Volume 117, Issue 17, 07 May 2015
Index of content:
- PROCEEDINGS OF THE 59TH ANNUAL CONFERENCE ON MAGNETISM AND MAGNETIC MATERIALS
- Soft Magnetic Materials
Influence of microstructure on the enhancement of soft magnetic character and the induced anisotropy of field annealed HITPERM-type alloys117(2015); http://dx.doi.org/10.1063/1.4906173View Description Hide Description
Hitperm-type rapidly quenched ribbons were submitted to field annealing, both longitudinal field (LF) and transversal field (TF) to the axis of the ribbon. LF annealing yields a reduction of the magnetic anisotropy and results can be explained in the frame of random anisotropy model. A coercivity of 3 A/m is obtained for Fe39Co39 Nb 6B15 Cu 1 alloy. The addition of Cu to these Nb-containing Hitperm-type alloys is a key factor to refine the microstructure in order to reach this very low coercivity value. TF annealing produces samples with sheared hysteresis loops suitable for sensor and high frequency applications.
117(2015); http://dx.doi.org/10.1063/1.4906766View Description Hide Description
Magnetoresistance (MR) reported in some non-magnetic semiconductors (particularly silicon) has triggered considerable interest owing to the large magnitude of the effect. Here, we showed that MR in lightly doped n-Si can be significantly enhanced by introducing two diodes and proper design of the carrier path [Wan, Nature 477, 304 (2011)]. We designed a geometrical enhanced magnetoresistance (GEMR) device whose room-temperature MR ratio reaching 30% at 0.065 T and 20 000% at 1.2 T, respectively, approaching the performance of commercial MR devices. The mechanism of this GEMR is: the diodes help to define a high resistive state (HRS) and a low resistive state (LRS) in device by their openness and closeness, respectively. The ratio of apparent resistance between HRS and LRS is determined by geometry of silicon wafer and electrodes. Magnetic field could induce a transition from LRS to HRS by reshaping potential and current distribution among silicon wafer, resulting in a giant enhancement of intrinsic MR. We expect that this GEMR could be also realized in other semiconductors. The combination of high sensitivity to low magnetic fields and large high-field response should make this device concept attractive to the magnetic field sensing industry. Moreover, because this MR device is based on a conventional silicon/semiconductor platform, it should be possible to integrate this MR device with existing silicon/semiconductor devices and so aid the development of silicon/semiconductor-based magnetoelectronics. Also combining MR devices and semiconducting devices in a single Si/semiconductor chip may lead to some novel devices with hybrid function, such as electric-magnetic-photonic properties. Our work demonstrates that the charge property of semiconductor can be used in the magnetic sensing industry, where the spin properties of magnetic materials play a role traditionally.
- Hard Magnetic Materials
Simulation of electric-field and spin-transfer-torque induced magnetization switching in perpendicular magnetic tunnel junctions117(2015); http://dx.doi.org/10.1063/1.4906201View Description Hide Description
Macrospin simulations are performed to model the magnetization switching driven by the combined action of electric-field and spin-polarized electric current (spin-transfer torque; STT) in MgO/CoFeB based magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy. The results indicate that at low current case, the free layer magnetization shows a fast toggle-like switching, the final parallel or antiparallel magnetization state is determined by the electric-field effect, and the STT just helps or resists it to reach the final state depending on the current direction. However, with the increase of current strength, the contribution of STT effect gradually increases, which eventually achieves a deterministic magnetization switching state. Simulations further demonstrate that by appropriately tuning the parameters of applied electric-field and current the power consumption can be easily reduced by two orders of magnitude.
117(2015); http://dx.doi.org/10.1063/1.4906285View Description Hide Description
The magnetic properties of Nd/Ni80Fe20 (Py) bilayer films with different layer thicknesses were investigated. Hysteresis loops were measured at different temperatures, an exchange bias effect is found at low temperature due to the Nd/Py interface. By fitting the temperature dependence of the saturation magnetization, the Néel point of Nd layer is obtained. Ferromagnetic resonance (FMR) experiments were carried out at room temperature as a function of Nd layer thickness. The damping parameter obtained by theoretical fitting of the FMR linewidth shows an increasing trend with increasing Nd layer thickness, which is taken as an indication of the spin pumping effect.
A crawling and drilling microrobot driven by an external oscillating or precessional magnetic field in tubular environments117(2015); http://dx.doi.org/10.1063/1.4906446View Description Hide Description
We propose a crawling and drilling microrobot actuated by an external precessional magnetic field (EPMF) to effectively unclog obstructed blood vessels. Conventional crawling microrobots can only generate crawling motions using an external oscillating magnetic field. The proposed microrobot can generate navigating (crawling) and drilling motions selectively or simultaneously by controlling the EPMFs. We prototyped the proposed microrobot, and conducted several experiments to verify the efficacy of the crawling and drilling ability of the microrobot in a tubular environment.
Persisting structures in almost saturated magnetic states and their implication for coercivity in a grain system revealed by a large-scale two-dimensional Ginzburg-Landau type simulation117(2015); http://dx.doi.org/10.1063/1.4906949View Description Hide Description
We investigate magnetization processes expected for thin films by using a two-dimensional Ginzburg-Landau type model augmented by a long-range dipole-dipole interaction. For homogeneous systems without any extrinsic effects, we find several persisting structures with opposite magnetization in almost saturated magnetic states. They take relatively stable forms, typically, a line, a honeycomb, and so on, which originate from the global connectivity of the initial maze in the sense of percolation. For systems with square-like grain structures, on the other hand, we find maze-like structures that are patterned beyond grain boundaries for zero field. For saturated fields, we again recognize the line-type structure in some grains but do not recognize it in the other grains, strongly depending on the initial pattern in each grain. When we decrease the field, such distinctions are maintained and the latter grains keep their magnetization down to some critical field. The result indicates the duality of the so-called magnetic separability as well as a mechanism of finite coercive fields in the presence of the grain boundaries.
- Magneto-Optic, Magnetoelastic, and Magnetocaloric Materials
Bit error rate performance for head skew angle in shingled magnetic recording using dual reader heads117(2015); http://dx.doi.org/10.1063/1.4906965View Description Hide Description
The two-dimensional magnetic recording (TDMR) exploits two-dimensional signal processing using the neighboring read-back waveforms. We allocate dual readers for the intended tracks and evaluate the effects of head skew angle on the bit error rate performance in partial response class-I maximum likelihood system with a two-dimensional finite impulse response filter using two read-back waveforms under TDMR R/W channel specifications of 4 Tbit/in.2. The results show that the effect of positive skew angle is larger than that of negative skew angle, and the center of skew angle should be shifted to minus direction.
- Magnetic Nanoparticles
Effect of interlayer interaction on domain structure of CoPt stacked films with perpendicular anisotropy117(2015); http://dx.doi.org/10.1063/1.4906200View Description Hide Description
The effect of interlayer magnetostatic interaction on the domain structure of CoPt (3 nm)/Pt (δ Pt nm)/CoPt (10 nm) stacked films having perpendicular anisotropy is investigated. The domain structure of the demagnetized CoPt stacked films is observed using magnetic force microscope. The Co80Pt20 stacked films with Pt interlayer thickness δ Pt less than about 20 nm have the maze domain similar to that of the film with δ Pt of 0 nm. This is because the top and bottom layers are connected by the magnetostatic interaction and the magnetization distribution of both layers is integrated. The domain structure of the films with δ Pt around 25 nm is mixture of the maze and irregular domains. For the films with δ Pt over about 30 nm, because the interaction between the top and bottom layers decreases, the irregular domain which is observed in the 3 nm thick CoPt single layer film appears. In the region where the domain structure changes from the maze domain to the irregular one, domain size steeply increases with increase of δ Pt.
117(2015); http://dx.doi.org/10.1063/1.4906433View Description Hide Description
Materials with perpendicular magnetic anisotropy (PMA) have been intensively studied for high-density nonvolatile memory such as spin-transfer-torque magnetic random access memory with low switching current density and high thermal stability. Compared with crystalline PMA multilayers, considerable works have been done on amorphous PMA multilayers because the amorphous materials are expected to have lower pinning site density as well as smaller domain wall width. This study is an overview of the PMA properties of amorphous [CoSiB/Pt]N multilayers with varying N, where the energy contribution is changed from domain wall energy to magnetostatic energy around N = 6. By measuring the field-induced domain wall motion, we obtain the creep exponent of μ = 1/4. These results in the amorphous PMA multilayers of [CoSiB/Pt]N demonstrate possible potential as a free layer for PMA-based memory devices.
Time-dependent domain wall nucleation probability in field-coupled nanomagnets with perpendicular anisotropy117(2015); http://dx.doi.org/10.1063/1.4906440View Description Hide Description
In this paper, the domain wall (DW) nucleation time and the DW nucleation probability of field-coupled magnets with perpendicular magnetic anisotropy are measured by experiment. A well-established Arrhenius model based on thermally activated magnetization reversal is applied to describe the time-dependent DW nucleation probability. Magneto-optical microscopy(MOKE) is used in the experiments to determine the DW nucleation time and the DW nucleation probability in a pNML inverter structure. The DW propagation speed is measured in order to calculate the required DW propagation time for entire magnetization reversal of pNML logic gates. Experimental results are compared to the derived model. Our results show that the interaction in pNML logic gates plays a significant role for the time-dependent DW nucleation probability and therefore for the reliability of field-coupled circuits.
Magnetic-tunnel-junction based low-energy nonvolatile flip-flop using an area-efficient self-terminated write driver117(2015); http://dx.doi.org/10.1063/1.4906760View Description Hide Description
A nonvolatile flip-flop (NV-FF) that makes it possible to minimize the backup energy before power-off is proposed. The use of a self-terminated write driver with continuous voltage monitoring enables automatic write-current termination to be performed immediately after the desired data are written, resulting in great energy saving. Moreover, by utilizing single-ended circuitry together with a symmetrical write current path structure, both a nonvolatile storage function and self-termination are compactly embedded in the NV-FF without performance degradation. As a result, the backup energy is reduced by 83% compared to that of the average of conventional magnetic tunnel junction (MTJ)-based NV-FFs with no self-termination. It is also demonstrated that total transistor counts are reduced by 43% compared to a conventional MTJ-based NV-FF with self-termination.
- Magnetic Recording
Investigation on high-temperature magnetic permeability of Si-rich nanocrystalline (Fe0.9Co0.1)74.5Nb2Si17.5B5Cu1 alloy117(2015); http://dx.doi.org/10.1063/1.4906297View Description Hide Description
The temperature dependence of initial permeability (μi -T curve) for as-quenched and annealed Si-rich (Fe0.9Co0.1)74.5 Nb 2Si17.5B5 Cu 1 alloy was investigated. The results showed that fewer Co doping into Fe74.5 Cu 1 Nb 2Si17.5B5 can reduce the onset primary crystallization temperature Tx1, enhance the Curie temperature of amorphous phase , and improve the high-temperature soft magnetic properties. After annealing at 480–650 °C, the common feature of μi -T curve is that the μi increases with T from room temperature and reaches a peak value at a certain temperature, which depends on the annealing temperature Ta , then μi decreases with T. This kind of μi -T curves is different from that of the common Co-contained Finemet-type alloys ever reported. The optimal high-temperature magnetic softness was obtained by annealing at 540 °C, the higher μi of 6500 at 10 kHz can be kept up to 500 °C. Such a magnetic softness at elevated temperature is superior to that of Fe74.5 Nb 2Si17.5B5 Cu 1 alloy and other Co-contained Finemet-type nanocrystalline alloys with higher Co content.
117(2015); http://dx.doi.org/10.1063/1.4906526View Description Hide Description
Nanometer (nm)-thick yttrium iron garnet (Y3Fe5O12, YIG) films present interest for spintronics. This work employs spectral ellipsometry and magneto-optic Kerr effect (MOKE) spectra to characterize nm-thick YIG films grown on single-crystal Gd3Ga5O12 substrates by magnetron sputtering. The thickness (t) of the films ranges between 10 nm and 40 nm. Independent on t, the polar MOKE hysteresis loops saturate in the field of about 1.8 kOe, consistent with the saturation magnetization in bulk YIG (4πM s ≈ 1.75 kG). The MOKE spectrum measured at photon energies between 1.3 eV and 4.5 eV on the 38-nm-thick film agrees with that measured on single-crystal YIG bulk materials. The MOKE spectrum of the 12-nm-thick film still preserves the structure of the bulk YIG but its amplitude at lower photon energies is modified due to the fact that the radiation penetration depth exceeds 20 nm. The t dependence of the MOKE amplitude is consistent with MOKE calculations. The results indicate that the films are stoichiometric, strain free, without Fe2+, and preserve bulk YIG properties down to t ≈ 10 nm.
Hybrid absorbers composed of Fe3O4 thin film and magnetic composite sheet and enhancement of conduction noise absorption on a microstrip line117(2015); http://dx.doi.org/10.1063/1.4906763View Description Hide Description
In response to develop wide-band noise absorbers with an improved low-frequency performance, this study investigates hybrid absorbers that are composed of conductive Fe3O4 thin film and magnetic composite sheets. The Fe3O4 films prepared via reactive sputtering exhibit a typical value of electrical resistivity of ≃10−4 Ωm. Rubber composites with flaky Fe-Si-Al particles of a high permeability and high permittivity are used as the magnetic sheet functioning as an electromagnetic shield barrier. Microstrip lines with a characteristic impedance of 50 Ω are used to measure the noise absorbing properties. For the Fe3O4 film with a low surface resistance and covered by the magnetic sheet, approximately 80% power absorption can be obtained at 1 GHz, which is significantly higher than that of the original magnetic sheet or Fe3O4 film. The high power absorption of the hybrid absorber is attributed to the enhanced ohmic loss of the Fe3O4 film through increased electric field strength bounded by the upper magnetic composite sheet. The noise absorption is further enhanced through increasing the electrical conductivity of the film containing more conductive phase (Fe3O4 + Fe), which can be prepared in a reduced oxygen partial pressure during reactive sputtering.
117(2015); http://dx.doi.org/10.1063/1.4906945View Description Hide Description
Field-induced unidirectional anisotropy of Co-based amorphous ribbons was discussed by magnetization measurement. The shifted hysteresis loops of Co58Fe5Ni10Si11B16 amorphous ribbons were obtained by annealing the samples in longitudinal magnetic field. Here, the feasibility of employing the longitudinal pulse field to tailor the anisotropy characteristic is demonstrated. It is found that the shifted loops can be technically controlled by enhancing the pulse field to modulate the magnetic anisotropy from unidirectional to uniaxial, and even back to unidirectional. The surface domains scan gives strong evidence that the pulse field can be one of the skillful methods to navigate the unidirectional anisotropy in the amorphous ribbons for potential applications.
- Patterned Films
117(2015); http://dx.doi.org/10.1063/1.4906527View Description Hide Description
In this work, we investigated the structural and magnetic properties of both as-deposited and post-annealed Cu-doped ZnO thin films for better understanding the possible mechanisms of room-temperature ferromagnetism (RT-FM) in ZnO-based diluted magnetic oxides. All of the films have a c-axis-oriented wurtzite structure and display RT-FM. X-ray photoelectron spectroscopy results showed that the incorporated Cu ions in as-deposited films are in 1+ valence state merely, while an additional 2+ valence state occurs in post-annealed films. The presence of Cu 2+ state in post-annealed film accompanies a higher magnetization value than that of as-deposited film and, in particular, the magnetization curves at 10 K and 300 K of the post-annealed film separate distinctly. Since Cu 1+ ion has a filled 3d band, the RT-FM in as-deposited Cu-doped ZnO thin films may stem solely from intrinsic defects, while that in post-annealed films is enhanced due to the presence of CuO crystallites.
- Ultra-thin and Multi-layer Films
117(2015); http://dx.doi.org/10.1063/1.4906207View Description Hide Description
Polycrystalline La0.7Sr0.3Mn1−xCoxO3 (x = 0.0–1.0) samples were synthesized by solid-state reaction. Structural and magnetization studies reveal the replacement of Co for Mn in La0.7Sr0.3Mn1−xCoxO3, and the decrease of the Curie temperature (T C) from 360 K (for x = 0) to 224 K (for x = 1). Positive slopes observed in the H/M versus M 2 curves prove all the samples undergo a second-order magnetic phase transition. By analysis of the M(H) data at temperatures around T C using the Kouvel-Fisher method, we obtained the values of critical parameters (T C, β, γ, and δ). The results suggest an existence of short-range FM order in the sample x = 0 with β = 0.377. Meanwhile, for the case of Co-doped samples, their β values in the range of 0.403–0.457 indicate a coexistence of short and long-range FM order. This means that Co-doping favors establishing FM long-range order in La0.7Sr0.3Mn1−xCoxO3. From M(H) data, we have also determined the magnetic entropy change (ΔS m) for the samples. We have found that the ΔS m(T) curves for different applied fields are collapsed onto a universal curve by normalizing the ΔS m(T) curves to their respective maximum value ΔS max (i.e., ΔS m(T)/ΔS max) and rescaling the temperature axis above and below T C with , where T r is the reference temperature.
117(2015); http://dx.doi.org/10.1063/1.4906281View Description Hide Description
Domain structures and magnetization reversal of (Co/Pd) and (CoFeB/Pd) multilayers with 7 and 14 repeats were investigated. The Co-based multilayers show much larger coercivities, a better squareness, and a sharper magnetization switching than CoFeB-based multilayers. From magnetic force microscopy observations, both structures show strong reduction in domains size as the number of repeats increases but the magnetic domains for Co-based multilayers are more than one order of magnitude larger than for CoFeB-based multilayers. By imaging domains at different times, breaks in the (CoFeB/Pd) multilayer stripes were observed within only few hours, while no change could be seen for (Co/Pd) multilayers. Although CoFeB single layers are suitable for magnetoresistive devices due to their large spin polarization and low damping constants, their lamination with Pd suffers mainly from thermal instability.
117(2015); http://dx.doi.org/10.1063/1.4906279View Description Hide Description
Motivated by the interesting magnetic anisotropy found in the heavy fermion family CeTX 2 (T = transition metal and X = pnictogen), here, we study the novel parent compound CeAu1− x Bi2− y by combining magnetization, pressure dependent electrical resistivity, and heat-capacity measurements. The magnetic properties of our nearly stoichiometric single crystal sample of CeAu1− x Bi2− y (x = 0.92 and y = 1.6) revealed an antiferromagnetic ordering at T N = 12 K with an easy axis along the c-direction. The field dependent magnetization data at low temperatures reveal the existence of a spin-flop transition when the field is applied along the c-axis (Hc ∼ 7.5 T and T = 5 K). The heat capacity and pressure dependent resistivity data suggest that CeAu0.92Bi1.6 exhibits a weak heavy fermion behavior with strongly localized Ce3+ 4f electrons. Furthermore, the systematic analysis using a mean field model including anisotropic nearest-neighbors interactions and the tetragonal crystalline electric field (CEF) Hamiltonian allows us to extract a CEF scheme and two different values for the anisotropic exchange parameters between the Ce3+ ions in this compound. Thus, we discuss a scenario, considering both the anisotropic magnetic interactions and the tetragonal CEF effects, in the CeAu1− x Bi2− y compounds, and we compare our results with the isostructural compound CeCuBi2.
117(2015); http://dx.doi.org/10.1063/1.4906432View Description Hide Description
The in-plane magnetization behaviors in TbB4 are theoretically studied using the frustrated classical XY model, including the exchange and biquadratic interactions, and the anisotropy energy. The magnetization curves at various temperatures are simulated, and the magnetic orders are uncovered by the tracking of the spin configurations. In addition, the effects of the in-plane anisotropy and biquadratic interaction on the magnetization curves are investigated in detail. The simulated results suggest that the magnetic anisotropy within the (001) plane owes to the complex interplay between these couplings, and the anisotropy term plays an important role.