NMR study of magnetic properties of Pr2Fe14B

Solid State Communications, Vol. 94, No. 8, pp. 619-621, 1995 Copyright 8 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/55 $9.50+.00 003&1098(95)00139-5 NMR STUDY OF MAGNETIC PROPERTIES OF mFel4B K. Shimizu and K. Ichinose Faculty of Education, Toyama University, 3 190 Gofuku, Toyama 930. Japan (Received 24 November 1994; accepted in revised fotm 11 January 1995 by P. Burlet)

14lPr NMR measurements have been done on Pr2Fel4B and (PrO.gLuO.l)2Fel4B

at

1.4K by the spin-echo method. The observed hypefine fields in Pr2Fel4B are 2574 and 2803kOe for the f and g sites, respectively, and in (PrO.9LuO.l)2Fe14B

2560 and

2804kGe for the f and g sites, respectively. The hyperfine fields due to the 4f shell of the Pr3+ ion were evaluated taking into account the transferred hyperfine field from the Fe sublattice. The magnetic moments of Pr 3+ in Pr2Fel4B were estimated to be 2.9 and 3.OpB for the f and g sites, respectively.

Keywords: A. magnetically

ordered materials, B. valence fluctuations,

E. nuclear

resonance

2. Results and Discussion

1. Introduction The magnetic properties of R2Fel4B (R=rare earth

We have performed 14lPr ( f=5/2 and Q= -0.059b)

element) which has two inequivalent rare earth sites f and g

NMR measurements on Pr2Fel4B and (Pr0.9Lug. 1) 14B at

can qualitatively be understood on the basis of single-ion

1.4K by the spin-echo method in zero external magnetic

anisotropy exchange

using a Hamiltonian and

consisting

crystalline-electric-field

field. The samples used in the present study were prepared

of R-Fe

by argon arc melting the appropriate constituents.

(CEF)

The

interactions[ 11.The CEF parameters of rare earth ion have

purity of the pure materials was 99.9% for Pr and Lu,

been obtained from the analysis of the magnetization

99.99% for Fe, and 99.5% for B. The ingot buttons were

curves. In Pr2Fel4B the direction of the easy magnetization

annealed at 900°C for two weeks and then rapidly cooled to

is along the c-axis and the Curie temperature Tc is 569K[2].

room temperature. The samples were confirmed to be single

The second-order CEF parameters of this compound have

phase with the tetragonal Nd2FelqB type (P42/mnm) of

been found to be smaller by about 50% than those of the

structure by X-ray powder diffraction

with graphite

other compounds[3]. The reduction of the CEF parameters

monochromated

has been attributed to an incipient valence instability of the

observed for the f and the g sites are shown in Fig.1. We

Pr ions in this compound. Such an anomaly has also been found in PrCo5[4]. The hypefine field and the magnetic

could observe neither resolved electric quadrupole splittings nor quadrupole modulation in the spin-echo decayi71,

moment of a Pr ion in the valence-fluctuating

Cu-Ka radiation. The Pr NMR spectra

which may be due to the small electric quadrupole moment

state are

expected to be smaller than for a normal trivalent Pr

of *4lPr and the magnetic hyperfine inhomogeneties

ioni5.61. In addition, 141Pr NMR in Pr2Fel4B has not yet

described later. The decays of the 141Pr spin-ho

been reported. In these respects, it is interesting determine the R hypetfime field in Pr2Fel4B.

were measured at peak positions of 3356.5 and 3656MHz

to

intensity

for the f and g sites, respectively. Assuming the decay of 619

620

NMR STUDY OF MAGNETIC PROPERTIES OF Pr*Fe,,B

Vol. 94, No.8

Table 1. Observed hyperfine fields, Hhf,hypetfine fields g site

due to 4f shell, Hqf, and magnetic moments

(6)

p(pB) of Pr for the f and g sites in Pr2Fe14B. The nuclear gyromagnetic ratio y/2x was taken to be 1.304MHz~kGe

Pr2Fe14B

f g

HhftkGe)

H4ftkGe)

25741t4 2803&4

3034 2.9 3153 3.0 33531t31 3.2

Pr3+ free ion 3200

3300

3400

3500 3600 Frequency (MHz)

3700

3800

Fig.1. Observed NMR spectra of 14lPr at 1.4K for the f and

g

sites

in

Pr2 Fe 1 4 B

Pro.cLuo.ikFei@

(a)

-

CI(~B)

and

in

W.

those of Gd[ll],

Tb[l2] and Er[12] in R2Fel4B,

we

obtain p - - 1.6MHz for the both sites. Here, we assumed that p4t=-2.6MHz and the asymmetry parameter fl is 0.35 for the f site[ 111.Then, over all electric quadrupole splitting

the spin-echo signal to follow a simple exponential function

12(2f-1)pl is about 13MHz. The observed values of the full

exp(-2z/T2), where ‘5is the separation between the first and

width at the half maximum height (WHM) of spectrum are

the second rf pulses, we obtain the spin-spin relaxation

60MHz and 48MHz for the f and g sites, repectively. in

time, T2, at 1.4K to be 1lpsec and 27pse.c for the f and g

Pr2Fet4B and are larger than the estimated values from the

sites, respectively. In Nd2Fet4B. the decay of the t45Nd

electric quadrupole interaction. In other R2Fel4B. average

spin-echo signal for the f site has been found to be non-

WHM values of the central lines for the f and g sites are

exponentialI81. The T2 values for the f site have been found

-16MHz for 159Tb [12] and -7MHz for 167Er[l3], which

to be 21 and 60psec in the first and end stage of the decay

correspond to the values from -2lMHz

curve, and that for the g-site to be 23psec with a single

141Pr. Thus, magnetic inhomogeneous broadening might

exponential behavior. These values are compared to our

smear out the quadrupole splittings of 141Pr. The hypertine field, Hhf, of Pr in Pr2Fel4B consists

results. Total electric quadrupole interaction parameter p,

to -69MHz for

of several contributions and may be written as

where 2p is the frequency difference between the adjacent Hhf = H4f + Hsp + $+I$

limes split by the electric quadrupole interaction, is given by p = p4f+

&t

[3COS28

-

1 + 17sin2f?COs2#]/2

(1)

(3)

where H4fis the intra-ionic contribution due to the 4f shell of the Pr ion, Hsp is the so-called self-polarization field

3eQ

‘lat = 41(21-l)h

which arises from the conduction electron polarization Fe caused by the Pr spin at the nuclear site, HP and H, are

(2)

““’

due to the conduction neighboring

where p4f is the contribution

from the 4f shell and is

electron

polarization

observed hyperfine fields for the f and the g sites are shown

-2.6MHz for Pr3+ free ion[9]. The electric field gradient(EFG), VZZ,arising mainly from the aspherical

in Table 1 together with the Pr3+ free-ion value[9].

charge distribution of the valence electrons[lO]. The direction of VZZis taken to be along the c-axis for the g site

(PrO.9LuB.t)2Fe14B

and in the c-plane for the f site[ 111.The polar angles 8 and @ are the angles between the Pr magnetic moment and the

by the

rare-earth and Fe spins, respectively. The

The

observed

respectively.

Hhf values for the f and g sites in are 2560f4

and 2804i4kOe.

These values are almost the same as the

2574~t4 and 2803i4kOe for the f and g sites, respectively, R in Pr2Fel4B. This indicates that the H, due to the

Vz z

neighboring Pr spins is very small. With respect to the

=13.8x1017V/cm2 and -89Ox10~7V/cm2 for the f and g

extra-ionic contributions such as the self polarization and the

sites, respectively, which were obtained by averaging over

transferred fields, the situation can be expected to be similar

principal

EFG

axis.

Using

the

values

of

Vol. 94, No.8

NMR STUDY OF MAGNETIC PROPERTIES OF Pr,Fe,,B Pr2Fe l4B

to the other R2Fel4B compounds. In the present case, then, the contributions

of Hsp and HP to Hhf are neglected

621

are 0.9 and 0.94 for the f and g sites ,

respectively, and are slightly small compared to the values 0.98 and 0.99 in Tb2FelqB,

similar to the case of Tb2Fel4B and Er2Fel4B[l2]. The Fe transferred field H, , which is antiparalell to the Fe

Er2Fe14B,

respectively[l2].

and 0.97 and 0.98 in However,

the ratios in

Pr2Fel4B are much larger than the 0.75 in PrCo5[6]. Thus, sublattice moment, can be deduced from the 89Y NMR

from the hyperfine-field analysis of Pr in Pr2Fel4B it seems

results on Y2Fel4B[14] taking into account the difference in the hypertine coupling constant between Y and Pr[ 151. We

likely that the anomaly as found in PrCo5 does not occur in

obtain Hf” to be 460 and 350kOe for the f and the g sites,

Pr2Fel4B. The magnetic moments of Pr in Pr2Fel4B were free estimated by means of the relation p=gJHqf/Hqf ,where

respectively,

gJ=3.2,

using the ratio of the hyperfine coupling

constant for Pr and for Y to be 2.0. The estimated H4f values for the f and g sites in Pr2Fel4B are shown in Table free 1. The ratios of H4f to Pr3+ free ion value, H4f/H4f ,in

to be 2.9 and 3.0uB for the f and g sites,

respectively. These values are in agreement with the average value of 3.lpB

over the f and g sites obtained

in

magnetization measurements[2]. References

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WI

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