Magnetic freezing transition in a CoO/Permalloy bilayer revealed by transverse ac susceptibility

Abstract

We utilize variable-temperature, variable-frequency magneto-optical transverse magnetic susceptibility technique to study the static and dynamical magnetic properties of a thin-film CoO/Permalloy bilayer. Our measurements demonstrate that in the studied system, the directional asymmetry of the hysteresis loop is associated mainly with the difference in the reversal mechanisms between the two reversed states of magnetization stabilized by the exchange-induced uniaxial anisotropy. The latter is found to be much larger than the exchange-induced unidirectional anisotropy of the ferromagnet. We also observe an abrupt variation of the frequency-dependent imaginary part of ac susceptibility near the exchange bias blocking temperature, consistent with the magnetic freezing transition inferred from the previous time-domain studies of magnetic aging in similar systems. The developed measurement approach enables precise characterization of the dynamical and static characteristics of thin-film magnetic heterostructures that can find applications in reconfigurable magnonic and neuromorphic circuits.Recent measurements of the time-dependence of the magnetization state in thin-film F/AF bilayers revealed slow power-law magnetic aging at low temperatures, which appears to be universal for such systems [13][14][15]. The dependence of aging on the magnetic history was inconsistent with the Arrhenius-type activation, instead indicating cooperative behaviors akin to the avalanche dynamics in glassy systems [16]. Since aging was observed only for thin films, this glassy dynamics was tentatively attributed to the emergence of HDS. The relationship between HDS and glasses was further elucidated arXiv:1807.05990v2 [cond-mat.mtrl-sci]

Experimental demonstration and analysis of random field effects in ferromagnet/antiferromagnet bilayers

“…Qualitatively, we attribute the surprising difference between the characteristics for the two opposite field directions to the exchange-spring effects in CoO, which may produce quasi-uniform exchange torque over length scales comparable to the grain size of polycrystalline CoO. We also note that our surprising observations are consistent with a recent observation, for similar Permalloy/CoO bilayers, of qualitatively different reversal mechanisms between the two opposite directions of Py magnetization 31 . Specifically, transverse ac susceptibility measurements showed that magnetization reversal from the magnetization direction opposite to the field-cooling direction into the direction aligned with the latter, occurs as soon as its energy becomes higher.…”

“…by reactive sputtering in ultrapure oxygen atmosphere, with the partial pressure of oxygen optimized as in our previous studies of CoO-based systems 29,31,38 .…”

Exchange bias without directional anisotropy in Permalloy/CoO bilayers

“…Ta(2) served as a capping layer protecting the films from oxidation. The thickness of 8 nm for CoO was above the transition to the Heisenberg domain state (HDS) at t ≈ 6 nm, minimizing the glassy magnetic dynamics of the AF magnetic order driven by the Py magnetization [18,33]. Nevertheless, our data provide evidence of spin flop reversal in AF, which is expected even for large AF thickness [35].…”

“…The multilayers were deposited in a 150 Oe in-plane magnetic field, which is known to facilitate magnetic ordering in CoO. Py and Ta were deposited by dc sputtering from the stoichiometric targets in 1.8 mTorr of ultrapure Ar, while CoO was deposited from a Co target by reactive sputtering in a mixture of ultrapure oxygen and Ar, with the partial pressure of oxygen optimized as in our previous studies of CoO-based systems [15,18,33,36]. Measurements below the Nèel temperature T N = 290 K of CoO, were performed after cooling from room temperature (RT) in the presence of an external magnetic field H dc = 0.5 kOe.…”

“…For small AF thicknesses, Malozemoff predicted a crossover to the "Heisenberg domain state" (HDS), in which the AF domain sizes become smaller than the domain wall (DW) widths, so the Nèel order becomes twisted everywhere [32]. Such a crossover was recently experimentally observed [18], and HDS was identified as a correlated spin glass state -a state that exhibits shortrange AF spin correlations but lacks long-range ordering -formed below a certain material-dependent thickness of AF [17,33].…”