Magnetic Force Microscopy Study of Zr₂Co₁₁-Based Nanocrystalline Materials: Effect of Mo Addition

1. Introduction

Nanocrystalline Zr₂Co₁₁-based materials are promising candidates for the development of rare-earth-free permanent magnets due to their good intrinsic magnetic properties with strong uniaxial anisotropy (11 Merg cm⁻³) and high Curie temperature (>500°C) [1–6]. The rhombohedral Zr₂Co₁₁ phase is identified to be responsible for the magnetic hardness in rapidly quenched alloys as discussed elsewhere [6–9]. The coercivity and maximum energy product of Zr₂Co₁₁-based nanocrystalline materials also were efficiently enhanced by the introduction of Mo element [10, 11]. The investigation of the magnetic microstructure and property relationship is important for the understanding of these improvements and, more generally, for the development of high-performance magnets. So far, how element additions affect magnetic microstructure and what is the correlation of magnetic microstructure and magnetism are still unclear.

In this work, how Mo addition affects the magnetic domain structures of nanocrystalline Zr₁₆Co_84−xMo_x (x = 0,0.5,1.0,1.5, and 2) alloys has been analyzed by Magnetic Force Microscopy (MFM). MFM provides the capability of observing magnetic domains with spatial resolutions down to 10 nm or better. It allows us to study in detail the evolution of local magnetic domain structures of Zr₂Co₁₁-based alloys with Mo additions and thus helps us to understand the magnetic microstructure-property relationship and further optimize the magnetic properties of such nanocrystalline materials.

2. Materials and Methods

Zr₁₆Co_84−xMo_x (x = 0,0.5,1, 1.5, and 2.0) alloys were prepared by arc melting of high-purity elements. The ingots were subsequently melt-spun in an argon atmosphere by ejecting molten ingots in a quartz tube onto the surface of a copper wheel with a rotating speed of 55 m/s. The obtained ribbons have typical size of 2 mm wide and 50 μm thick. The elemental compositions of the samples were determined by FEI Nova NanoSEM 450 equipped with an Energy Dispersive X-ray Spectrometer (EDS) to confirm the amount of Mo dopant. The SEM/EDS results identified Mo contents (atomic %) are 0.94, 1.32, and 1.87 for the samples x = 1,1.5, and 2, respectively, which are slightly smaller than the nominal concentration. For the simplicity, we still use x = 1,1.5, and 2 here to represent these samples in the paper.

The phase components of nanocrystalline Zr₁₆Co_84−xMo_x (x = 0 and 1.5) were examined by a Rigaku D/Max-B X-ray diffractometer (XRD) with Co K_α radiation. As shown in the XRD patterns (Figure 1), the diffraction peaks of the samples are identified as rhombohedral Zr₂Co₁₁ and orthorhombic Zr₂Co₁₁ structures. The relative intensity of the diffraction peaks of rhombohedral Zr₂Co₁₁ for x = 1.5 is higher than that for x = 0, while the orthorhombic Zr₂Co₁₁ peaks decrease with Mo addition, indicating that Mo addition increases the volume fraction of the rhombohedral phase. The sample with x = 1.5 mainly consists of the hard magnetic rhombohedral Zr₂Co₁₁ nanostructure.

The magnetic hysteresis loops were measured by a Superconducting Quantum Interference Device (SQUID) magnetometer with the maximum applied field of 7 T parallel to the length direction of ribbons at room temperature. The magnetic domain structures of samples were investigated in detail at the micron to nanometer scales by Bruker-ICON Magnetic Force Microscopy (MFM). All the samples are thermally demagnetized. The MFM uses a magnetic tip to measure the magnetic force gradient distribution on a sample’s surface by oscillating the cantilever normal to the surface at its resonant frequency. The magnetization of the Co-Pt MFM tip with high coercivity and high lateral resolution is perpendicular to the sample surface and points downward. All the MFM images were obtained at the same lift scan height of 20 nm, using the tapping/lift mode at room temperature.

3. Results and Discussion

Figure 2 shows hysteresis loops of the nanocrystalline Zr₁₆Co_84−xMo_x (x = 0,1.5, and 2.0) alloys at room temperature. It is evident that Mo addition improves the rectangularity shape of the hysteresis loop and increases the coercivity from 0.6 kOe for x = 0 to 2.9 kOe for x = 1.5. This leads to a significant increase of the maximum energy product (BH)_max. The best maximum energy product at room temperature of 4.2 MGOe has been achieved for the x = 1.5 sample, and then it decreases with excessive Mo addition (x = 2) due to the decrease of coercivity.

The magnetic properties (coercivity H_c, anisotropy field H_a, and maximum energy product (BH)_max) of four samples (x = 0,1, 1.5, and 2) deduced from hysteresis loops are listed in Table 1. The coercivity, anisotropy field, and maximum energy product are found to strongly depend on the Mo content. As discussed by Jin et al. [9], Mo addition increases the volume fraction of the hard magnetic phase, decreases the grain size of soft magnetic phase, and promotes the interphase exchange coupling action between hard magnetic and soft magnetic grains. The magnetocrystalline anisotropy field H_a = 2K/M₀ and the spontaneous magnetization M₀ can be determined by fitting the high-field part of the hysteresis loops using the law of approach to saturation method [12]. As shown in Table 1, the coercivity increases with increasing Mo content and reaches the maximum value of 2.9 Oe when x = 1.5. This is possibly due to the large increase of the anisotropy field of the hard magnetic phase from 18 kOe for x = 0 to 29 kOe for x = 1.5. This leads to a significant increase of the maximum energy product from 0.5 MGOe for x = 0 to 4.2 MGOe for x = 1.5.

Figure 3 shows the typical magnetic domain structures of Zr₁₆Co_84−xMo_x samples with x = 0,1, 1.5, and 2. All of these MFM images show bright-dark domain patterns with sharp contrast. The MFM image contrast represents magnetic force gradient mapping. Many magnetic grains participate in the formation of an interaction domain in which the grains exhibit the similar orientation of magnetization (up-down domains). Such domain patterns are well known to describe ferromagnetic structures [13]. Magnetic measurement results also indicate that all the samples are ferromagnetic at room temperature. From these MFM images we see that the domain size is on the nanometer scale and varies nonmonotonically with Mo content. It is easy to define the average domain size for simple domain configurations by visual methods. But for the complicated irregular interaction domains the magnetic correlation length L (the average domain width) can be estimated by several methods [13–17]. We have used the grain-size-analysis software incorporated in our MFM system to calculate L from the MFM images as an average over the lateral dimension of the domains sizes. The mean values of L determined from each MFM image are 225 nm, 154 nm, 128 nm, and 203 nm for x = 0,1, 1.5, and 2, respectively, as shown in Table 1. The given L values represent an average of magnetic domains in three 3 μm × 3 μm MFM images taken at various places on each sample surface. For the x = 1.5 sample, the domains show a more even size distribution with the relatively smaller domain size, while larger domains were observed for the x = 0 sample.

The magnetic correlation length L is basically related to the intergrain exchange coupling. The short magnetic correlation length indicates the formation of smaller magnetic domains, which is due to weaker intergrain exchange coupling among hard magnetic grains. So refinement of magnetic domain structure for the x = 1.5 sample can be well understood. The improvement of the maximum energy product of the samples upon the Mo addition can therefore be attributed to the refinement of the magnetic microstructure arising from the weaker intergrain exchange coupling action.

The values of Φ_rms of the MFM images of Zr₁₆Co_84−xMo_x samples (x = 0,1, 1.5, and 2) are listed in Table 1. Generally, the evolution of Φ_rms of the MFM images can be used to indicate the variation of the magnetic properties of the samples. It seems that there is a clear-cut correlation between Φ_rms and coercivity H_c or maximum energy product. The highest Φ_rms value of 0.66° for the sample with x = 1.5 is in good agreement with the largest coercivity of 2.9 kOe and the highest maximum energy product (BH)_max value of 4.2 MGOe. We find that the smaller the domain sizes and the larger the phase shift Φ_rms values the better the magnetic properties of the samples. This corresponds to a maximum energy product enhancement via improved magnetic domain refinement. The results indicate that the refinement and uniformity of the magnetic microstructure, small magnetic domain size, and weak intergrain exchange coupling play important roles in the achievement of better magnetic properties.

Atomic force microscopy (AFM) images show topographic information of sample surfaces (Figure 4), which provides complementary information to the corresponding MFM images (Figure 3). In order to know whether there is a correlation between the sample surface roughness and the transition width of the magnetization of domains, the root-mean-square surface roughness (RMS) has been calculated from AFM images. The results indicate RMS roughness is 8.1 nm, 3.1 nm, 5.4 nm, and 7.4 nm for x = 0,  1,1.5, and 2, respectively. It seems that there is no direct relationship between the magnetic domain sizes and the surface roughness of the samples, whereas the surface roughness of the samples with x = 1 and 1.5 is relatively smaller than that of the other two samples with x = 0 and x = 2. It suggests that suitable Mo addition leads to the formation of a smooth surface and thus narrows the grain size distribution, which may be favorable for the improvement of magnetic properties of such nanocrystalline Zr₂Co₁₁-based materials.

4. Conclusions

The effect of Mo addition on magnetic domain structures of nanocrystalline Zr₁₆Co_84−xMo_x (x = 0,1, 1.5, and 2) ribbons was investigated by Magnetic Force Microscopy. We find that element addition is a feasible way to modify magnetic domain structure and enhance coercivity for Zr₂(Co, Mo)₁₁ nanocrystalline alloys. The best magnetic properties were obtained with a significant increase in coercivity and isotropic maximum energy product for the x = 1.5 sample. The short magnetic correlation length of 128 nm and larger root-mean-square phase shift value of 0.66 were observed for this sample. The above results indicate that proper Mo addition weakens intergrain exchange coupling among hard magnetic grains helping in the refinement of the magnetic domain structure and thus leads to the enhancement of the coercivity and the maximum energy product of this class of rare-earth-free nanocrystalline permanent magnet materials.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.