Magnetic proximity effect in Pt/CoFe2O4 bilayers

Abstract

We observe the magnetic proximity effect (MPE) in Pt/CoFe 2 O 4 bilayers grown by molecular beam epitaxy (MBE). This is revealed through angle-dependent magnetoresistance measurements at 5 K, which isolate the contributions of induced ferromagnetism (i.e. anisotropic magnetoresistance) and spin Hall effect (i.e. spin Hall magnetoresistance) in the Pt layer. The observation of induced ferromagnetism in Pt via AMR is further supported by density functional theory calculations and various control measurements including insertion of a Cu spacer layer to suppress the induced ferromagnetism. In addition, anomalousHall effect measurements show an out-of-plane magnetic hysteresis loop of the induced ferromagnetic phase with larger coercivity and larger remanence than the bulk CoFe 2 O 4 . By demonstrating MPE in Pt/CoFe 2 O 4 , these results establish the spinel ferrite family as a promising material for MPE and spin manipulation via proximity exchange fields.* equal contributions **email: kawakami.15@osu.edu 2 Spin manipulation inside a nonmagnetic (NM) material using internal effective fields (spin orbit or exchange) is a very promising avenue toward the realization of next generation spintronic devices (spin transistors, magnetic gates, etc.) [1,2]. In particular, magnetic proximity effect (MPE) at the interface of a NM spin channel and a ferromagnetic insulator (FMI) is of great importance for generating exchange fields and induced ferromagnetism in the NM layer. Recently, spin manipulation by MPE has been realized in experiments that modulate spin currents in graphene on YIG (yttrium iron garnet) [3,4]. In addition, proximity exchange fields induced by FMI have been observed for graphene and monolayer transition transition metal dichalcogenides [5,6].Among FMIs, members of the spinel ferrite family (MFe 2 O 4 , with M=Co, Ni, etc.) are attractive because their magnetic properties can be tuned by alloy composition [7,8] as well as epitaxial strain [9][10][11]. In particular, CoFe 2 O 4 (CFO) is a hard ferrimagnetic insulator which exhibits high Curie temperature (728 K), spin-filtering properties [12][13][14], magneto-electric switching [15], and is readily integrated with practical spintronic materials (MgO, Fe, Cr). Unfortunately, all previous experiments have failed to observe MPE using CFO. To test for MPE in NM/FMI systems, Pt is widely used as the NM material due its closeness to fulfilling the Stoner criteria and thus allowing it to become ferromagnetically ordered at the interface with the FMI. Initial studies of Pt/CFO grown by pulsed laser deposition (PLD) utilized magnetotransport measurements and observed no MPE in the Pt layer [16]. Subsequent transport and element-specific magnetization measurements by x-ray magnetic circular dichroism (XMCD) also found no evidence for induced ferromagnetism in the Pt layer [17][18][19][20] contributing to a growing consensus that CFO cannot be used to obtain MPE. However, because the length scale of exchange interactions is on the order of angstroms, it should...

Investigating magnetic proximity effects at ferrite/Pt interfaces

“…Similar conclusion without any XMCD signal is obtained at room temperature, which allows to us rule out the presence of an interfacial compound with a magnetic transition at low temperature. The absence of magnetic proximity effects in our ferrite/Pt samples is in agreement with XMCD or X-ray resonant magnetic reflectivity measurements performed on the 29 In Fig. 2(c), we plot XANES and XMCD spectra at Pt L 2,3 edges for the Co/Pt reference sample.…”

“…This suggests that the absence of magnetic proximity effects at ferrite/Pt interfaces is not linked to the insulating character of the ferrites. We hope that our systematic investigation will trigger theoretical studies allowing to settle if the absence of induced magnetic moments at ferrite/Pt interfaces is a general rule and identify the possible role of defects and disorder in establishing magnetic moments or not since anisotropic magnetoresistance effects have been recently observed in the CoFe 2 O 4 /Pt bilayer, 29 suggesting the magnetic proximity effect.…”

Exploring interfacial exchange coupling and sublattice effect in heavy metal/ferrimagnetic insulator heterostructures using Hall measurements, x-ray magnetic circular dichroism, and neutron reflectometry

“…With the information from the AHE, we can extract the exchange coupling configuration in arbitrary HM/magnetic insulator (MI) bilayers. For instance, Zhou et al[6] and Amamou et al[29] observed the AHE signs due to MPE are negative and positive for the Pd/YIG and Pt/CoFe2O4 (CoFe2O4 is a MI), respectively, so that we can predict parallel exchange coupling for both Pd/YIG and Pt/CoFe2O4 by usingTable I. We also summarize results of the exchange coupling configurations in HM/magnet bilayers inTable II[6,26,27,[29][30][31],…”

Cyclotron motion without magnetic field

“…In summary, we demonstrate how fast pseudo-spin dynamics can be tracked through a pulsed non-adiabatic excitation, manifesting in ACM without magnetic field. ACM can be found in a variety of anomalous Hall materials for example intrinsic anomalous Hall systems in magnetic materials [25], ferromagnetic insulators: Cr-Ge-Te alloy films [26], CoFe 2 O 4 [27], EuO [28]; dilute magnetic semiconductors: (Ga, Mn)As [29]; magnetically doped topological insulators: Cr doped (Bi, Sb) 2 Te 3 [30], gapped 2D Dirac materials (graphene, silicene, germanene, and transition metal dichalcogenides) on top of magnetic substrates [21][22][23], and broken TRS 3D Dirac and Weyl semimetals. ACM displays a number of unusual characteristics including an intrinsic power-law decay, and real-space charge displacements that follow a spiral-like trajectory.…”