Xiufeng Han

Fabrication of high-magnetoresistance tunnel junctions using Co75Fe25 ferromagnetic electrodes

Temperature dependence of tunnel magnetoresistance (TMR) ratio, resistance, and coercivity from 4.2 K to room temperature and applied voltage dependence of the TMR ratio and resistance at room temperature for a tunnel junction, Ta (5 nm)/Ni79Fe21 (3 nm)/Cu (20 nm)/Ni79Fe21 (3 nm)/Ir22Mn78 (10 nm)/Co75Fe25 (4 nm)/Al (0.8 nm)-oxide/Co75Fe25 (4 nm)/Ni79Fe21 (20 nm)/Ta(5 nm), were investigated. TMR ratio, effective barrier height and width, and breakdown voltage of the junction can be remarkably enhanced after annealing at 300 °C for an hour. High TMR ratio of 49.7% at room temperature and 69.1% at 4.2 K were observed. The value of spin polarization of Co75Fe25, P=50.7%, deduced from the TMR ratio at 4.2 K was corresponding well to the experimental data measured at 0.2 K in a spin polarized tunneling experiment using a superconductor/insulator/ferromagnet tunneling junction.

Magnetoelectric coupling in the paramagnetic state of a metal-organic framework

Although the magnetoelectric effects - the mutual control of electric polarization by magnetic fields and magnetism by electric fields, have been intensively studied in a large number of inorganic compounds and heterostructures, they have been rarely observed in organic materials. Here we demonstrate magnetoelectric coupling in a metal-organic framework [(CH3)2NH2]Mn(HCOO)3 which exhibits an order-disorder type of ferroelectricity below 185 K. The magnetic susceptibility starts to deviate from the Curie-Weiss law at the paraelectric-ferroelectric transition temperature, suggesting an enhancement of short-range magnetic correlation in the ferroelectric state. Electron spin resonance study further confirms that the magnetic state indeed changes following the ferroelectric phase transition. Inversely, the ferroelectric polarization can be improved by applying high magnetic fields. We interpret the magnetoelectric coupling in the paramagnetic state in the metal-organic framework as a consequence of the magnetoelastic effect that modifies both the superexchange interaction and the hydrogen bonding.

Giant electrical modulation of magnetization in Co40Fe40B20/Pb(Mg1/3Nb2/3)0.7Ti0.3O3(011) heterostructure

We report a giant electric-field control of magnetization (M) as well as magnetic anisotropy in a Co40Fe40B20(CoFeB)/Pb(Mg1/3Nb2/3)0.7Ti0.3O3(PMN-PT) structure at room temperature, in which a maximum relative magnetization change (ΔM/M) up to 83% with a 90° rotation of the easy axis under electric fields were observed by different magnetic measurement systems with in-situ electric fields. The mechanism for this giant magnetoelectric (ME) coupling can be understood as the combination of the ultra-high value of anisotropic in-plane piezoelectric coefficients of (011)-cut PMN-PT due to ferroelectric polarization reorientation and the perfect soft ferromagnetism without magnetocrystalline anisotropy of CoFeB film. Besides the giant electric-field control of magnetization and magnetic anisotropy, this work has also demonstrated the feasibility of reversible and deterministic magnetization reversal controlled by pulsed electric fields with the assistance of a weak magnetic field, which is important for realizing strain-mediated magnetoelectric random access memories.

Electric Field Manipulation of Magnetization Rotation and Tunneling Magnetoresistance of Magnetic Tunnel Junctions at Room Temperature

Electric-field-controlled tunneling magnetoresistance (TMR) of magnetic tunnel junctions is considered as the milestone of ultralow power spintronic devices. Here, reversible, continuous magnetization rotation and manipulation is reported for TMR at room temperature in CoFeB/AlOx/CoFeB/piezoelectric structure by electric fields without the assistance of a magnetic field through strain-mediated interaction. These results provide a new way of exploring electric-field-controlled spintronics.

The perpendicular anisotropy of Co40Fe40B20 sandwiched between Ta and MgO layers and its application in CoFeB/MgO/CoFeB tunnel junction

Magnetic anisotropy of Co40Fe40B20 thin films sandwiched between Ta and MgO layers was investigated. Magnetic properties of CoFeB layers deposited on top and bottom of MgO layer are different. The thickness of the CoFeB layer and annealing temperature are the critical parameters to achieve and keep the perpendicular magnetic anisotropy. The phase diagram of perpendicular anisotropic strength of CoFeB layers on annealing temperatures and thicknesses of CoFeB layers is observed. According to phase diagrams, perpendicular CoFeB/MgO/CoFeB tunnel junctions were fabricated, and tunneling magnetoresistance (TMR) ratio was higher than 30% at low temperatures.

Nanoring magnetic tunnel junction and its application in magnetic random access memory demo devices with spin-polarized current switching (invited)

Current-driven magnetization switching was observed in the nanoring-shaped magnetic tunnel junctions (NR-MTJs) with key stack layers of both spin-valve-type antiferromagnetic/ferromagnetic/insulator/ferromagnetic and sandwich-type hard ferromagnetic/insulator/soft ferromagnetic structures. We successfully fabricated a series of ring-shaped MTJs with different ring-outer diameters of between 80nm and 4μm and different ring width of between 25nm and 2μm. Tunneling magnetoresistance ratio between 20% and 80% with different thickness of thin Al–O barrier was measured at room temperature as we apply a magnetic field or a pulsed current. When the electric current density exceeds a critical value of the order of 6×106A∕cm2, the magnetization of the two free and reference magnetic rings can be switched back and forth between parallel and antiparallel onion states. The experiments show that the spin transfer torque plays a main switching role in the magnetization reversal and the current-induced circular magnetic ...

Magnetic switching of ferromagnetic nanotubes

The magnetic switching of ferromagnetic nanotubes as function of geometrical parameters has been investigated. The modes of magnetization reversal are observed to depend on the geometry of the nanotubes. Time dependent magnetization properties reveal that the nanotubes have strong magnetic viscosity effects. The values of magnetic viscosity coefficient (S) for different applied fields are high near the coercive field.

Surface-plasmon-polariton assisted diffraction in periodic subwavelength holes of metal films with reduced interplane coupling

Metal films grown on an Si wafer perforated with a periodic array of subwavelength holes have been fabricated and anomalous enhanced transmission in the midinfrared regime has been observed. High order transmission peaks up to Si(2,2) are clearly revealed due to the large dielectric constant contrast of the dielectrics at the opposite interfaces. The Si(1,1) peak splits at oblique incidence both in TE and TM polarization, which confirms that anomalous enhanced transmission is a surface-plasmon-polariton (SPP) assisted diffraction phenomenon. Theoretical transmission spectra agree excellently with the experimental results and confirm the role of SPP diffraction by the lattice.

Patterned nanoring magnetic tunnel junctions

Patterned nanoring (NR) magnetic tunnel junctions (MTJs) with outer diameters between 100 and 400nm and narrow ring widths between 25 and 30nm were fabricated. The NR structure consists of CoFeB electrodes and Al-oxide barrier. The tunneling magnetoresistance (TMR) ratio in the range of 20%–50% for the NR-MTJs with the resistance-area product lower than 50Ωμm2 were observed at room temperature. These NR-MTJs allow current-induced magnetization switching with a low switching current density of around 9×106A∕cm2. Due to the small stray field and high TMR ratio, NR-MTJs offer superior prospects for very high density magnetic random access memory, recording medium, and other spintronic devices.

Magnetic tunnel junction sensor with Co/Pt perpendicular anisotropy ferromagnetic layer

Linear magnetoresistance is an important attribute for magnetic sensor design in space applications, three-dimensional detection of the magnetic field, and high field measurements. Here we demonstrate that a large linear magnetoresistance of up to 22% can be achieved in a magnetic tunnel junction that consists of two ferromagnetic layers, one with out of plane and one with in-plane magnetic anisotropy. The tunneling magnetoresistance with the electrical current perpendicular to the film plane and the magnetic configuration of the device are analyzed.