Youfeng Zheng

Ultrahigh density vertical magnetoresistive random access memory (invited)

In this paper, we present the vertical magnetoresistive random access memory (VMRAM) design based on micromagnetic simulation analysis. The design utilizes the vertical giant magnetoresistive effect of the magnetic multilayer. By making the memory element into a ring-shaped magnetic multilayer stack with orthogonal paired word lines, magnetic switching of the memory device becomes very robust. The design also adopts the readback scheme in pseudo spin valve MRAM so that only one transistor is needed for each bit line which can connect hundreds of memory elements, yielding a very high area density. It is estimated that the ultimate area density for the VMRAM is 400 Gbits/in.2. It is suggested that this memory design has the potential to not only replace the present semiconductor memory devices, such as FLASH, but also the potential to replace DRAM, SRAM, and even disk drives.

Switching field variation in patterned submicron magnetic film elements

In this article, a micromagnetic study of magnetic switching properties on submicron scale single layer and multilayer thin film elements is presented. Even at deep submicron scale, there exist various edge domain configurations at the saturation remanent state. It is found that the switching field of these patterned film elements can strongly depend on these edge domain configurations. If the edge domains are not controlled, switching field of a patterned magnetic film element can vary significantly during repeated switching processes.

End domain states and magnetization reversal in submicron magnetic structures

Patterned submicron magnetic thin films of various geometries have been systematically studied. We have observed end domain states in rectangular elements, which is in excellent agreement with micromagnetic simulation results. Significant deviation from single domain behavior has been found in low aspect ratio elements. We will show that this deviation is attributed to behavior of the end domains.

Magnetization vortices and anomalous switching in patterned NiFeCo submicron arrays

Switching characteristics of 20-nm-thick NiFeCo submicron arrays are systematically studied by means of magnetometry, magnetic force microscopy, and micromagnetic simulation based on solution of the Landau–Lifshitz–Gilbert equation. A switching anomaly found in these structures could not be described by the magnetization coherent rotation model. Both experimental data and simulation results indicate that trapped magnetization vortices are responsible for the switching anomaly.

Metal‐bonded Sm2Fe17‐N‐type magnets

A number of metal‐bonded Sm2Fe17‐N magnets have been fabricated. These magnets exhibit iHc = 5.1–17.0 kOe, Br = 6.4–8.4 kG, (BH)max=5.0–10.8 MGOe, Tc = 757 K, and ρ=6.2–6.7 g/cm3. Powder metallurgical techniques have been employed with a mixture of powdered Sm2Fe17‐N and Zn, Sn, or In. Heat treatment is carried out in the temperature range of 160–450 °C in a N2 atmosphere at pressures ranging from 0–900 psi. The effects of Zn, Sn, and In contents and heat treatment conditions on the magnetic properties have been studied. Zn as the binder significantly enhances the coercivity iHc from 1.8–2.5 kOe for Zn‐free magnets to 5–17 kOe for 9–20‐wt. % Zn‐containing magnets. The Fe‐Zn phase, FeZn4, and/or Fe3Zn7, formed during heat treatment, may play an important role in producing a high coercivity. Sn‐bonded magnets exhibit significant coercivity, whereas the In‐bonded materials do not. The coercivity behavior is discussed in terms of the chemistry of the system.

Characteristics of AP bias in spin valve memory elements

Spin valve memory element biased with a pair of antiparallel (AP) coupled ferromagnetic layer was analyzed and modeled via micromagnetic simulation. In an AP structure, an external field results in a torque, causing the antiparallel magnetization (AP) axis to rotate towards the direction orthogonal to the field. In addition, due to its strength difference between the two AP layers, the magnetostatic field from the free layer of the spin valve can lead to irreversible AP axis flipping. This irreversible flipping can be effectively prevented by applying an AF/F exchange pinning to one of the AP layers to overcome the differential field from the free layer.

Micromagnetics of small size patterned exchange biased Permalloy film elements (invited)

In this article, we present a study on the micromagnetics of exchange biased Permalloy films. Specifically, by combining magnetic force microscopy with micromagnetic modeling simulation, the magnetization reversal processes in exchange biased Permalloy films were studied. The bilayer films were lithographically patterned into micrometer scale rectangular elements. It is shown that the micromagnetic simulations accurately predict domain configurations during magnetization reversal of the exchange biased Permalloy film elements and provide detailed magnetization distributions and transient dynamic magnetization configurations that could not yet be obtained experimentally. The study found that, for both NiO/NiFe and FeMn/NiFe systems, the exchange bias field measured on individual patterned micrometer scale bilayer film elements can be significantly larger than that measured on the sheet film sample.

Micromagnetics of spin valve memory cells

This paper presents a systematic micromagnetic analysis on spin valve GMR memory elements. It is found that for submicron size spin valve elements, edge demagnetization field, arising from the pinned layer, results in significant magnetization curling at the end edges of the free layer. This edge demagnetization phenomenon yields significant degradation of device performance. It is proposed that by making the pinned film element slightly longer than the free layer so that the ends of the free and pinned layers are separated, the edge demagnetization in the free layer can be essentially eliminated.

Current induced noise in CPP spin valves

In this paper, an experimental investigation of current-induced noise in current perpendicular-to-plane spin-valve heads is reported. It is found that the sense current yields excitations of coherent spin waves as well as substantial 1/f-like noise when current exceeds a critical value. The noise is magnetic in nature. The criticality and current directional dependence of the noise excitation clearly show that the spin transfer is the cause.

Crystal structure and magnetic characteristics of alloys based on R‐Fe‐Si (R=Y, Nd, Gd, Dy, Ho, Er)

Crystal structure and magnetic properties of ternary compounds based on R2Fe14Si2 (off‐stoichiometric R2Fe17 type) were investigated. Results of the powder x‐ray‐diffraction indicated that the compounds examined in this study crystallize in the rhombohedral Th2Zn17‐type structure when R is Nd or Gd and in the hexagonal Th2Ni17‐type when R is Y, Dy, Ho, or Er. The Curie temperature TC and room‐temperature saturation magnetization Ms of R2Fe14Si2 system range from 465 to 572 K and 65 to 150 emu/g, respectively. The behavior of TC is found to follow the root of the de Gennes function and can therefore be ascribed to the strength of the rare‐earth–iron exchange interaction. The average magnetic moment of Fe in R2Fe14Si2 is estimated to be approximately 2.0 μB at 77 K. Addition of Si in the binary R‐Fe lattice induced a large uniaxial magnetic anisotropy at room temperature in some alloys. Results of M vs T for Er2Fe14Si2 indicate a spin reorientation transition at about 100 and 35 K. Dy2Fe14Si2 and Ho2Fe14Si2...