HAL Id: jpa-00217580
https://hal.archives-ouvertes.fr/jpa-00217580
Submitted on 1 Jan 1978
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of
sci-entific research documents, whether they are
pub-lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
SUPERCONDUCTING AND MAGNETIC
PROPERTIES OF ErRh4B4
H. Ott, W. Fertig, D. Johnston, M. Maple, B. Matthias
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C6, supplkment au
no
8, Tome 39, aoat 1978, page C6-375
SUPERCONDUCTING AND MAGNETIC PROPERTIES OF ErRh4B4
K is 3E 35%
H.R. Ott, W.A. Fertig
,
D.C. Johnston,
M.B. Maple and B.T. Matthias Labooratoriw fur Festktirperphysik, ET'HZ, 8093 Ziirich, SwitzerZand*
Physics Department, University o f CaZifornia, La JoZZa, U.S.A.**
Physics Department, University of CaZifornia, La JoZZa and Be22 Laboratories, Murray H i Z Z , U.S.A. RdsumB. Nous avonsmesur6 la dilatation thermique, l'aimantation, la dilatation magnCtostrictive et la rdsistivitd dlectrique du compose ErRh4B aux basses temperatures. Nous prdsentons les rdsultats sur l'influence des champs magnetiques sur 4es propri6tCs supraconductrices et magndtiques et nous discutons le diagramme de phase (H,T).Abstract.- We have measured the thermal expansion, the magnetization, the magnetostriction and the ac electrical resistance of ErRh,B, at low temperatures. Special emphasis is given to the influence of magnetic fields on the superconducting and magnetic properties of this compound and the resulting
(H,T) phase diagram is discussed.
The interplay of long range ordering effects has always been a subject of widespread interest and within the last year,the compound ErRh,B, has attrac-
ted a great deal of attention because of its pecu- liar superconducting and magnetic properties. At zero external magnetic field a transition from the normal to a superconducting state was found at 8 . 7 K
(Tcl) and at 0.9 K (TC2) a second transition backto a normal but magnetically ordered state was observed /1,2/. Neutron scattering experiments /3/ suggested that this ordered state is ferromagnetic and that the lower phase transition is not of first order. Recent calculations / 4 / have given a theoretical background and a partial explanation of the general behaviour of such compounds but several questions considering the very special behaviour of ErRh4B, remained to be answered. To study the interplay of the interactions stabilizing either the superconduc- ting or the magnetically ordered phase it seems of special interest to investigate the influence of ex- ternal magnetic fields on both the su~erconducting and the magnetic properties.
We therefore report on measurements of 1)the magnetic susceptibility and the magnetization between 1.4 K and 20 K in magnetic fields up to
100 kOe. 2) the thermal expansion and the magnetos- triction between 0.4 K and 13 K in magnetic fields up to 15 kOe and 3) the ac electrical resistivity between 50 mK and 10 K in magnetic fields up to
12 kOe. These experiments were made on two polycrys- talline samples of ErRh,B, : sample A for the mea- surements l) and 2) and sample B for the measure- ments 3) respectively. Experimental details and in-
formation concerning the sample preparation aregiven in reference 151.
An important question is that of the thermo- dynamic order of the phase transition at T
.
TheC 2
linear thermal expansion coefficient a shows a small though distinct cusp-like anomaly of the order of
K-', vary similar in shape as the previously found anomaly of the specific heat / 2 / . This
a
ano- maly clearly has to be attributed to the occurrence of the magnetic phase transition since for a usual normal to superconducting phase transition one ex- pects only a small discontinuity in a(T). The tem- pBrature dependence of thea
anomaly and the Lack of any observable hysteresis indicate that this ma- gnetic phase transition is not of first order, in agreement with the results of the neutron experi- ments. It is interesting to note that the peaks cf the specific-heat and the a anomalies are distinctly higher in temperature than the superconducting to normal transition observed in the resistance expe- riments at zero magnetic field. It thus appearsthat over a very small temperature range (% 50 mK) the ferromagnetic and the superconducting state truly coexist in ErRhbBb between T and T (the magneticC 2 m
ordering temperature).
A second and very interesting problem is that of the phase diagram of this material in the (H,T) plane, i.e. the investigation of the boundaries between the normal paramagnetic, the superconducting and the normal ferromagnetic phases. From low field magnetization, magnetostriction and resistivity mea- surements we are able to determine the temperature dependence of the critical field curve of the super-
conducting phase. Since ErRhrB, is a type-I1 super- conductor our measurements yield H (T). Below T
C 2 c1'
HC2 first increases, reaches a maximum of a few kOe around 5 K and then decreases again towards T .The
C 2
exact determination of H is greatly hampered in
C 2
all three cases mentioned above. In the magnetiza- tion and the magnetostriction, strong paramagnetic contributions set in above H and it is very dif-
C 1
ficult to define HC2. In these two cases we defined HC2 where the M(H) (see also figure 1) and E(H) cur- ves became irreversible. In the resistivity measu- rements one has the complication of surface current effects (H ) and this may lead to an overestimation
C 3 of HC2.
Fig. 1 : M(H) for ErRh4B4 at 1.6 K up to 100 kOe.At very low fields the negative magnetization is due to superconductivity.
There is hardly any doubt that the ferromagne- tic phase is due to the long range ordering of the ~ r ion moments. Neutron experiments /3/ revealed ~ + an ordered moment of only 5.6 /Er ion compared to
B
the Hund's rule value of 9pB/Er ion for the free ~ r ions. In figure 1 we show the field dependence ~ + of the magnetization M(H) at 1.6 K and up to 100 kOe At 100 kOe we measure a magnetization consistent with a moment of 7.65 pB/Er ion, again much lower
than the free-ion value. Even in the absence of ferromagnetic order we expect a moment of 8 . 9 9 ~
B
/ ~ r ion. Our result shows quite clearly that crystal- electric-field (CEF) effects are important.At this temperature the frozen flux due to supercurrents is quite remarkable and amounts to 2p /Er ion.B
. r .,
Fig. 2
:x-'
= H/M for ErRhlB4 between 1.4 K and 20 Kin various external magnetlc fields. For 0.5 K the data below Tcl are taken from low field magnetiza- tion results.
In figure 2 we show the inverse susceptibility
x-l = HIM as a function of temperature in different external magnetic fields. The saturation effects, becoming more and more pronounced in higher fields may either be due to Zeeman splitting of the lowest crystal-field level of the ground state multiplet (another manifestation of CEF effects) or they may also indicate theoccurrence of magnetic ordering. In the latter case it would suggest that the parama- gnetic to ferromagnetic phase boundary is shifted to higher temperatures with increasing magnetic fields.
This research was supported by the Schweize- rische Nationalfonds zur ~Zrderung der wissenschaf- tlichen Forschung and sponsored by ERDA contract No.ERDA E(04-3)-34PA227 and by U.S. Air Force con- tract No.F49620-77-C-0009. References /l/Matthias,B.T.,Corenzw&E.,Vandenberg,J.M. AND Barz,H.E.,Proc.Natl.Acad.Sci. U.S.A.
76
(1977) 1334. /2/Fertig,W.A.,Johnston,D.C.,Delong,.L.E.,McCallum R.W.,Maple,M.B. and Matthias,B.T.,Phys.Rev.Lett. 38 (1977) 987./
3 /Gncton ,D .E.
,Mc7&an ,D .E.
,Eckert ,J.
,
Shir ane ,G.and Thomlinson,W.,Phys.Rev.Lett.E (1977)1164.
141 Jarlborg,T.,Freeman,A.J. and Watson-Yang,T.J., Phys.Rev.Lett.
