Buschow K.H.J. (Ed.) Concise Encyclopedia of Magnetic and Superconducting Materials

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of a given metal or compound. The absorption of hy-

drogen in a metal or intermetallic compound leads to

drastic changes of the electronic structure not only due

to volume and structural changes but also due to

chemical effects (see Metal Hydrides: Electronic Band

Structure). In a simpliﬁed view the effect of hydrogen

absorption upon the magnetic properties in transition

metals and their intermetallics is twofold: First, the

presence of hydrogen perturbs the metal–metal bonds

by creation of metal–hydrogen interactions, and hy-

drogen contributes two electronic states but only one

electron which changes the Fermi level and hence

modiﬁes the transition metal moment in the band pic-

ture. Second, hydrogen absorption commonly results

in a volume expansion (i.e., negative pressure), which

inﬂuences not only the strength of the magnetic ex-

change interaction between localized/delocalized elec-

trons but increases also the extent of localization of

their associated magnetic moments. In other words, a

larger volume implies narrower bands which, on the

one hand, gives rise to more localized moments and,

on the other hand, may reduce the hybridization of

different electronic states present in the host com-

pound. Accordingly, the ground-state properties such

as the type of magnetic order (e.g., ferro-, ferri-, an-

tiferromagnetism, etc.), Pauli- or Van Vleck paramag-

netism or superconductivity as well as their

temperature dependences are significantly changed up-

on hydrogenation. For experimental and theoretical

details, we refer to the comprehensive review articles of

Buschow et al. (1982), Buschow (1984), Wiesinger and

Hilscher (1988, 1991), Vajda (1995), and Pourarian

(2002).

For the description of magnetic properties partic-

ularly the localized/delocalized nature of the electrons

appears to be an important factor. One has to con-

sider two extreme limits.

The localized limit for which the magnetic mo-

ments and their ﬂuctuations are localized in real

space (delocalized in reciprocal space). For these sys-

tems the Heisenberg model, at least as a prototype, is

appropriate where the phenomenological Weiss mo-

lecular ﬁeld is just the exchange interaction which

tends to align spins parallel (or antiparallel) when the

exchange integral is negative (or positive). Here the

carrier of magnetism is a spin and/or an angular

momentum yielding localized moments, and the mag-

netization vanishes at T

because of disorder between

the local moments due to thermal ﬂuctuations. Nev-

ertheless, their absolute value (or amplitude) remains

almost independent of temperature. Typical examples

are 3d-transition metal oxides that are insulators. In

these materials, the exchange interaction proceeds via

oxygen, which is called superexchange. Another type

of compounds with strongly localized moments are

rare-earth intermetallics where the localized moment

arises from the 4f-electrons that are well localized in

space and also well screened from the surrounding by

5s- and 5p-electrons. The magnetic interaction be-

tween the 4f-moments is mediated by the oscillatory

polarized conduction electrons and is called Ruder-

man–Kittel–Kasuya–Yosida (RKKY) exchange.

The itinerant limit for which the moments and

their ﬂuctuations are delocalized in real space (local-

ized in reciprocal space) with their amplitudes (or

magnitude) being strongly temperature dependent.

The simplest approach is the Stoner–Wohlfarth itin-

erant electron model where the magnetic moment is

determined by the number of unpaired electrons in

the exchange-split spin-up and spin-down bands.

With growing temperature the thermal excitations of

electron–hole pairs (single-particle excitations) reduce

the exchange splitting and thus favor the paramag-

netic state at elevated temperature. Consequently, the

magnetization disappears if the absolute value of the

magnetic moment goes to zero, which happens if the

exchange splitting is zero. This model sufﬁciently de-

scribes magnetism in metals at 0 K. Unfortunately, it

predicts Curie temperatures (T

) which are 5–10

times larger than those observed experimentally, be-

cause single-particle excitations are not (or only to a

reduced extent) responsible for metallic magnetism at

elevated temperatures. Accordingly, for quite a long

time 3d-magnetism has been a controversial topic,

where still some problems are not completely settled.

The reason for this controversy is the absence of a

general agreement upon the microscopic nature of the

magnetic excitations at ﬁnite temperatures in partic-

ular above and below T

To solve these inconsistencies, thermally induced

collective excitations of the spin system—as they are

well known for localized spins—are included by

Moriya to formulate a uniﬁed picture of magnetism.

A similar and very promising approach by Murata

and Doniach was to introduce local and random

classical ﬂuctuations of the spin system being excited

thermally which are called spin ﬂuctuations. Both

models become equivalent at high temperatures and

lead to a Curie–Weiss law above T

and allow the

calculation of realistic ab initio Curie temperatures

(for details see, e.g., Mohn (2003) and references

therein).

1. Binary Hydrides of d-metals

Most transition metals form hydrides at sufﬁciently

high pressures of hydrogen. The difﬁcult high-

pressure preparation technique and the rather limit-

ed hydrogen solubility of d-metals constrain the

available results, which are reviewed by Antonow

(2002). The following binary hydrides were found

to exhibit magnetic order: a-MnH

, g-MnH

and

e-MnH

order antiferromagnetically, the latter two

with a collinear spin structure (T

c300 K for

g-MnH

and T

E360 K for e-MnH

). Double h.c.p.

-FeH

(xE1) is a ferromagnet with T

exceed-

ing room temperature, while h.c.p. e-FeH

0.42

900

Metal Hydri des: Magnetic Properties

paramagnetic down to 4 K. Hydrides of Co are ferro-

magnets where, in the e-hydrides, the moments are

aligned along the c-axis and decrease with increasing

hydrogen content at a rate of 0.36m

per H atom. g

Ni hydride is ferromagnetic and T

decreases with

hydrogen content. The interpretation of the drastic

changes of the magnetic properties mainly involves

phenomenological and rather qualitative arguments

based on the rigid band model. Exceptions are binary

and ternary Pd-hydrides where superconductivity is

induced by hydrogen absorption (see Metal Hydrides:

Electronic Band Structure).

2. Binary Hydrides of Rare Earth and Actinides

(4f- and 5f-metals)

Contrary to d-metals the rare earths absorb hydrogen

readily and form solid solutions and binary hydrides

(RH

) with stoichiometries x ¼ 2 and x ¼ 3. The ge-

neric phase diagram according to Vajda (1995) is

displayed in Fig. 1. It consists of three basic parts: (i)

the metallic solid solution (a-RH

) with H inserted

into the tetrahedral interstices of the host metal; (ii)

the metallic dihydride b-RH

crystallizing in the

f.c.c. ﬂuorite system where two hydrogen atoms oc-

cupy the two available tetrahedral sites; (iii) the in-

sulating h.c.p. trihydride g-RH

3–d

where both

tetrahedral sites and one octahedral site are ﬁlled

up with H. In the case of x approaching 3, metallic

conductivity disappears, which has been attributed to

the formation of a low-lying s-band with the capacity

to hold six valence electrons which has been con-

ﬁrmed to a great extent by band structure calcula-

tions. In fact, in this simple view, the number of

electrons supplied to the conduction band by one R-

atom and three H-atoms is equal at x ¼ 3. Since this

low-lying bonding band is completely ﬁlled up with

electrons, in RH

conduction electrons are no longer

present, prohibiting interactions occurring via the

RKKY mechanism. This accounts for the suppres-

sion of magnetic interactions to below 3–5 K, where

magnetic dipole–dipole interactions become impor-

tant, which is fairly generally observed, indeed.

Considerable progress has been achieved with re-

spect to the details in off-stoichiometric b- and

g-phases since the discovery of the metal–insulator

transitions by Vajda (1995) and the associated optical

properties applicable to switchable mirrors (Huiberts

et al. 1996). The magnetic properties in the rather

extended solid solution (a-RH

) and dihydride

(b-RH

27d

) are significantly affected by ordering in-

teractions between the d-hydrogens. In the case of

magnetic rare earths, this gives rise to various fasci-

nating phenomena caused by the competition be-

tween magnetic anisotropy and RKKY interactions

on the one hand and the hydrogen sublattice via

crystal ﬁeld interactions on the other. The peculiar

quasilinear H–R–H ordering along the c-axis in the

a-phase of some h.c.p. rare earth (as well as in Sc and

Y) gives rise to the low-temperature ordered a

–phase

which affect the modulated magnetic structures (i.e.,

sinusoidal or conical) via Fermi surface topology

(Vajda 2000). In the b-RH

2 þd

phase, the excess

d-hydrogens occupying the octahedral sites in the

CaF

structure frequently order in a tetragonal hy-

drogen sublattice corresponding to an RH

2.25

stoic-

hiometry which modiﬁes the various commensurate

and incommensurate magnetic structures, sometimes

leading to the appearance of short-range magnetic

order or even yielding a breakdown of magnetism.

From the early actinides (Th, Pa, and U), only

a- and b-UH

order ferromagnetically below B 180 K

(for a summary, see Wiesinger and Hilscher 1991),

while the heavier rare-earth-like actinide hyd-

rides crystallize in the CaF

structure. Rather simi-

lar behavior is observed for AH

2 þd

systems

(A ¼ Np; Pu; Am) as for the isostructural b-RH

2 þd

(Vajda 2000).

3. Hydrides of Intermetallics Containing

d-transition Metals and Rare-earth Metals

When a transition metal (TM) is alloyed to a rare

earth (R) or a related metal as Y, Sc, and Zr, the

R–3d exchange interaction (3d–5d overlap) leads to a

significant reduction of the TM moment. The strong

hydrogen afﬁnity of the R-metals brings about a

Figure 1

Schematic phase diagram for R–H systems: a, solid

solution, a

, ordered H-sublattice in some R–H

solutions; b, dihydride; g, trihydride (reproduced by

permission of Vajda (1995) from Handbook on the

Physics and Chemistry of Rare Earths, Vol. 20, pp. 207–

29; r Elsevier).

901

Metal Hydri des: Magnetic Properties

decrease of the 3d–5d overlap in the hydrides. Thus,

the absorption of hydrogen frequently cancels this

moment depression to a certain degree. This partial

restoration of the 3d-moment is interpreted as a hy-

drogen-induced screening effect. In R–3d intermetal-

lics and their hydrides, where both the R- and the

3d-elements carry a magnetic moment, we can dis-

tinguish between three main types of magnetic inter-

actions which are quite different in nature: (i)

between the localized 4f-moments; (ii) between the

more itinerant 3d-moments; and (iii) between 3d-

and 4f-moments. Generally, it is observed that these

interactions decrease in the following sequence:

3d23d4 4f 23d4 4f 24f .

In contrast to binary 4f-hydrides, for ternary

R23d hydrides no similar straightforward arguments

can be given about the hydrogen-induced change of

the magnetic order. The only statement being gener-

ally valid is that upon hydrogen absorption the mag-

netic order of R–Co and R–Ni compounds is

considerably weakened. However, this is only rarely

observed in the case of R–Fe compounds. Thus, hy-

drogen absorption usually weakens the magnetic

coupling between 4f- and the 3d-moments via the re-

duction of the 4 f 23d exchange interaction that can

lead to substantial changes of the 3d-transition metal

moment in either way. This is explained by a reduced

overlap of the 3d-electron wave functions with the

5d-like ones due to the narrower bandwidth as a

consequence of the hydrogen-induced increase in

volume. Furthermore, concentration ﬂuctuations of

H-atoms over a few atomic distances may frequently

occur leading to a difference in electron concentra-

tion between one site and another. This can therefore

lead to a varying coupling strength and/or to com-

peting interactions, giving rise to frustration and

freezing phenomena at low temperatures. Addition-

ally, a disturbance of the lattice periodicity takes

place in the hydrides, reducing the mean free path of

the conduction electrons. This leads to a damping of

the RKKY conduction electron polarization, which

in turn decreases the magnetic coupling strength.

If the magnetic order in R-intermetallics is domin-

ated by 4f-moments, the concept of an R–H charge

transfer by analogy with binary rare-earth hydrides

has proved to be a reasonable ﬁrst approach for

explanations of hydrogen-induced changes in mag-

netism.

In the case where 3d-magnetism is dominant in the

R–3d compounds, no general rule can be given. Com-

monly, hydrogen absorption leads to a loss in 3d-

moment in Ni- and Co-based intermetallics, but to an

enhancement of the Fe-moment. For Mn-intermetal-

lics both changes from paramagnetism to ferromag-

netism and vice versa are obtained. In Fe-containing

intermetallic hydrides the 3d-states are localized to a

greater extent compared to the parent compound.

This leads to an enhancement of the molecular ﬁeld,

which, however, is opposed by the inﬂuence of the

increased Fe–Fe distance, tending to reduce it. As it is

observed experimentally, the former mechanism is

apparently the dominating one, yielding an increased

or at least an unchanged molecular ﬁeld constant n

RFe

upon hydrogenation.

When discussing the hydrogen-induced change of

the magnetic properties, one is, among other things,

faced with the problem of ﬁnding reliable moment

data. Frequently, one has to rely on magnetization

measurements. The latter may lead to wrong results

in those cases, where for experimental reasons (lack

of a high ﬁeld facility), only incomplete saturation

has been achieved. Particularly in the case of ternary

hydrides, magnetic saturation is difﬁcult to obtain.

An alternative way is offered by Mo

ssbauer meas-

urements. However, the problem of correlating the

hyperﬁne ﬁeld unambiguously with the magnetic mo-

ment (particularly in the case of the hydrides) still

remains. Only in few cases one can refer to reliable

data from neutron diffraction experiments. For a de-

tailed summary of the experimental data in this ﬁeld,

we refer to Yvon and Fischer (1988).

Subsequent to the availability of synchrotron radi-

ation (see Magnetism: Applications of Synchrotron

Radiation), x-ray absorption (XAS), and x-ray circular

magnetic dichroism (XCMD), several studies were

devoted to investigations of the electronic states in

hydrides. The former simpliﬁed approach, where

changes of magnetic properties upon hydrogenation

were frequently attributed to the inﬂuence of hydro-

gen on the 3d-sublattice only, has to be modiﬁed. On

the contrary, an important role is played by the rare

earth sublattice through the R(5d)–TM(3d)hybridiza-

tion, which is weakened upon hydrogen uptake.

XCMD experiments at the L

2,3

edges of a rare-earth

element and K-edge of a 3d-transition element are

particularly suitable to study hydrogen-induced

changes of the magnetic properties, since they allow

to probe directly the local states of the transition and

the rare-earth element. Furthermore, the valence of

the latter species can be determined, which in the case

of Ce and Yb compounds is of essential importance.

x-ray absorption near edge structure (XANES) studies

are capable for probing both the electronic unoccu-

pied states and the local structure at a selected site. In

particular, L

III

absorption spectroscopy yields infor-

mation on inner shell properties, 4f-occupation num-

bers, and valence shell properties of rare-earth systems

that is related to the magnetic properties of the system.

Due to an unstable valence, Ce and Yb compounds

are of particular interest in metal hydrogen studies.

4. Hydrides of Mn Compounds

Particularly for Mn-containing ternary hydrides—

where mainly RMn

and R

have been investi-

gated—no general prediction can be made for

the changes in magnetic properties upon hydrogen

902

Metal Hydri des: Magnetic Properties

uptake. Onset and complete loss of magnetic order

after hydrogen absorption are found, as well a sub-

stantial reduction of T

and the magnetization.

Moreover, spin glass behavior is frequently obtained

for Mn-rich ternary hydrides, which most probably

has to be related to the presence of Mn segregations.

The reason for these heterogeneous results probably

lies in the specific sensitivity of the magnetic prop-

erties of Mn compounds upon interatomic distances.

Particularly in Mn compounds a critical Mn–Mn

distance is of essential importance for the occurrence

of magnetic order. In contrast to parent RMn

com-

pounds, in the corresponding hydrides the Mn–Mn

magnetic interaction dominates and imposes magnet-

ic order in the rare-earth sublattice, with the result

that both the R- and Mn-sublattices order at the

same temperature. To completely understand the

complex process of magnetic ordering present in

these compounds, band structure calculations have to

be performed and considered.

The Laves-phase compounds RMn

have been in-

tensively studied because of their peculiar structural

and magnetic properties. Two magnetic sublattices

are present: the ﬁrst one is built up from well-local-

ized 4f-moments of the rare earth, whereas the second

one consists of unstable 3d Mn-moments, strongly

dependent on the Mn–Mn distance. The topological

frustration present in the parent compounds is re-

leased when hydrogen is absorbed by forming chang-

es of the local symmetry of the magnetic ions. The

hydrogen-induced lattice expansion stabilizes the

Mn-moments (Goncharenko et al. 1999). Large in-

creases of T

are observed upon hydrogen uptake.

The spin orientation in the hydrides commonly, dif-

fers from that observed in the parent compounds and

cannot be explained by a unique set of crystal ﬁeld

parameters. This leads to the suggestion that the hy-

drogen superstructure significantly inﬂuences the an-

isotropy (Cadavez-Peres et al. 2001). CeMn

1.8

0.2

absorbs more than 4.4 H atoms/f.u., thereby under-

going a volume expansion ( DV=V ¼ 42:5%, which is

the largest known among metal hydrides). Concom-

itantly, a metal atom site exchange occurs over a

distance of 2.7 A

at room temperature, which is the

ﬁrst of that type observed in Laves phases. The ex-

ceptional high mobility of the metal atom substruc-

ture is presumably related to the valence transition

þ4

to Ce

þ3

, which is accompanied by a change

from Pauli paramagnetism to antiferromagnetic or-

der below 75 K and Curie–Weiss susceptibility above;

see Fig. 2 (Filinchuk et al. 2003).

5. Hydrides of Fe Com pounds

In this case the hydrogen-induced inﬂuence on the

magnetic properties is less pronounced than in Mn-

compounds. Predominantly, the following stoichiome-

tries were studied: R

,RFe

X, R

B, R

X, RFe

,RFe

,andZr

Fe. Various neutron

diffraction experiments proved that there is an increase

of the Fe-sublattice moment after hydrogenation,

whereas the rare-earth sublattice moment is generally

reduced. The Curie temperature can show changes in

either direction; the behavior depends on the amount

of Fe in the compound. Compensation points, when-

ever present in the parent material, are lowered upon

the absorption of hydrogen, thus reﬂecting the reduced

rare-earth sublattice contribution to the total magnetic

order. In several cases spin reorientations upon hydro-

gen insertion were found. When the R–Fe exchange is

comparable to the local anisotropy energy of the rare

earth, the R-momen t is strongly coupled to the crystal

ﬁeld axis randomized by the presence of hydrogen.

This may lead to a fanning of the R-moments at low

temperatures. In that case a hydrogen-induced rise in

moment is observed; it only takes place up to a certain

hydrogen concentration, beyond which a complete loss

of any magnetic order is found, frequently accompa-

nied by complete damage of the lattice periodicity.

A systematic study of almost the whole series

was performed (see, e.g., Grandjean et al.

2002). Hydrogen uptake commonly increases the

magnetic ordering temperature as well as the satura-

tion magnetization. Spin reorientations present in the

parent compound are suppressed by the insertion of

hydrogen, an exception being Pr

. There, hy-

drogen uptake leads to a slight decrease of the

hyperﬁne ﬁeld. In Ce

, on the other hand, the

absorption of hydrogen changes the helical magnetic

structure into ferromagnetic alignment of the mag-

netic moments within the basal plane.

While in the context of metal hydrides predomi-

nantly polycrystalline materials (mainly powder) are

studied, in rare cases (as for RH

) single crystals of

hydrides could be prepared that allow us to study, for

Figure 2

Zero-ﬁeld cooled susceptibility (M/H) as a function of

temperature for CeMn

1.8

0.2

and CeMn

1.8

0.2

3.5

; the

antiferromagnetic order of CeMn

1.8

0.2

3.5

at about

75 K is shifted to below 10 K at 6 T.

903

Metal Hydri des: Magnetic Properties

example, the magnetocrystalline anisotropy in

(x ¼ 0; 3) crystals (Tereshina et al. 2003).

In one case (R ¼ Ho) hydrogen induces an FOMP-

type transition; for R ¼ Er this transition disappears

after the absorption of hydrogen. For R ¼ Tb an

easy cone transition is observed upon H-uptake. The

experimental results are interpreted on the basis of

the orientation of the 4f-shell quadrupolar moment

with respect to the direction of the electric ﬁeld gra-

dient due to the charges of the surrounding ions.

Ce exhibits intermediate valence behavior (v ¼ 3 :3)

in CeFe

,CeFe

,andCe

; this be-

havior is essentially unaffected by the absorption of

hydrogen. This is in contrast to the usual ﬁnding that

upon hydrogen absorption the significant volume ex-

pansion is associated with a change of tetravalent to

trivalent Ce. In this case, the increase of the magnet-

ization upon hydriding is not due to a significant Ce

4f-contribution via localization of the Ce 4f-moment.

From XCMD signals a complete 4f-delocalized band

is obvious, conﬁrming that the itinerant character of

the Ce 4f-states is also retained in the hydrides. This

has to be attributed to the variation of the 3d(Fe)–

5d(Ce) hybridization. It appears that although the

hydrogen uptake is due to the H–Ce interatomic

chemical attraction, the effect on the physical proper-

ties is rather due to a change of the Fe sublattice. The

magnetism on the Fe-sites is enhanced upon hydrogen

uptake, even if hydrogen is not attracted by Fe-atoms.

The large-hydrogen-induced volume increase cannot

be attributed to a significant change in the Ce valence;

it is mainly due to a relaxation of the stressed Fe–Fe

interatomic distances (Isnard et al. 1994).

In recent investigations on the high-performance

Nd–Fe–B magnets, a group of compounds with the

stoichiometry R

X was found as an intergranular

phase in sintered magnets. Since the Nd–Fe–B mag-

nets are frequently produced via the hydrogen

decrepitation (HDDP) process, R

X hydrides

became a matter of interest. Despite their large

Fe-content, the parent compounds exhibit a low net

magnetic moment below 1 m

/f.u. due to a ferrimag-

netic coupling of the moments. Upon hydrogen up-

take, these compounds behave as ferromagnetic,

exhibiting moments above 25 m

/f.u. at room tem-

perature (Leithe-Jasper et al. 1996). In the case of

X ¼ Sn, it was demonstrated by

119

Sn Mo

ssbauer

spectroscopy that a small hyperﬁne ﬁeld, being absent

in the parent compound, develops at the Mo

ssbauer

nuclei upon hydrogenation (Leithe-Jasper et al.

1997).

The itinerant metamagnetic transition associated

with large magnetocaloric effects in La(Fe

1x

)

triggered the investigation of the corresponding hyd-

rides (Fujita et al. 2003). Hydrogen absorption in-

creases both T

and the metamagnetic transition

significantly and thus makes La(Fe

1x

)

as one

of the most promising magnetic refrigerants working

in a wide temperature range around room temperature

and in fairly low magnetic ﬁelds (see Magnetic Re-

frigeration at Room Temperature).

6. Hydrides of Co Compounds

Compounds with various stoichiometries (R

B, RCo

, RCo

) were studied

extensively in the past, the results being treated in the

reviews cited. In contrast to the corresponding Fe-

systems, in all the Co-based systems hydrogen uptake

reduces the magnetic order of the Co-sublattice.

Values obtained for Curie temperatures and magnetic

moments (R as well as Co moments) in the hydride

are lower compared to those in their parent counter-

parts. The hydrides generally show a pronounced re-

sistance against magnetic saturation. With ultra-high

ﬁelds (up to 110 T) and high-pressure experiments

(P ¼ 12 kbar), it was possible to estimate the effects

of lattice expansion and changes of the electronic

structure in YCo

1.8

and Y(Co

0.88

0.12

)

where

itinerant metamagnetic transitions occur from the

low-moment to the high-moment state. Upon hydro-

gen uptake the critical ﬁeld B

is substantially re-

duced (see Fig. 3), whereas upon applying pressure

the critical ﬁeld B

increases at the rate of dB= dP ¼

1:46 T kbar

1

. From these data it was concluded that

both contributions—the lattice expansion and chang-

es of the electronic structure—are of similar magni-

tude (Bartashevich 2001), which should be proved by

band structure calculations.

Yamamoto et al. (2002) reported on an increase of

the equilibrium hydrogen pressure as a function of

magnetic ﬁelds in RCo

(R ¼ La; Pr; Sm) accord-

ing to the relation lnðP

Þ¼2BDM

=RT, where

Figure 3

Magnetization as a function of external ﬁeld for YCo

and YCo

1.8

; the arrows indicate the different critical

ﬁeld for the metamagnetic transition (reproduced by

permission of Bartashevich et al. (2001) from Physica B

294–295, 186–9; r Elsevier).

904

Metal Hydri des: Magnetic Properties

and P

are the equilibrium hydrogen pressures

under a magnetic ﬁeld B; R is the gas constant is and

is the change of the saturation magnetization

per desorbed mole of hydrogen. Figure 4 demon-

strates the correspondence between the ﬁeld variation

and changes of the hydrogen pressure, the effect of

ﬁeld being largest for the b þ g region in PrCo

4.4

In this context, it is worth noting that the magnetic-

ﬁeld-induced change in the heat of reaction was cal-

culated and experimentally proved for some RCo

systems by the same group (see, e.g., Yamaguchi et al.

2002). For a chemical reaction the magnetic ﬁeld

contributes to the chemical potential, yielding mag-

netothermodynamic effects: as a result, when a mag-

netic ﬁeld is applied, the heat of reaction increases in

the a þ b region, whereas it decreases in the b þ g

region for LaCo

7. Hydrides of Ni Compounds

In nearly all cases (RNi

, RNi

with R being mainly La and Y) a strong reduction of

the magnetic order after hydrogen absorption is ob-

tained, which frequently leads to a complete loss of

long-range order. Particularly in these Ni-containing

hydrides the precipitation of f.c.c. Ni clusters can be

observed. If the parent compound is paramagnetic

(as, e.g., LaNi

), the Ni cluster formation is readily

observed in an enhanced susceptibility or even by the

onset of magnetic order. The origin of the hydrogen-

induced ferromagnetism appears to be the exchange

between Ni clusters mediated by the metal–H matrix

(Blach and Gray 1997). In the case of Ce-containing

intermetallics, the insertion of hydrogen can lead to a

change from Kondo- to ferromagnetic behavior as,

for example, in CeNiInH

(Chevalier et al. 2002).

This is caused by the change of the Ce valence from

intermediate in the parent compound to trivalent in

the hydride, which is governed by the hydrogen-

induced decrease of the hybridization between the

Ce 4f-electrons and the conduction electrons due to

the large rise in volume.

See also: Magnetism in Solids: General Introduction;

Alloys of 4f (R) and 3d (T) Elements: Magnetism;

Density Functional Theory: Magnetism

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G. Hilscher and G. Wiesinger

Technische Universita

t, Wien, Austria

Metal Particle versus Metal

Evaporated Tape

Magnetic recording media make up a special class of

permanent magnets. Storing information involves

switching between different energy states so just as

in conventional magnets a high-energy product is

desirable. In addition, the need for a high storage

density imposes the requirement for media with ﬁne

microstructure capable of supporting magnetization

patterns on the submicrometer scale. A second, re-

lated, special requirement is for an extremely smooth

surface ﬁnish. This is because spatially varying mag-

netic ﬁelds decay rapidly with distance and therefore

the magnetic recording transducer must be kept close

to the medium. The design philosophy for ﬁnely

structured recording media is to take very small mag-

netic particles having a uniaxial magnetic anisotropy.

The particles are too small to support a domain wall

(single-domain particles) and essentially their rest

magnetization can be in only one of two distinct

states.

There are two very different commercial approaches

to producing media for tape recording. In the ﬁrst

approach, a nonmagnetic base ﬁlm is coated with

an ink containing the magnetic particles and other in-

gredients. This type of medium is called ‘‘particulate’’

and can be found as tape or as ﬂoppy disk. The mag-

netic particles in use are (i) g-Fe

,(ii)CrO

and

cobalt-doped g-Fe

, and (iii) metal particle (MP). It

is the general consensus that metallic particles are

the only particulate media that can sustain very high

recording densities.

In the second approach, the recording layer is pro-

duced by thin-ﬁlm deposition techniques, such as

evaporation. (Sputtering is not considered a viable

alternative because the achievable coating speed is

too low.) Metal evaporated (ME) tape is produced by

oblique evaporation in an oxygen atmosphere. ME

tape is the only thin-ﬁlm tape commercially available.

When ME tape ﬁrst appeared on the market at the

beginning of the 1990s, it greatly outperformed MP

tape and appeared to be the only tape medium that

could sustain a very high recording density.

1. Recording Potential of ME and MP Tape

Magnetic recording media of either particulate or

thin-ﬁlm type are granular in nature. Fundamentally,

the recording potential of a granular medium is

related to the number of grains that ﬁt into a bit.

Mallinson (1969) pointed out that the maximum sig-

nal-to-noise ratio (SNR) is proportional to the

particle density in a tape. At read-back, the average

magnetization of all magnetic particles in the volume

sensed by the read-back head determines the signal,

while the deviation from the mean magnetization is

noise. Tape media—including ME tape—have a rel-

atively small volumetric packing fraction, typically

smaller than 50%. (Even ME tape has a rather low

packing fraction (Richter 1993a).) According to

Mallinson (1991) the wide-band SNR ratio is given by

SNRE

nWl

2pð1  p

;

m ¼

ð1Þ

906

Metal Particle versus Metal Evaporated Tape

where l is the minimum wavelength occurring in the

recording system,

M is the mean magnetization

sensed by the head, p is the volumetric packing frac-

tion of the particles in the tape, and n is the number of

particles per unit volume. The tape volume sensed by

the head is proportional to Wl

, which directly ap-

pears in Eqn. (1).

Therefore, it is desirable to prepare media with

very small particles. Magnetically, extremely small

particles—although magnetically ordered—lose their

hysteresis, because the energy barriers, DE, separa-

ting the magnetization states are too small to with-

stand the thermal energy, k

T (k

¼1.38 

23

1

(Boltzmann’s constant) and T is the tem-

perature in kelvin). This leads to a thermally activat-

ed process with a relaxation rate, r, given by

r ¼ f

exp 



ð2Þ

where f

is the ‘‘attempt’’ frequency B10

Hz. At

zero applied ﬁeld, the energy barrier, DE,is

DE ¼ K

V ð3Þ

here K

is an anisotropy constant and V is the particle

volume.

Table 1 gives some material data for ME and MP

tape. The main constituent of MP tape particles is

iron. The most advanced MP particles are made of an

Fe–Co alloy rather than pure iron, because Fe–Co

has a somewhat higher saturation magnetization.

Moreover, alloying with cobalt improves the corro-

sion resistance of the metallic particles. MP particles

are made needle-like and thus possess shape aniso-

tropy. Since iron and Fe–Co alloys have cubic ele-

mentary cells the magnetocrystalline anisotropy can

be neglected and is close to zero anyway for the

higher cobalt-content Fe–Co alloys. The shape an-

isotropy ﬁelds in Table 1 are calculated assuming

elongated ellipsoids of revolution with a ratio of the

axes of ﬁve. Using the resulting anisotropy constant,

ðm

Þ, the minimum volume required to

achieve a stable magnetization can be calculated. In

the product m

we see the analogy with the en-

ergy product used to characterize permanent mag-

nets. For a lifetime of B10 years, Eqn. (2) yields a

required energy barrier of 40 k

T. The magnetic vol-

ume given in Table 1 yields an energy barrier of

80 k

T, which provides an adequate margin to toler-

ate demagnetizing ﬁelds and elevated temperatures.

Since pure metallic particles are pyrophoric, the par-

ticles are surface oxidized. The oxide shell is non-

magnetic and has a thickness of only 3 nm. Therefore,

the required (physical) particle volume is significantly

larger than the required magnetic volume.

A similar consideration applies to ME tape. Since

there is not much information available about the

shape of the magnetic subunits, the shape anisotropy

contribution has been set to zero. Since ME tape is

manufactured in the presence of oxygen, a part of the

magnetic layers is oxidized. In contrast to MP tape,

the oxidized volume fraction is considerably smaller.

The magnetic particle volumes for both types of tape

are rather similar, but the physical particle volumes

are considerably larger for MP tape. Evidently, the

balance between ME and MP tape shifts if the

amount of oxygen in either tape can be reduced.

The foregoing analysis is highly simpliﬁed and im-

plies that the magnetization reversal process follows

the Stoner–Wohlfarth model (Stoner and Wohlfarth

1948). In reality, the magnetization in MP tape

switches at lower ﬁelds, typically between 150 kAm

1

and 200 kAm

1

, which can be because of incoherent

switching processes such as curling (Aharoni 1986), or

deviations from the ellipsoidal shape (Seberino and

Bertram 1999). It has also been reported that the

metallic core of MP can consist of several crystallites

(Veitch et al. 1994), which leads to incoherent mag-

netization reversal and to a reduced thermal stability.

It should also be noted that the coercivity measured

in a magnetometer is lower than that at writing. This

is an effect of thermally activated magnetization proc-

esses and can be derived from Eqn. (2) (Sharrock and

McKinney 1981). ME tape does not follow the model

of coherent rotation either. Thin ﬁlms containing

Table 1

Magnetic properties of iron and Fe–Co as used for MP tape, and cobalt and Co

0.8

0.2

as used for ME tape. The

shape anisotropy values are calculated assuming an aspect ratio for the particle of ﬁve. The magnetic volume

represents the minimum volume of the magnetic material to achieve an energy barrier of 80 k

T. For MP tape, a

nonmagnetic oxide shell of thickness 3 nm is assumed, for ME tape the volume of the oxide is assumed to be 45% that

of the metallic core.

Material M

(kA m

1

) H

shape

ðkAm

1

Þ H

crystaline

ðkAm

1

Þ V

mag

(nm

) V

tot

(nm

) l

tot

(nm)

Fe 1710 712 433 3971 33

0.7

0.3

1926 802 341 3552 31

0.2

1170 584 770 1117

Co 1436 584 627 910

907

Metal Particle versus Metal Evaporated Tape

cobalt generally contain stacking faults, which reduce

the anisotropy and, in particular, the switching ﬁeld

down to values of 100–150 kAm

1

. Unlike particulate

tape, magnetic exchange interaction between the mag-

netic subunits cannot be entirely excluded.

Since neither of the two tape materials can be de-

scribed by the model of coherent rotation, the values

for the critical volumes given in Table 1 do not—

strictly speaking—apply. Nonetheless, the critical

volume for the ME tape ‘‘particles’’ can be expected

to be smaller owing to the lesser amount of oxygen

built into the tape. This gives ME tape an advantage

over MP tape, with an estimated factor of two in

critical volume. Applying Eqn. (1), one would there-

fore expect an advantage of 6 dB in SNR for ME tape

over MP tape. The potential gain of ME tape over

MP tape is therefore rather moderate (see also Rich-

ter and Veitch (1995)).

2. Structure and Manufacture of MP and ME

Tapes

2.1 MP Tape

Figure 1 shows a schematic view of the cross-section

of standard MP tape. The tape consists of a base ﬁlm,

a back coating, and the magnetic layer. In standard

MP tape, the magnetic powder accounts for roughly

40% of the volume of the magnetic layer. In addition

to the metal powder, the magnetic layer contains car-

bon black, abrasive particles of Al

, dispersants,

and lubricants, all held in a plastic binder system. It is

essential that the magnetic particles are well dispersed

in the coating ink. Very ﬁne MP particles can be better

dispersed if a high shear premix or kneading stage is

added to the process. Shortly after the coating process,

the still wet coating is magnetically oriented. After the

alignment, the coating has to dry until the chemical

reactions in the binder system are complete. The next

step is to ‘‘calander’’ the tape. A calander presses the

tape between polished rollers at a high temperature.

This treatment results in a compressed magnetic ﬁlm

and, most importantly, in a very smooth surface ﬁnish.

2.2 Double-layer Coating of MP Tape

Although double, or even multiple, coating is an old

idea, the exciting and reinvigorating developments

are more recent. Double-layer MP media have a very

thin magnetic layer and a nonmagnetic underlayer

that contains very small TiO

or a-Fe

particles

(Inaba et al. 1993). The two layers are coated simul-

taneously. The ﬁrst commercial product made on this

principle was a Hi-8 videotape with considerably im-

proved recording performance. It is not the thinness

of the magnetic layer itself—which is about 400 nm

for this tape—but the improved surface smoothness

that provides the additional output. It has been dem-

onstrated that magnetic layers as thin as 120 nm

(Saitoh et al. 1995, Richter and Veitch 1995) or even

100 nm (Inaba et al. 1998) can be achieved. MP tapes

can now virtually match the performance of ad-

vanced metal evaporated (AME) tapes of the kind

used in the consumer Digital Video system (Inaba

et al. 1998).

2.3 ME Tape

Figure 2 shows a schematic cross-section of ME tape.

The obvious difference between MP and ME tape is

the magnetic storage layer. ME tape is manufactured

by oblique evaporation of cobalt or Co

0.8

0.2

in an

oxygen atmosphere. At the beginning of the evapo-

ration of the layer, the vapor arrives at the substrate

at grazing incidence. A ﬁlm grown at grazing inci-

dence shows uniaxial anisotropy in an oblique direc-

tion. These ﬁlms are porous and it has been found

that ﬁnished ME tapes can contain as much as 50%

voids (Richter 1993a). Roll-coaters operate in a wide

range of evaporation angles (continuously varying

incidence, CVI (Shinohara et al. 1984)), whereby

deposition is started at grazing incidence to preserve

oblique anisotropy. A CVI process leads to a curved

columnar microstructure. The magnetically easy axis

is tilted out of the ﬁlm plane with a tilt angle roughly

coinciding with the angle at which the columns start

to grow on the base ﬁlm, typically around 351. For

improvement of particle separation and material

yield, the evaporation is performed in the presence

Figure 1

Cross-sectional view of MP tape. The magnetic particles

are needle-shaped. The bottom sketch indicates that

particulate tapes are magnetized longitudinally.

908

Metal Particle versus Metal Evaporated Tape

of oxygen. The oxidation of the magnetic material

largely removes exchange coupling in ME tape.

Apart from the magnetic layer, there are other dif-

ferences between ME and MP tape:

(i) For mechanical protection, ME tape typically

has a carbon overcoat, which is reactively sputtered.

This carbon layer is of the diamond-like carbon

(DLC) type and is 5–10 nm thick.

(ii) The lubricant has to be applied as a separate

layer on top of the thin-ﬁlm medium. In MP tape, the

pores in the coating serve as reservoirs for the lubricant.

(iii) The surface of the base ﬁlm, onto which the

magnetic layer is deposited in ME tape, carries a thin

coating with very small particles. After deposition

of the magnetic ﬁlm, the surface of the magnetic layer

then shows ‘‘nodules.’’ These nodules are about

10–20 nm high and have a density of 10–50 mm

2

The invention of the nodules has been the techno-

logical breakthrough for ME tape. The nodules im-

prove tribology at the expense of an additional

spacing loss that sacriﬁces a small amount of the

recording performance.

From a manufacturing point of view, MP tape is

very attractive because existing coating technology

can be used, whereas ME tape requires a switch to

vacuum technology, which is associated with a big

investment. Clearly, MP tape is much more econom-

ical, and many tape makers have opposed a technol-

ogy switch to ME tape.

3. Magnetic Properties of MP and ME Tape

Some magnetic parameters of ME and MP tape are

summarized in Table 2. Owing to the inclination of

the easy axis out of the ﬁlm plane, the magnetization

of the tape is tilted out of the ﬁlm plane. Initially,

little attention was paid to the demagnetization ef-

fects present in simple measurements of the magnetic

properties. Magnetic properties of regular particulate

tapes are measured in the longitudinal direction,

which coincides with the alignment direction (i.e., the

easy axis of the tape). For ME tape, this standard

procedure yields low values for the squareness (ratio

of remanent magnetization and saturation magneti-

zation) and large values for the switching ﬁeld dis-

tribution (SFD). The SFD is related to the

normalized slope of the hysteresis curve:

SFD ¼ 1 



at H¼H

Magnetically, ME tapes did not seem to be very

attractive.

If the tape magnetization has a perpendicular com-

ponent, there is a perpendicular demagnetization ﬁeld

that has to be added vectorially to the applied ﬁeld.

The magnetic ‘‘particles’’ in the tape therefore do not

‘‘see’’ the external ﬁeld alone, rather the vectorial sum

of the external ﬁeld,

, and the ﬁlm-demagnetizing

ﬁeld,

ð4Þ

Figure 2

Cross-sectional view of ME tape. The ﬁller particles

underneath the magnetic layer give rise to nodules

sticking out of the ﬁlm plane. The easy axis of

magnetization is tilted out of the ﬁlm plane.

Table 2

Magnetic parameters of ME and MP tapes.

Application Type M

(kAm

1

) H

(kAm

1

) Coating (mm) n (1000 mm

3

)

Hi8 tape MP 200 120–135 2–3 8

350 90 0.2 B125

DVC tape MP 4300 180 0.15 50

450 135 0.15 B125

a Intrinsic properties.

909

Metal Particle versus Metal Evaporated Tape