Session Y22: Focus Session: Coupled Thin-Film Structures for Magnetic Recording

Interface magnetocrystalline anisotropy induced by charge transfer on Pd/Ni/Cu films

We investigated microscopic mechanisms of interface magnetocrystalline anisotropy in epitaxial Pd/Ni/Cu films using x-ray magnetic circular dichroism and x-ray absorption spectroscopy at Ni $L_{\rm 2;3}$ edge and Pd $M_{\rm 2;3}$ edge. We found that a considerably charge transfer (from 4$d$ to 3$d$) of Pd and Ni dominantly occurs near the interface of Pd/Ni. The interfacial charge transfer result is enhanced the orbital magnetic moment, and leads to the interface magnetocrystalline anisotropy induced by about 19.1$\%$.   

Magnetic properties of L1$_{0}$ FePtCu:Au films for high-density perpendicular recording

We present fabrication methods for tunable magnetic properties of non-epitaxially grown, nanocomposite L1$_{0}$ FePtCu:Au films including a coupled FePt continuous gradient layer (CGL) for the application of high-density perpendicular recording media. The films are deposited with the structure FePt(x)/[FePtCuAu] on a thermally oxidized Si wafer and subsequently annealed. [FePtCuAu] is a nanocomposite layer (NL) with a fixed layer thickness, 12nm, and FePt(x) is the continuous gradient layer, with the thickness x varying from 0 to 10 nm, which is coupled to the NL. X-ray diffraction patterns show an enhanced degree of orientation for FePtCu:Au films with an increase of the CGL thickness. SQUID measurements show that magnetic properties of the films are tunable by changing the CGL layer thickness. When the CGL thickness is 10 nm, the magnetic properties of the film are M$_{s} \quad \cong $ 600 emu/cc, slope $\alpha $ (= 4$\pi d$M/$d$H) at the coercivity $\cong $ 1 and H$_{c} \quad \cong $ 6 kOe. These results are discussed in terms of exchange-coupled layers with differing properties, and demonstrate the possibility of fabricating FePt-based nanocomposite media with properties suitable for high-density perpendicular recording.   

Parallel and anti-parallel coupling of CoPt/Ru/CoFe trilayers investigated with magnetometry and ferromagnetic resonance

Due to their low resistivity ($\sim $30 $\mu \Omega $-cm) and critical thickness CoPt$_{x}$ (16 $\le $ x $\le $ 24 at {\%}) thin films are interesting pinning materials for current perpendicular to the plane (CPP) spin-valve sensors [1]. These properties help to minimize serial resistance and thus enhance the magneto-resistance. 4 nm thick CoPt films deposited on Cr were shown to exhibit coercivities up to 1.5 kOe. One requirement for using CoPt as a pinned layer in a CPP spin-valve is that anti-parallel (AP) coupling to a reference layer can be demonstrated to minimize magnetostatic coupling to the free layer to keep the free layer magnetically soft. Here we investigate the parallel (P) and AP coupling of CoPt$_{18}$(50)/Ru(x)/CoFe$_{16}$(36) trilayers with 0$\le $ x $\le $2.1 nm with magnetometry and ferromagnetic resonance (FMR). We found the coupling to be oscillatory with peaks at x = 0.7 nm (AP) 1.4 nm (P) and 2.0 nm (AP), respectively. These peaks were also observed using 35 GHz FMR in saturation. From the FMR technique the coupling strengths were determined to be 1150 (AP), 250 (P) and 660 (AP) Oe respectively. Variable temperature magnetometry reveals the exchange, coercivity and saturation fields increase monotonically with decreasing temperature. Supported at Miami by U.S. Dept. of Energy [1] S. Maat et al. J. Appl. Phys. Dec. 1 issue (2005)   

X-ray reflectivity of ruthenium nano-oxide layer in a CoFe-Ru-CoFe trilayer system

A grazing incidence X-ray reflectivity technique is used to determine electron density profile(EDP) as a function of depth in CoFe-Ru-CoFe and CoFe-Ru nano oxide layer(NOL)-CoFe trilayers. Four trilayers with ruthenium thicknesses of 8,8.5 and 9 \AA \hspace{.08cm} and one with Ru8.5\hspace{.05cm}\AA\hspace{.05cm}NOL, prepared by a dc planetary sputtering system, were investigated. For all samples, EDP shows a central peak which is related to the Ru layer. Natural oxidation in all samples introduces a graded EDP of the top CoFe layer that decreases gradually to zero. The large surface resistivity of Ru8.5 \AA\hspace{.05cm} NOL compared to Ru 8.5\AA\hspace{.08cm} can be related to the remarkable difference between their EDP.   

Systematic Investigation of Exchange Coupled Fe$_{x}$Co$_{1-x}$/CoPt Magnetic Bilayers Using the High-Throughput Approach

Exchange-coupled magnetic nanocomposites are being pursued for future permanent magnets with high energy products. To gain better understanding of the exchange coupling behavior between the soft and hard magnetic layers, we are using the high-throughput approach. Fe$_{x}$Co$_{1-x}$/CoPt magnetic bilayers are grown on MgO(110) substrates using the combinatorial electron-beam deposition. CoPt hard magnetic layer is epitaxially grown at 600$^{\circ}$ C. The Fe$_{x}$Co$_{1-x}$ soft layer is deposited at 200$^{\circ}$ C. To study the effect of changing soft phase parameters on exchange coupling, we have fabricated libraries where in one direction the composition of the soft-phase is continuously changing from Fe to Co and the thickness of the soft layer is changing in the other direction. The magnetic hysteresis loop for each soft layer composition and the thickness is measured using a magneto-optical Kerr effect system. From systematically changing hysteresis loops, we calculate the exchange field (H$_{ex})$ as a function of anisotropy and magnetization of the soft phase. Observed dependence of H$_{ex}$ on soft phase parameters are directly compared against theoretical models of the exchange mechanism. ONR N00014-05-1-0497, NSF DMR -0520471.   

Molecular Beam Epitaxial Growth and Magnetic properties of Fe(001)/Mn$_{3}$N$_{2}$(010) Thin Films

Exchange bias systems have attracted considerable attention due to their importance to magnetic sensor technology. Considering that T$_{Neel}$ of Mn$_{3}$N$_{2}$ (T$_{N}$=652$ ^\circ$C) is less than T$_{Curie}$ of Fe (T$_{C}$=770 $^\circ$C), and also since Mn-Mn spacing of Mn$_{3}$N$_{2}$(010) ($\simeq$2.86 angstrom) is closely matched to the Fe-Fe spacing of Fe(001)(2.87 angstrom), it is therefore of great interest to explore Fe epitaxy on Mn3N2(010). We have grown thin Fe films on Mn$_{3}$N$_{2}$(010)/MgO(001) using molecular beam epitaxy at 150$ ^\circ$C, and then following by annealing at 450$ ^\circ$C for 10 minutes. The growth is monitored by reflection high-energy electron diffraction, which shows c(2x2) reconstructed surface for the as-grown sample, and a change to (1x1) after annealing. Annealing leads to a smoothening of the film surface.The epitaxial orientation have been determined to be Fe [100] (001)// Mn3N2[101] (010)//MgO[110](001). Annealed samples are transferred to the in situ analysis chamber for scanning tunneling microscopy studies. Images show smooth terraces and atomic-height steps. Vibrating sample magnetometry measurements found in-plane anisotropy and hysteresis loop shifting after field cooling. The work is supported by NSF9983816 and 0304314.   

Novel materials and media concepts for thermally assisted magnetic recording.

Magnetic media using materials with high uniaxial magneto-crystalline anisotropy, K$_{U}$, combined with a thermal assist to overcome thermal stability and write field limitations are widely seen as a potential extension of current magnetic recording technology. Here we present an overview of recent work on adapting the thermo-magnetic properties of FePt-based high-K$_{U}$ materials for the requirements of such a recording system. In [1] we recently proposed a novel media structure consisting of two exchange coupled films, a high anisotropy film like, e.g., FePt, and a FeRh film. At close to equiatomic compositions FeRh is an antiferromagnet at low temperatures. Interestingly, upon heating beyond a critical temperature, T$_{AF-FM}$, FeRh becomes ferromagnetic for temperatures T$_{AF-FM}<$T$<$T$_{C}$. This opens interesting possibilities for media applications for thermally assisted recording: at a storage temperature, T$_{S}<$T$_{AF-FM}$, the magnetic information is stored in the high-K$_{U}$ FePt layer. For writing at increased temperature, T$_{AF-FM}<$T$_{W}<$T$_{C-FeRh}$, the FeRh becomes ferromagnetic, effectively lowering K$_{U}$ and increasing the total magnetic moment of the bilayer, thus lowering its coercivity via an exchange spring mechanism and helping magnetization reversal at temperatures well below T$_{C}$ of the FePt layer. A related area of great interest is the magnetization dynamics upon rapid heating and cooling of FeRh films using \textit{fs}-laser pump-probe techniques. First results indicate that the AF-FM transition can be driven on a timescale below 1 \textit{ps} [2], yielding interesting insight into the interaction of the spin, electron and lattice subsystems. \newline \newline [1] J.-U. Thiele, S. Maat, E. E. Fullerton, Appl. Phys. Lett. \textbf{82} (2003) p2859-2861 \newline [2] J.-U. Thiele, M. Buess, C. H. Back, Appl. Phys. Lett. \textbf{85} (2004) p2857-2859 and G. Ju \textit{et al.}, Phys. Rev. Lett. \textbf{93} (2004) 197403.   

Uncompensated Fe moments in positively exchange biased Ni/FeF$_{2}$ bilayers

The magnetization reversal of uncompensated Fe moments in exchange biased Ni/FeF$_{2}$ bilayers was determined using soft x-ray magnetic circular dichroism. Hysteresis loops resulting from Fe moments oriented parallel and perpendicular to the external field are almost identical to those of the ferromagnetic Ni layer. However, a small vertical loop shift indicates that some Fe moments are pinned antiparallel to small cooling fields leading to negative exchange bias but parallel to large cooling fields resulting in positive exchange bias. For intermediate cooling fields, loop bifurcation is observed. This is attributed to a coexistence of domains with uncompensated Fe moments pinned parallel and antiparallel to the cooling field. Furthermore, x-ray magnetic linear dichroism was employed to characterize the spin structure of the antiferromagnetically ordered FeF$_{2}$ but no indication for the formation of a parallel antiferromagnetic domain wall upon magnetization reversal in the ferromagnetic layer was found. The ALS at LBNL and work at UCSD are supported by DOE and work at UCD by ACS-PRF and Sloan Foundation.   

On the method of exchange bias measurement

Exchange bias is the observation of a hysteresis loop shift in ferromagnetic-antiferromagnetic compound structures [1]. Recently, such a shift was also observed in all ferromagnetic bilayer systems [2]. Conventionally, exchange bias fields are measured by determining the average coercive field of a hysteresis loop (two-point method). However, hysteresis loops in exchange biased systems can also change in shape and even exhibit asymmetrical reversal [3], which can affect the estimate of the exchange bias field significantly. A different method for measuring the exchange bias based on obtaining the ``center of mass'' for exchange biased hysteresis loops is proposed and analyzed in this work. The method is validated by numerical simulation, using a realistic model system composed of soft and hard ferromagnetic layers coupled antiferromagnetically. Our calculations allow us to determine the laterally averaged bias field $<$Hbias$>$ created by the hard layer grains. We find that $<$Hbias$>$ is in excellent quantitative agreement with the proposed ``center of mass'' measurement, but not with the conventional two-point method. [1] A. Berkowitz, K. Takano, J. Magn. Magn. Mater. 200,552 (1999) [2] A. Berger et. al., Appl. Phys. Lett. 85, 1571 (2004) [3] S. G. E. te Velthuis et. al., J. Appl. Phys. 87, 5046 (2001)   

Magnetic anisotropy in CrO$_{2}$ and CrO$_{2}$/Cr$_{2}$O$_{3}$ bilayer thin films

We have investigated the effective magnetic anisotropy in CVD-grown epitaxial CrO$_{2}$ films and Cr$_{2}$O$_{3}$/CrO$_{2}$ bilayers using a resonant RF transverse susceptibility (TS) technique. These materials are of interest due to their high spin polarization and the potential for exhibiting interesting magnetoelectric (ME) effects. In CrO$_{2}$, the measured values for the room temperature anisotropy constant for the films scaled with the film thickness and the data is influenced by magnetoelastic contributions at low temperature. TS in CrO$_{2}$/Cr$_{2}$O$_{3}$ bilayers revealed features associated with both the ferromagnetic and antiferromagnetic phases. In addition, a considerable broadening of the anisotropy fields and large K$_{eff}$ values were observed depending on the amount of Cr$_{2}$O$_{3}$. These anomalously large K$_{eff}$ values observed with Cr$_{2}$O$_{3}$ content cannot be accounted for by the variable thickness of CrO$_{2 }$alone and is indicative of intimate coupling between the two phases that has a significant effect on the magnetic properties.