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Submitted on 1 Jan 1979
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Ultrasonic study of magnetic structure of rare earth metals
S. Palmer, D. Jiles, C. Isci
To cite this version:
S. Palmer, D. Jiles, C. Isci. Ultrasonic study of magnetic structure of rare earth metals. Journal de
Physique Colloques, 1979, 40 (C5), pp.C5-33-C5-34. �10.1051/jphyscol:1979512�. �jpa-00218904�
JOURNAL DE PHYSIQUE
Colloque C5, supplkment au no 5, Tome 40, Mai 1979, page C5-33
Ultrasonic study of magnetic structure of rare earth metals
S. B. Palmer
(*),D. Jiles
(*)and C. Isci
(**) (*) Dept. of Applied Physics, University of Hull, Hull, England (**) Dept. of General Physics, University of Ege, Izmir, TurkeyRCsumB. - On a employ6 les valeurs de la constante tlastique c,, de Tb pour construire un diagramme de phase magnCtique de 1'61Cment. Les rCsultats sont cornparks
A
des mesures anttrieures sur Dy, Tb-50%
Ho et Gd-40%
Y.Abstract.
-
Measurements of the elastic constant c,, of T b have been used to construct a magnetic phase diagram of the element. The results are discussed in conjunction with earlier measurements on Dy, Tb-50%
Ho and Gd- 40%
Y.Ultrasonic measurements of elastic constants (cij) have been used to construct magnetic phase diagrams of the rare earth metals Tb-50 P:, Ho [I], Gd-40 % Y [2]
and Dy [3]. Anomalies in the temperature and magne- tic field dependence of the cij were correlated with the paramagnetic-spiral spin antiferromagnetic-ferro- magnetic-intermediate state phase changes. The rare earth element Tb has similar magnetic phases and we report here measurements of c,, as a function of temperature and magnetic field that yields a detailed magnetic phase diagram of Tb.
Two single crystals of Tb were used, both obtained from Metals Research, Cambridge, England. They were grown by float zoning techniques from similar quality start material. The final purity of the samples has not been established but, from vacuum fusion measurements of similar samples, is probably of the order of 99 "/, atomic with 0, N, H and C as major contaminents. The first sample (Sample I) had been subjected to a whole range of experiments extending over four years before the measurements reported here were carried out. These experiments included repeated thermal cycling down to helium temperatures.
Sample I1 was measured almost immediately it had been cut for the single crystal boule. One would therefore expect the major difference between the two samples to be due to internal strain in sample I produced by its thermal history.
The measurement of c , , was carried out by standard pulsed ultrasonic techniques [I] except that the velocity of sound was measured by the sing-around technique which allowed a point to point sensitivity of 1 part in lo6. The elastic constant c,, measured in zero field for sample I1 is shown in figure 1 where TN at - 226 K and T, at - 219 K can be clearly seen.
It is also evident that there is some structure in the AF state since the temperature dependence of the elastic constant is by no means smooth in this region between
TN and T,.
Fig. 1. - Variation of elastic constant c,, (Nm-') of Tb with temperature in zero applied field.
As examples of the isothermal plots used to produce the final phase diagram, figure 2a is takenat a tem- perature of 214 K while figure 26 is at 220 K. In figure 2a the knee at an applied field of 0.1 tesla marks the destruction of the A.F. spiral structure while the deep minimum centred about 0.4 tesla is associated with the conversion of the intermediate state to a ferromagnetic type of order. The behaviour is some- what different in figure 2b where there is no evidence of a knee in the curve and the sharp drop in c,, is taken as the destruction of the A.F. spiral and the subsequent sharp rise as the formation of ferromagne- tic order.
The phase diagrams for the two samples are shown in figure 3 where they are compared to measurements of ultrasonic attenuation [4]. In both cases the ferro- magnetic phase I can be clearly differentiated from a
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979512
C5-34 S. B. PALMER, D. JILES AND C. ISCI
- I
1
*. ..
0.20 0.40 0.60 0.80 1.00 Magnetic Fleld (tesla)
Fig. 2. - Variation of elastic constant c,, (Nm-2) o f Tb with magnetic field at 214 K and 220 K.
second ferromagnetic phase 11. It is notable that the only difference between the two samples is in the region of T,, which is 5 K higher for the sample with lower internal strains. This helps to explain the range of values for T , reported in the literature. Annealing the samples, even at 60 K [5j may well have a marked effect on T,. We intend to carry out similar measu- rements in the near future on a T b sample that has been grown by solid state electrotransport to further investigate the effects of purity and strain on the phase diagram.
The boundary at high temperatures between the paramagnetic and F(I1) phase is only seen as a broad
Fig. 3. - Magnetic phase diagram of Tb.
-
Sample I.Sample I1 where it differs from I.
- - -
Ultrasonic attenuation results [4].minimum in the field dependence of
c,,in agreement with theoretical approaches which predict a gradual change from the disordered to ordered phase in the presence of a magnetic field.
Finally the intermediate state between AF and F(I1) is the subject of great interest at present. Early theoretical work by Nagamiya [6] predicted that the A F spiral spin phase should collapse into a fan state at a critical magnetic field ( H , ) and that the fan should close smoothly as the magnetic field is increased above
H,. However, as in the early work [l, 2, 31 we notice that there is structure in the magnetic field dependence of
c,,in this intermediate state and the final production of ferromagnetic order is accompanied by a dramatic increase in
c,,to a saturation value. Taking these results in conjunction with the earlier work [ l , 2, 31 we would suggest that the intermediate state is either a distorted spiral or it is split into domains, partly ferromagnetic and partly distorted or non-distorted spiral.
Measurements on purer samples and the neutron diffraction work of Crangle et al. (private commu- nication) may well illuminate this problem.
References
[l] ISCI, C. and PALMER, S. B., J. Phys. Chem. Solids 38 (1977) 1253.
[2] PALMER, S. B., HUKIN, D. and ISCI, C., J. Phys. F, Metal Phys.
7 (1977) 2381.
[3] ISCI, C. PALMER, S. B., J. Phys. F, Metal Phys. 8 (1978) 247.
[4] MAEKAWA et al., Phys. Rev. B 13 (1976) 1284.
[5] PALMER, S. B. and GREENOUGH, R. D., J. Mag. and Mag. Mat. 1 (1976) 310.
[6] NAGAMIYA, T., J. Appl. Phys. 33 (1962) 1029.
