C. Hou

Effect of exchange coupling of polycrystalline antiferromagnetic layers on the magnetization behavior of soft magnetic layers

The complementary nature of the exchange bias field and the coercivity enhancement in ferromagnetic (F)-antiferromagnetic (AF) exchange-coupled layer systems is explained based on an extended Fulcomer and Charaps model, in which the directional distribution of the easy axes plays a decisive role in governing the magnetization behaviors. The model was further extended to deal with the AF-layer thickness ranging from zero to infinity taking into account of Neels spin fanning model. A comprehensive explanation of the influence of exchange coupling on the magnetization behavior of the F-layer is given. It is shown that a rotational magnetization process is inherent for the samples in which the exchange coupling effect is dominant. The same model explains the temperature dependence of the exchange bias field and coercivity and the memory effect observed after annealing. The varieties of phenomena are mostly ascribed to the existence of distribution in the magnetic properties, net spin moment at the AF-grain surface and AF-grain size.

Temperature dependence of the pinning field and coercivity of NiFe layers coupled with an antiferromagnetic FeMn layer

The pinning field Hp (the amount of the shift of the hysteresis loops) and the coercivity Hc of the samples of the form glass/Ta 120 A/(Cu 100 A)/NiFe 75 A/FeMn 150 A/Ta 50 A increase almost linearly with decreasing temperature down to 20 K, below which Hc increases sharply. The observed strong positive correlation between Hp and Hc, seems to be reasonably explained by a combination of a newly developed model in which a directional distribution of the pinning field caused by a random distribution of the crystalline orientations in the antiferromagnetic FeMn layer is taken into account and Hoffmann’s ripple theory in which the local anisotropy is assumed to be proportional to Hp, although the sharp increase in Hc at very low temperatures remains to be explained.

Characteristics of 360°-domain walls observed by magnetic force microscope in exchange-biased NiFe films

The characteristics of 360° domain walls have been investigated by magnetic force microscopy (MFM) in glass/Ta(10 nm)/NiFe(7.2 nm)/FeMn(8.0 nm)Ta(5 nm). The sample was prepared by radio-frequency-magnetron sputtering and characterized with a vibrating sample magnetometer. Both closed and line types of 360° domain walls were observed. The closed type (loop) walls include circular, ellipsoidal, and irregular shapes of domain wall, the long axis of which is generally perpendicular to the overall easy direction (pinned direction) of the exchange biased NiFe film. 360° wall loops were also classified by their chirality. Many 360° domain walls were observed near defects and the measured wall thickness ranged from 1.5 to 3 μm. The magnetic structure of the 360° loop was verified by simulations using a simplified model of the MFM imaging process. The formation of a 360° wall loops appears to originate from the rotational nature of the magnetization in the exchange-biased film, which comes from the distribution of...

Temperature dependence of exchange field and coercivity in polycrystalline NiO/NiFe film with thin antiferromagnetic layer: Role of antiferromagnet grain size distribution

The temperature dependence of the exchange bias Heb and coercivity Hc, has been measured for a NiO(6.5 nm)/Ni81Fe19(12 nm) bilayer which demonstrated at room temperature zero exchange bias, but an enhanced coercivity (Hc=65 Oe). Upon cooling the sample in a magnetic field (H=300 Oe), the exchange bias remains zero down to T=250 K, whereas the coercivity increases roughly linearly with decreasing temperature. Below this critical temperature an exchange shift of the hysteresis loop occurs that is accompanied by a decrease in the coercivity. Decreasing the temperature further results in an increase in both Heb and Hc. A thermal fluctuation model, where both fluctuating and stable AF grains contribute to the coercivity, with an additional assumption of increasing the density of NiO interfacial uncompensated spins at low temperature, predicts the correct temperature behavior of Heb and Hc.

Effect of F/AF coupling strength on the magnetization process of F/AF system

Abstract The effect of the exchange coupling strength J between NiFe and FeMn layers on the magnetization process in NiFe/FeMn structures was investigated. Samples with structures of NiFe/impurity (or Cu)/FeMn and NiFe/FeMn were prepared and measured to determine their hysteresis loop shift H p and coercivity H c . It is often assumed that H p is proportional to J . However, our results show that H p dependence on J is not monotonic. A phenomenological model is proposed to explain the results. A better method for estimation of J is also proposed.

Optical interference in magneto-optic Kerr-effect measurements of magnetic multilayers

Hysteresis loops of exchange-biased permalloy/Fe50Mn50/permalloy trilayers on glass were measured as a function of Fe50Mn50 and permalloy thicknesses with the longitudinal Kerr effect employing a coherent light source. Kerr signals originate from both permalloy layers and give a superposition of hysteresis loops. In vibrating sample magnetometer or looptracer measurements the contribution of a particular layer to a major hysteresis loop cannot be identified. With the Kerr setup presented it is possible to identify the contribution of each layer individually, since the finite optical path through the trilayer gives rise to optical attenuation and interference. For an increasing total thickness of the trilayer, the signal of the buried permalloy layer will become weaker due to attenuation. Kerr measurements of trilayers up to 40 nm show a superposition of two equally oriented loops. Major loops for trilayers of thickness greater than 40 nm show a superposition of two oppositely oriented hysteresis loops. Th...

Structural origin of coercivity enhancement and exchange-bias field in double antiferromagnet/ferromagnet bilayers

An experimental method to identify the structural origins of the coercivity enhancement and the exchange-bias field of polycrystalline ferro-antiferromagnetic (F/AF) coupled systems is proposed. The exchange-bias field and the coercivity of the FI layer of the samples of the structure FI/AFI are compared with those of the FII layer of the samples of the structure FII/AFII/FIII/AFIII. It is concluded that, for a given temperature, it is those AF grains which stop their growth at a critical thickness that contribute to the coercivity enhancement, and those which grow over the critical thickness that contribute to the exchange-bias field. Meanwhile the effective magnetic surface anisotropy introduced to the top surface of the AFII layer by the FIII layer can increase the exchange-bias field of the FII layer when the AFII layer is thin.

Thermal Effect On The Coercivity Of Ultrathin NiFe Films

The temperature dependence of the coercivity of ultrathin NiFe films was measured by a SQUID in the temperature range from 5 to 300 K. The change in coercivity with temperature was much greater for thinner films than for thicker ones. A domain wall motion model is proposed to explain the experimental results. It is concluded that the intrinsic coercivity increases rapidly with decreasing thickness, especially in the range of several nm, and that the low coercivity observed for very thin films at room temperature is ascribed primarily to the thermal effect.

Highly field-sensitive transverse-biased non-hysteretic spin valves

Transverse biasing effect on spin valves was studied by use of a uniform rotation model. The effect was proved to be substantial in improving non-hysteretic spin valves in the field sensitivity, although the dynamic range was sacrificed to some extent. An experimental test showed that compared with the conventional spin valve in which the pinning field direction is set perpendicular to the easy axis of the free layer, 2.5 times sensitivity was obtained with a dynamic range of 0.6 times compared to that of the ideal conventional case. Calculation showed that a self-biasing effect by tilting the pinning field direction of the pinned layer from the easy axis of the soft magnetic layer was also effective in enhancing the field sensitivity.

Theoretical and numerical model for estimating unknown magnetic parameters in studying ferromagnetic and antiferromagnetic coupled films

A new method of estimating unknown magnetic parameters for coupled thin films of ferromagnetic and antiferromagnetic materials is introduced. It combines convenient torque and spin angle measurements with strength of mathematical models and numerical techniques. Description of this approach is outlined and test results for several simulated examples are presented.