Second order anisotropy contribution in perpendicular magnetic tunnel junctions

Electric field control of perpendicular magnetic tunnel junctions with easy-cone magnetic anisotropic free layers

Magnetic tunnel junctions (MTJs) are the core element of spintronic devices. Currently, the mainstream writing operation of MTJs is based on electric current with high energy dissipation, and it can be notably reduced if an electric field is used instead. In this regard, it is promising for electric field control of MTJ in the multiferroic heterostructure composed of MTJ and ferroelectrics via strain-mediated magnetoelectric coupling. However, there are only reports on MTJs with in-plane anisotropy so far. Here, we investigate electric field control of the resistance state of MgO-based perpendicular MTJs with easy-cone anisotropic free layers through strain-mediated magnetoelectric coupling in multiferroic heterostructures. A remarkable, nonvolatile, and reversible modulation of resistance at room temperature is demonstrated. Through local reciprocal space mapping under different electric fields for Pb(Mg1/3Nb2/3)0.7Ti0.3O3 beneath the MTJ pillar, the modulation mechanism is deduced. Our work represents a crucial step toward electric field control of spintronic devices with non-in-plane magnetic anisotropy.

Impact of a rubrene buffer layer on the dynamic magnetic behavior of nickel layers on Si(100)

Interfaces of ferromagnetic/organic material hybrid structures refer to the spin interface that governs physical properties for achieving high spin polarization, low impedance mismatch, and long spin relaxation. Spintronics can add new functionalities to electronic devices by taking advantage of the spin degree of freedom of electrons, which makes understanding the dynamic magnetic properties of magnetic films important for spintronic device applications. Our knowledge regarding the magnetic dynamics and magnetic anisotropy of combining ferromagnetic layer and organic semiconductor by microwave-dependent magnetic measurements remains limited. Herein, we report the impact of an organic layer on the dynamic magnetic behavior of nickel/rubrene bilayers deposited on a Si(100) substrate. From magnetic dynamic measurements, opposite signs of effective magnetic fields between the in-plane (IP) and out-of-plane (OP) configurations suggest that the magnetization of Ni(x)/rubrene/Si prefers to coexist. A shift in OP resonance fields to higher values can mainly be attributed to the enhanced second-order anisotropy parameter K2 value. Based on IP measurements, a two-magnon scattering mechanism is dominant for thin Ni(x)/rubrene/Si bilayers. By adding a rubrene layer, the highly stable IP combined with the tunable OP ferromagnetic resonance spectra for Ni(x)/rubrene/Si bilayers make them promising materials for use in microwave magnetic devices and spintronics with controllable perpendicular magnetic anisotropy.

Comparative Study of Temperature Impact in Spin-Torque Switched Perpendicular and Easy-Cone MTJs

The writing performance of the easy-cone magnetic tunnel junction (MTJ) and perpendicularly magnetized MTJ (pMTJ) under various temperatures was investigated based on the macrospin model. When the temperature is changed from 273 K to 373 K, the switching current density of the pMTJ changes by 56%, whereas this value is only 8% in the easy-cone MTJ. Similarly, the temperature-induced variation of the switching delay is more significant in the pMTJ. This indicates that the easy-cone MTJ has a more stable writing performance under temperature variations, resulting in a wider operating temperature range. In addition, these two types of MTJs exhibit opposite temperature dependence in the current overdrive and write error rate. In the easy cone MTJ, these two performance metrics will reduce as temperature is increased. The results shown in this work demonstrate that the easy-cone MTJ is more suitable to work at high temperatures compared with the pMTJ. Our work provides a guidance for the design of STT-MRAM that is required to operate at high temperatures.

Growth, Magnetic Anisotropies and Exchange Bias of Thin Ni0.95Fe0.05/NiFeO Multilayers

Ni0.95Fe0.05/NiFeO multilayers were fabricated by radio frequency magnetron sputtering and natural oxidation. Doping of Ni by only 5 at. % Fe results in enhanced layering quality as X-ray reflectivity reveals. Due to magnetostatic anisotropy, the multilayers were found to be in-plane magnetized. The influence of mild thermal annealing (T = 525 K) on the magnetic properties of NiFe/NiFeO multilayers is also investigated. Annealing results in the enhancement of perpendicular magnetic anisotropy, mainly due to an increase in the uniaxial volume anisotropy term. Temperature-dependent hysteresis measurements between 4–400 K revealed considerable enhancement of coercivity and appearance of exchange bias effect.

Tailoring interfacial effect in multilayers with Dzyaloshinskii-Moriya interaction by helium ion irradiation

We show a method to control magnetic interfacial effects in multilayers with Dzyaloshinskii-Moriya interaction (DMI) using helium (He[Formula: see text]) ion irradiation. We report results from SQUID magnetometry, ferromagnetic resonance as well as Brillouin light scattering results on multilayers with DMI as a function of irradiation fluence to study the effect of irradiation on the magnetic properties of the multilayers. Our results show clear evidence of the He[Formula: see text] irradiation effects on the magnetic properties which is consistent with interface modification due to the effects of the He[Formula: see text] irradiation. This external degree of freedom offers promising perspectives to further improve the control of magnetic skyrmions in multilayers, that could push them towards integration in future technologies.

Perpendicular magnetic anisotropy and interfacial Dzyaloshinskii-Moriya interaction in as grown and annealed X/Co/Y ultrathin systems

The perpendicular magnetic anisotropy (PMA) and the interfacial Dzyaloshinskii-Moriya interaction (iDMI) are investigated in as grown and 300 °C annealed Co-based ultrathin systems. For this, Co films of various thicknesses (0.8 nm ⩽ t _Co ⩽ 5.7 nm) were deposited by magnetron sputtering on thermally oxidized Si substrates using Pt, W, Ir, Ti, Ru and MgO buffer or/and capping layers. X-ray diffraction was used to investigate their structural properties and vibrating sample magnetometry (VSM) was used to determine the magnetic dead layer thickness and the magnetization at saturation (M _s). VSM revealed that the M _s for the Pt and the Ir buffered and capped films is the largest. Microstrip line ferromagnetic resonance (MS-FMR), used to extract the gyromagnetic ratio of the thicker Co films, revealed the existence of a second order PMA term, which is thickness dependent. Brillouin light scattering (BLS) in the Damon-Eshbach configuration was used to investigate the thickness dependence of the iDMI effective constant from the spin wave vector dependence of the frequency difference between Stokes and anti-Stokes lines. BLS and MS-FMR techniques were combined to measure the spin wave frequency variation as a function of the in-plane applied magnetic field (where the second order PMA contribution vanishes). The thickness dependence of the effective magnetization was then deduced and used to investigate PMA. For all the systems, PMA results from interface and volume contributions that we determined. The largest interface PMA constants were obtained for Pt- and Ir-based systems due to the electron hybridization of Co with these heavy metals having high spin orbit coupling. Annealing at 300 °C increases both the interface PMA and iDMI for the Pt/Co/MgO most probably due to de-mixing of interpenetrating oxygen atoms from the Co layer and the formation of a sharp Co/O interface.

Enhanced Broad-band Radio Frequency Detection in Nanoscale Magnetic Tunnel Junction by Interface Engineering

Broad-band radio frequency (RF) detection is of great interest for its potential applications in wireless charging and energy harvesting. Here, we demonstrate that the bandwidth of broad-band RF detection in spin-torque diodes based on magnetic tunnel junctions (MTJs) can be enhanced through engineering the interface perpendicular magnetic anisotropy (PMA) between the CoFeB free layer and the MgO tunnel barrier. An ultrawide RF detection bandwidth of over 3 GHz is observed in the MTJs, and the broad-band RF detection behavior can be modulated by tuning the free layer PMA. Furthermore, a wide RF detection bandwidth (about 1.8 GHz) can be realized even without any external bias field for free layers with a thickness of about 1.65 nm. Finally, the dependence of the broad-band RF detection bandwidth on external magnetic field and RF power is discussed. Our results pave the way for RF energy harvesting for future portable nanoelectronics.

Symmetry broken spin reorientation transition in epitaxial MgO/Fe/MgO layers with competing anisotropies

The observation of perpendicular magnetic anisotropy (PMA) at MgO/Fe interfaces boosted the development of spintronic devices based on ultrathin ferromagnetic layers. Yet, magnetization reversal in the standard magnetic tunnel junctions (MTJs) with competing PMA and in-plane anisotropies remains unclear. Here we report on the field induced nonvolatile broken symmetry magnetization reorientation transition from the in-plane to the perpendicular (out of plane) state at temperatures below 50 K. The samples were 10 nm thick Fe in MgO/Fe(100)/MgO as stacking components of V/MgO/Fe/MgO/Fe/Co double barrier MTJs with an area of 20 × 20 μm². Micromagnetic simulations with PMA and different second order anisotropies at the opposite Fe/MgO interfaces qualitatively reproduce the observed broken symmetry spin reorientation transition. Our findings open the possibilities to develop multistate epitaxial spintronics based on competing magnetic anisotropies.

Critical switching current density induced by spin Hall effect in magnetic structures with first- and second-order perpendicular magnetic anisotropy

In this study, we derive analytical expressions for the critical switching current density induced by spin Hall effect in magnetic structures with the first- and second-order perpendicular magnetic anisotropy. We confirm the validity of the expressions by comparing the analytical results with those obtained from a macrospin simulation. Moreover, we find that for a particular thermal stability parameter, the switching current density can be minimized for a slightly positive second-order perpendicular magnetic anisotropy and the minimum switching current density can further be tuned using an external magnetic field. The analytical expressions are of considerable value in designing high-density magnetic random access memory and cryogenic memory.