FeMn Layer Research Articles

Asymmetry of nucleation density and its variation with Pt spacer thickness in exchange-biased [Pt/Co]5/Pt/FeMn multilayers

The magnetization reversal of perpendicular exchange-biased [Pt/Co]5/Pt/FeMn multilayers was investigated by magneto-optical Kerr microscopy. The effect of varying the Pt interlayer thickness between the topmost Co layer and the FeMn layer was studied. Our measurements reveal that coercivity (HC) and exchange bias field (HEB) vary with Pt spacer thickness, and have a maximum for a Pt thickness of 0.4 nm. Magnetization reversal measurements show that, for the unbiased multilayer, the reversal is dominated by domain wall propagation starting from a few nucleation centers. For the exchange-biased multilayers, magnetization reversal occurs by nucleation and domain wall propagation with a variation in the nucleation density. For 0 and 0.2 nm Pt spacer, no difference is observed in the nucleation density of the ascending and descending branches unlike for 0.4 and 0.6 nm Pt spacer where the measurements reveal a larger density of nucleation centers in the descending branch.

Stabilization of arsenic sludge with mechanochemically modified zero valent iron

Modified zero valent iron (ZVI) is obtained from commercial iron powder co-ground with manganese dioxide (MnO2) in intensive mechanical stress. The result indicates that the modified ZVI is very effective in arsenic sludge stabilization with a declination of arsenic leaching contraction from 72.50 mg/L to 0.62 mg/L, much lower than that of ordinary ZVI (10.48 mg/L). The involved process, including mechanochemical activation, corrosion and arsenic adsorption, is characterized explicitly to verify the improved arsenic stabilization mechanism. It shows that the mechanically formed Fe-Mn binary oxides layer results in an intensive corrosion extent, generating a mass of corrosion products. Moreover, as the emergence of Mn will restrain the process of iron (hydr)oxides crystallization, the ultimate corrosion products of the modified ZVI predominates in amorphous iron (hydr)oxides, performing much better in arsenic absorption. According to the BCR analysis, unstable arsenic in sludge is easily transformed to residual fraction by the help of amorphous iron (hydr)oxides, resulting in a restrained environmental availability of arsenic sludge after the modified ZVI stabilization.

Spin transport through the metallic antiferromagnet FeMn

We investigate spin transport through metallic antiferromagnets using measurements based on spin pumping combined with inverse spin Hall effects in Ni80Fe20/FeMn/W trilayers. The relatively large magnitude and opposite sign of spin Hall effects in W compared to FeMn enable an unambiguous detection of spin currents transmitted through the entire FeMn layer thickness. Using this approach we can detect two distinctively different spin transport regimes, which we associate with electronic and magnonic spin currents respectively. The latter can extend to relatively large distances (up to 9 nm) and is enhanced when the antiferromagnetic ordering temperature is close to the measurement temperature.

Effect of Ta underlayer on magnetic properties of FeMn/NiFe films

Abstract Effect of Ta underlayer on the magnetic properties of sputter-prepared NiFe(5 nm)/FeMn(20 nm) bilayer films have been studied. The magnetic properties of studied films are optimized by modification of working Ar pressure deposition of Ta (PTa) in the range of 2–12 mTorr and thickness of Ta (tTa) in the range of 0–25 nm. X-ray diffraction results show that the crystallinity of the FeMn(111) strongly depends on the PTa and tTa. All studied films exhibit smooth and flat surface with root-mean-square roughness below 1 nm due to deposition at RT. Large EB field (Heb) of 65–123 Oe with small coercivity (Hc) of 5–16 Oe is obtained. Besides, the change of Heb with various PTa and tTa are related to the crystallinity of FeMn(111) layer, interfacial roughness, and also strain/stress. Correlation between magnetic properties and microstructure is also discussed. This study suggests that proper Ta underlayer is crucial in the exchange bias for NiFe/FeMn bilayer system.

Fieldlike spin-orbit torque in ultrathin polycrystalline FeMn films

Field-like spin orbit torque in FeMn/Pt bilayers with ultra-thin polycrystalline FeMn has been characterized through planar Hall effect measurements. A large effective field is obtained for FeMn in the thickness range of 2 to 5 nm. The experimental observations can be reasonably accounted for by using a macro-spin model under the assumption that the FeMn layer is composed of two spin sublattices with unequal magnetizations. The large effective field corroborates the spin Hall origin of the effective field considering the much smaller uncompensated net moments in FeMn as compared to NiFe. The effective absorption of spin current by FeMn is further confirmed by the fact that spin current generated by Pt in NiFe/FeMn/Pt trilayers can only travel through the FeMn layer with a thickness of 1 to 4 nm. By quantifying the field-like effective field induced in NiFe, a spin diffusion length of 2 nm is estimated in FeMn, in consistence with values reported in literature by ferromagnetic resonance and spin-pumping experiments.

Geochemistry, geochronology, and tectonic setting of early Permian (~290 Ma) volcanic-hosted massive sulphide deposits of the Tasik Chini district, Peninsular Malaysia

ABSTRACTThe age and nature of Permian volcanism and sulphide mineralization in Malaysia are poorly understood. The Tasik Chini district is located in the Central Belt of Peninsular Malaysia and hosts the Bukit Botol and Bukit Ketaya volcanic-hosted massive sulphide (VHMS) deposits. These deposits are hosted by felsic-dominated Permian felsic volcanics that are part of a mixed volcano-sedimentary rock succession. Four mineralization zones were identified in both deposits: (1) stringer sulphide/zone; (2) massive sulphide zone; (3) barite zone; and (4) Fe-Mn and Fe-Si zones. The stringer and massive sulphides generally define the lower mineralized zones, whereas the barite, Fe-Mn, and Fe-Si layers define the stratigraphically upper mineralized zones. The main sulphide phases are pyrite, chalcopyrite, sphalerite, rare galena, and trace Sn-, Au-, and Ag-bearing minerals, with the latter two confined to the massive sulphide and barite zones. Trace and rare earth element data for the host rhyodacite-rhyolite at both deposits are consistent with formation in a volcanic arc palaeotectonic setting. In comparison, the trace element data for Triassic volcanic and intrusive rocks from the Tasik Chini area have moderate to low high field strength element (HFSE) concentrations along with transitional (Zr/Y = 4–7) to tholeiitic (Zr/Y = 2–4) affinities, but have similar magmatic arc signatures. Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) U–Pb zircon dating of rhyolites from the Bukit Botol deposit yields early Permian (286 ± 4 to 292 ± 3 Ma) ages. Similarly, the zircon U–Pb age results for Bukit Ketaya rhyolites reveal early Permian (286 ± 2 to 288 ± 4 Ma) ages. The differences in geochemical and geochronological results between the early Permian host and the later Triassic volcanic and intrusive rocks are likely due to tectonic progression from a volcanic arc environment to a collisional setting.

Anomalous exchange bias in FeMn/Gd bilayers with TC≪TN

Exchange coupling has been investigated in FeMn/Gd bilayers where the Curie temperature (TC) of Gd is much lower than the Néel temperature (TN) of FeMn. The coercivity (HC) exhibits unusual temperature dependence. As temperature increases, it decreases at first, then goes up to the maximum and keeps it in medium temperature range, finally drops rapidly and disappears at TN. The exchange bias field (HE) is negative initially and its magnitude decreases monotonically with increasing temperature, then it turns to be positive, finally disappears at high temperatures. The anomalous HC(T) behavior may be ascribed to the non-collinear coupling interaction at the FeMn/Gd interface. The positive HE can be attributed to the antiferromagnetic coupling between the interfacial Gd and FeMn moments. Moreover, the antiferromagnetic ordering adjacent to the FeMn layer may be responsible for inducing the exchange coupling at TC<T<TN.

Static and dynamic magnetic properties in exchange-biased NiFe/FeMn multilayers

In this work, NiFe(15 nm)/FeMn(t AF ) exchange-biased multilayers are fabricated by magnetron sputtering. The static magnetic properties are characterized using a vibrating sample magnetometer, while the dynamic properties are measured by a vector network analyzer in the frequency range of 0.2–5 GHz. The discrepancy between static and dynamic magnetic anisotropies, especially in the case of thin FeMn layers, for example t AF = 3 nm, can be explained by the rotational anisotropy (H rot ). Compared with the exchange-biased bilayers, the [NiFe(15 nm)/FeMn(t AF )]2 multilayers show a larger resonance frequency (f FMR) and effective damping coefficient (α eff ). Through analysis of the static $$\left( {H_{K}^{sta} } \right)$$ and dynamic $$\left( {H_{ \, K}^{dyn} } \right)$$ anisotropy fields in each NiFe layer, we find that H rot in the up NiFe layer is much larger than that in the down one.

Exchange bias in sputtered FeNi/FeMn systems: Effect of short low-temperature heat treatments

Short (5min) post-deposition thermal treatments under magnetic field at low temperature (up to 200°C) performed in exchange-coupled FeNi(40nm)/FeMn(20nm) bilayer thin films prepared by magnetron sputtering are shown to be effective to significantly modify their exchange field (from around 40Oe down to 27Oe) between FeNi and FeMn layers. A similar exchange field decrease was observed for the first deposited FeNi layer of the FeNi(40nm)/FeMn(20nm)/FeNi(40nm) trilayer films after the same thermal treatments. The exchange field value for the second FeNi layer was not substantially changed. The X-ray diffraction patterns indicates that such a heat treatment has no effect on the grain size and crystalline texture of the films, while atomic force microscope studies reveal an increase of the surface roughness after the treatment which is more noticeable in the case of the trilayer film. Analysis of the experimental results leads us to conclude that the variations of the exchange field after heat treatment are likely caused by a modification of interfacial roughness and/or interfacial magnetic structure, but unlikely by the changes in the microstructure and/or changes of composition of the antiferromagnetic FeMn layer.

Magnetic properties of sputtered anisotropic Pr–Fe–B thin films with different structures and antiferromagnetic materials

Anisotropic Pr–Fe–B films with soft-magnetic layer (Fe) and/or antiferromagnetic layer (Mn, FeMn or MnO) were prepared by direct-current (DC) magnetron sputtering on Si (100) substrates heated at 650 °C. The influence of four types’ different structures on the magnetic properties of Pr–Fe–B films was investigated. The phase and magnetic properties were characterized by means of X-ray diffraction (XRD) and superconducting quantum interference device (SQUID). Addition of antiferromagnetic layer enhances both the coercivity and the remanence ratios of Pr–Fe–B films with suitable structures. The interface number increases and the antiferromagnetic–ferromagnetic exchange interaction is likely to become stronger, which affect the improvement of magnetic properties. To further understand the influence of structures with soft-magnetic Fe layer and/or antiferromagnetic FeMn layer on the magnetic properties of Pr–Fe–B hard-magnetic films, the thickness of Pr–Fe–B layer was designed to decrease from 600 to 50 nm. The improvement of magnetic properties becomes obvious in Mo(50 nm)/Pr–Fe–B(25 nm)Mo(2 nm)FeMn(20 nm)Mo(2 nm)Pr–Fe–B(25 nm)/Mo(50 nm) film.

Giant Magnetoresistance in FeMn/NiCoFe/Cu/NiCoFe Spin Valve Prepared by Opposed Target Magnetron Sputtering

The giant magnetoresistance (GMR) effect in FeMn/NiCoFe/Cu/NiCoFe spin valve prepared by dc opposed target magnetron sputtering is reported. The spin valve thin films are characterized by Scanning Electron Microscopy (SEM), Vibrating Sample Magnetometer (VSM) and magnetoresistance ratio measurements. All measurements are performed in room temperature. The inserted 45 mm thickness FeMn layer to the NiCoFe/Cu/NiCoFe system can increase the GMR ratio up to 32.5%. The coercive field to be increased is compared with different FeMn layer thickness. Furthermore, the coercive field (Hc) decreases with increasing FeMn layer thickness. Magnitude of coercive field is 0.1 T, 0.09 T and 0.08 T for FeMn layer thickness is 30 nm, 45 nm and 60 nm, respectively. The FeMn layer is used to lock the magnetization in the ferromagnetic layer through the exchange anisotropy. This paper will describe the development of a GMR spin valve and its magnetic properties.

Ferromagnetic resonance study of the misalignment between anisotropy axes in exchange-biased NiFe/FeMn/Co trilayers

Exchange-biased NiFe/FeMn/Co trilayers were grown by dc magnetron sputtering and analyzed by in-plane ferromagnetic resonance using Q-band microwaves. The experiments revealed that distinct Co and NiFe resonance modes were excited by the microwave field. A misalignment between the anisotropy axes of the magnetic layers was deduced from the angular variations of the resonance fields, which also showed the effects of uniaxial and unidirectional anisotropies. A phenomenological model was used to fit the experimental results taking also into account a rotatable anisotropy field associated to the domain structure of the FeMn layer and the magnetic history of the films.

Exchange biased FeNi/FeMn bilayers with coercivity and switching field enhanced by FeMn surface oxidation

FeNi/FeMn bilayers were grown in a magnetic field and subjected to heat treatments at temperatures of 50 to 350 °C in vacuum or in a gas mixture containing oxygen. In the as-deposited state, the hysteresis loop of 30 nm FeNi layer was shifted. Low temperature annealing leads to a decrease of the exchange bias field. Heat treatments at higher temperatures in gas mixture result in partial oxidation of 20 nm thick FeMn layer leading to a nonlinear dependence of coercivity and a switching field of FeNi layer on annealing temperature. The maximum of coercivity and switching field were observed after annealing at 300 °C.

Enhancement of exchange bias field in top-pinned FeMn/Py bilayers with Ta/Cu hybrid underlayers

We compared the exchange bias field Hex of bottom-pinned Py/FeMn and top-pinned FeMn/Py bilayers with Ta/Cu hybrid underlayers for both as-deposited and annealed cases. Top-pinned bilayers show higher Hex than bottom-pinned bilayers. It is more than two times higher than that of bottom-pinned bilayers and even more than one order of magnitude higher than that of top-pinned bilayer with Ta single underlayers. Furthermore, top-pinned bilayers exhibit enhanced Hex after field cooling, while bottom-pinned bilayers remain almost unchanged. Dramatic increase in intensity of FeMn peak due to hybrid underlayer and FeMn/Py interface roughness dominantly result in the enhanced exchange bias field in the as-deposited case. On the other hand, the microstructural change like interface morphology and FeMn chemical composition at FeMn/Py local interface rather than crystallographic texture of FeMn layer play a key role in enhancing the exchange bias field in annealed case. Our results cannot be explained by the results from the previous studies on IrMn/CoFe bilayers. We suggest that the stoichiometric Fe50Mn50 ratio around FeMn/Py interface as well as FeMn(111) texture should be responsible for high Hex of top-pinned bilayers with Ta/Cu hybrid underlayers.

Lateral grain size effect on exchange bias in polycrystalline NiFe/FeMn bilayer films

This study explores the effect of lateral grain size on the exchange bias in the magnetron sputtered NiFe(5nm)/FeMn(20nm) bilayer films. The thicknesses of ferromagnetic (NiFe) and antiferromagntic (FeMn) layers were kept constant in the study. The lateral grain size variation was induced by increasing Ta buffer layer thickness. The cross-sectional transmission electron microscopy revealed that bilayers deposited on thicker Ta buffer layers have larger lateral grain diameter. Grain-to-grain epitaxy from buffer layer controls lateral grain size at NiFe/FeMn interface. A large increase in exchange bias field was observed with increasing thickness of Ta buffer layer, which is attributed to the enhanced ferromagnetic/antiferromagnetic coupling originated from larger interface lateral grain area.

Exchange bias field and coercivity of [NiFe/NiFeCuMo/NiFe]/FeMn multilayers

The exchange bias field (H EX ) and the coercivity (H C ) of the intermediate, super-soft magnetic NiFeCuMo layer were investigated for different thicknesses of the bottom NiFe layer. The H EX of the triple-pinned NiFe (4 nm)/NiFeCuMo (t NiFeCuMo = 1 nm)/NiFe (4 nm)/FeMn multilayer has a maximum value of 77 Oe, which is more than that of a single-pinned NiFe (8 nm)/FeMn layer (42 Oe). A multilayer system with the NiFeCuMo layer inserted between the pinned and the free NiFe layers can be used as a giant magnetoresistive spin-valve device for biosensors with improved magnetic sensitivity.

Direct observation of frozen moments in the NiFe/FeMn exchange bias system

We detect the presence of frozen magnetic moments in an exchange biased NiFe ferromagnet at the NiFe/FeMn ferromagnet/antiferromagnet interface by magnetic circular dichroism in x-ray absorption and resonant reflectivity experiments. Frozen moments are detected by means of the element-specific hysteresis loops. A weak dichroic absorption with unidirectional anisotropy can be linked to frozen magnetic moments in the ferromagnet. A more pronounced exchange bias for increasing the thickness of the FeMn layer correlates with an increase in orbital moment for interface Ni atoms carrying a frozen moment. These atoms compose about a single monolayer, but only a fraction of the atoms contributes by means of a strongly enhanced orbital moment to the macroscopic exchange bias phenomenon. The microscopic spin–orbit energy associated with these few interface frozen moment atoms appears to be sufficient to account for the macroscopic exchange bias energy.

Low Hysteresis FeMn-Based Top Spin Valve

FeMn-based top spin valves Ta/[FeNi/CoFe]/Cu/CoFe/FeMn/Ta with different Cu and FeMn layers thicknesses were prepared by DC magnetron sputtering at room temperature. It was shown that low field hysteresis due to free layer magnetization reversal can be reduced down to (0.1 divided by 0.2) Oe keeping the GMR ratio higher 8% by using both layers thicknesses optimization and non-collinear geometry of magnetoresistance measurements. Dependence of low field hysteresis and GMR ratio on the angle between applied magnetic field and pinning direction are presented.

Growth model of a hydrothermal low-temperature Si-rich chimney: Example from the CDE hydrothermal field, Lau Basin

The CDE hydrothermal field was first discovered during a Chinese cruise to the East Lau Basin Spreading Centre in 2007. Apart from significant amounts of loose Fe-Si-Mn (oxyhydr) oxide (referred to as oxide below) precipitates, a small Si-rich oxide chimney was also recovered on this cruise. In this study, we report on the mineralogical and geochemical analyses of this chimney and a model for its growth that has been developed. Based on the mineralogy and O isotope results, the chimney walls can be divided into four growth generations (layers) from the inner to the outer layers: amorphous opal and barite layer (precipitation temperature 68.5°C based on oxygen isotope determinations), a rod-like amorphous layer (precipitation temperature 39.6°C), a filamentous Fe-Si oxide layer, and an outer Fe-Mn oxide layer. Investigations based on SEM and EDS showed that neutrophilic Fe-oxidizing bacteria play an important role in the formation of this chimney, particularly in the outer two generations. In the first stage, the metabolic activity of the microbes results in the pervasive precipitation of the filamentous Fe-rich oxides inside a ring formed by some amorphous opal and barite; therefore, a loose porous layer forms. In the second stage, amorphous opal then precipitates inside this wall as a result of conductive cooling and gradually controls the mixing between the hydrothermal fluids and ambient seawaters. In the third stage, barite and some amorphous opal form from the higher temperature fluids at the summit of the chimney growth history. In the last stage, the chimney wall becomes thicker and denser and the exchange of hydrothermal fluids and seawater ceases. As a result, a Fe-Mn oxide layer precipitates onto the outer surface of the chimney wall as neutrophilic Fe-oxidizing bacteria reoccupy the surface of the chimney. This mineral sequence and the resultant growth generations are confirmed by the chemical characteristics of the chimney wall. Sr isotopes extracted from the Fe oxides of the four-generation wall generally show a decreasing trend of the 87Sr/86Sr ratios from the second layer to the inner layer (from 0.707008 to 0.705877) except for the outer layer (0.706502). The Sr isotope and chondrite normalized REE patterns of the corresponding bulk samples from the chimney wall also display a similar trend. Our study shows that the biogenic filament network plays a key role in the formation of the chimney in contrast to previous growth models of higher temperature chimneys, which often ignore the influence of biogenic factors.

Heating asymmetry induced by tunneling current flow in magnetic tunnel junctions

In this work, exchange bias was used as a probe to characterise the temperature profile induced by the inelastic relaxation of electrons tunnelling across a MgO barrier. Thermally assisted magnetic random access memory (TA-MRAM) cells comprising a magnetic tunnel junction (MTJ) with a reference pinned layer and a FeMn exchange biased storage layer were used. The pinning direction of the ferromagnetic storage layer is reversed when heated above the blocking temperature of the antiferromagnetic layer (FeMn). The power density required to reach this blocking temperature in the FeMn layer depends on the current polarity, indicating that the heat source term associated with the current flowing through the barrier depends itself on the current direction in contrast to simple Joule heating. This effect is due to the mechanism of energy dissipation in tunnelling. The tunnelling itself is ballistic i.e., without dissipation. However, after tunnelling, the hot electrons very quickly relax to the Fermi energy thereby loosing their excess energy in the receiving electrode. Therefore, the heat is essentially generated on one side of the barrier so that the whole profile of temperature throughout the pillar depends on the current direction. Full 3D thermal simulations also confirmed the temperature profile asymmetry. The proper choice of heating current direction (i.e., voltage polarity applied to the MTJ) can yield a reduction of about 10% in the heating power density required to enable writing in thermally assisted MRAM cells.