Field-induced non-collinear magnetic structures in amorphous R-Co-B alloys (R=Gd and Er)

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Journal of Magnetism and Magnetic Materials 119 (1993) 221-228 North-Holland

Field-induced non-collinear magnetic structures in amorphous R - C o - B alloys (R--Gd and Er)

R.J. Radwafiski a, J.J.M. Franse a, R. Krishnan b and H. Lassri b

° Van der Waals - Zeeman Laboratorium, University o f Amsterdam, Valckenierstraat 65, 1018 X E Amsterdam, The Netherlands

b Laboratoire de Magnetisme and Materiaux Magnetiques, CNRS, 92195 Meudon, France Received 3 April 1992

High-field magnetization studies performed at 4.2 K in magnetic fields up to 35 T on amorphous GdxCos0_xB20 and ErxCos0_xB20 alloys have revealed, for samples with stoichiometry close to that of a compensated ferrimagnet, a magnetic behaviour that is characteristic of a non-coUinear magnetic structure of the rare-earth and cobalt sublattices. From the non-collinear regime the exchange interactions between the 3d and rare-earth magnetic sublattices have ~ been accurately evaluated. The compositional dependence of the Co and rare-earth moments is discussed.

1. Introduction

Amorphous alloys containing rare-earth metals are of interest due to their industrial applications.

The alloys are also interesting from a scientific point of view as they offer the possibility to study various aspects of 3d and 4f magnetism. In con- trast with crystalline materials these aspects can be investigated in continuous concentrations of the rare-earth metal as well as in relation to the low local symmetry that is characteristic of the amorphous state. A review of the properties of amorphous alloys has recently been presented by Hansen [1]. Owing to the lack of long-range struc- tural order in the amorphous state, a random local anisotropy model has been developed in ref.

[2]. According to this model, the directions of the rare-earth ion moments in an amorphous alloy are randomly distributed in space leading to a sperimagnetic structure of the 3d and rare-earth moments. It has been suggested that a significant

Correspondence to: Dr. R.J. Radwafiski, Van der Waals - Zeeman Laboratorium, University of Amsterdam, Valckenier- straat 65, 1018 XE Amsterdam, The Netherlands. Tel.: +31- 20-5255647; telefax: +31-20-5255788.

non-collinearity of the 3d moments exists in amorphous Fe-based alloys in the absence of external fields [1, p. 317]. The sperimagnetic structure is manifested in the magnetization curve by (i) a gradual, but significant, increase to saturation associated with a continuous field:induced rotation of the moment directions to the applied field direction, and (ii) a value for the magnetization at low fields that is much lower than the saturation magnetization. In ref. [2], Coey has calculated the magnetization curves for an array of magnetic ions with random easy-axis anisotropy, and has shown that the low-field value for the magnetization is two times smaller than the saturated magnetization. The evaluation of the local magnetic moment from the magnetization measurements is still an open question owing to the fact that in reality one is always dealing with a limited part of tlie magnetization curve for which the magnetization process in not fully understood.

The aim of this paper is to study magnetization processes in rare-earth amorphous alloys, by studying alloys containing two types of R ions:

G d - C o - B alloys in which the Gd ion moment is

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222 R.J. RadwaJ~ski et al. / Magnetic structures in amorphous R-Co-:B alloys

Table 1

The spontaneous magnetization at 4.2 K of amorphous G d - C o - B alloys. Nominal and actual compositions are shown

Composition M s M s

nominal actual (Am2/kg) (/z n / f . u . ) Gd6COTaB20 Gd5.9Co74.1B20 46.5 46.0 GdsCo72B20 Gd 7.sC072.2 B20 24.3 a 24.8 Gdl0Co70B20 Gd9.2COT0.sB20 12.5 13.0 Cd14Co66B20 Gd 13.5C066.5B20 39.0 43.7 a Taken from ref. [3].

a spin-only moment, and E r - C o - B alloys in which the spin and orbital contributions to the magnetic moment of the Er ion moment are present. In this latter alloy, one expects an asperomagnetic structure within the Er magnetic sublattice.

2. Experimental

The various GdxCos0_xB2o alloys with x = 6, 10 and 14, and ErxCos0_x_yB20+y, with y---0 and xffi6, 8, 10, 14, were prepared by melt spinning of appropriate amounts of the metals in a pure argon atmosphere. The actual chemical compositions were determined by an inductively coupled plasma analysis. The nominal and actual compositions are shown in tables 1 and 2 for the Gd and Er compounds, respectively. In some cases the actual composition differs substantially from the nominal composition. In general, the actual stoichiometry is poorer in rare-earth metals, a fact that is explained by the significant evaporation of rare-earth elements during the melting process. The amorphous state was veri- fied by X-ray diffraction. Details of the prepara- tion and characterization of the samples have

Table 2:

Spontaneous magnetization at 4.2 K of amorphous E r - C o - B alloys. Nominal and actual compositions are shown

Composition M s M s

nominal actual (Am2/mg) (P'B/f.u.)

Er6Co74B2o Er5.sCo73.4B21.1 48.5 47.5 EraCo72B2o Er7.5Co71.eB2o.~ 25.5 26.0 ErloCo70B20 Er9.sCo69.5B21 3.5 3.7 Er14Co~B2o Er13.sCo66.5B20 33.0 37.8

6O

.. ~40

20

a-Gd~Co~B2o

4.2 K ~ GdloCo:,oB2 °

0 t | i I I I i I

0 20 40

B (19

Fig. 1. High-field magnetization curves at 4.2 K for a- GdxCoa0_xB20 alloys. The solid line shows the magnetization curve taken in the continous field mode up to 30 T. The dashed lines are guide for the eye. Nominal concentrations

are indicated.

been presented in previous papers [3,4]. Magneti- zation measurements were performed at 4.2 K in applied fields up to 35 T in the High-Field Instal- lation of the University of Amsterdam.

The investigated samples consist of small pieces with lengths between 1 and 5 mm and a width of i mm. The individual pieces have a limited, but still significant, freedom to rotate within the sam- ple holder into their minimum-energy direction during the measurements.

3. Results and discussion

The results of the measurements are shown in figs. 1 and 2 for the a - G d - C o - B and for a-Er- Co-B alloys, respectively. There is a clear evidence for magnetic transitions in high external fields. These transitions are observed in samples characterized by a small net magnetization. In general, the magnetization curves and the transitions observed in high magnetic fields in particu- lar, can be understood in terms of a field-induced non-collinearity of the rare-earth and 3d transition metal sublattice magnetizations. The model has been found to be applicable to crystalline rare-earth-3d transition metal compounds [5]. By applying this model to a number of the magneti-

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R.J. Radwa6ski et al. / Magnetic structures in amorphous R - C o - B alloys 223

150

:.-

~ / 0 0 /'" .

f o S

• y

• , : , , . , . a.Er~Co~.B2 ° 50 . . . " .." z'" 4.2 K

~CO,,.,~,o..'.- ...

f~r ~. . • ". ""

• "Erg.$Coo.sBzl

0 I i i i i i i ! _

o 20 40

B ( T }

Fig. 2. High-field magnetization curves at 4.2 K for a- ErxCos0_xB20 alloys. The dashed lines are guide for the eye.

zation curves in the high-field region measured on single-crystalline as well as on fine-particle specimens, the intersublattiee molecular-field coefficient that couples these two magnetic sublattices has been evaluated [6-8]. In order to discuss the observed high-field behaviour in detail, knowledge of these magnetic sublattices and, at least, of the values for the rare-earth and Co sublattice magnetizations is indispensible.

3.1. Magnetic moments

In fig. 3, experimental results for the spontaneous magnetization at 4.2 K are shown as a function of Gd concentrations. The spontaneous magnetization of the G d - C o - B compounds, which is derived by extrapolation to B = 0, rapidly decreases showing a nearly linear behaviour. This rapid decrease is associated with an antiferomag- netic coupling of the Co and Gd moments. Com- pensation occurs for x = 10.5, in agreement with previous studies [4]. In table 3, the spontaneous magnetization of the a - G d - C o - B alloys is split into the Co and Gd sublattice magnetizations.

The splitting is not a straightforward procedure as in magnetic measurements only the net magnetization is measured. The splitting shown in table 3 is made with two assumptions. The first assumption is that the Gd ion moment is equal to 7/~n, the theoretical value for the moment of the

E 40

20

a.Gd~Co~rB21 d

!o

5 10 \ 15

I X

\

I \

I I I I I I

Fig. 3. Dependence of the spontaneous magnetization at 4.2 K on the Gd concentration x in a-Gdxcos0_xB 2 alloys.

tfivalent ion in the ground-state multiplet given by Hund's rules. The second assumption is that the two magnetization~ are directed antiparallel.

In this way the Co moment is found to decrease from a value of 1.19/~ s in Gd6Co74B2o to 0.76/~ s in Gd14Co66B2o. The former value fits reasonably well, if extrapolated to x = 0, with the CO moment of 1.28/~ s observed in COs0B20 (ref. [1], p.

324). The variation of the CO moment with the gadolinium concentration is shown in fig. 4. The decrease of the Co moment with the Gd concentration can be understood as due to an increasing filling of the 3d spin-up band of the Co ion by the 6s2/5d electrons of Gd.

Table 3

Splitting of the spontaneous magnetization at 4.2 K of amorphous G d - C o - B alloys into the rare-earth and Co sublattice magnetization assuming the 'full Gd trivalent ion moment and an antiparallei Er and Co moment configuration. In the last column the derived CO ion magnetic moment is shown

Composition rood ~MGd Mco mco

(actual) (/z n / G d ) ,(/t B/f.u.) (/~s/f.u.) ( / ~ B / C o )

G d 5 . 9 C O 74.1B20 7.0 !41.3 87.3 1.18

Gd7.sCo72.2B2o 7.0 154.6 79.4 I.I0 Gd9.2CoT0.sB20 7.0 64.4 77.4 1.09 Gdla.5Co~.5B2 o 7.0 94.5 50.8 0.76

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,224 R.Z Radwatlski et al. / Magnetic structures in amorphous R - C o - B alloys

m(izB/Co-ion) 1 O.5

a-GdxCos~J~2o

4.2 K

[ / i . L

0 5 10 x

Fig. 4. Dependence of the CO ion moment in a-GdxCos0_xB2o alloys at 4.2 K on the Gd concentration. The value of 1.28/~ B

for x = 0 is taken from ref. [9].

Experimental results for the spontaneous magnetization at 4.2 K in E r - C o - B alloys are shown in table 2 as a function of the Er concentration.

As in G d - C o - B , rapid decreases in M s confirm the antiferromagnetic coupling of the Er and Co moments. Compensation occurs in the vicinity of x ffi 10, in agreement with ref. [3]. This composition is close to that observed in the a-Gd-CO-B alloys, suggesting that the Er ion moment is close to t h a t observed for the G d ion provided the Co moment has the same rare-earth concentration dependence. This is a rather striking result as one would expect the Er ion moment t o be close t o 9#B, the value oUthe trivalent ion in the ground- state muItiplet according to Hund's rules. We have made the analysis twice, always assuming that i n the absence of external magneti c fields rare-earth moments are opposite to the C o moments. The exchange interactions between the 3d spins are assumed' to be sufficiently strong to ensure ~collinearity of the 3d moments within'the 3d sublattice. In the first approach, the E r ion sublattice magnetization MEr has been calculated with an Er moment of 9.0/~ a corresponding to the collinear arrangement of the Er moments.

This analysis results in values for the CO magnetic moment in the a - E r ' C o - B alloys t h a t a r e only slightly decreasing with increasing Er concentration ,(see table 4). Such a behaviour cannot be rejected o u t r i g h t owing to the fact that a slight effect of the rare-earth ions on the CO moment at low R concentrations, up to x = 20, is observed in amorphous YxCOl00_x and Gdxcol0o_ x alloys.

For instance, the Co moment in Y2oCo2o amounts to 1.65/z B [9], a value that is only 3% lower than the Co moment in metallic cobalt. The same

Table 4

Splitting of the spontaneous magnetization magnetization at 4.2 K of amorphous Er-CO-B alloys into the Er and CO sublattice magnetization made with assumptions that there is (i) the full Er trivalent-ion moment, and (ii) the antiparallel Er and Co moment configuration. In the last column the derived CO ion magnetic moment is shown

Composition m Er MEr MCO m co

(actual) ( / ~ n / E r ) (/,~n/f.u.) (/~n/f.u.) (/,LB/CO) Ers.5CO 73.4B21.1 9.0 49.5 97.0 1.32 Er7.sCo71.6B2o.9 9.0 67.5 93.5 1.30 Erg.sCo69.sB21 9.0 85.5 89.2 1.28 Er13.sco66.sB20 9.0 121.5 83.7 1.26

behaviour is also observed in crystalline Y - C o or G d - C o compounds. The Co moment in YCo 5 and GdCos, compounds that can be regarded as Co alloys with x = 0.167, is close to 1.65/~ a. Com- paring the values for the Co moment in YEoCos0 and Con0B20 of 1.65 and ~1.28/~ B [1], respectively, gives evidence that primarily boron causes the reduction of the Co moment in a - Y - C o - B alloys.

In this view, however, it is difficult to explain the substantially different behaviour of the Co moment i n a - E r - C o - B and a - G d - C o - B alloys.

Thus, the combined action of R and B ions leads to a further decrease of the Co ion moment in R - C o - B alloys. In the second approach, we as- sume that the Co moment in a - E r - C o - B alloys behaves in the same way as in a - G d - C o - B alloys i.e. we accept that the variation of the Co moment in a - G d - C o - B alloys is representative for the CO moment in a , R - C O - B alloys. In that case the rare-earth and CO sublattice magnetizations are derived as shown in table 5. The resulting moment of t h e Er ion amounts to (7.3 + 0.3)/~B Table 5

Splitting of the spontaneous magnetization at 4.2 K of amorphous E r - C o - B alloys into the Er and Co suhlattice magnetization made with assumptions that (i) the CO ion moment is equal to that observed in amorphous G d - C O - B alloys, and (ii) the antiparallel Er and Co moment configuration exists. In the last column the derived Er ion magnetic moment is shown

Composition m co Mco MEr m Er

(actual) ( / ~ n / C o ) (/~B/f.u.) (lgn/f.u.) ( / z n / E r ) Er5.5Co73AB21.1 1.19 87.3 39.8 7.2 Er7.5Co71.6B~I 9 1.12 80.5 54.5 7.3 Erg, sCo69.5B21 1.06 73.7 70.0 7.4 Er13.sCo66.5 B20 0.76 50.5 88.3 6.5

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R.Z Radwatiski et al. / Magnetic structures in amorphous R-Co-B alloys ²²⁵

in the alloys with low Er concentrations of up to x---11. In the alloys with higher Er concentrations, values for the Er ion m o m e n t further decrease being 6.5/z a for x -- 13.5. This lower value compared with the full Er 3+ ion moment could indicate the existence of a speromagnetic structure within the Er sublattice with a fan angle of about 35 ° o r / a n d the reduction of the Er moment due to local crystal field effects.

3.2. High-field magnetization process

In the magnetization curve experimentally observed for the a-Gd9.2COTo.8B20, a-Er5.5Co73.4B21.1 and a-ErT.5COTL6B20.9 alloys, there is a clear magnetic transition that separates two regions in the applied field scale. The differential susceptibility characterizing these two states differs by at least one order of magnitude, In the f i r s t of the above-mentioned alloys, the differential susceptibility evaluated up to 13 T amounts to 0.08 A m 2 / T k g , whereas after the transition to 1.1 A m 2 / T k g . In order to understand these transitions we may recall here the magnetization curve expected for an ideal ferrimagnetic system con- sisting of two well-defined and well-established m a g n e t i c sublattices c o u p l e d by antiferromagnetic interactions. The well-established magnetic sublattice is not affected by external fields. It m e a n s that its internal structure and the moment constituting the sublattice are field independent.

The magnetization curve resulting from a consid- eration of the free energy of the ferrimagnetic system [7],

E -- M T • B - M R • B + nRTM r • MR,

is schematized in fig. 5. M r and M R denote the magnetization of the two sublattices. The slope of the magnetization curve observed in intermediate fields is a measure of the intersublattice molecular field coefficient naT. The magnetizations in t h e saturated state at low a n d high fields are equal to [M T - M R [ and I MT 71- M R l, respectively. In the alloys investigated the high-field saturated state (the forced ferromagnetic state) is realized i n fields exceeding 40 T. The m o d e l calculations with the derived parameters show

i ^MT+MR

l~eld

Fig. 5. The schematized magaetization curve for a ferrimagnet compound containing two magnetic sublattices.

that the Ers.5Co73.4B21~l alloy has the lowest upper critical field, amoUnting to 41 T. Indeed, the experimental curve for this alloy i n the highest measured fields shows an onset of t h e forced ferromagnetic state.

Values for the coefficient nRT derived from the slope of the magnetization curve in the high- field region are collected in table 6. Inspection of table 6 shows that these values are very similar for all Er alloys. In the G d alloy a three times larger value is found for the coefficient nRT compared with the Er alloys. This factor of three difference can be understood by taking into ac- count that the 3 d - 4 f interactions proceed via the spins. Indeed, the intrinsic exchange coupling pa-

Table 6

Evaluation of the exchange parameters in a-Gd-Co-B and a-Er-Co-B alloys by means of high-field magnetization studies at 4.2 K. nRT is the inverse of the slope of the magnetization curve observed in high magnetic fields. -PRT is' the exchange coupling parameter of the 4f spin with its 3d spin surroundings. BmRol ( ffi nRTMCo) is the molecular field experienced by the R magnetic moment. Values for Mco for the Er alloys have been taken from table 5

Composition n R T "/RT / k B BmolR

(actual) (Tkg/Am 2 ) (K) (T)

Er5.5Co 73.4B21.1 0.32 -97 28 Er7.5 Co 71.6 B20.9 0.32 -91 25

Er9.5Co69.sB21 0.32 - 84 22 Er13.5Co66.5B20 0.44 - 103 20 Gd9.2Co 70.sB20 1.11 - 101 82 Er15Fe65B12Si s 0.42 a - 94 44 a Calculated on basis of the magnetization curve shown in ref.

[11].

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226 R.J. Radwatiski et al. / Magnetic structures in amorphous R - C o - B alloys

r a m e t e r JRT between the 4f spin and its 3d spin surroundings is very similar. This parameter has been calculated directly from nRT using the expression [10]

gR 1 / Z 2 N T n R T ' J°RT ---- gR --

where N T is the number of 3d atoms per unit of mass. The minus sign has been introduced in order to get the negative value for J~tT in the case of antiferromagnetic alignment of the T and R moments. The increase of the coefficient nRT with the Er concentration visible for the Er13.5Co66.5B20 alloy mimics the variation of the strength of the 3d-4f exchange with the Er concentration observed in ref. [8] for crystalline Er compounds.

Transitions of the same type as observed in the compensated Co ferrimagnets with Er and Gd can also be identified in Fe-based amorphous alloys. Among the magnetization curves for a-Er- F e - B - S i alloys presented in fig. 2 of ref. [11], the curve for the a-ErlsFe65BnSi s alloy shows an anomalous behaviour and a transition at 4 T can be identified. The inverse of the susceptibility above 4 T gives a value of 0.42 T k g / A m 2, a value that is very close to the nRT value in E r - C o - B alloys with similar Er concentrations. As seen from table 6, the parameter J~tT in this Fe alloy is similar to that observed in the Co counterpart.

A critical inspection of the experimental curves and the ideal curve presented in fig. 5 reveals at least two shortcomings of the present analysis.

The high-field part of the magnetization curve should extrapolate to zero for B ffi 0. This occurs only for the a,Erg.5Co69.5B21 alloy that is close to the state of a fully compensated ferrimagnet. For other alloys there is an non-zero intercept. The intercept value is larger for larger net magnetizations. The same phenomenon has been observed in many experiments on free particles of crystalline RnT,, compounds [7,8]. The origin of the non-zero intercept is not clear yet. A second shortcoming is the non-zero susceptibility observed at low fields. A non-zero susceptibility is observed already in measurements on single- crystalline samples. For singie-crystalline samples

B e

150 O3

100

4.2 K CFF

NCFI

0 I ' - -

5 x 15

Fig. 6. Magnetic phase diagram for a-GdxCos0_xB20 alloys at 4.2 K. The full lines show the lower Be1 and upper Be2 critical fields calculated within the two-sublattice model with the experimentally derived parameters. CFI, collinear ferrimagnet; NCFI, non-collinear ferrimagnet; CFF, collinear field-

forced ferromagnet.

it is small and only slightly larger than expected for the Pauli susceptibility. In Ho2Co17 , for instance, the differential high-field susceptibility, Xhf, in the field range 5-20 T experimentally found amounts to 70 x 10-4~B/T fu. Attributing this susceptibility to the Co ions only, one obtains a value of 4 x 10-4/zB/T per Co ion, close to the value of 3.7 × 10-41zB/T observed in metallic Co.

In this respect, the value of 13 x 10-4/~e/T per Co ion found in the present measurements in the field range 2-13 T in a-Gd9.2Co70.sB21 is not extraordinarily large and also indicates the good stability of the Co and Gd moments in the amorphous state. This view is supported by magnetization measurements of two other Gd alloys with x - - 6 and 14 in which quite fiat magnetization

curves are observed in fields up to 35 T.

In figs. 6 and 7, the magnetic phase diagrams calculated for the Gd and Er alloys, respectively, show the dependence of the internal magnetic structure of these ferrimagnetic amorphous alloys on external fields. The concentration dependence of the lower critical field B¢1, above which a non-collinear magnetic structure appears, resem- bles very much the concentrations dependence of

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R.J. Radwaftski et al. / Magnetic structures in amorphous R - C o - B alloys 227

4O

a-ErxCos~B2o

4.2 g ~,,

NCFI

0 5 _X 15

Fig. 7. Magnetic phase diagram for a-ErxCos0_xB20 alloys at 4.2 K. The full lines show the lower Be1 and upper Bc2 critical fields calculated within the two-sublattice model with the experimentally derived parameters. CFI, collinear ferrimagnet; NCFI, non-collinear ferrimagnet; CFF, collinear field-

forced ferromagnet.

the spontaneous magnetization as the coefficient nRT is only weakly dependent on the Er concentration. The upper critical field Be2, in which the forced ferromagnetic state is attained, is for the Gd alloys more than a factor of three larger than in the Er alloys reflecting the fact that the 3d-4f exchange interactions in which the Gd ion moment is involved are approximately three times stronger that of the Er ion moment [12].

The values for the critical fields Be1 and Bee are very sensitive to the values for nRT. This value determines the molecular field experienced by the R ion moment B~o 1. If the effect of the R - R interactions is neglected due to its weakness and the small number of bonds, then B~o I ----

nRTMco. These fields, collected in Table 6, decrease with increasing Er concentrations from 28 to 20 T. The largest molecular field acts on the Gd ion moment due to its larger spin moment. It is again approximately a factor of three larger than in the Er alloys because the spin moment of the Gd 3+ ion is three times larger than that of the Er 3+ ion. It is worth noting that a value of 82 T obtained for Bmo I by means of the high-field Gd magnetization studies at 4.2 K is quite consistent with a value of 73 T that one can derive by the

mean-field analysis of the value of 230 K for the compensation temperature found experimentally for Gdt3.sCo66.sB20 [4].

Finally, it is worth comparing the strengths of the coupling between 3d and 4f spins JRT in amorphous and crystalline alloys. The parameter JRT is the exchange constant in a Heisenberg-like Hamiltonian. Following ref. [10] it is assumed that these interactions are restricted to the nearest neighbours. Then,i the parameters JgT and J~T are related via the expression

JRT = JRT//ZRT,

where Zg T is the number of nearest T-neighbours of the R atom. Accepting ZRT ~ - 12, one obtains a value for JRT/kB of --8 K. Values for an exchange coupling parameter of this size have been found in rare-earth-Bd transition metal compounds in the crystalline state. Values of - 6 . 8 and - % 8 K have been reported for Ho2Co17 and GdCo 5 compounds, respectively, on the basis of high-field magnetization studies on single-crystalline specimens [13,14]. Apparently the strength of the 3d-4f spin interactions does not depend strongly on the local symmetry.

4. Conclusions

We have observed in high magnetic fields a magnetic transition for some amorphous G d - Co-B and E r - C o - B alloys that occurs in the field range up to 35 T for systems with low net magnetizations. Such a transition is not expected in systems with a sperimagnetic structure for which a continuous and gradual approach to saturation, associated with an increasing fan angle of the sperimagnetic structure, is expected. The oc- currence of the transition can be understood within a model of two magnetic sublattices, formed by the rare-earth and 3d transition metal moments, as the formation of a non-collinear ferrimagnetic-like structure in high magnetic fields. The intrinsic magnetic parameters derived from the analysis of the magnetization curve of the amorphous G d - C o - B , E r - C o - B and E r - F e - B alloys are consistent, confirming the appli-

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228 R.J. Radwatlski et al. / Magnetic structures in amorphous R - C o - B alloys

cability of the model to describe the high-field magnetic behaviour of amorphous materials.

Moreover, the value for the intrinsic exchange parameter of - 8 K is very similar to that found for crystalline rare-earth compounds with 3d transition metals.

Acknowledgements

The work has been partly supported by Euro- pean Commission by its Research and Develop- ment programme within the Concerted European Action on Magnets (CEAM3) and Basic Interac- tions in Rare Earth Magnets (BIREM) projects.

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