Control of interlayer exchange coupling in Fe/Cr/Fe trilayers by ion beam irradiation

The manipulation of the antiferromagnetic interlayer coupling in epitaxial Fe/Cr/Fe(001) trilayers by 5 keV He ion beam irradiation has been investigated. It is shown that even for irradiation with low fluences a drastic change in strength of the coupling appears. For thin Cr spacers (below 0.6-0.7 nm) it decreases with fluence, becoming ferromagnetic for fluences above 2x10(14) ions/cm(2). The effect is connected with the creation of magnetic bridges in the layered system due to atomic exchange events caused by the bombardment. For thicker Cr spacers an enhancement of the antiferromagnetic coupling strength is found. A possible explanation of the enhancement effect is given.     

Direct-current induced dynamics in Co90 Fe10/Ni80 Fe20 point contacts

We have directly measured coherent high-frequency magnetization dynamics in ferromagnetic films induced by a spin-polarized dc current. The precession frequency can be tuned over a range of several gigahertz by varying the applied current. The frequencies of excitation also vary with applied field, resulting in a microwave oscillator that can be tuned from below 5 to above 40 GHz. This novel method of inducing high-frequency dynamics yields oscillations having quality factors from 200 to 800. We compare our results with those from single-domain simulations of current-induced dynamics.     

Magnetic properties of lithographically defined lateral Co/Ni80Fe20 wires

A novel micro-fabrication technique has been used to create an array of lateral magnetic multilayers consisting of micron-sized sputtered Co and Ni₈₀Fe₂₀ wires. The structures were fabricated using conventional optical lithography and a combination of hard and soft lift-off methods. For the field applied parallel to the wires intrinsic easy axis, we observed two switching fields corresponding to the distinct coercive field of the Ni₈₀Fe₂₀ wires (H_c1) and Co wires (H_c2) constituting the lateral multilayer wire array. A state of anti-parallel relative alignment of magnetization was observed when the applied field is greater than the switching field of Ni₈₀Fe₂₀ wires but less than the switching field of Co wires. We found the region of anti-parallel alignment of magnetization between the Co and Ni₈₀Fe₂₀ wires to be very sensitive to the relative orientation of the applied magnetic field.     

Effect of the ferromagnetic layer thickness on the interlayer interaction in Fe/Si/Fe trilayers

Three-layer magnetic film systems Fe/Si/Fe have been studied by the method of magnetic resonance. It is established that the ferromagnetic layer thickness affects the magnitude of the interlayer exchange interaction in this system. A mechanism explaining the observed effect is proposed.     

磁性隧道结Ni80Fe20/Al2O3/Co的制备和物性

用等离子体氧化形成绝缘层的方法,重复性地制备出了Ｎｉ８０Ｆｅ２０／Ａｌ２Ｏ３／Ｃｏ磁性隧道结。样品的隧道磁电阻（ＴＭＲ）比值在室温下最高可达６．０％,翻转场（ｓｗｉｔｃｈ ｆｉｅｌｄ）可低于８００Ａ／ｍ,平台宽度约２４００Ａ／ｍ。结电阻的变化范围从几百欧姆到几百千欧。     

Exchange coupling effects in NiO/Co and NiO/permalloy bilayers

NiO/Co and NiO/Ni₈₀Fe₂₀ bilayers were prepared at 293 onto SiO₂(1 0 1)/Si(1 1 1) and glass substrates using UHV (5×10⁻¹⁰ mbar) RF/DC magnetron sputtering. Results on magnetic measurements showed that the exchange biasing and coercive fields are inversely proportional to the Co and Ni₈₀Fe₂₀ (Py) layer thickness down to 2 nm. A maximal RT coupling energy for the NiO–Co and NiO–Py interface was estimated as 0.04 and 0.03 mJ/m² for the samples prepared onto SiO₂(1 0 1)/Si(1 1 1) substrates.     

Ru thickness-dependent interlayer coupling and ultrahigh FMR frequency in FeCoB/Ru/FeCoB sandwich trilayers

The antiferromagnetic (AFM) interlayer coupling effective field in a ferromagnetic/non-magnetic/ferromagnetic (FM/NM/FM) sandwich structure, as a driving force, can dramatically enhance the ferromagnetic resonance (FMR) frequency. Changing the non-magnetic spacer thickness is an effective way to control the interlayer coupling type and intensity, as well as the FMR frequency. In this study, FeCoB/Ru/FeCoB sandwich trilayers with Ru thickness ($t_{\rm Ru}$) ranging from 1 Å to 16 Å are prepared by a compositional gradient sputtering (CGS) method. It is revealed that a stress-induced anisotropy is present in the FeCoB films due to the B composition gradient in the samples. A $t_{\mathrm{Ru}}$-dependent oscillation of interlayer coupling from FM to AFM with two periods is observed. An AFM coupling occurs in a range of $2 {\rm Å} \le t_{\rm Ru} \le 8 {\rm Å}$ and over 16 $\mathrm{Å}$, while an FM coupling is present in a range of $t_{\rm Ru}< 2$ Å and $9 {\rm Å} \le t_{\rm Ru} \le 14.5 Å$. It is interesting that an ultrahigh optical mode (OM) FMR frequency in excess of 20 GHz is obtained in the sample with ${t}_{\mathrm{Ru}}= 2.5 \mathrm{Å}$ under an AFM coupling. The dynamic coupling mechanism in trilayers is simulated, and the corresponding coupling types at different values of $t_{\mathrm{Ru}}$ are verified by Layadi's rigid model. This study provides a controllable way to prepare and investigate the ultrahigh FMR films.     

Structure and interlayer coupling in Co/V multilayers

Two series of sputtered Co/V multilayers were investigated by X-ray diffraction, ferromagnetic resonance (FMR) and magnetic measurements. The multilayers are found severely alloyed, resulting in two FMR peaks of uniform mode. Spin-wave resonance (SWR) spectra were observed when the thickness of vanadium is thin, indicating interlayer coupling. The data of SWR were treated approximately respectively by volume inhomogeneity (VI) and volume homogeneity and entire surface pinning (SP) models. The relatively small calculated effective exchange constants showed weak exchange coupling between Co layers across V spacers.     

Hydrogen-controlled interlayer exchange coupling in Fe/LaHx multilayers

Magneto-optic Kerr magnetometry and neutron reflectometry reveal that Fe layers exhibit magnetic exchange coupling through LaH_x spacer layers. Ferromagnetic and antiferromagnetic coupling is observed on multilayers of these materials depending on the thickness of the hydride layers, but without oscillatory behavior. Starting from metallic La dihydride spacer layers the effect of dissolving increasingly more hydrogen was examined. Sign and value of the coupling depend crucially on the hydrogen content x. The coupling can be inverted from antiferromagnetic to ferromagnetic and vice versa. These alterations are due to modifications of the electronic structure of the hydride. When the hydrogen absorption saturates the hydride layers become insulating and the exchange coupling is likely to disappear. In this final state the multilayers are always characterized by a very soft ferromagnetic rectangular hysteresis curve. Upon removal of the hydrogen to the initial concentration the original magnetic structure is restored.     

Proximity effect in Fe/Pb/Fe trilayers

We have studied superconducting and magnetic properties of sputtered Fe/Pb/Fe-trilayers. For a fixed Pb thickness and with changing Fe thickness, , a monotonic decrease of the superconducting transition temperature was observed. Magnetization measurements clearly showed that Fe remains ferromagnetic down to the monolayer range. A quantitative comparison of with the theory of pair breaking by the exchange field reveals that the observed -suppression by the ferromagnetic Fe-layer is much weaker than expected. Possible reasons for the reduced -suppression in this system are discussed. Received: 30 June 1997 / Revised: 20 August 1997 / Accepted: 16 December 1997     

Coupled perpendicular magnetization in Fe/Cu/Fe trilayers

Ultrathin epitaxial Fe films on Cu(1 0 0) with perpendicular magnetization have been used as templates for the preparation of FCC Fe/Cu/Fe trilayers. The magnetic anisotropy and the coupling of these films have been studied by in-situ magneto optical Kerr effect measurements and Kerr microscopy. The magnetic coupling of both Fe layers is found to be dominated by magnetostatic interaction. Adsorbate-induced spin reorientation in the top layer also causes spin reorientation in the bottom layer. The governing role of the Fe-vacuum interface for the magnetism of the whole trilayer is demonstrated.     

Magnetic properties of Fe/ZnSe/Fe trilayers

The magnetic properties of Fe/Zn/Fe trilayers have been studied by ferromagnetic resonance and magnetization measurements. These measurements have been used to investigate the magnetic anisotropy of the iron layers and the magnetic coupling across the semiconductor spacer. The angular dependence of the resonance spectra has been measured in-plane and out-of-plane in order to deduce magnetic anisotropy constants of the samples. Experimental data were fitted by using an energy-density expression that includes bulk cubic anisotropy, growth-induced uniaxial in-plane anisotropy and perpendicular-surface anisotropy terms. A small ferromagnetic coupling is observed in the trilayers with spacer thickness up to .     

Oscillatory interlayer exchange coupling and its temperature dependence in [Pt/Co]3/NiO/[Co/Pt]3 multilayers with perpendicular anisotropy

Interlayer exchange coupling that oscillates between antiferromagnetic and ferromagnetic as a function of NiO thickness has been observed in [Pt(5 A)/Co(4 A)](3)/NiO(t(NiO) A)/[Co(4 A)/Pt(5 A)](3) multilayers with out-of-plane anisotropy. The period of oscillation corresponds to approximately 2 monolayers of NiO. This oscillatory behavior is possibly attributed to the antiferromagnetic ordering in NiO. The antiferromagnetic interlayer exchange coupling for the 11 A NiO layer shows an increase in coupling strength with increasing temperature, in agreement with the quantum interference model of Bruno for insulating spacer layers. A coexistence of exchange biasing and antiferromagnetic interlayer exchange coupling has been observed below T=250 K.     

Thickness-dependent magnetic properties of Ni81Fe19, Co90Fe10 and Ni65Fe15Co20 thin films

We present results of magnetization and magnetic anisotropy measurements in thin magnetic films of the alloys Ni₈₁Fe₁₉, Co₉₀Fe₁₀ and Ni₆₅Fe₁₅Co₂₀ that are commonly used in magnetoelectronic devices. The films were sandwiched between layers of Ta. At room temperature the critical thickness for all the films to become ferromagnetic is in the range 11–13 Å. In Co₉₀Fe₁₀ the coercivity and the anisotropy field both depend strongly on layer thickness.