Nanoparticles of superconducting YBa2Cu3O7-delta (YBCO) (Tc = 91 K) exhibit
ferromagnetism at room temperature while the bulk YBCO, obtained by heating the
nanoparticles at high temperature (940 degree C), shows a linear magnetization
curve. Across the superconducting transition temperature, the magnetization
curve changes from that of a soft ferromagnet to a superconductor. Furthermore,
our experiments reveal that not only nanoparticles of metal oxides but also
metal nitrides such as NbN (Tc = 6 - 12 K) and delta-MoN (Tc ~ 6 K) exhibit
room-temperature ferromagnetism.Comment: 11 pages, 6 figure

A. Gomathi

A. Sundaresan

Abraham

Blinov

C.N.R. Rao

Crespo

Esquinazi

Gomathi

Hong

Kugel

Pemmaraju

Shipra, 

Sundaresan

Venkatesan

Young

English

arXiv

Nanoparticles of superconducting YBa2Cu3O7&#8722;&#948; (YBCO) (Tc=91 K) exhibit ferromagnetism at room temperature while the bulk YBCO, obtained by heating the nanoparticles at high temperature (940&#176;C), shows a linear magnetization curve. Across the superconducting transition temperature, the magnetization curve changes from that of a soft ferromagnet to a superconductor. Furthermore, our experiments reveal that not only nanoparticles of metal oxides but also metal nitrides such as NbN (Tc=6-12 K) and &#948;-MoN (Tc~6 K) exhibit room-temperature ferromagnetism

Shipra,

Gomathi, A.

Sundaresan, A.

Rao, C. N. R.

Publications of the IAS Fellows

 1
Room-temperature ferromagnetism in nanoparticles of              
   Superconducting materials 
 
Shipra, A. Gomathi, A. Sundaresan* and C. N. R. Rao* 
    Chemistry and Physics of Materials Unit and 
Department of Science and Technology Unit on Nanoscience,  
Jawaharlal Nehru Centre for Advanced Scientific Research,  
Jakkur P. O., Bangalore 560 064 India 
       
Abstract 
 Nanoparticles of superconducting YBa2Cu3O7-δ (YBCO) (Tc = 91 K) exhibit 
ferromagnetism at room temperature while the bulk YBCO, obtained by heating the 
nanoparticles at high temperature (940o C), shows a linear magnetization curve. Across 
the superconducting transition temperature, the magnetization curve changes from that of 
a soft ferromagnet to a superconductor. Furthermore, our experiments reveal that not only 
nanoparticles of metal oxides but also metal nitrides such as NbN (Tc = 6 - 12 K) and δ-
MoN (Tc~ 6 K) exhibit room-temperature ferromagnetism. 
 
 
PACS:  75.30.Pd; 71.24.+q; 74.72.Bk  
Keywords: A. ferromagnetism; A. nanoparticles; A. superconductors 
 
*Electronic address: sundaresan@jncasr.ac.in 
*Electronic address: cnrrao@jncasr.ac.in 
 
 
 
 
 
 2
 
 
1. Introduction  
It is well known that the materials with partially filled d or f shells exhibit 
interesting magnetic properties. In the recent years, ferromagnetism above room 
temperature has been observed in several materials containing no unpaired d or f 
electrons. High-temperature ferromagnetism has been reported in alkaline-earth 
hexaboride.1 Ferromagnetic hysteresis and high Curie temperature was observed in 
proton irradiated graphite.2 Nanoparticles of gold capped with thiol were shown to 
exhibit ferromagnetism at room temperature.3 Thin films of many oxides with no 
unpaired electrons are also reported to be ferromagnetic at room temperature.4,5 It was 
suggested that defects or oxygen vacancies could be responsible for the observed 
ferromagnetism.  Ab initio electronic structure calculations using density functional 
theory in HfO2 have shown that isolated halfnium vacancies lead to ferromagnetism.6 
Meanwhile, there is a conflicting report attributing the ferromagnetism in HfO2 to 
possible iron contamination while using stainless-steel tweezers in handling thin films.7 
Very recently, we have reported room-temperature ferromagnetism in 
nanoparticles of nonmagnetic oxides such as CeO2 and Al2O3.8 The origin of 
ferromagnetism in these nanoparticles has been suggested to be due to magnetic moments 
arising from oxygen vacancies at the surfaces of the nanoparticles. We also suggested 
that all oxide nanoparticles would exhibit ferromagnetism at room temperature. These 
results motivated us to study whether the ferromagnetism occurs in nanoparticles of 
superconductors in the normal as well as the superconducting states, since the phenomena 
 3
of ferromagnetism and superconductivity are antagonistic. For this purpose, we selected 
the well known oxide high temperature superconductor, YBa2Cu3O7-δ (YBCO). We also 
examined whether the ferromagnetism exists in the nanoparticles of metal nitrides such as 
NbN and δ-MoN which are also known to be superconductors with low Tc. Interestingly, 
we found room-temperature ferromagnetism not only in the nanoparticles of oxide 
superconductor but also in the metal nitrides. 
2. Experimental 
Nanoparticles of YBCO were prepared by the citrate-gel method described in the 
literature except that the sample was cooled from 790o C to room temperature at a faster 
rate (5o C/min) than the reported one (1o C/min).9 The nanoparticles of NbN were 
prepared by heating NbCl5 with urea in 1:6 molar ratio at ~ 1000o C.10 The nanoparticles 
of δ-MoN were prepared by heating γ-Mo2N nanoparticles which in turn were prepared 
by the urea method described elsewhere.11 Powder x-ray diffraction (XRD) was used to 
identify the phase and its purity. The particle size and morphology were studied by Field 
Emission Scanning Electron Microscopy (FESEM) and Transmission Electron 
Microscopy (TEM). Magnetization measurements were carried out with vibrating sample 
magnetometer in physical property measuring system (PPMS, Quantum Design, USA). 
3. Results and discussion 
The XRD pattern of nanoparticles of YBCO showed the orthorhombic structure 
with a = 3.829 Å,   b = 3.874 Å and c = 11.670 Å. The XRD pattern of NbN 
nanoparticles was consistent with the cubic lattice with a = 4.44 Å. The structure of δ-
MoN was confirmed to be hexagonal with a = 5.68 Å and c = 5.56 Å. The particle size of 
 4
YBCO ranges from 100 - 200 nm as seen from the image of FESEM (Fig. 1). The 
particles size of δ-MoN and NbN were 20 and 30 nm, respectively.10 
Fig. 2 shows zero-field cooled (ZFC) and field cooled (FC) susceptibility data as a 
function of temperature. It can be seen that nanoparticles of YBCO exhibit 
superconducting diamagnetism with a critical temperature, Tc = 91 K similar to that of 
bulk YBCO despite the faster cooling rate adopted in this work. In contrast to bulk 
YBCO, the reversible region in the nanoparticles extends down to 40 K. This is due the 
increased threshold for flux penetration in the nanoparticles due to Bean-Livingston 
surface barrier.12 The magnetization versus field curve measured at 5 K is shown in the 
inset of the Fig. 2. The field decreasing part of the magnetization remains negative, 
consistent with the granular nature of the superconductor where the flux pinning is also 
weak compared to the bulk or single crystals. 
Intriguingly, the room-temperature magnetization of YBCO nanoparticles show 
hysteresis, with a coercivity of ~ 200 Oe, typical of ferromagnetic behavior as shown in 
Fig. 3. This observation is consistent with our prediction that all oxide nanoparticles 
would exhibit ferromagnetism.8 The origin of ferromagnetism is likely to be due to 
magnetic moments arising from the oxygen vacancies at the surfaces of the nanoparticles.  
A bulk YBCO was prepared by pressing the nanoparticles into rectangular bars and 
annealed at high temperatures. The magnetization measurement at room temperature 
revealed paramagnetic behavior (Fig. 3) which is typical of bulk YBCO. This supports 
the suggestion that the ferromagnetism is confined to the surface of the nanoparticles. It 
would be interesting to explore whether such surface ferromagnetism exists in the 
superconducting state of the nanoparticles. For this purpose we measured M(H) across 
 5
the superconducting transition. At Tc (= 91 K), the ferromagnetic hysteresis still remains 
with increased coercivity (300 Oe). At 90 K, the M(H) data is shown in the inset of Fig. 
3. It can be seen that the M(H) behavior is that of a superconductor rather than that of 
ferromagnetism. In fact, it would be difficult to infer the presence of ferromagnetism 
below Tc from the magnetization measurement.  
NbN nanoparticles showed Tc in the range 6 - 12 K depending on the preparative 
conditions. The diamagnetic susceptibility and M(H) curve in the superconducting state 
are shown in the inset of Fig. 4. More importantly, they also exhibit room-temperature 
ferromagnetism with coercivity ~ 500 Oe as seen in Fig. 4. In order to compare this result 
with bulk or bigger particles of NbN, the nanoparticles were pressed into bars and heated 
at high temperature (920oC) for a prolonged time in ammonia. This treatment gave an 
average particle size of ~ 200 nm. The results of the magnetic measurement are shown in 
Fig. 5.  It can be seen that the magnetic hysteresis measured in the superconducting state 
(T=3.3 K) is that of a bulk sample. Further, the magnetization measured at room-
temperature shows essentially a linear behavior but for a small hysteresis and step at very 
low fields which arises from the tape used to fix the sample with the quartz sample 
holder.  However, it is remarkable to note the M(H) behavior of smaller (Fig. 4) and 
bigger (Fig. 5)  particles of NbN which demonstrate that the ferromagnetism is associated 
with the smaller particles. Further, as there was no oxide impurity phase in the x-ray 
diffraction, we suggest that the observed ferromagnetism is intrinsic to the nanoparticles 
of NbN. This suggests that there can be vacancies of nitrogen at the surfaces of the 
nanoparticles that can also lead to creation of magnetic moment. In fact, we have 
observed room-temperature ferromagnetism in CdS and CdSe nanoparticles. Similarly, 
 6
room-temperature ferromagnetism and superconducting diamagnetism are observed in 
nanoparticles of δ-MoN as shown in Fig. 6. These results demonstrate that 
ferromagnetism may be a universal feature of nanoparticles of most compounds. 
 
4. Conclusions 
Room-temperature ferromagnetism is observed in nanoparticles of 
superconducting YBCO as well as metal nitrides. It requires further studies to explore the 
presence or absence of ferromagnetism in the superconducting state. The observation of 
room-temperature ferromagnetism in a variety of nanoparticles such as oxides, nitrides 
and chalcogenides suggests that ferromagnetism may be a universal feature of 
nanoparticles of most compounds. 
 
5. Acknowledgement 
We thank C. Madhu for his assistance in preparing some of the samples.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 7
References 
 
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Figure Captions 
 8
1. FESEM image of YBCO nanoparticles. Particle size ranges from 100 – 200 nm. 
2. Volume susceptibility as a function of temperature showing the superconducting 
transition around 91 K. Note the absence of irreversibility between ZFC and FC   
curves  down to 40 K. Inset shows the M(H) curve at 5 K. 
3. M(H) data of YBCO nanoparticles at 300 K and 91 K showing the ferromagnetic  
behavior. M(H) data of bulk YBCO is also shown for comparison. Inset shows the 
hysteresis at 90 K which is typical of a superconductor. 
4. M(H) data at 300 K for NbN nanoparticles (30 nm) showing ferromagnetic behavior. 
Inset at the bottom shows the superconducting diamagnetic onset around 6 K and the top 
inset shows the hysteresis in the superconducting state. 
5. M(H) data at 300 K for NbN particles with an average size of ~ 200 nm showing a 
paramagnetic behavior. Hysteresis at 3.3 K shown in the top inset is typical of a bulk 
superconductor. The diamagnetic transition is shown in the bottom inset. 
6. M(H) data at 300 K for δ-MoN. Inset shows the diamagnetic response under the  
    applied field of 5 Oe. 
 
 
 
 
 
 
 
 
 9
 
Fig1 
 
Fig2 
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Fig3 
 
Fig4 
 
 11
 
Fig5 
 
Fig6 


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Room-temperature ferromagnetism in nanoparticles of superconducting
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