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Some ferrimagnetic and antiferromagnetic materials atlow temperatures

R.M. Bozorth, Vivian Kramer

To cite this version:R.M. Bozorth, Vivian Kramer. Some ferrimagnetic and antiferromagnetic materials at low tem-peratures. J. Phys. Radium, 1959, 20 (2-3), pp.393-401. �10.1051/jphysrad:01959002002-3039300�.�jpa-00236056�

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SOME FERRIMAGNETIC AND ANTIFERROMAGNETIC MATERIALS AT LOW TEMPERATURES

By R. M. BOZORTH and VIVIAN KRAMER,Bell Telephone Laboratories, Inc. Murray Hill, New Jersey, U. S. A.

Résumé. 2014 Nous avons étudié l’aimantation de monocristaux et de polycristaux de quelquessubstances aux températures de 1,3 à 300 °K, dans les champs jusqu’à 12 500 0153rsteds. Nous avonsfait des mesures sur des monocristaux de LiMnPO4 (orthorhombique), Fe3(PO4)2.4H2O (mono-clinique) et HoFeO3 (orthorhombique) ; les substances polycristallines étudiées sont HoFeO3, Ho2O3et Er2O3, PbO.Fe122014xAlxO18, FeTiO3 et FeTiO3-Fe2O3 en solution solide, CoMnO3 et NiMnO3,CuF2.2H2O, MnF3, et CrF3. Pour les monocristaux ferromagnétiques, on observe une aiman-tation spontanée le long d’un axe cristallin spécifique ; dans le cas du Fe3(PO4)2.4H2O il ne futpas possible de faire dévier l’aimantation spontanée de la direction de l’axe monoclinique ; dansle cas de l’HoFeO3 il est possible aux basses températures de la faire dévier de l’axe privilégiéorthorhombique. L’aimantation spontanée de l’HoFeO3 augmente jusqu’à 3 magnétons de Bohrenviron si on baisse la température du cristal.

Abstract. 2014 The magnetization of single crystals and of polycrystals of a number of materialshas been investigated at temperatures from 1. 3 to 300 °K, in fields up to 12,500 oersteds. Measure-ments were made on single crystals of LiMnPO4 (orthorhombic), Fe3(PO4)2.4H2O (monoclinic),and HoFeO3 (orthorhombic) ; polycrystalline materials were HoFeO3, Ho2O3, and Er2O3,PbO.Fe122014xAlxO18, FeTiO3 and FeTiO3-Fe2O3 solid solutions, CoMnO and MiMnO3, CuF2.2H2O,MnF3, and CrF3. The ferromagnetic single crystals show spontaneous magnetization along onecrystallographic direction ; in Fe3(PO4)2.4H2O it was not possible to rotate the spontaneousmagnetization out of the direction of the axis, in HoFe3O it is readily rotated away from thepreferred orthorhombic axis at low temperatures. The spontaneous magnetization in HoFeO3rises to about 3 Bohr magnetons as the temperature is lowered.

LE JOURNAL DE PHYSIQUE ET LE RADIUM TOME 20, FÉVRIER-MARS 1959,

LiMnPO 4 Crystal

Mays [1] has reported shifts in the nuclear reso-nance of 31P with temperature in a number of

phosphates, and on the basis of the behavior ofthese shifts has suggested the presence of antiferro-magnetic or ferrimagnetic transitions. Miss Walshand the present authors have found by measu-rement of magnetic susceptibilitythat lithiophilite,Li(Mn, Fe)P04, is antiferromagnetic below 42 OK.A single crystal was cut from a natural crystalcorresponding approximately to the formulaLiMno.7Feo.3P04, and measurements were madealong the three orthorhombic axes.

Results are given in figure 1. The molar suscep-tibility, Xm, measured parallel to the a axis(a : b : c = 6.04 : 10.37 : 4.71) [2], decreases withdecreasing temperatures below the Néel point andextrapolates nearly to zero at 0 OK, according tothe theory for an antiferromagnetic with spinsparallel and antiparallel to the direction of theapplied field. The values of xm parallel to the band c axes are nearly constant below the Néeltemperature, ON, but have. a somewhat irregularcourse at these temperatures. No ferrimagnetismwas observed. Above ON the molar susceptibility,paralle] to the a-axis is given by

Xm = 4.65/(r+ 80),from which follows ueff = 6.1. This is slightlyhigher than that expected for Mn++(5.9). Measu-

FIG. 1. - Ferrimagnetism in lithiophilite. At low temper-atures spins are aligned parallel and antiparallel to [100].

rements parallel to [010] and [001] give slightlylower values of Xm and yefi.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphysrad:01959002002-3039300

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Since at 0 OK, Xm along the a axis approaches avalue almost zero, it may be concluded that thespin of each Mn ion (as distinguished from the netspin of all Mn ions) is aligned closely il [100]. Thisdirection lies in the (puckered) plane of the Mn++ions, parallel to the line connecting next-nearestMn++ neighbors.We are indebted to J. L. Durand for supplying us

with an oriented cryrtal of weight about 1 gram,and to J. M. Mays for discussion of the structure.

Ludlamite Crystal, FeS(P04)2.4H20

Ludlamite is monoclinic [3] (CI,,), the point-group having only one plane of symmetry whichis perpendicular to the two-fold axis, b. A crystalwas cut from the same specimen as that usedby Mays [1] in his study of nuclear resonance, and

was in the form of a short cylinder with its axis

FIG. 2. - Magnetization curves of ludlamite in the (001)plane, when H is inclined to a axis at angle 6.

FIG. 3. - Magnetization of ludlamite in the three principal planes perpendicular to the a, b and c’ axes.

Broken curves are those calculated from the observed values of spontaneous magnetization ao and para-magnetic magnetization X H at H = 10 800 Oe, the points are observed.

’

perpendicular to the cleavage plane, the ratio of gnated c’, is about 100 from the crystallographic clength to diameter being 0.84. This axis, desi- axis and is perpendicular to the a and b axes.

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Data were taken in the three planes perpendicularto a, b and c’. Measurements sbowed the Curiepoint to be 20 oR.Measurements in the two planes containing b

showed immediately that the crystal was ferro-magnetic along the b axis, and that in all directionsperpendicular to b the magnetization was propor-tional to the field. Some data for the magne-tization per gram, 6, in the ab plane are shownin figure 2. In figure 3 are plotted the valuesof a in the three planes for H = 10 800 andH = 0, the latter obtained by extrapolating aalong the straight portion of the a vs H lineto H = 0. The latter values, designated ao, plotas circles in the ab and bc’ planes ; this showsthat they exhibit characteristic ferromagnetic pro-perties in the b direction only. There is no appa-rent rotation of a,, out of the direction of the b axis ’in the highest fields used, 12 500 Oe, consequentlywe have not been able so far to place any limit onthe amount of anisotropy of the spontaneousmagnetization.The magnitude of a parallel to b corresponds

to nB = 0.8 Bohr magneton per atom of iron.Two plausible models may be suggested to accountfor this magnitude, in terms of the crystal struc-ture :The Fe++ ions in the structure occupy two non-

equivalent positions, and occur in straight lines ofthree ; the lines are " over " each other in the bdirection and form a plane parallel to tbe b axis.The iron ions in the line are equally spaced, andeach is surrounded by six oxygen atoms at thecorners of distorted octahedra. The end Fe++ions of the line of three are crystallographicallyequivalent to each other but not to the centerFe++ ion. If the three ions are ferromagneticallycoupled and are strictly parallel, the b componentof the magnetization is 4cos0b Bohr magnetonsper ion, where Ob is the angle between the spindirection and the b axis ; Ob must then be about 80°to agree with the observed value of 0.8. Otherlines of ions are so aligned in the crystal structurethat the components .1 b cancel to zero.

It seems more likely that the Fe++ ions in theline of three are antiferromagnetically coupled.In this case the component of the moment parallelto b is (4/3) cos 6b, and 6b must then be about 50°.The deviation of the alignment of the three spinsfrom exact parallelism with each other is, of course,not ruled out.

Since there is no plane of symmetry in thecrystal parallel to the b axis, the symmetry of thestructure does not require the minimum in suscep-tibility in the ac’ plane to have any special direc-tion. Therefore the observed direction of mini-mum susceptibility, parallel to the c’ axis, must benormal to the direction of the resultant spin, or atleast nearer to 900 to this direction than is any

other direction in the ac’ plane. This is shownschematically in figure 4.

FIG. 4. - Proposed alignments of moments in crystalsof ludlamite.

On the assumption of either antiferromagneticor ferromagnetic coupling, the spin directions arethus determined. These assumption, and the truespin directions, could be checked with neutron dif-fraction ; a study of this kind is being undertakenby Dr. S. C. Abrahams.We are indebted to J. L. Durand for supplying

the oriented crystal specimen, and to J. M. Maysfor discussion of the structure.

HoFe03 Crystal

Magnetic measurements have previously beenmade [4] on several of the rare-earth orthoferritesbetween room temperature and 1.3 OK, and

FIG. 5. - Magnetization vs temperature for H Il [010]in HoFe03 crystal.

Gilleo [5] has measured a single crystal of GdFe03at temperatures down to 77,DK. The present workon a HoFeO3 crystal at temperatures down

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to 1.3 °K shows spontaneous magnetization thatchanges in direction with change in temperatureand increases rapidly in magnitude when the tem-perature is lowered below 40 OK.

FIG. 6. - Magnetization vs temperature for [100] direc-tion, showing spontaneous magnetization of 3 Bohrmagnetons at low temperatures.

FIG. 7. - Solid curves, magnetization vs temperaturefor H Il [001]. Broken curves, same for [100] to indi-cate reversal of direction of easy magnetization. Also,ao for [001 ].

FIG. 8. - Magnetization curves of HoFeO3 in variouscrystallographic directions at 4.2 0 K.

The crystal is isomorphous with GdFe03, thestructure of which was determined by Geller [6].It is orthorhombic, and axes are chosen so thata b c. Magnetization vs temperature wasmeasured at several field strengths in four direc-tions in the orthorhombic crystal, namely [100],

[010], [001] and [110]. Results are given in

figures 5 to 8.The direction of spontaneous parasitic magne-

tization at room temperature is [001], and itsmagnitude is about nB = 0. 04 Bohr magneton permolecule. Superposed on this is the rare-earthparamagnetism, the total magnetization being :

as pointed out by Pauthenet and Blum [7]. On

cooling to about 60 °K, the direction of so changesand becomes parallel to [100], and its magnitudeincreases rapidly to about 3 Bohr magnetons at theJowest temperature.

In our experiments we measure only the com-ponent of magnetization parallel to the appliedfield, designated a. The direction in which a givenfield will give rise to the greatest 6 we will callthe direction of easy magnetization, and this willdepend not only on the spontaneous magneti-zation aobut also on the induced magnetization xH.At room temperature [001] is the direction of easymagnetization in all fields used. At 77 °K a isgreatest Il [100] in weak fields, and Il [010] instrong fields.At 4,DK the spontaneous magnetization is il [100]

and is about 2 Bohr magnetons per molecule. Atthis temperature the spontaneôus magnetizationcannot easily be rotated out of the [100] directioninto the [001] direction, but it can readily be rotat-ed into the [010].The higher values of a obtained in the [010] direc-

tion in high fields would be expected if the spinsof the Fe3+ ions lie in the (010) plane at ratherlarge angles to the [100] direction, and are held bysuch anisotropy forces that they can be rotatedmuch more easily toward [010] than toward [001].When H Il [010] the susceptibility X increases

with decreasing temperature down to about 5 OK,so it is concluded that the Ho3+ ions are respon-sible for the paramagnetism in the way generallyassumed. A value of t4ff = 10. 9 is derived fromthe curve of 1/x vs T when T > 60 OK.The maxima in the 6 vs T curves for the [010]

direction, at about 4 OK, suggest ordering ofthe H03+ ions belowthis temperature. They mightalso be accounted for by a change in the anisotropyin the (001) plane with temperature, a higher aniso-tropy corresponding to a stronger fixing of

Co Il [100] at lower temperatures, would mean thata given field [010] would cause less rotation inthis direction and consequently lower 6’s wouldbe observed. The curve of a vs T at H = 1 200oersteds bends downward below 4 OK and extrapo-lates approximately to zero at 0 OK, as one expectsbelow an antiferromagnetic transition.A specimen of polycrystalline HoFeO3 showed no

maxima on curves of 6 vs T, with constant H,and only a small amount of spontaneous magne

397

tization. The large values of x and the ease ofrotation of 6o apparently obscure the effects readilyobs erved in single crystals.

Koehler and his colleagues at Oak Ridge havefound by neutron diffraction that below 4 OK themoments of the Ho3+ ions are ordered Il [010],the Fe3+ ions being il [100]. These results fit in

nicely with our magnetic results, whichshow,ao Il [100]. We are indebted to Dr. Koehlerfor permission to quote these results.Measurements were also made on powdered

H0203 and Er203, in which Koehler et al. [8]. observed some kind of ordering at helium tempe-ratures by neutron diffraction. Our magnetic dataindicate the possibility of antiferromagnetic order-ing below 4 OK in Er203 but show no evidence ofthis in H02Q3. It seems probable, however, thatthe high value of x in the Ho 3+ ion would obscure atransition in the latter material just as it did inpowdered HoFe03. An indication of antiferro-magnetism in powdered ErFe03 at 4 °K has alreadybeen reported [4].

Experiments have also been made on single crys-tals of the orthoferrites of the rare-earth ions fromSm to Lu. We are greatlyr indebted to J. P. Remeika for

supplying the single crystals of HoFeO3 and otherorthoferrites, which were grown [9] from a Pb 0melt,. and to H. J. Williams, M. A. Gilleo,P. W. Anderson and R. C. Sherwood for discussionof the properties of HoFe03 and other rare earthorthoferrites.

Polycrystallîne PbO. AlxFe12-x018

E. W. Gorter [10] has proposed a model of thearrangement of spins in barium ferrite,BaO.6Fe20a, which has the same structure as

magnetoplumbite Pb0.6Fe203. By replacingvarious amounts of Fe in magnetoplumbite by Al,and by measuring the saturation magnetizationdown to liquid helium temperatures, it was hopedthat the model could be tested and the behaviorof Al in these crystals determined: Measurementsof magnetic saturation at room temperature insolid solutions of Al in Ca and Ba ferrites, by VanUitert and Swanekamp [11] have already indi-cated that Al replaces Fe in octahedral sites.inthese materials.A series of crystals was prepared by J. P. Re-

meika [9] by cristallization from molten PbO,with x = 0 to 8. For ease in attaining saturationthe specimen was prepared from rather coarsecrystalline powder ; this was placed in a strongfield at room temperature and vibrated to alignthe crystals as nearly as possible so that the direc-tion of easy magnetization was parallel to thedirection of the field to be used during measu-rement. Extrapolation of a to its value at T = 0

and = oothen left an uncertainty of only a fewper cent of the value for pure magnetoplumbite.The results are shown in figure 9. The broken

lines correspond to various§possibilities of replace-ment of Fe by Al, to be discussed below.According to Gorter’s model, based on the struc-

ture of Adelskôld [12], the Fe3+ ions in

PbO. Fe 12018 are arranged in layers perpendicularto the hexagonal axis, and the spins in each layerare parallel to each other and antiparallel to thespins in the adjacent iron layers. Starting withthe layer containing Pb, the number of Fe ions insuccessive layers, and the coordination number ofeach Fe ion in the layer, may be described as

follows : 1(5), 1(6), 3(6), 1(4), 1(6), 1(4), 3(6)and 1(6), then repeated. (The coordination num-ber of 5 gives only a rough idea of the oxygen envi-ronment of the iron atoms in the Pb plane.) Thespins corresponding to the 12 Fe ions in these1 ayers are then :

and the net moment is 20 Bohr magnetons. Ourobserved moment extrapolated to T = 0, H = oo,is about 19. For simplification of discussion letus designate the layers (beginning with the layercontaining Pb), and the number of Fe ions in otherlayers, together with their spin directions, in theorder of their distance from the first layers, as

follows :

If Al replaced Fe ions in the C positions (octa-hedral), the moments should lie on the straightline C of figure 9, as in fact they do until about

FIG. 9. - Variation of saturation in magnetoplumbitewith Ai content. See text for assumptions correspond-ings to broken lines.

x = 3 when half of the C ions are replaced. Repla-cement by Al of the tetrahedral iron ions in layerD or the octahedral ions in B would give line BD,and random replacement of Fe by Al would giveline R. In the magneto-plumbite structure the Alevidently prefers the octahedral position, if we

accept the assumptions of Gorter.

398

The observed moment goes to about zero whenx = 6. There are several ways of explaining thisf act, but the data are inconsistent with the repla-cement of all 6 C ions of Fe by Al.When x = 8 the moment is greater than for

x = 6. There are also a number of ways in whichthis may be explained if we do not have direct

knowledge, such as that obtainable by X-ray orneutron diffraction, of the positions of the Alatoms. It seems probable, however, that aa goesthrough zero near x = 6, and that therefore theorderly arrangement of Al ions persists beyond thispoint. It is planned to test this point by diffrac-tion studies, and to measure specimens with othervalues of x.The magnetoplumbite structure has not been

obtained from PbO melts beyond x =8. It maybe that the random distribution of Al and Featoms makes the structure unstable and that theoc-AI,0.3 structure, with some Fe2o3 in solid solution,is then the stable phase in PbO solution.

Ilmenite Structures

Bizette and Tsai [13] have shown that a naturalspecimen of ilmenite, FeTiOs, has a Néel point ofabout 65 °K. Shomate [14] had previously prepar-ed a specimen and observed a heat effect at about60 oR.

FIG. 10. - Magnetization of FeTio3 containing 2 and12 atomic per cent of Fe103.

Nagata, Akimoto and Ishikawa [15] in a seriesof articles, have shown that some of the solid solu-tions of FeTio3-Fe2o3 are strongly ferromagneticat room temperature. In the present measure-ments the temperature range has been extended to

1.3 OK, and Bohr magneton numbers determined.These results have already been reported brieflyand their implications discussed [16].Our measurements of prepared ilmenite contain-

ing by analysis an impurity of about 2 mole percent of Fe2O3, and of a solid solution containing12 mole per cent Fe 203 in FeTi03, are shown infigure 10. The Néel point of the former materialis about 40 OK, somewhat lower than that pre-viously reported, and ferromagnetism is observedbelow 4 OK. The 12 per cent material shows strongferromagnetism, and a marked maximum in thecurves at about 20 °K.

Values of the molar moment in a field of10 800 oersteds have been plotted for a number ofcompositions in figure 11 for the temperatures

FIG. 11. - Magnetization of solid solutions of FeTiO,-Fe203 series at various temperatures (H = 10 800).

300, 77, and 4 oK, and additional data of Akimotofor natural crystals at room temperature havebeen added. In determiningthe saturation magne-tization the procedure has been followed of extra-polating the moment at low temperatures to infi-nite field when the molar moment was large (great-er than 4 000 when H = 10 800) and to H = 0when the moment was small. This is based on theassumption, possibly not always valid, that para-magnetism is superposed on ferromagnetism whenthe moment is small, and that it is the anisotropywhich prevents saturation when the material is

strongly magnetic. Bohr magneton numbers

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determined from saturation at low temperaturesare given in figure 12.

Fm. 12. - Bohr magneton number of FeTi03-Fe2o3, ascompared with proposed spin model.

In the neighborhood of 40 mole per cent Fe20.,the saturation depends markedly on cooling rate,as illustrated in figure 13. In tbe quenched spe-

FIG. 13. - Effect of cooling rate on magnetization of

specimen with 42 atomic per cent Fe2o3.

cimen the maximum in the a vs T curve has disap-peared ; presumably the arrangement of Fe andTi ions is then more disordered and the resem-blance to the regular FeTio3 structure is less.

Several models of the ordering of spins in FeTi03and its solid solution have been proposed [17].Shirane, Nathans, and Pickart have investigatedthis material with neutron diffraction and havefound that the unit cell must be doubled in thedirection of the rhombohedral axis, as comparedwith the " X-ray " cell. This is to be expected ifthe (puckered) planes of Fe ions, perpendicular toaxis, are each ferromagnetically coupled withinthe plane, the planes being antiferromagnetically

coupled through intervening oxygen ions with

neighboring Fe planes. The planes of metal ionsthat cut one X-ray cell would then have the sequence(Ti) (+ Fe) (Ti) (- Fe) (Ti) (+ Fe) along the axis,and there would have to be twice as many planesin the unit cell :

to satisfy the spin-repetition distance.Shirane, Nathans, and Pickart have also found

that when a substantial amount of Ti is replaced byFe the spin pattern changes, as inferred from themagnetic data [16], and is consistent with the

sequence (+ Fe) (- Fe, Ti) (+ Fe) (- Fe, Ti)(+ Fe) - Fe, Ti). We are greatly indebted tothese authors for permission to quote their resultsbefore publication.As the composition of the solid solution changes,

the Bohr magneton numbers resulting from thelast sequence would fall on the straight line offigure 12, if we assign 4 Bohr units to each Fe++ion. This sequence may be expected to apply tothe FeTiOS-Fe2O3 solid solution series until theTi ions become too few to establish an orderedstructure. This is in qualitative accord with thedata.Two other compounds having the ilmenite struc-

ture, CoMnO3 and NiMn03, have been prepared bySwoboda, Toole and Vaughan [18] and measuredby Bozorth and Walsh [19]. Their Bohr magnetonnumbers are respectively 0. 72 and 0.61. Thesenumbers may be explained by the followingsequence of (puékered) planes of metal ions alongthe rhombohedral axis : (+ Co) (- Mn) (+ Co) (- Mn), the signs indi-

cating the sense of the spins. Although the spin-only moments of these ions are equal, the orbitalmoments of Co ions are difficult to quench and theeffective moments of the ions of the two metals,derived from paramagnetic data, differ by aboutthe observed moment of CoMnO3. This compoundmay thus be said to have " orbital ferrimagne-tism ".

If spin-only moments occurred in NiMnOg themoment would be one Bohr unit par molecule. Ifthe orbital moments of the Ni ions are not comple-tely quenched the moment becomes somewhat lessthan one unit per molecule. The observed valueof 0.61 is quite consistent with this point of view.

Prince and Abrahams have made a preliminarystudy of CoMnO3 by neutron diffraction, and havefound that the unit rhombohedral cell that isfound by neutrons is the same as that found byx-rays. The cell is not doubled as it is for FeTi03,and the sequence given above for CoMn03 is consis-tent with all known data. We appreciate permis-sion to quote the results of Prince and Abrahamsbefore publication.

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Fluorides

The fluorides examined were CuF2.2H2O, NnF3,and CrF3. The first of these, as reported byBozorth and Nielsen [20], becomes antiferromagne-tic at 26 OK. Susceptibilities above this tempe-rature follow the law X. = 0.46 j(T + 37), andueff =1.9, in good agreement with the momentsreported for other Cu++ salts. In accordance with

simple theory ,at 0 OK Xm has about 2/3 of itsvalue at the Néel point. Single crystals (mono-clinic) are Ebeing prepared for measurement in dif-ferent crystallographic directions.Antiferromagnetism in MnF3 was found [20]

after we were told informally by W. C. Koehlerand E. O. Wollan of Oak Ridge National Labora-tory that they, with H. R. Child and M. K. Wil-kinson, had detected magnetic ordering near 45 OKby means of neutron diffraction. According toour measurements Os = 47 OK, and above thistemperature the molar susceptibility is given byxm = 3. 10 i(T - 8), from which is derivedv,fi == 5. 0. The latter result is in fair agreementwith that of Klemm and Krose [21] who workedabove 90 OK, and is to be compared with the value4.9 reported by Nyholm and Sharpe [22]. It issomewhat unusual to find ON so much greater thanthe value of 6p = 8 appearing in the Curie-Weissexpression.CrF3 has been examined by Bizette and Tsai [23]

down to 70 °K and reported to become ferromagnet-ic at about 75 °K on cooling. Our measurementsenable us to determine the ferromagnetic momentextrapolated to 0 °K. Extrapolation against fieldstrength was to H = 0 ; this was done because theferromagnetic moment was small (parasitic) andQ in high fields varied linearly with H. Results,

FIG. 14. - Magnetization of CrF3, showing parasitic ferro-magnetism at low temperatures.

plotted in figure 14, show nB = 0. 03. Above theCurie point, 0 = 74 oK, Xm =2.07/(T + 160),corresponding to ueff = 4.1. This value is some-what higher than that derived from the data ofBizette and Tsai [23] and of Klemm and Krose [21].

For this material the intercept of the extrapo-

lated 1 /)(m vs T line on the T axis at -160 OK,an unusual situation for a material that becomesferromagnetic and more characteristic of an anti-ferromagnetic material. This material is bestconsidered as an antiferromagnetic in which, bysome mechanism not yet clear, the balance be-tween 2 sublattices is not exact and a small (para-sitic) ferrimagnetic moment persists.The work on CuF2.2H2O has been done in

cooperation with Dr. J. W. Nielsen. The MnF3was supplied by Dr. R. G. Shulman, and was thesame material as that used by him and Jacca-rino [24] in their measurements of nuclear reso-nance. We are indebted to Dr. W. C. Koehlerand Dr. E. 0. Wollan for making known to ustheir diffraction data for this material beforepublication. Investigation of CrF3 was carriedout in cooperation with Dr. K. Knox. who suppliedthe material and contributed to the discussion.

In aU of the work described in this paper wehave had the able assistance of A. J. Williams.

BIBLIOGRAPHY

[1] MAYS (J. M.), Phys. Rev., 1957,108,1090.[2] BJÔRLING (C. O.) and WESTGREN (A.), Geol. Fören.

Stockholm Fôrh., 1938, 60, 67 ; Strukturbericht, 1941,VI,107.

[3] ITO (T.) and MORI (H.), Acta Cryst., 1951, 4, 412.[4] BOZORTH, WILLIAMS and WALSH, Phys. Rev., 1956,

103, 572.[5] GILLEO (M. A.), J. Chem. Phys., 1956, 24, 1239.[6] GELLER (S.), J. Chem. Phys., 1956, 24, 1236.[7] PAUTHENET (R.) and BLUM (P.), C. R. Acad. Sc., 1954,

239, 33.[8] KOEHLER (W. C.), WOLLAN (E. O.), WILKINSON.

(M. K.) and CABLE (J. W.), Bull. Amer. Phys.Soc., II, 1957, 2, 127.

[9] REMEIKA (J. P.), J. Amer. Chem. Soc., 1956, 78, 4259[10] GORTER (E. W.), Philips Tech. Rev., 1952, 13, 194.[11] VAN UITERT (L. G.) and SWANEKAMP (F. W.), J. Appl.

Physics, 1957, 28, 482.[12] ADELSKÖLD (V.), Ark. Kemi Mineral. Geol., 1938,

12 A, n° 29, 1.[13] BIZETTE (H.) and TSAI (B.), C. R. Acad. Sc., 1956,

242, 2124.[14] SHOMATE (C. H.), J. Amer. Chem. Soc., 1946, 58, 964.[15] NAGATA (T.), Nature, 1953, 172, 850. ISHIKAWA (Y.)

and AKIMOTO (S.), J. Phys. Soc., Japan, 1957, 12,1083, and additional references therein.

[16] BOZORTH, WALSH and WILLIAMS, Phys. Rev., 1957,108, 157.

[17] See the discussion in ref. 16.[18] SWOBODA, TOOLE and VAUGHAN, J. Phys. Chem.

Solids, 1958, 5, 293.[19] BOZORTH and WALSH, J. Phys. Chem. Solids, 1958, 5,

299.[20] BOZORTH (R. M.) and NIELSEN (J. W.), Phys. Rev.,

1958, 110, 879.[21] KLEMM (W.) and KROSE (E.), Z. anorg. Chem., 1947,

253, 226.[22] NYHOLM (R. S.) and SHARPE (A. G.), J. Chem. Soc.,

1952, 3579.[23] BIZETTE and TSAI, C. R. Acad. Sc., 1940, 211, 252.[24] SHULMAN (R. G.) and JACCARINO (V.), Phys. Rev.,

1958, 109, 1084.

401

DISCUSSION

Dr. Koehler (Comments). - The neutron diffrac-tion measurements on HoFe03 and ErFe03 whichhave been made at the Oak Ridge National Labo-ratory are complementary to the results whichDr. Bozorth has presented. At temperaturesabove room temperature the Fe3+ ions in thosecompounds are ordered antiferromagnetically withthe axis of alignment parallel or nearly parallel tothe orthorhombic [100] direction. At tempera-tures below 43 °K the direction of alignment isobserved ton change ta a [110] plane The directionof ao is thus normal to the direction of antiferro-magnetic alignement. At very low temperatures,both the rare earth and iron ions are ordered,approximately antiferromagnetically, and it is

possible, from the interference of the scatteringby thé two systems to determine the directions oforientations of the moment and their magnitudes.Tbe. Er3+ ions in ErFe03 form a nearly ideal anti-ferromagnetic configuration in which a chain ofparauel moments is surrounded by four chains ofoppositely directed moments at nearest neigbbordistances. The erbium ion moments are paralleland antiparallel to [0011, approximately, and at1.25 oK have a magnitude of 5.8 PB. The ironions are oriented parallel, approximately, to [110].In HoFe03 the Ho3+ ions are found in a distortedantiferromagnetic array in which at 1.25 OK eachHo+8 ion moment with magnitude of 7.5 PB.makesan angle, in the (001) plane of about 270 with the[010] direction so as to produce a ferromagneticmoment of 3,4 uB parallel to [100]. The Fe3+moments are approximately parallel or antiparal-lel to [001].

Mr. Kawai. - I should like to make a shortcommunication in connection with one of Dr. Bo-zorth’s studies. About 4 years ago my colleagueDr. Kume and 1 were also interested in the magne-tic character of the solid solutions between hema-tite and ilmenite. In 1955 [1] Kume publishedthe results which are slightly different from whatyou have presented. just now. The difference isthat the point at which the intensity of magneticmoment begins to fait in the Ti-rich region of thesolid solution, slightly shifts towards pure ilmenite,the maximum therefore appearing at a point ofabout 5 mol % or so.The samples we used are, in most cases, natural

specimens which have been made in nature undervery high pressure but obviously at low tempera-ture. Ishikawa and Akimoto’s results as well asyour study indicate that the ordering of the solid-solutions makes as increase. On this point ofview, the natural samples we used are much morefavorable in getting the ordering, because both thetemperature and the time are quite favorable.

Taking this in mind, the difference above men-tioned is reasonably explained.

Next, we find Néel-point-like anomalies on thethermomagnetic curve over nearly the wholeregion of the solid-solution. Those points occurat about 2000 higher than the respective Curie-points. I think, in your results, similar kind ofanomalies seem to appear which is very interest-ing. Although we have not yet obtained the finalconclusions, have you some model explaining these .facts ?

[1] SHOICHI Kumz, Proc. Jap. Acad. 1955, 31, 709.Mr. Bozorth. - The shift of the maximum of

saturation toward lower Fe20S contents, in speci-mens subjected to geological aging, seems reason-able to nie. One expects the ordering to be morecomplete, and we have found slow cooling to givean effect in the same direction at higher Fe20.,concentrations.The Néel temperature in FeTiO.3 has an obvious

counterpart in slowly cooled (ordered) ferromagne-tic solid solùtions containing some Fe2O3, I believethat there is a close relation between these twomaxima in the a vs T curve, but 1 do not now knowthe relation in any detail.

Mr. Nagamiya. -r- As I have written in mymanuscript, Ishikawa observed the temperatureof ordering of Ti and Fe in hematite-ilmenite solu-tions. It decreases rapidly towards the ilmeniteside. It is possible, therefore, that the orderingin artificial samples is imperfect. Have you anyidea about the origin of the Néel-point-like ano-maly you also observed ?

Mr. Bozorth. - I believe the ordering in theartificial materials is not perfect, even when slowlycooled. The experiments show that slow cooling(ordering) enhances the Néel-point-like anomaly,and this suggests a close connection, about whichI have no detailed picture to suggest.Mr. McGuire. -- Did you notice any magnetic

field dependance of the susceptibility in the anti-ferromagnetic materials that you have measured,for example MnF3 ?

Mr. Bozorth. - No dependence of x on H wasnoted in any of the fluorides we investigated exceptCrF3, where the ferromagnetism was reported.Linearity was not tested at the lowest temperaturewhere saturation effects might be expected.

Mr. Hulliger. - Must the slight deviation ofthe spin direction from the ideal crystalline axesin HoFeO3 (and other perovskites) not be causedin fact by a real distorsion of the lattice ? (Youhave drawn the lattice as ideal).

Mr. Koehler. - Yes. The figure as shown doesnot clearly display the distorsions. The struc-ture is realy orthorhombic and there are in thestructure two sets of non-équivalent oxygen ions.