Role of interfacial structure on exchange-biased FeF2-Fe

Physical Review Letters, 2003

The exchange bias and magnetic anisotropies in a Co layer on a single-crystalline FeF 2 film have been determined between 30 and 300 K. By postulating that the coupling between the ferromagnet and the antiferromagnet persists above the Néel temperature (T N ) we develop a model that quantitatively describes the exchange bias and the anisotropies over the whole temperature range, both above and below T N . Using only the measured low temperature exchange bias and a distribution of blocking temperatures we explain (i) the temperature dependence of the bias, (ii) the magnitude of the anisotropies, (iii) the opposite sign of the first and second order anisotropies, (iv) the observed 1=T and 1=T 3 temperature dependencies of the first and second order uniaxial anisotropies above T N , and (v) the decrease of the anisotropies below T N .

Applied Physics Letters, 1996

Large exchange bias effects (ΔE∼1.1 erg/cm2) were observed in antiferromagnetic (FeF2)–ferromagnetic (Fe) bilayers grown on MgO. The FeF2 grows along the spin-compensated (110) direction. The FeF2–Fe interface roughness was characterized using specular and diffuse x-ray diffraction and atomic force microscopy. The magnitude of the exchange bias field HE increases as the interface roughness decreases. These results imply that magnetic domain creation in the antiferromagnet plays an important role.

Applied Physics Letters, 2009

A coexistence of lateral and in-depth domain walls in antiferromagnet/ferromagnet ͑AF/FM͒ thin films exhibiting double hysteresis loops ͑DHLs͒ is demonstrated. Comparison of single and DHLs together with local and global measurements confirms the formation of two oppositely oriented domains in the AF that imprint a lateral domain structure into the FM layer. Most significantly, the magnetization reversal mechanism within each opposite domain takes place by incoherent rotation of spring-like domain walls extending through the Ni thickness. Therefore, complex three-dimensional domain walls are created perpendicular and parallel to the AF/FM interface in exchange biased systems.

Phys Rev B, 2005

Exchange anisotropy in ferromagnet/antiferromagnet (FM/AF) films is usually introduced along the cooling field or FM magnetization direction. Here we investigate the dependence of the exchange anisotropy, loop bifurcation, and reversal mechanism on the cooling field direction using vector magnetometry. Three types of samples (FM=Fe, Ni /AF = FeF2, MnF2) have been studied where the AF layer is epitaxial (110), twinned (110), and polycrystalline. With an epitaxial AF which has one spin axis, the cooling field orients the exchange field along the spin axis. Applying the cooling field perpendicular to the spin axis results in bifurcated loops, whose shape evolves with the cooling field geometry and strength. With a twinned AF where there are two orthogonal spin axes, the exchange field direction is along the bisector of the spin axes that encompass the cooling field. With a polycrystalline AF, the exchange field direction is the same as the cooling field. Transverse hysteresis loops show that when the exchange field has a component perpendicular to the applied field, the magnetization reversal occurs by rotation in the direction of the perpendicular component. Our results demonstrate that in fluoride films, the exchange field is established primarily by the AF anisotropy direction, and only to a lesser extent the cooling field or the magnetization direction. The bifurcated loops are due to a distribution of AF anisotropies and large AF domain sizes. Furthermore, the magnetization reversal process is extremely sensitive to the exchange field direction.

Applied Physics Letters, 2007

Magnetization reversal via rotation is typical in ferromagnet/antiferromagnet exchange biased systems. The reversibility of the rotation is a manifestation of the microscopic reversal process. The authors have investigated the magnetization reversal in Fe/epitaxial-FeF2 thin films using vector magnetometry and first-order reversal curves. The reversal is predominantly by rotation as the applied field makes an angle with the antiferromagnet spin axis, mostly irreversible at small angles and reversible at larger angles. A modified Stoner-Wohlfarth model reproduces the overall trend of the irreversibility evolution. The remaining discrepancies between the modeled and measured irreversibilities may be attributed to local incomplete domain walls. (

Journal of Applied Physics, 1998

Two measurement techniques, both relying on reversible rotations of the magnetization, have been used to determine the magnitude of the interfacial exchange energy ͑IEE͒ between ferromagnetic and antiferromagnetic ͑F/AF͒ layers. One technique is to use the anisotropic magnetoresistance to determine rotations of the magnetization away from the unidirectional easy axis, where the rotation is accomplished by applying external magnetic fields less than the effective F/AF exchange field. The second technique uses measurements of the ac susceptibility as a function of the angle between the ac field and the unidirectional exchange field. Both of the reversible process techniques result in values of the IEE larger ͑by as much as a factor of 10 in Co/CoO bilayers͒ than the traditional irreversible technique of measuring a shift in the hysteresis loop. The ac susceptibility technique was also used to measure one Fe/FeF 2 bilayer. For this sample, the IEE values obtained by reversible and irreversible methods are equivalent.

Journal of Physics: Condensed Matter, 2012

The magnetic anisotropy of ferromagnetic (FM) Ni, Co, and Fe polycrystalline thin films grown on antiferromagnetic (AF) FeF 2 (110) epitaxial layers was studied, as a function of temperature, using ferromagnetic resonance. In addition to an in-plane anisotropy in the FM induced by fluctuations in the AF short-range order, a perpendicular (biquadratic) magnetic anisotropy, with an out-of-plane component, was found which increased with decreasing temperature above the AF Neél temperature (T N = 78.4 K). This is a surprising result given that the AF's uniaxial anisotropy axis was in the plane of the sample, but is consistent with prior experimental and theoretical work. The resonance linewidth had a strong dependence on the direction of the external magnetic field with respect to in-plane FeF 2 crystallographic directions, consistent with interface magnon scattering due to defect-induced demagnetizing fields. Below T N , the exchange bias field H E measured via FMR for the Ni sample was in good agreement with H E determined from magnetization measurements if the perpendicular out-of-plane anisotropy was taken into account. A low field resonance line normally observed at H ≈ 0, associated with domain formation during magnetization in ferromagnets, coincided with the exchange bias field for T < T N , indicating domain formation with the in-plane FM magnetization perpendicular to the AF easy axis. Thus, biquadratic FM-AF coupling is important at temperatures below and above T N .

Physical Review B, 2003

An analytical model of exchange anisotropy in epitaxial ferromagnetic/antiferromagnetic bilayers was developed. The model demonstrates that the high symmetry exchange anisotropy terms in ferromagnetic/ antiferromagnetic bilayers originate from a partial domain wall in the antiferromagnetic layer. Application of the model to the experimental data analysis enables one to separately determine the fraction of uncompensated interfacial spins in the antiferromagnetic layer and the interfacial exchange coupling energy between spins in the ferromagnet and in the antiferromagnet. The model provides a quantitative description of complex exchange anisotropy recently observed in Fe/MnF 2 bilayers.

Journal of Physics D: Applied Physics, 2011

Results from ferromagnetic resonance experiments carried out on epitaxially grown Fe/KNiF3/FeF2 trilayers are presented. Exchange coupling between the KNiF3, a weak anisotropy antiferromagnet, and the Fe leads to shifts in the resonance field of the ferromagnet. The field shifts can be described by a temperature-dependent exchange anisotropy . depends on the orientation direction of the applied field relative to the magnetic anisotropy axis, and a non-monotonic dependence on KNiF3 thickness. Three thickness regimes appear that correspond to different values of exchange bias in each region. A qualitative understanding of the basis for these three thickness regimes due to spin canting at the interfaces is presented. Our results illustrate a method to tune the value of exchange anisotropy using a combination of different antiferromagnets.

Physical Review B, 2002

The angular dependence of the magnetic anisotropy of exchange biased Fe/MnF 2 bilayers was measured. Below the Néel temperature of the antiferromagnetic MnF 2 layer, an exchange anisotropy is observed which consists of unidirectional, uniaxial, threefold and fourfold symmetry components. The threefold exchange anisotropy term is responsible for the asymmetric magnetization reversal process recently observed in this system.

Physical Review B, 1997

The temperature dependence of the exchange bias (H E ) near the FeF 2 Ne ´el temperature (ϳ 78.4 K͒ was correlated with structural measurements in FeF 2 -Fe bilayers. Low-angle x-ray diffraction and atomic force microscopy show that samples with larger height fluctuations have larger lateral grain sizes. Samples with larger lateral grain sizes exhibit a surface critical exponent (␤ S ϳ0.8) while samples with smaller grains and smaller height fluctuations have a decreased ␤ S , indicating a more three-dimensional-like phase transition or an increase in the FeF 2 surface exchange interaction. ͓S0163-1829͑97͒05230-2͔

Solid State Communications, 2000

Positive exchange bias (PEB) is a remarkable phenomenon, which was recently observed experimentally. Normal (negative) exchange bias (NEB) was discovered more than 40 years ago. Its signature is the shift of the hysteresis loop along the applied field axis by H E Ͻ 0; in systems where a ferromagnet (FM) is in close contact with an antiferromagnet (AFM). This occurs after the system is cooled below the Néel temperature in an external field H cf of a few kOe. As H cf is substantially increased H E adopts positive values. Here we explain this rather unexpected behavior on the basis of an incomplete domain wall model that develops in the FM, for Fe/FeF 2 and Fe/MnF 2 systems. A consistent and unified picture of both NEB and PEB, and satisfactory quantitative agreement with experimental results are obtained on the basis of our theory.

Applied Surface Science, 2008

2007

The exchange bias effect, discovered more than fifty years ago, is a fundamental interfacial property, which occurs between ferromagnetic and antiferromagnetic materials. After intensive experimental and theoretical research over the last ten years, a much clearer picture has emerged about this effect, which is of immense technical importance for magneto-electronic device applications. In this review we start with the discussion of numerical and analytical results of those models which are based on the assumption of coherent rotation of the magnetization. The behavior of the ferromagnetic and antiferromagnetic spins during the magnetization reversal, as well as the dependence of the critical fields on characteristic parameters such as exchange stiffness, magnetic anisotropy, interface disorder etc. are analyzed in detail and the most important models for exchange bias are reviewed. Finally recent experiments in the light of the presented models are discussed.

Physical Review B, 2002

The dependence of exchange bias on antiferromagnet thickness has been measured in FeF 2 /Fe and MnF 2 /Fe bilayers. The two fluoride systems have identical crystal structures, similar lattice constants, but anisotropy fields that differ by a factor of 20. Hence, by comparing the antiferromagnetic layer thickness dependence of the exchange bias in the two systems we are able to directly establish the effect of the antiferromagnet anisotropy. We find that the critical antiferromagnet thickness for the onset of exchange biasing is an order of magnitude smaller for the more anisotropic fluoride, confirming the often-used assumption that the anisotropy dictates the critical thickness. By measuring the temperature dependence of the exchange bias and the structural morphology of the layers we are able to prove that the effects we observe are not due to the blocking-temperature thickness dependence or the onset of discontinuity in thin antiferromagnet layers.