Exchange bias is one of the most extensively studied phenomena in magnetism, since it exerts a unidirectional anisotropy to a ferromagnet (FM) when coupled to an antiferromagnet (AFM) and the control of the exchange bias is therefore very important for technological applications, such as magnetic random access memory and giant magnetoresistance sensors. In this letter, we report the crystal structure manipulation of the exchange bias in epitaxial hcp Cr2O3 films. By epitaxially growing twined oriented Cr2O3 thin films, of which the c axis and spins of the Cr atoms lie in the film plane, we demonstrate that the exchange bias between Cr2O3 and an adjacent permalloy layer is tuned to in-plane from out-of-plane that has been observed in oriented Cr2O3 films. This is owing to the collinear exchange coupling between the spins of the Cr atoms and the adjacent FM layer. Such a highly anisotropic exchange bias phenomenon is not possible in polycrystalline films.
Exchange bias refers to the shift of the magnetic hysteresis loop of a ferromagnetic (FM) layer away from the zero magnetic field, resulting from the exchange interaction from an antiferromagnet (AFM) layer1,2,3,4. In practical applications such as magnetic random access memory and giant magnetoresistance sensors, etc.5,6, the exchange bias is used to “pin” the FM magnetization from switching in small magnetic fields so that the FM layer could serve as a fixed reference layer. In previous studies, it has been established that the exchange bias hinges on the spin orientations of the surface magnetic atoms in the AFM layer7,8,9,10,11,12,13. Since the direction of the spin orientations of the surface magnetic atoms is associated with the crystal structure of the AFM layer, the highly anisotropic exchange bias could be simply manipulated by the crystal orientation design. An ideal candidate AFM material to achieve this objective is the single crystalline Cr2O3 films with a hexagonal close packed (hcp) structure, of which the spin orientations of the Cr atoms are uniquely directed along the c axis and could dictate the direction of the exchange bias7,14,15.
In this letter, we report the manipulation of the exchange bias by controlling the surface spin configuration via crystal orientation design. By epitaxially growing oriented Cr2O3 films on (001) oriented rutile TiO2 substrates, we force the c axis to lie in the film plane, which results in a strong in-plane exchange bias between the Cr2O3 film and an adjacent permalloy (Py) layer, whilst the perpendicular exchange bias is completely suppressed. The perpendicular exchange bias was previously shown in (0001) oriented Cr2O3 films1,2,3,4,7,15,16. Our results along with the previous studies demonstrate crystal structure manipulations of the exchange bias based on the collinear exchange coupling between the spins of the Cr atoms and the adjacent FM layer.
Results and Discussion
The oriented Cr2O3 films are grown on the oriented rutile TiO2 substrates via laser molecular beam epitaxy (LMBE) with a base pressure of 2 × 10−8 mbar (see methods for details). Fig. 1a,b show the in-situ reflection high-energy electron diffraction (RHEED) characterization of the oriented TiO2 substrate’s surface viewed from the and directions. After the initial growth of 3 nm Cr2O3, the RHEED pattern of the TiO2 substrate disappears and that of Cr2O3 starts to appear, as shown in Fig. 1c,d. As the thickness of Cr2O3 film increases, its RHEED pattern becomes brighter. Fig. 1e–h show the RHEED patterns of 10 nm and 27 nm Cr2O3 films, respectively. To be noted, four satellite RHEED spots are observed around each main diffraction spot viewed from the direction of the TiO2 substrates (Fig. 1e,g).
The crystalline structural properties of these Cr2O3 films are further characterized by x-ray diffraction (XRD). The θ–2θ scans of the rutile TiO2 substrate, 10 nm, 20 nm, and 27 nm Cr2O3 films are shown in Fig. 2a. The peak at 2θ of ~63 degrees corresponds to the peak of the TiO2 substrates. For the 10 nm Cr2O3 film, a peak at 2θ of ~65 degrees is observed, which corresponds to the peak of the Cr2O3 crystal. As the thickness of Cr2O3 increases, the intensity of the peak at ~65 degrees becomes stronger. We note that for the 27 nm Cr2O3 film, only peak is detectable for the whole scan range (see supplementary information; Fig. S1), indicating good crystalline properties of the oriented Cr2O3 thin films. The surface morphology is characterized by atomic force microscopy (AFM). The root-mean-square (RMS) roughness is 0.09 nm for the rutile TiO2 substrate after annealing in the chamber (Fig. 2b). After the growth of 13 nm Cr2O3 film, the RMS roughness increases to 0.28 nm (Fig. 2c), indicating that the surface of the epitaxial Cr2O3 films is quite smooth.
The epitaxial growth of the oriented Cr2O3 films on TiO2 is quite interesting, given the fact that Cr2O3 and TiO2 belong to totally different space groups. Cr2O3 has a hexagonal crystal structure, which belongs to the group17, while rutile TiO2 has a cubic structure, which belongs to the P42 group18. However, the c lattice constant of Cr2O3 is 13.599 Å, and the a lattice constant of TiO2 is 4.584 Å, which results in a coincidental anion alignment with a lattice mismatch of only ~1.1% . Hence, the Cr2O3 films could be grown with the c axis lying in-plane and parallel to a or b axis of the TiO2 substrates (Fig. 3a). As the TiO2 crystal’s ab plane has four-fold symmetry, which could result in four-fold in-plane rotational symmetry of the crystalline structure of the Cr2O3 thin films. To investigate this, HRTEM is used to characterize the interfacial structure properties between Cr2O3 and TiO2 viewed from the direction of the TiO2 substrate. As shown in Fig. 3b, a sharp interface with TiO2 is observed, as indicated by the pink dashed line. For Cr2O3, two distinct zones of the crystalline structures are observed, one of which is denoted as zone , and the other one is denoted as zone . The boundary of these two zones is marked by a yellow dashed line. For zone , the c axis of Cr2O3 is parallel to the direction of the TiO2 substrate, and the six-fold symmetry pattern of the basal plane can be identified. For zone , the c axis is parallel to the direction of the TiO2 substrate. The crystal orientation of Cr2O3 films has been shown to be highly associated to the substrate crystalline structures. For example, preferential oriented growth of Cr2O3 is achieved on oriented Al2O3, Co, and oriented Cu and oriented Cr2O3 is grown on Fe films15,19,20. The growth mode of Cr2O3 film in our study is similar to that observed in an earlier report for the oriented Fe2O3 films grown on rutile .TiO2 substrates21.
Interestingly, with the c axis lying in the film plane, the spin orientations of the Cr atoms also lie in-plane in these oriented Cr2O3 film, as schematically shown in Fig. 3a. This is very different from previously reported (0001) oriented Cr2O3 films grown on Al2O3 substrates, of which both the spin orientations and the exchanges bias are perpendicular to the films7,15,16. To study the exchange bias of the epitaxial Cr2O3 films (see Methods for details), we deposit 10 nm Py on top of the oriented Cr2O3 films and measure the magnetic hysteresis loops by Magnetic Properties Measurement System (MPMS; Quantum Design) with both in-plane and out-of-plane magnetic fields at various temperatures (schematic drawings shown in Fig. 4a,b). The in-plane magnetic hysteresis loop are first measured. Prior to the measurement, the sample is cooled from 400 to 10 K in an in-plane magnetic field of 1000 Oe along the  direction of the TiO2 substrate, which is much smaller than the spin-flop field of several Tesla for Cr2O3 reported previously22. By cooling through the blocking temperature, the magnetization direction of the Py sets the surface spin configurations of the Cr2O3 films. Then, we measure the magnetization of the Py as a function of the in-plane magnetic field along the  direction of the TiO2 substrate (Fig. 4a) from 10 to 300 K. After subtracting a linear background which is mainly due to the diamagnetic response of the rutile TiO2 substrate, the shifted magnetic hysteresis loops of Py are displayed in Fig. 4c. At 10 K, as the magnetic field ramps from negative to positive, a sharp jump in magnetic moment occurs at ~110 Oe, but the jump occurs at ~ −400 Oe on the return sweep. These two switching fields are labeled as H1 and H2, respectively, as indicated in the top panel of the Fig. 4c. The exchange bias field (HB) is defined by the mean value of the H1 and H2, i.e. . As the temperature increases, the exchange bias field steadily decreases from the low temperature value.
To characterize the anisotropy of the exchange bias effect, magnetic hysteresis loops are measured with a magnetic field perpendicular to the films (Fig. 4b). The same field cooling procedure as with the in-plane magnetic fields is adopted. The out-of-plane magnetization curves measured at 10, 20, 40 and 60 K are shown in Fig. 4d. The symmetric magnetization hysteresis loops indicate a negligible perpendicular exchange, which is in stark contrast to the out-of-plane exchange bias observed in the oriented Cr2O3 films. The highly anisotropic exchange bias phenomenon can be attributed to the crystalline orientation difference of the Cr2O3 films. In hcp structures, the c-axis direction dictates the spin orientations of the magnetic atoms. In the oriented Cr2O3 films, the spin orientations of the Cr atoms are perpendicular to the films, whereas in the oriented Cr2O3 films, the spins of the Cr atoms lie in the film plane. These results further indicate the direct collinear exchange coupling between the spins of the Cr atoms and the adjacent Py layer.
Fig. 5a,b summarize the in-plane exchange bias field and in-plane conceive field (HC), where , for the sample consisting of the 13 nm Cr2O3 films and 10 nm Py as a function of the temperature. As the temperature increases from 10 to 60 K, the in-plane exchange bias field (Fig. 5a, Blue dots) decreases quickly from ~ −150 Oe to almost 0 Oe, where the sign depends on the magnetic field direction during magnetic cooling. No exchange bias is observable at and above 60 K, which implies a blocking temperature (TB) of ~60 K. An abrupt increase in Hc below 60 K is another property of exchange biased Py, which is due to the formation of the AFM order in this 13 nm Cr2O3 thin film2. As there are two crystalline zones of oriented Cr2O3, as indicated in zones and , we also measure the exchange bias in the direction along the TiO2 direction. Almost identical exchange biases are observed at each temperature (Fig. 5a, Green dots).
The measured TB of 13 nm Cr2O3 is ~60 K, which is much lower compared to the value reported on (0001) oriented bulk Cr2O3 single crystals15. In antiferromagnetic films, it has been known that TB is highly related to Neel temperature (TN), and is usually slightly lower than the TN. Both TB and TN increase as the AFM thickness increases due to finite-size effects2,12,23,24. To obtain the TB as a function of the thicknesses of the Cr2O3 thin films, the in-plane exchange bias for the samples consisting of 7, 10, 20 and 27 nm Cr2O3 films and 10 nm Py bilayer films are also measured. Fig. 6a,b show the exchange bias as a function of temperature for the bilayer structures consisting of Cr2O3 (7 nm)/Py (10 nm) and Cr2O3 (27 nm)/Py (10 nm), respectively. The blocking temperatures of these two structures are determined to be 40 K and 100 K. The blocking temperature increases as the thickness of the Cr2O3 films increases, as shown in Fig. 6c. For the 27 nm Cr2O3 film, the blocking temperature is only ~100 K, which is far below the TB of bulk Cr2O3. One possible reason is the non-trivial finite size effects arising from the grain boundaries or oxygen defects in the Cr2O325.
In summary, we have demonstrated the manipulation of the exchange bias of the Cr2O3 thin films by controlling the surface spin orientations of the Cr atoms via crystal orientation design. For the epitaxial growth of oriented Cr2O3 films, the spin configurations of Cr atoms give rise to only in-plane exchange bias at the interface between Py and the Cr2O3 thin films, while no perpendicular exchange bias is observed. Our results along with previous studies on (0001) oriented Cr2O3 films indicate the collinear exchange coupling between the spins of the Cr atoms and the adjacent FM layer.
Cr2O3 films growth
The oriented Cr2O3 films are grown on the (001) oriented rutile TiO2 substrates via laser molecular beam epitaxy (LMBE) with a base pressure of 2 × 10−8 mbar. Prior to the Cr2O3 growth, the substrate temperature is increased to 350 °C with a rate of 20 °C/min in the chamber with an oxygen partial pressure of 0.08 mbar. Then the Cr2O3 film is deposited from a Cr2O3 target with a laser power of (8.0 ± 0.2) mJ and a frequency of 2.0 Hz. The thickness of the Cr2O3 thin film (t) is determined from the cross section high resolution transmission electron microscopy.
Exchange bias measurement
A 10 nm Py is grown on top of the oriented Cr2O3 films by RF magnetron sputtering with a growth rate of 0.02 Å/s. A capping layer of 20 nm aluminum is deposited prior to taking the samples out of this sputtering chamber to prevent oxidization of Py. Magnetic Properties Measurement System (MPMS; Quantum Design) is used to measure the magnetic hysteresis loops to determine the exchange bias.
How to cite this article: Yuan, W. et al. Crystal Structure Manipulation of the Exchange Bias in an Antiferromagnetic Film. Sci. Rep. 6, 28397; doi: 10.1038/srep28397 (2016).
We acknowledge the funding support of National Basic Research Programs of China (973 Grants 2013CB921903, 2015CB921104, and 2014CB920902) and the National Natural Science Foundation of China (NSFC Grant 11574006). Wei Han also acknowledges the support by the 1000 Talents Program for Young Scientists of China.
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Physical Review B (2017)