Biquadratic Coupling Strengths Research Articles

Noncollinear interlayer exchange coupling across IrFe spacer layers

In this paper, we show that IrFe spacer layers enable the control of noncollinear interlayer exchange coupling between ferromagnetic Co layers over a spacer layer composition and thickness range that is larger than any observed before for other spacer layer materials. Moreover, in the studied Co|IrFe|Co structures, the bilinear coupling strength, J1, is above 2mJ/m2 and the biquadratic coupling strength, J2, is above 1mJ/m2 over a wide spacer layer composition and thickness range. Notably, the largest antiferromagnetic bilinear coupling strength ever observed across spacer layers of 0.6nm or thicker is achieved, with J1 peaking at 3.4mJ/m2. After annealing, orthogonal coupling is favoured over a wide spacer layer composition and thickness range, which could be useful for magnetic sensor devices. As the temperature is reduced below room temperature, the coupling parameters of the as-deposited films vary slowly, which could be a desirable feature for applications. Measurements of the saturation magnetization of Co|IrFe|Co structures reveal that IrFe has a magnetic moment in the spacer layer composition range for which noncollinear coupling is observed. This indicates that the origin of noncollinear coupling in Co|IrFe|Co could be attributed to the competition between the antiferromagnetic coupling of magnetic atoms across Ir atoms in the spacer layer and the ferromagnetic coupling of neighbouring magnetic atoms.

Effect of interlayer exchange coupling in spin-torque nano oscillator

The dynamics of the magnetization of the free layer in a spin-torque nano oscillator (STNO) influenced by a noncollinear alignment between the magnetizations of the free and pinned layers due to an interlayer exchange coupling has been investigated theoretically. The orientations of the magnetization of the free layer with that of the pinned layer have been computed through the macrospin model and they are found to match well with experimental results. The bilinear and biquadratic coupling strengths make the current to switch the magnetization between two states or oscillate steadily. The expressions for the critical currents between which oscillations are possible and the critical bilinear coupling strength below which oscillations are not possible are derived. The frequency of the oscillations is shown to be tuned and increased to or above 300 GHz by the current, which is the largest to date among nanopillar-shaped STNOs.

Effect of Variable Biquadratic Exchange Coupling on the Magnetic Hysteresis of Uniaxial Antiferromagnetic Co/Ru/Co Films

We investigated biquadratic exchange coupling strength in sputtered uniaxial antiferromagnetic Co/Ru/Co trilayers, which can be tuned by applying a magnetic field perpendicular to the sample surface during deposition. The perpendicular field induces a columnar grain structure that results in a perpendicular magnetic anisotropy for single Co layers. The perpendicular magnetic anisotropy is detected by combined magnetization and Brillouin light scattering measurements. In trilayers, increasing the perpendicular field during deposition results in a monotonic increase of the biquadratic exchange coupling strength, so these samples provide an experimental realization of the effect of variable biquadratic coupling strength on magnetic hysteresis behavior. The hard-axis remanence is a direct measure of biquadratic exchange coupling strength, which we demonstrated both by experiment and by a Stoner-Wohlfarth model that includes the relevant magnetic parameters.

Biquadratic exchange coupling in an unequal Fe/Cr/Fe(1 0 0) trilayer

We have investigated the magnetic properties of a (1 0 0)-oriented unequal trilayer, Fe(45 Å)/Cr(30 Å)/Fe(15 Å), by means of Brillouin light scattering and magnetization measurements. The experimental results show that this sample highlights the effect of biquadratic coupling which aligns the magnetization of the Fe layers at 90° to each other. We extracted the bilinear and biquadratic coupling strengths by fitting the experimental results with a theory that treats the static and dynamic responses on an equal footing. Our results confirm that the model describes both the static and dynamic properties even when the magnetization of the layers is aligned at 90°. The coupling strengths, and their temperature dependence, are discussed and compared with other results reported in the literature.

Biquadratic interlayer exchange coupling in epitaxial Fe/Si/Fe

We have studied the biquadratic exchange coupling in epitaxially grown Fe/Si/Fe. The temperature and thickness dependence of the biquadratic coupling strength were determined unambiguously by fitting the easy- and hard-axis magneto-optical Kerr effect loops. The origin of the biquadratic coupling can be fully understood in terms of Slonczewski’s loose spins mechanism.

Effect of biquadratic exchange coupling on magnetoresistance and magnetization process in magnetic bilayer systems

An improved model is proposed to deal with the magnetic bilayer systems taking into account the contribution of the anisotropy energy and biquadratic exchange coupling to elaborate on the evolution of the magnetoresistance (MR) ratio and magnetization process. The results indicate that the characteristic behavior of the MR ratio depends distinctly on both the biquadratic coupling constant and the layer thickness. The profile of the MR ratio was found to vary from an inverted bell shape to a concave pyramid with increasing biquadratic coupling strength, and decays sharply with the layer thickness. This model calculation helps us to provide a venue for further understanding the MR or giant magnetoresistance behavior of the magnetic multilayer system.

Oscillatory biquadratic coupling in Fe/Cr/Fe(001)

Polar Kerr measurements have been used to measure the dependence of the biquadratic coupling strength ${\mathrm{B}}_{12}$ on Cr thickness in an Fe/Cr/Fe trilayer. The overall behavior, which consists of a maximum coupling strength at ${\mathrm{d}}_{\mathrm{Cr}}$=5 \AA{} (3.5 ML) with a falloff at greater Cr thicknesses, is found to be consistent with in-plane Kerr and Brillouin light-scattering measurements performed on the same sample. The polar Kerr measurements suggest additionally that ${\mathrm{B}}_{12}$ increases from zero near zero Cr thickness, and that it oscillates in magnitude after the first peak, with a second peak in ${\mathrm{B}}_{12}$ occurring at about ${\mathrm{d}}_{\mathrm{Cr}}$=12 \AA{} (8.3 ML). The positions and heights of the first and second biquadratic coupling maxima, in relation to the first bilinear coupling maximum, show excellent agreement with previous measurements by K\"obler et al. of the biquadratic coupling behavior in Fe/Cr/Fe, and also show good agreement with the predictions of an intrinsic biquadratic coupling mechanism due to Edwards et al.

Brillouin light scattering study of Fe/Cr/Fe (211) and (100) trilayers.

The magnitude of the bilinear and biquadratic interlayer coupling strengths between Fe layers separated by Cr spacer layers is investigated by means of Brillouin light scattering, magneto-optic Kerr rotation, and magnetoresistance techniques. A data analysis scheme, which treats all three data sets on an equal footing, yields self-consistent anisotropy and interlayer coupling parameters extracted independently from the three techniques. The values of the bilinear and biquadratic coupling strengths are compared for simultaneously grown (211) and (100) Fe/Cr samples. The approach not only provides reliable values for the coupling strengths but also highlights the complementarity of these techniques in uniquely determining the magnetic parameters. \textcopyright{} 1996 The American Physical Society.

Exchange-coupled MBE-grown Co/Cu/Co(111) trilayers

It is shown that the high-field curvature in the in-plane and polar magnetization curves of exchange-coupled MBE-grown Co/Cu/Co(111) structures can be accurately fitted by assuming a sizeable biquadratic coupling in addition to the bilinear coupling, and the existence of a large areal fraction of ferromagnetic inclusions. Measurements on a sample with a single Co layer confirm that the curvature does not originate from the individual Co layers. Analysis of two Co/Cu/Co(111) trilayers with different CoCu interface qualities, as measured by NMR, shows significantly higher bilinear and biquadratic coupling strengths for the trilayer with the better defined CoCu interfaces. While NMR indicates reasonably good structural quality on a short lateral length scale, polar Kerr measurements as a function of interlayer thickness, and polarized neutron reflection measurements, indicate the existence of structural imperfections on a long lateral length scale.

Bilinear and biquadratic exchange coupling in epitaxial Fe/Cr/Fe/Ag/GaAs(100) structures

The interlayer exchange coupling has been investigated in epitaxial Fe(20 Å)/Cr/Fe(20 Å)/Ag/GaAs(100) structures that contain a wedge-shaped (0–40 Å) Cr layer. Longitudinal and polar magneto optical Kerr effect (MOKE) and Brillouin light scattering (BLS) measurements have been combined to determine values for the bilinear and biquadratic coupling strengths. While the phase and period of the oscillations in the interlayer coupling agree well with those reported by other researchers, the magnitude of the coupling strength is found to be reduced. The dependence of the biquadratic coupling strength upon the Cr thickness is well described by a simple power law and may provide a useful test of extrinsic biquadratic coupling models. We show that the combination of MOKE and BLS measurements are very effective in investigating the ultrathin Cr region and that ferromagnetic (FM) coupling produces large changes in the polar MOKE saturation field.

Interlayer exchange coupling in epitaxial Fe/Cr/Fe/Ag/GaAs(100) structures

The interlayer exchange coupling has been investigated in epitaxial Fe(20 Å)/Cr/Fe(20 Å)/Ag/GaAs(100) structures that contain a wedge-shaped (0–40 Å) Cr layer. Longitudinal and polar magneto-optical Kerr-effect (MOKE) and Brillouin light-scattering measurements have been combined to determine values for the relevant anisotropy constants and both the bilinear and biquadratic coupling strengths. The phase and period of the oscillations in the interlayer coupling are found to agree well with those reported by other researchers while the total coupling strength is found to be reduced. This reduction is presumably due to the presence of structural imperfections in our samples, and our results may therefore be of use in testing some of the recently proposed extrinsic biquadratic coupling mechanisms. Specifically, we find that for the Cr thicknesses studied the biquadratic coupling strength in our samples varies as d−1.4Cr where dCr is the thickness of the Cr layer. We also present results that show how the ultrathin Cr limit may be investigated. We show that the coercivity of the easy axis MOKE loops is sensitive to submonolayer coverages of Cr and that polar MOKE is sensitive to the strong ferromagnetic coupling found in the 0–4 Å Cr thickness range.

Studies of Exchange Coupling in Fe (001) Whisker/Cr/Fe Structures using BLS and Rheed Techniques.

ABSTRACTThe conditions for an almost perfect growth of smooth Cr (001) films on an iron whisker substrate have been investigated by means of reflection high energy electron diffraction (RHEED). The exchange interaction between 20 Monolayer thick Fe (001) films separated from a bulk whisker Fe (001) substrate by a variable number of Cr (001) Monolayers (ML) has been investigated by means of Brillouin light scattering experiments (BLS). These experiments show unambiguously that the exchange coupling strength between the iron film and the iron whisker can be described by a short wavelength oscillatory term superposed on a slowly varying antiferromagnetic background. The BLS data enabled one to separate the bilinear and the biquadratic contributions to the antiferromagnetic exchange coupling terms. Both the bilinear and the biquadratic coupling strengths exhibited a short period oscillatory dependence on the Cr interlayer thickness (∼2 Monolayers). Maxima in the bilinear antiferromagnetic coupling strength occur for an odd number of Cr Monolayers. This observation is not in agreement with first principles calculations. The first phase inversion has been found to occur between 4 and 5 ML of Cr.