Field-induced transformations of the spin ordering in Nd2CuO4

Journal of the Less-Common Metals, 164 & 165 (1990) 768-775

768

Field-induced

transformations

spin ordering D. Petitgrand, Laboratoire

of the

in NdaCu04

A.H. Moudden*,

P. Galez and P. Boutrouille

LBon Brillouin, CEA-CNRS,

CEN/Saclay,

F-91191 Gif/Yvette

Abstract Using elastic neutron scattering, we show first that at zero field NdpCuOq

undergoes

ated with the Cu++

successive magnetic spins .

phase transitions

A phase I with LaaNiOd

associ-

magnetic

type structure is stabilized between TN x 250K and Tl x 80K and a phase II with a La&u04 tween TI and Tz M 30K.

magnetic type structure is formed beBelow Tz a phase III with a magnetic

structure similar to that of phase I, is stabilized down to the lowest temperatures.

When a small magnetic field, up to about 2 Tesla,

is continuously

applied perpendicular

(1, -l,O),

the magnetic

continuously

reflections

to the tetragonal

(l/2,1/2,1)

axis along

with odd integers 1

vanish in the stability region of phases I and III , while

the intensity of those with even integers 1 increases . However in the region of phase II the opposite occurs: even 1 reflections continuously vanish and odd 1 reflections are enhanced. The results indicate that at relatively

small field of about 0.7 Tesla, a single magnetic

domain with collinear spins is stabilized.

Further, detailed studies of

the reversibility of these transformations

upon the applied field show

no significant hysteresis, suggestive of a continuous

transformation

from non collinear spins structures at zero field into their collinear analogues at high field.

* On leave of absence from Laboratoire

de Physique des Solides

Orsay. 0022-5088/90/$3.50

0 Elsevier Sequoia, Printed in The Netherlands

169

1. Introduction The magnetic

properties

NdzCu04

parents

It was shown magnetic

of the electron

and PrzCu04 in particular

phase transitions

doped

have been NdzCuOd

that

associated

superconductors’

extensively

studied2-?

undergoes

3

at least

with the ordering

of the Cu++

spins. A phase I with LazNi04 magnetic type structure is stabilized between TN M 25OK and T1 NN80K and a phase II with a La2Cu04 magnetic type structure is formed between Tl and T2 M 30K. Below Tz a phase III, with a magnetic stucture similar to that of phase I, is stabilized down behaviour, P&u04 La2NiOl

magnetic

temperatures. been

speculated7 phase

any

lattice have

distortion been

eracy

which

with

difficult

the magnetic

structures

domains

state

versibility

The

distor-

two systems

at all temperatures. spatial

degen-

somewhat

more

difficult

have been

used

to solve

however

because

averaging, should

of the

the question

of whether

be described

in terms

is still to be answered. a relatively

struc-

Using

small magnetic

elastic

of

neutron

field induces

spin reorientation,

and by a detailed

studies

of this transformation

we will show that

it is very likely to

be a transformation

from non collinear

structure

stable

magnetic

structure.

spins in a tetragonal

at zero field into a collinear

out on the Laboratoire The incident

spins

has

did not detect

introduces

two systems,

in Nd2CuOa

we will show that

a continuous

symmetry

structure

lowest

in La2Co04.

No detectable

as we115.

non collinear of these

of multi

or multi-q

scattering,

and

structure

problem

the magnetic single-q

makes

successive occur’

studies’

peaks.

structure

to the

in Nd2CuOd

which

diffraction

the tetragonal

down

an underlying

to those

in PrzCu04

of the nuclear

Collinear

the magnetic

X-ray

250K

reorientation

or superlattice

reported

basically

from

similar

high resolution

from

spin

as resulting

The tetragonality to solve.

structure

successive

transitions

Detailed tion

type

The

tural

remain

to the lowest temperatures. In contrast to this undergoes a unique magnetic transition5 into

neutron

The neutron

scattering

Leon Brillouin momentum

of the remagnetic

spins in an orthorhombic experiments

4F2 triple

were carried

axis spectrometer

was ki = 2.662A1-i

and a pyrolytic

graphite PG filter was used to suppress higher order contaminations. The single crystal of Nd2CuOd of about 10 x 10 x 10mm3 was placed in a superconducting magnet with the (1, -1,0) axis vertical so that magnetic reflections with indices (h/2, h/2,1), referring to the nuclear space group 14/mmm, can be measured.

.

2. Results and Discussions In figure 1 we show the temperature

variation of the integrated

intensity of the magnetic Bragg reflections (l/2,1/2,1) zero filed using scans along

I

(0, 0,1) direction. Clearly the anomalies t

’

, measured at

I

I

H=O

0 0

Fig I.

50

The temperature

100 150 200 250 300 Temperature [K]

dependence

of the integrated

intensity

of the

mug~etic (1/2,1/2,a) re~2ec tions at ~ero~e~d, shows the e~‘ste~ce of at least 9 different magnetic phases I, II and III. The intermediate phase

II is characterized

peak (l/2,1/2,0) perature phases

by the existence

of the

magnetic

wh’zch is absent in both the high and low temI and III. The solid lines are a guide for the

eye.

seen in the variation are indicative of 3 magnetic phases as reported earlier3~***. The phase I and III stable above 80K and below 30K respectively are compatible with a LasNi04 magnetic type structure. The intermediate phase II characterized by the presence of the magnetic Bragg peak (l/2,1/2,0) between 30 and 80K, is compatible with a LazCu04 magnetic type structure. In figure 2 we

report the analogous data when a magnetic field of 2 Tesla is applied parallel to (1, -1,0) . The magnetic reflection (l/2,1/2,0) is completely suppressed, however one can still distinguish between 3 different temperature regions. The high and low temperature regions (I, III) are now specifically characterized by magnetic reflections (l/2,1/2,1) with even integers 1 only, whereas the intermediate phase (II) is characterized by odd integers 1 only.

'H=ZT'

0-

1 2i

0

Fig 8. The temperature magnetic

50

100 150 200 250 300 Temperature [K]

dependence

(l/2,1/2,1)

of the integrated intensities

of the

re+!Iec tions is shown when a magnetic field

H = 2 Tesla is applied along the (1, - 1,O) direction.

The mag-

is ~~~preas~d~ however one can stdl netic resection (l/2,1/2,0) distinguish 3 diflerent temperature regions. The high and low temperature regions (I, III) are now specifically characterized by magnetic reflections (l/2,1/2, a) with even integer 1 only, whereas the intermediate phase (II) is characterized by odd integer I only.

112

This field-induced be accounted

selection

assuming,

netic structures

discussed

at H=2 Tesla correspond of type

the collinear

previously

LazCuOd

to a single domain

in the region

of collinear

Bragg

peaks.

the magnetic

of the integrated

A small

field of about

reflections

(l/2,1/2,1)

small

filed of about

peaks

I and III, and

II. temperature intensity

region

T x

of the magnetic

1 Tesla is sufficient with even integer

those corresponding to odd integer T M 1OOK , figure 4 shows the relatively

of the magspins and per-

in the regions

In figure 3 we show, in the intermediate 50K, the field dependence

version

at zero field. The observed

to the field, of type LanNiOd

pendicular

1 , can unambiguously

rule odd/even

exclusively

to suppress 1 and enhance

1 . In the high temperature region opposite selection rule. Again a

1 Tesla

suppresses

in this

region

the

1000

3

*i 800 $

600

0

Fig 3. The magnetic shown

1

2 3 4 Magnetic Field [T]

field dependence

in the intermediate

5

of the integrated

temperature

region

intensities

(50K)

is

, A small

field of about I Tesla is suficient to suppress the magnetic reflections (l/2,1/2,1) with even integer 1 and enhance those corresponding to odd integer 1 . The solid lines are a guide for the eye.

113

magnetic

reflections

(l/2,1/2,1)

with

odd

integers

1 and

enhances

the even-l-reflections. The variation upon

the

temperature scanning spins

magnetic

region up and

versibility

of the integrated

applied

(III).

down

No significant the magnetic

is very suggestive

at all temperature

picture

which considers

spins equally unlikely.

including

field within phases

I and

with their

field.

The I

I

expect

is observed 2 Tesla.

to observe

II. The

re-

alternating

with collinear

domains

seems very

irreversible

transdomain I

I

I

T= 100 K

1000

0

II

I

O

Fig 4. The magnetic shown

This

size of one antiferromagnetic I

low when

of the Cu++

domains

90” rotatedd

peaks

5 , for the

reorientation

at zero field magnetic

distributed under

of the magnetic in figure

hysteresis

of a continuous

In this case one would

formations

intensity

field is shown

1

field

I

I

I

2 3 4 Magnetic Field [T] dependence

in the high temperature

of the integrated region I .

L

5

intensities

is

1000

rh

‘2

600

3 600 a .$ 400 @lI 2 200

0

B B B

A

B

= 21 K = [i/2 l/2 13 Increasing H Decreasing H

B t.2

g

Is

4

0

0

0

Fig 5.

The

variation

of

0.5 1.0 1.5 magnetic Field [T] the

integrated intensities

2.0

of the magnetic

peaks

upon the ~~~~~e~magnetic field is shown in the low temperatature region

(III).

No significant hysteresis is observed when scanning

up and down the magnetic field within 2 Tesla.

would increase to the detriment of its 90’ counter part.

Further a

single domain with collinear spins could be stabilized at zero field after recycling the crystal at high field. This is not what we observe, the zero field state is always observed

with no odd/even

selection

rule. 3. Conclusion In conclusion we suggest that the magnetic phases I, II and III of Nd&.?uO~ at zero field have non collinear spins . The phases I and III must be a coherent superposition with its 90° rotated domain. superposition

of the La&u04

of the LaaNiO*

collinear structure

Whereas the phase II is a coherent collinear structure with its 90” counter

part . The field induced magnetic transformations we have observed in the three phases can be seen as a continuous reorientation of the spins from the non collinear magnetic

type structure at zero field

into their collinear version at high field. The magnetic field induces double-q to single-q tr~sformations.

References

1 Y. Tokura, H. Takagi and S. Uchida, Nature 377, 345 (1989); H. Takagi et al Phys. Rev. Lett. 39,1197, 2 G.M. Luke et al , Nature 338,49

(1989)

(1989)

3 J. Akimitsu et al, J. Phys. Sot. Japn. 58, 2646 (1989) 4 Y. Endoh, M. Matsuda, K. Yamada, K. Kakurai, Y. Hidaka, G. Shirane and R.J. Birgeneau, Phys. Rev. B. 40, 7023 (1989) 5 D.E. Cox, A.I. Goldman, M.A. Subramanian, J. Gopalakrishnan and A.W. Sleight, Phys. Rev. B. 40, 6998 (1989) 6 K. Yamada et al, Phys. Rev. B. 39, 2336 (1989) 7 S. Skanthakumar et al, Physica C 160, 120, (1989) 8 M. Matsuda et al (1990) ( to be published)