Spin glass transition in a thin-film NiO/permalloy bilayer

Abstract

We experimentally study magnetization aging in a thin-film NiO/Permalloy bilayer. Aging characteristics are nearly independent of temperature below the exchange bias blocking temperature TB, but rapidly vary above it. The dependence on the magnetic history qualitatively changes across TB. The observed behaviors are consistent with the spin glass transition at TB, with significant implications for magnetism and magnetoelectronic phenomena in antiferromagnet/ferromagnet bilayers.The properties of thin-film antiferromagnets (AFs) interfaced with ferromagnets (Fs) first became the subject of intense research in the context of exchange bias (EB) -unidirectional anisotropy acquired by F when the system is cooled through a certain blocking temperature T B [1,2]. They have recently attracted a renewed attention thanks to the high dynamical magnetization frequencies of AFs, enabling applications in fast interconversion and transmission of spin signals [3], and THz optics [4]. Additionally, vanishing magnetization of AFs can enable enhanced spin-transfer efficiency in electronic manipulation of the magnetic states for ultrahigh-density information storage, avoiding the constraints imposed by the angular momentum conservation and the dipolar fields ubiquitous to ferromagnetic systems [5].A number of novel phenomena have been recently observed or predicted for thin AF films, including antiferromagnetic spin-orbit torques [6][7][8], AF magnetoresistance [9], enhanced interconversion between electron spin current and spin waves [3,10], generation of THz signals [4,11], AF exchange springs [12,13], and topological effects [14][15][16]. While some of these phenomena are expected even for standalone AFs, strong exchange coupling at AF/F interfaces provides one of the most efficient approaches to controlling and analyzing the magnetization states of AFs, with the state of F controlled by the magnetic field or spin current, and characterized by the magnetoelectronic or optical techniques. However, despite intense ongoing research, little is known about the dynamical and even static magnetization states of thin AF films in F/AF bilayers.Here, we present measurements of magnetization aging, observed in a thin NiO film after magnetization reversal of an adjacent ferromagnet. Our main result is the observation of an abrupt transition between two qualitatively different aging regimes, which we identify as a glass transition in AF frustrated by the random exchange interaction at the F/AF interface. The insight provided by our findings may enable the implementation of new functionalities in magnetic nanodevices, facilitated by the controlled transition among the multidomain, spinliquid, and spin-glass states of thin-film magnetic systems.The AF in our study was a polycrystalline 15 nmthick NiO layer deposited by reactive sputtering on an oxidized 6 × 6 mm 2 Si substrate. A 10-nm-thick Ni 80 Fe 20 =Permalloy (Py) ferromagnet and a 3 nmthick capping SiO 2 layer were deposited on top. NiO has played a prominent role in recent studies of mag...

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

“…For Arrhenius activation, such a rapid variation of viscosity places very stringent constraints on the distribution of the activation barriers and the possible values of the activation attempt rates. For the studied thin-film Py/CoO bilayers, previously shown to exhibit non-Arrhenius cooperative magnetization dynamics [14], our observation of such a rapid viscosity variation is consistent with the magnetic freezing expected to occur at the putative magnetic glass transition in the Heisenberg domain state [15].…”

“…In a completely different physical realization -the spin glasses -a peak is observed in the imaginary component of susceptibility near the glass transition temperature [17]. Such a peak is therefore also expected to arise at the putative magnetic glass transition in the Heisenberg domain state, which was inferred from the previous time-domain measurements of magnetic aging in similar F/AF bilayers [14,15].…”

“…A similar magnetolectronic approach was utilized by us in measurements of magnetic aging in similar systems, and described in detail in Refs. [13][14][15]. Because of AMR, the resistance R of Py exhibits a 180 • -periodic dependence on the angle φ between the magnetization and the direction of current, characterized by the maximum value R max = 3.425 Ω in the studied sample at φ = 0, 180 • , T = 80 K, and the minimum value R min = 3.17 Ω at φ = ±90 • .…”

“…by the temperature dependence of aging in thin films of AF=NiO [15]. Aging was slow and independent of temperature T below the exchange bias blocking temperature T B , but abruptly accelerated above T B .…”

“…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].…”