Single Crystal Neutron Diffraction Studies of Hcp Rare Earth Thorium Alloys H

SINGLE CRYSTAL NEUTRON DIFFRACTION STUDIES OF HCP RARE EARTH THORIUM ALLOYS H. Child, W. Koehler

To cite this version:

H. Child, W. Koehler. SINGLE CRYSTAL NEUTRON DIFFRACTION STUDIES OF HCP RARE EARTH THORIUM ALLOYS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-1128-C1-1129. 10.1051/jphyscol:19711403. jpa-00214444

HAL Id: jpa-00214444 https://hal.archives-ouvertes.fr/jpa-00214444 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 Colloque C 1, supplément au n° 2-3, Tome 32, Février-Mars 1971, page C 1 - 1128

SINGLE CRYSTAL NEUTRON DIFFRACTION STUDIES OF HCP RARE EARTH THORIUM ALLOYS (*) H. R. CHILD and W. C. KOEHLER Solid State Division, Oak Ridge National Laboratory Oak Ridge, Tennessee

Résumé. — On a fait antérieurement des mesures par diffraction neutronique sur des échantillons en poudre d'alliages de Tb-terres rares. On a trouvé que l'addition du thorium dans les terres rares tendait de stabiliser la structure ferroma gnétique par rapport à la structure hélicoïdale. On rapporte ici des résultats plus précis qu'on a obtenus avec des mono cristaux. L'alliage 85 % Dy-15 % Th est ferromagnétique au-dessous de la température de Curie, 7c = 119 °K, sans aucune indication d'une structure hélicoïdale. Quand on ajoute le thorium à l'holmium, on trouve que la température de Néel aussi bien que l'angle entre l'aimantation en plans adjacents décroît. La structure conique est supprimée dans l'alliage 95 % Ho-5 % Th. Une phase ferromagnétique se forme ; dans cette phase les moments magnétiques se trouvent dans le plan mais avec une modulation faible superposée. Quand on augmente la concentration du thorium, cette modulation disparaît et la structure ferromagnétique est conservée. L'addition du thorium à l'erbium entraîne la décroissance de la température où s'ordonnent les composantes des moments magnétiques dans le plan, mais la température à laquelle la structure conique se développe augmente. L'alliage 95 % Er-5 % Th a les trois mêmes types de structure que l'erbium pur mais avec une séquence différente. Les alliages 90 % Er-10 % Th, et 85 % Er-15 % Th possèdent seulement des structures ferromagnétiques. Abstract. — Previous powder neutron diffraction measurements of polycrystalline hep Ho- and Er-Th alloys showed that the addition of the Th tended to enhance the ferromagnetic structure of the rare earths. We report here more detailed single crystal studies of representative alloys of rare earths with Th. An 85 % Dy-15 % Th crystal is ferromagnetic below Tc — 119 °K with no visible region of spiral structure. The addition of Th to Ho causes a drop in TN and a decrease in the interlayer turn angle of the spiral phase. The conical structure is suppressed with as little as 5 at. % Th and a phase with a large ferromagnetic base plane component and a small superimposed modulation is observed. As the concentration of Th is increased this extra modulation is eliminated and only the ferromagnetic component remains. Dilution of Er by Th causes a drop in the ordering temperature of the basal plane spiral component but a rise in the conical ferromagnetic transition temperature. A 95 at. % Er-5 % Th alloy shows all three regions of magnetic order of Er but their temperature sequence is changed. Alloys containing 90 and 85 at. % Er, however, have only a ferromagnetic structure.

Introduction. — Previous neutron diffraction expe 179 to 87 °K is not present in this alloy. The addition riments on polycrystalline samples of heavy rare of this relatively small amount of Th has thus altered earth-thorium alloys [1] showed that dilution by the magnetic structure of Dy so that it transforms thorium tended to enhance the ferromagnetic character spontaneously from a paramagnetic to a ferromagnetic of the rare earths as long as the hep structure was configuration without going through an intermediate retained. The limit of Th concentration in this structure spiral phase. The axial anisotropy of the pure-metal is 15 to 20 at. % and when this limit is exceeded, a is retained, however, since the moments in the alloy two phase region occurs followed by the fee Th phase lie in the base plane. when the Th concentration exceeds about 50 at. % [2]. Figure 1 illustrates the magnetic transition tempe Alloys with the fee phase show magnetic short range ratures observed for the Ho- and Er-Th crystals. order in their low temperature neutron patterns [3] There seems to be thermal hysteresis in some of the with behavior characteristic of antiferromagnetic transitions so the temperatures shown are those correlations but no long range magnetic order down measured on warming the sample whenever the diffe to 1.3 °K. In the course of the previous study some rence in the warming and cooling temperatures were details of the magnetic structures could not be well determined from the powder neutron patterns so single crystals of hep rare earth-Th alloys were grown by A. H. Millhouse using the strain anneal method and in this paper we report preliminary results of the study of these crystals. Most of the discussion will be devoted to the Ho and Er systems since these systems appear to be the most interesting. Attempts to grow crystals of the fee phases have so far been unsuccessful.

Experimental results. — An 85 at. % Dy-15 at. % Th alloy crystal exhibits a Curie temperature of 119 ± 3 °K, considerably higher than the Curie point of pure ORDERIN G TEMPERATUR E (°K ) Dy at 87 °K. Furthermore, no visible satellite reflec tions were observed at any temperature indicating that the spiral phase observed in pure Dy from

(*) Research sponsored by the TJ. S. Atomic Energy Com FIG. 1. — Magnetic transition temperatures of hep alloys mission under contract with the Union Carbide Corporation. of Th with Ho and Er.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711403 SINGLE CRYSTAL NEUTRON DIFFRACTION STUDIES OF HCP RARE EARTH THORIUM ALLOYS C 1 - 1129 outside of experimental error. Pure Ho [4] has a in the 95 an 90 at. % Ho alloys. This indicates that transition at TN = I33 OK to a spiral spin structure the simple spiral is not retained throughout this tem- in the base plane which transforms at 20OK into a perature region and instead some distortion of the conical configuration in which a ferromagnetic compo- spiral occurs [4]. nent of 1.7 pB is aligned along the c-axis while the The magnetic intensities from the 84 at. % Ho rest of the moment remains in the base plane spiral. alloy were put on an absolute basis by comparison With the addition of Th to Ho the magnetic structure with the nuclear intensities. Assuming the magnetic observed below T, is still a base plane spiral but the form factor calculated for Ho+~or the form factor turn angle between rnomcnts in adjacent layers is observed from Ho,O,, a least squares fit of the data reduced ; for example, from 500 in pure Ho to 430/layer yields a magnetic moment per Ho atom of 9.0 ? 0.2 p,. in the 95 % alloy at TN. The turn angle decreases to This is based on a ferromagnet with the moments in about the same value as pure Ho as the temperature the base plane and equal domain populations. This is lowered for this alloy but as the Th concentration value is lower by 10 0/, than the expected moment of

is increased, this reduction becomes greater. An 10 pR on the Ho + ion. This difference could indicate 84 % Ho alloy has an initial turn angle oi= 190 just a difference in the form factor for this alloy but the below TN and a final turn angle w, = 13.50 just above data seem to follow the form factor curves fairly well Tc. Turn angles this small could not be distinguished except for the scale factor. from w = 0 (a ferromagnet) in the former polycrys- Pure Er [5] has a transition at T, = 80 OK to a talline sample studies. magnetic structure in which the components of the The low temperature phases of the Ho-Th alloys moments oriented along the c-axis are modulated in are apparently of two types. The first occurs in the magnitude with a phase angle w between atoms in 95 and 90 at. % Ho alloys in which a large ferroma- adjacent c-axis layers. This configuration transforms gnetic moment is observed in the base plane with a- at T = 53 OK into a structure in which the base plane very small modulation superimposed on this predomi- components of the moment are arranged in a spiral nately ferromagnetic structure. This extra modulation and the c-axis moduiation begins squaring up into is shown by the presence of small satellite reflections an antiphase domain structure. Then, at Tc = 20 OK, at 40K with intensities corresponding roughly to the c-axis component (7.9 pB) becomes ferromagnetic 1 pB per atom if the structure were of the spiral type. while the base plane component (4.3 pB) retains the Furthermore, these satellites are present along the spiral arrangement. Thus the overall configuration is 001 zone indicating that the modulation is not solely conical and the total ordered moment is the expected along the c-axis. The second type of phase is present value of 9 pB. in the 84 at. % Ho alloy in which the dominant In the 95 % alloy, these three regions of magnetic ferromagnetism occurs in the base plane but without order are still observed but the ferromagnetic compo- any superimposed modulation. This interpretation that nent occurs at a temperature appreciably higher than there are different low temperature phases is supported the basal plane spiral. Thus as the temperature is by the behavior of the ordering temperatures which lowered below TN = 68 OK, magnetic satellites occur rise to 400 at 95 % Ho, fall to 240 at 90 % Ho, then except along the reciprocal c-axis indicating the c-axis rise again to 53 OK at 84 0/, Ho. Of course, there modulation. Then, at Tc = 460, magnetic intensity must be a third type of structure present over at least occurs at the nuclear lattice sites except for 001's and a small region of Th concentration : the conical phase the satellites disappear indicating a c-axis ferromagnet. present below 20 OK in pure Ho. However, it would Below TH = 260, satellites again appear and the be very difficult to see a small additional ferromagnetic satellites along the 001 zone are present as well which moment along the c-axis in the alloys due to the much indicates a base plane spiral arrangement. The inten- larger ferromagnetic component present in the base sities of the magnetic reflections show that the compo- plane. The two phases observed in these alloys are nent along the c-axis is still 7.9 + 0.3 pB/Er but the not similar to this conical structure since in them the base plane component is reduced from 4.3 pB to about ferromagnetic component is much greater and is 1.3 po/Er. The 90 and 85 "/, Er alloys show no modu- directed in the base plane instead of along the c-axis. lation of the moments and instead are ferromagnetic In the spiral phase above the low temperature along the c-axis with a moment about the same as transition, extra satellites with intensities of the order the c-axis component of pure Er. Thus the base plane of tenths of percent of the (100) nuclear are observed spiral component is suppressed entirely.

Refer [I] KORHLER(W. C.), CHZLD(H. R.), CABLE(J. W.), [3] CHILD(H. R.), KOEHLER(W. C.), and MILLHOUSE and MOON(R. M.), J. Appl. Phys., 1967, 38, (A. H.), J. Appl. Phys., 1968, 39, 1329. 1384. [4] KOEHLER(W. C.) et al., Phys. Rev., 1966, 151, 414. [2] EVANS(D. S.) and RAYNOR(G. V.), J. NucI. Matter, [5] CABLE(J. W.) et al., Phys. Rev., 1965, 140, A1896. 1960, 2, 209.