Search

Magnetic properties and structure of sputter-prepared FePd thin films on the glass substrates by underlayering with refractory elements M (M = Nb, Mo, and W) have been studied. The structural analysis shows that FePd films have a (111) preferred orientation. All studied FePd films exhibit in-plane magnetic anisotropy. For single-layer FePd films, the coercivity ( $H_{c})$ and magnetic energy product [(BH) $_{\mathrm {max}}]$ are increased with annealing temperature ( $T_{a})$ and reached the maximum value of 1.3 kOe and 3.8 MGOe in the sample annealing at 600 °C. With the further increase of $T_{a}$ , magnetic properties drop rapidly. Dependence of magnetic hardening on $T_{a}$ can be explained by dominant solid reactions in different region of $T_{a}$ including ordering and grain growth. Interestingly, magnetic properties of L10-FePd are largely improved by underlayering with 5 nm-thick Nb, Mo, and W layers. Both the coercivity and energy product are significantly increased to 1.6 kOe and 4.9 MGOe for M = Nb, 2.5 kOe and 6.4 MGOe for M = Mo, and 3.3 kOe and 8.7 MGOe for M = W, respectively. Structural analysis suggests that this magnetic enhancement is related to both higher ordering degree and refined microstructure.

Co and Ta inserted layers with thickness x varied from 0 to 20 nm on the dielectric permittivity of BaTiO3 (60 nm)/Co(x nm)/BaTiO3 (60 nm) (BCB) and BaTiO3 (60 nm)/Ta(x nm)/BaTiO3 (60 nm) (BTB) films fabricated on glass B270 substrates by the reactive sputtering technique was studied. The dielectric constant is around 7.6 for B270 glass substrate and BaTiO3 /B270 film. However, it is rapidly raised up to roughly 84.3-89.4 for BCB, and 75.5-88.2 for BTB samples with the thickness of the Co or Ta inserted layer larger than 2 nm. This large enhancement of the dielectric constant could be explained due to the growth mechanism of the metallic inserted layer from island clusters to continuous layer for samples with x roughly near 3 nm, and the conducting property of the metallic inserted layer. The adding of a Co or Ta interlayer redistributes the interface charges between metal and ferroelectric layers, and that enhances both the intrinsic polarization and its dielectric constant. For the magnetic induced dielectric variation, after comparing the variation of the dielectric properties of both Co and Ta inter layers, the magneto dielectric properties in BaTiO3 /Co/BaTiO3 films are manifested. This magnetic tunability of the dielectric constant is clearly attributed to the magnetic metallic Co layer in the films. From this study, the magneto dielectric properties are manifested in BTO/Co/BTO films and it has the potential for a ferroic sensor application.

Dielectric permittivity and loss of BiFeO3 (BFO) and 5 mol% lanthanum-substituted BFO [(Bi0.95La0.05)FeO3 or BFO-5% La] ceramics have been carried out as functions of temperature and frequency. A frequency-dependent and broad dielectric shoulder and maximum were observed in BFO and BFO-5% La near 600-700 K. These dielectric responses are likely due to the magnetoelectric coupling while the antiferromagnetic-paramagnetic transition takes place near the Neel temperature. As an approximation, a barrier model with intrinsic barriers B (in temperature unit) every lattice constant a and extrinsic barriers B +Δ every distance d is introduced to describe the low-frequency upturn in dielectric loss in the high-temperature region. Good qualitative fits are obtained for BFO and BFO-5% La. This work suggests that 5 mol% La substitution can enhance dielectric response and considerably reduce electric conductivity.

Room-temperature-deposited FePt thin films with thickness (t) ranged from 5 to 100 nm treated by rapid-thermal annealing (RTA) were studied. With annealing condition of 900°C for 60 seconds at heating rate of 80° C/sec., a metastable phase of FePt was observed in the films with t ≥ 40 nm. The phase is chemically ordered with a face-centered-cubic (fcc) structure. The lattice parameter of it is found identical to the planar spacing of L10 (100). The metastable structure is dominant in the film with t = 40 nm and gradually replaced by L10 phase with increasing t. The fcc phase is soft magnetic with saturation magnetization similar to that of disordered FePt. Drastic changes were also observed in surface morphology. The results infer the connection between the metastable transformation and internal strain. The formation of the fcc structure is sensitive to processing parameters except t . Slower heating rate, lower annealing temperature, and longer annealing time, tend to suppress the formation of it. The interesting findings provide additional knowledge for FePt thin films.

Development of (001) texture in FePt thin films deposited on glass substrates with different thickness (t) treated by rapid thermal annealing (RTA) is studied. A critical thickness of 30 nm is characterized: below which the (001) preferred orientation of the films develops with increasing t ; when t >; 30 , the films become isotropic. Remarkable perpendicular anisotropy in magnetic properties is achieved in the 30 nm thick sample with the best (001) texture. Discontinuous changes are also observed in surface morphology, microstructure, magnetic domain structure, and internal stress. Direct evidences are presented relating the formation of (001) texture to abnormal grain growth. The internal stress/strain analysis indicates that the residual tensile (σ) stress is proportional to the degree of (001) preferred orientation. A large value of σ of about 8.9 GPa is obtained in the film with t = 30, suggesting the driving force forming the texture.

Co inserted layer with different thickness on the permittivity of Ta2O5(60 nm)/Co(x nm)/Ta2O5(60 nm) trilayer films fabricated on the B270 glass substrates by the reactive sputtering technique was studied. The variation of dielectric constant of the samples is dependent on the thickness of the Co inserted layer observed from 1 kHz and 30 MHz. The dielectric constant of the Ta2O5(60 nm)/Co(x nm)/Ta2O5(60 nm) thin films was varied roughly from 7.9 up to 90.6 with the thickness (x) of the Co interlayer varied from 0 nm to 20 nm. The value of the dielectric constant is roughly near 7.9 for samples with x between 0 and 2 nm. However, it increases abruptly for samples with x larger than 2 nm, this large enhancement behavior of the dielectric constant could be explained due to the growth mechanism of the Cobalt inter-layer from island clusters to continuous Co layer for samples with x between 2 and 3 nm. For the magnetic induced ferroelectric variation, the variation of the dielectric constant increased with thickness of Co for samples with x larger than 3 nm. However this increase behavior is roughly saturated for applied magnetic field roughly above 20 Oe. This magnetic tunability of the dielectric constant is clearly attributed to the Co layer in the films. From the electric consideration, the adding of a Co inter-layer in Ta2O5/Co/Ta2O5 structures redistributes the interface charges between Co and Ta2O5 layers, and that enhances both the intrinsic polarization and its dielectric constant. The magnetoelectric properties in Ta2O5/Co/Ta2O5 films are manifested and it has potential for a ferroic sensor application.

Magnetization, grain size, x-ray diffraction, and dielectric permittivity of BiFeO3 (BFO) and 5 mol% lanthanum-substitution BFO [(Bi0.9BLa0.05)FeO3] ceramics have been carried out as a function of sintering temperature (910-950°C) BFO ceramic exhibits an antiferromagnetic behavior, whose magnetization curve is linear with the field at room temperature. A weak ferromagnetic behavior occurs in (Bi0.9BLa0.05)FeO3 (BLFO) ceramics at room temperature, in which remanent magnetization increases from 0.008 to 0.14 as sintering temperature raises from 910 to 950°C. This phenomenon can be correlated with the growth of grain size of BLFO ceramics, which grows with increasing sintering temperature. The dielectric permittivities of BFO and BLFO are about 33 and 57 at room temperature, respectively. The dielectric losses (tan δ) of BFO and BLFO ceramics vary respectively in the ranges of ~0.2-0.4 and ~0.002-0.004 below 150°C, indicating that 5 mol% La-substitution can efficiently reduce electric conductivity. A frequency-dependent and broad dielectric maximum was observed in both BFO and BLFO ceramics upon heating. This dielectric response is likely triggered by the antiferromagnetic-paramagnetic transition near the Neel temperature.

Magnetization, grain size, structure, and temperature-dependent dielectric permittivity of BiFeO3, (BiFeO3,0.95(BaTiO3)0.05 -(BaTiO3)0.05(5%BTO-BFO), and (Bi0.95Ba0.05)(Fe0.95Ti0.05)O3[5%(Ba, Ti)-BFO] ceramics have been carried out. BFO and 5%BTO-BFO exhibit an typical antiferromagnetic behavior, whose magnetization curve is linear with magnetic field at room temperature. The magnetic hysteresis loop of 5%(Ba,Ti)-BFO ceramic shows a weak ferromagnetic behavior with a small remanent magnetization of about 0.01 emu/g possibly due to a higher density in the grain matrix. This result also implies that the A-and B-site substitutions with Ba2+ and Ti4+ could efficiently suppress the spiral magnetic modulation. The magnetic susceptibilities of BFO, 5%BTO-BFO, and 5%(Ba,Ti)-BFO ceramics are about 1.9 × 10-5, 5.43 × 10-6, and 8.67 × 10-6 emu/gOe, respectively. Frequency-dependent dielectric maxima were observed in BFO, 5%BTO-BFO, and 5%(Ba,Ti)-BFO ceramics upon heating. This dielectric response is likely triggered by the antiferromagnetic-paramagnetic transition near the Neel temperature.

A method observing both radial and axial vibrations of time domain and frequency spectrum was studied to investigate the axial load effect of a flat-type brushless dc fan motor operating with and without the rated axial thrust force. When the developed small motor does not exhibit an intrinsically excited unbalanced magnetic force (UMF), the axial force leads to a minor contribution to the growth of the axial vibration. However, when the UMF is intrinsically excited, the preload can enormously decrease the undesired axial vibration. The ratio of the axial vibration of the prototype with the asymmetric magnetic sheet to the one without the sheet is experimentally observed around 55 percent. It demonstrates that axial load plays an effective and crucial factor in controlling the axial vibration of the mobile fan motor.

Ordered FePt continuous thin films with an amorphous SiO2 single capping layer have been fabricated on MgO (001) single-crystal substrates by the electron beam deposition technique at 400degC in order to investigate the effects of SiO2 capping layer on microstructure and magnetic reversal process of the FePt (001) thin films. The formation of nanogranular-like FePt films was directly obtained with this process due to the interpenetration of SiO2 which has a lower surface energy than that of pure Fe or Pt. Studies of angular dependent coercivity show a tendency of a domain-wall motion weaken towards rotation of reverse-domain type upon thickness of SiO2 capping layer on the FePt thin films. The intergrain interaction was confirmed from the Kelly-Henkel (deltaM) plot that indicated the strong exchange coupling between neighboring grains in the FePt continuous films without SiO2 capping layer. On the other hand, negative deltaM value was obtained when the FePt films with SiO2 capping layer, indicating the SiO2 capping layer, can lead to the reduction of intergrain exchange coupling thus presence of dipole interaction in the SiO2/FePt nanocomposite thin film structures.

In situ X-ray diffraction of BiFeO3 (BFO) ceramic and thin film has been carried out as a function of temperature (25degC-900degC) upon heating. The BFO thin film was deposited on Pt/TiOx/SiOx/Si(100) substrate by radio-frequency magnetron sputtering at 650degC. BFO thin film exhibits a structural transformation from rhombohedral to cubic in the region of 700-750degC upon heating. The magnetic property of BiFeO3/Co/BiFeO3 multilayer structure changes from diamagnetic to ferromagnetic phases with increasing thickness of Co layer. The dielectric permittivities (real and imaginary parts) of BiFeO3 ceramic exhibit a frequency-dependent relaxation-like behavior and its magnitude (real part) is higher than 1000 for f les 1 MHz. Surface morphology shows that the root-mean-square roughness of BFO thin film varies with sputtering power.

Fe53Pt47 and Fe58Pt42 thin films with 300 nm in thickness prepared using magnetron sputtering were investigated. The films were deposited on heated glass substrates and annealed at 400degC and 800degC for different time. Single phase FePt films with similar chemical ordering but different substructure size were produced in the two series of samples. The effect of microstructure refinement results in significant enhancement in remanence (Mr) of about 38.1% and 30.8% in the Fe53Pt47 and Fe58Pt42 films, respectively. The energy product, (BH)max, also showed a large increase of 25% and 72%; the maximum value of the two series of films are 16.2 MGOe and 19.6 MGOe, respectively. The large (BH)max was found to originate from the enhancement of M r and steep slope of demagnetization curve at coercive point (alpha). As the theoretical predictions, in an isotropic magnet, the magnetic moments in the transition region of magnetization near grain boundary are easy to be aligned by applied field. In this study, it is found that by increasing the magnetic transition region through microstructure refining, the remanence can be effectively enhanced meanwhile facilitate the collective magnetic reversal. Domain structure confirms that the refinement of microstructure effectively reduces the domain size.

The resistance bistability and the switching effect has been systematically studied in nickel oxide films with variable oxygen concentrations. Both the switching bias voltage and the resistance ratio of RESET to SET were found to be strongly concentration dependent. Meanwhile, the electrical characteristics can be exactly examined by an Schottky emission. By means of mathematical fitting, the resistance bistability seems to be related to the alternating of dielectric constant and the barrier height. Furthermore, the room temperature cycle endurance with a distinguishable operation window was found to exceed 800. This indicates that the nickel oxide has the potential to be a promising material on resistance random access memory

In this paper, we demonstrate high-performance giant magnetoresistive spin transistor (GMRST) and magnetotunneling resistive spin transistor (MTRST) which could be used as magnetic pickup head and magnetoresistive random access memory (MRAM). This kind of spin device is based on the technologies of giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) effects together with a p-n junction. The junction is used as a potential barrier to select spin electrons. In case of GMRST the collector current, I/sub C/, varies from 464 /spl mu/A at magnetically parallel state to 309 /spl mu/A at magnetically antiparallel state and gives a percentage change of about 50% at room temperature. As for the MTRST, the change of I/sub C/ is about 45% at room temperature with current varying from 2.9 to 4.2 /spl mu/A. Memory effect, which is the intrinsic behavior of magnetic materials, is also demonstrated.

In this paper, we report on domain wall (DW) pinning and depinning behavior using square notch and antinotch in NiFe nanowires. The presence of the DW in the samples was probed experimentally from focus magneto-optical Kerr effect (MOKE) measurement and anisotropic magnetoresistance (AMR). Pinning strengths as well as distribution of depinning field were measured. DW traps with antinotches were found to be more effective than notches, and the DW kept its structure during the depinning processes. Simulations of the magnetization orientation agreed with our results.

We demonstrate a non-volatile CMOS fully compatible embedded memory cell with a hybrid resistive switching material, TiO2-SiO2, in a 0.18 μm node CMOS process. Both voltage-and temperature-dependent transports indicate that a Schottky-type resistance switching model dominates the TiO2 based transition-metal-oxide resistive random access memory (TMO-RRAM) system. Data retention is improved by inserting a very thin SiO2 layer between the bottom electrode and TiO2 film to enhance the Schottky barrier height, while maintaining TiO2 based RRAM characteristics.

The magnetization reversal processes of single-layer nanoscale elliptical ring arrays are examined. The magnetic field was applied parallel or perpendicular to the long axis. The magnetization hysteresis loops showed a simultaneous-reversal single-step transition or two-step transition involving flux closure vortex states. For various aspect ratios and thicknesses, the transition between single-step and double-step magnetization reversals was measured to form phase diagrams. Simulations of the magnetization reversal behavior agreed with our results.

Cross-strip magnetic tunnel junctions (MTJs) were fabricated following this sequence: Si(100)/Ta(30 A)/Co(75 A)/AlOx(deltato)/Co (75 A)/ IrMn(90 A)/Ta(100 A) was deposited under an in-plane deposition field (h) = 500 Oe, where deltato = 12, 17, 22, and 26, and 30 A was the thickness of the AlO x layer. Tunneling magnetoresistance (TMR) rose initially from 21% to 36% and then fell to 24% as deltato increased monotonically. This means the spin tunneling weakened as deltato ges 26 A. The magnetostriction (lambdas) of the Co/AlOx/Co MTJs was generally in the range of -23 ppm to -12 ppm. Moreover, the lambdas vs. deltato plot shows the concave-up feature. High-resolution sectional transmission electron microscope (HR X-TEM) with nanoprobe energy dispersive x-ray (EDX) spectroscopy was used to analyze the Co, Al, and O concentrations across each AlOx layer. The different concentration dependences along deltato resulted in the lambdas feature. In addition, HR X-TEM, showed that in the microstructure of each Co/AlOx/Co MTJ, the interfaces of Co/AlO x and AlO x/Co are smooth. We find that the optimal MTJ in this series has the following properties: TMR = 36% and lambdas = -15 ppm.

We reported the synthesis and characterization of water-based FePt nanoparticles for biomedical applications. The superparamagnetic and hydrophilic FePt nanoparticles were synthesized by phase transfer method and the particles size were just only 2-4 nm. The functional FePt nanoparticles through ligand exchanging were achieved to bind the specific biomolecules. The streptavidin-biotin binding pair was used to demonstrate that water-based FePt nanoparticles could be further functionalized to provide a biotin moiety for specific interaction with streptavidin protein.

FePt/SiO2 nanogranular thin films have been prepared by molecular-beam epitaxy system on MgO (001) substrates with the method of insertion dual SiO2 layers into Fe/Pt multilayer films. We report the relationships between the inserting thickness of SiO2 layers and the microstructural and magnetic properties of FePt thin films. It indicated the nanogranular FePt thin films were successfully formed by inserting amorphous SiO2 layers into the Fe/Pt films. The reduction of grain/domain size and isolation of FePt particles can be achieved by such insertion and maintain (001) texture. The average grain size of FePt films with 5-nm SiO2 insert layers is estimated to be around 8 nm, while domain rotation is enhanced depicting a decoupling of intergrain interaction. The isolated grains are less magnetically coupled in the rotation mode and the reversal of magnetization is more independent

The objective of this paper is to propose a compact method using a novel bias magnet to induce an axial bias-magnetic force in order to reduce the radial vibration of a magnetic bearing (MB) motor. After applying the bias-magnetic force, the optimal radial vibration of the developed MB motor is only around 0.197 g (peak-peak, 1g = 9.8 m/s2 ) when the natural frequency of the bias magnet reaches the resonant frequency of 62 Hz. In this optimal condition, the ratio in the radial vibration of the MB motor with the bias-magnetic force to the original one is around 89%, and the driving power of 0.24 W is saved

Effect of Ag segregation on magnetic and structural properties of (FePt)77 Ag23 single-layer films were studied. Thin films were first sputtered on the heated glass substrates and cooled to room temperature, a post annealing at temperatures (Tp) from 200 degC to 400 degC for 60 min was then applied. Ll0 structure formed as substrate temperature (Ts) equals to 600 degC and 700 degC. For those ordered films, Ag was observed to precipitate as particles on the film surface and grow with the increasing Ts. After post annealing at 400 degC for 60min, those silver particles dissolved into the film, leading to changes in reversal behavior. The optimum condition is obtained in the Ts=600degC, Tp=400degC sample. Drastic reduction in coercive squareness ratio (from >0.8 to 0.41) and enhanced coercivity (from 4.7 to 6.4 kOe) indicate the reversal behavior alters from cooperative to independent. Negative deltaM curve suggests that this independent rotation results from the reduction of exchange interactions

Oscillating voltage (VOs), which depends on the frequency dependence of the magnetoimpedance (MI) effect, was applied to study a magnetic tunneling junction (MTJ) of Ru(5 nm)/Cu(10 nm)/Ru(5 nm)/IrMn(10 nm)/CoFeB(4 nm)/Al2 O3/CoFeB(4 nm)/Ru(5 nm) at frequencies up to 40 MHz. The MI ratio decreased as the VOs was increased. The MI ratio turned from positive to negative at a certain frequency. An equivalent circuit model was employed to analyze the results. The fact that MTJ can be regarded as the composition of a resistance component and two sets of parallel resistance (R) and capacitance (C) components in series has been utilized to describe the individual impedance contribution from the lead of cross pattern, barrier, and interface. The resistance (Rbarrier) and capacitance (Cbarrier) of the barrier effect are functions of VOs. The Rbarrier decreases as the VOs increases, However, C barrier behaves the opposite way. The tendency is for interfacial resistance Rinterface and interfacial capacitance Cinterface to have opposite results with increasing VOs . This work provides a detailed investigation of high-frequency transport behavior subjected to VOs, especially useful for MTJ characterization

Single-layer FePt, bilayer Mo/FePt and FePt/Mo, and trilayer Mo/FePt/Mo samples were prepared by rf magnetron sputtering. Each layer of FePt and Mo in the studied samples is 20 nm in thickness. Samples were deposited onto corning 1737 glass substrate at room temperature, then annealed at 350degC and 400degC for a period of 10-120 min. Coercivities of the Mo-layer introduced samples were higher than those of single-layer FePt. X-ray diffraction data also indicate higher degree of ordering for Mo-layer introduced samples. We suggest that the incoherent FePt/Mo interface may induce extra nucleation sites for ordering transformation, thus lower the threshold of ordering temperature

The 2-4 nm FePt nanoparticles were synthesized by chemical reduction and dispersed into the aqueous solution after the surface modification. The dielectric spectra were measured by an open-ended transmission line technique. An open-ended coaxial cable line was designed to transmit the microwave signals into nanoparticle-filled solutions. The reflection signal revealed the dielectric properties of hydrophobic-coated FePt-filled solution was received by the transmission line. The analysis showed that the effective permittivity was around 70 and down to 50 by the external magnetic field. The flexibility of nanoparticles' permittivity revealed in this study implicates their potential applications ranged from in microwave and millimeter-wave components to biomedical drug delivery system and positron emission tomography scan

The structural and exchange bias properties of CoFe/IrMn prepared on Si (100) and Si (111) with an Os/Cu buffer layer were investigated. Since the Os (0002) surface mesh has the same atomic arrangement as fcc (111) orientation, and the lattice mismatch between Os (0002) and IrMn (111) is as low as 2.6%, the CoFe/IrMn grown on H-Si(100) showed a strong IrMn (111) diffraction peak, while a very weak IrMn (111) peak appeared on H-Si (111). With increasing Os thickness (dOs), the IrMn (200) peak was weakened, while the IrMn (111) became strong on H-Si (100). For the CoFe/IrMn grown on H-Si(111), no obvious structural change appeared. Os plays an important role on tuning the IrMn to result in the exchange bias. On the other hand, CoFe/IrMn showed an exchange field (Hex) on both H-Si(100) and H-Si(111) with the Os buffer layer; however, the magnetization switching process was different due to different the crystalline degree. A sharp magnetization switching process occurs for IrMn(111) on Os/Cu/H-Si(100) with a square hysteresis loop. A 370 and 310 Oe of Hex was found in textured CoFe/IrMn on Os/Cu/H-Si(100) and Os/Cu/H-Si(111), respectively

Thermally activated processes have intrinsically stochastic, exponentially distributed waiting times. A simulation algorithm based on this property is used here to model the effect of a recording head on an initially demagnetized, thermally relaxing particulate medium. Coherent rotation of magnetization and dipolar interactions are assumed and it is found that interactions enhance transition noise and the width of the transition region.

The ac properties of a pseudo spin valve (PSV) consisting of NOL top/Co/Cu/Co/Ni80Fe20/NOLbottom with a top and bottom nano-oxide layer (NOL) were studied as functions of NOL and thickness, d, by magneto impedance spectroscopy. The NOLs were formed by natural oxidizing the top and bottom ferromagnetic or Ta layers in situ. The increase in the thickness of NOL resulting in the increment of effective capacitance caused the shift of roll-off frequency from low to high. The frequency dependence behavior of this NOLtop/PSV/NOLbottom system can be modeled by equivalent circuit theory. PSV with a different NOL structure can be checked by such a nondestructive analysis method

The torque of magnetic coupling with different magnetic poles has been investigated by theoretical computations with two dimensional modeling processes of finite element analysis. The torque is sensitive to the number of magnetic poles, the material of the magnetic gears, and the distance between the magnetic gears. For a 2 mm separation distance of two magnetic gears with 20 mm radius, the maximum torque occurs between 6 and 20 poles which is dependent on the thickness of the iron yoke inside the magnets, and on the magnetization configuration within a pole of the magnets. This is explained by analyzing the magnetic coupling strength of each pole. Finally, the computer calculation is tested using an experimental data taken from a sintered NdFeB magnetic gear.

We report on the magnetic properties of CuO nanoparticles prepared by the sol-gel method. M (T) curves show a blocking temperature at T/sub B/=135 K and 160 K for 13 nm and 17 nm size particles, respectively, in measuring field H=100 Oe. The M-H data for both the samples show a reversal magnetization at 300 K, whereas below T/sub B/ it exhibit symmetrical hysteresis loops with a coercive field of 200 Oe (155 Oe) and a remanence 0.306 emu/mole (0.171 emu/mole) for 13 nm (17 nm) at 5 K. Presence of superparamagnetism and net magnetic moments of CuO nanoparticles are attributed to the uncompensated Cu/sup 2+/ions at the surface of the particles.

When demagnetized La /sub 1-x/Sr/sub x/MnO/sub 3/ (LSMO) thin films are magnetized by a local magnetic field from a scanning magnetic tip without an external magnetic field, a magnetic ripple pattern appears on the LSMO surface. The magnetic ripple pattern, consisting of magnetic ripples with varying magnetic periods, is different from the conventional pattern magnetized by an external field. The magnetic ripples with longer magnetic periods have a larger magnitude of magnetic moment. However, the magnitude of the remanent magnetization induced by the magnetic force microscopy (MFM) tip is close to that induced by the external field. Since the MFM images of the magnetic ripples with varying periods can be converted to inverse images and spectra by a fast Fourier transformation (FFT), the inverse FFT images and spectra are also used to probe the magnitude of magnetic moment of the periodic magnetic ripples.

The Fe/sub 3/O/sub 4/ powder was mixed with Cu-ferrite powder then sintered in an argon atmosphere at temperatures between 1000/spl deg/C and 1250/spl deg/C for 4 h. The Cu-ferrite content in the mixture powder was between 0 mol% and 30 mol%. The effects of Cu-ferrite content and sintering temperature on the magnetoresistance (MR) and microstructure of the sintered ferrite were investigated. It was found that the Cu ions were dispersed uniformly in all sintered samples by the analysis of Cu element mapping of the scanning electron microscopy. The maximum MR ratio of the sintered ferrite at room temperature is about 7.3% when Cu-ferrite content was 7.28 mol% and sintering temperature was 1200/spl deg/C. From the plot of log /spl rho/ versus T/sup -1/2/ curves and the measurement of the current versus voltage curves of the sintered samples, it can be known that the magnetoresistance mechanisms in these sintered samples are the contribution of spin-polarized tunneling effect.

The room temperature stability of a silicon base magnetic tunneling transistor fabricated by a magnetic tunneling junction (MTJ) with a p-n junction barrier prepared on a Si (100) substrate is reported. The collector current and the change of magnetocurrent (MC) at room temperature as a function of magnetic field at the emitter bias is investigated. The collector current varies from 4.2 /spl mu/A at magnetically parallel state to 2.89 /spl mu/A at magnetically anti-parallel state. The emitter voltage dependence of emitter currents and the transfer ratio at the parallel state were observed to increase as the emitter voltage (V/sub E/) is increased. MC of emitter and base currents decreases as V/sub E/ goes up while the MC of the collector current reaches a maximum at V/sub E/ of 1.2 V.

The current-induced domain-wall motion has been observed by using a U-shaped permalloy wire. We observed two magnetic states in a U-shaped pattern. One is the vortex domain wall at the center of semicircular arc region of the U-shaped pattern, and the other is the continuous magnetic state without the domain wall in between. In general, the current density of the order of /spl sim/10/sup 7/ A/cm/sup 2/ is needed to drive a magnetic domain wall. In this paper, the critical current for domain-wall motion increases as the bias field increases. The bias field means the field deviated from the switching field of the wire.

Ag submicrometer pyramidal islands with a (100) truncated top and four (111) sidewalls were successfully fabricated on Si(100) substrates at a deposition temperature of 500/spl deg/C. The magnetization reversal of 30 nm thick Fe films grown on Ag pyramidal islands was studied by magnetic force microscopy. The magnetization reversal of Fe films was first started at the sidewalls of pyramidal islands. Then, the formation and propagation of wall-like structures were observed under a higher external magnetic field, namely 95 Oe. The magnetic domain images indicated that the Fe films grown on the sidewalls of pyramidal islands and the strength of external field played important roles on the magnetization reversal of Fe films.

Both face-centered cubic (111) single- and bi-crystal [Co(3/spl Aring/)/Pt(10/spl Aring/)/sub 23/ multilayers have been successfully fabricated on sapphire (0001) and yttria-stabilized cubic zirconia (100) substrates, respectively, via molecular-beam epitaxy. All as-deposited samples displayed a perpendicular magnetic anisotropy with coercivity of around 1000 Oe. After post annealing, bi-crystal films possessed a higher coercive field and better squareness than the other. Under post annealing at 400/spl deg/C, the single crystal lost its perpendicular magnetic anisotropy because of the diffusion of Pt buffer layers and the destruction of Co/Pt multilayered structures. The coercive force of bicrystal films remained unchanged after post annealing. However, a 500/spl deg/C post annealing is enough to reduce the perpendicular magnetic anisotropy of bicrystal films. The rough surface and grain boundaries may play important roles on the magnetic hardness in bicrystal systems.

The torque of magnetic coupling with different magnetic poles has been investigated by theoretical computations with both 2D and 3D modelling processes of the finite element analysis. The torque can be increased by increasing the number of poles for magnetic coupling between magnetic gears with 2 mm separation distance, provided that the number of poles is less than 10. However, the torque is inversely proportional to the number of poles when its number is larger than 10. This phenomenon is explained by analyzing the magnetic coupling strength of different poles. The 3D computer simulations are in excellent agreement with the experimental values. The 2D computer simulations can be improved by a "two magnetization vectors within a pole" technique; i.e. by properly rotating the angle between the two magnetization vectors.

The torque of magnetic coupling with different magnetic poles has been investigated by theoretical computations with both FEA and torque formula techniques. The torque of magnetic coupling is sensitive to the number of magnetic poles, and it decreases with increasing the distance between the magnetic rings. From our theoretical computations, we find that we can increase the torque by increasing the number of poles for magnetic coupling between magnetic rings with short distance; however, this is not true for magnetic rings with large separation. We also demonstrate that the theoretical computations of both MagNet software and torque formula established by Furlani (1993) are quite consistent and useful for computing the torque of various magnetic couplings. Finally, the theoretical calculation has been successfully tested by using an experimental data taken from a NdFeB magnetic coupling. >

Exchange anisotropy for an ordered tetragonal PtMn/Ni/sub 80/Fe/sub 20/(110) quad-crystal film grown on an Mo(001) step surface was studied. The PtMn/Ni/sub 80/Fe/sub 20/ bilayers grown at 300/spl deg/C show a unidirectional exchange anisotropy with the easy axis parallel to the underlying Mo step-edge direction. The azimuthal distribution of the coercive fields displays a fourfold symmetry. The azimuthal distribution of the coercive fields can be explained by the fourfold crystalline-induced interfacial strain between Ni/sub 80/Fe/sub 20/ and Mo. The azimuthal distribution of the exchange fields was mainly attributed to the step-induced unidirectional anisotropy and the biquadratic coupling between antiferromagnetic-ferromagnetic interface.

For studying the influence of the current passed through a metal line for the magnetic cells on semiconductors, we prepared two types of the devices. Case 1 is that only one patterned permalloy cell on top of the insulated metal strip, and two cells are beside the strip. Case 2 is that all three patterned magnetic cells are on top of the strip. The magnetic field needed to reverse the magnetization of a submicrometer-size permalloy single domain cell with aspect ratio of 6 is larger than that of a unpatterned millimeter-size permalloy thin film due to the dimension effect. Magnetic force microscopy images of the patterned cells before and after applying various electrical currents were investigated. We have observed that: 1) the magnetic field produced by the word line will not change the magnetic configuration of the magnetic cells near the wires; 2) the magnetic field produced by the word line is quite uniform; and 3) for small aspect ratio of the submicrometer magnetic cells (

Single-layer Fe films 800 A thick, with and without a 10-A-thick Ag buffer layer, were prepared at various growtht emperature,Tg, on GaAs(001) substrates by molecular-beam epitaxy. For Tg around 200 °C, high-quality single crystal films were obtained without the Ag buffer layer. For Tg ⩽100 °C, a defect structure was observed in the iron layer. However, the addition of an ultrathin Ag buffer layer between GaAs and Fe has the desirable effects of eliminating the defect structure and significantly enhancing the FMR absorption.

Perpendicular-anisotropic Nd-Fe-B films can be obtained as-deposited at high temperature. High deposition temperatures enhance the growth of columnar grain structure. Isotropic Nd-Fe-B films were obtained by deposition at room temperature followed by post annealing in high vacuum. Two kinds of microstructure were observed in cross-sectional TEM micrographs. Magnetic domains of nanocrystalline Nd-Fe-B films were investigated by MFM to explain the magnetization reversal behavior.

Nanometer size Fe/sub 3/O/sub 4/ particles were fabricated by chemical coprecipitation technique. The particle shape and size are affected by the PH value of the reactive solutions. The ferrofluids were fabricated with the Fe/sub 3/O/sub 4/ particles as magnetic particles, ammonium oleate as surfactant, and de-ionized water as solvent. Optical transmission of Fe/sub 3/O/sub 4/ nanoparticle ferrofluids was investigated as a function of incident optical wavelengths between 450 and 750 nm and applied magnetic fields up to 150 Oe. In general, for samples with lower PH value, the particles are small and agglomerate together, however, for samples with higher PH value, the particles are larger and distributed uniformly. Samples precipitated with higher PH value show larger variation of the transmittance (/spl Delta/T). This can be understood by our transmission electron microscope and X-ray diffraction pattern studies. The behavior of agglomeration for samples with lower PH value relates to the smaller variation of the optical transmission to the magnetic field.

The magnetic properties of ultrathin Co/Ag/Si[111] films were studied by surface magneto-optic Kerr effect. A magnetic phase diagram for binary-metal Co/Ag films deposited on Si[111] surfaces has been successfully established. The phase diagram can be divided into four regions: nonferromagnetic Co-Si compound; low Curie temperature Co film; canted out-of-plane anisotropy; in-plane anisotropy. The ferromagnetic inactive layers at the Co/Si interface are formed due to intermixing of Co and Si; and can be efficiently reduced by the Ag buffer layer. We have demonstrated that the inactive layer thickness is reduced from 2.1 ML for Co/Si[111], to 1.0 for Co/2.8 MLAg/Si[111], and 0.0 for Co/5.6 MLAg/Si[111].

A simulation algorithm is proposed which reproduces, at a great CPU time saving, solutions of multilevel master equations used to describe thermally driven dynamics of interacting particle arrays. Hysteresis loops for planar particle arrays are computed and the stability of intermediate, partially demagnetized configurations and their dependence on array size are discussed.

A high efficiency and low cogging torque spindle motor has been designed by varying the magnetization profiles of the rotor and the geometry shape of the teeth of the stator in motor. Both radial and fixed magnetization profiles for the magnetic ring in a spindle motor were studied by computer simulation and experimental measurement. After smoothing the edge of the teeth of the stator, we can reduce the cogging torque tremendously. Experimental data are in good consistent with theoretical calculation.

We present the magnetoresistance and magnetization behavior in melt-spun Cu/sub 80/Ni/sub 16/Fe/sub 4/ ribbons. The as-spun ribbon exhibits magnetoresistance (MR) ratios of 13% and 0.1% at 5 K and room temperature, respectively. The MR behavior of the ribbon is found to be mostly related to the magnetic fluctuation scattering at temperatures well above the blocking temperature and the spin-dependent scattering at lower temperatures. The latter mechanism can be understood in terms of the blocking of small magnetic particles and paramagnetic-ferromagnetic transformation at lower temperatures, as supported by the magnetization data. When the microstructure is modified by proper heat treatment, the ribbon exhibits significant giant magnetoresistance behavior both at room temperature and 5 K.

The torque of the radial magnetic coupling between perpendicular magnetic gears with different magnetic poles has been investigated experimentally. The torque of magnetic coupling decreases with increasing the distance between the magnetic gears. For different multipole magnetic couplings with the same magnetic field strength, the torque of magnetic coupling increases as the number of magnetic poles increases for distances smaller than a critical separation distance d/sub c/, but it is reversed as the separation distance becomes larger. This critical separation distance for our sintered NdFeB magnet system with an iron yoke of 2 mm thickness is roughly between 9 and 11 mm. This phenomenon is explained by analyzing the magnetic coupling strength of different poles of magnets on the perpendicular magnetic gears.

Correlation between the thickness of the nonmagnetic layer and the magnetic anisotropy of epitaxial Co(11~00)/Cr(211)/MgO(110) bilayers was studied experimentally and analyzed theoretically. The Co thickness was held constant at 200 /spl Aring/ while the thickness of the Cr layer was varied between 6 and 100 /spl Aring/. Coercivity was found to decrease with increasing thickness of a continuous Cr layer: at low temperatures the observed data are in good agreement with a model of roughness induced magnetostatic anisotropy energy.

Ultrafine Gd particles in a Cu metallic matrix have been obtained by post-annealing of as-sputtered metastable Gd/sub x/Cu/sub 1-x/ alloys at temperatures higher than 250/spl deg/C. The results of resistivity measurements show that the resistivities of as-sputtered metastable Gd/sub 0.2/Cu/sub 0.8/ and Gd/sub 0.3/Cu/sub 0.7/ alloys exhibit a resistivity minimum at low temperatures, an indication of the Kondo effect. In addition the temperature of the resistivity minimum rises with the increasing Gd content. Conversely, the resistivity of films after formation of Gd granules levels off at low temperatures. The magnetic results indicate that the metastable GdCu alloys are essentially a spin-glass at low temperature with the transition temperatures at around 3.5/spl sim/5 K. However, magnetization data at 80 K shows that the as-sputtered samples contain some small Gd ferromagnetic clusters in the paramagnetic matrix. The magnetic particles of granular films begin to be thermally blocked at T/sub B/ and become magnetically ordered below T/sub B/. Due to larger grain sizes, T/sub B/ is much higher for x=0.3 than for 0.2. The variation of transport and magnetic properties for samples with different annealing processes are described in detail.