Moderate dc magnetic fields of less than 1 T allow tuning the exchange bias in an epitaxially grown Fe 10 nm/Cr2O3 2.7 nm/Fe 10 nm trilayer between negative and positive bias fields. Remarkably, this tunable exchange bias is observed at least up to 395 K which exceeds the Néel temperature of bulk Cr2O3 (307 K). The presence of spontaneous exchange bias and the absence of training effects at room temperature suggest the existence of stable interface moments independent of antiferromagnetic long range order in Cr2O3. Furthermore, the coercivity remains constant, independent of the exchange bias field. In contrast, large training associated with nonequilibrium spin configurations of antiferromagnetically ordered Cr2O3 appears below 50 K

Belashchenko, Kirill D.

Binek, Christian

Mukherjee, Tathagata

Sahoo, Sarbeswar

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Christian Binek Publications Research Papers in Physics and Astronomy 10-30-2007 Isothermal low-field tuning of exchange bias in epitaxial Fe/Cr2O3/Fe Sarbeswar Sahoo University of Nebraska-Lincoln, sarbeswar@gmail.com Tathagata Mukherjee University of Nebraska-Lincoln, tatha.muk@gmail.com Kirill D. Belashchenko University of Nebraska-Lincoln, belashchenko@unl.edu Christian Binek University of Nebraska-Lincoln, cbinek@unl.edu Follow this and additional works at: https://digitalcommons.unl.edu/physicsbinek  Part of the Physics Commons Sahoo, Sarbeswar; Mukherjee, Tathagata; Belashchenko, Kirill D.; and Binek, Christian, "Isothermal low-field tuning of exchange bias in epitaxial Fe/Cr2O3/Fe" (2007). Christian Binek Publications. 33. https://digitalcommons.unl.edu/physicsbinek/33 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Christian Binek Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Isothermal low-field tuning of exchange bias in epitaxial Fe/Cr2O3/FeS. Sahoo, T. Mukherjee, K. D. Belashchenko, and Ch. BinekaDepartment of Physics and Astronomy and the Nebraska Center for Materials and Nanoscience, Universityof Nebraska, Lincoln, Nebraska 68588-0111, USAReceived 13 August 2007; accepted 2 October 2007; published online 23 October 2007Moderate dc magnetic fields of less than 1 T allow tuning the exchange bias in an epitaxially grownFe 10 nm/Cr2O3 2.7 nm/Fe 10 nm trilayer between negative and positive bias fields. Remarkably,this tunable exchange bias is observed at least up to 395 K which exceeds the Néel temperature ofbulk Cr2O3 307 K. The presence of spontaneous exchange bias and the absence of training effectsat room temperature suggest the existence of stable interface moments independent ofantiferromagnetic long range order in Cr2O3. Furthermore, the coercivity remains constant,independent of the exchange bias field. In contrast, large training associated with nonequilibriumspin configurations of antiferromagnetically ordered Cr2O3 appears below 50 K. © 2007 AmericanInstitute of Physics. DOI: 10.1063/1.2801519The majority of today’s realized spintronic devices1–3utilize the exchange bias4 EB effect.5–8 The latter is a fun-damental proximity effect taking place at the interfaces ofmagnetic heterostructures including ferromagnetic F/antiferromagnetic AF, F/ferrimagnetic, soft F/hard F sys-tems, and others. Conventionally, EB is established by fieldcooling the heterostructure below the Néel temperature TN,or by applying a magnetic field during the fabrication pro-cess. EB manifests itself in the shift of the hysteresis loopalong the magnetic field axis and a concomitant increase inthe coercivity 0Hc. The EB magnitude 0Heb depends onintrinsic parameters such as the exchange coupling at theF/AF interface, interface roughness, individual F and AF mi-cromagnetic structure, or their thicknesses. However, it hasbeen reported that EB can be tuned by various extrinsic pa-rameters. In this context, a broad spectrum of experimentalprotocols has been employed: cooling in different magneticfields,9 cooling in zero field from different magnetizationstates,10 cooling in combinations of dc and ac fields,11 orsaturating the F in large negative fields and then measuringthe remaining loop with different waiting times.12,13 Otherirreversible approaches such as high-temperature annealing14or ion irradiation15 have also been used. Nogués et al.,16reported that EB can be tuned at room temperature by sub-jecting the system to pulsed fields as high as 55.6 T. Allthese attempts to tune EB externally involve tedious experi-mental treatments or impractically large magnetic fields noteasily available, which make them unsuitable for practicalapplications.In this letter, we present an epitaxially grown Fe10 nm/Cr2O3 2.7 nm/Fe 10 nm trilayer structure and dem-onstrate that EB can be tuned at room temperature by sub-jecting the system to moderate dc magnetic fields, −10Hdc1 T. The motivation of using Cr2O3 as the AFlayer is based on our recent observation of an appreciablemoment in single Cr2O3 films.17 There the direction of theinduced moment could be controlled by an applied dc field.Coupling of a tunable pinning moment to an adjacent F filmis anticipated to provide tunable EB.Moreover, EB above TN of the AF has recently beenobserved in other F-AF systems such as Co/NiF2 Ref. 18,Fe3O4/CoO Ref. 19, and Ni80Fe20/Co3O4 Ref. 20. Thereit is primarily attributed to either strain or proximity effectsat the F/AF interfaces. In our trilayer, the EB is stable up tothe highest measured temperature T=395 K, which is farabove the bulk Néel temperature of Cr2O3, TN=307 K. Here,we explore the possibility of using dc fields to control the EBeffect. Unlike previous studies, our system does not requireany specific field-cooling procedure.The trilayer system, Fe 10 nm/Cr2O3 2.7 nm/Fe 10 nm,is prepared in a molecular beam epitaxy chamber at a basepressure of 110−10 mbar. The 0001-oriented c-Al2O3substrate is heated to and maintained at 573 K during thedeposition process. Note that no external magnetic field waspresent during the deposition. A stoichiometric Cr2O3 filmwas grown by thermal evaporation of Cr metal in an O2partial pressure of 2.210−6 mbar.18 Growth rates of Fe andCr2O3 were 0.37 and 0.28 nm/min.Analysis of the large-angle x-ray diffraction patternMax-B, Rigaku D in Fig. 1 illustrates that single-crystallineFe and Cr2O3 films are formed. The 110-oriented bcc Fepeak shows pronounced Laue oscillations originating fromcoherently scattering Fe 110 lattice planes. The distributionof scattered intensity around the central maximum at 2=44.67° is fitted by the one-dimensional Laue diffractionfunctionaElectronic mail: cbinek2@unl.eduFIG. 1. Color online X-ray diffraction pattern of Fe 10 nm/Cr2O32.7 nm/Fe 10 nm trilayer. Solid lines are the fit to the Laue function accord-ing to Eq. 1. The inset shows small angle x-ray reflection data circles andthe best fit line by using LEPTOS-2 program.APPLIED PHYSICS LETTERS 91, 172506 20070003-6951/2007/9117/172506/3/$23.00 © 2007 American Institute of Physics91, 172506-1Downloaded 23 Oct 2007 to 129.93.36.58. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspL = L0sin2NKd/2sin2Kd/2, 1where N is the number of coherently scattering planes, d isthe spacing between the Fe 110 planes, K=2k0 sin 2 with being the scattering angle, k0=2 / the magnitude ofwave vector and Cu K=0.154 nm the wavelength of thex-ray source. Best fit of the experimental data to Eq. 1 withfixed =0.154 nm yields N=79±0.02 and d=0.204±0.001 nm. The value of d is in close agreement withthe bulk d110=0.203 nm. This infers that the Fe film thick-ness contributing to the coherent scattering is Nd=16 nm.The high texture quality of the films was furthermore cor-roborated by small values 	0.04° of the full width at halfmaximum observed in the rocking curve scan performedaround Fe 110 peak. Analysis of the Cr2O3 film peak re-veals 112¯0-textured growth. The inset of Fig. 1 shows thesmall angle x-ray reflectivity SAXR diagram AXS-D8Discover, Bruker of the trilayer structure. Fit of these databy using LEPTOS-2 software indicates a Fe 10±0.4 nm/Cr2O32.72±0.18 nm/Fe 10.8±0.3 nm trilayer structure, which isin excellent agreement with the desired system.The magnetic measurements were carried out using thesuperconducting quantum interference device SQUID mag-netometer MPMS-XL, Quantum Design with fields appliedin the sample plane. Figure 2a shows hysteresis loops mea-sured at T=300 K after subjecting the system to zero or fi-nite dc fields. The exchange biased hysteresis loops haveremanent to saturation magnetization ratio Mr /Ms=1. Theyalso exhibit asymmetry with respect to the coercive fields inthe respective up and down branches. It may originate fromthe small difference in 0Hc values of the top and bottom Felayers resembling the scenario of a thick-thin layer switchingprocess.21Remarkably, Fig. 2a shows that nonzero EB is presentfor zero 0Hdc. This spontaneous EB is observed in the vir-gin sample after preparation at 573 K without any magneticfield treatment which could have magnetized the Fe films.The hysteresis loop shift EB always means that the mag-netic structure of the system has a reversible component thatlifts the degeneracy of the state magnetized in the oppositedirection. In our case, EB free from any training effect per-sists well above bulk TN of Cr2O3, and therefore, barringlarge proximity effects, it appears unlikely that the EB origi-nates from exchange coupling with the antiferromagnet. Itmay be explained by the presence of magnetically hard re-gions that are only reversed by strong dc fields. Such regionsmay appear, for example, due to the formation of chemicallymodified interface regions such as Fe3O4 with high Curietemperatures. The presence of magnetically hard regionsreaching even into the F film is consistent with recent neu-tron reflectometry studies.22We find that the sign of 0Heb can be controlled exclu-sively by 0Hdc in our trilayer. This is reminiscent of thescenario of changing the sign of 0Heb depending on thestrength of the cooling fields.9 Note, however, that our EBtuning takes place isothermally where the pinning magneti-zation is reversed by a magnetic field only without breakingand reestablishing AF long range order during a field coolingprocess. Competition between AF exchange coupling of thepinning and the Fe magnetizations with the Zeeman interac-tion energy of the pinning system determines the sign of0Heb. For 0Hdc0.2 T, AF coupling remains dominantover the Zeeman energy, resulting in regular negative EB.For larger 0Hdc, the Zeeman interaction overcomes the AFexchange coupling which aligns the pinning moments paral-lel to larger 0Hdc, leading to positive EB. Note that recentlyCheon et al. pointed out that EB fields can be reversed in allEB systems below the blocking temperature via reversal ofthe uncompensated pinning magnetization.23 However, theisothermal field tuning described here takes place at easilyaccessible magnetic fields of less than 1 T, and does notdepend on long range AF order.Figure 2b shows 0Heb=0H++H− /2 and 0Hc=0H+−H− /2 vs 0Hdc, where 0H+ and 0H− are thefield values at which the magnetization becomes zero onright and left branch of the hysteresis loop, respectively.While 0Hc remains unchanged to within 1% of its meanvalue of 2 mT, 0Heb varies in the range −0.90Heb1.2 mT depending on the dc field. Note that identical EBfields have been determined from alternating gradient forcemagnetometry using electromagnets to generate the dc fields.Hence, trapped flux in the superconducting coils of theSQUID magnetometer is unambiguously excluded. It is alsoimportant to mention that the results are independent of theduration the system is subjected to the dc field. Indeed, wemeasured the same hysteresis loops with exposure times of10, 100, and 300 s. The results presented in this letter corre-spond to exposure duration of 100 s.Training of the exchange bias effect offers a unique toolto test the deviation from the equilibrium of the AF layer.24To this end, we measured the training effect after tuning theexchange bias by various dc fields. Shown in Fig. 3a arethe results at 300 K with 0Hdc=1 and −1 T, respectively.Note that these loop shifts resemble positive EB. Clearly, thehysteresis loops remain unchanged implying the absence ofany aging phenomena and, hence, indicating that this un-usual high temperature exchange bias phenomenon origi-nates from a stable interface moment. The latter is indepen-dent of the AF long range order which establishes at lowertemperatures.FIG. 2. Color online a Representative hysteresis loops at T=300 K aftersubjecting the trilayer to various dc fields. The inset shows a loop at T=395 K and 0Hdc=0. b 0Heb and 0Hc vs 0Hdc. Lines are guide to theeye.172506-2 Sahoo et al. Appl. Phys. Lett. 91, 172506 2007Downloaded 23 Oct 2007 to 129.93.36.58. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspIn contrast to the results at 300 K, EB measured at 10 Kreveals huge training effects associated with the nonequilib-rium AF configuration of the Cr2O3 film below T*50 K,which leaves its fingerprints in the M /0H vs T curve insetof Fig. 4a measured at a field of 10 mT. Figure 4a pre-sents selected hysteresis loops at 10 K within a training se-quence after field cooling from 395 K in a field of 50 mT.The gradual decrease of 0Heb open squares and 0Hccircles with the subsequent loop number n plotted in Fig.4b shows the usual training behavior, indicating AF longrange order and its spin configurational relaxation. This ag-ing process continues until the AF reaches its quasi-equilibrium state in the limit n→. The solid squares repre-sent the best fit of 0Heb vs n data to a recently developedphenomenological theory,240Hebn + 1 − Hebn = − 0Hebn − Hebe 3, 2where =210−4 mT−2 and 0Hebe=0.4 mT were obtainedas the two fitting parameters.  contains the interface ex-change coupling and the damping constant governing therelaxation dynamics of the AF spin configuration while0Hebe is the quasiequilibrium exchange bias field.24In conclusion, we found that exchange bias resultingfrom pinned interface magnetization in an epitaxialFe/Cr2O3/Fe trilayer is isothermally tunable by moderate dcmagnetic fields at room temperature. This behavior is distinctfrom regular exchange bias in the sense that it does not relyon field-cooling treatments and shows no training effect. Itindicates that the pinning magnetization is independent ofthe long range AF order in the Cr2O3 spacer. Regular ex-change bias with pronounced training behavior is, however,recovered below 50 K. 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Wolf, J. M.Gaines, R. W. Erwin, and M. A. Verheijen, Phys. Rev. Lett. 84, 61022000.20J. van Lierop, K.-W. Lin, J.-Y. Guo, H. Ouyang, and B. W. Southern, Phys.Rev. B 75, 134409 2007.21T. Ambrose and C. L. Chien, Appl. Phys. Lett. 65, 1967 1994.22M. R. Fitzsimmons, B. J. Kirby, S. Roy, Z.-P. Li, I. V. Roshchin, S. K.Sinha, and I. K. Schuller, Phys. Rev. B 75, 214412 2007.23M. Cheon, Z. Liu, and D. Lederman, Appl. Phys. Lett. 90, 012511 2007.24Ch. Binek, Phys. Rev. B 70, 014421 2004.FIG. 3. Color online a Room temperature training cycles after subjectingthe trilayer to 0Hdc=50 mT indicating the absence of any training effect.b Two consecutive loops measured after subjecting the trilayer to 0Hdc=1 T cirlces and −1 T squares, respectively.FIG. 4. Color online a Regular training effect at T=10 K after fieldcooled from T=395 K in 0H=50 mT. The inset shows M /0H vs T mea-sured at 0H=10 mT. b 0Heb open squares and 0Hc circles vs loopnumber, n. The solid symbols are the best fit of Eq. 2 to the training data.Lines are guide to the eye.172506-3 Sahoo et al. Appl. Phys. Lett. 91, 172506 2007Downloaded 23 Oct 2007 to 129.93.36.58. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Isothermal low-field tuning of exchange bias in epitaxial Fe/Cr2O3/Fe

S. Sahoo

T. Mukherjee

K. D. Belashchenko

Ch. Binek

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Isothermal low-field tuning of exchange bias in epitaxial Fe∕Cr2O3∕Fe

https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1034&amp;context=physicsbinek

Isothermal low-field tuning of exchange bias in epitaxial Fe/Cr2O3/Fe

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