Alex Panchula

Magnetically engineered spintronic sensors and memory

The discovery of enhanced magnetoresistance and oscillatory interlayer exchange coupling in transition metal multilayers just over a decade ago has enabled the development of new classes of magnetically engineered magnetic thin-film materials suitable for advanced magnetic sensors and magnetic random access memories. Magnetic sensors based on spin-valve giant magnetoresistive (GMR) sandwiches with artificial antiferromagnetic reference layers have resulted in enormous increases in the storage capacity of magnetic hard disk drives. The unique properties of magnetic tunnel junction (MTJ) devices has led to the development of an advanced high performance nonvolatile magnet random access memory with density approaching that of dynamic random-access memory (RAM) and read-write speeds comparable to static RAM. Both GMR and MTJ devices are examples of spintronic materials in which the flow of spin-polarized electrons is manipulated by controlling, via magnetic fields, the orientation of magnetic moments in inhomogeneous magnetic thin film systems. More complex devices, including three-terminal hot electron magnetic tunnel transistors, suggest that there are many other applications of spintronic materials.

Spin-transfer torque switching in magnetic tunnel junctions and spin-transfer torque random access memory

We present experimental and numerical results of current-driven magnetization switching in magnetic tunnel junctions. The experiments show that, for MgO-based magnetic tunnelling junctions, the tunnelling magnetoresistance ratio is as large as 155% and the intrinsic switching current density is as low as 1.1 ? 106?A?cm?2. The thermal effect and current pulse width on spin-transfer magnetization switching are explored based on the analytical and numerical calculations. Three distinct switching modes, thermal activation, dynamic reversal, and precessional process, are identified within the experimental parameter space. The switching current distribution, write error, and read disturb are discussed based on device design considerations. The challenges and requirements for the successful application of spin-transfer torque as the write scheme in random access memory are addressed.

Spin transfer switching in dual MgO magnetic tunnel junctions

Dual magnetic tunnel junction (MTJ) structures consisting of two MgO insulating barriers of different resistances, two pinned reference layers aligned antiparallel to one another, and a free layer embedded between the two insulating barriers have been developed. The electron transport and spin dependent resistances in the dual MTJ structures are accounted for by sequential tunneling with some spin-flip relaxation in the central electrode (the free layer). With a tunneling magnetoresistance ratio of 70%, a switching current density Jc (at 30ms) of 0.52MA∕cm2 is obtained, corresponding to an intrinsic value of Jc0 (at 1ns) of 1.0MA∕cm2. This value of Jc0 is 2–3 times smaller than that of a single MgO insulating barrier MTJ structure and results from improvements in the spin-transfer torque efficiency. The asymmetry between JcAP→P and JcP→AP is significantly improved, which widens the read-write margin for memory device design. In addition, the experimental results show that the switching current density can...

Spin transfer switching and spin polarization in magnetic tunnel junctions with MgO and AlOx barriers

We present spin transfer switching results for MgO based magnetic tunneling junctions (MTJs) with large tunneling magnetoresistance (TMR) ratio of up to 150% and low intrinsic switching current density of 2–3×106A∕cm2. The switching data are compared to those obtained on similar MTJ nanostructures with AlOx barrier. It is observed that the switching current density for MgO based MTJs is 3 to 4 times smaller than that for AlOx based MTJs, and that can be attributed to higher tunneling spin polarization (TSP) in MgO based MTJs. In addition, we report a qualitative study of TSP for a set of samples, ranging from 0.22 for AlOx to 0.46 for MgO based MTJs, and that shows the TSP (at finite bias) responsible for the current-driven magnetization switching is suppressed as compared to zero-bias tunneling spin polarization determined from TMR.

Spin-transfer switching in MgO-based magnetic tunnel junctions (invited)

We present spin-transfer switching results for MgO-based magnetic tunneling junctions (MTJs) with large tunneling magnetoresistance ratio of up to 150% and low intrinsic switching current density (Jc0) of (2–3)×106A∕cm2. The low intrinsic switching current density is attributed to high tunneling spin polarization (TSP) in MgO-based MTJs. The current switching data are discussed based on a qualitative study of TSP in MgO-based MTJs. Additional film stack modification needed to decrease the switching current to meet the requirement of advanced magnetoresistive random access memory application is also discussed.We present tight-binding calculations of the spin torque in non-collinear magnetic tunnel junctions based on the non-equilibrium Green functions approach. We have calculated the spin torque via the effective local magnetic moment approach and the divergence of the spin current. We show that both methods are equivalent, i.e. the absorption of the spin current at the interface is equivalent to the exchange interaction between the electron spins and the local magnetization. The transverse components of the spin torque parallel and perpendicular to the interface oscillate with different phase and decay in the ferromagnetic layer (FM) as a function of the distance from the interface. The period of oscillations is inversely proportional to the difference between the Fermi-momentum of the majority and minority electrons. The phase difference between the two transverse components of the spin torque is due to the precession of the electron spins around the exchange field in the FM layer. In absence of applied bias and for a relatively thin barrier the perpendicular component of the spin torque to the interface is non-zero due to the exchange coupling between the FM layers across the barrier.

Structure, materials and shape optimization of magnetic tunnel junction devices : Spin-transfer switching current reduction for future magnetoresistive random access memory application

We present a systematic study of spin transfer switching in magnetic tunneling junctions (MTJs). Several ways to decrease the switching current density through material and stack engineering and MTJ element shape optimization are explained in detail. The data are presented for switching on MgO-based MTJ with high tunnel magnetoresistance (TMR) of 150% and low intrinsic switching current density Jc0 of (2–3)×106 A/cm2. Micromagnetic modeling is used to study the spin transfer switching mechanism in nanosecond regime for spin transfer torque random access memory (STT-RAM) pillar. The importance of current-induced Oersted field on the initial onset of precession is discussed.

Magnetic tunnel junctions with ZnSe barriers

Magnetic tunnel junctions with ZnSe barriers were fabricated with a combination of magnetron sputtering, ion beam sputtering, and effusion cell evaporation. Tunneling magnetoresistance values of ∼10% are observed at room temperature. The temperature and barrier thickness dependences of the junction resistance and tunneling magnetoresistance are consistent with a predominant direct tunneling mechanism when the barrier thickness is less than ∼10 nm thick.

Spin polarization and magnetotransport of Mn–Sb alloys in magnetic tunnel junctions

The spin polarization of MnxSb1−x for x=0.35–0.45 has been explored via magnetic tunnel junctions using CoFe counterelectrodes and via superconducting tunneling spectroscopy using Al counterelectrodes. MnxSb1−x with x∼0.45 shows a tunneling spin polarization of ∼30% at 0.25 K, and a tunneling magnetoresistance of ∼18% at 10 K both of which are very similar to previously reported data on NiMnSb alloys. These results support the notion that surface segregation of Mn and Sb reduces the spin polarization of the purported half-metal NiMnSb.

Physical properties of CeGe2−x (x = 0.24) single crystals

We present data on the anisotropic magnetic properties, heat capacity and transport properties of CeGe2-x (x=0.24) single crystals. The electronic coefficient of the heat capacity, γ∼110 mJ mol(-1) K(-2), is enhanced; three magnetic transitions, with critical temperatures of ≈7, ≈5 and ≈4 K are observed in thermodynamic and transport measurements. The ground state has a small ferromagnetic component along the c-axis. Small applied field, below 10 kOe, is enough to bring the material to an apparent saturated paramagnetic state (with no further metamagnetic transitions up to 55 kOe) with a reduced, below 1.2 μB, saturated moment.