Optical imaging of antiferromagnetic domains in ultrathin CoO(001) films

Antiferromagnetic (AFM) domains in ultrathin CoO(001) films are imaged by a wide-field optical microscopy using magneto-optical birefringence effect. The magnetic origin of observed optical contrast is confirmed by the spin orientation manipulation through exchange coupling in Fe/CoO(001) bilayer. The finite size effect of ordering temperature for ultrathin single crystal CoO film is revealed by the thickness and temperature dependent measurement of birefringence contrast. The magneto-optical birefringence effect is found to strongly depend on the photon energy of incident light, and a surprising large polarization rotation angle up to 168.5 mdeg is obtained from a 4.6 nm CoO film with a blue light source, making it possible to further investigate the evolution of AFM domains in AFM ultrathin film under external field.


I. Introduction
Antiferromagnet is attracting considerable attention due to its potential in future spintronic applications [1][2][3][4][5][6][7]. Antiferromagnet has been applied as the pinning layer in spintronics devices for decades, and recently was considered to replace the ferromagnet as the active spin-dependent information element in the next generation of spintronic devices, due to the robustness against perturbation from external magnetic fields, the absence of stray fields, ultrafast dynamics, and considerable magneto-transport effects in antiferromagnetic (AFM) materials with wide variety [8]. Current-induced switching of AFM spins in both metallic [2][3][4] and insulating [5][6][7] AFM systems has been recently reported, making it possible to store information in AFM spintronics devices. Most experimental investigations on the current-induced switching of AFM domains were utilized by the anisotropic magnetoresistance, spin Hall magnetoresistance, and the related planar Hall resistance, but the electronic signals induced by the current pulse are not necessary correlated with the evolution of AFM domain states [9][10][11]. So, there is an urgent need to directly measure the AFM domain in real space during the operation of electric current for further understanding the mechanism of current-induced AFM domain switching.
To date, the most common technique to study the AFM domains is the photoemission electron microscopy (PEEM) based on X-ray magnetic linear dichroism effect (XMLD) [3,5,12] However, the XMLD-PEEM measurement requires the access to large synchrotron facilities, and is also difficult to be incorporated simultaneously with the external electric current and magnetic fields.
Recently, we reported that the magneto-optical birefringence effect can be applied to image the AFM domains in NiO thin films by a tabletop Kerr microscope [13], which is more accessible than XMLD-PEEM and can work under external fields. Thus, it would be fundamentally interesting to explore whether this imaging technique can be applied to the other AFM materials.
CoO is another important AFM material with a bulk Né el temperature ( ) of 293 K [14][15][16]. Although the AFM spins in bulk CoO align along the <117> directions, 3 the CoO films show different spin structures due to the strain effect from the substrate [17,18]. Single crystalline CoO film can be well epitaxied on MgO(001) substrates [17,19], with the spins aligning in the film plane due to the tensile stress in the CoO film [19]. The CoO AFM spins in Fe/CoO(001) bilayers has been measured by XMLD-PEEM, where the Fe FM spins and CoO AFM spins are perpendicularly coupled [20,21]. The AFM domain nucleation and propagation in Fe/CoO(001) under magnetic field has been interpreted indirectly through the measurement of ferromagnetic (FM) properties of Fe layer utilizing the magneto-optical Kerr effect [19]. Thus, to further understand the exchange coupling between CoO and FM layer, a direct imaging of CoO AFM domain in the presence of an external field can provide pivotal information. Recently, utilizing a femtosecond pump-probe method, Zhen et al. demonstrate the Né el vector dynamics in CoO film due to large magneto-optical Voigt effect [22]. The optical birefringence effect has been applied to image the AFM domains in bulk CoO crystal by light transmissions [15,17,23], thus it is also feasible that the AFM domain structure of ultrathin CoO film can be studied by the optical birefringence effect in the reflection geometry.
In this work, we report the studies on the AFM domains in single crystal CoO thin films grown on MgO(001) substrates with the magneto-optical birefringence effect. By manipulating the AFM Né el vector through the exchange coupling in Fe/CoO bilayer, we confirm the AFM order based origin of the observed optical contrast. Through the systematical studies on the temperature and thickness dependence of the magnetic contrast, we prove that the AFM domains can be observed down to 1.5 nm, and the thickness dependent of CoO film follows the finite size effect. Finally, we discover that the magneto-optical birefringence effect has strong dependence on the wavelength of the incident light, and the large polarization rotation angle up to 168.5 mdeg is quantified for a 4.6 nm CoO film. Our studies demonstrate that the magneto-optical birefringence effect is an effective tool to study the spin properties in AFM ultrathin films under external field, which could be helpful for the development of AFM spintronics. 4
The CoO AFM domains are imaged with a commercial Evico magneto-optic Kerr microscope [25,26]. A white-color light-emitting diode (LED) with a wide wavelength range between 420 nm and 650 nm is mostly used as the light source in this study. During the measurements, the sample temperature can be varied between 77 K and 330 K. The magnetic field up to 1000 Oe is applied by a rotatable electromagnet. Our Kerr microscope is also equipped with a red LED source with the wavelength of ~650 nm and a blue LED source with the wavelength of ~455 nm, thus we also measured the optical birefringence contrasts with different LED sources. The contrast due to the optical birefringence effect can be better identified by calculating the asymmetry, i.e. , as shown in Fig.1(d), and clear black and white domains can be observed [13]. The contrast can be attributed to the AFM domains with the AFM magnetization aligned along <110> and <1 0> axis due to the in-plane four-fold symmetry of CoO(001).

III. Results and discussion:
Next, we further confirm that the measured optical contrast of CoO films in Fig. 1 originates from the AFM domains. One common way is to directly compare the optical image with the XMLD-PEEM image [13], which requires the XMLD-PEEM beamtime in the synchrotron facilities. It is well known that CoO has the G-type AFM spin structure with a compensated (001) surface, and in an Fe/CoO(001) system the Fe FM spins and CoO AFM spins are perpendicularly coupled [19], thus the CoO spin orientation in Fe/CoO(001) can be manipulated by field cooling. Next, we prepare a 5 nm thick CoO film, and half of it is covered with a 2 nm Fe film for comparison, see 6 sample structure in Fig. 2(a). We measure the optical birefringence effect near the boundary between pure CoO film and Fe/CoO bilayer after the field cooling. The field cooling is performed from 330 K down to 77 K within a field of 1000 Oe along different directions, then the birefringence images are measured at zero field, as shown in Figs. 2(b)-(d). For H FC || [110] in Fig. 2(b), there is only uniform dark contrast in the Fe/CoO region, but the multi-domains can be observed in the pure CoO region. For H FC || [ 10] in Fig. 2( We further quantify the thickness dependence of the contrast at 77 K and 290 K, as shown in Fig. 4(a). The domain contrast is almost linearly dependent on the CoO thickness. The optical birefringence effect in NiO/MgO(001) [13] also has the similar linear thickness dependence. We found that the contrast of the 10 nm CoO film is about 2% at 290 K, and reaches about 9.3% at 77 K, but the birefringence contrast of a 20 nm NiO film is only ~2.5% at room temperature [13], so the CoO film contains much stronger optical birefringence effect than the NiO film grown on MgO(001).
Such difference may be attributed to the different spin orientation in NiO and CoO film, since the CoO AFM spins lie in the film plane [19,29], and the NiO AFM spins align along the canting directions with an angle of ~10 away from the normal direction [13]. Fig. 4 We further determine as a function of CoO film thickness, shown in Fig. 4(c).
The thickness dependent is expected to follow the finite-size scaling relation as [18,33,34] .
Here, is the Né el temperature in the bulk, is the of the film with a finite thickness d, is the extrapolated spin-spin correlation length at zero 8 temperature, and is the shift exponent for the finite-size scaling [18,33]. Our data can be well fitted with Eq. (1), resulting in the fitting parameters of , and . Fig. 4(d) shows the log-log plot of as a function of CoO thickness, and further proves the power law dependence of in Eq. (1). The finite size effect of the CoO ordering temperature has been proved by the specific heat measurements in polycrystalline CoO films [34], magnetic susceptibility in CoO/SiO 2 superlattices [18], inverse spin Hall effect in YIG/CoO/Pt heterostructures [35], and the exchange coupling in Fe/CoO bilayers [19] . Our results prove that the magneto-optical birefringence effect can be used to identify the AFM finite size effect from a wedge film, which will potentially benefit further study on the AFM thin films. We note that the fitted for CoO in our study is higher than the reported values ranging between 293 K and 315 K in polycrystalline CoO thin film [34][35][36] or in CoO/oxide superlattices [18,33], which is likely induced by the epitaxial strain from the MgO substrate, since theoretical calculations indicated that the lattice strain can enhance the of AFM Cr 2 O 3 [37,38].
We also find that the optical birefringence effect of CoO film strongly depends on the wavelength of incident light. Figures 5(a)-(c) show the same AFM domains of a 4.6 nm thick CoO measured by the three LED sources with different colors. We found that the domain contrast is the strongest for the blue LED, but the weakest for the red LED. The polarization rotation angle can be quantified by measuring the asymmetry as a function of the analyzer's offset angle due to the relation of [13].  [13]. It should be noted that of CoO film is much larger than the 9 longitudinal Kerr angles from Fe [39] or Co [40] thick films, which are usually less than 21 mdeg.  [2][3][4][5][6][7] or AFM domain wall motion [42,43] induced by current or field pulses, which is helpful for future developments of AFM spintronics devices.