Actual Research Topics

Thin Film Multilayer Systems with interfacial Dzyaloshinskii-Moriya Interaction

The Dzyaloshinskii-Moriya interaction (DMI) induced by the interfaces of thin-film structures has been shown to support the existence of skyrmions at room temperature [1]. The average Dzyaloshinskii-Moriya (DM) interaction D, and the exchange stiffness A can be extracted from MFM data of the domain structure obtained after different demagnetization procedures. We have recently shown that even the sign of D can be measured because the magnetic field distribution above (and below) a thin film with a chiral magnetization structure depends on the sense of rotation of the magnetic moments inside the Néel walls [2]. Local values of the DMI can be obtained from fitting model skyrmion magnetization structures to MFM data. We find that the local values of D are substantially larger than the average value, indicating that in our system, the skyrmions are strongly pinned [3]. Apart from the domains and skyrmions MFM can also detect small field variations arising from a local variation of the areal magnetic moment density that can be attributed to a corresponding variation of the Co layer thickness that lead to skyrmion pinning.

 

References:

[1] C. Moreau-Luchaire, et al., Nature Nanotech. 11, 444 (2016)

[2] M.A. Marioni & H.J. Hug et al., Nano Lett. 18, 2263 (2018)

[3] M. Bacani & H.J. Hug et al., arXiv:1609.01615 [cond-mat]

 

Beating the recording quadrilemma using Curie temperature modulated structures

Heat-assisted recording (HAMR) combined with bit-patterned media (BPM) is one of the candidate technologies to overcome present limits in magnetic recording and to possibly extend magnetic recording to storage densities of several tens of Tb/in2. BMP are required to reduce the transition jitter noise that would be present in granular recording media, where the bit transitions are invariably irregular given that each bit requires about 30 irregular magnetic grains of the recording media. To ensure stability of the magnetic information over time, high anisotropy is engineered, which gives rise to a large coercivity. In turn heat assistance is necessary to raise the temperature during writing thereby reducing the medium coercivity to levels that can be written. However, elevated temperatures also lower the magnetization which substantially increases thermally induced recording errors. One of the co-applicants (D. Suess) has proposed a composite media structure, consisting of two exchange-coupled layers with different Curie temperatures, to overcome the above limitations.

 

L10-ordered FePt is one of the few material systems with ultra-high magnetic anisotropy providing sufficient thermal stability. However the preparation of materials of this class requires either high-temperature epitaxial growth or annealing at elevated temperatures to obtain the L10 phase. Moreover, the Curie temperature of about 750 K requires challenging heat management strategies for both the recording media as well as for the write-head.

 

The goals of this proposal are to develop a new exchange-coupled double layer prototype system suitable for HAMR/BPM and to demonstrate recording at densities beyond current limits with a viable extrapolation to several tens of Tb/in2. To achieve these goals, we will fabricate an optimized [Co/Ni]N/TbxFe1-x-yCoy bilayer system. The [Co/Ni]N-multilayer serves as a high Curie temperature, low-anisotropy write layer which also generates sufficient stray field for the readout process. The amorphous ferrimagnetic TbxFe1-x-yCoy layer serves as a high anisotropy storage layer. With the Co content the Curie temperature of the TbxFe1-x-yCoy layer can tuned within the interval from 400 K to 600 K and is hence considerably smaller than that of L10-ordered FePt. This allows lower writing temperatures that reduce writing error rates, increases the lifetime of near field transducers of the write heads, and generally simplifies heat management issues. Further, the damping parameter in amorphous TbFeCo films depends on the Tb content reaching values of to 0.5 significantly larger than those obtained in the FePt system. According to our preliminary work, a large damping parameter is essential for a reliable magnetization switching process. Due to large damping in the Tc modulated structure containing TbFeCo thermally written in errors in BPM is expected to decrease from about 5% to close to zero. Furthermore, the amorphous structure of the TbFeCo storage layer will be advantageous for a narrow switching field distribution in BPM.

 

In summary, the project addresses a novel exchange-coupled ferro-/ferrimagnetic composite Curie temperature modulated bilayer system for HAMR/BPM. Unique experimental methods are available in the two experimental groups with a thorough background in magnetic thin film research. The experimental work will be supported by a theory group having a long-standing experience in the field of magnetic recording systems.

Exchange Bias

An obstacle to understanding the exchange bias (EB) effect is that only a subset of the pinned uncompensated spins (pinUCS),  those pinned and coupled to the ferromagnet (F) are responsible for the EB effect. Experimental methods that measure the pinUCS density distribution with spatial resolution comparable to the materials grain size are needed. Here we use quantitative, high-resolution magnetic force microscopy (MFM) to measure the local areal density of pinned uncompensated spins (pinUCS) and to correlate the F-domain structure in a perpendicular anisotropy Co/Pt multilayer with the pinUCS density in the CoO antiferromagnet [1]. Larger applied fields drive the receding domains to areas of proportionally higher pinUCS aligned antiparallel to F-moments. This confirms our prior results [2] that these antiparallel pinUCS are responsible for the EB effect, while parallel pinUCS coexist. The experimentally observed domain evolution with field could be matched well with a 2D phase-field model that incorporates the 10 nm-resolved measured local biasing characteristics of the antiferromagnet [3]. Frustration limits the exchange bias field in typical ferromagnet/antiferromagnet EB systems. Frustration can be avoided if the antiferromagnet is replaced by a rare-earth ferromagnetic layer and exchange bias fields larger than 1T were observed [4]. Because of the strong coupling between the ferromagnetic and ferrimagnetic thin films, a different magnetization process of the ferromagnetic film is observed [5]: instead of domain nucleation followed by a lateral motion of the domain walls over the landscape of pinUCS, an increasing number of isolated CoPt grains reverse their magnetization and a high-energy in-plane domain wall is formed at the ferromagnet/rare earth ferrimagnet interface.  

 

References:

[1] I. Schmid et al. PRL, 105 (2010) 197201

[2] I. Schmid et al. EPL, 81 (2008) 17001

[3] A. Benassi et al. Sientific Reports 4 (2014) 4508

[4] S. Romer et al. Appl. Phys. Lett. 101 (2012) 222404

[5] X. Zhao et  al. in preparation (2018)