Iron-rich (Fe1-x-yNixCoy)88Zr7B4Cu1 nanocrystalline magnetic materials for high temperature applications with minimal magnetostriction

As inductor technology advances, greater efficiency and smaller components demand new core materials. With recent developments of nanocrystalline magnetic materials, soft magnetic properties of these cores can be greatly improved. FeCo-based nanocrystalline magnetic alloys have resulted in good soft magnetic properties and high Curie temperatures; however, magnetoelastic anisotropies persist as a main source of losses. This investigation focuses on the design of a new Fe-based (Fe,Ni,Co)88Zr7B4Cu1 alloy with reduced magnetostriction and potential for operation at elevated temperatures. The alloys have been processed by arc melting, melt spinning, and annealing in a protective atmosphere to produce nanocrystalline ribbons. These ribbons have been analyzed for structure, hysteresis, and magnetostriction using X-Ray diffraction, vibrating sample magnetometry (VSM), and a home-built magnetostriction system, respectively. In addition, Curie temperatures of the amorphous phase were analyzed to determine the best performing, high-temperature material. Our best result was found for a Fe77Ni8.25Co2.75Zr7B4Cu1 alloy with a 12 nm average crystallite size (determined from Scherrer broadening) and a 2.873 A lattice parameter determined from the Nelson-Riley function. This nanocrystalline alloy possesses a coercivity of 10 A/m, magnetostrictive coefficient of 4.8 ppm, and amorphous phase Curie temperature of 218°C.As inductor technology advances, greater efficiency and smaller components demand new core materials. With recent developments of nanocrystalline magnetic materials, soft magnetic properties of these cores can be greatly improved. FeCo-based nanocrystalline magnetic alloys have resulted in good soft magnetic properties and high Curie temperatures; however, magnetoelastic anisotropies persist as a main source of losses. This investigation focuses on the design of a new Fe-based (Fe,Ni,Co)88Zr7B4Cu1 alloy with reduced magnetostriction and potential for operation at elevated temperatures. The alloys have been processed by arc melting, melt spinning, and annealing in a protective atmosphere to produce nanocrystalline ribbons. These ribbons have been analyzed for structure, hysteresis, and magnetostriction using X-Ray diffraction, vibrating sample magnetometry (VSM), and a home-built magnetostriction system, respectively. In addition, Curie temperatures of the amorphous phase were analyzed to determine the bes...


INTRODUCTION
2][3] The most efficient means to miniaturization of these components is to increase the switching frequency. 4However, this miniaturization can only occur if the component losses remain small.With increased switching frequencies, high current inductors require an improved magnetic core that maintains high efficiency (i.e.lower core losses) under these operation conditions.In addition, magnetic components in many power applications must be able to operate at high temperatures especially in high current applications.
Nanocrystalline soft magnetic materials have been developed to have low losses at higher switching frequencies up to 20 kHz, 5 making them of interest for electric vehicle technologies.These materials are comprised of ferromagnetic nanocrystallites surrounded by a ferromagnetic residual amorphous phase.When grains are randomly oriented, have sizes smaller than the exchange correlation length, and are exchange-coupled through the amorphous phase; an effective magnetocrystalline anisotropy is averaged by exchange interactions. 6This effective magnetocrystalline anisotropy is small in magnitude and results in lower losses and high permeability with the potential for large a E-mail: anthony.martone@case.edu;matthew.willard@case.edumagnetizations. 7Because the reduced magnetocrystalline anisotropy is dependent on maintaining exchange-coupling interaction between crystallites, losses are very dependent on the Curie temperature of the residual amorphous phase, T c am .This has limited many Fe-based nanocrystalline materials, such as Finemet 8 and Nanoperm, 9 to lower operating temperatures (below 250 • C from T c am ).High temperature soft magnetic materials such as HiTperm (Fe 44 Co 44 Zr 7 B 4 Cu 1 ) have been developed to increase the operating temperature with relatively low losses. 10,11While the addition of cobalt increases the Curie temperature, it also increases the coercivity and core losses due to increased magnetostriction.The partial substitution of nickel for cobalt in HiTperm alloys has proven to reduce the magnetostriction and therefore coercivity and core loss of FeCo-based nanocrystalline materials. 12,13In studies of the (Fe 1-2x Ni x Co x ) 88 Zr 7 B 4 Cu 1 alloy system, Fe 77 Ni 5.5 Co 5.5 Zr 7 B 4 Cu 1 had the best soft magnetic properties 13 with a coercivity of 12.1 A/m and magnetostriction of 9.4 ppm.This study focuses on refinement of FeNiCo-based nanocrystalline magnetic alloys to reduce magnetostriction and therefore lower coercivity while maintaining a relatively high Curie temperature (>200 • C).

EXPERIMENTAL PROCEDURE
Five nanocrystalline (Fe 1-x-y Ni x Co y ) 88 Zr 7 B 4 Cu 1 alloys were created and analyzed to explore two paths within the (Fe,Ni,Co) 88 Zr 7 B 4 Cu 1 quasi-ternary compositional diagram. 14The two alloy design paths intersect at the best performing alloy previously studied from the (Fe 1-2x Ni x Co x ) 88 Zr 7 B 4 Cu 1 system, namely Fe 77 Ni 5.5 Co 5.5 Zr 7 B 4 Cu 1 . 12,13One path, P 1 , explores a greater nickel to cobalt ratio while holding the iron percentage constant (therefore the total cobalt and nickel percentage remains constant, too).Along path, P 1 , the nickel percentage increases by an eighth of the total nickel and cobalt percentage (i.e. in steps of 1.375at%) at the expense of cobalt while holding the iron percentage constant at 77at%, including the following alloys: Nanocrystalline alloys were produced through arc melting, melt spinning, and isothermal annealing.Twenty-five gram homogeneous ingots were produced by arc melting highly pure elemental constituents of at least 99.95 at% purity in a gettered argon atmosphere using a Thermal Technology LLC Model BJ5 system with a Miller Gold Star 652 power source and diffusion pump.Ingots were melted and inverted at least 3 times to ensure homogeneity, then melt spun using a single-roller jet casting technique on a Yein Tech Rapid Solidification Processing System.Alloys were melted with induction heating and ejected out of a 0.6-0.65 mm orifice onto a copper quenching wheel with a surface velocity of 40-45 m/s.Melt spinning produced amorphous ribbons which were 2-3 mm in width and 20 µm in thickness. 12,13rystallization temperatures of the as-spun ribbons were determined with Differential Scanning Calorimetry (DSC).A Netzsch 404 F3 Pegasus instrument was used with a heating rate of 10 • C/min from room temperature to 800 • C using alumina crucibles.Based on the crystallization temperatures of the ribbons, an annealing temperature was chosen from the processing window above the primary crystallization temperatures and below any subsequent crystallization temperatures.Alloys were annealed at 550 • C for 3600 s in an argon-filled ampoule using a Lindberg Model 92731 six zone tube furnace followed by water quenching.
X-Ray Diffraction (XRD) was performed on both as-spun and heat-treated ribbons to determine phases present, lattice parameters, and crystallite sizes.As-spun ribbons were analyzed with a Rigaku D/Max 2200 and heat-treated ribbons were analyzed with a Bruker D8 Discover Series II, both using Cu Kα radiation with a two theta (2θ) from 20 • -120 • with a step size of 8.33x10 -3 • /s (0.500 • /min).The lattice parameter of the crystallite phase was analyzed with the Nelson-Riley function, and the crystallite size was determined with the Scherrer analysis. 15 Lakeshore Model 7410 VSM was used to measure quasistatic magnetic hysteresis loops with a maximum applied field of 1.19x10 6 A/m (15 kOe).This instrument with furnace attachment was also used to perform thermomagnetic measurements on as-spun ribbons at a fixed field of 3.18x10 5 A/m (4 kOe) from room temperature to 550 • C. The Curie temperature of the amorphous phase was determined by a two tangent method.A tangent line was plotted (with the greatest R 2 value) for the linear region of the curve where the magnetization drastically decreases.Another tangent line was plotted (with the greatest R 2 value) for the horizontal linear region following the drastic decrease in magnetization.The intersection of these two tangents was determined to be the Curie temperature of the amorphous phase. I all VSM experiments, ribbon samples were aligned parallel to the applied field to reduce demagnetization effects.In addition, a home-built, strain gauge system was used to determine the magnetostrictive coefficient of all ribbon samples.16

RESULTS AND DISCUSSION
As-spun samples were confirmed to be amorphous from the presence of a single broad peak in their X-ray diffractograms.Heat-treated samples were confirmed to have a nanocrystalline bodycentered cubic (BCC) structured phase of α-(Fe,Ni,Co) with the presence of a residual amorphous phase; an example of diffractograms for the as-spun and heat-treated Fe 77 Ni 8.25 Co 2.75 Zr 7 B 4 Cu 1 alloys are shown in Figure 1.Nelson-Riley function analysis of the five diffraction peaks in Figure 1 for the heat-treated Fe 77 Ni 8.25 Co 2.75 Zr 7 B 4 Cu 1 sample indicates a lattice parameter for the BCC phase of 0.2873 nm ± 0.0003 nm.The crystallite size was determined to be 12 nm ± 3 nm by Scherrer analysis of the XRD peak broadening (averaged from the five peaks).These values were typical for all heat-treated alloys; having crystallite sizes ranging from 12-16 nm and lattice parameters ranging from 0.2872-0.2876nm.
Quasistatic hysteresis loops for as-spun and heat-treated samples were measured and interpreted for coercivity and saturation magnetization.Heat-treated Fe 77 Ni 8.25 Co 2.75 Zr 7 B 4 Cu 1 had the lowest coercivity of 10.5 A/m with a saturation magnetizations of 166.2 (A•m 2 )/kg.Along path P 1 , the coercivity decreased as nickel percentage increased from 12.1-10.5A/m.Along path P 2 , the coercivity increased as nickel percentage increased from 12.1-14.9A/m.The trend of coercivity based on nickel percentage follows directly with the trend of magnetostriction as the magnetoelastic anisotropy dominates the coercivity of these nanocrystalline alloys.The saturation magnetization of the alloys were all above 165 (A•m 2 )/kg, ranging from 166.2-176.7 (A•m 2 )/kg.The saturation magnetizations roughly followed a Slater-Pauling curve 17,18 as a function of average magnetic valence electrons per atom with a peak around 8.25 valence electrons per atom.
Magnetostrictive coefficients were determined from strain gauge measurements (voltage) versus time as ribbon sections subjected to a rotating saturating magnetic field.The voltage change is caused by the expansion and contraction of the strain gauge as the ribbons exhibit magnetostriction with regards to the direction of the rotating field.The voltage multiplied by a factor of 3.0x10 -5 (determined from experimental constants including amplifier gain, voltage bias, system resistances, and strain gauge factor) is equal to the strain in the sample along the strain gauge lengthwise direction.The maximum (peak) strain can be related to the saturation magnetostrictive coefficient by multiplying by a factor of 4/3, 1+υ (Poisson's ratio for typical metal).A plot of the strain (in microstrain units   C from the two tangent method.The Curie temperatures of the amorphous phase increased from 218-348 • C as the fraction of cobalt increased from 2.75-7.333at%.Curie temperatures of the amorphous phase, crystallite sizes, and other magnetic properties of the nanocrystalline alloys are shown in Table I. A plot of Curie temperature versus the fraction of cobalt is shown in Figure 4. Alloys from this study as well as Knipling alloys 12,13 are plotted with magnetostrictive coefficients of their nanocrystalline form annotated.  10 The significantly larger coercivity in HiTperm is due to the much greater contribution of magnetoelastic anisotropy exhibited as magnetostriction (>20 ppm for HiTperm). 10

CONCLUSION
Five (Fe 1-x-y Ni x Co y ) 88 Zr 7 B 4 Cu 1 nanocrystalline magnetic materials have been created along two compositional design paths.All of these alloys have been shown to have low coercivities (<15 A/m), low magnetostrictive coefficients (<12.5 ppm), and relatively high Curie temperatures (>200 • C).This study has shown a strong dependence of coercivity on magnetostriction in the nanocrystalline (Fe,Co,Ni) 88 Zr 7 B 4 Cu 1 alloy system.This dependence of coercivity suggests that magnetoelastic anisotropy dominates the coercivity in this nanocrystalline alloy system.For the alloys studied, nanocrystalline Fe 77 Ni 8.25 Co 2.75 Zr 7 B 4 Cu 1 resulted in the smallest magnetostrictive coefficient (4.8 ppm) and lowest coercivity (10.5 A/m) amongst the alloys having a Curie temperature above 200 • C.
Fe 77 Ni 8.25 Co 2.75 Zr 7 B 4 Cu 1 is shown to have the lowest magnetostrictive coefficient (4.8 ppm) for all alloys having a Curie temperature of the amorphous phase above 200 • C. As the coercivities of these alloys have a strong dependence on magnetostriction, this alloy also has the lowest coercivity (10.5 A/m).While nanocrystalline Fe 77 Ni 8.25 Co 2.75 Zr 7 B 4 Cu 1 has a lower Curie temperature of the amorphous phase than HiTperm, nanocrystalline Fe 44 Co 44 Zr 7 B 4 Cu 1 (>500 • C), the coercivity, 10.5 A/m, is significantly lower (80-200 A/m for HiTperm).

TABLE I .
Summary of structural (crystallite size, D) and magnetic properties (saturation specific magnetization, Ms, coercivity, Hc, magnetostriction coefficient, λ, and Curie temperature of the amorphous phase, T c am ) of nanocrystalline (Fe,Ni,Co) 88 Zr 7 B 4 Cu 1 alloys annealed at 550 • C for 3600 s. 77 Ni 8.25 Co 2.75 Zr 7 B 4 Cu 1 can be seen in Figure 3.The T c am of Fe 77 Ni 8.25 Co 2.75 Zr 7 B 4 Cu 1 was determined to be 218 FIG. 4. Curie temperature of the amorphous phase versus cobalt fraction of magnetic transition metals for (Fe,Ni,Co) 88 Zr 7 B 4 Cu 1 alloys from the present study (circles) with comparison to values in Ref. 12 (triangles).Magnetostrictive coefficients of the nanocrystalline alloys annotated.Fe