Magnetic excitations of ordered Pd3Fe and disordered Pd Fe alloys

Journal of Magnetism

MAGNETIC

and Magnetic

Materials

EXCITATIONS

D.M’K.

PAUL*,

* Dqwrtntrrtt

of Ph.vsrc:v, Utwer.sit~ UK

Edrntxqh.

54-57

(1986)

OF ORDERED

P.W. MITCHELL of Wuruwk,

1171

1171-1172

Pd,Fe

** and S.A. HIGGINS C‘orvntr\,

-PdFe

AND DISORDERED

ALLOYS

**

C V4 7A L. L’K * * Deppcrrtmrm

of Ph\wc .s. ~,~~rvr.s,tv i,t Edrtd~ur,qh,

Neutron scattering studies of the magnetic excitatmns of slngle crystal specimens of Pd 1Fe and some PdFe allqs have been made. using several triple axis spectrometers (TAS) at the ILL. Grenoble. For Pd,Fe. the renormalisatio>~wth temperature of the damped excitations in the “Stoner Continuum” has been observed. The influence of stoichiometry and chemical order has been examined by studies of partially disordered Pd,Fe and Pd,, Fe,,. For low concentrations of Fe 111Pd heavily damped excitations with unusual dispersion have been measured.

,iL:i”‘i

1. Introduction

The influence of single particle (“Stoner”) excitations on the dispersion and lifetimes of the spin wave excitations of itinerant magnets is a well-known phenomenon [ 11. Such effects have only been observed for a limited number of materials. e.g. Fe [2], Ni [3], MnSi [4] and the antiferromagnetic alloy Mn,,Ni,, [S]. Further, the experimental data suggests that there is little temperature dependence for the excitations which propagate with finite lifetimes in the “Stoner region”. 2. Ordered

1

1

1

t ro.97

TIet_ 10 a

Pd,Fe

The ordered compound Pd,Fe is a useful material for the examination of such effects. The dispersion of the magnetic excitations from the ground state of this ferromagnet is highly anisotropic. Heavily broadened excitations have been observed in the [loo] direction near to the Brillouin zone boundary, but there is no damping in the other symmetry directions [6]. The reduced spin wave stiffness of this intermetallic compound relative to the elemental itinerant magnets. permits observations to be made over the entire Brillouin zone. Our measurements of the temperature dependence of magnetic inelastic scattering of Pd,Fe have been made over a range of temperatures from 293 K to above the Curie temperature (7; = 499 K). The technique used for these studies was neutron inelastic scattering, using the DS TAS of the ILL, with polarisation analysis of both the incident and scattered neutron beams. The use of polarised neutrons allows an unambiguous separation of the magnetic scattering from the nuclear component. This study is an extension of previous measurements by Holden [7]. Agreement between the two experiments is in general good; however Holden’s analysis appears to underestimate the magnon widths near to the zone boundary. Fig. 1 presents the magnetic excitation dispersion relation of ordered Pd,Fe in the [loo] direction as a function of reduced temperature. relative to c. This

0304-8853/86/$03.50

I

0 Elsevier Science Publishers

6 4 2 0 0

0.1

Q

0.2

0.3

a4

0.5

(wavevector)

Fig. 1. Magnetic excitation dispersion relation and fwhm for ordered Pd ,Fe in the [IOO] direction at three temperatures

diagram also contains the observed fwhm of a Lorentzian function describing the scattered intensity. The data clearly demonstrates the extension of the damped region towards the zone centre and the renormalisation of the excitation energies with increasing temperature. Such effects would be expected for a “Stoner magnet” due to the closure of the band gap on approaching the Curie temperature. Above T a purely diffusive behaviour was observed. within the statistical accuracy of the data and allowing for the influence of instrumental resolution and the applied magnetic field. 3. Stoichiometry

and chemical

order

The influence of chemical order and stoichiometry on the broadened excitations of Pd,Fe was examined by measuring the spin wave spectra for a partially disordered sample of Pd,Fe and the random alloy Pd,, Fe,,. The disordered sample was produced by quenching from above the order-disorder transition.

B.V.

From

the superlattice

that

159

onI>

~mplea an

excitation

tion of either relation.

using

There

lo\\

For

which high

of

observed

arc considered

density

materials

iron

techniques giant

the

crystal

measured

as

limited

persion

with

Such that

Stoner energies

peaked

chemical

over

’

Our

dis-

and

expected

the

was broadened

Brillouin

in frequency

excitations the

disorder

ih due

at Iow energy. iron

concentration and

Brillouin

of

and

gested

that

to

band-structure

of the

PdFe

exhibit

Debye wa

is

local

which

cxwith

by assum-

environment

density

move

ix reduced.

of

to tower However.

effects

tions

which

be useful Ni.

itinerant

model

of Pd,Fe

will be a subject

would

closes

he

at T.

magnets

and

of the properties alloys

have

are directly

ordered unusual of future

of

the properties

of disordered

the properties alloys

the band-gap

in understanding

Studies

dependence

state.

Low

dispersion

sug-

related conrela-

study.

=erc

width.

explained

the

centration

Pd,iFe,,, which

to a high

Fe

temperature

of other

I

‘_

can be qualitatively broadening

quantitative

would

spectrum

and

studies

a

Pd ,Fe

which

The

from

over-

excitations

tone.

where

an

measured

propcr-

of Pd 1 Fe are what

that

For

to the unusuat

the

suggests

by THz.

for ;1 system

differs

for

excitations

ther-

the entire

direction.

measurements

of the magnetic

using

Pd,,Fe,,,

over

spectrum

at 0.7 A

the

For

contribute

5. Conclusion

w+ich

for Pd,,Fe,,,

measured

of (0.7 + 0.3)

structure

effects

as

U ~‘a:,

quadratic

well-described

in fig. 2. Highly

the maximum ing

;I

also

alloys.

of the

This

was

a width

across

;I

region

was 0.1 A

were

of wavevector

presented

The

was observed

excitation

ohb,erLed

for Pd,,Fe,,S.

following

the IN8 TAS.

response

hi/arre

hibits

A’.

excitations

response This

function

energy;

were observed.

and

independent ;I

to describe

must

ties of these

was

the inclusion

excitations

energy

neutrons

damped

II

A’ and

with

moments)

scattering

I for Pd,,Fe,,,.

0.25 A Higher

angle

Pd,,Fe,,,.

required

(26 k 3) THz

relation

(D) ol’

factor

of intensity

resolution

For

nab

oil oi2 ai 0!4 & Q (wavevector)

of

samples

stiffness

k 0.6) THz

form

i

pal-

at low energies.

single

TAS.

in

excitation

to the presence

excitations

were

IN12

to be (11.2 moment

variation

/one.

1

magnetic

magnetic

to be due

of Stoner

examined

on

estimated

mal

I

dispersion

the

dissolved

unusual

and PdqjFe,,,. The spin wave Pd,,Fe,,, thew alloys was measured using small

and

6

T ;-

material.

concentrations

we have

spectra

;I

1--

no indica-

have of

8,

up to

unpolarised

\\as

broadening

Pd95Feo5(Q,Q,Q)T=5K

both

alloys

ladium

The

For

measured

in the spin

lifetime

of elther

4. PdFe

;I

any

were

TAS.

estimated

sample.

15 THL.

any anisotropy

or

excitations

of

the IN8

and

it wax

in the

excitations

frequency

hams

intensities

remained

the magnetic

neutron

of

Bragg

order

(giant

References [l] [2] [3] [4]

J.F Cooke. J. Magn. Magn. Mat. 14 (1979) 117. J.W. Lynn. Phys. Rev. Bll (1975) 2625. J.W. Lynn and H.A. Mook. Phys. Rev. 023 (19X1) 198. Y. Ishikawa, G. Shirane. J.A. Tarvin and M. Kohgi. Phg\. Re\. 816 (1977) 4956. [5] I>. M’K. Paul and B.D. Ralnford (1985). to he published. [6] A.J. Smith. W.G Stirling and T.M. Holden. J. Phvs. F7 (1977) 2411. [7] T.M. Holden. J. Phy\. F6 (1976) 433.