Digital data, generated by corporate and individual users, is growing day by day due to a vast range of digital applications. Magnetic hard disk drives (HDDs) currently fulfill the demand for storage ...space, required by this data growth. Although flash memory devices are replacing HDDs in applications like mobile phones, laptops, and desktops, HDDs cover the majority of digital data stored in the cloud and servers. Since the capacity growth of HDDs is slowing down, it is essential to look for a potential alternative. One such alternative is domain wall (DW) memory, where magnetic domains in the form of two-dimensional or three-dimensional wires are used to store the information. DW memory (DWM) devices should satisfy the four basic operations, such as writing (nucleating domains or inserting DWs in memory element), storing (stabilizing DWs), shifting (moving DWs), and reading (reading magnetization direction). An external magnetic field or spin-transfer torque can be used to write the information. Spin–orbit torque or electric field may be used for shifting the DWs. The information can be read using tunneling magnetoresistance. The domains may be stored along the tracks using artificial pinning potentials. The absence of moving parts makes the DWM consume less power as compared to HDDs, and be more robust. The potential to stack many layers to store information in three dimensions makes them potentially a large storage capacity device. In addition to memory, DW devices also offer a route for making synaptic devices for neuromorphic computing.
Despite these potential advantages of DWM, significant advances in research are needed before DWM could become commercially viable. One of the major challenges associated with DWM is DW dynamics. Many problems, such as controlled DW motion, the stability of domains, reducing the dimensions of the DW devices are still to be addressed. Artificial pinning sites fabricated through either geometrical or non-geometrical methods have been proposed for controlling DW motion. This review paper presents a survey of the investigations carried out so far and the future perspective of such devices.
Organic semiconductor/ferromagnetic bilayer thin films can exhibit novel properties due to the formation of spinterface at the interface. Buckminsterfullerene (C60) has been shown to exhibit ...ferromagnetism at the interface when it is placed next to a ferromagnet (FM) such as Fe or Co. Formation of a spinterface occurs due to the orbital hybridization and spin-polarized charge transfer at the interface. In this work, we have demonstrated that one can enhance the magnetic anisotropy of the low Gilbert damping alloy CoFeB thin film by introducing a C60 layer. We have shown that anisotropy increases by increasing the thickness of C60, which might be a result of the formation of a spinterface. However, the magnetic domain structure remains the same in the bilayer samples as compared to that in the reference CoFeB film.
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 (K S), 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 K S. The SCCE is found to be high when the value of K S is small.
Abstract
Ferromagnetic materials exhibiting low magnetic damping (
α
) and moderately high-saturation magnetization are required from the viewpoints of generation, transmission, and detection of spin ...waves. Since spin-to-charge conversion efficiency is another important parameter, high spin mixing conductance
$$({g_{r}^{\uparrow \downarrow}})$$
(
g
r
↑
↓
)
is the key for efficient spin-to-charge conversion. Full Heusler alloys, e.g., Co
2
Fe
0.4
Mn
0.6
Si (CFMS), which are predicted to be 100% spin-polarized, exhibit low
α
. However,
$$g_r^{ \uparrow \downarrow }$$
g
r
↑
↓
at the interface between CFMS and a paramagnet is not fully understood. Here, we report investigations of spin pumping and the inverse spin Hall effect in CFMS/Pt bilayers. Damping analysis indicates the presence of significant spin pumping at the interface of CFMS and Pt, which is also confirmed by the detection of an inverse spin Hall voltage. We show that in CFMS/Pt,
$$g_r^{ \uparrow \downarrow }$$
g
r
↑
↓
(1.70 × 10
20
m
−2
) and the interface transparency (83%) are higher than the values reported for other ferromagnetic/heavy metal systems. We observed a spin Hall angle of ~0.026 for the CFMS/Pt bilayer system.
Domain wall (DW) statics and dynamics have been immensely studied for the last few decades in a disordered medium due to rich fundamental physics and spintronics applications. Here, we have studied ...the DW dynamics in Pt/CoFeB/MgO thin film with perpendicular magnetic anisotropy. We have observed an unusual behaviour in the DW dynamics which cannot be only explained via universal creep or depinning law. At low driving force, the sample follows the universal creep law; however, near the depinning field (
H
d
), an excess velocity has been observed in DW dynamics in comparison to the usual creep law. We propose that, due to additional relaxation events near
H
d
, the DW propagation length is increased and hence the DW velocity. The excess velocity has been modelled to a modified creep law.
Abstract The interlayer exchange coupling (IEC) between two ferromagnetic (FM) layers separated by a non-magnetic (NM) spacer layer gives rise to different types of coupling with the variation of ...spacer layer thickness. When the NM is metallic, the IEC is attributed to the well-known Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction which shows an oscillatory decaying nature with increasing thickness. Due to this, it is possible to tune the coupling between the two FM to be either ferromagnetic or antiferromagnetic. In this work we have studied a Pt/Co/Ir/Co/Pt system where the Co thickness has been taken in the strong perpendicular magnetic anisotropy regime which is much less than the spin reorientation transition thickness. By tuning the Ir thickness to 2.0 nm, a canted state of magnetization reversal in the system is observed which gives rise to a possibility of nucleating topologically non-trivial spin textures like skyrmions. Further, with the combination of transport and magnetic force microscopy (MFM) measurements, we have confirmed the presence of skyrmions in our system. These findings may be useful for potential applications in emerging spintronic and data storage technologies using skyrmions.
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.
The electrochemical growth of Heusler alloy film with good morphological quality and crystalline order using single-crystalline substrate is demonstrated. Static magneto optical Kerr effect studies ...are employed to reveal the surface magnetization reversal of the films. An understanding of the intrinsic nature of the magnetization dynamics in this class of electrochemically grown materials is presented using time-resolved magneto optical Kerr effect measurements, under femtosecond laser excitation. Excitation laser fluence dependence study reveals the ultrafast demagnetization time, fast remagnetization time, and magnetic damping parameter as well as their correlation.
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.
We deposited Fe/NiFe bilayers using magnetron sputtering and studied their static and dynamic properties. We performed PNR measurements and investigated whether the interfacial exchange coupling is responsible for the tuning of the magnetic properties.
PDF
