G.H. Yu

Interface reaction of NiO/NiFe and its influence on magnetic properties

Ta/NiO/NiFe/Ta multilayers were prepared by rf reactive and dc magnetron sputtering. The exchange coupling field between NiO and NiFe reached 120 Oe. The composition and chemical states at the interface region of NiO/NiFe were studied using the x-ray photoelectron spectroscopy (XPS) and peak decomposition technique. The results show that there are two thermodynamically favorable reactions at NiO/NiFe interface: NiO+Fe=Ni+FeO and 3NiO+2Fe=3Ni+Fe2O3. The thickness of the chemical reaction as estimated by angle-resolved XPS was about 1–1.5 nm. These interface reaction products are magnetic defects, and we believe that the exchange coupling field Hex and the coercivity Hc of NiO/NiFe are affected by these defects.

Large enhancement of the anomalous Hall effect in Co/Pt multilayers sandwiched by MgO layers

We report a large enhanced anomalous Hall effect (AHE) in the Co/Pt multilayers sandwiched by two MgO layers. The Hall resistivity (ρxy) was over an order of magnitude larger than that in pure Co/Pt multilayers. By optimizing the thickness of MgO layers, a high field sensitivity value of 2445 V/Au2009T for Hall sensors was achieved. The enhancement of AHE is mainly attributed to the MgO–Pt interfacial effect.

Designed synthesis of materials for high-sensitivity geomagnetic sensors

A structure of Ta/MgO/NiFe/MgO/Ta was designed and synthesized, which combines the advantages of both tunnel magnetoresistance materials with high magnetic field sensitivity and anisotropic magnetoresistance materials with high directional sensitivity. The magnetoresistance ratio in the device with structure of Ta(5)/MgO(4)/NiFe(10)/MgO(3)/Ta(3) (thicknesses in nanometers) increases with an increase in annealing temperature, reaching a maximum value of 3.5% at 450u2009°C, and then decreases with a further increase in annealing temperature. Meanwhile, a high sensitivity of 2.1%/Oe is obtained. The higher magnetoresistance ratio and sensitivity come from the significant specular reflection of electrons at both interfaces due to the crystalline MgO layer together with the sharp interfaces with the NiFe layer.

Effect of interfacial structures on anomalous Hall behavior in perpendicular Co/Pt multilayers

A large enhancement of anomalous Hall resistivity was obtained in the perpendicular [Co/Pt]3 multilayers sandwiched by MgO/CoO hybrid bilayers. For example, the saturation Hall resistivity (ρxy) is greatly increased, which is 250% and 67% larger than that in pure [Co/Pt]3 multilayers and that in [Co/Pt]3 multilayers sandwiched by pure MgO layers, respectively. Meanwhile, the perpendicular magnetic anisotropy in the multilayers with MgO/CoO hybrid bilayers was enhanced. The large enhancement of ρxy originates from the modified metal/oxide interfacial structures, together with improved crystallization of core [Co/Pt]3 multilayers, due to the insertion of hybrid bilayers.

Interface reaction of Ta/Ni81Fe19 or Ni81Fe19/Ta and its suppression

Ta/Ni81Fe19 and Ni81Fe19/Ta structures are commonly used in the magnetic multilayers with giant magnetoresistance. For a Ta/Ni81Fe19/Ta fundamental structure, Ta seed and Ta cap layers resulted in a loss of moment equivalent to a magnetically dead layer of thickness 1.6±0.2u2009nm. In order to find out the reason, the composition and chemical states at the interface regions of Ta/Ni81Fe19 and Ni81Fe19/Ta were studied using the x-ray photoelectron spectroscopy and peak decomposition technique. The results show that there are thermodynamically favorable reactions at the Ta/Ni81Fe19 and Ni81Fe19/Ta interfaces: 2Ta+Ni=NiTa2. However, the thickness of a magnetically dead layer was significantly reduced by the insertion of a small amount of Bi in the Ta/Ni81Fe19/Ta structure. This result indicates that a surfactant Bi can suppress the interface reaction in multilayers.

Interlayer segregation in magnetic multilayers and its influence on exchange coupling

Experimental results show that the exchange coupling field (H-ex) of NiFe/FeMn for Ta/NiFe/FeMn/Ta multilayers is higher than that for spin-valve multilayers Ta/NiFe/Cu/NiFe/FeMn/Ta. X-ray photoelectron spectroscopy shows that Cu atoms segregate to the NiFe/FeMn interface for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers. While studying Ta/X(X=Bi,Pb,Ag,In)/NiFe/FeMn multilayers, we also find that X atoms segregate to the NiFe/FeMn interface, which results in a decrease of the H-ex. However, a small amount of Bi, Pb, etc. deposited between Cu and pinned NiFe layer for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers can increase H-ex

Spin transfer in antisymmetric exchange-biased spin-valves

In this letter, we report on measurements of current-induced magnetization switching (CIMS) in current-perpendicular-to-plane exchange-biased spin-valves (ESPVs). The structures of the ESPVs are all “antisymmetric,” but with different thickness of a ruthenium (Ru) layer. It is confirmed that the “antisymmetric” structures largely enhance the spin transfer effect and therefore reduce critical current densities for the CIMS. The effect of the Ru layer on the spin transfer in the ESPVs is also systematically studied. With a decrease of the Ru layer’s thickness, the critical current densities can be further reduced. The lowest critical current we achieved in an “antisymmetric” structure is 1×106A∕cm2, which realizes a reduction of more than one order of magnitude compared with all the reported works.

Perpendicularizing magnetic anisotropy of full-Heusler Co2feAl films by cosputtering with terbium

In this letter, we fabricated Co2FeAl films with perpendicular-to-plane magnetic anisotropy by cosputtering with terbium (Tb). The as-prepared (Tb+Co2FeAl) films (TCFA) consists of nanocrystalline L21 Co2FeAl and amorphous alloy of Tb(Co, Fe, and Al). The coercivity field (Hc) of the TCFA films is adjustable from 200 to 800 Oe. After annealing, the Hc decreases to 70 Oe. A perpendicularly magnetized spin valve with the TCFA films as free and reference layers shows a current-perpendicular-to-plane magnetoresistance of 1.8% at room temperature. Our result opens a way to fabricate perpendicularly magnetized full-Heusler alloys and makes it possible to realize faster and simple structured magnetic storage bits in the future.

Configuration of the uncompensated moments at the FM/AFM interface with a NM spacer

The pinned uncompensated moments are believed to play the key role in forming the exchange bias at the FM/AFM interface. However, most of the uncompensated moments are reversible under the dragging torque from the FM layer. We produced a ruined AFM surface by inserting and doping Pt at the FM/FeMn interface and found that the ruined AFM surface is a source of extra uncompensated moments. The configuration of the uncompensated moments changes considerably with the distance between the Pt/FeMn interface and the FM surface. The whole amount and pinned fraction of the uncompensated moments in the samples with the proper spacer thickness are simultaneously enriched.

Effect of Cu surface segregation on properties of NiFe/FeMn bilayers

The films of NiFe/FeMn with Ta and Ta/Cu buffer layers were prepared by magnetron sputtering. Results show that the exchange bias field of NiFe/FeMn films with Ta/Cu buffer is lower than that of the films with Ta buffer. The crystalline texture, surface roughness and element distribution of these two sets of samples were examined, and there is no apparent difference for the texture and roughness. However, the segregation of Cu atoms on the surface of NiFe in the trilayer of Ta/Cu/NiFe has been observed by using the angle-resolved X-ray photoelectron spectroscopy. The decrease of the exchange bias field for NiFe/FeMn films with Ta/Cu buffer layers is mainly caused by the diffusion of Cu atoms through NiFe layer, which stayed at the interface of NiFe/FeMn film or even intruded into FeMn layer. The present results indicate that Cu segregation through NiFe layer should be suppressed in order to improve the exchange bias field in giant magnetoresistance spin valves with Cu spacer.