Hard Magnetic Alloys Research Articles

Noncollinear spin structure in Fe3+xCo3−xTi2 ( x=0,2,3 ) from neutron diffraction

Neutron powder diffraction has been used to investigate the spin structure of the hard-magnetic alloy $\mathrm{F}{\mathrm{e}}_{3+x}\mathrm{C}{\mathrm{o}}_{3\ensuremath{-}x}\mathrm{T}{\mathrm{i}}_{2}$ ($x=0,2,3$). The materials are produced by rapid quenching from the melt, they possess a hexagonal crystal structure, and they are nanocrystalline with crystallite sizes $D$ of the order of 40 nm. Projections of the magnetic moment onto both the crystalline $c$ axis and the basal plane were observed. The corresponding misalignment angle exhibits a nonlinear decrease with $x$, which we explain as a micromagnetic effect caused by Fe-Co site disorder. The underlying physics is a special kind of random-anisotropy magnetism that leads to the prediction of $1/{D}^{1/4}$ power-law dependence of the misalignment angle on the crystallite size.

Effect of mixing duration and sintering temperature on the magnetic properties of hard magnetic powder alloy Fe-30Cr-8Co (wt.%)

Effect of mixing duration and sintering temperature on the magnetic properties of hard magnetic powder alloy Fe-30Cr-8Co (wt.%) was studied. The obtained magnetic hysteresis properties of the hard magnetic powder alloy are better than as for industrial Fe-Cr-Co alloy which cobalt content is twice bigger. The optimum sintering temperature was 1350-1400 °C. At a sintering temperature of 1400 °C for 2.5 hours the mixing duration does not affect on the magnetic hysteresis properties. This technological operation is not critical.

Influence of heat treatment on the magnetic properties of Fe-28%Cr-7%Co-2%Mo-0.5%Si powder hard magnetic alloy

The hard magnetic alloy Fe-28%Cr-7%Co-2%Mo-0.5%Si (in wt. %) was prepared by the methods of powder metallurgy. The maximum magnetic properties on the alloy was received after the TMT temperature of 540 °C and was the following: residual induction Br up to 1.3 T, coercive force Hc up to 40 kAm−1 and maximum energy product (BH)max up to 31 kJm−3. The hardness of the alloy increases from 228 to 250 HV 30 in the temperature range of spinodal decomposition at 600 – 660 °C. The coefficient of thermal expansion of the alloy is 13,1*10−6 K−1. It was found that the additional annealing of the alloy till 460 °C can increase the coercive force approximately for 6 kAm−1.

Phase transformations and properties of concentration-inhomogeneous magnetic materials based on the Fe–30%Cr–27%Co system

The phase and structural transformations of a powder magnetically hard alloy of the system Fe–30%Cr–27%Co–1%Si–0,07%B with a metastable α1+α2 phase composition, a high content of cobalt, and a high level of magnetic properties were studied. The density and coefficient of variation of the concentration of the main elements of the sintered blanks at the level of deformable analogs are achieved by sintering in the α-phase with contact melting in the presence of a «vanishing» liquid phase formed due to the addition of silicon and boron ferroalloys. A kinetic approach to the development of a competitive hard magnetic alloy with a high proportion of the strongly magnetic phase is proposed. The effect of boron additions on the incubation period of the formation of an undesirable σ-phase and the temperature range of the concentration stratification of the α-solid solution on the strongly magnetic α1-phase and the weakly magnetic α2-phase were established. Optical microscopy, X-ray phase analysis and differential scanning calorimetry were used to determine temperature and time parameters of heat treatment of the alloy, including hardening, thermomagnetic treatment (TMT) and final aging, providing the required combination of Hc and Br by increasing the stability of the metastable α-phase up to 20 minutes in the interval temperatures of spinodal decomposition α → α1 + α2. The greatest increase in magnetic properties after TMT, observed at the 1st and 2nd steps of final aging, is related to the decomposition of the α-solid solution and the formation of subgrain boundaries. Elements of the obtained structure are characterized by submicron and nanometric sizes, which correlate with the research results on deformable alloys based on the Fe – Cr – Co system. The anisotropic α1+α2-structural state achieved by thermomagnetic treatment provided an increase in the magnetic properties of the studied 30H27KSR powder alloy to 30 % and the squareness coefficient of the magnetic hysteresis loop equal to 0,82 .

Crystallization Studies of Hard Magnetic Pr9Fe56Co13Zr1Ti3B18 Alloys Ribbons of Various Thicknesses

Crystallization Studies of Hard Magnetic Pr₉Fe₅₆Co₁₃Zr₁Ti₃B₁₈ Alloys Ribbons of Various Thicknesses

Magnetic Hysteretic Properties of a Powdered Fe–27Cr–10Co–1Mo Hard Magnetic Alloy

The magnetic hysteretic properties of an Fe–27Cr–10Co–1Mo (27Kh10K1M) magnetic hard alloy are studied. Residual induction Br (up to 1.36 T), coercive force HcB (up to 45.4 kA/m), and maximum energy product (BH)max (up to 44 kJ/m3) have been evaluated using the Statgraphics Centurion XVI software package after optimum heat treatment. The obtained data are higher than those of a commercial iron–chromium–cobalt 25Kh15KA alloy (Russian Standard GOST 24897–81) by 15–20% on average. Different optimum heat treatment of the alloy is shown to be required to obtain the optimum values of its residual induction Br, coercive force HcB, and maximum energy product (BH)max.

Coercivity and Phase Evolution in Mechanically Milled (FeCo)2B-Type Hard Magnetic Alloy

Two types of re-substituted (FeCo)2B-type alloys with stoichiometric (Fe0.675Co0.3Re0.025)2B and 1.5 at% boron-excess compositions were prepared by suction casting and subsequent mechanical milling and annealing. Phase evolution in the (FeCo)2B-type alloys in the course of mechanical milling and annealing was investigated. Single-phase re-substituted (FeCo)2B-type material was prepared from stoichiometric (Fe0.675Co0.3Re0.025)2B by the combination of suction casting and mechanical milling. The two alloys had the same phase constitution of (Fe, Co)2B-type and amorphous phases in heavily milled state. However, the heavily milled alloys had different phase constitution after annealing; the stoichiometric alloy consisted of single (Fe, Co)2B-type phase, while the boron-excess alloy consisted of two-phase mixture of (Fe, Co)2B-type and (Fe, Co)B-type phases. The two heavily milled alloys had similar coercivity of around 1 kOe after full annealing.

Magnetic Hysteresis and Mechanical Properties of Hard Magnetic Fe–27Cr–15Co–2Mo–Si–Ti–V Alloy

Magnetic hysteresis and mechanical properties of a hard magnetic alloy Fe–27Cr–15Co–2Mo–V–Si–Ti in an isotropic and anisotropic state are investigated with the help of the method of design of experiments for the purpose of developing a new hard magnetic material with increased values of residual induction B r (isotropic state) and coercive force HcB (anisotropic state). Optimization of the modes of heat treatment is performed by the creation of central composite design 23 + star points. Statistical analysis of the data obtained makes it possible to ascertain the standard Pareto charts for B r , HcB, and (BH)max values for isotropic and anisotropic alloys. Nonlinear regression equations for dependences of B r , HcB, and (BH)max on variation factors are obtained, on the basis of which optimal regimes of heat treatment are determined. Magnetic hysteresis properties of Fe–27Cr–15Co–2Mo–V–Si–Ti alloy are enhanced significantly after an optimal heat treatment. As a result, a new hard magnetic material on the basis of the Fe–27Cr–15Co system is developed. In the anisotropic state, the values of residual induction, coercive force, and maximum energy product are B r > 1 T, HcB > 50 kA/m, and (BH)max > 24 kJ/m3, respectively, whereas in the isotropic alloy they are B r ~ 0.8 T, HcB ~ 40 kA/m, and (BH)max > 11.5 kJ/m3. Analysis of magnetic properties of specimens from the produced pilot batch of anisotropic Fe–27Cr–15Co–2Mo–V–Si–Ti magnets in the number of 51 pieces shows that the yield of magnets with HcB > 45 kA/m is 100% and with HcB > 50 kA/m is 83%.

Magnetic properties of powder hard magnetic Fe–27Cr–10Co–0.5Mo and Fe–27Cr–10Co–2Mo alloys

Magnetic hysteresis properties of powder hard magnetic Fe–27Cr–10Co alloys with 0.5 and 2.0 wt % molybdenum were studied. For Fe–27Cr–10Co–0.5Mo alloy: Br = 1.35 T, HcB = 45.6 kA/m and (BH)max = 41 kJ/m3. For Fe–27Cr–10Co–2Mo alloy: Br = 1.29 T, HcB = 41 kA/m, (BH)max = 34.8 kJ/m3. It was shown that an alloying with 2 wt. % molybdenum of the low-cobalt powder hard magnetic Fe–27Cr–10Co alloy unlike high-cobalt hard magnetic FeCrCo alloys did not increase its magnetic hysteresis properties, reduced the camber of a demagnetization curve in a second quadrant.

Coercivity mechanism of rare-earth free MnBi hard magnetic alloys

In this work, we present and discuss results concerning the hard magnetic behavior of rare earth-free MnBi alloys obtained by suction casting technique. The physics of coercivity for these type of alloys is based on the nucleation process of reverse domains, which in turn is determined by the alloy microstructure features such as phase distribution, morphology, grain size and in particular, defects, which are characteristic ofreal materials. The microstructure of the as-cast alloy presented here comprises the formation of the Low Temperature Intermetallic Phase (LTIP)-MnBi, interspersed within Bi- and Mn-rich areas. A considerable intrinsic coercivity field of 238 kA/m together with a saturation magnetization of 0.04 T were observed. The nucleation controlled mechanism of this alloy was described in terms of the Kronm¨uller equation, which incorporates the detrimental effect of microstructure defects through fitting parameters associated to reduced intrinsic magnetic properties at grain size boundaries, interfaces and local demagnetizing fields. A notorious switching of coercivity mechanism associated with domain wall pinning was found to be produced upon annealing of the alloy at 583 K for 24 hrs, yielding a drastic reduction of coercivity (down to 16 kA/m). The key microstructural feature determining the switching of coercivity mechanism is the formation/suppression of Bi-rich areas, which promotes the nucleation and growth of LTIP.

Response of Magnetic Force Microscopy Probes under AC Magnetic Field

In this paper, magnetic force microscopy (MFM) probes with different coating materials were characterized under AC magnetic field. A perpendicular magnetic write head similar to those used in hard disk drives was employed as the AC magnetic field generator. In order to measure a response of MFM probes to AC magnetic field, a MFM probe under test was scanned, at a scan height of 10 nm, across the surface of the magnetic write head. During MFM imaging, the write head was biased by a sufficient magnitude of AC current, approximately 30 mA. A spectral analysis for a frequency sweep from 1 kHz to 100 MHz was extracted from post-processing MFM images. As expected, a MFM probe coated with hard magnetic alloys, i.e. FePt, has the lowest response to AC magnetic fields. MFM probes coated with soft magnetic alloys, i.e. NiFe and NiCoCr, have a relatively high and flat response across the frequency range. Ni coated MFM probe has the highest response to AC magnetic fields. In addition, CoCr and NiCo coated MFM probes show lower response than NiFe and NiCoCr probes at low frequencies; however, theirs response to AC magnetic field increase for the AC magnetic field with a frequency above 50 kHz. This can be implied that those MFM probes are a good candidate for being used to study the high-frequency performance of perpendicular magnetic write heads. Noting that response of all MFM probes significantly decreased when driven frequencies above 1 MHz due to the limitation of the hardware, i.e. response of quadrant photodiode and op-amp in a pre-amplifier.

On the limits of coercivity in permanent magnets

The maximum coercivity that can be achieved for a given hard magnetic alloy is estimated by computing the energy barrier for the nucleation of a reversed domain in an idealized microstructure without any structural defects and without any soft magnetic secondary phases. For Sm1–zZrz(Fe1–yCoy)12–xTix based alloys, which are considered an alternative to Nd2Fe14B magnets with a lower rare-earth content, the coercive field of a small magnetic cube is reduced to 60% of the anisotropy field at room temperature and to 50% of the anisotropy field at elevated temperature (473 K). This decrease of the coercive field is caused by misorientation, demagnetizing fields, and thermal fluctuations.

Production of alloys in the Fe–Co system with an oxygen concentration below 10 ppm

A fundamentally new solution of the challenge of producing alloys in the Fe–Co system with an oxygen concentration below 10 ppm (10–3%) has been for the first time justified and demonstrated. A thermodynamic analysis showed that decreasing the pressure of the gas phase over the melt significantly increases the deoxidizing power of carbon. At cobalt and carbon contents characteristic of soft- and hard-magnetic alloys and a total pressure of 0.01 atm, the oxygen concentration was 10–1 ppm (10–3–10–4%). With increasing cobalt content of the melt, the deoxidizing power of carbon increases. The curves of the oxygen solubility in carbon-containing iron–cobalt melts pass through a minimum, which shifts toward lower carbon contents with increasing cobalt content of the melt.

Exchange coupled L10 FeCuPt/Fe heterostructures: Magnetic properties and reversal behavior at elevated temperatures

Exchange-coupled heterostructures were prepared by combining hard magnetic L10 FeCuPt alloy films with soft magnetic Fe thin films. For the FeCuPt thin film, the addition of Cu allows adjusting magnetic properties such as perpendicular magnetic anisotropy, coercivity, saturation magnetization, and Curie temperature. The coupling to thin Fe layers, providing high saturation magnetization and high Curie temperature, leads to lowering of the switching field of the hard magnetic FeCuPt layer and to a reduction of thermally induced recording errors. We have investigated the influence of the Fe layer thickness on the switching behavior of the hard layer, revealing a strong reduction for Fe layer thicknesses larger than the exchange length of Fe of about 2nm. Furthermore, superior performance of bilayer structures in comparison to single FeCuPt layers has been demonstrated by recording experiments at temperatures below 150°C, employing a scanning magnetoresistive microscope.

A Study of the Causes of Failure of Permanent Magnets from Cast Hard Magnetic Alloys

The microstructure and magnetic properties of permanent magnets fabricated from cast hard magnetic alloys of the Fe – Ni – Al system with cobalt addition of type YuNDK are studied. The causes of failure of such magnets in operation are determined. Recommendations are given for correcting the melting process and the modes of heat treatment of magnets in order to provide the required properties without disturbing the integrity of the magnets.

Concertation-Inhomogeneous Hard Magnetic Alloys of the Fe – Cr – Co System with Elevated Content of Cobalt and Boron

The structure and magnetic properties of a powder alloy based on the Fe – Cr – Co system with elevated content of cobalt and boron are studied. The dependences of the magnetic induction and of the coercivity on the aging temperature are determined experimentally. It is shown that the introduction of boron into the alloy hinders precipitation of σ-phase and makes it possible to raise the cobalt content by 4% as compared to standard alloy 30Kh23K.

Hybrid Electroless-Electrolytic Process for Metallization of 3D Printed Functional Microdevices

3D printing is currently finding an increasing number of applications in many different fields. This technique, relegated for years to high-end rapid prototyping due to its high costs, is now acquiring industrial relevance for the direct fabrication of objects. The well-known advantages of scalability, production rate and the possibility to produce shapes otherwise impossible to manufacture with conventional techniques are at the base of the exponential market growth of additive manufacturing technologies. Such advantages apply not only to the production of macroscopic structures, a practice already well implemented in the current practice, but also to the manufacturing of microscopic objects. 3D printing of functional devices in the millimetric and sub-millimetric dimensional range can be challenging due to the high resolution needed, in the order of tens of micrometers. This result is possible using only some of the technologies available: stereolithography (SLA), two photons lithography (2PL) or selective laser sintering (SLS). However, all these techniques are characterized by the same drawback: the difficulty to work with metals. Metallic parts are used when the functional structures produced must interact with magnetic fields or must be electrically/thermally conductive. Only selective laser sintering (SLS) is able to work on metallic parts with sufficient precision, but typically the objects does not have the same properties of a bulk counterpart, either forged or machined. A possible solution to get metallic properties on 3d printed objects is to impart them only to the surface via external metallization. Particularly advantageous from this point of view are the wet metallization techniques, thanks to their low cost and high productivity rates. The handling of very small parts is however difficult and, for this reason, suitable approaches must be developed. An approach to the plating of millimetric objects have already been investigated [1]. In that case, Cu and a CoNiP alloy were applied to freestanding 3D printed microdevices using a fully electroless process. The approach presented is characterized however by significant drawbacks, the first one being that only electroless alloys can be deposited. As an example, high performances hard magnetic alloys like CoPt are impossible to deposit. A second disadvantage is connected to the plating rate: electroless deposition must be carried out at low temperatures due to the physical characteristics of the 3D printing resins, and this strongly limits plating rate. Another possible approach to apply a metallic layer on a 3D printed microstructure is the application of an hybrid electroless/electrolytic metallization route. A first layer of a metal is electroless applied, according to previous literature [2], to make the surface of the resin conductive. Once a first layer of metal covers the surface, electrolytic deposition can be used to deposit the functional coating. This technique is described in the present work, where two methods are used to contact the object with the current generator. The first one is the exploitation of the 3D printing supports, generally used during the 3D printing steps, as electrical contacts. The second one is the use of a modified version of barrel plating. Functionality and performances of the two approaches are studied on prototypical microdevices printed via SLS. Finally, an applicative example is presented. The method described can find application in many sectors, including the production of magnetically controlled micromanipulators [3] or the additive manufacturing of MEMS. [1] ECS Meeting MA2016-01 1105 [2] R. Bernasconi, C. Credi, G. Natale, M. Tironi, F. Cuneo, M. Levi and L. Magagnin; ECS Trans. 72, issue 21 (2016) 9-21 [3] S. Kim, F. Qiu, S. Kim, A. Ghanbari, C. Moon, L. Zhang, B. J. Nelson and H. Choi; Adv. Mater.25 (2013) 5863-5868

Low Temperature Electroless Deposition of Hard Magnetic Alloys for the Metallization of Additive Manufactured Functional Microstructures

Low temperature electroless deposition of hard magnetic CoNiP alloys is presented in the article. Process compatibility with photocurable resins used in micro 3D printing is investigated, together with the influence of plating parameters on compositional, microstructural and magnetic properties. In-situ Raman spectroscopy is used to analyze the chemical processes occurring during electroless metallization. Such processes are correlated to the properties obtained in the final coatings. Finally, the plating process is applied to the metallization of prototypical stereolithography 3D printed micrometric samples.

Magnetic and Mechanical Properties of Hard Magnetic Alloys 30Kh21K3M and 30Kh20K2M2V

The magnetic and mechanical properties of two hard magnetic alloys (30Kh21K3M and 30Kh20K2M2V) of the Fe – Cr – Co system in anisotropic and isotropic conditions produced by traditional induction melting and by the method of powder metallurgy with subsequent pressure treatment of the metal are studied. The obtained regression equations for B r , H cB and (BH )max describe the quantities adequately in the used range of variation of factors. The response surfaces and their sections in the phase space of the heat treatment factors are plotted. The results of the analysis of the proportion of residual induction in anisotropic and isotropic states are used to infer that the model of the mechanism of magnetization reversal of hard magnetic alloys of the class considered needs amending.

Magnetic Properties of Hard Magnetic Alloy Fe – 28% Cr – 13.4% Co – 2% Mo – 0.5% Si

The method of regression analysis is used to obtain equations describing the dependences of magnetic hysteresis properties of magnetically hard powder alloy Fe – 28% Cr – 13.4% Co – 2% Mo – 0.5% Si on regimes of thermomagnetic treatment (the temperatures of the start of the treatment and the rates of cooling in magnetic field). The determined treatment modes make it possible to obtain in an alloy with a coercive force H c up to 40 kA/m, a residual induction B r up to 1.2 T, and a maximum energy product (BH)max up to 25 kJ/m3. The alloy may find application in the production of rotors of synchronous hysteresis-reluctance motors.