Complex Study of Magnetization Reversal Mechanisms of FeNi/FeMn Bilayers Depending on Growth Conditions

Magnetization reversal processes in the NiFe/FeMn exchange biased structures with various antiferromagnetic layer thicknesses (0–50 nm) and glass substrate temperatures (17–600 °C) during deposition were investigated in detail. Magnetic measurements were performed in the temperature range from 80 K up to 300 K. Hysteresis loop asymmetry was found at temperatures lower than 150 K for the samples with an antiferromagnetic layer thickness of more than 10 nm. The average grain size of FeMn was found to increase with the AFM layer increase, and to decrease with the substrate temperature increase. Hysteresis loop asymmetry was explained in terms of the exchange spring model in the antiferromagnetic layer.


Introduction
For thin film materials with exchange-coupled ferro-(FM) and antiferromagnetic (AFM) layers, below the Neel temperature and at the induced uniaxial anisotropy, magnetic hysteresis loops are shifted. This phenomenon is called the exchange bias and is widely used in modern spintronics, magnetic recording, and magnetic sensorics, for example, in giant magnetoresistance (GMR)-based or giant magnetoimpedance (GMI)-based elements [1][2][3]. Depending on the application, the most important property of an exchange bias system can be either the effect value [4], or layer thickness [2,5], or hysteresis loop shape [5][6][7].
Experimental studies show that the magnetic properties of an exchange bias system are in the direct dependence on layer thickness, roughness, grain size, and deposition sequence [8]. The above characteristics of thin films prepared by magnetron sputtering can be varied by changing the pressure or substrate temperature in the chamber during deposition [9,10].
Ni 80 Fe 20 alloy (Permalloy) is one of the most useful materials for exchange bias systems due to its high initial and maximum magnetic permeability, as well as corrosion resistance, when compared with other materials. For using it in GMR sensors, an optimal

Materials and Methods
The samples of Ta/NiFe/FeMn structures were prepared on Corning glass substrates using an ATC ORION-8 (AJA INTERNATIONAL) magnetron sputtering system in RF and DC mode. The pressure of the Ar atmosphere was 1.6 mTorr. The buffer Ta layer of 5 nm was preliminarily deposited on the substrate for better quality of other layer growth [3]. The compositions of ferro-and antiferromagnetic layers were Ni 81 Fe 19 and Fe 50 Mn 50 , respectively. During the deposition, an in-plane magnetic field of 250 Oe was used to create uniaxial magnetic anisotropy in the samples. The thickness of the films was measured with a Dektak 150 stylus profilometer and the sputtering rate of NiFe, FeMn, and Ta was 1.2, 0.9, and 0.3 Å/s.

Results
The Selected Area Electron Diffraction (SAED) and TE study the influence of the substrate temperature on the film s the growth of the AFM layer at the substrate temperature o matted with the FeMn phase. At 600 °С (Figure 2b,c), there i FeMn one: FeMn3, FeMn4, pure Fe phase, and Fe3O4 oxide ph strate temperatures higher than 100 °С, the migration of atom the formation of these phases. This can happen because Mn relatively good diffusion mobility at a thermal treatment [25,2 having an increased oxidation ability [27]. Formation of the ad the amount of the antiferromagnetic FeMn phase that can l crease as the temperature increases [18]. From the dark-field be observed that an average grain size at the substrate temper Structural properties of the samples were carried out by transmission electron microscopy (TEM) using a Hitachi HT7700 at 100 kV accelerating voltage. Planar-view samples were prepared using a Hitachi FB 2100 (FIB) single-beam focused ion beam system. Powder X-ray diffraction (XRD) analysis was performed using the Phillips X'Pert Pro MPD with CuKα1 and CuKα2 radiation. The measurements were carried out in a 2θ range of 10-120 • (0.02 • step, 12 s per step) with 40 kV and 40 mA tube settings. Magnetic properties of all samples were investigated in the temperature range 80-300 K using the vibrating sample magnetometer (VSM) by Lake Shore 7400. In-plane hysteresis loops were obtained along the easy and hard magnetization axis.

Results
The Selected Area Electron Diffraction (SAED) and TEM techniques were used to study the influence of the substrate temperature on the film structure. It was revealed that the growth of the AFM layer at the substrate temperature of 100 • C (Figure 2a,b) is formatted with the FeMn phase. At 600 • C (Figure 2b,c), there is a set of phases beyond the FeMn one: FeMn 3 , FeMn 4 , pure Fe phase, and Fe 3 O 4 oxide phase. Apparently, at the substrate temperatures higher than 100 • C, the migration of atoms during deposition leads to the formation of these phases. This can happen because Mn atoms are known to have a relatively good diffusion mobility at a thermal treatment [25,26], as well as the FeMn layer having an increased oxidation ability [27]. Formation of the additional phases may reduce the amount of the antiferromagnetic FeMn phase that can lead to an exchange bias decrease as the temperature increases [18]. From the dark-field images (Figure 2b,d), it can be observed that an average grain size at the substrate temperature of 100 • C is larger than at 600 • C. XRD analysis was used to find out whether FeNi and FeMn layer grain size changes with the increasing substrate temperature. Figure 3 shows diffractograms of the films deposited at substrate temperatures of 17, 100, 200, 400, 500, and 600 °С. The XRD patterns show peaks corresponding to the FeMn and NiFe layers to have the fcc structure with (111) orientation that is usually observed in such structures [28][29][30][31]. Moreover, one can see that the peak intensity of the FeMn layer decreases with the increasing substrate temperature, while the width of the FeMn peaks increases with the increasing TSUB. The average grain size d of the layers was calculated using the Scherrer equation d = kλ/ Lcosθ [32], where k (0.89) is the Scherrer constant, λ is the X-ray wavelength, L is the relative value of the full width at half maximum (FWHM) of a peak, and θ is the Bragg angle. XRD analysis was used to find out whether FeNi and FeMn layer grain size changes with the increasing substrate temperature. Figure 3 shows diffractograms of the films deposited at substrate temperatures of 17, 100, 200, 400, 500, and 600 • C. The XRD patterns show peaks corresponding to the FeMn and NiFe layers to have the fcc structure with (111) orientation that is usually observed in such structures [28][29][30][31]. Moreover, one can see that the peak intensity of the FeMn layer decreases with the increasing substrate temperature, while the width of the FeMn peaks increases with the increasing T SUB . The average grain size d of the layers was calculated using the Scherrer equation d = kλ/ Lcosθ [32], where k (0.89) is the Scherrer constant, λ is the X-ray wavelength, L is the relative value of the full width at half maximum (FWHM) of a peak, and θ is the Bragg angle.  The grain size d (FeMn) of FeMn (111) phase as a function of the substrate temperature during deposition is presented in Table 1. For the FeMn layer, the average grain size is 12 ± 1 nm at the substrate temperatures of 17 °С and 100 °С. The average grain size of the FeMn layer decreased down to 7 ± 1 nm as the substrate temperature is increased from 200 °С to 500 °С. There is no visible peak of the FeMn phase in the XRD pattern at the substrate temperature of 600 °С. This means probably that the FeMn layer has too small average grain size. The observed tendency can be due to the found changes of the AFM layer with the increasing temperature, described above. Namely, it is possible that the occurrence of the Fe, FeMn3, and FeMn4 phases discourages an increase in the FeMn grain size.
For the NiFe layer, the average grain size is 21 ± 1 nm and does not show any changes as a function of substrate temperature. The obtained values of grain size for both layers are consistent with previously obtained ones for similar structures [29,31].
Thus, based on the above diffraction studies, we can suppose that with an increase of the substrate temperature, an amount of the pure FeMn phase decreases, as well as its grain size decreases that is accompanied with an increase in the pure Fe phase.
For the structure of Ta (5 nm)/NiFe (40 nm)/FeMn (20 nm) deposited at different substrate temperatures, hysteresis loops along the easy axis have a rectangular shape ( Figure  4), i.e., the magnetization reversal goes by a sharp domain wall propagation, while along the hard axis it goes by a gradual rotation of the magnetization. This behavior is typical for the NiFe/FeMn systems [33]. The coercivity along the hard axis is about zero for all samples except for the film deposited at the substrate temperature of 600 °С. With increas- The grain size d (FeMn) of FeMn (111) phase as a function of the substrate temperature during deposition is presented in Table 1. For the FeMn layer, the average grain size is 12 ± 1 nm at the substrate temperatures of 17 • C and 100 • C. The average grain size of the FeMn layer decreased down to 7 ± 1 nm as the substrate temperature is increased from 200 • C to 500 • C. There is no visible peak of the FeMn phase in the XRD pattern at the substrate temperature of 600 • C. This means probably that the FeMn layer has too small average grain size. The observed tendency can be due to the found changes of the AFM layer with the increasing temperature, described above. Namely, it is possible that the occurrence of the Fe, FeMn 3 , and FeMn 4 phases discourages an increase in the FeMn grain size.
For the NiFe layer, the average grain size is 21 ± 1 nm and does not show any changes as a function of substrate temperature. The obtained values of grain size for both layers are consistent with previously obtained ones for similar structures [29,31].
Thus, based on the above diffraction studies, we can suppose that with an increase of the substrate temperature, an amount of the pure FeMn phase decreases, as well as its grain size decreases that is accompanied with an increase in the pure Fe phase.
For the structure of Ta (5 nm)/NiFe (40 nm)/FeMn (20 nm) deposited at different substrate temperatures, hysteresis loops along the easy axis have a rectangular shape (Figure 4), i.e., the magnetization reversal goes by a sharp domain wall propagation, while along the hard axis it goes by a gradual rotation of the magnetization. This behavior is typical for the NiFe/FeMn systems [33]. The coercivity along the hard axis is about zero for all samples except for the film deposited at the substrate temperature of 600 • C. With increasing substrate temperature, the slope of magnetization curves along the hard axis decreases. Thus, the last two findings indicate the weakening of the uniaxial anisotropy in the samples with the increasing substrate temperature. ing substrate temperature, the slope of magnetization curves along the hard axis decreases. Thus, the last two findings indicate the weakening of the uniaxial anisotropy in the samples with the increasing substrate temperature. Dependences of the easy axis coercivity HC (TSUB) and the exchange bias HEX (TSUB) on the substrate temperature are shown in Figure 5. The maximal exchange bias of 23 Oe is achieved for the sample with the substrate kept at room temperature during deposition. With the increase in the substrate temperature, the exchange bias decreases inconsistently, meaning there is a large decline between TSUB of 100 and 200 °С, and a small decline at the TSUB range of 200-500 °С. We suppose it could be correlated with the observed change in the FeMn (111) phase grain size, indicated in Table 1, but this needs an additional study. At 600 °С, exchange bias almost vanishes. Thus, with the increasing substrate temperature the interfacial exchange energy weakens as it is in direct ratio with the exchange bias [8], wherein the increase in the substrate temperature does not change the magnetization reversal mechanism. In addition, coercivity of the samples does not change significantly as the substrate temperature increases, although the slight decrease up to 8 Oe is observed starting from 400 °С, which is related to a weakening of the AFM layer anisotropy. As the anisotropy weakens, AFM spins can follow FM spin at the interface during magnetization reversal, thus leading to the increase in the coercivity. Dependences of the easy axis coercivity H C (T SUB ) and the exchange bias H EX (T SUB ) on the substrate temperature are shown in Figure 5. The maximal exchange bias of 23 Oe is achieved for the sample with the substrate kept at room temperature during deposition. With the increase in the substrate temperature, the exchange bias decreases inconsistently, meaning there is a large decline between T SUB of 100 and 200 • C, and a small decline at the T SUB range of 200-500 • C. We suppose it could be correlated with the observed change in the FeMn (111) phase grain size, indicated in Table 1, but this needs an additional study. At 600 • C, exchange bias almost vanishes. Thus, with the increasing substrate temperature the interfacial exchange energy weakens as it is in direct ratio with the exchange bias [8], wherein the increase in the substrate temperature does not change the magnetization reversal mechanism. In addition, coercivity of the samples does not change significantly as the substrate temperature increases, although the slight decrease up to 8 Oe is observed starting from 400 • C, which is related to a weakening of the AFM layer anisotropy. As the anisotropy weakens, AFM spins can follow FM spin at the interface during magnetization reversal, thus leading to the increase in the coercivity. Thus, it was found that as the substrate temperature increases, the FeMn grain size decreases as well as the exchange bias. This can be explained in terms of the thermal stability model [34,35], according to which at small grain sizeses below the critical dCRIT, an AFM layer contains a large fraction of thermally unstable grains (Figure 6). At grain sizes Thus, it was found that as the substrate temperature increases, the FeMn grain size decreases as well as the exchange bias. This can be explained in terms of the thermal stability model [34,35], according to which at small grain sizeses below the critical d CRIT , an AFM layer contains a large fraction of thermally unstable grains (Figure 6). At grain sizes larger than some d SET , an AFM contains a portion of thermally stable grains but not set under applied magnetic field. Only at d in between these two critical values a larger fraction of AFM spins align with FM. In our case, at the given AFM layer thickness of 20 nm, the lower substrate temperature was during the film deposition, the smaller grain size was formed in the antiferromagnetic layer. This results in the formation of a larger fraction of thermally unstable grains in the FeMn layer, which leads to a decrease in the interfacial exchange coupling. A decrease in the exchange bias with decreasing FeMn grain size was also reported previously [36]. nm)/FeMn (20 nm) structures.
Thus, it was found that as the substrate temperature increases, th decreases as well as the exchange bias. This can be explained in terms bility model [34,35], according to which at small grain sizeses below t AFM layer contains a large fraction of thermally unstable grains (Figur larger than some dSET, an AFM contains a portion of thermally stable under applied magnetic field. Only at d in between these two critical v tion of AFM spins align with FM. In our case, at the given AFM layer t the lower substrate temperature was during the film deposition, the was formed in the antiferromagnetic layer. This results in the formation of thermally unstable grains in the FeMn layer, which leads to a decrea exchange coupling. A decrease in the exchange bias with decreasing Fe also reported previously [36].  NiFe (47 nm)/FeMn (t nm) structures were deposited at room temperature to study the effect of FeMn layer thickness on the sample properties. Figure 7 shows the diffractograms of the samples with 0, 5, 10, 20, 30, 40, and 50 nm thicknesses of the FeMn layer. Both layers, NiFe and FeMn, exhibit an fcc structure with (111) plane orientation.
NiFe (47 nm)/FeMn (t nm) structures were deposited at room temperature to s the effect of FeMn layer thickness on the sample properties. Figure 7 shows the diffr grams of the samples with 0, 5, 10, 20, 30, 40, and 50 nm thicknesses of the FeMn la Both layers, NiFe and FeMn, exhibit an fcc structure with (111) plane orientation. The average grain size of NiFe layer is 18 ± 1 nm for all studied samples of this s that correlates with the structural features of NiFe layer observed by V. Vas'kovskiy [3]. The estimated value of average grain size of the NiFe layer is less than in the prev series (NiFe (40 nm)/FeMn (20 nm)) due to the difference in layer thicknesses (47 n this series vs. 40 nm in the previous one). The grain size of the FeMn layer simultaneo increases with an increase in the film thickness, and the calculated values of the grain are comparable with those previously obtained [37][38][39]. The grain size d (FeMn) o FeMn phase as a function of the layer thickness is presented in Table 2. Hysteresis loops for the samples with 0, 5, 10, 30, and 50 nm of the AFM layer th ness are shown in Figure 8. All the loops along the easy axis have a rectangular shape corresponds to the magnetization reversal by a domain wall propagation. Furtherm along the hard axis, the slope of the hysteresis loops increases as the t (FeMn) incre Therefore, the uniaxial magnetic anisotropy of the NiFe/FeMn system increases with increase in the AFM layer thickness, wherein magnetization reversal mechanisms do change. At 5 nm of the FeMn layer thickness, the coercivity along the hard axis is enha up to 5 Oe, and the exchange bias is only 2 Oe. This means that during magnetiza reversal the interfacial AFM layer spins follow the FM layer spins under the torque The average grain size of NiFe layer is 18 ± 1 nm for all studied samples of this series that correlates with the structural features of NiFe layer observed by V. Vas'kovskiy et al. [3]. The estimated value of average grain size of the NiFe layer is less than in the previous series (NiFe (40 nm)/FeMn (20 nm)) due to the difference in layer thicknesses (47 nm in this series vs. 40 nm in the previous one). The grain size of the FeMn layer simultaneously increases with an increase in the film thickness, and the calculated values of the grain size are comparable with those previously obtained [37][38][39]. The grain size d (FeMn) of the FeMn phase as a function of the layer thickness is presented in Table 2. Hysteresis loops for the samples with 0, 5, 10, 30, and 50 nm of the AFM layer thickness are shown in Figure 8. All the loops along the easy axis have a rectangular shape that corresponds to the magnetization reversal by a domain wall propagation. Furthermore, along the hard axis, the slope of the hysteresis loops increases as the t (FeMn) increases. Therefore, the uniaxial magnetic anisotropy of the NiFe/FeMn system increases with the increase in the AFM layer thickness, wherein magnetization reversal mechanisms do not change. At 5 nm of the FeMn layer thickness, the coercivity along the hard axis is enhanced up to 5 Oe, and the exchange bias is only 2 Oe. This means that during magnetization reversal the interfacial AFM layer spins follow the FM layer spins under the torque [23]. In this case, anisotropy energy of the antiferromagnetic layer is not enough for a high exchange bias. In this case, anisotropy energy of the antiferromagnetic layer is not enough for a high exchange bias. The dependences of exchange bias and coercivity on the FeMn layer thickness are presented in Figure 9 (for easy axis). HEX is 2 Oe for the 5 nm-thick FeMn layer and 22 Oe for the 10 nm thick layer. A further increase in the antiferromagnetic layer thickness has no significant effect on the exchange bias. This behavior of the exchange bias vs. AFM layer thickness is typical for NiFe/FeMn bilayered systems [38]. After reaching a critical thickness, which in our case is 10 nm, the exchange bias does not noticeably increase. It can be explained via the observed AFM grain size change. Particularly, when the layer thickness is 5 nm, it contains small grains, and the AFM spins are thermally unstable, so they cannot contribute to a strong exchange bias. With the FeMn layer thickness increase up to 10 nm, a portion of unstable grains reduces, so the exchange bias correspondingly increases. As for the coercivity, it changes insignificantly in the range of 2-5 Oe, except at 5 nm for the AFM layer thickness, where HC reaches the value of 11 Oe. This can happen due to the fact that unstable AFM spins are at the interface with FM ones, and upon magnetization reversal, affect the coercivity [8]. The dependences of exchange bias and coercivity on the FeMn layer thickness are presented in Figure 9 (for easy axis). H EX is 2 Oe for the 5 nm-thick FeMn layer and 22 Oe for the 10 nm thick layer. A further increase in the antiferromagnetic layer thickness has no significant effect on the exchange bias. This behavior of the exchange bias vs. AFM layer thickness is typical for NiFe/FeMn bilayered systems [38]. After reaching a critical thickness, which in our case is 10 nm, the exchange bias does not noticeably increase. It can be explained via the observed AFM grain size change. Particularly, when the layer thickness is 5 nm, it contains small grains, and the AFM spins are thermally unstable, so they cannot contribute to a strong exchange bias. With the FeMn layer thickness increase up to 10 nm, a portion of unstable grains reduces, so the exchange bias correspondingly increases. As for the coercivity, it changes insignificantly in the range of 2-5 Oe, except at 5 nm for the AFM layer thickness, where H C reaches the value of 11 Oe. This can happen due to the fact that unstable AFM spins are at the interface with FM ones, and upon magnetization reversal, affect the coercivity [8].  The dependences of exchange bias and coercivity on the FeMn layer thickness are presented in Figure 9 (for easy axis). HEX is 2 Oe for the 5 nm-thick FeMn layer and 22 Oe for the 10 nm thick layer. A further increase in the antiferromagnetic layer thickness has no significant effect on the exchange bias. This behavior of the exchange bias vs. AFM layer thickness is typical for NiFe/FeMn bilayered systems [38]. After reaching a critical thickness, which in our case is 10 nm, the exchange bias does not noticeably increase. It can be explained via the observed AFM grain size change. Particularly, when the layer thickness is 5 nm, it contains small grains, and the AFM spins are thermally unstable, so they cannot contribute to a strong exchange bias. With the FeMn layer thickness increase up to 10 nm, a portion of unstable grains reduces, so the exchange bias correspondingly increases. As for the coercivity, it changes insignificantly in the range of 2-5 Oe, except at 5 nm for the AFM layer thickness, where HC reaches the value of 11 Oe. This can happen due to the fact that unstable AFM spins are at the interface with FM ones, and upon magnetization reversal, affect the coercivity [8]. Magnetization reversal processes were studied using VSM at temperatures of 300 K, 150 K, and 80 K. Figure 10 shows hysteresis loops along the easy axis for the NiFe (47 nm)/FeMn (t nm) films deposited at room temperature. At 5 and 10 nm of the FeMn layer (Figure 10a,b), the hysteresis loop shape is rectangular, no asymmetry is observed, and the magnetization reversal process is irreversible [20,40] (i.e., domain wall propagation) and remains the same in the given temperature range. At t (FeMn) = 30 nm (Figure 10c) hysteresis loops are asymmetrical. At 80 K and 150 K, it is clearly seen that the hysteresis loops have regions with a more rounded shape (empty symbols in Figure 10c) which represents a reversible magnetization switching (i.e., gradual rotation of magnetic moments). Figure 9. Dependences of the exchange bias and coercivity for easy axis on the antiferromagnetic layer thickness.
Magnetization reversal processes were studied using VSM at temperatures of 300 K, 150 K, and 80 K. Figure 10 shows hysteresis loops along the easy axis for the NiFe (47 nm)/FeMn (t nm) films deposited at room temperature. At 5 and 10 nm of the FeMn layer (Figure 10a,b), the hysteresis loop shape is rectangular, no asymmetry is observed, and the magnetization reversal process is irreversible [20,40] (i.e., domain wall propagation) and remains the same in the given temperature range. At t (FeMn) = 30 nm (Figure 10c) hysteresis loops are asymmetrical. At 80 K and 150 K, it is clearly seen that the hysteresis loops have regions with a more rounded shape (empty symbols in Figure 10c) which represents a reversible magnetization switching (i.e., gradual rotation of magnetic moments). To distinguish whether the observed hysteresis loop asymmetry is just an effect of the grain size increase, or if it is caused also by the AFM layer thickness increase, we performed the study of magnetization reversal processes in the NiFe (40 nm)/FeMn (20 nm) sample deposited at the substrate temperature of 600 °С. This sample has an average grain size of the FeMn (111) phase less than 7 nm and the nominal layer thickness t (FeMn) = 20 nm. Figure 11 shows both the normalized hysteresis loops (Figure 11a) along the easy axis, and corresponding switching field distributions (SFDs) (Figure 11b) for the NiFe (40 nm)/FeMn (20 nm) films deposited at the substrate temperature of 600 °С. At 80 K, HEX is 11 Oe and HC is 16 Oe. The hysteresis loops at 80 K and 150 K are asymmetrical. At the descending magnetic field, the corresponding SFDs have a complex shape and are much more extended than at the ascending field. To distinguish whether the observed hysteresis loop asymmetry is just an effect of the grain size increase, or if it is caused also by the AFM layer thickness increase, we performed the study of magnetization reversal processes in the NiFe (40 nm)/FeMn (20 nm) sample deposited at the substrate temperature of 600 • C. This sample has an average grain size of the FeMn (111) phase less than 7 nm and the nominal layer thickness t (FeMn) = 20 nm. Figure 11 shows both the normalized hysteresis loops (Figure 11a) along the easy axis, and corresponding switching field distributions (SFDs) (Figure 11b) for the NiFe (40 nm)/FeMn (20 nm) films deposited at the substrate temperature of 600 • C. At 80 K, H EX is 11 Oe and H C is 16 Oe. The hysteresis loops at 80 K and 150 K are asymmetrical. At the descending magnetic field, the corresponding SFDs have a complex shape and are much more extended than at the ascending field.
Let us compare two samples: NiFe (47 nm)/FeMn (10 nm) deposited at room temperature substrate, and NiFe (40 nm)/FeMn (20 nm) deposited at T SUB =600 • C. They have grain size of FeMn (111) phase 7 nm and less than 7 nm, respectively. For the first sample, the asymmetry of the hysteresis loop does not appear (Figure 10b), while for the second sample the hysteresis loop asymmetry is clearly observed (Figure 11a). Thus, the existence of the asymmetry for our samples is probably determined by the thickness of the antiferromagnetic layer, taking into account also the above-observed AFM layer phase separation. The observed type of the asymmetry can be attributed to the occurrence of an exchange spring in the AFM layer [41], because it is energetically more favorable to deform the antiferromagnet rather than to break the interfacial exchange coupling. In this case, the domain wall parallel to the interface in the AFM layer can contribute to the magnetization reversal [19,22,29,42,43]. In general, this effect is possible for promotion of a partial domain wall [23]. In our case, the observed hysteresis loop asymmetry is observed for the samples with AFM layer thicknesses above 10 nm. In this case, the appearance of the asymmetry with decreasing temperature can be explained by a decrease in grain thermal fluctuations, which leads to the stabilization of spins. Let us compare two samples: NiFe (47 nm)/FeMn (10 ature substrate, and NiFe (40 nm)/FeMn (20 nm) depos grain size of FeMn (111) phase 7 nm and less than 7 nm, r the asymmetry of the hysteresis loop does not appear (F sample the hysteresis loop asymmetry is clearly observed of the asymmetry for our samples is probably determine romagnetic layer, taking into account also the above-obs

Conclusions
In this work, we study the influence of temperature of the glass substrate and the thickness of the antiferromagnetic layer on structural and magnetic properties of NiFe/FeMn exchange biased thin films. It was found that the grain size of the AFM layer decreases with the increase of substrate temperature from 17 • C up to 600 • C. In addition, the AFM grain size increases with increasing layer thickness. Exchange bias behavior vs. FeMn layer thickness, as well as vs. the substrate temperature, was explained in terms of the thermal stability model. In addition, the features of magnetization reversal of the samples were studied in the temperature range from 80 K up to room temperature. Asymmetry of the hysteresis loops at 80 K and 150 K was observed for the sample with the FeMn layer thickness more than 10 nm. This effect was explained by the appearance of the AFM domain walls parallel to the interface. In addition, the observed features can be useful for developing novel methods [44] for tuning the exchange spring in the AFM.