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Magnetic tunnel junctions (MTJs) have been identified as promising candidates for the development of high-performance, ultra-low field magnetometers due to their high sensitivity, low cost, low power consumption, and small size. However, 1/f noise is often quite large at low frequencies and inevitably becomes one of the most difficult issues in developing a magnetic field sensor with ultrahigh sensitivity. Low-frequency 1/f noise can have both electric and magnetic origins, and it is a result of complex non-linear interactions between many degrees of freedom inside a sensor. Therefore, a reduction of the 1/f noise can be expected for the magnetic sensor with very small dimensions. Here, the dependence of the 1/f noise on voltage and strong hard-axis bias field in deep submicrometer-sized MgO-based MTJs is investigated with various junction sizes. The noise spectra were measured by using a home-built low-frequency noise measurement setup with maximum frequency up to 30 kHz. We find that the noise spectral power density is 1/f-like at low frequencies. The experimental results suggest a relative reduction of 1/f noise with respect to the intrinsic thermal noise in small-sized MTJs. The results may open a new approach for reducing the 1/f noise level in MTJ nanosensors. [ABSTRACT FROM AUTHOR]

This work presents a technique to calibrate the spin torque oscillator (STO) measurement system by utilizing the whiteness of shot noise. The raw shot noise spectrum in a magnetic tunnel junction based STO in the microwave frequency range is obtained by first subtracting the baseline noise, and then excluding the field dependent mag-noise components reflecting the thermally excited spin wave resonances. As the shot noise is guaranteed to be completely white, the total gain of the signal path should be proportional to the shot noise spectrum obtained by the above procedure, which allows for an accurate gain calibration of the system and a quantitative determination of each noise power. The power spectral density of the shot noise as a function of bias voltage obtained by this technique was compared with a theoretical calculation, which showed excellent agreement when the Fano factor was assumed to be 0.99. [ABSTRACT FROM AUTHOR]

The magnetorefractive effect (MRE) has become an established technique for the contactless measurement of giant and colossal magnetoresistance and has also been extended to the study of thick granular tunnelling films. Here, the MRE is used to study the behaviour of a MgO(100)/Mg0(20 nm)/Fe(20 nm)/MgO(1.5 nm)/Fe(5 nm)/Co(3 nm)/Au(3 nm) magnetic tunnel junction patterned into square pillars of lateral dimension: 50/[micro]m and 5 [micro]m. A significant MRE is observed, enhanced by the presence of an absorption edge associated with MgO, although the exact origin of the effect is unclear. Index Terms--Infrared, magnetoresistance, tunnelling.

Effects of porosity and pore orientation on the magnetization process were studied for lotus-type porous nickel and cobalt. It was confirmed that the saturation magnetization of porous metals varied linearly with the porosity, independent of the pore growth directions. Anisotropic behavior was clearly observed for the magnetization curves of lotus-type porous nickel, i.e., the magnetization perpendicular to the pore growth direction is always smaller than that parallel to the pore growth direction. The observed anisotropy is significantly large in the low magnetic field range below 2.5 kOe and is attributed to the anisotropic forms of the pores. Furthermore, it was found that the porosity dependence of magnetization at a certain applied field can be expressed by the empirical formula called Archie’s law, and the values of their exponents in the parallel and perpendicular directions at around 200 Oe were determined to be 1.1 and 1.8, respectively. These values are coincident with those reported for electrical conductivity of lotus-type porous nickel in the parallel and perpendicular directions. [ABSTRACT FROM AUTHOR]

L10-FePt (001) and L10-FePt (110) dot arrays with well-defined geometry were fabricated through the use of electron beam lithography and Ar ion etching. The lateral size of dots was varied in the range from 0.2×0.2 to 5×5 μm2. Coercivity (Hc) for the perpendicularly magnetized FePt (001) dots increases with decreasing the dot size. In the case of the FePt (110) dots with in-plane magnetization, on the other hand, the dot size dependence of Hc is completely different from that for FePt (001) dots: Hc shows a slight decrease as the dot size decreases. After annealing at 600 °C, the values of Hc for both FePt (001) and FePt (110) dots are remarkably enhanced although the dot size dependence shows similar behavior to that before annealing. The magnetization reversal for all the dots occurs through the nucleation of reversed domains and subsequent domain wall propagation. [ABSTRACT FROM AUTHOR]

The spin-torque diode effect in submicron-scale [Co.sub.60][Fe.sub.20][B.sub.20]/MgO/[([Co.sub.x][Fe.sub.1-x]).sub.80][B.sub.20] magnetic tunnel junctions (MTJs) under perpendicular fields is examined. The studies have shown that the Fe-rich CoFeB free layers have displayed a perpendicular magnetic anisotropy, which has agreed well for the reduced switching current density in the MTJs.

The magnetization process of epitaxial [Co.sub.2]MnSi/Cr/[Co.sub.2]MnSi trilayers with oscillatory interlayer exchange coupling is investigated. The 90 degree Celsiusoupling energy and the anisotropy constant has disclosed that the strength of 90 degree Celsiusoupling decreases exponentially with the spacer layer thickness.

[L1.sub.0]-FePt (001) and [L1.sub.0]-FePt (110) dot arrays with well-defined geometry are fabricated by using electron beam lithography and Ar ion etching and their dot size dependence of magnetic properties are investigated before and after annealing. The values of coercivity [(H.sub.c]) for both FePt (001) and FePt (110) dots are enhanced although the dot size dependence has shown similar behavior to that before annealing and the magnetization reversal for all the dots has occurred through the nucleation of reversed domains and subsequent domain wall propagation.

FePt/Au/FePt nanopillars with current-perpendicular-to-plane geometries are fabricated, and current induced magnetization switching is demonstrated. Resistance measured with sweeping current show the resistance change due to the change of the magnetization alignment of the two FePt layers, which is interpreted as the magnetization switching by spin-polarized current.

The relationship between Coulomb staircase and tunnel magnetoresistance (TMR), employing current-perpendicular-to-plane (CPP) geometry measurements in CoAlO granular films is reported. It is concluded that clear Coulomb staircase is observed, and TMR shows oscillatory behavior associated with the Coulomb staircase.

Measurements of thermally excited ferromagnetic resonance were performed on spin torque oscillators having a perpendicularly magnetized free layer and in-plane magnetized reference layer (abbreviated as PMF-STO in the following) for the purpose of obtaining magnetic properties in the PMF-STO structure. The measured spectra clearly showed a large main peak and multiple smaller peaks on the high frequency side. A Lorentzian fit on the main peak yielded Gilbert damping factor of 0.0041. The observed peaks moved in proportion to the out-of-plane bias field. From the slope of the main peak frequency as a function of the bias field, Lande g factor was estimated to be about 2.13. The mode intervals showed a clear dependence on the diameter of the PMF-STOs, i.e., intervals are larger for a smaller diameter. These results suggest that the observed peaks should correspond to eigenmodes of lateral spin wave resonance in the perpendicularly magnetized free layer. [ABSTRACT FROM AUTHOR]

Numerical simulation is carried out to investigate the magnetization process of epitaxial Co2MnSi/Cr/Co2MnSi trilayers with oscillatory interlayer exchange coupling. A series of the magnetization curves is successfully fitted by using a very simple model, in which the absent 180° coupling is assumed while 90° coupling and the lowest order cubic anisotropy are considered. The 90° coupling energy and the anisotropy constant can be uniquely determined, and it is revealed that the strength of 90° coupling decreases exponentially with the spacer layer thickness. [ABSTRACT FROM AUTHOR]

We fabricated magnetic tunnel junctions with a 3d-transition material(X)-doped MgO (Mg-X-O) barrier, and evaluated the effect of the doping on magnetoresistance (MR) and microstructure. Among the variations of X (X = Fe, Co, Ni, Cr, Mn, Ti, V, and Zn), X = Fe and Mn showed a high MR ratio of more than 100%, even at a low resistance-area product of 3 Ωμm2. The microstructure analysis revealed that (001) textured orientation formed for X = Fe and Mn despite substantial doping (about 10 at%). The elemental mappings indicated that Fe atoms in the Mg-Fe-O barrier were segregated at the interfaces, while Mn atoms were evenly involved in the Mg-Mn-O barrier. This suggests that MgO has high adaptability for Fe and Mn dopants in terms of high MR ratio. [ABSTRACT FROM AUTHOR]

In this work we have studied the effect of external applied magnetic field on the locking range of spintronic feedback nano oscillator. Injection locking of spintronic feedback nano oscillator at integer and fractional multiple of its auto oscillation frequency was demonstrated recently. Here we show that the locking range increases with increasing external magnetic field. We also show synchronization of spintronic feedback nano oscillator at integer (n=1,2,3) multiples of auto oscillation frequency and side band peaks at higher external magnetic field values. We have verified experimental results with macro-spin simulation using similar conditions as used for the experimental study. [ABSTRACT FROM AUTHOR]

Perpendicularly magnetized magnetic tunnel junction (p-MTJ) is a key element for developing high-density spin-transfer torque switching magnetoresistive random access memory. Recently, a large exchange coupling (IEC) in the synthetic antiferromagnetic reference layer with Ir interlayer was observed in p-MTJs. The evaluation of the IEC is, however, difficult due to the electrostatic breakdown of MTJs. This study demonstrates the evaluation of the IEC with Ir interlayer in giant magnetoresistive (GMR) nanopillar. We fabricated three kinds of perpendicularly magnetized GMR elements; bottom-free structures with Cu or Ir spacer, and top-free structure with Ir spacer. The magnetoresistance (RH) loops of all samples show sharp changes of the magnetoresistance at the magnetic fields over ±10 kOe, indicating the existence of the large IECs. In particular, a sharp change of the magnetoresistance at the field over ±20 kOe was found for the element with Cu of 2 nm thickness. [ABSTRACT FROM AUTHOR]

Examines the temperature and bias-voltage dependence of tunnel-type giant magnetoresistance in cobalt (Co), aluminium (Al) and oxygen (O) insulating granular films. What the insulating granular systems consist of; How the Co-Al-O granular films were prepared; Measurement of the bias-voltage dependence of magnetoresistance in the films; Effect of Coulomb blockade in granular systems.

Static and dynamic control of the chirality and polarity of a magnetic vortex confined in a magnetic tunnel junction is demonstrated. The modes associated with the four chirality/polarity vortex configurations are first explored by resonant excitation with a low power rf current. When the rf power is increased, both the chirality and polarity of the vortex can be resonantly switched, which--as shown by micromagnetic simulations--involves vortex expulsion and renucleation. This tunable resonant switching of the vortex parameters are an exciting step forward for the viability of magnetic vortex-based applications. [ABSTRACT FROM AUTHOR]

We investigate the microwave response of a spin transfer vortex based oscillator in a magnetic tunnel junction with an in-plane reference layer combined with a spin valve with an out-of-plane magnetization spin polarizing layer. The main advantage of this perpendicular spin polarizer is to induce a large spin transfer force even at zero magnetic field, thus leading to a record emitted power (up to 0.6 μW) associated to a very narrow spectral linewidth of a few hundreds of kHz. The characteristics of this hybrid vortex based spin transfer nano-oscillator obtained at zero field and room temperature are of great importance for applications based on rf spintronic devices as integrated and tunable microwave source and/or microwave detector. [ABSTRACT FROM AUTHOR]

Highly sensitive microwave devices that are operational at room temperature are important for high-speed multiplex telecommunications. Quantum devices such as superconducting bolometers possess high performance but work only at low temperature. On the other hand, semiconductor devices, although enabling high-speed operation at room temperature, have poor signal-to-noise ratios. In this regard, the demonstration of a diode based on spin-torque-induced ferromagnetic resonance between nanomagnets represented a promising development, even though the rectification output was too small for applications (1.4 mV mW−1). Here we show that by applying d.c. bias currents to nanomagnets while precisely controlling their magnetization-potential profiles, a much greater radiofrequency detection sensitivity of 12,000 mV mW−1 is achievable at room temperature, exceeding that of semiconductor diode detectors (3,800 mV mW−1). Theoretical analysis reveals essential roles for nonlinear ferromagnetic resonance, which enhances the signal-to-noise ratio even at room temperature as the size of the magnets decreases. [ABSTRACT FROM AUTHOR]

The temperature dependence of a vortex-based nano-oscillator induced by spin transfer torque (STVO) in magnetic tunnel junctions (MTJ) is considered. We obtain emitted signals with large output power and good signal coherence. Due to the reduced non-linearities compared to the uniform magnetization case, we first observe a linear decrease of linewidth with decreasing temperature. However, this expected behavior no longer applies at lower temperature and a bottom limit of the linewidth is measured. [ABSTRACT FROM AUTHOR]

Phase locking experiments on vortex based spin transfer oscillators with an external microwave current are realized. We present clear evidence of synchronization, i.e., phase locking, frequency pulling, as well as fractional synchronization in this system with a minimum peak linewidth of only 3 kHz in the locked state. Large locking ranges are achieved (up to 1/3 of the oscillator frequency) allowing us to demonstrate the simultaneous phase locking of two independent oscillators connected in series with the external source. [ABSTRACT FROM AUTHOR]

Ultrathin [Co/Pt]n and [Co/Pd]n superlattice films consisting of 0.14-0.20-nm-thick Co and Pt(Pd) layers were deposited by sputtering. A large perpendicular magnetic anisotropy [(3-9)×106 ergs/cm3] and an ideal square out-of-plane hysteresis loop were attained even for ultrathin superlattice films with a total thickness of 1.2-2.4 nm. The films were stable against annealing up to 370 °C. MgO-based perpendicular magnetic tunnel junctions with this superlattice layer as the free layer showed a relatively high magnetoresistance ratio (62%) and an ultralow resistance-area product (3.9 Ω μm2) at room temperature. The use of these films will enable the development of gigabit-scale nonvolatile memory. [ABSTRACT FROM AUTHOR]

Spin-dependent single electron tunneling was investigated in a magnetic double tunnel junction including Au nanoparticles as a center electrode. Tunnel magnetoresistance (TMR) clearly emerged with increasing spin-polarized current injected into Au nanoparticles and reached a maximum value of about 12% at 4.2 K. The observation indicates that spin accumulation occurs in Au nanoparticles and causes TMR. The spin relaxation time in Au nanoparticles, as estimated from the critical current for the appearance of TMR, is of the order of 10 ns, which is much longer than that in the bulk state. [ABSTRACT FROM AUTHOR]

Interlayer exchange coupling (IEC) in trilayers, which consist of a full Heusler Co2MnSi (CMS) phase as ferromagnetic layers separated by a Cr spacer layer, has been investigated. The shape of magnetization loops shows unusual oscillatory behavior with the thickness of Cr. The oscillation period is about 3.3–3.5 nm. The charecteristics of magnetization curves show that 90° coupling plays a dominant role in IEC between CMS layers. Moreover, the strength of 90° coupling turns out to be very high (up to -1.85 ergs/cm2) around the first oscillation peak. [ABSTRACT FROM AUTHOR]

The effect of spin-transfer torque in 90° magnetization configuration on current-induced magnetization reversal was studied in current-perpendicular-to-plane giant magnetoresistance pillars of in-plane magnetized FePt/Au/FePt trilayers combined with an L10-FePt perpendicular spin polarizer. The resistance change associated with the transition of the alignment of the two in-plane magnetizations was observed, showing no significant difference in the magnitude from that of a pillar without the perpendicular spin polarizer. However, the spin-transfer torque from the perpendicular spin polarizer reduced the current density for the magnetization reversal of the in-plane magnetization. [ABSTRACT FROM AUTHOR]