Introduction

The discovery of exchange bias (EB) effect by Meiklejohn and Bean1 has garnered enormous interest from the scientific community for its intriguing fundamental and technological aspects. Recent impetus on EB have resulted in diverse tantalizing avenues as the modern day electronic devices include its usage in spin valves, magnetic recording read heads, giant magnetoresistive sensors, etc2,3. The EB is usually characterized by an asymmetric shift in the magnetic hysteresis loop along the field axis when a ferromagnetic (FM)-antiferromagnetic (AFM) layered or a composite system is cooled in a static magnetic field through the Nèel temperature (TN) of the AFM phase4. The magnitude of the loop shift (HEB) depends on various factors such as the interfacial roughness, characteristics of the FM-AFM layers involved, the complex spin structure at the interface, the uncompensated moments at the interface, etc4,5,6. Usually for FM-AFM systems, the shift of the hysteresis loop is opposite to the cooling field (HCF) direction and is termed as negative exchange bias (NEB). On the other hand, the shift of hysteresis loop along the same sign of HCF is termed as positive exchange bias (PEB)5,6. The PEB, a rarely observed phenomenon, was first reported for FeF2/Fe bilayer thin-films5,6. It is attributed to the AFM exchange coupling with its sign and magnitude strongly dependent on the HCF5,6. The AFM exchange coupling at the interface was also reported for two FM perovskite oxides, namely, La2/3Sr1/3MnO3 and SrRuO37. The Cu1-xMnx/Co bilayers exhibited PEB in the vicinity of blocking temperature which subsequently vanishes at lower temperatures resulting in NEB due to the coexistence of FM and AFM interface coupling8. More recently, the PEB for Ni81Fe19/Ir20Mn80 bilayers was observed and explained in the framework of meta-stable magnetic disorder at the FM-AFM interface induced by the magnetic training effect (TE)9.

Initially, most of the scientific quest to unravel the EB phenomenon was seen on metallic systems1,3,4,5,6,7,8,9,10,11. Recently, however, this phenomenon is also being explored and tuned in the magnetic perovskite oxides7,12,13,14,15,16. Understanding the evolution of EB in perovskites oxide bilayers and multilayers is essential as these systems present a greater degree of freedom for tunability of EB at the interface via strain, orbital reconstruction, charge-transfer, etc. Their suitable combinations with structural compatibility at the FM-AFM interface might unveil many potent facets of EB. Observation of EB in the disordered-ordered magnetic interfaces, i.e., in paramagnetic (PM) LaNiO3 and FM LaMnO3 superlattice and the PM CaRuO3 and AFM CaMnO3 superlattices are clearly the recent important discoveries in this area12,13. More recently, strain engineered unexpected EB with the emergence of a self assembled spin glass like phase of LaSrMnO4 at the film/substrate interface was reported for (La,Sr)MnO3 single thin-films17. All endeavours are focussed on controlling and manipulation of EB by the interfacial interactions, thickness and number of layers of the FM and AFM phases and the type of AFM order in the superlattice structures14,15. Overall the progress in EB has been two-fold. First, the EB has been addressed in unconventional heterostructures/bilayers with FM-PM, AFM-PM and collinear-noncollinear magnetic heterostructures7,12,13,16. This has challenged our present understanding of EB which is generally observed in conventional FM-AFM heterostructures14,15. The second focus has been to tune and realize the novel EB properties beyond NEB. For instance, the realization of PEB and its reversal to NEB with critical role played by both the extrinsic and the intrinsic factors in controlling PEB, are essential components yet to be explicitly realized and understood.

In this communication, we report a novel and unique set of EB properties in orthoferrtite-ruthenate bilayers La0.3Sr0.7FeO3/SrRuO3 (LSFO/SRO) fabricated on mosaic and non-mosaic SrTiO3 (STO) (111) substrate. These samples, henceforth will be referred to as LSMosaic and LSNon-mosaic, respectively. The proximity of the magnetic transition temperatures of the G-type AFM LSFO (TN ~ 190 K) and the FM SRO (TC ~ 160 K) makes them a suitable combination for investigation of EB properties in bilayer thin-film18,19,20,21. The (111) orientation of STO was chosen as it presents opportunity for increased interactions at the interface as compared to the conventional (100) STO substrate. This occurs as the [Fe3+/Fe5+] ions in the AFM LSFO will be surrounded by three of the same type and three of the other type i.e. Ru4+ ions of the FM SrRuO312. We observe a low-field PEB, its sign reversal by both extrinsic and intrinsic factors and achieved a gigantic vertical magnetization shift. In this bilayer system, G-type AFM structure of LSFO coupled with FM SRO present an opportunity to control the EB by intriguing intrinsic factors such as nearest neighbour spin compensation, spin-flop coupling and competing superexchange (SE) interactions between FM and AFM resulting in a spin glass like interface. Whereas, the mosaicity of the substrate introduces external factors such as modulated spin structure at domain walls, random defects and interface roughness to control and manipulate the EB. Formation of LSFO/SRO bilayers on both the mosaic and non-mosaic STO (111) substrate helps extract the contribution of extrinsic and intrinsic factors responsible for novel features of EB. A unique exhibition of diverse EB properties in LSFO/SRO observed here has been explained in the framework of modulation of the interfacial AFM spin structure with HCF and training induced subsequent runs.

Results

A simplified illustration of the spins at the interface in the LSFO/SRO (FM/AFM) bilayers is shown in figure 1. The ordered and the disordered interfaces typically arise from the non-mosaic and the mosaic STO substrates, respectively [figure 1]. The θ − 2θ XRD scans confirmed the phase purity of LSMosaic and LSNon-mosaic samples [figure 2(a)]. In-plane epitaxial relationship was established by extracting the azimuthal-ϕ scans along the various peaks, i.e. (104) for LSFO, (400) for SRO and (110) for STO in the LSMosaic [figure 2(b)]. Three peaks in ϕ-scans with a separation of 120 degrees are observed for LSFO, SRO and STO which is expected to arise from the three fold symmetry of the STO (111) substrate. The mosaicity of the LSMosaic is distinctly evident in the reciprocal space map (RSM) scans around the asymmetric (330) peak. It shows that the STO substrate peak is split into multiple spots [figure 2(c) and supplementary S1]. This typically depicts that the substrate surface consists of several small crystalline blocks and each block corresponds to one of the reflection of the substrate in the RSM map as shown in figure 2(c). Further, corresponding to each substrate reflection there exists a reflection of the coherently strained LSFO and SRO epitaxial layers for the LSMosaic. Such exhibition of multiple epitaxial peaks is absent in the LSNon-mosaic sample which is formed on non-mosaic STO substrate [figure 2(d)]. The bulk pseudo-cubic lattice parameter of the LSFO is 3.87 Å, SRO is 3.93 Å and the STO is 3.905 Å. The out-of-plane lattice constant for the LSFO is 3.85 Å and the SRO is 3.945 Å. This suggests that the LSFO is under tensile strain, whereas, the SRO is under compressive strain. Overall, we can recognize qualitatively different crystal structures of the same substrate on which the LSFO/SRO bilayers namely, LSMosaic and LSNon-mosaic, were fabricated and their respective implications on the EB properties studied.

Figure 1
figure 1

Schematic of an idealized illustration of the spins (arrows) for La0.3Sr0.7FeO3/SrRuO3 (AFM/FM) bilayer in, (a) an ordered interface on non-mosaic SrTiO3 substrate (LSNon-mosaic) and (b) disordered interface on the mosaic SrTiO3 substrate (LSMosaic) [where, cooling field (HCF) is parallel to the film-plane].

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Figure 2
figure 2

(a) shows θ − 2θ scan for LSMosaic and LSNon-mosaic sample, (b) ϕ-scans along the peaks (104) for LSFO, (400) for SRO and (110) for STO substrate, (c–d) shows the reciprocal space maps for LSMosaic and LSNon-mosaic along the asymmetric (330) orientation of the mosaic and non-mosaic STO (111) substrate, respectively.

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Magnetization (M) versus temperature (T) at a magnetic field (H) of 500 Oe in the field cooled cooling (FCC) protocol shows a TC ~ 150 K for LSMosaic and LSNon-mosaic [inset figure 3(a)]. This is slightly lower than the bulk TC ~ 160 K of the SRO, presumably, due to strain in the thin film14,15. The M versus H loops at 2 K for zero-field cooling (ZFC) and in different HCF for LSMosaic are shown in figure 3(a). It may be seen that the M-H loops for LSMosaic exhibits dissimilar manifestation of the HEB with HCF. On one hand, we observe PEB for LSMosaic at low cooling field (HCF) ~ 1 T [>Hc] while, on the other hand, a HCF of ~7 T dramatically supplants this PEB to a NEB regime [figure 3(a)]. This, in essence, is displayed in figure 3(b), where an unusual crossover from PEB to NEB ~5 T is observed. In contrast to this the LSNon-mosaic sample exhibits only NEB at various HCF which saturates in a field of ~5 T [figure 3(b)]. Overall, the EB properties of LSMosaic are novel and unusual, whereas, the EB for LSNon-mosaic is rather conventional and is commonly observed for FM-AFM systems.

Figure 3
figure 3

(a) Magnetization (M) versus magnetic field (H) loops of LSMosaic in zero field cooling (ZFC) and at various cooling fields (HCF), inset shows M versus temperature (T) plot in field cool warming protocol (H = 500 Oe) for LSMosaic, LSNon-mosaic and LSFO and (b) shows HCF dependence of exchange bias (HEB) for LSMosaic and LSNon-mosaic sample, inset depicts the training induced decrease in coercivity (HC) of LSMosaic.

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In the LSMosaic sample the mosaicity of the substrate induces topographic modulations which results in randomly oriented AFM easy axis of AFM grains in LSFO layer with a FM SRO layer coupled on to it. These sporadic distributions of magnetic inhomogeneities, having imperfections and defects at the interface result in various spin frustrated ensembles with a mixture of FM, AFM and spin flop coupling regimes22,23. The resultant of these microscopic FM-AFM exchange interactions at the interface and at the grain boundaries is understood to govern the dynamics of the system. The HCF drives the LSMosaic in two ways, namely, i) at low HCF [HC < HCF < 5 T], the microscopic AFM superexchange (AFM-SE) interactions dominate the FM double exchange at the interface and result in the PEB [figure 2(a)] and ii) as the HCF is increased above 5 T, FM double exchange gets strengthened and dominates the microscopic AFM exchange at the interface giving NEB. Thus, a PEB → NEB crossover can be tuned via subtle interplay of surface AFM spin correlations with HCF.

To gain deeper insight of AFM spin correlations, we performed a multistage training cycles on the LSMosaic and the LSNon-mosaic sample24,25,26. This was experimentally realized in the following sequence; LSMosaicA (initial cycle) → LSMosaicB (after 15 cycles) → LSMosaicC (after 15cycles) → LSMosaicD (after 12 cycles), while for nonmosaic LSNon-mosaic (12 cycles) [1 cycle is the loop recorded at 2 K with HCF = +7 T]. Training from LSMosaicA to LSMosaicB, causes a marginal increase in the PEB with a slight decrease in HC [inset figure 3(b)]. Further, training results in vanishing of the PEB with a complete emergence of NEB regime for LSMosaicC [figure 4(c)]. This NEB for the LSMosaicC is associated with an increased HC and a decreased Mav [] compared to that for LSMosaicA [figure 4(a)] suggesting enhanced spin-flop coupling for LSMosaicC16. The subsequent training cycle yields to LSMosaicD, which shows a transition in shape of the hysteresis loop as a function of HCF at 2 K [figure 4(b)]. It may be seen for LSMosaicD the HCF of −3 T yields a NEB loop [figure 4(b)]. As this HCF is increased to −5 T the loop manifests with a lesser HC [step1 to 2] with a marked increase in overall M [step 2–3]. Another loop recorded with HCF of −6 T displays an entirely different shape as switching field (HC) decreases, as compared to the loop recorded with HCF of −3 T [figure 4(b)]. This indicates that the pinning defects in the AFM layer are undergoing changes not only with training runs but have HCF sensitivity as well.

Figure 4
figure 4

(a) Coercivity (HC) (closed symbols) and average saturation magnetization (Mav) (open symbols) versus cooling field (HCF) at a temperature of 2 K, (b) Magnetization (M) versus magnetic field (H) loops at different HCF for LSMosaicD, (c) Exchange bias (HEB) with number of cycles (n) [solid line is the fit as per equation. 1] for LSMosaicC and LSNon-mosaic, inset shows vertical shift (Mshift) versus HCF for LSMosaic (LSMosaicA → LSMosaicD) and LSNon-mosaic samples and (d) shows the maximum Mshift (~35%) for the optimized bilayer [LSFO (110 nm)/SRO(10 nm)].

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The disorder induced in the LSMosaic is quite intriguing, as training causes HEB to traverse from PEB (LSMosaiA–B) to NEB (LSMosaicC–D) regime, whereas its counterpart LSNon-mosaic exhibits NEB regime only. The TE is essential signature and can unveil the microstructural spin rearrangements along with the possible mechanisms driving the HEB. To understand the underlying intricacies, we compared the influence of training in the NEB regime of LSMosaic C with that of the LSNon-mosaic. The training leads to irreversible changes in the interfacial domain configurations, which causes the magnetization of the LSFO pinning layer to be nonconserved26. Such relaxation effects in the nonconserved order parameters can be addressed using Landau-Khalatnikov expression which was successfully employed to describe the TE in LSMO/SRO heterostructures26. The phenomenological expression used to model the cycle dependence (n) with HEB is,

where, K and are the crucial fitting parameters, HEB(1) is the first loop HEB value. The equation (1) can also be written as 26. The value of K usually lies in the range −1 ≤ K ≤ 026. When K = 0, it yields HEB(n + 1) = HEB(n) implying no training, whereas for K = −1, it is which yields a step like change in HEB between the first two data points with no TE for n > 226. Equation (1) was successfully fitted to both LSMosaicC and LSNon-mosaic, with the values of K as −0.52 and −0.97, respectively. For n ≥ 2, the HEB for LSMosaicC keeps on decreasing with n, whereas, the HEB for LSNon-mosaic exhibits a negligible change.

The contrasting training behaviour for LSMosaicC and LSNon-mosaic, plausibly indicates different training mechanisms governing both the samples. We attribute the initial large decrease in HEB for both the samples to a ‘Hoffmann’ like behaviour, where the major changes after the first reversal can be ascribed to a transformation from an initial noncollinear arrangement of the AFM spins to a more relaxed collinear arrangement27. Furthermore, as per Hoffmann's model, the TE should cease for n ≥ 227. This is displayed by LSNon-mosaic, whereas, LSMosaicC shows a continuous decrease in HEB even beyond n ≥ 2. This decrease in HEB (n ≥ 2) for LSMosaic, typically indicates that along with the Hoffman's component (which largely trains out after the first cycle), a second contribution to training may be present. This seems to arise from the thermally activated depinning of the uncompensated AFM spins28,29. Thus, the LSMosaic and the LSNon-mosaic can explicitly be distinguished via field training, as the former exhibits a combination of a Hoffman and thermally activated depinning mechanism, whereas, the later trains out via ‘Hoffman’ mechanism27,28.

We also observed a positive vertical magnetization shift in the hysteresis loop along the same sign as of the HCF for both the samples LSMosaic and LSNon-mosaic [inset figure 4(c)]. Interestingly, vertical shift also displays the TE as it decreases from LSMosaicA → LSMosaicD [inset figure 4 (c)]. Vertical shift can be calculated using, , where, and are positive and negative saturation values of the hysteresis loop. Observation of vertical shift is rare and usually points towards the uncompensated spins at the FM-AFM interface or that are in the bulk AFM14,15,30,31,32. Further, this rare and intriguing observation of vertical shift present in our bilayer system on STO (111) was found to vary with the thickness of AFM LSFO layer [unpublished data]. Thickness variation in AFM or FM phase of a FM/AFM bilayer system is an essential component to control the HEB, HC and can also be used to tune the vertical shift33,34,35. We noted a maximum vertical shift of 35% for our optimized bilayer sample with LSFO(110 nm)/SRO(10 nm) on non-mosaic STO(111) [figure 4(d)].

For further analysis of the sign reversal of the EB of LSMosaic, the loop asymmetries (dM/dH) were derived from the hysteresis data and are shown in figure 59. It may be seen that for low positive HCF (1 T) the first loop reversal is sharper than the second reversal of the loop [figure 5(a)] and yields PEB. As the HCF is increased to +7 T the peak height is reversed and yields a transformation to a NEB regime for the LSMosaicA [figure 5(b)]. This shows the sensitivity of the AFM spin structure to the HCF and points towards a change in the microscopic AFM to FM exchange interaction at the interface [see schematic in figure 5(a) (AFM interface coupling) → 5 (b) (FM interface coupling)]. The shape of the subsequent hysteresis loops after training is more symmetric and rounded for LSMosaicC [not shown and is similar to figure 5(c)]. Furthermore, a peak in the vicinity of H = 0 T for LSMosaicD [figure 5(d)] shows that the FM spins have now softened and are very sensitive to any reversal of the direction of sweeping field. This scenario is in good congruence with that discussed earlier for figure 4 (b) in which we observed an enhanced saturation M with a decreased HC. The loop asymmetries as described above portrays the significant deviations in the pinning AFM layer with the HCF and training runs resulting in PEB → NEB transition [Inset figure 5(a–d)].

Figure 5
figure 5

Asymmetry in hysteresis loop (dM/dH) versus magnetic field (H) for LSMosaicA and LSMosaicD at different cooling field (HCF).

Inset boxes with orange, green and blue colour depicts the spin configurations of La0.3Sr0.7FeO3/Interface/SrRuO3, respectively.

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Figure 6(a) illustrates the temperature dependence of the HEB for the LSMosaic sample after various training runs. The blocking temperature for LSMosaic and LSNon-mosaic is nearly the same 130 K [Supplementary figure S2]. We find that for LSMosaicA exhibiting PEB, the HEB increases slightly for a temperature upto 50 K and then it shows a decrease with increasing temperature [Figure 6(a)]. In the NEB regime for LSMosaicC and LSMosaicD the HEB exhibits an exponential type of decrease with increasing temperature. This usually signifies the frustrated spin state at the interface36,37. To substantiate this the HEB data of LSMosaicC and LSMosaicD were fitted to the equation , where is the extrapolation of HEB at absolute zero and TA is a constant [figure 6 (a)]36,37. We obtained convincing fits with, and −0.063 T with TA = 30 K and 21 K for LSMosaicC and LSMosaicD, respectively. Further, inset figure 6(a) depicts the temperature variation in the HC and Mav for LSMosaic sample. We observed an enhanced overall Mav for LSMosaicD, as compared to that of LSMosaic(A–C) in the entire temperature range [figure 6(b,c and d)]. This suggests that the training causes a temperature independent retention of the irrecoverable permanent spin rearrangements in the AFM layer for the LSMosaicD.

Figure 6
figure 6

(a) Exchange Bias (HEB) versus temperature (T) for LSMosaic at a cooling field of +3 T (solid symbols) and −3 T (hollow symbols), dashed line is fit as per equation , while the solid line is guide to the eye. Inset depicts temperature variation of HC and Mav for LSMosaic sample, (b), (c) & (d) shows the temperature variation of hysteresis loop shapes for LSMosaicA, LSMosaicC and LSMosaicD.

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Discussions

In this section we will discuss the key observations of the LSMosaic sample, in the following sequence, i) competing exchange interactions at the LSFO/SRO interface and the possible EB model for the observed PEB, ii) dynamics of the training induced dissimilar hysteresis loop shape transitions and iii) the vertical magnetization shift.

The subtle interplay of FM-SE and AFM-SE interactions at the LSFO/SRO interface drives the PEB → NEB transition in the LSMosaic sample. The transition may be attributed to a potential crossover from AFM to FM exchange coupling [figure 3(b)]. This occurs as the mosaicity induces a disorder at the LSmosaic interface, thus, inducing the competition between FM-AFM exchange interactions. On one hand, LSFO grain boundaries exhibit FM-SE interaction in Fe5+-O-Fe3+ and AFM-SE interaction in Fe3+-O-Fe3+ in the [001] plane20. On the other hand, across the FM-AFM interface Ru4+-O-Fe3+ and Ru4+-O-Fe5+ exhibits a FM double exchange interaction. The increasing HCF overcomes the localized AFM-SE interaction and strengthens the FM double exchange resulting in a crossover from PEB to NEB regime. Several models were proposed to explain the EB effect22,23,38,39,40,41,42,43,44,45. The EB in mosaic LSMosaic sample is suggestive of a scenario in which the interface domain wall (IDW) develops as a result of competition between AFM coupling and the Zeeman energy44,45. Presently, IDW can manifest between different crystallite ensembles, consisting of independent AFM grain boundaries with a coupled FM layer on to it. The IDW can provide AFM coupling at the interface which will yield PEB for LSMosaicA. Also, IDW shows training and HCF sensitivity. Thus, as the HCF is increased thickness of IDW may decrease due to domain wall compression, yielding a complete NEB regime for LSMosaic C–D44,45.

At this point, it is imperative to discuss the possibility of charge transfer at the LSFO-SRO interface. Charge transfer was found to be associated with the observed unidirectional anisotropy in LSMO/YBCO46,47. In contrast, for the La2CuO4/LSMO bilayers, it was demonstrated that charge transfer is not a key factor, as the HC was found to exhibit a AFM thickness dependence [keeping FM thickness constant]. In the present case too, the HC was found to vary with the LSFO thickness for the LSFO/SRO bilayers on nonmosaic STO substrate [unpublished data]. This further bolsters the dominant role of SE interaction at the LSFO/SRO spin-glass like interface.

The TE introduces irreversible changes in the LSFO layer and at the LSFO/SRO interface, which manifests in the form of a magnetic reorientation from a square loop [LS1A–B] to a stepped hysteresis loop [LS1C–D] [figure 6 (b)]48. Interestingly, this loop shape variation may be associated with an enhancement in spin-flop coupling strength (Jex). For the LSMosaic sample, the strength of spin-flop coupling at the interface can be estimated using, Jex = HEBtFMMS, (where, tFM is thickness of FM SRO layer and MS is saturation magnetization)48. The deduced value of Jex (2 K) for LSMosaic(A–B) → LSMosaicC → LSMosaicD varies as 0.2 → 0.66 → 0.57 erg/cm2. Apparently higher value of Jex substantiates the enhanced spin-flop coupling in LSMosaicC–D which yields a stepped hysteresis loop, whereas a low Jex favours a square loop in LSMosaicA–B.

Now, we further discuss the implications of the multistage training runs and switching of the hysteresis loops in the LSMosaic sample [figure 6 (b)]49,50,51. The LSMosaicA sample exhibits a coherent reversal of the hysteresis loop in the whole temperature range [figure 6(c)]. On the other hand, this coherent reversal of the SRO spins is hindered at HC2 for the LSMosaicC–D and the loop closes at HC3. This emergence of HC2 can be associated with the domain wall depinning processes which may be training or thermally assisted28,29,50,51. Further the TE largely alters the pinned spin concentration from LSMosaicA to LSMosaicD. This is evident as the relative changes in HC1 with temperature are quite pronounced for LSMosaicA and LSMosaicC. In contrast, the LSMosaicD exhibits a negligible change in HC1. This indicates that the pinning defects concentration have been drastically reduced for LSMosaicD with subsequent training runs. Furthermore, the HC1 was found to decrease from −0.4 T (LSMosaicC) to +0.1 T (LSMosaicD). This points towards a sharp reversal of the SRO spins even before H = 0. Remarkably, this was also evident in the loop asymmetries, as a sharp peak was observed near H = 0 [figure 5(d)]. The nearly temperature independent trend of HC1 for LSMosaicD suggests that the LSFO interfacial spins have now been depinned and have started reversing with the FM SRO spins. This causes drastic reduction in HC for LSMosaicD, which is also accompanied with a huge increase (64%) in Mav of the loops [figure 3(b)]. This excess M in LSMosaicD may have contributions from, i) the interfacial AFM ions Fe5+ (~1.5 μB) and Fe3+ (~3.5 μB) which have started rotating coherently with the FM layer20, ii) the, FM SRO might break into mixture of different regions (hard and soft), for large HC hard regions out number their softer counterparts and vice versa52.

Finally, we comment on another important observation, which is the vertical magnetization shift [inset figures 4(c) and 4(d)]. The observation of vertical shift along the same sign as of the HCF usually indicates FM coupling at the interface5,6. We observed a positive vertical shift for LSMosaicA and LSNon-mosaic which suggests FM coupling at interface. But, interestingly, LSMosaicA also exhibits a PEB, which point towards the AFM coupling at the interface. Nevertheless, similar contrasting scenario was well addressed by Fritzimmons et al., as they showed that a microscopic AFM coupling at the interface is likely possible and can manifest along with a positive vertical shift30. This is seen for LSMosaicA sample. Moreover, a giant vertical shift of about 35% for our optimized sample suggests that a large number of uncompensated AFM spins exists when the bilayer is grown along (111) orientation of STO [figure 4(d)]. This may occur as LSFO is known to exhibit an intriguing quasi-2D charge ordering on STO (111) rather than a perfect 3D charge ordered regime with a charge-disproportionate Fe3+ and Fe5+ ions along (111)19. The latent defects and imperfections in the film may give rise to uncompensated spins in the bulk along with the surface AFM spins resulting in massive EB.

To summarize, we report a novel method of mosaicity induced disorder to obtain a rare phenomenon of PEB, magnetic annealing and HCF induce PEB → NEB transition and accompanying loop shape transitions. While the mosaic-disorder induces AFM exchange coupling at the interface which causes PEB, the uncompensated spins arising from the intrinsic nature of the magnetic order of LSFO yield the huge vertical shift. These studies open up new avenues for obtaining the otherwise elusive PEB for FM/AFM systems and an innovative way to tune giant vertical shift in magnetic oxides.

Methods

The bilayers of LSFO as bottom layer and SRO as top layer were fabricated on STO (111) single crystal substrates by pulsed laser deposition (PLD) technique using a 248 nm KrF excimer laser. Deposition was carried out at a repetition rate of 4 Hz with laser energy of 1.7 J/cm2 at the target with a substrate temperature of 700°C, oxygen partial pressure of 25 Pa and a post-deposition annealing for 5 minutes in 1.5 kPa of O2. Thickness of the bilayers with LSFO (37 nm) and SRO (20 nm) for LSMosaic and LSNon-mosaic were measured using a surface profiler. The X-ray diffraction (XRD) measurements were carried out using PANalytical Empyrean. Magnetization measurements were performed on a SQUID magnetometer (Quantum design, USA).