Superconductivity and magnetic interactions in the pseudoternary bct system Ho(Rh1−xRux)4B4

Physica 148B (1987) 76-79 North-Holland, Amsterdam

SUPERCONDUCTIVITY AND MAGNETIC INTERACTIONS IN THE PSEUDOTERNARY bct SYSTEM Ho(RhI_xRu/)4B4 H. A D R I A N , A. T H O M A , A. M E I N E L T , R. B L A N K , B. H E N S E L , G. A D R I A N and M. S T E I N E R * Physikalisches Institut, Friedrich-Alexander-Universitiit Erlangen-Niirnberg, D-8520 Erlangen, Fed. Rep. Germany * Hahn-Meitner-lnstitut, D-IO00 Berlin, Fed. Rep. Germany Received 6 August 1987 We report temperature- and magnetic-field-dependent neutron diffraction experiments on polycrystalline samples of the pseudoternary bct Ho(Rh~_xRu,)4B4 system. The x-dependent variation of the electronic density of states at the Fermi level N(Ev) , which causes the well-known characteristic superconducting phase diagram, also leads to a change of the magnetic order from a non-collinear antiferromagnet in coexistence with superconductivity for x <0.40 to a canted ferromagnet for x > 0.80. A transition between these two magnetic structures which is accompanied by a destruction of superconductivity is also induced by an external field. The observed hysteresis in the H~2(T) data below 0.6 K and magnetic relaxation phenomena in )t,,c(T) for 0.40~
1. Introduction and experimental Since the discovery of superconductivity in the R R h a B 4 series considerable work has been dedicated to the study of the basic interactions in superconducting c o m p o u n d s with a regular sublattice of magnetic ions. Whereas several systems are known where antiferromagnetism coexists with superconductivity, uniform ferromagnetic order is found to lead to a destruction of superconductivity and therefore to reentrant behavior. The interplay between the local magnetization due to the magnetic ions and superconducting persistent currents is evident from the slight depression of Tin, the t e m p e r a t u r e for uniform magnetic ordering, and the formation of spatially modulated magnetic structures in a small temperature interval [1]. Despite the large n u m b e r of studies, there is still some controversy about the relevant magnetic interactions. Although it is widely accepted that the R K K Y interaction is responsible for magnetic ordering, there exists also contradictory experimental evidence, e.g. in the pt-series R E R h a B 4 T m is not proportional to the de Gennes factor. As T m in these systems is of the order of a few degrees one has also to consider the direct d i p o l e - d i p o l e interaction for 0378-4363/87/$03.50 © Elsevier Science Publishers BN. (North-Holland Physics Publishing Division) and Yamada Science Foundation

which one estimates T m ~-0.6 K using magnetic m o m e n t s of 10.6 Ix~ ( H o 3+) at a distance of 0.5 nm and neglecting angular dependences [2]. In order to contribute to this question we studied the t e m p e r a t u r e and magnetic field dependence of the magnetically ordered structures of polycrystalline bct Ho(Rhl_xRux)4B4 samples by neutron diffraction. The electronic properties of this system as function of the composition p a r a m e t e r x have been intensively studied by photoemission and superconductive measurements [3]. From this it is known that the electronic density of states N ( E ) can be described by a rigid band model leading to a shift of E v to lower energies with increasing x according to the reduction of the n u m b e r of valence electrons. Consequently N(EF) is shifted from a position in an isolated peak through a minimum into a different peak of the d-band structure resulting in the characteristic superconducting phase diagram of these pseudoternary systems. The variation of N ( E v ) with x should also be directly reflected in Tm(x ), if the magnetic order is due to R K K Y interaction, whereas the dipole-dipole interaction is not expected to be changed by x. The neutron scattering experiments were carried out at the reactor B E R I I of the Hahn-

H. Adrian et al. / Superconductivity and magnetism in Ho(Rh~ ~Ru~)4B 4

Meitner-Institut in Berlin with powdered samples of Ho(Rhl_xRu~)4B 4 with x = 0 . 1 0 , 0.35, 0.50, 0.80, 0.90, and 1.00. The samples were prepared by arc-melting the appropriate amounts of the elements using enriched HB (98%). The wavelength of the monochromatized neutrons was 0.239 nm. The samples were usually mounted in a 3He-cryostat with a superconducting magnet providing temperatures down to 0.5 K and magnetic fields up to 5 T.

2.

77

~RhI_xRuxI4B

4

B =0

X=0.10

_ ~

/]

o,~.

.IIIjLI....

II,!!ri

I..II B=0.2T

Results

The magnetic neutron scattering intensities for x = 0 . 1 0 (upper part of fig. 1) and x = 0 . 9 0 (upper part of fig. 2) at 0.5 K were obtained by subtracting the scattering intensities measured at 4.2 K, where only nuclear Bragg peaks of the bct phase were observed. In case of x - - 0 . 1 0 the diffraction lines can be grouped into three sets, labeled B (very weak), C and D which correspond each to a purely antiferromagnetic orthogonal component of the magnetic moment (/"£(B) < 2 / ~ , /Z(c ) = / Z ( D ) ~ 5/z~), resulting in total in a non-collinear, antiferromagnetic structure with T m ~ 1.6 K as proposed in fig. 3. Similarly for x t> 0.90 the lines can be grouped into two sets A and B, where again B corresponds to an antiferromagnetic component within the basal plane, whereas A results from a perpendicular ferromagnetic component (/[Z(A)~ ~['~(B) ~ 6 / X B ) - It is easy to see that this structure, which is found below T m ~ 2.5 K, can alternatively be described by two orthogonal ferromagnetic sublattices, and is therefore addressed in the following as canted ferromagnet. In fig. 3 the positions of the distorted R h / R u - and B-tetrahedra, which exist in 2 different orientations are represented by shaded and unshaded cubes [4]. For x = 0.35 we find a similar line pattern except for the absence of the set labeled B and T m ~ 1.4 K. The most notable result for x = 0.50 is that only the most prominent lines of the sets C and D could be identified, indicating a considerably lower T~ than for x ~< 0.35. Surprisingly, for x = 0.80 we again find large line intensities of the sets C and D just as for x = 0.35. Therefore the transition from the

.....

..

,~

ii

o " '

• ..

B=0.ST

; -

I

~

B = 1.0 T

I

•

;..

~, i

~

' .i

~il ~ ~_~~

°

/i

' i'-

. . . .

B : 4.0T

~"

,~

Ho{RhI_xRux)4Bt. X=0.10

• . - " -

I0

-

.' ~ !

~l!

'....~'.i

.,I

..

i-li .... . ~ .

B =OAT

~-i~ c,.

•1 :~ ..

._.-.

, .! _ ~

~

"

• ..-..

!

-i "

~

:i

15

~

[I

~

(~

I

~ ~

..- t

20

25

30

I I i .

3~

40

.....

45

SCATTERING ANGLE Fig. 1. M a g n e t i c scattering intensities for Ruo.lo)4B 4 at 0.5 K in v a r i o u s m a g n e t i c fields.

,i~.i

50

55

20 (deg) Ho(Rh~

78

_ 0 tO I--" Z Z3 0 C)

H. Adrian et al. / Superconductivity and magnetism in Ho(Rh~ ~RUx)~B~

3000-

B=O

25OO2000-

1500-

I(0.7K} -I{42 K)

~

B I --

300

I

~

AB

--I

t

A

I

I

B=4T

j,

,i

/BL

BA

t -----~"

I

~

2500-

i

2000-

I! /..,,

PLi i'i

•

o-

~c~-

Ho(Rhl_xRux) 4 [34

~"

X:

,oo,_

~.

0,90

I(B=4T) -liB=O)

Ii

/i

I . . . .

O-

-

i

-50@-

i0

1~

2@

i ,2.

..... •

~

ducting (T~ = 2.5 K) one has to conclude that the onset of ferromagnetic order prevents superconductivity in Ho(Rhl_xRux)4B 4 for x close to 1. Therefore it is interesting to study the influence of external fields on the magnetically ordered structures. Fig. 2 shows the change in the diffraction pattern for the canted ferromagnet by an external field o f / x 0 H = 4 T. One can clearly see that the ferromagnetic contribution (A) increases, whereas the antiferromagnetic (B) decreases. More dramatic changes occur in case of the non-collinear antiferromagnetic structure as shown in fig. 1. The strong antiferromagnetic lines of sets C and D disappear and the sets A and B appear, with the transition occurring between 0.4 and 0.6 T. This shows that an external magnetic field changes the magnetic structure in a non-trivial manner from the described compli-

;

30

3@

40

4@

50

SCATTERING ANGLE 20(deg) Fig.

2. Magnetic scattering intensities for Ho(Rh010 ~ a t 0.5 K at 0 and 4 T. The lower section shows the difference spectrum• RU0.9o)aB

non-collinear antiferromagnet to the canted ferromagnet occurs between x = 0.80 and x = 0.90. In many respects very similar results have been reported for Dy(Rh~ x R U x ) 4 B 4 [5]. For samples with x = 0.35 superconductivity with T c-~6.3 K coexists with antiferromagnetism. The transition into an antiferromagnetically ordered state is reflected in a shallow minimum in the temperature dependence of the upper critical field as shown in fig. 7 of ref. [6]. A further noteworthy feature of the H~2(T ) data is the occurrence of a hysteresis between data taken in increasing and decreasing magnetic fields below 0 . 6 K . As the non-magnetic compounds RRu4B 4 with R = Y or Lu are supercon-

Fig. 3. 'Proposed magnetic structure of Ho(Rh0 9o Ru 0 10)4B4. The Ho-moments are represented by 3 orthogonal components B, C, and D (not to scale). For Ho(RholoRuo90)nB4 the proposed structure consists of an antiferromagnetic component (B) and a ferromagnetic component (A).

H. Adrian et al. / Superconductivity and magnetism in Ho(Rh 1 xRttx)4B4

cated non-collinear antiferromagnet to the canted ferromagnet. In case of x -- 0.35 this transition occurs at about 0.25 T. A comparison with the Hcz(T) [6] shows that the destruction of superconductivity is due to this transition to a structure with a ferromagnetic c o m p o n e n t [7]. Finally, the ac susceptibility Xac measured down to 5 0 m K shows a simple superconducting behavior for x ~< 0.35 and clear ferromagnetic peaks for x i> 0.85. H o w e v e r , for the composition range 0.40 ~< x ~< 0.70 significant discrepancies between cooling and warming cycles, as well as magnetic relaxation effects at 5 0 m K over several hours have been reported [6]. Similar p h e n o m e n a have been observed for Hoo.95Ero.25M06S 8 [8].

3. Discussion and conclusions The large variety of experimental results can be consistently explained on the basis of two competing magnetic interactions. If we assume that antiferromagnetic ordering for x~<0.35 is due to RKKY interaction, we expect T m ~ N ( E F ) . The reduction of N(EF) around x/> 0.35, which leads to an abrupt drop of T c due to its exponential dependence, is reflected in the observed decrease of Tm. A competing second magnetic interaction manifests itself in the observed hysteresis in H c 2 ( T ) below 0.6 K. The contribution of the d i p o l e - d i p o l e interaction which is expected to be independent of x may also be essential for the occurrence of the complicated magnetic structures instead of collinear ones. The existence of competing magnetic interactions favoring different magnetic structures also

79

explains the metamagnetic transition in an external field. This interpretation is supported by the unusual Xac(T) data for 0.40 ~< x ~< 0.70. In this composition range N ( E v ) is expected to be reduced to about 1/3 of its value for x - - 0 . 1 0 . Consequently the R K K Y and d i p o l e - d i p o l e interactions would lead to different magnetic structures at comparable ordering temperatures. The resulting magnetic behavior, which may be attributed as frustrated magnetism, gives a natural explanation for the magnetic relaxation effects in Xac at 50 mK.

Acknowledgement The authors acknowledge financial support by the Bundesminister f/ir Forschung und Technologie.

References [1] S.K. Sinha, G.W. Crabtree, D.G. Hinks and H. Mook, Phys. Rev. Lett. 48 (1982) 950. [2] P.W. Anderson, Phys. Rev. Lett. 55 (1985) 1805. [3] R. Knauf, A. Thom/i, H. Adrian and R.L. Johnson, Phys. Rev. B 27 (1984) 2477. R. Knauf, H. Adrian, A. Meinelt and R.L. Johnson, Phys. Rev. B 32 (1985) 2895. [4] L.D. Woolf, D.C. Johnston, H.B. MacKay, R.W. McCallum and M.B. Maple, J. Low Temp. Phys. 35 (1979) 651. [5] K. Motoya, C.F. Majkrzak, G. Shirane, H.C. Hamaker and M.B. Maple, Phys. Rev. B 30 (1984) 3743. [6] A. Thomfi, H. Adrian and A. Meinelt, J. Low Temp. Phys. 64 (1986) 329. [7] H. Iwasaki and Y. Muto, Physica 135B (1985) 326. [8] L. Brossard, M.S. Torikashvili, M.B. Maple, H. Suhl, C.F. Majkrzak, G. Shirane and M. Tachiki, Phys. Rev. B 30 (1984) 1188.