SINGLE CRYSTAL NEUTRON DIFFRACTION STUDIES OF HCP RARE EARTH THORIUM ALLOYS

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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�

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- riments on polycrystalline samples of heavy rare earth-thorium alloys [1] showed that dilution by thorium tended to enhance the ferromagnetic character of the rare earths as long as the hep structure was retained. The limit of Th concentration in this structure is 15 to 20 at. % and when this limit is exceeded, a two phase region occurs followed by the fee Th phase when the Th concentration exceeds about 50 at. % [2].

Alloys with the fee phase show magnetic short range order in their low temperature neutron patterns [3]

with behavior characteristic of antiferromagnetic correlations but no long range magnetic order down to 1.3 °K. In the course of the previous study some 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 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

179 to 87 °K is not present in this alloy. The addition of this relatively small amount of Th has thus altered the magnetic structure of Dy so that it transforms spontaneously from a paramagnetic to a ferromagnetic configuration without going through an intermediate spiral phase. The axial anisotropy of the pure-metal is retained, however, since the moments in the alloy lie in the base plane.

Figure 1 illustrates the magnetic transition tempe- ratures observed for the Ho- and Er-Th crystals.

There seems to be thermal hysteresis in some of the transitions so the temperatures shown are those measured on warming the sample whenever the diffe- rence in the warming and cooling temperatures were

ORDERIN G TEMPERATUR E (°K )

FIG. 1. — Magnetic transition temperatures of hep alloys of Th with Ho and Er.

(*) Research sponsored by the TJ. S. Atomic Energy Com- mission under contract with the Union Carbide Corporation.

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 transition at TN = I33 OK to a spiral spin structure in the base plane which transforms at 20OK into a conical configuration in which a ferromagnetic compo- nent of 1.7 ^pB is aligned along the c-axis while the rest of the moment remains in the base plane spiral.

With the addition of Th to Ho the magnetic structure observed below T, is still a base plane spiral but the turn angle between rnomcnts in adjacent layers is reduced ; for example, from 500 in pure Ho to 430/layer in the 95 % alloy at TN. The turn angle decreases to about the same value as pure Ho as the temperature is lowered for this alloy but as the Th concentration is increased, this reduction becomes greater. An 84 % Ho alloy has an initial turn angle oi = 190 just below TN and a final turn angle w, = 13.50 just above Tc. Turn angles this small could not be distinguished from w = 0 (a ferromagnet) in the former polycrys- talline sample studies.

The low temperature phases of the Ho-Th alloys are apparently of two types. The first occurs in the 95 and 90 at. % Ho alloys in which a large ferroma- gnetic moment is observed in the base plane with a- very small modulation superimposed on this predomi- nately ferromagnetic structure. This extra modulation is shown by the presence of small satellite reflections at 40K with intensities corresponding roughly to 1 pB per atom if the structure were of the spiral type.

Furthermore, these satellites are present along the 001 zone indicating that the modulation is not solely along the c-axis. The second type of phase is present in the 84 at. % Ho alloy in which the dominant ferromagnetism occurs in the base plane but without any superimposed modulation. This interpretation that there are different low temperature phases is supported by the behavior of the ordering temperatures which rise to 400 at 95 % Ho, fall to 240 a t 90 % Ho, then rise again to 53 OK at 84 0/, Ho. Of course, there must be a third type of structure present over at least a small region of Th concentration : the conical phase present below 20 OK in pure Ho. However, it would be very difficult to see a small additional ferromagnetic moment along the c-axis in the alloys due to the much larger ferromagnetic component present in the base plane. The two phases observed in these alloys are not similar to this conical structure since in them the ferromagnetic component is much greater and is directed in the base plane instead of along the c-axis.

In the spiral phase above the low temperature transition, extra satellites with intensities of the order of tenths of percent of the (100) nuclear are observed

Refer [I] KORHLER (W. C.), CHZLD (H. R.), CABLE (J. W.),

and MOON (R. M.), J. Appl. Phys., 1967, 38, 1384.

[2] EVANS (D. S.) and RAYNOR (G. V.), J. NucI. Matter, 1960, 2, 209.

in the 95 an 90 at. % Ho alloys. This indicates that the simple spiral is not retained throughout this tem- perature region and instead some distortion of the spiral occurs [4].

The magnetic intensities from the 84 at. % Ho alloy were put on an absolute basis by comparison with the nuclear intensities. Assuming the magnetic form factor calculated for H o + ~ or the form factor observed from Ho,O,, a least squares fit of the data yields a magnetic moment per Ho atom of 9.0 ? 0.2 p,.

This is based on a ferromagnet with the moments in the base plane and equal domain populations. This value is lower by 10 0/, than the expected moment of 10 ^pR on the Ho

ion. This difference could indicate a difference in the form factor for this alloy but the data seem to follow the form factor curves fairly well except for the scale factor.

Pure Er [5] has a transition at T, = 80 OK to a magnetic structure in which the components of the moments oriented along the c-axis are modulated in magnitude with a phase angle w between atoms in adjacent c-axis layers. This configuration transforms at T = 53 OK into a structure in which the base plane components of the moment are arranged in a spiral and the c-axis moduiation begins squaring up into an antiphase domain structure. Then, at Tc = 20 OK, the c-axis component (7.9 pB) becomes ferromagnetic while the base plane component (4.3 ^pB) retains the spiral arrangement. Thus the overall configuration is conical and the total ordered moment is the expected value of 9 pB.

In the 95 % alloy, these three regions of magnetic order are still observed but the ferromagnetic compo- nent occurs at a temperature appreciably higher than the basal plane spiral. Thus as the temperature is lowered below TN = 68 OK, magnetic satellites occur except along the reciprocal c-axis indicating the c-axis modulation. Then, at Tc ⁼ 460, magnetic intensity occurs at the nuclear lattice sites except for 001's and the satellites disappear indicating a c-axis ferromagnet.

Below TH = 260, satellites again appear and the satellites along the 001 zone are present as well which indicates a base plane spiral arrangement. The inten- sities of the magnetic reflections show that the compo- nent along the c-axis is still 7.9 + ^0.3 pB/Er but the base plane component is reduced from 4.3 ^pB to about 1.3 po/Er. The 90 and 85 "/, Er alloys show no modu- lation of the moments and instead are ferromagnetic along the c-axis with a moment about the same as the c-axis component of pure Er. Thus the base plane spiral component is suppressed entirely.

[3] CHILD (H. R.), KOEHLER (W. C.), and MILLHOUSE ( A . H . ) , J . Appl. Phys., 1968, 39, 1329.

[4] KOEHLER (W. C.) et al., Phys. Rev., 1966, 151, 414.

[ 5 ] CABLE (J. W.) et al., Phys. Rev., 1965, 140, A1896.