Cheryl L. Bowman

Glenn Research Center, Cleveland, Ohio

Magnetic Materials Suitable for Fission Power

Conversion in Space Missions

NASA/TM—2012-217752

December 2012
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Cheryl L. Bowman

Glenn Research Center, Cleveland, Ohio

Magnetic Materials Suitable for Fission Power

Conversion in Space Missions

NASA/TM—2012-217752

December 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

This work benefi ted from input and discussions by a myriad of colleagues. Drs. Keith Leonard, Jeremy Busby and Ali Sayir

were integral to the initial evaluation of Stirling alternator radiation sensitivity and contributed to this literature review. Dr. Omar 

Mireles pondered many of these issues as background to his Ph.D. thesis. Steven Geng and Janis Niedra provided invaluable 

insight in the characterization of high temperature alternator magnets. I would also like to thank Dr. Jon Goldsby and Debra 

Goodenow for reviewing the manuscript and making numerous clarifying suggestions. This work was supported by the Fission 

Surface Power project part of the Game Changing Technologies Program, formerly under the Exploration Technology Development 

Program (ETDP).

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Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217752 1 

Magnetic Materials Suitable for Fission Power Conversion 

in Space Missions 

Cheryl L. Bowman 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Terrestrial fission reactors use combinations of shielding and distance to protect power conversion 

components from elevated temperature and radiation. Space mission systems are necessarily compact and 

must minimize shielding and distance to enhance system level efficiencies. Technology development 

efforts to support fission power generation scenarios for future space missions include studying the 

radiation tolerance of component materials. The fundamental principles of material magnetism are 

reviewed and used to interpret existing material radiation effects data for expected fission power 

conversion components for target space missions. Suitable materials for the Fission Power System (FPS) 

Project are available and guidelines are presented for bounding the elevated temperature/radiation 

tolerance envelope for candidate magnetic materials. 

Introduction 

The Fission Power Systems (FPS) Project has been developing the technological foundations for 

power generation on the surface of the Moon, an asteroid, or Mars (Ref. 1). These technology 

development tasks include an investigation of the radiation tolerance of the power system components. 

NASA guidelines for establishing radiation tolerance with respect to a particular mission specify first to 

define the environment, then to review the anticipated radiation behavior, and finally to determine that the 

subsystems will perform as required in the predicted environment (Ref. 2). The Fission Surface Power 

System Initial Concept Definition document (Ref. 1) provided the baseline mission and environment 

definition. Other reports have identified candidate materials-of-construction that may be sensitive to the 

radiation environment resulting from a fast-spectrum fission reactor (Refs. 3 and 4) and reviewed 

experimental studies relevant to the mission specific radiation tolerance of polymeric components 

(Ref. 5). The objective of this summary is to provide a more complete review of the magnetic materials 

that may degrade in this particular space-mission-relevant radiation environment. 

The goal of this paper is both to review the relevant radiation literature and also to provide sufficient 

background to explain how the conclusions can be applied to different mission scenarios. Since the 

existing radiation-effects experimental data was generated for temperature and/or radiation conditions not 

necessarily the same as the current mission scenario, this report includes a review of basic phenomena 

and guidelines for extending the terrestrial focused database to space-mission relevant conditions. The 

FPS components and component materials have been described previously (Refs. 1 and 3). To 

summarize, a linear alternator is envisioned which will generate electricity from the relative motion of 

hard magnets with high coercive force strength/high remanent magnetic flux density, such as NdFeB class 

or SmCo class, and soft magnetic windings made from FeCo class alloys (Ref. 6). Specific property 

requirements depend on the details of the alternator design, but the radiation response of NdFeB, SmCo, 

and FeCo magnetic alloys are important factors to consider when designing a space power system 

designed for operating extended times in a radiation environment. 
NASA/TM—2012-217752 2 

Basis of Magnetism 

The foundations of material magnetism are based on the electronic behavior of the atoms. Each 

electron in an atom has a unique quantum number, which characterizes the electron shell (K, L, M, N, 

etc.), the subshell (either s, p, d, or f), the energy state, and the spin moment (either +1⁄2 or –1⁄2) (Ref. 7). 

The magnetic response of a material is fundamentally related to these electron positions and especially the 

spin behavior. All materials respond to applied magnetic fields, but the degree and the usefulness of the 

magnetic response vary greatly. Atoms that have full electron shells and subshells have only a weak, 

diamagnetic response to an applied magnetic field. This negative response is due to the applied field’s 

influence on the angular momentum of orbiting electrons (Ref. 8). From a practical standpoint, these 

materials are often considered non-magnetic. Atoms without completely filled shells/subshells have a 

permanent magnetic moment based on incomplete cancelation of the electron spin and can be either 

paramagnetic or ferromagnetic. The atomic-level magnetic moments for paramagnetic materials are 

random and these materials have no macroscopic magnetic strength except in the presence of an external 

field. Ferromagnetic materials, however, may possess a permanent magnetic moment in the absence of an 

external field due to the physical alignment of electron-spin moments in adjacent atoms. The adjacent 

atoms with aligned spin moments are called atomic dipoles and are the fundamental building block of a 

magnetic domain. Observations have shown a clear trend between valence electron structure and 

magnetic moment strength that results from a compromise between Coulomb attraction and kinetic 

energy (Ref. 8). Narrow d-shell bands have been found to favor magnetism and are prevalent in select 

transition metals and rare earth metals. 

The ferromagnetic materials that are useful in motor or generator power systems are those classified 

as both soft and hard. In this sense, soft and hard magnetic properties are analogous to ductile and strong 

in a mechanical system. Soft magnetic materials have magnetic domains that readily align to a magnetic 

field with minimal wasted energy. Conversely, hard magnetic materials resist change in their magnetic 

field alignment and can behave as a magnetic spring pushing back on an applied field. The term 

“permanent magnet” is sometimes used interchangeably with hard magnetic material, although no 

magnetic properties are permanent in a strict sense. Sufficiently high thermal energies, external magnetic 

fields, or combinations thereof will lead to magnetic realignment and, hence, loss of “permanence”. 

The following terms are useful for describing magnetic material properties. The magnetization, M, is 

defined either as the net magnetic moment per unit volume in the material or as the contribution of the 

intrinsic material magnetization to the total magnetic field. The maximum possible magnetic saturation 

for any material is defined by its atomic chemistry although the measured magnetic saturation, Ms, for a 

material changes as a function of temperature. The Curie temperature, Tc, represent the temperature at 

which the magnetic saturation drops abruptly to zero. 

The magnetic field strength, H, has the same units as M but represents the strength of the external 

magnetic field. The magnetic induction, B, or the magnetic flux density, takes into account both the 

external field and the material contributions. Induction strength is defined as B = 0H + 0M where 0 is 

the permeability in a vacuum. In general, permeability, μ, and susceptibility, χ, are related ways to 

describe the material magnetic response and μ = 1+χ. There are slightly different unit conventions are 

associated with the SI (tesla) or cgs (gauss) unit systems so care must be taken when comparing equations 

in the literature. 

Figure 1 illustrates the response of a ferromagnetic material starting in a demagnetized state (B = 

H = 0). The application of a positive external field, H, leads to an increase in measured magnetic field, B. 

A similar plot can be drawn for the material magnetic field response, M, versus external field. As the 

external field strength increases there is a realignment of magnetic domains until saturation is reached, Bs 

or Ms. When the applied field is reversed, the magnetic domains once again rotate along easy directions 

first and can produce a response curve with a hysteresis. The initial slope of the curve is the initial 

permeability, μi. The residual induction, Br, or remanence, Mr, represent the remaining magnetic field 

strength when there is no applied field. The coercivity, Hc, is the reverse (or negative) field strength that 
NASA/TM—2012-217752 3 

must be applied to return B to zero. Similarly the intrinsic coercive field, iHc, is the applied field needed 

to return M to zero for a H versus M relation. The shape of this B-H curve and the values of Bs, Br, Hc, 

and μi describe the engineering properties of the magnetic material. 

As the magnetic domain (region of aligned dipoles) increases, the magnetic strength of the material 

grows. There is a maximum possible magnetic strength, or magnetic saturation, of each ferromagnetic 

material based on its atomic structure. Therefore, chemistry changes either can raise or lower the 

magnetic saturation of a given metal alloy depending on how the minor element(s) atoms incorporate into 

the host lattice. The magnetic saturation is also a function of temperature because the increasing thermal 

energy causes atomic vibration which opposes the magnetic alignment forces. The Curie temperature is 

the temperature at which the long-range ordering of dipole spin alignment no longer occurs in 

ferromagnetic materials. Although the measurable magnetic strength disappears at the Curie temperature, 

it is interesting to note that neuron scattering experiments show that some very localized magnetic 

moments still exist (Ref. 8). 

The structure of the magnetic material at the atomic level, also known as the crystallographic 

structure, influences many of the magnetic parameters. Magnetic domains are bounded by domain walls 

that are similar to crystallographic grain boundaries in that they move more freely as temperature rises 

and their motion can be limited, or pinned, by microstructural features. Limiting grain boundary size is 

one common way to control domain size. Magnetic permeability or susceptibility (the proportional 

constant between M & H) and coercivity (remnant field at B = 0) are affected by microstructure as well as 

chemistry and temperature. The influence of the crystal structure on magnetic reorientation has to do both 

the metallic crystal symmetry and the atomic spin orbital shape. Domain wall movement, and therefore 

magnetic field reorientation, is easier along some crystallographic directions than others. For example, the 
NASA/TM—2012-217752 4 

easy direction, or easy axis, for body center cubic iron is along the [100]-type direction, which is the cube 

edges. The magnetic anisotropy constants, Ki, represent the difference in energy density required for 

magnetic saturation along an easy direction and a hard direction in the same alloy. K1~0 is best for soft 

magnets (e.g., ordered FeCo) and negative K1 implies a easy plane (family of directions), not just one 

easy direction. 

Microstructural pinning features that restrict domain wall movement typically are desirable for “hard” 

magnets and undesirable for “soft” magnets. These microstructural features include grain size, the 

presence of secondary phases or precipitates, and the presence of cold work. Crystallographic texture that 

results from non-random grain orientations can either improve or impede magnetic response depending 

on the orientation of the texture with respect to the magnetic field lines. Induced electrical currents also 

lead to energy loss in soft magnetic materials, so higher electrical resistivity is a desirable material 

property for soft magnets. 

Soft Magnetic Materials 

When domain wall movement and domain magnetic orientation occur in weak applied fields, Hc 

1000 Amp/m, then the material is considered a soft magnet (Ref. 8). In addition to low coercivity, high 

initial permeability is desired to have a tight B-H hysteresis loop. Soft magnetic material performance is 

improved by alignment of crystallographic directions which allow easy movement of magnetic domains 

with the anticipated applied field directions. Soft magnetic material performance is improved by 

properties that increase remanence and lessen magnetic anisotropy. Anisotropy in the crystal lattice is 

important because the slope near saturation is dominated by the last remnants of alignment and because 

increasing anisotropy impedes domain wall movement. Impurities, especially in the form of precipitates, 

and residual stress can also pin domain walls and decrease soft magnetic performance. Proper alloying 

benefits magnetic properties by decreasing resistivity or improving other properties such as grain size. If 

grain size is too large, there are too few domain walls and micro-eddy current losses become high. Grain 

sizes that are too small restrict domain movement. 

Soft magnetic alloys based on equiatomic FeCo alloys have a very high saturation strength, ~2.4 T, 

and relatively low magnetic anisotropy. This class is the preferred soft magnetic material for high flux 

density, high temperature applications and Hiperco 50 is a commercially available alloy in this class. 

Proper heat treatment produces a B2-type long range order which gives Hiperco 50 a high yield strength 

even at elevated temperature as well as high magnetocrystalline anisotropy (Refs. 9 and 10). Unfortunately the ordered microstructure does have low ductility. Because this composition goes through an 

order-disorder phase transformation around 730 °C, the magnetic and mechanical properties are highly 

sensitive to manufacturing processes. Additions of up to 2 percent V and 4 percent Ni can improve 

fabricability and mechanical properties with minimal negative impact on magnetic properties. For sheet 

products, this alloy class is hot rolled above 912 °C, quenched for cold rolling without long-range order, 

and then heat treated to regain long range order. Rolling into thin sheets for laminations can produce 

crystallographic textures that impede alignment of the easy magnetization directions with applied fields. 

Annealing and intermediate cooling rates after annealing are important to minimize adverse 

crystallographic texture and optimize magnetic properties (Ref. 8). 

Hard Magnetic Materials 

Hard or “permanent” magnetic materials are those which retain a relatively strong magnetic 

alignment and resist realignment from a reverse applied field as the result of a large residual induction, 

Br, and coercivity, Hc. Common permanent magnets have coercivities in the range of 104

 to 106 Amp/m 

(Ref. 8). It should be noted that for soft magnetic materials, the B-H curve and M-H curve are functionally equivalent, but that there is a distinction in hard magnetic materials. The intrinsic coercivity derived 

from the M-H curve is the shape independent description of coercivity. The B-H knee, or (B-H)max in the 
NASA/TM—2012-217752 5 

second quadrant (upper left in Fig. 1) is a common descriptor to characterize the strength or energy 

density of hard magnetic materials. Magnetic permanence is enhanced by and magnetic reversal 

suppressed by: 

1. Using chemistry or microstructure to increase magnetic anisotropy, 

2. Maximize remanence through preferred grain texture, 

3. Pinning domain walls or making single domain particles or grains, 

4. Minimizing opportunities for domain exchange between particles or grains, such as 

non-magnetic grain boundaries. 

Magnetic coupling of transition metal elements and lanthanide series (rare earth) elements vary in a 

systematic and predictable manner based on standard quantum physics (Ref. 8). There are several useful 

alloys of the RT5 and R2T17 series, where R = rare earth and T = transition metal. The phase boundary 

separation in the Sm-Co phase diagram is such that most SmCo alloys are in fact a combination of both 

SmCo5 and Sm2Co17 and will be generically referred to here as SmCo except when it is necessary to 

distinguish the phases. Based on a hexagonal structure (rather than a cubic structure), SmCo has a large 

magnetic anisotropy, but domain walls move readily unless pinned. However, limited substitution of Fe 

for Co and microstructure refining additions of Cu & Zr have led to SmCo alloys with B-H energy 

products up to 24 to 30 MOe. In summary, the simple alloys have coercivity that is grounded in 

anisotropy and nucleation of reverse domains is the limiting step. The more complex, higher energyproduct SmCo alloys rely on complex microstructure for grain refinement, domain pinning and thus are 

tailored based on both chemistry and heat treatment (Ref. 11). 

Alloys based on Nd2Fe14B1 have a high anisotropy based on a tetragonal crystal phase, a large 

magnetism due to the Rare Earth-Transition Metal coupling of Nd-Fe, and the ability to grow nonmagnetic B-rich and Nd-rich phases to decouple the magnetic grains. Very small grains and preferred 

grain orientation are required for optimal magnetic properties. Magnetic reversal appears to be dominated 

by nucleation and growth of reverse domains at temperatures up to 175 °C. Additional refinement options 

include partial substitution of Co for Fe to increase Tc and substitution of Tb for Nd to stabilize magnetic 

anisotropy to higher temperatures. 

The Curie temperature is higher for the Co-based rare earth magnets than the Fe-based magnets. 

Tc also increases with increasing Co content and decreases with increasing Fe content. Apparently an 

anitferromagnetic exchange dominates Fe-Fe bonds at smaller bonding separation which also explains 

why increasing pressure can also depress Tc in Fe-based alloys (Ref. 8). 

Radiation Damage to Magnetic Properties 

The Fission Surface Power System Initial Concept Definition document provided a baseline radiation 

environment description for system. Shielding was proposed that would limit dose at the power 

conversion components to 5 Mrad gamma radiation and 21014 n/cm2

 high-energy neutron fluence 

(Ref. 1). The nominal magnetic materials operating temperature, 150 °C, can also be obtained from this 

system description. Therefore this review can focus on radiation damage at specific fluence and 

temperature ranges. The magnetic materials relevant to this program are metallic alloys with secondary 

phases that can be intermetallic or ceramic. Radiation damage processes and effects on metal alloys are 

covered in great detail by Was (Ref. 12) and are summarized here. Gamma radiation is a type of ionizing 

radiation because the gamma photons have insufficient energy to affect an atom nucleus but can displace 

electrons and thus ionize atoms. Typically, ionizing radiation has minimal effect on alloys since the 

electrons redistribute readily in the metallically bonded crystal structure. Conversely, neutron radiation 

has sufficient incident energy to damage metallic materials. 

When a high-energy neutron strikes a material, a large energy transfer can take place. The struck 

atom is referred to as the primary knock-on atom (pka) and its energy is distributed to other nearby atoms 

by a series of secondary collisions in what is called a displacement cascade. These collisions produce 
NASA/TM—2012-217752 6 

empty lattice sites (vacancies) and atoms injected into the interstices between lattice sites (interstitials). 

Fortunately, most vacancies and interstitials recombine; those that do not are responsible for radiationinduced microstructural and mechanical property changes. The continuous production and subsequent 

diffusion of point defects lead gradually to the formation of point-defect clusters and other extended 

defects. The evolution of such extended defects in turn is responsible for the physical and mechanical 

property changes of the material. Through this basic damage process, incident radiation changes defect 

diffusion rates, dislocation microstructures, void/bubble formation, and elemental segregation resulting in 

localized chemistry changes and changes in phase stability (Ref. 12). Note that this is a stochastic process 

and radiation can pass through a material without striking any atoms at all. 

Radiation Studies With Soft Magnetic Materials 

In the early 1960s, Gordon and Sery studied the effect of radiation on numerous soft magnetic 

materials. Figure 2 shows their results for in-pile magnetic property measurements on a range of soft 

magnetic materials up to 21018 n/cm2

 (fast fluence ~1017 n/cm2) and sample temperatures of ~80 °C 

(Ref. 13). Alloy “2V Premendur” in these plots is chemically similar to the modern Hiperco 50. The ironnickel alloys optimized for very high initial permeability appeared to be most sensitive to irradiation 

damage. The iron-cobalt alloy appeared to be unaffected. Gordon and Sery continued to study the ironnickel alloys in various radiation fields and found that combinations of radiation and applied magnetic 

field could affect the magnetic domain alignment in much the same way as the combination of elevated 

temperature and magnetic fields (Refs. 14 and 15). These studies suggest that radiation can influence 

microstructural-controlled magnetic properties analogously to thermal treatments even when the bulk 

temperature remains low. Research specific to modern Hiperco 50 has shown that operating above 450 °C 

induces magnetic property degradations resulting from long-range order instability (Ref. 16). 
NASA/TM—2012-217752 7 

In his chapter on phase stability, Was (Ref. 12) reviewed irradiation induced order-disorder reactions for 

several alloys and suggest that the influence of radiation on the ordering parameter was dependent on 

more factors than just fluence or dose rate. A combination of elevated temperature and radiation 

environment would likely have synergistic impact on the phase stability in Hiperco 50 and magnetic 

property degradation seen reported by Lin (Ref. 16) may occur at lower bulk temperatures under 

irradiation. 

Radiation Studies With Hard Magnetic Materials 

Early radiation studies on SmCo magnets were performed using proton beam and proton induced 

neutron spallation radiation. Brown et al. found negligible deterioration in residual induction in SmCo 

after exposure to fluences up to 1018 n/cm2

 for neutrons with E  0.1 MeV (Ref. 17). Yet nearly 

concurrent studies in France subjecting similar SmCo alloys to lower fluences (~1016/cm2

 or 109 rad) of 

400 GeV high energy protons resulted in nearly complete demagnetization (Ref. 18). Blackmore 

performed similar studies using fluences from 106

 to 1010 rad of lower energy, 500 MeV, protons and 

distinguished the higher stability of the Sm2Co17 alloys compared to the earlier SmCo5 alloys (Ref. 19). 

These results, shown in Figure 3, confirmed that radiation energy level is a key factor and that both bulk 

chemistry (SmCo5 versus Sm2Co17) as well as specific chemistry/processing (Crucore 18 versus other 

SmCo5 magnets) are important. 

Cost et al. studied the effects of neutron irradiation on Nd-Fe-B magnetic properties (Ref. 20). NdFeB 

permanent magnets from two different manufacturers were irradiated in the Omega West Reactor at Los 

Alamos National Laboratory with fast neutrons, E  5 eV, at temperatures of 77 and 153 °C (350 and 

426 K) to fluences up to 61016 n/cm2

. The Curie temperature for both magnets was stated as 312 °C 

(585 K), therefore the exposure temperatures were nominally 60 and 73 percent of the Curie temperature. 

At intervals during the irradiation the samples were removed from the reactor and remanence measured at 

room temperature. Cadmium shielding around the port was used to minimize the thermal induced (n, ) 

 
NASA/TM—2012-217752 8 

reaction with the 10B isotope. The authors believed this damage would be different than that caused by 

higher energy neutron irradiation alone. The authors stated that the initial loss of remanence for 

irradiation to 1015 n/cm2

 was 10 percent at the higher temperature (~73 percent TC) and roughly 5 percent 

at the lower temperature (~60 percent TC). The loss rates were nearly the same for samples obtained from 

two different manufactures. The authors concluded that these losses are due to irradiation since the 

remanence did not decay with thermal exposure alone at 153 °C. Cause for greater decay at the higher 

temperature was speculated to be that a collision cascade has a higher probability of nucleating a reverse 

domain when the exposure temperature is closer to the Curie temperature. Also there was an increase in 

coercivity after radiation and re-magnetization, presumably due to agitation and relaxation of domain wall 

boundaries. This research demonstrated that there was a decrement in hard magnet permanence due to fast 

neutron exposure. 

Brown and Cost extended this research by focusing on fast neutron, E  5 MeV, flux of 41012 n/cm2

/s 

and a temperature of 77 °C for several NdFeB alloys with various length-to-diameter (L/D) ratios and 

grain sizes (Ref. 21). It is well established that the aspect ratio of a magnetic material coupon affects the 

magnetic field shape and influences magnetization/demagnetization processes (Ref. 8). As expected, the 

rate of remanence decay in this research decreased monotonically with increasing L/D. The authors tried 

various comparisons to find correlation between material characteristics and decreasing remanence. They 

could not correlate the grain size and radiation-induced decay; however, the range of grain sizes 

investigated was not large. They found that remanence decay rate did decrease with increasing intrinsic 

coercitivity. The linear correlation coefficient (R) between Hci and remanence decay rate was 0.78. A 

slightly better correlation was found between the decay and knee field (r = 0.83). The authors specifically 

point out that these correlations are at best rough guidelines due to the scatter in the results. These results 

confirm that irradiation induced degradation is sensitive to chemistry, processing, and the magnetic field 

shape/strength during irradiation. This highlights the need to understand the experimental specifics when 

attempting to compare results from various studies. 

Luna et al. exposed commercially available Sm2Co17 and Nd2Fe14B magnets and an experimental 

PrFeB alloy to gamma, electron beam, and mixed beam (electron/photon) radiation fields (Ref. 22). 

Exposure of Sm2Co17 and Nd2Fe14B magnets to a 60Co gamma source resulted in no measurable 

remanence loss. Direct exposure of Sm2Co17 and Nd2Fe14B magnets to an 82 MeV electron beam to a 

cumulative dose of 36 krad resulted in no measurable remanence loss in the SmCo but a 1.5 percent loss 

in the NdFeB. Samples of SmCo, NdFeB, and PrFeB were also exposed to a mixed electron/photon 

radiation field produced by the electron beam striking a tungsten alloy target. Results were slightly 

confused by the loss of water cooling during part of the experiments, but the SmCo was virtually 

unchanged even without active cooling and yet there was remanence loss in the NdFeB samples even 

when actively cooled. The PrFeB suffered the largest remanence loss as a result of irradiation. This paper 

also presents an often-referenced summary table which ranked alloy radiation hardness as Sm2Co17, 

SmCo5, followed by Nd2Fe14B as the least radiation tolerant. 

Zeller made some interesting comparisons between the character of demagnetization induced by 

elevated temperature and by irradiation (Ref. 23). He illustrated the field shape effects discussed 

previously by measuring magnetic field as a probe traverse a specimen normal to the magnetization 

direction. Elevated temperature exposures up to 54 °C produces more severe demagnetization in the 

center of the specimen than near the edges. This same shape relation was noted after exposure to 60Co 

gamma radiation and to a more notable extent after irradiation with 106 MeV 2

H charged particles. He 

noted that the presence of light elements such as boron should increase the susceptibility of the magnets 

to radiation-induced thermal spikes since the recoil energy is so much higher for light elements than 

typical metals. The author did not note any temperature measurements made during irradiation exposure 

and thus the possible effects of temperature and radiation damage cannot be separated. 

Finnish researchers published radiation studies in the early 1990s using 20 MeV protons (Refs. 24 to 

26) and delved further into the mechanisms of irradiation induced demagnetization. Talvitie et al. 

compared Nd-Fe-B to Nd-Fe-B doped with Nb and Dy, irradiated from –258 °C to room temperature, and 

with magnetization along either the short or long sample dimension, followed by positron defect 
NASA/TM—2012-217752 9 

measurements (Ref. 24). Their observations, including a 1000 times increase in sensitivity between 

–258 °C and room temperature as well as insensitivity of hysteresis curves, suggested that the magnetic 

losses were temperature driven and not related to defect formation. In a companion paper, they expanded 

their results with cube shaped specimens and introduced a simple theoretical frame work for modeling the 

irradiation induced changes (Ref. 25). The model suggests that the energy of the impinging radiation 

particle is transferred to the lattice in a spherical region surrounding the primary knock-on atom. The 

temperature then locally rises above the Curie temperature and the presence of a demagnetizing field can 

nucleate a reverse magnetic domain. They calculated the critical radius needed for reverse magnetic 

domain nucleation as a function of impacted atom type, initial temperature, grain size (actually domainlimiting microstructural feature size), and magnitude of the reverse magnetic field. In 1994 Kähkönen et 

al. published additional results that included magnetic packages rather than individual magnets, varied the 

impinging proton energy from 14 to 20 MeV, and studied the effects of alpha particle radiation at 

numerous temperatures from 22 to 295 K (Ref. 26). Their theoretical work suggests that the right 

combination of low demagnetization field and low global specimen temperature will preclude irradiationinduced demagnetization. They calculated that there would be no irradiation induced demagnetization in 

the fine grained, sintered Nd2Fe14B magnets they studied for temperatures less that 50 percent of the Tc

when magnetization is perpendicular to the short dimension of thin rectangular magnets, B/oH = –10. 

They reported that the irradiation induced demagnetization resulting from alpha particle radiation was 

also temperature dependent, although correlation rate was different and was attributed to a stronger 

energy transfer from the alpha particles to the lattice compared to the lighter proton particles. 

In the early 2000s researchers from Argonne National Laboratory (Refs. 27 and 28) and Fermi 

National Accelerator Laboratory (Refs. 29 to 31) published numerous studies addressing radiation 

tolerance of permanent magnets. Alderman and Job irradiated NdFeB magnets with x-rays, gamma-rays, 

fast neutrons up to 1.611014 n/cm2

, and with thermal neutrons up to 3.341012 n/cm2 (Refs. 27 and 28). 

Subsequent to irradiation, sample flux densities were measured. The fast neutron irradiation from 1 to 

2 MeV 252Ca exposure revealed residual induction measurements with no significant changes in flux 

densities after 11012 n/cm2

 exposure, but there was a 0.6 and 10 percent flux density degradation 

respectively after the 21013 and 1.611014 n/cm2

 steps. Thermal neutron irradiation was achieved with 

polyethylene-moderation of the 252Ca and fluences of up to 3.341012 n/cm2

 had no statically significant 

effect on residual induction. The authors claimed that the irradiations were performed at ambient (room) 

temperature, however it did not appear that there was active cooling or in situ temperature measurement. 

Volk reviewed permanent magnet radiation studies, emphasized the theoretical framework presented by 

Kähkönen et al., and proposed research to test the Kähkönen theory (Ref. 29). He proposed radiation testing 

magnetic materials with the same grain size and know magnetization direction but with significantly 

different coercivity. He emphasized testing magnetic materials in component configuration so that the 

demagnetization fields would be representative. A subsequent paper provided additional background 

discussing why existing literature did not have sufficient detail to confirm or refute the Kähkönen theory and 

they proposed a model magnet configuration that was a variation on the offset-quadrupole magnet design. 

This multipole would consists of three dipoles and a variable gap (Ref. 30). The arrangement of the dipoles 

would allow control of the self-demagnetization fields. Several papers presented preliminary radiation 

results for individual magnet blocks (Refs. 31 and 32). Unfortunately the research required to confirm the 

basic understanding was not completed due to insufficient funding (Ref. 33). 

Chen et al. studied the effect of neutron irradiation effect on Sm2Co17 and NdFeB magnets using a 

water-moderated reactor with a broad, heavily thermal, energy range. They showed that Sm2Co17-type 

magnets had significantly higher resistance to neutron irradiation than Nd-Fe-B-type magnets (Ref. 34). 

Their results supported the Kähkönen thesis that radiation-induced thermal spike leading to an increase in 

localized temperature, TL. For both Nd13Dy2Fe77B8 and SmCo magnets the temperature raised by thermal 

spike was dependent on the neutron flux, or reactor’s power, rather than neutron fluence. The recorded 

temperature in the NdFeB sample was 266 °C for the neutron flux of 2.11013 n/cm2

/s. The NdFeB 

magnets lost all their magnetic strength, and authors concluded that the local temperature at samples 
NASA/TM—2012-217752 10 

interior must exceed Tc = 309 °C. The Sm2Co17 magnets, however, had imperceptible change in magnetic 

flux after up to ~1020 n/cm2

. 

Additionally, there are a number of papers that summarize either magnet degradation as the result of 

in situ operation or test individual magnets destined for a specific application (Refs. 35 to 38). As was 

observed by previous authors (Ref. 30), simply cataloging the existing data does not provide the basic 

understanding to provide fundamental guidance. Very few of the papers report factors such as grain size 

and specimen temperature during irradiation. Most of the data is reported on commercially available 

magnets which have been found to vary chemically and in physical characteristics. Thus the community 

still has only a general guidelines to predict radiation induced demagnetization. 

The Effect of Gamma—Irradiation on the Magnetic Properties 

Boockmann et al. studied the effect of -radiation from a 60Co-source on the magnetic properties of 

sintered Rare earth- magnets. The magnetic properties of sintered SmCo5, Sm2(Co,Cu,Fe,Zr)17 and 

(Nd,Dy)15Fe77B8 magnets were not affected by -radiation from a 60Co-source up to accumulated dose of 

50 Mrad (Ref. 39). In addition to studying neutron irradiation, Alderman et al. performed parallel studies 

on NdFeB using x-radiation and  radiation (Ref. 27). Even with doses of 280 and 700 Mrad, respectively, 

the photon radiation had negligible impact on magnetic properties. Furthermore researchers study a 

synchrotron beam source detected no magnetic degradation due to x-ray exposure estimated to be 

12 Mrad (Ref. 40). Therefore, permanent magnetic degradation appears minimal due to x- or -radiation. 

The Effect of Electron—Irradiation on the Magnetic Properties 

Bizen et al. (Refs. 41 and 42) investigated the magnetic field change of undulator magnets when 

exposed to a 2.0 GeV electron beam. They characterized magnetic field intensity for specimen 

magnetized both parallel and perpendicular to the aspect ratio as well as individual and stacked magnets. 

Magnetic intensity decreased monotonically with accumulation of electron dose; approximately 

2.5 percent intensity loss for the accumulated electron dose of 401013 (Ref. 41). Subsequent experiments 

utilized vacuum and air thermal stabilization treatments prior to irradiation. Heat treatment at 142 °C for 

up to 24 hr led to a loss of magnetic intensity of up to 0.69 percent, but greatly reduced the subsequent 

irradiation-induced changes of the Nd2Fe14B magnets (Ref. 42). This result is similar to the increased 

stability seen with thermal treatments (Ref. 8) or controlled irradiation (Ref. 15). 

Summary and Conclusions 

The research for the radiation effect of permanent magnets was sparse before the 1980s, and primarily 

performed at the Department of Energy laboratories in order to evaluate the insertion devices for the 

synchrotron radiation facilities. In recent decades, numerous researchers investigated rare earth magnets 

exposed to neutrons, electron beams, protons, gamma-radiation, and X-radiation. Studies have shown that 

accumulation of neutron or high energy particle radiation can degrade magnetic properties. Numerous 

studies suggest that radiation damage affects both soft and hard magnetic properties in a manner that is 

analogous to and additive with thermal damage. The parallel or anit-parallel spin alignment of the orbiting 

electrons is sensitive to energy balance. Increases in kinetic energy in the material increases the mobility of 

magnetic domains and increases the likelihood of reverse domain nucleation, which promotes a realignment 

along external magnetic field lines. Increased kinetic energy can also alter the crystal structure of the 

magnetic material by standard processes such as grain growth, precipitation or solutioning. Alloys with 

highly engineered crystal structures can achieve the most desirable magnetic properties, but also have 

greater driving force to destabilize in response to increased thermal/radiological energy. 

 
NASA/TM—2012-217752 11 

Currently no physics-based models or experimental data are available to fully predict the effect of 

thermal and radiation conditions of a FPS mission on baseline magnetic materials. Significantly more 

studies have addressed the radiation tolerance of hard magnetic materials and theoretical models have 

been proposed, but not verified. There is a general consensus in the literature that bulk temperature, 

radiation, and demagnetization fields all influence magnet radiation hardness. A guideline of staying 

below 50 percent of Curie temperature (T300 K when Tc = 583 K) for small demagnetization condition 

was suggested for minimal radiation-induced degradation in specific NdFeB-type permanent magnets 

(Ref. 26). Note that this theoretical estimate with very specific assumptions including the size of reverse 

magnetic domains and the strength of the demagnetizing field. 

Although there is no direct experimental verification for the Kähkönen model, the trends in the 

literature support it. Figure 4 summarizes some examples of neutron irradiation experiments for rare earth 

permanent magnets. The measured bulk temperatures of three NdFeB experiments were 77, 153, and 

267 °C, which represented approximately 60, 73, and 86 percent of the Curie temperature. There was 

measurable post-irradiation remanence degradation in each case. In similar SmCo material studies, 

magnet bulk temperatures did not exceed ~40 percent of their Curie temperature and experience no 

measurable degradation. Thus it is suggested that, in absence of targeted experiments to define precise 

temperature/radiation limits, the permanent magnets should not be used at a bulk temperature exceeding 

50 percent of their Curie temperature in conjunction with neutron fluences that exceed 1016 n/cm2

. If the 

demagnetization field of the magnetic component is not low, then the bulk temperature should be lower 

than 50 percent of Tc. Modern Sm2Co17-type alloys have Curie temperatures on the order of 800 C 

(1073 K) (Ref. 34). These alloys should be functional in the modest neutron environments envision for 

the FPS convertors at temperatures up to ~265 °C (535 K). 

 
NASA/TM—2012-217752 12 

Radiation damage in soft magnetic materials has not been studied as extensively as the damage in 

hard magnetic materials. The most extensive radiation review suggested that the more highly engineered 

alloys with the highest initial permeability were the most susceptible to radiation induced degradation 

(Ref. 13). However, the data was reported for fluences of 1018 n/cm2

 and higher, which is well above the 

approximately 1014 n/cm2

 mission fluence currently considered. A reasonable supposition would be that 

the degradation mechanisms affecting soft magnetic materials would be dominated by the energy transfer. 

The published Curie temperature for candidate alloy Hiperco 50 is 938 °C (1211 K). If the 50 percent Tc 

limit discussed relative to hard magnets could be applied, the combined conservative radiationtemperature limit would be ~327 °C. However Hiperco 50 experiences discontinuities in the B-H curve 

after very short times at 580 °C and induction loss after extended times at 450 °C (Ref. 16). In a review of 

phase transformations in radiation environments, the influence of thermal spike distribution on orderdisorder transformation was described. The fundamental mechanism appears to be the same as that 

driving reverse domain nucleation in the hard magnetic material, although the particular temperature/ 

fluence relation need not be the same. In the absence of specific temperature/radiation interaction data, 

limiting Hiperco 50 application in a radiation environment to a bulk temperature of 160 °C (~50 percent 

of the general temperature limit) should be a conservative guideline to maintain optimum magnetic 

properties. Higher temperature applications are possible if the design can accommodate higher core losses 

resulting from decreased induction and increased coercivity. Studies on magnetic loss due to temperature 

effects alone can provide non-conservative guideline (Refs. 16 and 43) and additional performance losses 

can be anticipated due to radiation interactions for temperatures greater than 160 °C. The magnitude of 

the additional degradation could be estimated from modeling the radiation capture thermal distribution or 

could be measured experimentally. 

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Cambridge, England, 1994. 
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12. Was, Gary S.: Fundamentals of Radiation Materials Science: Metals and Alloys. Springer, Berlin, 

Germany, 2007. 

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no. 8, 1996, pp. 4578-4580. 

17. Brown, R.D.; Brush Jr., E.D.; and Hunter, W.T.: Radiation Effects on Samarium-Cobalt Permanent 

Magnets. LA—9437-MS, 1982. (also Proceedings 6th International Workshop on Rare Earth-Cobalt 

Permanent Magnets, Baden/Vienna, Austria, 1982). 

18. Coninckx, F.; et al.: Radiation Effects on Rare-Earth Cobalt Permanent Magnets. CERN/SPS 83-1 

(EBs) TIS-RP/IR/83-07, 1983. 

19. Blackmore, E.W.: Radiation Effects of Protons on Samarium-Cobalt Permanent Magnets. IEEE 

Trans. Nuc. Sci. vol. NS-32 no. 5, 1985, pp. 3669-3671. 

20. Cost, J. J.; et al.: Effect of Neutron Irradiation on Nd-Fe-B Magnetic Properties. IEEE Trans. Mag., 

vol. 24 no. 3, 1988, pp. 2106-2019. 

21. Brown, R.D.; and Cost, J. R.: Radiation-Induced Chances in Magnetic Properties of Nd-Fe-B 

Permanent Magnets,” IEEE Trans. on Mag., vol. 25 no. 3, 1989, pp. 3117-3120. 

22. Luna, Henry B.; et al.: Bremsstrahlung Radiation Effects in Rare Earth Permanent Magnets. Nuclear 

Instruments and Methods in Physics Research, vol. A285, 1989 pp. 349-354. 

23. Zeller, A.F.: Radiation Damage Mechanisms in NdFeB. Proceedings of the Eleventh International 

Workshop on Rare-Earth Magnets and Their Applications, Carnegie Mellon University, Pittsburg, 

PA, 1990, paper W6.2. 

24. Talvitie, M.; et al.: Magnetic flux loss in Nd-Fe-B magnets irradiated with 20 MeV protons. 

J. Magn. Magn. Mater., vol. 102, 1991, pp. 323-330. 

25. Kähkönen, O-P.; et al.: Radiation damage in Nd-Fe-B magnets: temperature and shape effects. 

J. Phys.: Condens. Matter, vol. 4, 1992, pp. 1007-1014. 

26. Kähkönen, O-P.; et al.: Effects of proton and α irradiations on permanent magnets. Phys. Review B, 

vol. 49, no. 9, 1994, pp. 6052-6057. 

27. Alderman, J.; et al.: Measurement of Radiation-induced Demagnetization of Nd-Fe-B Permanent 

Magnets. Nucl. Instr. and Methods in Phys. Res., vol. A481, 2002, pp. 9-28. 

28. Job, P.K.; et al.: Study of The Radiation Damage of Nd-Fe-B Permanent Magnets. RPSD Topical 

Meeting, American Nuclear Society, Santa Fe, NM, 2002. (also report no. ANL/AOD/CP-106006) 

29. Volk, J.T.: Summary of radiation damage studies on rare earth permanent magnets. eConf C010630, 

Stanford Linear Accelerator Center, SLAC-R-599 www.slac.stanford.edu/econf/C010630/ 

proceedings.shtml Accessed April 30, 2012. 

30. Spencer, James; and Volk, James: Permanent Magnets for Radiation Damage Studies. Proceedings 

of the Particle Accelerator Conference, Institute of Electrical and Electronics Engineers, Portland, 

OR, 2003, pp. 2180-2182. 

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of the 9th European Particle Accelerator Conference, European Physical Society Accelerator Group, 

Lucerne, Switzerland, 2004. (also report no. SLAC-PUB-10534). 

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Accelerator Conference, Institute of Electrical and Electronics Engineers, Knoxville, TN, 2005, 

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33. Volk, James T.: private communication, May 22, 2012. 
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34. Chen, C.H.; et al.: The Effect of Neutron Irradiation on Nd-Fe-B and Sm2Co17-Based HighTemperature Magnets. IEEE Trans. on Mag., vol. 41, no. 10, 2005, pp. 2106-2019. 

35. Pflüger, J.; Heintze, G.; and Vasserman, I.: Search for possible radiation damage on a NdFeB 

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36. Krebs, Gary F.; and Holmes, Michael: Measurement of the Radiation Incident on ALS NdFeB 

Permanent Magnet Insertion Device Structures and a Determination of Their Lifetime. Proceedings 

of the Particle Accelerator Conference Institute of Electrical and Electronics Engineers, Vancouver, 

Canada, 1997, pp. 796-798. 

37. Spencer, James; Mao, Stan; and Spencer, Cherrill: Further Experience with SLC Permanent 

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Demagnetization of Insertion Device Permanent Magnets. Proceedings of the 11th Biennial 

European Particle Accelerator Conference, European Physical Society Accelerator Group, Genoa, 

Italy, 2008. (also report no. BNL‐81453‐2008‐CP) 

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Mater., vol. 101, 1991, 345-346. 

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42. Bizen, T.; et al.: Baking effect for NdFeB magnets against demagnetization induced by high-energy 

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899. 

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Magnetic Materials Suitable for Fission Power Conversion in Space Missions 

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14. ABSTRACT

Terrestrial fission reactors use combinations of shielding and distance to protect power conversion components from elevated temperature 

and radiation. Space mission systems are necessarily compact and must minimize shielding and distance to enhance system level 

efficiencies. Technology development efforts to support fission power generation scenarios for future space missions include studying the 

radiation tolerance of component materials. The fundamental principles of material magnetism are reviewed and used to interpret existing 

material radiation effects data for expected fission power conversion components for target space missions. Suitable materials for the 

Fission Power System (FPS) Project are available and guidelines are presented for bounding the elevated temperature/radiation tolerance 

envelope for candidate magnetic materials. 

15. SUBJECT TERMS

Magnets; Permanent magnets; Power converters; Nuclear electric power generation; Radiation tolerance 

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