Large Saturation Fields Research Articles

Flexible Manipulation of Operating Range in Giant Magnetoresistance Unipolar Switch Based on Double-Pinned Spin Valve

Giant magnetoresistance (GMR) magnetic switches have been widely investigated due to their attractive advantages such as good stability, small size, large saturation field, and high integration. However, in currently available commercial GMR unipolar switches, the operation and release points are limited to a narrow range. In this work, a modified double pinned spin valve (SV) structure is proposed to achieve a wide operating range and easy modulation of magnetic unipolar switches. In particular, a modified SV structure of Ta/NiFe/IrMn/CoFe/Ru/CoFe/Cu/CoFe/IrMn/Ta was chosen, where the large and small exchange bias fields were obtained from the bottom synthetic antiferromagnetic (SAF) structure and the top CoFe/IrMn layers, respectively. Accordingly, the operating range can be easily modulated by adjusting the thickness of top pinned CoFe layer. It is worthwhile to note that the controllable range exceeds 100Oe, which is much larger than that exhibited by current commercialized SV structure. Furthermore, in the patterned sample having the modified double pinned SV, the operation and release points are independent of length and width. The reason is attributed to the exchange bias field, which weakens the impact of shape anisotropy. In addition, this result also facilitates the simplification of the design process. That is to say, the proposed modified double pinned SV offers a broad application region for GMR magnetic unipolar switches and reveals a new paradigm for the development of next-generation magnetic sensors.

Direct Observation of Strong Anomalous Hall Effect and Proximity-Induced Ferromagnetic State in SrIrO3.

The 5d iridium-based transition metal oxides have gained broad interest because of their strong spin-orbit coupling, which favors new or exotic quantum electronic states. On the other hand, they rarely exhibit more mainstream orders like ferromagnetism due to generally weak electron-electron correlation strength. Here, a proximity-induced ferromagnetic (FM) state with TC ≈ 100 K and strong magnetocrystalline anisotropy is shown in a SrIrO3 (SIO) heterostructure via interfacial charge transfer by using a ferromagnetic insulator in contact with SIO. Electrical transport allows to selectively probe the FM state of the SIO layer and the direct observation of a strong, intrinsic, and positive anomalous Hall effect (AHE). For T≤ 20 K, the AHE displays unusually large coercive and saturation field, a fingerprint of a strong pseudospin-lattice coupling. A Hall angle, σxy AHE /σxx , larger by an order of magnitude than in typical 3d metals and an FM net moment of about 0.1 μB /Ir, is reported. This emphasizes how efficiently the nontrivial topological band properties of SIO can be manipulated by structural modifications and the exchange interaction with 3d TMOs.

Asymmetric Hysteresis Loops in Structured Ferromagnetic Nanoparticles with Hard/Soft Areas

Horizontally shifted and asymmetric hysteresis loops are often associated with exchange-biased samples, consisting of a ferromagnet exchange coupled with an antiferromagnet. In purely ferromagnetic samples, such effects can occur due to undetected minor loops or thermal effects. Simulations of ferromagnetic nanostructures at zero temperature with sufficiently large saturation fields should not lead to such asymmetries. Here we report on micromagnetic simulations at zero temperature, performed on sputtered nanoparticles with different structures. The small deviations of the systems due to random anisotropy orientations in the different grains can not only result in strong deviations of magnetization reversal processes and hysteresis loops, but also lead to distinctly asymmetric, horizontally shifted hysteresis loops in purely ferromagnetic nanoparticles.

The Synthesis and Characterisation of a Molecular Sea-Serpent: Studies of a {Cr24 Cu7 } Chain.

A finite chain of thirty‐one paramagnetic centers is reported, synthesized by reaction of hydrated chromium fluoride, copper carbonate and pivalic acid in the presence of 1,4,7,10‐tetrazacyclododecane (cyclen). Magnetic studies show predominantly anti‐ferromagnetic exchange leading to a high density of low‐lying spin states and large saturation field.

The Synthesis and Characterisation of a Molecular Sea‐Serpent: Studies of a {Cr24Cu7} Chain

AbstractA finite chain of thirty‐one paramagnetic centers is reported, synthesized by reaction of hydrated chromium fluoride, copper carbonate and pivalic acid in the presence of 1,4,7,10‐tetrazacyclododecane (cyclen). Magnetic studies show predominantly anti‐ferromagnetic exchange leading to a high density of low‐lying spin states and large saturation field.

Preparation and orientation mechanism analysis of (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> magneto-optical single crystal film with out-of-plane orientation

Liquid-phase epitaxy (LPE) is one of the best techniques for the preparation of single crystal garnet films. However, the specific Faraday rotation angle of Yttrium iron garnet (YIG) is small, and its easy magnetization axis is parallel to the film surface. The YIG requires a large external saturation field, which cannot meet the development needs of magneto-optical devices. It is found that Bi-substituted YIG(Bi:YIG) film has a larger specific Faraday angle. By adjusting the easy magnetization axis of Bi: YIG perpendicular to the film surface, the saturation magnetization of Bi: YIG can be reduced, so that it can work under a small external magnetic field. This meets the development needs of miniaturization and energy saving of magneto-optical device. The saturation magnetization of garnet film can be effectively reduced by substituting Ga<sup>3+</sup> for YIG crystal, mainly for Fe<sup>3+</sup> at the 24d position of its tetrahedron. And the lattice constants of Gd<sub>3</sub>Ga<sub>5</sub>O<sub>12</sub> (GGG) and YIG are 1.2383 nm and 1.2376 nm, respectively. However, the radius of Bi<sup>3+</sup> (10.8 nm) is larger than that of Y<sup>3+</sup> (9.0 nm), the lattice mismatch of garnet film increases with the incorporation of Bi<sup>3+</sup>. In order to neutralize the lattice expansion caused by Bi<sup>3+</sup>, Tm<sup>3+</sup> (8.69 nm) with a radius smaller than that of Y<sup>3+</sup> (9.0 nm) is selected. Based on the theoretical analysis of the magnetocrystalline anisotropy of garnet film, (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> mono-crystalline films with different growth temperatures and different thickness values are grown by LPE on GGG (111) substrates. The experimental results show that when the thickness of epitaxial film is greater than 1 μm, the influence of shape anisotropy on magnetocrystalline anisotropy can be ignored. With the increase of growth temperature, the substitution number of Bi<sup>3+</sup> ions decreases gradually, the lattice constant of epitaxial film decreases gradually, and the lattice mismatch first decreases and then increases. Then, the state of compressive stress gradually changes into that of tensile stress. Compared with growth-induced anisotropy, the stress-induced anisotropy is dominant in the change of magnetocrystalline anisotropy. The Verdet constant of (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> film is 11.8 × 10<sup>4</sup> rad/Tm@1064 nm. The results show that the prepared (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> mono-crystalline films have great development potential in magneto-optical devices.

Tailoring the magnetic properties of galfenol film grown on single-crystal diamond

The thin film properties are often compromised but can be tuned by optimizing the growth parameters. Galfenol (FeGa), a magnetostrictive material can be used in MEMS technology particularly due to its large magnetostriction coefficient, low saturation and switching fields. Hence the fabrication of high quality FeGa thin film is important. Recently, the growth of FeGa thin film on semiconductor substrate offer a promising platform for micromechanical sensor. Here, we report the successful growth and high magnetic properties of FeGa thin film on single crystal diamond substrate. In order to obtain FeGa thin film with high soft magnetic properties, the variations of the growth parameters including working pressure, Ar flow rate, sputtering power and growth duration are investigated. It is shown that the microstructure, surface morphology, and magnetic properties of the FeGa thin film can be tailored by the growth parameters. Crystallized FeGa thin films with surface roughness less than 1 nm, low coercivity (Hc) of 26.2 Oe, and low saturation magnetization field (Hs) of 450 Oe and high remanence ratio (Mr/Ms) of 0.9 are obtained. The integration of FeGa thin film on diamond promotes the development of device based on magneto-strictive film integrated with semiconductors.

Cluster spin glass state caused by antiphase boundaries in NiFeGa alloys

We report an observation of magnetic glassy properties in NiFeGa ferromagnetic shape memory alloys. A cluster spin glass transition was observed at approximately 27 K in Ni50Fe21Ga29. The glassy property originates from the freezing of fine magnetic domains separated by antiphase boundaries (APBs). Due to the pinning effect of the APBs, depression of the magnetization just below the Curie temperature was observed. The pinned domains induce very large saturation fields and a nearly random distribution of magnetization in low field. Based on dynamic magnetic measurements, the frequency dependence of the spin-glass transition was investigated by using the Vogel-Fulcher law and a power law. Below the freezing temperature, an aging phenomenon was observed and described by stretched exponential functions. These theoretical investigations confirm the cluster spin-glass state, which originates from a strong interaction between the frustrated magnetic domains.

Strong perpendicular magnetic anisotropy in [Co/Pt]n ultrathin superlattices

Ultrathin [Co/Pt]n superlattice films consisting of 0.18–0.60-nm-thick Co and Pt sublayers were deposited by sputtering. A large in-plane saturation field (Hs) of ∼39 kOe and a very large effective perpendicular magnetic anisotropy (Keff) with a magnitude of 107 erg/cm3 were attained. The highest Keff was ∼1.40 × 107 erg/cm3. These films are promising candidates for the reference layer of the p-MgO magnetic tunnel junction in Gb-scale magnetic random-access memory.

Structural and Magnetic Properties of Amorphous and Nanocrystalline CoFeSiB Thin Films

This study examined the structural, magnetic, and transport properties of CoFeSiB films with various Co compositions. The main focus was on two samples, amorphous Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">74</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> and nanocrystalline Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">78</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> thin films. The results show that the amorphous film is a typical soft magnetic material, while the nanocrystalline film has a large saturation field. It is believed that in a nanocrystalline thin film, a large saturation field is caused by antiferromagnetic exchange at the boundary between the amorphous and nanocrystalline phases.

Magnetoresistance of mesoscopic granular ferromagnets

We have performed magnetoresistance (MR) measurements of granular ferromagnets having lateral dimensions smaller than 0.5 $\mu$m and containing a small number of grains (down to about 100). Compared to macroscopic samples, these granular samples exhibit unusually large saturation fields and MR amplitudes. In addition, the evolution of the magnetoresistance curve as the intergrain distance decreases is qualitatively different than that of large samples. We discuss these results and suggest that they reflect a transition from percolation to quasi single-channel dominated transport.

Magnetic, transport and structural properties of Co/Ir multilayers grown by molecular beam epitaxy

We report on the structural properties of a [Co30/Ir10]10× (Å) superlattice, as well as on the magnetic and transport properties of a Co15/Ir5/Co30 (Å) artificial ferrimagnetic system. The samples were grown by molecular beam epitaxy (MBE) on MgO(001) substrates covered with a Ir130 (Å) buffer layer. High resolution cross section and plan view transmission electron microscopy (TEM) images present a high quality epitaxial stack [100]MgO(001) ‖ [100]Ir(001) ‖ [100]Co(001), with a tetragonalization of the Co fcc structure, due to strains induced by the Ir buffer. TEM images also show that the Co/Ir interfaces are flat, while the layers are continuous and free of bridges. These observations are consistent with zero field nuclear magnetic resonance measurements which indicate an fcc structure of the Co layers, and an interface mixing between Co and Ir limited to one atomic layer. As a consequence the antiferromagnetically coupled Co/Ir/Co sandwich presents large saturation and coercive fields which exceed 20 kOe and 220 Oe, respectively. Annealing made on the same sandwich indicate that the magnetic and transport properties are stable up to 300 °C. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin-dependent CPP transport properties of semiconductive-oxide/ferromagnet multilayers

The spin-dependent CPP (current perpendicular to plane) transport properties in semiconductive-oxide(MO x )/ferromagnet multilayers consisting of CoPt/Co/MO x /Co/MO x /Co/CoPt, where M=Ti, Sn, were investigated by magneto-transport measurement. The XRD patterns reveal that the structures of sputtered MO x films are amorphous or nanocrystalline. The MR ratio of a TiO x -based multilayer (0.43%) was much smaller than that of a SnO x -based one (2.0%) at T=77 K. As inferred from the shape of MR curves with a large saturation field above 4 kOe, suppressed MR change in the TiO x -based multilayer is caused by the interlayer diffusion of Co atoms into the TiO x layer. The same behavior was not observed in the SnO x -based one. The MR ratio was reduced by half on increasing t SnO x from 4 to 8 nm. This decrease can be ascribed to the scattering of the spin-polarized electron in the SnO x layer.

Magnetic and electrical properties of Co/Si multilayer thin films

This paper reports on the effects of interlayer mixing and a solid-state alloying reaction on the properties of a Co/Si multilayer system. The compositionally modified structure changes from a CoSi alloy to Co/CoSi to CoSi/Si layered systems as the nominal thickness of Si is increased over that of Co. The temperature-dependent magnetic properties of the multilayers are complex, with ferromagnetic and also thermally unstable superparamagnetic properties characterized by a blocking temperature behavior typical of small, low anisotropy particles. The electrical resistivity charts a transition from metallic to nonmetallic character as a function of increasing Si content. Resistivity minima and large magnetic saturation fields at low temperatures suggest the presence of Kondo-like behavior typical of dilute alloys.

Exchange anisotropy in epitaxial NiO(001)-fcc Co bilayers

We have sputter deposited NiO–Co bilayers on MgO (001) substrates. NiO grows epitaxially on MgO at 800 °C and subsequently room temperature deposited Co films have a fcc crystal structure in epitaxy with the oxide underlayer. Bias fields higher than 700 Oe at 5 K were reported. Polar Kerr effect measurements evidence saturation fields for perpendicular magnetization up to 15 T at low temperature, proof for a strong NiO–Co exchange interaction. The formation of a domain wall in the Co layer is a possible mechanism for such large saturation fields.

Magneto-impedance of glass-coated Fe–Ni–Cu microwires

The magneto-impedance (MI) of glass-coated Fe–Ni–Cu microwires was investigated for longitudinal radio-frequency (rf) currents up to a frequency of 200 MHz using a rf lock-in amplifier method. The MI, defined as ΔZ/Z=[Z(H)−Z(H=0.3 T)]/Z(H=0.3 T), displays a peak structure (negative MI) at zero field for rf currents with frequencies less than 20 MHz and this crosses over to a sharp dip (positive MI) at higher frequencies. This crossover behavior is ascribed to the skin-depth-limited response primarily governed by the field-dependence of the permeability. Large saturation fields (300 to 600 Oe) and other anomalies indicate the possible influence of giant magneto-resistance (GMR) on the MI.

1/f noise in saturated GMR heads

The extra-low-frequency noise of GMR heads was measured at large saturation fields (/spl sim/10 kOe). It was 1/f in nature. The head resistance increased exponentially with applied cumulative voltage pulses when the amplitude was larger than 1 V. However, the noise did not change even after the head resistance increased by 18%. This could be explained if the noise was generated mostly from the contiguous junction areas. A supra-linear dependence of noise voltage on sense current was observed. The supra-linearity increased after the sensor height was reduced by lapping. Hooge's formula was shown to apply at low currents. The excessive noise at high currents and at reduced sensor height was attributed to current crowding, in particular, current crowding caused by current heating.

Origin of the Anomalous Magnetic Behavior in Single CrystalFe3O4Films

Antiphase boundaries (APBs) were observed in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ single crystal films grown on MgO. The APBs are an intrinsic consequence of the nucleation and growth mechanism in films. Across an APB, the intrasublattice superexchange coupling is greatly strengthened, while the intersublattice superexchange coupling is weakened, reversing the dominant interaction from that found in the bulk. Thus the APB separates oppositely magnetized regions, consistent with Lorentz microscopy measurements. The APBs induce very large saturation fields and nearly random magnetization distribution in zero field.

Effects of lamination on soft magnetic properties of FeN films on sloping surfaces

We have found that the soft magnetic properties of FeN/AlN laminated films do not degrade on sloping surfaces. The easy axis coercivities of FeN/AlN multilayer films are approximately 1.5 Oe and have little variation with slope angle, α. The anisotropy fields slightly vary from 13 to 17 Oe when the slope angles (α) change from 0° to 60°. In contrast, single layer FeN films show a significant degradation of soft magnetic properties with the slope angle. Residual stress does not have a significant correlation with magnetic properties. The large coercivities and saturation fields in the single layer FeN films can be ascribed to a change in the (110) texture of the films.

Domain structures and magnetic properties of FeN films sputter-deposited on sloping surfaces

We obtained soft magnetic properties of FeN on sloping substrates by lamination with thin AlN interlayers. Single layer FeN films on sloping substrate have large saturation field and small squareness. The cause of the degradation of soft magnetic properties in single layer films is ascribed to the perpendicular anisotropy. The existence of the perpendicular anisotropy of FeN sputter-deposited on sloping surfaces is confirmed by the observation of stripe domains with magnetic force microscopy. The observed values of the stripe domain width and the critical film thickness are compared with the calculated values. The stripe domains are parallel to the applied field direction when a saturation field is applied prior to the observation. The stripe domains rotate with the applied field direction.