Triggering of spin-flipping-modulated exchange bias in FeCo nanoparticles by electronic excitation

The exchange coupling between ferromagnetic (FM)-antiferromagnetic (AF) interfaces is a key element of modern spintronic devices. We here introduce a new way of triggering exchange bias (EB) in swift heavy ion (SHI) irradiated FeCo-SiO2 films, which is a manifestation of spin-flipping at high irradiation fluence. The elongation of FeCo nanoparticles (NPs) in SiO2 matrix gives rise to perpendicular magnetic anisotropy at intermediate fluence. However, a clear shift in hysteresis loop is evident at the highest fluence. This reveals the existence of an AF exchange pinning domain in the NPs, which is identified not to be oxide shell from XANES analysis. Thermal spike calculations along with first-principles based simulations under the framework of density functional theory (DFT) demonstrate that spin flipping of 3d valence electrons is responsible for formation of these AF domains inside the FM NPs. EXAFS experiments at Fe and Co K-edges further unravel that spin-flipping in highest fluence irradiated film results in reduced bond lengths. The results highlight the possibility of miniaturization of magnetic storage devices by using irradiated NPs instead of conventionally used FM-AF multilayers.

range, nuclear stopping power, and electronic stopping power of 120 MeV Au +9 ions (i.e. SHIs used in our study) in SiO 2 are respectively 15 μm, 0.2 keV/nm and 14.7 keV/nm respectively as derived from SRIM 2006 software. Thus, it is very unlikely that these Au SHIs get stuck or implanted inside the ≈ 200 nm thicker films. We have further cross-checked from (Energy Dispersive X-ray Spectroscopy) EDX 17 and (X-ray Photoelectron Spectroscopy) XPS measurements that there is no signature of Au in the composite thin films. Several mechanisms viz. evolution of thermal spike 16,18 , ion-hammering effect 19 , creep deformation 14 , shears stress induced deformation 20 etc are described to explain SHI induced micro and macro structural changes in different materials. We noted that the shear stress in 200 MeV Iodine (I) ion irradiated Co NPs for 10 13 ions/cm 2 fluence is calculated only to be ≈ 0.19 GPa 21 , which is rather insufficient for producing ion-hammering effect in metal NPs. In view of this, the elongation of metal NPs (inside an insulating media) in the direction of ion beam is mostly explained by thermal spike mechanism as described in our former work 22 . Detailed thermal-pressure profiles are calculated for I irradiated Co/ SiO 2 nanocomposites and Pb irradiated SnO 2 along with the thermal spike induced temperature distributions in refs 14 and 23 respectively. They have shown that alongside of the molten metal flow along the beam path at very high thermal spike temperatures (above 1000 K), this thermal pressure also plays a role in particle elongation.
In equiatomic FeCo alloys, magnetic to non-magnetic phase transformation is described under 35-45 GPa pressure 24 . Qiu et al. have also found a meta-stable AF phase for FeCo system 25 . These ideas can therefore be extended to tune the magnetic phase or structure of FeCo NPs for EB/MA studies with optimized SHI irradiation, which can give a fresh insight into the SHI-induced magnetization phenomenon. We found that the shape induced MA is predominant at intermediate fluence; while FM to AF phase transition takes over at the highest fluence where EB effect is observed.
In this article we, presumably for the first time, address triggering of the EB effect along with variation of MA in SHI irradiated NPs by state-of-the-art experimental techniques combined with first-principles-based calculations under the frame work of density functional theory (DFT).

Experimental Section
Fe and Co foils, glued on a SiO 2 target, were co-sputtered in a high vacuum sputtering chamber by 1.5 keV Ar fast atom beam (FAB) for depositing FeCo-SiO 2 nanocomposite films on Si substrate. The Ar source was mounted at an angle of 45° facing towards the sputtering cathode. The substrate holder was rotated continuously for uniform deposition of the films. The relative area of the metal pieces w.r.t. the quartz plate exposed to the atom beam determines the amount of metal fraction (here 20%) in the film. The formation of FeCo alloy phase in the nanocomposite was further ensured by 2 hours of annealing in H 2 atmosphere in a tubular furnace at 600 °C 17 . The as-grown and annealed samples were then subjected to different fluences of 120 MeV Au +9 SHI irradiation viz. 5.0 × 10 13 (5e13), 7.5 × 10 13 (7.5e13) and 1.0 × 10 14 (1e14) ions/cm 2 fluences at the 15UD Palletron accelerator of IUAC, New Delhi, India. Cross-sectional TEM (XTEM) samples were prepared following conventional procedure and images were recorded with FEI TITAN 80-300 microscope operating at accelerating voltage of 300 kV. Both in-plane and out-of-plane magnetic measurements were carried out in a Quantum Design MPMS SQUID magnetometer. X-ray absorption data at near and far edges (for both Fe and Co K-edges) were collected at XAFS beamline, Elettra, Italy.

Results and Discussions
Observation of magnetic anisotropy and exchange bias effects. In Fig. 1(a-d) we have shown the in-plane and out-of-plane M-H characteristics for unirradiated and different fluence irradiated FeCo-SiO 2 thinfilms. We noted that the out-of-plane magnetic coercivity (Fig. 2a) is increased about ≈ 350 times in 5e13 film as compared to the unirradiated film. Further irradiating the films at higher fluences results in reduction of out of plane magnetic coercivity. XTEM images of unirradiated and irradiated films, as shown in insets of Fig. 1(a,b), depict spherical to ellipsoid-like particle elongation in the direction of ion beam. Therefore we can say that this enhanced MA along the SHI beam direction in 5e13 film is a direct consequence of NP elongation due to the shape anisotropy introduced in the system. At higher fluences beyond 5e13, though the aspect ratio of the NPs increases, but due to fragmentation/dissolution of FeCo NPs the overall MA reduces [see Fig. 3] 17 . Next, a shift of the out-of-plane M-H loop towards positive field values is observed in 1e14 film (Fig. 2b,c), indicating the presence of an exchange bias (EB) effect. The variation of out-of-plane exchange field with SHI fluence is shown Fig. 2(d). Note that, in the in-plane M-H measurements for 1e14 film ( Fig. 1(d)), we observe a slanted loop, characteristic of a magnetic hard axis. There is no EB field since the exchange coupling has been established in the perpendicular direction of the film plane. The higher out-of-plane coercivity and hence the larger out-ofplane anisotropy, is therefore responsible for the direction-dependent exchange bias in 1e14 film. We noted from our ZFC-FC data [see Fig. S1 in supporting information] that the blocking temperature rises gradually as one increases the SHI fluence and reaches ≈ 230 K in 1e14 film: implying systematic increase in magnetic ordering with SHI fluence. Thus this improved degree of magnetic ordering at the highest fluence (i.e. 1e14 film) has played a crucial role in triggering the EB effect. We would like to mention here that no exchange bias effect is observed from the room temperature M-H data. Note that, Chen et al. 26 have reported shift in high temperature (100-400 K) M-H loops and proposed it as an EB-like phenomenon in superparamagnetic Ni NPs, having superparamagnetic blocking temperature ≈ 50-60 K. Keeping this possibility in mind and to avoid the occurrence of superparamagnetic phases, the M-H measurement temperature is always kept fixed to 5 K in our study that is well below the superparamagnetic blocking temperature of our all irradiated FeCo NPs [see Fig. S1].
Understanding from electronic structure: XANES. Occurrence of AF FeCo phase has been reported before by Qiu et al. 25 . Thus, the interaction of AF domains with FM domains can be a probable reason for this anomalous triggering of EB effect in irradiated FeCo NPs in 1e14 film. One of the possibilities is formation of AF oxides at the shell of the elongated NPs. To cross-check this, we have performed X-ray absorption near edge spectroscopy (XANES) [see Fig. 4(a,b)] measurements at both Co and Fe K-edges. Note that a prominent pre-edge feature is present in unirradiated and in all irradiated films, which are similar to that of the corresponding metal foils. The presence of these pre-edge peaks, representing 1s to 3d transitions, clarifies that the NPs predominantly sustain T d symmetry like metals and not O h symmetry as oxides. However, having a closer look at the pre-edge  (inset Fig. 4b) at Fe K-edge, we find that there is an edge shift in all irraidated films confirming partial oxidation in post-irradiated FeCo NPs. But, the overlap of 5e13 and 1e14 XANES spectra ascertains that oxide content is more or less same in both the films. Thus formation of AF oxides can not be a viable reason for the observed EB effect only in 1e14 film. Moreover, our core level photo-emission data (not shown) has ruled out the possibility of silicide formation in the 1e14 film. Also, no signature of oxide shell formation is evident from XTEM images.  Understanding from electronic structure: DFT and thermal spike modelling. To understand the reason behind this emerging EB effect only at the highest fluence and how the 3d electrons are playing a role in it, we needed to have a look at the electronic structure of the systems. First we have calculated the lattice temperature (T l ) 18 and pressure (P) 23 profiles in films irradiated at different fluences using thermal spike model. The electronic (T e ) and lattice (T l ) temperatures are found by solving the set of partial differential equations: l l l l l e l C e , C l , K e , K l are the specific heats and thermal conductivities of electronic and lattice sub-systems respectively. ρ l is the material density. r A( , t) is the energy transferred to the electrons from heavy ion at a time t and at a distance r from the ion's path. ; we adapted the lattice specific heat and thermal conductivity values of metals and SiO 2 from the works of Wang et al. 27 and Kumar et al. 13 . The expression for A r t ( , ) is taken as described by Meftah et al. 18 . More details are described in ref. 22. Thereafter, the thermal pressure profile P(r, t) is calculated in order to understand the pressure effect into the system. Note that the increase in pressure is l where, α, χ are the volume expansion co-efficient and adiabatic compressibility of FeCo respectively and Δ T l is the increase in lattice temperature. Therefore, gives us the thermal pressure profile of the system. To calculate the lattice temperature profiles, the statistically averaged minor dimensions of the NPs in 5e13 and 1e14 films are taken as 6 and 3 nm respectively from grazing incident small angle X-ray scattering measurement 22 . Upon solving Equations 1 and 2 we find the lattice temperature goes upto 4000 K in 5e13 and 6000 K in 1e14 film (see Fig. 5). From our calculated P(r, t) profiles as in Fig. 5, we observe thermal pressure rises up to 1.31 and 2.02 GPa in 5e13 and 1e14 films respectively. It's been reported that Fe 0.5 Co 0.5 undergoes a pressure-induced bcc FM to hcp non-FM phase transition for an applied pressure of 30-40 GPa 24 . Based on this observation, we conclude that the thermal pressure developed in our system is not sufficient enough for magnetic phase transformation. Therefore, origin of the observed EB effect requires more fundamental understanding at the atomistic label. The formation of stable bcc FeCo phase in our unirradiated NPs was confirmed from detailed extended X-ray absorption fine structure (EXAFS) analysis 17 . Keeping this in mind, we started our calculations, with a bcc FeCo cluster (viz. (FeCo) 8 ). We have also confirmed that this structure is the global minimum structure at that size using cascade genetic algorithm implementation 28,29 . Note that Wu et al. have shown that the binding energy/ cell (− 0.51 eV/cell) is minimum for (FeCo) 8 cluster when it is in bcc configuration 30 . Following the thermal spike temperatures raised in respective fluences, we have performed ab initio molecular dynamics (MD) simulations under the frame work of DFT to first understand the structural changes of the bcc (FeCo) 8 cluster at those temperatures (i.e. fluences). For this we have used all electron based FHI-aims code, which uses numerical atom centered basis set 31 . The exchange and correlation functional is taken from generalized gradient approximations (GGA) as in PBE 32 implementation which is duly validated with more advanced HSE06 33 hybrid functionals. To draw analogy with our annealing conditions, we have chosen 800 K for the MD run of the (FeCo) 8 cluster corresponding to the unirradiated film. We then raised the temperature up to 4000 and 6000 K as per our thermal spike model findings for 5e13 and 1e14 films respectively. At each temperature the system was kept for 8 ps MD using Nose-Hoover thermostat and following that it was cooled to room temperature [T = 300 K] via 4 ps MD simulation. Thus, a fast quenching was introduced in our model system to see the structural deformation when the FeCo NPs are irradiated under different fluences. Following this the structure is optimized to its nearest minima. Finally to investigate the directional magnetism of these structures, we have performed non-collinear magnetic calculations using Vienna ab initio simulation package 34,35 with Projector-augmented wave (PAW) pseudopotential. There is very minimal (less than 0.001 Å in bond length) structural changes in the fully relaxed optimized structures using plane-wave based VASP and all-electron based FHI-aims.
The atom-wise spin-Density of states (DOS) is plotted (near the Fermi energy) in Fig. 6a for the DFT structures corresponding to 5e13 and 1e14 films respectively (see Fig. 6b). This shows that parallel orientation of atom-spins (of a few Co atoms) in T = 4000 K structure (i.e. 5e13 film) is flipped in T = 6000 K structure (i.e. 1e14 film). Further, we noted from the total DOS and experimental XPS valence band data in ref. 22 that the density of valence electrons is maximum for 5e13 film: indicating more polarization of 3d electrons therein. It's therefore inferred that this is the reason behind higher MA in the T = 4000 K structure (5e13 film). The spin flipping in the T = 6000 K structure is more prominent from the spin-moment plot (see Fig. 6b), where the hirshfeld spin moments of all individual atoms are shown when the magnetic field is applied along y-dir (out-of-plane). This not only reduces the total magnetization but also form few AF domains within the FeCo cluster. We therefore Figure 7. The spin-density distribution is shown for T = 6000 K structure when the magnetic field is applied (a) along x-direction (in-plane) and (b) along y-direction (out-of-plane) in our non-collinear calculations. conclude that the interaction between these AF and FM domains is responsible for the observed EB in 1e14 film. In order to establish that the directional nature of EB is a direct consequence of strong perpendicular anisotropy in the FeCo NPs, we have plotted the variation of the spin-density distribution under the applied magnetic field both in x-direction and y-direction of the T = 6000 K structure (see Fig. 7). It's clearly evident that strong perpendicular anisotropy in the FeCo NPs results the out-of-plane distorted nature of the electronic spin density. The later validates the reason behind the directional nature of the observed EB effect.
Validation from local atomic structure: EXAFS. To validate our theoretical conclusion, we have further analyzed the local atomic environment with EXAFS. The k 3 weighted EXAFS at both Co and Fe K-edges and the corresponding Fourier transform (FT) spectra are presented in Fig. 8(a-d). This clearly shows that the first nearest neighbour bond length (BL) gets reduced as one goes from unirradiated (UI) to 1e14 film. Because of high fluence (i.e. temperature) and disorder, the BL in principle, should increase in 1e14 film. This off-the-trend reduction in BL can't be justified on the basis of partial oxidation. One of the possible reasons for this is spin-flipping. The attraction between the anti-parallel spins dominates the atomic arrangement and reduces the nearest neighbour BL. Note that the pair-correlation function (see Fig. 8e,f) of our respective MD structures also show the similar trend of BL reduction as our EXAFS FT findings: thus justifying our explanation about the emergence of EB effect at the highest fluence. Note that, our observation of presence of AF domains in T = 6000 K cluster is also checked and validated in model clusters including oxygen atoms as well.

Conclusion
In conclusion, we have tailored the magnetic shape anisotropy and have triggered a directional EB effect in FeCo NPs by monitoring SHI fluences for magnetic device applications. The MA first increases in 5e13 film due to NP elongation along SHI beam direction and improved polarization of 3d electrons. The out-of-plane coercivity then reduces due to fragmentation and dissolution of NPs beyond 7.5e13. The observed EB effect in the highest fluence irradiated film is explained by formation of AF domains as a consequence of spin-flipping at higher thermal spike temperature.