Magnetic Properties of Single Crystals of Europium, Neodymium, and Praseodymium T

MAGNETIC PROPERTIES OF SINGLE CRYSTALS OF EUROPIUM, NEODYMIUM, AND PRASEODYMIUM T. Johansson, K. Mc Ewen, P. Touborg

To cite this version:

T. Johansson, K. Mc Ewen, P. Touborg. MAGNETIC PROPERTIES OF SINGLE CRYSTALS OF EUROPIUM, NEODYMIUM, AND PRASEODYMIUM. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-372-C1-374. 10.1051/jphyscol:19711128. jpa-00213945

HAL Id: jpa-00213945 https://hal.archives-ouvertes.fr/jpa-00213945 Submitted on 1 Jan 1971

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE CoIIoque C I, supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 372

MAGNETIC PROPERTIES OF SINGLE CRYSTALS OF EUROPIUM, NEODYMIUM, AND PRASEODYMIUM

T. JOHANSSON, K. A. Mc EWEN and P. TOUBORG Laboratory for Electrophysics, The Technical University, Lyngby, Denmark

RbumB. - Les moments magnktiques de monocristaux de Eu, Nd et Pr ont 6te mesures entre 2 OK et 300 OKdans des champs jusqu'a 60 kG. Les moments effectifs de la phase paramagnktique ont ete dktermines. Les transitions magne- tiques de Nd a basse temperature donnent des anomalies de la susceptibilitk en champ faible. La susceptibilit6 de Pr a basse tempkrature est beaucoup plus grande dans le plan de base que selon l'axe c et tend a se saturer dans les grands champs. Eu donne, par une transition du premier ordre, un plan magnetique ordonne B 94,3 OK ; en dessous de cette temperature la susceptibilite est anisotrope et non linkaire.

Abstract. - The magnetic moments of Eu, Nd and Pr single crystals have been measured between 2 OK and 300 OK in fields up to 60 kG. The effective moments in the paramagnetic phase have been obtained. The magnetic transitions in Nd at low temperatures are observable as anomalies in the low field susceptibility. The low temperature susceptibility of h is much greater in the basal plane than along the c-axis, and tends to saturate at high fields. Eu has a first-order transition to a magnetically ordered phase at 94.3 OK ; below this temperature the susceptibility is anisotropic and non- linear.

1. Introduction. - The magnetic properties of the light rare earth metals are of particular interest because of the widely different role that the exchange and crystal field energies play in determining their magnetic structures. In divalent bcc Eu the crystal field is very small compared with the exchange, but in the cases of Nd and Pr the crystal field splittings of the magnetic energy levels in the double hexagonal structure are comparable with the exchange energies. These effects have been investigated by single crystal magnetization experiments using a vibrating sample magnetometer, a Faraday balance and an integrating fluxmeter. The results for each metal will be described in turn. 2. Europium. - The magnetization of polycrystalline Eu has previously been measured [I] in fields up to 12 kG and neutron diffraction studies [21 on Eu powder indicate a transition at about 90 OKto a helically ordered structure with [I001 as the rotation axis and a slightly temperature dependent interlayer turn angle of about -SO0. In the present experiments, spherical samples were cut from single crystals grown by the Czochralski technique. In the paramagnetic phase the magnetization is isotropic with an effective moment of 8.48 ,uB/atom. Since the moment is derived from the Eu++ ion in an S-state with spin S = 712, the effective Bohr magneton number is 7.9. Hence some conduction electron enhancement of the moment is indicated. FIG. 1. - The variation of moment with magnetic field applied At 94.3 OK there is a first-order transition to a isothermal in the El001 direction in Eu. magnetically ordered phase, as evidenced by the dis- continuous change in magnetic moment [3]. Below the ordering temperature the magnetization is anisotro- be characterized by high-field and low-field suscepti- pic with [loo] as the hard and [I101 the easy directions. bilities. Although the former is only weakly tempera- The magnitude of the discontinuity ranges from 4.5 % ture dependent, nevertheless it has a maximum value to 1 % between hard and easy directions respectively. at approximately 26 OK. There is no sign of any Magnetization isotherms for temperatures down saturation of the magnetization : indeed a slight to 4.2% are illustrated in figure I for the [I001 upward curvature is apparent at the highest fields direction. The non-linear form of these isotherms may and lowest temperatures.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711128 MAGNETIC PROPERTIES OF SINGLE CR.YSTALS OF EUROPIUM, NEODYMIUM C 1 - 373

The low field susceptibility is up to six times larger than the high field susceptibility and its temperature dependence has been determined from isofield measurements. Starting from 4.2 OK the lowest isofield plot (2.52 kG) shows an initial magnetization increasing slowly to a maximum value at 14.5 OK before dropping by some 20 % over the next lo0 or so. In this new phase, the magnetization decreases slowly until about 65 OK when it decreases more rapidly before levelling off at around 82OK until the first order transition at 94.3 OK. As the applied field is increased, the separate ordered phases become less readily distinguished, because of the smearing of the magnetic transitions, and are barely observable at 50 kG. I...... _ Measurements have also been made in the case of I 10 20 30 40 50 60 70 80 90 100 the magnetic field along the [I101 easy direction. These TEMPERATURE [KI results are qualitatively identical to the [loo] results, for instance the anisotropy at 4.2 OK varies bet- FIG. 2. -The low-field volume susceptibility (MKSA units) of Pr and Nd in the [I101 and the [OOl] directions as a function ween 5 % and 9 % for different fields, and changes of temperature. little with increasing temperature. The effect of a magnetic field on a screw spin system has been discussed by Nagamiya, Nagata and Kitano At 20K the magnetic moment of Nd [V] shows [4], [5]. In the absence of anisotropy, the helical struc- alignment of domains at about 9 kG. In the [I101 ture becomes simply distorted, in the direction of the direction transitions are observed at 21 and 33 kG applied field, by an appropriate tilting of the moments whereas no transitions are observed in the [OOl] until, at a critical field H,, a transition to a fan struc- direction and in fields above 10 kG the moment in ture occurs. As the field increases further, the moments this direction remains substantially smaller than the close to a ferromagnetic alignment. Using the magneti- moment in the [I101 direction. The domain alignment zation data, and a value of the spin turn angle (500) shows magnetic hysteresis which is about 2 kG broad. from [2], to obtain molecular field model exchange When the temperature is increased to 6 OK little domain parameters, the calculated low field susceptibility alignment effect is observed and the two transitions from [4] is some 50 % greater than the measured in the [I101 direction coincide at a field of 27 kG. value. However the results are not compatible with This single transition is still present at 11 OK but has fan and ferromagnetic alignments. It is hoped that disappeared at 18 OK. Above 50 OK the magnetic neutron diffraction studies in a magnetic field, which moment for both directions is a linear function of are being undertaken, will reveal the magnetic struc- field in the field range studied. The curves of the ini- tures. tial susceptibility for the [I101 and the [OOl] directions in Nd (see Fig. 2) lie close together and show ano- 3. Praseodymium and Neodymium. - !Measure- malies at the two transition temperatures 7.5 OK ments of the susceptibility of dhcp Pr and Nd have and 19.20K. been performed by Lock [6] on polycrystalline samples The reciprocal susceptibility in the [I101 direction and Behrendt et al. [7] have measured the magnetic for both Pr and Nd obeys the Curie-Weiss law above properties of Nd single crystals in fields up to 19 kG. 50 OK but the reciprocal susceptibility in the [Wl] More recently the magnetic structure of single crys- direction remains slightly non-linear with tempera- tals has been studied by magnetization and by neu- ture until approximately 90 OK. In the upper tempera- tron diffraction measurements [8]. ture range the effective paramagnetic moment per We have measured the magnetic moment of single atom is found to be 3.45 pB and 3.59 pB for Pr in the crystal spheres (about 4 mm in diameter) of dhcp [I101 and [OOl] direction respectively and 3.39 ,u, Pr and Nd over the temperature range 2 OK to 100 OK, and 3.50 p, for Nd in the [I101 and [001] direction in fields up to 60 kG, with an accuracy of approxima- respectively. These values may be compared with tely 1 %. the theoretical value of 3.58 p, for the free Pr3+ ion The magnetic moment of Pr [9] varies smoothly and 3.62 ,u, for the free Nd3+ ion. with field and shows a tendency to saturate at high The magnetization and susceptibility curves of fields and Iow temperatures. Above 35 OK the moment Pr and Nd may be understood [8], [ll], 1123 by is a linear function of field in the whole field range assuming that Pr is an induced moment system in studied. The moment is highly anisotropic, [OOl] which the large magnetic anisotropy is dominated being the hard and [I101 the easy directions respec- by the anisotropy of the effective exchange. Nd on tively. The reciprocal initial susceptibility is shown the other hand is an ordered moment system in zero in figure 2. The form of the curve for the [Wl] direc- field at low temperatures and the abrupt changes in tion reveals a tendency towards antiferromagnetism the magnetization by an applied magnetic field may below 30 OK [8], [lo]. be due to the crossing of crystal field levels. T. JOHANSSON, K. A. Mc EWEN AND P. TOUBORG

References [I] BOZORTH(R.) and VAN VLECK (J.), Phys. Rev., [7] BEHRENDT(D. R.), LEGVOLD(S.) and SPEDDING(F.), 1960, 118, 1493. Phys. Rev., 1957, 106, 723. 121 NERESON(N.), OLSEN(C.) and ARNOLD(G.), Phys. [8] JOHANSSON(T.), LEBECH(B.), NIELSEN(M.), M~L- Rev., 1964, 135, A176. LER (H. B), and MACKINTOSH(A. R.), Phys. Rev. 131 See figure 1 of MACKINTOSH(A.), this issue, (Magne- Letters, 1970, 25, 524. tism in the light rare earth metals). [9] See figure 2 of MACKINTOSH(A.), this issue. [4] NAGAMWA(T.), NAGATA(K.) and KITANO(Y.), [lo] CABLE(J. W.), MOON(R. M.), KOEHLER(W. C) and Prog. Theor. Phys., 1962, 27, 1253. WOLLAN(E. 0), Phys. Rev. Letters, 1964, 12, 553. [5] KITANO(Y.) and NAGAMIYA(T.), Prog. Theor. Phys., [ll] LEBECH(B.), and RAINFORD(B. D) this issue, (The 1964, 31, 1. magnetic structures of Neodymium and Praseo- [6] LOCK(J. M.), Proc. Phys. Soc., 1957, 70 B, 566. dymium).