"Temperature-dependent magnetic depth profiles of epitaxial films with graded and oscillatory exchange coupling structure"

Who: Lorenzo Fallarino, Nanomagnetism Group

Place: nanoGUNE seminar room, Tolosa Hiribidea 76, Donostia - San Sebastian

Date: Monday, 5 September 2016, 11:00

Precise tailoring of magnetic interfaces allows for the control of exchange coupling strength and effective Curie temperature (TC) over tens of nm [1]. In this regard, Cobalt-Chromium alloys, Co1-xCrx, constitute an interesting model system having magnetic properties that are easily tunable by changing the dopant concentration x. We have grown a series of graded Co/Co1-xCrx/Co (10-10) epitaxial thin films on top of a Si(110)/Ag(110)/Cr(211) template structure, with x having an overall U-shaped concentration profile. The structural XRD analysis verifies the epitaxial nature and crystallographic quality for the entire set of samples, with in-plane c-axis orientation. Temperature and field-dependent magnetic depth profiles have been measured using polarized neutron reflectometry (PNR). The effective TC is found to vary as a function of depth, exhibiting a minimum in the center of the structure, verifying the graded modulation. The coupling in between the two outer Co layers results dependent on the magnetization of the central Co1-xCrx spacer, and thus continuously tunable via temperature T. For xc = 0.28, magnetometry measurements reveal square hysteresis loops below 260 K, which change to a two-steps loop behavior for higher temperatures, suggesting an abrupt transition from coupled to uncoupled behavior. We have therefore studied the temperature dependence of the magnetization reversal process via PNR for this sample, measurements that confirm the magnetic decoupling of top and bottom sample parts at elevated temperatures. In addition to this U-shaped concentration profile, we have explored an ?oscillatory effective Curie temperature? structure of different wavelength l produced by a triangular Cr content profile, oscillating in between x = 0.22 and x = 0.28. PNR data prove the achievement of the desired oscillatory pattern and by changing l, we have explored the viability of compositional effects being transferred into modulated magnetic states.

[1] B.?J. Kirby, H.?F. Belliveau, D.?D. Belyea, P.?A. Kienzle, A.?J. Grutter, P. Riego, A. Berger, and Casey W. Miller, Phys. Rev. Lett. 116, 047203 (2016).

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