SPIN

Nanomagnets with small in-plane shape anisotropy energy barriers on the order of the thermal energy have unstable magnetization that fluctuates randomly in time. They have recently emerged as promising hardware platforms for stochastic computing and machine learning because the random magnetization states can be harnessed for probabilistic bits. Here, we have studied how the statistics of the magnetization fluctuations (e.g., the power spectral density) is affected by (i) moderate variations in the barrier height of the nanomagnet (caused by small size variations) and (ii) the presence of structural defects — both localized and delocalized — in order to assess how robust the stochastic computing platform based on Low Barrier Nanomagnets (LBM) is. We found that the power spectral density is relatively insensitive to moderate barrier height change and also relatively insensitive to the presence of small localized defects. However, extended (delocalized) defects, such as thickness variations over a significant fraction of the nanomagnet, affect the power spectral density very noticeably. That means extended defects can significantly alter the fluctuation rate of the magnetization in low barrier nanomagnets. Since the fluctuation rate is crucial for stochastic computing applications, this has very serious implications for the latter. Thickness variations are difficult to avoid in real nanomagnets with in-plane anisotropy since they must be thin to keep the barrier height small and the substrate on which they are fabricated may have surface roughness comparable to the nanomagnet thickness. This raises questions about the viability of stochastic computing with low barrier nanomagnets possessing in-plane anisotropy. Our results establish that small variations in the shape (causing small variations in the barrier height), or small localized defects, are relatively innocuous and tolerable but extended defects are not. The latter must be avoided for stochastic computing applications.

In this paper, we investigate the structural, electronic and magnetic properties of CoFeCrZ ($\mathrm{\text{Z}}=\mathrm{\text{P}}$, As,Sb) quaternary Heusler alloy, using the first-principles full potential linear muffin-tin orbital (FP-LMTO) method within the spin gradient generalized approximation (GGA) for the exchange and correlation potential. Our results demonstrate that in ferromagnetic phase, the all alloys CoFeCrZ are stable in type-1 configuration and are half-metallic ferromagnets (HMF) with gaps of 0.99eV, 0.57eV and 0.70Ev, respectively. The obtained negative formation energy shows that CoFeCrZ alloys have strong structural stability. The calculated total magnetic moment, $M_T$ for all alloys exhibit Slater-Pauling rule, $M_T=(N_V-24){\mu }_B$. At zero pressure, the three alloys shown 100% spin-polarization at Fermi–level $E_F$ with high Curie temperatures $Tc$. Our calculation indicate also that the half-metallicity and high magnetic moment of CoFeCrP, CoFeCrAs and CoFeCrSb are robust against the lattice compression (up to 7.80%, 5.40% and 11%, respectively). On the basis of these results, it is suggested that the CoFeCrZ Heusler could be suitable for spintronics devices applications.

Generation of random numbers is one of the most important steps in cryptographic algorithms. High endurance, high performance and low energy consumption are the attractive features offered by the Magnetic Tunnel Junction (MTJ) devices. Therefore, they have been considered as one of the promising candidates for next-generation digital integrated circuits. In this paper, a new circuit design for true random number generation using MTJs is proposed. Our proposed circuit offers a high speed, low power and a truly random number generation. In our design, we employed two MTJs that are configured in special states. Generated random bit at the output of the proposed circuit is returned to the write circuit to be written in the relevant cell for the next random generation. In a random bitstream, all bits must have the same chance of being “0”or “1”. We have proposed a new XOR-based method in this paper to resolve this issue in multiple random generators that produce truly random numbers with a different number of ones and zeros in the output stream. The simulation results using a 45nm CMOS technology with a special model of MTJ validated the advantages offered by the proposed circuit.

We report here a tight-binding model study of frequency-dependent ferromagnetic spin susceptibility of the graphene system. The tight-binding Hamiltonian consists of electron hoppings up to third-nearest-neighbors, substrate and impurity effects in the presence of Coulomb interaction of electrons separately at two in-equivalent A and B sub-lattices of graphene. To calculate magnetic susceptibility, we calculate the two-particle electron Green’s functions by using Zubarev’s double time Green’s function technique. The electron occupations at A and B sub-lattices for both up and down spins are computed numerically and self-consistently. The frequency-dependent real part of ferromagnetic susceptibility of the system is computed numerically by taking $1000\times 1000$ grid points of the electron momentum. The susceptibility displays a sharp peak at the neutron momentum transfer energy at low energies and another higher energy resonance peak appearing at substrate-induced gap. The $q$-peak shifts to a higher energy with the increase of momentum $q$. The susceptibility shows that the high energy peak shifts to higher energies due to the corresponding increase of substrate-induced gap observed experimentally. It is observed that the Coulomb interaction suppresses the substrate-induced gap, but the impurity doping at A site enhances the substrate-induced gap, while doping at B site suppresses it.

The electronic structure, magnetism and thermal proprieties of the full-Heusler alloys Co₂YGa (Y $=$ V, Cr and Mn) have been investigated by first-principles calculations based on density functional theory with the generalized gradient approximation (GGA). Our obtained results of lattice parameters show reasonable agreement to the previously reported experimental and other theoretical studies. The calculations show that all Co₂YGa (Y $=$ V, Cr and Mn) alloys belong to half-metallic compound with a magnetic moment of 2.00, 3.00 and 4.00 ${\mu }_{\mathrm{\text{B}}}$ at their respective equilibrium lattice constants which is in good agreement with the Slater–Pauling rule and perfect 100% spin polarization at the Fermi level. The thermal effect on the macroscopic properties of these alloys is presented such as the thermal expansion coefficient, heat capacity and Debye temperature, based on the quasi-harmonic Debye model.

A nonequilibrium Green’s function (NEGF) formulation to study the transport characteristics of magnetic tunnel junctions (MTJs) that contains impurities at the barrier region and many-body interaction at the electrode region is presented. The formulation makes use of the already developed NEGF method for MTJs without any impurities and the impurity Green’s function is obtained using Haldane’s approach that explicitly takes into account the on-site Coulomb interaction ($U$) of the impurity. The formulation is used to obtain the spin-dependent tunnel current of model MTJs as a function of the applied bias for different values of $U$ corresponding to both the parallel and antiparallel configuration of the end electrodes of the MTJ. The tunnel currents are used to obtain the tunnel magnetoresistance (TMR) of the MTJ and we observed from our study that the TMR is strongly influenced by the impurities. The TMR is found to be reduced compared to that of the MTJ without impurities and the bias dependence of the TMR is found to be strongly influenced by $U$. Our studies revealed that the MTJ can exhibit almost completely spin polarized current at certain values of $U$.

In this study, we carried out ab-initio calculations of structural, electronic, optical and thermo-electric properties of CaTaO₂N compound in Pnma orthorhombic structure, using the full-potential linearized augmented plane wave method (FP-LAPW), within the framework of density functional theory (DFT). The calculated structural parameters are found to be in good agreement with the experimental results. Moreover, we have studied the electronic band structure, total and partial density of states in order to explain the origin of band gaps and the nitrogen anion contribution in the valence and the conduction bands. The CaTaO₂N band structure has shown a direct band gap in the direction $\mathrm{\Gamma }\to \mathrm{\Gamma }$ (with the value 2.32eV). The optical properties represented by the dielectric functions for CaTaO₂N compound have revealed that the Pnma structure absorbs the light at a large window in the edge UV-Vis regions. In order to explain the thermo-electric properties, we have calculated Seebeck coefficient, electrical conductivity, thermal conductivity and the factor figure of merit in this temperature range 100–1000 K. The factor figure of mérit (ZT) of CaTaO₂N takes a maximum value of 0.775 at $T=1000$K.

Exchange bias and intrinsic Gilbert damping ${\alpha }_0$ are two important parameters in magnetic multilayers. To explore the correlation between exchange coupling and damping, the samples Ta (2.6nm)/Pt (2.5nm)/Co${}_{62}$Fe${}_{25}$B${}_{13}$ (0.63nm)/Pt (0.5nm)/Mn${}_{78}$Ir${}_{22}$ ($t$)/Ta (4nm) are deposited by using pulsed-DC magnetron sputtering. The static and dynamic magnetic responses with different thickness of antiferromagnetic MnIr are investigated using vibrating sample magnetometer (VSM) and the pump-probe time-resolved magneto-optical Kerr effect (TR-MOKE). The exchange bias initially increases and then decreases with increasing thickness of MnIr layer, which is associated with the interfacial exchange coupling. The dynamic results demonstrate that the Gilbert damping of CoFeB/Pt/MnIr can increase by up to 40% compared with CoFeB/Pt. Based on the fact that the spin pumping has been excluded because of the Pt interlayer, we attribute the enhancement in Gilbert damping to the exchange coupling between AFM and FM.

Co/Mn multilayers were prepared using evaporation technique under ultra-high vacuum conditions and their structural and magnetic properties were studied. The temperature dependence of the spontaneous magnetization $M_s(T)$ was recorded for different cobalt layer thickness ($t_{\mathrm{\text{Co}}}$) ranging from 19Å to 70Å. Bloch’s law describes satisfactory the thermal change of magnetization for all samples. In addition, the change of the spin-wave (SW) parameter $B_{3∕2}$ versus $t_{\mathrm{\text{Co}}}$ was discussed. The torque magnetometry was utilized to extract the effective anisotropy ($K_{\mathrm{\text{eff}}}$) and the interlayer coupling constant ($J_I$) was determined from the in-plan M–H loops. An updated model based on SW theory was applied to describe the $M(T)$ variation and to calculate the approximate values of exchange interactions for samples with different Co-layer thicknesses.