Magnetization reversal, giant exchange bias effect and magnetoresistance in oxygen vacancy ordered Sr$_{4}$Fe$_{3}$CoO$_{11}$

TL;DR

This study explores the magnetic, transport, and structural properties of Sr₄Fe₃CoO₁₁, revealing complex magnetic reversal, giant exchange bias, and magnetoresistance effects driven by magnetic frustration and anisotropy.

Contribution

It reports the first detailed investigation of the magnetic reversal, exchange bias, and magnetoresistance in oxygen vacancy ordered Sr₄Fe₃CoO₁₁, highlighting the role of magnetic frustration.

Findings

Magnetic reversal occurs at 47 K with increased anisotropy.

Negative magnetoresistance reaches 80% at 25 K in 7 T.

Giant exchange bias observed below T_N due to magnetic frustration.

Abstract

We report the structural, magnetic, exchange bias and magnetotransport effect in Sr$_{4}$Fe$_{3}$CoO$_{11}$. The material crystallizes in the orthorhombic $\textit{Cmmm}$ space group. It shows antiferromagnetic (G-type) transition (T$_{N}$ = 255 K) along with interesting temperature induced magnetization reversal (T$_{Comp.}$= 47 K measured at 100 Oe). The magnetic reversal can be elucidated considering the increased magnetocrystalline anisotropy with Co substitution. Magnetoresistance measurements shows an interesting crossover from negative to positive side at $\sim$ 100 K. The negative magnetoresistance reaches 80 $\%$ at 25 K in 7 T magnetic field. Giant exchange bias effect is observed below T$_{N}$ under field cooling condition. The origin of the negative magnetoresistance and giant exchange bias in this sample can be attributed to the magnetic frustration.

Equations

$$H_{EB}(n)-H_{E\infty }=k/\sqrt{n},$$

$$H_{EB}(n+1)-H_{EB}(n)=\gamma (H_{EB}(n)-H_{E\infty }{)}^3,$$

Full text

I Introduction

Strongly correlated transition metal oxide (TMOs) material systems has attracted widespread attention as a plethora of exotic phenomena such as high-temperature superconductivity (SC, cuprates) 1 , colossal magnetoresistance (manganites) 2 , exchange bias 3 ; 4 and multiferroicity (bismuth compounds) 5 arises from correlated electron physics in 3d TMOs. Indeed, the strong correlation among electrons is a connecting feature of TMOs exhibiting extraordinary electronic and magnetic properties, and that can develop into a promising technological application (such as in electronics and spintronic field). At the same time, purely by changing the valence state of the transition metal ions, unusual physical properties can emerge in TMOs This is realized to be one of the critical issues in understanding the exotic properties observed in high-temperature superconductivity6 , multiferroics 7 , and magnetoresistive compounds 8 .

Among transition metals, iron is a very attractive transition metal element due to its potential to exhibit several oxidation states and spin states in different environments which often induce interesting properties. Oxygen deficient strontium ferrites are such systems displaying valence changes. In recent years a great number of studies has been done due to their diverse structural and physicochemical properties relevant for potential applications 9 ; 10 ; 11 ; 12 ; 13 ; 14 ; 15 . They belong to a class of anion-deficient perovskite-like compounds, which are known for the SrnFenO3n-1 (or SrFeO3-δ) system. This system is well recognized to display a rich phase diagram depending on the oxygen content, ranging from 3.0 to 2.0, which includes cubic SrFeO314 , tetragonal Sr8Fe8O23 (SrFeO2.87) 9 , orthorhombic Sr4Fe4O11 (SrFeO2.75) 11 , Sr2Fe2O5 (SrFeO2.5) 16 and SrFeO2 phase 15 . The square planer oxygen deficient end member exhibits Fe2+ cations with an above room temperature magnetic ordering 15 . The other end phase cubic SrFeO3 possesses an unusually high Fe4+ oxidation state 14 . The intermediate member Sr4Fe4O11 ( orthorhombic 2$\sqrt{2}$ap $\times$ 2ap $\times$ $\sqrt{2}$ap, ap = lattice parameter of the cubic perovskite sub-cell) have an equal number of tetravalent and trivalent iron. Also, Sr4Fe4O11 shows a variety of interesting properties, such as anomalous thermoelectric power and unusual magnetic behavior 11 ; 13 . Hitherto, all the structural studies have indicated that Fe ions alternatively occupy square pyramidal and octahedral sites and undergo a charge ordering below 675 K with an equal amount of Fe4+ and Fe3+ alternatively arranged in the lattice 11 ; 12 ; 13 ; 17 ; 18 ; 19 . It is reported that one Fe site (Fe2, Octahedral) forms a long-range AFM order (TN = 240 K), whereas the other (Fe1, square pyramidal) forms a magnetically frustrated spin-glass-like configuration below TN, which originate unusual magnetic properties in Sr4Fe4O11.

In this paper, we have shown the structure, complex magnetic and transport properties of the oxygen-deficient perovskite-related compound with composition Sr4Fe3CoO11. Giant exchange bias effect and a sizable magnetoresistance are observed below the magnetic transition of the material. Detailed structural and complex magnetic properties are presented and discussed here.

II Experimental Details

Polycrystalline samples of composition Sr4Fe3CoO11 were synthesized by standard solid state reaction method using stoichiometric amount of high purity SrCO3, Fe2O3 , Co3O4 at 1473 K. Phase purity was confirmed by X-Ray diffraction (XRD) using Cu K$\alpha$ ($\lambda$ = 1.54056 Å) using Philips 1050 diffractometer. Powder neutron diffraction (NPD) measurements were done at PSI, Switzerland with temperature using wavelengths of $\lambda$ = 1.494 Å and $\lambda$ = 1.8857 Å for room temperature and low temperature respectively. Fullprof suite program used to refine the diffraction patterns. The temperature and magnetic field dependent magnetisation and transport properties measurements were performed using Squid Quantum Design XL-MPMS, MPMS-3 magnetometer and PPMS.

III Results and discussion

a Structural Characterization

Preliminary Rietveld refinement of the XRD pattern indicate that the compound can be isolated as single phases, crystallizing in an orthorhombic symmetry, space group (S. G.) Cmmm ( $\sim$ 2$\sqrt{2}$ap $\times$ 2ap $\times$ $\sqrt{2}$ap). Figure 1 shows the final plot of joint Rietveld refinement of the RT XRD and RT NPD pattern of the material. The results obtained from the joint refinement of the RT NPD/RT XRD data, are provided in the supporting information file, and they are similar to the reported one 19 .

Figure 2 shows the crystal strucrure of Sr4Fe3CoO11. Similar to the previous we have found full occupancies for the oxygen positions19 . Site occupancy refinement indicates a random distribution of Co in both Fe sites. Sr4Fe3CoO11 shares the same vacancy ordered structure of Sr4Fe4O11.

b Magnetic Structure and Magnetic Properties

To get the magnetic structure, NPD patterns were taken down to 48 K. Figure 3 shows the temperature evolution of new reflections of magnetic origin for Sr4Fe3CoO11. We have used the Shubnikov space group Cm’m’m (propagation vector k = 0, 0, 0, $\Gamma$7) to refine the low temperature NPD patterns of the present material. It shows a G-type AFM ordering below 250 K. The temperature evolution of the magnetic moments is shown as an inset in figure 3.

Figure 4(a) shows the temperature dependence of the FC and ZFC susceptibility ($\chi$) collected at 100 Oe for Sr4Fe3CoO11. An AFM ordering (TN = 255 K $\pm$ 2 K) and then a large irreversibility between the FC and ZFC susceptibility is apparent in the figure. The sudden rise of the FC susceptibility curve below TN highlights the existence of weak ferromagnetic (WFM) correlations below the AFM transition temperature. Below TN, the FC susceptibility curve bifurcate from the ZFC with a broad maximum at $\sim$ 165 K $\pm$ 3 K and then on further cooling, it continuously dropping towards a negative value by crossing the $\chi_{ZFC}$ at TCross $\sim$ 70 K $\pm$ 4 K and reaches the magnetic compensation point ($\chi_{FC}$ = 0) at TComp = 47 K.

Below this compensation temperature, the FC susceptibility is negative down to the lowest measured temperature (2 K), i.e., the resultant magnetization is opposite to the applied magnetic field. Thus, the FC magnetization exhibit temperature induced magnetization reversal phenomenon. The ZFC susceptibility curve shows a peak at $\sim$60 K, indicates a complex magnetic interaction/magnetic frustration at that temperature. To see the characteristics of negative magnetization observed in the sample, we have measured the magnetization at different applied magnetic fields. Figure 4(b) shows the temperature dependence of FC susceptibility data collected under different magnetic field. It shows that the compensation temperature is shifted towards the lower temperature side with increasing magnetic field and when the applied magnetic field increases to 500 Oe, the FC susceptibility switches to the positive side. This suggests that the net magnetization, originally opposite to the direction of the applied magnetic field is oriented along the direction of the applied magnetic field at a higher magnetic field. It is important to recall here that the isostructural compounds, such as Sr4Fe4O11 and the samples with lower Co concentration do not show any magnetization reversal 12 ; 13 ; 18 . In this structure, there are two different Fe sites, square pyramidal Fe1 sites and octahedral Fe2 sites (see figure 2). The Fe2 sub lattice form a long range ordered AFM spin structure 11 ; 17 .

Each Fe1 will have two Fe2 neighbors with up-spin and two with down spin . As a result, the Fe1 magnetic moments are topologically frustrated due to a ferromagnetic Fe1-O-Fe2 double exchange and AFM Fe1-O-Fe1 super-exchange interactions. Also, in the present material Co and Fe ions are randomly sharing two different crystallographic (Fe) sites with a net AFM interaction. Therefore, the doped cobalt cations also give rise to additional canting of Fe moments; this in fact is observed by the sudden rise of $\chi_{FC}$ below TN. In low symmetric antiferromagnetic materials, the existence of WFM is mainly due to the single-ion magnetocrystalline anisotropy or DM interactions or both 20 ; 21 . Also, in certain cases, where the net moment caused by these mechanisms can be oriented in the opposite field direction 20 ; 21 ; 22 , and they can have different temperature dependencies. Under this circumstances, negative magnetization can arise under low applied magnetic fields. In such a case, the observed magnetization reversal in the present sample can be attributed to the increase of the magnetocrystalline anisotropy with Co doping. As we reach the 25$\%$ substitution level (Sr4Fe3CoO11), the magnetocrystalline anisotropy dominates and the net magnetic moment is aligned opposite to the applied magnetic field.

c Exchange Bias and Training effect

Recently, the complex magnetic materials have attracted remarkable interest for exploring the exchange bias effect (EB) 4 ; 23 ; 24 ; 25 ; 26 . The EB phenomenon is generally demonstrated as a shift 27 in the isothermal magnetization loop with respect to the field axes. It can be utilized in several technological applications such as magnetic recording read heads 28 , random access memories 29 and other spintronic devices 30 ; 31 . Figure 5(a) shows the magnetic hysteresis loop (M-H) between $\pm$90 kOe for the Sr4Fe3CoO11) at 10 K, measured in FC condition (cooling field, HFC = $\pm$40 kOe). For positive cooling field (HFC = 40 kOe), the M-H loop is shifted towards the left field axis and it is opposite to the M-H loop measured in negative cooling field (HFC = -40 kOe) at 10 K. This clearly indicates that the observed EB effect has not emerged from the unsaturated minor loop. The EB field is calculated from the M-H loop shift ((HEB = -(HC(L) + HC(R))/2, where HC(L) and HC(R) are the left and right intercepts with the field axis). A very high value of HEB $\sim$ 1.5 kOe is observed at 10 K. The suitable cooling field was chosen by measuring the M-H loops in various cooling fields (HFC) ranging from 0.5 kOe to 60 kOe at 4 K. To study the nature of exchange bias properties, and to know its origin, we have measured the temperature variation of the exchange bias effect. Figure 5(b) shows the temperature dependence of the exchange bias field (HEB). The EB effect is started to emerge from the AFM ordering temperature (HEB = 271 Oe at 250 K). Then, at 25 K the HEB shows a sharp rise and shows a very high value of 5.8 kOe at 4 K.

Sr4Fe3CoO11 shows training effect. In training effect, the uncompensated spin at the interface shows relaxation due to the repetition of the M-H measurement. As a result the exchange bias field shows variation on the number of successive loops studied. Figure 6 highlights the variation of HEB with number of field cycles (n). In this instance, the exchange bias field shows a decreasing tendency as the cycle number (n) increases. The decrease in HEB can be described by fitting the data to the following empirical power law for n $\geq$ 2 32 .

[TABLE]

Where HEB (n) = exchange field in the nth cycle. HE∞ = exchange field for n = ${\infty}$, and k is a system dependent constant. The obtained value of HE∞ is 0.3 kOe, the remnant HEB at 10 K. Also, our experimental data follow the recursive formula suggested by Binek 33 i.e.,

[TABLE]

where $\gamma$ = 1/2k2. Notably, the Eq. 1 is applicable for data starting from n = 2, while Eq. 2 is also valid for for n = 1.

As discussed previously, the Fe1 site is topologically frustrated due to a ferromagnetic Fe1-O-Fe2 double exchange and AFM Fe1-O-Fe1 super-exchange interactions. Also, the random distribution of the Co cation in two different Fe sites in having a net AFM interaction originates additional frustration in the sample. Therefore, the giant EB effect in this material could be due to the existence of different magnetic interactions in the layered structure.

d Magnetoresistance

Figure 7(a) shows temperature dependence of the electrical resistivity [$\rho(T)$] at different magnetic fields. All curves exhibit a semiconducting-type behavior. We do not observe any change in the resistivity data measured in the presence of a magnetic field down to 250 K. However, below 250 K $\rho(T)_{H}$ start to deviate from $\rho(T)_{H=0kOe}$ and becomes higher down to 100 K. Then, with further lowering the temperature, an interesting crossover from positive to negative magnetoresistance is observed. The temperature variation of magnetoresistance (MR) (MR = [$\rho(T)_{H}$-$\rho(T)_{H=0kOe}$]/$\rho(T)_{H=0kOe}$) is highlighted in figure 7(b). We have observed a huge value of negative MR over a wide temperature range (100 - 20 K), and at 25 K the MR is -80$\%$. Above 100 K, the magnetoresistance turns into the positive side ($\sim$ 10$\%$ at 150 K). In order to confirm the MR behavior and its crossover, resistivity ($\rho$(H)) was measured in the presence of magnetic field at different fixed temperatures. Similar to the $\rho(T)$ measurements at different magnetic fields, it confirms the crossover of the magnetoresistance. However, the small positive magnetoresistance observed above 100 K is found to decrease with increasing magnetic field. The opening up of the band gap due to the dominant antiferromagnetic interactions at higher temperature could be the origin of the positive magnetoresistance in this sample. A similar situation is already observed in a few iron based compounds 34 ; 35 . Then with further lowering the temperature, magnetic frustration increases and this develops the spin-dependent scattering of carriers which in turn originate the conventional (negative) MR in this sample. At a lower temperature, the magnetic disorder due to the existence of different magnetic interactions in the layered structure enhances the spin-dependent scattering of the carriers and therefore the electrical resistance in zero magnetic field.

IV Conclusions

We have investigated the structural, magnetic, exchange bias and magneto-transport properties for the oxygen deficient Sr4Fe3CoO11 materials. The compound shows orthorhombic symmetry, crystallizing in the Cmmm space group, isostructural with Sr4Fe4O11. The onset of the antiferromagnetic (G-type) transition is observed at TN = 255 K. Then, with further lowering the temperature, the material shows a range of exciting physical properties. Temperature induced magnetization reversal is observed at a compensation temperature (TComp.) of 47 K. The magnetization reversal is observed due to the increased magnetocrystalline anisotropy with Co substitution. Also, two technologically important phenomena, magnetoresistance and exchange bias is observed near room temperature in this compound. The exchange bias is observed below the AFM ordering temperature when the sample is cooled with a magnetic field. The large exchange bias effect in this sample is attributed to the magnetic frustration in the structure. The magnetoresistance starts to appear below TN = 255 K and reaches a huge value of 80$\%$ at 25 K in 7 T. An interesting crossover in the magnetoresistance value from negative to positive side is observed $\sim$ 100 K. The opening up of the band gap due to the dominant antiferromagnetic interactions at higher temperature could be the origin of the positive magnetoresistance in the sample. On the other hand, increasing magnetic frustration in the structure with lowering the temperature develops the spin-dependent scattering of carriers which originate the conventional (negative) magnetoresistance.

Acknowledgements

PM and SM have contributed equally. SM, CM and OT thank CNRS, Université Bordeaux, and SOPRANO project (Seventh Framework Programme FP7/ 2007-2013 under Grant Agreement No. 214040) for funding this work. R. P. S. acknowledges Science and Engineering Research Board (SERB), Government of India for the Ramanujan Fellowship through Grant No. SR/S2/RJN-83/2012. S. M acknowledges Science and Engineering Research Board, Government of India for the NPDF fellowship (Ref. No. PDF/2016/000348). This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. The authors also thank Dr. Vladimir Pomjakushin for his help in collecting the neutron powder diffraction patterns.