Session T15: Focus Session: Magnetic Nanostructures-Exchange Bias and Exchange-Coupled Systems

Investigation of CoO/Fe/Ag(001) and NiO/Fe/Ag(001) epitaxial thin films X-ray magnetic dichroism

Interfacial coupling at antiferromagnetic(AFM)/ferromagnetic(FM) interface is less understood because of the so-called magnetic frustration and the lack of direct measurement on AFM thin films. These difficulties have been partial overcome by recent development of X-ray Magnetic Dichroism on single crystalline magnetic thin films. In this talk, I will report our study on epitaxial CoO/Fe/Ag(001) system using X-ray Magnetic Circular Dichroism (XMCD) and X-ray Magnetic Linear Dichroism (XMLD). XMCD was used to measure the Fe hysteresis loops and XMLD was used to measure the response of the AFM CoO spins in response to the Fe magnetization reversal. We find that the CoO spins consist of rotatable and frozen spins with respect to the Fe spin rotation, and only the Fe uniaxial magnetic anisotropy follows the CoO frozen spins. By using focused ion beam to pattern the films into microstructures, we also observed vortex state in CoO disk imprinted from the Fe vortex state.   

Mirror symmetry in magnetization reversal and coexistence of positive and negative exchange bias in Ni/FeF$_{2}$ heterostructures

Positively and negatively exchange biased (PEB and NEB) magnetoresistance (MR) loops in Ni/FeF$_{2}$ ferromagnetic/antiferromagnetic (AF) heterostructures proceed through exactly the same reversal mechanisms. The MR curves exhibit striking mirror symmetry: the increasing (decreasing) field branch of the PEB (NEB) loop is identical to the decreasing (increasing) branch of the NEB (PEB) loop. The latter suggests that, on average, similar but opposite sign EB domain configurations are reached for low or high cooling fields, with equal interfacial density of pinned uncompensated AF spins responsible for NEB or PEB. Micromagnetic simulations are in agreement with experimental results and imply the coexistence of EB domains of opposite sign for all cooling fields, which results in a new reversal mechanism not previously reported. The persistence of NEB (PEB) domains even at high (low) cooling fields may be produced by the size distribution and density of pinned uncompensated moments in the bulk of the AF. The support of the Spanish MICINN, Catalan DURSI, IKERBASQUE and the US Department of Energy are recognized.   

Phase separation and giant exchange bias in Mn-based binary alloys

The ability to tailor, control and modify the magnetic properties of Mn-based alloys opens the possibility of developing exchange bias systems for permanent magnet and sensor technologies. Rapid solidification of AgMn, CuMn and AlMn manganese-based alloys, with significant Mn concentration ($\sim $ 30-70 at{\%}), has produced alloys that all exhibit a remarkable exchange bias at $T$ = 10 K, of the order of $\sim $ 10 kOe. Structural characterization confirms the formation of a phase-separated nanostructure (\textit{fcc} for AgMn and CuMn and \textit{hcp} for AlMn) of 40-80 nm in all alloys as characterized by phase-specific crystallographic texture and lattice parameters. The observed exchange bias is highly reduced upon moderate annealing ($T$ = 250 $^{\circ}$C) accompanied by homogenization of the Mn concentrations in the alloys. These results are tentatively attributed to different metastable incorporation of Mn within the two phases, yielding slightly different unit cell volumes, which provides an antiferromagnetic (Mn-rich phase) and ferromagnetic (Mn-poor phase) character in these two phases.   

Exchange Bias and Large Vertical Magnetization Shift in FM/V$_{2}$O$_{3}$ Interfaces

We have investigated exchange bias in different combinations of V$_{2}$O$_{3}$ thin films with ferromagnetic layers. The exchange bias is accompanied by a large vertical shift in the magnetization. These effects are only observed when V$_{2}$O$_{3}$ is grown on top of Ni$_{80}$Fe$_{20}$ permalloy (Py). The magnitude of the vertical shift is as large as 60{\%} of the total magnetization which has never been reported in any system. The exchange bias and the vertical shift are related to the formation of a Fe$_{3}$O$_{4}$ interlayer. We will show evidence that the Fe$_{3}$O$_{4}$ Verwey transition is responsible for the appearance of the exchange bias and the vertical shift in the magnetization.   

Exchange Bias in Ni/Co Multilayers with Perpendicular Anisotropy

We have studied exchange bias in Ta(5nm)/Au(10)/[Ni(t)/Co(0.4)]$\times $5/IrMn(8)/Ta(5) multilayers for Ni thicknesses of 0.8-1.2 nm. The samples were deposited via sputtering and no deposition field was used. The samples were annealed at 200 C in an applied field of 1500 Oe for different durations, then measured by polar Magneto-Optical Kerr Effect at room temperature. We find that the field annealing significantly alters the hysteresis loop shape, giving it more single domain character while simultaneously inducing exchange bias in the direction of the annealing field. After 34 (2 and 32) hrs annealing, the exchange bias of the samples each reach a maximum value ranging from 100 Oe for the thickest Ni to 35 Oe for the thinnest Ni. We find that the samples with the thinner Ni layers approach their exchange bias maximum values for shorter annealing times.   

Electrical measurement of antiferromagnetic moments in exchange-coupled IrMn/NiFe stacks

A great attention is currently focused on the development of new spintronic devices. One crucial technological issue is the coupling phenomena between thin antiferromagnetic (AFM) and ferromagnetic (FM) layers. Despite the exchange coupling was for the first time observed already in 1956, yet the effect is not fully understood. Certainly, the zero macroscopic magnetic moment in the AFM materials hinders the investigation, in contrast to the exhaustive studies on the FM counterpart. Only a very few experiments at large scale facilities explored the status of the AFM layer. Here, we will address the study of both FM and AFM moments in the archetypical IrMn/NiFe exchange-coupled system. Using common laboratory magnetization and transport tools, we found a direct link between reversal of FM moments in NiFe and rotation and pinning of AFM moments in IrMn [1]. We will show that a full rotation of AFM moments in IrMn occurs at high temperatures, in contrast to only partial rotation of AFM at low temperatures. We will discuss the experimental results in the framework of different exchange coupling models. [1] http://arxiv.org/abs/1108.2189   

Exchange bias of conetic thin films

In this work, we study the exchange bias and coercivity of Ni$_{77}$Fe$_{14}$Cu$_{5}$Mo$_{4}$ (Conetic, also known as mu-metal) exchange coupled with FeMn as functions of Conetic thickness and buffer layer material. The samples studied were BL(30nm)/Conetic(9nm-30nm)/FeMn(10nm)/Ta(5nm), where BL = Cu or Ta. All samples were grown by magnetron sputtering in a deposition field of $\sim $150 Oe during growth to set the exchange bias axis. Room temperature hysteresis loops were measured by a magneto-optical Kerr effect magnetometer as a function of applied-field angle. ~For each variety of sample, the exchange bias and coercivity were inversely proportional to Conetic thickness. With Cu buffer layers grown on Si, the H$_{eb}$ decreased from 300 Oe to 62 Oe, and H$_{c}$ decreased from 99 Oe to 9 Oe. ~Similar results were found when the Cu buffer layer was grown on SiOx, though the maximum coercivity was only 67 Oe. For the samples grown on Si(001)/Ta(5nm), the exchange bias decreased from 80 Oe to 14 Oe, while the coercivity increases only slightly from 2 Oe to 10 Oe. These results indicate a trade-off between preserving the softness of the ferromagnet and having a large exchange, which may be useful for tuning the performance of low-field sensing materials   

Effect of Cu Layer on Exchange Bias in FeMn

One of the most important puzzles related to the mechanism of exchange bias is the origin and role of uncompensated magnetization in the antiferromagnet. We study effects of a Cu layer on the uncompensated magnetization and exchange bias in FeMn, the material with a high potential for applications in magnetic recording. The multilayers of Ta(50 {\AA})/[FeMn(50 {\AA} -- 150 {\AA})/Cu(50 {\AA})]$_{10}$/Ta(50 {\AA}) are deposited by UHV DC magnetron sputtering on top of Si/SiO$_{x}$ 3 mm x5 mm substrates. Samples with a single layer of FeMn of the same thickness, Ta(50 {\AA})/FeMn(50 {\AA} -- 150 {\AA})/Ta(50 {\AA}) are used as control samples. The samples are cooled in a field of 7 T and their magnetization is measured using a SQUID magnetometer. All the samples have uncompensated magnetization that exhibits a hysteresis at 10 K. The hysteresis loops for FeMn/Cu multilayers are exchange bias shifted, while FeMn without Cu exhibits no exchange bias. Dependence of coercive field (H$_{c})$, exchange bias (H$_{e})$, and saturated magnetization (M$_{s})$ on the FeMn thickness and on temperature will be discussed. Work is supported by Texas A{\&}M University, TAMU-CONACYT Collaborative Research Program, and by NSF (USF).   

Interface coupling between ferromagnets and random and dilute antiferromagnets

Depth profiles for pinned and unpinned magnetizations were determined across the interface between a ferromagnet (F) and random and dilute antiferromagnets (RAF and DAF) exemplified by Fe$_{0.45}$Ni$_{0:55}$F$_{2}$/Co and Fe$_{0:34}$Zn$_{0:66}$F$_{2}$/Co bilayers, respectively, using polarized neutron reflectivity (PNR). PNR measurements were complemented by magnetometry using applied fields as large as 160 kOe to assure saturation of the entire sample, including magnetic moments that are normally pinned at lower fields. The locations of pinned and unpinned magnetization in the ferro- and antiferromagnets were identified. The origin of exchange bias in the RAF system is noticeably different than that of the DAF system. In the RAF system, a domain wall is formed at the RAF/F interface when the ferromagnet's magnetization is reversed. In the DAF system, some domains within the bulk of the DAF are reversed upon reversal of the ferromagnet with others remaining pinned, while the interface magnetization is entirely reversed. This work was supported by the National Science Foundation.   

A study of the magnetic interlayer coupling between CoO(NiO) and Fe films across MgO spacer layer in CoO(NiO)/MgO/Fe/Ag(001) using MOKE and XMLD

CoO(NiO)/MgO/Fe/Ag(001) films were grown epitaxially and studied by Magneto Optical Kerr Effect (MOKE) and X-ray Magnetic Linear Dichroism (XMLD). The enhancement of the Fe layer coercivity due to its coupling to the AFM (NiO) overlayer was studied as a function of the MgO spacer layer thickness using MOKE. We found that the Fe coercivity enhancement persists to $\sim$2 ML MgO thickness, after which the AFM-FM interlayer coupling becomes too weak to affect the Fe ceorcivity. Below 2ML MgO thickness, the Fe coercivity enhancement depends both on the MgO and NiO thicknesses. To further understand the effect of interlayer coupling, the rotatable CoO spins in CoO/MgO/Fe/Ag(001) after field cooling along the Fe(100) axis were determined using the XMLD measurement as a function of the MgO thickness. We found that 2 ML MgO thickness sets a critical value beyond which the CoO/MgO/Fe interlayer coupling no longer rotates the CoO spins.   

Modification of thickness dependent magnetic properties of perpendicular anisotropy Co/Pd multilayer upon hydrogenation

We have studied the change in saturation magnetization ($M_{S})$ and effective perpendicular anisotropy ($K_{eff}$ ) upon hydrogenation at room temperature and a pressure of one atmosphere in (Co/Pd)$_{25}$ multilayers, with Co thickness \textit{$\le $ }5 {\AA} and Pd thickness ranging from 0 {\AA} to 25 {\AA}. The change in $M_{S}$ and $K_{eff}$ was studied as a function of the x-ray scattering length density profile, generated from the x-ray reflectivity fits. The results show that when the Pd thickness \textit{$\le $ }10 {\AA}, the films were highly interdiffused, resulting in no measurable change in $M_{S}$ and $K_{eff}$ . As the thickness of Pd increases, the contrast between the Co and Pd layers increases, leading to a decrease in $M_{S}$ and an increase in the component of magnetization in the plane of the samples and hence causing $K_{eff}$ to decrease. The results clearly demonstrate that the solubility of hydrogen in the multilayer samples decreases with increasing alloying effects as it decreases the vacancy in the Pd 4$d $band leading to no electronic transfer from the hydrogen atoms to the Pd.   

Momentum transfer resolved memory in a magnetic system with perpendicular anisotropy

We have used resonant, coherent soft x-ray scattering to measure wave vector resolved magnetic domain memory in Co/Pd multilayers. The technique uses angular cross correlation functions and can be applied to any system with circular annuli of constant values of scattering wave vector {\bf q}. In our Co/Pd film, the memory exhibits a maximum at {\bf q}=0.0384 nm$^{\rm -1}$ near initial reversal that decreases in magnitude as the magnetization is further reversed. The peak is attributed to bubble domains that nucleate reproducibly near initial reversal and which grow into a labyrinth domain structure that is not reproduced from one magnetization cycle to the next.   

Measure of magnetization anisotropy by AMR in electrodes of Co/Pd multilayers

We studied the anisotropic magnetoresistance (AMR) properties of multilayered Co/Pd thin film electrodes as function of magnetic field. The perpendicular magnetization anisotropy (PMA) in these films is found to modify the AMR. The magnetoresistance (MR) for fields out-of-plane ($\rho_{op}$) is considerably different than for in-plane fields transverse to current direction ($\rho_{ip}$), although in both cases current is perpendicular to the magnetic field. Moreover, opposed to other thin films, where $\rho_{op}$ is smaller than $\rho_{ip}$, our films show an opposite effect, the origin of which is not clear. We can understand the AMR properties of the electrodes by an expanded Stoner-Wolfarth model, where we introduce an additional energy scale related to the PMA. Through a numerical refinement process, we can extract anisotropic energy constants of the films. This is done by reconstructing the MR behavior of the electrodes, using the linear terms in the dependence of resistivity on magnetization orientation. Our anisotropic constants coincide remarkably with other bulk measurements. Thus, our refinement process is an excellent method to extract anisotropic constants also in nano-scale systems, which cannot be measured otherwise.