S.W. 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.

Mn diffusion effect in the exchange biased NiFe/FeMn/NiFe trilayers

We have fabricated NiFe/Mn/FeMn/Mn/NiFe multilayers using ultrahigh vacuum ion-beam deposition system to study the diffusion effect of the inserted Mn on exchange biasing. As the thickness of the Mn layer was changed from 0 to 1.5 nm, the exchange biasing field Hex(top) at the top interface of FeMn/Mn/NiFe was decreased from 258 Oe to 24 Oe. On the other hand, the Hex(bottom) was slightly decreased 103 Oe to 78 Oe without a change in the coercive field Hc. Above 1.2 nm, a reverse phenomena of Hex was observed. After annealing at 200 °C, the Hex(top) was increased by almost a factor of 2; however, the Hex(bottom) did not change. The analyses of x-ray patterns and Auger spectroscopy showed that the abnormal tendency of Hex(bottom) originated from the diffusion of Mn atoms at bottom interface into the FeMn layer during film growth, and that the Mn was concentrated at the interface by annealing.

Determination of rotatable anisotropy in exchange-biased bilayers using anisotropic magnetoresistance technique

We improved the anisotropic magnetoresistance model to determine the quantity and direction of rotatable anisotropy in FeMn/NiFe structures independent of the thickness of the antiferromagnet layer. In our model, there are two important parameters, the magnitude α=Hrot/Hex and direction β=θrot/θa of rotatable anisotropy, where Hrot and θrot are an effective magnetic field and angle of a rotatable anisotropy, and Hex and θa are the exchange bias field and angle of applied field. As FeMn thickness decreased from 20 nm to 3.5 nm, α value increased from about 0.15 to 0.55. This proves that weak exchange coupling regions increase as the antiferromagnet thickness decreases. The α and β almost did not change as the NiFe thickness increases from 7 nm to 15 nm, even if the Hex was reduced from 170 Oe to 70 Oe.

High bias voltage dependence in tunneling magnetoresistance of a ramp-type junction

A ramp-type tunneling magnetoresistance (TMR) junction having structure NiO(60 nm)/pinned Co(10 nm)/NiO(60 nm)/barrier Si3N4(2–6 nm)/free NiFe(10 nm) with the 15° slope was investigated. We obtained nonlinear I–V characteristics for ramp-type tunneling junctions that are distinctively different with and without an applied magnetic field. In the barrier with a Si3N4 thickness of 4 nm, the bias voltage dependence of TMR was stable up to 10 V with a negative TMR ratio of about −10%. The negative TMR is very peculiar for an asymmetric tunneling process between a wedge Co pinned layer and a free NiFe layer.

Laser annealing in exchanged-biased films with out-of-plane and in-plane magnetic anisotropy

The local magnetization reversal of an exchange-biased in-plane FeMn/NiFe bilayers and out-of plane [Pd/Co]5/FeMn multilayers as a function of exposed region is investigated. The multilayer films are deposited using ion-beam and dc magnetron sputtering method. Laser annealing using a diode pumped solid state (Nd:YAG) laser is then utilized to locally reverse the unidirectional anisotropy in the films. The hysteresis loops for the exchange biased films are also analyzed by the magnetic optical Kerr effect and the extraordinary Hall effect.

Thickness and annealing temperature dependence of perpendicular exchange biasing in [Pd/Co]/FeMn multilayers

The thickness of Pd, Co, FeMn, and buffer (Ta, Pd) layers and annealing temperature dependences on perpendicular exchange biasing field (H/sub ex/) and coercivity field (H/sub c/) in Ta(Pd)/[Pd/Co]/sub n//FeMn/Ta(Pd) multilayers are investigated. Hysteresis loops with perpendicular magnetization curves were measured using extraordinary Hall effect. Increase in the thickness of buffer Ta layer lead to the enhancement of both H/sub ex/ and H/sub c/. The H/sub ex/ of 0.1 nm Pd sample increased slightly from 125 Oe to 145 Oe, and vanished at 0.5 nm Pd. As the Co thickness decreases, the H/sub ex/ and H/sub c/ increased up to 360 Oe and 633 Oe in 0.48 nm and 0.32 nm Co sample, respectively. H/sub ex/ increased up to 130 Oe as the FeMn thickness increases to 12 nm, and decreased to 90 Oe at 21 nm FeMn thickness. H/sub ex/ is also observed to increase as the annealing temperature increases. At 330 /spl deg/C, the perpendicular anisotropy disappeared.

Thickness dependence of rotatable anisotropy in exchange-biased NiFe/FeMn bilayers

In this paper, we improved the AMR model to determinate the quantity and direction of rotatable anisotropy in the FeMn/NiFe with a thickness of AF and FM layers.