Magnetic ordering in PuBi

Journal of Magnetism and Magnetic Materials 63 & 64 (1987) 145-147 North-Holland, Amsterdam

145

M A G N E T I C O R D E R I N G IN PuBi P B UR LET, S Q U E Z E L , J ROSSAT-MIGNOD, J - C SPIRLET*, J REBIZANT* and O V O G T + Centre d'Emdes Nucl~awes, DRF/ SPh-MDN, 85 X, 38041 Grenoble C~dex, France * European Instuut for Transuramum Elements, Post[ach 2340, D- 7500 Karlsruhe, Fed Rep Germany + Laboratonum fur Festkorperphyszk, E T H Zurich, CH-8093 Zurich, Swttzerland

The magnetic ordering m PuBJ has been studied by neutron diffraction In zero apphed magnetic field PuB1 orders at any temperature below TN = 58 K m a long period commensurate phase of wavevector k = [0,0,3/13] corresponding to a sequence (4-~5) of ferromagnetic (001) planes This phase is independent of the magnetlc field up to large crmcal field where a paramagnetlc state is reduced The magnetic phase dmgram of PuBJ is presented

1. Introduction

2. Experimental

The actlnlde monopntctldes AnX (X = N, P, As, Sb, BI), with the simple NaCl-type structure, form large series of compounds allowing comparative studies when anions or actlnide ~ons are changed Uranium compounds have been studied, first on powdered samples and more recently on single crystals They exhibit quite complex magnetic properties which have been summarized in ref [1] Much less work has been performed on neptunium and plutonium compounds, however the success of the recent program developed at the Transuranium Instltut of Karlsruhe for growing single crystal of neptunium and plutonium monopnictldes allowed to perform detailed neutron scattering experiments In the last few years conclusive results have been reported, from single crystal experiments, on PuAs, PuSh [2] and NpAs [3] Neutron diffraction studies on such single crystal samples are very ~mportant because It is the only technique which can give definitive results on the microscopic nature of the magnetically ordered phases and accurate information on phase transitions In this paper we present the results of a neutron d~ffraction investigation of the magnetic phase diagram of PuBI, which was found, by magnetic measurements to order antiferromagnetlcally below about 60 K [4]

For safety reasons and to prevent any chemical contamination of the sample, the single crystal prepared at the European Institut for Transuranium Elements has been encapsulated in a double wall aluminlum can, inside the helium glove box just after the opening of the tungsten crucible Neutron diffraction experiments have been performed at the Siloe reactor of the CENGrenoble The crystal was put in a cryomagnet assembly with a [001] axis vertical and parallel to the magnetic field 3. Results and discussion

3 1 Zero field magnenc ordenng Neutron scans have been performed along the symmetry directions to determine the wavevector of the magnetic structure At any temperature below T N = ( 5 8 ± 1)K a magnetic intensity is observed at Q = [2,0 232 + 0 001,0] Furthermore, the intensity of the Bragg peak [0k0] is zero which implies a longitudinal (mkllk) polarization of the modulation Scans along equivalent symmetry directions [k20] and [20k] give also magnetic Bragg peaks but with a much lower intensity (about 1/5) than for [2k0] indlcatlng that a nearly single-domain state is

0304-8853/87/$03 50 O Elsevier Science Pubhshers B V (North-Holland Physics Publishing Division)

146

P Butler et al / Magnetic ordenng in PuBz

estabhshed Therefore, we conclude that the magnetic ordering ts of smgle-k type ] e colhnear Moreover, third-order harmontcs [2,3 k,0] are observed At T = 4 2 K , the ratio of the amphtudes takes the value A k / A 3 k = 4 ± 1 5 md]catmg a complete squarmg up of the modulation implying that all Pu 3+ magnettc moments have the same value Measurements of mtegrated nuclear and magnetic mtensmes yield, using the Fermi lengths b p u = 0 85 × 10 -12cm and b B , = 0 8 2 × 1 0 -~2cm, to an amphtude A k = (0 6 ± 0 1)/XB at T = 4 2 K Assuming a complete squaring of the modulation a magnetic moment value m = (~r/4) Ak = (0 5 + 0 1)/zB, per Pu 3+ ion can be deduced, m quite good agreement with magnetization measurements which give m = 0 5 / x B a t T = 4 0 K and H = 8 0 k O e

I

I

PuBi

I

I

,ooo-

I

4000

I

I

11[ool]

3ooo

2

g 5000

~~e~.

4000 ~ 3000

Oe

H

2o0o

= 18kOe

o"'---"-----.o~ ¢-

0

4000 5ooo 3000

o.--

~ = H o - - 5 °

2ooo

kOe

4000 ~

=

3000

OkOe

2000 3 2 Magnettc phase d~agram

1000

0

Whatever the value of the magnetic field applied along a [100] direction this modulated phase remains unaffected and the wave-vector keeps the same value than m zero field As shown m fig 1, m low field ( H < 20 kOe) the magnetic intensity follows, m decreasmg temperature, a continuous curve mdIcatmg a second order transmon at TN However, m larger fields the N6el temperature decreases and the transition becomes of first order T h e variation of the [2k0] reflection with increasing field at T = 4 2 K is reported m fig 2 T h e modulated phase ]s destroyed In field of 7 5 k O e and the saturated paramagnet]c state ts reached These results are summarized m the phase diagram given m fig 3 The absence of any temperature and field dependence of the value of the wave vector, and the large stability of the modulated magnettc phase strongly suggest that the magnetic ordering ts actually commensurate with the lattice The observed value k = (0 2 3 2 2 0 001)flu Is, within the experimental accuracy equal to 3/13 = 0 231 This value is very near to 1/4 which would correspond to a (4 + 4 - ) sequence of ferromagnetic (001) planes from which the ordering m PuB] Is certainly derwed Actually, the commensurate structure with k = 3/13 corresponds to the sequence (4 +,

I

I 20

10

I

I

30

I

40

I

50

Temperoture

60

I

"70 SO

(K)

Fig 1 Intensity of the magnetic Bragg peak [2k0] as a function of t e m p e r a t u r e for different values of a magnetic field applied along [001]

I

I

I

'

I

'

7O0 ¢-

sool 500

E~

¢-

r 0

T=4

300

2K

HII[O01] 200

c0

PuBi o=[2 o 232 o]

100

o o

2.0 40 60 80 1O0 Mognetlc field (kOe)

Fig 2 Intensity of the magnetic Bragg peak [2k0] at T = 4 2 K as a function of a magnetic field a p p h e d along [001]

147

P Burlet et al / MagneOc ordenng m PuB, 90

I

I

I

I

PuBi

.0-

I

I

-II[o0 ]

Table 1 Magnetic propertms of plutonmm monopmctldes

--o

" a (A)

I1) 7O 0 PuP PuAs PuSb PuSb PuBi

60 "o N.--

(,.) .-

40

TN (K)

k (rlu)

Moment Ordered direction moment

85

0, ferro 0, ferro 0 13 0, ferro 0 232

(100) (100) (100) (100) {100)

126 125 Tl~ = 70

6 35

58

0 75 067 0 74 0 50

~---[o,o,3/~3]

¢o'~ 0

5 55 5 78 6 24

Tc (K)

30

(4+,4-,5+,4-,4+,5-) 20

<4=5>

10

0 0

I I0

I 20

I 30

I 40

I 50

60

Temperature (K) F,g 3 (H, T) magnetic phase dtagram of PuBt

4-,5+,4-,4+,5-) of ferromagnetic (001) planes noted (425) In PuBl, as in all the uranmm, neptuntum and p l u t o m u m monopmcUdes already studied, the magnetic ordering Is always characterized by a wave v e c t o r k along the cubic axis w~th a longitudinal polarization (mkllk) T h e r e is also a c o m m o n feature cons,stmg in strongly amsotrop~c exchange mteractlons resulting from the hybndlzatmn of 5f electrons with band electrons

[5] A comparison of the Pu m o n o p n l c u d e s ~s given in table 1 From PuP to PuB1 the ordering t e m p e r a t u r e decreases as the lattice p a r a m e t e r increases whereas the magnetic m o m e n t s are always strongly ahgned along the cubic axes and keep nearly a constant value PuP and PuAs are ferromagnetic at any temperatures below Tc but an antfferromagnet~c character starts to develop in PuSb which exhibits an Incommensurate modulated phase below TN down to a lock-m

transltton toward a ferromagnettc phase A m o n g the monopnlctldes, PuBt ts the only one which rem a m s with a long period magnetic structure o v e r the whole t e m p e r a t u r e range These results ind i c a t e that the mteractlons between (001) planes are becoming more antlferromagnetlc from PuAs to PuB~ while the nearest ne~ghbour m t e r a c t m n remains stdl ferromagneUc to stabdlze the wavevector value (k < 1/2) Furthermore, it is surprisIng that m PuB~ the second order transmon at TN leads directly to the c o m m e n s u r a t e (425) phase and not to an incommensurate one with a subsequent lock-m transition as it ~s often observed m these c o m p o u n d s Th~s ~s m d l c a t w e that the mteractmns are actually of rather long range type References

[l] J Rossat-MJgnod, G H Lander and P Burlet, m Handbook on the Physic and Chemistry of the Actmldes, vol 1, eds A J Freeman and G H Lander (North-Holland, Amsterdam, 1985) p 415 [2] P Burlet, S Quezel, J Rossat-Mlgnod, J - C Sp,rlet, J Reblzant, W Muller and O Vogt, Phys Rev B30 (1984) 6660 [3] P Burlet, S Quezel, M Kuzmetz, D Bonnlsseau, J Rossat-Mlgnod, J C Splrlet, J Reblzant and O Vogt, J Less-Common Metals 121 (1986) [4] J - C Splrlet, J Rebmant and O Vogt, 13eme Journees des Actlnldes, Elat, Israel (1983), unpubhshed [5] B R Cooper, R Smmann, D Yang, P Thayamballl and A Banerjea, m Handbook on the Physics and Chemistry of the Actmldes, vol 2, eds A J Freeman and G H Lander (North-Holland, Amsterdam, 1985) p 435