Shu-Fan Cheng

Growth and magnetic properties of single crystal Co/sub 2/MnX (X=Si,Ge) Heusler alloys

Half-metallic (HM) ferromagnetic materials have recently drawn intense interests due to their potential use in magnetoelectronic devices. Co-based Heusler alloys of the type CO/sub 2/MnX (X=Si,Ge), predicted to be HM by first principles band structure calculations, are of particular interest since they alone possess Curie temperatures in excess of 900 K. Since the spin polarization (P) is believed to be sensitive to antisite defects that are likely to occur in vapor-quenched thin film synthesis, single crystals of Co/sub 2/MnX (X=Si, Ge) were prepared using the tri-arc Czochralski method. X-ray diffraction, including Laue backscattering, was employed to determine the high crystalline quality of these crystals. SQUID magnetometry measured a magnetic moment per formular unit that is close to the calculated value indicating that these alloys may in fact be HM. However, point contact Andreev reflection, a technique that has reliably measured high P in CrO/sub 2/, measures P values for these crystals of 50-60%, well below their theoretical values. The reduced spin polarization may be due to the effects of crystal symmetry breaking at the surface or the presence of anti-site defects, or that these materials are not truly half-metallic.

Structure of melt-spun Fe-Ga based magnetostrictive alloys

Fe/sub 1-x/Ga/sub x/ (x=0.17, 0.21, 0.25 and 0.3) alloys were rapidly quenched by the melt-spinning technique to determine the maximum Ga concentration that retains the disordered bcc (/spl alpha/-Fe) phase at room temperature. The texture of the ribbons as a function of melt-spinning parameters and annealing was also extensively studied. All of the as-spun samples were found to be crystalline. For x=0.17, only the disordered bcc phase was found and for x=0.25 and 0.3, primarily DO/sub 3/ phase was found, while for x=0.21, the phase is bcc at high wheel speed and DO/sub 3/ at low speed. For the free side of the as-spun ribbons, the easy growth axis of /spl alpha/-Fe or of DO/sub 3/ tilts about 10/spl deg/ to 20/spl deg/ from the ribbon normal and is along the ribbon length. This texture does not change significantly with wheel speed. The texture on the wheel side of the ribbon is close to random at high speed but increases with slower speed. For Fe/sub 83/Ga/sub 17/, 900/spl deg/C annealing reduces the tilt angle while 1100/spl deg/C annealing recrystallizes the grains and produces strong out of plane texture but randomizes the directionality in the plane. Annealing with sulfur addition was found to enhance the {100} texture but did not produce the desired along the longitudinal direction of the ribbon. The saturation magnetostriction, /spl lambda//sub s/, of as-spun Fe/sub 79/Ga/sub 21/ and Fe/sub 83/Ga/sub 17/ both having a disordered bcc structure and strong {100} texture was measured as 98 and 130 ppm, respectively, along the ribbon length.

Grain size dependence of magnetic properties in shock synthesized bulk Pr2Fe14B∕α-Fe nanocomposites

The structural and magnetic properties of the melt-spun Pr2Fe14B∕α-Fe nanocomposite powders consolidated via shock-wave compression and subjected to postshock thermal treatment were investigated. Shock compression results in grain refinement, which leads to a reduction of an effective anisotropy and therefore an increase in the ferromagnetic exchange length, resulting in an enhanced exchange coupling in fully consolidated bulk magnets. A small amount of amorphous phase formed during the shock compression were observed to crystallize into Pr2Fe14B upon annealing above 600°C. The heat treatment also results in the recovery of coercivity partially lost during the consolidation, which can be related directly to the dependence of the effective anisotropy on the grain size, as illustrated by the transmission electron microscopy observation of grain refinement in the shock-consolidated bulk samples. A uniform grain morphology is suggested as a means for further increasing the magnetic properties of bulk nanocomp...

Characterization of as-deposited and sintered yttrium iron garnet thick films formed by aerosol deposition

We have employed the aerosol deposition method (ADM) to direct-write 39-µm-thick polycrystalline films of yttrium iron garnet at room temperature onto sapphire at a rate of 1–3 µm/min as a first step toward integration into microwave magnetic circuits. The resulting randomly oriented polycrystalline films are composed of fractured compact nanosized crystallites. Upon postdeposition sintering at 1280 °C the density and magnetic properties become closer to those of the bulk. These results suggest that the ADM shows promise for depositing very thick films at high deposition rates to produce structurally and magnetically uniform films that retain the structural properties of the starting powder.

Advances in magnetometry through miniaturization

Recent innovations may lead to magnetic sensors that are smaller, more sensitive, and/or cost less than current magnetometers. Examples of this are the chip scale atomic magnetometer, magnetic tunnel junctions with MgO barriers, and a device for minimizing the effect of 1∕f noise, the microelectromechanical system (MEMS) flux concentrator. In the chip scale atomic magnetometer, researchers have been able to fabricate the light source, optics, heater, optical cell, and photodiode detector in a stack that passes through a silicon wafer. Theoretical and subsequent experimental work has led to the observation of magnetoresistance values of 400% at room temperature in magnetic tunnel junctions with MgO barriers. This large magnetoresistance occurs because electrons in the majority band can tunnel more easily through the MgO barrier than electrons in the minority band. The MEMS flux concentrator has the potential to increase the sensitivity of magnetic sensors at low frequencies by more than an order of magnitu...

Aerosol Deposition of Yttrium Iron Garnet for Fabrication of Ferrite-Integrated On-Chip Inductors

We have employed aerosol deposition (AD) to deposit 39 μm thick polycrystalline films of yttrium iron garnet at room temperature onto sapphire at a rate of 1-3 μm/min as an initial investigation of utilizing AD for fabricating ferrite-integrated on-chip inductors. We characterize the structural and magnetic properties of the as-received starting powder, as-deposited film, and a pressed puck formed from the starting powder. Results show that the films are comprised of randomly oriented polycrystalline grains with structural and magnetic properties that closely resemble that of the starting powder. Results from coating a gold single-turn inductor show an increase in inductance of 79% up to ~300 MHz without affecting the Q-factor. These results demonstrate AD as a promising technique for depositing thick ferrite films at high deposition rates for low-temperature fabrication of ferrite-integrated on-chip inductors.

Growth and magnetic properties of single crystal Fe1−xCoxS2 (x=0.35–1)

The pyrite Fe1−xCoxS2 (0.25⩽x⩽0.9) was predicted to be half metal, insensitive to Fe–Co disorder. To verify its half metallicity, we have grown single crystals of Fe1−xCoxS2 (x=0.35, 0.5, 0.75, 0.9, and 1). Crystals of up to a few mm in size by temperature gradient (923–873 K) solution growth from Te melts, which showed high crystalline quality by scanning electron microscopy, x-ray diffraction, and magnetic measurements. A half-metallic magnetization of 1±0.03μB per Co atom was found for x=0.35–0.9. Point contact Andreev reflection measurements, however, showed spin polarization of 47%–61% with the maximum around x=0.5, well below predictions. Ferromagnetic resonance measurements give g∼2.08 with large intrinsic damping, which indicates a reduced half-metallicity. Inductively coupled plasma and energy dispersive x-ray spectroscopy results confirm a sulfur deficiency of ∼1.5%–10% [(Fe,Co)S2−y,0.03⩽y⩽0.2] and small amounts of Te. Electronic structure calculations show the extra electron provided by Co subs...

Underlayer Effect on Perpendicular Magnetic Anisotropy in Co 20 Fe 60 B 20 /MgO Films

Perpendicular magnetic tunneling junctions with TaCoFeBMgO have been extensively studied in recent years. However, the effects of the underlayer on the formation of the CoFeB perpendicular magnetic anisotropy (PMA) are still not well understood. Here, we report the results of our systematic use of a wide range of elements (Ti, V, Cr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, and Au) encompassed by columns IVA, VA, VIA, VIIA, and VIIIA of the periodic table as the underlayer in an underlayerCo20Fe60B20MgO stack. Our goals were to survey more elements that could conceivably create a PMA in CoFeB and thereby to explore the mechanisms enabling these underlayers to enhance or create the PMA. We found that underlayer elements having an outer shell of both 4d electrons (Zr, Nb Mo, and Pd) and 5d electrons (Hf, Ta, W, Re, Ir, and Pt) resulted in the development of a PMA in the MgO-capped Co20Fe60B20. Hybridization between the 3d electrons of the Fe or Co (in the Co20Fe60B20) at the interface with the 4d or 5d electrons of the underlayer is thought to be the cause of the PMA development.

Advances in magnetometry

Innovations may lead to magnetic sensors with superior performance. Examples of this are the chip scale atomic magnetometer, magnetic tunnel junctions with MgO barriers, and a device for minimizing the effect of 1/f noise, the MEMS flux concentrator. In the chip scale atomic magnetometer, researchers have been able to fabricate the light source, optics, heater, optical cell, and photodiode detector in a stack that passes through a silicon wafer. Theoretical and subsequent experimental work has led to the observation of magnetoresistance values of 400% at room temperature in magnetic tunnel junctions with MgO barriers. The MEMS flux concentrator has the potential to increase the sensitivity of magnetic sensors at low frequencies by more than an order of magnitude. The MEMS flux concentrator does this by shifting the operating frequency to higher frequencies where the 1/f noise is much smaller. The shift occurs because the motion of flux concentrators on MEMS flaps modulates the field at kHz frequencies at the position of the sensor.

Effects of flux concentrators on the low frequency noise in magnetoresistive sensing devices

This paper investigated the effects of flux concentrators (FCs) on magnetic 1/f noise in magnetoresistive sensing devices. This is done by comparing results for the cases of bare (without FCs) sensors and those with FCs. It is shown that FCs can be a source of broad-band low frequency noise in AC MEMS flux concentrator device when the signal modulation amplitudes are sufficiently high.