NiFe Magnetization Research Articles

Significant Role of Interfacial Spin–Orbit Coupling in the Spin-to-Charge Conversion in Pt/NiFe Heterostructure

For the spin-to-charge conversion (SCC) in heavy metal/ferromagnet (HM/FM) heterostructure, the contribution of interfacial spin-orbit coupling (SOC) remains controversial. Here, we investigate the SCC process of the Pt/NiFe heterostructure by the spin pumping in YIG/Pt/NiFe/IrMn multilayers. Due to the exchange bias of NiFe/IrMn structure, the NiFe magnetization can be switched between magnetically unsaturated and saturated states by opposite resonance fields of YIG layer. The spin-pumping signal is found to decrease significantly when the NiFe magnetization is changed from the saturated state to the unsaturated state. Theoretical analysis indicates that the interfacial spin absorption is enhanced for the above-mentioned NiFe magnetic state change, which results in the increased and decreased spin flow in the Pt layer and across the Pt/NiFe interface, respectively. These results demonstrate that in our case the interfacial SOC effect at the Pt/NiFe interface is dominant over the bulk inverse spin Hall effect in the Pt layer. Our work reveals a significant role of interfacial SOC in the SCC in HM/FM heterostructure, which can promote the development of high-efficiency spintronic devices through interfacial engineering.

Investigation of magnetization dynamic damping in NiFe/Pd bilayer at room temperature

We report the magnetization dynamics behavior of NiFe/Pd bilayer structure by varying the thickness of NiFe layer using broadband ferromagnetic resonance set up. We found linear dependence of the Gilbert damping parameter (α) with the inverse of thickness of FM (NiFe) layer, which indicates possible signature of spin pumping. The calculated value of spin mixing conductance is found to be 1.47×1019m-2. The saturation magnetization of NiFe in NiFe/Pd samples are found to be 1.34±0.03.

Inducing high resonant frequency in exchange-coupled bilayer via fast annealing

The reversed magnetization of NiFe (10 nm)/FeMn (8 nm) bilayers was carried out in an applied external magnetic field (Ha) with the direction opposite to the deposition magnetic field (Hd). The exchange bias field (He), static magnetic anisotropic field (Hksta), and resonant frequency (fr) of the bilayers before and after annealing were characterized. Part of the AFM moments were fixed at an opposite direction to another part by controlling the capacitor voltage. At the critical capacitor voltages 37 and 43 V, the AFM moments and corresponding FM moments in Ha direction were almost the same as in Hd direction, respectively, and were responsible for the lower Hksta and He. This demonstrates that the strongest competitive effect between the AFM/FM exchange coupled with opposite direction, and thus more unstable AFM grains occurred, causing the rotational anisotropy field (Hrot), fr, and the dynamic magnetic anisotropic field (Hkdyn) to reach their maximum.

Zero-field electron Larmor precession in NiFe/SiN/GaAs nanohybrid structure at room temperature

We investigate the spin dynamics of a NiFe/SiN/GaAs nanohybrid structure, in which the volume of NiFe is significantly reduced by ion milling. The photogenerated GaAs electron spins induce the electron Larmor precession with a long decay component (LDC) with a time constant τLDC of 550 ps under zero field at room temperature, indicating the presence of an effective magnetic field. The LDC reflects the relaxation of NiFe magnetization, whose time constant is modulated by the inner product of NiFe magnetization and GaAs electron spins in the external field. These results indicate the existence of an exchange interaction between the NiFe magnetization and GaAs electron spins.

Separation of inverse spin Hall effect and anomalous Nernst effect in ferromagnetic metals

Inverse spin Hall effect (ISHE) in ferromagnetic metals (FM) can also be used to detect the spin current generated by longitudinal spin Seebeck effect in a ferromagnetic insulator YIG. However, anomalous Nernst effect (ANE) in FM itself always mixes in the thermal voltage. In this work, the exchange bias structure (NiFe/IrMn) is employed to separate these two effects. The exchange bias structure provides a shift field to NiFe, which can separate the magnetization of NiFe from that of YIG in M-H loops. As a result, the ISHE related to magnetization of YIG and the ANE related to the magnetization of NiFe can be separated as well. By comparison with Pt, a relative spin Hall angle of NiFe (0.87) is obtained, which results from the partially filled 3d orbits and the ferromagnetic order. This work puts forward a practical method to use the ISHE in ferromagnetic metals towards future spintronic applications.

Dynamic in situ observation of voltage-driven repeatable magnetization reversal at room temperature

Purely voltage-driven, repeatable magnetization reversal provides a tantalizing potential for the development of spintronic devices with a minimum amount of power consumption. Substantial progress has been made in this subject especially on magnetic/ferroelectric heterostructures. Here, we report the in situ observation of such phenomenon in a NiFe thin film grown directly on a rhombohedral Pb(Mg1/3Nb2/3)0.7Ti0.3O3(PMN-PT) ferroelectric crystal. Under a cyclic voltage applied perpendicular to the PMN-PT without a magnetic field, the local magnetization of NiFe can be repetitively reversed through an out-of-plane excursion and then back into the plane. Using phase field simulations we interpret magnetization reversal as a synergistic effect of the metastable ferroelastic switching in the PMN-PT and an electrically rotatable local exchange bias field arising from the heterogeneously distributed NiO clusters at the interface.

Role of the antiferromagnetic pinning layer on spin wave properties in IrMn/NiFe based spin-valves

Brillouin light scattering (BLS) was exploited to study the spin wave properties of spin-valve (SV) type samples basically consisting of two 5 nm-thick NiFe layers (separated by a Cu spacer of 5 nm), differently biased through the interface exchange coupling with an antiferromagnetic IrMn layer. Three samples were investigated: a reference SV sample, without IrMn (reference); one sample with an IrMn underlayer (10 nm thick) coupled to the bottom NiFe film; one sample with IrMn underlayer and overlayer of different thickness (10 nm and 6 nm), coupled to the bottom and top NiFe film, respectively. The exchange coupling with the IrMn, causing the insurgence of the exchange bias effect, allowed the relative orientation of the NiFe magnetization vectors to be controlled by an external magnetic field, as assessed through hysteresis loop measurements by magneto-optic magnetometry. Thus, BLS spectra were acquired by sweeping the magnetic field so as to encompass both the parallel and antiparallel alignment of the NiFe layers. The BLS results, well reproduced by the presented theoretical model, clearly revealed the combined effects on the spin dynamic properties of the dipolar interaction between the two NiFe films and of the interface IrMn/NiFe exchange coupling.

Effects of a NiFe/MgO interface on the thermoelectric transport behavior of spin-polarized electrons

The effects of an NiFe/MgO interface on the thermoelectric transport behavior of spin-polarized electrons were studied by measuring the Seebeck coefficient (S) and anisotropic magneto-thermopower (AMT) of an MgO/NiFe/MgO multilayer structure. Thermally annealing this multilayer was found to increase S (343% at 450°C) through crystallization of interfacial MgO, which allows conductive electrons to achieve better specular scattering at the interface. The AMT was also increased by annealing (139% at 450°C), but this is ascribed to a chemical metallization of interfacial Fe in which the deposition-induced oxygen atoms migrate out of the NiFe layer. This helps to recover the magnetization of NiFe, thereby causing spin-dependent thermoelectric transport behavior at the interface.

Co/Pd]4–Co–Pd–NiFe spring magnets with highly tunable and uniform magnetization tilt angles

By varying the Pd thickness (tPd) from 0 to 8nm in [Co/Pd]4/Co/Pd(tPd)/NiFe exchange springs, we demonstrate (i) continuous tailoring of the exchange coupling between a [Co/Pd]4/Co layer with perpendicular anisotropy, and a NiFe layer with an in-plane easy axis, (ii) tuning of the NiFe out-of-plane magnetization angle from 20○ to 80○, and (iii) an up to two-fold increase in the NiFe damping. The partial decoupling also results in a highly uniform NiFe magnetization. These properties make [Co/Pd]4/Co/Pd(tPd)/NiFe spring magnets ideal candidates for use as tilted polarizers, by combining stable and well-defined spin directions of its carriers with a high degree of angular freedom.

Direct observation of Oersted-field-induced magnetization dynamics in magnetic nanostripes

We have used time-resolved x-ray photoemission electron microscopy to investigate the magnetization dynamics induced by nanosecond current pulses in NiFe/Cu/Co nanostripes. A large tilt of the NiFe magnetization in the direction transverse to the stripe is observed during the pulses. We show that this effect cannot be quantitatively understood from the amplitude of the Oersted field and the shape anisotropy. High-frequency oscillations observed at the onset of the pulses are attributed to precessional motion of the NiFe magnetization about the effective field. We discuss the possible origins of the large magnetization tilt and the potential implications of the static and dynamic effects of the Oersted field on current-induced domain-wall motion in such stripes.

THE EXCHANGE-BIASING IN THE TOP-CONFIGURATION Ni80Fe20/Ir20Mn80 SYSTEM

In this work, top-configuration exchange-biasing NiFe ( y )/ IrMn (90Å) systems have been investigated with three different conditions: (a) the substrate temperature ( T s ) was kept at room temperature (RT) only, (b) T s at RT with an in-plane field ( h ) = 500 Oe deposition during deposition, and (c) T s = RT with h during deposition and postdeposition annealing in the field at T A = 250° C for 1h, with the samples field cooled to RT. High resolution electron cross-sectional transmission electron microscopy (HR X-TEM) and x-ray results reveal that the IrMn (111) texturing plays a key role in the exchange-biasing field ( H ex ) and interfacial energy ( J k ). The H ex versus y result shows that H ex increases when y decreases. Since J k = H ex M s y , where M s is NiFe magnetization, it is easy to derive H ex = J k /( M s y ). Therefore, if H ex is inversely proportional to y , with J k / M s constant, we find H ex y = constant. In short, the y dependence of J k is similar to that of M s for each curve. The H c is inversely proportional to y because of the surface pinning effects from the Ta / NiFe and NiFe / IrMn interfaces. Finally, the optimal values for H ex and J k are 220 Oe and 0.075 erg/cm2, respectively.

Field-dependent anisotropic magnetoresistance and magnetization measurements of NiFe/FeMn exchange-biased bilayers

The exchange-bias effect was investigated through the analyses of the magnetic hysteresis loops and anisotropic magnetoresistance measurements of NiFe/FeMn bilayers. The angular variations of the exchange-bias field and coercivity were fitted in the framework of a domain-wall formation model which assumes formation of a planar domain wall at the antiferromagnetic side of the interface. The anisotropy parameters of both layers were extracted from the fits and were also used to obtain the orientation of the NiFe magnetization for different magnetic field in-plane directions. The angular dependence of the anisotropic magnetoresistance was then calculated and a comparison with the experimental measurements was made. The agreement between these curves was good for magnetic field much higher than the exchange-bias field and only reasonable for magnetic field of the order of or lower than the exchange-bias field.

On the role of the soft layer in exchange‐spring hard/soft magnetic bilayers

Bilayers of SmCo/NiFe and SmCo/Co with well-defined in-plane uniaxial anisotropy were grown by dc magnetron sputtering on glass substrates. The magnetization reversal process was investigated by magneto-optic Kerr effect from both sides of the samples, obtaining the hard and soft response separately. The NiFe layers turn out to be exchange coupled to the SmCo films and, for soft layer thicknesses above 30 nm, display reversible demagnetization loops expected from exchange-spring magnets. The experimental NiFe magnetization curve is in good agreement with the theoretical curve obtained by minimizing the magnetic energy in the soft film for an ideal hard/soft bilayer. The soft layer critical thickness above which the exchange spring behavior sets up and its presumed independence on soft layer characteristics are discussed, by comparing the two systems under investigation. We find that SmCo/Co does not fulfil the predictions of the currently accepted theories and that the magnetization reversal of the bilayer is dominated by the Co layer. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lorentz transmission electron microscopy and magnetic force microscopy characterization of NiFe/Al-oxide/Co films

Magnetization reversal process of NiFe/Al-oxide/Co junction films was observed directly using Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM). In situ magnetizing experiments performed in both LTEM and MFM were facilitated by a pair of electromagnets, which were mounted on the sample stages. A two-stage magnetization reversal process for the junction film was clearly observed in LTEM with NiFe magnetization reversed first via domain wall motion followed by Co magnetization reversal via moment rotation and domain wall motion. Reversal mechanism and domain characteristics of the NiFe and Co layers showed very distinctive features. The magnetization curve of the junction film measured using alternating gradient force magnetometry showed a nonzero slope at the antiparallel magnetization configuration region, which implies that magnetization directions of the NiFe and Co layers were not exactly antiparallel due to Co moment rotation existed in that region. After the magnetization reversal of the Co was complete, MFM images revealed some magnetic contrast, which suggests that an out-of-plane magnetization component remained in the Co layer. Such magnetic contrast disappeared at higher magnetic fields when the Co moments further rotated and aligned parallel to the applied field direction.

Direct observation of domains and magnetization switching processes in NiFe/alumina/Co trilayer junctions

Transmission Lorentz microscopy was used to observe the effect of an applied field on the domain structure and magnetization process of NiFe/alumina/Co junctions. The alumina layer was prepared either by oxidizing a sputtered Al layer in air, or by direct sputtering of Al 2O 3. Two distinct magnetization reversal processes, where NiFe reverses first, followed by the magnetization reversal of Co, were observed with increasing applied field. The range of field values over which the magnetization of NiFe and Co are in an antiparallel configuration is approximately the same for both films. The alumina thickness, and waviness of the interfaces, can affect the magnetization process of the junctions.

Pressure dependence of the magnetization of NiFe3N

Abstract The experimental result of the decrease of the magnetization with pressure within the ordered nitride NiFe3N is addressed using the A.S.W. (augmented spherical wave) method. Among the two magnetic sublattices i.e. Ni and Fe in the structure only the latter is found responsible of the pressure dependence of the magnetization.