Correlation between phase composition and exchange bias in CoFe/MnN and MnN/ CoFe polycrystalline films

Magnetic properties and phase composition of both MnN/CoFe (MnN at top of bilayer) and CoFe/MnN films (MnN at bottom of bilayer) bilayers through annealing at various temperatures (T a = 300-450 ○ C) and then cooling to room temperature under the application of an external magnetic field of 1.5 kOe are compared. The exchange bias field (H E ), the magnitude of magnetic hysteresis loop shift, of the studied films is highly related to phase composition of antiferromagnetic (AF) layer. The increase of H E with increasing T a in the range of 300-375 ○ C possibly results from the improvement of magnetocrystalline anisotropy of AF related to the promoted crystallinity and stress relaxation of tetragonal face-centered θ -MnN phase. The reduction of H E at higher T a is due to the decreased volume fraction or disappearance of θ -MnN phase and the formation of impurity phases, such as Mn 4 N and Mn. The induction of impurity phases is possibly related to the diffusion of part of N out of MnN phase at higher T a . Higher H E for CoFe/MnN than MnN/CoFe at T a = 300-375 ○ C might be attributed to larger amount and higher degree of stress relaxation for θ -MnN phase. For CoFe/MnN film annealed at 375 ○ C, the highest H E = 562 Oe is attained, and the corresponding interfacial exchange energy of 0.47 mJ/m 2 in this study is comparable to that reported by Meinert et al. [Phys. Rev. B 92 , 144408 (2015)].


I. INTRODUCTION
Nowadays spin electronics is becoming a significant technology and the foundation of spin-valve based devices, like giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) and so on. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The spin-valve based devices mainly consist of a free layer and a pinned layer. In general, as a pinned layer, strong exchange bias (EB) between a FM layer and antiferromagnetic (AF) layer is requisite. [1][2][3]6 EB, characterized by a shift of magnetic hysteresis loop, results from the interaction between the FM and AF layers. 4,5 For practical EB system, the main requirements for AF layer are large magnetocrystalline anisotropy constant (K AF ), high Néel temperature (T N ), good corrosion resistance, ease of manufacturing, environmental safety and low price. 6,7 Recently, MnN reported could meet the above requirements. AF MnN crystallizes at θ phase with tetragonal face-centered NaCl structure at room temperature (RT). 6,7,13 Its T N is about 660 K where this magnetic transition accompanies with a phase transformation from tetragonal to cubic. 6,7 Magnetic properties of MnN-based exchange bias system have been studied by Meinert. 6 7 As EB effect is mostly an interface phenomenon, EB field (H E ) is highly sensitive to interface roughness, morphology, crystallinity, thickness and grain size of both FM and AF. [14][15][16] In this work, we compare magnetic properties and phase composition of both MnN/CoFe (MnN at top of bilayer) and CoFe/MnN (MnN at bottom of bilayer) films with various annealing temperatures (Ta) in the range of 300-450 ○ C under the application of an external magnetic field of 1.5 kOe.

II. EXPERIMENT
CoFe/MnN and MnN/CoFe films with 5-nm-thick CoFe layer and 40-nm-thick MnN layer were prepared on 10-nm-thick Ta underlayer buffered glass substrates (Corning 1737) at RT by magnetron sputtering at the external in-plane magnetic field of 350 Oe induced by the NdFeB sintered magnet. The base pressure of the system was better than 5×10 −7 torr. MnN films were prepared from elemental Mn target at the ratio of Ar to N 2 atmosphere of 1:1. Subsequently, 3-nm-thick Ta was sputtered onto each samples to avoid oxidation. The post-annealing was performed at base pressure better than 5×10 −6 torr and various Ta in the range of 300-450 ○ C for 15 minutes, and then cooling to RT at the applied magnetic field of 1.5 kOe to align spins to form AF ordering and thus form inplane EB. The structural characterization was carried out by x-ray diffractometer (XRD) using Cu Kα radiation. Magnetic properties were measured by an alternating gradient magnetometer (AGM). The thickness and surface morphology of the sample were measured by atomic force microscopy (AFM).

XRD patterns of MnN/CoFe (MnN at the top layer) and
CoFe/MnN (MnN at the bottom layer) films annealed at different temperature (Ta) within the applied magnetic field of 1.5 kOe are shown in Fig. 1(a) and (b), respectively. θ-MnN(002) phase is found in both as-deposited films, while no diffraction peak from CoFe and Ta layers is detected due to too thin thickness. Distinct phase composition for two series films with Ta is observed. In MnN/CoFe films, with increasing Ta, MnN(002) diffraction peak is shifted to higher angle attributed to the stress relaxation. The estimated d-spacing of (002) is reduced from 2.14 Å for as-deposited film to 2.09 Å for the film annealed at 400 ○ C. The strengthened intensity and the narrowed full width at half maximum (FWHM) of MnN(002) peak with the increase of Ta reveals grain growth. Mn 4 N and Mn phases appear to coexist with MnN phase at Ta in range of 350-400 ○ C. At higher Ta = 450 ○ C, Mn 4 N phase prevails.
On the other hand, for CoFe/MnN films, the peak shift to higher angle, the strengthened intensity and the narrowed FWHM for MnN(002) texture with increasing Ta are similar to above MnN/CoFe series films. The d-spacing of (002) is reduced from 2.151 Å for as-deposited film to 2.089 Å for the film annealed at 400 ○ C. Unlike above MnN/CoFe, no impurity phase is signed at Ta in the range of 300-375 ○ C for CoFe/MnN films. Mn 4 N and Mn phases coexist with MnN phase at Ta = 400 ○ C. At higher Ta = 450 ○ C, Mn phase prevails. Magnetic properties of two series films are summarized and shown in Figure 3(a) and (b). As shown in Fig. 3(a)  the formation of impurity phases, such as Mn 4 N and Mn, as shown in Fig. 1. The induction of impurity phases is possibly related to the diffusion of part of N out of MnN phase at higher Ta. Higher H E for CoFe/MnN than MnN/CoFe at Ta = 300-375 ○ C might be attributed to larger amount and higher degree of stress relaxation for θ-MnN phase. As shown Fig. 3(b), Hc increases with increasing Ta for 2 series films. Hc increases from 21 Oe at as-deposited state to 400 Oe at Ta = 450 ○ C for MnN/CoFe films, while Hc arises from 201 Oe at as-deposited state to 640 Oe at Ta = 450 ○ C for CoFe/MnN films. The change of Hc with Ta for two series films is possibly related to roughness of the interface between CoFe and MnN layers: in addition to phase composition, the main difference between MnN/CoFe and CoFe/MnN films is considered to the roughness of the interface between MnN and CoFe.
In order to infer the roughness of the interface, AFM images of the top surface for as-deposited CoFe and MnN films on Ta underlayers are studied and shown in Fig. 4 (a) and (b), respectively. The surface roughness (R) of MnN (R = 0.4 nm) is larger than CoFe (R = 0.2 nm). Higher interfacial roughness in CoFe/MnN than MnN/CoFe results in higher coercivity for CoFe/MnN. Fig. 4(c)-(f) show AFM images of the studied films. Clearly, very flat surface with low R, and therefore, good EB is induced. R is summarized in Fig. 4(g). With increasing Ta, R of below 1 nm is observed

ARTICLE
scitation.org/journal/adv at Ta lower than 375 ○ C, and therefore, large H E is attained. However, higher Ta leads to the intermixing and the diffusion of part of N out of MnN phase, sharply increases R, and therefore decreases H E and increases Hc. Accordingly, the interfacial morphology also contributes to affect magnetic properties of the presented CoFe/MnN system with Ta in addition to phase composition.

IV. CONCLUSIONS
Structure and magnetic properties of thin film stacks containing CoFe and MnN layer deposited in different sequence are reported. Two series films annealed at Ta below 400 ○ C mainly consist of θ-MnN(002) phase, and the stress relaxation and grain growth of MnN(002) with increasing Ta are found. Distinct phase composition is found for two series films annealed at higher Ta. For MnN/CoFe films, additional Mn 4 N and Mn phases appear at Ta in range of 350-400 ○ C, and Mn 4 N phase prevails at higher Ta = 450 ○ C. On the other hand, for CoFe/MnN films, additional Mn 4 N and Mn phases also appear at Ta = 400 ○ C, but Mn phase prevails at higher Ta = 450 ○ C. With annealing, the coercivity increase results from 3 facts: phase segregation, interfacial intermixing, and grain growth. Besides, when MnN is at the bottom, H E is larger due to less phase segregation, while Hc is also larger due to larger intermixing roughness.