Magnetic Domain Structure of Nanocrystalline Zr 18-x Hf x Co 82 Ribbons: Effect of Hf

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1 Mater. Res. Soc. Symp. Proc. Vol Materials Research Society DOI: /opl Magnetic Domain Structure of Nanocrystalline Zr 18-x Hf x Co 82 Ribbons: Effect of Hf Lanping Yue 1, I. A. Al-Omari 1,2,3, Wenyong Zhang 1,2, Ralph Skomski 1,2, and D. J. Sellmyer 1, 2 1 Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE Department of Physics and Astronomy, University of Nebraska, Lincoln, NE U.S.A. 3 Department of Physics, Sultan Qaboos University, PC 123, Muscat, Sultanate of Oman ABSTRACT The effect of Hf on the permanent magnetism of nanocrystalline Zr 18-x Hf x Co 82 ribbons (x = 0, 2, 4, and 6) was investigated by magnetic properties measurement and magnetic force microscopy (MFM). Emphasis is on the local magnetic domain structures in polycrystalline rapidly solidified Zr 18-x Hf x Co 82 ribbons for four different samples with small fractions of Hf dopants (x 6). The investigation of the magnetic properties of the Zr 18-x Hf x Co 82 ribbons revealed that all the samples under investigation are ferromagnetic at room temperature, and the corresponding MFM images show bright and dark contrast patterns with up-down magnetic domain structures. It is found that the saturation magnetization and the coercivity depend on Hf doping concentration x in the samples. For a sample with Hf concentration x = 4, the maximum energy product (BH) max value is 3.7 MGOe. The short magnetic correlation length of 131 nm and smallest root-mean-square phase shift value of were observed for x = 4, which suggests the refinement of the magnetic domain structure due to weak intergranular exchange coupling in this sample. The above results indicate that suitable Hf addition is helpful for the magnetic domain structure refinement, the coecivity enhancement, and the energy-product improvement of this class of rare-earth-free nanocrystalline permanent-magnet materials. INTRODUCTION The limited resources and supplies of rare earths have led to a renewed interest in rareearth-free magnetic materials [1], and our research [2-4] is a part of this trend. Among the considered materials are Zr 2 Co 11 -based rare-earth-free permanent-magnet alloys. The additional elements (such as Mo, B, Al, Fe, Ni, etc.) added to Zr-Co alloys have been used to optimize the structures and improve magnetic properties [4-10]. We have recently reported that there are positive effects of substituting Zr by limited amounts of Hf on the coerecivity enhancement and thus improvements of the hard magnetic properties of nanocrystalline Zr 18-x Hf x Co 82 ribbons [3]. The investigation of the magnetic microstructure is important for the understanding of these improvements and, more generally, for the development of high-performance magnets. In this paper, emphasis is on local magnetic domain structures in four different samples of rapidly solidified Zr 18-x Hf x Co 82 ribbons with small fractions of Hf (x = 0, 2, 4, and 6). A detailed characterization of magnetic domain-size effects in Zr 18-x Hf x Co 82 ribbons was performed using high resolution magnetic force microscopy (MFM).

2 EXPERIMENTAL Rapidly solidified nanocrystalline Zr 18-x Hf x Co 82 ribbons (x = 0, 2, 4, and 6) were prepared by arc melting, followed by melt spinning on a rotating copper wheel at a speed of 40 m/s under argon atmosphere. The typical size of the ribbons is approximately 2 mm wide and 50 m thick. The magnetic properties were measured using a Superconducting Quantum Interference Device (SQUID) magnetometer with the maximum applied field up to 7 T parallel to the length direction of ribbons at room temperature. The local domain structures of the samples were studied using a Digital Instruments D3100 magnetic force microscope. The magnetization of the Co-Pt 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. RESULTS and DISCUSSION The magnetic properties (saturation magnetization M s, coercivity H c, anisotropy field H a, and maximum energy product (BH) max ) of four samples measured from hysteresis loops are listed in Table I. Magnetic measurement results indicate that all the samples are ferromagnetic at room temperature. The saturation magnetization and the coercive force are found to depend on the Hf concentration. Hf addition is helpful for the coercivity enhancement. The maximum energy product (BH) max at room temperature increases with increasing hafnium concentration and reaches a maximum value of 3.7 MGOe for x = 4, then it decreases with x. Table I. Dependence of the magnetic properties (saturation magnetization M s, coercivity H c, anisotropy field H a, and maximum energy product (BH) max ) and of the parameters of MFM images (root-mean-square values rms of the phase shift of the images and average magnetic correlation length L) on the Hf concentration x in nanocrystalline Zr 18-x Hf x Co 82 ribbons. Sample X (Hf) M s (emu/g) H c (koe) H a (koe) (BH) max (MGOe) rms ( 0 ) L (nm) The coercivity increase with Hf content is largely a consequence of the reduced magnetization, because H c = 2 K/M s [11], with some concentration dependence of. This describes the "geometrical" parameter and also includes some other factors, for example, the grain-boundary stoichiometry and the polycrystalline texture, etc. The magnetocrystalline

3 anisotropy parameter K and the spontaneous magnetization M 0 can be determined by fitting the hysteresis loops using the law of approach to saturation method [12]. The value of K of these samples was calculated to be 1.1 MJ/m 3 and independence of Hf concentration [3]. The anisotropy field was calculated as H a = 2K /M 0 [4, 11]. As shown in Table I, the coercivity slightly increases from 3.1 koe for x =4 to 3.3 koe for x = 6, this is due to the large increase of the anisotropy field of the hard magnetic phase from 34.8 koe for x = 4 to 40.4 koe for x = 6. Figure 1 shows the typical magnetic domains of these samples. All of the MFM images show bright-dark domain structures with sharp contrast. Such domain patterns are well-known to describe ferromagnetic structures. From these MFM images we see that the domain size is on the nanometer scale and varies non-monotonically with Hf concentration. For the sample with x = 4, the domains have a more even size distribution with a smaller domain size, while larger domains were observed in the sample with x = 6. 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 [13-18]. Figure 1. Typical MFM images of nanocrystalline Zr 18-x Hf x Co 82 ribbons with different Hf concentrations (x = 0, 2, 4, and 6). Each image is 3 x 3 m 2 in size. 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. We chose the ten largest sizes of magnetic domains to calculate the mean value of L as shown in

4 Figure 2. The mean values of L estimated at a threshold of 50% of the highest peak (the largest frequency shift) determined from each MFM image are 168 nm, 157 nm, 131 nm, and 180 nm for x = 0, 2, 4, and 6 respectively, as shown in Table I. The given L values represent an average of ten largest sizes of magnetic domains in three 3 m x 3 m MFM images taken at various places on the each sample surface. For the sample with Hf doping of x = 4, the smallest L of 131 nm was estimated. In general, the correlation length L reflects magnetic domain structure features on a length scale much larger than the domain-wall width. It seems that there is no clearcut correlation between correlation length L and coercivity Hc or magnetization Ms. However, the magnetic correlation length is basically related to the intergranular exchange coupling between the magnetic grains, and weak exchange can be expected to result in the formation of smaller magnetic domains. The relatively short magnetic correlation length indicates that the intergranular exchange coupling between the magnetic grains in the sample with x = 4 is weaker than that in the other samples. The improvement of the energy product of the samples upon the Hf addition can therefore be attributed to the refinement of the magnetic microstructure giving rise to the reduced intergranular exchange. Figure 2. Illustration of the magnetic correlation length L estimated from the average values of the ten largest sizes of magnetic domains (red) in 3 m x 3 m MFM images (at a threshold of 50% of the largest frequency shift). 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 MFM phase image measures the phase lag between the drive voltage and the cantilever response. Vertical gradients in the magnetic force cause a shift Δf 0 in the resonance frequency. In this case, the drive frequency shifts lead to phase shifts ΔΦ which then gives an image of the magnetic force gradients. The root-mean-square values of phase shift Φ rms of the MFM images is the standard deviation of the phase shift Φ within the given scan area, and it can be calculated by the following equation [19]: Here Φ ave is the average Φ value within the scan area of the MFM image, Φ i is the current ith Φ value, and N is the number of points within a given area.

5 The values of Φ rms of the MFM images are listed in Table I. The smallest Φ rms value of for the sample with x = 4 is in good agreement with the relatively short magnetic correlation length of 131 nm and the highest energy product (BH) max value of 3.7 MGOe of the sample. It seems that the smaller the domain sizes and 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 grain refinement. The results indicate that the refinement and uniformity of the magnetic microstructure, small magnetic domain size, and low magnetic inter-granular exchange coupling play important roles on the achievement of better magnetic performance. The root-mean-square surface roughness (RMS) on these samples grown under similar conditions has also been calculated from atomic force microscopy (AFM) images in order to know whether there is a correlation between the sample surface roughness and the transition width of the magnetization of domains. The results indicate an RMS roughness of about 6 nm for samples with x = 2 and 4, whereas the surface roughness of the samples with x = 0 and 6 is smaller, RMS ~ 4.3 nm. The results indicate that there is no direct relationship between the magnetic domain size and the surface roughness of the samples. CONCLUSIONS This study shows that magnetic properties and domain structures are strongly influenced by Hf doping concentration x in nanocrystalline Zr 18-x Hf x Co 82 ribbons. The coercivity of the samples increases with Hf addition x. The maximum energy product (BH) max increases with increasing x, from 2.8 MGOe for x = 0 to a maximum value of 3.7 MGOe for x = 4. The smallest domain size with a relatively short magnetic correlation length of 131 nm and smallest rootmean-square phase shift Φ rms value of are observed for the Hf concentration x = 4, which suggests the refinement of the magnetic domain structure due to weak intergranular exchange coupling in this sample. The above results indicate that suitable Hf addition is helpful for the refinement of the magnetic domain structure and improvement of the energy product of the rareearth-free nanocrystalline permanent-magnet ribbons. ACKNOWLEDGMENTS This research is supported by DOE/Ames/BREM under grant DE-AC02-07CH11358 and the Nebraska Center for Materials and Nanoscience. Al-Omari acknowledges Sultan Qaboos University for the support provided in this study under the research grant number IG/SCI/PHYS/12/02. REFERENCES 1. N. Jones, Nature 472, 22 (2011). 2. R. Skomski, J. Shield, and D. J. Sellmyer, Magn. Techn. Int. 222, 22 (2011). 3. I. A. Al-Omari, W. Y. Zhang, L. P. Yue, R. Skomski, J. E. Shield, X. Li, and D. J. Sellmyer, IEEE Trans. Magn. 49 (7), 3394 (2013). 4. W. Y. Zhang, S. R. Valloppilly, X. Li, R. Skomski, J. E. Shield, and D. J. Sellmyer, IEEE Trans. Magn. 48, 3603 (2012).

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