Magnetic and electrical transport properties of Ru doped cobalt ferrite thin films with perpendicular magnetic anisotropy

Epitaxial ferrimagnetic thin films of (Co, Ru) Fe2O4 were grown on MgO (001) substrate using pulsed laser deposition technique. Ruthenium substitution in cobalt ferrite has increased the conductivity by orders of magnitude, but it has a minimal effect on magnetic properties. The film has a high coercivity and perpendicular magnetic anisotropy (PMA), where the magnetic easy axis points perpendicular to the film surface. We report the magnetic and electrical transport properties here. The temperature variation of resistivity showed different conduction mechanisms at high and low-temperature regimes. Room temperature Hall measurement reveals “n” type carrier with a carrier concentration of 4×10/cm. The film showed negative MR and a linear decrement with the magnetic field without any saturation. © 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/9.0000169


INTRODUCTION
Room temperature magnetic semiconductors have drawn significant attention due to their potential applications in spin-based spintronic devices such as MRAM, 1,2 ferromagnetic field-effect transistor (FET). [3][4][5] The metallic ferromagnetic films having PMA are primarily used in ultra-high-density magnetic recording. 6,7 There is a lack of sufficient oxide-based magnetic semiconductors with PMA. Cobalt ferrite (CFO) is a suitable magnetic oxide with combined traits of a tunable magnetic semiconductor and PMA. Cobalt ferrite has an inverse spinel structure, and the metal ions occupy two different sites, namely oxygen tetrahedral (A) and oxygen octahedral (B). The transition metal ion Co 2+ occupies the B site while an equal number of Fe 3+ occupy both A and B sites. The insulating nature of cobalt ferrite is due to a lack of hopping conduction at the B site, where all Fe ions exist with a +3 valence state. It has a high Curie temperature (Tc ∼860 K) due to antiferromagnetic superexchange interaction between A and B sites. Cobalt ferrite has a high positive first-order magnetocrystalline anisotropy constant accompanied by a considerable value of magnetostriction due to a large orbital moment of Co 2+ at the octahedral site. 8 In thin films, the strain related distortion mostly caused by lattice mismatch between film and substrate coupled with high magnetostriction leads to PMA. 9,10 Even though CFO has PMA and essential magnetic properties, the insulating nature of CFO restrict its use in many spintronics application. So, to realize spin-polarized carriers in an oxidebased magnetic semiconductor with PMA, we have substituted Co 2+ ion with dopants having a valence state more than +3. In this case, it is Ru 4+ . Ruthenium preferentially occupies the B site with a low spin state. 11 So doping retains its hard-magnetic behavior and PMA while modifying the electrical property significantly. Ru doping results in Fe 3+ /Fe 2+ mixed-valence state at B site to maintain charge neutrality. This supports the hopping of spin-polarized 3d-t 2g electrons and transforms insulating cobalt ferrite to semiconducting. In this work, we have fabricated a ferromagnetic semiconductor that is a ruthenium doped cobalt ferrite thin film with PMA. We have focused on magnetic, electrical, and magnetotransport properties and tried to understand the primary conduction mechanism. This work demonstrates the potential use of an oxide-based magnetic semiconductor with spin-polarized carriers in spintronics based devices.

EXPERIMENTAL METHODS
We have deposited Ruthenium doped cobalt ferrite (Co 0.8 RuxFe 2 O 4 -CRFO) thin film using pulsed laser deposition (PLD) technique. The PLD system used a KrF excimer laser (λ=248 nm, 20 ns pulses). The target was prepared by the conventional solid-state reaction method using high-quality precursors Fe 3 O 4 , RuO 2 , Co 3 O 4 from Sigma Aldrich. We have used a singlecrystalline MgO (001) substrate, which has a lattice mismatch of -0.4 % (tensile). The films were deposited at a frequency of 3 Hz and with an energy fluence of 1.2 J/cm 2 . We have maintained a substrate temperature of 600 ○ C during the deposition process. The film was in situ annealed at 450 ○ C for 1 hour. We did the film deposition and annealing at a high vacuum of 2× 10 -5 mbar. The crystal structure and thickness of the film were analyzed using a high-resolution X-Ray diffractometer (Rigaku smartLab). We investigated the surface roughness and magnetic domain structure using an AFM machine. The in-plane (IP) and out-of-Plane (OOP) magnetic hysteresis loops were measured using a superconducting quantum interference device (SQUID, Quantum Design) with a maximum applied field of ± 7 T. We fabricated a Hall bar pattern using a shadow mask during the deposition for transport measurements and carried out the measurements in a four-probe configuration. We performed the electrical and magnetotransport measurements using a physical property measurement system (PPMS, Quantum Design) with a magnetic field applied perpendicular to the film plane. DISCUSSION   Fig 1(a) shows the high-resolution 2θ-ω measurement for CRFO/MgO film. The appearance of only the CRFO (004) peak suggests phase-pure growth and the film's highly oriented nature along (00l) direction. The out-of-plane lattice constant was calculated from the 2θ position using Bragg's law, giving a value of 8.36 Å. We examined the film's crystallinity quality by performing an omega scan about the CRFO (004) peak. The peak had a minimal FWHM value of ω = 0.3 ○ , shown in Fig 2(b). This shows the highly crystalline nature of the film. We did in situ annealing to improve the crystallinity of the film and also to minimize the defects. The thickness of the film was in the range of 40-60 nm. The film's surface morphology and the corresponding MFM image are shown in Fig 1(c) and Fig 1(d), respectively. The surface roughness was measured using the AFM system in a scan area of 2 μm ×2 μm. It showed an rms roughness of as low as 0.2 nm showing an extremely smooth surface. The magnetic microstructure of the film was measured using MFM in the corresponding area. The MFM image was taken in a demagnetized (as prepared sample) state and at room temperature. We have used a magnetic tip coated with Co/Cr thin film at a lift height of 90 nm for MFM measurement. Before measurement, we have magnetized the tip vertically. The MFM image shows domain points in both up and down directions shown by a purple and yellow color, respectively. The magnetic domain image showed a cluster like feature and had different shape and size. The cluster type domain is mostly found in samples with perpendicular anisotropy and with large coercivity. 12   9,13,14 It is noteworthy to point out that with Ru doping, the film's magnetic properties are slightly affected. The lower value of Ms on MgO substrate was due to the formation of the antiphase boundary (APB). The presence of APB deteriorates the magnetic propertied drastically. In this case, APBs formed due to the high symmetry nature of the MgO substrate than the CRFO film result in many nucleation sites. 15 The OOP M-H loop has a squareness (Mr/Ms) of 35 %. The smaller squareness is due to the presence of in-plane domains and domains with easy axis tilted away from the normal direction in a remanent state. 16 We could notice shrinkage in the M-H loop at the lower field.

RESULTS AND
We obtained a similar loop for CFO films of thickness ∼40 nm on MgO. [17][18][19] The shrinkage may be arising because while sweeping the field from positive to negative field, there could be nucleation of domain walls due to the presence of an antiphase boundary. 19 We have also studied the temperature dependence of magnetic properties of CRFO thin film. The OOP magnetic hysteresis loop was measured at different temperatures as plotted in Fig 2(b). Fig 2(c) summarizes the variation of remanence (Mr/Ms) and Ms with temperature. It showed that both remanence and Mr have a higher value at a lower temperature due to perfect spin alignment with reduced thermal agitation. The value decreases as we approach room temperature. The large coercivity of CRFO film is due to the high magnetocrystalline anisotropy constant of Co 2+ in the octahedral site. The variation of coercivity with temperature is shown in Fig 2(d). It shows that the coercivity decreases rapidly with the increase of temperature. The magnetic anisotropy is usually very sensitive to temperature and has a high value at low temperature. Coercivity variation with temperature reflects temperature dependence of magnetic anisotropy. 20 Fig 3(a) shows the temperature dependence of electrical resistivity (ρ) of CRFO thin film in a temperature range of 140 -400 K. The increasing trend of resistivity at lower temperature indicates semiconducting nature. With ruthenium doping, the resistivity of cobalt ferrite has decreased by orders of magnitude. We found that the room temperature resistivity of CRFO film was ∼ 2×10 -1 Ω-cm, while that of CFO had a room temperature resistivity of 10 5 -10 6 Ω-cm. 21 In CRFO film, Ru 4+ substitute Co 2+ and may be preferentially occupying octahedral (B) site due to larger ionic radii. 20,22 It converts Fe 3+ to Fe 2+ to maintain charge neutrality and hence increases conduction by hopping of single spin state t 2g electrons at B site shown in the inset of Fig 3(a). Adding to Ru doping, the high vacuum growth of CRFO film also facilitated Fe 3+ to Fe 2+ conversion due to oxygen deficiency and, hence increasing the conductivity. We examined the doping-induced change in the valence state of Fe using XPS measurement (data are shown in supplementary material). The ρ vs. T data was fitted to different models to understand the underlying conduction mechanism. In Fig 3(b) fitted the data to Arrhenius type equation defined as where ρ 0 , Ea, K B is a preexponential constant, activation energy for allowing hopping of electron from Fe 2+ to Fe 3+ and the Boltzmann constant, respectively. From the slope of ln ρ vs. 1000/T graph, we found the Ea value to be 89 meV. 23 The Arrhenius plot showed good linearity throughout the temperature range of 140 -400 K, giving clear evidence of thermally activated conduction. 24,25 The electron conduction in some other spinel ferrites were explained by the small polaron hopping model 26,27 Which is defined as where ρ 0 , wp, KB are preexponential term, potential barrier for hopping, and Boltzmann constant, respectively. 26 Fig 3(c) shows ln(ρ/T 3/2 ) vs. 1000/T graph was fitted to two lines (of a different slope) at low (140-210 K) and high (210-400 K)-temperature range despite one line in the whole temperature range. Fitting to a single line shows discrepancy at a lower temperature, as shown in the inset of Fig 3(c). This reflects a change in the conduction mechanism at ∼ 210 K. We obtained the fitting parameter W PL = 107 meV and W PH = 130 meV corresponding to low and high temperature, respectively. 26,28 It was also possible to fit the data by assuming Mott's variable range hopping (VRH) in the whole temperature range, as shown in Fig 3(d). The equation was defined as Where T 0 is the mott characteristic temperature. 29 Again, the fitting resulted in two slopes with values of T 0L =7×10 -7 K and T 0H = 6×10 -6 K at the low (140-233 K) and high (233-400 K) temperature range, respectively. 23,30 The inset shows a bad fitting at a lower temperature when fitted with a single line. Fitting with various models, we found that the transport mechanism of the magnetic semiconductor is well understood using the small polaron hopping model and Mott's variable range hopping (VRH). There is a cross-point temperature in the range of 210 K -233 K, where a change of conduction mechanism occurs. The carriers have a thermally activated behavior with two activation energy below and above this temperature. This temperature dependence ρ with two activation energies resembles the other spinel ferrite thin films. 30 This double activation energy is due to defects associated with the film. 31 In Fig 4(a-d), we have demonstrated the magnetotransport properties to investigate the intriguing magnetic properties of the magnetic semiconductor. Fig 4(a) showed the schematic illustration of the Hall bar device used for electric and magnetic transport measurements. The magnetic field was applied perpendicular to the film plane. In a ferromagnetic material, the hall resistivity is defined by an empirical relation defined as The first term in Eq. (4) represents the ordinary Hall effect with R 0 as the normal Hall coefficient. This results from the magnetic Lorentz force. The second term represents the anomalous Hall effect (AHE), and it depends on the perpendicular magnetization of the sample with R A as the anomalous Hall coefficient. Usually, AHE has a more considerable contribution than the ordinary Hall effect. The room temperature variation of Hall resistivity and out-of-plane magnetization with the magnetic field is shown in Fig 4(b). The loops overlapped at lower field values (H < 2 T) confirms the occurrence of AHE. AHE at room temperature signifies the presence of spinpolarized carriers. Fig 4(d) present ρxy variation with the magnetic field at 300 K and at 200 K. R A value calculated to be 3.06 ×10 -7 m 3 /As and 6.7 ×10 -7 m 3 /As at 300 K and 200 K, respectively. It shows an increment of spin polarization at low temperatures. From Hall measurement, we found the carrier type to be "n" type with a carrier concentration of 4.4 ×10 20 /cm 3 and 2.7 ×10 20 /cm 3 and mobility of 0.23 cm 2 /Vs and 0.06 cm 2 /Vs at 300 K and 200 K, respectively. This implies that both carrier concentration and mobility have lower values at a lower temperature. 23 We measured the change in resistance with the magnetic field, magnetoresistance (MR) of the CRFO film with magnetic field variation. The highest applied field was up to ± 9 T in the perpendicular direction.
The MR is defined here as Where R(H) and R(0) are the resistance in the presence and absence of a magnetic field, respectively. The MR has a negative value and showed a linear decrement with a magnetic field without saturation. A negative MR arises from defects created in the film during the growth process like APB, stacking defects. 32,33 The APBs couple antiferromagnetically with the adjacent domains. The presence of magnetic field forces parallel alignment of domains results in increased conductance and a negative MR. 34 The MR value was measured at different temperatures, as shown in Fig 4(c). The MR % increased systematically towards the lower temperature and had the highest value of 2.8 % at 150 K, shown in the inset of Fig 4(c). 35 This may be due to temperature dependence of tunneling of spin-polarized electrons across APBs. 36,37

CONCLUSION
We have successfully fabricated ferrimagnetic semiconductor, ruthenium doped cobalt ferrite thin film by PLD. The film has high crystallinity and a smooth surface. Ruthenium doping has transformed insulating cobalt ferrite into a magnetic semiconductor without much affecting its magnetic properties. CRFO/MgO has perpendicular magnetic anisotropy produced by stress anisotropy. The Mr/Ms and Hc have a higher value at a lower temperature, and the value decreases as we approach room temperature. We investigated the activated conduction mechanism in the entire temperature range. The AHE demonstrates spin-polarized carriers at room temperature, and it increases at a lower temperature. It also showed a negative MR with the highest value of 2.8 % at 150 K. This shows the potential of our system for spintronics applications.

SUPPLEMENTARY MATERIAL
See supplementary material for the conversion of Fe 3+ to Fe 2+ with ruthenium doping without altering the valence state of cobalt.