Magnetic proximity effect in $\mathrm{Pt}/{\mathrm{CoFe}}_2{\mathrm{O}}_4$ bilayers

We observe the magnetic proximity effect (MPE) in $\mathrm{Pt}/{\mathrm{CoFe}}_2{\mathrm{O}}_4$ bilayers grown by molecular beam epitaxy. This is revealed through angle-dependent magnetoresistance measurements at 5 K, which isolate the contributions of induced ferromagnetism (i.e., anisotropic magnetoresistance) and the spin Hall effect (i.e., the spin Hall magnetoresistance) in the Pt layer. The strong evidence for induced ferromagnetism in Pt via the anisotropic magnetoresistance is supported further by density functional theory calculations and various control measurements including the insertion of a Cu spacer layer to suppress the induced ferromagnetism. In addition, anomalous Hall effect measurements show an out-of-plane magnetic hysteresis loop of the induced ferromagnetic phase with larger coercivity and larger remanence than the bulk ${\mathrm{CoFe}}_2{\mathrm{O}}_4$. By demonstrating the MPE in $\mathrm{Pt}/{\mathrm{CoFe}}_2{\mathrm{O}}_4$, these results establish the spinel ferrite family as a promising material for the MPE and spin manipulation via proximity exchange fields.

Figure 1

(a) RHEED image of a 40-nm CFO film grown on a MgO(001) substrate, taken along the [110] in-plane direction. (b) Representative atomic force microscopy image taken over a $10\times 10\mu {\mathrm{m}}^2$ scan size with rms roughness of 0.14 nm. (c) A $\theta \text{−}2\theta$ x-ray diffraction scan of a Pt(1.7-nm)/CFO(40-nm)/MgO heterostructure. The inset: A fine scan showing a clear CFO(004) peak. (d) and (e) HAADF STEM micrographs of the CFO/MgO interface and Pt/CFO interface, respectively. (f) Magnetic hysteresis loops of Pt(1.7 nm)/CFO(40 nm)/MgO for both in-plane (red dashed line) and out-of-plane (black solid line) applied magnetic fields.

Figure 2

(a) Measurement geometry for the AMR $(\gamma$ scan on the $n\text{−}j$ plane). (b) Measurement geometry for the SMR $(\beta$ scan on the $n\text{−}t$ plane). (c) $\gamma$ dependence of $\mathrm{\Delta }{\rho }_{\mathrm{xx}}/{\rho }_0$ of Pt/CFO taken with ${\mu }_{0}H=10\mathrm{T}$, showing the presence of the AMR. Inset: $\gamma$ dependence of $\mathrm{\Delta }{\rho }_{xx}/{\rho }_0$ of Pt/MgO (the green line) and Cu/CFO (the black crosses), showing OMR. (d) $\beta$ dependence of $\mathrm{\Delta }{\rho }_{\mathrm{xx}}/{\rho }_0$ of Pt/CFO taken with ${\mu }_{0}H=10\mathrm{T}$, showing the presence of the SMR. (e) Hall resistance for Pt(1.7 nm)/CFO (the red circles), Pt(5 nm)/MgO (the green line), and Cu(8 nm)/CFO (the black crosses). All measurements are taken at $T=5$ K.

Figure 3

(a) Hall resistivity of Pt(1.7 nm)/CFO (the red circles) and Pt(1.7 nm)/Cu(2 nm)/CFO (the blue line) with a linear OHE background subtracted. Addition of the Cu spacer heavily suppresses the magnitude and coercivity. (b) The SMR $\beta$ scans of Pt/CFO (the red circles) and Pt/Cu/CFO (the blue line), (c) The AMR $\gamma$ scans of Pt/CFO (the red circles) and Pt/Cu/CFO (the blue line). All measurements are taken at $T=5$ K. The magnetic field was fixed at 10 T for the $\gamma$ and $\beta$ scans.

Figure 4

(a) Average magnetic moment per Pt atom in ${\mu }_B$ for Pt layers adjacent to the CFO/Pt(001)/(001) interface, calculated with DFT. Layer averages decrease sharply to zero in one to two layers from the interface. (b) The magnetic moment isosurfaces at $0.0025{\mu }_B$ for the Pt atoms in the eight-layer calculation cell. Strong negative moments (red) are observed only in the layers closest to the interface, diminishing to zero by the fourth layer of the Pt atoms. (c) Orbital-resolved $d$-DOS of a Pt atom located directly over a magnetic Fe atom at the CFO/Pt interface. The thin lines are spin-up (positive values) and spin-down (negative values) DOS, and the bold line is their sum.