Session D14: Focus Session: MRAM and Magnetic Devices

Microstrip phase shifter using Fe as the active element

In this work, we investigate the performance of microstrip phase shifters using FMR effects of an iron film. Using standard techniques, Fe films of thickness varying from 100nm to 800nm were placed between the signal line and dielectric. This geometry may be constructed in one technological step. For a microstrip line with 100 nm iron the frequencies providing the maximum performance were at 8 and 27 GHz over a magnetic field range of 0.08 to 2.77 kOe. For an Fe film of 800 nm, the best performance was found at 10 and 45 GHz. In this case, the high-frequency differential phase shift had a weaker frequency dependence suitable for broadband applications.   

Nonlinear effects in iron based microstrip structures.

Nonlinear effects in magnetic films are a subject of growing interest. The onset of parametric instability translates into practical power limits for microwave devices. Nearly all high power studies were done in ferrites; recently An et al investigated Permalloy. No work has been performed on planar devices or on iron films. Here we investigate the transmission of cw-microwaves in a 6 mm x 13 micron, 200 nm iron based microstrip notch filter in the frequency domain. There are three regions in the transmission response. Up to a threshold power of P = 90 mW, the differential absorption of ferromagnetic resonance (FMR) is nearly constant as a function of input power. Above P, the sample absorption decreases significantly as the power is increased. In addition we observe a subsidiary absorption (SA) peak at a frequency above that of the FMR. In comparison to Fe, a 7.2 micron YIG film in the transducer geometry has P at 1$\sim $2 mW; for a permalloy 128 nm film it is 10 mW. This indicates that an Fe microstrip has a much higher power handling capability. Finally, our structures can also be used as a power limiter. The SA can be significantly increased at high powers, thus limiting the transmission in the frequency range where the SA occurs   

Injection and Propagation of Magnetic Domain Walls in Thin Film Nanowires

The fields required for injection and propagation of magnetic domain walls in thin-film nanowires were studied using high-bandwidth scanning Kerr polarimetry. This method has recently been used for studying the field- and current-driven dynamics of magnetic domain walls [1, 2]. Nanowires of various widths were fabricated from a 20 nm-thick film of Permalloy (Ni$_{80}$Fe$_{20})$ etched using a focused ion beam (FIB). These wires were joined at their ends by large-area continuous film regions. Domain walls were introduced into a wire by applying an injection field sufficient to ``inject'' a nucleated wall from the continuous film into the geometrically-constrained wire neck. After injection, a dc propagation field, typically less than the injection field, is capable of driving the wall at a constant velocity. The influence of nanowire geometry on the injection and propagation fields, and the variation of these fields induced by a dc spin-polarized electric current, will be discussed. [1] G.S.D. Beach, C. Nistor, C. Knutson, M. Tsoi, J.L. Erskine, Nat. Mater. \textbf{4}, 741 (2005). [2] G.S.D. Beach, C. Knutson, C. Nistor, M. Tsoi, J.L. Erskine, Phys. Rev. Lett. \textbf{97}, 057203 (2006).   

Switching of magnetic domain structure in permalloy microstructures using 2D electron gas.

We demonstrate the ability to monitor and change the magnetization state of microscopic permalloy element deposited on the active area of a 2DEG Hall probe. While sweeping the external magnetic field recorded Hall voltage signal provides information on local magnetization of the ferromagnetic element. Simultaneously, the exact magnetization state of permalloy element is imaged with a magnetic force microscope. Applying short, but intense current pulses through the Hall probe we can change the magnetization state of the permalloy ellipse. Such hybrid semiconductor-ferromagnet structures could offer novel direction for non-volatile memory storage elements.   

Systematic tuning of magnetization reversal properties in Permalloy nanowires using sloped ends

The magnetization reversal of Permalloy (NiFe) nanowires was investigated by Magneto Optic Kerr effect (MOKE) magnetometry, where one end of the wire exhibits a slope in the thickness. Arrays of straight nanowires with a thickness of 8nm, widths of 150nm and 100um length were prepared by electron-beam lithography. The sloped ends were achieved by using penumbra shadow masks during NiFe deposition. The topography of the wires was studied by atomic force microscopy. One finds that the slope profile can be tuned by the position under the mask, mask-to-sample distance and angle of deposition. Corresponding MOKE hysteresis loops show a systematic reduction of the coercive field with increasing length of the sloped part. E.g. wires, where the slope has a length of 30um exhibit a coercive field of 11 Oe, whereas nanowires without sloped ends show 107 Oe. One can conclude that the coercivity can be controlled not only by modifying the lateral shape of magnetic nanoelements but also by their thickness profile.   

First principles Modeling of Magnetoresistance in Magnetic Memory devices

We have performed first principles calculations of the zero- bias conductance and TMR for crystalline Fe-MgO-Fe MTJs, and studied the effects of different oxide layers in the Fe/MgO interface, and the effects of structural ``disorder'' in the device. We find that such ``defects'' in the atomic structure have strong effects on the conductance. We use the result of the calculations to rationalize recent experimental findings. The simulations have been done with the software package Atomistix ToolKit (ATK), which is based on density functional theory (DFT) and non-equilibrium Green's functions (NEGFs).   

Nanomagnetic Bit Cells for MRAM Applications

Magnetoresistive Random Access Memory (MRAM) combines magnetic tunnel junction devices with standard silicon-based microelectronics to obtain the combined attributes of non-volatility, high-speed operation, and unlimited read/write endurance not found in any other existing memory technology. The first MRAM product to market, Freescale's 4Mb MR2A16A, is built on 180 nm CMOS technology with magnetic bit cells of 300 nm minimum dimensions integrated in the upper layers of metal. At these dimensions, both the magnetic switching and magnetoresistive property distributions are governed by a combination of material and patterning variations. One of the keys to controlling these distributions and insuring manufacturability was the invention of the Toggle Write mode. This mode uses a balanced synthetic antiferromagnetic free layer combined with a phased write pulse sequence to achieve robust magnetic switching margin by eliminating the half-select disturb issue found in conventional approaches. Another crucial solution was the ability to deposit and pattern high-quality, high-TMR magnetic tunnel junctions with narrow bit-to-bit resistance variation, low defect density and long-term reliability. In this talk, I will present details of each of the above technology elements, the performance and bit cell reliability, and the scaling behavior to the reduced dimensions of advanced technology nodes.   

Permanent-Magnet Free Biasing of MR Sensors with Tunable Sensitivity

Exchange coupling$^{1}$ has been previously observed in a trilayer structure of ferromagnet (FM)/non-magnetic/antiferromagnet (AFM) and the exchange bias was found to be a function of the thickness of the buffer layer.$^{2,3,4}$ This unique coupling is used as a stabilizing bias for the sense layer with the additional ability to tailor the magnetic gain of the sensor for various applications. The elimination of permanent magnet bias results in the elimination of one patterning and one deposition step. Ruthenium (Ru) is used as the buffer layer and is self aligned with the FM and AFM layers and the thickness is varied to change the slope of the transfer curve in the linear region. Sensor devices are fabricated with a bipolar output, a medium sensitivity, and a wide field range. The results show that this biasing scheme is well suited for barber pole and soft adjacent layer (SAL) anisotropic magnetoresistance (AMR) stripes used in magnetic field sensors with a FM layer of Permalloy (NiFe) and an AFM layer of Iridium-Manganese (IrMn). Applications include a 256 channel read head used for magnetic forensics. 1N.J. Gokemeijer, T. Ambrose, C.L. Chien, N. Wang and K.K. Fung, J. Appl. Phys. \textbf{81 }(8), 4999, 15 April 1997. 2W.H. Meiklejohn and C.P. Bean, Phys. Rev. \textbf{102}, 1413 1956; \textbf{105}, 904, 1957. 3L. Thomas, A.J. Kellock and S.S.P. Parkin, J. Appl. Phys. \textbf{87 }(9), 5061, 1 May 2000. 4D. Wang, J. Daughton, C. Nordman, P. Eames and J. Fink, J. Appl. Phys. \textbf{99}, 2006.   

Effect of Interfacial Disorder on 1/f Noise in Magnetic Tunnel Junctions

Magnetic tunnel junctions (MTJs) have the potential for low field ( 1pT/Hz$^{0.5 }$@ 1 Hz) magnetic sensors. However, 1/f noise limits their performance. Here we correlate measured 1/f noise with dynamic Lorentz imaging and high-frequency ferromagnetic resonance (FMR) measurements. The measurements show that a large fraction of the 1/f noise is due to thermal fluctuations of nano-scale magnetic ripple structure which arises from a combination of disorder in the antiferromagnetic exchange bias layer and interfacial roughness in the tunnel barrier. We have changed the interfacial properties by varying growth conditions and by inserting nano-oxides. The samples show varying amounts of disorder that manifests itself as increased ripple structure, increased 1/f noise, and a broadened FMR linewidth. Time dependent Lorentz imaging has been used to directly observe nano-scale thermal fluctuations that give rise to 1/f noise.   

IrMn pinning rings to suppress closure domains in the sense layer of magnetic tunnel junctions

We have investigated very soft magnetic materials as sense films for use in magnetic tunnel junctions. One problem MTJs face is that the soft layer tends to break up into domains. Flux closure at the edges seems to be the driving force. Such domains are incompatible with low noise MTJs. We have found what appears to be a solution to this problem using an IrMn ring at the edges of the sense layer. Using a soft-film circle $\sim $100 microns in diameter and an IrMn ring $\sim $10 microns in width at the edge of the circle, we have a central circle $\sim $80 microns in diameter that is nearly as soft as a large area film. The pinning of the soft layer appears to die off within a few microns of the IrMn edge, leaving ample area for an MTJ structure on a single-domain soft layer. In addition, the hard axis of the soft film (which is used to attain a sensor with linear response) has very little hysteresis.   

Position dependence of thin layer notch filter waveguides

Because of the importance of processing high frequency electromagnetic waves for communication, there has been interest in developing ultra-small thin-layer magnetic notch filters. These filters operate in the 5-40 GHz range. In the past theoretical work has concentrated on a structure where the magnetic film was right next to the one of the conductive films in a waveguide. Here we present a theoretical model, which investigates the properties of a waveguide with two dielectric films and one magnetic film placed between two conductive layers. We find that this more general structure produces a deeper attenuation and a narrower peak compared to the earlier structure. The additional attenuation is on the order of 20 dB/cm for the same parameters. The comparison between experiment and theory is presented.   

Subwavelength Magnetic Plasmon Waveguides

A one-dimensional magnetic plasmon propagating in a linear chain of single split ring resonators is proposed. The subwavelength size resonators interact mainly through exchange of conduction current, resulting in stronger coupling as compared to the corresponding magnetoinductive interaction. Finite-difference time-domain simulations in conjunction with a developed analytical theory show that efficient energy transfer with signal attenuation of less then 0.57dB/$\mu $m and group velocity higher than 1/4c can be achieved. The proposed novel mechanism of energy transport in the nanoscale has potential applications in subwavelength transmission lines for a wide range of integrated optical devices.