Phase Change-Induced Magnetic Switching through Metal–Insulator Transition in VO2/TbFeCo Films

The ability to manipulate spins in magnetic materials is essential in designing spintronics devices. One method for magnetic switching is through strain. In VO2 on TiO2 thin films, while VO2 remains rutile across the metal–insulator transition, the in-plane lattice area expands going from a low-temperature insulating phase to a high-temperature conducting phase. In a VO2/TbFeCo bilayer, the expansion of the VO2 lattice area exerts tension on the amorphous TbFeCo layer. Through the strain effect, magnetic properties, including the magnetic anisotropy and magnetization, of TbFeCo can be changed. In this work, the changes in magnetic properties of TbFeCo on VO2/TiO2(011) are demonstrated using anomalous Hall effect measurements. Across the metal–insulator transition, TbFeCo loses perpendicular magnetic anisotropy, and the magnetization in TbFeCo turns from out-of-plane to in-plane. Using atomistic simulations, we confirm these tunable magnetic properties originating from the metal–insulator transition of VO2. This study provides the groundwork for controlling magnetic properties through a phase transition.


I. INTRODUCTION
With the rapid developments of automation, the need for fast processing and compact data storage has promptly increased.Spintronic devices have the potential to serve as the building blocks of speedy data processors and high-density memory 1-6 .In spintronics, magnetic moments are the key components for reading and writing data.Being able to control magnetic moments is crucial in designing spintronic devices [3][4][5][6] .Several methods, such as current and laser pulses, can switch magnetic moments in multilayer thin films [7][8][9] .Investigating other methods to manipulate spins is critical for future developments in spintronics.
Among many mechanisms to control magnetism, straintronics, which employs strain-mediated effects for switching, presents an intriguing opportunity.It can serve as a foundation for energy-efficient devices [10][11][12] .One possibil-ity of is using strain arises from the metal-insulator transi-tion (MIT).For example, MIT in Vanadium dioxide (VO2) has drawn interest from both fundamental theories and technological applications 13,14 .Recent studies have shown possible applications in ultrafast optics and electronic devices for sensing and switching [15][16][17][18][19] .In bulk VO , MIT occurs at main rutile, the lattice parameters change along in-plane and out-of-plane directions 27 .Furthermore, in a similar V2O3 system, this coexistence of nanoscale phases near MIT leads to changes in magnetic properties in V2O3/Ni bilayers 28,29 .Moreover, magnetism in paramagnetic centers is found to be affected by MIT in VO2 due to magnetoelastic anisotropy 30 .In these samples, the changes in lattice parameters of VO2 serve as the most important mechanism for tuning magnetic properties.Because of their high magnetostrictions, ferrimagnetic rare-earth (RE) transitional-metal (TM) alloys such as TbFeCo are promising materials to study the effect on magnetism from MIT.
Amorphous ferrimagnetic RE-TM thin films have been widely studied for their applications in high-density lowcurrent spintronics devices 31 , sub-ps ultrafast magnetic switching 8,9,[32][33][34] , and a host for magnetic skyrmions with tunable Dzyaloshinskii-Moriya Interaction [35][36][37][38][39] .These ferrimagnetic films exhibit strong perpendicular magnetic anisotropy (PMA) and can be synthesized at roomtemperature requiring no epitaxial growth 40,41 .Magnetic properties, such as magnetization and coercivity, are greatly influenced by the compensation temperature, which can be tuned by varying composition and thickness 42,43 .These prop-340K 20 2 and it is accompanied by abrupt changes in structural erties make TbFeCo a good material to reveal the effect on magnetism from MIT. and electronic properties.Across MIT, bulk VO2 undergoes a structural transition from a low-temperature monoclinic to a high-temperature rutile phase.In VO2 thin films under uniaxial strain, recent reports reveal a complex mix of structural phases near MIT [21][22][23][24][25][26][27] .When VO2 films are epitaxially grown on TiO2 substrates, due to epitaxial bi-axial strains, the transitions are isostructural.In addition, MIT occurs at different temperatures, for VO2 films grown on different orientations of the TiO2 substrates.In VO2/TiO2, although VO2 films re-In this work, the impact on magnetic properties from MIT is investigated in VO2/TbFeCo bilayer.Amorphous TbFeCo films are grown on epitaxial VO2 samples and Si/SiO2 substrate.Comparison of magnetic properties reveals changes in magnetic anisotropy and magnetization in TbFeCo near MIT of VO2.Furthermore, atomistic simulations are employed to incorporate the strain effect induced by VO2 on TbFeCo near MIT.These results can serve as a foundation for devel-oping techniques to control magnetic properties through MIT for device applications.More importantly, since properties of VO2 [15][16][17][18][19] and RE-TM 8,9,[32][33][34] can be controlled through an ultrafast laser, these results open up the possibility of high-speed data processing using RE-TM on VO2.

II. MATERIALS AND METHODS
∼100 nm VO2 thin films were grown on (011), and (100) TiO2 substrates by reactive biased target ion beam deposition (RBTIBD).Details of growth conditions can be found in a each other in a 3 x 3 x 9 configuration to expand the simulation's size to 4.8 nm x 4.8 nm x 14.4 nm, and 20250 atoms in total.The parameters used in the simulation are listed in Table I.The anisotropy axis for each atom is distributed ran-domly within a 30-degree cone, with the axis of cone pointing along the out-of-plane direction.The exchange interactions are benchmarked based on Oslter et al. 45 and our experiments to maintain the same Curie temperature and compensation temperature for a given composition.Using stochastic Landau-Lifshitz-Gilbert (LLG) equation 46 , hysteresis loops were simulated and compared to experiments.The strain anisotropy (Kstrain) is given by previous publication 44 .15  Structural characterization of the samples was performed by X-ray diffraction (XRD) using a SmartLab system (Rigaku Inc.) in the 2θ range between 20 degrees and 80 degrees.Thin thickness measurements were performed by X-ray reflectivity (XRR) technique in the SmartLab.The film surface morphology was characterized via atomic force microscopy (AFM) by Cypher (Asylum Research Inc.).The magnetic properties at various temperatures were performed by vibrating sample magnetometer (VSM) option in a Versa Lab system (Quantum Design Inc.).The magneto-transport properties at various temperatures were performed by the electric transport option in the Versa Lab system.Temperatures were varied from 250 K to 350 K and applied magnetic fields were varied from -2 T to 2 T for these measurements.An atomistic simulation was employed to study the change in magnetic hysteresis due to strain.A handmade atomistic code was used for the atomistic simulations.Since Fe and Co atoms belong to the same TM sublattice in the RE-TM ferrimagnet, Co atoms are treated as Fe atoms.Tb and Fe atoms are distributed in a 1.6 nm x 1.6 nm x 1.6 nm RE25TM75 amorphous structure.We placed replicas of this box next to where λ = 100 ppm is the magnetostriction of amorphous TbFeCo, Ey = 100 GPa is the Young's Modulus of TbFeCo and ε is the strain exerted on TbFeCo by MIT of VO2.In the case of TbFeCo thin films, a positive Kstrain leads to perpendicular magnetic anisotropy, while a negative Kstrain leads to in-plane magnetic anisotropy.The percentage of atoms that experience strain ε varies with the phase distribution in VO2 as VO2 undergoes MIT, based on the fraction of metallic phase obtained from experiments.From Laverock et al. 26 , VO2's transition is not abrupt across MIT.Near the MIT, there is a mixture of a low-temperature insulating phase and a hightemperature metallic phase present in the sample.To model this behavior, we approximated the fraction of atoms experiencing strain from VO2's MIT, based on the fraction of metallic phase obtained from the experiment by Laverock et al. 26 at various temperatures.For example, in TbFeCo on VO2/TiO2(011), no atoms experience strain at 250 K, 25% of atoms experience strain at 300 K and 75% of atoms experience strain at 320 K.

III. RESULTS AND DISCUSSIONS
VO2 films were grown on TiO2 substrates with three different orientations.Fig. 1(a) presents XRD patterns of VO2/TiO2(011) (green), (100) (blue) films measured at room temperature.The 2θ peaks are indexed using rutile VO2 (R-VO2) and TiO2.Different orientations of R-VO2 are found in samples grown on different orientations of TiO2 substrates.R-VO2 (101), R-VO2 (002), and R-VO2 (200) peaks are observed in VO2/TiO2 (101), and VO2/TiO2 (100), respectively.These correspond to the epitaxial growth of VO2 films in each TiO2 orientation.The rutile phase in VO2 at room temperature is consistent with the findings in a previous publication by Kittiwantanakul et al. 27 .In VO2 thin films epitaxially grown on TiO2 substrates, due to epitaxial bi-axial strains, VO2 remains rutile in both the low-temperature insulating phase and the high-temperature conducting phase.Although VO2 remains rutile, temperature-dependent XRD shows a change in relative lattice spacing across MIT.Above the MIT, the relative lattice space in VO2 on TiO2(100) becomes comparable to that of bulk VO2 47 .Thus, in VO2/TiO2(011), the in-plane lattice area , defined as A = a x c expands from 12.66 Å 2 , where a and c are lattice contants equal to 4.41Å and 2.87Å respectively, to 12.99 Å 2 , where a and c are lattice contants equal to 4.56Å and 2.85Å respectively, going from the low-temperature insulating phase to the high-temperature conducting phase.On the other hand, in VO2/TiO2(100), the in-plane lattice area compresses from 13.03 Å 2 , where a and c are lattice contants equal to 4.51Å and 2.89Å respectively, to 12.99 Å 2 , where a and c are lattice contants equal to 4.56Å and 2.85Å respectively, across MIT.
To characterize the MIT of VO2/TiO2, resistance measurements from 240 K to 400 K are shown in Fig. 1 (b).Across the MIT, VO2/TiO2 films show several orders of magnitude decrease in resistance, confirming the transition to a metallic state from an insulating state.Different orientations of VO2/TiO2 have different MIT temperatures between 310 K and 350 K.The MIT temperature is found in VO2/TiO2 (011) at ∼320 K, followed by VO2/TiO2(100) at ∼350 K.
Hysteresis-like behavior is present near MIT in all three orientations, where sharp changes in resistance occur at different temperatures under heating and cooling.The shift in MIT is due to different epitaxial bi-axial strains in VO2/TiO2 for different orientations 27 .
To study the strain effect on magnetic properties from VO2 across MIT, we deposited 15 nm thick TbFeCo with 5 nm thick Ta capping on top of various VO2/TiO2 films at the same time.Fig. 2 (a) shows a schematic diagram of the heterostructure investigated in this work.We studied the surface morphology and roughness in these films by AFM.Fig. 2 (b)-(e) show the AFM images of TbFeCo/VO2/TiO2 before and after the depositions of TbFeCo and Ta capping layer.Before the deposition of TbFeCo, the RMS roughnesses of the samples are 1.19 nm in VO2/TiO2(011), and 0.66 nm in VO2/TiO2(100).After the deposition of TbFeCo and Ta capping layer, the RMS roughnesses of the sam-ples are 1.29 nm in TbFeCo/VO2/TiO2(011), and 0.81 nm in TbFeCo/VO2/TiO2(100).This means that the changes in roughnesses after the deposition of TbFeCo are rather small.Furthermore, the AFM images show little changes to the samples' surfaces.These indicate the TbFeCo layers with Ta capping deposited on VO2/TiO2 maintained the same roughnesses and uniformity for each sample.
To investigate if there is any magnetic switching of TbFeCo due to MIT in VO2, we fabricated each sample into Hall bar configurations and performed the anomalous Hall effect measurements on the patterned films.Anomalous Hall effect is considered here instead of direct hysteresis loops.This is because the TbFeCo films here have a low magnetiza-tion of about 1 × 10 5 A/m, resulting in a small magnetic moment signal in M-H loops measurements.Thus, anoma-lous Hall effect is considered here for clearer results from measurements.Fig. 3 (a)-(c) show normalized Hall resistance as a function of out-of-plane applied magnetic field of (a) TbFeCo/SiO2/Si, (b) TbFeCo/VO2/TiO2(011), and (c) TbFeCo/VO2/TiO2(100).For higher temperatures, above 330 K, increases in noise are observed in both Fig. 3 (a) and  (c).We suspect this is due to the temperature effect in the patterned films.In Fig. 3 (a), the Hall resistance of TbFeCo/SiO2/Si shows very minor changes from 300 K to 350 K, which is expected.Since SiO2/Si substrate has no transitions within this temperature range, the only source of strain acting on TbFeCo arises from the difference in thermal expansion between TbFeCo and SiO2/Si substrate.The thermal expansion coefficient of SiO2/Si substrate is 0.24 ppm/K.In comparison, the thermal expansion coefficient of amorphous TbFeCo near 300 K is about 10 ppm/K, estimated from amorphous TbFe alloy 48 .From 300 K to 350 K, ε due to thermal expansion is ∼ 500 ppm, which is 5 x 10 − 4 .Using Eq. 1, this gives Kstrain of about -7.5 x 10 3 J/m 3 , much smaller than Ku of 1 x 10 5 J/m 3 in TbFeCo.As shown in Fig. 3 (a),ε of 5 x 10 − 4 is too small to have any effects on magnetic anisotropy in TbFeCo, and the magnetic moments of TbFeCo remain pointing in the out-of-plane directions at zero fields.These minor changes in hysteresis loops are likely due to an increase in temperature.The lack of significant changes in TbFeCo's outof-plane loops shows that the magnetic anisotropy of TbFeCo is near constant around room temperature.
Next, we focus on the behavior of TbFeCo on VO2/TiO2 near room temperatures.From Fig. 3 (b), normalized Hall resistance as a function of out-of-plane applied magnetic field of TbFeCo/VO2/TiO2(011) shows a clear loss of PMA going from 250 K to 320 K.The magnetic hysteresis loops become less squared and the magnetic moments of TbFeCo switch from out-of-plane to in-plane.From Fig. 1 (b), the MIT of VO2/TiO2(011) (green line) occurs near 320 K.This means that the loss of PMA in TbFeCo corresponds to the MIT of VO2 near 320 K.As the temperature goes up from 250 K to 320 K, VO2's in-plane lattice area expands across the MIT of VO2/TiO2(011).From Kittiwantanakul et al. 27 , the in-plane lattice area expands from 12.66 Å 2 in the low-temperature phase to 12.99 Å 2 in the high-temperature phase.This cor-responds to ε of 2.6 x 10 − 2 and Kstrain of -3.9 x 10 5 J/m 3 using Eq. 1, greater than Ku of 1 x 10 5 J/m 3 in TbFeCo.Besides strain from VO2's in-plane lattice expansion, another source of strain arises from the difference in the thermal expansion between TbFeCo and VO2.As discussed earlier, the thermal expansion coefficient of amorphous TbFeCo near 300 K is about 10 ppm/K.On the other hand, the thermal expansion coefficient of VO2 near 300 K is about 21.1 ppm/K 49 .Thus, ε due to thermal expansion going from 250 K to 320 K is ∼ 800 ppm, which is 8 x 10 − 4 .This is over an order of magnitude smaller than the ε of 2.6 x 10 − 2 , arises from VO2's MIT.Moreover, from Laverock et al. 26 , VO2 films are not homogeneous.The MIT of VO2 films involves a mixture of a low-temperature insulating phase and a high-temperature conducting phase across a temperature range.The means that TbFeCo on VO2 is experiencing a gradual change in strain across MIT.This is supported by the progressive loss of PMA in TbFeCo going from 250 K to 320 K, as seen in Fig. 3 (b).This shows that the switching of TbFeCo from out-of-plane to in-plane is likely due to the tensile strain that arises from VO2's in-plane lattice expansion across MIT.
The Hall effect of TbFeCo/VO2/TiO2(100) reveals the absence of PMA in TbFeCo throughout the measured temperature.This is probably due to the presence of tensile strain acting on TbFeCo by VO2/TiO2(100).The in-plane lattice area of the low-temperature insulating phase in VO2/TiO2(100) is 13.03 Å 227 , which is larger compared to the in-plane lattice area in VO2/TiO2(011) (12.66 Å 2 ).This means that the underlayer of VO2/TiO2(100) is most likely applying a tensile interfacial strain on the TbFeCo atoms in these multilayer thin films.Since amorphous TbFeCo has positive magnetostriction, a tensile strain will lead to an additional in-plane anisotropy contribution.In contrast, in TbFeCo/VO2/TiO2(011), the smaller in-plane lattice area at the low-temperature insulating phase in VO2/TiO2(011) is creating a compressive interfacial strain on TbFeCo, resulting in PMA in TbFeCo.Furthermore, when the in-plane lattice area of VO2/TiO2(011) expands to 12.99 Å 2 across the MIT, TbFeCo on VO2/TiO2(011) lost PMA.This shows that the in-plane lattice area of 12.99 Å 2 or larger is supplying a tensile interfacial strain on TbFeCo.
From Fig. 3 (c), as the temperature changes from 300 K to 350 K, there are no changes in the magnetic anisotropy of TbFeCo, magnetic moments of TbFeCo remain in-plane at zero external fields throughout the measured temperatures.From Fig. 1 (b), the MIT of VO2/TiO2(100) (blue line) occurs near 350 K.This means that across the MIT of VO2, magnetic properties of TbFeCo remain unaffected.This can be explained by the change in the in-plane lattice area of VO2/TiO2(100) across MIT.In VO2/TiO2(100), the in-plane lattice area shrank from 13.03 Å 2 in the low-temperature phase to 12.99 Å 2 in the high-temperature phase 27 .This corresponds to an ε of -3 x 10 − 3 .Note that the negative sign here corresponds to the compressive strain exerted on TbFeCo, compared to tensile strain in the other samples.The strain in VO2/TiO2(100) is almost 10 times smaller than  the strain in VO2/TiO2(011), which is 2.6 %.Therefore, it makes sense that the change in magnetic anisotropy of TbFeCo is only observed in TbFeCo/VO2/TiO2(011), but not in TbFeCo/VO2/TiO2(100).
To verify that the strain from VO2's MIT is the source of magnetic switching in TbFeCo, an atomistic model is employed.In this model, strain anisotropy is given by Eq. 1. Fig. 4 (a) and (b) show the comparison of measured out-ofplane anomalous Hall effect and simulated hysteresis loops from 300 K to 350 K in TbFeCo on SiO2/Si substrate, respectively.In this sample, no strain anisotropy is included in the simulations because SiO2/Si substrate does not undergo transition across these temperatures.Results indicate the minor changes in measured anomalous Hall effect from 300 K to 350 K are due to an increase in temperature.A discrepancy in the coercivity between measurements and simulations is observed in Fig. 4 (a) and (b).We suspect the discrep-ancy originates from the complex cone-shaped anisotropy in amorphous rare-earth transition-metal films 40 .Next, we investigated hysteresis loops of TbFeCo on VO2/TiO2(011) using atomistic simulations.Fig. 4 (c) and (d) show the com-parison of measured out-of-plane anomalous Hall effect and simulated out-of-plane hysteresis loops from 250 K to 320 K in TbFeCo/VO2/TiO2(011), respectively.With the incorporated model of the strain anisotropy, the measured and simulated hysteresis loops are in good agreement.Both show the gradual loss of PMA in TbFeCo from 250 K to 320 K and the magnetic moments turn from out-of-plane to in-plane at zero fields going from 250 K to 320 K.This confirms that strain from VO2's MIT is the source of magnetic switching in TbFeCo.

IV. CONCLUSIONS
In summary, 15 nm thick amorphous TbFeCo films were deposited on VO2/TiO2 to study the strain effect of metalinsulator transition (MIT) on magnetic properties.Us-ing TbFeCo on thermally oxidized Si substrate as a ref-erence sample, changes in magnetic anisotropy were ob-served in TbFeCo/VO2/TiO2(011) film.
Near the MIT of VO2/TiO2(011), a decrease in magnetic anisotropy was found in TbFeCo and the magnetization of TbFeCo switched from out-of-plane to in-plane at zero external fields.This decrease in magnetic anisotropy originated from the tensile strain arising from the transition of VO2/TiO2(011), where the in-plane lattice area of VO2 expands.Furthermore, atomistic simulations of TbFeCo with strain anisotropy from VO2 were in agreement with measurements, confirming that the in-plane lattice expansion in VO2/TiO2(011) across MIT is sufficient to switch magnetic moments in TbFeCo.These results offer a platform for using the phase transition to achieve magnetic switching in spintronics devices for desirable applications.The data that support the findings of this study are available from the corresponding upon reasonable request.
The authors declare no conflict of interest.
C.T.M, S.K, A.S, and Y.W.: sample fabrication, and measurements.C.T.M and M.G.M: modeling, writing-review, and editing.A.W.G and S.J.P.: supervision and writing-review.All authors have read and agreed to the published version of the manuscript.This research received no external funding.