Search

Abstract: In this paper, a detailed study of the magnetic behaviour particularly at the Verwey transition is presented for some stoichiometric magnetite samples, as a function of their shape (film/bulk) and crystalline structure (single/polycrystal). Travelling solvent floating zone technique was first used to obtain crystalline bulk magnetite samples (110) oriented, while molecular beam epitaxy allowed us to grow a 68nm Fe3O4 (111) crystalline film on -Al2O3(0001) substrate. A 64nm Fe3O4 polycrystalline film grown on an Si/SiO2 substrate using DC reactive magnetron sputtering method completed the sample series. An anomalous magnetisation behaviour has been evidenced in the Verwey temperature vicinity that is shown to depend on the applied magnetic field. A magneto-electronic model explains the origin of this behaviour. The maximum in the demagnetisation at the Verwey transition has been noticed very near 0.1T (i.e. 90kA/m, as found out on bulk magnetite). The amount and nature of structural defects have been proven to be determinant in the magnetisation process, and the approach to saturation is well described by the relation, with in bulk magnetite crystal and in epitaxial magnetite film, even across and below the Verwey transition after paraprocess corrections. These dependences exist in the 10–300K temperature range, excluding isotropic point (near 130K), and demonstrate that the antiphase boundaries (APBs) are responsible for the much more progressive approach to saturation observed in thin films than in bulk samples which is consistent with previous observations. On the epitaxial film, n is found to decrease down to 0.33 at isotropic point due to an enhanced super-exchange interaction through APBs. [Copyright &y& Elsevier]

Abstract: We theoretically studied spin-dependent electron transport perpendicular-to-plain (CPP) in magnetic laminated multilayered structures by using Kubo formalism. We took into account not only bulk scattering, but the interface resistance due to both specular and diffuse reflection and also spin conserving and spin-flip processes. It was shown that spin-flip scattering at interfaces substantially reduces the value of giant magnetoresistance (GMR). This can explain the experimental observations that the CPP GMR ratio for laminated structures only slightly increases as compared to non-laminated ones even though lamination induces a significant increase in CPP resistance. [Copyright &y& Elsevier]

Abstract: The interlayer exchange coupling between Co/Pt perpendicular-to-plane magnetized layers across a thin IrMn spacer layer was experimentally studied. In contrast to earlier studies on interlayer coupling through antiferromagnetic NiO, which revealed an oscillatory coupling behavior as a function of NiO thickness, a ferromagnetic coupling was observed here in the range of IrMn thickness between 0.6 and 1.5nm and antiferromagnetic between 1.5 and 2.5nm. The antiferromagnetic coupling is attributed to an orange peel magnetostatic mechanism whereas the ferromagnetic coupling is attributed to an out-of-plane polarization of the antiferromagnetic IrMn layer induced by the interfacial exchange interaction with the adjacent out-of-plane ferromagnetic layers. Measurements of hysteresis loops versus temperature show that the coupling vanishes at 510K for t IrMn=1nm. This critical temperature is far below the Néel temperature of bulk IrMn, but above the blocking temperature of IrMn/Co bilayers at such thickness. Using a one-dimensional model describing a partial domain wall in the antiferromagnet, we explain the coupling in terms of an out-of-plane tilt of the Mn moments at the IrMn/(Co/Pt) interfaces yielding a weak net polarization of the IrMn. Finally, the non-oscillatory decay of the coupling was attributed to the compensated spin structure of the IrMn in the parallel to the interfaces. [Copyright &y& Elsevier]

Abstract: The minimum applied field HSW required to reverse the magnetic moment of the ferromagnetic/antiferromagnetic storage layer of a thermally assisted magnetic random access memory (TA-MRAM) device during the application of a heating electric pulse is investigated as a function of pulse power PHP and duration δ. For the same power of the heating pulse PHP (or, equivalently, for the same temperature of the storage layer), HSW increases with decreasing heating time δ. This behavior is consistently interpreted by a thermally activated propagating domain-wall switching model, corroborated by a real-time study of switching. The increase of HSW with decreasing pulse width introduces a constraint for the minimum power consumption of a TA-MRAM where writing combines heating and magnetic field application. [Copyright &y& Elsevier]

Abstract: We investigate thermally activated domain wall creep in a system consisting of two ultrathin Co layers with perpendicular anisotropy coupled antiferromagnetically through a 4nm thick Pt spacer layer. The field driven dynamics of domain walls in the softer Co layer have been measured while keeping the harder Co layer negatively saturated. The effect of the interlayer interaction on the soft layer is interpreted in terms of an effective coupling field, , which results in an asymmetry between the domain wall speeds measured under positive and negative driving fields. We show that creep theory remains valid to describe the observed wall motion when the effective coupling field is included in the creep velocity law as a component of the total field acting on the wall. Using the resultant modified creep expression, we determine a value for the effective coupling field which is consistent with that measured from the shift of the soft layer''s minor hysteresis loop. The net antiferromagnetic coupling is attributed to a combination of RKKY and orange-peel coupling. [Copyright &y& Elsevier]

Abstract: X-ray spectroscopy measurements have been performed on a series of trilayers to investigate the role of Co oxidation in the perpendicular magnetic anisotropy of the interface. It is observed that high temperature annealing modifies the magnetic properties of the Co layer, inducing an enhancement of the perpendicular magnetic anisotropy. The microscopic structural properties are analyzed via X-ray Absorption Spectroscopy, X-ray Magnetic Circular Dichroism and X-ray Photoelectron Spectroscopy measurements. It is shown that annealing enhances the amount of interfacial oxide, which may be at the origin of a strong perpendicular magnetic anisotropy. [Copyright &y& Elsevier]

Abstract: Magnetoresistive heads used in computer disk drives are nowadays based on the giant magnetoresistance of spin-valves. In the present technology, the current is flowing parallel to the interfaces of the spin-valve stack (the so-called “current-in-plane geometry”). However, due to the steady increase in the storage areal density of disk drives, this technology is close to its physical limit. Consequently, more and more efforts are made aiming at using the current-perpendicular-to-plane geometry which offers larger signal amplitude and better spatial resolution. However, new physical effects appear when the current flows perpendicular to the plane in a magnetic multilayer. Of particular interest are the spin transfer effects which were predicted by Slonczewski and Berger in 1996. When they traverse the pinned layer of the spin-valve, the conduction electrons acquire a spin polarization. This spin current exerts a torque on the magnetization of the free layer which induces very peculiar dynamical phenomena. This spin torque can induce the switching of the magnetization of the free layer or the generation of magnetic excitations which contribute to the noise of the head. Therefore in the context of magnetoresistive heads, it is important to understand and master these effects which perturb the behavior of the heads. On the contrary, these effects can be used in a positive way as a new write scheme in non-volatile magnetic random access memories or to generate a steady precession of the magnetization in RF oscillators. [Copyright &y& Elsevier]

Abstract: In conventional high-density magnetic media, information is stored in the form of magnetic domains oriented in two possible remanent states along an easy axis of anisotropy. In order to further increase the areal density keeping the same bit size, multilevel magnetic media consisting of N decoupled ferromagnetic storage layers (N ⩾2) with different coercivities can be used. In such a case, the recording media exhibit up to 2 N stable remanent states. In this study, decoupled (Co/Pt) multilayers systems with perpendicular anisotropy have been sputtered on continuous silicon substrates and arrays of pre-patterned silicon nanodots in order to achieve magnetic continuous and patterned multilevel media. These media were characterized from a magnetic point of view. [Copyright &y& Elsevier]

Abstract: A sputtered Ta3nm/[[Pt2nm/Co0.4nm)]3/IrMn7nm]7/Pt10nm multilayer has been analyzed by laser-assisted tomographic atom probe, allowing to perform three-dimensional reconstructions of the layers and to determine their chemical composition at the atomic scale. From the concentration profiles, we show that the Co layer in contact with IrMn is strongly mixed. The Co/IrMn and IrMn/Pt interfaces are thus non-symmetric, the Co/IrMn interface being more diffuse than the IrMn/Pt one. This study demonstrates that the LATAP technique is extremely well suited for atomic scale structural and chemical characterizations of magnetic multilayers in relationship with their magnetic properties. [Copyright &y& Elsevier]

Abstract: The dynamic state diagram for spin current-induced magnetization dynamics is presented for an out-of-plane magnetized (perpendicular) polarizer and an in-plane magnetized (planar) free layer as obtained from macrospin simulations. The state boundaries are compared upon using a constant spin polarization factor with an angular-dependent spin polarization factor g(θ MP) as introduced by Slonczewski. While two critical boundaries remain unaffected by the angular dependence of the polarization, the transition from precession to an out-of-plane stable state is drastically shifted and shows a strong asymmetry with respect to the sign of the spin-polarized current. [Copyright &y& Elsevier]

• Monolithic heterogeneous integration of magnetic functions for Internet of Things (IoT) platforms. • Co-integration of digital and analog functions from a single magnetic tunnel junction stack. • Spin transfer torque (STT) multifunctional stack. • Sensor, non-volatile memory and RF oscillator functions. • Perpendicular magnetic anisotropy in multifunctional magnetic tunnel junction. The objective of this study is to co-integrate multiple digital and analog functions together within CMOS by developing a universal magnetic tunneling junction stack (MTJ) capable of realizing logic, memory, and analog functions, within a single baseline technology. This will allow monolithic heterogeneous integration, fast and low-power processing, and high integration density, particularly useful for Internet of Things (IoT) platforms. This unique spintransfer-torque (STT) MTJ is called Multifunctional Standardized Stack (MSS). This paper presents the progress regarding memory, oscillator and sensor functionalities targeted for the technology. We show that a single magnetic stack deposition can be used to obtain these three functionalities on the same wafer. [ABSTRACT FROM AUTHOR]

• New mechanism of magnetooptical phenomena has been proposed. • Frequency dependence of the off-diagonal elements of conductivity tensors and Polarizability is calculated. • Comparison with the common mechanism is carried out. We propose a new mechanism of magneto-optical Faraday and Kerr effects. Our mechanism is based on spin–orbit interaction of conducting electrons of ferromagnetic material with the electric field of the radiation. We estimate the magnitude of magneto-optical effects originated from this mechanism with the magnitude obtained in the framework of standard mechanism, based on spin–orbit scattering of the conducting electrons at the potential of crystal lattice. We discuss the possibility to distinguish this new contribution in the experiment. [ABSTRACT FROM AUTHOR]

Abstract: A theoretical study is presented concerning the relationship between spin transfer torque and absolute current-perpendicular-to-plane magnetoresistance in metallic spin-valves (SV) and magnetic tunnel junctions. In a first step, using Valet and Fert Boltzmann-like theory extended to any metallic magnetic multilayers with non-collinear magnetizations, linear relationships between spin torque and absolute current-perpendicular-to-plane giant magnetoresistance are derived numerically, when varying the parameters of the structure one by one. The obtained results are compared with an extension of J.C. Slonczewski''s circuit theory. The latter model gives an analytical expression of this linear dependence. In a second step, using an out-of-equilibrium perturbation formalism (Keldysh technique), we study the tunnel magnetoresistance (TMR) dependence of the spin torque amplitude in magnetic tunnel junctions when varying the parameters one by one in the junction. Once again, linear dependences are obtained, with different characteristics when the TMR vanishes. It is shown that these two equivalent behaviours are associated with different origins of the spin torque. [Copyright &y& Elsevier]