C.O. Kim

Exchange bias in NiFe∕FeMn∕NiFe trilayers

NiFe∕FeMn∕NiFe trilayer structure forms an integral part of many conventional and tunneling magnetoresistance spin valve structures with FeMn antiferromagnetic layer. A systematic investigation of the exchange bias variations of the seed and top pinned NiFe layers in the NiFe∕FeMn∕NiFe trilayer structure is reported as a function of thickness of all the three constituting layers, in multilayers prepared by rf magnetron sputtering. X-ray diffraction patterns show the (111) texture for the NiFe and FeMn layers, necessary for the development of antiferromagnetic γ-fcc phase. In thickness variation studies of all the three magnetic layers, seed NiFe layer shows greater bias (150Oe) than the top pinned NiFe layer (80Oe only). The exchange bias shows the expected 1∕t behavior for increasing NiFe layer thickness after initial maxima at low thickness. In the FeMn antiferromagnet layer thickness variation on the other hand, the large bias values attained around 5nm thickness is nearly retained up to a thickness of...

Temperature dependence of magnetoimpedance effect in amorphous Co66Fe4NiB14Si15 ribbon

The temperature dependence of the magnetoimpedance (MI) effect is important both for scientific study and for thermal stability of MI sensors. We have performed the measurement of MI effect in amorphous Co66 Fe4 NiB14 Si15 (Metglas 2714A) ribbon from a cryogenic chamber where the temperature of the sample can vary from 10 to 300 K. The ac current was fixed at 10 mA for all measured frequencies ranging from 100 kHz to 10 MHz. The magnetoimpedance ratio (MIR) was revealed the drastic increment as a function of MIR(T)=MIR(0)exp(cT2), where c is a constant. The measured MIR values at room temperature are usually 2–3 times larger than the data measured at 10 K for all measured frequencies. However, the shapes of the MIR curves are remained. This result shows the potential application of the MI effect for a temperature sensor.

Giant magnetoimpedance in composite wires with insulator layer between non-magnetic core and soft magnetic shell

Abstract A method for calculation of the magnetoimpedance in composite wires having an insulator layer between non-magnetic core and soft magnetic shell is described. It is assumed that the magnetic shell has a helical anisotropy and the driving current flows through the core only. The distribution of eddy currents and expressions for the impedance are found by means of a solution of Maxwell equations taking into account the magnetization dynamics within the shell governed by the Landau–Lifshitz equation. The effect of the insulator layer on the magnetoimpedance is analyzed.

Current distribution and giant magnetoimpedance in composite wires with helical magnetic anisotropy

Abstract The giant magnetoimpedance effect in composite wires consisting of a non-magnetic inner core and soft magnetic shell is studied theoretically. It is assumed that the magnetic shell has a helical anisotropy. The current and field distributions in the composite wire are found by means of a simultaneous solution of Maxwell equations and the Landau–Lifshitz equation. The expressions for the diagonal and off-diagonal impedance are obtained for low and high frequencies. The dependences of the impedance on the anisotropy axis angle and the shell thickness are analyzed. Maximum field sensitivity is shown to correspond to the case of the circular anisotropy in the magnetic shell. It is demonstrated that the optimum shell thickness to obtain maximum impedance ratio is equal to the effective skin depth in the magnetic material.

Thickness dependence of parallel and perpendicular anisotropic resistivity in Ta/NiFe/IrMn/Ta multilayer studied by anisotropic magnetoresistance and planar Hall effect

Ferromagnetic layer thickness dependence of anisotropic magnetoresistivities in Ta∕NiFe(t)∕IrMn (10 nm)∕Ta has been investigated for t=3, 4, 5, 7, 8, 10, 12, 15, and 20 nm by the method of anisotropic magnetoresistance and planar Hall effect. Our results revealed that the parallel and perpendicular resistivity components performed a varying function with increment in NiFe thickness. Both the resistivities at first were observed to increase when the NiFe thickness increases from 3 to 10 nm; then for the NiFe thicknesses from 10 to 20 nm, the resistivities of NiFe layer decrease as the NiFe thickness increases. However, the anisotropic resistivity change, which is the difference between parallel and perpendicular resistivities, was observed to increase for the whole range of thicknesses when the NiFe thickness increases. The measured quantities were found to be in good agreement with the theoretically estimated parameters using single domain model; thus these behaviors are well explained based on the modern...

Magnetic tunnel junctions with Hf oxide and modified Hf oxide tunnel barriers

Magnetic tunnel junctions (MTJ’s) with Hf oxide and modified Hf oxide barriers were fabricated by ozone oxidation. The tunnel magnetoresistance (TMR) ratio in Hf oxide junction was 13% at room temperature and 21% at 77 K. In order to understand the low TMR ratio in MTJ’s with Hf oxides compared to those with Al oxides, tunnel barriers were modified by inserting a thin Al oxide layer of 0.3 nm at the interfaces between ferromagnetic electrodes and Hf oxide insulating layers. As the Al layer of 0.3 nm was inserted at top and bottom interfaces, the TMR ratio was restored to the value of the junctions with Al oxides. This implies that the polarization of CoFe contacted with Al oxide is larger than that of CoFe contacted with Hf oxide and the low TMR ratio in MTJ’s with Hf oxides may be attributed to the reduction of spin polarization of the CoFe electrodes due to CoFe/Hf oxide interface interaction.

Magnetostatic properties of heterogeneous Co-based amorphous/crystalline phases

Abstract Using the magneto-optical Kerr effect, magnetic hysteresis loops are measured on annealed amorphous Co 66 Fe 4 B 15 Si 15 samples, to characterize the magnetostatic properties of the heterogeneous crystalline/amorphous phases. A gradual change, on microscale inhomogeneous, change of the magnetic properties with respect to thickness is revealed on HCP-Co, FCC-Co crystalline phases near the surface. The inner amorphous phase exhibits an irregular variation of the local magnetic properties, presumably due to the occurrence and distribution of microcrystallites. The effective field, exerted by the crystalline layer on the amorphous phase, is opposite to the surface magnetization, indicating that there is an antiferromagnetic coupling between surface and inner amorphous phases.

Coercivity enhancement by Zn addition in hot deformed NdFeB magnets

Anisotropic NdFeB magnets were fabricated by the single-stage hot deformation method from commercial isotropic MQPA powder. The effect of Zn addition on the magnetic properties was investigated and the considerable increase of coercivity was observed. The increment of coercivity was about 57% in the case of 0.4 wt% Zn addition compared with that for additive-free magnets.

Hysteretic characteristics of giant magnetoimpedance due to the exchange coupling in annealed amorphous materials

Experiments of the giant magnetoimpedance (GMI) profile have been performed in annealed amorphous Co66Fe4B15Si15 ribbons in open air to characterize the role of the bias field on the GMI. The GMI ratio profile measured at 0.1 MHz exhibits a drastic step-like change, the so-called GMI valve, in an 8 h annealed sample at 380 °C. The GMI valve is related to exchange coupling of the bias field with magnetization of the soft amorphous phase, where the bias field is caused by hcp-Co and fcc-Co crystalline phases on the surface B and Si depleted layer. The bias field is stable for an external field less than 100 Oe, but its direction is entirely changed according to the external field over 400 Oe. The GMI profile begins to reveal hysteresis for increasing and decreasing cyclic fields when the maximum field strength exceeds 200 Oe. The peak for the increasing field is positioned in the negative field region, but in the positive field region for decreasing field. This behavior is opposite to general magnetic hyste...

Modeling of torsion stress giant magnetoimpedance in amorphous wires with negative magnetostriction

Abstract A model describing the influence of torsion stress on the giant magnetoimpedance (GMI) in amorphous wires with negative magnetostriction is proposed. The wire impedance is found by means of the solution of Maxwell equations together with the Landau−Lifshitz equation, assuming a simplified spatial distribution of the magnetoelastic anisotropy induced by the torsion stress. The impedance is analyzed as a function of the external magnetic field, torsion stress and frequency. It is shown that the magnetoimpedance ratio torsion dependence has an asymmetric shape, with a sharp peak at some value of the torsion stress. The calculated field and stress dependences of the impedance are in qualitative agreement with results of the experimental study of the torsion stress GMI in Co-based amorphous wires.