simulations using object oriented micromagnetic framework (OOMMF) which reproduced the experimental results even considering antidot arrays in nano dimension. In literature it has been reported that in MAL arrays with in-plane anisotropy the domain propagation gets significantly hindered by the presence of the holes. However here we show that in perpendicularly magnetized Co/Pt the propagation of the domain walls is not restricted by the presence of the antidots. Further we have performed magnetic relaxation study and found that the global relaxation time for the MAL arrays of Co/Pt is faster as compared to it's parent thin film. This behavior is opposite to what has been observed in literature for in-plane magnetized MAL arrays.

Effect of magnetic fullerene on magnetization reversal created at the Fe/C60 interface.

Probing the hybridized magnetic interface between organic semiconductor (OSC) and ferromagnetic (FM) layers has drawn significant attention in recent years because of their potential in spintronic applications. Recent studies demonstrate various aspects of organic spintronics such as magnetoresistance, induced interface moment etc. However, not much work has been performed to investigate the implications of such OSC/FM interfaces on the magnetization reversal and domain structure which are the utmost requirements for any applications. Here, we show that non-magnetic Fullerene can obtain non-negligible magnetic moment at the interface of Fe(15 nm)/C60(40 nm) bilayer. This leads to substantial effect on both the magnetic domain structure as well as the magnetization reversal when compared to a single layer of Fe(15 nm). This is corroborated by the polarized neutron reflectivity (PNR) data which indicates presence of hybridization at the interface by the reduction of magnetic moment in Fe. Afterwards, upto 1.9 nm of C60 near the interface exhibits magnetic moment. From the PNR measurements it was found that the magnetic C60 layer prefers to be aligned anti-parallel with the Fe layer at the remanant state. The later observation has been confirmed by domain imaging via magneto-optic Kerr microscopy.

Multiferroic and magnetoelectric materials--novel developments and perspectives.

Magnetoelectric (ME) materials are of utmost interest in view of both fundamental understanding and novel desirable applications. Despite its smallness, the linear ME effect has been shown to control spintronic devices very efficiently, e.g., by using the classic ME antiferromagnet Cr2O3. Similar nano-engineering concepts exist also for type-I multiferroic single phase materials like BiFeO3 and BiMnO3. Record high ME response has been realized in stress-strain coupled multiphase magnetoelectrics like PZT/FeBSiC composites, enabling applications in sensors. In type-II multiferroics, whose ferroelectricity is due to modulated magnetic ordering, the ME coupling is of fundamental interest. Higher-order ME response characterizes disordered systems, which extend the conventional multiferroic scenario toward ME multiglass (e.g., Sr(1-x)MnxTiO3).