Synthetic Antiferromagnetic Multilayers Research Articles

Exchange coupling in (Co/Pt)2/Nb/(Pt/Co)2 multilayers induced by the Yu–Shiba–Rusinov bound states

The Ruderman–Kittel–Kasuya–Yosida (RKKY) theory of oscillatory exchange interaction in synthetic antiferromagnetic multilayers results in the oscillation behavior of giant magnetoresistance ratio. In addition, the formation of Yu–Shiba–Rusinov (YSR) bound states will induce an extra antiferromagnetic interaction, which is expected to mediate stronger exchange coupling than conventional RKKY interaction. However, the YSR bound states have only been studied in some superconducting hosts with magnetic impurities. The narrow range of material systems that are not compatible with the device integration architecture limits the practical application of YSR interaction. Here, we observe the RKKY interaction and provide the evidence for YSR interaction in Pt/(Co/Pt)2/Nb/(Pt/Co)2/Pt synthetic antiferromagnetic multilayers by quantitatively determining the coupling energy via ferromagnetic resonance technique.

Current-induced dynamics of skyrmion tubes in synthetic antiferromagnetic multilayers

Topological spin textures can be found in both two-dimensional and three-dimensional nanostructures, which are of great importance to advanced spintronic applications. Here we report the current-induced skyrmion tube dynamics in three-dimensional synthetic antiferromagnetic (SyAF) bilayer and multilayer nanostructures. It is found that the SyAF skyrmion tube made of thinner sublayer skyrmions is more stable during its motion, which ensures that a higher speed of the skyrmion tube can be reached effectively at larger driving current. In the SyAF multilayer with a given total thickness, the current-induced deformation of the SyAF skyrmion tube decreases with an increasing number of interfaces; namely, the rigidity of the SyAF skyrmion tube with a given thickness increases with the number of ferromagnetic (FM) layers. For the SyAF multilayer with an even number of FM layers, the skyrmion Hall effect can be eliminated when the thicknesses of all FM layers are identical. Larger damping parameter leads to smaller deformation and slower speed of the SyAF skyrmion tube. Larger fieldlike torque leads to larger deformation and a higher speed of the SyAF skyrmion tube. Our results are useful for understanding the dynamic behaviors of three-dimensional topological spin textures and may provide guidelines for building SyAF spintronic devices.

Thermal and field-rate dependence of multi-step magnetization switching of synthetic antiferromagnetic multilayers

Synthetic antiferromagnetic (SAF) multilayers are a kind of functional materials with great potential for spintronic devices. Many excellent properties of SAF multilayers have been widely reported in previous works. Here, we experimentally found that the magnetization switching steps n of [Pt/Co]2/Ru/[Co/Pt]4 SAF multilayers with perpendicular magnetic anisotropy are highly dependent on the temperature and magnetic field sweeping rate by magneto-optic Kerr effect and anomalous Hall resistance measurement. It is found that the contribution of the upper and lower layers of the SAF multilayers to the anomalous Hall coefficients is different. The switching steps n change from 2 to 3 by reducing the thickness of the non-magnetic spacer, decreasing the sweeping rate of the magnetic field, and increasing the temperature of the measuring environment. Our findings could provide a method to achieve multi-step magnetization switching for functional spintronic devices.

An Artificial Skyrmion Platform with Robust Tunability in Synthetic Antiferromagnetic Multilayers

AbstractMagnetic skyrmions are topologically nontrivial spin structures, and their existence in ferromagnetically coupled multilayers has been widely reported with a disordered arrangement. Here, a nucleation scenario of ordered skyrmions in nanostructured synthetic antiferromagnetic (SAF) multilayers is proposed and experimentally demonstrated using direct magnetization imaging, indirect magnetometer and magnetoresistance measurement, and micromagnetic simulation. Instead of relying on Dzyaloshinskii–Moriya interaction, the antiferromagnetic interlayer exchange coupling in the SAF multilayers fulfills the role of nucleation and stabilization of skyrmions. The robustness of the proposed skyrmion nucleation scenario is examined against temperature from 4.5 to 300 K and device size from 400 to 1200 nm. Interestingly, these synthetic skyrmions still behave well with a size less than 100 nm. The higher stability than generic magnetic domains can be attributed to topological protection. The results thus provide an artificial skyrmion platform to meet the functional needs of high density and designable arrangement in magnonic and spintronic applications.

Annealing effect on laser-induced magnetization dynamics in Co/Ni-based synthetic antiferromagnets with perpendicular magnetic anisotropy

We report a comprehensive study of annealing treatment modulation on the laser-induced ultrafast magnetic behaviors in antiferromagnetically exchange-coupled [Ni/Co]4/Ru/[Co/Ni]3 multilayers with perpendicular magnetic anisotropy (PMA). Magnetic hysteresis loops indicate that the uniaxial PMA field Hkeff decreases monotonously with the increase in annealing temperature Ta, but the variation of interlayer coupling field Hex is rather complicated. Time-resolved magnetic-optical Kerr effect (MOKE) measurements demonstrate that the laser-excited demagnetization and precession process relies significantly on Ta. Upon laser impulsion, the MOKE signal immediately shows a nonchanging transient increase and decrease with H increasing for low Ta, but only the ultrafast decreasing behavior for high Ta. From the subsequent dynamic precession spectra, the optical and acoustic precession modes are identified. By fitting the field-dependent frequency curves via the deduced dispersion relations, both Hkeff and Hex are determined and their variation trends agree well with the results from the static magnetic measurement. Moreover, it is found that the critical field where the ultrafast signal decrease occurs is dependent on the co-effect of Hkeff and Hex, whereas the maximum field at which the optical mode precession disappears shares the same trend as Hex. The magnetic damping of acoustic mode is seen to increase with Ta due to the increased inhomogeneities. Our findings provide a deep understanding of the magnetic properties in synthetic antiferromagnetic multilayers with high annealing temperatures, which will be helpful for designing advanced spintronic devices.

Temperature-dependent interlayer exchange coupling strength in synthetic antiferromagnetic [Pt/Co]2/Ru/[Co/Pt]4 multilayers**Project supported by the National Natural Science Foundation of China (Grant No. 11704191), the Jiangsu Specially-Appointed Professor, the Natural Science Foundation of Jiangsu Province of China (Grant No. BK20171026), and the Six-Talent Peaks Project in Jiangsu Province, China (Grant No. XYDXX-038).

In this work, we experimentally investigated the thermal stability of the interlayer exchange coupling field (Hex) and strength (–Jiec) in synthetic antiferromagnetic (SAF) structure of [Pt(0.6)/Co(0.6)]2/Ru(tRu)/[Co(0.6)/Pt(0.6)]4 multilayers with perpendicular anisotropy. Depending on the thickness of the spacing ruthenium (Ru) layer, the observed interlayer exchange coupling can be either ferromagnetic or antiferromagnetic. The Hex were studied by measuring the magnetization hysteresis loops in the temperature range from 100 K to 700 K as well as the theoretical calculation of the –Jiec. It is found that the interlayer coupling in the multilayers is very sensitive to the thickness of Ru and temperature. The Hex exhibits either a linear or a non-linear dependence on the temperature for different thickness of Ru. Furthermore, our SAF multilayers show a high thermal stability even up to 600 K (Hex = 3.19 KOe, –Jiec = 1.97 erg/cm2 for tRu = 0.6 nm, the unit 1 Oe = 79.5775 A·m–1), which was higher than the previous studies.

Ionic liquid gating control of RKKY interaction in FeCoB/Ru/FeCoB and (Pt/Co)2/Ru/(Co/Pt)2 multilayers

To overcome the fundamental challenge of the weak natural response of antiferromagnetic materials under a magnetic field, voltage manipulation of antiferromagnetic interaction is developed to realize ultrafast, high-density, and power efficient antiferromagnetic spintronics. Here, we report a low voltage modulation of Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction via ionic liquid gating in synthetic antiferromagnetic multilayers of FeCoB/Ru/FeCoB and (Pt/Co)2/Ru/(Co/Pt)2. At room temperature, the distinct voltage control of transition between antiferromagnetic and ferromagnetic ordering is realized and up to 80% of perpendicular magnetic moments manage to switch with a small-applied voltage bias of 2.5 V. We related this ionic liquid gating-induced RKKY interaction modification to the disturbance of itinerant electrons inside synthetic antiferromagnetic heterostructure and the corresponding change of its Fermi level. Voltage tuning of RKKY interaction may enable the next generation of switchable spintronics between antiferromagnetic and ferromagnetic modes with both fundamental and practical perspectives.

Magnetization Reversal Process Study in Exchange Coupled Synthetic Antiferromagnetic Multilayers and Patterned Structures

FeCo/Ru/FeCo exchange coupled synthetic antiferromagnetic (SAF) multilayers were prepared with two sputtering modes. One is continuous sputtering mode and the other is layer-by-layer sputtering mode. The former mode implies that the growth of the entire FeCo layer has no intervals, while the latter implies that the growth of the entire FeCo has many intervals. It is found that the exchange coupling filed $H_{\mathrm {ex}}$ of sample sputtered by layer-by-layer mode is higher than the one sputtered by continuous mode. Magnetic properties of samples deposited by layer-by-layer mode were studied versus sputtering power. We can conclude that the bilinear exchange constant increases and the biquadratic exchange constant decreases with the increase of the sputtering power. The magnetization reversal process was studied by the magnetic force microscopy (MFM) measurement. We can get that the magnetization reversal process is a domain-wall move process. When the applied magnetic filed is smaller than $H_{\mathrm {ex}}$ , both the two magnetic layers have same domain structure and the domain structure of the two layers are reverse correspondingly. It is also found that when the applied field is smaller than $H_{\mathrm {ex}}$ and changing, the domain wall of the two layers move correspondingly at the same time. From MFM measurement of exchanging coupling square elements, we can conclude that the single domain size of SAF multilayers is larger than 5 $\mu \text{m}$ when the applied magnetic field is smaller than the $H_{\mathrm {ex}}$ .

Study of Magnetization Reversal Process in FeCo/Ru/FeCo Exchange Coupled Synthetic Antiferromagnetic Multilayers

FeCo/Ru/FeCo exchange coupled synthetic antiferromagnetic multilayers were prepared with two sputtering modes. One is continuous sputtering mode, and the other mode is layer-by-layer sputtering mode. The former mode implies that substrate faced the target and film growth process was continuous when FeCo layers were sputtering, whereas the latter implies that substrate was rotating with the mask at a speed of 5 rpm when FeCo layers were sputtering. It was found that the exchange coupling fieldHexof sample sputtered by layer-by-layer mode was higher than the one sputtered by continuous mode. Domain structures were measured with applying varied in-plane magnetic fields along the easy axis in order to study the magnetization reversal process. We found it is a domain wall move process. When the applied field is smaller thanHex, both the two magnetic layers have domain structure and the domain structure of the two layers is reversed correspondingly. When the applied field is varying in the range of-HextoHex, the domain wall of the two layers moves correspondingly at the same time.

Field Detection in Spin Valve Sensors Using CoFeB/Ru Synthetic-Antiferromagnetic Multilayers as Magnetic Flux Concentrators

This work compares the performance of spin valve sensors comprising magnetic flux concentrators (MFCs) composed of Co93Zr3Nb4 (CZN) or (Co70Fe30)80B20-based synthetic-antiferromagnet (SAF) multilayer stacks. In addition, the influence of a tapered MFC tip is also studied. When compared to CZN films, SAFs have the disadvantage of lower magnetic susceptibility (χ = 196 for SAF vs. χ = 753 for CZN), which affects negatively the field gain in gap (10 for SAF vs. 43 for CZN). However, from the overall noise spectrum, one can conclude that the magnetic field detections for sensors incorporating CoFeB/Ru multilayers as MFCs are close to the ones obtained with CZN, being mainly determined by the sensor intrinsic properties instead. For low frequencies, field detection levels at 10 Hz improved from ~ 61 nT/Hz0.5 for single spin valve sensors down to ~ 1.8 nT/Hz0.5 when CZN concentrators with a steep-profile are used.

Exchange Coupling in Synthetic Antiferromagnetic Multilayers for Magnetic Write Head

Fine tuning of exchange coupling constants was demonstrated in synthetic antiferromagnetic multilayers consisting of three ferromagnetic layers and two spacer layers. The effects of interfacial roughness on both the bilinear and biquadratic exchange coupling constants were studied. A model was developed to simulate the behavior of such synthetic antiferromagnetic multilayer system. Detailed exchange coupling constants, both bilinear and biquadratic, were extracted from experimental data by using the model and compared for different samples.

Effect of interface roughness on exchange coupling in synthetic antiferromagnetic multilayers

FeCo/Ru/FeCo synthetic antiferromagnetic (SAF) multilayers with different FeCo/Ru interface roughnesses were prepared by varying the sputtering conditions. The films were characterized by hysteresis loops and torque magnetometry. A large remanence observed in the hard direction for the SAF with interface root-mean-square (RMS) roughness of 0.83 nm suggested a large biquadratic exchange coupling, while the SAF with RMS interface roughness of 0.49 showed primarily bilinear behavior. From field dependent torque curves, assuming coherent rotation, the values of J/sub 1/ and J/sub 2/ were found to be 0.35 and 0.05 erg/cm/sup 2/, respectively, in the smooth SAF compared to 0.13 and 0.16 erg/cm/sup 2/ in the rough SAF. The results are in reasonable agreement with the values obtained from the hysteresis loops and are consistent with the model for apparent biquadratic exchange arising from ferromagnetic coupling through pinholes in the Ru spacer layer.