M.Y. Kim

Exchange biasing field of NiFe/[IrMn–Mn]/CoFe multilayers depending on Mn content

Exchange bias by inserting a submonolayer of Mn between two neighboring IrMn layers in NiFe/[IrMn–Mn]/CoFe multilayers was investigated. As-deposited CoFe pinned layers with an [IrMn–Mn] layer resulted in larger exchange biasing field (Hex) and blocking temperature (Tb) than when pure Ir22Mn78 was used. Hex and Tb improved with content of 76.8–78.1 vol % Mn, but Hex and Tb of NiFe/IrMn/CoFe dropped considerably with addition of more than 0.6 vol % Mn. The Hex obtained for as-grown two multilayer samples with 77.5 and 78.7 vol % Mn, were 259 and 150 Oe, respectively. In the case of IrMn with 77.5 vol % Mn, Hex increased to 475 Oe at 350 °C but decreased to 200 Oe at 450 °C. The magnetic properties and thermal stabilities of the NiFe/[IrMn–Mn]/CoFe multilayer were enhanced by suitable additions of Mn. In applications where higher Hex and Tb were required, the proper contents of Mn could be used.

Magnetization switching and tunneling magnetoresistance effects of MTJs with synthetic antiferromagnet free layers consisting of amorphous CoFeSiB

Magnetic tunnel junctions (MTJs), which consisted of amorphous CoFeSiB layers, were investigated. The CoFeSiB layers were used to substitute for the traditionally used CoFe and/or NiFe layers with an emphasis given on understanding the effect of the amorphous free layer on the switching characteristics of the MTJs. CoFeSiB has a lower saturation magnetization (M/sub s/:560 emu/cm/sup 3/) and a higher anisotropy constant (K/sub u/:2 800 erg/cm/sup 3/) than CoFe and NiFe, respectively. An exchange coupling energy (J/sub ex/) of -0.003 erg/cm/sup 2/ was observed by inserting a 1.0 nm Ru layer in between CoFeSiB layers. In the Si-SiO/sub 2/-Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlO/sub x//CoFeSiB 7 or CoFeSiB (t)/Ru 1.0/CoFeSiB (7-t)/Ru 60 (in nanometers) MTJs structure, it was found that the size dependence of the switching field originated in the lower J/sub ex/ using the experimental and simulation results. The CoFeSiB synthetic antiferromagnet structures were proved to be beneficial for the switching characteristics such as reducing the coercivity (H/sub c/) and increasing the sensitivity in micrometer size, even in submicrometer sized elements.

Dependence of Natural Oxidation Spin-Dependent Tunneling Junction on Junction Area

The top-type tunneling magnetoresistance (TMR) multilayer films with a structure of Ta5/NiFelO/ Ta5/NiFe10/FeMn10/NiFe2,/CoFe2/Al 2 O 3 1/CoFe3/NiFe20 (thickness in nm) with in-situ natural AlO x oxidation were deposited by a sputtering system with a base pressure of 10 -9 Torr. Junctions with sizes from 4 to 80 μm 2 were fabricated by using a conventional photolithography process. The as-deposited junction showed TMR of 16% at room temperature with resistance of 14-15 Ω, dependent on junction area.

Effect of Interface Roughness on Magnetoresistance and Magnetization Switching in Double-Barrier Magnetic Tunnel Junction

The three kinds of double-barrier magnetic tunnel junction (DMTJ) with or without amorphous ferromagnetic Co70.5Fe4.5Si15B10 (in at. %) free-layer were investigated to understand the effect of the free-layer on the bias voltage dependence of tunneling magnetoresistance (TMR) ratio. The DMTJ structure consisted of Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlOx/free-layer 7/AlOx/CoFe 7/IrMn 10/Ru 60 (thickness in nm). Various free layers, such as CoFe 7, CoFeSiB 7, and CoFe 1.5/CoFeSiB 4/CoFe 1.5 were prepared and compared. The roughness values of the interface between free-layer and tunnel barrier were confirmed by using the techniques of X-ray reflectivity and transmission electron microscopy. As a result, the amorphous free-layer made the interface roughness of DMTJ smoother, reducing the interlayer coupling field and suppressing the bias voltage dependence of TMR ratio at a given voltage.

High exchange-coupling field and thermal stability of antiferromagnetic Ir-Mn pinned spin-valve films

IrMn pinned spin-valve (SV) films with stacks of Ta-NiFe-IrMn-CoFe-Cu-CoFe-NiFe-Ta were prepared by dc sputtering onto thermally oxidized Si[111] substrates at room temperature under a magnetic field of about 100 Oe. The annealing cycle number and temperature dependence of the exchange-coupling field (H/sub ex/), magnetoresistance (MR) ratio, and coercivity (H/sub c/) were investigated. By optimizing the process of deposition and the post-thermal annealing condition, we obtained the IrMn-based SV films with MR ratio of 3.6%, H/sub ex/ of 1180 Oe for the pinned layer. The H/sub ex/ is stabilized after the second annealing cycle, and it is thought that this SV reveals high thermal stability. The H/sub ex/ maintained its strength of 600 Oe in operation up to 240/spl deg/C and decreased monotonically to zero at 270/spl deg/C.

Magnetoresistance of [Ni(Pt)/Mn] Superlattice‐Based Spin Valves

The magnetoresistance (MR) of [Ni/Mn] superlattice-based spin valves was compared with that of [Pt/Mn] superlattice. The [Ni/Mn] superlattice after a thermal treatment at 240 °C showed a fct (face centered tetragonal) structure with an antiferromagnetic phase. The thermal stability of its spin valves increased gradually with the annealing treatment. As-deposited [Pt/Mn] superlattice-based spin valves showed an exchange coupling field (H ex ) of 17 Oe and an MR ratio of 1.2% without deposition field. The surface morphologies of the two superlattice-based spin valves were dominantly different after annealing.

Cu thickness dependence of interlayer coupling for free layer on NiMn alloy-pinned spin valve films

Annealing cycle number and nonmagnetic layer thickness dependences of interlayer coupling field (H inf ) and coercivity (H cf ) of free magnetic layer on NiMn alloy-pinned spin valve films (SVF), glass/Ni 81 Fe 19 (70 A)/Co(10 A)/Cu(t A)/Co(15A)/Ni 81 Fe 19 (35 A)/Ni 25 Mn 75 (250 A)/Ta(50 A) prepared by magnetron sputtering, were investigated. At a Cu thickness of SVF of 35 A and peak exchange coupling field (H ex ) of 620 Oe, coercivity H c = 280 Oe and MR ratio = 2.5%. For every SVF H inf and H cf increased up to stabilized values with the increase of annealing cycle number up to 15, which were 120 Oe for H inf and 75 Oe for H cf . The increase of H cf with the annealing cycle number seems to be caused by the effective reduction of Cu layer thickness due to the increase of interfacial mixing of Cu layer and Co layer. The effect of interfacial mixing is analysed to be increased or weakend when the Cu layer becomes thinner or thicker than the Cu layer thickness of 35 A, respectively.

Magnetoresistance in hybrid type YBCO-NiO/NiFe/Cu/NiFe film structure

The magnetoresistance properties of NiO/NiFe/Cu/NiFe spin valve film deposited on MgO(100) substrate with YBa₂Cu₃O_7(YBCO) film were investigated at room temperature and at 77 K. The magnetoresistance (MR) curves of the hybrid superconductor-magnetoresistor film structure showed an exchange coupling field of 300 Oe and an inverse magnetoresistance ratio of -6.5%. The magnetization configurations of the two magnetic layers in the NiO spin valve were antiparallel due to an increment in the conduction electron flow to superconductor YBCO film. This sample showed an inverse MR ratio.

Basic magnetic properties of Nd based permanent magnet Nd/sub 2/Fe/sub 12.7/Cr/sub 1.3/B by bipolar pulse-type hysteresis loop tracer

Introduction The user-friendly and economical bipolar pulsetype hysteresis loop tracers which are able to measure four quadrant 1-H loop of Nd based hi.& coercivity magnet with a single expnimental run were constmucted in our laboratory[l]-[2]. In the later one of two loop hacers. an alternative pulse magnetic field with peak value of about 10 T is generated by a way of c o r n l i n g 3 monopolar pulse generators to an air solenoid magnet by using a sequential ignition circuit aod a magnet exciting circuit in room tempeatore. In this work, using the Imp tracer, basic magnetic properties such as the major and minor I-H Imps with initial magnetization curves, magnetic anisahopy and anisotropic field of rad type Nd based magnet N d r F e t K n J with length of 5 mm and diameter of 3.6 mm, are investigated.